W-doped ZnO films are synthesized by pulsed laser deposition on Al2O3(1120) substrate, and their gas-sensing properties are investigated. Significantly improved sensitivity and response time to ethanol vapor are achieved. In order to study the relation between high sensing performance and local structure around the dopant W site, we carry out two-dimensional photoelectron diffraction on W-doped ZnO thin films at two different annealing temperatures (600 and 1000 °C). The photoelectron-intensity angular-distribution (PIAD) patterns of Zn 3p and W 4f core levels show the presence of forward-focusing peaks of very different relative intensities in the bulk and surface. From the PIAD patterns and their circular dichroism, we find clear evidence for the substitution of a segregated W atom into a Zn site in the second atomic layer. The result shows a direct relationship between the drastic increase in gas-sensing performance via W doping and the segregation of W at the surface. It also demonstrates that this photoelectron-diffraction measurement is a powerful tool for the study of the local structure of dopant sites in gas-sensing materials.
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