Spin-Layer- and Spin-Valley-Locking in CVD-Grown AA'- and AB-Stacked Tungsten-Disulfide Bilayers

TL;DR

This study investigates the valleytronic and optical properties of CVD-grown WS2 bilayers with different stacking configurations, revealing how stacking influences spin-valley locking and optical dichroism, with implications for 2D material optoelectronics.

Contribution

It provides the first detailed analysis of how stacking order in WS2 bilayers affects their spin-valley locking and optical dichroism, combining experimental and theoretical insights.

Findings

AB stacking exhibits pronounced circular and linear dichroism.

Spin-valley locking persists in AB stacking, while AA' shows spin-layer-locking.

Experimental excitonic shifts agree with theoretical predictions.

Abstract

Valley-selective optical selection rules and a spin-valley locking in transition-metal dichalcogenide (TMDC) monolayers are at the heart of "valleytronic physics", which exploits the valley degree of freedom and has been a major research topic in recent years. In contrast, valleytronic properties of TMDC bilayers have not been in the focus so much by now. Here, we report on the valleytronic properties and optical characterization of bilayers of WS2 as a representative TMDC material. In particular, we study the influence of the relative layer alignment in TMDC homo-bilayer samples on their polarization-dependent optical properties. Therefore, CVD-grown WS2 bilayer samples have been prepared that favor either the inversion symmetric AA' stacking or AB stacking without inversion symmetry during synthesis. Subsequently, a detailed analysis of reflection contrast and photoluminescence spectra under different polarization conditions has been performed. We observe circular and linear dichroism of the photoluminescence that is more pronounced for the AB stacking configuration. Our experimental findings are supported by theoretical calculations showing that the observed dichroism can be linked to optical selection rules, that maintain the spin-valley locking in the AB-stacked WS2 bilayer, whereas a spin-layer-locking is present the inversion symmetric AA' bilayer instead. Furthermore, our theoretical calculations predict a small relative shift of the excitonic resonances in both stacking configurations, which is also experimentally observed.