Raman spectra of twisted bilayer graphene close to the magic angle

TL;DR

This study investigates how the Raman spectra of twisted bilayer graphene vary with twist angle, revealing electron-phonon interactions near the magic angle and proposing a model for 2D band lineshape variations.

Contribution

It provides new insights into the Raman spectral features of twisted bilayer graphene near the magic angle, including a geometrical model for 2D lineshape variations.

Findings

Electron-phonon interaction affects G band linewidth near the magic angle.

2D band lineshape depends on lattice structure and strain regions.

Estimated soliton width approaching the magic angle.

Abstract

In this work, we study the Raman spectra of twisted bilayer graphene samples as a function of their twist-angles ($\theta$), ranging from 0.03$^\circ$ to 3.40$^\circ$, where local $\theta$ are determined by analysis of their associated moire superlattices, as imaged by scanning microwave impedance microscopy. Three standard excitation laser lines are used (457, 532, and 633 nm wavelengths), and the main Raman active graphene bands (G and 2D) are considered. Our results reveal that electron-phonon interaction influences the G band's linewidth close to the magic angle regardless of laser excitation wavelength. Also, the 2D band lineshape in the $\theta$ < 1$^\circ$ regime is dictated by crystal lattice and depends on both the Bernal (AB and BA) stacking bilayer graphene and strain soliton regions (SP). We propose a geometrical model to explain the 2D lineshape variations, and from it, we estimate the SP width when moving towards the magic angle.