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

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-WSe2/graphene heterostructure. Depending on the nature of the optically prepared state, we find the different dominating transfer mechanisms: while electron injection from graphene to WSe2 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 WSe2. 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 WSe2 to valence band transitions in graphene. Microscopic calculations find interfacial dipole-monopole coupling underlying the Meitner-Auger energy transfer to dominate over conventional Förster- 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.

Dipolar Coupling at Interfaces of Ultrathin Semiconductors, Semimetals, Plasmonic Nanoparticles, and Molecules

“…[ 51 ] As a new effect beyond the electron‐tunneling process, the combination of a gapped semiconductor and a semimetal leads to the intriguing interfacial Meitner–Auger energy transfer corresponding to a dipole–monopole coupling, as observed in ref. [51]. The process we address is sketched in Figure 6b.…”

“…To study the Meitner–Auger energy transfer, Figure 6b, we derive the equation of motion for the incoherent () exciton occupation in the K ‐valley , see Equation () [ 51 ] …”

“…[ 12 ] Also for detuning below the optical bandgap, a substantial amount hot excitons with large center‐of‐mass momentum is formed, which can contribute to the interfacial Meitner–Auger energy transfer. [ 12,51 ]…”

“…3) Other widely considered heterostructures are interfaces of TMDC monolayers and graphene, where many studies have focused on the individual role of charge and energy transfer between both layers. [ 45–51 ] We discuss a new type of interlayer energy transfer in Section 5, namely the Meitner–Auger coupling. [ 51 ] Here, the energy is transferred from the TMDC layer to graphene via non‐radiative recombination of a TMDC exciton which excites an intraband plasmon in graphene.…”

“…[ 45–51 ] We discuss a new type of interlayer energy transfer in Section 5, namely the Meitner–Auger coupling. [ 51 ] Here, the energy is transferred from the TMDC layer to graphene via non‐radiative recombination of a TMDC exciton which excites an intraband plasmon in graphene.…”