Donald S. Bethune

Vibrational Raman and infrared spectra of chromatographically separated C60 and C70 fullerene clusters

Abstract We report vibrational Raman and infrared spectra for chemically separated C 60 and C 70 fullerenes. Thin film samples were prepared by subliming the chromatographically separated species onto appropriate substrates. The C 60 Raman spectrum shows eight clear lines and two weaker ones. If C 60 in fact that has the proposed buckminsterfullerene structure (as is strongly indicated by recent experiments), the present Raman measurements together with the four observed IR frequencies give a complete set of Raman and infrared active fundamental frequencies for this molecule. A comparison of this set with the calculated spectrum for buckminsterfullerene shows satisfactory agreement.

On the efficacy of electrocatalysis in nonaqueous Li-O2 batteries.

Heterogeneous electrocatalysis has become a focal point in rechargeable Li-air battery research to reduce overpotentials in both the oxygen reduction (discharge) and especially oxygen evolution (charge) reactions. In this study, we show that past reports of traditional cathode electrocatalysis in nonaqueous Li-O(2) batteries were indeed true, but that gas evolution related to electrolyte solvent decomposition was the dominant process being catalyzed. In dimethoxyethane, where Li(2)O(2) formation is the dominant product of the electrochemistry, no catalytic activity (compared to pure carbon) is observed using the same (Au, Pt, MnO(2)) nanoparticles. Nevertheless, the onset potential of oxygen evolution is only slightly higher than the open circuit potential of the cell, indicating conventional oxygen evolution electrocatalysis may be unnecessary.

Bond lengths in free molecules of buckminsterfullerene, c60, from gas-phase electron diffraction.

Electron diffraction patterns of the fullerene C60 in the gaseous state have been obtained by volatilizing it from a newly designed oven-nozzle at 730�C. The many peaks of the experimental radial distribution curve calculated from the scattered intensity are completely consistent with icosahedral symmetry for the free molecule. On the basis of this symmetry assumption, least-squares refinement of a model incorporating all possible interatomic distances led to the values rg(C1-C2) = 1.458(6) angstroms (�) for the thermal average bond length within the five-member ring (that is, for the bond fusing five- and six-member rings) and rg(C1-C6) = 1.401(10) � for that connecting five-member rings (the bond fusing six-member rings). The weighted average of the two bond lengths and the difference between them are the values 1.439(2) � and 0.057(6) �, respectively. The diameter of the icosahedral sphere is 7.113(10) �. The uncertainties in parentheses are estimated 2σ values.

The vibrational Raman spectra of purified solid films of C60 and C70

Abstract A technique to produce samples consisting primarily of C 60 and C 70 by fractional sublimation of carbon soot was found and used to produce solid films of these molecules. Film compositions were determined using a surface analytical mass spectrometric technique. Vibrational Raman spectra of the purified films were measured and vibrational lines of both C 60 and C 70 are identified. A C 60 line at 273 cm −1 is observed, in agreement with theoretical predictions for the lowest frequency H g “squashing” mode of Buckminsterfullerene. The two strongest C 60 lines, found at 1469 and 497 cm −1 , can consistently be assigned to the two totally symmetric A g modes on the basis of their frequencies and measured depolarization ratios.

The sticking of O2 on a Pt(111) surface

This paper reports detailed molecular beam measurements of the sticking coefficient at zero coverage for O2 on a Pt(111) surface as a function of initial energy (Ei), angle of incidence (θi), and surface temperature (Ts). Under most conditions the sticking coefficient measures the probability for dissociative chemisorption. These results demonstrate that both precursor mediated and quasi‐direct dissociation can be observed, depending upon the initial conditions. The quasi‐direct process is revealed by a step increase in the sticking with Ei. This feature scales intermediately between Ei and the normal component En, and is weakly dependent on Ts. The precursor mediated sticking is well described by standard precursor kinetic models. At low Ei and Ts, sticking measures trapping into a molecularly adsorbed state. This trapping decreases more rapidly with Ei than anticipated from simple models and scales intermediately between Ei and En. The sticking results are discussed in terms of likely dynamic processes ...

