Germanene reformation from oxidized germanene on Ag(111)/Ge(111) by vacuum annealing

Suzuki, Seiya; Katsube, Daiki*; Yano, Masahiro; Tsuda, Yasutaka; Terasawa, Tomoo; Ozawa, Takahiro*; Fukutani, Katsuyuki; Kim, Y.*; Asaoka, Hidehito; Yuhara, Junji*; et al.

Small Methods, p.2400863_1 - 2400863_9, 2024/00

https://doi.org/10.1002/smtd.202400863

Step unbunching phenomenon on 4H-SiC (0001) surface during hydrogen etching

Sakakibara, Ryotaro*; Bao, J.*; Yuhara, Keisuke*; Matsuda, Keita*; Terasawa, Tomoo; Kusunoki, Michiko*; Norimatsu, Wataru*

Applied Physics Letters, 123(3), p.031603_1 - 031603_4, 2023/07

https://doi.org/10.1063/5.0153565

We here report a step unbunching phenomenon, which is the inverse of the phenomenon of step bunching. When a 4H-SiC (0001) surface is annealed at a high temperature, step bunching arises due to the different velocities of the step motion in adjacent steps, resulting in steps with a height of more than several nanometers. We found that the bunched steps, thus, obtained by hydrogen etching in an Ar/H atmosphere were "unbunched" into lower height steps when annealed subsequently at lower temperatures. This unbunching phenomenon can be well explained by the consequence of the competition between energetics and kinetics. Our findings provide another approach for the surface smoothing of SiC by hydrogen etching and may give significant insight into the application of SiC power devices and two-dimensional materials growth techniques in general.

Efficient hydrogen isotope separation by tunneling effect using graphene-based heterogeneous electrocatalysts in electrochemical hydrogen isotope pumping

Yasuda, Satoshi; Matsushima, Hisayoshi*; Harada, Kenji*; Tanii, Risako*; Terasawa, Tomoo; Yano, Masahiro; Asaoka, Hidehito; Gueriba, J. S.*; Dio, W. A.*; Fukutani, Katsuyuki

ACS Nano, 16(9), p.14362 - 14369, 2022/09

https://doi.org/10.1021/acsnano.2c04655

The fabrication of hydrogen isotope enrichment system is essential for the development of industrial, medical, life science, and nuclear fusion fields, therefore alternative enrichment techniques with high separation factor and economic feasibility have been still explored. Herein, we report the fabrication of heterogeneous electrode with layered structures consisting of palladium and graphene layers for polymer electrolyte membrane electrochemical hydrogen pumping for the hydrogen isotope enrichment. We demonstrated significant bias voltage dependence of hydrogen/deuterium (H/D) separation ability and its high H/D at lower bias voltage. Theoretical analysis also demonstrated that the observed high H/D at low bias voltage stems from hydrogen isotopes tunneling through atomically-thick graphene during the electrochemical reaction, and the bias dependent H/D results in a transition from the quantum tunneling regime to classical over- barrier regime for hydrogen isotopes transfer via the graphene. These findings provide new insight for a novel economical methodology of efficient hydrogen isotope enrichment.

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.

Development of low energy H gun for evaluation of H permeability of graphene

Terasawa, Tomoo; Fukutani, Katsuyuki; Yasuda, Satoshi; Asaoka, Hidehito

no journal, , 

Hydrogen isotope separation by tunneling effect used graphene-based heterogeneous electrocatalysts in electrochemical hydrogen pumping

Yasuda, Satoshi; Matsushima, Hisayoshi*; Terasawa, Tomoo; Yano, Masahiro; Asaoka, Hidehito; Gueriba, J.*; Dio, W.*; Fukutani, Katsuyuki

no journal, , 

We fabricated heterogeneous electrode with layered structures consisting of palladium and graphene layers and reported D2 enrichment from hydrogen and deuterium mixture gas using polymer electrolyte membrane electrochemical hydrogen pumping used the fabricated electrode. Mass spectroscopic analysis demonstrated significant bias voltage dependence of hydrogen/deuterium (H/D) separation ability and the H/D value decreases as the bias voltage increases. Theoretical analysis showed that the observed high H/D at low bias voltage stems from hydrogen isotopes tunneling through atomically-thick graphene during the electrochemical reaction, and the bias dependent H/D results in a transition from the quantum tunneling regime to classical over- barrier regime for hydrogen isotopes transfer via the graphene. These findings provide new insight for a novel economical methodology of efficient hydrogen isotope enrichment.

Growth and properties of graphene on Hex-Au(001) reconstructed surface

Terasawa, Tomoo

no journal, , 

Graphene, a two-dimensional material consisting of a honeycomb lattice of C atoms, has attracted much attention from basic physics to applications because of its extremely high carrier mobility and half-integer quantum Hall effect. Since a choice of substrate affects the properties of graphene, the synthesis and properties of graphene on various substrates have been the subject of surface science research. The interface between graphene and Au is expected in the field of spintronics because Au has a large atomic number and a large spin-orbit interaction. On the other hand, the atomic configuration in this interface is often unknown, and therefore, the angle-resolved photoemission spectroscopy (ARPES) experiments and density functional theory (DFT) calculations do not match each other for this interface. Here, we report the band structure of graphene on the Hex-Au(001) reconstructed surface using ARPES and DFT calculations. Since the solubility of C in Au is very low, graphene can be grown on Au surfaces by chemical vapor deposition (CVD). Hex-Au(001) reconstruction was kept even after the CVD growth of graphene. Therefore, the ARPES measurement was carried out for graphene on the well-known atomic configuration of Hex-Au(001). The ARPES intensity map shows the bandgap in the graphene pi band close to the Au 6sp band. The DFT calculated band structure shows the bandgap at the crossing point of the graphene pi and Au 6sp bands. We discuss that the bandgap originates from the hybridization between graphene and Au, similar to the case of graphene and Au interface on the SiC substrate. As the Rashba splitting of 100 meV was observed in the graphene and Au interface on the SiC substrate, we expect that the hybridization between graphene and Au is essential for the future applications of graphene for spintronic devices.