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.