Sol–gel Derived ZnAl 2 O 4 Nanopowders Co-Doped with Cr 3+ , Er 3+ and Yb 3+ Ions

Three series of nanocrystalline phosphors based on ZnAl 2 O 4 spinel co-doped with Cr 3+ , Er 3+ and Yb 3+ ions in the dopant concentration ratios 1:1:3, 1:1:5 and 1:1:7 were obtained by sol–gel citrate method. Unlike conventional high-temperature solid-state synthesis, the applied method makes it possible to obtain single-phase cubic spinel materials for all three series at a minimal annealing temperature of 700 °C. In this way, fine powders with an average grain size of 31–33 nm were obtained. The lattice parameters of the phosphors triply co-doped with Cr 3+ , Er 3+ and Yb 3+ ions are in the range of 8.094–8.087 Å, systematically higher than the corresponding values for single Cr-doped ZnAl 2 O 4 , thus proving incorporation of larger Er and Yb ions into the ZnAl 2 O 4 spinel structure. Photoluminescence properties of ZnAl 2 O 4 :Cr,Er,Yb nanocrystalline phosphors were studied at the ambient conditions. The phosphors exhibit a characteristic photoluminescence related to d–d transitions in Cr 3+ ions and up-conversion emission of Er 3+ ions. The intensity of these emissions is dependent on the concentrations ratio of the activators and the annealing temperature.