First-principles approach to the charge-transport characteristics of monolayer molecular-electronics devices: Application to hexanedithiolate devices

We report on the development of an accurate first-principles computational scheme for the charge transport characteristics of molecular monolayer junctions and its application to hexanedithiolate (C6DT) devices. Starting from the Gaussian basis set density-functional calculations of a junction model in the slab geometry and corresponding two bulk electrodes, we obtain the transmission function using the matrix Green’s function method and analyze the nature of transmission channels via atomic projected density of states. Within the developed formalism, by treating isolated molecules with the supercell approach, we can investigate the current-voltage characteristics of single and parallel molecular wires in a consistent manner. For the case of single C6DT molecules stretched between $\mathrm{Au}\left(111\right)$ electrodes, we obtain reasonable quantitative agreement of computed conductance with a recent scanning tunneling microscope experiment result. Comparing the charge transport properties of C6DT single molecules and their monolayer counterparts in the stretched and tilted geometries, we find that the effect of intermolecular coupling and molecule tilting on the charge transport characteristics is negligible in these devices. We contrast this behavior to that of the $\pi$-conjugated biphenyldithiolate devices we have previously considered and discuss the relative importance of molecular cores and molecule-electrode contacts for the charge transport in those devices.

Figure 1

(Color online) Schematic illustration of the device modeling. (a) The device is constructed by sandwiching a molecular monolayer between electrode 1 and electrode 2. It is semi-infinite along the electrode surface normal direction. The resulting infinite problem setup is reduced into a finite one via the MGF formulation. Independent DFT calculations are performed for (b) the finite 2D device model including the molecules and part of electrode atoms (represented by the dashed lines), and (c) two 3D bulk cells corresponding to the electrodes 1 and 2 (represented by dashed lines). These three DFT outputs are combined via the MGF method to extract the information on the semi-infinite device. Dots in (a) and (c) denote the periodicity along the surface normal direction. Periodicity along the surface parallel direction is implicitly assumed.

Figure 2

(Color online) Top and side views of the (a) $(\sqrt{3}\times \sqrt{3})R30°$ $\mathrm{Au}\left(111\right)$ cell corresponding to the bottom electrode. Top views of the (b) $2\times 2(\sqrt{3}\times \sqrt{3})R30°$ $\mathrm{Au}\left(111\right)$ cell used for the monolayer calculations and (c) $(2\sqrt{3}\times 2\sqrt{3})R30°$ $\mathrm{Au}\left(111\right)$ supercell used for the single molecule calculations. In (b) and (c), S atoms that bridge the C6 molecular core to the fcc hollow site of the bottom electrode are shown together.

Figure 3

(Color online) (a) Ball and stick representation of the stretched single C6DT device model. Three-layer $\mathrm{Au}\left(111\right)$ slabs [orange (gray)] were used for the energy minimization, and they were later augmented by another two $\mathrm{Au}\left(111\right)$ layers [magenta (dark gray)] for the calculation of the charge transport characteristics. Partitioning scheme of “electrodes” and “molecule” for the transmission calculations is also shown (see the text). The procedure of generating from (b) the single device model (c) the corresponding monolayer device model.

Figure 4

(Color online) (a) Transmission function of a single stretched C6DT molecule and (b) its PDOS. (c) Transmission/molecule of the corresponding C6DT monolayer and (d) its PDOS. In (b) and (d), solid lines are molecular core (C6) projections and dashed lines are S bridge atoms (DT) projections.

Figure 5

(Color online) Ball and stick representation of (a) the optimized C6DT SAM, (b) the device model based on the SAM structure, and (c) the corresponding single molecule device model.

Figure 6

(Color online) (c) Transmission/molecule of the optimized C6DT monolayer and (d) its PDOS. (a) Transmission function of the corresponding single C6DT molecule and (b) its PDOS. In (b) and (d), solid lines are C6 projections, and dashed lines are DT projections.

Figure 7

(Color online) Per molecule $I\text{−}V$ characteristics of devices based on the stretched single C6DT molecule (solid lines) and optimized C6DT SAM (dashed lines). For comparison, per molecule $I\text{−}V$ characteristics of herringbone-packing $\theta \approx 15°$ BPDT SAM device (long dashed lines) and parallel-packing $\theta \approx 30°$ BPDT SAM device (dot-dashed lines) from Ref. 10 are shown together.