M. Gheorghe

Theoretical tools for transport in molecular nanostructures

We have developed a quantum simulation tool to investigate transport in molecular structures. The method is based on the joint use of a density functional tight binding (DFTB) and of a Greens function technique which allows us the calculation of current flow through the investigated structures. Typical calculations are shown for carbon-nanotube-based field effect transistors and for DNA fragments. Transport; molecular structures

Vibrational effects in the linear conductance of carbon nanotubes

We study the influence of structural lattice fluctuations on the elastic electron transport in single-wall carbon nanotubes within a density-functional-based scheme. In the linear-response regime, the linear conductance is calculated via configurational averages over the distorted lattice. Results obtained from a frozen-phonon approach as well as from molecular-dynamics simulations are compared. We further suggest that the effect of structural fluctuations can be qualitatively captured by the Anderson model with bond disorder. The influence of individual vibrational modes on the electronic transport is discussed as well as the role of zero-point fluctuations.

Molecular Devices Simulations Based on Density Functional Tight-Binding

We have developed a quantum simulation tool to investigate transport in molecular structures. The method is based on the joint use of a Density functional tight-binding (DFTB) and of a Greens function technique which allows us the calculation of current flow through the investigated structures. Typical calculations are shown for carbon-nanotube-based field effect transistors, sensors and for DNA fragments.

The influence of thermal fluctuations on the electronic transport of alkeno-thiolates

In this paper, we investigate the influence of molecular vibrations on the tunneling of electrons through alkeno-thiolates of varying lengths sandwiched in between two gold contacts. The study is confined to the elastic scattering. 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. The quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. The calculations show no temperature dependence for the tunneling current in the regime between 270-350 K.

Electronic transport in molecular devices: the role of coherent and incoherent electron?phonon scattering

The Green function for the coupled electron–phonon system is derived from a quantized version of the SSH Hamiltonian and is solved exactly for relevant modes of vibration. This Green function is used to compute the elastic and inelastic current components thought a di-thio-phenylene molecule in between Au contacts.

Influence of the electron-phonon interactions on the transport properties at the molecular scale

In the present work we 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 uctuations calculated at each step of a molecular dynamics (MD) simulation. 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. At low temperatures the quantum-mechanical treatment is necessary in order to correctly include the zero-point fluctuations. However, for temperatures higher than few hundreds Kelvin the simple harmonic approximation which leads to the phonon modes breaks because the oscillation amplitudes of the lowest energy modes become large.