2D materials such as graphene and its derivatives and broad class of transition metal dichalcogenides attracted significant attention of the research community during last decade. Tip enhanced optical spectroscopy ( TEOS) that includes tip enhanced Raman spectroscopy (TERS) and tip enhanced photoluminescence (TEPL) allows characterization of defects and inhomogeneities in these materials at nanometer scale, something conventional confocal Raman or photoluminescence microscopy can not do.
We observed that the gap mode TERS and TEPL reponse in grpahene, graphene oxide and TMDCs gets significantly enhanced over the wrinkles in 2D sheets. Despite similarity in behavior, the nature of this increased intensity is different for graphene and TMDCs. In case of 2D carbon, D,G,2D modes, all in-plain vibrations, got enhanced over wrinkles due to increased coupling of the optical electric field normal to the sample plain and the vertically aligned portions of 2D sheet in the wrinkles. In case of TMDCs such as WS2, or others, mechanical strain in wrinkles results in funneling of defects and excitons into those areas, which leads to increased concentration of defect bound excitons that demonstrate strongly enhanced and significantly red-shifted PL response, which should be expected taking into account that the binding energy of defect-bound excitons is lower compared to free excitons in 2D materials.
Different nature of increased TEOS response in wrinkles of 2D sheets of carbon and TMDCs is further supported by the fact that TERS signal of flat graphene transferred to gold is negligible, since the in-plain modes do not couple to the electric field in the tip-substrate gap, while the TERS signal of flat sheets of TMDCs on gold or silver is very strong, specifically for the out-of-plain modes, which will be illustrated with examples of TERS maps of mono-to few-layer sheets of WS2, MoS2, and MoSe2.