Highly Localized Strain in a MoS₂/Au Heterostructure Revealed by Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) has been rapidly improved over the past decade and opened up opportunities to study phonon properties of materials at the nanometer scale. In this Letter, we report on TERS of an ultrathin MoS₂ flake on a nanostructured Au on silicon surface forming a two-dimensional (2D) crystal/plasmonic heterostructure. Au nanostructures (shaped in triangles) are prepared by nanosphere lithography, and then MoS₂ is mechanically exfoliated on top of them. The TERS spectra acquired under resonance conditions at 638 nm excitation wavelength evidence strain changes spatially localized to regions as small as 25 nm in TERS imaging. We observe the highest Raman intensity enhancement for MoS₂ on top of Au nanotriangles due to the strong electromagnetic confinement between the tip and a single triangle. Our results enable us to determine the local strain in MoS₂ induced during heterostructure formation. The maximum frequency shift of E_2g mode is determined to be (4.2 ± 0.8) cm^-1, corresponding to 1.4% of biaxial strain induced in the MoS₂ layer. We find that the regions of maximum local strain correspond to the regions of maximum topographic curvature as extracted from atomic force microscopy measurements. This tip-enhanced Raman spectroscopy study allows us to determine the built-in strain that arises when 2D materials interact with other nanostructures.