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by monolayer hexagonal boron nitride coating for improved photo- and thermionic-cathodesYamaguchi, Hisato*; Yusa, Ryunosuke*; Wang, G.*; Pettes, M. T.*; Liu, F.*; Tsuda, Yasutaka; Yoshigoe, Akitaka; Abukawa, Tadashi*; Moody, N. A.*; Ogawa, Shuichi*
Applied Physics Letters, 122(14), p.141901_1 - 141901_7, 2023/04
Times Cited Count：3 Percentile：81.68(Physics, Applied)A lowering of work function for LaB by monolayer hexagonal BN coating is reported. Photoemission electron microcopy (PEEM) and thermionic emission electron microscopy (TEEM) both revealed that the hBN coated region of a LaB(100) single crystal has lower work function compared to the bare (i.e., non-coated) and graphene coated regions. A larger decrease of work function for the hBN coated LaB(100) compared to graphene coated LaB(100) was qualitatively supported by our density functional theory (DFT) calculations. Adding an oxide layer in the calculations improved consistency between the calculation and experimental results. We followed up our calculations with synchrotron-radiation X-ray photoelectron spectroscopy (SR-XPS) and confirmed the presence of an oxide layer on our LaB.
Ogawa, Shuichi*; Yamaguchi, Hisato*; Holby, E. F.*; Yamada, Takatoshi*; Yoshigoe, Akitaka; Takakuwa, Yuji*
Journal of Physical Chemistry Letters (Internet), 11(21), p.9159 - 9164, 2020/11
Times Cited Count：3 Percentile：17.73(Chemistry, Physical)Atomically thin layers of graphene have been proposed to protect surfaces through the direct blocking of corrosion reactants such as oxygen with low added weight. The long term efficacy of such an approach, however, is unclear due to the long-term desired protection of decades and the presence of defects in as-synthesized materials. Here, we demonstrate catalytic permeation of oxygen molecules through previously-described impermeable graphene by imparting sub-eV kinetic energy to molecules. These molecules represent a small fraction of a thermal distribution thus this exposure serves as an accelerated stress test for understanding decades-long exposures. The permeation rate of the energized molecules increased 2 orders of magnitude compared to their non-energized counterpart. Graphene maintained its relative impermeability to non-energized oxygen molecules even after the permeation of energized molecules indicating that the process is non-destructive and a fundamental property of the exposed material.
Hozumi, Hideaki*; Yamaguchi, Hisato*; Kaga, Toshihide*; Eda, Goki*; Mattevi, C.*; Ogawa, Shuichi*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.
no journal, ,
In order to clarify the time evolution of the chemical bonding state during thermal reduction of graphene oxide (GO), real-time photoelectron spectroscopy was employed for observing the thermal reduction kinetics of GO. The GO was prepared by the modified Hummer method. The experiments were performed using the surface reaction analysis apparatus placed at the BL23SU of SPring-8. The XPS measurements were performed simultaneously during the annealing at 473 K, 673 K, 873 K, and 1073 K. The C1s photoelectron spectra are decomposed by 8 components. The 
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transition loss peak intensity is propotional to the intensity of sp
graphene components with temperature elevation. In addition, defect intensity increased in proportion with the sp graphene intensity. These facts indicate that defects were formed on the graphene during reduction and these defects cause the recovery of electric conductivity, that is, the appearance of Fermi edge.
Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Kaga, Toshihide*; Eda, Goki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.
no journal, ,
no abstracts in English
Watanabe, Daiki*; Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Eda, Goki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.
no journal, ,
no abstracts in English
Watanabe, Daiki*; Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Eda, Goki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.
no journal, ,
Watanabe, Daiki*; Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Eda, Goki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.
no journal, ,
no abstracts in English
Watanabe, Daiki*; Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Eda, Goki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.
no journal, ,
no abstracts in English
Watanabe, Daiki*; Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Eda, Goki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; et al.
no journal, ,
no abstracts in English
Watanabe, Daiki*; Ogawa, Shuichi*; Yamaguchi, Hisato*; Hozumi, Hideaki*; Mattevi, C.*; Yoshigoe, Akitaka; Ishizuka, Shinji*; Teraoka, Yuden; Yamada, Takatoshi*; Chhowalla, M.*; et al.
no journal, ,
Takabayashi, Susumu*; Fukuda, Akito*; Tsukazaki, Ryuta*; Koga, Hisashi*; Yamaguchi, Hisato*; Ogawa, Shuichi*; Takakuwa, Yuji*; Tsuda, Yasutaka; Yoshigoe, Akitaka
no journal, ,
Graphene has a wide variety of potential applications, including as a channel material in field-effect transistors and as a gas barrier film. The chemical modification and reformation of graphene was studied using photoemission-assisted plasma. In the C 1s photoelectron spectra of graphene/Cu substrates treated under Townsend discharge conditions, which prevent ion damage, both sp2 carbon (sp2 C-C, sp2 C-H) of graphene and a sp3 carbon (sp3 C-H) were observed by synchrotron radiation XPS, indicating that the six-membered ring structure of graphene was cleaved and hydrogen was bonded to it. On the other hand, annealing in a hydrogen-reducing atmosphere resulted in the disappearance of the sp3 C-H and sp2 C-H peaks and the appearance of the sp3 C-C peak, corresponding to hydrogen desorption.
