CHCl dissociation, CH abstraction, and Cl adsorption from the dissociative scattering of supersonic CHCl on Cu(111) and Cu(410)

Makino, Takamasa*; Tsuda, Yasutaka; Yoshigoe, Akitaka; Dio, W. A.*; Okada, Michio*

Applied Surface Science, 642, p.158568_1 - 158568_6, 2024/01

https://doi.org/10.1016/j.apsusc.2023.158568

Band gap formation in graphene by hybridization with Hex-Au(001) reconstructed surface

Terasawa, Tomoo; Matsunaga, Kazuya*; Hayashi, Naoki*; Ito, Takahiro*; Tanaka, Shinichiro*; Yasuda, Satoshi; Asaoka, Hidehito

Vacuum and Surface Science, 66(9), p.525 - 530, 2023/09

https://doi.org/10.1380/vss.66.525

As Au (001) surfaces exhibit a quasi-one-dimensional corrugated structure, Hex-Au(001), its periodicity was predicted to change the electronic structure of graphene when graphene was grown on this surface. Furthermore, the hybridization between graphene and Au is known to introduce bandgap and spin polarization into graphene. Here, we report angle-resolved photoemission spectroscopy and density functional theory calculation of graphene on a Hex-Au(001) surface. A bandgap of 0.2 eV in the graphene Dirac cone was observed at the crossing point of the graphene Dirac cone and Au 6sp bands, indicating that the origin of the bandgap formation was the hybridization between the graphene Dirac cone and Au 6sp band. We discussed the hybridization mechanism and anticipated spin injection into the graphene Dirac cone.

Probing strain and doping along a graphene wrinkle using tip-enhanced Raman spectroscopy

Balois-Oguchi, M. V.*; Hayazawa, Norihiko*; Yasuda, Satoshi; Ikeda, Katsuyoshi*; Nguyen, T. Q.*; Escao, M. C.*; Tanaka, Takuo*

Journal of Physical Chemistry C, 127(12), p.5982 - 5990, 2023/03

https://doi.org/10.1021/acs.jpcc.2c08529

Micrometer-sized wrinkles in graphene are known to affect the electronic properties of graphene due to their shape and the strain variations they create. Here, we analyze the strain distribution and doping of a graphene wrinkle having 1.9 nm width using tip-enhanced Raman spectroscopy (TERS) in ambient conditions. We found a strong correlation between the TERS images of the graphene wrinkle and the electronic Raman scattering (eRS) of the Au(111) substrate. Our work demonstrates that the as-fabricated physical and electronic properties of nanometer-sized features, such as wrinkles, can be probed and studied in detail with TERS which is essential for nanodevice characterization.

Evaluation of doped potassium concentrations in stacked two-Layer graphene using real-time XPS

Ogawa, Shuichi*; Tsuda, Yasutaka; Sakamoto, Tetsuya*; Okigawa, Yuki*; Masuzawa, Tomoaki*; Yoshigoe, Akitaka; Abukawa, Tadashi*; Yamada, Takatoshi*

Applied Surface Science, 605, p.154748_1 - 154748_6, 2022/12

https://doi.org/10.1016/j.apsusc.2022.154748

Immersion of graphene in KOH solution improves its mobility on SiO/Si wafers. This is thought to be due to electron doping by modification with K atoms, but the K atom concentration C in the graphene has not been clarified yet. In this study, the C was determined by XPS analysis using high-brilliance synchrotron radiation. The time evolution of C was determined by real-time observation, and the C before irradiation of synchrotron radiation was estimated to be 0.94%. The C 1s spectrum shifted to the low binding energy side with the desorption of K atoms. This indicates that the electron doping concentration into graphene is decreasing, and it is experimentally confirmed that K atoms inject electrons into graphene.

Electrochemically driven specific alkaline metal cation adsorption on a graphene interface

Yasuda, Satoshi; Tamura, Kazuhisa; Kato, Masaru*; Asaoka, Hidehito; Yagi, Ichizo*

Journal of Physical Chemistry C, 125(40), p.22154 - 22162, 2021/10

https://doi.org/10.1021/acs.jpcc.1c03322

Understanding electrochemical behavior of the alkaline metal cation-graphene interface in electrolyte is essential for understanding the fundamental electrochemical interface and development of graphene-based technologies. We report comprehensive analysis of the electrochemical behavior of both alkaline metal cations and graphene using electrochemical surface X-ray diffraction (EC-SXRD) and Raman (EC-Raman) spectroscopic techniques in which the interfacial structure of cations and the charging state and mechanical strain of the graphene can be elucidated. EC-SXRD and cyclic voltammetry demonstrated electrochemically driven specific adsorption and desorption of cations on the graphene surface involved in the dehydration and hydration process. This study provides new insight for understanding fundamental electrochemical behavior of the alkaline metal cation-graphene interface and contributes to the development of carbon-based novel applications.

Structure of quasi-free-standing graphene on the SiC (0001) surface prepared by the rapid cooling method

Sumi, Tatsuya*; Nagai, Kazuki*; Bao, J.*; Terasawa, Tomoo; Norimatsu, Wataru*; Kusunoki, Michiko*; Wakabayashi, Yusuke*

Applied Physics Letters, 117(14), p.143102_1 - 143102_5, 2020/10

https://doi.org/10.1063/5.0021071

A systematic structural study of epitaxial graphene samples on the SiC (0001) surface has been performed by the surface X-ray diffraction method, which is a non-contact technique. For samples with only a buffer layer, one layer graphene, and multilayer graphene, the distances between the buffer layer and the surface Si atoms were found to be 0.23 nm. This value is the same as reported values. For quasi-free-standing graphene samples prepared by the rapid cooling method, there was no buffer layer and the distance between the quasi-free-standing graphene and the surface Si atoms was 0.35 nm, which is significantly shorter than the value in hydrogen-intercalated graphene and slightly longer than the interplane distance in graphite. The Si occupancy deviated from unity within 1 nm of the SiC surface. The depth profile of the Si occupancy showed little sample dependence, and it was reproduced by a simple atomistic model based on random hopping of Si atoms.

