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Dieter M Kolb

Publications and source records attributed to Dieter M Kolb.

5 recordsLinked to original sources
PubMed2006

Local density of states effects at the metal-molecule interfaces in a molecular device.

Clarifying the nature of interactions between metal electrodes and organic molecules still represent one of the challenging problems in molecular electronics that needs to be solved in order to optimize electron transport through a molecular device. For this purpose, electronic properties at metal-molecule interfaces were studied by combining experimental and theoretical methods. Applying a novel electrochemical approach, strictly two-dimensional Pd islands were prepared on top of 4-mercaptopyridine self-assembled monolayers (4MP-SAMs) which, in turn, were deposited on (111)-oriented Au single crystals. Electron spectroscopy together with density functional theory calculations revealed strong interactions between the molecules and the islands due to Pd-N bonds, resulting in a drastically reduced density of states (DOS) at the Fermi level EF for a nearly closed Pd monolayer, and even non-metallic properties for nanometre-sized islands. Similarly, a significantly reduced DOS at EF was observed for the topmost Au layer at the Au-SAM interface due to Au-S interactions, suggesting that these effects are rather general.

Journal Article↗
PubMed2004

Structure and electrochemistry of 4,4'-dithiodipyridine self-assembled monolayers in comparison with 4-mercaptopyridine self-assembled monolayers on Au(111).

4,4'-Dithiodipyridine (PySSPy) monolayers on Au(111) were investigated by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). The studies were performed in solutions of different anions and pHs (0.1 M H2SO4, 0.1 M HClO4, 0.1 and 0.01 M Na2SO4, 0.1 and 0.01 M NaOH). The cyclic current-potential curves in H2SO4 show current peaks at about 0.4 V, which are absent for all other electrolytes at this potential. The XPS data suggest that PySSPy adsorbs via the S endgroup on the gold surface and the S-S bond breaks during adsorption. From the chemical shift of the N(ls) peak, it is concluded that in acidic media the self-assembled monolayer (SAM) is fully protonated, whereas in basic solution it is not. The pKa is estimated to be 5.3. STM studies reveal the existence of highly ordered superstructures for the SAM. In Na2SO4 and H2SO4, a (7 x mean square root of 3) structure is proposed. However, whereas in Na2SO4 solutions the superstructure does not change with potential, in 0.1 M H2SO4 the superstructure is observed only negative of the current peak at +0.4 V. At more positive potentials, the film becomes disordered. The results are compared to those for 4-mercaptopyridine (PyS) SAMs. XPS experiments and current-potential curves indicate that both molecules adsorb in the same manner on Au(111), that is, even in the case of PySSPy the adspecies is PyS. The STM results, however, call for a more subtle interpretation. While in Na2SO4 solutions the observed superstructures are the same for both SAMs, markedly different structures are found for PySSPy and PyS SAMs in 0.1 M H2SO4.

Journal Article↗
PubMed2004

Self-assembled monolayers of 4-mercaptopyridine on Au(111): a potential-induced phase transition in sulfuric acid solutions.

In situ scanning tunneling microscopy images of self-assembled monolayers (SAMs) of 4-mercaptopyridine (4-MPy) on Au(111) recorded in neat 0.1 M H2SO4 solutions provided evidence for a potential-induced phase transition over the range 0.40-0.15 V versus saturated calomel electrode. Analysis of the data was consistent with the presence of a (5 x square root(3)) and (10 x square root(3)) superstructure (phase A) at the positive end, that is, 0.40 V, for which the local coverage, theta(loc), is about 0.2 (two 4-MPy molecules per unit cell), which compresses at the negative end, that is, 0.15 V, to yield a much denser superstructure (phase B, theta(loc) ca. 0.5). This behavior is unlike that reported for the 4-MPy-Au(111) SAM prepared by identical means, in 0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 x square root(3)) superstructure was observed over the same potential range. The compression associated with the phase A to phase B transition is attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPy layer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing of adsorbed 4-MPy species, which are aided by intermolecular pi-pi ring interactions, resulting in higher local coverages.

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