Performance information

• Dimensionality-Induced Transition from Degenerate to Nondegenerate States in Nb-Doped WSe₂
  Kaito Kanahashi; Itsuki Tanaka; Tomonori Nishimura; Kohei Aso; Anh Khoa Augustin Lu; Satoru Morito; Limi Chen; Takafumi Kakeya; Satoshi Watanabe; Yoshifumi Oshima; Yukiko Yamada-Takamura; Keiji Ueno; Amin Azizi; Kosuke Nagashio
  ACS Nano, Mar. 2025, [Reviewed]
  American Chemical Society (ACS), Scientific journal
  DOI：https://doi.org/10.1021/acsnano.4c17660
  DOI ID：10.1021/acsnano.4c17660, ISSN：1936-0851, eISSN：1936-086X
• Spatial and reconfigurable control of photoluminescence from single-layer MoS2 using a strained VO2-based Fabry–Pérot cavity
  Koyo Nakayama; Shota Toida; Takahiko Endo; Mitsuru Inada; Shingo Sato; Hiroshi Tani; Kenji Watanabe; Takashi Taniguchi; Keiji Ueno; Yasumitsu Miyata; Kazunari Matsuda; Mahito Yamamoto
  Applied Physics Letters, Volume：125, Number：22, Nov. 2024, [Reviewed]
  We investigated the photoluminescence (PL) from single-layer MoS2 on VO2 platelets grown on SiO2, where the insulating and metallic phases can coexist above a bulk transition temperature of 340 K, due to the inhomogeneous strain. We found that the intensity of PL from MoS2 on metallic VO2 is higher than that on the insulating counterpart, resulting in spatially varying PL even at the sub-micrometer scale. In contrast to the intensity, the PL peak energies were observed to be nearly identical on insulating and metallic VO2, indicating that the influences of charge transfer, strain, and dielectric screening on MoS2 are comparable, regardless of the phase state. Thus, the observed difference in PL intensity is due to the difference in refractive indices of insulating and metallic VO2, leading to the phase-dependent Fabry–Pérot interference effect. We performed numerical simulations for the emission from MoS2 supported on the VO2-based Fabry–Pérot interferometer. The calculated emission intensity ratio on insulating and metallic VO2 well reproduces the experimental observations. These results suggest a strategy for controlling PL from two-dimensional semiconductors in a spatial and reconfigurable manner.
  AIP Publishing, Scientific journal
  DOI：https://doi.org/10.1063/5.0236517
  DOI ID：10.1063/5.0236517, ISSN：0003-6951, eISSN：1077-3118
• Contact Properties on a Semiconducting MoTe₂ Crystal Using Polymorphic Structures
  Tianshun Xie; Mengnan Ke; Peter Krüger; Keiji Ueno; Nobuyuki Aoki
  ACS Applied Electronic Materials, Volume：6, Number：9, First page：7026, Last page：7034, Sep. 2024, [Reviewed]
  American Chemical Society (ACS), Scientific journal
  DOI：https://doi.org/10.1021/acsaelm.4c01369
  DOI ID：10.1021/acsaelm.4c01369, ISSN：2637-6113, eISSN：2637-6113
• Realization of single MoTe2 crystal in-plane TFET by laser-induced doping technique　　　　　　　　　　　　　　　
  Tianshun Xie; Mengnan Ke; Keiji Ueno; Kenji Watanabe; Takashi Taniguchi; Nobuyuki Aoki
  Applied Physics Letters, May 2024, [Reviewed]
  Scientific journal
  DOI：https://doi.org/10.1063/5.0197172
  DOI ID：10.1063/5.0197172, ORCID：160223643
• Realization of MoTe2 CMOS inverter by contact doping and channel encapsulation　　　　　　　　　　　　　　　
  Tianshun Xie; Mengnan Ke; Keiji Ueno; Kenji Watanabe; Takashi Taniguchi; Nobuyuki Aoki
  Japanese Journal of Applied Physics, Feb. 2024
  Scientific journal
  DOI：https://doi.org/10.35848/1347-4065/ad16bc
  DOI ID：10.35848/1347-4065/ad16bc, ORCID：148977672
• Single-Gate MoS₂ Tunnel FET with a Thickness-Modulated Homojunction
  Tomohiro Fukui; Tomonori Nishimura; Yasumitsu Miyata; Keiji Ueno; Takashi Taniguchi; Kenji Watanabe; Kosuke Nagashio
  ACS Applied Materials & Interfaces, Volume：16, Number：7, First page：8993, Last page：9001, Feb. 2024, [Reviewed]
  American Chemical Society (ACS), Scientific journal
  DOI：https://doi.org/10.1021/acsami.3c15535
  DOI ID：10.1021/acsami.3c15535, ISSN：1944-8244, eISSN：1944-8252
• Coherent optical response driven by non-equilibrium electron–phonon dynamics in a layered transition-metal dichalcogenide
  Takumi Fukuda; Kotaro Makino; Yuta Saito; Paul Fons; Atsushi Ando; Takuya Mori; Ryo Ishikawa; Keiji Ueno; Jessica Afalla; Muneaki Hase
  APL Materials, Volume：12, Number：2, Feb. 2024, [Reviewed]
  Layered transition-metal dichalcogenides (TMDs) are model systems to explore ultrafast many-body interactions and various nonlinear optical phenomena. For the application of TMD-based optoelectronic devices capable of ultrafast response, it is essential to understand how characteristic electron–hole and electron–phonon couplings modify ultrafast electronic and optical properties under photoexcitation. Here, we investigate the sub-picosecond optical responses of layered semiconductor 2H–MoTe2 in the presence of an electron–hole (e–h) plasma and a long-lived coherent phonon. Transient reflectivity measurements depending on photon energy reveal that the optical response for short-time delays (< 1ps) was significantly modified by band-gap renormalization and state filling due to the presence of the e–h plasma. Furthermore, octave, sum, and difference phonon frequencies transiently appeared for the early time delays (< 2ps). The emergent multiple phonon frequencies can be described as higher-order optical modulations due to deformation-potential electron–phonon coupling under resonant photoexcitation conditions. This work provides comprehensive insights into fundamental physics and the application of non-equilibrium quasiparticle generations on TMDs under time-periodic phonon driving forces.
  AIP Publishing, Scientific journal
  DOI：https://doi.org/10.1063/5.0188537
  DOI ID：10.1063/5.0188537, eISSN：2166-532X
• Silicon–van der Waals heterointegration for CMOS-compatible logic-in-memory design
  Mu-Pai Lee; Caifang Gao; Meng-Yu Tsai; Che-Yi Lin; Feng-Shou Yang; Hsin-Ya Sung; Chi Zhang; Wenwu Li; Jun Li; Jianhua Zhang; Kenji Watanabe; Takashi Taniguchi; Keiji Ueno; Kazuhito Tsukagoshi; Ching-Hwa Ho; Junhao Chu; Po-Wen Chiu; Mengjiao Li; Wen-Wei Wu; Yen-Fu Lin
  Science Advances, Volume：9, Number：49, Dec. 2023, [Reviewed]
  Silicon CMOS-based computing-in-memory encounters design and power challenges, especially in logic-in-memory scenarios requiring nonvolatility and reconfigurability. Here, we report a universal design for nonvolatile reconfigurable devices featuring a 2D/3D heterointegrated configuration. By leveraging the photo-controlled charge trapping/detrapping process and the partially top-gated energy band landscape, the van der Waals heterostacking achieves polarity storage and logic reconfigurable characteristics, respectively. Precise polarity tunability, logic nonvolatility, robustness against high temperature (at 85°C), and near-ideal subthreshold swing (80 mV dec ⁻¹ ) can be done. A comprehensive investigation of dynamic charge fluctuations provides a holistic understanding of the origins of nonvolatile reconfigurability (a trap level of 10 ¹³ cm ⁻² eV ⁻¹ ). Furthermore, we cascade such nonvolatile reconfigurable units into a monolithic circuit layer to demonstrate logic-in-memory computing possibilities, such as high-gain (65 at V_dd = 0.5 V) logic gates. This work provides an innovative 3D heterointegration prototype for future computing-in-memory hardware.
  American Association for the Advancement of Science (AAAS), Scientific journal
  DOI：https://doi.org/10.1126/sciadv.adk1597
  DOI ID：10.1126/sciadv.adk1597, eISSN：2375-2548
• Soft x-ray photoelectron momentum microscope for multimodal valence band stereography
  Fumihiko Matsui; Kenta Hagiwara; Eiken Nakamura; Takayuki Yano; Hiroyuki Matsuda; Yasuaki Okano; Satoshi Kera; Eri Hashimoto; Shinji Koh; Keiji Ueno; Takahiro Kobayashi; Emi Iwamoto; Kazuyuki Sakamoto; Shin-ichiro Tanaka; Shigemasa SUGA
  Review of Scientific Instruments, Volume：94, Number：8, Aug. 2023, [Reviewed]
  The photoelectron momentum microscope (PMM) in operation at BL6U, an undulator-based soft x-ray beamline at the UVSOR Synchrotron Facility, offers a new approach for μm-scale momentum-resolved photoelectron spectroscopy (MRPES). A key feature of the PMM is that it can very effectively reduce radiation-induced damage by directly projecting a single photoelectron constant energy contour in reciprocal space with a radius of a few Å−1 or real space with a radius of a few 100 μm onto a two-dimensional detector. This approach was applied to three-dimensional valence band structure E(k) and E(r) measurements (“stereography”) as functions of photon energy (hν), its polarization (e), detection position (r), and temperature (T). In this study, we described some examples of possible measurement techniques using a soft x-ray PMM. We successfully applied this stereography technique to μm-scale MRPES to selectively visualize the single-domain band structure of twinned face-centered-cubic Ir thin films grown on Al2O3(0001) substrates. The photon energy dependence of the photoelectron intensity on the Au(111) surface state was measured in detail within the bulk Fermi surface. By changing the temperature of 1T-TaS2, we clarified the variations in the valence band dispersion associated with chiral charge-density-wave phase transitions. Finally, PMMs for valence band stereography with various electron analyzers were compared, and the advantages of each were discussed.
  {AIP} Publishing, English, Scientific journal
  DOI：https://doi.org/10.1063/5.0154156
  DOI ID：10.1063/5.0154156, ISSN：0034-6748, eISSN：1089-7623, ORCID：140830833
• Photoinduced Structural Dynamics of 2H-MoTe₂ Under Extremely High-Density Excitation Conditions
  Takumi Fukuda; Uta Ozaki; Samuel Jeong; Yusuke Arashida; Kaito En-ya; Shoji Yoshida; Paul J. Fons; Jun-ichi Fujita; Keiji Ueno; Muneaki Hase; Masaki Hada
  The Journal of Physical Chemistry C, Jul. 2023
  American Chemical Society (ACS), Scientific journal
  DOI：https://doi.org/10.1021/acs.jpcc.3c02838
  DOI ID：10.1021/acs.jpcc.3c02838, ISSN：1932-7447, eISSN：1932-7455
• Ultrafast melting of charge-density wave fluctuations at room temperature in 1T-TiSe2 monitored under non-equilibrium conditions
  Yu Mizukoshi; Takumi Fukuda; Yuta Komori; Ryo Ishikawa; Keiji Ueno; Muneaki Hase
  Applied Physics Letters, Volume：122, Number：24, Jun. 2023
  We investigate the ultrafast lattice dynamics in 1T-TiSe2 using femtosecond reflection pump–probe and pump–pump–probe techniques at room temperature. The time-domain signals and Fourier-transformed spectra show the A1g phonon mode at 5.9 THz. Moreover, we observe an additional mode at ≈ 3 THz, corresponding to the charge-density wave (CDW) amplitude mode (AM), which is generally visible below Tc≈200 K. We argue that the emergence of the CDW amplitude mode at room temperature can be a consequence of fluctuations of order parameters based on the additional experiment using the pump–pump–probe technique, which exhibited suppression of the AM signal within the ultrafast timescale of ∼0.5 ps.
  AIP Publishing, Scientific journal
  DOI：https://doi.org/10.1063/5.0153161
  DOI ID：10.1063/5.0153161, ISSN：0003-6951, eISSN：1077-3118
• Terahertz emission from transient currents and coherent phonons in layered MoSe2 and WSe2
  Jessica Afalla; Joselito Muldera; Semmi Takamizawa; Takumi Fukuda; Keiji Ueno; Masahiko Tani; Muneaki Hase
  Journal of Applied Physics, Volume：133, Number：16, Apr. 2023
  Terahertz (THz) time-domain emission spectroscopy was performed on layered 2H-MoSe 2 and 2H-WSe 2. The THz emission shows an initial cycle attributed to surge currents and is followed by oscillations attributed to coherent interlayer phonon modes. To obtain the frequencies of the interlayer vibrations, an analysis of the THz emission waveforms was performed, separating the two contributions to the total waveform. Results of the fitting show several vibrational modes in the range of 5.87–32.75 cm − 1 for the samples, attributed to infrared-active interlayer shear and breathing modes. This study demonstrates that THz emission spectroscopy provides a means of observing these low-frequency vibrational modes in layered materials.
  AIP Publishing, Scientific journal
  DOI：https://doi.org/10.1063/5.0146489
  DOI ID：10.1063/5.0146489, ISSN：0021-8979, eISSN：1089-7550
• Multilayer In-Plane Heterostructures Based on Transition Metal Dichalcogenides for Advanced Electronics　　　　　　　　　　　　　　　
  Hiroto Ogura; Seiya Kawasaki; Zheng Liu; Takahiko Endo; Mina Maruyama; Yanlin Gao; Yusuke Nakanishi; Hong En Lim; Kazuhiro Yanagi; Toshifumi Irisawa; Keiji Ueno; Susumu Okada; Kosuke Nagashio; Yasumitsu Miyata
  ACS Nano, Volume：17, Number：7, First page：6545, Last page：6554, Feb. 2023
  In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS2 from the edges of mechanically exfoliated multilayer flakes of WSe2 or NbxMo1-xS2. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS2 on the exfoliated flakes. For the WSe2/MoS2 sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the NbxMo1-xS2/MoS2 in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS2. The formation of a staggered gap band alignment of NbxMo1-xS2/MoS2 is also supported by first-principles calculations.
  Scientific journal
  DOI：https://doi.org/10.1021/acsnano.2c11927
  Scopus：https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=85149104150&origin=inward
  Scopus Citedby：https://www.scopus.com/inward/citedby.uri?partnerID=HzOxMe3b&scp=85149104150&origin=inward
  DOI ID：10.1021/acsnano.2c11927, ISSN：1936-0851, eISSN：1936-086X, ORCID：129773033, PubMed ID：36847351, SCOPUS ID：85149104150
• Enhanced contact properties of MoTe₂-FET via laser-induced heavy doping
  Tianshun Xie; Kazuki Fukuda; Mengnan Ke; Peter Krüger; Keiji Ueno; Gil-Ho Kim; Nobuyuki Aoki
  Japanese Journal of Applied Physics, Volume：62, Number：SC, First page：SC1010, Last page：SC1010, Dec. 2022
  Abstract
  
  The doping technique is vital for applying two-dimensional (2D) materials such as transition metal dichalcogenide (TMDC)-based field effect transistors (FETs), which can control the channel polarity and improve the performance. However, as conventional doping techniques for silicon-based FET are not suitable for 2D materials, the realization of heavy doping of TMDC materials is challenging, especially for n-type heavy doping. This study reports a simple, regioselective, controllable, and chemically stable heavy doping method for 2H-MoTe₂ crystal via high-density laser irradiation. The polarity of the doping can be controlled by changing the irradiation environment. For the MoTe₂-nFET, good performance with enhanced contact properties was obtained using the contact doping method via laser irradiation in a vacuum environment.
  IOP Publishing, Scientific journal
  DOI：https://doi.org/10.35848/1347-4065/aca67e
  DOI ID：10.35848/1347-4065/aca67e, ISSN：0021-4922, eISSN：1347-4065
• Is the Bandgap of Bulk PdSe2Located Truly in the Far-Infrared Region? Determination by Fourier-Transform Photocurrent Spectroscopy
  Wataru Nishiyama; Tomonori Nishimura; Masao Nishioka; Keiji Ueno; Satoshi Iwamoto; Kosuke Nagashio
  Advanced Photonics Research, Volume：3, Number：11, First page：2200231, Last page：2200231, Nov. 2022
  Wiley, Scientific journal
  DOI：https://doi.org/10.1002/adpr.202200231
  DOI ID：10.1002/adpr.202200231, ISSN：2699-9293, eISSN：2699-9293
• Diffused beam energy to dope van der waals electronics and boost their contact barrier lowering　　　　　　　　　　　　　　　
  Che-Yi Lin; Mu-Pai Lee; Yuan-Ming Chang; Yi-Tang Tseng; Feng-Shou Yang; Mengjiao Li; Jiann-Yeu Chen; Ciao-Fen Chen; Meng-Yu Tsai; Yi-Chun Lin; Keiji Ueno; Mahito Yamamoto; Shun-Tsung Lo; Chen-Hsin Lien; Po-Wen Chiu; Kazuhito Tsukagoshi; Wen-Wei Wu; Yen-Fu Lin
  ACS applied materials & interfaces, Aug. 2022, [Reviewed]
  American chemical society ({ACS}), English, Scientific journal
  DOI：https://doi.org/10.1021/acsami.2c07679
  DOI ID：10.1021/acsami.2c07679, ISSN：1944-8244, ORCID：118159145
• Photo‐Induced Tellurium Segregation in MoTe 2
  Takumi Fukuda; Ryota Kaburauchi; Yuta Saito; Kotaro Makino; Paul Fons; Keiji Ueno; Muneaki Hase
  physica status solidi (RRL) – Rapid Research Letters, Volume：16, Number：9, First page：2100633, Last page：2100633, Jun. 2022
  Wiley, Scientific journal
  DOI：https://doi.org/10.1002/pssr.202100633
  DOI ID：10.1002/pssr.202100633, ISSN：1862-6254, eISSN：1862-6270, ORCID：114313321
• Mist chemical vapor deposition of Al1−xTixOy thin films and their application to a high dielectric material　　　　　　　　　　　　　　　
  Arifuzzaman Rajib; Abdul Kuddus; Kojun Yokoyama; Tomohiro Shida; Keiji Ueno; Hajime Shirai
  Journal of Applied Physics, Volume：131, Number：10, First page：105301, Last page：105301, Mar. 2022
  {AIP} Publishing, Scientific journal
  DOI：https://doi.org/10.1063/5.0073719
  DOI ID：10.1063/5.0073719, ORCID：109527683
• Intrinsic Electronic Transport Properties and Carrier Densities in PtS2 and SnSe2: Exploration of n(+)-Source for 2D Tunnel FETs　　　　　　　　　　　　　　　
  Yuichiro Sato; Tomonori Nishimura; Dong Duanfei; Keiji Ueno; Keisuke Shinokita; Kazunari Matsuda; Kosuke Nagashio
  ADVANCED ELECTRONIC MATERIALS, Volume：7, Number：12, Dec. 2021, [Reviewed]
  Van der Waals 2D heterostructures are the ideal platform for tunnel field-effect transistors (TFETs) because of dangling-bond-free heterointerfaces. However, the limited selection of n(+)-source materials restricts 2D-TFET research. In this study, intrinsic electron transport properties and carrier density (n) of bulk PtS2 are experimentally examined by Hall measurements to explore its use as an n(+)-source material suitable for TFETs in comparison with SnSe2. Despite the similar maximum depletion widths (W-Dm) and apparently metallic I-D-V-G curves at room temperature for both bulk samples, the Hall measurements elucidate that n approximate to 3.6 x 10(17) cm(-3) in PtS2 is much smaller than approximate to 4.7 x 10(18) cm(-3) in SnSe2. They also reveal that this difference comes from the depth of the donor level below the conduction band. Therefore, although band-to-band tunneling current is successfully observed in the n-PtS2/p-WSe2 TFET, V-G modulates the n-PtS2 source as well as p-WSe2 channel due to the nondegenerate doping of PtS2, resulting in a degraded subthreshold swing. The analysis of the W-Dm-donor concentration (N-D) relation elucidates that N-D is not evaluated only by W-Dm but is largely affected by the energy gap. The general W-Dm-N-D relation for different energy gaps provides a guide to select 2D materials suitable for TFETs.
  WILEY, English, Scientific journal
  DOI：https://doi.org/10.1002/aelm.202100292
  DOI ID：10.1002/aelm.202100292, ISSN：2199-160X, Web of Science ID：WOS:000681339800001
• State-of-the-Art of Solution-Processed Crystalline Silicon/Organic Heterojunction Solar Cells: Challenges and Future
  Jaker Hossain; A. T. M. Saiful Islam; Koji Kasahara; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  Challenges and Advances in Computational Chemistry and Physics, First page：33, Last page：56, May 2021
  Springer International Publishing, In book
  DOI：https://doi.org/10.1007/978-3-030-69445-6_2
  DOI ID：10.1007/978-3-030-69445-6_2, ISSN：2542-4491, eISSN：2542-4483
• Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory　　　　　　　　　　　　　　　
  Taro Sasaki; Keiji Ueno; Takashi Taniguchi; Kenji Watanabe; Tomonori Nishimura; Kosuke Nagashio
  ACS NANO, Volume：15, Number：4, First page：6658, Last page：6668, Apr. 2021
  Two-dimensional heterostructures have been extensively investigated as next-generation nonvolatile memory (NVM) devices. In the past decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate (FG) voltage measurements. Systematic comparisons of MoTe2, WSe2, and MoS2 channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and p-n junction in the channel. Furthermore, the control experiment indicated that the access region in the device structure is required to achieve 2D channel/FG tunneling by preventing electrode/FG tunneling. The present understanding suggests that the ambipolar 2D-based FG-type NVM device with the access region is suitable for further realizing potentially high electrical reliability.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/acsnano.0c10005
  DOI ID：10.1021/acsnano.0c10005, ISSN：1936-0851, eISSN：1936-086X, Web of Science ID：WOS:000645436800061
• AlOxThin Films Synthesized by Mist Chemical Vapor Deposition, Monitored by a Fast-Scanning Mobility Particle Analyzer, and Applied as a Gate Insulating Layer in the Field-Effect Transistors　　　　　　　　　　　　　　　
  Arifuzzaman Rajib; Abdul Kuddus; Tomohiro Shida; Keiji Ueno; Hajime Shirai
  ACS Applied Electronic Materials, Volume：3, Number：2, First page：658, Last page：667, Feb. 2021
  We investigated the synthesis of aluminum oxide (AlOx) thin films using mist chemical vapor deposition (mist-CVD) from aluminum acetylacetonate (Al(acac)3) and methanol/water (MeOH/H2O) mixture (volume ratio, 7:3). Different deposition parameters, such as the flow rate (Fd) of dilution gas N2, furnace temperature (Tf), solution concentration, and mesh bias (Vm), were optimized via the analysis of the size distribution of mist precursors using a fast-scanning mobility particle analyzer. The film morphology, rigidity of the AlOx network, and junction property at the AlOx/n-type crystalline Si (n-Si) were dominated by the size distribution of the mist precursors determined by the deposition parameters. Further, the mesh bias supply during film growth promoted the miniaturization of the size distribution of the charged mist particles. Consequently, a marked increase in the number density of the mist particles resulted in an increased refractive index (n) of the AlOx thin films with small surface roughness values. Furthermore, such property of the AlOx films improved the junction property at the AlOx/n-Si interface. In this study, the correlation between the size distribution of mist particles, which is dependent on the deposition parameters, and the film and interface properties, is presented together with metal-oxide-semiconductor field-effect transistor (MOS-FET) performance for the AlOx thin films obtained by mist-CVD.
  Scientific journal
  DOI：https://doi.org/10.1021/acsaelm.0c00758
  Scopus：https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=85099954273&origin=inward
  Scopus Citedby：https://www.scopus.com/inward/citedby.uri?partnerID=HzOxMe3b&scp=85099954273&origin=inward
  DOI ID：10.1021/acsaelm.0c00758, eISSN：2637-6113, SCOPUS ID：85099954273
• Enhanced Exciton-Exciton Collisions in an Ultraflat Monolayer MoSe2 Prepared through Deterministic Flattening.　　　　　　　　　　　　　　　
  Takato Hotta; Akihiko Ueda; Shohei Higuchi; Mitsuhiro Okada; Tetsuo Shimizu; Toshitaka Kubo; Keiji Ueno; Takashi Taniguchi; Kenji Watanabe; Ryo Kitaura
  ACS nano, Dec. 2020, ［International magazine］
  Squeezing bubbles and impurities out of interlayer spaces by applying force through a few-layer graphene capping layer leads to van der Waals heterostructures with the ultraflat structure free from random electrostatic potential arising from charged impurities. Without the graphene capping layer, a squeezing process with an AFM tip induces applied-force-dependent charges of Δn ∼ 2 × 1012 cm-2 μN-1, resulting in the significant intensity of trions in photoluminescence spectra of MoSe2 at low temperature. We found that a hBN/MoSe2/hBN prepared with the "graphene-capping-assisted AFM nano-squeezing method" shows a strong excitonic emission with negligible trion peak, and the residual line width of the exciton peak is only 2.2 meV, which is comparable to the homogeneous limit. Furthermore, in this high-quality sample, we found that the formation of biexciton occurs even at extremely low excitation power (Φph ∼ 2.3 × 1019 cm-2 s-1) due to the enhanced collisions between excitons.
  English
  DOI：https://doi.org/10.1021/acsnano.0c08642
  DOI ID：10.1021/acsnano.0c08642, arXiv ID：arXiv:2006.02640, PubMed ID：33356145
• All 2D Heterostructure Tunnel Field-Effect Transistors: Impact of Band Alignment and Heterointerface Quality
  Keigo Nakamura; Naoka Nagamura; Keiji Ueno; Takashi Taniguchi; Kenji Watanabe; Kosuke Nagashio
  ACS Applied Materials & Interfaces, Volume：12, Number：46, First page：51598, Last page：51606, Nov. 2020
  American Chemical Society (ACS), Scientific journal
  DOI：https://doi.org/10.1021/acsami.0c13233
  DOI ID：10.1021/acsami.0c13233, ISSN：1944-8244, eISSN：1944-8252
• Flat bands in twisted bilayer transition metal dichalcogenides　　　　　　　　　　　　　　　
  Zhiming Zhang; Yimeng Wang; Kenji Watanabe; Takashi Taniguchi; Keiji Ueno; Emanuel Tutuc; Brian J. LeRoy
  Nature Physics, Volume：16, Number：11, First page：1093, Last page：1096, Nov. 2020
  The crystal structure of a material creates a periodic potential that electrons move through giving rise to its electronic band structure. When two-dimensional materials are stacked, the resulting moiré pattern introduces an additional periodicity so that the twist angle between the layers becomes an extra degree of freedom for the resulting heterostructure. As this angle changes, the electronic band structure is modified leading to the possibility of flat bands with localized states and enhanced electronic correlations1–6. In transition metal dichalcogenides, flat bands have been theoretically predicted to occur for long moiré wavelengths over a range of twist angles around 0° and 60° (ref. 4) giving much wider versatility than magic-angle twisted bilayer graphene. Here, we show the existence of a flat band in the electronic structure of 3° and 57.5° twisted bilayer WSe2 samples using scanning tunnelling spectroscopy. Our direct spatial mapping of wavefunctions at the flat-band energy show that the localization of the flat bands is different for 3° and 57.5°, in agreement with first-principles density functional theory calculations4.
  Nature Research, English, Scientific journal
  DOI：https://doi.org/10.1038/s41567-020-0958-x
  DOI ID：10.1038/s41567-020-0958-x, ISSN：1745-2481, SCOPUS ID：85087652297
• Understanding the Memory Window Overestimation of 2D Materials Based Floating Gate Type Memory Devices by Measuring Floating Gate Voltage　　　　　　　　　　　　　　　
  Taro Sasaki; Keiji Ueno; Takashi Taniguchi; Kenji Watanabe; Tomonori Nishimura; Kosuke Nagashio
  SMALL, Volume：16, Number：47, Nov. 2020
  The memory window of floating gate (FG) type non-volatile memory (NVM) devices is a fundamental figure of merit used not only to evaluate the performance, such as retention and endurance, but also to discuss the feasibility of advanced functional memory devices. However, the memory window of 2D materials based NVM devices is historically determined from round sweep transfer curves, while that of conventional Si NVM devices is determined from high and low threshold voltages (V(th)s), which are measured by single sweep transfer curves. Here, it is elucidated that the memory window of 2D NVM devices determined from round sweep transfer curves is overestimated compared with that determined from single sweep transfer curves. The floating gate voltage measurement proposed in this study clarifies that the V(th)s in round sweep are controlled not only by the number of charges stored in floating gate but also by capacitive coupling between floating gate and back gate. The present finding on the overestimation of memory window enables to appropriately evaluate the potential of 2D NVM devices.
  WILEY-V C H VERLAG GMBH, English, Scientific journal
  DOI：https://doi.org/10.1002/smll.202004907
  DOI ID：10.1002/smll.202004907, ISSN：1613-6810, eISSN：1613-6829, Web of Science ID：WOS:000583797900001
• Exciton diffusion in hBN-encapsulated monolayer MoSe2　　　　　　　　　　　　　　　
  Takato Hotta; Shohei Higuchi; Akihiro Ueda; Keisuke Shinokita; Yuhei Miyauchi; Kazunari Matsuda; Keiji Ueno; Takashi Taniguchi; Kenji Watanabe; Ryo Kitaura
  PHYSICAL REVIEW B, Volume：102, Number：11, Sep. 2020
  Excitons, quasiparticles composed of an electron and a hole, play an important role in optical responses in low-dimensional nanostructures. In this work, we have investigated exciton diffusion in monolayer MoSe2 encapsulated between flakes of hexagonal boron nitride (hBN/MoSe2/hBN). Through photoluminescence imaging and numerical solution of the two-dimensional diffusion equation, we revealed that temperature dependence of exciton mobility, mu(ex), in hBN/MoSe2/hBN shows a nonsaturating increase at low temperature, which is qualitatively different from those of quantum wells composed of compound semiconductors. The ultraflat structure of monolayer MoSe2 in hBN/MoSe2/hBN probably leads to the suppression of charged-impurity scattering and surface-roughness scattering.
