Unraveling the Mystery of the 1.5 layer Raman Response in Exfoliated MoS2.

Recently a new method of exfoliation of large area monolayers of transition metal dichalcogenides (TMDC) was proposed¹. Confocal Raman characterization of exfoliated monolayers of MoS2 showed A’ peak splitting, the components of this spilt peak corresponding to a standard A’ peak of the monolayer and A_1g peak of the bi-layer correspondingly. We further performed tip enhanced Raman Scattering (TERS) imaging of these exfoliated crystals. Thanks to greatly improved spatial resolution of TERS imaging, we discovered that some areas of what seemed to be a uniform monolayer, actually contained a network of tiny islands (few tens of nm across) having spectral signature closely matching TERS response of the bilayer while TERS spectra of areas in between these islands closely resembled standard Raman spectrum of the monolayer. Thanks to these observations we came to the conclusion that the gold-assisted exfoliation of MoS2 may result in inhomogeneous crystals featuring a network of tiny bilayer islands, and consequently, greatly increased crystal edge length per unit of area, a feature that may be useful for (photo)catalytic applications of TMDCs.
References
Velický, M.; et.al. ACS Nano 2018, 12, 10463-10472.