Anomalous interlayer vibrations in few-layer InSe induced by weak intralayer bonding

Abstract

Atomically thin InSe is a promising semiconductor that possesses exceptional plasticity, high electron mobility, and wide bandgap tunability, which are thought to be highly sensitive to interlayer coupling. Since the interlayer vibration modes can provide direct access to the interlayer coupling strength, in this study, we systematically investigated the interlayer modes in few-layer InSe using low-frequency Raman spectroscopy. We found that the commonly used linear chain model (LCM), which treats the single layer as a rigid entity, is inadequate in accurately describing the frequencies of interlayer shear modes in InSe due to the influence of weak in-plane intralayer In–In bonding. This issue can be addressed with a modified model that accounts for both the in-plane interlayer coupling between InSe layers and the in-plane intralayer interaction within InSe layers. However, the out-of-plane intralayer In–In bonding is strong enough so that it has negligible impact on the frequency of the interlayer layer-breathing modes, which can be well understood by the LCM. Our study reveals how the weak intralayer bonding in two-dimensional materials gives a non-negligible contribution to the corresponding interlayer vibrations.