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

We report on the MoO oxides and their derivatives on microscopic 2H MoS 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 MoS flakes deposited on Si substrates. We detect MoO oxides mostly by measuring selected nanomechanical properties of the MoO layer, such as its compressive mechanical stress at the plastic yield. We discuss basal surface coverage of the single MoS flakes by the MoO oxides. We discuss conditions for appearance of all possible MoO oxide derivatives, such as molybdenum(VI) hydroxyoxides and MoO hydrates. Our findings agree with an expected mechanistic switch in thermal oxidation in water vapors vs. air.