Research On Preparation And Optical Properties Of Co-doped ZnO Nanocrystals

Nanoscale zinc oxide (ZnO) is an important wide band gap (Eg=3.37 eV) semiconductor material with excellent piezoelectric and optical properties. Due to the large exciton binding energy of 60 meV, which ensures the high efficient excitonic emission at room temperature, it is regarded as one of the most promising material for UV light-emitting diodes (LED), UV detector, gas sensors, flat panel displays, etc. The properties of ZnO nanoparticles can be modulated by appropriate element doping techniques. Co is a typical transition metal with a very rich electronic shell structure. It can effectively regulate the surface state of ZnO nano-structures and thus tune the properties and applications. In addition, Co²⁺ and Zn²⁺ is very close to the ionic radius. So the position of Zn²⁺ can be easily occupied by Co²⁺, which induce the modification of properties of ZnO matrix material. To research the effects of Co dopant on the optical properties of ZnO, in this paper, we prepared ZnO:Co nanoparticles via a co-precipitation method and a low-temperature aging method and investigated their microstructures morphologies and optical properties by XRD, FESEM, XPS, UV, PL .1. ZnO:Co nanoparticles were prepared by a co-precipitation method. The influences of calcining time, and the temperature, dropping order of reagents on the products were studied. The optimal conditons for the preparation of ZnO:Co nanoparticles were determined. ZnO:Co nanoparticles with different Co doping levels were prepared based on the above conditions. The influences of Co-doping on the microstructure and optical properties of ZnO were investigated in detail. Results show that cobalt ions, in the oxidation state of Co²⁺, replace Zn²⁺ ions in the ZnO lattice without changing its wurtzite structure. Co-doping has no significant effects on the morphology of the particles, but slightly reduces the particle size. Co-doping leads to an obvious decrease in the band gap, which is responsible for the red shifts of the wavelength in UV absorption and PL emissions.2. Low-temperature aging method were also carried out to fabricate ZnO:Co nanoparticles. The influences of the molar ratio of Zn²⁺/OH-, the dropping order of reagents and the prestiring temperature on the products were studied to determine the optimum conditions. Based on the optimum way, ZnO:Co nanoparticles with different Co doping levels were prepared by a one-step and a two-step methods respectively. The structure, morphology and optical properties of ZnO:Co nanoparticles were characterized. Results show that Co²⁺ replaces Zn²⁺ ions into the ZnO lattice using the above two solution methods. The doping behavior occurs on the surface of the particles, and does not change the general morphology of ZnO. The band gap decreasing, leads to the red shifts of the wavelength in UV absorption. The UV emission at 380 nm has no obvious change. The spectra shows a new emission peak at 450 nm.While the visible luminescence (480～600 nm) disappeared, which is due to the passivation layer formed on the surface of crystals.