X-ray diffraction as a tool for the determination of the structure of double-walled carbon nanotube batches

The average structure of double-walled carbon nanotube DWCNT samples can be determined by x-ray diffraction XRD. We present a formalism that allows XRD patterns of DWCNTs to be simulated and we give researchers the tools needed to perform these calculations themselves. Simulations of XRD patterns within this formalism are compared to experimental data obtained on two different DWCNT samples, produced by chemical vapor deposition or by peapod conversion i.e., high-temperature peapod annealing. For each sample, we are able to determine structural aspects such as the number of walls, the diameter distribution of inner and outer tubes, the intertube spacing, and the bundled structure

Symmetry-breaking in the H2@C60endofullerene revealed by inelastic neutron scattering at low temperature

The fine structure of the rotational ground state of molecular ortho-hydrogen confined inside the fullerene cage C60 is investigated by inelastic neutron scattering (INS). The INS line corresponding to transitions between the three sub-levels comprising the ortho ground state to the non-degenerate para ground state was studied as a function of temperature down to 60 mK in neutron energy gain. The experiments show that at ambient pressure the three ortho sub-levels are split into a low energy non-degenerate level and a high energy doubly degenerate level separated by 0.135 ± 0.010 meV. This observation is consistent with hydrogen molecules being located at sites with axial symmetry superseding the icosahedral symmetry of isolated rigid C60 cages in the solid phase. To gain insight into the role of inter-cage interactions in determining the symmetry breaking potential, the effects of hydrostatic pressure on the fine structure of the line was also investigated. The analysis of the INS spectra shows that the potential and the energy levels of H2 are sensitive to the orientation of neighbouring cages, consistent with the low-temperature crystalline phase of C60

Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment

The inelastic scattering of neutrons by nanoparticles due to acoustic vibrational modes (energy below 10 meV) confined in nanoparticles is calculated using the Zemach-Glauber formalism. Such vibrational modes are commonly observed by light scattering techniques (Brillouin or low-frequency Raman scattering). We also report high resolution inelastic neutron scattering measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling the observation of such vibrations are discussed. These include a narrow nanoparticle size distribution which minimizes inhomogeneous broadening of the spectrum and the presence of hydrogen atoms oscillating with the nanoparticle surfaces which enhances the number of scattered neutrons.Comment: 3 figures, 1 tabl

The Endofullerene HF@C 60 : Inelastic Neutron Scattering Spectra from Quantum Simulations and Experiment, Validity of the Selection Rule and Symmetry Breaking

Accurate quantum simulations of the low-temperature inelastic neutron scattering (INS) spectra of HF@C60 are reported for two incident neutron wavelengths. They are distinguished by the rigorous inclusion of symmetry-breaking effects in the treatment and having the spectra computed with HF as the guest, rather than H2 or HD, as in the past work. The results demonstrate that the precedent-setting INS selection rule, originally derived for H2 and HD in near-spherical nanocavities, applies also to HF@C60, despite the large mass asymmetry of HF and the strongly mixed character of its translation–rotation eigenstates. This lends crucial support to the theoretical prediction made earlier that the INS selection rule is valid for any diatomic molecule in near-spherical nanoconfinement. The selection rule remains valid in the presence of symmetry breaking but is modified slightly in an interesting way. Comparison is made with the recently published experimental INS spectrum of HF@C60. The agreement is very good, apart from one peak for which our calculations suggest a reassignment. This reassignment is consistent with the measured INS spectrum presented in this work, which covers an extended energy range

Competition between covalent and non-covalent grafting of fluorescein isothiocyanate on double-walled carbon nanotubes: a quantitative approach

The functionalization of carbon nanotubes with fluorescent molecules is a standard procedure in many toxicity studies aiming at knowing their distribution within cells or whole organisms. Nevertheless, there is a lack of knowledge concerning the efficiency of the grafting processes, and more specifically concerning the question of the competition between covalent and non-covalent grafting. In this work, we investigated the grafting process of the fluorescein isothiocyanate (FITC) onto double-walled carbon nanotubes (DWNTs) using X-ray photoelectron spectroscopy, inelastic neutron scattering spectroscopy and computational simulations.We demonstrated that both covalent and non-covalent grafting occurred during the functionalization with the FITC. Moreover, we showed that a significant fraction of the fluorophore remained simply adsorbed onto the DWNTs despite thorough washing steps, which raises concerning questions about the use of this fluorophore in some toxicity studies and its possible ability to mislead their conclusions

Magnetic Lattice Dynamics of the Oxygen-Free FeAs Pnictides: How Sensitive are Phonons to Magnetic Ordering?

To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin-density wave. The study covers both the 122 (AFe2As2; A=Ca, Sr, Ba) and 1111 (AFeAsF; A=Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating computationally and experimentally the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe2As2 originating from different aspects of structure and bonding

Confirming a predicted selection rule in inelastic neutron scattering spectroscopy: the quantum translator-rotator H2 entrapped inside C60

We report an inelastic neutron scattering (INS) study of H2 molecule encapsulated inside the fullerene C60 which confirms the recently predicted selection rule, the first to be established for the INS spectroscopy of aperiodic, discrete molecular compounds. Several transitions from the ground state of para-H2 to certain excited translation-rotation states, forbidden according to the selection rule, are systematically absent from the INS spectra, thus validating the selection rule with a high degree of confidence. Its confirmation sets a precedent, as it runs counter to the widely held view that the INS spectroscopy of molecular compounds is not subject to any selection rules

Investigation of the grafting of fluorophores onto double-walled carbon nanotubes: the influence of the geometry of the molecules

The functionalization of nanoparticles is nowadays a standard procedure for many applications. Carbon nanotubes are no exception, and their functionalization has become an important part of the research effort these past decades. Nevertheless, there is still a lack of knowledge concerning the efficiency of the grafting processes, and more specifically concerning the question of the competition between covalent and non-covalent grafting when functionalized for example with aromatic fluorescent molecules. In a previous article, we investigated the grafting of fluorescein isothiocyanate onto double-walled carbon nanotubes. In order to explore the influence of the geometry of the fluorophore on the efficiency of the grafting processes, this work investigated the grafting of a streptocyanine onto the same carbon nanotubes using a combination of X-ray photoelectron spectroscopy, inelastic neutron scattering spectroscopy and computational simulations. The selected streptocyanine presents a very different geometry compared to FITC. Nevertheless, we show that, as in the case of FITC, both covalent and non-covalent grafting occurred simultaneously and that a comparable fraction of the fluorophore remained simply adsorbed onto the DWNTs despite thorough washings, indicating that the geometry of the streptocyanine did not exhibit the expected influence on the grafting process efficiency

Experimental, theoretical and computational investigation of the inelastic neutron scattering spectrum of a homonuclear diatomic molecule in a nearly spherical trap: H2@C60

In this paper we report a methodology for calculating the inelastic neutron scattering spectrum of homonuclear diatomic molecules confined within nano-cavities of spherical symmetry. The method is based on the expansion of the confining potential into multipoles of the coupled rotational and translational angular variables. The Hamiltonian and the INS transition probabilities are evaluated analytically. The method affords a fast and computationally inexpensive way to simulate the inelastic neutron scattering spectrum of molecular hydrogen confined in fullerene cages. The potential energy surface is effectively parametrized in terms of few physical parameters comprising an harmonic term, anharmonic corrections and translation–rotation couplings. The parameters are refined by matching the simulations against the experiments and the excitation modes are identified for transfer energies up to 215 meV