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10 September 2008 Frequency and percolation dependence of the observed phase transition in nanostructured and doped VO2 thin films
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The response to applied electric fields of vanadium dioxide thin films above and below the phase transition is shown experimentally to depend on the size of grains if below ~200nm across, and on aluminum doping above a critical concentration. Tc drops as doping level increases, but does not depend on grain size. The observed phase transition undergoes a remarkable qualitative shift as the applied field goes from optical to low frequencies. The expected insulator to metal transition is found at optical frequencies, but at low frequencies an insulator-to-insulator transition occurs. Optical switching at both T < Tc and T > Tc is nearly independent of doping level and grain size. In contrast dc properties in both phases are quite sensitive to both factors. The band gaps predicted by optical and dc data differ, and densities of states change with doping level. Lattice or electron dynamics alone cannot yield such behaviour, but it can arise if there is a transient phase change. The way doping and grain size can support such a phase is discussed. Only individual nanograins need to switch phases coherently to explain data, not the whole sample. Resistance as a function of composition across the transition was derived using effective medium compositional analysis of optical data at temperatures in the hysteresis zone. Expected percolation behaviour does not arise in such an analysis, with the observed thresholds different when heating and cooling, and they occur at temperatures which differ from the usual Tc values.
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A. R. Gentle, G. B. Smith, and A. I. Maaroof "Frequency and percolation dependence of the observed phase transition in nanostructured and doped VO2 thin films", Proc. SPIE 7041, Nanostructured Thin Films, 70410J (10 September 2008);

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