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TU Dresden » Faculty of Mechanical Science and Engineering » Institute for Materials Science » Chair of Materials Science and Nanotechnology



Thursday, 11 December 2003
(at 15:15 in room Phy 4.1.13)
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Quantum transport through metal/organic/metal systems

Marieta Gheorghe

Dipartimento di Ingegneria Elettronica
Universita di Roma Tor Vergata
  Italy  






We have developed a quantum simulation tool to investigate transport in molecular structures. The method is based on the joint use of density functional tight-binding (DFTB) and of a Green's function technique which allows us the calculation of the current flow through the investigated structures.
Green's function methods have been applied to study the transport properties of single-walled carbon nanotubes reacting with C6H4 (benzene) molecules and nitrine (NH(CH2)2) group. The nanotube is ideally subdivided into three parts: a finite section on which the benzene molecules are adsorbed, sandwiched in between two semi-infinite lead contacts. The ballistic current flowing across the two contacts is monitored during a molecular dynamics (MD) simulation of the central section of the nanotube plus C6H4 molecules. The steady state current along the nanotube with the adsorbed molecules is strongly reduced with respect to the current flowing along the unperturbed nanotube. The current reduction depends strongly on the orientation of the molecule with respect to the nanotube axis. The adsorbtion of the benzene molecule involves bonding with two Carbon atoms of the nanotube. The phase coherence of the transmitted wave is strongly perturbed when the two carbons lie at different positions with respect to the nanotube axis. A much smaller perturbation of the current is predicted when the molecule is adsorbed orthogonally to the axis of the nanotube.
We also investigate the influence of molecular vibrations on the tunneling of electrons through a molecule sandwiched between two metal contacts. The study is confined to the elastic scattering only, but beyond the harmonic approximation. The problem is tackled both from a classical and a quantum-mechanical point of view. The classical approach consists in the computation of the time-dependent current fluctuations along the step of a molecular dynamics (MD) simulations. On the other hand, the vibrational modes are treated quantum-mechanically and the tunneling current is computed as an ensemble average over the distribution of the atomic configurations obtained by a suitable approximation of the density matrix for the normal mode oscillators. We show that the lattice fluctuations modify the electron transmission. However, the answers obtained with the two methods are different. At low temperatures the quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. For temperatures higher than a few hundred Kelvin the simple harmonic approximation which leads to the phonon modes breaks down because the oscillation amplitudes of the lowest energy modes become very large. In this regime beyond the harmonic approximation, higher order terms should be considered leading to phonon-phonon interactions and classical MD simulations prove to be simpler and give better results.

Invited by G. Cuniberti (MC seminar)

last modified: 2018.10.24 Mi
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