Electronic Tunneling through self-assembled monolayers of phenylene oligomers on Au(111)

We investigated the electrical properties of self-assembled monolayers (SAMs), which are composed of a single component of benzylic derivatives of oligophenylene thiols, HSCH2(C6H4)(m-1)C6H5 (m = 1, 2, 3) on an An(111) substrate, using conducting atomic force microscopy (AFM). The use of an Au-coated AFM tip with a decanethiol SAM resulted in a bilayer junction at the contact area between the Au-coated tip with the decanethiol SAM and the Au(111) substrate covered with one of the phenylene oligomer SAMs, i.e., Au-decanethiol SAM/oligophenylene SAM-Au(111). The electrical properties of the phenylene oligomers were studied in terms of the dependence of the observed tunneling current on the length of the oligomers at the same constant sample bias voltage. An exponential decrease in the tunneling current was observed with increasing length of the oligomers. The decay constant, beta, of the tunneling current through the SAMs is related to the electronic structure of the molecules. The present study gave a beta value of 5.5 nm(-1) for the phenylene oligomers by measuring the current-voltage (I-V) curves of the bilayer junctions and plotting the resulting current (at +1.0 V) as a function of the length of the oligomers. The I-V curves were symmetrical around V = 0. The value of beta did not depend significantly on the applied voltage and was close to reported ones of 6.6 nm(-1) for electron transfer through thin organic films of a series of corresponding Hg-hexadecanethiol SAM/oligophenylene SAM-Ag junctions and of 4.0 nm(-1) for photoinduced electron transfer in bis-porphyrin donor-acceptor compounds with polyphenylene spacers. The slight difference among these three values was attributed to the difference in barrier height not for electron transfer but for hole transfer through the phenylene oligomers.