Lowell D. Lamb

Valence and core photoelectron spectroscopy of C60, buckminsterfullerene

Abstract The MgKα X-ray (XPS) and He(I)/He(II) ultraviolet (UPS) photoelectron spectra of several thin films of C 60 (one to three monolayers) prepared by vapor deposition on gold are reported. The core XPS spectrum shows a single narrow carbon 1s ionization at 285.1 eV. The valence UPS spectra show very sharp valence ionization bands, comparable to gas phase spectra, with wide separations between the lowest ionization energy features. The ionizations are consistent with theoretical calculations based on the highly symmetric truncated icosahedral structure. The first vertical ionization energy relative to the vacuum level is estimated to be 7.6 ± 0.2 eV from these solid state measurements.

The infrared emission spectrum of gas-phase C60 (buckmisterfullerene)

Abstract The gas-phase infrared spectrum of C 60 has been observed in emission with the National Solar Observatory Fourier transform spectrometer at Kitt Peak. Bands attributable to the C 60 molecule are found at 527.1, 570.3, 1169.1 and 1406.9 cm −1 . Additional emission features are tentatively assigned to C 70 or combination bands of C 60 . A new, strong emission is observed at 1010.2 cm −1 belonging to an unknown molecule. None of these features can be associated with any of the strong emission bands observed so far in astronomical sources.

Extraction and STM Imaging of Spherical Giant Fullerenes

High-temperature, high-pressure extracts of soot produced by the Kr�tschmer-Huffman technique (KH carbon) were characterized by mass spectrometry and imaging with scanning tunneling microscopes (STMs). The mass spectra of these samples are similar to those of ambient-pressure, high-boiling-point solvent extractions, supporting the idea that solvent temperature and possibly pressure are key parameters in extraction of the giant fuilerenes. The STM images show that the giant fullerenes in these samples are roughly spherical in shape and range in diameter from approximately 1 to 2 nanometers, corresponding to fullerenes containing 60 to 330 atoms. No evidence of bucky tubes was found.

Large odd-numbered carbon clusters from fullerene-ozone reactions

The odd-numbered carbon clusters C119, C129, and C139 have been observed in the mass spectra of toluene extracts of fullerene soots and of the products of ozone-fullerene reactions. Specifically, ozone-C60 reactions yield C119, ozone-C70 reactions yield C139, and ozone-(C60/C70) reactions produce C119, C129, and C139. These unexpected species correspond to dimers of C60, C60/C70, and C70, respectively, less one carbon atom, and are stable gas-phase ions with behavior similar to that of fullerenes. The results suggest a new route to functionalization and derivatization of fullerenes through controlled ozone-catalyzed cage-opening reactions.

Gas-phase infrared emission spectra of C60 and C70. Temperature-dependent studies

Abstract The gas-phase infrared emission spectra of C60 and C70 were recorded by Fourier-transform spectroscopy. The measurements were carried out in the temperature range 500–950°C and the band positions were extrapolated to 0 K. In addition to the strong fundamental bands, numerous weak features were observed. We attribute these weak bands primarily to binary combination bands. Our measurements should prove useful for astronomical searches, low-temperature laboratory measurements and for detailed force-field calculations.

The Ionizations of C 60 in the Gas Phase and in Thin Solid Films.

The high-resolution He I photoelectron spectrum of C 60 in the gas phase is reported and compared with the photoelectron spectrum of C 60 as a thin film prepared by vapor deposition (one to three monolayers) on gold. The spectra show low valence ionization bands that are very sharp and well-separated for a molecule of this size, consistent with the highly symmetric truncated icosahedral structure and theoretical calculations. The total band widths of the valence ionizations from the thin film samples are comparable to those from the gas phase species, showing that the electronic interactions between the molecules and with the surface do not significantly influence these measurements of the molecular electronic structure. The gas phase photoelectron spectra also show vibrational fine structure in the first and second ionization bands with spacings that are consistent with the two totally symmetric vibrational modes of C 60 . The first vertical ionization energy relative to the vacuum level is determined to be 7.61 ± 0.02 eV from these gas phase measurements.

Evidence for solid-state effects in the electronic structure of C60 films: a resonance-Raman study

Abstract The Raman excitation profiles for C 60 films grown on silicon reveal the existence of optical transitions near 2.4 eV, well below the lowest dipole-allowed absorption line predicted for isolated icosahedral molecules. These transitions are not observed in C 60 dissolved in CS 2 . We propose an explanation in terms of a breakdown of the icosahedral inversion symmetry in C 60 crystals.

Characterization of carbon nanotubes by scanning probe microscopy

Abstract Carbon nanotubes, fabricated by the Ebbesen-Ajayan method, were imaged using scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in air and were compared to images obtained with high-resolution transmission electron microscopy (HRTEM). The HRTEM images revealed an abundance of elongated structures ranging in diameter from 3.0 to 30 nm, and with lengths of up to 0.8 μm. Many of the structures possessed several graphitic shells as if the tubes were nested one in the other. Reproducible images of the tubular structures, typically 20 nm in diameter and with a large variation in length, were obtained with both STM and AFM when the nanotubes were deposited on hydrogen-terminated Si(111), confirming that the nested structures observed with HRTEM do indeed have a tubular morphology. No single-walled, bare nanotubes or spherical fullerenes (typical of the Kratschmer-Huffman process) were observed.

High resolution images of single C60 molecules on gold (111) using scanning tunneling microscopy

Abstract The electronic interactions of fullerene molecules with metals, with other molecules, and with themselves are important to the chemical and conductive properties of these materials. We demonstrate high resolution scanning tunneling microscopy images of C 60 molecules condensed on epitaxial gold (111) films on mica, in which the C 60 molecules are isolated from each other. The C 60 molecules were locked in position to the gold substrate by an ordered layer of methyl isobutyl ketone. The images of the C 60 molecules exhibit intramolecular contrast indicating a significant electronic interaction with the gold substrate. Current versus voltage measurements show that both the C 60 and the thin film of methyl isobutyl ketone have conductances comparable to that of the gold substrate.

Buckyball-substrate interactions probed by STM and AFM

Abstract Submonolayers of buckyball molecules (C 60 ) on a gold substrate, deposited under ultrahigh-vacuum (UHV) conditions, have been imaged in air using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). STM images show intramolecular contrasts within most individual C 60 molecules. AFM images of the C 60 molecules, often grouped in a hexagonal arrangement, show no atomic-scale features. A possible explanation for the presence or absence of the intramolecular contrasts in the images obtained with the STM and AFM, respectively, is given by a molecular orbital calculation.