The unique properties of DNA molecules (self assembling and self recognition) which are essential for its function as carrier of the genetic code have attracted in the past years the attention of the molecular electronics community. Two potential applications have been up to now envisioned for DNA: (i) as template in molecular electronic circuits, and (ii) as wiring system (molecular wire). Concerning the latter, a fundamental functioning precondition is the capability to support an electric current. The partially contradictory electrical transport experiments in the last years have presented a challenge to theoreticians.
Many factors have turned out to be essential in determining charge propagation in DNA: the quality of the molecule-metal contacts, the specific base-pair sequence, environmental effects, and structural fluctuations, among others. In our research, we have mainly addressed the influence of the environment and internal vibrational excitations on the quantum transport characteristics of short DNA molecular wires. Structural fluctuations mediated by the previous two physical factors can be a source of dissipation and decoherence for propagating charges and thus dramatically affect the transport properties of the molecule.
In view of the complexity of the target system, we have adopted a Hamiltonian-model approach, which allows to identify and filter out individual factors controlling charge transport in DNA. Our results have demonstrated e.g. the emergence, inside the molecular HOMO-LUMO gap, of environment-induced virtual states which may strongly modify the low-energy transport in DNA wires. Presently, we are working along three main lines to further develop our approach: (i) Hamiltonian parametrization via density-functional calculations of the electronic structure of realistic DNA configurations, (ii) implementation of charge-vibron interactions in the non-equilibrium regime via Keldysh Green function techniques, (iii) phenomenological description of charge transport in noisy environments with strong non-Markovian character. The first line will help to reduce the arbitrariness of the model Hamiltonian, the second one will lead to a realistic treatment of the finite voltage transport regime. Finally, the last line will allow for the phenomenological inclusion of strong nonlinear and long-time memory effects in the nuclear dynamics of the DNA molecular wire. These three research lines will allow a closer and more realistic contact to DNA electrical transport experiments.
Carbon nanotubes (CNTs) possess unique electronic properties which made them soon after their discovery the target of possible nanoelectronic applications. The implementation of CNTs as active components of molecular electronic circuits is thus extremely promising. The theoretical challenges in studies of systems related to CNTs involve the handling of large systems of up to several thousand atoms in a rigid, quasi-1D-crystal structure using tools at various levels of precision ranging from classical mechanical models and quantum mechanical tight binding models to ab initio methods, Green function decimation techniques and Landauer/Keldysh transport calculations. We study the effect of extended metal-CNT contacts, the electronic properties of CNT three terminal devices, telescopic CNTs, phonons and vibrons in CNTs and their coupling to electronic transport channels, spin-transport, magneto-transport in periodic and disordered CNTs and gold nanotubes, as recently observed in experiments.
Transport through single molecules is in the focus of both experimental and theoretical investigations not only because of the possible application in electronic devices but also in view of the fundamental understanding of fascinating new quantum phenomena. Depending on the quality of the molecule/leads contacts one could observe a whole range of physical effects ranging from coherent transport to sequential tunneling (the Coulomb blockade effect). Molecules, however, are typically very flexible objects and to be able to operate molecular devices at room temperature and at finite voltage it is extremely important to understand the role of molecular vibrations on quantum-transport. Thus investigation of interplay of correlation and vibration effects is the main theoretical challenge here.
To accurately describe quantum transport in realistic molecular systems one needs a reliable method to characterize the underlying electronic structure. For this reason, we work at combining the density functional theory and the tight-binding approach with the powerful technique of nonequilibrium Green functions in the presence of the electron-vibron interaction for describing inelastic transport through molecular junctions. This method is first tested on simple single and double level systems and then applied to real molecules (e.g. as we have done for a hydrogen molecule clamped between two platinum electrodes or for alkeno-thiolate molecules between gold contacts). We are now concentrating in building a unified theory for strong-to-weak couplings to the electrodes and studying current-voltage curves, as well as conductance and noise characteristics, of a single benzene ring, bi- and ter-phenyl junctions. In the last case current switching effect through mutual ring rotation are expected to have a very large effect onto quantum transport.