A versatile electrospray method was utilized for deposition of thin ZnO films doped with Co (5%) (CZO) or co-doped with Co (2.5%) and Al (2.5%) (CAZO). Thin polycrystalline films with approximate thickness of 200 nm and high transmittance (more than 80%) were obtained. No additional XRD peaks due to dopant impurities or dopant oxides were observed. The cobalt doping led to decrease in grain size and increase in crystallite size from 22 nm to 29 nm in the (101) direction. Smaller changes were observed for the CAZO films. Surface roughness of the films was measured using a 3D optical profiler. Surface roughness of the doped films was from 5 nm to 9 nm higher than that of the pure films. Refractive index, extinction coefficient and thickness of the films were calculated using ellipsometric measurements and were further used for determination of optical band gap and Urbach energy. Refractive index and optical band gap increased with doping from 1.86 and 3.29 eV for pure ZnO to 2.00 and 3.35 eV for CZO and 1.97 and 3.33 eV for CAZO films. Through calculation of Urbach energy (119 meV for pure ZnO, 236 meV for CZO and 138 meV for CAZO), it was demonstrated that doping leads to an increase in structural disorder, most pronounced in the case of Co doping.

This study focuses on the influence of electrospray deposition parameters on the morphology, topography, optical and sensing properties of ZnO films deposited on gold electrodes of quartz crystal resonators. The substrate temperature, precursor feed rate and emitter’s voltage were varied. Zinc acetate dehydrate dissolved in a mixture of deionized water, ethanol and acetic acid was used as a precursor. The surface morphology and average roughness of the films were studied by scanning electron microscopy (SEM) and 3D optical profilometry, respectively, while the optical properties were investigated by diffuse reflectance and photoluminescence measurements. The sensing response toward ammonia was tested and verified by the quartz crystal microbalance (QCM) method. The studies demonstrated that electrospray deposition parameters strongly influence the surface morphology, roughness and gas sensing properties of the films. The deposition parameters were optimized in order for the highest sensitivity toward ammonia to be achieved. The successful implementation of the electrospray method as a simple, versatile and low-cost method for deposition of ammonia-sensitive and selective ZnO films used as a sensing medium in QCM sensors was demonstrated and discussed.

Polycrystalline ZnO thin films were prepared on silicon substrates using electrospray method with vertical setup. Water and ethanol were used as solvents for zinc acetate dehydrate and no postdeposition annealing was required for formation of ZnO. The influence of substrate temperature in the range of 150–250°C on surface morphology and roughness was studied by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and optical profilometry. An improvement of surface quality and smoothing of the films with temperature were obtained. X-ray diffraction measurements revealed that, at all investigated substrate temperatures, the films were polycrystalline with crystallites’ sizes decreasing with temperature. Besides, the preferred crystal orientation varies with the substrate temperature. The analysis of surface chemical composition and oxidation state was performed with X-ray photoelectron spectroscopy (XPS). It was shown that, at substrate temperature of 200°C, the deposited ZnO films were closest to the stoichiometric ones. In general, the films at 150°C were oxygen-deficient, while at other studied temperatures, the films had excess of oxygen more pronouncedly at 200°C. Spectral ellipsometric measurements confirmed that the structural disorder is the highest at 150°C and improves with temperature. Refractive indexes for films at 200°C and 250°C are almost the same, 1.97 and 1.93, respectively, at wavelength of 600 nm, while for the sample deposited at 150°C, the refractive index is substantially lower, 1.67. The optical band gap is slightly influenced by the substrate temperature: 3.27 eV at 150°C and 3.32 eV at 200°C.