Optical and Sensing Properties of Cu Doped ZnO Nanocrystalline Thin Films

Undoped and Cu doped ZnO films of two different molarities deposited by spray pyrolysis using zinc nitrate and cupric chloride as precursors show polycrystalline nature and hexagonal wurtzite structure of ZnO. The crystallite size varies between 10 and 21 nm. Doping increases the transmittance of the films whereas the optical band gap of ZnO is reduced from 3.28 to 3.18 eV. With increment in doping the surface morphology changes from irregular shaped grains to netted structure with holes and then to net making needle-like structures which lends gas sensing characteristics to the films. Undoped ZnO shows maximum sensitivity at 400 degrees C for higher concentration of CO2. The sensitivity of Cu doped sample is maximum at 200 degrees C for all CO2 concentrations from 500 to 4000 ppm.