Reactivity of Diazonium Salts on Single- and Multilayer MoS2 on Au(111)

There is significant interest in using single- and few-from the ability to tune its optoelectronic properties. One method investigated to tune the mechanical and electronic properties of MoS2 is through aryl radical addition reactions with compounds such as 4-nitrobenzenediazonium tetrafluoroborate (4-NBD). Here we investigated this for single-layer MoS2 (SLM) and multilayer MoS2 (MLM) on Au(111) substrates. The optical, chemical, and tribological properties of SLM and MLM on Au(111) were investigated before and after functionalization with an aqueous solution of 4-NBD. Atomic force microscopy (AFM) was used to characterize the pristine and functionalized MoS2 on Au(111) and illustrated the formation of a film on the MoS2 surfaces from the diazonium compound. This film density was greater on SLM than on MLM, likely due to the increased electronic coupling between SLM and the Au(111) substrate. Interestingly, we find that the films formed appear to be weakly bound on the surface of SLM and MLM and were easily worn away through AFM when contact forces exceeded 5 and 3 nN, respectively, suggesting limited covalent binding to the surfaces. While some of the diazonium reacts with the surfaces, the remaining diazonium film on the surface of SLM and MLM is stabilized through electron transfer from the underlying Au(111) substrate yielding a self-terminating dendritic growth on the surface with a film thickness of about 1.5 nm, regardless of the thickness of the MoS2 or the initial concentration of the diazonium solution.