Electron transport in a gold nanoparticle assembly structure stabilized by a physisorbed porphyrin derivative

Gold nanoparticles stabilized by meso-5,10,15,20-tetrakis(2-thienyl)porphyrin (2T) via physisorption (2T-AuNP) were synthesized, and the electronic transport of assemblies of these films was studied. The adsorption mechanism of 2T on gold nanoparticles was examined using UV-vis-NIR, IR, Raman, and H1-NMR spectroscopy, which showed no evidence of any covalent bonding between 2T and the gold nanoparticles. In temperature-dependent resistivity measurements, a crossover from thermally assisted hopping to Efros-Shklovskii-type variable-range hopping (ES-VRH) was observed around 50 K on decreasing the temperature. At higher temperatures, the 2T-AuNP assembly structure followed an Arrhenius plot (E_A=15meV) with ohmic I-V characteristics at each measurement point. On the other hand, the activation energy at lower temperatures decreased nonlinearly in a T^-1 plot, and the logarithm of the resistance obeyed a T^-1/2 law, corresponding to an ES-VRH mechanism, which is predicted for disordered materials as a variable-range hopping mechanism influenced by strong Coulomb interactions. ES-VRH behavior has been observed previously in saturated molecule/gold nanoparticle assemblies and was confirmed in our 2T-AuNP assembly. Electronically active conjugated molecules were successfully incorporated between the nanoparticles, keeping the electronic structure of the gold nanoparticle and 2T moieties isolated from each other.