Network structures made of p-conjugated molecular wires of oligothiophene 3mer, or 9mer carrying thiol groups at a,v-positions, and gold nanoparticles with average diameter of 4 nm were prepared on interdigitated gold electrodes. Observation of the resultant assemblies by means of FE-SEM and TEM revealed that the gold nanoparticles were connected by p-molecular wires to form a network. The networks exhibited thermally activated electron transport at room temperature with activation energies of 21and 45 meV for 3mer-and 9mer-networks, respectively, and these values were almost the same as those of networks connected with non-conjugated molecules having similar lengths. However, the activation energy became very small (y0.1 meV) at temperatures lower than 30 K and non-linear current-voltage characteristics (I 3 V 3 ) appeared in p-conjugated networks at 4.2 K. These results suggest that the gold nanoparticles in the networks work as Coulomb islands and the temperature-independent behavior at lower temperatures can be interpreted in terms of a co-tunneling mechanism.

As a bottom-up approach toward spintronics, a network structure of gold nanoparticles connected with spin-polarized wire molecules has been studied. A spinless network is prepared as a reference system. The network of gold nanoparticles with an average diameter of 4 nm form granules ͑average diameter of 100 nm͒, which in turn, connect themselves with each other to bridge 2 m-gap gold electrodes. Since the charging energy of a 4-nm gold nanoparticle amounts to 160 meV, it works as a Coulomb island and the conduction through the network is dominated by Coulomb blockade effect at room temperature. Thermal-activation-type conduction is found in a temperature range of 300 K-30 K, below which cotunneling is suggested to dominate. Important findings reported here are as follows: ͑1͒ The cotunneling occurs at elevated temperatures as high as T = 30 K due to the small size of gold nanoparticles: Nonlinear characteristics featured by I-V 3 are found, suggesting that the number of tunnel junctions relevant to the cotunneling is two. ͑2͒ The cotunneling current is substantially smaller in spin-polarized network than in spinless network, suggesting that spin-flip scattering caused by localized spins on wire molecules suppresses cotunneling process: The interpretation is supported by negative magnetoresistance observed in spin-polarized networks.

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