Disentangling the Evolution of Electrons and Holes in photoexcited ZnO nanoparticles

TL;DR

This study investigates the ultrafast dynamics of electrons and holes in photoexcited ZnO nanoparticles using spectroscopy and simulations, revealing rapid charge carrier processes and lattice interactions occurring within femtoseconds to picoseconds.

Contribution

It combines ultrafast spectroscopy with ab-initio simulations to elucidate the distinct timescales and mechanisms of charge carrier evolution in ZnO nanoparticles.

Findings

Electrons cool and excitons form in <500 fs.

Holes migrate and get trapped at oxygen vacancies within ~1.4 ps.

Impulsive hole trapping causes ultrafast lattice expansion.

Abstract

The evolution of charge carriers in photoexcited room temperature ZnO nanoparticles in solution is investigated using ultrafast ultraviolet photoluminescence spectroscopy, ultrafast Zn K-edge absorption spectroscopy and ab-initio molecular dynamics (MD) simulations. The photoluminescence is excited at 4.66 eV, well above the band edge, and shows that electron cooling in the conduction band and exciton formation occur in <500 fs, in excellent agreement with theoretical predictions. The X-ray absorption measurements, obtained upon excitation close to the band edge at 3.49 eV, are sensitive to the migration and trapping of holes. They reveal that the 2 ps transient largely reproduces the previously reported transient obtained at 100 ps time delay in synchrotron studies. In addition, the X-ray absorption signal is found to rise in ~1.4 ps, which we attribute to the diffusion of holes through the lattice prior to their trapping at singly-charged oxygen vacancies. Indeed, the MD simulations show that impulsive trapping of holes induces an ultrafast expansion of the cage of Zn atoms in <200 fs, followed by an oscillatory response at a frequency of ~100 cm-1, which corresponds to a phonon mode of the system involving the Zn sub-lattice.