Basal-Plane Ligand Functionalization on Semiconducting 2H-MoS Monolayers.

Molybdenum disulfide (MoS) is a two-dimensional material promising for electronic, optical, and catalytic applications. To fully harness its potential, functionalization is essential to controlling its properties. However, MoS functionalization has been mostly limited to either 1T-phase MoS or the edges of 2H-phase MoS, and the chemistry of covalent functionalization on the basal plane of 2H-MoS is poorly understood. Here, we report a facile approach to covalently functionalize chemical vapor deposition (CVD) grown 2H-MoS monolayers (MLs), as well as mechanically exfoliated MoS, via thiol conjugation at sulfur vacancies on the basal plane. Thorough characterization confirmed the functionalization by thiol molecules on MoS MLs, and we experimentally proved that sulfur vacancies in MoS MLs play a key role in the functionalization of basal planes. By the controlling of the amount of sulfur vacancies via sulfur annealing, the degree of MoS functionalization was effectively tuned. Because thiol conjugation partially repairs or passivates sulfur vacancies, enhanced photoluminescence response and decreased active sites for hydrogen evolution catalysis were observed for functionalized MoS. Moreover, such functionalization can be utilized for making MoS-based heterostructures, an example of which was demonstrated using a dithiol molecule to link MoS layers and PbSe quantum dots. These results provide new understanding and insights on the surface chemistry of MoS and open up more opportunities for MoS MLs with well-controlled properties and broader applications.