Metal-insulator transition and size dependent electrical percolation in onion-like carbon/polydimethylsiloxane composites

Dielectric/electric properties of onion-like carbon (OLC)/polydimethylsiloxane composites were investigated over very wide frequency (20 Hz-3 THz) and temperature (26-500K) ranges. The percolation threshold in these composites strongly dependents on the OLC aggregate sizes and was lowest for the composites with the smallest OLC aggregate sizes (similar to 40 nm). Interestingly, the transition into the insulator state of the composites occurred at higher temperatures. The transition temperature increases with OLC aggregate concentration. Above the percolation threshold, the electrical conductivity in the composites occurs mainly due to electron tunneling between OLC clusters and quasi-one-dimensional hopping inside the clusters. The hopping almost vanishes at frequencies above 100 GHz where the phonon contribution dominates. (C) 2014 AIP Publishing LLC.