Signature of lattice dynamics in twisted 2D homo/hetero-bilayers

TL;DR

This study uses Raman spectroscopy and theoretical calculations to reveal how lattice dynamics in twisted bilayer WSe2 and other TMDCs are reflected in optical properties, especially the Raman intensity ratios, across different twist angles.

Contribution

It demonstrates that Raman intensity ratios serve as signatures of lattice dynamics in twisted 2D bilayers and extends findings to various TMDC hetero-bilayers.

Findings

Raman intensity ratio correlates with moiré period and twist angle.

Exponential profile of Raman intensity ratio matches moiré superlattice behavior.

Lattice dynamics signatures are consistent across different TMDC bilayer materials.

Abstract

Twisted 2D bilayer materials are created by artificial stacking of two monolayer crystal networks of 2D materials with a desired twisting angle $\theta$. The material forms a moir\'e superlattice due to the periodicity of both top and bottom layer crystal structure. The optical properties are modified by lattice reconstruction and phonon renormalization, which makes optical spectroscopy an ideal characterization tool to study novel physics phenomena. Here, we report a Raman investigation on a full period of the twisted bilayer (tB) WSe$_2$ moir\'e superlattice (\textit i.e. 0{\deg} $\leq \theta \leq$ 60{\deg}). We observe that the intensity ratio of two Raman peaks, $B_{2g}$ and $E_{2g}/A_{1g}$ correlates with the evolution of moir\'e period. The Raman intensity ratio as a function of twisting angle follows an exponential profile matching the moir\'e period with two local maxima at 0{\deg} and 60{\deg} and a minimum at 30{\deg}. Using a series of temperature-dependent Raman and photoluminescence (PL) measurements as well as \textit{ab initio} calculations, the intensity ratio $I_{B_{2g}}/I_{{E_{2g}}/{A_{1g}}}$ is explained as a signature of lattice dynamics in tB WSe$_2$ moir\'e superlattices. By further exploring different material combinations of twisted hetero-bilayers, the results are extended for all kinds of Mo- and W-based TMDCs.