Evolution of Interlayer Coupling in Twisted MoS2 Bilayers

TL;DR

This study investigates how interlayer coupling in twisted MoS2 bilayers varies with twist angle, revealing that stacking configuration influences bandgap and phonon frequency due to steric effects, with implications for 2D material engineering.

Contribution

First experimental and theoretical analysis of twist-angle-dependent interlayer coupling evolution in MoS2 bilayers, highlighting steric effects as a key factor.

Findings

Bandgap size varies with twist angle, largest redshift in AA and AB stacking.

Out-of-plane phonon frequency depends on twist angle.

Interlayer separation differences are driven by steric repulsion.

Abstract

Van der Waals (vdW) coupling is emerging as a powerful method to engineer and tailor physical properties of atomically thin two-dimensional (2D) materials. In graphene/graphene and graphene/boron-nitride structures it leads to interesting physical phenomena ranging from new van Hove singularities1-4 and Fermi velocity renormalization5, 6 to unconventional quantum Hall effects7 and Hofstadter's butterfly pattern8-12. 2D transition metal dichalcogenides (TMDCs), another system of predominantly vdW-coupled atomically thin layers13, 14, can also exhibit interesting but different coupling phenomena because TMDCs can be direct or indirect bandgap semiconductors15, 16. Here, we present the first study on the evolution of interlayer coupling with twist angles in as-grown MoS2 bilayers. We find that an indirect bandgap emerges in bilayers with any stacking configuration, but the bandgap size varies appreciably with the twist angle: it shows the largest redshift for AA- and AB-stacked bilayers, and a significantly smaller but constant redshift for all other twist angles. The vibration frequency of the out-of-plane phonon in MoS2 shows similar twist angle dependence. Our observations, together with ab initio calculations, reveal that this evolution of interlayer coupling originates from the repulsive steric effects, which leads to different interlayer separations between the two MoS2 layers in different stacking configurations.