Ultrafast Charge-Doping via Photo-Thermionic Injection in van der Waals Devices

TL;DR

This paper demonstrates ultrafast charge doping in van der Waals heterostructures through photo-thermionic emission, enabling rapid and controllable charge injection in moiré devices with potential for advanced optoelectronic applications.

Contribution

It introduces a novel ultrafast photodoping mechanism via photo-thermionic emission in twisted WSe2 bilayers, expanding control over charge dynamics in 2D heterostructures.

Findings

Photo-thermionic emission causes rapid hole injection into the bilayer.

Injected holes can be tuned by excitation energy, fluence, and gate bias.

Persistent optical signatures indicate charge diffusion and local charge buildup.

Abstract

Van der Waals (vdW) heterostructures of two-dimensional (2D) materials have become a rich playground for the exploration of correlated quantum phases, and recent studies have begun to probe their non-equilibrium dynamics under femtosecond laser excitation. In a time-resolved experiment, optical excitation of the multilayer structure can lead not only to rich dynamic responses from the target layers, such as moir\'e interfaces, but also to additional device functionality from the layer degree of freedom. Here, we investigate optical excitation in a prototypical moir\'e device of dual-gated twisted WSe2 bilayers, with few-layer graphite gates and hexagonal boron nitride (hBN) spacers. We establish an ultrafast photodoping mechanism in the moir\'e bilayer from photo-thermionic emission of the graphite gates. Using transient reflectance experiments, we reveal photo-induced hole injection evidenced by: (i) a shift of gate voltages at which optical signatures of correlated insulators are observed, (ii) a persistent optical signature indicative of charge diffusion at microsecond timescales and local charge buildup from pulse-to-pulse accumulation, and (iii) photoinduced absorption due likely to transient formation of correlated insulators. We further demonstrate that the injected holes can be selectively controlled by tuning the excitation energy, fluence, and gate bias.