  AMER PHYSICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1103/PhysRevB.102.115424
  DOI ID：10.1103/PhysRevB.102.115424, ISSN：2469-9950, eISSN：2469-9969, Web of Science ID：WOS:000571391100006
• Facile and reversible carrier-type manipulation of layered MoTe2 toward long-term stable electronics
  Mengjiao Li; Che-Yi Lin; Yuan-Ming Chang; Shih-Hsien Yang; Mu-Pai Lee; Ciao-Fen Chen; Ko-Chun Lee; Feng-Shou Yang; Yi Chou; Yi-Chun Lin; Keiji Ueno; Yumeng Shi; Yi-Chia Chou; Kazuhito Tsukagoshi; Yen-Fu Lin
  ACS Applied Materials & Interfaces, Aug. 2020
  American Chemical Society (ACS), Scientific journal
  DOI：https://doi.org/10.1021/acsami.0c09922
  DOI ID：10.1021/acsami.0c09922, ISSN：1944-8244, eISSN：1944-8252
• Effect of thermally annealed atomic-layer-deposited AlOx/chemical tunnel oxide stack layer at the PEDOT:PSS/n-type Si interface to improve its junction quality　　　　　　　　　　　　　　　
  Md Enamul Karim; Yuki Nasuno; Abdul Kuddus; Tomofumi Ukai; Shunji Kurosu; Masahide Tokuda; Yasuhiko Fujii; Tatsuro Hanajiri; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  JOURNAL OF APPLIED PHYSICS, Volume：128, Number：4, Jul. 2020
  We investigated the effects of thermal annealing on an atomic layer deposition-fabricated AlOx/chemical tunnel oxide (ch-SiOx) stack layer, for passivating and enhancing the field-inversion at the poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/n-type crystalline Si (n-Si) interface. Annealing in N-2/H-2 forming gas at 560 degrees C for 30min increased the effective minority carrier lifetime (tau(eff)) of the AlOx/ch-SiOx stack layer to 300-331 mu s, which decreased sheet resistance and enhanced the built-in potential and open-circuit voltage in PEDOT:PSS/n-Si heterojunction solar cells to 750mV and 645mV, respectively. These improvements originate from the local chemical bond configuration of the tunnel oxide ch-SiOx, which determines the passivation ability and band alignment at the AlOx/n-Si interface.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/5.0007918
  DOI ID：10.1063/5.0007918, ISSN：0021-8979, eISSN：1089-7550, Web of Science ID：WOS:000556325800003
• Synthesis of AlOx thin films by atmospheric-pressure mist chemical vapor deposition for surface passivation and electrical insulator layers
  Arifuzzaman Rajib; Karim Md Enamul; Shunji Kurosu; Tomofumi Ukai; Masahide Tokuda; Yasuhiko Fujii; Tatsuro Hanajiri; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  Journal of Vacuum Science & Technology A, Volume：38, Number：3, First page：033413, Last page：033413, May 2020
  American Vacuum Society, Scientific journal
  DOI：https://doi.org/10.1116/1.5143273
  DOI ID：10.1116/1.5143273, ISSN：0734-2101, eISSN：1520-8559
• Ultrafast dynamics of the low frequency shear phonon in 1T′- MoTe 2
  Takumi Fukuda; Kotaro Makino; Yuta Saito; Paul Fons; Alexander V. Kolobov; Keiji Ueno; Muneaki Hase
  Applied Physics Letters, Volume：116, Number：9, First page：093103, Last page：093103, Mar. 2020
  AIP Publishing, Scientific journal
  DOI：https://doi.org/10.1063/1.5143485
  DOI ID：10.1063/1.5143485, ISSN：0003-6951, eISSN：1077-3118
• Improved efficiency of methylammonium-free perovskite thin film solar cells by fluorinated ammonium iodide treatment　　　　　　　　　　　　　　　
  Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  Organic Electronics, Volume：78, First page：105596, Last page：105596, Mar. 2020
  Elsevier BV, Scientific journal
  DOI：https://doi.org/10.1016/j.orgel.2019.105596
  DOI ID：10.1016/j.orgel.2019.105596, ISSN：1566-1199
• Low-temperature growth of crystalline tungsten disulfide thin films by using organic liquid precursors　　　　　　　　　　　　　　　
  Yukihiro Ikeda; Keiji Ueno
  Japanese Journal of Applied Physics, Volume：59, Feb. 2020
  A low-temperature growth method for crystalline thin films of transition metal dichalcogenides (TMDCs) is urgently required to use various materials as substrates. In this paper, we report the growth of crystalline tungsten disulfide (WS2) thin films at a low growth temperature of 400 °C by using organic liquid precursors: bis(tert-butylimido)bis(dimethylamido)tungsten(VI) [(t-BuN)2W(NMe2)2] and di-tert-butyl disulfide thin films by the two-step reaction of an amorphous (a-)PbIxBr2-x layer and FAI(x) Br1-x solution diluted in IPA. An optical dispersion model was developed to extract the complex refractive index N (=n+ik), optical transition, and film thickness of FAPbI(x) Br3-x perovskites by SE analysis as a function of immersion time in a solution of FAI(x)Bri(1-x) diluted in IPA. SE combined with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning microscopy (SEM) revealed that IPA promoted film crystallization of a-PbIxBr2-x accompanied by the formation of surface roughness, grain boundaries, and voids, followed by enhanced diffusion of FAI(x)Br(1-x) into the grain boundaries/voids in the mesoporous crystallized PbIxBr2-x network. These processes contribute synergistically to the growth of the perovskite structure.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/acs.jpcc.6b07527
  DOI ID：10.1021/acs.jpcc.6b07527, ISSN：1932-7447, SCOPUS ID：84994884896, Web of Science ID：WOS:000387737900022
• Nafion-Modified PEDOT:PSS as a Transparent Hole-Transporting Layer for High-Performance Crystalline-Si/Organic Heterojunction Solar Cells with Improved Light Soaking Stability　　　　　　　　　　　　　　　
  Jaker Hossain; Qiming Liu; Takuya Miura; Koji Kasahara; Daisuke Harada; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  ACS Applied Materials & Interfaces, Volume：8, Number：46, First page：31926, Last page：31934, Nov. 2016
  We demonstrate the chemistry of amphiphilic perfluorosulfonic copolymer Nafion-coated conductive poly(3,4-ethyelenedioxythiophene):poly-(styrenesulfonate) (PEDOT:PSS) and its effect on the photovoltaic performance of PEDOT:PSS/crystalline Si (c-Si) heterojunction solar cells. The highly hydrophilic sulfonate group of insulating, chemically stable Nafion interacts with PSS in PEDOT:PSS, which reduce the Coulombic interaction between PEDOT and PSS. The highly hydrophobic fluorocarbon backbone of Nafion favorably interacts with hydrophobic PEDOT of PEDOT:PSS. These factors give rise to the extension of pi-conjugation of PEDOT chains. Silver paste used as a top grid electrode diffused into the Nafion layer and contacted with underneath Nafion-modified PEDOT:PSS layer. As a consequent, solution-processed Nafion-coated PEDOT:PSS/c-Si heterojunction solar cells exhibited a higher power conversion efficiency of 14.0% with better stability for light soaking rather than that of the pristine PEDOT:PSS/c-Si device by adjusting the layer thickness of Nafion. These findings originate from the chemical stability of hydrophobic fluorocarbon backbone of Nafion, diffusivity of silver paste into Nafion and contact with PEDOT:PSS, and Nafion as an antireflection layer.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/acsami.6b10272
  DOI ID：10.1021/acsami.6b10272, ISSN：1944-8244, SCOPUS ID：84999115078, Web of Science ID：WOS:000388913900053
• Monolayer 1 T-NbSe2 as a Mott insulator　　　　　　　　　　　　　　　
  Yuki Nakata; Katsuaki Sugawara; Ryota Shimizu; Yoshinori Okada; Patrick Han; Taro Hitosugi; Keiji Ueno; Takafumi Sato; Takashi Takahashi
  NPG ASIA MATERIALS, Volume：8, Nov. 2016
  The emergence of exotic quantum phenomena is often triggered by a subtle change in the crystal phase. Transition metal dichalcogenides (TMDs) exhibit a wide variety of novel properties, depending on their crystal phases, which can be trigonal prismatic (2H) or octahedral (1T). Bulk NbSe2 crystallizes into the 2H phase, and the charge density wave and the superconductivity emerge simultaneously and interact with each other, thereby creating various anomalous properties. However, these properties and their interplay in another polymorph, 1T-NbSe2, have remained unclear because of the difficulty of synthesizing it. Here we report the first experimental realization of a monolayer 1T-NbSe2 crystal grown epitaxially on bilayer graphene. In contrast with 2H-NbSe2, monolayer 1T-NbSe2 was found to be a Mott insulator, with an energy gap of 0.4 eV. We also found that the insulating 1T and metallic 2H phases can be selectively fabricated by simply controlling the substrate temperature during epitaxy. The present results open a path to crystal-phase engineering based on TMDs.
  NATURE PUBLISHING GROUP, English, Scientific journal
  DOI：https://doi.org/10.1038/am.2016.157
  DOI ID：10.1038/am.2016.157, ISSN：1884-4049, eISSN：1884-4057, SCOPUS ID：84994571030, Web of Science ID：WOS:000387402500001
• Synthesis of High-Quality Large-Area Homogenous 1T ' MoTe2 from Chemical Vapor Deposition　　　　　　　　　　　　　　　
  Lin Zhou; Ahmad Zubair; Ziqiang Wang; Xu Zhang; Fangping Ouyang; Kai Xu; Wenjing Fang; Keiji Ueno; Ju Li; Tomas Palacios; Jing Kong; Mildred S. Dresselhaus
  ADVANCED MATERIALS, Volume：28, Number：43, First page：9526, Last page：+, Nov. 2016
  High-quality large-area few-layer 1T' MoTe2 films with high homogeneity are synthesized by the controlled tellurization of MoO3 film. The Mo precursor plays a key role in determining the quality and morphology of the 1T' MoTe2. Furthermore, the amount of Te strongly influences the phase of the MoTe2. The growth method paves the way toward the scalable production of 1T' MoTe2-based applications.
  WILEY-V C H VERLAG GMBH, English, Scientific journal
  DOI：https://doi.org/10.1002/adma.201602687
  DOI ID：10.1002/adma.201602687, ISSN：0935-9648, eISSN：1521-4095, SCOPUS ID：84987638165, Web of Science ID：WOS:000391175000010
• Effect of substrate bias on mist deposition of conjugated polymer on textured crystalline-Si for efficient c-Si/organic heterojunction solar cells　　　　　　　　　　　　　　　
  Tatsuya Ohki; Koki Ichikawa; Jaker Hossein; Yasuhiko Fujii; Tatsuro Hanajiri; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE, Volume：213, Number：7, First page：1922, Last page：1925, Jul. 2016
  We investigated the chemical mist deposition (CMD) of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on textured crystalline silicon (c-Si) substrate for efficient c-Si/PEDOT:PSS heterojunction solar cells. The Mie scattering observation and differential mobility analysis characterization revealed that the average size of mist particle decreased from 330 to 30 '10 nm with increasing the number density and traveling velocity of mist precursor when positive dc bias of 10 kV was applied to the c-Si substrate. The uniform deposition of PEDOT:PSS films was performed on textured c-Si with the increased adhesion by suitably adjusting V-s and substrate temperature. A power conversion efficiency (PCE) of 11.57% was obtained with J of 36.1 mA cm(-2), Vo, of 0.49 V, and 1-1- of 0.66, when a positive V-s of 10 kV was applied to c-Si substrate. Furthermore, PCE increased to 12.5% with J of 35.6 mA cm(-2), V-oc of 0.53 V, and FF of 0.67 by adding a 20-nm-thick AR coating layer of molybdenum oxide MoOx, (refractive index, n = 2.1) fabricated by CMD using 12 molybdo(VI) phosphoric acid n-hydrate [H-3(PMo12O40) nH(2)O] as a precursor. These findings imply that CMD using negatively charged mist precursor is a possible method for the uniform deposition of PEDOT:PSS on textured c-Si substrate for efficient c-Si/PEDOT:PSS heterojunction solar cells. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  WILEY-V C H VERLAG GMBH, English, Scientific journal
  DOI：https://doi.org/10.1002/pssa.201532951
  DOI ID：10.1002/pssa.201532951, ISSN：1862-6300, eISSN：1862-6319, SCOPUS ID：84963730570, Web of Science ID：WOS:000385222900040
• Correlation between the fine structure of spin-coated PEDOT:PSS and the photovoltaic performance of organic/crystalline-silicon heterojunction solar cells　　　　　　　　　　　　　　　
  Shuji Funda; Tatsuya Ohki; Qiming Liu; Jaker Hossain; Yoshihiro Ishimaru; Keiji Ueno; Hajime Shirai
  JOURNAL OF APPLIED PHYSICS, Volume：120, Number：3, Jul. 2016
  We investigated the relationship between the fine structure of spin-coated conductive polymer poly(3,4-ethylenedioxythiphene):poly(styrene sulfonate) (PEDOT:PSS) films and the photovoltaic performance of PEDOT:PSS crystalline-Si (PEDOT:PSS/c-Si) heterojunction solar cells. Real-time spectroscopic ellipsometry revealed that there were two different time constants for the formation of the PEDOT:PSS network. Upon removal of the polar solvent, the PEDOT:PSS film became optically anisotropic, indicating a conformational change in the PEDOT and PSS chain. Polarized Fourier transform infrared attenuated total reflection absorption spectroscopy and Raman spectroscopy measurements also indicated that thermal annealing promoted an in-plane pi-conjugated C-alpha-C-beta configuration attributed to a thiophene ring in PEDOT and an out-of-plane configuration of -SO3 groups in the PSS chain with increasing composition ratio of oxidized (benzoid) to neutral (quinoid) PEDOT, I-qui/I-ben. The highest power conversion efficiency for the spin-coated PEDOT:PSS/c-Si heterojunction solar cells was 13.3% for I-qui/I-ben -9-10 without employing any light harvesting methods. Published by AIP Publishing.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4958845
  DOI ID：10.1063/1.4958845, ISSN：0021-8979, eISSN：1089-7550, SCOPUS ID：84979299024, Web of Science ID：WOS:000381382500003
• Carrier Polarity Control in alpha-MoTe2 Schottky Junctions Based on Weak Fermi-Level Pinning　　　　　　　　　　　　　　　
  Shu Nakaharai; Mahito Yamamoto; Keiji Ueno; Kazuhito Tsukagoshi
  ACS APPLIED MATERIALS & INTERFACES, Volume：8, Number：23, First page：14732, Last page：14739, Jun. 2016
  The polarity of the charge carriers injected through Schottky junctions of a-phase molybdenum ditelluride (alpha-MoTe2) and various metals was characterized. We found that the Fermi-level pinning in the metal/alpha-MoTe2 Schottky junction is so weak that the polarity of the carriers (electron or hole) injected from the junction can be controlled by the work function of the metals, in contrast to other transition metal dichalcogenides such as MoS2. From the estimation of the Schottky barrier heights, we obtained p-type carrier (hole) injection from a Pt/alpha-MoTe2 junction with a Schottky barrier height of 40 meV at the valence band edge. n-Type carrier (electron) injection from Ti/alpha-MoTe2 and Ni/alpha-MoTe2 junctions was also observed with Schottky barrier heights of 50 and 100 meV, respectively, at the conduction band edge. In addition, enhanced ambipolarity was demonstrated in a Pt-Ti hybrid contact with a unique structure specially designed for polarity-reversible transistors, in which Pt and Ti electrodes were placed in parallel for injecting both electrons and holes.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/acsami.6b02036
  DOI ID：10.1021/acsami.6b02036, ISSN：1944-8244, eISSN：1944-8252, SCOPUS ID：84975299230, Web of Science ID：WOS:000378195000054
• Self-Limiting Oxides on WSe2 as Controlled Surface Acceptors and Low-Resistance Hole Contacts　　　　　　　　　　　　　　　
  Mahito Yamamoto; Shu Nakaharai; Keiji Ueno; Kazuhito Tsukagoshi
  NANO LETTERS, Volume：16, Number：4, First page：2720, Last page：2727, Apr. 2016
  Transition metal oxides show much promise as effective p-type contacts and dopants in electronics based on transition metal dichalcogenides. Here we report that atomically thin films of under-stoichiometric tungsten oxides (WOx with x < 3) grown on tungsten diselenide (WSe2) can be used as both controlled charge transfer dopants and low barrier contacts for p-type WSe2 transistors. Exposure of atomically thin WSe2 transistors to ozone (O-3) at 100 degrees C results in self-limiting oxidation of the WSe2 surfaces to conducting WOx films. WOx-covered WSe2 is highly hole-doped due to surface electron transfer from the underlying WSe2 to the high electron affinity WOx. The dopant concentration can be reduced by suppressing the electron affinity of WOx by air exposure, but exposure to O-3 at room temperature leads to the recovery of the electron affinity. Hence, surface transfer doping with WOx is virtually controllable. Transistors based on WSe2 covered with WOx show only p-type conductions with orders of magnitude better on-current, on/off current ratio, and carrier mobility than without WOx, suggesting that the surface WOx, serves as a p-type contact with a low hole Schottky barrier. Our findings point to a simple and effective strategy for creating p-type devices based on two-dimensional transition metal dichalcogenides with controlled dopant concentrations.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/acs.nanolett.6b00390
  DOI ID：10.1021/acs.nanolett.6b00390, ISSN：1530-6984, eISSN：1530-6992, SCOPUS ID：84964914034, Web of Science ID：WOS:000374274600088
• Investigating the chemical mist deposition technique for poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) on textured crystalline-silicon for organic/crystalline-silicon heterojunction solar cells　　　　　　　　　　　　　　　
  Jaker Hossain; Tatsuya Ohki; Koki Ichikawa; Kazuhiko Fujiyama; Keiji Ueno; Yasuhiko Fujii; Tatsuro Hanajiri; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：55, Number：3, Mar. 2016
  Chemical mist deposition (CMD) of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was investigated in terms of cavitation frequency f, solvent, flow rate of nitrogen, substrate temperature T-s, and substrate dc bias V-s as variables for efficient PEDOT:PSS/crystalline silicon (c-Si) heterojunction solar cells. The high-speed-camera and differential mobility analysis characterizations revealed that the average size and flux of PEDOT: PSS mist depend on f, type of solvent, and V-s. Film deposition occurred when positive V-s was applied to the c-Si substrate at T-s of 30-40 degrees C, whereas no deposition of films occurred with negative V-s, implying that the film is deposited mainly from negatively charged mist. The uniform deposition of PEDOT: PSS films occurred on textured c-Si(100) substrates by adjusting T-s and V-s. The adhesion of CMD PEDOT: PSS film to c-Si was greatly enhanced by applying substrate dc bias V-s compared with that of spin-coated film. The CMD PEDOT: PSS/c-Si heterojunction solar cell devices on textured c-Si(100) in 2 x 2 cm(2) exhibited a power conversion efficiency eta of 11.0% with better uniformity of the solar cell parameters. Furthermore, eta was increased to 12.5% by adding an AR coating layer of molybdenum oxide MoOx formed by CMD. These findings suggest that CMD with negatively charged mist has great potential for the uniform deposition of organic and inorganic materials on textured c-Si substrates by suitably adjusting T-s and V-s. (C) 2016 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.7567/JJAP.55.031601
  DOI ID：10.7567/JJAP.55.031601, ISSN：0021-4922, eISSN：1347-4065, SCOPUS ID：84961990487, Web of Science ID：WOS:000370491100017
• Solution-processed crystalline silicon double-heterojunction solar cells　　　　　　　　　　　　　　　
  Ramesh Devkota; Qiming Liu; Tatsuya Ohki; Jaker Hossain; Keiji Ueno; Hajime Shirai
  APPLIED PHYSICS EXPRESS, Volume：9, Number：2, Feb. 2016
  Crystalline silicon double-heterojunction solar cells were fabricated using Si/organic and Si/Cs2CO3 heterojunctions. The front heterojunction is formed by spin-coating conductive polymer poly(3,4-ethyenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on n-type Czochralski (CZ) (100) silicon, which separates the photogenerated carriers and blocks the electron dark current while allowing the photocurrent to pass through. The rear heterojunction, formed by spin-coating Cs2CO3 and polyethylenimine (PEI) dissolved in 2-ethoxyethanol and Al metal evaporation, functions as a back surface field that reduces the hole dark current while allowing the electron photocurrent to pass through. The double-heterojunction device showed a power conversion efficiency of 12.7% under AM1.5G simulated solar light exposure. (C) 2016 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.7567/APEX.9.022301
  DOI ID：10.7567/APEX.9.022301, ISSN：1882-0778, eISSN：1882-0786, SCOPUS ID：84956984570, Web of Science ID：WOS:000371297800013
• 19aAJ-4 Electronic structure of Bi ultrathin film on transition-metal dichalcogenide 1T-TaS_2 studied by high-resolution ARPES　　　　　　　　　　　　　　　
  Yamada K.; Trang C.; Souma S.; Sugawara K.; Sato T.; Ueno K.; Takahashi T.
  Meeting Abstracts of the Physical Society of Japan, Volume：71, First page：2470, Last page：2470, 2016
  The Physical Society of Japan (JPS), Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.71.1.0_2470
  DOI ID：10.11316/jpsgaiyo.71.1.0_2470, ISSN：2189-079X, CiNii Articles ID：110010058409, CiNii Books ID：AA12721570
• 20pBE-2 Thickness dependence of electronic states in NbSe_2 thin film studied by high-resolution ARPES　　　　　　　　　　　　　　　
  Nakata Y.; Yamada K.; Kimizuka H.; Tanaka Y.; Sugawara K.; Souma S.; Sato T.; Shimizu R.; Hitosugi T.; Ueno K.; Takahashi T.
  Meeting Abstracts of the Physical Society of Japan, Volume：71, First page：1868, Last page：1868, 2016
  The Physical Society of Japan (JPS), Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.71.1.0_1868
  DOI ID：10.11316/jpsgaiyo.71.1.0_1868, ISSN：2189-079X, CiNii Articles ID：110010057838, CiNii Books ID：AA12721570
• Electronic structure of atomic layer NbSe₂ thin film studied by high-resolution ARPES　　　　　　　　　　　　　　　
  Nakata Y.; Sugawara K.; Souma S.; Sato T.; Shimuzu R.; Hitosugi T.; Ueno K.; Takahashi T.
  Meeting Abstracts of the Physical Society of Japan, Volume：71, First page：1171, Last page：1171, 2016
  

  遷移金属ダイカルコゲナイドNbSe_2_の原子層化に伴う電子状態の変化を明らかにするために、分子線エピタキシー法を用いて2層グラフェン上に単層NbSe_2_薄膜を作製し、その電子状態を高分解能ARPESにより測定した。その結果、単層NbSe_2_において、バルクとは大きく異なる特異な電子状態を観測した。講演では、バルクの実験結果およびバンド計算との比較から、特異な電子状態の起源について議論する。

  
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.71.2.0_1171
  DOI ID：10.11316/jpsgaiyo.71.2.0_1171, CiNii Articles ID：130006243828
• High-resolution ARPES study of NbSe₂ thin film grown on Graphene　　　　　　　　　　　　　　　
  Nakata Y.; Sugawara K.; Souma S.; Sato T.; Shimuzu R.; Hitosugi T.; Ueno K.; Takahashi T.
  Meeting Abstracts of the Physical Society of Japan, Volume：71, First page：1393, Last page：1393, 2016
  

  バルクの2H-NbSe_2_は約33K以下でCDW相へ転移することが知られているが、膜厚の減少に伴うCDWの変化については未解明である。今回我々は、2層グラフェン上にMBE法により単層及び多層のNbSe_2_薄膜を作製し、それらの電子状態を高分解能ARPESによって測定した。講演では、詳細な電子構造の実験結果を報告するとともに、膜厚に依存したCDWの変化について議論する。

  
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.71.2.0_1393
  DOI ID：10.11316/jpsgaiyo.71.2.0_1393, CiNii Articles ID：130006244003
• Anomalous band structure of Bi ultrathin film on 1T-TaS₂ studied by high-resolution ARPES　　　　　　　　　　　　　　　
  Yamada K.; Trang C.; Souma S.; Sugawara K.; Sato T.; Ueno K.; Takahashi T.
  Meeting Abstracts of the Physical Society of Japan, Volume：71, First page：2310, Last page：2310, 2016
  

  V族のBi薄膜は、黒リン同様に興味深い相変態を示すことが知られており、(111)方位だけではなく、(110)方位の超薄膜においても2次元のトポロジカル絶縁体(TI)相の発現が期待され、注目を集めている。今回我々は、1T-TaS_2_やSi基板上にBi(110)超薄膜を作製し、電子構造を高分解能ARPESで測定した。その結果、TI相を予測するバンド計算と類似した電子構造を見出した。講演では、Bi(110)超薄膜のバンド構造における基板・温度依存性を詳しく議論する。

  
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.71.2.0_2310
  DOI ID：10.11316/jpsgaiyo.71.2.0_2310, CiNii Articles ID：130006244991
• Introduction to the Growth of Bulk Single Crystals of Two-Dimensional Transition-Metal Dichalcogenides　　　　　　　　　　　　　　　
  Keiji Ueno
  JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN, Volume：84, Number：12, Dec. 2015
  Semiconducting two-dimensional transition-metal dichalcogenides (MX2) are attracting much attention as promising materials for a new generation of optical and electronic devices. MX2 compounds are complementary or competitive to graphene because of the existence of a native band gap. The growth of large and high-quality bulk single crystals is one of the critical issues for the application of MX2 compounds, whose bulk crystals are generally grown by the chemical vapor transport (CVT) method. In the present review, I introduce experimental techniques required for the CVT growth of high-quality MX2 single crystals.
  PHYSICAL SOC JAPAN, English, Scientific journal
  DOI：https://doi.org/10.7566/JPSJ.84.121015
  DOI ID：10.7566/JPSJ.84.121015, ISSN：0031-9015, SCOPUS ID：84957074331, Web of Science ID：WOS:000365804100015
• Origin of Noise in Layered MoTe2 Transistors and its Possible Use for Environmental Sensors　　　　　　　　　　　　　　　
  Yen-Fu Lin; Yong Xu; Che-Yi Lin; Yuen-Wuu Suen; Mahito Yamamoto; Shu Nakaharai; Keiji Ueno; Kazuhito Tsukagoshi
  ADVANCED MATERIALS, Volume：27, Number：42, First page：6612, Last page：+, Nov. 2015
  Low-frequency current fluctuations are monitored and the mechanism of electric noise investigated in layered 2H-type a-molybdenum ditelluride transistors. The charge transport mechanism of electric noise in atomically thin transitionmetal dichalcogenides is studied under different environments; the development of a new sensing functionality may be stimulated.
  WILEY-V C H VERLAG GMBH, English, Scientific journal
  DOI：https://doi.org/10.1002/adma.201502677
  DOI ID：10.1002/adma.201502677, ISSN：0935-9648, eISSN：1521-4095, SCOPUS ID：84946781252, Web of Science ID：WOS:000364700200007
• Changes in structure and chemical composition of alpha-MoTe2 and beta-MoTe2 during heating in vacuum conditions　　　　　　　　　　　　　　　
  Keiji Ueno; Koji Fukushima
  APPLIED PHYSICS EXPRESS, Volume：8, Number：9, Sep. 2015
  Molybdenum ditelluride (MoTe2) is known to have two different crystal phases: alpha-MoTe2 and beta-MoTe2. alpha-MoTe2 is the polytype stable below 815 degrees C, and beta-MoTe2 is stable above 900 degrees C. Here, we report changes in the structure and composition of each single crystal during heating in low and high vacuum conditions. Contrary to the results of a recently published paper, we did not observe alpha-MoTe2 transforming to beta-MoTe2 at elevated temperatures. In a low vacuum, the alpha-MoTe2 sample was mainly oxidized, forming MoO2. In a high vacuum, it partially decomposed with surface oxidation. The recently reported "reversible phase transition" of alpha-MoTe2 is the misinterpretation of X-ray diffraction data. (C) 2015 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.7567/APEX.8.095201
  DOI ID：10.7567/APEX.8.095201, ISSN：1882-0778, eISSN：1882-0786, SCOPUS ID：84940992838, Web of Science ID：WOS:000362184700026
• Highly Efficient Solution-Processed Poly(3,4-ethylenedio-xythiophene):Poly(styrenesulfonate)/Crystalline-Silicon Heterojunction Solar Cells with Improved Light-Induced Stability　　　　　　　　　　　　　　　
  Qiming Liu; Ryo Ishikawa; Shuji Funada; Tatsuya Ohki; Keiji Ueno; Hajime Shirai
  ADVANCED ENERGY MATERIALS, Volume：5, Number：17, Sep. 2015
  © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.p-Toluenesulfonic acid/dimethyl sulfoxide (PTSA/DMSO)-treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/crystalline-silicon (c-Si) hybrid solar cells with a solution-processed anti-reflection coating of TiO<inf>2</inf> exhibit a record power conversion efficiency of 15.5%. Additionally, the performance stability of the hybrid device for both air storage and light exposure is improved due to removal of the PSS matrix from the PTSA/DMSO-treated PEDOT:PSS thin film.
