Interfacial Charge Transfer Circumventing Momentum Mismatch at Two-Dimensional van der Waals Heterojunctions

Abstract

ABSTRACT:Interfacial charge separation and recombination at heterojunctions of monolayer transition metal dichalcogenides (TMDCs) are of interest to two dimensional optoelectronic technologies.These processes can involve large changes in parallel momentum vector due to the confinement of electrons and holes to the K-valleys in each layer. Since these high-momentum valleys are usually not aligned across the interface of two TMDC monolayers, how parallel momentum is conserved in the charge separation or recombination process becomes a key question. Here we probe this question using the model system of a type-II heterojunction formed by MoS 2 and WSe 2 monolayers and the experimental technical of femtosecond pump-probe spectroscopy. Upon photo-excitation specifically of WSe 2 at the heterojunction, we observe ultrafast (<40 fs) electron transfer from WSe 2 to MoS 2 , independent of the angular alignment and, thus, momentum mismatch between the two TMDCs. The resulting interlayer charge transfer exciton decays via nonradiatively recombination, with rates varying by up to three-orders of magnitude from sample to sample, but with no correlation with inter-layer angular alignment. We suggest that the initial interfacial charge separation and the subsequent interfacial charge recombination processes circumvent momentum mismatch via excess electronic energy and via defect-mediated recombination, respectively.

Exciton broadening in WS2 /graphene heterostructures

“…We see that, within experimental uncertainty, the broadening does not have a strong dependence on graphene thickness up to tetra-layer graphene and ranges between 5 and 10 meV. Overall, the broadening is 4 to 5 times less than that seen in TMDC/TMDC heterostructures [42,51,61], which was attributed to interlayer (i.e. not static) charge transfer.…”

2D Layered Material‐Based van der Waals Heterostructures for Optoelectronics

“…Raman, PL, and absorption spectra, and exciton dynamics are the optimum methods to probe the optical properties of vdWHs due to the efficient, accurate, and nondestructive measurements . Javey and co‐workers fabricated vdWH based on monolayer WSe 2 and MoS 2 and a distinguished PL peak at 1.55 eV ( Figure a) was observed, which is lower than both the excitonic PL peaks at 1.87 and 1.64 eV for the monolayer MoS 2 and WSe 2 , respectively.…”

Interlayer hybridization and moiré superlattice minibands for electrons and excitons in heterobilayers of transition-metal dichalcogenides

“…By contrast, in twisted TMD heterobilayers such as those discussed in Secs. III to V, the ground-state excitons are formed by electron and hole states confined to opposite layers, known as interlayer excitons (IXs) 14,51,57,58 . The wave vector mismatch between the electron and hole band edges shown in, e.g., Fig.…”