Laser heating induced phase changes of VO2 crystals in air monitored by Raman spectroscopy

We have studied the effect of intense visible light on vanadium dioxide crystals in air by measuring micro-Raman spectra for increasing laser powers. A laser wavelength of 632.8 nm was used. For power densities above 500 MW/m2 laser heating triggers the phase transition, which is known to occur at around 68° C, from the room temperature insulating VO2(M) monoclinic phase to the high temperature semiconductor VO2(R) tetragonal rutile phase. The changes are fully reversible and the crystal goes back to the monoclinic phase when the laser power is lowered to the initial value. This transition is also observed in electron diffraction experiments under vacuum, heating in this case produced by the electron beam. For laser power densities above 1300 MW/m2 the change is irreversible and laser heating provokes a reaction with the oxygen in air and an oxidation into V2O5. For comparison we have also done thermogravimetric and differential scanning calorimetry experiments from 20° C to 600° C.