Jenő Kürti

Structure and energetics of neutral and negatively charged C60 dimers

Abstract Ground state optimized geometries and total energies have been determined for the neutral and the doubly negative charged C 60 dimers of various structures. Semi-empirical calculations have been carried out using MNDO, AM1 and PM3 parametrization. For the neutral and the charged dimers different structures have been found at the global energy minima. In the neutral case, the well known 66/66 cycloaddition product with D 2h symmetry was found to be the most stable configuration. In contrast, the charged dimer had a different structure: the AM1 and PM3 calculations showed that it is bound through a single carbon atom on each C 60 moiety, the two balls being in trans position (C 2h symmetry). This is in accord with X-ray results on the quenched phase of A 1 C 60 (A = K, Rb).

Oxygen-isotope labeled titania: Ti18O2

(18)O-isotope labelled titania (anatase, rutile) was synthesized. The products were characterized by Raman spectra together with their quantum chemical modelling. The interaction with carbon dioxide was investigated using high-resolution FTIR spectroscopy, and the oxygen isotope exchange at the Ti(18)O(2)/C(16)O(2) interface was elucidated.

Density of states deduced from ESR measurements on low‐dimensional nanostructures; benchmarks to identify the ESR signals of graphene and SWCNTs

Electron spin resonance (ESR) spectroscopy is an important tool to characterize the ground state of conduction electrons and to measure their spin-relaxation times. Observing ESR of the itinerant electrons is thus of great importance in graphene and in single-wall carbon nanotubes. Often, the identification of CESR signal is based on two facts: the apparent asymmetry of the ESR signal (known as a Dysonian lineshape) and on the temperature independence of the ESR signal intensity. We argue that these are insufficient as benchmarks and instead the ESR signal intensity (when calibrated against an intensity reference) yields an accurate characterization. We detail the method to obtain the density of states from an ESR signal, which can be compared with theoretical estimates. We demonstrate the success of the method for K doped graphite powder. We give a benchmark for the observation of ESR in graphene.

Dimensional changes as a function of charge injection for trans-polyacetylene: A density functional theory study

Charge-induced dimensional changes allow conducting polymers and single walled carbon nanotubes to function as electromechanical actuators. The unit cell of the prototypical conducting polymer, trans-polyacetylene, was calculated as a function of charge injection using density functional theory in combination with ultrasoft pseudopotentials using the solid-state Vienna ab initio simulation package. Test calculations on the charged pyridinium molecular ion give results in good agreement with the experimental geometry. Strain versus charge relationships are predicted from dimensional changes calculated using a uniform background charge (“jellium”) for representing the counterions, which we show provides results consistent with experiment for doped polyacetylenes. These jellium calculations are consistent with further presented calculations that include specific counterions, showing that hybridization between the guest dopant ions and the host polyacetylene chains is unimportant. The lack of guest–host orbit...

Preparing local strain patterns in graphene by atomic force microscope based indentation

Patterning graphene into various mesoscopic devices such as nanoribbons, quantum dots, etc. by lithographic techniques has enabled the guiding and manipulation of graphene’s Dirac-type charge carriers. Graphene, with well-defined strain patterns, holds promise of similarly rich physics while avoiding the problems created by the hard to control edge configuration of lithographically prepared devices. To engineer the properties of graphene via mechanical deformation, versatile new techniques are needed to pattern strain profiles in a controlled manner. Here we present a process by which strain can be created in substrate supported graphene layers. Our atomic force microscope-based technique opens up new possibilities in tailoring the properties of graphene using mechanical strain.

Resonance Raman Optical Activity of Single Walled Chiral Carbon Nanotubes

Resonance (vibrational) Raman Optical Activity (ROA) spectra of six chiral single-walled carbon nanotubes (SWCNTs) are studied by theoretical means. Calculations are performed imposing line group symmetry. Polarizability tensors, computed at the π-electron level, are differentiated with respect to DFT normal modes to generate spectral intensities. This computational protocol yields a ROA spectrum in good agreement with the only experiment on SWCNT, available at present. In addition to the conventional periodic electric dipole operator we introduce magnetic dipole and electric quadrupole operators, suitable for conventional k-space calculations. Consequences of the complex nature of the wave function on the scattering cross section are discussed in detail. The resonance phenomenon is accounted for by the short time approximation. Involvement of fundamental vibrations in the region of the intermediate frequency modes is found to be more notable in ROA than in Raman spectra. Calculations indicate exceptionally strong resonance enhancement of SWCNT ROA signals. Resonance ROA profile of the (6,5) tube shows an interesting sign change that may be exploited experimentally for SWCNT identification.

I-band-like non-dispersive inter-shell interaction induced Raman lines in the D-band region of double-walled carbon nanotubes

Non-dispersive, inter-layer interaction induced Raman peaks (I bands)—in the region of the D band—have been observed recently for bilayer graphene, when the two layers were rotated with respect to each other. Here, similar observations for double-walled carbon nanotubes are theoretically predicted. The prediction is based on double resonance theory, involving non-zone-centered phonons, and the effect of disorder is replaced by interaction between the two tubes.

The growth of new extended carbon nanophases from ferrocene inside single-walled carbon nanotubes

The Raman response of new structures grown after filling SWCNTs with ferrocene and transformation t moderate high temperatures is demonstrated to be very strong, even stronger than the response from the tubes. Transmission electron microscopy demonstrates that the new objects are flat and exhibit a structure similar to short fragments of nanoribbons. The growth process is controlled by two different activation energies for low and high transformation temperatures, respectively. Immediately after filling Raman pattern from a precursor molecule are detected. Two different types of nanoribbons were identified by selecting special laser energies for the Raman excitation. These ribbons have the signature of quaterrylene and terrylene, respectively.

Controlled Isotope Arrangement in 13C Enriched Carbon Nanotubes

We report the synthesis of a novel isotope engineered 13C–12C heteronuclear nanostructure: single-wall carbon nanotubes made of 13C enriched clusters which are embedded in natural carbon regions. The material is synthesized with a high temperature annealing from 13C enriched benzene and natural C60, which are coencapsulated inside host SWCNTs in an alternating fashion. The Raman 2D line indicates that the 13C isotopes are not distributed uniformly in the inner tubes. A semiempirical method based modeling of the Raman modes under 13C isotope enrichment shows that experimental data is compatible with the presence of 13C rich clusters which are embedded in a natural carbon containing matrix.

The role of Van Hove singularities in disorder induced Raman scattering

The disorder induced D band in the Raman spectrum of graphite and carbon nanotubes has been shown to originate from a double resonance process, involving a scattering by a phonon, and a scattering by a defect. In the case of carbon nanotubes, the Van Hove singularities are expected to have a significant effect on the D band. We present a detailed study of this effect on the case of a simple one dimensional semiconducting model system, with quadratic electronic and phonon dispersion relations. The D band is calculated by exact integration of the perturbation formulas and the dependence of the position and the intensity on the laser excitation energy and the damping factor describing the finite lifetime is examined in detail. The intensity of the D band shows an extra enhancement when the energy of the incoming or the outgoing photon matches or nearly matches the energy of a Van Hove singularity of the nanotube.