Molecular junctions composed of oligothiophene dithiol-bridged gold nanoparticles exhibiting photoresponsive...

Three oligothiophene dithiols with different numbers of thiophene rings (3, 6 or 9) have been synthesized and characterized. The X-ray single crystal structures of terthiophene 2 and sexithiophene 5 are reported herein to show the exact molecular lengths, and to explain the difference between their UV-visible spectra arising from the different packing modes. These dithiols with different chain lengths were then treated with 2-dodecanethiol-protected active gold nanoparticles (Au-NPs) by means of in situ thiol-to-thiol ligand exchange in the presence of 1 microm gap Au electrodes. Thus the molecular junctions composed of self-assembled films were prepared, in which oligothiophene dithiol-bridged Au-NPs were attached to two electrodes by means of Au-S bonded contacts. The morphologies and current-voltage (I-V) characteristics of these films were studied by SEM and AFM approaches, which suggested that the thickness of the films (3-4 layers) varied within the size of one isolated Au-NP and typical distance-dependent semiconductor properties could be observed. Temperature dependent I-V measurements for these molecular junctions were performed in which the films served as active elements in the temperature range 6-300 K; classical Arrhenius plots and subsequent linear fits were carried out to give the activation energies (deltaE) of devices. Furthermore, preliminary studies on the photoresponsive properties of these devices were explored at 80, 160, and 300 K, respectively. Physical and photochemical mechanisms were used to explain the possible photocurrent generation processes. To the best of our knowledge, this is the first report in which oligothiophene dithiols act as bridging units to link Au-NPs, and also the first report about functionalized Au-NPs exhibiting photoresponse properties in the solid state.