Kyushu University Academic Staff Educational and Research Activities Database
List of Papers
Ago Hiroki Last modified date:2024.04.09

Professor / Interdisciplinary Graduate School of Engineering Sciences / Faculty of Engineering Sciences


Papers
1. M. Nakatani, S. Fukamachi, P. Solís-Fernández, S. Honda, M. Harada, K. Kawahara, Y. Tsuji, Y. Sumiya, M. Kuroki, K. Li, Q. Liu, Y.-C. Lin, A. Uchida, S. Oyama, H. Ji, K. Okada, K. Suenaga, Y. Kawano, K. Yoshizawa, A. Yasui, H. Ago, Ready-to-transfer two-dimensional materials using tunable adhesive force tapes, Nature Electronics, 10.1038/s41928-024-01121-3, 7, 119, 2024.02, [URL].
2. Y.-C. Lin*, R. Matsumoto, Q. Liu, P. Soslís-Fernández, M.-D. Siao, P.-W. Chiu, H. Ago, K. Suenaga, Alkali metal bilayer intercalation in graphene, Nature Communications, 10.1038/s41467-023-44602-3, 15, 425, 2024.01, [URL].
3. C. J. Knill*, S. Douyon, K. Kawahara, H. Yamaguchi, G. Wang, H. Ago, N. Moody, S. Karkare, Effects of nonlinear photoemission on mean transverse energy from metal photocathodes, ACS Nano, 10.1021/acsnano.3c06958, 17, 23659, 2023.11.
4. D. Inukai, T. Koyama, K. Kawahara, H. Ago, H. Kishida, Complex third-order nonlinear susceptibility of single-layer graphene governing third-harmonic generation, Phys. Rev. B, 10.1103/PhysRevB.108.075408, 108, 075408 , 2023.08.
5. S. Hirokawa, H. Teshima, H. Kawamoto, P. Solis-Fernandez, H. Ago, Q. Y. Li, K. Takahashi, Random but limited pressure of graphene liquid cells, Ultramicroscopy, 10.1016/j.ultramic.2023.113747, 250, 113747 , 2023.04.
6. S. Fukamachi, P. Solis-Fernandez, K. Kawahara, D. Tanaka, T. Ohtake, Y.-C. Lin, K. Suenaga, H. Ago, Large-area synthesis and transfer of multilayer hexagonal boron nitride for enhanced graphene device arrays, Nature Electronics, 10.1038/s41928-022-00911-x, 6, 126-136, 2023.02, [URL], Multilayer hexagonal boron nitride (hBN) can be used to preserve the intrinsic physical properties of other two-dimensional materials in device structures. However, integrating the material into large-scale two-dimensional heterostructures remains challenging due to the difficulties in synthesizing high-quality large-area multilayer hBN and combining it with other two-dimensional material layers of the same scale. Here we show that centimetre-scale multilayer hBN can be synthesized on iron–nickel alloy foil by chemical vapour deposition, and then used as a substrate and as a surface-protecting layer in graphene field-effect transistors. We also develop an integrated electrochemical transfer and thermal treatment method that allows us to create high-performance graphene/hBN heterostacks. Arrays of graphene field-effect transistors fabricated by conventional and scalable methods show an enhancement in room-temperature carrier mobility when hBN is used as an insulating substrate, and a further increase—up to a value of 10,000 cm2 V−1 s−1—when graphene is encapsulated with another hBN sheet..
7. H.-L. Liu, B. D. Annawati, N. T. Hung, D. P. Gulo, P. Solis-Fernandez, K. Kawahara, H. Ago, R. Saito, Interference of excitons and surface plasmons in the optical absorption spectra of monolayer and bilayer graphene, Phys. Rev. B, 10.1103/PhysRevB.107.165421, 107, 165421, 2023.03.
8. S. Nakajima, Y. Wasai, K. Kawahara, N. Nabatova-Gabain, P. Gomasang, H. Ago, H Akinaga, K. Ueno, Structure analysis of graphene-capped copper by spectroscopic ellipsometry for humidity reliability assessment, Jpn. J. Appl. Phys., 10.35848/1347-4065/acb77a, 62, SC1092, 2023.03.
9. C. J. Knill, H. Yamaguchi, K. Kawahara, G. Wang, E. Batista, P. Yang, H. Ago, N. Moody, S. Karkare, Near-Threshold Photoemission from Graphene-Coated Cu(110), Phys. Rev. Appl., 10.1103/PhysRevApplied.19.014015, 19, 014015 , 2023.01.
10. T. Vincent, K. Kawahara, V. Antonov, H. Ago, O. Kazakova, Data cluster analysis and machine learning for classification of twisted bilayer graphene, Carbon, 10.1016/j.carbon.2022.09.021, 201, 141-149, 2023.01.
11. D. Inukai, T. Koyama, M. Araidai, K. Kawahara, H. Ago, H. Kishida, Fermi energy dependence of ultrafast photoluminescence from graphene, J. Appl. Phys., 10.1063/5.0092558, 132, 134301 , 2022.10.
12. Y. Hsin, P. Solís-Fernández, H. Hibino, H. Ago, Surface etching and edge control of hexagonal boron nitride assisted by triangular Sn nanoplates, Nanoscale Adv., 10.1039/D2NA00479H, 4, 3786-3792 ,
, 2022.08.
13. Y. Araki, P. Solís-Fernández, Y.-C. Lin, A. Motoyama, K. Kawahara, M. Maruyama, G. Yanlin, R. Matsumoto, K. Suenaga, S. Okada, H. Ago, Twist angle-dependent molecular intercalation and sheet resistance in bilayer graphene, ACS Nano, 10.1021/acsnano.2c03997, 16, 9, 14075-14085 , 2022.08.
14. P. Solis-Fernandez, H. Ago, Machine learning determination of the twist angle of bilayer graphene by Raman spectroscopy: Implications for van der Waals heterostructures, ACS Appl. Nano Mater., 10.1021/acsanm.1c03928, 5, 1, 356-1366, 2022.01.
15. R. Das, P. Solís-Fernández, D. Breite, A. Prager, A. Lotnyk, A. Schulze, H. Ago, High flux and adsorption based non-functionalized hexagonal boron nitride lamellar membrane for ultrafast water purification, Chem. Eng. J., 10.1016/j.cej.2020.127721, 420, 2, 127721, 2021.09.
16. S. Hirokawa, H. Teshima, P. Solis-Fernandez, H. Ago, Q.-Y. Li, K. Takahash, Pinning in a Contact and Noncontact Manner: Direct Observation of a Three-Phase Contact Line Using Graphene Liquid Cells, Langmuir, 10.1021/acs.langmuir.1c01589, 37, 42, 12271-12277, 2021.10.
17. Tom Vincent, Kenji Kawahara, Vladimir Antonov, Hiroki Ago, Olga Kazakova, Data cluster analysis and machine learning for classification of twisted bilayer graphene, Carbon, 10.1016/j.carbon.2022.09.021, 201, 141-149, 2022.09, [URL].
18. Y.-C. Lin, A. Motoyama, S. Kretschmer, S. Ghaderzadeh, M. Ghorbani-Asl, Y. Araki, A. V. Krasheninnikov, H. Ago, K. Suenaga, Polymorphic Phases of Metal Chlorides in the Confined 2D Space of Bilayer Graphene, Adv. Mater., 10.1002/adma.202105898, 33, 52, 2105898, 2021.10.
19. Y.-C. Lin, A. Motoyama, R. Matsumoto, H. Ago, K. Suenaga, Coupling and Decoupling of Bilayer Graphene Monitored by Electron Energy Loss Spectroscopy, Nano Lett., 10.1021/acs.nanolett.1c03689, 21, 24, 10386-10391, 2021.12.
20. H. Ago, S. Okada, Y. Miyata, K. Matsuda, M. Koshino, K. Ueno, K. Nagashio, Science of 2.5 dimensional materials: paradigm shift of materials science toward future social innovation, Sci. Tech. Adv. Mater., 10.1080/14686996.2022.2062576, 23, 1, 275-299, 2022.02.
21. S. Fukushima, S. Fukamachi, M. Shimatani, K. Kawahara, H. Ago, S. Ogawa, Graphene-based deep-ultraviolet photodetectors with ultrahigh responsivity using chemical vapor deposition of hexagonal boron nitride to achieve photogating, Opt. Mater. Express, 10.1364/OME.457545, 12, 5, 2090-2101, 2022.05.
22. H. G. Ji, U. Erkılıç, P. Solís-Fernández, H. Ago, Stacking orientation-dependent photoluminescence pathways in artificially stacked bilayer WS2 nanosheets grown by chemical vapor deposition: Implications for spintronics and valleytronics, ACS Applied Nano Materials, 10.1021/acsanm.1c00192, 3717-3724, 2021.03.
23. R. Das, P. Solís-Fernández, D. Breite, A. Prager, A. Lotnyk, A. Schulze, H. Ago, High flux and adsorption based non-functionalized hexagonal boron nitride lamellar membrane for ultrafast water purification, Chemical Engineering Journal, 10.1016/j.cej.2020.127721, 2020.11.
24. D. Inukai, T. Koyama, K. Kawahara, H. Ago, H. Kishida, Electronic states of electrochemically doped single-layer graphene probed through Fano resonance effects in Raman scattering, Journal of Physical Chemistry C, 10.1021/acs.jpcc.0c06566, 26428-26433, 2020.11.
25. U. Erkılıç, H. Ago, Type-I heterostructure and improved phase stability of formamidinium lead iodide perovskite grown on WS2, Evergreen, 323-328, 2020.09.
26. Y. Uchida, K. Kawahara, S. Fukamachi, H. Ago, Chemical Vapor deposition growth of uniform multilayer hexagonal boron nitride driven by structural transformation of metal thin film, ACS Applied Electronic Materials, 10.1021/acsaelm.0c00601, 3270-3278, 2020.09.
27. U. Erkılıç, H. G. Ji, E. Nishibori, H. Ago, One-step vapour phase growth of two-dimensional formamidinium-based perovskite and its hot carrier dynamics, Physical Chemistry Chemical Physics, 10.1039/D0CP02652B, 21512-21519, 2020.08.
28. High output voltage generation of over 5 V from liquid motion on single-layer MoS2.
29. P. Solís-Fernández, Y. Terao, K. Kawahara, W. Nishiyama, T. Uwanno, Y.-C. Li, K. Yamamoto, H. Nakashima, K. Nagashio, H. Hibino, K. Suenaga, H. Ago, Isothermal Growth and Stacking Evolution in Highly Uniform Bernal-Stacked Bilayer Graphene, ACS Nano, 10.1021/acsnano.0c00645, 2020.05.
30. S. Hirokawa, H. Teshima, P. Solís-Fernández, H. Ago, Y. Tomo, Q. Li, K. Takahashi, Nanoscale Bubble Dynamics Induced by Damage of Graphene Liquid Cells, ACS Omega, 10.1021/acsomega.0c01207, 5, 11180-11185, 2020.04.
