Excitation Laser Energy Dependence of the Gap-Mode TERS Spectra of WS2 and MoS2 on Silver

In this work, we present a systematic study of the dependence of the gap-mode tip-enhanced Raman scattering (TERS) response of the mono- and bilayer WS2 and MoS2 on silver as a function of the excitation laser energy in a broad spectral range from 473 to 830 nm. For this purpose, we collected consecutive TERS maps of the same area in the sample containing mono- and bilayer regions with the same TERS probe with 6 different excitation lasers. To decrease the number of collected TERS maps, we used for the first time, to the best of our knowledge, concurrent excitation and collection with two lasers simultaneously. We found that the E2g/A1g peak intensity ratio for the bilayer WS2@Ag and the ratio of the A '/A1g peak intensity of the out-of-plane mode for the mono- and the bilayer change in a significantly nonmonotonous way as the excitation laser energy is swept from 1.58 to 2.62 eV. The former ratio increases at energies corresponding to A and B excitons (similar to 2.0 and 2.4 eV, respectively) in bilayer WS2. The absolute intensity of the A ' peak in the monolayer, and correspondingly the A '/A1g ratio, is surprisingly high at lower excitation energies but dips dramatically at the energy corresponding to the A exciton, being restored partially in between A and B excitons, but still showing the descending trend as the excitation laser energy increases. A somewhat similar picture was observed in mono- and bilayers of MoS2@Ag, though the existing set of excitation lasers did not match the excitonic profile of this material as nicely as for the case of WS2. We attribute the observed behavior to the presence of 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 the formation of intermediate/strong coupling between the excitonic resonances in TMDs and the nanocavity reevaluates the role of various resonances in gap-mode TERS, and should become an important factor to be considered by TERS practitioners during experiment planning. Finally, based on the observed phenomena and their explanation, we propose the "ideal" substrate for efficient TERS and tip-enhanced photoluminescence (TEPL) measurements.