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Two-dimensional (2D) materials beyond graphene such as transition metaldichalcogenides (TMDs) have unique mechanical, optical and electronicproperties with promising applications in flexible devices, catalysis andsensing. Optical imaging of TMDs using photoluminescence and Raman spectroscopycan reveal the effects of structure, strain, doping, defects, edge states,grain boundaries and surface functionalization. However, Raman signals areinherently weak and so far have been limited in spatial resolution in TMDs to afew hundred nanometres which is much larger than the intrinsic scale of theseeffects. Here we overcome the diffraction limit by using resonant tip-enhancedRaman scattering (TERS) of few-layer MoS2, and obtain nanoscale optical imageswith ~ 20 nm spatial resolution. This becomes possible due to electric fieldenhancement in an optimized subnanometre-gap resonant tip-substrateconfiguration. We investigate the limits of signal enhancement by varying thetip-sample gap with sub-Angstrom precision and observe a quantum quenchingbehavior, as well as a Schottky-Ohmic transition, for subnanometre gaps, whichenable surface mapping based on this new contrast mechanism. This quantumregime of plasmonic gap-mode enhancement with a few nanometre thick MoS2junction may be used for designing new quantum optoelectronic devices andsensors.

Quantum limit in subnanometre-gap tip-enhanced nanoimaging of few-layer MoS2