  WILEY-V C H VERLAG GMBH, English, Scientific journal
  DOI：https://doi.org/10.1002/aenm.201500744
  DOI ID：10.1002/aenm.201500744, ISSN：1614-6832, eISSN：1614-6840, SCOPUS ID：84941186958, Web of Science ID：WOS:000361226700013
• Construction of van der Waals magnetic tunnel junction using ferromagnetic layered dichalcogenide　　　　　　　　　　　　　　　
  Miho Arai; Rai Moriya; Naoto Yabuki; Satoru Masubuchi; Keiji Ueno; Tomoki Machida
  APPLIED PHYSICS LETTERS, Volume：107, Number：10, Sep. 2015
  We investigate the micromechanical exfoliation and van der Waals (vdW) assembly of ferromagnetic layered dichalcogenide Fe0.25TaS2. The vdW interlayer coupling at the Fe-intercalated plane of Fe0.25TaS2 allows exfoliation of flakes. A vdW junction between the cleaved crystal surfaces is constructed by dry transfer method. We observe tunnel magnetoresistance in the resulting junction under an external magnetic field applied perpendicular to the plane, demonstrating spin-polarized tunneling between the ferromagnetic layered material and the vdW junction. (C) 2015 AIP Publishing LLC.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4930311
  DOI ID：10.1063/1.4930311, ISSN：0003-6951, eISSN：1077-3118, SCOPUS ID：84941356974, Web of Science ID：WOS:000361640200042
• Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2　　　　　　　　　　　　　　　
  Lin Zhou; Kai Xu; Ahmad Zubair; Albert D. Liao; Wenjing Fang; Fangping Ouyang; Yi-Hsien Lee; Keiji Ueno; Riichiro Saito; Tomas Palacios; Jing Kong; Mildred S. Dresselhaus
  JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Volume：137, Number：37, First page：11892, Last page：11895, Sep. 2015
  The controlled synthesis of large-area, atomically thin molybdenum ditelluride (MoTe2) crystals is crucial for its various applications based on the attractive properties of this emerging material. In this work, we developed a chemical vapor deposition synthesis to produce large-area, uniform, and highly crystalline fewlayer 2H and 1T' MoTe2 films. It was found that these two different phases of MoTe2 can be grown depending on the choice of Mo precursor. Because of the highly crystalline structure, the as-grown few-layer 2H MoTe2 films display electronic properties that are comparable to those of mechanically exfoliated MoTe2 flakes. Our growth method paves the way for the large-scale application of MoTe2 in high-performance nanoelectronics and optoelectronics.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/jacs.5b07452
  DOI ID：10.1021/jacs.5b07452, ISSN：0002-7863, SCOPUS ID：84942239142, Web of Science ID：WOS:000361930000010
• Growth of single-layer graphitic carbon nitride on single crystal substrates　　　　　　　　　　　　　　　
  Katoh Tokio; Obata Seiji; Ueno Keiji; Saiki Koichiro
  JSAP Annual Meetings Extended Abstracts, Volume：2015.2, First page：3367, Last page：3367, Aug. 2015
  The Japan Society of Applied Physics, Japanese
  DOI：https://doi.org/10.11470/jsapmeeting.2015.2.0_3367
  DOI ID：10.11470/jsapmeeting.2015.2.0_3367, eISSN：2436-7613
• Electrostatically Reversible Polarity of Ambipolar alpha-MoTe2 Transistors　　　　　　　　　　　　　　　
  Shu Nakaharai; Mahito Yamamoto; Keiji Ueno; Yen-Fu Lin; Song-Lin Li; Kazuhito Tsukagoshi
  ACS NANO, Volume：9, Number：6, First page：5976, Last page：5983, Jun. 2015
  A doping-free transistor made of ambipolar cc-phase molybdenum ditelluride (alpha-MoTe2) is proposed in which the transistor polarity (p-type and n-type) is electrostatically controlled by dual top gates. The voltage signal in one of the gates determines the transistor polarity, while the other gate modulates the drain current. We demonstrate the transistor operation experimentally, with electrostatically controlled polarity of both p- and n-type in a single transistor.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/acsnano.5b00736
  DOI ID：10.1021/acsnano.5b00736, ISSN：1936-0851, eISSN：1936-086X, SCOPUS ID：84935017328, Web of Science ID：WOS:000356988500038
• Double resonance Raman modes in monolayer and few-layer MoTe2　　　　　　　　　　　　　　　
  Huaihong Guo; Teng Yang; Mahito Yamamoto; Lin Zhou; Ryo Ishikawa; Keiji Ueno; Kazuhito Tsukagoshi; Zhidong Zhang; Mildred S. Dresselhaus; Riichiro Saito
  PHYSICAL REVIEW B, Volume：91, Number：20, May 2015
  We study the second-order Raman process of mono- and few-layer MoTe2, by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using 532, 633, and 785 nm excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the M point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van Hove singularity of the electronic density of states. The double resonance Raman process with intervalley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M-point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy-dependent Raman shifts in the experiment.
  AMER PHYSICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1103/PhysRevB.91.205415
  DOI ID：10.1103/PhysRevB.91.205415, ISSN：1098-0121, eISSN：1550-235X, SCOPUS ID：84929598374, Web of Science ID：WOS:000354365100005
• Self-Limiting Layer-by-Layer Oxidation of Atomically Thin WSe2　　　　　　　　　　　　　　　
  Mahito Yamamoto; Sudipta Dutta; Shinya Aikawa; Shu Nakaharai; Katsunori Wakabayashi; Michael S. Fuhrer; Keiji Ueno; Kazuhito Tsukagoshi
  NANO LETTERS, Volume：15, Number：3, First page：2067, Last page：2073, Mar. 2015
  Growth of a uniform oxide film with a tunable thickness on two-dimensional transition Metal dichalcogenides is of great importance for electronic and optoelectronic applications. Here we demonstrate homogeneous surface oxidation of atomically thin WSe2 with a self-limiting thickness from single- to trilayers. Exposure to ozone (O-3) below 100 degrees C leads to the lateral growth of tungsten oxide selectively along selenium zigzag-edge orientations on WSe2. With further O-3 exposure, the oxide regions coalesce and oxidation terminates leaving a uniform thickness oxide film on top of unoxidized WSe2. At higher temperatures, oxidation evolves in the layer-by-layer regime up to trilayers. The oxide films formed on WSe2 are nearly atomically flat. Using photoluminescence and Raman spectroscopy, we find that the underlying single-layer WSe2 is decoupled from the top oxide but hole-doped. Our findings offer a new strategy for creating atomically thin heterostructures of semiconductors and insulating oxides with potential for applications in electronic devices.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/nl5049753
  DOI ID：10.1021/nl5049753, ISSN：1530-6984, eISSN：1530-6992, SCOPUS ID：84924630786, Web of Science ID：WOS:000351188000095
• Optical microscope observation of Diacetylene thin film growth on Si(111) substrate　　　　　　　　　　　　　　　
  Kamata Yuto; Obata Seiji; Ueno Keiji; Saiki Koichiro
  JSAP Annual Meetings Extended Abstracts, Volume：2015.1, First page：2166, Last page：2166, Feb. 2015
  The Japan Society of Applied Physics, Japanese
  DOI：https://doi.org/10.11470/jsapmeeting.2015.1.0_2166
  DOI ID：10.11470/jsapmeeting.2015.1.0_2166, eISSN：2436-7613
• Fabrication of organic/inorganic hybrid CMOS devices using solution-processed graphene electrodes
  Koichi Suganuma; Hajime Shirai; Keiji Ueno
  IEEJ Transactions on Electronics, Information and Systems, Volume：135, Number：2, First page：156, Last page：159, Feb. 2015
  Complementary metal-oxide-semiconductor (CMOS) inverter circuits were fabricated using solution-processed field-effect transistors (FETs)
  p-type FET of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and n-type FET of Li-doped ZnO. As the source/drain electrodes of these FETs, reduced graphene oxide (RGO) films were prepared also by the solution process. The fabricated CMOS inverter device showed the voltage gain of 10.
  Institute of Electrical Engineers of Japan, Japanese, Scientific journal
  DOI：https://doi.org/10.1541/ieejeiss.135.156
  DOI ID：10.1541/ieejeiss.135.156, ISSN：1348-8155, SCOPUS ID：84922239834
• 17pAH-4 High-resolution ARPES study of NbSe_2 thin film　　　　　　　　　　　　　　　
  Nakata Y.; Yamada K.; Kimizuka H.; Tanaka Y.; Sugawara K.; Souma S.; Sato T.; Ueno K.; Takahashi T.
  Meeting Abstracts of the Physical Society of Japan, Volume：70, First page：1592, Last page：1592, 2015
  The Physical Society of Japan (JPS), Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.70.2.0_1592
  DOI ID：10.11316/jpsgaiyo.70.2.0_1592, ISSN：2189-079X, CiNii Articles ID：110010029283, CiNii Books ID：AA12721570
• 18aCE-3 Bi ultrathin film on 1T-TaS_2 studied by high-resolution ARPES　　　　　　　　　　　　　　　
  Yamada K.; C. Trang; Souma S.; Sugawara K.; Sato T.; Ueno K.; Takahashi T.
  Meeting Abstracts of the Physical Society of Japan, Volume：70, First page：1230, Last page：1230, 2015
  The Physical Society of Japan (JPS), Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.70.2.0_1230
  DOI ID：10.11316/jpsgaiyo.70.2.0_1230, ISSN：2189-079X, CiNii Articles ID：110010029080, CiNii Books ID：AA12721570
• Retraction to Poly(3-hexylthiophene) films by electrospray deposition for crystalline silicon/organic hybrid junction solar cells [Phys. Status Solidi C, 2071, (2012) 10-11], DOI: 10.1002/pssc.201200129
  Taiga Hiate; Naoto Miyauchi; Zeguo Tang; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  Physica Status Solidi (C) Current Topics in Solid State Physics, Volume：12, First page：1191, Last page：1191, Jan. 2015
  © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.The above article, published online on 25 July 2015 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the authors, the journal Editor-in-Chief, Stefan Hildebrandt, and Wiley-VCH GmbH & Co. KGaA. The retraction has been agreed since the contents of the article have already been published by the corresponding author in Jpn. J. Appl. Phys. 51, 061602 (2015) on 4 June 2015. The manuscript submitted to physica status solidi (c) was part of the E-MRS 2015 Spring Meeting, Symposium A, Proceedings. The authors regret the dual publication and express their sincere apologies to readers. (
  DOI：https://doi.org/10.1002/pssc.201570094
  DOI ID：10.1002/pssc.201570094, ISSN：1862-6351, SCOPUS ID：84939653832
• Efficient organic/polycrystalline silicon hybrid solar cells　　　　　　　　　　　　　　　
  Qiming Liu; Tatsuya Ohki; Dequan Liu; Hiromitsu Sugawara; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  NANO ENERGY, Volume：11, First page：260, Last page：266, Jan. 2015
  We firstly investigated efficient poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS)/n-type polycrystalline silicon (p-Si) heterojunction solar cells fabricated by chemical mist deposition (CMD) using a high-pressure H2O-vapor-treated p-Si prior to organic film deposition. High-pressure H2O vapor treatment of the p-Si efficiently suppressed grain boundary defects and improved carrier transport at the PEDOT:PSS/p-Si interface. Furthermore, compared to spin coated devices, the CMD devices demonstrated a more uniform photovoltaic performance. The power conversion efficiency of the PEDOT:PSS/p-Si heterojunction solar cells was 9.7% with a short-circuit current density of 33.5 mA/cm(2), an open-circuit voltage of 0.54 V, and a fill factor of 0.53. These findings suggest that CMD with a negatively charged mist precursor provides uniform adhesion of PEDOT:PSS on p-Si, resulting in increased photovoltaic performance. (C) 2014 Elsevier Ltd. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/j.nanoen.2014.10.032
  DOI ID：10.1016/j.nanoen.2014.10.032, ISSN：2211-2855, eISSN：2211-3282, SCOPUS ID：84911930097, Web of Science ID：WOS:000351194300030
• van der Waals Junctions of Layered 2D Materials for Functional Devices　　　　　　　　　　　　　　　
  Tomoki Machida; Rai Moriya; Miho Arai; Yohta Sata; Takehiro Yamaguchi; Naoto Yabuki; Sei Morikawa; Satoru Masubuchi; Keiji Ueno
  2015 IEEE INTERNATIONAL ELECTRON DEVICES MEETING (IEDM), Volume：2016-February, 2015
  We fabricated van der Waals (vdW) junctions of two-dimensional (2D) crystals using mechanical exfoliation and transfer technique of atomic layers. By connecting cleaved flakes of Fe0.25TaS2, van der Waals magnet tunnel junction was constructed, exhibiting tunneling magnetoresistance effect. In addition, large current modulation with ON-OFF current ratio exceeding 10(5) and ON current density of 10(4) A/cm(2) was achieved in metal/MoS2/graphene vertical FET. Josephson effect was demonstrated in van der Waals NbSe2/NbSe2 junctions.
  IEEE, English, International conference proceedings
  DOI：https://doi.org/10.1109/IEDM.2015.7409783
  DOI ID：10.1109/IEDM.2015.7409783, ISSN：0163-1918, SCOPUS ID：84964038603, Web of Science ID：WOS:000380472500177
• Erratum: Optical and carrier transport properties of graphene oxide based crystalline-Si/organic Schottky junction solar cells (Journal of Applied Physics (2013) 114 (234506))
  I. Khatri; Z. Tang; T. Hiate; Q. Liu; R. Ishikawa; K. Ueno; H. Shirai
  Journal of Applied Physics, Volume：116, Number：23, Dec. 2014
  American Institute of Physics Inc., English, Scientific journal
  DOI：https://doi.org/10.1063/1.4900524
  DOI ID：10.1063/1.4900524, ISSN：1089-7550, SCOPUS ID：84919683738
• Improved performance of poly(3,4-ethylenedioxythiophene):poly(stylene sulfonate)/n-Si hybrid solar cell by incorporating silver nanoparticles　　　　　　　　　　　　　　　
  Ishwor Khatri; Qiming Liu; Keiji Ueno; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：53, Number：11, Nov. 2014
  We report an enhancement in the efficiency of poly(3,4-ethylenedioxythiophene):poly(stylene sulfonate) (PEDOT:PSS)/n-Si hybrid solar cell by incorporating silver nanoparticles (AgNPs) with PEDOT:PSS. AgNPs were prepared by reducing silver nitrate in green-tea solution, which showed characteristic absorption peak due to the surface plasmonic resonance effect. AgNPs incorporated PEDOT:PSS/n-Si hybrid device shows power conversion efficiency (eta) of 10.21%, which is comparatively higher to the performance of pristine device without AgNPs. Here, we noticed that incorporation of AgNPs decreases sheet resistance and enlarged surface roughness of PEDOT:PSS film for the efficient collection of charges, rather than plasmonic effect. (C) 2014 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.7567/JJAP.53.110305
  DOI ID：10.7567/JJAP.53.110305, ISSN：0021-4922, eISSN：1347-4065, SCOPUS ID：84909957557, Web of Science ID：WOS:000346462200005
• Improved photovoltaic response by incorporating green tea modified multiwalled carbon nanotubes in organic-inorganic hybrid solar cell　　　　　　　　　　　　　　　
  Ishwor Khatri; Qiming Liu; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  CANADIAN JOURNAL OF PHYSICS, Volume：92, Number：7-8, First page：849, Last page：852, Jul. 2014
  In this work, multiwalled carbon nanotubes (MWCNTs) are separated and cut into short pipes using a green tea solution and embedded at interface of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) (PEDOT:PSS)/n-Si hybrid solar cells. Devices fabricated with embedding green tea modified MWCNTs show much better performance (10.02%) than that of a device without MWCNTs (9.2%) due to better hole transportation, easy exciton splitting, and suppression of charge recombination.
  CANADIAN SCIENCE PUBLISHING, NRC RESEARCH PRESS, English, Scientific journal
  DOI：https://doi.org/10.1139/cjp-2013-0506
  DOI ID：10.1139/cjp-2013-0506, ISSN：0008-4204, eISSN：1208-6045, SCOPUS ID：84904297439, Web of Science ID：WOS:000339379500064
• Self assembled silver nanowire mesh as top electrode for organic-inorganic hybrid solar cell　　　　　　　　　　　　　　　
  Ishwor Khatri; Qiming Liu; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  CANADIAN JOURNAL OF PHYSICS, Volume：92, Number：7-8, First page：867, Last page：870, Jul. 2014
  We prepare transparent, selfassembled polygonal silver nanowire (AgNW) mesh by bubble template and use as top electrode for a poly (3,4ethylenedioxythiophene):poly(stylenesulfonate) (PEDOT:PSS)/n-Si hybrid solar cell. Devices were fabricated by pressing the self-assembled AgNW and ITO electrodes onto the surface of the PEDOT: PSS and device performances were compared. In identical transmittances of ITO and self-assembled AgNW (i.e., 87% transmittance at wavelength of 550 nm), the self-assembled AgNW mesh electrodes shows lower sheet resistance (8 Omega/square) with enhanced transparency in the ultraviolet and infrared regions. As a result, a device performance with an efficiency of 9.60% was obtained with the self-assembled electrode compared to 9.07% efficiency from the indium-tin oxide (ITO) electrode under 100 mW/cm(2) of AM 1.5 illumination. This study suggests the potential application of a self-assembled AgNW electrode as the transparent conducting electrode for future optoelectronic devices.
  CANADIAN SCIENCE PUBLISHING, NRC RESEARCH PRESS, English, Scientific journal
  DOI：https://doi.org/10.1139/cjp-2013-0564
  DOI ID：10.1139/cjp-2013-0564, ISSN：0008-4204, eISSN：1208-6045, SCOPUS ID：84904317577, Web of Science ID：WOS:000339379500068
• Self-assembled silver nanowires as top electrode for poly(3,4-ethylenedioxythiophene): poly(stylenesulfonate)/n-silicon solar cell　　　　　　　　　　　　　　　
  Ishwor Khatri; Ayo Hoshino; Fumiya Watanabe; Qiming Liu; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  THIN SOLID FILMS, Volume：558, First page：306, Last page：310, May 2014
  We prepare transparent, self-assembled polygonal silver nanowire (AgNW) mesh by bubble template and transferred as top electrode in poly(3,4-ethylenedioxythiophene): poly(stylenesulfonate) (PEDOT:PSS)/n-silicon hybrid solar cell, which shows power conversion efficiency of 10.62% under 100 mW/cm(2) of AM 1.5G illuminations. The self-assembled AgNW mesh electrode exhibited a low sheet resistance (50 Omega/square) with high transmittance (87%) at a wavelength of 550 nm. In self-assembled AgNW mesh, AgNWs are well packed making continuous path for charge collection and transportation. Nevertheless, studies have also been carried out to transfer AgNWs from donor (glass or plastic) to receiver substrates (thermal release tape) and an expected reason for the transfer of AgNWs from thermal release tape to the top of PEDOT: PSS has also been proposed. (C) 2014 Elsevier B. V. All rights reserved.
  ELSEVIER SCIENCE SA, English, Scientific journal
  DOI：https://doi.org/10.1016/j.tsf.2014.02.073
  DOI ID：10.1016/j.tsf.2014.02.073, ISSN：0040-6090, SCOPUS ID：84898785080, Web of Science ID：WOS:000334314100047
• Ambipolar MoTe2 Transistors and Their Applications in Logic Circuits　　　　　　　　　　　　　　　
  Yen-Fu Lin; Yong Xu; Sheng-Tsung Wang; Song-Lin Li; Mahito Yamamoto; Alex Aparecido-Ferreira; Wenwu Li; Huabin Sun; Shu Nakaharai; Wen-Bin Jian; Keiji Ueno; Kazuhito Tsukagoshi
  ADVANCED MATERIALS, Volume：26, Number：20, First page：3263, Last page：+, May 2014
  We report ambipolar charge transport in a-molybdenum ditelluride (MoTe2) flakes, whereby the temperature dependence of the electrical characteristics was systematically analyzed. The ambipolarity of the charge transport originated from the formation of Schottky barriers at the metal/MoTe2 contacts. The Schottky barrier heights as well as the current on/off ratio could be modified by modulating the electrostatic fields of the back-gate voltage (V-bg) and drain-source voltage (V-ds). Using these ambipolar MoTe2 transistors we fabricated complementary inverters and amplifiers, demonstrating their feasibility for future digital and analog circuit applications.
  WILEY-V C H VERLAG GMBH, English, Scientific journal
  DOI：https://doi.org/10.1002/adma.201305845
  DOI ID：10.1002/adma.201305845, ISSN：0935-9648, eISSN：1521-4095, SCOPUS ID：84901493242, Web of Science ID：WOS:000336999800012
• Strong Enhancement of Raman Scattering from a Bulk-Inactive Vibrational Mode in Few-Layer MoTe2　　　　　　　　　　　　　　　
  Mahito Yamamoto; Sheng Tsung Wang; Meiyan Ni; Yen-Fu Lin; Song-Lin Li; Shinya Aikawa; Wen-Bin Jian; Keiji Ueno; Katsunori Wakabayashi; Kazuhito Tsukagoshi
  ACS NANO, Volume：8, Number：4, First page：3895, Last page：3903, Apr. 2014
  Two-dimensional layered crystals could show phonon properties that are markedly distinct from those of their bulk counterparts, because of the loss of periodicities along the c-axis directions. Here we investigate the phonon properties of bulk and atomically thin alpha-MoTe2 using Raman spectroscopy. The Raman spectrum of alpha-MoTe2 shows a prominent peak of the in-plane E-2g(1) mode, with its frequency upshifting with decreasing thickness down to the atomic scale, similar to other dichalcogenides. Furthermore, we find large enhancement of the Raman scattering from the out-of-plane B-2g(1) mode in the atomically thin layers. The B-2g(1) models Raman inactive in the bulk, but is observed to become active in the few-layer films. The intensity ratio of the B-2g(1) to E-2g(1) peaks evolves significantly with decreasing thickness, in contrast with other dichalcogenides. Our observations point to strong effects of dimensionality on the phonon properties of MoTe2.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/nn5007607
  DOI ID：10.1021/nn5007607, ISSN：1936-0851, eISSN：1936-086X, SCOPUS ID：84899451693, Web of Science ID：WOS:000334990600087
• Preparation of graphene films from solid state precursors on Cu foils by Joule-heating method　　　　　　　　　　　　　　　
  Baba Tomoya; Saiki Koichiro; Ueno Keiji
  JSAP Annual Meetings Extended Abstracts, Volume：2014.1, First page：3606, Last page：3606, Mar. 2014
  The Japan Society of Applied Physics, Japanese
  DOI：https://doi.org/10.11470/jsapmeeting.2014.1.0_3606
  DOI ID：10.11470/jsapmeeting.2014.1.0_3606, eISSN：2436-7613
• Real-time measurement of optical anisotropy during film growth using a chemical mist deposition of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)　　　　　　　　　　　　　　　
  T. Hiate; N. Miyauchi; Q. Liu; R. Ishikawa; K. Ueno; H. Shirai
  JOURNAL OF APPLIED PHYSICS, Volume：115, Number：12, Mar. 2014
  Real-time monitoring of optical anisotropy during growth by the chemical mist deposition of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films was carried out using spectroscopic ellipsometry. The microstructure of the grown films was found to be primarily determined by the DC bias applied to the mesh electrode. The ellipsometry results revealed that uniaxial anisotropy appeared for film thicknesses of about 5 nm and above, which corresponds to the average size of PEDOT crystallites. The extraordinary refractive index was found to be strongly correlated with the carrier mobility. Both the degree of optical anisotropy and the carrier mobility could be controlled during film growth by adjusting the DC bias. (C) 2014 AIP Publishing LLC.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4869956
  DOI ID：10.1063/1.4869956, ISSN：0021-8979, eISSN：1089-7550, SCOPUS ID：84898045854, Web of Science ID：WOS:000333901100023
• 原子層エレクトロニクスに向けたカルコゲナイド系層状物質の基礎物性と薄膜形成手法　　　　　　　　　　　　　　　
  上野啓司; 塚越一仁
  応用物理, Volume：83, Number：4, First page：274, Last page：278, 2014, [Reviewed]
  応用物理学会, Japanese
  ISSN：0369-8009, CiNii Articles ID：40020062119, CiNii Books ID：AN00026679, ORCID：35732259
• Solution-processed graphene oxide as a possible material for photovoltaic device　　　　　　　　　　　　　　　
  Shirai, H.; Ueno, K.
  Journal of the Institute of Electronics, Information and Communication Engineers, Volume：97, Number：3, First page：215, Last page：221, Jan. 2014
  Scientific journal
  ISSN：0913-5693, ORCID：33901965, SCOPUS ID：84901430610
• Optical and carrier transport properties of graphene oxide based crystalline-Si/organic Schottky junction solar cells
  I. Khatri; Z. Tang; T. Hiate; Q. Liu; R. Ishikawa; K. Ueno; H. Shirai
  Journal of Applied Physics, Volume：114, Number：23, Dec. 2013
  We investigated the graphene oxide (GO) based n-type crystalline silicon (c-Si)/conductive poly(ethylene dioxythiophene):poly(styrenesulfonate)(PEDOT: PSS) Schottky junction devices with optical characterization and carrier transport measurement techniques. The optical transmittance in the UV region decreased markedly for the films with increasing the concentration of GO whereas it increased markedly in the visible-infrared regions. Spectroscopic ellipsometry revealed that the ordinary and extraordinary index of refraction increased with increasing the concentration of GO. The hole mobility also increased from 1.14 for pristine film to 1.85 cm2/V s for the 12-15 wt. % GO modified film with no significant increases of carrier concentration. The highest conductivity was found for a 15 wt. % GO modified PEDOT:PSS film: the c-Si/PEDOT:PSS:GO device using this sample exhibited a relatively high power conversion efficiency of 11.04%. In addition, the insertion of a 2-3 nm-thick GO thin layer at the c-Si/PEDOT:PSS interface suppressed the carrier recombination efficiency of dark electron and photo-generated hole at the anode, resulting in the increased photovoltaic performance. This study indicates that the GO can be good candidates for hole transporting layer of c-Si/PEDOT:PSS Schottky junction solar cell. © 2013 AIP Publishing LLC.
  English, Scientific journal
  DOI：https://doi.org/10.1063/1.4847515
  DOI ID：10.1063/1.4847515, ISSN：0021-8979, SCOPUS ID：84891389994
• Improved photovoltaic performance of crystalline-Si/organic Schottky junction solar cells using ferroelectric polymers　　　　　　　　　　　　　　　
  Q. Liu; I. Khatri; R. Ishikawa; A. Fujimori; K. Ueno; K. Manabe; H. Nishino; H. Shirai
  APPLIED PHYSICS LETTERS, Volume：103, Number：16, Oct. 2013
  The effect of inserting an ultrathin layer of ferroelectric (FE) poly(vinylidene fluoridetetrafluoroethylene) P(VDF-TeFE) at the crystalline (c-)Si/poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) interface of a c-Si/PEDOT:PSS Schottky junction solar cell is demonstrated. P(VDF-TeFE) is a highly resistive material that exhibits a large, permanent, internal polarization electric field by poling of molecular dipole among the polymer chains. Because of these properties, performance can be enhanced by adjusting the thickness of the FE layer and subsequent poling process. Inserting a 3-nm-thick FE layer increases the power conversion efficiency g from 10.2% to 11.4% with a short-circuit current density J(sc) of 28.85 mA/cm(2), an open-circuit voltage V-oc of 0.57 V, and a fill factor FF of 0.692. Subsequent poling of the FE layer under a reverse DC bias stress increased g up to 12.3% with a J(sc) of 29.7 mA/cm(2), a V-oc of 0.58 V, and an FF of 0.71. The obtained results confirm that the spontaneous polarization of the FE layers is responsible for the enhancement of g, and that the polarization-based enhancement works if the FE layer is highly crystalline. These findings originate from efficient charge extraction to the electrodes and a suppression of non-radiative recombination at the c-Si/PEDOT: PSS interface. (C) 2013 AIP Publishing LLC.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4826323
  DOI ID：10.1063/1.4826323, ISSN：0003-6951, eISSN：1077-3118, SCOPUS ID：84886421198, Web of Science ID：WOS:000326148700086
• Optical anisotropy in solvent-modified poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) and its effect on the photovoltaic performance of crystalline silicon/organic heterojunction solar cells　　　　　　　　　　　　　　　
  Qiming Liu; Takashi Imamura; Taiga Hiate; Ishwor Khatri; Zeguo Tang; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  APPLIED PHYSICS LETTERS, Volume：102, Number：24, Jun. 2013
  An investigation was carried out into the effect of uniaxial optical anisotropy in poly (3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) on the photovoltaic performance of crystalline Si/PEDOT: PSS heterojunction solar cells fabricated by spin coating using either a methanol (MeOH) solvent alone or using MeOH and ethylene glycol (EG) as cosolvents. Spectroscopic ellipsometry revealed that the extraordinary index of refraction increased by the use of the cosolvents. In contrast, the ordinary index of refraction indicated metallic properties and was almost independent of the concentration of MeOH or EG. The highest conductivity was found for a (PEDOT: PSS):(MeOH):(EG) weight ratio of 1:1:0.1, and this sample exhibited a relatively high power conversion efficiency of 11.23%. These findings suggest that the increase in the extraordinary index of refraction leads to an enhancement of the hole mobility in PEDOT: PSS, resulting in improved photovoltaic performance. (C) 2013 AIP Publishing LLC.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4811355
  DOI ID：10.1063/1.4811355, ISSN：0003-6951, SCOPUS ID：84879820777, Web of Science ID：WOS:000320962400113
• Effects of molybdenum oxide molecular doping on the chemical structure of poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) and on carrier collection efficiency of silicon/poly(3,4-ethylenedioxythiophene): poly(stylenesulfonate) heterojunction solar cells　　　　　　　　　　　　　　　
  Q. Liu; I. Khatri; R. Ishikawa; K. Ueno; H. Shirai
  APPLIED PHYSICS LETTERS, Volume：102, Number：18, May 2013
  The effects of MoO3 molecular doping in poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate) (PEDOT:PSS) on the chemical structure and, in turn, on the carrier collection efficiency of c-Si/PEDOT:PSS heterojunction solar cells are demonstrated. Scanning electron microscopy revealed that the hydrophilic PSS polymer chain was intercalated into the interlayer van der Waals gap of MoO3 flake sheets, which modified the chemical structure of PEDOT: PSS. MoO3 exhibited intense photoluminescence in the 350-550 nm region, which enhanced the carrier collection efficiency of c-Si/PEDOT:PSS heterojunction solar cells with no significant changes. These findings suggest that the intense photoluminescence of MoO3 and its light wavelength conversion contribute to the increased carrier collection efficiency. (C) 2013 AIP Publishing LLC.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4804298
  DOI ID：10.1063/1.4804298, ISSN：0003-6951, eISSN：1077-3118, SCOPUS ID：84877775124, Web of Science ID：WOS:000320439900081
• Efficient Organic Photovoltaic Cells Using MoO3 Hole-Transporting Layers Prepared by Simple Spin-Cast of Its Dispersion Solution in Methanol　　　　　　　　　　　　　　　
  Mayuko Kishi; Yosuke Kubo; Ryo Ishikawa; Hajime Shirai; Keiji Ueno
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：52, Number：2, Feb. 2013
  MoO3 is one of the efficient hole-transporting materials for organic photovoltaic cells (OPVs). Here, a facile method of preparing the MoO3 buffer layer will be introduced. MoO3 powder was added into methanol and ultrasonication of the dispersion solution was carried out. Then the solution was centrifuged, and the supernatant was spin-cast on an indium-tin-oxide anode. On it, a photoconversion layer was prepared by spin-casting a poly(3-hexylthiophene):[6,6]-phenyl-C-61 butyric acid methyl ester (P3HT:PCBM) composite solution. The fabricated OPVs revealed an efficiency as high as 3.05%, which is better than that of P3HT:PCBM OPV with a poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) hole-transporting layer. (C) 2013 The Japan Society of Applied Physics
  JAPAN SOC APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.7567/JJAP.52.020202
  DOI ID：10.7567/JJAP.52.020202, ISSN：0021-4922, SCOPUS ID：84874145843, Web of Science ID：WOS:000314466800002
• Green-tea modified multiwalled carbon nanotubes for efficient poly(3,4-ethylenedioxythiophene):poly(stylenesulfonate)/n-silicon hybrid solar cell　　　　　　　　　　　　　　　
  Ishwor Khatri; Zeguo Tang; Qiming Liu; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  APPLIED PHYSICS LETTERS, Volume：102, Number：6, Feb. 2013
  Poly(3,4-ethylenedioxythiophene): poly(stylenesulfonate) (PEDOT: PSS)/n-Si hybrid solar cells were studied with and without embedding green-tea modify multiwalled carbon nanotubes (MWCNTs) at interface. Devices fabricated with embedding green-tea modified MWCNTs show much better performance than that of a device without MWCNTs with short circuit current density (J(sc)), open circuit voltage (V-oc), fill factor, and power conversion efficiency (eta) as 30.31 mA/cm(2), 0.54V, 0.66, and 10.93%, respectively. Here, we believe that green-tea disperse MWCNTs bundles to individual and its incorporation improved built-in potential (V-b) of the device for better hole transport, easy exciton splitting, and suppression of charge recombination, thereby improving photovoltaic response. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4792691]
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4792691
  DOI ID：10.1063/1.4792691, ISSN：0003-6951, SCOPUS ID：84874255755, Web of Science ID：WOS:000315053300101
• High-performance top-gated monolayer SnS2 field-effect transistors and their integrated logic circuits　　　　　　　　　　　　　　　
  H. S. Song; S. L. Li; L. Gao; Y. Xu; K. Ueno; J. Tang; Y. B. Cheng; K. Tsukagoshi
  NANOSCALE, Volume：5, Number：20, First page：9666, Last page：9670, 2013
  Two-dimensional (2D) layered semiconductors are very promising for post-silicon ultrathin channels and flexible electronics due to the remarkable dimensional and mechanical properties. Besides molybdenum disulfide (MoS2), the first recognized 2D semiconductor, it is also important to explore the wide spectrum of layered metal chalcogenides (LMCs) and to identify possible compounds with high performance. Here we report the fabrication of high-performance top-gated field-effect transistors (FETs) and related logic gates from monolayer tin disulfide (SnS2), a non-transition metal dichalcogenide. The measured carrier mobility of our monolayer devices reaches 50 cm(2) V-1 s(-1), much higher than that of the back-gated counterparts (similar to 1 cm(2) V-1 s(-1)). Based on a direct-coupled FET logic technique, advanced Boolean logic gates and operations are also implemented, with a voltage gain of 3.5 and output swing of >90% for the NOT and NOR gates, respectively. The superior electrical and integration properties make monolayer SnS2 a strong candidate for next-generation atomic electronics.
