Probing phonon modes in reconstructed twisted homo- and hetero-bilayer system

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 moiré superlattices. At small twist angles, significant lattice relaxation results in the emergence of domain structures separated by one-dimensional (1D) soliton networks, influencing electronic band structures and phonon modes. In this study, we systematically investigate the impact of lattice reconstruction on phonon renormalization in twisted bilayer graphene (TBLG) and graphene-hBN moiré superlattices, representing homo- and hetero-bilayer system, respectively. Using Raman spectroscopy, we identify distinct phonon behaviors 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°), attributed to moiré-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 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 (AB′), 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. These findings pave the way for controlling phononic, optoelectronic, and heat flow properties in next generation van der Waals heterostructures.