31. Y.-C. Lin, H. G. Ji, L.-J. Chang, Y.-P. Chang, Z. Liu, G.-D. Lee, P.-W. Chiu, H. Ago, K. Suenaga, Scanning Moiré Fringe Method: A Superior Approach to Perceive Defects, Interfaces, and Distortion in 2D Materials, ACS Nano, 10.1021/acsnano.0c01729, 2020.04.
32. Alexandre Budiman Taslim, Hideaki Nakajima, Yung Chang Lin, Yuki Uchida, Kenji Kawahara, Toshiya Okazaki, Kazu Suenaga, Hiroki Hibino, Hiroki Ago, Synthesis of sub-millimeter single-crystal grains of aligned hexagonal boron nitride on an epitaxial Ni film, Nanoscale, 10.1039/c9nr03525g, 11, 31, 14668-14675, 2019.08, Hexagonal boron nitride (h-BN), an insulating two-dimensional (2D) layered material, has attracted increasing interest due to its electrical screening effect, high-temperature-resistant gas barrier properties, and other unique applications. However, the presence of grain boundaries (GBs) in h-BN is a hindrance to obtain these properties. Here, we demonstrate the epitaxial growth of monolayer h-BN by chemical vapor deposition (CVD) on Ni(111) thin films deposited on c-plane sapphire. The Ni(111) films showed higher thermal stability than Cu(111) and Cu-Ni(111) alloy films, allowing us to perform CVD growth at a high temperature of 1100 °C. This resulted in an increase of the h-BN grain sizes to up to 0.5 millimeter, among the highest reported so far, and in a well-defined triangular grain shape. Low-energy electron microscopy (LEEM) revealed the epitaxial relationship between h-BN and the underlying Ni(111) lattice, leading to a preferential alignment of the h-BN grains. Both the large grain size and the alignment are expected to facilitate the synthesis of h-BN with a low density of GBs. We also found that the addition of N2 gas during the CVD improves the crystalline shape of the h-BN grains, changing from an irregular, truncated to a sharp triangle. The growth behavior of monolayer h-BN is further discussed in terms of the dependences on growth temperature and pressure, as well as on the structural evolution of the Ni metal catalyst. Our findings not only help understand the h-BN growth mechanism but also offer a new route to grow high-quality, monolayer h-BN films..
33. Hyun Goo Ji, Pablo Solís-Fernández, Daisuke Yoshimura, Mina Maruyama, Takahiko Endo, Yasumitsu Miyata, Susumu Okada, Hiroki Ago, Chemically Tuned p- and n-Type WSe2 Monolayers with High Carrier Mobility for Advanced Electronics, Advanced Materials, 10.1002/adma.201903613, 31, 42, 2019.10, Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post-silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom-thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p- and n-type semiconductors is essential for various device applications, such as complementary metal-oxide-semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe2 is demonstrated by chemical doping. Two different molecules, 4-nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe2 field-effect transistors (FETs) to p- and n-type, respectively. Moreover, the chemically doped WSe2 show increased effective carrier mobilities of 82 and 25 cm2 V−1s−1 for holes and electrons, respectively, which are much higher than those of the pristine WSe2. The doping effects are studied by photoluminescence, Raman, X-ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe2 FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (≈0.17 nW). Furthermore, a p-n junction within single WSe2 grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe2 will contribute to the development of TMDC-based advanced electronics..
34. Ufuk Erklllç, Pablo Solís-Fernández, Hyun Goo Ji, Keisuke Shinokita, Yung Chang Lin, Mina Maruyama, Kazu Suenaga, Susumu Okada, Kazunari Matsuda, Hiroki Ago, Vapor Phase Selective Growth of Two-Dimensional Perovskite/WS2 Heterostructures for Optoelectronic Applications, ACS Applied Materials and Interfaces, 10.1021/acsami.9b13904, 11, 43, 40503-40511, 2019.01, Organic-inorganic hybrid perovskites have attracted increased interest owing to their exceptional optoelectronic properties and promising applications. Monolayers of transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS2), are also intriguing because of their unique optoelectronic properties and their atomically thin and flexible structures. Therefore, the combination of these different types of materials is very attractive in terms of fundamental science of interface interaction, as well as for the realization of ultrathin optoelectronic devices with high performance. Here, we demonstrate the controlled synthesis of two-dimensional (2D) perovskite/WS2 heterostructures by an all vapor-phase growth approach. This involves the chemical vapor deposition (CVD) growth of monolayer WS2, followed by the vapor-phase selective deposition of 2D PbI2 onto the WS2 with the successive conversion of PbI2 to organic-inorganic perovskite (CH3NH3PbI3). Moreover, the selective growth of the perovskite on prepatterned WS2 enables the direct synthesis of patterned heterostructures, avoiding any damage to the perovskite. The photodetectors utilizing the perovskite/WS2 heterostructure show increased responsivities compared with isolated thin perovskite obtained by conventional solution methods. The integration of 2D perovskite with TMDCs opens a new avenue to fabricate advanced devices by combining their unique properties and overcoming current processing difficulties of perovskites..
35. Yuki Uchida, Sho Nakandakari, Kenji Kawahara, Shigeto Yamasaki, Masatoshi Mitsuhara, Hiroki Ago, Controlled Growth of Large-Area Uniform Multilayer Hexagonal Boron Nitride as an Effective 2D Substrate, ACS nano, 10.1021/acsnano.8b03055, 12, 6, 6236-6244, 2018.06, Multilayer hexagonal boron nitride (h-BN) is an ideal insulator for two-dimensional (2D) materials, such as graphene and transition metal dichalcogenides, because h-BN screens out influences from surroundings, allowing one to observe intrinsic physical properties of the 2D materials. However, the synthesis of large and uniform multilayer h-BN is still very challenging because it is difficult to control the segregation process of B and N atoms from metal catalysts during chemical vapor deposition (CVD) growth. Here, we demonstrate CVD growth of multilayer h-BN with high uniformity by using the Ni-Fe alloy film and borazine (B3H6N3) as catalyst and precursor, respectively. Combining Ni and Fe metals tunes the solubilities of B and N atoms and, at the same time, allows one to engineer the metal crystallinity, which stimulates the uniform segregation of multilayer h-BN. Furthermore, we demonstrate that triangular WS2 grains grown on the h-BN show photoluminescence stronger than that grown on a bare SiO2 substrate. The PL line width of WS2/h-BN (the minimum and mean widths are 24 and 43 meV, respectively) is much narrower than those of WS2/SiO2 (44 and 67 meV), indicating the effectiveness of our CVD-grown multilayer h-BN as an insulating layer. Large-area, multilayer h-BN realized in this work will provide an excellent platform for developing practical applications of 2D materials..
36. Hyun Goo Ji, Mina Maruyama, Adha Sukma Aji, Susumu Okada, Kazunari Matsuda, Hiroki Ago, Van der Waals interaction-induced photoluminescence weakening and multilayer growth in epitaxially aligned WS2, Physical Chemistry Chemical Physics, 10.1039/c8cp04418j, 20, 47, 29790-29797, 2018.01, Recently, transition metal dichalcogenides (TMDCs) have attracted great interest due to their unique electronic and optical properties. Chemical vapor deposition (CVD) has been regarded as the most promising method for the synthesis of large-area TMDCs with high reproducibility. Having similar hexagonal crystal structures with many TMDCs, c-plane sapphire is commonly used as a growth substrate in CVD. However, few studies have been reported on the influence of the sapphire substrate on the growth behavior and physical properties of TMDCs. In this work, we demonstrate that higher strain is induced in epitaxially grown WS2 grains via van der Waals interactions with sapphire as compared with misaligned WS2 grains. In addition, this strain was found to enhance overlayer deposition on monolayer WS2, while multilayer growth was not observed in non-epitaxial WS2. Photoluminescence (PL) of the epitaxially grown WS2 grains was reduced, reflecting the effective van der Waals interaction with sapphire. Moreover, low-temperature PL measurements revealed strong influence of the c-plane sapphire surface on the optical properties of WS2. Density functional theory (DFT) calculation supports that the aligned WS2 grains are more strongly bound to the sapphire surface, as compared with misaligned WS2. Our work offers a new insight into the understanding of the influence of the substrate on the CVD-grown TMDC materials..
37. Kenshiro Suenaga, Hyun Goo Ji, Yung Chang Lin, Tom Vincent, Mina Maruyama, Adha Sukma Aji, Yoshihiro Shiratsuchi, Dong Ding, Kenji Kawahara, Susumu Okada, Vishal Panchal, Olga Kazakova, Hiroki Hibino, Kazu Suenaga, Hiroki Ago, Surface-Mediated Aligned Growth of Monolayer MoS2 and In-Plane Heterostructures with Graphene on Sapphire, ACS nano, 10.1021/acsnano.8b04612, 2018.01, Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS2) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS2 grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS2-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS2, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS2 growth on sapphire was further developed to the rational synthesis of an in-plane MoS2-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS2 parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS2 was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures..
38. Dong Ding, Hiroki Hibino, Hiroki Ago, Grain Boundaries and Gas Barrier Property of Graphene Revealed by Dark-Field Optical Microscopy, Journal of Physical Chemistry C, 10.1021/acs.jpcc.7b10210, 122, 1, 902-910, 2018.01, We demonstrate that dark-field (DF) optical microscopy is a powerful tool to visualize grain boundaries (GBs) and grain structure of graphene grown by chemical vapor deposition (CVD). Copper oxide nanoparticles sparsely formed along the graphene GBs by postgrowth mild oxidation allow one to determine the position and structure of the GBs by the DF microscope. As DF imaging offers a much higher sensitivity than bright-field (BF) microscopy, some GBs were observed even without the postgrowth oxidation. We found that periodic Cu steps formed below graphene can be also used to visualize the grain structure of the as-grown graphene by DF microscopy. Moreover, DF imaging is applicable to study of the gas barrier property of CVD graphene. Interestingly, the dissolved oxygen inside Cu foil enhanced oxidation of the Cu surface below graphene in spite of the fact that the graphene protects the underlying Cu from the exterior gas. Our work highlights the wide availability of DF optical microscopy in characterizing graphene and related two-dimensional materials grown on metal substrates..