  ROYAL SOC CHEMISTRY, English, Scientific journal
  DOI：https://doi.org/10.1039/c3nr01899g
  DOI ID：10.1039/c3nr01899g, ISSN：2040-3364, eISSN：2040-3372, SCOPUS ID：84884862198, Web of Science ID：WOS:000325005500032
• Top-Contacted Organic Field-Effect Transistors with Graphene Electrodes Prepared by Laminate Transfer Method　　　　　　　　　　　　　　　
  Koichi Suganuma; Takuya Gotou; Keiji Ueno
  APPLIED PHYSICS EXPRESS, Volume：5, Number：12, Dec. 2012
  Top-contacted organic field-effect transistors (OFETs) with graphene electrodes were fabricated by the laminate transfer method. Graphene electrodes were prepared on a different glass substrate and transferred onto an organic semiconductor layer of poly(3-hexylthiophene) (P3HT) using a double-layer laminate film of polyacrylonitrile ( PAN) and poly( methyl methacrylate) ( PMMA). The fabricated top-contacted OFET with graphene electrodes covered by the polymer film showed good p-type transistor characteristics with hole mobility of (3.7 +/- 0.5) x 10(-2) cm(2) V-1 s(-1), and 92% of the mobility was maintained even after exposure to ambient air for 250 h. (C) 2012 The Japan Society of Applied Physics
  JAPAN SOC APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/APEX.5.125104
  DOI ID：10.1143/APEX.5.125104, ISSN：1882-0778, SCOPUS ID：84871267851, Web of Science ID：WOS:000312000800021
• Increased Organic Photovoltaic Cell Efficiency by Incorporating a Nonionic Fluorinated Surfactant Cathode Interlayer　　　　　　　　　　　　　　　
  Ryo Ishikawa; Hajime Shirai; Keiji Ueno
  APPLIED PHYSICS EXPRESS, Volume：5, Number：12, Dec. 2012
  The performance of an organic thin film photovoltaic cell was improved using a solution-processed fluorinated surfactant Zonyl (R) FS-300 (Zonyl) cathode interlayer. Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C-71-butyric acid methyl ester ([70]PCBM) were used as the donor and acceptor, respectively, in the bulk-heterojunction photoactive layer. The spin-casting of the appropriate concentration of the Zonyl solution on the photoactive layer significantly and simultaneously improved the short-circuit current, open-circuit voltage, and fill factor. The optimized Zonyl treatment increased the power conversion efficiency from 3.53 to 4.76%. (C) 2012 The Japan Society of Applied Physics
  JAPAN SOC APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/APEX.5.121601
  DOI ID：10.1143/APEX.5.121601, ISSN：1882-0778, SCOPUS ID：84871261851, Web of Science ID：WOS:000312000800003
• Crystalline Silicon/Graphene Oxide Hybrid Junction Solar Cells　　　　　　　　　　　　　　　
  Qiming Liu; Fumiya Wanatabe; Aya Hoshino; Ryo Ishikawa; Takuya Gotou; Keiji Ueno; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：51, Number：10, Oct. 2012
  Soluble graphene oxide (GO) and plasma-reduced (pr-) GO were investigated using crystalline silicon (c-Si) (100)/GO/pr-GO hybrid junction solar cells. Their photovoltaic performances were compared with those of c-Si/GO/pristine conductive poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) heterojunction and c-Si/PEDOT:PSS:GO composite devices. The c-Si/GO/pr-GO and conductive PEDOT:PSS/Al heterojunction solar cells showed power conversion efficiencies of 6.5 and 8.2%, respectively, under illumination with AM 1.5 G 100 mW/cm(2) simulated solar light. A higher performance of 10.7% was achieved using the PEDOT:PSS:GO (12.5 wt%) composite device. These findings imply that soluble GO, pr-GO, and the PEDOT:PSS:GO composite are promising materials as hole transport and transparent conductive layers for c-Si/organic hybrid junction solar cells. (C) 2012 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.51.10NE22
  DOI ID：10.1143/JJAP.51.10NE22, ISSN：0021-4922, eISSN：1347-4065, SCOPUS ID：84869120444, Web of Science ID：WOS:000310707800114
• Electrospray-Deposited Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) for Poly(3-hexylthiophene):Phenyl-C-61-Butyric Acid Methyl Ester Photovoltaic Cells　　　　　　　　　　　　　　　
  Taiga Hiate; Tomohisa Ino; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：51, Number：10, Oct. 2012
  Electrospray-deposited poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was investigated using several solvent solutions as a hole-transporting layer for spin-coated poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C-61 butyric acid methyl ester (P3HT:PCBM) mixture organic thin-film solar cells (OSCs). Among them, the uniform deposition of PEDOT:PSS films was realized using 70% N, N-dimethylformamide solvent solution. The P3HT:PCBM solar cells with electrospray-deposited PEDOT:PSS as a hole transport layer showed a relatively high performance with an open-circuit voltage V-oc of 0.47 V, a short-circuit current J(sc) of 8.9 mA/cm(2), a fill factor FF of 0.48, and a power conversion efficiency of 2.0%, which were almost the same as those with a spin-coated PEDOT:PSS layer. These findings imply that the electrospray-deposited PEDOT:PSS is a possible material for use as a hole transport layer for conjugated polymer-based OSCs. (C) 2012 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.51.10NE30
  DOI ID：10.1143/JJAP.51.10NE30, ISSN：0021-4922, eISSN：1347-4065, SCOPUS ID：84869115119, Web of Science ID：WOS:000310707800122
• Real-time ellipsometric characterization of the initial growth stage of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) films by electrospray deposition using N,N-dimethylformamide solvent solution　　　　　　　　　　　　　　　
  Tomohisa Ino; Taiga Hiate; Takeshi Fukuda; Keiji Ueno; Hajime Shirai
  JOURNAL OF NON-CRYSTALLINE SOLIDS, Volume：358, Number：17, First page：2520, Last page：2524, Sep. 2012
  The initial growth stage of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films by the electrospray deposition (ESD) method was investigated using 70% N,N-dimethylformamide (DMF) as a solvent solution through the real-time spectroscopic ellipsometric (SE) characterization. The uniaxial anisotropic optical property was prominent for the film growth on c-Si wafer, whereas the homogeneous growth occurred with random molecular orientation on indium-tin-oxide (ITO) coated glass. These findings probably originate from a preferential orientation of the polymer chains as obtained in the preparation process, sort of substrate, its morphology, and film thickness. (C) 2012 Elsevier B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/j.jnoncrysol.2012.01.055
  DOI ID：10.1016/j.jnoncrysol.2012.01.055, ISSN：0022-3093, eISSN：1873-4812, SCOPUS ID：84865702464, Web of Science ID：WOS:000310394700142
• Electrospray Deposition of Poly(3-hexylthiophene) Films for Crystalline Silicon/Organic Hybrid Junction Solar Cells　　　　　　　　　　　　　　　
  Tomohisa Ino; Masahiro Ono; Naoto Miyauchi; Qiming Liu; Zeguo Tang; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：51, Number：6, Jun. 2012
  The electrospray deposition (ESD) of poly(3-hexylthiophene) (P3HT) films for use in crystalline silicon/organic hybrid heterojunction solar cells on a crystalline silicon (c-Si) wafer was investigated using real-time characterization by spectroscopic ellipsometry (SE). In contrast to the nonuniform deposition of products frequently obtained by spin-coating, a uniform deposition of P3HT films was achieved on flat and textured hydrophobic c-Si wafers by adjusting the deposition conditions. Similar findings were also obtained for the deposition of conductive poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) on P3HT. The c-Si/P3HT/PEDOT:PSS heterojunction solar cells exhibited efficiencies of 4.1 and 6.3% on flat and textured c-Si wafers, respectively. These findings suggest that ESD is a promising method for the uniform deposition of P3HT and PEDOT: PSS films on flat and textured hydrophobic substrates. (c) 2012 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.51.061602
  DOI ID：10.1143/JJAP.51.061602, ISSN：0021-4922, eISSN：1347-4065, SCOPUS ID：84863329516, Web of Science ID：WOS:000305135200010
• Highly efficient crystalline silicon/Zonyl fluorosurfactant-treated organic heterojunction solar cells　　　　　　　　　　　　　　　
  Qiming Liu; Masahiro Ono; Zeguo Tang; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  APPLIED PHYSICS LETTERS, Volume：100, Number：18, Apr. 2012
  We demonstrate a highly efficient hybrid crystalline silicon (c-Si) based photovoltaic devices with hole-transporting transparent conductive poly-(3,4-ethlenedioxythiophene):poly(styrenesufonic acid) (PEDOT:PSS) films, incorporating a Zonyl fluorosurfactant as an additive, compared to non additive devices. The usage of a 0.1% Zonly treated PEDOT: PSS improved the adhesion of precursor solution on hydrophobic c-Si wafer without any oxidation process. The average power conversion efficiency eta value was 10.8%-11.3%, which was superior to those of non-treated devices. Consequently, c-Si/Zonyl-treated PEDOT: PSS heterojunction devices exhibited the highest eta of 11.34%. The Zonyl-treated soluble PEDOT: PSS composite is promising as a hole-transporting transparent conducting layer for c-Si/organic photovoltaic applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709615]
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.4709615
  DOI ID：10.1063/1.4709615, ISSN：0003-6951, eISSN：1077-3118, SCOPUS ID：84862542199, Web of Science ID：WOS:000303598600067
• Efficient Crystalline Si/Poly(ethylene dioxythiophene): Poly(styrene sulfonate): Graphene Oxide Composite Heterojunction Solar Cells　　　　　　　　　　　　　　　
  Masahiro Ono; Zeguo Tang; Ryo Ishikawa; Takuya Gotou; Keiji Ueno; Hajime Shirai
  APPLIED PHYSICS EXPRESS, Volume：5, Number：3, Mar. 2012
  Efficient crystalline silicon (c-Si) heterojunction solar cells with conductive poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT: PSS) and graphene oxide (GO) composite are demonstrated using a structure of Ag/PEDOT:PSS/PEDOT:PSS:GO composite/c-Si (100)(rho: 3-5 Omega.cm)/Al. The power-conversion efficiency eta increased to 10.7% under illumination of AM1.5 100mW/cm(2) simulated solar light by adjusting the PEDOT:PSS and GO mixing concentration ratio. The GO addition to conductive PEDOT:PSS suppressed electron recombination and/or promoted the hole current at the anode. The soluble PEDOT:PSS:GO composite is promising as a hole-transporting transparent conducting layer for c-Si photovoltaic applications. (c) 2012 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.1143/APEX.5.032301
  DOI ID：10.1143/APEX.5.032301, ISSN：1882-0778, eISSN：1882-0786, SCOPUS ID：84858675339, Web of Science ID：WOS:000301344400014
• Ionic liquid-mediated epitaxy of high-quality C-60 crystallites in a vacuum　　　　　　　　　　　　　　　
  Yoko Takeyama; Shingo Maruyama; Hiroki Taniguchi; Mitsuru Itoh; Keiji Ueno; Yuji Matsumoto
  CRYSTENGCOMM, Volume：14, Number：15, First page：4939, Last page：4945, 2012
  Ionic liquid-mediated vacuum deposition was demonstrated as a novel growth technique for high-quality C-60 crystallites. In this process the ionic liquid worked as a solvent, i.e. the gas phase C-60 precursors were continuously fed into the ionic liquid on a substrate in a vacuum chamber, from which the nucleation and the subsequent growth of C-60 crystallites proceeded. The impact of varying the substrate on the growth behaviour was intensively examined in terms of not only the lattice-mismatch between C-60 and the substrates, but also the wetting behavior of the ionic liquid on the substrates. This led to a significant improvement of the C-60 crystallinity and epitaxial growth was successfully found on certain substrates. In particular, molecularly smooth and thick C-60 hexagonal-shaped crystallites epitaxially grew on a MoS2 substrate, the average size of which reached as large as 10 mm.
  ROYAL SOC CHEMISTRY, English, Scientific journal
  DOI：https://doi.org/10.1039/c2ce25163a
  DOI ID：10.1039/c2ce25163a, ISSN：1466-8033, SCOPUS ID：84863705273, Web of Science ID：WOS:000305999500009
• Chemical mist deposition of graphene oxide and PEDOT:PSS films for crystalline Si/organic heterojunction solar cells　　　　　　　　　　　　　　　
  Ishwor Khatri; Takashi Imamura; Akira Uehara; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 10-11, Volume：9, Number：10-11, First page：2134, Last page：2137, 2012
  The chemical mist deposition of graphene oxide (GO) and poly(3,4-ethylenedioxythiophene): poly(stylene-sulfonate) (PEDOT:PSS) were investigated on the crystalline silicon (c-Si) substrate for the n-type c-Si/organic heterojunction solar cells. The uniform coating of GO flake with an average size of 1-5 mu m was obtained on the hydrophobic c-Si wafer using deionized water as a solvent. The GO layer thickness could be controlled from a monolayer to a few tens of nanometres by varying mist deposition time. The c-Si/GO/PEDOT:PSS heterojunction solar cells shows the efficiency improved from 8.75 to 9.27% with insertion of GO flakes a the PEDOT:PSS and c-Si interface owing to the enhanced suppression of electron recombination and/or promotion of hole current at the anode. Furthermore, the CMD was also applied to coat conductive PEDOT: PSS films on the textured c-Si. The. of 7.75% was obtained with a short-circuit current of 34.22 mA/cm(2), a open-circuit voltage of 0.46 V, and a fill factor of 0.48. These findings imply that CMD is a promising method for the uniform deposition of both nanomaterials (GO) and conjugated polymer for the c-Si/organic heterojunction solar cells. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  WILEY-V C H VERLAG GMBH, English, International conference proceedings
  DOI：https://doi.org/10.1002/pssc.201200132
  DOI ID：10.1002/pssc.201200132, ISSN：1862-6351, SCOPUS ID：84867947008, Web of Science ID：WOS:000314688000059
• Efficient crystalline Si/organic hybrid heterojunction solar cells　　　　　　　　　　　　　　　
  Qiming Liu; Ishwor Khatri; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 10-11, Volume：9, Number：10-11, First page：2101, Last page：2106, 2012
  Efficient crystalline silicon (c-Si) heterojunction solar cells with conductive poly(ethylene dioxythiophene): poly(styrene sulfonate)(PEDOT:PSS) and graphene oxide (GO) are demonstrated using device structures of c-Si/GO/PEDOT:PSS/Ag and cSi/PEDOT:PSS:GO composite/Al. The power-conversion efficiency eta increased from 8.76 to 9.27% under illumination of AM1.5 100 mW/cm(2) simulated solar light by adjusting the GO thickness for the nSi/GO/PEDOT:PSS heterojunction solar cells.. also increased to 10.6% for the n-Si/PEDOT:PSS:GO composite devices by adjusting the GO mixing concentration. The GO addition to conductive PEDOT:PSS suppressed electron recombination and/or promoted the hole current at the anode. The soluble PEDOT: PSS: GO composite is promising as a hole-transporting transparent conducting layer for c-Si/organic junction photovoltaic applications. The uniform coating of poly(3-hexylthiophene-2,5-dily) (P3HT) and PEDOT:PSS on textured hydrophobic c-Si wafer was observed by the electrospray deposition. The textured related enhancement of the device performance is demonstrated. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  WILEY-V C H VERLAG GMBH, English, International conference proceedings
  DOI：https://doi.org/10.1002/pssc.201200131
  DOI ID：10.1002/pssc.201200131, ISSN：1862-6351, SCOPUS ID：84867952936, Web of Science ID：WOS:000314688000051
• Poly(3-hexylthiophene) films by electrospray deposition for crystalline silicon/organic hybrid junction solar cells　　　　　　　　　　　　　　　
  Taiga Hiate; Naoto Miyauchi; Zeguo Tang; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 10-11, Volume：9, Number：10-11, First page：2071, Last page：2074, 2012
  The electrospray deposition (ESD) of poly(3-hexylthiophene) (P3HT) and conductive poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) on P3HT for use in crystalline silicon/organic hybrid heterojunction solar cells on CZ crystalline silicon (c-Si) (100) wafer was investigated using real-time characterization by spectroscopic ellipsometry (SE). In contrast to the nonuniform deposition of products frequently obtained by conventional spin-coating, a uniform deposition of P3HT and PEDOT:PSS films were achieved on flat and textured hydrophobic c-Si(100) wafers by adjusting the deposition conditions. The c-Si/P3HT/PEDOT:PSS heterojunction solar cells exhibited efficiencies of 4.1 and 6.3% on flat and textured c-Si(100) wafers, respectively. These findings suggest that ESD is a promising method for the uniform deposition of P3HT and PEDOT: PSS films on flat and textured hydrophobic substrates. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  WILEY-V C H VERLAG GMBH, English, International conference proceedings
  DOI：https://doi.org/10.1002/pssc.201200129
  DOI ID：10.1002/pssc.201200129, ISSN：1862-6351, SCOPUS ID：84867942138, Web of Science ID：WOS:000314688000044
• Optical properties and carrier transport in c-Si/conductive PEDOT:PSS(GO) composite heterojunctions　　　　　　　　　　　　　　　
  Zeguo Tang; Qiming Liu; Ishwor Khatri; Ryo Ishikawa; Keiji Ueno; Hajime Shirai
  PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 9, NO 10-11, Volume：9, Number：10-11, First page：2075, Last page：2078, 2012
  We investigated the crystalline silicon (c-Si)/conductive poly(ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS): graphene oxide (GO) composite heterojunction devices with optical characterization and carrier transport measurement techniques. The optical transmittance in the UV region also decreased markedly for the films with GO content above 20 wt%, whereas it increased markedly in the visible-infrared regions. The solar cell devices consisting of spin-coated PEDOT: PSS(GO) composite on n-type c-Si(100) wafer exhibited a maximum efficiency of 10.3%, a short-circuit current J(sc) of 28.9 mA/cm(2), a open-circuit voltage V-oc of 0.548 V, and a fill factor FF of 0.675 at a GO content of 12.5 wt%. The ideality factor deduced from current density-voltage (J-V) plots in the dark increased with GO content from 1.12 for pristine PEDOT:PSS to 2.91. Temperature-dependent dark J-V measurements suggest that the carrier transport in the devices is controlled by diffusion and recombination in the space-charge region for the devices. The role of the GO addition into conductive PEDOT: PSS is demonstrated for the c-Si(100)/-PEDOT:PSS (GO) composite junction solar cells in terms of the optical and carrier transport properties. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  WILEY-V C H VERLAG GMBH, English, International conference proceedings
  DOI：https://doi.org/10.1002/pssc.201200130
  DOI ID：10.1002/pssc.201200130, ISSN：1862-6351, SCOPUS ID：84867935974, Web of Science ID：WOS:000314688000045
• Solution Processed Graphene Transparent Conductive Film
  Keiji Ueno
  2012 19TH INTERNATIONAL WORKSHOP ON ACTIVE-MATRIX FLATPANEL DISPLAYS AND DEVICES (AM-FPD): TFT TECHNOLOGIES AND FPD MATERIALS, First page：49, Last page：52, 2012
  This article introduces the method to fabricate a transparent graphene conductive film from graphite powder by a solution process. Chemically oxidized graphite powder was dispersed in water, and it was exfoliated by repeated centrifugation and washing processes to obtain graphene oxide (GO). Successively, a GO film was prepared by coating a transparent substrate with the GO solution, and it was chemically and thermally deoxidized to produce a transparent graphene conductive film. To date, organic thin film photovoltaic cells with the graphene transparent electrodes have shown the power conversion efficiency as high as 1.2%. Novel application of GO to organic/inorganic heterojunction solar cells will be also presented.
  JAPAN SOCIETY APPLIED PHYSICS, English, International conference proceedings
  SCOPUS ID：84867908879, Web of Science ID：WOS:000326894500013
• Surface Modification of Poly(3,4-ethylene dioxthiophene):Poly(styrene sulfonic acid) (PEDOT:PSS) Films by Atmospheric-Pressure Argon Plasma for Organic Thin-Film Solar Cells　　　　　　　　　　　　　　　
  Tomohisa Mo; Tatsuya Hayashi; Takeshi Fukuda; Keiji Ueno; Hajime Shirai
  JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, Volume：11, Number：9, First page：8035, Last page：8039, Sep. 2011
  Highly-conductive poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) films obtained by the addition of dimethylsulfoxide (DMSO) and the argon plasma exposure were used as a transparent conductive anode (TCA) for copper-phthalocyanine (CuPc)/C-60 organic thin-film solar cells (OSCs). The CuPc/C-60 OSCs on as-grown DMSO added PEDOT:PSS layer showed a power efficiency of 0.6%, whereas it was improved markedly to 1.34% after the atmospheric-pressure argon plasma exposure, which was comparable to that formed on indium-tin-oxide layer. Effects of the DMSO addition and the argon plasma exposure in the spin-coated PEDOT:PSS films is demonstrated in terms of the in-depth characterization of optical and electrical properties.
  AMER SCIENTIFIC PUBLISHERS, English, Scientific journal
  DOI：https://doi.org/10.1166/jnn.2011.5065
  DOI ID：10.1166/jnn.2011.5065, ISSN：1533-4880, eISSN：1533-4899, SCOPUS ID：84856850669, Web of Science ID：WOS:000296209900070
• Real-Time Ellipsometric Characterization of the Initial Growth Stage of Poly(3,4-ethylene dioxythiophene): Poly(styrene sulfonic acid) Films by Electrospray Deposition　　　　　　　　　　　　　　　
  Tomohisa Ino; Takashi Asano; Takeshi Fukuda; Keiji Ueno; Hajime Shirai
  JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, Volume：11, Number：9, First page：8030, Last page：8034, Sep. 2011
  Kinetic spectroscopic ellipsometry have been used to study the initial growth stage of poly(3,4-polyethylene dioxythiophene):poly(styrenesulfonic acid)(PEDOT:PSS) films by the electrospray deposition (ESD) method. The real-time spectra analysis revealed that the surface overlayer decreased in thickness once the first bulk layer monolayer was formed, indicating a smoothening effect as the nucleation-related microstructure coalesced into the bulk layer. Once the coalescence was completed and the nucleation-induced surface roughness layer was stabilized, the underlying bulk layer increased linearly with time. These results originate from the degrees of the evaporation of solvent material during the transferring the precursors to the surface and/or of the diffusion of deposition precursors after sticking at the growing surface.
  AMER SCIENTIFIC PUBLISHERS, English, Scientific journal
  DOI：https://doi.org/10.1166/jnn.2011.5064
  DOI ID：10.1166/jnn.2011.5064, ISSN：1533-4880, eISSN：1533-4899, SCOPUS ID：84856894365, Web of Science ID：WOS:000296209900069
• Depth Profile Characterization of Spin-Coated Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonic acid) Films for Thin-Film Solar Cells during Argon Plasma Etching by Spectroscopic Ellipsometry　　　　　　　　　　　　　　　
  Tomohisa Ino; Tatsuya Hayashi; Keiji Ueno; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：50, Number：8, Aug. 2011
  Depth profiles of the optical constants, carrier mobility, and carrier density of spin-coated poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) films were investigated by real-time characterization by spectroscopic ellipsometry (SE) during argon plasma etching at atmospheric pressure. Spectral analysis revealed that homogeneous etching occurred within 10-15 nm of the top surface, followed by the appearance of a conductive PEDOT phase and surface roughening, which originated from the depth profile of the PEDOT-to-PSS molar concentration ratio. The use of the plasma-etched PEDOT:PSS layer relatively improved the performance of the cupper phthalocyanine (CuPc)/C-60 organic thin-films solar cells as a hole-transport layer with higher optical transmittance by adjusting the plasma etching condition. (C) 2011 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.50.08JG02
  DOI ID：10.1143/JJAP.50.08JG02, ISSN：0021-4922, eISSN：1347-4065, SCOPUS ID：80051969829, Web of Science ID：WOS:000294339500045
• Real-Time Ellipsometric Characterization of Initial Growth Stage of Poly(3,4-ethylene dioxythiophene):Poly(styrene sulfonic acid) Films by Electrospray Deposition　　　　　　　　　　　　　　　
  Tomohisa Ino; Takashi Asano; Takeshi Fukuda; Keiji Ueno; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：50, Number：8, Aug. 2011
  Kinetic spectroscopic ellipsometry has been used to study the initial growth stage of poly(3,4-ethylene dioxythiophene): poly(styrene sulfonic acid) (PEDOT:PSS) films by the electrospray deposition (ESD) method. The real-time spectra analysis revealed that the surface overlayer decreased in thickness, once the first bulk layer monolayer was formed, indicating a smoothening effect as the nucleation-related microstructure coalesced into the bulk layer. Once the coalescence was completed and the nucleation-induced surface roughness layer was stabilized, the underlying bulk layer increased linearly with time. These results originate from the degrees of the evaporation of solvent material during the transfer of the precursors to the surface and/or of the diffusion of deposition precursors after sticking at the growing surface. (C) 2011 The Japan Society of Applied Physics
  IOP PUBLISHING LTD, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.50.081603
  DOI ID：10.1143/JJAP.50.081603, ISSN：0021-4922, eISSN：1347-4065, SCOPUS ID：80051974504, Web of Science ID：WOS:000294336600032
• Atmospheric-pressure argon plasma etching of spin-coated 3,4-polyethylenedioxythiophene:polystyrenesulfonic acid (PEDOT:PSS) films for cupper phtalocyanine (CuPc)/C-60 heterojunction thin-film solar cells　　　　　　　　　　　　　　　
  Tomohisa Ino; Tatsuya Hayashi; Keiji Ueno; Hajime Shirai
  THIN SOLID FILMS, Volume：519, Number：20, First page：6834, Last page：6839, Aug. 2011
  Depth profiles of the optical constants, carrier mobility, and carrier density of spin-coated 3,4-polyethylenedioxythiophene:polystyrenesulfonic acid (PEDOT:PSS) films were investigated by real-time characterization by the spectroscopic ellipsometry (SE) during argon plasma etching at atmospheric pressure. Spectral analysis revealed that homogeneous etching occurred within 10-15 nm of the top surface, followed by the appearance of a conductive PEDOT phase and surface roughning, which originated from the depth profile of the PEDOT-to-PSS molar concentration ratio. The use of the plasma-etched PEDOT:PSS layer improved relatively the performance of the copper phtalocyanine (CuPc)/C-60 organic thin-films solar cells as a hole-transport layer with higher optical transmittance by adjusting the plasma etching condition. (C) 2011 Elsevier B.V. All rights reserved.
