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

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.