39. Ploybussara Gomasang, Takumi Abe, Kenji Kawahara, Yoko Wasai, Nataliya Nabatova-Gabain, Nguyen Thanh Cuong, Hiroki Ago, Susumu Okada, Kazuyoshi Ueno, Moisture barrier properties of single-layer graphene deposited on Cu films for Cu metallization, Japanese Journal of Applied Physics, 10.7567/JJAP.57.04FC08, 57, 4, 2018.04, The moisture barrier properties of large-grain single-layer graphene (SLG) deposited on a Cu(111)/sapphire substrate are demonstrated by comparing with the bare Cu(111) surface under an accelerated degradation test (ADT) at 85°C and 85% relative humidity (RH) for various durations. The change in surface color and the formation of Cu oxide are investigated by optical microscopy (OM) and X-ray photoelectron spectroscopy (XPS), respectively. First-principle simulation is performed to understand the mechanisms underlying the barrier properties of SLG against O diffusion. The correlation between Cu oxide thickness and SLG quality are also analyzed by spectroscopic ellipsometry (SE) measured on a non-uniform SLG film. SLG with large grains shows high performance in preventing the Cu oxidation due to moisture during ADT..
40. H. Nakajima, T. Morimoto, Y. Okigawa, T. Yamada, Y. Ikuta, K. Kawahara, H. Ago, T. Okazaki, Imaging of local structures affecting electrical transport properties of large graphene sheets by lock-in thermography, Science Advances, 10.1126/sciadv.aau3407, 5, 2, 2019.02, The distribution of defects and dislocations in graphene layers has become a very important concern with regard to the electrical and electronic transport properties of device applications. Although several experiments have shown the influence of defects on the electrical properties of graphene, these studies were limited to measuring microscopic areas because of their long measurement times. Here, we successfully imaged various local defects in a large area of chemical vapor deposition graphene within a reasonable amount of time by using lock-in thermography (LIT). The differences in electrical resistance caused by the micrometer-scale defects, such as cracks and wrinkles, and atomic-scale domain boundaries were apparent as nonuniform Joule heating on polycrystalline and epitaxially grown graphene. The present results indicate that LIT can serve as a fast and effective method of evaluating the quality and uniformity of large graphene films for device applications..
41. Kaito Kanahashi, Naoki Tanaka, Yoshiaki Shoji, Mina Maruyama, Il Jeon, Kenji Kawahara, Masatou Ishihara, Masataka Hasegawa, Hiromichi Ohta, Hiroki Ago, Yutaka Matsuo, Susumu Okada, Takanori Fukushima, Taishi Takenobu, Formation of environmentally stable hole-doped graphene films with instantaneous and high-density carrier doping via a boron-based oxidant, npj 2D Materials and Applications, 10.1038/s41699-019-0090-x, 3, 1, 2019.12, Large-area graphene films have substantial potential for use as next-generation electrodes because of their good chemical stability, high flexibility, excellent carrier mobility, and lightweight structure. However, various issues remain unsolved. In particular, high-density carrier doping within a short time by a simple method, and air stability of doped graphene films, are highly desirable. Here, we demonstrate a solution-based high-density (>1014 cm−2) hole doping approach that promises to push the performance limit of graphene films. The reaction of graphene films with a tetrakis(pentafluorophenyl)borate salt, containing a two-coordinate boron cation, achieves doping within an extremely short time (4 s), and the doped graphene films are air stable for at least 31 days. X-ray photoelectron spectroscopy reveals that the graphene films are covered by the chemically stable anions, resulting in an improved stability in air. Moreover, the doping reduces the transmittance by only 0.44 ± 0.23%. The simplicity of the doping process offers a viable route to the large-scale production of functional graphene electrodes..
42. Ploybussara Gomasang, Kenji Kawahara, Kenta Yasuraoka, Mina Maruyama, Hiroki Ago, Susumu Okada, Kazuyoshi Ueno, A novel graphene barrier against moisture by multiple stacking large-grain graphene, Scientific reports, 10.1038/s41598-019-40534-5, 9, 1, 2019.12, The moisture barrier properties of stacked graphene layers on Cu surfaces were investigated with the goal of improving the moisture barrier efficiency of single-layer graphene (SLG) for Cu metallization. SLG with large grain size were stacked on Cu surfaces coated with CVD-SLG to cover the grain-boundaries and defective areas of the underneath SLG film, which was confirmed to be oxidized by Raman spectroscopy measurements. To evaluate the humidity resistance of the graphene-coated Cu surfaces, temperature humidity storage (THS) testing was conducted under accelerated oxidation conditions (85 °C and 85% relative humidity) for 100 h. The color changes of the Cu surfaces during THS testing were observed by optical microscopy, while the oxidized Cu into Cu 2 O and CuO was detected by X-ray photoelectron spectroscopy (XPS). The experimental results were accord with the results of first-principle simulation for the energetic barrier against water diffusion through the stacked graphene layers with different overlap. The results demonstrate the efficiency of SLG stacking approach against moisture for Cu metallization..
43. Adha Sukma Aji, Ryohei Nishi, Hiroki Ago, Yutaka Ohno, High output voltage generation of over 5 V from liquid motion on single-layer MoS2, Nano Energy, 10.1016/j.nanoen.2019.104370, 68, 2020.02, Water stores a large amount of clean energy in its dynamic forms. Owing to the wide availability of water and the rising demand for clean energy sources, the direct electricity generation from liquid motion by low-dimensional materials such as graphene has gained much interest. Electricity generation is induced by the motion of an electric double layer at the interface between the liquid droplet and solid surface. However, the output voltage with materials such as graphene is still limited to several hundred millivolts. In this report, we present a large-area single-layer MoS2 film grown by chemical vapor deposition as a nanogenerator, which is capable of generating a large output voltage of more than 5 V from the motion of an aqueous NaCl droplet. Our findings indicate that the high shunt resistance of MoS2 results in a large generated voltage. Furthermore, we demonstrate the possibility to scale up the MoS2 nanogenerator by arranging them in series and parallel connections, which respectively increase the output voltage and current, with a three-times increase with an array of three MoS2 nanogenerators. Our work opens the application of single-layer MoS2 for harvesting electricity from the dynamic movement of liquid, such as the capability to harvest ocean wave energy, which is also demonstrated here..
44. Y. Miyoshi, Y. Fukazawa, Y. Amasaka, R. Reckmann, T. Yokoi, K. Ishida, K. Kawahara, H. Ago, H. Maki, High-speed and on-chip graphene blackbody emitters for optical communications by remote heat transfer, Nature Communications, 1279, 2018.03.
45. Hyun Goo Ji, Yung Chang Lin, Kosuke Nagashio, Mina Maruyama, Pablo Solís-Fernández, Adha Sukma Aji, Vishal Panchal, Susumu Okada, Kazu Suenaga, Hiroki Ago, Hydrogen-Assisted Epitaxial Growth of Monolayer Tungsten Disulfide and Seamless Grain Stitching, Chemistry of Materials, 10.1021/acs.chemmater.7b04149, 30, 2, 403-411, 2018.01, Recently, research on transition metal dichalcogenides (TMDCs) has been accelerated by the development of large-scale synthesis based on chemical vapor deposition (CVD). However, in most cases, CVD-grown TMDC sheets are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs) which deteriorate the physical properties of the TMDC. Here, we demonstrate the epitaxial growth of monolayer tungsten disulfide (WS2) on sapphire by introducing a high concentration of hydrogen during the CVD process. As opposed to the randomly oriented grains obtained in conventional growth, the presence of H2 resulted in the formation of triangular WS2 grains with the well-defined orientation determined by the underlying sapphire substrate. Photoluminescence of the aligned WS2 grains was significantly suppressed compared to that of the randomly oriented grains, indicating a hydrogen-induced strong coupling between WS2 and the sapphire surface that has been confirmed by density functional theory calculations. Scanning transmission electron microscope observations revealed that the epitaxially grown WS2 has less structural defects and impurities. Furthermore, sparsely distributed unique dislocations were observed between merging aligned grains, indicating an effective stitching of the merged grains. This contrasts with the GBs that are observed between randomly oriented grains, which include a series of 8-, 7-, and alternating 7/5-membered rings along the GB. The GB structures were also found to have a strong impact on the chemical stability and carrier transport of merged WS2 grains. Our work offers a novel method to grow high-quality TMDC sheets with much less structural defects, contributing to the future development of TMDC-based electronic and photonic applications..
46. Masaharu Tsuji, Kanako Matsuda, Mayu Tanaka, Satsuki Kuboyama, Keiko Uto, Nozomi Wada, Hirofumi Kawazumi, Takeshi Tsuji, Hiroki Ago, Jun Ichiro Hayashi, Enhanced Photocatalytic Degradation of Methyl Orange by Au/TiO2 Nanoparticles under Neutral and Acidic Solutions, ChemistrySelect, 10.1002/slct.201702664, 3, 5, 1432-1438, 2018.02, A comparative study was carried out on the degradation of methyl orange (MO) by TiO2 and Au/TiO2 photocatalysts in neutral and acidic solutions. Au/TiO2 photocatalysts with an Au : Ti atomic ratio of 1.5±0.1% : 98.5±0.1% were prepared by using a microwave-polyol method in the presence of P25 TiO2. Initial degradation rates of MO by TiO2 and Au/TiO2 were 0.13 and 0.22 min−1 at pH 7, whereas they increased to 0.96 and 3.06 min−1 at pH 2, respectively. These results indicate that the MO degradation rates are enhanced by loading Au nanoparticles on TiO2 in neutral and acidic solutions by factors of 1.7 and 3.2, respectively. Mass spectroscopic studies lead us to conclude that major reaction products are formed by demethylation and OH addition of benzenoid form of MO in neutral solutions, whereas they are produced through ring opening and carboxylation of quinonoid form of MO after scission of a center N-NH bond in acidic solutions. The relative importance of electron trapping and surface plasmon resonance (SPR) effects of Au nanoparticles is discussed for the enhancement of photocatalytic activity of Au/TiO2..
47. Adha Sukma Aji, Masanori Izumoto, Kenshiro Suenaga, Keisuke Yamamoto, Hiroshi Nakashima, Hiroki Ago, Two-step synthesis and characterization of vertically stacked SnS-WS2 and SnS-MoS2 p-n heterojunctions, Physical Chemistry Chemical Physics, 10.1039/c7cp06823a, 20, 2, 889-897, 2018.01, We demonstrate the synthesis of unique heterostructures consisting of SnS and WS2 (or SnS and MoS2) by two-step chemical vapor deposition (CVD). After the first CVD growth of triangular WS2 (MoS2) grains, the second CVD step was performed to grow square SnS grains on the same substrate. We found that these SnS grains can be grown at very low temperature with the substrate temperature of 200 °C. Most of the SnS grains nucleated from the side edges of WS2 (MoS2) grains, resulting in the formation of partly stacked heterostructures with a large overlapping area. The SnS grains showed doped p-type transfer character with a hole mobility of 15 cm2 V-1 s-1, while the WS2 and MoS2 grains displayed n-type character with a high on/off ratio of >106. The SnS-WS2 and SnS-MoS2 heterostructures exhibited clear rectifying behavior, signifying the formation of p-n junctions at their interfaces. This heterostructure growth combined with the low temperature SnS growth will provide a promising means to exploit two-dimensional heterostructures by avoiding possible damage to the first material..
