The End Is Different than The Middle: Spatially Dependent Dynamics in ZnO Rods Observed by Femtosecond Pump-Probe Microscopy

Pump-probe transient absorption microscopy was used to follow the electron-hole (e-h) recombination dynamics at different points within individual needle-shaped ZnO-rods to characterize spatial differences in dynamical behavior. The results from pump-probe experiments are correlated with spatially resolved ultrafast emission measurements, and scanning electron microscopy provides structural details. Dramatically different e-h recombination dynamics are observed in the narrow tips compared to the interior, with the ends exhibiting a greater propensity for electron-hole plasma (EHP) formation and faster recombination of carriers across the band gap that stem from a physical confinement of the charge carriers. In the interior of the rod, a greater fraction of the e-h recombination is trap-mediated and occurs on a significantly longer time scale.