Electrical properties of gas-phase synthesized nanoparticles

Zinc oxide (ZnO) doped with aluminum receives increasing attention for being an alternative material for the established but much more expensive indium tin oxide (ITO) due to the fact, that it has comparable electrical and optical properties. The electrical properties of mechanically compacted pellets prepared from nanosized ZnO powders are investigated using impedance spectroscopy. The impedance of the samples is measured in hydrogen and in synthetic air between room temperature and 400¿C. In both atmospheres, the measurements show two different electrical transport processes depending on the temperature and the doping level. In synthetic air, the conductivity increases for doping concentrations up to 7.74% of aluminum and collapses for higher doping levels. In hydrogen atmosphere, the conductivity decreases with rising doping level of Al. This behavior can be explained by generation of free charge carriers due to the incorporation of hydrogen and doping with aluminum, respectively. At higher temperatures and at high doping concentrations, scattering processes at grain boundaries as well as lattice defects increasingly affect the charge carrier transport processes leading to a decreasing overall conductivity. The electrical conductivity shows reversible behavior when the atmosphere is changed from hydrogen to ambient conditions and back. To replace ITO in applications, transparent conductive layers with good electrical and optical properties are required. ZnO dispersions are prepared and printed on pre-structured substrates by ink-jet printing to investigate the electrical and sensing properties of printed films. The properties are measured without any annealing steps from room temperature up to 200¿C in ambient conditions and in hydrogen atmosphere using impedance spectroscopy. Compared to the measurements in air, the resistance in hydrogen decreases by a factor of five even at room temperature. The ink-jet printed ZnO films with nanosized particles can be used as sensor without any annealing or post-processing for sensing.