D.J. Ewing, M. Casper, A. Stramel, B. Morick, B. Cook, J. Wu
The Department of Energy's Kansas City National Security Campus,
Keywords: zinc oxide, ZnO, inkjet printing, chemical sensing, UV sensing
Summary:Zinc oxide (ZnO) nanostructures show great promise for a variety of applications, such as UV detectors, chemical, and gas sensors. Zinc oxide has a direct bandgap (3.4 eV), a high electron mobility, and environmental stability. When fabricated as a nanostructure, such as a quantum dot or nanoparticle, the large surface to volume ratio and large active area improve optical absorption and chemical sensitivity. Nanoparticles of ZnO can also be placed in solution, enabling simple fabrication methods such as drop casting and ink jet printing. Ink jet printing offers a number of advantages over traditional thin film fabrication of bulk ZnO, such as low cost, high volume manufacturing, monolithic integration with CMOS devices, creation of unique device shapes, ability to print on various substrates (silicon, quartz, plastic), and the ability to create multi-functional devices by printing different materials side by side. In this study, we investigate the chemical and optical response of resistive sensors fabricated using zinc oxide (ZnO) nanoparticle thin films. Sensors were fabricated by either spin coating or ink jet printing ZnO nanoparticles onto a pre-patterned substrate consisting of metal electrodes and a graphene channel. This configuration is a simple photoconductive or chemo-resistive sensor, in which either light or an absorbed chemical changes the electrical conductivity or resistivity of the ZnO film. The resulting change in output current is measured, indicating the presence of UV light or a targeted chemical. Both the ink jet printed and drop cast ZnO sensors exhibit response times of less than 30 seconds when exposed to 10 torr of ammonia gas. The sensors fabricated by drop casting ZnO displayed sensitivity to ammonia as low as 3 mT. Optical sensors fabricated by ink jet printing display photoresponsivity to UV light of 0.01 and 16.8 A/W for devices consisting of one printed layer and three printed layers, respectively. Response times for the ink jet printed devices were less 30 seconds. In addition to performance data, the underlying physical mechanism that allow for a diverse sensing options will be discussed. Due to the low cost, ease of manufacturing and integration with CMOS, ink jet printing of ZnO and other conductive metal oxide sensors have potential use for a wide range of applications in the defense, national security, and commercial sectors.