High resolution electron energy loss spectroscopy has been used to investigate the vibrational and electronic excitations of single-phase, crystalline AxC60 films. Substantial changes in intramolecular mode frequencies and intensities with alkali concentration arise from charge transfer effects as well as free carrier screening. For x=4, splitting of the tu band is observed, resulting in an insulating phase with an estimated gap of 0.3-0.4eV. Screening in the x=3 phase results in enhanced surface sensitivity and deviations from bulk stoichiometry at the surface of metallic films are observed.

In situ Raman scattering, UPS and HREELS measurements have been used to study the fee to orthorhombie phase transition in RbCeo thin films. Large changes in the Raman spectra are interpreted in terms of increased interfullerene coupling in the orthorhombie phase. However, the data do not support the proposed polymer model for this phase. Photoemission measurements indicate only small differences in the electronic states between the two phases in contrast with photopolymerized Ceo where additional states are observed. Low energy metallic-like excitations and screening of intramolecular vibrations are observed in HREELS.

Photoemission and Raman spectroscopy performed on alkali-doped C-60 has revealed the origin of an additional feature often seen between the conduction bands derived from the highest occupied molecular orbital and lowest unoccupied molecular orbital. Changes in these bands due to variation of temperature, stoichiometry, and alkali metal are correlated with the behavior of a disorder-induced intramolecular Raman-scattering peak. Loss of interstitial site symmetry due to thin-film disorder is suggested to be the origin of the additional electronic structure and associated Raman band. This would imply the effects of intermolecular interactions on the electronic properties of alkali-doped C-60 are still not completely understood. [S0163-1829(98)01048-0].

Electron energy loss spectroscopy (EELS) has been used to observe size dependent changes in the electronic states of isolated, nanocrystalline carbon particles supported on SiO2 substrates. The structure and size distribution of the particles have been characterized by Raman scattering, Anger electron spectroscopy, and electron microscopy. EELS observations indicate that a semimetal to semiconductor transition occurs for particles smaller than 1 nm. In addition, hydrogen adsorption is found to significantly alter the electronic states, demonstrating that both finite size and dangling bond effects modify the properties of small carbon nanocrystallites.

A large number of elemental and compound semiconductors in bulk form become metallic upon melting due to changes in the local atomic structure occurring in the transition from the solid to the liquid state. We report studies of the vibrational properties of liquid Bi clusters that suggest that for the smaller sizes this atomic structure transition does not occur. Using interference enhanced Raman spectroscopy, in situ studies of bismuth ultrathin films, supported on weakly interacting carbon substrates, have been performed. Changes in the vibrational spectra with liquid cluster size have been correlated with ex situ transmission electron microscopy to estimate a critical size below which densification of the structure does not occur. For various models this yields clusters of diameter in the range of similar to 45-75 Angstrom.

