Microscopic study of electrical transport through single molecules with metallic contacts: Organic molecules and finite
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Microscopic study of electrical transport through single molecules with metallic contacts: Organic molecules and finite carbon nanotube Yongqiang Xue and Mark A. Ratner Department of Chemistry and Materials Research Center Northwestern University Evanston, Illinois 60208 ABSTRACT We present microscopic study of electronic and transport properties of single molecules sandwiched between two metallic contacts using Green’s function based modeling approach within both ab initio and self-consistent semi-empirical framework. The methods are applied to thiol-based organic molecules and finite-size single-wall carbon nanotubes respectively. Results on electrostatics, transmission and current-voltage characteristics are presented. INTRODUCTION Exploring the use of individual molecules as active components in electronic devices has been at the forefront of nanoelectronics research in recent years. Numerous useful device characteristics including molecular rectifying diodes, negative differential resistance and fieldeffect transistors have been demonstrated using molecular-scale structures including small conjugated molecules [1,2], single- and multi-wall carbon nanotubes [3,4]. The electrical characteristics of molecular devices are usually measured by sandwiching the molecules between two metallic electrodes [1-8]. A major task of theoretical study of molecular electronics is therefore to predict the electronic and transport properties through such metal-molecule-molecule junctions given the structures of the molecule, the electrodes and their geometrical arrangement. For short organic molecules, transport through the metal-molecule-metal junctions can be modeled using fully self-consistent First-principles based method. But for long wire-shaped molecules like carbon nanotube which typically contains hundreds and thousands of atoms, a semi-empirical approach which takes into account self-consistently the charge transfer induced by the metal-molecule coupling and the applied electrical field will be more versatile. The purpose of this paper is to present both approaches and their application to molecular electronic transport with metallic contacts. As an example of heterostructured molecule where the molecule is connected to the metallic electrodes through end group chemically different to the molecule core, two thiol-based molecules - phenyl dithiolate (PDT) and biphenylene dithiolate (BDT) molecule – sandwiched between two gold electrode is studied using the first-principles based self-consistent Green’s function method. As an example of homogenous molecule, finite-size (10,0) carbon nanotube end-contacted to two gold and titanium electrodes is studied using our recently developed self-consistent semi-empirical method. FIRST-PRICIPLES BASED SELF-CONSISTET GREEN’S FUNCTION APPROACH Our first-principles based self-consistent Green’s function (SCGF) method has been discussed in detail elsewhere [7], so we give only a brief description here to motivate the discussion of the self-consistent semi-empirical method in the n
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