Probing Phonon Modes in Reconstructed twisted Homo and Hetero Bilayer System

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 atomic rearrangement strongly depend on the twist angle, leading to the formation of periodic moiré superlattices. In this study, we systematically investigate the impact of lattice reconstruction on phonon renormalization in twisted bilayer graphene (TBLG) and graphene-hBN moiré superlattices. 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 (0.3-1 degrees). At large twist angles, the peaks gradually merge back into a single feature, reflecting the reduced impact of lattice reconstruction. Similarly, in hBN-graphene moiré superlattices, we detect moiré-induced Raman peaks above and below the G peak, while the central G peak remains largely invariant to twist angle variation.  The theoretical calculations based on classical force-field uncover moiré phonon modes originating from different stacking regions, including AB (BA), AA, and SP configurations, providing insights into phonon renormalization driven by lattice reconstruction. Our results establish a direct link between twist angle, lattice reconstruction, moiré phonons, and interlayer coupling, offering a fundamental framework for understanding phonon engineering in twisted bilayer systems.