Importance of Multiple Excitation Wavelengths for TERS Characterization of TMDCs and Their Vertical Heterostructures

Tip-enhanced Raman scattering (TERS)-based nanoimaging is gaining ever-increasing popularity for nanoscale spectroscopic imaging of 2D materials due to its much improved sensitivity and spatial resolution compared to conventional confocal Raman microscopy. In this work we report a successful application of TERS for spectroscopic imaging of the WS2-WSe2 vertical heterostructures transferred to gold. Our approach affords the unambiguous identification of the composition of vertical heterobi- and heterotrilayers. Collecting TERS maps of the same area with various excitation wavelengths showed that 671 and 785 nm excitations yield a strong enhancement of Raman bands for both WS2 and WSe2 sublayers in vertical heterostructures. On the other hand, 638 nm excitation seems insensitive to the WSe2 component in vertical heterobilayers. In this context, the use of a single wavelength for the TERS characterization of heterobilayers leads to wrong conclusions when it comes to the composition of this important class of low-dimensional quantum materials. Additionally, in TERS spectra recorded from monolayer WS2 collected with 671 nm excitation, we observe the appearance of a strong resonant band at 324 cm(-1), which was absent in TERS spectra collected with 638 and 785 nm excitations. This observation further bolsters the importance of varied excitation wavelengths to match the electronic/excitonic resonances of 2D materials probed through TERS.