  ELSEVIER SCIENCE SA, English, Scientific journal
  DOI：https://doi.org/10.1016/j.tsf.2011.04.042
  DOI ID：10.1016/j.tsf.2011.04.042, ISSN：0040-6090, SCOPUS ID：80051547199, Web of Science ID：WOS:000294790900043
• Bulk heterojunction organic photovoltaic cell fabricated by the electrospray deposition method using mixed organic solvent　　　　　　　　　　　　　　　
  Takeshi Fukuda; Kenji Takagi; Takashi Asano; Zentaro Honda; Norihiko Kamata; Keiji Ueno; Hajime Shirai; Jungmyoung Ju; Yutaka Yamagata; Yusuke Tajima
  PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS, Volume：5, Number：7, First page：229, Last page：231, Jul. 2011
  A high-efficiency bulk heterojunction organic photovoltaic cell (OPV) was achieved by the electrospray deposition method. The surface roughness of the P3HT: PCBM thin film can be reduced using the mixed solvent consisting of o-dichlorobenzene (o-DCB) and acetone. The effect of acetone concentration is related to its dielectric constant. Under an optimized concentration of acetone in o-DCB (20 vol%), the P3HT/PCBM active layer with a smooth surface can be formed, and the power conversion efficiency of the OPV was 1.9%. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  WILEY-BLACKWELL, English, Scientific journal
  DOI：https://doi.org/10.1002/pssr.201105232
  DOI ID：10.1002/pssr.201105232, ISSN：1862-6254, SCOPUS ID：79959962240, Web of Science ID：WOS:000293595100005
• Efficient Organic Photovoltaic Cells Using Hole-Transporting MoO3 Buffer Layers Converted from Solution-Processed MoS2 Films　　　　　　　　　　　　　　　
  Shogo Kato; Ryo Ishikawa; Yosuke Kubo; Hajime Shirai; Keiji Ueno
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：50, Number：7, Jul. 2011
  We introduce a new method to fabricate a MoO3 hole-transporting layer for organic photovoltaic cells (OPVs). We fabricated a MoS2 film from its solution and converted it to MoO3. MoS2 has a lamellar crystal structure similar to graphite, and it can be exfoliated into monolayer MoS2 dispersible in water. Li atoms were first intercalated into van der Waals gaps of MoS2, and the compound was immersed in water to generate H-2 bubbles, which broke the van der Waals bond between adjacent MoS2 layers. The produced solution of MoS2 was spin-casted on an indium tin oxide substrate, and the film was oxidized by ozone. On the converted MoO3 hole-transporting layer, an organic photoconversion layer was fabricated by spin-casting a poly(3-hexylthiophene):[6,6]-phenyl-C-61 butyric acid methyl ester (P3HT:PCBM) composite solution. Fabricated OPVs revealed a power conversion efficiency as large as 3.14%, which was superior to that of P3HT/PCBM OPV with a poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) hole-transporting layer. (C) 2011 The Japan Society of Applied Physics
  JAPAN SOC APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.50.071604
  DOI ID：10.1143/JJAP.50.071604, ISSN：0021-4922, SCOPUS ID：79960694667, Web of Science ID：WOS:000292878200045
• Fabrication of Transparent and Flexible Organic Field-Effect Transistors with Solution-Processed Graphene Source-Drain and Gate Electrodes　　　　　　　　　　　　　　　
  Koichi Suganuma; Shunichiro Watanabe; Takuya Gotou; Keiji Ueno
  APPLIED PHYSICS EXPRESS, Volume：4, Number：2, Feb. 2011
  Transparent organic field-effect transistors with solution-processed graphene source-drain and gate electrodes were fabricated on flexible substrates. Graphene electrodes were formulated by chemical and thermal reduction of solution-processed graphene oxide films. A graphene gate electrode, a gate dielectric layer, and an organic active layer were coated on a transparent flexible polyimide film. Next, graphene source-drain electrodes, which had been formed on another glass substrate, were peeled off and transferred onto the active layer using a thermal release tape. The transparent and flexible poly(3-hexylthiophene) transistor with graphene electrodes showed p-type characteristics with hole mobility of 2.3 x 10(-2) cm(2).V(-1).s(-1). (C) 2011 The Japan Society of Applied Physics
  JAPAN SOC APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/APEX.4.021603
  DOI ID：10.1143/APEX.4.021603, ISSN：1882-0778, SCOPUS ID：79951588162, Web of Science ID：WOS:000287378800011
• Depth profile characterization of spin-coated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) films by spectroscopic ellipsometry　　　　　　　　　　　　　　　
  Tomohisa Ino; Tatsuya Hayashi; Takeshi Fukuda; Keiji Ueno; Hajime Shirai
  PHYSICA STATUS SOLIDI C: CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 8, NO 10, Volume：8, Number：10, First page：3025, Last page：3028, 2011
  Depth profiles of the optical constants, carrier mobility, and carrier density of spin-coated poly(3,4-ethylenedioxythiophene):(polystyrene sulfonic acid) (PEDOT: PSS) films were investigated by real-time spectroscopic ellipsometry (SE) characterization during argon plasma etching at atmospheric pressure. Spectra analysis revealed that homogeneous etching occurred within 10-20 nm of the top surface, followed by the appearance of conductive PEDOT phase with a surface roughening. These findings originated from the depth profile of the PEDOT-to-PSS molar concentration ratio. The use of a 10-20-nm etched PEDOT: PSS layer improved relatively the performance of a hole-transport layer for CuPc/C-60 organic thin-films solar cells with higher optical transmittance. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
  WILEY-V C H VERLAG GMBH, English, International conference proceedings
  DOI：https://doi.org/10.1002/pssc.201001218
  DOI ID：10.1002/pssc.201001218, ISSN：1862-6351, SCOPUS ID：80053599990, Web of Science ID：WOS:000301591500013
• Origin of the ambipolar operation of a pentacene field-effect transistor fabricated on a poly(vinyl alcohol)-coated Ta2O5 gate dielectric with Au source/drain electrodes　　　　　　　　　　　　　　　
  Satoko Takebayashi; Shigeomi Abe; Koichiro Saiki; Keiji Ueno
  APPLIED PHYSICS LETTERS, Volume：94, Number：8, Feb. 2009
  Pentacene field-effect transistors (FETs) were fabricated on a poly(vinyl alcohol) (PVA)-coated Ta2O5 gate dielectric using Au source/drain electrodes. Their device characteristics were examined before and after annealing the FETs in a vacuum. Before annealing, the pentacene FET showed an ambipolar operation with hole mobility of 0.1 cm(2) V-1 s(-1) and electron mobility of 0.005 cm(2) V-1 s(-1). After annealing at 50 degrees C, however, the p-type operation characteristics almost diminished and an enhanced n-type operation was observed. Na+ ions in the PVA are supposed to change the character of the injection barrier at the Au/pentacene junction and enable the n-type operation.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.3089692
  DOI ID：10.1063/1.3089692, ISSN：0003-6951, SCOPUS ID：61349094005, Web of Science ID：WOS:000263804400079
• 27aYH-8 Reduction of ultra thin graphene oxide films and its electric properties　　　　　　　　　　　　　　　
  小幡 誠司; 佐藤 裕樹; 白石 淳子; 服部 功三; 吉田 雅史; 上野 啓司; 斉木 幸一朗
  Meeting Abstracts of the Physical Society of Japan, Volume：64.1.4, First page：812, 2009
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.64.1.4.0_812_3
  DOI ID：10.11316/jpsgaiyo.64.1.4.0_812_3, eISSN：2189-0803, CiNii Articles ID：110007372761
• Step-bunched Bi-terminated Si(111) surfaces as a nanoscale orientation template for quasisingle crystalline epitaxial growth of thin film phase pentacene　　　　　　　　　　　　　　　
  Toshihiro Shimada; Manabu Ohtomo; Tadamasa Suzuki; Tetsuya Hasegawa; Keiji Ueno; Susumu Ikeda; Koichiro Saiki; Miho Sasaki; Katsuhiko Inaba
  APPLIED PHYSICS LETTERS, Volume：93, Number：22, First page：223303, Dec. 2008
  We developed a nanoscale orientation template for quasisingle crystalline epitaxial growth of thin film phase pentacene. By using alpha-3x3 Bi termination of surface dangling bonds on step-bunched vicinal surface of Si(111), thin film phase epitaxial pentacene was grown with the crystal axes aligned to the surface steps. Alignment occurred when the step height was higher than the molecular height. The mechanism of the alignment was examined by calculating the energy of the crystal edge.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.3040309
  DOI ID：10.1063/1.3040309, ISSN：0003-6951, eISSN：1077-3118, CiNii Articles ID：80020068364, SCOPUS ID：57349166529, Web of Science ID：WOS:000261430600074
• Nucleation on the substrate surfaces during liquid flux-mediated vacuum deposition of rubrene　　　　　　　　　　　　　　　
  Toshihiro Shimada; Yui Ishii; Keiji Ueno; Tetsuya Hasegawa
  JOURNAL OF CRYSTAL GROWTH, Volume：311, Number：1, First page：163, Last page：166, Dec. 2008
  We studied the effect of the substrate materials on the solution-mediated vacuum deposition of organic semiconductor rubrene using liquid flux with a low vapor pressure. We found that bis-2-ethylhexyl phthalate was suitable as the liquid flux. It was revealed that the microscopic crystal shape completely changed when different substrates were used. This result clearly indicates the nucleation of the crystals occurs on the substrate surface submerged in the solution. Epitaxially oriented needle crystals as long as 5 mm were obtained by using Au(111) substrate. (C) 2008 Elsevier B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/j.jcrysgro.2008.10.096
  DOI ID：10.1016/j.jcrysgro.2008.10.096, ISSN：0022-0248, SCOPUS ID：57649215856, Web of Science ID：WOS:000262796500032
• Nanotransfer of the Polythiophene Molecular Alignment onto the Step-Bunched Vicinal Si(111) Substrate　　　　　　　　　　　　　　　
  Ryo Onoki; Genki Yoshikawa; Yuki Tsuruma; Susumu Ikeda; Koichiro Saiki; Keiji Ueno
  LANGMUIR, Volume：24, Number：20, First page：11605, Last page：11610, Oct. 2008
  A poly(3-dodecylthiophene-2,5-diyl) film having in-plane anisotropic molecular arrangement was successfully fabricated by transferring its Langmuir-Blodgett film onto a step-bunched Si(111) substrate. Polarized near-edge X-ray absorption fine structure measurements revealed that the polythiophene main chains are preferentially orientated along periodic facet/terrace nanostructures on the step-bunched substrate, whereas less anisotropy was found on a flat substrate. The step-bunched Si substrate has been proved to be effective for controlling the in-plane molecular arrangement in the polymer thin film.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/la8016722
  DOI ID：10.1021/la8016722, ISSN：0743-7463, CiNii Articles ID：80019928706, PubMed ID：18778089, SCOPUS ID：55549094431, Web of Science ID：WOS:000260049300039
• Molecular layer-by-layer growth of C-60 thin films by continuous-wave infrared laser deposition　　　　　　　　　　　　　　　
  Seiichiro Yaginuma; Kenji Itaka; Masamitsu Haemori; Masao Katayama; Keiji Ueno; Tsuyoshi Ohnishi; Mikk Lippmaa; Yuji Matsumoto; Hideomi Koinuma
  APPLIED PHYSICS EXPRESS, Volume：1, Number：1, First page：015005-1-015005-3, Jan. 2008
  The observation of reflection high energy electron diffraction (RHEED) oscillations has been proved to be a key to open the nano-world of materials, since it definitely verifies that the film growth proceeds in layer-by-layer mode with each layer thickness controllable by simply counting the number of oscillations. This enabled the fabrication of nano-engineered hetero-junctions and devices as commonly practiced for conventional semiconductors and metals. Here we report on the first observation of clear RHEED intensity oscillation in thin film fabrication of pi-conjugated molecular solid. The observation has been achieved by coupling a novel deposition method using a continuous-wave infrared laser for evaporation and a high sensitive RHEED detector, in addition to the combinatorial optimization of film deposition parameters that facilitated our preceding first success in the layer-by-layer growth of oxide thin films. Some details of system design and experimental conditions are presented to discuss the key factors for atomically controlled film growth of molecular solids. (C) 2008 The Japan Society of Applied Physics.
  JAPAN SOC APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/APEX.1.015005
  DOI ID：10.1143/APEX.1.015005, ISSN：1882-0778, SCOPUS ID：57049104631, Web of Science ID：WOS:000255452500034
• Effect of organic buffer layer on performance of pentacene field-effect transistor fabricated on natural mica gate dielectric　　　　　　　　　　　　　　　
  Akira Matsumoto; Ryo Onoki; Susumu Ikeda; Koichiro Saiki; Keiji Ueno
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS, Volume：46, Number：36-40, First page：L913, Last page：L916, Oct. 2007
  The mobility of pentacene-based organic field-effect transistors (OFETs) fabricated on natural mica substrates has been highly improved using poly(methyl methacrylate) (PMMA) or octadecyltrimethoxysillane (OTMS) as a buffer layer between the mica dielectric and the pentacene active layer. The field-effect mobility increased from 2.4 x 10(-3) to 0.14 or 0.31 cm(2)center dot V-1 center dot s(-1) by inserting the PMMA or OTMS buffer layer, respectively. The suppression of the unfavorable effect of potassium ions using the OTMS buffer layer confirmed the feasibility of using natural mica as a transparent gate dielectric of OFETs.
  INST PURE APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.46.L913
  DOI ID：10.1143/JJAP.46.L913, ISSN：0021-4922, CiNii Articles ID：120001370972, SCOPUS ID：36048995921, Web of Science ID：WOS:000250535600017
• Orientation control of standing epitaxial pentacene monolayers using surface steps and inplane band dispersion analysis by angle resolved photoelectron spectroscopy
  Tadamasa Suzuki; Toshihiro Shimada; Keiji Ueno; Susumu Ikeda; Koichiro Saiki; Tetsuya Hasegawa
  Materials Research Society Symposium Proceedings, Volume：965, First page：380, Last page：385, Dec. 2006
  ABSTRACT
  
  We prepared quasi single crystalline pentacene monolayer films on Bi-terminated Si(111) by using bunched steps on vicinally-cut surfaces as an orientation template. Band dispersion in the conduction plane of pentacene was clearly observed by angle-resolved photoelectron spectroscopy.
  Springer Science and Business Media LLC, Scientific journal
  DOI：https://doi.org/10.1557/proc-0965-s06-19
  DOI ID：10.1557/proc-0965-s06-19, ISSN：0272-9172, eISSN：1946-4274, SCOPUS ID：40949147723
• Structure of organic thin films grown on surface-modified tantalum oxide　　　　　　　　　　　　　　　
  Ryo Onoki; Shigeomi Abe; Keiji Ueno; Hiroo Nakahara; Koichiro Saiki
  CHEMISTRY LETTERS, Volume：35, Number：7, First page：746, Last page：747, Jul. 2006
  We fabricated an ordered monolayer film of a long-chain fatty acid or a perfluorofatty acid onto an anodized tantalum oxide (Ta2O5) amorphous surface by the Langmuir-Blodgett (LB) method, and grew a pentacene or C-60 thin film on it. Remarkable improvements of morphology and crystallinity of the organic thin films were observed on the CH3-terminated monolayer film of the fatty acid.
  CHEMICAL SOC JAPAN, English, Scientific journal
  DOI：https://doi.org/10.1246/cl.2006.746
  DOI ID：10.1246/cl.2006.746, ISSN：0366-7022, eISSN：1348-0715, SCOPUS ID：33749340028, Web of Science ID：WOS:000240439900026
• In-situ measurement of molecular orientation of the pentacene ultrathin films grown on SiO2 substrates　　　　　　　　　　　　　　　
  Genki Yoshikawa; Tetsuhiko Miyadera; Ryo Onoki; Keiji Ueno; Ikuyo Nakai; Shiro Entani; Susumu Ikeda; Dong Guo; Manabu Kiguchi; Hiroshi Kondoh; Toshiaki Ohta; Koichiro Saiki
  SURFACE SCIENCE, Volume：600, Number：12, First page：2518, Last page：2522, Jun. 2006
  Molecular orientations of pentacene ultrathin films grown on SiO2 substrates were studied without the influence of the atmosphere by vacuum atomic force microscopy (V-AFM) and near edge X-ray absorption fine structure (NEXAFS). The experimental processes from deposition of pentacene to characterization of films were performed under vacuum condition without exposure to the atmosphere. V-AFM and NEXAFS measurements showed that pentacene molecules tend to grow on SiO2 surface with their molecular long axes perpendicular to the substrate surfaces (standing-mode) irrespective of preparation procedure of SiO2 substrate. (c) 2006 Elsevier B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/j.susc.2006.04.012
  DOI ID：10.1016/j.susc.2006.04.012, ISSN：0039-6028, CiNii Articles ID：120001370595, SCOPUS ID：33744992043, Web of Science ID：WOS:000238893800016
• Anisotropic polymerization of a long-chain diacetylene derivative Langmuir-Blodgett film on a step-bunched SiO2/Si surface　　　　　　　　　　　　　　　
  R Onoki; K Ueno; H Nakahara; G Yoshikawa; S Ikeda; S Entani; T Miyadera; Nakai, I; H Kondoh; T Ohta; M Kiguchi; K Saiki
  LANGMUIR, Volume：22, Number：13, First page：5742, Last page：5747, Jun. 2006
  Alternating facet/terrace nanostructures were fabricated on a SiO2 surface by step-bunching and thermal oxidation of a vicinal Si(111) substrate, and their influence upon the polymerization direction of a long-chain diacetylene derivative monolayer film was investigated by angle-dependent polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that the peak intensity of the C 1s-d* transition was stronger when the electric vector plane of the incident X-ray was parallel to the direction of the periodic facet/terrace structures rather than perpendicular to them. On the contrary, a polymer film fabricated on a flat SiO2 surface showed no in-plane anisotropy of the peak intensity. These results indicate that the diacetylene groups in the diacetylene derivative monolayer are preferentially photopolymerized in the direction not across but along the periodic one-dimensional structures on the step-bunched and thermally oxidized SiO2/Si(111) surface.
  AMER CHEMICAL SOC, English, Scientific journal
  DOI：https://doi.org/10.1021/la060482d
  DOI ID：10.1021/la060482d, ISSN：0743-7463, SCOPUS ID：33745746059, Web of Science ID：WOS:000238217000039
• Fabrication of an organic field-effect transistor on a mica gate dielectric　　　　　　　　　　　　　　　
  Akira Matsumoto; Ryo Onoki; Keiji Ueno; Susumu Ikeda; Koichiro Saiki
  CHEMISTRY LETTERS, Volume：35, Number：4, First page：354, Last page：355, Apr. 2006
  We fabricated pentacene-based organic field-effect transistors (OFETs) using single-crystalline natural mica substrates as the gate dielectric. Substrate temperatures during the deposition of pentacene films were varied from room temperature (RT) to 90 degrees C. Epitaxial growth of the pentacene film on the mica surface was observed even at RT. The FET working characteristics on the mica gate dielectric have been observed for the first time.
  CHEMICAL SOC JAPAN, English, Scientific journal
  DOI：https://doi.org/10.1246/cl.2006.354
  DOI ID：10.1246/cl.2006.354, ISSN：0366-7022, eISSN：1348-0715, SCOPUS ID：33745840149, Web of Science ID：WOS:000237721500005
• Anodization of electrolytically polished Ta surfaces for enhancement of carrier injection into organic field-effect transistors　　　　　　　　　　　　　　　
  K Ueno; S Abe; R Onoki; K Saiki
  JOURNAL OF APPLIED PHYSICS, Volume：98, Number：11, First page：114503-1-114503-5, Dec. 2005
  Electrolytically polished surfaces of Ta sheets were anodized in an aqueous KI solution to form Ta2O5 films with a relatively high dielectric constant and a high breakdown electric field. Organic field-effect transistors (OFETs) were fabricated on these Ta2O5/Ta substrates with a bottom gate configuration. The Ta2O5 films anodized at 100 V were about 200 nm thick, and the dielectric strength was as high as 5 MV/cm. A p-type pentacene OFET fabricated on this bottom gate showed the good field-effect mobility of 0.35 cm(2)/V s. By using this high-k gate substrate, it becomes possible to inject a higher amount of charge carriers into organic active layers than on the SiO2/Si gate conventionally used in most OFETs.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.2138807
  DOI ID：10.1063/1.2138807, ISSN：0021-8979, eISSN：1089-7550, SCOPUS ID：29144535658, Web of Science ID：WOS:000234119600088
• Bulk-like pentacene epitaxial films on hydrogen-terminated Si(111)　　　　　　　　　　　　　　　
  T Shimada; H Nogawa; T Hasegawa; R Okada; H Ichikawa; K Ueno; K Saiki
  APPLIED PHYSICS LETTERS, Volume：87, Number：6, First page：061917, Aug. 2005
  The epitaxial growth of pentacene on hydrogen-terminated Si(111) is reported. Reflection high energy electron diffraction (RHEED) revealed that the crystal packing resembles that in the bulk crystal even at a monolayer thickness, which was maintained in multilayers. A ripening effect was clearly observed by atomic force microscopy (AFM). These results are important to obtain oriented crystalline films of pentacene combined with silicon microdevices with reduced defect densities. (c) 2005 American Institute of Physics.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.2008371
  DOI ID：10.1063/1.2008371, ISSN：0003-6951, eISSN：1077-3118, SCOPUS ID：23944444818, Web of Science ID：WOS:000231016900031
• 26pXC-7 Direction-controlled polymerization and NEXAFS analysis of a long-chain diacetylene derivative LB film　　　　　　　　　　　　　　　
  Onoki R.; Ueno K.; Nakahara H.; Yoshikawa G.; Ikeda S.; Kiguchi M.; Nakai I.; Kondoh H.; Ohta T.; Saiki K.
  Meeting Abstracts of the Physical Society of Japan, Volume：60.1.4, First page：870, 2005
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.60.1.4.0_870_3
  DOI ID：10.11316/jpsgaiyo.60.1.4.0_870_3, eISSN：2189-0803, CiNii Articles ID：110004537383
• Morphological change of C60 monolayer epitaxial films under photoexcitation　　　　　　　　　　　　　　　
  Y. Yamamoto; H. Ichikawa; K. Ueno; A. Koma; K. Saiki; T. Shimada
  Physical Review B - Condensed Matter and Materials Physics, Volume：70, Number：15, First page：1, Last page：155415, Oct. 2004, [Reviewed]
  Thin films of C60 were grown under laser irradiation at various intensities 0-200 mW/mm2 and their growth shapes were investigated by ex situ atomic force microscopy (AFM) observation. The nucleation density of the first layer decreases with increasing laser intensity, probably due to the temperature rise of the migrating clusters. In addition to this gradual laser intensity dependence, an anomalous enhancement of the nucleation density was observed on irradiation at 50 mW/mm2, which was attributed to the influence of photopolymerization. As for the second layer, there Was a threshold laser intensity (200 mW/mm2) at which the nucleation density increased and the shape of the domains became irregular. This is due to the combined effect of hindered migration caused by the polymerized first layer and photopolymerization of the migrating molecules themselves. Energy transfer from the excited substrate to the migrating C60 molecules is strongly suggested.
  English, Scientific journal
  DOI：https://doi.org/10.1103/PhysRevB.70.155415
  DOI ID：10.1103/PhysRevB.70.155415, ISSN：0163-1829, SCOPUS ID：37649031320
• Methyl-terminated Si(111) surface as the ultra thin protection layer to fabricate position-controlled alkyl SAMs by using atomic force microscope anodic oxidation　　　　　　　　　　　　　　　
  R Okada; T Miyadera; T Shimada; A Koma; K Ueno; K Saiki
  SURFACE SCIENCE, Volume：552, Number：1-3, First page：46, Last page：52, Mar. 2004
  Methyl-terminated Si(111) (Me-Si(111)) and bilayer-GaSe terminated Si (BGS) surfaces were investigated to explore their usefulness as the protection layer in the fabrication process of nanometer-scale self-assembled monolayers (SAMs). BGS was not stable in the reagent involved in the alkyl SAM formation, whereas Me-Si(111) was suitable for the protection layer. Anodization using atomic force microscope (AFM) and following HF and hexadecene treatments were performed to form nano-scale alkyl SAM lines on the Me-Si(111) surface. It was found from AFM observation and frictional force measurement that only the patterned areas were selectively covered with hexadecyl SAMs. (C) 2004 Elsevier B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/j.susc.2004.01.026
  DOI ID：10.1016/j.susc.2004.01.026, ISSN：0039-6028, eISSN：1879-2758, CiNii Articles ID：80016549565, SCOPUS ID：1342264254, Web of Science ID：WOS:000220123000009
• Scanning tunneling microscopy and spectroscopy study of LiBr/Si(001) heterostructure　　　　　　　　　　　　　　　
  M Katayama; K Ueno; A Koma; M Kiguchi; K Saiki
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS, Volume：43, Number：2A, First page：L203, Last page：L205, Feb. 2004
  LiBr/Si(001) heterostructure has been investigated by scanning tunneling microscopy and spectroscopy (STM and STS). In the initial stage of LiBr growth, rectangular islands are observed consisting of accumulation of about 0.2 nm-thick unit layers. The STM results indicate that LiBr grows on Si(001) in a single layer fashion. The STS measurement shows a wide band gap region in I-V curve and the energy gap of the LiBr film shows no thickness dependence down to a nominal thickness of 1.2 monolayer (ML).
  INST PURE APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.43L203
  DOI ID：10.1143/JJAP.43L203, ISSN：0021-4922, CiNii Articles ID：150000012948, SCOPUS ID：1942475767, Web of Science ID：WOS:000220092900027
• Accumulation and depletion layer thicknesses in organic field effect transistors　　　　　　　　　　　　　　　
  M Kiguchi; M Nakayama; K Fujiwara; K Ueno; T Shimada; K Saiki
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS, Volume：42, Number：12A, First page：L1408, Last page：L1410, Dec. 2003
  We present a simple but powerful method to determine the thicknesses of the accumulation and depletion layers and the distribution curve of injected carriers in organic field effect transistors. The conductivity of organic semiconductors in thin film transistors was measured in situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. Using this method, the thicknesses of the accumulation and depletion layers of pentacene were determined to be 0.9 nm (V-G = - 15 V) and 5 nm (V-G = 15 V), respectively.
  INST PURE APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.42.L1408
  DOI ID：10.1143/JJAP.42.L1408, ISSN：0021-4922, CiNii Articles ID：150000012372, SCOPUS ID：0742286297, Web of Science ID：WOS:000187509800002
• Visible light photoemission and negative electron affinity of single-crystalline CsCl thin films　　　　　　　　　　　　　　　
  G Yoshikawa; M Kiguchi; K Ueno; A Koma; K Saiki
  SURFACE SCIENCE, Volume：544, Number：2-3, First page：220, Last page：226, Oct. 2003
  CsCl has been known to show extraordinary photoemission by visible (VIS) light only after ultraviolet (UV) light irradiation. To elucidate this phenomenon, photo yield and photoemission spectrum of a single-crystalline CsCl thin film were measured in ultrahigh vacuum. Photo yield measurements confirmed that photoelectrons were emitted from F-centers induced in the CsCl thin film by preceding UV irradiation. Absence of the similar phenomenon in the NaCl film indicated that VIS photoemission in alkali halides was caused not only by defect formation but also by negative electron affinity. (C) 2003 Elsevier B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/j.susc.2003.08.016
  DOI ID：10.1016/j.susc.2003.08.016, ISSN：0039-6028, CiNii Articles ID：80016315513, SCOPUS ID：0141959022, Web of Science ID：WOS:000186017900015
• Nano-scale anodic oxidation on a Si(111) surface terminated by bilayer-GaSe　　　　　　　　　　　　　　　
  K Ueno; R Okada; K Saiki; A Koma
  SURFACE SCIENCE, Volume：514, Number：1-3, First page：27, Last page：32, Aug. 2002
  Nano-scale oxide patterns were fabricated on a bilayer-GaSe terminated Si(1 1 1) surface using an atomic force microscope (AFM) in air. The Si(1 1 1) surface regularly terminated by bilayer-GaSe is very stable in air, although it can be oxidized through the electrochemical reaction when positive sample bias voltage is applied between the surface and a conductive cantilever tip of AFM. It has been revealed that higher sample bias voltage, slower tip velocity and/or higher ambient humidity produce wider and/or thicker oxide protrusions. Then, nano-scale oxide patterns as narrow as 50 nm have been successfully drawn on the terminated surface by adjusting these experimental conditions. These oxide lines can be etched away by dipping the sample into aqueous HF solution, and nano-scale grooves can be fabricated on the bilayer-GaSe terminated Si(1 1 1) surface. (C) 2002 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/S0039-6028(02)01603-5
  DOI ID：10.1016/S0039-6028(02)01603-5, ISSN：0039-6028, eISSN：1879-2758, SCOPUS ID：0037055541, Web of Science ID：WOS:000177832500005
• Fabrication of GaAs quantum dots on a bilayer-GaSe terminated Si(111) substrate　　　　　　　　　　　　　　　
  上野啓司; 斉木幸一朗; 小間篤
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, Volume：40, Number：3B, First page：1888, Last page：1891, Dec. 2001
  We have developed a novel method to fabricate self-assembled quantum dots (QDs) of compound semiconductors on a Si(111) substrate using the so-called 'droplet epitaxy' technique. In order to fabricate QDs on a Si substrate by droplet epitaxy, we examined the termination of a Si (111) surface with a bilayer-GaSe. This surface is formed by depositing 1 monolayer Ga on a Si(111)-7×7 surface and annealing in a Se flux at 520°C. Then Ga atoms are deposited to form Ga droplets on this surface, and the sample is annealed in an As flux to transform Ga droplets into GaAs QDs. It is revealed that GaAs QDs with a diameter as small as 10nm and a height of 5nm can be formed on the bilayer-GaSe/Si(111) substrate at a maximum density of 8.4 × 1010 cm-2. Using this method a new technique will be available to fabricate QDs of many kinds of compound semiconductors on the Si(111) substrate. © 2001 The Japan Society of Applied Physics.
