Electrical transport properties and their reproducibility for linear porphyrin arrays

We present an investigation of electrical transport through fused and orthogonal dithiolated porphyrin arrays of different molecular lengths. Length dependence measurements show that conductance decreases much more slowly with molecular length than the exponential dependence expected from a simple theoretical model. From the temperature dependence of I_SD - V_SD curves, the thermal activation energy E_a is estimated to be about 0.35 eV at zero-bias voltage, independent of molecular conformation and length. At high-bias voltages, however, the decrease in E_a as a function of bias voltage is more significant for the longer porphyrin arrays. We have also studied the thermal cycling effects. After thermal cycling, dithiolated porphyrin arrays are found to aggregate into clusters between nanoelectrodes. This is probably due to the diffusion of porphyrin arrays on the SiO₂ substrate at low temperatures, which enhances the conductance.