Sub-nm range momentum-dependent exciton transfer from a 2D semiconductor to graphene

TL;DR

This study investigates exciton transfer mechanisms in 2D TMD/graphene heterostructures, revealing a sub-nanometer, momentum-dependent transfer process primarily governed by charge tunneling, with implications for optoelectronic applications.

Contribution

It provides direct experimental evidence of ultrafast exciton transfer dynamics and clarifies the roles of tunneling and dipolar interactions in 2D heterostructures.

Findings

Exciton transfer time is approximately 2.5 ps at cryogenic temperature.

Transfer vanishes with a 1 nm dielectric spacer, indicating tunneling dominance.

Dipolar interactions have negligible impact on bright excitons but affect hot excitons.

Abstract

Van der Waals heterostructures made from atomically thin transition metal dichalcogenides (TMD) and graphene have emerged as a building block for optoelectronic devices. Such systems are also uniquely poised to investigate interfacial coupling as well as photoinduced charge and energy transfer in the 2D limit. Recent works have revealed efficient photoluminescence quenching and picosecond transfer in TMD/graphene heterostructures. However, key questions regarding the transfer mechanisms remain. Here, employing time-resolved photoluminescence spectroscopy with 1~ps resolution in MoSe$_2$ monolayer directly coupled to a few-layer ``staircase-like'' graphene flake, we consistently observe an exciton transfer time of $\approx 2.5~\mathrm{ps}$ at cryogenic temperature that is marginally affected by the number of graphene layers. Remarkably, exciton transfer vanishes in samples consisting in an MoSe$_2$ monolayer separated from graphene by a thin dielectric spacer of hexagonal boron nitride, as soon as the spacer thickness reaches 1~nm. These results suggest that charge tunnelling processes govern exciton dynamics. Other mechanisms mediated the dipolar interactions (F\"orster-type energy transfer) have no measurable impact on bright excitons (with near-zero center of mass momentum) but may accelerate the relaxation of finite momentum ``hot'' excitons, leading to larger photoluminescence quenching than anticipated based on the measurements of the photoluminescence decay rates. Our work provides important insights into charge and energy transfer in van der Waals materials with direct implications for energy harvesting and funneling.