Detailed work function and structural investigations of layered MoO3 onto SiO2 and MoS2 in air

Abstract

MoS2, its oxides and heterostructures are increasingly utilized in nanoscale opto-electronics and catalysis, where electrical properties such as work function (WF) affect their performance. Herein, a detailed study of the thickness-dependent work function in the case of thermally produced layered Mo oxides onto SiO2 and MoS2 substrates in air is presented. First, the effects of typical contaminants on the mechanically exfoliated MoS2 substrates are investigated. Then, high resolution structural characterization by atomic force microscopy supported by X-ray photoelectron spectroscopy shows that thermal MoS2 oxidation in humid air produces layered amorphous MoO3 with nano-crystallites. Kelvin-probe force microscopy analysis shows that the first MoO3 layer on both substrates is negatively charged and that the MoO3 work function depends on the oxide thickness. The thickness-dependent WF is explained by presence of screened electric field at the MoO3/substrate interface. The negative doping to MoO3 by the studied substrates is confirmed through density functional theory simulations. Moreover, an electrical device from amorphous MoO3 monolayer onto MoS2 is built and shows rectification behavior due to p-type doping of the MoS2 by the oxide layer. Overall, this study provides an insight to understand and manipulate electrical properties of MoO3 and MoO3/MoS2 heterostructures in various devices.