Quantum length dependence of conductance in oligomers: First-principles calculations

In recent experiment it was found that for a quantum length dependence of conductance of oligothiophene-CH2 molecules under low bias [ Xu et al. Nano Lett. 5 1491 (2005)], the longer molecule has larger conductance. Due to the experimental motivation, we calculated the conductance of a similar organic compound, oligothiophene, by means of the first-principles method. Our calculations show a similar quantum length dependence of conductance in the low bias region and an oscillated length dependence of conductance in the high bias region. The transport behaviors are determined by the distinct electronic structures of the molecular compounds. The length dependencies of conductance for several other oligomers are calculated to show the diversity in the transport behaviors of molecular wires. The results show that Ohm’s law is not valid for the molecular conductance anymore, and for the low bias region, Magoga’s law is not applicable for some molecular wires, for example, oligothiophene dithiolates.