In situ work-function measurement during chemical transformation of MoS₂ to MoO₃ by ambient-pressure x-ray photoelectron spectroscopy

In this study, the oxidation of a two-dimensional (2D) MoS₂ was performed as an alternative route for the synthesis of a 2D-layered MoO₃ structure with high work function (WF) and hole mobility. The proposed method can also be used to tune the electronic properties (WF and bandgap) of MoO₃/MoS₂ composite-based semiconductors. By ambient pressure x-ray photoelectron spectroscopy (AP-XPS), in situ monitoring of the WF and chemical state of the surface was carried out during the oxidation of MoS₂ to MoO₃ layers. By heating the MoS₂ sample in an O₂ + Ar gas environment, the chemical transformation of the MoS₂ to a MoO₃/MoS₂ composite layer and eventually to MoO₃ was observed. The chemically transformed MoO₃ film had a properly layered structure, according to cross-sectional transmission electron microscopy and high-resolution grazing-incidence x-ray diffraction analyses. During the oxidation, the WF change according to the change in surface chemical state was simultaneously measured using Ar gas as a surface potential probe. This study demonstrates the capability of AP-XPS for the monitoring and optimization of the conditions for chemical transformation (oxidation) to achieve desired physical properties (e.g. WF).