Activation of methane dissociation on a Pt(111) surface

This paper reports detailed molecular beam measurements of the dissociative chemisorption probability for methane on a Pt(111) surface. We find large increases in the dissociative chemisorption probability S0 with increases in Ei cos2 θi (the normal component of translational energy), Ev (the vibrational energy of the incident methane), and Ts (surface thermal energy). The comparable activation of the reaction by addition of any of these three forms of energy cannot be accounted for by any single model for C–H bond activation proposed to date. A large kinetic isotope effect is also observed, with S0 decreasing significantly for CD4 relative to CH4.

C60 Rotation in the Solid State: Dynamics of a Faceted Spherical Top

The rotational dynamics of C60 in the solid state have been investigated with carbon-13 nuclear magnetic resonance (13C NMR). The relaxation rate due to chemical shift anisotropy (1/9T1CSA1) was precisely measured from the magnetic field dependence of T1, allowing the molecular reorientational correlation time, τ, to be determined. At 283 kelvin, τ = 9.1 picoseconds; with the assumption of diffusional reorientation this implies a rotational diffusion constant D = 1.8 x 1010 per second. This reorientation time is only three times as long as the calculated τ for free rotation and is shorter than the value measured for C60 in solution (15.5 picoseconds). Below 260 kelvin a second phase with a much longer reorientation time was observed, consistent with recent reports of an orientational phase transition in solid C60. In both phases τ showed Arrhenius behavior, with apparent activation energies of 1.4 and 4.2 kilocalories per mole for the high-temperature (rotator) and low-temperature (ratchet) phases, respectively. The results parallel those found for adamantane.

Scandium clusters in fullerene cages.

The production and spectroscopic characterization of fullerene-encapsulated metal-atom clusters is reported. In particular, both solution and solid-state electron paramagnetic resonance (EPR) spectra of Sc3C82 have been obtained. ScC82 also gives an EPR spectrum, but Sc2Cn species—the most abundant metallofullerenes in the mass spectrum—are EPR-silent even though Sc2 is EPR-active in a rare-gas matrix at 4.2 K. The results suggest that the three scandium atoms in Sc3C82 form an equilateral triangle—as was previously suggested for Sc3 molecules isolated in a cryogenic rare-gas matrix. The spectrum of ScC82 has features similar to those found earlier for LaC82 and YC82, suggesting that it can also be described as a +3 metal cation within a -3 fullerene radical anion. An implication of this work is that production of macroscopic quantities of clustercontaining fullerenes may make possible the fabrication of exotic new structures with regular arrays of metal-atom clusters isolated in fullerene molecules, resulting in a new type of host/guest nanostructured material.

Carbon nanotubes with single-layer walls

Macroscopic quantities of single-layer carbon nanotubes have recently been synthesized by co-condensing atomic carbon and iron group or lanthanide metal vapors in an inert gas atmosphere. The nanotubes consist solely of carbon, sp 2 -bonded as in graphene strips rolled to form closed cylinders. The structure of the nanotubes has been studied using high-resolution transmission electron microscopy. Iron group catalysts, such as Co, Fe, and Ni, produce single-layer nanotubes with diameters typically between 1 and 2 nm and lengths on the order of micrometers. Groups of shorter nanotubes with similar diameters can grow radially from the surfaces of lanthanide carbide nanoparticles that condense from the gas phase. If the elements S, Bi, or Pb (which by themselves do not catalyze nanotube production) are used together with Co, the yield of nanotubes is greatly increased and tubules with diameters as large as 6 nm are produced. Single-layer nanotubes are anticipated to have novel mechanical and electrical properties, including very high tensile strength and one-dimensional conductivity. Theoretical calculations indicate that the properties of single-layer tubes will depend sensitively on their detailed structure. Other novel structures, including metallic crystallites encapsulated in graphitic polyhedra, are produced under the conditions that lead to nanotube growth.

Laser deposition of carbon clusters on surfaces: A new approach to the study of Fullerenes

We have accumulated large pure carbon clusters (Fullerenes) on a surface. These clusters were produced by laser vaporization of graphite in a static rare‐gas atmosphere. Their presence on the surface was confirmed by an isotope scrambling experiment using a laser‐desorption/laser‐ionization mass spectrometric detection scheme.