Temperature dependence of optical and transport properties in VO2 with high temperature anomalies

Thermochromic VO2 is of interest for energy efficient glazing, and for fast telecommunications because it optically switches in the near IR. Despite extensive study several aspects of its apparently diverse behaviour have not been explained satisfactorily. The visible–NIR permittivity and dc electrical conductivity of high quality thin films of VO2, across the metal–insulator phase transition and well into the metallic phase to temperatures up to 100 °C above Tc are studied as a function of temperature and grain size. Experimental behaviour is partly explained with effective medium models, existing band structures and classical transport theory. Anomalies however include: unphysically fast relaxation rate, counter-intuitive and significant differences between optical and dc, and bulk and thin film parameters; and residual “non-metallic” features above the transition in highly oriented films. Residual, but transient high temperature d-electron singlet pairing on V dimers, which is sensitive to nanostructure, is examined as a source of some anomalies.