Mechanistic Insights into MoS₂ Functionalization via Thiol Groups of 4-Aminothiophenol

Functionalization of transition metal dichalcogenides is a key process for future technological applications. To date, various routes, primarily chemical, have been proposed for efficient functionalization with thiol moieties. Previous functionalization protocols have generally taken advantage of intrinsic defects typically present on the surface. However, the specific nature of the molecule-substrate bond, covalent or noncovalent, has always remained controversial. In this work, we present a study of the functionalization of MoS₂ by physical vapor deposition in ultrahigh vacuum conditions of the 4-aminothiophenol molecule. We have investigated its interaction with the intrinsic sulfur vacancies present in MoS₂ and with additional vacancies generated by ion bombardment. We observe that the molecule is found in planar configuration on the surface and the thiol moiety remains intact. Atomic force microscopy measurements show high diffusion of molecules through the surface, along with a strong interaction between the thiol moieties and the tip itself. Interestingly, this interaction leads to a contrast inversion in the acquired AFM images, which is a key observation that leads to consider that molecules are not covalently bonded. Density functional theory calculations rule out thiol dehydrogenation, attributing the observed contrast inversion to molecular tip functionalization. These findings provide compelling evidence of a nonspecific physisorption interaction, confirming the absence of covalent bonding between the molecule and the surface.