Probing Phonon Modes in Reconstructed twisted Homo and Hetero Bilayer System

TL;DR

This study investigates how twist angle-induced lattice reconstruction affects phonon modes in twisted bilayer graphene and graphene-hBN heterostructures, revealing angle-dependent phonon behaviors and Moire phonon modes through Raman spectroscopy and theoretical analysis.

Contribution

It provides a systematic analysis of phonon renormalization due to lattice reconstruction in twisted bilayer systems, linking twist angle, Moire patterns, and phonon behavior with experimental and theoretical insights.

Findings

Phonon peak broadening and splitting at small twist angles in TBLG.

Moire-induced Raman peaks observed in hBN-graphene heterostructures.

Theoretical identification of Moire phonon modes from different stacking regions.

Abstract

Twist angle engineering in van der Waals homo and hetero-bilayers introduces profound modifications in their electronic, optical and mechanical properties due to lattice reconstruction. In these systems, the interlayer coupling and atomic rearrangement strongly depend on the twist angle, leading to the formation of periodic Moire superlattices. At small twist angles, significant lattice relaxation results in the emergence of domain structures separated by one dimensional soliton networks, influencing electronic band structures and phonon modes. Here we systematically investigate the impact of lattice reconstruction on phonon renormalization in twisted bilayer graphene (TBLG,homo) and graphene-hBN Moire superlattices(hetero). Using Raman spectroscopy, we identify distinct phonon behaviours across different twist angle regimes. In TBLG, we observe the evolution of the G peak, including broadening, splitting, and the emergence of additional peaks in the small angle range 0.3 to 1 degree, attributed to Moire modified phonon interactions. At large twist angles, the peaks gradually merge back into a single feature, reflecting the reduced impact of lattice reconstruction. Similarly, in hBN graphene Moire superlattices, we detect Moire induced Raman peaks above and below the G peak, while the central G peak remains largely invariant to twist angle variation. The theoretical calculations uncover Moire phonon modes originating from different stacking regions providing insights into phonon renormalization. Our results establish a direct link between twist angle, lattice reconstruction, Moire phonons, and interlayer coupling, offering a fundamental framework for understanding phonon engineering in twisted bilayer systems. These findings pave the way for controlling phononic, optoelectronic and heat flow properties in next generation van der Waals heterostructures.