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2009

2013     2012     2011     2010     2009

 

Research Highlights 2009

 

L. Lafferentz, F. Ample, H. Yu, S. Hecht, C. Joachim, and L. Grill

Conductance of a Single Conjugated Polymer as a Continuous Function of its Length

Science 323, 1193 (2009)

Abstract. The development of electronic devices at the single-molecule scale requires detailed understanding of charge transport through individual molecular wires. To characterize the electrical conductance, it is necessary to vary the length of a single molecular wire, contacted to two electrodes, in a controlled way. Such studies usually determine the conductance of a certain molecular species with one specific length. We measure the conductance and mechanical characteristics of a single polyfluorene wire by pulling it up from a Au(111) surface with the tip of a scanning tunneling microscope, thus continuously changing its length up to more than 20 nanometers. The conductance curves show not only an exponential decay but also characteristic oscillations as one molecular unit after another is detached from the surface during stretching.

 

 

M. Piantek, G. Schulze, M. Koch, K.J. Franke, F. Leyssner, A. Krüger, C. Navío, J.Miguel, M. Bernien, M. Wolf, W. Kuch, P. Tegeder, and J. I. Pascual

Reversing the thermal stability of a molecular switch on a gold surface: ring-opening reaction of nitro-spiropyran

J. Am. Chem. Soc. 131, 12729 (2009)

Abstract. he ring-opening/closing reaction between spiropyran (SP) and merocyanine (MC) is a prototypical thermally and optically induced reversible reaction. However, MC molecules in solution are thermodynamically unstable at room temperature and thus return to the parent closed form on short time scales. Here we report contrary behavior of a submonolayer of these molecules adsorbed on a Au(111) surface. At 300 K, a thermally induced ring-opening reaction takes place on the gold surface, converting the initial highly ordered SP islands into MC dimer chains. We have found that the thermally induced ring-opening reaction has an activation barrier similar to that in solution. However, on the metal surface, the MC structures turn out to be the most stable phase. On the basis of the experimentally determined molecular structure of each molecular phase, we ascribe the suppression of the back reaction to a stabilization of the planar MC form on the metal surface as a consequence of its conjugated structure and large electric dipole moment. The metal surface thus plays a crucial role in the ring-opening reaction and can be used to alter the stability of the two isomers.

 

 

C. Nacci, S. Fölsch, K. Zenichowski, J. Doki, T. Klamroth, and P. Saalfrank

Current versus Temperature-Induced Switching in a Single-Molecule Tunnel Junction: 1,5 Cyclooctadiene on Si(001)

Nano Lett. 9, 2996–3000 (2009)

Abstract. The biconformational switching of single cyclooctadiene molecules chemisorbed on a Si(001) surface was explored by quantum chemical and quantum dynamical calculations and low-temperature scanning tunneling microscopy experiments. The calculations rationalize the experimentally observed switching driven by inelastic electron tunneling (IET) at 5 K. At higher temperatures, they predict a controllable crossover behavior between IET-driven and thermally activated switching, which is fully confirmed by experiment.

 

 

M. Bernien, J. Miguel, C. Weis, Md. E. Ali, J. Kurde, B. Krumme, P. M. Panchmatia, B. Sanyal, M. Piantek, P. Srivastava, K. Baberschke, P. M. Oppeneer, O. Eriksson, W. Kuch, and H. Wende

Tailoring the Nature of Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Substrates

Phys. Rev. Lett. 102, 047202-1-4 (2009)

Abstract. We demonstrate that an antiferromagnetic coupling between paramagnetic Fe-porphyrin molecules and ultrathin Co and Ni magnetic films on Cu(100) substrates can be established by an intermediate layer of atomic oxygen. The coupling energies have been determined from the temperature dependence of x-ray magnetic circular dichroism measurements. By density functional theory+U calculations the coupling mechanism is shown to be superexchange between the Fe center of the molecules and Co surface-atoms, mediated by oxygen.

 

 

N. Severin, S. Kirstein, I.M. Sokolov, and J.P. Rabe

Rapid trench channeling of graphenes with catalytic silver nanoparticles

Nano Lett. 9, 457-461 (2009)

Abstract. We demonstrate trench channeling of mono- and multilayer graphenes with silver nanoparticles with high speed in ambient environment and at elevated temperatures. A silver nanoparticle located at a graphene edge catalyzes oxidation of neighboring carbon atoms, thereby burning a trench into the graphene layer. High-resolution scanning tunneling microscopy imaging reveals that the trench edges are very smooth with a peak-to-peak roughness below 2 nm. We discuss the channeling mechanism and demonstrate that channeling speeds of up to 250 nm/s and the smoothness of the resulting trenches indicate the prospect of a “catalytic pen” for high-precision lithography on graphenes.

 

 

C. Bombis, F. Ample, L. Lafferentz, H. Yu, S. Hecht, C. Joachim, and L. Grill

Single molecular wires connecting metallic and insulating surface areas

Angew. Chem. Int. Ed. 48, 9966 (2009)

Abstract. he ring-opening/closing reaction between spiropyran (SP) and merocyanine (MC) is a prototypical thermally and optically induced reversible reaction. However, MC molecules in solution are thermodynamically unstable at room temperature and thus return to the parent closed form on short time scales. Here we report contrary behavior of a submonolayer of these molecules adsorbed on a Au(111) surface. At 300 K, a thermally induced ring-opening reaction takes place on the gold surface, converting the initial highly ordered SP islands into MC dimer chains. We have found that the thermally induced ring-opening reaction has an activation barrier similar to that in solution. However, on the metal surface, the MC structures turn out to be the most stable phase. On the basis of the experimentally determined molecular structure of each molecular phase, we ascribe the suppression of the back reaction to a stabilization of the planar MC form on the metal surface as a consequence of its conjugated structure and large electric dipole moment. The metal surface thus plays a crucial role in the ring-opening reaction and can be used to alter the stability of the two isomers.

 

 

2013     2012     2011     2010     2009