Thermochromatic vanadium dioxide (VO2) thin films synthesized by atomic layer deposition and post-treatments

As a thermochromatic material showing metal-insulator transition behavior below 70 degrees C, crystalline films of vanadium dioxide (VO2) have potential applications in many fields. In this work, amorphous vanadium oxide (VxOy) films were deposited on glass substrates by atomic layer deposition (ALD). The film growth mechanism and growth rate were studied by quartz crystal micro balance, and a constant growth rate of about 0.5 angstrom/cycle was obtained. X-ray diffraction and X-ray photoelectron spectroscopy characterizations demonstrated that the as-deposited ALD VxOy film was amorphous with mostly V4+ oxidation state. Crystalline VO2O5 films were prepared by annealing the as-deposited ALD VxOy films at 300 degrees C under O-2 flow. Subsequently, different annealing methods and conditions were tested for transforming V2O5 films to VO2 films. It turned out that high purity crystalline VO2 films could be obtained by heating the V2O5 films in an oxidizing environment at a reduced pressure. Results of transmittance measurements revealed that these crystalline VO2 films had excellent thermochromatic properties with a reversible change in transmittance at about 65 degrees C.