X-ray diffraction studies of the growth of vanadium dioxide nanoparticles

We have characterized for the first time in situ the growth of vanadium dioxide nanoparticulate films prepared by pulsed-laser deposition, using a five-circle x-ray diffractometer, in order to provide structural information as the films are grown. A vanadium metal target was ablated in the presence of a pulsed N2O reactive gas source, and films were grown on Si(001) and Al2O3(0001) substrates. Optical measurements confirmed that the films deposited in this way exhibit the well-known VO2 metal–insulator transition at approximately 70 °C. The VO2 films grown at room temperature on silicon substrates are amorphous and extremely smooth. These become considerably rougher upon thermal annealing, as the VO2 phase crystallizes out in the form of hemispherical islands. These films also contain traces of a V2O5 nanoparticle phase in the first few monolayers, although the degree of VO2 crystallinity in the nanoparticles is quite high. In contrast, the VO2 nanoparticles grown on the sapphire substrates are in the form of preferentially oriented rodlike islands, with a maximum rod width of about 100 nm. This in-plane texture is in keeping with the relatively good lattice match between film and substrate, indicating good potential for the growth of VO2 epitaxial films and single-crystal nanoparticles on such substrates.