Polytypism and Unexpected Strong Interlayer Coupling of two-Dimensional Layered ReS2

TL;DR

This study reveals unexpected strong interlayer coupling and polytypism in multilayer ReS2, an anisotropic 2D material, challenging previous assumptions of weak interlayer interactions and demonstrating stable stacking orders through spectroscopy and theoretical calculations.

Contribution

It uncovers two stable stacking orders in multilayer ReS2 and demonstrates unexpectedly strong interlayer coupling using spectroscopy and first-principles calculations.

Findings

Two stable stacking orders (aa and a-b) identified in multilayer ReS2.

Strong interlayer coupling with force constants 55-90% of MoS2.

Observation of multiple interlayer shear modes indicating anisotropic and isotropic stacking.

Abstract

The anisotropic two-dimensional (2D) van der Waals (vdW) layered materials, with both scientific interest and potential application, have one more dimension to tune the properties than the isotropic 2D materials. The interlayer vdW coupling determines the properties of 2D multi-layer materials by varying stacking orders. As an important representative anisotropic 2D materials, multilayer rhenium disulfide (ReS2) was expected to be random stacking and lack of interlayer coupling. Here, we demonstrate two stable stacking orders (aa and a-b) of N layer (NL, N>1) ReS2 from ultralow-frequency and high-frequency Raman spectroscopy, photoluminescence spectroscopy and first-principles density functional theory calculation. Two interlayer shear modes are observed in aa-stacked NL-ReS2 while only one interlayer shear mode appears in a-b-stacked NL-ReS2, suggesting anisotropic-like and isotropic-like stacking orders in aa- and a-b-stacked NL-ReS2, respectively. The frequency of the interlayer shear and breathing modes reveals unexpected strong interlayer coupling in aa- and a-b-NL-ReS2, the force constants of which are 55-90% to those of multilayer MoS2. The observation of strong interlayer coupling and polytypism in multi-layer ReS2 stimulate future studies on the structure, electronic and optical properties of other 2D anisotropic materials.