Ultrafast exciton formation at the ZnO(10${\overline{\textbf{1}}}$0) surface

TL;DR

This study investigates ultrafast exciton formation at the ZnO surface using femtosecond spectroscopy, revealing rapid surface exciton creation, effects of excitation density, and stability for energy transfer applications.

Contribution

It provides new insights into the ultrafast dynamics of surface excitons in ZnO and their stability, which was not previously characterized in detail.

Findings

Surface excitons form within 200 fs after excitation.

High excitation densities reduce exciton formation due to Coulomb screening.

Surface excitons are stable against hydrogen-induced work function changes.

Abstract

We study the ultrafast quasiparticle dynamics in and below the ZnO conduction band using femtosecond time-resolved two-photon photoelectron spectroscopy. Above band gap excitation causes hot electron relaxation by electron-phonon scattering down to the Fermi level $E_\text{F}$ followed by ultrafast (200 fs) formation of a surface exciton (SX). Transient screening of the Coulomb interaction reduces the SX formation probability at high excitation densities near the Mott limit. Located just below the surface, the SX are stable with regard to hydrogen-induced work function modifications and thus the ideal prerequisite for resonant energy transfer applications.