Janus monolayer transition metal dichalcogenides (TMDs), created by post-growth substitution of the top chalcogen layer, represent a new direction for engineering 2D crystal properties. However, their rapid ambient degradation and the difficulty of obtaining large-area monolayer samples have limited the available experimental probes, leaving their detailed electronic structure near the Fermi level largely unexplored. In this work, by performing micro-focused angle-resolved photoemission spectroscopy ({\mu}-ARPES) on an identical sample transformed from monolayer WSe2 to Janus WSSe via a H2 plasma-assisted chalcogen-exchange method, we reveal the evolution of its electronic band structure. We observe ARPES signature consistent with the Rashba-type spin splitting due to broken horizontal mirror symmetry, and a significant upward shift of the highest valence band at the {\Gamma}-point by approximately 160 meV. These direct observations clarify the key electronic modifications that govern the material's properties and provide a pathway for band engineering in Janus TMDs.
@article{arxiv.2510.04113,
title = {Direct observation of band structure modifications from monolayer WSe2 to Janus WSSe},
author = {Masato Sakano and Shunsuke Akatsuka and Takato Yamamoto and Tianyishan Sun and Dingkun Bi and Hiroto Ogura and Naoya Yamaguchi and Fumiyuki Ishii and Natsuki Mitsuishi and Kenji Watanabe and Takashi Taniguchi and Miho Kitamura and Koji Horiba and Kenichi Ozawa and Katsuaki Sugawara and Seigo Souma and Takafumi Sato and Yuta Seo and Satoru Masubuchi and Tomoki Machida and Toshiaki Kato and Kyoko Ishizaka},
journal= {arXiv preprint arXiv:2510.04113},
year = {2025}
}