Proximity Growth of Monolayer MoS₂ Films via Concurrent O₂ Etching and Sulfurization

The synthesis of homogeneous monolayer molybdenum disulfide (MoS₂) thin films has been an important benchmark for enabling its usage in industrial scale electronic and optoelectronic devices. Recently, molecular O₂ has emerged as a reagent for scalable production of MoS₂ films through either gradual etching of an Mo source or evolution from MoO₃ preceding sulfurization. However, oxidation of MoS₂ and its precursors is not well understood and is a frequent subject of study. Here, we invert the order of traditional MoS₂ sulfurization to demonstrate that balancing the etch rate of partially formed MoS₂ films as opposed to an unreacted Mo source is an alternative pathway towards producing homogeneous monolayer MoS₂ films. Further, this growth enables adjacent regions of either multilayer or monolayer MoS₂ to be patterned dependent on whether the respective region was deposited with a Mo metal film or was left as exposed substrate. We study growth effects at 550 °C and 650 °C as well as different O₂ flow rates of 1 sccm and 5 sccm. This study suggests that MoS₂ formation through O₂ etching traverses two separate mechanisms based on the pristine quality of the partially formed MoS₂ source film. As suggested in theoretical studies, defect-filled MoS₂ lowers the activation energy of O₂ adsorption to 0.8 eV and leads to growth of monolayer-bilayer MoS₂ flakes. Alternatively, O₂ adsorption onto pristine MoS₂ raises the activation energy to 1.59 eV and generates monolayer MoS₂ films that spread homogeneously across a 1x5 cm area. The films were characterized as MoS₂ using Raman spectroscopy. XPS analysis also confirms peaks for MoS₂ that are shifted to higher energies due to the presence of trace MoO₃. This demonstrates that the joint adsorption of oxygen and sulfur species onto Mo is advantageous to the creation of the observed homogeneous monolayer MoS₂. Beyond this, Mo-O bonds present in the film could be a significant factor in increasing its photoluminescence compared to traditional MoS₂ syntheses.