Formation and phase transition of VO 2 precipitates embedded in sapphire
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Formation and phase transition of VO2 precipitates embedded in sapphire Laurence A. Gea, J. D. Budai, and L. A. Boatner Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 31 July 1998; accepted 9 February 1999)
Crystallographically coherent precipitates of vanadium dioxide (VO2 ) have been formed in the near-surface region of single crystals of sapphire (Al2 O3 ) using a combination of ion implantation and thermal treatments. As in the case of either bulk VO2 single crystals or thin films of VO2 , the thermally induced semiconductor-to-metal phase transition of the embedded VO2 precipitates is accompanied by a large hysteretic change in the infrared optical transmission. The VO2 precipitate transition temperature (Tc 72 to 85 ±C) is higher than that of bulk VO2 (Tc 68 ±C) and is sensitive to the implantation conditions. The present results show that the damage resulting from the coimplantation of vanadium and oxygen into an Al2 O3 host lattice dictates the final microstructure of the VO2 precipitates and, consequently, affects the transition temperature, as well as the optical quality of the VO2yAl2 O3 surface-nanocomposite precipitate system.
I. INTRODUCTION
Materials whose intrinsic properties enable them to perform both sensing and actuating functions are of current interest for use in a wide range of applications. Among materials of this type, those whose sensing and actuating mechanisms are based on phase transitions are particularly intriguing, since the associated property changes induced by external perturbations can be rapid, reversible, and may span several orders of magnitude. These features have provided the motivation for a number of previous investigations of the property changes associated with phase transitions in thin films of the vanadium oxides, VO2 and V2 O3 ,1–4 and applications have been developed that involve the thermally induced semiconductor-to-metal (Tc 68 ±C), or insulator-tometal (Tc 2128 ±C), respectively, resistivity change of these materials.5–9 Recently, we have shown that it is possible to form coherent precipitates of V2 O3 10 or VO2 11,12 that are embedded in the near-surface region of an a –Al2 O3 (sapphire) matrix by using an ion-implantation and thermal-treatment method. For the case of VO2 precipitates, Fig. 1 illustrates the temperature dependence of the optical transmission of (020)-oriented VO2 nanocrystals (implantation dose 2 3 1017 V cm–2 ) embedded in the near-surface region of a (001)-oriented sapphire substrate.3,4 As shown in the figure, the infrared optical transmission drops sharply with increasing temperature (top trace), and as the temperature is decreased (bottom trace), the optical transmission returns to its original value. The observed hysteresis is associated with a first-order phase transition13 that is described as a crystallographic distortion from a tetragonal structure to a 2602
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 6, Jun 1999
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