Interlayer charge transport controlled by exciton\u2013trion coherent coupling

Sangeeth Kallatt,Sarthak Das,Suman Chatterjee,Kausik Majumdar

doi:10.1038/s41699-019-0097-3

Abstract

The possibility of electrical manipulation and detection of a charged exciton (trion) before its radiative recombination makes it promising for excitonic devices. Using a few-layer graphene/monolayer WS2/monolayer graphene vertical heterojunction, we report interlayer charge transport from top few-layer graphene to bottom monolayer graphene, mediated by a coherently formed trion state. This is achieved by using a resonant excitation and varying the sample temperature; the resulting change in the WS2 bandgap allows us to scan the excitation around the exciton–trion spectral overlap with high spectral resolution. By correlating the vertical photocurrent and in situ photoluminescence features at the heterojunction as a function of the spectral position of the excitation, we show that (1) trions are anomalously stable at the junction even up to 463 K due to enhanced doping, and (2) the photocurrent results from the ultrafast formation of a trion through exciton–trion coherent coupling, followed by its fast interlayer transport. The demonstration of coherent formation, high stabilization, vertical transportation, and electrical detection of trions marks a step toward room-temperature trionics.

Highlights

A trion is a charged exciton constituting of two electrons and one hole (X−) or two holes and one electron (X+), which exhibits a binding energy on the order of 30 meV3,7 in monolayer TMDCs—a number that is about an order of magnitude higher than that which is observed in III–V semiconductor quantum wells.[8–10]
Since the binding energy is higher than kBT at 300 K, trions in monolayer TMDCs are stable at room temperature
The radiative lifetime of a trion has been reported in a broad range from a few picoseconds to tens of picoseconds,[11–14] but it is longer than that of the exciton,[11,15–17] providing us time for electrical manipulation before it radiatively recombines in a spontaneous manner

Summary

Introduction

Excitons are bound pairs of an electron and a hole, and play a crucial role in a variety of optoelectronic devices. In this work, exploiting efficient coherent coupling between the exciton and trion driven by resonant excitation, we demonstrate a strong photocurrent in a few-layer graphene (FLG)/monolayer WS2/monolayer graphene (MLG) vertical junction. The photocurrent results from electron hopping from top FLG to bottom MLG mediated through the coherently formed trion state in the WS2 sandwiched layer.

Results

Conclusion