Tuning the electrical conductivity of Pt-containing granular metals by postgrowth electron irradiation

We fabricated Pt-containing granular metals by focused electron beam-induced deposition from the (CH3)(3)CH3C5H4Pt precursor gas. The granular metals are made of platinum nanocrystallites embedded in a carbonaceous matrix. We exposed the as-grown nanocomposites to low-energy electron beam irradiation and measured the electrical conductivity as a function of irradiation dose. Postgrowth electron beam irradiation transforms the matrix microstructure and thus the strength of the tunneling coupling between Pt nanocrystallites. For as-grown samples (weak tunnel coupling regime) we find that the temperature dependence of the electrical conductivity follows the stretched exponential behavior characteristic of the correlated variable-range hopping transport regime. For briefly irradiated samples (strong tunnel coupling regime) the electrical conductivity is tuned across the metal-insulator transition. For long-time irradiated samples the electrical conductivity behaves like that of a metal. In order to further analyze changes of the microstructure as a function of the electron irradiation dose, we carried out transmission electron microscope (TEM), micro-Raman spectroscopy, and atomic force microscopy (AFM) investigations. TEM pictures reveal that crystallite size in long-time irradiated samples is larger than that in as-grown samples. Furthermore, we do not have evidence of microstructural changes in briefly irradiated samples. By means of micro-Raman spectroscopy we find that by increasing the irradiation dose the matrix changes, following a graphitization trajectory between amorphous carbon and nanocrystalline graphite. Finally, by means of AFM measurements we observe a reduction of the volume of the samples with increasing irradiation time, which we attribute to the removal of carbon molecules. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3559773]