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Manfred M Kappes

Publications and source records attributed to Manfred M Kappes.

25 records · Page 2Linked to original sources
PubMed2005

Frequency dependence of the dielectrophoretic separation of single-walled carbon nanotubes.

Dielectrophoresis on single-walled carbon nanotubes in surfactant suspensions has been demonstrated to separate metallic from semiconducting tubes by their different electric field-induced polarisabilities. Here we report that the interaction between SWNTs and the surfactant induces a nanotube surface conductance which gives rise to a unique electric field frequency dependence of the dielectrophoretic force acting on semiconducting SWNTs. We observe a surfactant concentration dependent crossover frequency enabling separation of metallic from semiconducting SWNTs at high frequency and deposition of metallic and semiconducting SWNTs at low frequency. Proof for the effectiveness of separation is given by a comparative Raman spectroscopy study on dielectrophoretically deposited tubes excited with two different wavelengths.

Crystallization↗
PubMed2004

Electronic photodissociation spectroscopy of Aun- x Xe (n = 7-11) versus time-dependent density functional theory prediction.

Electronic (one-photon) photodepletion spectra were recorded for gold cluster anions complexed with one xenon atom over the photon energy range 2.1-3.4 eV. Clusters were generated by pulsed laser vaporization and probed under collisionless molecular beam conditions. The spectra obtained are highly structured with the narrowest features--assigned to individual electronic transitions--having bandwidths of less than 40 meV. Time-dependent density functional theory predictions of optically allowed transitions for the most stable--planar--isomers of the corresponding bare metal cluster anions are generally consistent with the experimental observation.

Journal Article↗
PubMed2004

Photoelectron spectroscopy of isolated multiply negatively charged oligonucleotides.

Ultraviolet photoelectron spectroscopy in an ion beam was used to investigate the electronic properties of isolated DNA oligonucleotides [dA(5)-4H](4-) and [dT(5)-4H](4-), carrying four excess negative charges. We find the fourth adiabatic electron affinity to be slightly negative for [dA(5)-4H](4-), while it is positive for [dT(5)-4H](4-). This implies a significant influence of the base composition on energetics, which is in turn relevant for analytic applications and also for charge transport properties.

Anions↗
PubMed2004

Structures of mixed gold-silver cluster cations (Ag(m)Au(n)+, m+n<6): ion mobility measurements and density-functional calculations.

The collision cross sections of Ag(m)Au(n)+ (m+n)<6 cluster ions were determined. For bimetallic clusters, we observe a significant intracluster charge transfer leaving most of the ions positive charge on the silver atoms. The mixed trimeric ions Ag2Au+ and AgAu2+ are triangular like the pure gold and silver trimers. Most of the tetrameric clusters are rhombus shaped, with the exception of Ag3Au+, which has a Y structure with the gold atom in the center. Among the pentamers we find distorted X structures for all systems. For Ag2Au3+ we find an additional isomer which is a trigonal bipyramid. These findings are in line with predictions based on density-functional theory calculations, i.e., all these structures either represent the global minima or are within less than 0.1 eV of the predicted global minimum.

Journal Article↗
PubMed2003

Photoelectron spectroscopy of C(84) dianions.

We report the laser photoelectron spectra of doubly negatively charged C84 (D2 and D(2d)) using 532 nm and 355 nm radiation. From these spectra, values for the second electron affinity and vertical detachment energy, as well as upper and lower limits for the repulsive Coulomb barrier, are obtained. These values are discussed in the context of classical electrostatic models. The experimental spectra are compared with the accessible excited states of the C-84 product ion calculated in the framework of time dependent density functional theory.

Journal Article↗
PubMed2003

Separation of metallic from semiconducting single-walled carbon nanotubes.

We have developed a method to separate metallic from semiconducting single-walled carbon nanotubes from suspension using alternating current dielectrophoresis. Our method takes advantage of the difference of the relative dielectric constants of the two species with respect to the solvent, resulting in an opposite movement of metallic and semiconducting tubes along the electric field gradient. Metallic tubes are attracted toward a microelectrode array, leaving semiconducting tubes in the solvent. Proof of the effectiveness of separation is given by a comparative Raman spectroscopy study on the dielectrophoretically deposited tubes and on a reference sample.

Journal Article↗