Annealing effects on the physical properties of electrodeposited ZnO/CdSe core-shell nanowire arrays

Annealing effects on the physical properties of ZnO/CdSe core-shell nanowires have been analyzed in the range of 150-400 degrees C. Annealing at temperatures higher than 350 degrees C induce a structural transition in nanocrystalline CdSe nanowire shell, from cubic zinc blende to hexagonal wurtzite structure. This transition takes place at temperatures higher than those in CdSe bulk crystals (95 degrees C), underlying that the CdSe structure is determined not only by the temperature but also by the crystal size. The role of free surface energy in low-dimensional CdSe systems is emphasized. Our hypothesis explains the behavior observed in CdSe nanowire shell, as well as the results from other authors in CdSe nanocrystals. Annealing at temperatures >= 350 degrees C also result in the increase of particle size constituting the CdSe nanowire shell from 3 nm for the as-deposited to >= 9 nm. This considerably enhances the electronic properties of the ZnO/CdSe nanowires. The improvement may result from an easier charge carrier transport in CdSe nanowire shell promoted by the loss of quantum confinement, which is revealed by optical spectroscopy. High external quantum efficiencies (> 70%) in ferro-/ferricyanide solutions have been obtained for ZnO/CdSe core-shell nanowire arrays annealed at 400 degrees C, demonstrating their potential in nanostructured solar cells.