Physical Adsorption and Oxidation of Ultra-Thin MoS₂ Crystals

The oxidation mechanism of atomically thin molybdenum disulfide (MoS₂) is critical to its nanoelectronics, optoelectronics and catalytic applications as these devices dissipate Joule heating and often operate in oxidative environments. Herein, we systematically investigate the oxidation of mono- and few-layer MoS₂ flakes in air with temperature of 23-525 °C and relative humidity of ~10-60% using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy. Our study reveals the formation of a uniform nanometer-thick physical adsorption layer on the surface of MoS₂, which is attributed to the adsorption of ambient moisture. This physical adsorption layer acts as a thermal shield of the underlying MoS₂ lattice to enhance its thermal stability and can be effectively removed by an AFM tip scanning in contact mode or annealing at 400 °C. Our study also shows that high-temperature thermal annealing and AFM tip-based cleaning result in chemical adsorption on sulfur vacancies in MoS₂, leading to p-type doping. Our study highlights the importance of humidity control in ensuring reliable and optimal performance for MoS₂-based electronic and electrochemical devices and provides crucial insights into the surface engineering of MoS₂, which are also relevant to the study of other two-dimensional transition metal dichalcogenide (TMD) materials and their applications.