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Journal Articles

Work function lowering of LaB$$_{6}$$ by monolayer hexagonal boron nitride coating for improved photo- and thermionic-cathodes

Yamaguchi, 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

https://doi.org/10.1063/5.0142591
 Times Cited Count：6 Percentile：80.55(Physics, Applied)

A lowering of work function for LaB$$_{6}$$ 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$$_{6}$$(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$$_{6}$$(100) compared to graphene coated LaB$$_{6}$$(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$$_{6}$$.

Journal Articles

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
 Times Cited Count：6 Percentile：59.05(Chemistry, Physical)

Immersion of graphene in KOH solution improves its mobility on SiO$$_{2}$$/Si wafers. This is thought to be due to electron doping by modification with K atoms, but the K atom concentration C$$_{K}$$ in the graphene has not been clarified yet. In this study, the C$$_{K}$$ was determined by XPS analysis using high-brilliance synchrotron radiation. The time evolution of C$$_{K}$$ was determined by real-time observation, and the C$$_{K}$$ 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.

Oral presentation

PEEM observation of work function changes on LaB$$_{6}$$ coated by 2D materials

Ogawa, 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$$_{6}$$) 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$$_{6}$$(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$$^{circ}$$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.

Oral presentation

Work function changes by 2D material coatings on LaB$$_{6}$$

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$$_{6}$$ 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$$_{6}$$ by coating with two-dimensional materials (graphene and hexagonal boron nitride (hBN)). A larger decrease of work function for the hBN coated LaB$$_{6}$$(100) compared to graphene coated LaB$$_{6}$$(100) was observed and qualitatively explained by our density functional theory (DFT) calculations.

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