Tailoring excitonic complexes in layered materials

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 lm. While in the 1H monolayer TMD inversion symmetry is broken, and the reflection symmetry is maintained but, in the bilayer, it is reversed. Trions are excitonic species with a positive or negative charge, and thus, unlike neutral excitons, the flow of trions can generate a net detectable charge current. Trions under favourable doping conditions can be created in a coherent manner using resonant excitation. The neutral biexciton (bound state of two excitons) can assemble further to create a charged state with another electron or hole. Generally, in W-based TMDs these ve-particle quinton states dominate the population density and this can also be engineered to produce photocurrent at cryogenic temperature. In the firrst work, we show that in a few-layer TMDC lm, the wave functions of the conduction and valence-band-edge states contributing to the K(K0) valley are spatially con ned in the alternate layers - giving rise to direct (quasi-)intralayer bright exciton and lower-energy interlayer dark excitons. Depending on the spin and valley con figuration, 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 intralayer nature of the bright exciton manifests as a relatively weak dependence of the exciton binding energy on the thickness of the few-layer lm, and the binding energy is maintained up to 50 meV in the bulk limit - which is an order of magnitude higher than conventional semiconductors...