48. Adha Sukma Aji, Pablo Solís-Fernández, Hyun Goo Ji, Kenjiro Fukuda, Hiroki Ago, High Mobility WS2 Transistors Realized by Multilayer Graphene Electrodes and Application to High Responsivity Flexible Photodetectors, Advanced Functional Materials, 10.1002/adfm.201703448, 27, 47, 2017.12, The electrical contact is one of the main issues preventing semiconducting 2D materials to fulfill their potential in electronic and optoelectronic devices. To overcome this problem, a new approach is developed here that uses chemical vapor deposition grown multilayer graphene (MLG) sheets as flexible electrodes for WS2 field-effect transistors. The gate-tunable Fermi level, van der Waals interaction with the WS2, and the high electrical conductivity of MLG significantly improve the overall performance of the devices. The carrier mobility of single-layer WS2 increases about a tenfold (50 cm2 V−1 s−1 at room temperature) by replacing conventional Ti/Au metal electrodes (5 cm2 V−1 s−1) with the MLG electrodes. Further, by replacing the conventional SiO2 substrate with a thin (1 µm) parylene-C flexible film as insulator, flexible WS2 photodetectors that are able to sustain multiple bending stress tests without significant performance degradation are realized. The flexible photodetectors exhibited extraordinarily high gate-tunable photoresponsivities, reaching values of 4500 A W−1, and with very short (2, graphene, and the very thin polymer film will find applications in various flexible electronics, such as wearable high-performance optoelectronics devices..
49. Hiroki Kinoshita, Il Jeon, Mina Maruyama, Kenji Kawahara, Yuri Terao, Dong Ding, Rika Matsumoto, Yutaka Matsuo, Susumu Okada, Hiroki Ago, Highly Conductive and Transparent Large-Area Bilayer Graphene Realized by MoCl5 Intercalation, Advanced Materials, 10.1002/adma.201702141, 29, 41, 2017.11, Bilayer graphene (BLG) comprises a 2D nanospace sandwiched by two parallel graphene sheets that can be used to intercalate molecules or ions for attaining novel functionalities. However, intercalation is mostly demonstrated with small, exfoliated graphene flakes. This study demonstrates intercalation of molybdenum chloride (MoCl5) into a large-area, uniform BLG sheet, which is grown by chemical vapor deposition (CVD). This study reveals that the degree of MoCl5 intercalation strongly depends on the stacking order of the graphene; twist-stacked graphene shows a much higher degree of intercalation than AB-stacked. Density functional theory calculations suggest that weak interlayer coupling in the twist-stacked graphene contributes to the effective intercalation. By selectively synthesizing twist-rich BLG films through control of the CVD conditions, low sheet resistance (83 Ω ▫−1) is realized after MoCl5 intercalation, while maintaining high optical transmittance (≈95%). The low sheet resistance state is relatively stable in air for more than three months. Furthermore, the intercalated BLG film is applied to organic solar cells, realizing a high power conversion efficiency..
50. Pablo Solís-Fernández, Mark Bissett, Hiroki Ago, Synthesis, structure and applications of graphene-based 2D heterostructures, Chemical Society Reviews, 10.1039/c7cs00160f, 46, 15, 4572-4613, 2017.08, With the profuse amount of two-dimensional (2D) materials discovered and the improvements in their synthesis and handling, the field of 2D heterostructures has gained increased interest in recent years. Such heterostructures not only overcome the inherent limitations of each of the materials, but also allow the realization of novel properties by their proper combination. The physical and mechanical properties of graphene mean it has a prominent place in the area of 2D heterostructures. In this review, we will discuss the evolution and current state in the synthesis and applications of graphene-based 2D heterostructures. In addition to stacked and in-plane heterostructures with other 2D materials and their potential applications, we will also cover heterostructures realized with lower dimensionality materials, along with intercalation in few-layer graphene as a special case of a heterostructure. Finally, graphene heterostructures produced using liquid phase exfoliation techniques and their applications to energy storage will be reviewed..
51. Dong Ding, Pablo Solís-Fernández, Rozan Mohamad Yunus, Hiroki Hibino, Hiroki Ago, Behavior and role of superficial oxygen in Cu for the growth of large single-crystalline graphene, Applied Surface Science, 10.1016/j.apsusc.2017.02.250, 408, 142-149, 2017.06, Decreasing the nucleation density of graphene grown on copper (Cu) foil by chemical vapor deposition (CVD) is essential for the synthesis of large-area single-crystalline graphene. Here, the behavior of the copper oxide layer and its impact on the graphene growth have been investigated. We found that a small amount of oxygen dissolves into the Cu when the oxide layer decomposes during the heating up in a non-reducing Ar environment. The remaining oxygen in the Cu foil can play an important role in decreasing the graphene nucleation density. The dissolved oxygen can withstand at high temperatures even in reducing H2 environments without completely losing its effectiveness for maintaining a low graphene nucleation density. However, heating up in a H2 environment significantly reduces the copper oxide layer during the very first moments of the process at low temperatures, preventing the oxygen to dissolve into the Cu and significantly increasing the nucleation density. These findings will help to improve the graphene growth on Cu catalyst by increasing the grain size while decreasing the grain density..
52. Yuki Uchida, Tasuku Iwaizako, Seigi Mizuno, Masaharu Tsuji, Hiroki Ago, Epitaxial chemical vapour deposition growth of monolayer hexagonal boron nitride on a Cu(111)/sapphire substrate, Physical Chemistry Chemical Physics, 10.1039/c6cp08903h, 19, 12, 8230-8235, 2017.02, Hexagonal boron nitride (h-BN), an atomically thin insulating material, shows a large band gap, mechanical flexibility, and optical transparency. It can be stacked with other two-dimensional (2D) materials through van der Waals interactions to form layered heterostructures. These properties promise its application as an insulating layer of novel 2D electronic devices due to its atomically smooth surface with a large band gap. Herein, we demonstrated the ambient-pressure chemical vapour deposition (CVD) growth of high-quality, large-area monolayer h-BN on a Cu(111) thin film deposited on a c-plane sapphire using ammonia borane (BH3NH3) as the feedstock. Highly oriented triangular h-BN grains grow on Cu(111), which finally coalescence to cover the entire Cu surface. Low-energy electron diffraction (LEED) measurements indicated that the hexagonal lattice of the monolayer h-BN is well-oriented along the underlying Cu(111) lattice, thus implying the epitaxial growth of h-BN, which can be applied in various 2D electronic devices..
53. Dong Ding, Pablo Solís-Fernández, Hiroki Hibino, Hiroki Ago, Spatially Controlled Nucleation of Single-Crystal Graphene on Cu Assisted by Stacked Ni, ACS Nano, 10.1021/acsnano.6b06265, 10, 12, 11196-11204, 2016.12, In spite of recent progress of graphene growth using chemical vapor deposition, it is still a challenge to precisely control the nucleation site of graphene for the development of wafer-scale single-crystal graphene. In addition, the postgrowth patterning used for device fabrication deteriorates the quality of graphene. Herein we demonstrate the site-selective nucleation of single-crystal graphene on Cu foil based on spatial control of the local CH4 concentration by a perforated Ni foil. The catalytically active Ni foil acts as a CH4 modulator, resulting in millimeter-scale single-crystal grains at desired positions. The perforated Ni foil also allows to synthesize patterned graphene without any postgrowth processing. Furthermore, the uniformity of monolayer graphene is significantly improved when a plain Ni foil is placed below the Cu. Our findings offer a facile and effective way to control the nucleation of high-quality graphene, meeting the requirements of industrial processing..
54. Naohiro Fujita, Daisuke Matsumoto, Yuki Sakurai, Kenji Kawahara, Hiroki Ago, Taishi Takenobu, Kazuhiro Marumoto, Direct observation of electrically induced Pauli paramagnetism in single-layer graphene using ESR spectroscopy, Scientific Reports, 10.1038/srep34966, 6, 2016.10, Graphene has been actively investigated as an electronic material owing to many excellent physical properties, such as high charge mobility and quantum Hall effect, due to the characteristics of a linear band structure and an ideal two-dimensional electron system. However, the correlations between the transport characteristics and the spin states of charge carriers or atomic vacancies in graphene have not yet been fully elucidated. Here, we show the spin states of single-layer graphene to clarify the correlations using electron spin resonance (ESR) spectroscopy as a function of accumulated charge density using transistor structures. Two different electrically induced ESR signals were observed. One is originated from a Fermi-degenerate two-dimensional electron system, demonstrating the first observation of electrically induced Pauli paramagnetism from a microscopic viewpoint, showing a clear contrast to no ESR observation of Pauli paramagnetism in carbon nanotubes (CNTs) due to a one-dimensional electron system. The other is originated from the electrically induced ambipolar spin vanishments due to atomic vacancies in graphene, showing a universal phenomenon for carbon materials including CNTs. The degenerate electron system with the ambipolar spin vanishments would contribute to high charge mobility due to the decrease in spin scatterings in graphene..
55. Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroki Ago, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Yasuyuki Takata, A general method of fabricating free-standing, monolayer graphene electronic device and its property characterization, Sensors and Actuators, A: Physical, 10.1016/j.sna.2016.05.002, 247, 24-29, 2016.08, We demonstrate a general process for fabricating graphene nanoelectronic devices that have next several features: free-standing, micrometer-sized monolayer graphene with high quality, arbitrarily-shaped metallic electrodes or sensors. In contrast to the normal routes, a gas etching process is used to create a deep trench in silicon for suspending the whole graphene device in a much larger area. User-designed electrodes or sensors are fabricated on the suspended graphene at the same time for realizing multiple functions. In this work, a suspended gold nanofilm sensor is designed to measure the intrinsic electrical and thermal properties of graphene on site. The sensor serves as both electrode and precise resistance thermometer at the same time. By simply changing the metallic electrode shape and electrical circuit, the free-standing graphene can be made into different devices, such as single-molecule detector or nano-resonator. In order to test the robustness of graphene device, a high electrical current is applied to heat the graphene in vacuum until it breaks. The breakdown current density is measured to be 1.86 mA/μm. More importantly, this method is not only limited to graphene, but also can be applied to any other two-dimensional materials..
