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TranSIESTA: A SPICE for Molecular Electronics

Kurt Stokbro, Jeremy Taylor, and Mads Brandbyge
Mikroelektronik Centret, DK

Keywords: First principles modelling, molecular electronics, rectifiers, switches

With our newly developed method,TranSIESTA[1], it has become possible to model complex molecular electronics devices under operation conditions. In this presentation I will describe the basic principles behind the method and applications of the method to model recent experimental data for molecular rectifiers and molecular devices displaying negative differential resistance. The TranSIESTA method is based on density functional theory, and calculates the selfconsistent electronic structure of a molecule coupled to 3-dimensional electrodes with different electrochemical potentials, using a full atomistic ab initio description of both the electrodes and the molecular structure. The calculations reveal information about the scattering states, transmission coefficients, electron current and nonequilibrium forces in the system. We have used the method to investigate the IV characteristics of an Aviram-Ratner type molecular diode and compares the theoretical results with experimental data. The calculations reveal that the electrode coupling plays an important role, and that an asymmetric coupling to the external electrodes is the origin of the rectification observed experimentally. To investigate the effect of electrode coupling we have calculated the IV characteristics of a symmetric molecule, a phenylene-ethynylene oligomer, for different electrode couplings. From these calculations we extract a simple model for the diode characteristics as function of the molecular coupling to the external electrodes.[3] These relations can rationalize several experimental results, and may be used to design molecular diodes with improved properties. Other experiments of Phenylene-ethynylene with nitro side groups[4] have shown Negative Differential Resistance. We have calculated the IV characteristics of Phenylene-ethynylene oligomers with a varity of sidegroups, and based on these calculations we discuss the origin of the complex electrical properties of such systems. [1] M. Brandbyge, J. L. Mozos, P. Ordejon, J. Taylor, K. Stokbro, Phys. Rev. B. 65, 165401 (2002). [2] SIESTA: D. Sanchez-Portal, P. Ordejon, E. Artacho and J. Soler, Int. J. Quantum Chem. 65, 453 (1997). [3] J. Taylor, M. Brandbyge, and K. Stokbro. Phys. Rev.Lett. (in press). [4] J. Chen, M.A. Reed, A.M. Rawlett, J. M. Tour, Science 286, 1550 (1999).

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