Experimental verification of SO 2 and S desorption contributing to defect formation in MoS 2 by thermal desorption spectroscopy.

The defect-free surface of MoS 2 is of high importance for applications in electronic devices. Theoretical calculations have predicted that oxidative etching could be responsible for sulfur vacancy formation. No direct experimental evidence, however, points out the role of adsorbed oxygen on sulfur vacancy formation for MoS 2 , especially on an insulating SiO 2 /Si substrate. Herein, by applying thermal desorption spectroscopy, we found that sulfur loss can be tightly coupled to adsorbed oxygen, as confirmed by observation of SO 2 desorption. With annealing MoS 2 , even under ultrahigh vacuum, oxygen molecules adsorbed on MoS 2 assist the sulfur atom in dissociating from MoS 2 , and thus, defects are formed as the result of SO 2 desorption from 200 °C to 600 °C. At higher temperatures (over 800 °C), on the other hand, direct sulfur desorption becomes dominant. This finding can be well explained by combining the morphology investigation enabled by atomic layer deposition at defective sites and optical transitions observed by photoluminescence measurements. Moreover, a preannealing treatment prior to exfoliation was found to be an effective method to remove the adsorbed oxygen, thus preventing defect formation.