coated by 2D materialsOgawa, Shuichi*; Yusa, Ryunosuke*; Wang, G.*; Pettes, M. T.*; Liu, F.*; Tsuda, Yasutaka; Yoshigoe, Akitaka; Abukawa, Tadashi*; Moody, N. A.*; Yamaguchi, Hisato*
no journal, ,
Lanthanum hexaboride (LaB) has a low work-function and is widely used as a thermionic cathode. For practical application, further reduction of its work-function and high durability have been required. In this study, the effect of 2D material coating materials (graphene and hexagonal boron nitride (hBN)) prepared by a wet-transfer method on the work-function of LaB(100) was studied by using photoelectron emission microscopy (PEEM), synchrotron radiation photoemission spectroscopy, Raman spectroscopy, atomic force microscopy and DFT calculations. PEEM images for samples after 905
C heating clearly showed strong photoemission in the hBN coating region. DFT calculations indicated that the work-function increases in graphene due to the inward dipole formation, while the work function decreases in hBN due to the outward dipole forming at the interface.
Fukuda, Akito*; Takabayashi, Susumu*; Yamaguchi, Hisato*; Ogawa, Shuichi*; Takakuwa, Yuji*; Tsuda, Yasutaka; Yoshigoe, Akitaka
no journal, ,
Chemical modification and modification of graphene is widely required. In this presentation, we report the results of XPS analysis of graphene treated in a photoemission assisted Townsend discharge (PATD) regime, which can prevent ion damage. The XPS peaks assigned with sp2 carbon (sp2 C-C, sp2 C-H) and sp3 carbon (sp3 C-H) were observed, suggesting that the six-membered ring structure of graphene was cleaved and hydrogen was bonded to the cleaved structure. After annealing at 700C for 20 min under an H2-reduction, sp3 C-H and sp2 C-H peaks disappeared and the sp3 C-C peak appeared. Hydrogen is thought to be desorbed by the annealing treatment.
Fukuda, Akito*; Takabayashi, Susumu*; Naito, Haruhiro*; Tanaka, Shuto*; Yamaguchi, Hisato*; Ogawa, Shuichi*; Takakuwa, Yuji*; Tsuda, Yasutaka; Yoshigoe, Akitaka
no journal, ,
Although modification or chemical modification is necessary in graphene applications, the damage due to the modification processes is a serious problem. In this conference, it is reported that the defects confirmed by D-band peak in Raman spectroscopy can be selectively controlled by using the Townsend discharge region of photoemission-assisted plasma, which can prevent ion damage.
Takabayashi, Susumu*; Fukuda, Akito*; Naito, Haruhiro*; Tanaka, Shuto*; Yamaguchi, Hisato*; Ogawa, Shuichi*; Takakuwa, Yuji*; Tsuda, Yasutaka; Yoshigoe, Akitaka
no journal, ,
Modification and chemical modification are required in graphene applications. Photoemission-assisted plasma has attracted attention as a novel process method to prevent ion damage. In only Ar ambient, the sp3 CH peak was observed in the XPS spectrum in addition to the peak attributed to the basal structure of graphene (C-C, C-H). The six-membered ring structure of graphene is considered to be cleaved and hydrogen is added. Annealing in H
ambient caused the disappearance of the sp3 C-H and sp2 C-H peaks and the appearance of a new sp3 C-C peak. This result suggests that hydrogen desorption seems to take place during annealing.
Fukuda, Akito*; Naito, Haruhiro*; Tanaka, Shuto*; Yamaguchi, Hisato*; Ogawa, Shuichi*; Takakuwa, Yuji*; Tsuda, Yasutaka; Yoshigoe, Akitaka; Takabayashi, Susumu*
no journal, ,
Graphene is a two-dimensional material. Its outstanding characteristics such as gas-barrier properties have been remarked. To improve these characteristics or find new one, its modification is indispensable; however, the existing methods may destroy or reduce properties: implantation, adsorption, and chemical modification. We have been using low-energy ion attack of photoemission-assisted Townsend discharge (PATD) plasma. In conventional radio-frequency discharge plasma, a sheath electric field may cause severe damage to graphene. Because of the displacement current, both current and voltage are difficult to measure independently and precisely. Power in watt, which is a product of current and voltage, is used as a variable. However, the current is an extensive variable and is a factor of kinetics of chemical reactions. The voltage is an intensive variable and is a factor of thermodynamics. PATD is a DC plasma. Thus, we can expect precisely-controlled graphene by PATD.
Ogawa, Shuichi*; Yusa, Ryunosuke*; Wang, G.*; Pettes, M. T.*; Liu, F.*; Tsuda, Yasutaka; Yoshigoe, Akitaka; Abukawa, Tadashi*; Moody, N. A.*; Yamaguchi, Hisato*
no journal, ,
LaB has been used as a thermionic cathode due to its low work function, but it is also expected to be used as a photocathode by further lowering the work function. Here, we report photoemission electron microcopy (PEEM), thermionic emission electron microscopy (TEEM) and synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS) study on the work function change of LaB by coating with two-dimensional materials (graphene and hexagonal boron nitride (hBN)). A larger decrease of work function for the hBN coated LaB(100) compared to graphene coated LaB(100) was observed and qualitatively explained by our density functional theory (DFT) calculations.
