Spatial mapping of a low-frequency combination Raman mode in twisted bilayer graphene

Abstract

The exotic properties of twisted bilayer graphene (tBLG) are profoundly affected by interlayer interactions, which can be sensitively probed by the low-frequency Raman modes. However, conventional Raman spectroscopy lacks spatial resolution and, hence, can hardly disclose spatial-dependent inhomogeneous properties of interest. Herein, we demonstrate spatial mapping of low-frequency Raman modes in tBLG via micro-Raman spectroscopy (mRS) imaging. In our constructed tBLG, the shear (C) and out-of-plane breathing (ZO′) modes are directly mapped out, and, more importantly, a regional-dependent, highly localized awakening of the combination C+ZO′ mode is clearly visualized. Through a set of mRS-based analyses, including correlation of Raman G- and 2D-mode frequencies, G-band splitting, and polarization-dependent Raman intensity distributions, we reveal that its turning on/off correlates with the mechanical strain distributions in a sensitive fashion. The spatial mapping of this unique combination mode offers an indicator for scrutinizing the elusive strain in the tBLG system or the like.