Surface-Bound and Volatile Mo Oxides Produced During Oxidation of Single MoS2 Crystals in Air and High Relative Humidity.

Robert Szoszkiewicz,Paweł Dąbrowski,Maciej Rogala

doi:10.3390/ma13143067

Abstract

We report on the MoO3 oxides and their derivatives on microscopic 2H MoS2 flakes oxidized in air and high relative humidity at a moderate temperature range below 410 °C. We combine XPS and AFM measurements such as topography, friction, creation of nanoscale ripples and scratches on the MoS2 flakes deposited on Si substrates. We detect MoO3 oxides mostly by measuring selected nanomechanical properties of the MoO3 layer, such as its compressive mechanical stress at the plastic yield. We discuss basal surface coverage of the single MoS2 flakes by the MoO3 oxides. We discuss conditions for appearance of all possible MoO3 oxide derivatives, such as molybdenum(VI) hydroxyoxides and MoO3 hydrates. Our findings agree with an expected mechanistic switch in thermal oxidation in water vapors vs. air.

Highlights

Occurring MoS2 crystals, molybdenites, have been widely used as solid lubricants [1].The most common 2H molybdenite has three other polytypes: 3R, 1T and 1T’, which differ in structure and electronic properties [2,3]. 2H and 3R molybdenites can be peeled off mechanically to yield atomically flat MoS2 crystals with thickness down to one monolayer [2]
With help of X-ray photoelectron spectroscopy (XPS) and Atomic force microscopy (AFM) measurements we reported on microscopic details associated with the oxidation of thick MoS2 flakes deposited on silicon at temperatures between 205 and 410 ◦ C in dry and humid air
We observed for samples oxidized at 350 ◦ C that triangular etch pits started to coexist with MoO3 islands and layers

Summary

Introduction

Occurring MoS2 crystals, molybdenites, have been widely used as solid lubricants [1]. In one of our earlier AFM and micro-Raman studies we pointed out that during initial 10–15 min of heating at temperatures ranging from 320 to 390 ◦ C, in air, a predominantly observed outcome on thick microscopic MoS2 flakes was triangular etch pits within the basal MoS2 surface [11]. The MoO3 layers produced during gentle oxidation of single 2H MoS2 crystals are transparent, thin and difficult to be differentiated both physically and chemically from an underlying MoS2 substrate. Such MoO3 layers might contain MoO3 derivatives, such as molybdenum(VI) hydroxyoxides and/or. We study sublimation of the MoO3 oxide layer and its formation in high relative humidity conditions

Materials and Methods

Removing thethe surface-bound layeron onaasingle singleMoS with an AFM

Sublimation of oxide

Thermal oxidation

Conclusions