Conductance through single biphenyl molecules: symmetric and asymmetric coupling to electrodes

The contacts and the chemical bonds formed between metallic electrodes and molecules determine to a large extent the conductive properties of single molecular junctions, which represent the smallest possible active elements in an electronic circuit. We therefore investigated in a comparative study, using the break junction technique (MCBJ), the conductive properties of [1,1'-biphenyl]-4,4'-dithiol (M1) and of 4'-mercapto-[1,1'-biphenyl]-4-carbonitrile (M2) between gold electrodes. As a function of electrode separation, characterized by the conductance close to 0 V, we found several plateaus of relative stability, with those close to 0.01G0 being the most pronounced. The overall conductance of symmetric and asymmetric molecules were surprisingly similar, only the range of stability was smaller for M2. While M1 yielded symmetric I-V-curves, only small asymmetries were detected for M2. These are also reflected in the comparable values for coupling parameters using the single level resonance model. The high conductance for the asymmetric molecule is interpreted as a result of coherent coupling of electronic states through the whole molecule, so that the outcome cannot be predicted just by adding conductive properties of individual molecular groups.