Current-voltage characteristics through a single light-sensitive molecule

A light-sensitive molecular switch based on single azobenzene molecule has been proposed recently [C. Zhang, M. H. Du, H. P. Cheng, X. G. Zhang, A. E. Roitberg, and J. L. Krause, Physical Review Letters 92, 158301 (2004)]. Here we investigate the stability of the molecular switch under finite bias. Using a first-principles method that combines the nonequilibrium Green’s function technique and density functional theory, we compute the current-voltage curves for both trans and cis configurations of the azobenzene molecule connected to two gold leads between bias voltages of 0 and $1\mathrm{V}$. We find that the current through the trans configuration is significantly higher than that through the cis configuration for most biases, suggesting that the molecular switch proposed previously is stable under the finite bias. A negative differential conductance (NDR) is found for the cis configuration at $0.8\mathrm{V}$. Analysis of the band structure of the leads and the molecular states reveals that the transmission through the highest occupied molecular orbital state of the molecule is suppressed significantly at this bias voltage, which causes the NDR.

Figure 1

(Color online) Panel (a) is the device structure of the molecular switch that consists of two gold leads and an azobenzene molecule. Top: the trans configuration. Bottom: the cis configuration. (b) Band structure of the model quasi-one-dimensional gold wire. Note that only one band goes across the Fermi energy.

Figure 2

(Color online) The difference in electrostatic potential between 0 and $1\mathrm{V}$ biases for the cis configuration. The $x\text{−}z$ plane is parallel to the ${\mathrm{N}}_2$ bond, cutting through the midpoint of the Au unit cell.

Figure 3

(Color online) Transmission probability as a function of energy and the external bias voltage for the cis configuration.

Figure 4

(Color online) Current as a function of bias voltage for both trans and cis configurations. The top line represents the current for trans and the bottom line is for cis.

Figure 5

(Color online) The differential conductance as a function of bias voltage for both trans and cis configurations.

Figure 6

(Color online) Transmission coefficient as a function of energy of the cis configuration at external bias voltages of 0.64, 0.72, 0.8, and $0.88\mathrm{V}$, in solid, dotted, dashed, and dotted-dashed lines, respectively. The solid vertical line indicates the Fermi energy.