  We present a systematic study of the dependence of gap mode tip-enhanced
Raman scattering (TERS) of mono- and bi-layer WS$_2$ and MoS$_2$ as a function
of excitation laser energy. We collected consecutive TERS maps of mono-and
bi-layer regions with 6 different excitation lasers. To decrease the
acquisition time, we used for the first time concurrent excitation and
collection with two lasers simultaneously. We found that the E$_{2g}$/A$_{1g}$
peak intensity ratio for bilayer WS$_2$@Ag and the A'/A$_{1g}$ peak intensity
ratio of the out-of-plane modes for mono- and bilayer change in a significantly
non-monotonous way with excitation laser energies from 1.58 to 2.62 eV. The
former ratio increases at energies corresponding to A and B excitons in bilayer
WS$_2$. The intensity of the A peak in the monolayer, and hence the A/A$_{1g}$
ratio, is surprisingly high at low excitation energies, dips dramatically at
energy corresponding to the A exciton, and is restored partially in between A
and B excitons, though still showing a descending trend with increasing energy.
A similar picture was observed in mono- and bi-layer MoS$_2$, though the
existing set of lasers did not match its excitonic profile as nicely as for
WS$_2$. We attribute the observed behavior to intermediate (Fano resonance) or
strong (Rabi splitting) coupling between the excitons in transition metal
dichalcogenides (TMDs) and the plasmons in the tip-substrate nanocavity. This
is akin to the so-called Fano (Rabi) transparency experimentally observed in
far field scattering from TMDs between two plasmonic metals. The possibility of
intermediate/strong coupling between excitonic resonances in TMDs and the
nanocavity re-evaluates the role of resonances in gap-mode TERS and should
become an important factor to be considered by TERS practitioners when planning
experiments. Finally, we propose the ideal substrate for efficient TERS and tip
enhanced photoluminescence measurements.
