Single and couple doping ZnO nanocrystals characterized by positron techniques

Zinc oxide (ZnO) nanocrystals have been synthesized using a mild hydrothermal process using low temperatures and pressures with the advantages of free growth catalyst, low cost and alternative technology. Positron annihilation lifetime spectroscopy and coincidence Doppler broadening (CDB) spectroscopic methods have been used to investigate the roles of single- and co-dopants and native defects of the ZnO nanocrystals controlled by the synthesis process. It is shown that single Ag¹⁺ and Pd²⁺ dopants occupy interstitial sites of the ZnO lattice and single Ru³⁺ doping replaces Zn vacancies substitutionally with a significant effect on the CDB momentum ratio curves when compared using ZnO as the reference spectrum. The co-doping of the ZnO lattice with (Sn⁴⁺ + Co²⁺) shows similar CDB ratios as Ru³⁺ single-doping. Also co-doping with (Ag¹⁺ + Pd²⁺) or (Ag¹⁺ + W⁶⁺) shows significant decreases in the band gap energy up to about 12.6% compared to single doping. The momentum ratio curves, referenced to undoped ZnO, indicate dopants in interstitial and substitutional sites. The presence of transition metal ions interstitially will trap electrons which resist the recombination of electrons and in turn affect the conductivity of the material. (paper)