Nanograin VO2 in the metal phase:: a plasmonic system with falling dc resistivity as temperature rises

Thin films of vanadium dioxide with grain sizes smaller than 60 nm have a metallic phase with excellent plasmonic response, but their dc resistivity falls as temperature rises to values well above the metal-insulator transition. At the transition optical switching is complete, but the switch in dc resistance is incomplete. In the metallic phase, nanograin and large grain samples have similar values of both plasma frequency and relaxation rate. However, plasmonic response in nanograins is stronger due to the absence of a low energy interband transition found in large grain films. Conductivity rises with thermal activation energy of 108 meV, which is well below that in the semiconductor phase. Possible mechanisms for 'non-metal-like' dc behaviour in this plasmonic system are briefly discussed. They include fluctuations, which are coherent in nanograins but incoherent for larger grains. Nanoscale systems seem preferable for optical switching applications and large grain structures for dc switching work.