56. Yuichiro Takesaki, Kenji Kawahara, Hiroki Hibino, Susumu Okada, Masaharu Tsuji, Hiroki Ago, Highly Uniform Bilayer Graphene on Epitaxial Cu-Ni(111) Alloy, Chemistry of Materials, 10.1021/acs.chemmater.6b01137, 28, 13, 4583-4592, 2016.07, Band gap opening in bilayer graphene (BLG) under a vertical electric field is important for the realization of high performance graphene-based semiconductor devices, and thus, the synthesis of uniform and large-area BLG is required. Here we demonstrate the synthesis of a highly uniform BLG film by chemical vapor deposition (CVD) over epitaxial Cu-Ni (111) binary alloy catalysts. The relative concentration of Ni and Cu as well as the growth temperature and cooling profile was found to strongly influence the uniformity of the BLG. In particular, a slow cooling process after switching off the carbon feedstock is important for obtaining a uniform second layer, covering more than 90% of the total area. Moreover, low-energy electron microscopy (LEEM) study revealed the second layer grows underneath the first layer. We also investigated the stacking order by Raman spectroscopy and LEEM and found that 70-80% of bilayer graphene has Bernal stacking. The metastable 30°-rotated orientations were also observed both in the upper and lower layers. From our experimental observations, a new growth mode is proposed; the first layer grows during the CH4 supply on Cu-Ni alloy surface, while the second layer is segregated from the bulk alloy during the cooling process. Our work highlights the growth mechanism of BLG and offers a promising route to synthesize uniform and large-area BLG for future electronic devices..
57. Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroki Ago, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Yasuyuki Takata, Simultaneous measurement of electrical and thermal conductivities of suspended monolayer graphene, Journal of Applied Physics, 10.1063/1.4954677, 119, 24, 2016.06, We measured both in-plane electrical and thermal properties of the same suspended monolayer graphene using a novel T-type sensor method. At room temperature, the values are about 240 000 Ω-1 m-1 and 2100 W m-1 K-1 for the electrical and thermal conductivities, respectively. Based on the Wiedemann-Franz law, the electrons have negligible contribution to the thermal conductivity of graphene, while the in-plane LA and TA modes phonons are the dominant heat carriers. In monolayer graphene, the absence of layer-layer and layer-substrate interactions enhances the contribution of long wave-length phonons to the heat transport and increases the thermal conductivity accordingly. The reported method and experimental data of suspended monolayer graphene are useful for understanding the basic physics and designing the future graphene electronic devices..
58. Takeshi Koyama, Kenta Mizutani, Hiroki Ago, Hideo Kishida, Two-Step Excitation Triggered by One-Photon Absorption on Linear Dispersion in Monolayer Graphene, Journal of Physical Chemistry C, 10.1021/acs.jpcc.6b01490, 120, 20, 11225-11229, 2016.05, We report the decrease in absorption caused by a near-infrared laser pulse in the visible region in monolayer graphene. This absorption decrease shows the existence of a two-step excitation process of carriers, in which one-photon absorption and Auger recombination sequentially occur. This process results from the linear dispersion nature of monolayer graphene. In addition, the monolayer graphene shows the ultrafast decay of carrier population. The observed properties are of importance for ultrafast optical switching utilizing the optical nonlinearity induced by carrier excitation..
59. Masaharu Tsuji, Daisuke Shimamoto, Keiko Uto, Masashi Hattori, Hiroki Ago, Enhancement of catalytic activity of AgPd@Pd/TiO2 nanoparticles under UV and visible photoirradiation, Journal of Materials Chemistry A, 10.1039/c6ta05699g, 4, 38, 14649-14656, 2016.01, The effects of photoirradiation for the production of hydrogen from the decomposition of formic acid (FA) were studied using Ag93Pd7@Pd/TiO2 (anatase (A) or P25 (P)) nanocatalysts. The catalytic activity was enhanced by a factor of 1.5-1.6 under UV and visible (vis) photoirradiation at room temperature for both TiO2 supports. It was explained by the formation of an electron-rich Pd shell because of the migration of photogenerated electrons from the TiO2 surface. The catalytic activity of Ag93Pd7@Pd/TiO2 (A) was 1.7-7.3 times higher than that of Ag93Pd7@Pd/TiO2 (P) without and under photoirradiation at 27-90 °C. The catalytic activity of Ag93Pd7@Pd/TiO2 (A) under photoirradiation at 27 °C with 468 mmol H2 g per catalyst per h, is the best value ever reported out of all of the heterogeneous catalysts using TiO2 (A) as a photocatalyst at room temperature..
60. Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Hiroki Ago, Yasuyuki Takata, A simple method for fabricating free-standing large area fluorinated single-layer graphene with size-tunable nanopores, Carbon, 10.1016/j.carbon.2015.12.070, 99, 564-570, 2016.04, As a solid-state membrane with only one-atom thickness, nano-porous graphene has attracted intense attention in many critical applications. Here, the key challenge is to suspend a single-layer graphene (SLG) and drill nanopores with precise dimensions. Here, we report a simple and reliable route for making suspended fluorinated SLG with size-tunable nanopores. Our method consists of two steps: 1. a free-standing SLG ribbon was created between two gold pads after deep dry etching of silicon substrate by xenon difluoride. The SLG was fluorinated by 5-13%. Superior to the normal wet etching method, the dry etching process is much simpler and results in less hole-defect and edge deformation. A large area fluorinated SLG can be suspended due to the sufficient etch depth. 2. a focused ion beam was introduced to drill nanopores in graphene with an initial diameter around 20 nm. Followed by an electron beam induced carbon deposition, the diameter of nanopore was gradually decreased to sub-10 nm. By changing the deposition time, the size of nanopore can be precisely controlled. High-cost transmission electron microscope is no longer needed. Our method provides a simple and effective way for preparing free-standing fluorinated SLG ribbon suitable for single-molecule detection..
61. S. Dushenko, Hiroki Ago, K. Kawahara, T. Tsuda, S. Kuwabata, T. Takenobu, T. Shinjo, Y. Ando, M. Shiraishi, Gate-Tunable Spin-Charge Conversion and the Role of Spin-Orbit Interaction in Graphene, Physical Review Letters, 10.1103/PhysRevLett.116.166102, 116, 16, 2016.04, The small spin-orbit interaction of carbon atoms in graphene promises a long spin diffusion length and the potential to create a spin field-effect transistor. However, for this reason, graphene was largely overlooked as a possible spin-charge conversion material. We report electric gate tuning of the spin-charge conversion voltage signal in single-layer graphene. Using spin pumping from an yttrium iron garnet ferrimagnetic insulator and ionic liquid top gate, we determined that the inverse spin Hall effect is the dominant spin-charge conversion mechanism in single-layer graphene. From the gate dependence of the electromotive force we showed the dominance of the intrinsic over Rashba spin-orbit interaction, a long-standing question in graphene research..
62. Hiroki Ago, Satoru Fukamachi, Hiroko Endo, Pablo Solís-Fernández, Rozan Mohamad Yunus, Yuki Uchida, Vishal Panchal, Olga Kazakova, Masaharu Tsuji, Visualization of Grain Structure and Boundaries of Polycrystalline Graphene and Two-Dimensional Materials by Epitaxial Growth of Transition Metal Dichalcogenides, ACS Nano, 10.1021/acsnano.5b05879, 10, 3, 3233-3240, 2016.03, The presence of grain boundaries in two-dimensional (2D) materials is known to greatly affect their physical, electrical, and chemical properties. Given the difficulty in growing perfect large single-crystals of 2D materials, revealing the presence and characteristics of grain boundaries becomes an important issue for practical applications. Here, we present a method to visualize the grain structure and boundaries of 2D materials by epitaxially growing transition metal dichalcogenides (TMDCs) over them. Triangular single-crystals of molybdenum disulfide (MoS2) epitaxially grown on the surface of graphene allowed us to determine the orientation and size of the graphene grains. Grain boundaries in the polycrystalline graphene were also visualized reflecting their higher chemical reactivity than the basal plane. The method was successfully applied to graphene field-effect transistors, revealing the actual grain structures of the graphene channels. Moreover, we demonstrate that this method can be extended to determine the grain structure of other 2D materials, such as tungsten disulfide (WS2). Our visualization method based on van der Waals epitaxy can offer a facile and large-scale labeling technique to investigate the grain structures of various 2D materials, and it will also contribute to understand the relationship between their grain structure and physical properties..
63. Haidong Wang, Kosaku Kurata, Takanobu Fukunaga, Hiroshi Takamatsu, Xing Zhang, Tatsuya Ikuta, Koji Takahashi, Takashi Nishiyama, Hiroki Ago, Yasuyuki Takata, In-situ measurement of the heat transport in defect-engineered free-standing single-layer graphene, Scientific Reports, 10.1038/srep21823, 6, 2016.02, Utilizing nanomachining technologies, it is possible to manipulate the heat transport in graphene by introducing different defects. However, due to the difficulty in suspending large-area single-layer graphene (SLG) and limited temperature sensitivity of the present probing methods, the correlation between the defects and thermal conductivity of SLG is still unclear. In this work, we developed a new method for fabricating micro-sized suspended SLG. Subsequently, a focused ion beam (FIB) was used to create nanohole defects in SLG and tune the heat transport. The thermal conductivity of the same SLG before and after FIB radiation was measured using a novel T-type sensor method on site in a dual-beam system. The nanohole defects decreased the thermal conductivity by about 42%. It was found that the smaller width and edge scrolling also had significant restriction on the thermal conductivity of SLG. Based on the calculation results through a lattice dynamics theory, the increase of edge roughness and stronger scattering on long-wavelength acoustic phonons are the main reasons for the reduction in thermal conductivity. This work provides reliable data for understanding the heat transport in a defective SLG membrane, which could help on the future design of graphene-based electrothermal devices..
64. Pablo Solís-Fernández, Susumu Okada, Tohru Sato, Masaharu Tsuji, Hiroki Ago, Gate-Tunable Dirac Point of Molecular Doped Graphene, ACS Nano, 10.1021/acsnano.6b00064, 10, 2, 2930-2939, 2016.02, Control of the type and density of charge carriers in graphene is essential for its implementation into various practical applications. Here, we demonstrate the gate-tunable doping effect of adsorbed piperidine on graphene. By gradually increasing the amount of adsorbed piperidine, the graphene doping level can be varied from p-to n-type, with the formation of p-n junctions for intermediate coverages. Moreover, the doping effect of the piperidine can be further tuned by the application of large negative back-gate voltages, which increase the doping level of graphene. In addition, the electronic properties of graphene are well preserved due to the noncovalent nature of the interaction between piperidine and graphene. This gate-tunable doping offers an easy, controllable, and nonintrusive method to alter the electronic structure of graphene..
65. Rozan M. Yunus, Hiroko Endo, Masaharu Tsuji, Hiroki Ago, Vertical heterostructure of MoS2 and graphene nanoribbons by two-step chemical vapor deposition for high-gain photodetectors, Physical Chemistry Chemical Physics, 17, 25210-25215, 2015.09.
66. Hiroki Ago, Yujiro Ohta, Hiroki Hibino, Daisuke Yoshimuara, Rina Takizawa, Yuki Uchida, Masaharu Tsuji, Toshihiro Okajima, Hisashi Mitani, Seigi Mizuno, Growth Dynamics of Single-Layer Graphene on Epitaxial Cu Surfaces, CHEMISTRY OF MATERIALS, 10.1021/acs.chemmater.5b01871, 27, 15, 5377-5385, 2015.08.
