Tip-Enhanced Raman Spectroscopy Coherence Length of 2D Materials: An Application to Graphene

Herein, a protocol is presented for determining the tip-enhanced Raman spectroscopy (TERS) coherence length (Lc) of the two main Raman bands of graphene. This method involves performing approach curve experiments and plotting the normalized areas of the G and 2D bands as a function of tip-sample distance. The resultant data are fitted using a specific TERS formula to extract Lc. The results indicate different coherence lengths for the G and 2D bands in graphene as LcG=(13 +/- 2)$L_{\text{c}}<^>{\text{G}} = \left(\right. 13 \pm 2 \left.\right)$ and Lc2D=(10 +/- 1)$L_{\text{c}}<^>{2 \text{D}} = \left(\right. 10 \pm 1 \left.\right)$ nm. While this protocol is specifically done for graphene, the underlying theoretical framework, based on mode symmetry, can be extended to other 2D materials, such as transition metal dichalcogenides. This demonstrates the versatility and potential of TERS in exploring the coherence properties of various 2D materials. (a) Approach curve experiment schematic with the tip (empty triangles) being pulled up. The red arrow shows the laser interacting with the sample and tip. (b,d) Averaged normalized G and 2D band areas vs. tip-sample distance. The theory (red line) gives the coherence lengths LcG=(13 +/- 2)$L_{\text{c}}<^>{\text{G}} = \left(\right. 13 \pm 2 \left.\right)$ and Lc2D=(10 +/- 1)$L_{\text{c}}<^>{2 \text{D}} = \left(\right. 10 \pm 1 \left.\right)$ nm.image (c) 2024 WILEY-VCH GmbH