Layer degree of freedom for excitons in transition metal dichalcogenides

TL;DR

This paper reveals how layer index acts as an additional degree of freedom in TMDCs, affecting exciton properties, and demonstrates the coexistence of intra- and inter-layer excitons with implications for optical behavior.

Contribution

It introduces the concept of layer degree of freedom in TMDC excitons and shows how it influences exciton states and optical properties in few-layer films.

Findings

Bright excitons are superpositions of layer-specific states.

Exciton binding energy remains high (~50 meV) regardless of thickness.

Layer-dependent photoluminescence broadening indicates energy transfer dynamics.

Abstract

Layered transition metal dichalcogenides (TMDCs) host a variety of strongly bound exciton complexes that control the optical properties in these materials. Apart from spin and valley, layer index provides an additional degree of freedom in a few-layer thick film. Here we show that in a few-layer TMDC film, the wavefunctions of the conduction and valence band edge states contributing to the K (K') valley are spatially confined in the alternate layers - giving rise to direct (quasi-)intra-layer bright exciton and lower-energy inter-layer dark excitons. Depending on the spin and valley configuration, the bright exciton state is further found to be a coherent superposition of two layer-induced states, one (E-type) distributed in the even layers and the other (O-type) in the odd layers. The intra-layer nature of the bright exciton manifests as a relatively weak dependence of the exciton binding energy on the thickness of the few-layer film, and the binding energy is maintained up to 50 meV in the bulk limit - which is an order of magnitude higher than conventional semiconductors. Fast stokes energy transfer from the intra-layer bright state to the inter-layer dark states provides a clear signature in the layer-dependent broadening of the photoluminescence peak, and plays a key role in the suppression of the photoluminescence intensity observed in TMDCs with thickness beyond monolayer.