Mark S. Crowder

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.

Multiple species of La@C82 and Y@C82. Mass spectroscopic and solution EPR studies

Abstract Two dominant electron paramagnetic resonance (EPR) hyperfine patterns for both La and Y fullerenes have been identified and linked to the mass spectroscopic (MS) peaks of La@C 82 and Y@C 82 , respectively, as also observed by Suzuki. Additionally, lower intensity La@C n EPR multiplets are identified which have not, as yet, been correlated with specific MS peaks. The intensity ratios of the two dominant EPR multiplets depend on various parameters such as solvent and temperature, suggesting the species possess different chemical properties.

Characterization of fullerenes and doped fullerenes

Abstract The results of a variety of experiments used to characterize fullerenes, metallofullerenes and alkali-metal intercalated C 60 are reported. It is shown that differential scanning calorimetry and NMR characterization of the orientational phase transition in C 60 provide sensitive means to assess the purity and crystallinity of fullerene samples. The results of a mass-spectrometric investigation of metallofullerene samples produced by co-vaporization of carbon and metals are described. An account is given of some electron paramagnetic resonance results obtained for LaC 82 and solid-state NMR results obtained on an alkali-intercalated fullerite, Rb 3 C 60 , which show for the first time the presence of magnetically inequivalent carbons in underivatized C 60 . Together these experiments yield a great deal of information about the phase purity, molecular dynamics and structure of a variety of fullerene materials.

Paramagnetic centers of amorphous carbon thin films: Influence of hydrogen content and O2 permeation

Electron paramagnetic resonance (EPR) studies of sputtered amorphous hydrogenated carbon (a‐C:H) films have been conducted in several gaseous ambients, at three microwave frequencies, and as a function of film H content. Unpaired electron concentrations are found to decrease with increasing H content while EPR linewidths are shown to be a function of H content; unresolved proton hyperfine interaction is the linewidth determining factor for H content greater than 24% while conductivity is suggested to be the most significant linewidth determining factor for H content less than 24%. Furthermore, the EPR linewidth of a‐C:H films broadens in the presence of gaseous O2 and demonstrates that these films are permeable to O2 and other gases.

Metal atoms and clusters in fullerene cages

Fullerenes containing metal atoms and clusters can be formed by the arc-vaporization method. The electronic structure of these metallofullerenes can be probed using magnetic resonance techniques. Electron paramagnetic resonance (EPR) spectra of LaC82, YC82, ScC82 and Sc3C82 have been obtained. Metallofullerenes containing a single metal atom (MC82 with M = La, Y, or Sc), have small hyperfine couplings and g-values close to 2, implying that they can be described as + 3 metal cations within — 3 fullerene radical anion cages. In the La and Y cases, additional EPR active MC82 species have been found. The EPR spectrum of Sc3C82 shows that the metal atoms are equivalent, suggesting that they may form a triangular molecule. No EPR spectrum is found for Y2C82 or for Sc2C2n species (with 2n = 82,84,86), suggesting that they are diamagnetic. Sc NMR spectra of a solution containing Sc2C2n species have been obtained which confirm the diamagnetism of the discandium metallofullerenes.

Hydrogen Passivation of Grain Boundaries in Polycrystalline Silicon Deposited by Molecular Beams

Grain boundary properties of polysilicon deposited by molecular beams have been investigated by electron spin resonance and conductivity measurements. The variations of the conductivity activation energy with doping can be explained by a density of states model consisting mainly of two exponential bandtails, implying that dangling bonds play a minor role. A hydrogen plasma treatment at 500 °C reduces the spin density by a factor of three but also passivates weak Si-Si bonds thus leading to steeper bandtails. The possibility of hydrogen-related intra-grain gap states is also discussed.