The conduction properties of cx,w-diaminoalkanes and hydrazine bridging gold electrodes

The conduction properties of respectively (see example structures in Fig. I) . The pyramids consist of four layers, with 1·, 3, 6, and 10 atoms in the layers from the inside to the outside. The electronic st ructures are then obtained to an accuracy better than 106 Hartree in the total energy. To calculate the conduc285 tance in this open system, we divide it into three parts (left (L), extended-molecule (M), and right (R)), where the extended-molecule part consists of the molecule and the inner two layers of each pyramid. The conductance is then ascertained using Green's function techniques [28,39,40]. One requirement for this method to be valid is that the overlap and Hamiltonian matrix elements coupling the left and right side vanish. Here, these elements of the overlap are always smaller than 107 and the ones of the Hamiltonian smaller than 107 Hartree. Results for zero-bias conductance obtained from the transmission at the Fermi energy are shown in a semi-logarithmic plot in Fig. 2 as a function of the length N of the junction. Also shown in this figure are the low bias conductances measured by Venkataraman et al. [24] and Chen et al. [23]. In absolute magnitude, our calculated values are very similar to those observed by Chen et aI., these being ca. twice those measured by Venkataraman et al. ; however, the absolute accuracy expected from these calculations is insufficient to discriminate between the two sets of experimental results. An exponential fit to the calculated data for N = 110 methylene gives an inverse decay length of fJ = 0.93 per methylene (equivalent.to 0.73 A I), also in good agreement with the experimental results of 0.81 ± 0.01 [23] and 0.91 ± 0.03 [24] as well as with the calculated result of 0.98 by Fagas and Greer [30]. Interestingly, extrapolation of our calculated results for the diaminoalkanes yields a contact resistance l/Gc of two amineAu bonds as ~ 150 kn, close to the calculated value of 116 kn for the AuhydrazineAu junction. This indicates that the alkanes act purely as spacers, meaning that they do not have a strong influence on the electronic structure of the Auamine interface. We also investigate the symmetry of conduction. The conductance point group is the geometric point group of the junction reduced by its end-to-end symmetry [31]. Therefore, the conductance point group of all junctions investigated here is C,; the mirror plane is shown in red Fig. I. Example geometries of AlH x,w-diaminoalkaneAu junctions: (left) AU2odiaminomethaneAu2o in C2v symmet ry. (right) AU 2oI,2-diaminoethaneAu2D in C2h symmetry. The symmetry element of the C.,. conductance point group is the red plane. The nitrogen atoms are represented in blue, the ca rbon atoms in black , the go ld atoms in yell ow, and the hydrogen .a toms in grey. (For interpreta tion of the references to colo r in this fi gure legend , the reade r is referred to the web version of thi s article .)