Anomalous Ultrafast Thermalization of Photoexcited Carriers in Two-Dimensional Materials Induced by Orbital Coupling

TL;DR

This study investigates ultrafast carrier thermalization in 2D materials using rt-TDDFT, revealing distinct behaviors in graphene and PtTe2 that influence photo-thermionic emission and device efficiency.

Contribution

It demonstrates how orbital coupling differences cause anomalous thermalization in PtTe2, providing new insights for designing better photodetectors.

Findings

Graphene shows Fermi-Dirac thermalization consistent with experiments.

PtTe2 exhibits high-energy tails deviating from Fermi-Dirac distributions.

Orbital coupling differences explain the thermalization anomalies.

Abstract

Understanding the dynamics of photoexcited carriers is essential for advancing photoelectronic device design. Photon absorption generates electron-hole pairs, and subsequent scatterings can induce ultrafast thermalization within a picosecond, forming a quasi-equilibrium distribution with overheated electrons. The high-energy tail of this distribution enables carriers to overcome energy barriers, thereby enhancing quantum efficiency--a phenomenon known as photo-thermionic emission (PTE). Despite its importance, the onset and mechanisms of PTE remain under debate. Using real-time time-dependent density functional theory (rt-TDDFT), we investigate ultrafast carrier thermalization in two-dimensional materials graphene and PtTe2, and the results reveal distinct differences. In graphene, both electrons and holes thermalize into Fermi-Dirac distributions with good agreement to experiment, while PtTe2 exhibits anomalous high-energy tails for both electrons and holes, deviating significantly from Fermi-Dirac behavior. We attribute this anomaly to differences in orbital coupling between the two materials, from which we derive design principles for identifying optimal PTE candidates and, ultimately, improving photodetector performance.