Current-voltage characteristics through a single light-sensitive molecule Chun Zhang,* Yao He, and Hai-Ping Cheng † Department of Physics and Quantum Theory Project, University of Florida, Gainesville, Florida 32611 Yongqiang Xue College of Nanoscale Science and Engineering, University at Albany—SUNY, Albany, New York 12203 Mark A. Ratner Department of Chemistry, Northwestern University, Evanston, Illinois 60208 X.-G. Zhang and Predrag Krstic Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received 31 July 2005; revised manuscript received 3 October 2005; published 31 March 2006 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 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 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. DOI: 10.1103/PhysRevB.73.125445 PACS numbers: 85.65.+h, 73.63.-b, 82.37.Gk I. INTRODUCTION Electronic devices based on single molecules have been considered as one of the most promising technologies to ex- tend today’s semiconductor-based electronics. 1–10 Many po- tentially useful molecular electronic devices have been proposed. 2,3,11–15 Most prominent among these is a single- molecule switch 13–15 since a switch is a critical element of any modern design of logical and memory circuits. In a pre- vious work, 14 we proposed a light-driven molecular switch consisting of a single azobenzene molecule connected to two semi-infinite Au leads via two linker S atoms. The azoben- zene molecule has two stable configurations: the trans and the cis states. In the linear response regime, the trans con- figuration was shown to have a significantly higher conduc- tance than the cis configuration. Since the molecule can be switched reversibly from one configuration to the other by photoexcitation, 16–18 it is a promising candidate for the light- driven molecular switch. In this paper we will examine the stability of this switching behavior under a finite bias volt- age. Applying a finite bias voltage V b , the Fermi energies deep in both leads are shifted by ±V b /2, respectively, which changes the electrostatic potential and correspondingly the effective single-particle potential in the device area. We in- clude a sufficiently large proportion of the leads into the active device region. The electronic potential deep in the two leads far away from the device region is, then, not affected by the electron interactions in the device area. The nonlinear transport characteristics are computed using first-principles methods, 15,19–21 which combine the nonequilibrium Green’s functions technique of quantum transport 22,23 with the den- sity functional theory DFT of the device electronic structure. 24 II. THEORETICAL APPROACH Transport studies deal with open systems that are con- nected to two or more external reservoirs. Within the re- gime of coherent transport, this is studied using the scatter- ing theory introduced by Landauer and Buttiker. 25,26 In this approach, the transport system under study is divided into three parts—left lead, right lead, and the scattering region or the device area—which also includes portions of two elec- trodes to take into account of the molecule-lead coupling and the lead screening effect. Applying a finite bias voltage shifts the Fermi energies of the two leads relative to each other V b =  R -  L without losing generality, we assume that  R   L , where  R and  L are the Fermi energies of right and left leads, respectively. The bias voltage enters the calcula- tion via shifts in the electrostatic potential in the left lead by the amount of -V b /2 and the right lead by V b /2, which forms the boundary condition for studing the charge and po- tential response of the device region. 20 The electronic struc- ture of the leads is obtained through two separate lead cal- culations, from which we obtain two self-energy terms,  L and  R , due to the left and right leads, respectively, using standard procedures. 27,28 A self-consistent procedure based on DFT is used to calculate the effective single-particle po- PHYSICAL REVIEW B 73, 125445 2006 1098-0121/2006/7312/1254455/$23.00 ©2006 The American Physical Society 125445-1