# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1905.02391