Photoexcited Small Polaron Formation in Goethite ({\alpha}-FeOOH) Nanorods Probed by Transient Extreme Ultraviolet Spectroscopy

TL;DR

This study investigates how ligand field strength affects small polaron formation in goethite nanorods using transient extreme ultraviolet spectroscopy, revealing how excitation energy influences formation time and probability.

Contribution

It provides new insights into the dependence of small polaron formation dynamics on excitation energy and ligand environment in goethite nanorods.

Findings

Polaron formation time increases with excitation photon energy.

Polaron formation probability remains constant at about 85%.

Ligand and crystal structure modifications could control polaron formation.

Abstract

Small polaron formation limits the mobility and lifetimes of photoexcited carriers in metal oxides. As the ligand field strength increases, the carrier mobility decreases, but the effect on the photoexcited small polaron formation is still unknown. Extreme ultraviolet transient absorption spectroscopy is employed to measure small polaron formation rates and probabilities in goethite ({\alpha}-FeOOH) crystalline nanorods at pump photon energies from 2.2 to 3.1 eV. The measured polaron formation time increases with excitation photon energy from 70 {\pm} 10 fs at 2.2 eV to 350 {\pm} 30 fs at 2.6 eV, whereas the polaron formation probability (85 {\pm} 10%) remains constant. By comparison to hematite ({\alpha}-Fe2O3), an oxide analog, the role of ligand composition and metal center density in small polaron formation time is discussed. This work suggests that incorporating small changes in ligands and crystal structure could enable the control of photoexcited small polaron formation in metal oxides.