67. 服部 真史, Daisuke Shimamoto, Hiroki Ago, Masaharu Tsuji, AgPd@Pd/TiO2 nanocatalyst synthesis by microwave heating in aqueous solution for efficient hydrogen production from formic acid, JOURNAL OF MATERIALS CHEMISTRY A, 10.1039/c5ta01434d, 3, 20, 10666-10670 , 2015.04.
68. Hiroki Ago, Hiroko Endo, SOLIS FERNANDEZ PABLO, Rina Takizawa, Yujiro Ohta, Yusuke Fujita, Kazuhiro Yamamoto, Masaharu Tsuji, Controlled van der Waals Epitaxy of Monolayer MoS2 Triangular Domains on Graphene, ACS Applied Materials & Interfaces, 7, 9, 5265-5273, 2015.02.
69. Qin-Yi Li, Koji Takahashi, Hiroki Ago, Zhang, Xing, Ikuta, Tatsuya, Takashi Nishiyama, Kenji Kawahara, Temperature dependent thermal conductivity of a suspended submicron graphene ribbon, JOURNAL OF APPLIED PHYSICS, 10.1063/1.4907699, 117, 6, 065102, 2015.02.
70. SOLIS FERNANDEZ PABLO, MARK ALEXANDER BISSETT, Masaharu Tsuji, Hiroki Ago, Tunable doping of graphene nanoribbon arrays by chemical functionalization, NANOSCALE, 10.1039/c4nr07007k, 7, 8, 3572-3580 , 2015.01.
71. MARK ALEXANDER BISSETT, Yuichiro Takesaki, Masaharu Tsuji, Hiroki Ago, Increased chemical reactivity achieved by asymmetrical 'Janus' functionalisation of graphene, RSC Advances, 4, 52215-52219, 2014.10.
72. Kawasaki, Tetsuya, Sugawara, Kenta, Dobroiu, Adrian, Eto, Takanori, Kurita, Yuki, Kojima, Kazuki, Yabe, Yuhei, Sugiyama, Hiroki, Watanabe, Takayuki, Suemitsu, Tetsuya, Ryzhii, Victor, Iwatsuki, Katsumi, Fukada, Youichi, Kani, Jun-ichi, Terada, Jun, Yoshimoto, Naoto, Kenji Kawahara, Hiroki Ago, Otsuji, Taiichi, Graphene-channel FETs for photonic frequency double-mixing conversion over the sub-THz band, SOLID-STATE ELECTRONICS, 10.1016/j.sse.2014.07.009, 103, 216-221 , 2015.01.
73. R. Ohshima, A. Sakai, Y. Ando, T. Shinjo, K .Kawahara, Hiroki Ago, S. Shiraishi, Observation of spin-charge conversion in CVD-grown single-layer graphene, Applied Physics Letters, 105, 162410, 2014.10.
74. Hiroki Ago, Yasumichi Kayo, SOLIS FERNANDEZ PABLO, Kazuma Yoshida, Masaharu Tsuji, Synthesis of high-density arrays of graphene nanoribbons by anisotropic metal-assisted etching, CARBON, 10.1016/j.carbon.2014.07.010, 78, 339-346, 2014.11.
75. Hiroki Ago, Rozan M. Yunus, Masahiro Miyashita, SOLIS FERNANDEZ PABLO, Masaharu Tsuji, Hiroki Hibino, Formation of Oriented Graphene Nanoribbons over Heteroepitaxial Cu Surfaces by Chemical Vapor Deposition, CHEMISTRY OF MATERIALS, 10.1021/cm501854r, 26, 18, 5215-5222 , 2014.09.
76. Yui Ogawa, Katsuyoshi Komatsu, Kenji Kawahara, Masaharu Tsuji, Kazuhito Tsukagoshi, Hiroki Ago, Structure and transport properties of the interface between CVD-grown graphene domains, Nanoscale, 6, 13, 7288-7294, 2014.07.
77. Hu BaoShan, ZiDong Wei, Hiroki Ago, Jin Yan, Xia MeiRong, Luo ZhengTang, Pan QingJiang, Liu YunLing, Effects of substrate and transfer on CVD-grown graphene over sapphire-induced Cu films, Science China Chemistry, 10.1007/s11426-014-5073-3, 57, 6, 895-901, 2014.06.
78. MARK ALEXANDER BISSETT, Masaharu Tsuji, Hiroki Ago, Strain Engineering the Properties of Graphene and Other Two-Dimensional Crystals, Physical Chemistry Chemical Physics, 16, 23, 11124-11138 , 2014.06.
79. Hiroki Ago, Izumi Tanaka, Yui Ogawa, Rozan M. Yunus, Masaharu Tsuji, Hiroki Hibino, Strain Engineering the Properties of Graphene and Other Two-Dimensional Crystals, ACS Nano, 10825-10833, 2014.04.
80. Hiroki Ago, Izumi Tanaka, Yui Ogawa, Rozan M. Yunus, Masaharu Tsuji, Hiroki Hibino, Lattice-oriented catalytic growth of graphene nanoribbons on heteroepitaxial nickel films, ACS Nano, 7, 12, 10825-10833, 2013.12.
81. MARK ALEXANDER BISSETT, Satoru Konabe, Susumu Okada, Masaharu Tsuji, Hiroki Ago, Enhanced chemical reactivity of graphene induced by mechanical strain, ACS Nano, 7, 11, 10335-10343, 2013.11.
82. Yui Ogawa, Tianchao Niu, Swee Liang Wong, Masaharu Tsuji, Andrew Thye Shen Wee, Wei Chen, Hiroki Ago, Self-assembly of polar phthalocyanine molecules on graphene grown by chemical vapor deposition, Journal of Physical Chemistry C, 117, 42, 21849-21855, 2013.10.
83. SOLIS FERNANDEZ PABLO, Kazuma Yoshida, Yui Ogawa, Masaharu Tsuji, Hiroki Ago, Dense arrays of highly aligned graphene nanoribbons produced by substrate-controlled metal-assisted etching of graphene, Advanced Materials, 25, 45, 6562-6568, 2013.12.
84. Hiroki Ago, Kenji Kawahara, Yui Ogawa, Shota Tanoue, MARK ALEXANDER BISSETT, Masaharu Tsuji, Hidetsugu Sakaguchi, Roland J. Koch, Felix Fromm, Thomas Seyller, Katsuyoshi Komatsu, Kazuhito Tsukagoshi, Epitaxial growth and electronic properties of large hexagonal graphene domains on Cu(111) thin film, Applied Physics Express, 6, 7, 75101-1-75101-4, 2013.07.
85. Xiaoyang Hu, Yingjiu Zhang, Hiroki Ago, Huihua Zhou, Xiao Li, Lili Fan, Bin Cai, Xinjian Li, Minlin Zhong, Kunlin Wang, Dehai Wu, Hongwei Zhu, Ultra-fast synthesis of graphene by melt spinning, Carbon, 61, 299-304, 2013.09.
86. GE WANYIN, kenji Kawahara, Masaharu Tsuji, Hiroki Ago, Large-scale synthesis of NbS2 nanosheets with controlled orientation on graphene by ambient pressure CVD, Nanoscale, 5, 13, 5773-5778, 2013.07.
87. Z. Tang, E. Shikoh, Hiroki Ago, K. Kawahara, Y. Ando, T. Shinjo, M. Shiraishi, Dynamically-generated pure spin current in single-layer graphene, Phys. Rev. B (Rapid Communication), 87, 14, 140401-1-5, 2013.04.
88. T. Koyama, Y. Ito, K. Yoshida, Masaharu Tsuji, Hiroki Ago, H. Kishida, A. Nakamura, Near-infrared photoluminescence in the femtosecond time region in monolayer graphene, ACS Nano, 7, 3, 2335-2443, 2013.03.
89. MARK ALEXANDER BISSETT, Masaharu Tsuji, Hiroki Ago, Mechanical strain of chemically functionalized chemical vapor deposition grown graphene, J. Phys. Chem. C, 117, 6, 3152-3159, 2013.02.
90. MARK ALEXANDER BISSETT, Wataru Izumida, Riichiro Saito, Hiroki Ago, Effect of domain boundaries on the Raman spectra of mechanically strained graphene, ACS Nano, 6, 11, 10229-10238, 2012.10.
91. Hiroki Ago, Yui Ogawa, Masaharu Tsuji, Seigi Mizuno, Hiroki Hibino, Catalytic growth of graphene: towards large-area single-crystalline graphene, J. Phys. Chem. Lett., 3, 16, 2228-2236, 2012.08.
92. Hiroki Ago, Yoshito Ito, Masaharu Tsuji, Ken-ichi Ikeda, Step-templated CVD growth of aligned graphene nanoribbons supported by single-layer graphene film, Nanoscale, 4, 16, 5178-5182, 2012.08.
93. C. M. Orofeo, H. Hibino, K. Kawahara, Y. Ogawa, M. Tsuji, K. Ikeda, S. Mizuno, H. Ago, Influence of Cu metal on the domain structure and carrier mobility in single-layer grap, Carbon, 50, 6, 2189-2196, 2012.06.
94. H. Ago, Y. Kayo, M. Tsuji, Growth of horizontally-aligned single-walled carbon nanotubes on sapphire surface by needle-scratching method, Jpn. J. Appl. Phys., 51, 4, 04DN02-1-4, 2012.04.
95. Y. Ogawa, B. Hu, C. M. Orofeo, M. Tsuji, K. Ikeda, S. Mizuno, H. Hibino, H. Ago, Domain structure and boundary in single-layer graphene grown on Cu (111) and Cu (100) films, J. Phys. Chem. Lett., 3, 2, 219-226, 2012.01.
96. B. Hu, H. Ago, C. M. Orofeo, Y. Ogawa, M. Tsuji, On the nucleation of graphene in chemical vapor deposition, New J. Chem., 36, 1, 73-76, 2012.01.
97. B. Hu, H. Ago,* Y. Ito, K. Kawahara, M. Tsuji, E. Magome, K. Sumitani, N. Mizuta, K. Ikeda, S. Mizuno, Epitaxial growth of large-area single-layer graphene over Cu(111)/sapphire by atmospheric pressure CVD, Carbon, 50, 1, 57-65, 2012.01.
98. H. Ago, T. Ayagaki, Y. Ogawa, M. Tsuji, Ultra-high vacuum-assisted control of metal nanoparticles for horizontally-aligned single-walled carbon nanotubes with extraordinary uniform diameters, J. Phys. Chem. C, 115, 27, 13247-13253, 2011.07.
99. C. M. Orofeo, H. Ago, B. Hu, M. Tsuji, Synthesis of large-area, homogeneous, single layer graphene by annealing amorphous carbon on Co and Ni, Nano Res., 4, 6, 531-540, 2011.06.
100. H. Ago, Y. Nakamura, Y. Ogawa, M. Tsuji, Combinatorial catalyst approach for high-density growth of horizontally aligned single-walled carbon nanotubes on sapphire, Carbon, 49, 1, 176-186, 2011.01.
