Recently, Professor Jie Chen's team in the School of Physical Sciences and Engineering published a review article entitled "Phonon physics in twisted two-dimensional materials" in Applied Physics Letters, a prestigious journal of the American Physical Society.

Twisted two-dimensional (2D) materials have attracted wide attention in both academic studies and engineering applications, thanks to their unique and adjustable physical and chemical properties. Compared with the strong intralayer covalent bonding, the relatively weak interlayer van der Waals (vdW) interaction in 2D materials makes it possible to stack and assemble a vdW homogeneous/heterogeneous system layer by layer, which provides a powerful means for designing and manufacturing 2D stacking materials with different structures and peculiar properties. With the continuous advancements in the synthesis technology and characterization techniques, a growing number of twisted 2D materials have been prepared.

As the most extensively investigated twisted 2D materials so far, twisted graphene has been the focus of recent experimental and theoretical studies. A series of characteristic peaks caused by interlayer coupling phonon modes and moire phonons sensitive to twisted angle have been observed in previous studies on the Raman spectra of twisted bilayer graphene, which has greatly inspired the interest in studying the angle-dependent phonon properties in twisted few-layer graphene.

This paper reviews the recent developments and discoveries on phonon properties in twisted two-dimensional stacking homogeneous and heterogeneous systems and focuses on the impacts of the interlayer twisted angle on phonon dispersion. Meanwhile, we introduced the recent research on the influence of the interlayer twisted angle on phonon transport behavior along the in-plane and out-of-plane directions. In addition, the theoretical and experimental open questions and challenges faced in the phonon characteristics of twisted two-dimensional materials are discussed, and some possible solutions are put forward.

FIG. 1. Phonon energy dispersions in (a) single layer graphene, (b) AA-stacked bilayer graphene, and (c) 2D phonon DOS for the frequency range, and the illustration is 2D phonon DOS as a function of the extended frequency range; (d) Stokes/anti-Stokes Raman spectra in the shear (C) and interlayer breathing modes (LBMs) peak region in the Brillouin center zone for twisted four-layer graphene.

FIG. 2. (a) Snapshot figures with different interlayer twisted angles of graphene/h-BN vdW heterostructures. (b) Z position deviation (D) for the supported single-layer graphene. (c) Room temperature j. (d) Phonon relaxation time vs twisted angle.

FIG. 3. (a) The out-of-plane thermal conductance for the h-BN/graphene/h-BN vdW heterostructures, where NL denotes the number of h-BN layers on each side of graphene. (b) Surface fluctuation in graphene layer vs the interlayer twisted angle. (c) The spectral phonon transmission for various twisted angles at 500 K. (d) Out-of-plane vibrational DOS in graphene and interfacial h-BN layer at 500 K

Weijun Ren, a PhD student at Tongji University, is the first author of the article, and Professor Jie Chen of Tongji University and Professor Gang Zhang of the High Performance Computing Research Centre of the Singapore Science and Technology Research Agency are co-corresponding authors.

Title:Phonon physics in twisted two-dimensional materials

Link:https://doi.org/10.1063/5.0106676