How Annealing and Charge Scavengers Affect Visible Emission from ZnO Nanocrystals

Simultaneous transmission infrared (IR) absorption and photoluminescence (PL) spectroscopies are used to reveal the correlation of free electron and defect densities during the stepwise annealing of ZnO nanocrystals in high vacuum. For increasing annealing temperatures between 700 and 1000 K, the free electron density increases with a negligible increase in PL yield. With increased annealing temperature above 1000 K, the free electron density decreases and the PL yield increases in inverse proportion. Accompanying the free electron loss are indications of increased bound charges and changes in the multiphonon bands in the infrared spectrum, which collectively suggest that structural change and defect formation accompanies the loss of free electrons and the increase in PL. Exposure of the previously annealed sample to electron (O2) and hole (MeOH) scavengers shows that the buildup of holes quenches visible emission, while additional electrons have a marginal effect on the PL yield. Given that certain neutral donors bind excitons and facilitate energy transfer to visible emitting sites, the buildup of free holes appears to quench PL intensity by ionizing those neutral donors.