Exciton Emission and Light‐Induced Charge Separation in Colloidal ZnO Nanocrystals

Adsorption of organic molecules at ZnO nanoparticle surfaces enables the transfer of energy or charge across resulting organic–inorganic interfaces and, consequently, determines the optoelectronic performance of ZnO‐based hybrids. We investigated adsorption‐induced changes with photoluminescence (PL) and electron paramagnetic resonance (EPR) spectroscopy on aqueous colloidal ZnO dispersions. Citrate and acetate ion adsorption increases or decreases radiative exciton annihilation at hν=3.3 eV and at room temperature, respectively. Searching for a correspondence between PL emission and the yield of trapped charge carriers originating from exciton separation (using photon energies of hν=4.6 eV and fluxes of ṄPh=1014 cm−2 s−1 for excitation), we found that there is a negligible fraction of paramagnetic products that originates from exciton separation. Upon polychromatic excitation with significantly higher photon fluxes (ṄPh=1016 cm−2 s−1), ZnO‐specific shallow defects trap unpaired electrons in citrate‐ and acetate‐functionalized samples. The adsorption‐dependent PL intensity changes and the excitation parameter dependent yield of separated charges (EPR) in colloidal ZnO nanoparticles underline that the distribution over the different exciton annihilation channels sensitively depends on interface composition and the intensity of the photoexcitation light.