  INSTITUTE OF PURE AND APPLIED PHYSICS, English
  DOI：https://doi.org/10.1143/JJAP.40.1888
  DOI ID：10.1143/JJAP.40.1888, ISSN：0021-4922, CiNii Articles ID：150000038119, CiNii Books ID：AA10457675, SCOPUS ID：0035267624
• Electron-energy-loss spectroscopy of C-60 monolayer films on active and inactive surfaces　　　　　　　　　　　　　　　
  K Iizumi; K Ueno; K Saiki; A Koma
  APPLIED SURFACE SCIENCE, Volume：169, First page：142, Last page：146, Jan. 2001, [Reviewed]
  The characteristics of interaction between C-60 molecules and Si(111)-7 x 7, Ag/Si(111)-root3 x root3 R30 degrees and layered material MoS2 surfaces have been investigated using electron-energy-loss spectroscopy (EELS), The EEL spectrum of C-60/Si(111)-7 x 7 shows a new peak at loss energy of 2.7 eV. This indicates the existence of charge transfer from the substrate to C-60 molecules. The EEL spectrum of a C-60 monolayer film grown on a cleaved surface of MoS2 is almost the same as that of bulk C-60 The EEL spectrum of a C-60 monolayer film on an Ag/Si(111) surface is quite different from that on a clean Si(111)-7 x 7 surface, although the films on those substrates have the same epitaxial arrangement. Furthermore, intensities of energy-loss peaks of C-60/Ag/Si(111) are slightly smaller than those of C-60/MoS2 in spite of having the same loss-energy. This suggests that the interaction between C-60 molecules and the Ag/Si(111) surface is stronger than that between C-60 molecules and the MoS2 surface. (C) 2001 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/S0169-4332(00)00740-6
  DOI ID：10.1016/S0169-4332(00)00740-6, ISSN：0169-4332, Web of Science ID：WOS:000167087700028
• Fabrication of compound-semiconductor quantum dots on a Si(111) substrate terminated by bilayer-GaSe　　　　　　　　　　　　　　　
  K Ueno; K Saiki; A Koma
  PROCEEDINGS OF THE 25TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS, PTS I AND II, Volume：87, First page：385, Last page：386, 2001, [Reviewed]
  We have developed a novel method to fabricate self-assembled quantum dots (QDs) of compound semiconductors on a Si(111) substrate, whose surface is terminated by bilayer-GaSe. This surface is formed by depositing 1 monolayer Ga on a clean Si(111)-7 x 7 surface and successive annealing in a Se flux at 520 degreesC. Then the surface was irradiated with a Ga flux to form Ga droplets and annealed in an As flux at 250 degreesC. It has been revealed that GaAs nano-crystals with the average size of 30 nm diameter and 10 nm height are formed on the substrate with density of 1.2 X 10(10)cm(-2). This method will open a new way to fabricate QDs of many kinds of compound semiconductors.
  SPRINGER-VERLAG BERLIN, English, International conference proceedings
  ISSN：0930-8989, Web of Science ID：WOS:000171592800177
• Low-energy electron energy loss spectroscopy of monolayer and thick La@C-82 films grown on MOS2 substrates　　　　　　　　　　　　　　　
  K Ueno; Y Uchino; K Iizumi; A Koma; K Saiki; Y Inada; K Nagai; Y Iwasa; T Mitani
  NANONETWORK MATERIALS: FULLERENES, NANOTUBES AND RELATED SYSTEMS, Volume：590, First page：485, Last page：488, 2001, [Reviewed]
  Monolayer and multilayer La@C-82 films were epitaxially grown on MoS2 substrates, and their growth features together with electronic states were investigated using atomic force microscopy and electron energy loss spectroscopy. It was found that the interaction between a La@C-82 molecule and a MoS2 surface is stronger than that between La@C-82 Molecules. The electronic state of the initial La@C-82 layer on the MoS2 surface differs from that of the successively grown bulk-like layers due to excess electrons in the carbon cage of La@C-82.
  AMER INST PHYSICS, English, International conference proceedings
  ISSN：0094-243X, Web of Science ID：WOS:000176150700115
• Electron-energy-loss spectroscopy of KxC60 and K-halides: comparison in the K-3p excitation region　　　　　　　　　　　　　　　
  K Ueno; Y Uchino; K Iizumi; K Saiki; A Koma
  APPLIED SURFACE SCIENCE, Volume：169, First page：184, Last page：187, Jan. 2001
  We have investigated the electronic structure of KxC60 (x = 0-6) using low-energy electron energy loss spectroscopy (LEELS), especially focusing on the K-3p core-electron excitation spectra. It is found that the structure of the K-3p-excitation spectrum of KxC60 quite differs from that of KCI. Furthermore, the K-3p-excitation LEELS of K3C60 has been revealed to be different from that of K6C60. K-3p electrons are excited into K-4s- and K-3d-derived empty states in both KxC60 and KCI, but in the case of KxC60 the K-3d-derived empty states have a rather complicated structure where several levels are not well separated. Consequently, K-3p-excitation LEEL spectra of KxC60 show wide-spread plateau-like structures. The difference in the K-3p- excitation spectra of K3C60 and K6C60 is considered to originate from the different crystal field around K+ cations in the C-60 molecular crystal. (C) 2001 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/S0169-4332(00)00729-7
  DOI ID：10.1016/S0169-4332(00)00729-7, ISSN：0169-4332, CiNii Articles ID：120001370985, SCOPUS ID：18844471293, Web of Science ID：WOS:000167087700037
• Investigation of the growth mechanism of an InSe epitaxial layer on a MoS2 substrate　　　　　　　　　　　　　　　
  T Hayashi; K Ueno; K Saiki; A Koma
  JOURNAL OF CRYSTAL GROWTH, Volume：219, Number：1-2, First page：115, Last page：122, Oct. 2000
  Sub-monolayer films of layered semiconductor InSe were grown on MoS2 substrates by molecular beam epitaxy, and the change in their growth features with Se/In flux ratio was investigated using scanning tunneling microscope in vacuum. It was found that InSe domains grown at 340 degrees C have a hexagonal shape when the Se/In ratio is about 17. Detailed images of the hexagonal InSe domains have revealed that adjacent sides of the hexagon have different structures; one is a straight edge and the other is a disordered edge. When the Se/In ratio was decreased, the disordered edges became predominant, the straight edges disappeared, and the InSe domain became triangular. On the contrary, when the Se/In ratio was increased, InSe domains became triangular ones consisting of only straight edges. The growth mechanism of InSe domains is discussed by considering the crystal structure of InSe and the reactivity of each side with incoming atoms. It is suggested that the balance of incorporation rate of In and Se atoms determines the structure of InSe domains. (C) 2000 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/S0022-0248(00)00627-8
  DOI ID：10.1016/S0022-0248(00)00627-8, ISSN：0022-0248, SCOPUS ID：0037743369, Web of Science ID：WOS:000089675700015
• Highly sensitive reflection high-energy electron diffraction measurement by use of micro-channel imaging plate　　　　　　　　　　　　　　　
  K Saiki; T Kono; K Ueno; A Koma
  REVIEW OF SCIENTIFIC INSTRUMENTS, Volume：71, Number：9, First page：3478, Last page：3479, Sep. 2000
  Micro-channel plate (MCP) was used to intensify the image on the fluorescent screen in reflection high-energy electron diffraction. The primary electron current could be reduced by four orders of magnitude to get the image of the same intensity as in the measurement without MCP. The reduction of electron beam enabled observation of ionic crystal surfaces without causing electrolysis and charging effect. (C) 2000 American Institute of Physics. [S0034- 6748(00)02309-1].
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.1287625
  DOI ID：10.1063/1.1287625, ISSN：0034-6748, CiNii Articles ID：120001370930, SCOPUS ID：0141916970, Web of Science ID：WOS:000089036700036
• Electron energy loss spectroscopy of a La@C_<82> thin film.　　　　　　　　　　　　　　　
  Uchino Y.; Ueno K.; Iizumi K.; Saiki K.; Koma A.; Inada Y.; Nagai K.; Iwasa Y.; Mitani T.
  Meeting Abstracts of the Physical Society of Japan, Volume：55.1.4, First page：740, 2000
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.55.1.4.0_740_2
  DOI ID：10.11316/jpsgaiyo.55.1.4.0_740_2, eISSN：2189-0803, CiNii Articles ID：110001916237
• Electron spectroscopy of Ga quantum dots fabricated on bilayer-GaSe.　　　　　　　　　　　　　　　
  Nakahara T.; Ueno K.; Saiki K.; Koma A.
  Meeting Abstracts of the Physical Society of Japan, Volume：55.2.4, First page：778, 2000
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.55.2.4.0_778_2
  DOI ID：10.11316/jpsgaiyo.55.2.4.0_778_2, eISSN：2189-0803, CiNii Articles ID：110002150713
• Growth and characterization of Ga2Se3/GaAs(100) epitaxial thin films　　　　　　　　　　　　　　　
  K Ueno; M Kawayama; ZR Dai; A Koma; FS Ohuchi
  JOURNAL OF CRYSTAL GROWTH, Volume：207, Number：1-2, First page：69, Last page：76, Nov. 1999, [Reviewed]
  Single crystalline Ga2Se3 thin films were grown on GaAs(1 0 0) by molecular beam epitaxial (MBE) under Se-rich atmosphere. Reflection high-energy electron diffraction (RHEED) analysis suggests that the Ga2Se3 thin film is epitaxially grown on GaAs(1 0 0) with their [1 0 0] crystallographic directions aligned with each other. The electronic structure determined by the ultraviolet photoelectron spectroscopy (UPS) and electron energy loss spectroscopy (EELS) revealed the forbidden band gap of 2.7 eV. Transmission electron microscopy has shown that vacancies are distributed on one set of {1 1 1} crystal planes of alpha-Ga2Se3 structure by forming a root 3 x root 3 configuration, resulting in the three times larger modulation periodicity along one of the [1 1 0] crystal directions into vacancy ordered beta-Ga2Se3 structure. The crystal structure was consistent to a model proposed by Lubbers and Leute (J. Solid State Chem. 43 (1982) 339). There are two sets of planar defects, microtwin with {1 1 1} type twin planes and stacking faults. High population of planer defects observed in the thin film was regarded as a result of Ga vacancy ordering in the crystal structure of Ga2Se3. (C) 1999 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  ISSN：0022-0248, eISSN：1873-5002, Web of Science ID：WOS:000083594900011
• A Novel Method to Fabricate a Molecular Quantum Structure: Selective Growth of C60 on Layered Material Heterostructures　　　　　　　　　　　　　　　
  Keiji Ueno; Kentaro Sasaki; Koichiro Saiki; Atsushi Koma
  Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers, Volume：38, Number：1 B, First page：511, Last page：514, 1999
  We propose a new method to fabricate nano-scale devices consisting of C60. C60 molecules have a spherical shape with a diameter of 0.7 nm. It may be possible to consider each molecule as a quantum dot to fabricate a single-electron-tunneling device which will work at room temperature. In order to control the position of a C60 molecule on a device substrate, we have developed a "selective growth" technique on layered material heterostructure substrates, which utilizes the difference in the nucleation process of C60, molecules on various kinds of layered materials. To clarify the selective growth mechanism, the density of C60 islands grown on different layered material substrates was measured using atomic force microscopy. It has been suggested that differences in the adsorption and diffusion energies of a C60 molecule on layered materials influence the selective growth.
  Japan Society of Applied Physics, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.38.511
  DOI ID：10.1143/JJAP.38.511, ISSN：0021-4922, CiNii Articles ID：110003955735, SCOPUS ID：0032621059
• Van der Weals type buffer layers: epitaxial growth of the large lattice mismatch system CdS/InSe/H-Si(111)　　　　　　　　　　　　　　　
  T Loher; K Ueno; A Koma
  APPLIED SURFACE SCIENCE, Volume：130, First page：334, Last page：339, Jun. 1998, [Reviewed]
  The lattice mismatch at the heterostructure CdS/Si(111) is close to 8%. To relieve the structural stress at this interface, an InSe van der Waals-type buffer layer was grown between the Si substrate and the CdS film. After the Si(111) substrate surface was passivated by hydrogen, the InSe-buffer layer was grown on top by molecular beam epitaxy with high crystalline quality. The lattice orientations of the InSe film are in registry with the Si substrate, as shown by reflection high energy electron diffraction. As seen in the atomic force microscope, the InSe films have roughly the same terrace width of 50-70 nm as the Si substrate due to the wafer miscut. By contrast the step edges of the InSe film are more regular than those of the silicon. The CdS-film was grown on the InSe. The crystal axis of the film is in full registry with the InSe substrate as monitored by reflection high energy electron diffraction. The film morphology is determined as simultaneous growth of multilayers observed by atomic force microscope. (C) 1998 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  ISSN：0169-4332, Web of Science ID：WOS:000074407600059
• Fabrication of C(60) nanostructures by selective growth on GaSe/MoS(2) and InSe/MoS(2) heterostructure substrates　　　　　　　　　　　　　　　
  K Ueno; K Sasaki; T Nakahara; A Koma
  APPLIED SURFACE SCIENCE, Volume：130, First page：670, Last page：675, Jun. 1998
  C(60) molecules were deposited on a submonolayer InSe film which was grown on a MoS(2) substrate. In the previous experiment on the growth of a C(60) thin film on a GaSe/MoS(2) heterostructure, C(60) grew only on exposed MoS(2) regions and never nucleated on GaSe domains at substrate temperature above 180 degrees C. In the present case, however, C(60) molecules grow only on InSe domains and do not nucleate on the exposed MoS(2) when the substrate temperature is higher than 80 degrees C. Using this method, C(60) domains whose dimension is smaller than 100 nm could be fabricated on each InSe domain. The selectivity of the C(60) growth is supposed to originate not from the surface morphology of those heterostructures, but from the difference in adsorption energy and surface diffusion energy of C(60) molecules on the surfaces of three different layered materials and a C(60) film. (C) 1998 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/S0169-4332(98)00136-6
  DOI ID：10.1016/S0169-4332(98)00136-6, ISSN：0169-4332, CiNii Articles ID：120001370933, SCOPUS ID：17444440854, Web of Science ID：WOS:000074407600116
• Nanostructure fabrication using selective growth on nanosize patterns drawn by a scanning probe microscope　　　　　　　　　　　　　　　
  K Sasaki; K Ueno; A Koma
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, Volume：36, Number：6B, First page：4061, Last page：4064, Jun. 1997
  A novel method of fabricating organic material nanostructures using selective growth on patterned layered material surfaces has been developed. First, an epitaxial monolayer film of layered semiconductor GaSe was gown on a cleaved face of MoS2. Then, nanosize patterns were drawn by scratching only the grown GaSe film using an atomic force microscope (AFM). Next, C-60 molecules were deposited on the surface. It has been found that if a substrate temperature is appropriately chosen, C-60 molecules nucleate only on the bare MoS2 surface and fill up the carved nanostructures. This combination of AFM lithography and selective growth enables the formation of C-60 nanostructures as small as 10 nm.
  JAPAN J APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.36.4061
  DOI ID：10.1143/JJAP.36.4061, ISSN：0021-4922, CiNii Articles ID：110003947120, SCOPUS ID：0004786354, Web of Science ID：WOS:000073628300065
• Investigation of the growth mechanism of layered semiconductor GaSe　　　　　　　　　　　　　　　
  K Ueno; N Takeda; K Sasaki; A Koma
  APPLIED SURFACE SCIENCE, Volume：113, First page：38, Last page：42, Apr. 1997
  Heteroepitaxial films of lamellar semiconductor GaSe were grown on MoS2 cleaved surfaces by molecular beam epitaxy and its growth mechanism was investigated by atomic force microscope. At the initial stage of the growth, triangular islands of GaSe with a unit layer thickness are formed, There exist two types of domains, which have opposite orientations of the triangles, It is suggested that the difference in the orientation corresponds to the difference in stacking positions of Se-Ga-Ga-Se four hexagonal atomic sheets in the unit layer. It is also revealed that stacking polytypes of GaSe can he determined by observing the orientation of triangular islands in each unit layer in a multilayer film. Most of GaSe films have the epsilon- and/or gamma-type slacking. But some domains really have the beta-type one which rarely appear in a hulk single-crystal of GaSe.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/S0169-4332(96)00837-9
  DOI ID：10.1016/S0169-4332(96)00837-9, ISSN：0169-4332, SCOPUS ID：0031547190, Web of Science ID：WOS:A1997WW85000008
• Nanostructure fabrication by selective growth of molecular crystals on layered material substrates　　　　　　　　　　　　　　　
  K Ueno; K Sasaki; N Takeda; K Saiki; A Koma
  APPLIED PHYSICS LETTERS, Volume：70, Number：9, First page：1104, Last page：1106, Mar. 1997
  Nanostructures consisting of C-60 molecules were fabricated on a GaSe/MoS2 heterostructure. A submonolayer film of a lamellar compound semiconductor GaSe was grown on a MoS2 substrate to form nanoscale holes or grooves surrounded by monolayer steps. Atomic force microscope (AFM) observation indicates that C-60 molecules grow only in the bare MoS2 nanoregions at a substrate temperature of 180 degrees C. C-60 molecules fill up those holes and grooves, and nanoscale C-60 domains with polygonal shapes can be formed. This selective growth method can be combined with nanoscale patterning made by a scanning-tunneling microscope or AFM to produce nanostructures of molecular crystals with designed shapes. (C) 1997 American Institute of Physics.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1063/1.118498
  DOI ID：10.1063/1.118498, ISSN：0003-6951, CiNii Articles ID：120001370929, SCOPUS ID：0343095886, Web of Science ID：WOS:A1997WL14700015
• Preparation of GaS thin films by molecular beam epitaxy　　　　　　　　　　　　　　　
  H Yamada; K Ueno; A Koma
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS, Volume：35, Number：5A, First page：L568, Last page：L570, May 1996
  Layered semiconductor GaS thin films have been grown by molecular beam epitaxy on hydrogen-terminated Si(111) and GaSe(0001) substrates. Elemental sulfur is so volatile that we used a valved cell as a sulfur source. Two similar to 100 monolayer of GaS films were obtained which have sharp reflection high-energy electron diffraction (RHEED) patterns. The GaS thin film grown on GaSe(0001) was a single-domain one, whereas that on hydrogen-terminated Si(111) was a multidomain one. Both films have been proved to be stoichiometric and to have the same electronic structure as that of the bulk GaS.
  JAPAN J APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.35.L568
  DOI ID：10.1143/JJAP.35.L568, ISSN：0021-4922, SCOPUS ID：0030142630, Web of Science ID：WOS:A1996UM42300012
• SCANNING TUNNELING MICROSCOPE OBSERVATION OF THE METAL-ADSORBED LAYERED SEMICONDUCTOR SURFACES　　　　　　　　　　　　　　　
  H ABE; K KATAOKA; K UENO; A KOMA
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS, Volume：34, Number：6B, First page：3342, Last page：3345, Jun. 1995
  Two metal elements, Sn and Na, have been deposited onto layered semiconductor MoS2(0001) surfaces and observed with a scanning tunneling microscope. The metal-adsorbed area shows a very large dark spot (3-5nm diameter) when the sample bias is positive, and the atomic image of the MoS2 substrate can be clearly seen inside the dark area. The same area, on the other hand, shows a bright horseshoe-shaped spot when the sample bias is negative.
  JAPAN J APPLIED PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.34.3342
  DOI ID：10.1143/JJAP.34.3342, ISSN：0021-4922, SCOPUS ID：0029327415, Web of Science ID：WOS:A1995RH68400007
• VAN-DER-WAALS EPITAXY ON HYDROGEN-TERMINATED SI(111) SURFACES AND INVESTIGATION OF ITS GROWTH-MECHANISM BY ATOMIC-FORCE MICROSCOPE　　　　　　　　　　　　　　　
  K UENO; M SAKURAI; A KOMA
  JOURNAL OF CRYSTAL GROWTH, Volume：150, Number：1-4, First page：1180, Last page：1185, May 1995, [Reviewed]
  NH4F treatment of a Si(lll) substrate gives a very flat and chemically inactive surface because of the regular termination of each active dangling bond by a hydrogen atom. On this inactive surface. layered semiconductor GaSe and molecular crystal C-60 grow through weak Van der Waals interaction. As a result their epitaxial films can be grown with their own lattice structures in spite of the large difference in their lattice constants from the substrate. Atomic force microscope (AFM) analysis of the GaSe thin film revealed many triangular spiral structures. indicating screw dislocation growth mechanism. In contrast, AFM observation of the C-60 film showed that it grows in the island growth mode, and each island has a two-dimensional dendritic shape.
  ELSEVIER SCIENCE BV, English, Scientific journal
  ISSN：0022-0248, Web of Science ID：WOS:A1995RD43300095
• Substrate effects on the nucleation and growth of GaSe layers by van der Waals epitaxy　　　　　　　　　　　　　　　
  LE Rumaner; JL Gray; FS Ohuchi; K Ueno; A Koma
  STRUCTURE AND PROPERTIES OF MULTILAYERED THIN FILMS, Volume：382, First page：101, Last page：106, 1995, [Reviewed]
  MATERIALS RESEARCH SOC, English, International conference proceedings
  ISSN：0272-9172, Web of Science ID：WOS:A1995BE43P00016
• HETEROEPITAXIAL GROWTH OF LAYERED GASE FILMS ON GAAS(001) SURFACES　　　　　　　　　　　　　　　
  H ABE; K UENO; K SAIKI; A KOMA
  JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS, Volume：32, Number：10A, First page：L1444, Last page：L1447, Oct. 1993, [Reviewed]
  Epitaxial films of layered GaSe with (0001) surfaces have been grown on GaAs(001) substrates in spite of the large difference in their crystal symmetry. In situ observation of reflection high-energy electron diffraction has revealed that a number of facets grow on the surface of GaAs(001) under Se irradiation. The existence of those facets is essential to the single-domain growth of a layered GaSe film on a GaAs(001) surface.
  JAPAN SOC APPLIED PHYSICS, English, Scientific journal
  ISSN：0021-4922, Web of Science ID：WOS:A1993MC04400023
• Heteroepitaxial growth of layered semiconductor gase on a hydrogen-terminated si(111) surface
  Kuang-Yu Liu; Keiji Ueno; Yasunori Fujikawa; Koichiro Saiki; Atsushi Koma
  Japanese Journal of Applied Physics, Volume：32, Number：3 B, First page：L434, Last page：L437, 1993
  Layered III-VI semiconductor GaSe has been heteroepitaxially grown on HF-treated Si(111) surfaces. The HF- treated Si surface is chemically inactive because of the hydrogen termination of active dangling bonds. GaSe can be grown on such a surface through weak van der Waals interaction, which relaxes the lattice matching requirement. With careful control of the substrate temperature, thermal desorption of surface hydrogen atoms was prevented so that a single-domain film of GaSe could be grown with good crystallinity. Auger electron spectra and high-resolution electron energy loss spectra revealed high quality of the grown GaSe film. © 1993 The Japan Society of Applied Physics.
  English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.32.L434
  DOI ID：10.1143/JJAP.32.L434, ISSN：1347-4065, SCOPUS ID：0345497522
• Hétéroépitaxial growth of layered gase films on gaas(Ool) surfaces
  Hideki Abe; Keiji Ueno; Koichiro Saiki; Atsushi Koma
  Japanese Journal of Applied Physics, Volume：32, Number：10 A, First page：1444, Last page：1447, 1993
  Epitaxial films of layered GaSe with (OOOl) surfaces have been grown on GaAs(OOl) substrates in spite of the large difference in their crystal symmetry. In situ observation of reflection high-energy electron diffraction has revealed that a number of facets grow on the surface of GaAs(OOl) under Se irradiation. The existence of those facets is essential to the single-domain growth of a layered GaSe film on a GaAs(OOl) surface. © 1993 The Japan Society of Applied Physics.
  English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.32.L1444
  DOI ID：10.1143/JJAP.32.L1444, ISSN：1347-4065, SCOPUS ID：0027677089
• Heteroepitaxy of layered III-VI semiconductor GaSe on hydrogen-terminated Si(111) surfaces　　　　　　　　　　　　　　　
  Keiji Ueno; Liu Kuang-Yu; Yasunori Fujikawa; Koichiro Saiki; Atsushi Koma
  Conference on Solid State Devices and Materials, First page：680, Last page：682, Dec. 1992
  Layered III-VI semiconductor GaSe films have been grown heteroepitaxially on HF-treated Si(111) surfaces. Active dangling bonds on the Si(111) surface are regularly terminated by hydrogen atoms. The GaSe film grows through weak van der Waals interaction with the substrate, relaxing the lattice-matching condition drastically. Single crystalline GaSe films can be grown from the initial stage in spite of the large difference in their crystal structures. High resolution electron energy loss spectroscopy reveals a good quality of the grown GaSe films.
  SCOPUS ID：0026957774
• HETEROEPITAXY OF LAYERED COMPOUND SEMICONDUCTOR GASE ONTO GAAS-SURFACES FOR VERY EFFECTIVE PASSIVATION OF NANOMETER STRUCTURES　　　　　　　　　　　　　　　
  K UENO; H ABE; K SAIKI; A KOMA; H OIGAWA; Y NANNICHI
  SURFACE SCIENCE, Volume：267, Number：1-3, First page：43, Last page：46, Apr. 1992
  Growth of layered III-VI compound semiconductor GaSe on a GaAs(111)B substrate has been investigated for the purpose of an effective passivation of the GaAs surface. Although GaSe and GaAs have completely different lattice structures, it has been found that a good GaSe film having its own lattice constant grows epitaxially. The growth proceeds by weak Van der Waals forces between each layer of GaSe so that the lattice matching condition is drastically relaxed. The photoluminescence intensity of the GaAs(111)B surface covered by the GaSe film is larger than that of an as-etched surface, even after the sample has been left in air for a month. The decay of the photoluminescence intensity of the GaSe-covered surface has been found to be slower than that of the sulfur-passivated surface, indicating its excellent ability for the passivation. This seems to result from the stability of covering GaSe film in addition to the proper termination of dangling bonds by selenium atoms at the GaSe/GaAs interface.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/0039-6028(92)91084-O
  DOI ID：10.1016/0039-6028(92)91084-O, ISSN：0039-6028, SCOPUS ID：0026850793, Web of Science ID：WOS:A1992HM90100013
• GROWTH OF MOSE2 THIN-FILMS WITH VANDERWAALS EPITAXY　　　　　　　　　　　　　　　
  FS OHUCHI; T SHIMADA; BA PARKINSON; K UENO; A KOMA
  JOURNAL OF CRYSTAL GROWTH, Volume：111, Number：1-4, First page：1033, Last page：1037, May 1991, [Reviewed]
  The concept of Van der Waals epitaxy that has been recently introduced removes severe lattice matching requirement by using materials which only have strong bonding in two dimensions. We demonstrate that an epilayer of MoSe2 deposited on various substrates can produce films of high crystalline quality despite of large mismatch. RHEED oscillation, observed in-situ for growing MoSe2 epilayers, shows a layer-by-layer growth with evidence for for bilayer type growth, from which the 2H(b) polytype is determined. STM provides real space images of the morphology of the epilayer, and shows novel structures resulting from the large lattice mismatch where the epilayer atoms are commensurated.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/0022-0248(91)91127-V
  DOI ID：10.1016/0022-0248(91)91127-V, ISSN：0022-0248, Web of Science ID：WOS:A1991FT19000191
• HETEROEPITAXIAL GROWTH BY VANDERWAALS INTERACTION IN ONE-DIMENSIONAL, 2-DIMENSIONAL AND 3-DIMENSIONAL MATERIALS　　　　　　　　　　　　　　　
  A KOMA; K UENO; K SAIKI
  JOURNAL OF CRYSTAL GROWTH, Volume：111, Number：1-4, First page：1029, Last page：1032, May 1991
  The lattice matching condition usually encountered in heteroepitaxial growth has been found to be relaxed greatly when the interface between constituent materials has Van der Waals nature and forms no direct chemical bonds. Layered transition metal dichalcogenides are the typical materials having that nature, and a variety of heterostructures can be grown by using them. This kind of approach has been proved to be applied also to heteroepitaxial growth between such quasi-one-dimensional materials as telurium and selenium, that onto dangling-bond terminated three-dimensional material substrates, and that of organic materials forming Van der Waals type crystals.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/0022-0248(91)91126-U
  DOI ID：10.1016/0022-0248(91)91126-U, ISSN：0022-0248, SCOPUS ID：0026413337, Web of Science ID：WOS:A1991FT19000190
• Heteroepitaxy of layered semiconductor gase on a gaas(III)b surface
  Keiji Ueno; Hideki Abe; Koichiro Saiki; Atsushi Koma
  Japanese Journal of Applied Physics, Volume：30, Number：8, First page：L1352, Last page：L1354, 1991
  Growth of a III-VI compound semiconductor GaSe on a GaAs(lll)B substrate has been tried by the molecular beam epitaxy technique. Although GaSe and GaAs have completely different lattice structures, it has been found that a good GaSe him having its own lattice constant grows with its c-axis normal to the GaAs substrate surface. The growth proceeds via van der Waals-like weak forces between each layer of GaSe, relaxing the lattice-matching condition drastically. The heteroepitaxial growth of GaSe will be applied to effective surface passivation of GaAs. © 1991 The Japan Society of Applied Physics.
