Insulator coated metal nanoparticles with a core/shell geometry exhibit a temperature sensitivity similar to advanced spinels

The preparation of carbon-coated copper nanoparticles with different carbon layers resulted in materials with a highly sensitive pressure and temperature dependent conductivity. The core/shell geometry of these carbon/metal composites afforded two distinctly different electrical behaviors depending on the carbon layer properties. Graphene layers with a predominant sp(2) character showed an ill-defined bandgap structure as evidenced by UV-vis diffuse reflectance spectroscopy. The resulting composites were weak conductors with low sensitivity. Use of predominately insulating carbon layers with a well-defined bandgap of above 1.9 eV resulted in composites with a material constant beta of over 4700 K which is comparable to currently used commercial spinels. A theoretical analysis and detailed material characterization by Raman spectroscopy, X-ray diffraction, C-13 NMR spectroscopy and thermoanalysis suggested a tunneling based conduction mechanism in these core/shell materials. This interpretation was supported by a good correlation between experimental data and the estimated effects arising from the theoretical analysis of the tunneling effects. (c) 2007 Elsevier B.V. All rights reserved.