Contact-structure dependence of transport properties of a single organic molecule between Au electrodes

The contact-structure dependence of the transport properties are studied for the junction systems of a benzene dithiolate (BDT) molecule sandwiched between Au(111) atomic electrodes using the nonequilibrium Green’s function method based on the density functional theory. The transport properties for the hollow, bridge, and on-top contact structures are calculated. We show clearly that molecular orbitals of the isolated BDT molecule are responsible for the transmission peaks near the Fermi level. The contribution of these molecular orbitals, observed as broadened and shifted peaks in the projected density of states, is determined by the strength of their interaction with the Au electrodes, and the strength of the molecule-electrode interaction is clearly dependent on the contact structures. In the case of hollow contact, which has the strongest molecule-electrode interaction, we observe a linear potential drop across the junction for the various bias voltages. In the on-top contact structure that has the weakest interaction, on the other hand, the nonlinear potential drop and a weak negative differential conductance are observed. We obtain the largest current for the bridge contact structure, which has a moderate interaction. In this way, the transport properties strongly depend on the strength of the molecule-electrode interaction caused by a change of the contact structure and the unusual behavior is obtained for the contact structure that has the weakest interaction.

Figure 1

(Color online) Schematic atomic structure for the Au/BDT/Au junction systems. (a) Hollow contact structure, (b) bridge contact structure, and (c) on-top contact structure.

Figure 2

(Color) The eigenvalues and the molecular orbitals (MOs) from the $\mathrm{HOMO}-3$ to the $\mathrm{LUMO}+2$ of the isolated BDT molecule. The atomic structure for each MO are shown in the lower left panel [(a) a top view and (b) a side view]. For the plots of these MOs, we use the same threshold values. The $\mathrm{HOMO}-1$ and HOMO are related to $\sigma$ orbitals, while the $\mathrm{HOMO}-3$, $\mathrm{HOMO}-2$, LUMO, and $\mathrm{LUMO}+2$ are related to conjugate $\pi$ orbitals.

Figure 3

(Color) The transmission for the Au/BDT/Au junction system and the projected density of states (PDOS) with respect to the molecular orbitals from the $\mathrm{HOMO}-3$ to the $\mathrm{LUMO}+2$ for the isolated BDT molecule at zero bias voltage. The Fermi energy is set to be $0\mathrm{eV}$. Lower panels show the eigenvalues of the isolated BDT molecule. The eigenvalue of 22th state is set to be energy of the PDOS for the $\mathrm{LUMO}+1$. (a) The hollow contact structure, (b) the bridge contact structure, and (c) the on-top contact structure.

Figure 4

(Color) The transmission for the Au/BDT/Au junction system and the PDOS with respect to the molecular orbitals from the $\mathrm{HOMO}-3$ to the $\mathrm{LUMO}+2$ of the isolated BDT molecule at zero bias voltage for various rotational angle. The Fermi energy is set to be $0\mathrm{eV}$. (a) The hollow contact structure, (b) the bridge contact structure, and (c) the on-top contact structure.

Figure 5

(Color online) Potential drops and difference of the charge density between the system under bias and at zero bias voltage. (a) The hollow contact structure, (b) the bridge contact structure, and (c) the on-top contact structure.

Figure 6

(Color) The transmission and the PDOS with respect to the molecular orbitals from the $\mathrm{HOMO}-3$ to the $\mathrm{LUMO}+2$ for the isolated BDT molecule under bias. (a) The hollow contact structure, (b) the bridge contact structure, and (c) the on-top contact structure. The Fermi energy of the left electrode is set to be $0\mathrm{eV}$.

Figure 7

(Color online) The bias voltage dependence of the energy of the center of the peak of the PDOS with respect to the molecular orbitals from the $\mathrm{HOMO}-3$ to the $\mathrm{LUMO}+2$ for the isolated BDT molecule. (a) The hollow contact structure, (b) the bridge contact structure and (c) the on-top contact structure. The dotted lines are shown the line to be proportional to $V∕2$.

Figure 8

(Color online) Current $I$ against bias voltage $V$ for various contact structures shown in Fig. 1.