  Two-dimensional (2D) materials beyond graphene such as transition metal
dichalcogenides (TMDs) have unique mechanical, optical and electronic
properties with promising applications in flexible devices, catalysis and
sensing. Optical imaging of TMDs using photoluminescence and Raman spectroscopy
can reveal the effects of structure, strain, doping, defects, edge states,
grain boundaries and surface functionalization. However, Raman signals are
inherently weak and so far have been limited in spatial resolution in TMDs to a
few hundred nanometres which is much larger than the intrinsic scale of these
effects. Here we overcome the diffraction limit by using resonant tip-enhanced
Raman scattering (TERS) of few-layer MoS2, and obtain nanoscale optical images
with ~ 20 nm spatial resolution. This becomes possible due to electric field
enhancement in an optimized subnanometre-gap resonant tip-substrate
configuration. We investigate the limits of signal enhancement by varying the
tip-sample gap with sub-Angstrom precision and observe a quantum quenching
behavior, as well as a Schottky-Ohmic transition, for subnanometre gaps, which
enable surface mapping based on this new contrast mechanism. This quantum
regime of plasmonic gap-mode enhancement with a few nanometre thick MoS2
junction may be used for designing new quantum optoelectronic devices and
sensors.
