Electron transport in multiterminal molecular devices: A density functional theory study

The electron transport properties of a four-terminal molecular device are computed within the framework of density functional theory and nonequilibrium Keldysh theory. The additional two terminals lead to new properties, including a pronounced negative differential resistance not present in a two-terminal setup, and a pseudogating effect. In general, quantum interference between the four terminals and the central molecule leads to a complex nonlinear behavior of the current, which depends on the alignment of individual molecular states under bias and their coupling to the leads.

Figure 1

(Color online) $I\text{−}V$ curves of the four-terminal molecular device. For comparison, the $I\text{−}V$ curve $\left(I_2\right)$ of the corresponding two-terminal 1–2 setup is also shown. In the inset, the bias geometry of the four-terminal system is schematically displayed. Au, S, C, and H atoms are shown in yellow (in the lead region), red (black), cyan (large gray), and gray (small gray), respectively. The semi-infinite leads are built out of Au(111) nanowires.

Figure 2

(Color) (a) Position-dependent density of states along the left-to-right direction at biases of 0.0, $-0.2$, $-0.5$, and $-0.8\text{V}$. The DOS is averaged in the plane perpendicular to the current. Note that the electronic states originating from the vertical leads are not included in the figure. The chemical potentials of the left and right leads are shown as white lines. The zero of energy is at $\left({\mu }_1+{\mu }_2\right)/2$. The dotted and dashed ovals enclose the pseudomolecular states that originate from the highest occupied and lowest unoccupied molecular orbitals, respectively. To make it clearer, the bottom panel (b) illustrates only the main features of the DOS, and the alignment and broadening of the LUMO and HOMO states.

Figure 3

(Color) Changes in the pseudomolecular states with the bias voltage. The densities of states close to the left, right, and bottom molecule-lead interfaces (of leads 1, 2, and 3) are shown. Due to the symmetry in the bias geometry, the electronic structures at the top and bottom interfaces are identical. The red arrow indicates the position of the LUMO, which mainly drives the current.

Figure 4

(Color online) Currents in the four-terminal geometry shown in (a) as a function of pseudogate voltage $V_{\text{g}}$. (b) Currents $I_{ij}$ passing through terminals $i$ and $j$. A positive (negative) $I_{ij}$ indicates electrons flow from electrode $i$ $\left(j\right)$ to electrode $j$ $\left(i\right)$. (c) Total currents $I_i$ passing through terminals $i$. The currents $I_3$ and $I_4$ are equal by symmetry. $I_i>0$ indicates electrons entering electrode $i$ and $I_i<0$ indicates electrons leaving electrode $i$.

Figure 5

Illustration of LUMO shift under different pseudogate voltages. The largest part of the broadened LUMO state (shaded oval) is in the bias window for positive pseudogate voltages. When the pseudogate voltage is negative, the LUMO state moves up and only a small part of the broadened LUMO is in the bias window.