Quantum Transport on the Molecular Scale
- Thomas Frauenheim (University of Bremen, Germany)
- Abraham Nitzan (Tel Aviv University, Israel)
- Heiko Weber (University of Erlangen-Nürnberg, Germany)
<p>Molecular Electronics is recognized as a key candidate to succeed the silicon based technology once we have arrived at the end of the semiconductor roadmap. The use of organic molecules in nanoscale nonlinear circuits offers many opportunities for new types of devices, which will differ in fabrication, functionality, and architecture. But even the fundamental question how elec-trical current flows across a single molecule is not satisfactorily understood. The common goal of this workshop to bring together experimental and computational groups to stimulate exchange and to develop a full physical picture of molecular-scale charge transport.</p>
<h2>State of the art</h2>
<p>On the computational side, the workshop will combine two major directions: Model-based approaches familiar from mesoscopic physics (quantum dots) and theoretical chemistry (intramolecular charge transfer) over the years have been adapted to the molecular electronics context. In particular, this implies that analytical approaches and solutions have been developed by explicitly considering molecular properties and by accessing new parameter regimes appropriate for molecules [1,2]. Complementary, first-principle-based atomistic approaches to quantum transport of complete molecular devices (including coupling to leads/substrates) have been developed to feed the model type calculations and to achieve a predictive power relevant to experiment . The latter include Density-Functional-Theory methods with improved functionals and more advanced many-particle/quasiparticle as well as fully time-dependent methods for accurate calculations of energies, couplings, and conductances. While the molecule itself has to be treated as a strongly inhomogeneous many-body system, the coupling to the leads in-duces non-equilibrium conditions. On the experimental side, the field of single-molecule contacts has been successful in contacting individual molecules using various techniques like mechanically controlled break junctions, electromigrated break junctions, nanoparticle dimers, Scanning tunneling microscopes, and more. The parameters range from ultralow temperatures to room temperature, with experiments in vacuum, in solvent or in electrolyte. Each technique has advantages and limitations, and the overall picture is only accessible by careful comparison of the entity of available data. Simultaneously, the community has experienced that structure property relationships are very interesting on the qualitative level, but have difficulties in reproducing results due to the nearly unavoidable sample-to-sample fluctu-ations . This is also a major handicap for delivering reliable data for the comparison with theoretical predictions. Techniques to master these problems have matured [5,6] and now noise, Kondo physics, molecular magnetism, molecular spin crossover and other effects become visible on the single-molecule level, which is a very desirable development for addressing improved quantitative understanding by computational studies.</p>
 G. Cuniberti, G. Fagas, K. Richter, eds. Introducing Molecular Electronics Lecture Notes in Physics 680 (Springer, 2005).
 M. Galperin, M.A. Ratner and A. Nitzan, Molecular Transport Junctions: Vibrational Effects, J. Phys.: Condens. Matter 19, 103201 (2007).
 A. Pecchia and A. Di Carlo, Rep. Prog. Phys 67 (2004) 1497; A. Di Carlo, A. Pecchia, L. Latessa, T. Frauenheim, and G. Seifert, Tight-binding DFT for Molecular Electronics (gDFTB)ï¿½, in: Introducing Molecular Electronics, Springer Series: Lecture Notes in Physics vol. 680; eds. G. Cuniberti, G. Fa-gas, K. Richter, New York, (2005) Chapter 5.
 L. Venkataraman, J. E. Klare, C. Nuckolls, M. S. Hybertsen and M. L. Steigerwald, Dependence of single-molecule junction conductance on mo-lecular con-formation, Nature 442, 904-907 (2006).
 E. Lï¿½rtscher, H. B. Weber, H. Riel, Statistical Approach to Investigating Transport through Single Molecules, Phys. Rev. Lett. 176807 (2007).
 R. Ward, N. J. Halas, J. W. Ciszek, J. M. Tour, Y. Wu, P. Nordlander, and D. Natelson, Simultaneous measurements of electronic conduction and Ra-man response in molecular junctions, Nano Lett. 8, 919-924 (2008).