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C4.11: Theory and Simulation of Molecular Matrials and Functionality

Subproject Leader:

Ferdinand Evers Institut für Nanotechnologie, KIT
Peter Wölfle Institut für Nanotechnologie, KIT
 

Contributing Scientists:

Present: Soumya Bera, Guillaume Geranton

 

Figure 1:  Graphene flake with hydrogene termination near the edges, size N=5. The flake buckles due to the application of isotropic pressure.
Figure 1: Graphene flake with hydrogene termination near the edges, size N=5. The flake buckles due to the application of isotropic pressure.
Figure 2: Results of our  DFT  transport study. [5] Top panels: Transmission function for parallel (a) and antiparallel (b) alignment of electrode spins. The LUMO based transport resonance is clearly visible near 0.3eV.
Figure 2: Results of our DFT transport study. [5] Top panels: Transmission function for parallel (a) and antiparallel (b) alignment of electrode spins. The LUMO based transport resonance, is clearly visible near 0.3 e.V.
The orbital plots illustrate how the down shift in energy of the Co-based d-band for the majority carriers makes it more difficult for the corresponding molecule based LUMO orbital to hybridize with the electrodes.
The orbital plots illustrate how the down shift in energy of the Co-based d-band for the majority carriers makes it more difficult for the corresponding molecule based LUMO orbital to hybridize with the electrodes.

Project C4.11 focuses on the theory and simulation of molecular materials and devices along two major strands of activity which are both closely intertwined with research, either experimental or theoretical, of other CFN groups. The first set of activities studies material properties of graphene flakes, the second set combines with STM experiments in the group of Wulf Wulfhekel and is devoted to Molecular Electronics.

Graphene: Since its fabrication in clean form has become technologically feasible, graphene has been in the focus of frontier research. One of its most celebrated properties are its massless low energy excitations ("Dirac fermions''), which emanate from the symmetries of the honeycomb lattice. While epitaxial methods to grow graphene allow to achieve relatively large, clean and homogeneous films, chemistry technologies open up a way into mass production of smaller graphene flakes. Indeed, the electronic properties of such flakes can be quite different from bulk graphene, e.g., due to the finite size and the presence of edges.

Our first activity in C4.11 addresses the elastic properties of small graphene flakes by means of elaborate calculations based on the density functional theory as implemented in the TURBOMOLE package and analytical considerations [1]. We find that the edge can compress the chemical bonds internal to the flake so much, that the bond length decreases by ~1% for the smallest flakes that were investigated. As a consequence many phenomenological parameters of the flakes, e.g., the Lamee parameters deviate significantly from their bulk values, up to 30%.

Our second activity focuses on the effect of vacancies on the density of states and the mobility of charge carriers in graphene films. Indeed, a single vacancy introduces a state with energy at the boundary between the HOMO/LUMO level. Such zero modes tend to be very important for the low energy response of graphene flakes. Our investigations demonstrate this for the special example of the conductivity. [2] With increasing number of vacancies the conductivity first increases reaching a maximum value beyond which it starts decreasing again until it drops to zero in the percolation limit.

The last topic is motivated by the observation that the effect of many defects in graphene sheets ("disorder") cannot be understood by considering a single defect, e.g., a single vacancy alone. This is because in the presence of many defects electronic states form a complicated interference pattern which is accompanied by very large fluctuations of the wavefunction amplitude. These fluctuations become especially interesting in the presence of a strong magnetic field, i.e. in the quantum Hall effect. Then, the moments of the wavefunctions <|Ψ(r)|2q> scale with the flake size, N, in a power law manner,   <|Ψ(r)|2q>~Nτq, where each moment scales with its own exponent τq . We investigate the influence of such "multifractal" scaling behavior on the effective Coulomb interaction and other flake poperties.   [3]

Molecular Electronics: In Molecular Electronics molecules are investigated with respect to their properties concerning electronic functionality as, e.g., transistors or memory elements. In recent years we have been collaborating with several experimental groups in order to understand how the molecular design influences the current voltage characteristics. [4]

With an eye on molecular switches, the response of molecule to external stimuli, e.g., a gate potential or a change of magnetization of the contacts is of particular interest.Wulf Wulfhekel and Stefan Schmaus have been able to measure the magnetoconductance of a single Phtalocyanine (Pc) molecule, see subproject C4.10. In the case of the H2Pc molecule a value for the giant magnetoconductance (GMR) of 60% was reported. This value is extremely large, especially in view of the fact that H2Pc does not contain any magnetic atoms. Motivated by this observation we have performed transport calculations which reveal that it is the spin selective broadening of the ligand based LUMO levels of H2Pc which induces such a big GMR value. [5]

References

[1]

S. Bera, A. Arnold, F. Evers, R. Narayanen, and P. Woelfle, Elastic properties of graphene flakes: Boundary effects and lattice vibrations, Phys. Rev. B 82, 195445 (2010).

[2]

P. M. Ostovsky, M. Titov, S. Bera, I. V. Gornyi, and A. D. Mirlin, Diffusion and Criticality in Undoped Graphene with Resonant Scatterers, Phys. Rev. Lett. 105, 266803 (2010).

[3]

I. Burmistrov, S. Bera, F. Evers, I. V. Gornyi, A. D. Mirlin, Wavefunction multifractality and dephasing at metal-insulator and quantum Hall transitions, arXiv:1011.3616 (2011).

[4]

A. Mischchenko, D. Vonlanthen, M. Buerkle, J. Vilas, F. Pauly, V. Meded, A. Bagrets, C. Li, I. Pobelov, F. Evers, M. Mayor and T. Wandlowski, STM-break junction electric conductance measurement of single molecule biphenyl dithiol derivatives, Nano Letters 10, 156 (2010).

[5]

S. Schmaus, A. Bagrets, Y. Nahas, T. K. Yamada, A. Bork, M. Bowen, E. Beaurepaire, F. Evers and W. Wulfhekel, Magneto-resistance through single molecules using a spin-polarized STM, Nature Nanotechnology 6, 185 (2011).

 

List of Publications 2006-2011 as PDF

Subproject Report 2006-2010 as PDF