Momentum Conserved Ultrafast Charge Transfer Dynamics of Interlayer Excitons in vdW Heterostructures

TL;DR

This study reveals that momentum conservation significantly influences ultrafast charge transfer in van der Waals heterostructures, with aligned layers exhibiting much faster transfer rates than misaligned ones, impacting 2D device efficiency.

Contribution

It demonstrates the critical role of momentum conservation in charge transfer dynamics of TMDC heterostructures, using advanced electron energy loss spectroscopy and microscopy techniques.

Findings

Charge transfer rate is about ten times faster in aligned heterostructures.

Momentum conservation governs charge transfer dynamics in 2D heterostructures.

Layer rotation angle critically affects charge transfer efficiency.

Abstract

Heterostructures comprising van der Waals (vdW) stacked transition metal dichalcogenide (TMDC) monolayers are a fascinating class of two-dimensional (2D) materials with unique properties. The presence of interlayer excitons, where the electron and the hole remain spatially separated in the two layers due to ultrafast charge transfer, is an intriguing feature of these heterostructures. Inevitably, the efficiency of 2D heterostructure devices is critically dependent on the charge transfer dynamics. However, the role of the relative rotation angle of the constituent layers on this charge transfer dynamics is hitherto unknown. Here, we investigate MoS$_2$/WSe$_2$ vdW heterostructures (hMWs) using monochromated low-loss electron energy loss (EEL) spectroscopy combined with aberration-corrected scanning transmission electron microscopy (STEM), and report that momentum conservation is a critical factor in the charge transfer dynamics of TMDC vdW heterostructures. The low-loss EEL spectra of the heterostructures with various rotation angles reveal that the charge transfer rate can be about one order-of-magnitude faster in the aligned (or anti-aligned) case than the misaligned cases. These results provide a deeper insight into the role of the fundamental principle of momentum conservation in 2D vdW heterostructure charge transfer dynamics.