  English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.30.L1352
  DOI ID：10.1143/JJAP.30.L1352, ISSN：1347-4065, SCOPUS ID：0026203505
• EPITAXIAL-GROWTH OF TRANSITION-METAL DICHALCOGENIDES ON CLEAVED FACES OF MICA　　　　　　　　　　　　　　　
  K UENO; K SAIKI; T SHIMADA; A KOMA
  JOURNAL OF VACUUM SCIENCE & TECHNOLOGY A-VACUUM SURFACES AND FILMS, Volume：8, Number：1, First page：68, Last page：72, Jan. 1990
  We have grown ultrathin films of layered transition metal dichalcogenides (MoSe2, NbSe2) heteroepitaxially on cleaved faces of mica (muscovite). This is the first success in the heteroepitaxial growth between highly heterogeneous layered materials having different crystal structures and lattice constants that differ by as much as 58%. The lattice matching condition is greatly loosened in those cases because the growth proceeds with weak van der Waals forces between the substrate and the grown layer. This opens a new way to fabricate a heterostructure composed of many kinds of layered materials having various physical and chemical properties. © 1990, American Vacuum Society. All rights reserved.
  AMER INST PHYSICS, English, Scientific journal
  DOI：https://doi.org/10.1116/1.576983
  DOI ID：10.1116/1.576983, ISSN：0734-2101, CiNii Articles ID：120001370926, SCOPUS ID：84967850146, Web of Science ID：WOS:A1990CL59400010
• Heteroepitaxial growth of layered transition metal dichalcogenides on sulfur-terminated GaAs{111} surfaces　　　　　　　　　　　　　　　
  Keiji Ueno; Toshihiro Shimada; Koichiro Saiki; Atsushi Koma
  Applied Physics Letters, Volume：56, Number：4, First page：327, Last page：329, 1990
  Layered transition metal dichalcogenides (MoSe2, NbSe 2) have been heteroepitaxially grown on (NH4)2 Sx (x≅2) treated GaAs(111)Ga, GaAs(∼(111))As surfaces in spite of the large difference in their crystal structures. The in situ observation of reflection high-energy electron diffraction has shown that the grown film has its own lattice constant even from the first layer. The lattice matching condition, which is severely restricting in the usual heteroepitaxial growth case, is greatly relaxed in the present system because only weak van der Waals forces exist between the grown film and the substrate. This results from the fact that sulfur atoms regularly terminate dangling bonds on the GaAs surface after the (NH4)2Sx treatment.
  English, Scientific journal
  DOI：https://doi.org/10.1063/1.102817
  DOI ID：10.1063/1.102817, ISSN：0003-6951, CiNii Articles ID：120001370925, SCOPUS ID：0000367334
• APPLICATION OF VANDERWAALS EPITAXY TO HIGHLY HETEROGENEOUS SYSTEMS　　　　　　　　　　　　　　　
  K SAIKI; K UENO; T SHIMADA; A KOMA
  JOURNAL OF CRYSTAL GROWTH, Volume：95, Number：1-4, First page：603, Last page：606, Feb. 1989
  Van der Waals epitaxy was applied to the fabrication of a highly heterogeneous system such as transition metal dichalcogenides (TX2) on mica. TX2 were grown epitaxially on the mica with a preferential orientation of TX2[1120] {norm of matrix} mica[100] on TX2(0001) {norm of matrix} mica(001). Uniform overgrowth was observed from a thickness as thin as 1 nm. © 1989.
  ELSEVIER SCIENCE BV, English, Scientific journal
  DOI：https://doi.org/10.1016/0022-0248(89)90475-2
  DOI ID：10.1016/0022-0248(89)90475-2, ISSN：0022-0248, SCOPUS ID：0024605783, Web of Science ID：WOS:A1989T639300138
• Characteristic secondary electron emission from graphite and glassy carbon surfaces
  Keiji Ueno; Takao Kumihashi; Atsushi Saiki; Atsushi Koma
  Japanese Journal of Applied Physics, Volume：27, Number：5A, First page：L759, Last page：L761, 1988
  Secondary electron emissions (SEE) from graphite and glassy carbon surfaces have been measured. The spectra give several characteristic peaks whose energies are independent of incident electron energies from 20 to 2500 eV. It has been proved that the SEE spectra show good agreement with core electron excitation energy loss spectra and with the calculated density of state (DOS) of the conduction band. Thus, the SEE spectra directly reflect the conduction band DOS above vacuum level, and the difference between the spectra of graphite and glassy carbon comes from the difference in their conduction band structures. © 1988 The Japan Society of Applied Physics.
  English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.27.L759
  DOI ID：10.1143/JJAP.27.L759, ISSN：1347-4065, SCOPUS ID：0024011602
• Low-energy electron energy loss spectroscopy on YBa2Cu3O7-y
  Atsushi Ando; Koichiro Saiki; Keiji Ueno; Atsushi Koma
  Japanese Journal of Applied Physics, Volume：27, Number：3A, First page：L304, Last page：L307, 1988
  Electronic structures of the high-Tcsuperconductor YBa2Cu3O7-yhave been investigated by low-energy electron energy loss spectroscopy. Loss peaks due to interband transitions, plasmon excitation and core electron excitations have been observed. It has been found from the observed spectra that a high-density-of-state portion of the first conduction band is located at 2−4.7 eV above the Fermi level and that of the higher conduction band is located at around 10 eV above the first conduction band. © 1988 The Japan Society of Applied Physics.
  English, Scientific journal
  DOI：https://doi.org/10.1143/JJAP.27.L304
  DOI ID：10.1143/JJAP.27.L304, ISSN：1347-4065, SCOPUS ID：0023983227

• Transport properties of ferromagnetic layered chalcogenide Cr_1/3TaS₂　　　　　　　　　　　　　　　
  Yamasaki Y.; Arai M.; Moriya R.; Masubuchi S.; Pyon S.; Tamegai T.; Ueno K.; Machida T.
  Meeting Abstracts of the Physical Society of Japan, Volume：72.1, First page：1803, Last page：1803, 2017
  遷移金属ダイカルコゲナイドは、層間に異なる元素をインターカレートすることにより強磁性や超伝導などの様々な物性を発現する。我々は劈開可能な層状物質強磁性体の候補として、2H-TaS_2_にCrをインターカレートしたCr_1/3_TaS_2_に着目し、その電気伝導及び磁化特性を調べた。さらにこの物質を劈開した試料における伝導特性についても報告する。
  The Physical Society of Japan, Japanese
  DOI：https://doi.org/10.11316/jpsgaiyo.72.1.0_1803
  DOI ID：10.11316/jpsgaiyo.72.1.0_1803, eISSN：2189-0803, CiNii Articles ID：130006709619
• Depth Profile Characterization of Spin-Coated Poly(3,4-ethylenedioxythiophene): Poly(styrene sulfonic acid) Films for Thin-Film Solar Cells during Argon Plasma Etching by Spectroscopic Ellipsometry (Retraction of Vol 50, art no 08JG02, 2011)　　　　　　　　　　　　　　　
  Tomohisa Ino; Tatsuya Hayashi; Keiji Ueno; Hajime Shirai
  JAPANESE JOURNAL OF APPLIED PHYSICS, Volume：55, Number：8, Aug. 2016
  IOP PUBLISHING LTD, English, Others
  DOI：https://doi.org/10.7567/JJAP.55.089201
  DOI ID：10.7567/JJAP.55.089201, ISSN：0021-4922, eISSN：1347-4065, Web of Science ID：WOS:000403923600001
• Graphene oxide/Crystalline Si solar cells　　　　　　　　　　　　　　　
  ONO Masahiro; INO Tomohisa; ISHIKAWA Ryo; GOTOU Takuya; UENO Keiji; SHIRAI Hajime
  Technical report of IEICE. OME, Volume：111, Number：369, First page：29, Last page：33, 21 Dec. 2011
  Efficient crystalline Si heterojunction solar cells with poly(ethylenehiophene)(PEDOT)-polystyrene sulfonate (PSS) and graphene oxide (GO) composite are demonstrated using a structure of Ag/(PEDOT:PSS:GO)/c-Si (100)(ρ: 3-5 Ω・cm)/Al. The power conversion efficiency enhanced up to 6.5% under illumination of AM1.5 100 mW/cm^2 light. Because of ease of preparation, soluble PEDOT:PSS:GO composite will be promising for hole-transporting layer and transparent conducting layer for crystalline Si photovoltaic applications.
  The Institute of Electronics, Information and Communication Engineers, English
  ISSN：0913-5685, CiNii Articles ID：110009466889, CiNii Books ID：AN10013334
• Organic field-effect transistors with solution-processed graphene electrodes　　　　　　　　　　　　　　　
  SUGANUMA Koichi; SAIKI Koichiro; GOTOU Takuya; UENO Keiji
  Technical report of IEICE. OME, Volume：111, Number：369, First page：35, Last page：39, 21 Dec. 2011
  Solution-processed electrodes for organic field effect transistor (OFET) were fabricated using a graphene oxide (GO) solution. The GO solution was synthesized by oxidation and exfoliation of natural graphite powder. Fabricated graphene electrodes for OFET were available as either bottom- or top-contacted ones. Graphene electrode OFETs showed better OFET performance and higher mobility than those of thermally deposited Au electrode OFETs.
  The Institute of Electronics, Information and Communication Engineers, Japanese
  ISSN：0913-5685, CiNii Articles ID：110009466890, CiNii Books ID：AN10013334
• 21aTA-6 Comparison of different methods for fabricating graphene thin films using the chemical exfoliation　　　　　　　　　　　　　　　
  Yoshida M.; Saiki K.; Ueno K.
  Meeting abstracts of the Physical Society of Japan, Volume：63, Number：2, First page：807, Last page：807, 25 Aug. 2008
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110006985487, CiNii Books ID：AA11439205
• 低次元構造半導体を用いた新奇薄膜電界効果トランジスタ形成に関する研究　　　　　　　　　　　　　　　
  上野啓司; 小野木亮; 阿部重臣; 松本晃; 川端ちひろ; 平岡和; 保戸塚梢; 白石淳子; 高橋新; 武林聡子; 坪倉陽
  Volume：平成17-18年度, First page：1, Last page：45, 2007
• 直鎖π共役電子系の電気伝導度測定に関する研究　　　　　　　　　　　　　　　
  上野啓司
  Volume：第5号(18年度), First page：553, Last page：554, 2007
• 低次元構造半導体を用いた新奇薄膜電界効果トランジスタ形成に関する研究　　　　　　　　　　　　　　　
  上野啓司; 小野木亮; 阿部重臣; 松本晃; 川端ちひろ; 平岡和; 保戸塚梢; 白石淳子; 高橋新; 武林聡子; 坪倉陽
  科学研究費補助金(基盤研究(C)) 研究成果報告書, Volume：平成17-18年度, First page：1, Last page：45, 2007
• 直鎖π共役電子系の電気伝導度測定に関する研究　　　　　　　　　　　　　　　
  上野啓司
  総合研究機構研究プロジェクト研究成果報告書, Volume：第5号(18年度), First page：553, Last page：554, 2007
• 層状化合物を用いた高移動度フレキシブル電界効果トランジスタの開発に関する研究　　　　　　　　　　　　　　　
  上野啓司
  Volume：第4号(17年度), 2006
• Preparation of organic devices by the solution process　　　　　　　　　　　　　　　
  上野啓司; 小野木亮; 高橋新; 森朋彦
  Volume：7, Number：7, First page：49, Last page：52, 2006
  A thin film of 2, 5, 11, 14-tetradodecylhexabenzocoronene (HBC4D) was fabricated by the horizontal lifting transfer of its Langmuir film onto a hydrophobic substrate surface, and characterized by UV-vis optical absorption spectroscopy, X-ray diffraction and atomic force microscopy. A field effect transistor (FET) of HBC4D was also fabricated by the wet process, and its performances were compared with HBC4D-FET fabricated by the vacuum deposition. Highly-ordered growth of the HBC4D thin film was achieved by the wet process, but its FET performances were worse than those of the vacuum-deposited FET.
  Japanese
  ISSN：1347-4758, CiNii Articles ID：120001371309, CiNii Books ID：AA11808968
• 層状化合物を用いた高移動度フレキシブル電界効果トランジスタの開発に関する研究　　　　　　　　　　　　　　　
  上野啓司
  総合研究機構研究プロジェクト研究成果報告書, Volume：第4号(17年度), 2006
• 溶液系の有機デバイス作製技術検討　　　　　　　　　　　　　　　
  上野啓司; 小野木亮; 高橋新; 森朋彦
  埼玉大学地域共同研究センター紀要, Volume：7, Number：7, First page：49, Last page：52, 2006
  A thin film of 2, 5, 11, 14-tetradodecylhexabenzocoronene (HBC4D) was fabricated by the horizontal lifting transfer of its Langmuir film onto a hydrophobic substrate surface, and characterized by UV-vis optical absorption spectroscopy, X-ray diffraction and atomic force microscopy. A field effect transistor (FET) of HBC4D was also fabricated by the wet process, and its performances were compared with HBC4D-FET fabricated by the vacuum deposition. Highly-ordered growth of the HBC4D thin film was achieved by the wet process, but its FET performances were worse than those of the vacuum-deposited FET.
  Japanese
  ISSN：1347-4758, CiNii Articles ID：120001371309, CiNii Books ID：AA11808968
• 27aWB-5 Molecular orientation in pentacene ultra thin films grown on SiO_2 substrates　　　　　　　　　　　　　　　
  Yoshikawa G.; Miyadera T.; Ueno K.; Onoki R.; Nakai I.; Entani S.; Ikeda S.; Kiguchi M.; Kondoh H.; Ohta T.; Saiki K.
  Meeting abstracts of the Physical Society of Japan, Volume：60, Number：1, First page：875, Last page：875, 04 Mar. 2005
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110004537400, CiNii Books ID：AA11439205
• 26pYK-6 Time of Flight Measurement of Organic Thin Film FETs　　　　　　　　　　　　　　　
  Yamamoto Y.; Shimada T.; Ueno K.; Saiki K.
  Meeting abstracts of the Physical Society of Japan, Volume：60, Number：1, First page：803, Last page：803, 04 Mar. 2005
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110004537122, CiNii Books ID：AA11439205
• 高誘電率ゲート酸化膜を用いた電界効果トランジスタによる有機薄膜の物性変調　　　　　　　　　　　　　　　
  上野啓司
  Volume：16年度, 2005
• 高誘電率ゲート酸化膜を用いた電界効果トランジスタによる有機薄膜の物性変調　　　　　　　　　　　　　　　
  上野啓司
  総合研究機構研究プロジェクト研究成果報告書, Volume：16年度, 2005
• Morphological change of C-60 monolayer epitaxial films under photoexcitation　　　　　　　　　　　　　　　
  Y Yamamoto; H Ichikawa; K Ueno; A Koma; K Saiki; T Shimada
  PHYSICAL REVIEW B, Volume：70, Number：15, First page：155415-1-155415-6, Oct. 2004
  Thin films of C-60 were grown under laser irradiation at various intensities 0-200 mW/mm(2) and their growth shapes were investigated by ex situ atomic force microscopy (AFM) observation. The nucleation density of the first layer decreases with increasing laser intensity, probably due to the temperature rise of the migrating clusters. In addition to this gradual laser intensity dependence, an anomalous enhancement of the nucleation density was observed on irradiation at 50 mW/mm(2), which was attributed to the influence of photopolymerization. As for the second layer, there was a threshold laser intensity (200 mW/mm(2)) at which the nucleation density increased and the shape of the domains became irregular. This is due to the combined effect of hindered migration caused by the polymerized first layer and photopolymerization of the migrating molecules themselves. Energy transfer from the excited substrate to the migrating C-60 molecules is strongly suggested.
  AMER PHYSICAL SOC, English
  DOI：https://doi.org/10.1103/PhysRevB.70.155415
  DOI ID：10.1103/PhysRevB.70.155415, ISSN：2469-9950, eISSN：2469-9969, Web of Science ID：WOS:000224855900091
• 13pWH-8 Operating characteristics of an organic FET having a high-k gate oxide layer(FET)　　　　　　　　　　　　　　　
  Ueno K.; Onoki R.; Abe S.; Saiki K.
  Meeting abstracts of the Physical Society of Japan, Volume：59, Number：2, First page：761, Last page：761, 25 Aug. 2004
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002049847, CiNii Books ID：AA11439205
• 13pWH-8 Operating characteristics of an organic FET having a high-k gate oxide layer(FET)　　　　　　　　　　　　　　　
  Ueno K; Onoki R; Abe S; Saiki K
  Meeting abstracts of the Physical Society of Japan, Volume：59, Number：2, First page：506, Last page：506, 25 Aug. 2004
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002054167, CiNii Books ID：AA11439205
• STM observation of alkall hallde/metal heterostructures　　　　　　　　　　　　　　　
  Katayama M.; Inoue H.; Kiguchi M.; Ueno K.; Saiki K.
  Meeting abstracts of the Physical Society of Japan, Volume：58, Number：2, First page：789, Last page：789, 15 Aug. 2003
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002064118, CiNii Books ID：AA11439205
• Organic FET using pentaccne thin films grown under anisotropic condition　　　　　　　　　　　　　　　
  Ueno K.; Saiki K.
  Meeting abstracts of the Physical Society of Japan, Volume：58, Number：2, First page：752, Last page：752, 15 Aug. 2003
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002063824, CiNii Books ID：AA11439205
• Film Thickness Dependence of Field Effect Mobility in Pentacene Thin Film　　　　　　　　　　　　　　　
  Kiguchi M.; Nakayama M.; Fujiwara S.; Ueno K.; Shimada T.; Saiki K.
  Meeting abstracts of the Physical Society of Japan, Volume：58, Number：2, First page：752, Last page：752, 15 Aug. 2003
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002063826, CiNii Books ID：AA11439205
• STM observation of LiBr thin films　　　　　　　　　　　　　　　
  Katayama M.; Ueno K.; Kiguchi M.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：58, Number：1, First page：880, Last page：880, 06 Mar. 2003
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002223859, CiNii Books ID：AA11439205
• 表面不活性化シリコン基板上における異方的有機薄膜成長　　　　　　　　　　　　　　　
  上野啓司; 斉木幸一朗小間篤
  Volume：72, Number：3, First page：322, Last page：326, 2003
• 表面不活性化シリコン基板上における異方的有機薄膜成長　　　　　　　　　　　　　　　
  Volume：28, Number：3, First page：46, Last page：55, 2003
• 表面不活性化シリコン基板上における異方的有機薄膜成長　　　　　　　　　　　　　　　
  上野啓司; 斉木幸一朗小間篤
  応用物理, Volume：72, Number：3, First page：322, Last page：326, 2003
• 表面不活性化シリコン基板上における異方的有機薄膜成長　　　　　　　　　　　　　　　
  応用物理, Volume：28, Number：3, First page：46, Last page：55, 2003
• STM observation of LiBr/Si(001) heterostructure　　　　　　　　　　　　　　　
  Katayama M.; Ueno K.; Kiguchi M.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：57, Number：2, First page：779, Last page：779, 13 Aug. 2002
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110009719873, CiNii Books ID：AA11439205
• Mechanism of the visible light photoemission from alkali halide thin films induced by UV irradiation　　　　　　　　　　　　　　　
  Yoshikawa G.; Kiguchi M.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：57, Number：2, First page：779, Last page：779, 13 Aug. 2002
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110009719872, CiNii Books ID：AA11439205
• Highly stable passivation of a Si(111) surface using bilayer-GaSe　　　　　　　　　　　　　　　
  K Ueno; H Shirota; T Kawamura; T Shimada; K Saiki; A Koma
  APPLIED SURFACE SCIENCE, Volume：190, Number：1-4, First page：485, Last page：490, May 2002
  As a stable and 'epitaxial' passivation of a Si surface, we propose the bilayer-GaSe termination of a Si(1 1 1) surface. This surface is fabricated by depositing one monolayer of Ga on a clean Si(1 1 1) surface and subsequent annealing in a Se flux at around 520 degreesC, which results in unreconstructed 1 x 1 termination of the Si(1 1 1) surface by bilayer-GaSe. We found by scanning tunneling microscopy observation that slow cooling of the clean Si(1 1 1) surface from 850 to 520 degreesC with simultaneous deposition of a Ga flux results in better termination of the Si(1 1 1) surface. It was also found that this surface is stable against heating around 400 degreesC in O-2 atmosphere of 3 x 10(-3) Pa. By utilizing these properties of the bilayer-GaSe terminated surface, we have succeeded in fabricating ZnO quantum dots on this substrate. (C) 2002 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English
  DOI：https://doi.org/10.1016/S0169-4332(01)00923-0
  DOI ID：10.1016/S0169-4332(01)00923-0, ISSN：0169-4332, CiNii Articles ID：120001370987, Web of Science ID：WOS:000176520700090
• Epitaxial growth of a vacancy-ordered Ga2Se3 thin film on a vicinal Si(001) substrate　　　　　　　　　　　　　　　
  K Ueno; S Tokuchi; K Saiki; A Koma
  JOURNAL OF CRYSTAL GROWTH, Volume：237, Number：2, First page：1610, Last page：1614, Apr. 2002
  We have fabricated a vacancy-ordered Ga2Se3 epitaxial film by molecular beam epitaxy on a vicinal Si(0 0 1) surface tilted toward the [1 1 0] direction by 4degrees. It was found by reflection high-energy electron diffraction observations that Ga2Se3 films grown around 470degreesC with Se/Ga flux ratios higher than 100 show excellent ordering of Ga vacancies, The vacancy ordering was not observed when the substrate temperature was higher than 490degreesC. In addition, polycrystalline film growth was dominant when the substrate temperature was lower than 450degreesC, and no vacancy ordering was observed when the Se/Ga ratio was 10. Only a poor crystallinity Ga2Se3 film was obtained on a nominal Si(0 0 1) substrate, which suggests that the single-domain 2 x I reconstruction on the vicinal Si(0 0 1) surface is essential to the growth of the vacancy-ordered Ga2Se3 film. (C) 2002 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English
  DOI：https://doi.org/10.1016/S0022-0248(01)02353-3
  DOI ID：10.1016/S0022-0248(01)02353-3, ISSN：0022-0248, Web of Science ID：WOS:000176512900139
• Epitaxial growth and electronic structure of a C-60 derivative prepared by using a solution spray technique　　　　　　　　　　　　　　　
  T Shimada; H Nakatani; K Ueno; A Koma; Y Kuninobu; M Sawamura; E Nakamura
  JOURNAL OF APPLIED PHYSICS, Volume：90, Number：1, First page：209, Last page：212, Jul. 2001
  We demonstrate and analyze the epitaxial film formation of a molecular material that cannot be evaporated in vacuum due to thermal decomposition. A solution of the material is sprayed onto single crystalline van der Waals surfaces using a pulse valve under controlled vapor pressure of the solvent. Monolayer epitaxial films are obtained and we propose that the growth is mediated by liquid ultrathin films formed on the surface. Molecular arrangement and electronic structure of C-60(CH3)(5)H films are studied by reflection high energy electron diffraction and ultraviolet photoelectron spectroscopy, respectively. The present technique will be useful to study the electronic structure of recently synthesized molecular materials. (C) 2001 American Institute of Physics.
  AMER INST PHYSICS, English
  DOI：https://doi.org/10.1063/1.1379052
  DOI ID：10.1063/1.1379052, ISSN：0021-8979, CiNii Articles ID：120001370931, Web of Science ID：WOS:000169361100030
• STM observation of a bilayer-GaSe terminated Si(111) surface.　　　　　　　　　　　　　　　
  Shirota H.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：56, Number：1, First page：820, Last page：820, 09 Mar. 2001
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002164817, CiNii Books ID：AA11439205
• 表面不活性基板を利用した有機・無機ナノ構造の形成(<小特集>ナノ結晶)　　　　　　　　　　　　　　　
  上野啓司; 小間篤
  Volume：28, Number：3, First page：164, Last page：173, 2001
  DOI：https://doi.org/10.19009/jjacg.28.3_164
  DOI ID：10.19009/jjacg.28.3_164
• 表面不活性基板を利用した有機・無機ナノ構造の形成(<小特集>ナノ結晶)　　　　　　　　　　　　　　　
  上野啓司; 小間篤
  日本結晶成長学会誌, Volume：28, Number：3, First page：164, Last page：173, 2001
  New-type nanostructures of organic molecules and inorganic semiconductors have been successfully fabricated on 'inactive' substrate surfaces such as layered material surfaces and dangling-bond terminated semiconductor surfaces. In this article we introduce two examples of our methods. One is the fabrication of C_<60> molecular nanostructures on a GaSe/MoS2 heterostructure substrate by the selective growth method. The other is the fabrication of self-organized compound semiconductor quantum dots on a bilayer-GaSe terminated Si(111) substrate. The use of the inactive substrate surfaces opens a new way to fabricate position-controlled nanostructures, which have been difficult to form by conventional self-organization methods.
  The Japanese Association for Crystal Growth (JACG), Japanese
  DOI ID：10.19009/jjacg.28.3_164, ISSN：0385-6275, CiNii Articles ID：110002715468, CiNii Books ID：AN00188386
• Layered Material Substrates　　　　　　　　　　　　　　　
  UENO Keiji; KOMA Atsushi
  Journal of the Surface Science Society of Japan, Volume：21, Number：11, First page：716, Last page：723, 10 Nov. 2000
  Japanese
  ISSN：0388-5321, CiNii Articles ID：10005348918, CiNii Books ID：AN00334149
• Investigation of the growth mechanism of an InSe epitaxial layer on a MoS2 substrate　　　　　　　　　　　　　　　
  T Hayashi; K Ueno; K Saiki; A Koma
  JOURNAL OF CRYSTAL GROWTH, Volume：219, Number：1-2, First page：115, Last page：122, Oct. 2000
  Sub-monolayer films of layered semiconductor InSe were grown on MoS2 substrates by molecular beam epitaxy, and the change in their growth features with Se/In flux ratio was investigated using scanning tunneling microscope in vacuum. It was found that InSe domains grown at 340 degrees C have a hexagonal shape when the Se/In ratio is about 17. Detailed images of the hexagonal InSe domains have revealed that adjacent sides of the hexagon have different structures; one is a straight edge and the other is a disordered edge. When the Se/In ratio was decreased, the disordered edges became predominant, the straight edges disappeared, and the InSe domain became triangular. On the contrary, when the Se/In ratio was increased, InSe domains became triangular ones consisting of only straight edges. The growth mechanism of InSe domains is discussed by considering the crystal structure of InSe and the reactivity of each side with incoming atoms. It is suggested that the balance of incorporation rate of In and Se atoms determines the structure of InSe domains. (C) 2000 Elsevier Science B.V. All rights reserved.
  ELSEVIER SCIENCE BV, English
  DOI：https://doi.org/10.1016/S0022-0248(00)00627-8
  DOI ID：10.1016/S0022-0248(00)00627-8, ISSN：0022-0248, CiNii Articles ID：120001370934, Web of Science ID：WOS:000089675700015
• C(1s) core-electron excitation of C_<60>/Si(111) systems.　　　　　　　　　　　　　　　
  Iizumi K.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：55, Number：2, First page：749, Last page：749, 10 Sep. 2000
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002150110, CiNii Books ID：AA11439205
• Surface structure of a bilayer-GaSe teminated Si(111) substrate and its application to thin film growth.　　　　　　　　　　　　　　　
  Shirota H.; Iizumi K.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：55, Number：2, First page：777, Last page：777, 10 Sep. 2000
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002150687, CiNii Books ID：AA11439205
• Investigation of epitaxial arrangement and electronic structure of a La@ C-82 film grown on an MoS2 surface　　　　　　　　　　　　　　　
  K Iizumi; Y Uchino; K Ueno; A Koma; K Saiki; Y Inada; K Nagai; Y Iwasa; T Mitani
  PHYSICAL REVIEW B, Volume：62, Number：12, First page：8281, Last page：8285, Sep. 2000
  Molecular arrangement and electronic structure of a La@C-82 film epitaxially grown on an MoS2 surface have been studied using reflection high-energy electron diffraction and electron energy-loss spectroscopy (EELS). It was revealed that La@C-82 molecules form a close-packed hexagonal lattice on a cleaved face of MoS2 with the intermolecular distance of 1.13+/-0.03 nm. EELS of the La@C-82 film in the valence excitation region indicated seven peaks coming from pi-->pi* transitions together with the pi-plasmon excitation. The absence of a distinct band gap means that the La@C-82 epitaxial him is not semiconducting, but metallic or semimetallic. From the EELS result, we propose an electronic structure diagram of the La@C-82 epitaxial film.
  AMER PHYSICAL SOC, English
  DOI：https://doi.org/10.1103/PhysRevB.62.8281
  DOI ID：10.1103/PhysRevB.62.8281, ISSN：1098-0121, eISSN：1550-235X, Web of Science ID：WOS:000089593400085
• Investigation of epitaxial arrangement and electronic structure of a La@ C-82 film grown on an MoS2 surface
  上野 啓司; 小間 篤
  PHYSICAL REVIEW B, Volume：62, Number：12, First page：8281, Last page：8285, Sep. 2000
  Copyright notice(c)2000 American Physical Society. All rights reserved. Publisher's version: http://link.aps.org/abstract/PRB/v62/p8281Molecular arrangement and electronic structure of a La@C-82 film epitaxially grown on an MoS2 surface have been studied using reflection high-energy electron diffraction and electron energy-loss spectroscopy (EELS). It was revealed that La@C-82 molecules form a close-packed hexagonal lattice on a cleaved face of MoS2 with the intermolecular distance of 1.13+/-0.03 nm. EELS of the La@C-82 film in the valence excitation region indicated seven peaks coming from pi-->pi* transitions together with the pi-plasmon excitation. The absence of a distinct band gap means that the La@C-82 epitaxial him is not semiconducting, but metallic or semimetallic. From the EELS result, we propose an electronic structure diagram of the La@C-82 epitaxial film.