Confinement of hydrogen molecules at graphene-metal interface by electrochemical hydrogen evolution reaction

Yasuda, Satoshi; Tamura, Kazuhisa; Terasawa, Tomoo; Yano, Masahiro; Nakajima, Hideaki*; Morimoto, Takahiro*; Okazaki, Toshiya*; Agari, Ryushi*; Takahashi, Yasufumi*; Kato, Masaru*; et al.

Journal of Physical Chemistry C, 124(9), p.5300 - 5307, 2020/03

https://doi.org/10.1021/acs.jpcc.0c00995

Confinement of hydrogen molecules at graphene-substrate interface has presented significant importance from the viewpoints of development of fundamental understanding of two-dimensional material interface and energy storage system. In this study, we investigate H confinement at a graphene-Au interface by combining selective proton permeability of graphene and the electrochemical hydrogen evolution reaction (electrochemical HER) method. After HER on a graphene/Au electrode in protonic acidic solution, scanning tunneling microscopy finds that H nanobubble structures can be produced between graphene and the Au surface. Strain analysis by Raman spectroscopy also shows that atomic size roughness on the graphene/Au surface originating from the HER-induced strain relaxation of graphene plays significant role in formation of the nucleation site and H storage capacity.

Effect of hydrogen on chemical vapor deposition growth of graphene on Au substrates

Terasawa, Tomoo; Taira, Takanobu*; Yasuda, Satoshi; Obata, Seiji*; Saiki, Koichiro*; Asaoka, Hidehito

Japanese Journal of Applied Physics, 58(SI), p.SIIB17_1 - SIIB17_6, 2019/08

https://doi.org/10.7567/1347-4065/ab19ae

Chemical vapor deposition (CVD) on substrates with low C solubility such as Cu and Au is promising to grow monolayer graphene selectively in a large scale. Hydrogen is often added to control the domain size of graphene on Cu, while Au does not require H since Ar is inert against oxidation. The effect of H should be revealed to improve the quality of graphene on Au. Here we report the effect of H on the CVD growth of graphene on Au substrates using in situ radiation-mode optical microscopy. The in situ observation and ex situ Raman spectroscopy revealed that whether H was supplied or not strongly affected the growth rate, thermal radiation contrast, and compressive strain of graphene on Au. We attributed these features to the surface reconstruction of Au(001) depending on H supply. Our results are essential to achieve the graphene growth with high quality on Au for future applications.

In-situ microscopic observation of crystal growth of graphene using thermal radiation

Terasawa, Tomoo; Saiki, Koichiro*; Yasuda, Satoshi; Asaoka, Hidehito

Dai-39-Kai Nihon Netsu Bussei Shimpojiumu Koen Rombunshu (CD-ROM), p.262 - 264, 2018/11

Graphene, monolayer graphite, has been expected as one of the new materials targeting the next generation electronics since its first isolation in 2004, due to the ultrahigh carrier mobility up to 100,000 cm/Vs and high transparency of 97.7%. The high transparency of graphene make it invisible on various substrates. Particularly, graphene on Cu, one of the common growth substrates for high-quality graphene, cannot be observed by optical microscopes. Here, we report the optical microscopic method to visualize graphene using thermal radiation. We observed a Cu surface by a zoom-lens and a CMOS camera during the growth of graphene by chemical vapor deposition. When graphene was grown on Cu substrates, the thermal radiation intensity increased at the area covered with graphene. The thermal radiation contrast between Cu surfaces with and without graphene showed that the thermal radiation intensity increased as the number of graphene layers in a layer-by-layer manner. We quantitatively analyzed the thermal radiation contrasts at various temperatures. We found the thermal radiation contrast was independent from the sample temperature. This result suggests that the emissivity of graphene is independent from the temperature, which is consistent with the theory of the optical properties of graphene. Our findings are essential for the discussion of the thermal radiation from the atomically thin materials including graphene.

In situ SR-XPS observation of Ni-assisted low-temperature formation of epitaxial graphene on 3C-SiC/Si

Hasegawa, Mika*; Sugawara, Kenta*; Suto, Ryota*; Sambonsuge, Shota*; Teraoka, Yuden; Yoshigoe, Akitaka; Filimonov, S.*; Fukidome, Hirokazu*; Suemitsu, Maki*

Nanoscale Research Letters, 10, p.421_1 - 421_6, 2015/10

https://doi.org/10.1186/s11671-015-1131-9

Graphene has attracted much attention as a promising material in electronics and photonics. The graphitization temperature of 1473 K or higher of graphene-on-silicon(GOS), however, is still too high to be fully compatible with the Si technology. Here, the first application of Ni-assisted formation of graphene to the GOS method was reported. We demonstrate that the graphene formation temperature can be reduced by more than 200 K by this method. Moreover, solid-phase reactions during heating/annealing/cooling procedures have been investigated in detail by using synchrotron-radiation X-ray photoelectron spectroscopy. As a result, we clarify the role of Ni/SiC reactions, in which not only Ni silicidation and but also Ni carbonization is suggested as a key process in the formation of graphene.