William J. Romanow

Orientational disorder in solvent-free solid c70.

The high-temperature structure of solvent-free C70 has been determined with high-resolution x-ray powder difraction and electron microscopy. Samples crystallized from solution form hexagonal close-packed crystals that retain an appreciable amount of residual toluene, even after prolonged heating. Samples prepared by sublimation, which contain no detectable solvent, are primarily face-centered cubic with some admixture of a hexagonal phase. The relative volume of the hexagonal phase can be further reduced by annealing. The structures of both phases are described by a model of complete orientational disorder. The cubic phase contains an appreciable density of stacking faults along the [111] direction.

X-Ray Diffaction Evidence for Nonstoichiometric Rubidium-C60 Intercalation Compounds

Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions χ in RbχC60 is reported. In addition to the face-centered cubic (fcc) (χ = 3, superconducting), body-centered tetragonal (χ = 4), and body-centered cubic (bcc) (χ = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 x c ≤ 1, and for a substoichiometric bcc phase, χ ∼ 5, is presented. In contrast, χ = 3 and χ = 4 appear to be line phases with nearly zero solubility of Rb vacancies and interstitials at 300 K. These results are summarized in a provisional binary phase diagram.

Identification of a growth defect in solid C 60 by electron diffraction

The origin of an anomalous sawtooth-shaped feature in x-ray powder diffraction of solid C 60 is explained via electron diffraction analysis. Films sublimed on holey carbon crystallize with close-packed (111) planes parallel to the surface. Rods of diffuse scattering are found along the 〈111〉 axis normal to the surface but not along other 〈111〉 axes. Powder averaging of these rods, coupled with the x-ray form factor of spherical shells with 3.5 A radius, accounts for the sawtooth feature. We attribute this phenomenon to planar defects parallel to close-packed layers, which form during the growth of solid C 60 by sublimation. A possibly related consequence of the growth mode is the observation of strong macroscopic (111) preferred orientation in films sublimed on a variety of substrates.

C71H2 cyclopropanes and annulenes: synthesis and characterization

Addition of diazomethane to a toluene solution of C70 affords a mixture of isomeric pyrazolines, which, upon photolysis, furnish two isomeric C71H2 cyclopropanes; thermolysis of the mixture produces two isomeric annulenes.

Photoluminescence in solid films of C60

Abstract Photoluminescence has been measured for solid films of nominally pure C 60 and C 60 /C 70 mixtures, as a function of temperature and excitation energy. The photoluminescence spectrum shows vibronic structure, and there is evidence for the involvement of three modes. Trace amounts ( 70 appear to be clearly visible in the spectrum of the nominally pure C 60 film.

The dependence of the symmetry (spin) forbidden photoluminescence of C60 molecules on their proximity to metal

Abstract We find out that a monolayer of C 60 molecules in close proximity (0–4 nm) to an Au substrate exhibits a striking photoluminescence in both the visible and near-IR due to the radiative recombination of singlet and triplet excitions, respectively. The luminescence is observed even when the C 60 molecules are in direct contact with the metal. This is the first observation of a molecular triplet luminescence that is induced by a metal substrate, which is all the more remarkable since a metal substrate normally quenches the luminescence of adsorbed molecules.

Optical properties of fullerence C60 film in the vicinity of orientational phase transition probed by ellipsomerty

Abstract In this article we report the effect of dynamic disorder in the vicinity of orientational phase transition (Tc=250–260 K) in solid C 60 on reduction of molecular local symmetry. We probe these phenomena by ellipsometry at the wavelength 520 nm, which corresponds to orbitally forbidden molecular absorption of fullerene. The temperature dependence of optical constants of solid fullerene has been compared with its luminescent properties and depolarization of the reflected light. It has been shown that all data exhibit hysteresis-like behavior nearby Tc.