Showing papers by "Hans-Gerd Boyen published in 2009"

Chemical Interactions at Metal/Molecule Interfaces in Molecular Junctions—A Pathway Towards Molecular Recognition†

Abstract: Adv. Mater. 2008, 20, 1–5 2008 WILEY-VCH Verlag Gmb Small organic molecules as potential building-blocks for future nanoelectronic devices will require new types of sensors able to identify/quantify molecules in appropriate solutions on a single-molecule level. Recently, an elegant new method has been proposed that allows detection of small aromatic units like 4aminothiophenol (4-ATP) molecules by measuring the tunneling resistance between two metal electrodes separated by a short distance (a few nanometers). While, from a simple point of view, an increase in conductivity should be expected because of the bridging of the tunnelling gap by one or more molecules (thus offering molecular orbitals as additional transport channels), a decreased conductivity was observed experimentally with a reduction factor that depended on the type of molecule present in the solution. Here, we report on combined experimental and theoretical efforts aimed at unravelling this phenomenon by studying the electronic properties of one of themetal electrodes in such a molecular junction. For this purpose, a 4-ATP self-assembledmonolayer (SAM) has been prepared on top of a Au(111) crystal, which, in a second step, has been metallized by a nearly closed Pd overlayer of monoatomic height by means of a recently developed electrochemical approach. Photoelectron spectroscopy together with density functional theory (DFT) taking into account all contributing parts of the molecular junction finally allowed analysis of its structural setup and its electronic properties. Angle-resolved X-ray photoelectron spectroscopy (XPS) reveals that the 4-ATP SAM actually consists of a minimum of two molecular layers. Most importantly, using ultraviolet photoelectron spectroscopy (UPS) and DFT simulations, strong chemical interactions between the metal overlayer and the amino groups are found to play a decisive role in determining the overall electronic properties, and thus the transport properties of the SAM/metal contact, as will be demonstrated in the following. It is well-known that 4-ATP has a strong tendency to form multilayers on Au(111), which lead to scanning tunnelling microscopy (STM) images with considerable height variations and of blurred contrast when it comes to molecular-scale resolution. Even for highly diluted solutions (sub-millimolar concentrations of the 4-ATP), more than just one layer is generally formed. On the other hand, reductive desorption of thiols from gold surfaces is known to occur at electrode potentials negative of 0.1 V vs. standard calomel electrode (SCE), which may be considered an appropriate means for desolving any thiol in excess of the first layer. In Figure 1 cyclic voltammograms are shown for Au(111) in 0.1 M H2SO4, after the electrode had been immersed for 15min in a 0.1 10 3 M 4-ATP/0.1 M H2SO4 modification solution. While the first cycle (dash-dotted line, start at þ0.2 V vs. SCE in the negative direction) was restricted to the stability range of the 4-ATP adlayer, i.e., 0 and þ0.4 V, the second cycle (solid line) was extended to much more negative potentials to explore partial reductive desorption. The latter process is clearly reflected in a cathodic current that peaks at 0.18V. The third cycle (dotted line) now shows double-layer charging behaviour up

Transition from anomalous kinetics towards Fickian diffusion for Si dissolution into amorphous Ge

Abstract: Over the last years several experimental and theoretical studies of diffusion kinetics on the nanoscale have shown that the time evolution differs from the classical Fickian law (kc=05) However, all work was based on crystalline samples or models, so far In this letter, we report on the diffusion kinetics of a thin amorphous-Si layer into amorphous-Ge to account for the rising importance of amorphous materials in nanodevices Employing surface sensitive technics, the initial kc was found at 07+-01 Moreover, after some monolayers of Si dissolved into the Ge, kc changes to the generally expected classical Fickian law with kc=05