Surface morphology-optical properties relationship in thermochromic VO2 thin films obtained by air oxidation of vanadium nitride

In this paper, vanadium nitride (VN) thin films have been deposited on Al substrates by reactive magnetron sputtering. Thermochromic VO2 films have been obtained by air oxidation of VN samples performed at three temperatures (450, 525 and 550 °C) at various durations (lower than 50 min). X-ray diffraction and Raman spectrometry of the VN oxidized films indicate that VO2 and V2O5 are the only phases produced during the oxidation process. Vanadium dioxide is the first oxide formed. It coexists with VN in a long period at 450 °C or suddenly disappears at 525 and 550 °C. Whatever the temperature, V2O5 is exclusively detected after the total oxidation of VN. This oxide is detrimental to the thermochromic performance of films. The emissivity-switching properties of the oxidized films were analyzed by infrared camera in the 7.5–13 μm range. The comparison among all the samples exhibiting a thermochromic behavior shows that the maximum emissivity switch is independent of the oxidation temperature and the surface morphology of the samples. These results could open a new strategy in the investigation of VN oxidation as a method to obtain VO2, along with an insight into the correlation between surface morphology and optical properties. [Display omitted] •Thermochromic VO2 thin films have been obtained by air oxidation of VN films.•The VO2 morphology is strongly dependent on the annealing temperature.•The annealing duration has to be optimized to guarantee VO2 films exempt of V2O5 phase.•The emissivity switch of the VO2 films is independent of the film morphology.