Material-Dependent Functionalization of CVD-Grown TMDC Monolayers Probed by Vibrational Nanospectroscopy

Understanding the molecular-level mechanisms governing covalent functionalization of two-dimensional semiconductors remains a major challenge. Here, we investigate the chemical vapor deposition (CVD) growth of monolayer MoSe₂, WSe₂, and MoSe₂-WSe₂ in-plane heterostructures, which were subsequently transferred onto conductive Au(111) substrates using a contamination- and defect-free protocol. Tip-enhanced Raman spectroscopy (TERS) and photo-induced force microscopy (PiFM) are high-resolution scanning probe techniques, enabling colocalized topographical and vibrational spectroscopic mapping of surface structures and bound-molecules, providing sub-diffraction of light spatial resolutions. Using TERS, we provide unique Raman nanospectroscopy analyses and imaging of lateral and vertical-lateral MoSe₂-WSe₂ nanodomains and alloyed heterojunctions. Applying PiFM, we directly visualize electrodeposited aryl diazonium salts, providing key direct nanoscale chemical evidence of 4-carboxyphenyl (4-CP) and 4-nitrophenyl (4-NP) covalent functionalization on transition metal dichalcogenides (TMDCs) and their respective heterostructures in protic and organic solvents, revealing material-dependent reactivity. Atomistic molecular dynamics simulations reveal that 4-nitrobenzenediazonium cations (4-NBD⁺) interact more strongly with MoSe₂ than WSe₂, exhibiting persistent interfacial hydration and reduced mobility, which localizes precursors at the interface. This sustained interfacial availability rationalizes the experimentally observed higher reduction probability and grafting efficiency on MoSe₂. Together, these findings provide a robust molecular-level framework for solvent- and material-dependent covalent functionalization of two-dimensional TMDC monolayer materials.