Electronic conductance through organic nanowires

Quantum-mechanical-based methods are used to obtain the current-voltage $\left(I\text{−}V\right)$ characteristic curves of a molecular nanowire bridging two metallic electrodes. The effect of the molecular electronic structure and mechanism of coupling to the metallic contacts on these curves is investigated for conjugated sulfur-based compounds, oligothiophen. The molecular nanowire is connected to two Au clusters, mimicking the electrodes. The electronic structure of the molecules in the wire was determined using ab initio–like calculations. The Hamiltonian matrix elements associated with these molecules were calculated via the density functional theory and the extended Hückel theory methods. The effect of the number of Au contact atoms on the $I\text{−}V$ curves was also investigated using two models; single-atom contact at each electrode and three-atom contact, through the hollow site of the Au(111) plane, at each electrode. We have also investigated the influence of the number of thiophen rings in the wire on the conductance properties. The shapes of the computed $I\text{−}V$ curves are in agreement with the available experimental data.

Figure 1

Schematic illustration of a molecular nanobridge, composed of a conjugated molecule (2TT) chemisorbed onto Au electrodes via the thiol terminal group. The electrodes and the central cluster are shown.

Figure 2

Schematic diagram for the energy levels of the 2TT molecule, and the relative positions of Fermi level of the Au contacts.

Figure 3

(a) The $I\text{−}V$ curve for the 2TT molecule. Broken line for single atom contact and solid line for hollow site contact. (b) The voltage drop through the 2TT molecule calculated under $2\mathrm{V}$ applied voltage bias. The horizontal axis is $Z$. (The curves obtained using DFT based calculation.)

Figure 4

Calculated density of states (DOS) and transmission function for the 1TT, 2TT, 3TT, 4TT, and 5TT molecules (calculated using EHT method).

Figure 5

Calculated $I\text{−}V$ characteristic curves for the molecules in Fig. 5 (calculated using the EHT method). The Fermi energies for 1TT, 2TT, 3TT, 4TT, and 5TT molecules are $-9.83$, $-8.60$, $-9.12$, $-8.48$, and $-9.65\mathrm{eV}$, respectively.

Figure 6

Conductance in molecules in Fig. 5 as a function of the bias potential applied to the Au contacts (calculated using the EHT method).