Atomic Mechanism of MoS2 Oxidation Induced by

Ultraviolet-Ozone Treatment

Yeongdong Lee1, Han Seul Kim2, Juho Lee2, Sufyan Ramzan2, Seungwoo Son1, Jung Hwa Kim1, Na Yeon Kim1, Yong-Hoon Kim2 and Zonghoon Lee1,3

1School of Material Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea

2Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea

3Center for Multidimensional Carbon Materials, Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea

E-mail: zhlee@unist.ac.kr

Recently, oxidation of Transition Metal Dichalcogenides (TMDCs) has aroused a lot of interest due to its tunability of electronic and optoelectronics properties. It has been reported that molybdenum oxide is formed on the surface of MoS2 after oxidation. The resultant modified characteristics of TMDCs have been reported in several articles. However, only a few reports have elucidated the oxidation mechanism. In this study, surface oxidation of MoS2 is performed by ultraviolet-ozone (UVO) treatment and an oxidation mechanism is proposed. The single-layer is oxidized without macroscopic preferential sites, while few-layers oxidation starts from the edges and progresses gradually inwards. We can rarely detect the triangular nucleation which starts from surface defect of MoS2. To illustrate this, DFT calculations were performed. The oxygen substitution energies for various S atom sites have similar values, which means that the difference originates from kinetics rather than thermodynamics. Few-layer MoS2 have MoS2-MoS2 interactions which limits the exposure to the reactive oxygen species in addition to single-layer MoS2-SiO2 interface. After oxygen substitution of topside MoS2, bending occurs. Reactive oxygen permeates in this way which differs the oxidation behavior to start from the edges. It leads to the difference oxidation process in single-layer and few-layer MoS2. The understanding of the oxidation mechanism will allow the fabrication of atomically precise MoO3/MoS2 heterostructure and the development of novel strategies to prevent the oxidation of MoS2.

Keyword: MoS2, oxidation, ultraviolet-ozone treatment, first-principles simulations