Dual metal-insulator and insulator-insulator switching in nanoscale and Al doped VO2

Thin films of VO(2) doped with aluminium, or with nanoscale grain sizes, have been produced. They display semiconductor resistive behaviour above the transition temperature T(c), but a metallic and plasmonic optical response. All samples optically switch over almost identical large ranges at the transition, but have quite variable resistive switching. At fixed grain size a rigorous new quantitative correlation is found between semiconductor resistivity below T(c) and the activation energy above T(c) as Al doping level varies. Large crystals doped with Al also display this dual behaviour. A possible mechanism is discussed involving fast local fluctuations on neighbouring V(4+) ions involving transient dimers with no net spin. Such fluctuations would then need to interact and correlate their motion over the scale of a nanograin within the lifetime of the dimer excitation.