Excitonic signatures of ferroelectric order in parallel-stacked MoS$_2$

TL;DR

This study directly probes ferroelectricity in few-layer MoS₂ using optical spectroscopy, revealing how excitonic properties are affected by ferroelectric order and enabling optical control of polarization states.

Contribution

It provides the first direct optical evidence of ferroelectricity in 3R-MoS₂ and uncovers the asymmetric response of excitons to ferroelectric order based on stacking configurations.

Findings

Intralayer excitons are sensitive to ferroelectric order.

Interlayer hybridized excitons remain decoupled from ferroelectricity.

Optical readout and control of multi-state polarization achieved.

Abstract

Interfacial ferroelectricity, prevalent in various parallel-stacked layered materials, allows switching of out-of-plane ferroelectric order by in-plane sliding of adjacent layers. Its resilience against doping potentially enables next-generation storage and logic devices. However, studies have been limited to indirect sensing or visualization of ferroelectricity. For transition metal dichalcogenides, there is little knowledge about the influence of ferroelectric order on their intrinsic valley and excitonic properties. Here, we report direct probing of ferroelectricity in few-layer 3R-MoS$_2$ using reflectance contrast spectroscopy. Contrary to a simple electrostatic perception, layer-hybridized excitons with out-of-plane electric dipole moment remain decoupled from ferroelectric ordering, while intralayer excitons with in-plane dipole orientation are sensitive to it. Ab initio calculations identify stacking-specific interlayer hybridization leading to this asymmetric response. Exploiting this sensitivity, we demonstrate optical readout and control of multi-state polarization with hysteretic switching in a field-effect device. Time-resolved Kerr ellipticity reveals a direct correspondence between spin-valley dynamics and stacking order.