Basal-Plane Ligand Functionalization on Semiconducting 2H-MoS₂ Monolayers

Molybdenum disulfide (MoS₂) 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, MoS₂ functionalization has been mostly limited to either 1T-phase MoS₂ or the edges of 2H-phase MoS₂, and the chemistry of covalent functionalization on the basal plane of 2H-MoS₂ is poorly understood. Here, we report a facile approach to covalently functionalize chemical vapor deposition (CVD) grown 2H-MoS₂ monolayers (MLs), as well as mechanically exfoliated MoS₂, via thiol conjugation at sulfur vacancies on the basal plane. Thorough characterization confirmed the functionalization by thiol molecules on MoS₂ MLs, and we experimentally proved that sulfur vacancies in MoS₂ 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 MoS₂ 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 MoS₂. Moreover, such functionalization can be utilized for making MoS₂-based heterostructures, an example of which was demonstrated using a dithiol molecule to link MoS₂ layers and PbSe quantum dots. These results provide new understanding and insights on the surface chemistry of MoS₂ and open up more opportunities for MoS₂ MLs with well-controlled properties and broader applications.