Controlling carrier densities in photochemically reduced colloidal ZnO nanocrystals: size dependence and role of the hole quencher.

Photodoped colloidal ZnO nanocrystals are model systems for understanding the generation and physical or chemical properties of excess delocalized charge carriers in semiconductor nanocrystals. Typically, ZnO photodoping is achieved photochemically using ethanol (EtOH) as a sacrificial reductant. Curiously, different studies have reported over an order of magnitude spread in the maximum number of conduction-band electrons that can be accumulated by photochemical oxidation of EtOH. Here, we demonstrate that this apparent discrepancy results from a strong size dependence of the average maximum number of excess electrons per nanocrystal, <nmax>. We demonstrate that <nmax> increases in proportion to nanocrystal volume, such that the maximum carrier density remains constant for all nanocrystal sizes. <nmax> is found to be largely insensitive to precise experimental conditions such as solvent, ligands, protons or other cations, photolysis conditions, and nanocrystal or EtOH concentrations. These results reconcile the broad range of literature results obtained with EtOH as the hole quencher. Furthermore, we demonstrate that <nmax> depends on the identity of the hole quencher, and is thus not an intrinsic property of the multiply reduced ZnO nanocrystals themselves. Using a series of substituted borohydride hole quenchers, we show that it is possible to increase the nanocrystal carrier densities over 4-fold relative to previous photodoping reports. When excess lithium and potassium triethylborohydrides are used in the photodoping, formation of Zn(0) is observed. The relationship between metallic Zn(0) formation and ZnO surface electron traps is discussed.