Tip-enhanced nanoscopy of two-dimensional transition metal dichalcogenides: progress and perspectives

Abstract

We reviewed the recent progress of TERS and TEPL in the applications of transition metal dichalcogenides and their heterojunctions.

Local phonon imaging of AlN nanostructures with nanoscale spatial resolution

“…Typically, in the literature TERS enhancement factor is defined as a ratio where I tip down is the TERS intensity in the contact mode and I tip up is the Raman intensity when the tip is lifted up to the distance of about 50 nm. 52 However, in this case, determination of the TERS enhancement factor using a tip-up/down approach requires averaging over a significant number of measurements because the Raman spectra are very much site dependent resulting in a wide spread of enhancement coefficients. Therefore, here the contrast was used for visualization of single nanocrystals which was calculated as an average of 81 TERS spectra on AlN nanocrystal and on the AlN layer (see Fig.…”

“…Both these conditions are fulfilled in our TERS experiment. The where 𝐼 𝑡𝑖𝑝 𝑑𝑜𝑤𝑛 is the TERS intensity in the contact mode and 𝐼 𝑡𝑖𝑝 𝑢𝑝 is the Raman intensity when the tip is lifted up to the distance of about 50 nm 52 . However, in this case, determination of TERS enhancement factor using tip up/down approach requires averaging over significant number of measurements because the Raman spectra are very much site dependent resulting in a wide spread of enhancement coefficients.…”

Tip-Enhanced Raman Spectroscopy of Monolayer MoS2 on Au(111)

“…Monolayer transition metal dichalcogenides (TMDs) are promising materials for advanced applications in high-end electronics, optoelectronics, spintronics, and sensing. , However, the atomic-scale structures such as defects or local strain in TMDs can exert a substantial influence on their material properties, potentially limiting device performance. , As device dimensions continue to shrink, the imperative to detect and locate defects and local strain at the nanoscale becomes increasingly critical. Tip-enhanced Raman spectroscopy (TERS) emerges as a facile and nondestructive tool for addressing these requirements, with the ability to visualize local defects, grain boundaries, edges, and strains. − A typical TERS spectrum of monolayer 2H-MX 2 (M = Mo, W, X = S, Se) includes peaks from first-order Raman modes E ′ and A 1 ′ and the second-order Raman mode 2LA­(M). − ,− Some also report Raman inactive mode A 2 ″ due to the local plasmonic field employed in TERS. ,, The E ′, A 1 ′ , and A 2 ″ originate from phonons at the Γ point of the Brillouin zone. In the presence of defects, additional defect-induced phonon modes, such as LA­(M), ZO­(M), etc.…”

Disentangling doping and strain effects at defects of grown MoS2 monolayers with nano-optical spectroscopy

“…Recently, tip-enhanced PL and Raman spectroscopies (TEPL and TERS, respectively) have also been used to study defective optical features in TMDs. [29][30][31][32] As the tip amplifies the light signal due to its plasmonic field, the spatial resolution of these tip-enhanced techniques is in general determined by the tip size. Thus, investigations of excitonic effects, 33,34 grain boundaries, [35][36][37] edge defects, [38][39][40] strain, 41 wrinkles, 42,43 lateral interfaces, [44][45][46][47] hybrid heterostructures 48 and defect-bound localized states 40,49 have been carried out by TEPL and TERS measurements.…”