Self-consistent-field study of conduction through conjugated molecules - art. no. 035416

Current-voltage (I-V) characteristics of individual molecules connected by metallic leads are studied theoretically. Using the Pariser-Parr-Pople quan tum chemical method to model the molecule enables us to include electron-el ectron interactions in the Hartree approximation. The self-consistent-field method is used to calculate charging together with other properties for th e total system under bias. Thereafter the Landauer formula is used to calcu late the current from the transmission amplitudes. The most important param eter to understand charging is the position of the chemical potentials of t he leads in relation to the molecular levels. At finite bias, the main part of the potential drop is located at the molecule-lead junctions. Also, the potential of the molecule is shown to partially follow the chemical potent ial closest to the highest occupied molecular orbital (HOMO). Therefore, th e resonant tunneling steps in the I-V curves are smoothed giving a I-V rese mbling a ''Coulomb-gap." However, the charge of the molecule is not quantiz ed since the molecule is small with quite strong interactions with the lead s. The calculations predict an increase in the current at the bias correspo nding to the energy gap of the molecule irrespective of the metals used in the leads. When the bias is increased further, charge is redistributed from the HOMO level to the lowest unoccupied molecular orbital of the molecule. This gives a step in the I-V curves and a corresponding change in the pote ntial profile over the molecule. Calculations were mainly performed on poly ene molecules. Molecules asymmetrically coupled to the leads model the I-V curves for molecules contacted by a scanning tunneling microscopy tip. I-V curves for pentapyrrole and another molecule that show negative differentia l conductance are also analyzed. The charging of these two systems depends on the shape of the molecular wave functions.