  Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) like
molybdenum disulfide (MoS2) are generating significant excitement due to their
unique electronic, chemical, and optical properties. Covalent chemical
functionalization represents a critical tool for tuning the properties of TMDCs
for use in many applications. However, the chemical inertness of semiconducting
TMDCs has thus far hindered the robust chemical functionalization of these
materials. Previous reports have required harsh chemical treatments or
converting TMDCs into metallic phases prior to covalent attachment. Here, we
demonstrate the direct covalent functionalization of the basal planes of
unmodified semiconducting MoS2 using aryl diazonium salts without any
pretreatments. Our approach preserves the semiconducting properties of MoS2,
results in covalent C-S bonds, is applicable to MoS2 derived from a range of
different synthesis methods, and enables a range of different functional groups
to be tethered directly to the MoS2 surface. Using density functional theory
calculations including van der Waals interactions and atomic-scale scanning
probe microscopy studies, we demonstrate a novel reaction mechanism in which
cooperative interactions enable the functionalization to propagate along the
MoS2 basal plane. The flexibility of this covalent chemistry employing the
diverse aryl diazonium salt family is further exploited to tether active
proteins to MoS2, suggesting future biological applications and demonstrating
its use as a versatile and powerful chemical platform for enhancing the utility
of semiconducting TMDCs