  American Physical Society, English
  ISSN：0163-1829, CiNii Articles ID：120001370971
• Epitaxy of fullerene molecules on Si substrates terminetaed by foreign atoms　　　　　　　　　　　　　　　
  UENO Keiji
  Technical report of IEICE. OME, Volume：100, Number：245, First page：51, Last page：56, 21 Jul. 2000
  There are active dangling bonds on a clean surface of Si(111), but the surface can be passivated by termintating those bonds using appropriate atoms. The surface regularly terminated by foreign atoms is as inactive as the cleaved surface of a layered material such as MoS_2 or GaSe, and the surface energy is quite low. Then it is possible to utilize this surface as the substrate to grow an epitaxial thin film of such organic molecular crystals as fullerene and metal-phthalocyanine via the van der Waals interaction. In this paper I will explain the method for terminating a Si(111) surface by hydrogen, arsenic or bilayer-GaSe, and report on the growth of C_<60> thin films on these dangling bond terminated Si(111) surfaces. In addition, I will report about the measurement of the electronic structure of grown C_<60> films.
  The Institute of Electronics, Information and Communication Engineers, Japanese
  ISSN：0913-5685, CiNii Articles ID：110003301168, CiNii Books ID：AN10013334
• Electronic structure of C_<60> molecules adsorbed on Si(111) surfaces terminated by foreign atoms　　　　　　　　　　　　　　　
  Ueno K.; Shirota H.; Iizumi K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：55, Number：1, First page：782, Last page：782, 10 Mar. 2000
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110001916742, CiNii Books ID：AA11439205
• Electron Energy Loss Spectroscopy of (C_<60>,K)/Si(111) systems.　　　　　　　　　　　　　　　
  Iizumi K.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：55, Number：1, First page：749, Last page：749, 10 Mar. 2000
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110001916354, CiNii Books ID：AA11439205
• Growth mechanism of a III-VI layered compound semiconductor thin film grown on a MoS_2 substrate　　　　　　　　　　　　　　　
  Hayashi T.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：55, Number：1, First page：759, Last page：759, 10 Mar. 2000
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110001916471, CiNii Books ID：AA11439205
• 24aZ-6 Electron Energy Loss Spectroscopy of C_<60> monolayer films grown on Si(111)-7×7 and Si(100)-2×1 surfaces　　　　　　　　　　　　　　　
  Iizumi K.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：54, Number：2, First page：785, Last page：785, 03 Sep. 1999
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110001994330, CiNii Books ID：AA11439205
• 24aZ-1 Photoyield measurement on alkali halide / semiconductor heterostructures (II)　　　　　　　　　　　　　　　
  Kono T.; Saiki K.; Ueno K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：54, Number：2, First page：783, Last page：783, 03 Sep. 1999
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110001994325, CiNii Books ID：AA11439205
• 26aPS-34 Epitaxial growth and electron spectroscopy measurement of Ga_2Se_3 thin films on Si substrates　　　　　　　　　　　　　　　
  Ueno K; Nakahara T; Iizumi K; Saiki K; Koma A; Dai Z.R.; Ohuchi F
  Meeting abstracts of the Physical Society of Japan, Volume：54, Number：2, First page：820, Last page：820, 03 Sep. 1999
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110001994464, CiNii Books ID：AA11439205
• 26aPS-13 High Resolution Electron Energy Loss Spectroscopy of C_<60> monolayer films grown on a Si(111)-√<3>x√<3>Ag surface　　　　　　　　　　　　　　　
  Iizumi K; Ueno K; Saiki K; Koma A
  Meeting abstracts of the Physical Society of Japan, Volume：54, Number：2, First page：815, Last page：815, 03 Sep. 1999
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110001994444, CiNii Books ID：AA11439205
• 31a-S-12 Electron Energy Loss Spectroscopy of C-<60> monolayer films grown on inactive surfaces　　　　　　　　　　　　　　　
  Iizumi K.; Ueno K.; Saiki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：54, Number：1, First page：338, Last page：338, 15 Mar. 1999
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002220666, CiNii Books ID：AA11439205
• 28p-S-9 Photoyield measurement on alkali halide / semiconductor heterostructures　　　　　　　　　　　　　　　
  Kono T.; Saiki K.; Ueno K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：54, Number：1, First page：306, Last page：306, 15 Mar. 1999
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002220202, CiNii Books ID：AA11439205
• Structure of C_<60> thin films epitaxially grown on various layered material substrates.　　　　　　　　　　　　　　　
  UENO K.; IIZUMI K.; SAIKI K.; KOMA A.
  Meeting abstracts of the Physical Society of Japan, Volume：53, Number：2, First page：396, Last page：396, 05 Sep. 1998
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002056924, CiNii Books ID：AA11439205
• Growth mechanism of InSe thin films grown on MoS_2 substrates.　　　　　　　　　　　　　　　
  HAYASHI T.; UENO K.; SAIKI K.; KOMA A.
  Meeting abstracts of the Physical Society of Japan, Volume：53, Number：2, First page：378, Last page：378, 05 Sep. 1998
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002056781, CiNii Books ID：AA11439205
• Fabrication of Molecular Crystal Nanostructures by a Selective Growth Method　　　　　　　　　　　　　　　
  UENO Keiji; SHIMADA Toshihiro; KOMA Atsushi
  Journal of the Surface Science Society of Japan, Volume：19, Number：1, First page：14, Last page：20, 10 Jan. 1998
  Japanese
  ISSN：0388-5321, CiNii Articles ID：10009861740, CiNii Books ID：AN00334149
• 表面を選ぶサッカーボール　　　　　　　　　　　　　　　
  上野啓司
  Volume：13, Number：11, First page：47, Last page：50, 1998
• 表面を選ぶサッカーボール　　　　　　　　　　　　　　　
  上野啓司
  物理科学雑誌パリティ, Volume：13, Number：11, First page：47, Last page：50, 1998
  Japanese
  ISSN：0911-4815, CiNii Articles ID：120001370937, CiNii Books ID：AN10017131
• 7a-PS-58 Selective Growth of Organic Mlecular Crystals on Layered Material Substrates　　　　　　　　　　　　　　　
  Ueno K.; Sasaki K.; Koma A.
  Meeting abstracts of the Physical Society of Japan, Volume：52, Number：2, First page：369, Last page：369, 16 Sep. 1997
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002061306, CiNii Books ID：AA11439205
• 31a-T-12 2D layered material buffer layers in large lattice mismatch heterostructures:The system CdS/InSe/Si　　　　　　　　　　　　　　　
  Loher T; Ueno K; Koma A
  Meeting abstracts of the Physical Society of Japan, Volume：52, Number：1, First page：364, Last page：364, 17 Mar. 1997
  The Physical Society of Japan (JPS), English
  ISSN：1342-8349, CiNii Articles ID：110002237728, CiNii Books ID：AA11439205
• 29a-PS-31 SPM observation of heteroepitaxially grown III-VI layered semiconductor thin film surface.　　　　　　　　　　　　　　　
  Ueno K; Sasaki K; Koma A
  Meeting abstracts of the Physical Society of Japan, Volume：52, Number：1, First page：338, Last page：338, 17 Mar. 1997
  The Physical Society of Japan (JPS), Japanese
  ISSN：1342-8349, CiNii Articles ID：110002237630, CiNii Books ID：AA11439205
• Initial growth mechanism of C_<60> epitaxial thin films　　　　　　　　　　　　　　　
  Sugai M.; Saiki K.; Koma A.
  Abstracts of the meeting of the Physical Society of Japan. Sectional meeting, Volume：1996, Number：2, First page：572, Last page：572, 13 Sep. 1996
  The Physical Society of Japan (JPS), Japanese
  CiNii Articles ID：110001985810, CiNii Books ID：AN10453836
• 27p-PSA-7 Characterization of heteroepitaxially grown layerd materials by HREELS.　　　　　　　　　　　　　　　
  Ueno K; Fujikawa Y; Wei Gao; Saiki K; Koma A
  Volume：1992, Number：2, First page：491, Last page：491, 14 Sep. 1992
  The Physical Society of Japan (JPS), Japanese
  CiNii Articles ID：110001999247, CiNii Books ID：AA11823236
• 29p-PS-18 Van der waals Epitaxy on Hydrogen-terminated Si Substrate.　　　　　　　　　　　　　　　
  Liu K; Ueno K; Saiki K; Koma A
  Volume：47, Number：2, First page：467, Last page：467, 12 Mar. 1992
  The Physical Society of Japan (JPS), Japanese
  CiNii Articles ID：110002173617, CiNii Books ID：AN10453938
• 30p-BPS-2 STM Observation of Heteroepitaxially Grown Layered Materials　　　　　　　　　　　　　　　
  Ueno K.; Yamamoto H.; Mori T.; Yoshii K.; Saiki K.; Koma A.
  Volume：46, Number：2, First page：462, Last page：462, 12 Sep. 1991
  The Physical Society of Japan (JPS), Japanese
  CiNii Articles ID：110002154207, CiNii Books ID：AN10453938
• 27p-R-6 Growth and Characterization of (GaSe/MoSe_2)_n Superlattice on GaAs　　　　　　　　　　　　　　　
  Ueno K.; Abe H.; Saiki K.; Koma A.
  Volume：1991, Number：2, First page：514, Last page：514, 11 Mar. 1991
  The Physical Society of Japan (JPS), Japanese
  CiNii Articles ID：110001905439, CiNii Books ID：AA11403638
• PERIODIC LATTICE-DISTORTIONS AS A RESULT OF LATTICE MISMATCH IN EPITAXIAL-FILMS OF 2-DIMENSIONAL MATERIALS　　　　　　　　　　　　　　　
  BA PARKINSON; FS OHUCHI; K UENO; A KOMA
  APPLIED PHYSICS LETTERS, Volume：58, Number：5, First page：472, Last page：474, Feb. 1991
  Epilayers of transition metal dichalcogenides (TMDs) with two-dimensional structures can be grown with molecular beam epitaxy onto other TMDs substrates without regard to lattice matching. Although there is no strong bonding between the epilayer and the substrate, the van der Waals interaction between the two hexagonally closet packed lattices results in a periodic distortion which, due to electronic effects, is prominently imaged with the scanning tunneling microscope.
  AMER INST PHYSICS, English
  DOI：https://doi.org/10.1063/1.104611
  DOI ID：10.1063/1.104611, ISSN：0003-6951, CiNii Articles ID：120001370928, Web of Science ID：WOS:A1991EW47700014
• 2a-E-12 STM observation of the heterostructure grown by Van der Waals epitaxy method.　　　　　　　　　　　　　　　
  Ueno K.; Saiki K.; Koma A.; Parkinson B.A.; Ohuchi F.
  Volume：1990, Number：2, First page：418, Last page：418, 12 Sep. 1990
  The Physical Society of Japan (JPS), Japanese
  CiNii Articles ID：110002023554, CiNii Books ID：AA11823236
• VANDERWAALS EPITAXIAL-GROWTH AND CHARACTERIZATION OF MOSE2 THIN-FILMS ON SNS2　　　　　　　　　　　　　　　
  FS OHUCHI; BA PARKINSON; K UENO; A KOMA
  JOURNAL OF APPLIED PHYSICS, Volume：68, Number：5, First page：2168, Last page：2175, Sep. 1990
  AMER INST PHYSICS, English
  DOI：https://doi.org/10.1063/1.346574
  DOI ID：10.1063/1.346574, ISSN：0021-8979, CiNii Articles ID：120001370927, Web of Science ID：WOS:A1990DW26700034

• 2024
• 2024

• Present
• Present
• Present
• Present

• Integration of 2 dimensional tunnel FET for ultra-low power consumption system　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (S), 27 Apr. 2022 - 31 Mar. 2027
  The University of Tokyo
  Grant amount(Total)：200070000, Direct funding：153900000, Indirect funding：46170000
  Grant number：22H04957
• Synthesis of Ultrathin Functional Oxides Using Atomically Thin Transition Metal Dichalcogenides as Oxidation Precursors　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (B), 01 Apr. 2022 - 31 Mar. 2026
  Kansai University
  Grant amount(Total)：17290000, Direct funding：13300000, Indirect funding：3990000
  Grant number：23K23182
• Synthesis of Ultrathin Functional Oxides Using Atomically Thin Transition Metal Dichalcogenides as Oxidation Precursors　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (B), 01 Apr. 2022 - 31 Mar. 2026
  Kansai University
  Grant amount(Total)：17290000, Direct funding：13300000, Indirect funding：3990000
  Grant number：22H01914
• 触媒効果による層状化合物原子膜の低温単結晶成長実現　　　　　　　　　　　　　　　
  01 Apr. 2021 - 31 Mar. 2024
  Grant amount(Total)：4160000, Direct funding：3200000, Indirect funding：960000
  Grant number：21K04826
• Low-temperature van der Waals epitaxy using high vapor pressure sources　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (B), 01 Apr. 2018 - 31 Mar. 2021
  Ueno Keiji, Saitama University
  Grant amount(Total)：16510000, Direct funding：12700000, Indirect funding：3810000
  In order to epitaxially grow single-crystalline ultra-thin films of layered transition metal dichalcogenides at low substrate temperatures, we have set up a molecular beam epitaxy system using highly reactive high-vapor pressure precursor compounds as source materials and proceeded with deposition experiments. As a result, tungsten disulfide, niobium disulfide, and niobium diselenide thin films were successfully epitaxially grown on synthetic mica and other substrates.
  In deposition experiments using an atomic layer deposition system, it was found that crystalline tungsten disulfide thin films can be grown not only on the metal film but also on the external substrate surface at lower temperatures than those without the metal film, by depositing a small metal film on a thermally oxidized silicon substrate in advance.
  Grant number：18H01822
• International supports of atomic layered materials and promoting the collaborated research　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), 06 Nov. 2015 - 31 Mar. 2019
  Saito Riichiro; NISHINA yuta; YANAGI kazuhiro; KAWANO yukio, Tohoku University
  Grant amount(Total)：45890000, Direct funding：35300000, Indirect funding：10590000
  Atomic layered materials, which are the thinnest material in the nature, have shown unique and significant properties and thus the research of the atomic layered materials are very active in the world for recent decades. In particular, hexagonal boron nitride (h-BN) is an important atomically-flat materials which can produce only in Japan. In this project, we support the supply of hBN samples internationally to enhance the international collaboration, which is very efficient for promoting the activity not only domestic but also international research. Further, sending or inviting many young researchers for the international collaborating research on atomic layer materials by this project, we encourage young scientists to establish a new network of the research. This is an important results of the project for our continuing the research for the next generation.
  Grant number：15K21722
• Understanding and device application of the hetero-atomic layers　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), 28 Jun. 2013 - 31 Mar. 2018
  NAGASHIO Kosuke; WATANABE Kenji; YAMADA Takatoshi, The University of Tokyo
  Grant amount(Total)：204230000, Direct funding：157100000, Indirect funding：47130000
  In the research of atomic layers for Scientific Research on Innovative Areas, (1) the transport properties of 2 dimensional layered semiconductors, (2) fabrication technique for the layered heterostructure, and its transport properties and (3) application to the OLED were studied. Compared with the conventional heteroepitaxy, layered heterostructure can be fabricated without considering the lattice mismatch. The layered heterointerface is proved to be electrically inert. Further development can be expected.
  Grant number：25107004
• Promotion of Science Atomic layers　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area), 28 Jun. 2013 - 31 Mar. 2018
  Saito Riichiro, Tohoku University
  Grant amount(Total)：181740000, Direct funding：139800000, Indirect funding：41940000
  In this project, we have synthesized the thinnest material in the history of human, that is, atomic layered materials. Using the synthesized, atomic layered materials, we have developed electronic or optical devices that require minimum consumption of the power, which enable us the possible applications for the commercial market. Further, stacking these atomic layered materials one by one, we make a completely new materials which enables a new properties such as two-dimensional superconductivity or quantum devices. This research activity is enhanced by the international collaboration of the researches.
  Grant number：25107001
• Development of high performance organic thin film devices by utilizing the junction between graphene and organic semiconductor　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), 2010 - 2012
  UENO Keiji, Saitama University
  Grant amount(Total)：4290000, Direct funding：3300000, Indirect funding：990000
  In the present research, it has been tried to improve the performance of solar cells and field-effect transistors by using chemically exfoliated solubilized graphene as the constituent material of these devices. By adding solubilized graphene, photoconversion efficiency of the solution-processed thin film solar cell was improved to be higher than 12 %. It was also realized to obtain three times higher mobility of an organic thin film field-effect transistor by adding the solubilized graphene, and to fabricate a transparent transistor by using graphene transparent electrodes.
  Grant number：22560002
• 層状物質の薄片剥離およびヘテロ積層による新奇物性発現　　　　　　　　　　　　　　　
  2007 - 2008
  Grant amount(Total)：3300000, Direct funding：3300000
  Grant number：19656011
• Fabrication of novel thin film transistors by using low-dimensional structure semiconductors　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), 2005 - 2006
  UENO Keiji, Saitama University
  Grant amount(Total)：3600000, Direct funding：3600000
  In this research, I tried to fabricate a "flexible" and "high-quality" thin film transistor by using such "anisotropic" structure low-dimensional materials as organic macromolecule polymers and inorganic layered structure semiconductors. For two years research, following results have been obtained.
  (1)Optimization of the fabrication process of a SiOx gate substrate surface that has an anisotropic template structure.
  (2)Establishment of the fabrication process of a highly ordered thin film of a diacetylene derivative compound.
  (3)Successful fabrication of highly ordered and defect-free ;t-conjugated systems by the anisotropic polymerization of a highly-ordered diacetylene derivative thin film.
  (4)Successful fabrication of an organic field effect transistor on a mica gate dielectric.
  (5)Successful transfer of the anisotropic template surface structure by the imprinting method onto an amorphous surface.
  (6)Successful fabrication of a. highly ordered thin film of a polythiophene derivative compound onto the template substrate surface by the horizontal lifting method.
  (7)Successful position-selective fabrication of C_<60>domains on the SiOx template substrate surface that has the one-dimensional periodic structures.
  (8)Improvement in the performances of the organic field effect transistor fabricated on the mica gate dielectric by inserting a PMMA or OTMS shielding layer between the mica surface and the organic film.
  (9)Investigation of the exfoliation method of such layered materials as MoS_2, WSe_2, graphite, etc.
  Grant number：17550165
• 高度界面制御有機・無機複合構造による量子物性の発現と応用　　　　　　　　　　　　　　　
  2002 - 2006
  Grant amount(Total)：526370000, Direct funding：431780000, Indirect funding：94590000
  Grant number：14GS0207
• 有機超伝導薄膜からのコヒーレント光超放射に関する研究　　　　　　　　　　　　　　　
  2002 - 2003
  Grant amount(Total)：3400000, Direct funding：3400000
  Grant number：14750006
• Research of "vacancy epitaxy"　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), 2000 - 2001
  UENO Keiji, The University of Tokyo
  Grant amount(Total)：3600000, Direct funding：3600000
  (1) I tried to grow Ga vacancy-ordered Ga_2Se_3 epitaxial films on Si (100) substrates. In order to obtain a single-domain epitaxial film I used a vicinal Si (001) substrare, whose clean surface has the 2×1 single-domain reconstruction. In addition, the lattice constant of the Si is almost same with that of Ga_2Se_3. It has been found that Ga vacancy-ordered single-domain Ga_2Se_3 films can be grown on the vicinal Si (001) substrates with the growth condisions ; substrate temperature = 480℃, VI/III ratio > 100. At the substrate temperature higher than 500℃, no Ga vacancy ordering was observed. At lower substrate temperature than 420 ℃, polycrystalline growth occurred. At the VI/III ratio of 10, no Ga vacancy-ordering was found, too. Photoluminescence spectra taken at 6.5K showed very btoad luminescence peak around 650 〜 1000 nm. No polarized photoluminescence was observed from the Ga vacancy-ordered Ga2Se3 film, which suggests that the grown film on the vicinal Si (001) substrate has the monoclinic crystal structure.
  Grant number：12650006
• フラーレンおよびその化合物のヘテロ構造化による新物性の探求　　　　　　　　　　　　　　　
  1999 - 2000
  Grant amount(Total)：3700000, Direct funding：3700000
  Grant number：11165212
• 分子性結晶を用いた室温動作単電子トンネル素子の研究　　　　　　　　　　　　　　　
  1998 - 1999
  Grant amount(Total)：2100000, Direct funding：2100000
  Grant number：10750006
• Fabrication of their properties　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (A), 1998 - 1999
  KOMA Atsushi; SHIMADA Toshihiro; UENO Keiji; SAIKI Koichiro, The University of Tokyo
  Grant amount(Total)：38900000, Direct funding：38900000
  This project has aimed at establishing techniques necessary to fabricate highly heterogeneous crystalline structures among various materials from inorganic to inorganic and from insulators to superconductors. Novel physical properties are being found from those exotic structures.
  (1) A complex heterostructure to achieve a rocksalt oxide film on GaAs
  A single-crystalline MgO film was grown on GaAs (001) by constructing a complex heterostructure with two alkali halide buffer layers. The growth temperature was decreased to 150℃ as compared with direct growth of MgO on GaAs (001). Electron energy loss spectrum of the grown film agreed well with that of bulk MgO, indicating that surface stoichiometry was maintained. The structure was stable up to 600℃ against heating in, UHV condition. The concept of a complex heterostructure will help fabrication of functional oxide layers on GaAs substrates and lead to oxide/semiconductor integrated devices.
  (2) Epitaxial growth and phase transition of liquid crystal monolayers
  Monolayer films of liquid crystals on single-crystalline inorganic substrates are expected to reveal novel properties involving phase transitions in two dimensions. It was found that liquid crystal 12CB can be grown epitaxially on alkali halide (OO1) surfaces. Temperature dependence of the film structure was investigated by using high-sensitive reflection high energy electron diffraction and atomic force microscopy. The shape of the monolayer boundary drastically changes with the substrate temperature near the transition temperature of 12CB.
  (3) Selective growth of molecular materials for quantum structures
  Selective growth is the only practical method to fabricate well-defined nanostructures of molecular materials in large scales. Difference in the stabilization energies at film-substrate interfaces is essential to achieve high selectivity. Two guiding principles have been established. One is the use of lattice matching of the grown film and the substrate materials which is found among ionic substrate materials. The other is the use of different van der Waals interaction which is found using layered materials.
  (4) Physical properties of ultrathin films and heterostructures
  Physical properties of fabricated structures were measured and novel properties are being revealed. For example, MnPc ultrathin film shows magnetic properties different from bulk single crystals due to different molecular arrangement.
  Grant number：10305002
• Fabrication of novel functional surfaces through ionic crystal / semiconductor heterostructurse and their properties　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (B), 1997 - 1998
  SAIKI Koichiro; UENO Keiji, The University of Tokyo
  Grant amount(Total)：13900000, Direct funding：13900000
  The results of the present research are summarized.
  (1) Initial oxidation process of Mg and Ba films studied by electron spectroscopies
  MgO and BaO, which are lattice matched to GaAs and Si, could be expected to be key materials to integrate functional perovskite oxides with highly developed semiconductor technologies. In the present work we have revealed the initial oxidation process of Mg and Ba films using Auger electron spectroscopy, electron energy loss spectroscopy and ultraviolet photoelectron spectroscopy. The conditions of oxidation such as critical oxygen exposure, appropriate substrate temperature, etc. have been clarified.
  (2) Evaluation of alkali halide hetero-interface by high sensitive RHEED analysis
  The interface of alkali halide heterostructures have been analyzed by reflection high energy electron diffraction (RHEED). The disadvantage of this method is that the probing electron causes damage to the growing film. In the present work we have devised high sensitive RHEED apparatus using micro channel plate (MCP), which could reduce the probing current by as much as 1/8000. With this apparatus we could observe the initial growth process of alkali halide films and clarified the mechanism in detail.
  (3) A complex heterostructure to achieve a rocksalt oxide film on GaAs
  A single-crystalline MgO film was grown on GaAs(001) by constructing a complex heterostructure with two alkali halide buffer layers. The growth temperature was decreased to 150 ﾟC as compared with direct growth of MgO on GaAs(001). Electron energy loss spectrum of the grown film agreed well with that of bulk MgO, indicating that surface stoichiometry was maintained. The structure was stable up to 600ﾟC against heating in UHV condition. The concept of a complex heterostructure will help fabrication of functional oxide layers on GaAs substrates and lead to oxide/semiconductor integrated devices.
  (4) Photoyield measurement of alkali halide/semiconductor heterostructure
  We have devised a new apparatus to measure a photoyield from the film surface. The photoyield from the NaCl/GaAs heterostructure with various NaCl thickness have been measured up to now. This provides the photo threshold for the electron emission and additive information on the band alignment of the exotic heterostructure such as NaCl/GaAs.
  Grant number：09450006
• STM/AFMによる層状物質基板上への超微細構造の作製　　　　　　　　　　　　　　　
  1996 - 1996
  Grant amount(Total)：1000000, Direct funding：1000000
  Grant number：08750009
• 原子モアレ変調構造の走査トンネル分光　　　　　　　　　　　　　　　
  1995 - 1995
  Grant amount(Total)：2000000, Direct funding：2000000
  Grant number：07225204
• ファンデアワールス・エピタキシ-法によるフラーレン超薄膜の作製とその物性の解明　　　　　　　　　　　　　　　
  1995 - 1995
  Grant amount(Total)：2000000, Direct funding：2000000
  Grant number：07213207
• 表面をSTM/AFMで修飾した基板上への結晶成長に関する研究　　　　　　　　　　　　　　　
  1994 - 1994
  Grant amount(Total)：900000, Direct funding：900000
  Grant number：06750004
• ファンデアワールス・エピタキシ-法によるフラーレン超薄膜の作製とのその物性の解明　　　　　　　　　　　　　　　
  1994 - 1994
  Grant amount(Total)：1700000, Direct funding：1700000
  Grant number：06224206
• 原子モアレ変調構造の走査トンネル分光　　　　　　　　　　　　　　　
  1994 - 1994
  Grant amount(Total)：2300000, Direct funding：2300000
  Grant number：06236204
• Fabrication and Physical Properties of Organic-Inorganic Superlattice　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for General Scientific Research (B), 1993 - 1994
  KOMA Atsushi; SHIMADA Toshihiro; UENO Keiji, The University of Tokyo
  Grant amount(Total)：7300000, Direct funding：7300000
  This project was aimed at the fabrication and the pursuit of novel physical properties of organic-inorganic epitaxial superlattices. The method of fabrication was van der Waals epitaxy, which utilizes molecular beam epitaxy technique to van der Waals interfaces. The results obtained are as follows : (1) The first example of organic-inorganic EPITAXIAL superlattice has been presented using KBr-VOPc (vanadyl phthalocyanine) system.(2) By using AFM (atomic force microscope), it has been found that the morphology of the grown organic films can be controlled by changing the substrate temperature and molecular beam flux. This result is important to fabricate atomically uniform superlattices.(3) It was demonstrated that the molecular orientation can be controlled by regularly-spaced steps of vicinal substrates. This method is expected to be useful to suppress the formation of multi-domain structure which deteriorates the crystallinity of organic-inorganic superlattices. It will also be useful to fabricate 'quantum wells' , or onedimensionally ordered structure of organic molecules.(4) Capping of organic micro structure by inorganic material for protection is achieved for the first time by using the idea of organic-inorganic superlattice.(5) As new materials combinations for organic-inorganic heteroepitaxy, metal phathalocyanines on Se-terminated GaAs (111) substrates, and squaric acid (C_4H_2O_4) on alkali halides were introduced.
  Grant number：05452093
• 異種構造物質間でのヘテロエピタキシ-に関する研究　　　　　　　　　　　　　　　
  1993 - 1993
  Grant amount(Total)：900000, Direct funding：900000
  Grant number：05750005
• ファンデアワールス・エピタキシー法によるフラーレン超薄膜の作製とその物性の解明　　　　　　　　　　　　　　　
  1993 - 1993
  Grant amount(Total)：3000000, Direct funding：3000000
  Grant number：05233208
• ファンデアワールス相互作用によるヘテロエピタキシャル成長と界面構造の解明　　　　　　　　　　　　　　　
  1993 - 1993
  Grant amount(Total)：3100000, Direct funding：3100000
  Grant number：05211206
• 原子モアレ変調構造の走査トンネル分光　　　　　　　　　　　　　　　
  1993 - 1993
  Grant amount(Total)：3700000, Direct funding：3700000
  Grant number：05245203
• Exploring Study of Novel Properties in Artificially Stacked Materials Grown by van der Waals Epitaxy　　　　　　　　　　　　　　　
  Japan Society for the Promotion of Science, Grants-in-Aid for Scientific Research, Grant-in-Aid for Specially Promoted Research, 1990 - 1992
  KOMA Atsushi; UENO Keiji; TADA Hirokazu; SAIKI Koichiro; AOKI Hideo, University of Tokyo
  Grant amount(Total)：220000000, Direct funding：220000000
  This project has been fulfillled to create new artificially stacked materials made of highly heterogeneous layrs and to explore novel properties they show. By extending the idea of van der Waals epitaxy developed by us we have succeeded to grow various kinds of heterostructures. The concept of dangling bond termination has made it possible to grow many layred materials on such ordinary three-dimensional materials as GaAs and Si. Good epitaxial films of fullerene and many organic molecular meterials have also been fabricated successfully. Success in heteroepitaxy between lattice-mismatched alkali halides or polar organic materials on alkali halide by using electrostatic interaction has also increased the possible combination of materials in heteroepitaxy. New properties found and investigated in the present project include moiree-type modulation in STM image, polytype and charge density state formations in ultrathin films, control of optical properties of phthalocyanien films by stacking and anomaly in surface polariton dispersion in ultrathin epitaxial films of alkali halides. Crystal growth, crystal structure, electronic properties, superconductivity dielectric properties in the artificially stacked structures have been investigated theoretically by means of Monte Carlo study, Band, calculation, Quantum Monte Carlo study of analytical method. The occurrence of superconductivity due to nonretarded attraction has been demonstrated in a class of repulsively interacting electron systems that consist of a carrier band interacting with an insulting band.
  Grant number：02102002
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  Competitive research funding
• -　　　　　　　　　　　　　　　
  Competitive research funding