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Frank Hennrich

Publications and source records attributed to Frank Hennrich.

7 recordsLinked to original sources
PubMed2005

Spectroscopy of single- and double-wall carbon nanotubes in different environments.

Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.

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PubMed2005

On the electron-phonon coupling of individual single-walled carbon nanotubes.

We show that the phonon coupling to the electronic system in individual metallic single-walled carbon nanotubes is not due to coupling to low-energy plasmons. The evidence stems from the measured Raman-Stokes G-mode, which for metallic and semiconducting tubes could be fitted well by the superposition of only two Lorentzian lines associated with vibrational modes along the nanotube axis and the nanotube circumference. In the case of metallic tubes the lower-energy G mode is significantly broadened, however maintaining the Lorentzian line shape, in contrast to the theoretically expected asymmetric Breit-Wigner-Fano line shape from phonon-plasmon coupling. The results were obtained by studying 25 individual metallic and semiconducting single-walled carbon nanotubes with atomic force microscopy, electron transport measurements, and resonant Raman spectroscopy.

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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.

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PubMed2003

Functionalization of single-walled carbon nanotubes with (R-)oxycarbonyl nitrenes.

Sidewall functionalization of single-walled carbon nanotubes (SWCNTs) via the addition of (R-)oxycarbonyl nitrenes allows for the covalent binding of a variety of different groups such as alkyl chains, aromatic groups, dendrimers, crown ethers, and oligoethylene glycol units. Such additions lead to a considerable increase in the solubility in organic solvents such as 1,1,2,2-tetrachloroethane (TCE), dimethyl sulfoxide (DMSO), and 1,2-dichlorobenzene (ODCB). The highest solubilities of 1.2 mg/mL were found for SWCNT adducts with nitrenes containing crown ether of oligoethylene glycol moieties in DMSO and TCE, respectively. The presence of chelating donor groups within the addends allowed for the complexation of Cu(2+) and Cd(2+). Atomic force microscopy (AFM) and transmission electron microscopy (TEM) revealed that the functionalized tubes form thin bundles with typical diameters of 10 nm. The presence of thin bundles in solution is supported by (1)H NMR spectroscopy. The elemental composition of the functionalized SWCNT was determined by X-ray photoelectron spectroscopy (XPS). The use of Raman and electron absorption spectroscopy (UV/Vis-nIR) showed that the electronic properties of the SWCNTs are mostly retained after functionalization, indicating a low degree of addition within this series of SWCNT derivatives.

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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.

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