Jonathan P. Hare

Electronic spectra and transitions of the fullerene C60

Abstract Absorption spectra of C60 have been measured in the ranges (a) 190–700 nm in n-hexane solutions at 300 K, (b) 390–700 nm in n-hexane and in 3-methylpentane solutions at 77 K. 40 vibronic bands were observed. They exhibit a large range of bandwidths and intensities, whose significance is discussed. Assignment of electronic transitions has been carried out using the results of theoretical calculations. Vibronic structures have been analyzed within the framework of theories of electronic transitions of polyatomic molecules applied to the Ih symmetry group. Nine allowed 1T1u−1Ag transitions have been assigned in the 190–410 nm region. Observed and calculated allowed transition energies and oscillator strengths are compared. Detailed vibronic analyses of the 1 1T1u−1 1Ag and 2 1T1u−1 1Ag transitions illustrate the role of Jahn-Teller couplings. Orbitally forbidden singlet-singlet transitions are observed between 410 and 620 nm. Their vibronic structures were analyzed in terms of concurrent Herzberg-Teller and Jahn-Teller vibronic interactions. The 77 K spectra provided useful information on hot bands and on other aspects of the analyses. Vibronic bands belonging to triplet←singlet transitions were detected between 620 and 700 nm.

Tungsten oxide tree-like structures

Abstract An interesting micrometer scale tree-like structure has been generated by heating a W foil, partly covered by a SiO 2 plate, in an Ar atmosphere at ca. 1600°C. Upon sonication, the trees are broken into nanoneedles (ca. 5–50 nm wide and 20–200 nm long) and planar polyhedral nanoparticles (ca. 10–50 nm cross-section). Structural analysis, using ED, EDX, XRD, and HRTEM, showed that: (1) the trees consist of well-crystallised WO x phases ( x =0–3); (2) the nanoneedles are composed mainly of monoclinic W 18 O 49 phases; and (3) the nanoparticles consist primarily of WO 3 . The tree growth is thought to arise from the intrinsic crystalline feature of WO x , the planar defect or the shear structure of which is responsible for the breakdown of the trees.

A SIMPLE ROUTE TO SILICON-BASED NANOSTRUCTURES

.Powders of the nanoparticles were obtained from the colloidal solutions by removing the water with a rotary evaporator (bath temperature 50 C). Transmission electron micrographs of the samples were taken using a Philips CM 300 UT electron microscope, working at 300 kV acceleration voltage. A Philips Xpert system was used to measure the X-ray diffraction pattern of powder samples. UV-vis absorption spectra of the colloidal solutions were recorded with a Lambda 40 spectrometer (Perkin‐Elmer). Photoluminescence spectra were recorded with a Spex Fluoromax 2 spectrometer having a spectral resolution of 0.5 nm.

3D Silicon oxide nanostructures: from nanoflowers to radiolaria

Novel flower-like nanostructures consisting of silicon oxide nanofibers, radially attached to a single catalytic particle, were generated by solid-solid and gas-solid reactions under a temperature gradient. In this process, a mixture of SiC and Co powders, deposited on silica substrates and heated under an Ar/CO atmosphere at ca. 1500C, produced material with unusual three-dimensional (3D) networks of nanofibers of uniform diameter (ca. 20-120nm) and length (ca. 10-250mu;m). Scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction and energy dispersive X-ray (EDX) analyses reveal that the nanofibres are amorphous and consist only of silicon oxide, generated from the reaction of CO with SiC. Nanostructure formation is catalyzed by Co particles, which act as nucleation sites and templates for 3D growth. Experiments using Si3N4 and Si in conjunction with other catalysts (e.g. Fe, Ni and CoO) yield similar results and confirm that the resulting SiOx fibres display virtually unique and remarkable radial growth starting from single metal particles. These structures exhibit morphologies comparable to radiolarian and diatom skeletons and may provide insight into the formation of microbiological systems.

Inelastic neutron scattering spectrum of the fullerene C60

Abstract The vibrational spectrum of C 60 has been measured by inelastic neutron scattering at 20 K. All 14 infrared and Raman active vibrational modes can be identifeid together with many optically inactive and low-energy lattice modes. The spectrum extends smoothly to ≈ 200 MeV. Fairly good agreement with existing models of the vibrational properties of C 60 is found.

SiC–SiOx heterojunctions in nanowires

Novel 2-D nanoscale SiC networks have been generated by heating SiC–Fe–Co mixtures under a CO atmosphere. Examination of the products by EDX and HRTEM showed that most of the nanowires consist of β-SiC elongated single crystals, wrapped in amorphous SiOx sheaths. Intriguing crystalline features and defects associated with SiC nanowires were observed. We believe that a binary Fe–Co catalyst is responsible for the SiC network: Fe catalyses the formation of the SiC inner cores and Co the SiOx (x = 1–2) outer shell. A two-step growth mechanism, involving a vapour–liquid–solid (V–L–S) step, is thought to account for SiC nanowire creation.

Synthesis of nanotubes via catalytic pyrolysis of acetylene: A SEM study

Pyrolysis of acetylene over quartz plates coated with various metal catalysts resulted in the formation of all-carbon nanostructures which were observed directly by scanning electron microscopy (SEM). The nanotubes were studied in detail using transmission electron microscopy (TEM), and appear to grow as ultra-thin tubes with a central hollow core and considerable thickening due to secondary pyrolytic deposition. In nearly all samples the nanotube diameter was uniform. In some cases catalytic particles were entrapped within the tubes. The efficiency of the catalysts was evaluated semi-quantitatively.

Enthalpies of formation of buckminsterfullerene (C60) and of the parent ions C60+, C602+, C603+ and C60–

The standard enthalpy of formation of buckminsterfullerene (C60) in the crystalline state has been determined as 2278.1 ± 14.4 kJ mol–1 using a microcombustion calorimeter. Statistical mechanical calculations of the heat capacity of gaseous C60 as a function of the temperature and a critical survey of the literature data for the enthalpy of sublimation, the heat capacity in the crystalline state, the first, second and third ionization energies and the electron affinity of C60, enabled the enthalpy of formation of gaseous C60 and the enthalpies of formation of the ions C60+, C602+, C603+ and C60– to be derived.

The vibrational Raman spectra of C60 and C70

Abstract Vibrational Raman spectra have been obtained at 1 cm−1 resolution, for C60 and C70. For C60 the positions of the bands were in good agreement with theoretical predictions. For C70 27 bands were observed out of the total of 53 that were theoretically predicted. These data will permit refinement of calculations of vibrational lines appropriate to closed-cage carbon clusters. The technique for producing appreciable quantities of high purity C70 is described.

Fluorination of buckminsterfullerene

Fluorination of solid C60 with fluorine gas proceeds in a stepwise manner to give, after twelve days (10 mg scale), a colourless derivative indicated by a single line 19F NMR spectrum to be C60F60; reaction time is quantity dependent.