Conformations and charge transport characteristics of biphenyldithiol self-assembled-monolayer molecular electronic...

We report a computational study of conformations and charge transport characteristics of biphenyldithiol (BPDT) monolayers in the (sqrt.3 x sqrt.3)R30 degrees packing ratio sandwiched between Au(111) electrodes. From force-field molecular-dynamics and annealing simulations of BPDT self-assembled monolayers (SAMs) with up to 100 molecules on a Au(111) substrate, we identify an energetically favorable herringbone-type SAM packing configuration and a less-stable parallel packing configuration. Both SAMs are described by the (2sqrt.3 x sqrt.3)R30 degrees unit cell including two molecules. With subsequent density-functional theory calculations of one unit cell of the (i) herringbone SAM with the molecular tilt angle theta approximately 15 degrees , (ii) herringbone SAM with theta approximately 30 degrees , and (iii) parallel SAM with theta approximately 30 degrees, we confirm that the herringbone packing configuration is more stable than the parallel one but find that the energy variation with respect to the molecule tilting within the herringbone packing is very small. Next, by capping these SAMs with the top Au(111) electrode, we prepare three molecular electronic device models and calculate their coherent charge transport properties within the matrix Green's function approach. Current-voltage (I-V) curves are then obtained via the Landauer-Buttiker formula. We find that at low-bias voltages (|V| < or = 0.2 V) the I-V characteristics of models (ii) and (iii) are similar and the current in model (i) is smaller than that in (ii) and (iii). On the other hand, at higher-bias voltages (|V| > or 0.5 V), the I-V characteristics of the three models show noticeable differences due to different phenyl band structures. We thus conclude that the BPDT SAM I-V characteristics in the low-bias voltage region are mainly determined by the -Au [corrected] interaction within the individual molecule-electrode contact, while both intramolecular conformation and intermolecular interaction can affect the BPDT SAM I-V characteristics in the high-bias voltage region.