101. H. Ago, Y. Ito, N. Mizuta, K. Yoshida, B. Hu, C. M. Orofeo, M. Tsuji, K. Ikeda, S. Mizuno, Epitaxial chemical vapor deposition growth of single-layer graphene over cobalt film crystallized on sapphire, ACS Nano, 4, 12, 7407-7414, 2010.12.
102. C. M. Orofeo, H. Ago, N. Yoshihara, M. Tsuji, Methods to horizontally align single-walled carbon nanotubes on amorphous substrate
, J. Novel Carbon Resourse Sci., 2, 36-40, 2010.09.
103. Y. Ogawa, H. Ago, M. Tsuji , Epitaxial growth of faceted Co nanoparticles on sapphire surfaces, Chem. Lett., 39, 9, 964-965, 2010.09.
104. H. Ago, T. Nishi, K. Imamoto, N. Ishigami, M. Tsuji, T. Ikuta, K. Takahashi , Orthogonal growth of horizontally-aligned single-walled carbon nanotube arrays, J. Phys. Chem. C, 114, 30, 12925-12930, 2010.08.
105. C. M. Orofeo, H. Ago, T. Ikuta, K. Takahashi, M. Tsuji , Growth of horizontally aligned single-walled carbon nanotubes on anisotropically etched silicon substrate, Nanoscale, 2, 9, 1708-1714, 2010.09.
106. J. Onoda, S. Mizuno, H. Ago, STEM observation of tungsten tips sharpened by field-assisted oxygen etching
, Surf. Sci., 604, 13-14, 1094-1099, 2010.07.
107. H. Ago, I. Tanaka, M. Tsuji, K. Ikeda, Patterned growth of graphene over epitaxial catalyst, Small, 6, 11, 1226-1233, 2010.06.
108. H. Ago, C. M. Orofeo, N. Ishigami, N. Yoshihara, M. Tsuji, Recent development in technology for horizontally-aligned growth of single-walled carbon nanotubes, Sens. Mater., 21(7), 321-330, 2009.12.
109. H. Ago, R. Ohdo, M. Tsuji, T. Ikuta, and K. Takahashi, Effective patterning of metal nanoparticles on sapphire surface for aligned growth of single-walled carbon nanotubes, J. Nanosci. Nanotech., 10(6), 3867-3872., 2010.06.
110. B. Hu, H. Ago, N. Yoshihara, and M. Tsuji, Effects of water vapor on diameter distribution of SWNTs grown over Fe/MgO based catalysts, J. Phys. Chem. C, 114(9), 3850-3856., 2010.03.
111. A. Nakamura, N. Hikosaka, S. Imamura, Y. Takahashi, H. Ago, H. Kishida, Third-order nonlinear optical response in double-walled carbon nanotubes, J. Luminescence, 129(12), 1722-1725., 2009.12.
112. H. Ago, K. Imamoto, T. Nishi, M. Tsuji, T. Ikuta, and K. Takahashi, Direct growth of bent carbon nanotubes on surface engineered sapphire, J. Phys. Chem. C, 113(30), 13121-13124., 2009.07.
113. N. Yoshihara, H. Ago, M. Tsuji, T. Ikuta, and K. Takahashi, Horizontally aligned growth of single-walled carbon nanotubes on surface modified silicon wafer, J. Phys. Chem. C, 113(19), 8030-8034, 2009.05.
114. C. M. Orofeo, H. Ago,* N. Yoshihara, and M. Tsuji, Top-down approach to align single-walled carbon nanotubes on silicon substrate, Appl. Phys. Lett., 94(5), 053113-1-3, 2009.02.
115. N. Ishigami, H. Ago, T. Nishi, K. Ikeda, M. Tsuji, T. Ikuta, and K. Takahashi , Unidirectional growth of single-walled carbon nanotubes, J. Am. Chem. Soc. , 130(51), 17264-17265, 2008.12.
116. H. Ago, N. Ishigami, K. Imamoto, T. Suzuki, K. Ikeda, M. Tsuji, T. Ikuta, and K. Takahashi, Horizontally-aligned single-walled carbon nanotubes on sapphire, J. Nanosci. Nanotech., 8(11), 6165-6169, 2008.11.
117. H. Ago, I. Tanaka, M. Tsuji, K. Ikeda, and S. Mizuno , Hole-doping to aligned single-walled carbon nanotubes from sapphire induced by heat treatment, J. Phys. Chem. C, 112(47), 18350-18354, 2008.10.
118. N. Ishigami, H. Ago, K. Imamoto, M. Tsuji, K. Iakoubovskii, and N. Minami, Crystal plane dependent growth of aligned single-walled carbon nanotubes on sapphire, J. Am. Chem. Soc., 130(30), 9918-9924 (2008). , 2008.07.
119. H. Ago, N. Ishigami, K. Imamoto, T. Suzuki, K. Ikeda, M. Tsuji, T. Ikuta, and K. Takahashi, Horizontally-alinged single-walled carbon nanotubes on sapphire, J. Nanosci. Nanotech., J. Nanosci. Nanotech., in press (2008)., 2008.06.
120. N. Yoshihara, H. Ago, and M. Tsuji, Mechanism of carbon nanotube growth over gold-supported catalysts, Jpn. J. Appl. Phys., 47(4), 1944-1948 (2008).
, 2008.04.
121. H. Ago, N. Ishigami, N. Yoshihara, K. Imamoto, K. Ikeda, M. Tsuji, T. Ikuta, and K. Takahashi, Visualization of horizontally-aligned single-walled carbon nanotube growth with 13C/12C isotopes, J. Phys. Chem. C, J. Phys. Chem. C (Letter), 112(6), 1735-1738 (2008.2). selected as cover, 2008.02.
122. N. Yoshihara, H. Ago, and M. Tsuji, Chemistry of water-assisted carbon nanotube growth over Fe-Mo/MgO catalyst, J. Phys. Chem. C, J. Phys. Chem. C, 111(31), 11577-11582 (2007)., 2007.08.
123. N. Ishigami, H. Ago, Y. Motoyama, M. Takasaki, M. Shinagawa, K. Takahashi, K. Takahashi, and M. Tsuji, Microreactor utilizing a vertically-aligned carbon nanotube array grown inside the channels, Chem. Commun., 1626-1628, 2007.03.
124. H. Ago, K. Imamoto, N. Ishigami, R. Ohdo, K. Ikeda, and M. Tsuji, Competition and cooperation between lattice-oriented growth and step-templated growth on aligned carbon nanotubes on sapphire, Appl. Phys. Lett., 90(12), 123112-1-3, 2007.03.
125. X. Zhang, H. Xie, M. Fujii, H. Ago, K. Takahashi, T. Ikuta, H. Abe, and T. Shimizu, Thermal and electrical properties of a suspended nanoscale thin film, Int. J. Heat Mass Transfer, Int. J. Thermophys., 28(1), 33-43 (2007.2)., 2007.02.
126. H. Ago, E. Uchimura, T. Saito, S. Ohshima, N. Ishigami, M. Tsuji, M. Yumura, and M. Miyake, Mechanical Immobilization of Hela Cells on aligned carbon nanotube array, Mater. Lett, 60(29-30), 3851-3854, 2006.12.
127. H. Ago, N. Uehara, N. Yoshihara, M. Tsuji, M. Yumura, N. Tomonaga, and T. Setoguchi, Gas analysis of CVD process for high yield growth of carbon nanotubes over metal-supported catalysts, Carbon, 44(14), 2912-2918 , 2006.11.
128. X. Zhang, H. Xie, M. Fujii, K. Takahashi, T. Ikuta, H. Ago, H. Abe, and T. Shimizu, Experimental study on thermal characteristics of suspended platinum nanofilm sensors, Int. J. Heat Mass Transfer, 49(21-22), 3879-3883, 2006.10.
129. A. Nakamura, T. Tomikawa, M. Watanabe, Y. Hamanaka, Y. Saito, and H. Ago, Nonlinear optical response and relaxation dynamics in double-walled carbon nanotubes, J. Luminescence, 119-120, 8-12, 2006.07.
130. H. Ago, N. Uehara, K.Ikeda, R. Ohdo, K. Nakamura, and M. Tsuji, Synthesis of horizontally-aligned single-walled carbon nanotubes with controllable density and polarized Raman spectroscopy, Chem. Phys. Lett., 421(4-6), 399-403, 2006.04.
131. T. Saito, W. C. Xu, S. Ohshima, H. Ago, M. Yumura, and S. Iijima, Supramoleclar catalysts for the gas-phase synthesis of single-walled carbon nanotubes, J. Phys. Chem. B, 110(12), 5849-5853, 2006.03.
132. M. Fujii, X. Zhang, H. Xie, H. Ago, K. Takahashi, T. Ikuta, H. Abe, and T. Shimizu, Measuring the thermal conductivity of a single carbon nanotub, Phys. Rev. Lett., 10.1103/PhysRevLett.95.065502, 95, 6, 95(6), 065502-1-4, 2005.08.
133. T. Saito, S. Ohshima, W. C. Xu, H. Ago, M. Yumura, and S. Iijima, Size-control of metal nanoparticle catalysts for the gas-phase synthesis of single-walled carbon nanotubes, J. Phys. Chem. B, 10.1021/jp044200z, 109, 21, 10647-10652, 109(21), 10647-10652 (2005)., 2005.07.
134. H. Ago, K. Nakamua, K. Ikeda, N. Uehara, N. Ishigami, and M. Tsuji, Aligned growth of isolated single-walled carbon nanotubes programmed by atomic arrangement of substrate surface, Chem. Phys. Lett., 10.1016/j.cplett.2005.04.054, 408, 4-6, 433-438, 408(4-6), 433-438 (2005)., 2005.06.
135. H. Ago, S. Imamura, T. Okazaki, T. Saito, M. Yumura, and M. Tsuji, CVD growth of single-walled carbon nanotubes with a narrow diameter distribution and their optical properties", J. Phys. Chem. B, 10.1021/jp050307q, 109, 20, 10035-10041, 109(20), 10035-10041 (2005)., 2005.05.
136. X. Zhang, H. Xie, M. Fujii, H. Ago, K. Takahashi, T. Ikuta, H. Abe, and T. Shimizu, Thermal and electrical conductivity of a suspended platinum nanofilm, Appl. Phys. Lett., 10.1063/1.1921350, 86, 17, 86(17), 171912-171912-3 (2005)., 2005.04.
137. H. Ago, S. Ohshima, K. Tsukagoshi, M. Tsuji, and M. Yumura, Formation mechanism of carbon nanotubes in the gas-phase synthesis from colloidal solutions of nanoparticles, Curr. Appl. Phys., 10.1016/j.cap.2004.06.004, 5, 2, 128-132, 5(2), 128-132 (2005)., 2005.02.
