The effect of pressure and W-doping on the properties of ZnO thin films for NO2 gas sensing

Pure and W-doped ZnO thin films were obtained using magnetron sputtering at working pressures of 0.4 Pa and 1.33 Pa. The films were deposited on glass and alumina substrates at room temperature and subsequently annealed at 400oC for 1 hour in air. The effects of pressure and W-doping on the structure, chemical, optical and electronic properties of the ZnO films for gas sensing were examined. From AFM, the doped film deposited at higher pressure (1.33 Pa) has spiky morphology with much lower grain density and porosity compared to the doped film deposited at 0.4 Pa. The average gain size and roughness of the annealed films were estimated to be 65 nm and 2.2 nm, respectively with slightly larger grain size and roughness appeared in the doped films. From XPS the films deposited at 1.33 Pa favored the formation of adsorbed oxygen on the film surface and this has been more pronounced in the doped film which created active sites for OH adsorption. As a consequence the W-doped film deposited at 1.33 Pa was found to have lower oxidation state of W (35.1 eV) than the doped film deposited at 0.4 Pa (35.9 eV). Raman spectra indicated that doping modified the properties of the ZnO film and induced free-carrier defects. The transmittance of the samples also reveals an enhanced free-carrier density in the W-doped films. The refractive index of the pure film was also found to increase from 1.7 to 2.2 after W-doping whereas the optical band gap only slightly increased. The W-doped ZnO film deposited at 0.4 Pa appeared to have favorable properties for enhanced gas sensing. This film showed significantly higher sensing performance towards 5-10 ppm NO2 at lower operating temperature of 150oC most dominantly due to increased free-carrier defects achieved by W-doping.

Germination biology and occurrence of pol-yembryony in two forms of cats claw creeper vine, Dolichandra unguis-cati (Bignonia-ceae): implications for its invasiveness and management

Cat’s claw creeper, Dolichandra unguis-cati (L.) Lohmann (syn. Macfadyena unguis-cati (L.) Gentry) is a major environmental weed in Australia. Two forms (‘long’ and ‘short’ pod) of the weed occur in Australia. This investigation aimed to evaluate and compare germination behavior and occurrence of polyembryony in the two forms of the weed. Seeds were germinated in growth chambers set to 10/20 °C, 15/25 °C, 20/30 °C, 30/45 °C and 25 °C. Germination and polyembryony were monitored over a period of 12 weeks. For all the treatments in this study, seeds from the short pod form exhibited significantly higher germination rates and higher occurrence of polyembryony than those from the long pod form. Seeds from the long pod form did not germinate at the lowest temperature of 10/20 °C; in contrast, those of the short pod form germinated under this condition, albeit at a lower rate. Results from this study could explain why the short pod form of D. unguis-cati is the more widely distributed form in Australia, while the long pod form is confined to a few localities. The results have implication in predicting future ranges of both forms of the invasive D. unguis-cati, as well as inform management decisions for control of the weed.