Quantum Transport at the Molecular Scale

Molecular electronics – the use of single molecules as functional entities in electronic devices – is often heralded as a replacement for the conventional CMOS approach which faces physical limits in further miniaturization. Even if consumer products based on this emerging technology will not be available in the near future, single molecule devices offer a unique test bed to explore fundamental aspects of both physics and chemistry today and to develop experimental techniques for controlling quantum transport on the molecular scale in a reproducible way.

Experiments can now directly probe the properties of a finite molecule with discrete level structure coupled to macroscopic metallic leads at substantial voltage bias, i.e. at strong nonequilibrium. Electronic transport mediated by charge and spin degrees of freedom is measured and found to be strongly influenced by electron–electron, as well as electron–photon interactions. These interactions give rise to effects like spin– and Frank–Condon blockade of the current, device heating or light–induced transport, to name just a few.

Over the last six years, 29 groups in Germany, together with collaboration partners worldwide, interlinked their efforts to advance the field of “Quantum Transport at the Molecular Scale” under the auspices of the German Science Foundation in the eponymous DFG Priority Program SPP 1243.

The exploration of molecular junctions as unit elements of electronic devices has been widely investigated since the pioneering works of Mark Ratner and collaborators. There has been considerable experimental and theoretical progress in the field, to which research work within the DFG Priority Program 1243 significantly contributed. The transport mechanism in single molecule junctions is most commonly coherent tunneling and an effective qualitative description of the underlying physics can be formulated in terms of Landauer–Büttiker theory. In the last decade, a more accurate parameter–free description of molecular junctions has been achieved by combining steady state Non Equilibrium Green's Function (NEGF) with Density Functional Theory (DFT) and beyond. Getting more mature, these techniques gained large popularity and stimulated collaboration between experimental and computational groups in Germany but also worldwide. Understanding the merits and shortcomings of the methodology, now allows achieving a quantitative understanding of charge transport through short molecules. Within this approach, coherent transport and elastic scattering can be efficiently described also beyond the Landauer picture. Finite bias effects can be taken into account by rigorous calculation of non–equilibrium populations, therefore including space charge effects and bias–dependent level pinning. Open boundary conditions can be included within a non–perturbative approach by means of self–energies and account for the coupling with semi–infinite leads, therefore correctly including the spectral density of the electronic bath beyond the wide band approximation. Experiments in good agreement are routinely compared with computational results making molecular transport today to be well understood, qualitatively and quantitatively, near the Landauer limit and going beyond. These schemes can be applied to systems with a relatively large number of degrees of freedom (typically below one hundred atoms) and have been applied to a plethora of systems including single molecules in break junctions, single molecules and clusters under electrochemical conditions, molecular wires, point contacts and molecules probed by scanning spectroscopy, optically driven molecular switches.

The final progress report of the participating groups in the SPP 1243 has been given at the international CECAM Workshop “Molecular electronics: Quo vadis” held at University of Bremen, Germany from March 4th to 8th, 2013.

The major goal of this meeting was to reflect what has been achieved and frankly disclose open problems. Besides members of the SPP, leading international experts have been invited to discuss about the future of the field and formulate the great challenges ahead: It turned out that the slogan “Molecular electronics: Quo vadis?” did not only mark the end of a successful joint research effort, but rather indicated the plethora of open scientific questions that arose. In that sense, this special issue might be a preliminary conclusion, but also be suited as the starting point for many novel research directions.

Thomas Frauenheim

Bremen, November 2013