138. H. Ago, K. Nakamura, N. Uehara, and M. Tsuji, Roles of meal-support interaction in growth of single- and double-walled carbon nanotubes studied with diameter-controlled iron particles supported on MgO, J. Phys. Chem. B, 108 (49), 18908-18915 (2004)., 2004.12.
139. H. Ago, K. Nakamura, S. Imamura, and M. Tsuji, Growth of double-wall carbon nanotube with diameter-controlled iron oxide nanoparticles supported on MgO, Chemical Physics Letters, 10.1016/j.cplett.2004.04.110, 391, 4-6, 308-313, 391(4-6), 308-313 (2004)., 2004.06.
140. Y. Zhang, H. Ago, J. Liu, M. Yumura, K. Uchida S. Ohshima, S. Iijima, J. Zhu, X. Zhang, The synthesis of In, In2O3 nanowires and In2O3 nanoparticles with shape-controlled, Journal of Crystal Growth, 264, 363-368 (2004)., 2004.04.
141. Y. Zhang, H. Ago, M. Yumura, S. Ohshima, K. Uchida, T. Komatsu, and S. Iijima, Study of the growth of boron nanowires synthesized by laser ablation, Chemical Physics Letters, 385, 177-183 (2004)., 2004.01.
142. H. Ago, R. Azumi, S. Ohshima, H. Kataura, and M. Yumura, STM study of molecular adsorption on single-wall carbon nanotube surface, Chemical Physics Letters, 10.1016/j.cplett.2003.11.053, 383, 5-6, 469-474, 383, 469-474 (2004)., 2004.01.
143. H. Ago, J. Qi, K. Tsukagoshi, K. Murata, S. Ohshima, Y. Aoyagi, and M. Yumura, Catalytic growth of carbon nanotubes and their patterning based on ink-jet and lithographic techniques, Journal of Electroanalytical Chemistry, 559, 25-30 (2003)., 2003.11.
144. K. Tsukagoshi, At. Suzuki, I. Yagi, E. Watanabe, Y. Aoyagi, H. Ago, S. Ohshima, and M. Yumura, High density current operation in nanographite fiber synthesized by chemical vapor deposition, Journal of Applied Physics, 2003.09.
145. H. Ago, K. Murata, M. Yumura, J. Yotani, and S. Uemura, Ink-Jet Printing of Nanoparticle Catalyst for Site-Selective Carbon Nanotube Growth, Applied Physics Letters, 82(5), 811-813 (2003)., 2003.02.
146. Y. Zhang, H. Ago, M. Yumura, T. Komatsu, S. Ohshima, K. Uchida, and S. Iijima, Synthesis of crystalline boron nanowires by laser ablation, Chemical Communications, 2806 (2002)., 2002.12.
147. T. Kimura, H. Ago, M. Tobita, S. Ohshima, M. Kyotani, and M. Yumura, Polymer Composites of Carbon Nanotubes Aligned by a Magnetic Field, Advanced Materials, 14(19), 1380-1383 (2002)., 2002.10.
148. H. Ago, S. Ohshima, K. Uchida, T. Komatsu, and M. Yumura, Carbon nanotube synthesis using colloidal solution of metal nanoparticles, Physica B, 323(1-4), 306-307 (2002)., 2002.10.
149. S. Ohshima, H. Ago, H. Inoue, and M. Yumura, Development of mass-production technology for multiwalled carbon nanotubes, New Diamond and Frontier Carbon Technology, 11(6), 437-448 (2001)., 2001.12.
150. H. Ago, S. Ohshima, K. Uchida, and M. Yumura, Gas-phase synthesis of single-wall carbon nanotubes from colloidal solution of metal nanoparticles, The Journal of Physical Chemistry B, 105(43), 10453-10456 (2001)., 2001.11.
151. B. W. Alphenaar, K. Tsukagoshi, and H. Ago, Spin electronics using carbon nanotubes, Physica E, 6(1-4), 848-851 (2000)., 2000.02.
152. H. Ago, T. Komatsu, S. Ohshima, Y. Kuriki, and M. Yumura, Dispersion of metal nanoparticles for aligned multiwall carbon nanotube arrays, Applied Physics Letters, 77(1), 79-81 (2000)., 2000.07.
153. H. Ago, M. S. P. Shaffer, D.S. Ginger, A. H. Windle, and R. H. Friend, Electronic interaction bewteen photo-excited poly(p-phenylene vinylene) and carbon nanotubes, Physical Review B, 61(3), 2286-2290 (2000)., 2000.01.
154. H. Ago, K. Petritsch, M. S. P. Shaffer, A. H. Windle, and R. H. Friend, Composites of carbon nanotubes and conjugated polymers for photovoltaic devices, Advanced Materials, 11(15), 1281-1285 (1999)., 1999.10.
155. K. Tsukagoshi, B. W. Alphenaar, and H. Ago, Coherent transport of electron spin in a ferromagnetically contacted carbon nanotube, Nature, 401, 572-574 (1999)., 1999.10.
156. H. Ago, Th. Kugler, F. Cacialli, W. R. Salaneck, M. S. P. Shaffer, A. H. Windle, and R. H. Friend, Work functions and surface functional groups of multiwall carbon nanotubes, The Journal of Physical Chemistry B, 103(38), 8116-8121 (1999)., 1999.09.
157. H. Ago, M. Kato, K. Yahara, K. Yoshizawa, K. Tanaka, and T. Yamabe, Ab initio study on interaction and stability of lithium-doped amorphous carbons, Journal of the Electrochemical Society, 146(4), 1262-1269 (1999)., 1999.04.
158. A. K. Bakhshi, Y. Yamaguchi, H. Ago, and T. Yamabe, Theoretical design of donor-acceptor polymers with low bandgaps, Journal of Molecular Structure (THEOCHEM), 427, 211-219 (1998)., 1998.03.
159. H. Ago, Th. Kugler, F. Cacialli, K. Petritsch, R. H. Friend, W. R. Salaneck, Y. Ono, T. Yamabe, and K. Tanaka, Workfunction of purified and oxidized carbon nanotubes, Synthetic Metals, 103, 2494-2495 (1999)., 1999.11.
160. H. Ago, K. Tanaka, T. Yamabe, T. Miyoshi, K. Takegoshi, T. Terao, S. Yata, Y. Hato, and N. Ando, Structural analysis of polyacenic semiconductor (PAS) materials with 129Xe NMR measurements, Carbon, 35, 1781-1787 (1997)., 1997.12.
161. H. Ago, K. Tanaka, T. Yamabe, K. Takegoshi, T. Terao, S. Yata, Y. Hato, and N. Ando, 7Li NMR study of Li-doped polyacenic semiconductor (PAS) materials, Synthetic Metals, 89, 141 (1997)., 1997.08.
162. H. Ago, K. Nagata, K. Yoshizawa, K. Tanaka, and T. Yamabe, Theoretical study of Li-doped polycyclic aromatic hydrocarbons, Bulletin of the Chemical Society of Japan, 70, 1717-1726 (1997)., 1997.07.
163. H. Ago, T. Kuga, T. Yamabe, K. Tanaka, A. Kunai, and M. Ishikawa, Electronic properties of p-type doped copolymers consisting of oligothienylene and disilanylene units, Chemistry of Materials, 9, 1159-1165 (1997)., 1997.05.
164. H. Ago, T. Kuga, T. Yamabe, K. Tanaka, S. Yata, Y. Hato, and N. Ando, ESR study of alkali-doped polyacenic semiconductor (PAS) materials prepared by
thermal decomposition of azides, Carbon, 35, 651-656 (1997)., 1997.12.
165. K. Tanaka, H. Ago, Y. Matsuura, T. Kuga, S. Yata, Y. Hato, and N. Ando, ESR study of Li-doped polyacenic semiconductor (PAS) materials, Synthetic Metals, 89, 133-139 (1997)., 1997.08.
166. K. Tanaka, H. Aoki, H. Ago, T. Yamabe, and K. Okahara, Interlayer interaction of two graphene sheets as a model of double-layer carbon nanotube, Carbon, 35, 121-125 (1997)., 1997.05.
167. K. Tanaka, H. Ago, T. Yamabe, K. Okahara, and M. Okada, Bond alternation in carbon nanotubes including s-electrons, International Journal of Quantum Chemistry, 63, 637-644 (1997)., 1997.06.
168. T. Yamabe, K. Tanaka, H. Ago, K. Yoshizawa, and S. Yata, Structure and properties of deeply Li-doped polyacenic semiconductor (PAS), Synthetic Metals, 86, 2411-2414 (1997)., 1997.02.
169. A. K. Bakhshi, H. Ago, K. Yoshizawa, K. Tanaka, and T. Yamabe, Electronic properties of polymers based on thienothiadiazole and thiophene, The Journal of Chemical Physics, 104, 5528-5538 (1996)., 1996.04.
170. K. Tanaka, H. Aoki, H. Ago, and T. Yamabe, Electronic property of polyacene in a constant magnetic field perpendicular to the condensed aromatic-rings plane, Synthetic Metals, 79, 145-148 (1996)., 1996.05.
171. K. Yoshizawa, M. Hatanaka, H. Ago, K. Tanaka, and T. Yamabe, Magnetic properties of 1,3,5-tris[bis(p-methoxyphenyl)amino]benzene cation radicals, Bulletin of the Chemical Society of Japan, 69, 1417-1422 (1996)., 1996.05.
172. A. K. Bakhshi, Y. Yamaguchi, H. Ago, and T. Yamabe, Design of novel donor-acceptor polymers with low band gaps, Synthetic Metals, 79, 115-120 (1996)., 1996.05.
173. J. Ohshita, T. Watanabe, D. Kanaya, H. Ohsaki, M. Ishikawa, H. Ago, K. Tanaka, and T. Yamabe, Polymeric organosilicon systems. 22: synthesis and photochemical properties of poly[(disilanylene)oligophenylenes] and poly[(silylene)biphenylenes], Organometallics, 13, 5002-5012 (1994), 1994.09.
174. K. Tanaka, H. Ago, T. Yamabe, M. Ishikawa, and T. Ueda, Electronic structures of organosilicon polymers containing thienylene units, Organometallics, 13, 3496-3501 (1994)., 1994.12.
175. K. Tanaka, H. Ago, and T. Yamabe, Design of ferromagnetic polymers involving organosilicon moieties, Synthetic Metals, 72, 225-229 (1995)., 1995.06.
176. K. Tanaka, H. Ago, T. Yamabe, J. Li, and K. Kitazawa, An ESR analysis of C60S16, Chemical Physics Letters, 235, 217-220 (1995)., 1995.03.
177. A. K. Bakhshi, Y. Yamaguchi, H. Ago, and T. Yamabe, Electronic structures of donor-acceptor polymers based on polythiophene, polyfuran, and polypyrrole, Molecular Engineering, 6, 239-248 (1996)., 1996.03.