Giant gap-plasmon tip-enhanced Raman scattering of MoS₂monolayers on Au nanocluster arrays

Abstract

In this article, we present the results of a gap-plasmon tip-enhanced Raman scattering study of MoS monolayers deposited on a periodic array of Au nanostructures on a silicon substrate forming a two dimensional (2D) crystal/plasmonic heterostructure. We observe a giant Raman enhancement of the phonon modes in the MoS monolayer located in the plasmonic gap between the Au tip apex and Au nanoclusters. Tip-enhanced Raman mapping allows us to determine the gap-plasmon field distribution responsible for the formation of hot spots. These hot spots provide an unprecedented giant Raman enhancement of 5.6 × 10 and a spatial resolution as small as 2.3 nm under ambient conditions. Moreover, due to strong hot electron doping in the order of 1.8 × 10 cm, we observe a structural change of MoS from the 2H to the 1T phase. Owing to the very good spatial resolution, we are able to spatially resolve those doping sites. To the best of our knowledge, this is the first time reporting of such a phenomenon with nm spatial resolution. Our results will open the perspectives of optical diagnostics with nanometer resolution for many other 2D materials.

Experimental Studies and Modeling of “Starlike” Plasmonic Nanostructures for SERS Application

“…With increasing of the solution concentration by an order of magnitude the amount of molecules on a gold NP also increases by an order. A factor that causes non‐linear dependence in this case can be the occurance of “hot spots.” Indeed, it was shown earlier that for certain types of SERS substrates the main contribution to signal enhancement is made by the “hot spots.” This was clearly demonstrated in TERS (tip‐enhanced Raman scattering) experiments, where the hot spots occurring between the plasmonic substrate and metallic tip can be directly mapped as the Raman intensity of the analyte …”

Resonant tip-enhanced Raman scattering by CdSe nanocrystals on plasmonic substrates

“…31 Recently we employed gap-mode AFM-TERS for determining the local strain in mechanically transferred monolayers of MoS 2 on Au nanostructures. 8,26,32 The biaxial strain in an ultrathin MoS 2 layer induces a shi of the E 2g phonon mode, which is probed by TERS spectroscopy with a spatial resolution of less than 25 nm. 8 A dramatic Raman enhancement factor of 5.6 Â 10 8 accompanied by a high spatial resolution of 2.3 nm was achieved in gap-mode TERS for monolayer MoS 2 on Au nanodisks and allowed us to observe, for instance, the MoS 2 phase transition from 2H to 1T during TERS mapping.…”

Tip-enhanced Raman spectroscopy: principles, practice, and applications to nanospectroscopic imaging of 2D materials

“…[227] Specifically, due to strong hot electron doping through the plasmonic coupling between MoS 2 and Au nanoclusters, their TERS spectra indicated that there is a structural change of MoS 2 from the 2H to the 1T phase during TERS imaging. [227] These examples illustrate that TERS can be used to investigate local structural heterogeneities in such inorganic 2D materials at the nanoscale, which is of great importance for potential device applications. sheet by TERS imaging with a reasonably high spatial resolution (<60 nm) and a significantly improved sensitivity (contrast factor of >4000) compared to confocal Raman microscopy ( Fig.…”