Low temperature NO2 gas sensing with ZnO nanostructured by laser interference lithography

ZnO conductometric gas sensors have been widely studied due to their good sensitivity, cost-efficiency, long stability and simple fabrication. This work is focused on NO₂ sensing, which is a toxic and irritating gas. The developed sensor consists of interdigitated electrodes covered by a ZnO sensing layer. ZnO has been grown by means of the aerosol assisted chemical vapor deposition technique and then nanostructured by laser interference lithography with a UV laser. The SEM and XRD results show vertically oriented growth of ZnO grains and a 2D periodic nanopatterning of the material with a period of 800 nm. Nanostructuring lowers the base resistance of the developed sensors and modifies the sensor response to NO₂. Maximum sensitivity is obtained at 175 °C achieving a change of 600% in sensor resistance for 4 ppm NO₂versus a 400% change for the non-nanostructured material. However, the most relevant results have been obtained at temperatures below 125 °C. While the non-nanostructured material does not respond to NO₂ at such low temperatures, nanostructured ZnO allows NO₂ sensing even at room temperature. The room temperature sensing capability possibly derives from the increase of both the surface defects and the surface-to-volume ratio. The long stability and the gas sensing under humid conditions have also been tested, showing improvements of sensitivity for the nanostructured sensors.

ZnO gas sensing improvement due to laser interference nanostructuration.

Study of deposition parameters and growth kinetics of ZnO deposited by aerosol assisted chemical vapor deposition

Aerosol-assisted Chemical Vapor Deposition (AACVD) is a thermally activated CVD technique that uses micro-droplets as deposition precursors. An AACVD system with a custom-designed reaction chamber has been implemented to grow ZnO thin films using zinc chloride as a precursor. The present work aims to study the impact of the deposition parameters on the thin film, as well as the microstructure evolution and growth kinetics. Aerosol flow has an effect on the density of nucleation sites and on the grain size. The temperature affects the morphology of the grown ZnO, showing a preferential orientation along the c-axis for 350 °C, 375 °C and 400 °C substrate temperatures. The microstructural evolution and the growth kinetics are also presented. A different evolution behavior has been observed for 350 °C, where nucleation site density is the highest at the early stages and it decreases over time in contrast with the cases of 375 °C and 400 °C, where there is an initial increase and a subsequent decrease. The activation energy of the chemical reaction is 1.06 eV. The optical characterization of the material has been performed through reflection measurements showing a relationship between the spectrum and the ZnO film thickness. The electrical characterization has been done by means of an interdigital capacitor, with which it is possible to measure the grain and grain boundary resistance of the material. Both resistances are of the order of 10⁵–10⁶ Ω.

Impact of deposition parameters, microstructure and growth kinetics analysis of ZnO grown by Aerosol-assisted Chemical Vapor Deposition (AACVD).