Research Highlights

Quantum Chemistry

The role of the anchoring groups in single molecule conduction

One of the key problems in molecular electronics is to elucidate the role of the anchoring groups that bind the molecules to the metallic electrodes. In the BIMORE project ER Linda Zotti (UAM) collaborated with the experimental groups of Elke Scheer (University of Konstanz) and Thomas Wandlowski (University of Bern) to address this fundamental issue.

Our theoretical calculations have shown that the anchoring groups not only determine the strength of the metal-molecule coupling, but more importantly, they also control the position of the molecular levels that are relevant for the transport. In this sense, we have shown, in good agreement with the experiments, that the use of different anchoring groups (like thiol, amine or nitrile) to attach the same molecule can give rise to very different transport properties. In other words, we have proven that conductance of a single-molecule junction can be largely tuned by chemical means, which paves the way for engineering these nanoscale electronic circuits. The results of ER Linda Zotti and her colleagues have been recently published:

“Revealing the role of anchoring groups in the electrical conduction through single-molecule junctions”, L.A. Zotti, T. Kirchner, J.C. Cuevas, F. Pauly, T. Huhn, E. Scheer, and A. Erbe, Small 6, 1529 (2010).

“Single molecule junctions based on nitrile- terminated biphenyls: A promising new anchoring group”, A. Mishchenko, L.A. Zotti, D. Vonlanthen, M. Bürkle, F. Pauly, J.C. Cuevas, M. Mayor, and T. Wandlowski, J. Am. Chem. Soc. 133, 184 (2011).

Molecular Biology

Relating structure and functionality of light harvesting complexes using single molecule spectroscopy

BIMORE Researchers from University of Glasgow in collaboration with University of Bayreuth studied individual light harvesting complexes (LH2) from purple bacteria grown under low light conditions. Single-molecule spectroscopy reveals that LH2 complexes from Rhodopseudomonas palustris 2.1.6. have a heterogeneous polypeptide composition.

Tatas H.P. Brotosudarmo, Vladimíra Moulisová, et, al., Biophys. J., 97(11), 3019–3028(2009).

Condensed matter photophysics

Exciton size and mobility in (6,5) carbon nanotubes

We studied the fundamental properties of excitons (bound electron-hole pairs) in carbon nanotubes. Taking advantage of a highly defined sample and of the high temporal resolution available at Politecnico di Milano, we found that the “exciton size” (electron-hole correlation length) is close to 2 nm, and that the exciton diffusion length in these samples is only on the order of 10 nm.

L. Lüer, S. Hoseinkhani, D. Polli, J. Crochet, T. Hertel, G. Lanzani, Nature Physics 5, 54 - 58 (2009).

The results are of high importance because i) they validate theoretical descriptions of excitons in carbon nanotubes, that predicted a similar exciton size and ii) they show that many excitons can coexist on an isolated nanotube without the risk of exciton annihilation, making them apt for photonic applications like, lasers and optical switches.

Coherent Phonon Dynamics in Semiconducting Carbon Nanotubes:A Quantitative Study of Electron-Phonon Coupling

L. Lüer, C. Gadermaier, J. Crochet, T. Hertel, D. Brida, G. Lanzani, Phys. Rev. Lett. 102, 127401 (2009).

We excite and detect coherent phonons in semiconducting (6,5) carbon nanotubes via a sub-10-fs pump-probe technique. Simulation of the amplitude and phase profile via time-dependent wave packet theory yields excellent agreement with experimental results under the assumption of molecular excitonic states and allows determining the electron-phonon coupling strength for the two dominant vibrational modes.

Highly conductive molecular junctions based on direct binding of benzene to platinum electrodes.

M. Kiguchi, O. Tal, S. Wohlthat, F. Pauly, M. Krieger, D. Djukic, J.C. Cuevas, J. M. van Ruitenbeek, Phys. Rev. Lett. 101, 046801 (2008). See accompanying Viewpoint Physics 1, 5 (2008).

Juan Carlos Cuevas from Universidad Autonoma de Madrid calculated the properties (geometrical structure, conductance, vibrational energy) of single benzene molecules interfaced with Platinum electrodes via single atomic contacts. The results explained experimental facts obtained by collaborating groups outside of Bimore. The results were selected for a “Viewpoint in Physics”. They are of outstanding importance for two reasons: i) they show that stable contacts between metal electrodes and single molecules can be achieved without the need of a sulphur bridge; this opens our synthetic partners in Denmark and Israel new possibilities of collaborations with the single molecule conduction groups Reserved Area IBM and UAM and ii) they show that the theoretical formalism deployed in Madrid describes correctly the geometrical and electronic structure of single molecules between atomic contacts to metal electrodes. Full list of publications by the Bimore consortium.

© 2011 Last Modified: September