PEEM observation of work function changes on LaB 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*

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 905C 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.