Observation of ultrafast interfacial Meitner-Auger energy transfer in a van der Waals heterostructure

TL;DR

This study uncovers ultrafast interfacial energy transfer mechanisms in van der Waals heterostructures, revealing a Meitner-Auger process that could inform future hot-carrier device designs.

Contribution

It provides the first layer- and momentum-resolved observation of Meitner-Auger energy transfer in a monolayer WSe₂/graphene heterostructure, supported by microscopic calculations.

Findings

Identification of Meitner-Auger energy transfer as a dominant mechanism

Distinction between electron injection and exciton-based energy transfer

Microscopic coupling analysis showing dipole-monopole interactions dominate

Abstract

Atomically thin layered van der Waals heterostructures feature exotic and emergent optoelectronic properties. With growing interest in these novel quantum materials, the microscopic understanding of fundamental interfacial coupling mechanisms is of capital importance. Here, using multidimensional photoemission spectroscopy, we provide a layer- and momentum-resolved view on ultrafast interlayer electron and energy transfer in a monolayer-WSe$_2$/graphene heterostructure. Depending on the nature of the optically prepared state, we find the different dominating transfer mechanisms: while electron injection from graphene to WSe$_2$ is observed after photoexcitation of quasi-free hot carriers in the graphene layer, we establish an interfacial Meitner-Auger energy transfer process following the excitation of excitons in WSe$_2$. By analysing the time-energy-momentum distributions of excited-state carriers with a rate-equation model, we distinguish these two types of interfacial dynamics and identify the ultrafast conversion of excitons in WSe$_2$ to valence band transitions in graphene. Microscopic calculations find interfacial dipole-monopole coupling underlying the Meitner-Auger energy transfer to dominate over conventional F\"orster- and Dexter-type interactions, in agreement with the experimental observations. The energy transfer mechanism revealed here might enable new hot-carrier-based device concepts with van der Waals heterostructures.