Advanced Materials, 34, 2200643 (2022)

Multiple 2D Phase Transformations in Monolayer Transition Metal Chalcogenides

Jinhua Hong¹, Xi Chen^{2, 3}, Pai Li⁴, Masanori Koshino¹, Shisheng Li⁵, Hua Xu⁶, Zhixin Hu^{2, 3}, Feng Ding^{4, 7} and Kazu Suenaga^{1, 8}

1 Nanomaterials Research Institute National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba 305-8565, Japan.

2 Center for Joint Quantum Studies and Department of Physics, Institute of Science, Tianjin University, Tianjin 300350, China.

3 Center for Joint Quantum Studies and Department of Physics Institute of Science Tianjin University Tianjin 300350, China.

4 Center for Multidimensional Carbon Materials (CMCM) Institute for Basic Science (IBS) Ulsan 689-798, Republic of Korea.

5 International Center for Young Scientists (ICYS) National Institute for Materials Science (NIMS) Tsukuba 305-0044, Japan.

6 Key Laboratory of Applied Surface and Colloid Chemistry School of Materials Science and Engineering Shaanxi Normal University Xi’an 710119, China.

7 School of Materials Science and Engineering Ulsan National Institute of Science and Technology (UNIST) Ulsan 689-798, Republic of Korea.

8 The Institute of Scientific and Industrial Research (ISIR-SANKEN) Osaka University Osaka 567-0047, Japan.

* zhixin.hu@tju.edu.cn, suenaga-kazu@sanken.osaka-u.ac.jp

Abstract

Phase transformation lies at the heart of materials science because it allows for the control of structural phases of solids with desired properties. It has long been a challenge to manipulate phase transformations in crystals at the nanoscale with designed interfaces and compositions. Here in situ electron microscopy is employed to fabricate novel 2D phases with different stoichiometries in monolayer MoS2 and MoSe2. The multiphase transformations: MoS2 → Mo4S6 and MoSe2 → Mo6Se6 which are highly localized with atomically sharp boundaries are observed. Their atomic mechanisms are determined as chalcogen 2H ↔ 1T sliding, cation shift, and commensurate lattice reconstructions, resulting in decreasing direct bandgaps and even a semiconductor– metal transition. These results will be a paradigm for the manipulation of multiphase heterostructures with controlled compositions and sharp interfaces, which will guide the future phase engineered electronics and optoelectronics of metal chalcogenides.