Surface morphology-optical properties relationship in thermochromic VO_(2) 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 VO_(2) films have been obtained by air oxidation of VN samples performed at three temperatures(450,525 and 550℃)at various durations(lower than 50 min).X-ray diffraction and Raman spectrometry of the VN oxidized films indicate that VO_(2) 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℃ or suddenly disappears at 525 and 550℃.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 7.5e13 mm 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 VO_(2),along with an insight into the correlation between surface morphology and optical properties.