Gas sensing properties of pulsed laser deposited vanadium oxide thin films with various crystal structures

Vanadium oxide thin films were fabricated by pulsed laser deposition. The crystal structure and symmetry of the deposited films were studied with X-ray diffraction and Raman spectroscopy, respectively. The film microstructure was also studied with atomic force microscopy and scanning electron microscopy. The thin film crystal structures varied between almost pure V2O5 phase and another phase, suggested being V7O16, generally found in samples composed of nanotubes and identified as VOx-NT. The measured optical transmission spectra of the films also supported the existence of two different phases. The electrical resistivity of the films as a function of temperature behaved like in a typical semiconductor. The gas sensing properties of the films were characterized for different NOx, CO and H2 concentrations. The results showed a response to NOx and H2, which varied from oxidative to reducing according to the film composition and gas background environment. Only very small response was seen towards CO. Gas response and resistivity measurements indicated that the VOx-NT-type phase has both n-type and p-type conduction mechanisms.