Metal-to-nonmetal transition in copper nanoshells grown on copper oxide nanoparticles

Copper oxide nanoparticles of median diameter 17 nm were synthesized by a chemical method. By subjecting these to a reduction treatment at 373 K for a duration extending to 1 h copper shells of thicknesses in the range 2-6.5 nm were grown. Optical absorption measurements were carried out on these core-shell structured nanosystems in the temperature range 274-343 K. Analyses of the data show that below a shell thickness of 3 nm the copper shell conductivity has a value below the Mott's minimum metallic conductivity and shows a semiconducting behavior. Electrical conductivity measurements on pellets of the nanostructured powders show a resistivity variation at low temperatures with activation energies in satisfactory agreement with those obtained from the optical studies