Current rectification in molecular junctions produced by local potential fields

The transport properties of an octane-dithiol (ODT) molecule coupled to Au(001) leads are analyzed using density-functional theory (DFT) and nonequilibrium Green’s functions. It is shown that a symmetric molecule can turn into a diode under influence of a local electric field created by an external charged probe. The origin of the asymmetry of the current-voltage $\left(I\text{−}V\right)$ dependence is traced back to the appearance of a probe-induced quasilocal state in the pseudogap of the ODT molecule. The induced state affects electron transport provided it is close to the Fermi level of the leads. An asymmetric placement of the charged probe along the alkane chain makes the induced quasilocal state in the energy gap very sensitive to the bias voltage and results in rectification of the current. The results based on DFT are supported by independent calculations using a simple one-particle model Hamiltonian.

Figure 1

(Color online) The ball-and-stick model of the junction with the ODT molecule and fragments of the gold leads used in our DFT computations. The regular balls indicate the positions of the atoms. The irregular contours in the figure show isosurfaces of LDOS (see text in the end of Sec. 3 for a discussion) near the energy $E_{\text{F}}+1.2\text{eV}$ integrated in the energy window of width 1 eV. The left figure is for the junction without the probe and the right one is for the junction with the probe in position C1. The probe is shown with an additional (red) ball in the right figure.

Figure 2

(Color online) Evolution of the transmission versus energy near the Fermi level $\left(E_{\text{F}}\equiv 0\right)$ as a function of the bias voltage for the different values and the positions of the probe charge obtained from SMEAGOL for the junction with ODT. All the plotted curves but one correspond to $4k$ computations. Left column, upper row: the junction without the probe; $4k$ results for $0<V<2\text{V}$ are compared with $1k$ for $V=0$. Left column, lower row: transmission function for various values of the charge probe in the extended energy scale. Middle column: the probe is positioned near the leftmost C atom (position C1, as in Fig. 1) of the ODT chain at a distance ca. $4\text{Å}$ from the carbon atom ($V<0$: upper row; $V>0$: lower row). Right column: the probe is shifted to the position of third C atom from the left, C3 ($V<0$: upper row; $V>0$: lower row). The heavy lines show the results for the limiting values of voltage indicated with arrows and the thin ones are for intermediate values of voltage.

Figure 3

$I\text{−}V$ dependence for the ODT molecular junction. Left figure: without the charge probe. Middle figure: the $+2\left|e\right|$ charge probe is near the leftmost C atom (C1). Right figure: the $+2\left|e\right|$ charge probe is near the third C atom from the left (C3).

Figure 4

(Color online) PDOS (summed over the sulfur valence orbitals) for the sulfur atoms of ODT molecule coupled to the leads at $V=0$.

Figure 5

(Color online) Evolution of the transmission in the vicinity of the Fermi level $\left(E_{\text{F}}=0\right)$ with the bias voltage for the molecular chain of 8 mers described by the Hamiltonian ${\mathcal{H}}_M$ coupled to the leads with ${\Gamma }_{a,b}=0.2$. The used model parameters are as follows: $E_a=3$, $E_b=-3$, $t_b=-t_a=1$, $t_{ab}=0.11$, $L=2$, and $E_S=2.5$. Left: transmission for the system without the probe; right: transmission for the system with the probe corresponding to $E_L=-2.1$ located near the $L=2$ atom.

Figure 6

$I\text{−}V$ dependence of the model system described by the Hamiltonian ${\mathcal{H}}_M$ for the same parameters as in Fig. 5.