Substrate-related optical activity in monolayer WS 2 ${\rm WS}_2$ and MoSe 2 ${\rm MoSe}_2$ : A tip-enhanced Raman spectroscopy study.

Transition metal dichalcogenides (TMDs) are promising two-dimensional materials whose properties are strongly influenced by substrate interactions. While conventional Raman spectroscopy probes these effects, its diffraction-limited resolution often averages out local variations such as strain, masking intrinsic behaviours. Here, we employ tip-enhanced Raman spectroscopy (TERS) to investigate the vibrational properties of monolayer ${\mathrm{WS}}_2$ and ${\mathrm{MoSe}}_2$ on top of glass and glass/hBN substrates. TERS offers nanometric spatial resolution, allowing direct correlation between Raman features and topographical inhomogeneities. Our results reveal that local variations in strain, doping, and dielectric screening that vary across the substrate interface are often accompanied by nanoscale structural features such as wrinkles, which locally modulate the vibrational response.