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



Monday, 17 March 2003
(at 11:15 in room Phy 4.1.13)
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Transport properties of molecular electronics devices:
a self-consistent density-functional tight-binding approach


Aldo Di Carlo

Dipartimento di Ingegneria Elettronica
Università di Roma Tor Vergata
  Italy  






Technological advances in fabrication, characterization and control at the nanoscale level have enabled the manufacturing of a variety of new organic-inorganic heterostructures with a good degree of reproducibility. In many cases the prototypical devices involve just one single molecule as active component, hence the name of molecular electronics which has been given to this new field. In recent years we have seen the first striking successes of this technology and the demonstration of its potentialities.[1] Such a new class of devices requires new simulation approaches, since the inherent quantum-mechanical physics involved must be treated properly. The debate over the exact nature of the transport mechanisms in many of such systems still remains open.
We have developede[2] a new code for transport computations based on the self-consistent density-functional tight-binding (DFTB) method, [3] on which we have implemented the non-equilibrium Green's function approach (NEGF) [4] for the computation of the charge density. This scheme allows treatment of systems comprising a large number of atoms and enables the computation of the tunneling current flowing between two contacts in a fully self-consistent manner with the open boundary conditions that naturally arise in transport problems. The key ingredient of the self-consistent loop is the solution of the Hartree potential of the density functional Hamiltonian which is calculated by solving the corresponding three-dimensional Poisson's equation involving the non-equilibrium charge density with the correct boundary conditions. We give a full description of our methodology and show applications to molecular rectifiers, molecular FETs, and transport in Carbon Nanotubes.
Phonon scattering is an important issue for transport properties in nanometer scale systems. We analyse the effect of elastic scattering with coherent phonons in a -conjugated phenylene-ethynylene molecule sandwiched in between two Au contacts. We compute the tunneling current by performing a quantum Monte Carlo average over the ensemble of the configurations of the atomic coordinates. We show that in organic molecular devices we may expect the presence of low frequency vibrational modes which arise from internal degrees of freedom involving the collective motion of large submolecular sections. Such vibrations could occur, as in our example, from torsional oscillations around triple bonds which have low energy barriers. However, such low energy modes may have relevant implications on electron-phonon scattering. Since these modes become quickly populated as the temperature grows, we show that the simplest phononic approximation leads to incorrect results already at moderate temperatures. In contrast, classical Molecular Dynamics (MD) simulations do not suffer from artifacts due to the first order expansion of the molecular vibrations into eigenmodes and fully account for the non linearities involved in the regime of large oscillation amplitudes. The zero-point phononic fluctuations, not accounted properly in the classical MD, are shown to be important only at low temperatures. We demonstrate that the phononic fluctuations can account for up to an order of magnitude discrepancy from the tunneling currents computed at 0 K.
[1] P.J.Kuekes, R.S.Williams and R.J.Heath: "Molecular wire crossbar memory", U.S. Patent #6.128.214, October 3, 2000
[2] A. Di Carlo e t al., Physica B, 314, 86 (2002).
[3] M.Elstner et al., Phys. Rev. B. 58, 7260 (1998).
[4] L.V. Keldysh, Sov. Phys. JEPT 20, 1018 (1965).

Invited by G. Cuniberti (MC seminar)

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