Ultrafast Relaxation and Charge Transfer in ZnO-Ag Nanoheterodimers

Abstract By combining plasmonic and semiconducting components, metal/semiconductor nanosystems offer unique opportunities to enhance charge separation and energy transfer across interfaces, with interesting prospects in photocatalysis, energy conversion, and photodetection. We report the synthesis of ZnO-Ag nanoheterodimers (NHDs) based on an optofluidic approach that controls the photodeposition of single silver nanodots onto ZnO nanoparticles of controlled morphology and size. Femtosecond transient absorption spectroscopy reveals efficient charge transfer from Ag to ZnO, accompanied by NHD-specific picosecond-scale relaxation channels with decay times of 1–2 ps, distinct from the electron–phonon relaxation and electron–hole recombination observed in the isolated components. Our results reveal the importance of morphology: NHDs based on longer ZnO nanorods exhibit slower dynamics than those based on shorter rods or spherical ZnO particles. The observed morphological dependence does not originate from a change in the ZnO-Ag contact area, which varies opposite to the measured rate constants, but instead reflects geometric confinement of carriers within the ZnO domain: in smaller NHDs, both injected electrons and residual holes remain spatially close to interfacial recombination sites. These findings demonstrate the critical role of ZnO morphology and carrier diffusion paths to the metal–semiconductor contact in tuning charge relaxation dynamics in nanostructures.