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

Abstract

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 MoS 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 MoS 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 MoS 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 MoS induced during heterostructure formation. The maximum frequency shift of E mode is determined to be (4.2 ± 0.8) cm, corresponding to 1.4% of biaxial strain induced in the MoS 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.

Van der Waals interfacial reconstruction in monolayer transition-metal dichalcogenides and gold heterojunctions

“…S emiconducting 2D TMDs, such as MoS 2 and WS 2 , have shown exceptional electronic 1 , optical 2 , and catalytic 3 properties superior to their bulk counterparts. To employ 2D TMDs in electronic devices, one of the essential processes is to connect the atomically-thick crystals with bulk metals that serve as electrodes, connectors and supporting substrates [4][5][6] . The structure and chemistry of 2D semiconductor-metal interfaces significantly influence the performances of 2D electronic devices 7 .…”

Characterization of two‐dimensional materials from Raman spectral data

“…This can be done through the analysis of the output of the fitting modules as shown in Figure . The physical effects that can be profiled at this stage using the tool are doping in graphene, strain in graphene, disorder in graphene, and strain in MoS 2 …”

Advanced Characterization Methods for Electrical and Sensoric Components and Devices at the Micro and Nano Scales

“…However, Raman spectroscopy provides a nanometer resolution only in tip‐enhanced Raman spectroscopy (TERS) configuration by applying a plasmonic nanoantenna that localizes and enhances the electric field, as explained in more detail in Section . Using an AFM tip with a metal coating acting as a plasmonic nanoantenna, the TERS principle was used to map strain in CNTs, MoS 2 , graphene, and in Si/Ge quantum dots …”