Metal semiconductor phase transition in vanadium dioxide nanocrystals

The goal of this research was to improve the understanding of the submicron VO₂ formation in the near surface of a host material and to explore the possibility of size effects in the mechanics of the semiconductor to metal phase transition as well as in the optical properties of VO₂. By means of ion implantation and thermal processing, we were able to produce variable-sized nanoscale VO₂ precipitates embedded in SiO ₂. The transition temperatures were found to be correlated with the size of the precipitates, in such a way that for smaller particles, both transitions were thermally delayed. A review of the energy barriers and other features involved in the transition, led us to conclude that regardless of that exact mechanism, the phase transition must proceed in a heterogeneous fashion. Smaller particles were expected to have a lower chance of containing a nucleation site and thus, they need a greater thermal driving force in order to activate them. VO₂ precipitates were not only controlled in size but as an unexpected result they turned out to be produced in elongated shapes oriented mainly along the implanted surface. This morphology, which was explained in terms of the Bravais-Friedel law of crystal growth, allowed us to understand the optical properties of the precipitates. We concluded that the optical behavior shown by the particles in the SiO₂ matrix, was result of a surface plasmon resonance due to the dielectric confinement and metallic character of the VO₂ in the high temperature phase. Beside these contributions to material and physical sciences, we have shown that established results for VO₂ doping can be applicable to our submicron particles. We were able to successfully control the width of the hysteresis loop by adding Ti ions before the precipitation. We also reached lower switching temperatures by implanting small quantities of W. Ion implantation also proved to be an easy and convenient way to incorporate VO₂ nanoparticles into an optical fiber and thin film Si/SiO₂ technologies.