Recent advances in TERS/TEPL characterization of lateral and vertical heterostructures of TMDCs

Abstract

Despite the pandemic, recent two years showed significant progress in application of Tip Enhanced Raman Spectroscopy (TERS) and tip enhanced Photoluminescence (TEPL) to nanoscale spectroscopic characterization of lateral and vertical heterostructures of transition metal dichalcogenides (TMDCs). In this talk I'll discuss how TERS imaging revealed existence of the alloyed transition area at the junction of MoS2-WS2 heteromonolayers, it's width varying greatly along the junction line from over 500nm to less then 25nm ( pixel limited resolution in those experiments). Next, I'll demonstrate how ultra low frequency TERS imaging that can directly probe interlayer phonons in twisted vertical heterobilayers of WS2 and WSe2 can sense the layer decoupling in the nanobubbles formed within these heterostructures. In addition, I'll demonstrate that the Stokes/ anti-Stokes ratio of the first and second order modes in TERS spectra of WSe2 and WS2-WSe2 heterostructures can be dramatically different even when those bands are separated by less than 10 cm-1 in spectral space. Possible physical mechanisms of such dramatic difference will be discussed. Finally, I'll demonstrate a crucial importance of varied excitation wavelengths for TERS characterization of TMDCs and their vertical heterostructures. In case of the CVD-grown WS2-WSe2 vertical heterostructures, with 785 nm excitation the characteristic Raman bands of both constituents can be clearly seen, while TERS spectra of the same crystal obtained with the same TERS probe but with 638nm excitation showed greatly suppressed intensity of the WSe2 bands. In addition, I'll show that the TERS spectra of the monolayer WS2 collected with 671 nm excitation, contain a resonant band at 324 cm-1, which is completely absent in TERS spectra of the same crystal collected with 638 or 785 nm excitation, which assumes the presence of the resonance at this wavelength.