We report ultraviolet (uv) Raman scattering studies of hydrogen-free, diamondlike amorphous carbon thin films with a wide range of tetrahedral bonding. The uv Raman spectra are shown to provide direct evidence for the presence of sp 3 -bonded C atoms in these materials. The experimental results are found to be in excellent agreement with theoretical predictions and contribute to an improved understanding of the mechanism by which the diamondlike fraction develops within the amorphous carbon network. [S0031-9007(97)03420-0] For over a decade, diamondlike amorphous carbon (DLC) has stimulated great interest from both scientific and industrial perspectives. Hydrogen-free DLC has interesting and useful properties [1], such as high hardness, chemical inertness, thermal stability, wide optical gap of ,2 eV, and negative electron affinity. Therefore, this material is important for coating technology and electronic device applications. Typically, it is produced by vacuum arc [2,3] or pulsed laser deposition [4] methods. In contrast to conventional amorphous carbon (a-C) prepared by evaporation or sputtering which consists mostly of threefold or sp 2 -bonded atoms, DLC contains significant fractions (up to 80 at. %) of fourfold or sp 3 -bonded C atoms. In spite of extensive experimental work on DLC, evidence for the presence of sp 3 C atoms is somewhat indirect and measurements of the sp 3 C content tend to be empirical in nature. Although neutron [5] and electron diffractions studies [3] of DLC have been performed, information about the sp 3 C bonding cannot be readily extracted from the measurements. Estimates of the sp 3 C fraction in DLC are usually made by transmission electron energy loss spectroscopy (EELS) which relies on the loss of transitions from the 1s level to the empty p p states [2,6] associated with the presence of sp 2 C atoms. While vibrational spectroscopies in principle can probe changes in bonding more directly, most of the available experimental techniques have not been successful in studies of DLC. Nuclear magnetic resonance (NMR) can detect sp 3 C atoms [7,8] but requires thick samples which are rather difficult to make in the case of DLC due to the high stress and consequent delamination. Inelastic neutron scattering also requires very thick samples. Typically, Raman scattering is a convenient tool for vibrational characterization of amorphous solids, in which case it represents the phonon density of states (PDOS), weighted by a coupling parame

Electron energy loss spectroscopy (EELS) and ultraviolet photoemission have been used to study the electronic structure of ZnSe(100) surfaces. Changes in both the occupied states and electronic transitions are observed upon passing from the disordered, Se-saturated (1 × 1) surface to ordered Se-stabilized (2 × 1) and Zn-terminated c(2 × 2) reconstructions. In particular, the (2 × 1) surface is found to exhibit a lower onset of interband transitions than both the c(2 × 2) phase and bulk ZnSe. The smaller surface band-gap for this surface can be attributed to transitions involving Se dimer states lying within the bulk gap.

Dark-field imaging in a scanning transmission electron microscope using a thin annular detector, with a ratio of detector width to detector radius of only about 10%, rather than the commonly-used broad detector, is shown to be especially valuable for the detection of crystallites of diameter 1–3 nm supported on relatively thick amorphous films, even when the crystallites are of light-atom material. Nanocrystals of carbon of average diameter 1.0 nm have been clearly imaged on films of amorphous silica 6 nm thick by using the thin detector to select the diffraction maximum corresponding to the inter-layer graphitic spacing of 0.34 nm. Also, nanoparticles in this size range, whether crystalline or amorphous, may be imaged with the thin annular detector set to detect small-angle scattering. A simple theoretical treatment suggests how the image contrast may vary with the particle size, image resolution and support thickness. Observations on the nanoparticles of carbon on amorphous silica confirm the theoretical predictions.

Small isolated nanocrystallites of carbon have been prepared on amorphous SiO2 and studied by interference enhanced Raman scattering (IERS) and scanning transmission electron microscopy (STEM). A new mode of dark field STEM, using a thin annular detector, has allowed imaging of 1-2 nm graphite-like particles using (002) diffraction. For such small particles, the Raman spectra provide the first evidence for changes in the phonon density of states of a nanocrystalline system.

Pair distribution functions (PDFs), obtained from powder neutron diffraction measurements have been used to construct structural models for the body centered orthorhombic (bco) and fcc phases of RbC60. The PDF exhibits small differences, primarily due to the shortened inter-fullerene distance in the bco phase. The bco-RbC60 system is well fit to a model derived from first-principles quantum molecular dynamics in which the C-C interfullerene distance is 1.57 Å. While the geometry of the interfullerene linkages is similar to a previously proposed model, the present results imply less distortion of the C60 molecules and larger interfullerene distances. The PDF analysis also indicates significant orientational disorder between chains of linked molecules.

High resolution electron energy loss spectroscopy was used to observe variations in the vibrational states of amorphous carbon films with differences in preparation methods. Diamondlike films made by pulsed laser deposition (PLD) exhibit differences in the weighted phonon density of states relative to sputtered films. These differences are due to an increase of fourfold bonded atoms in the PLD films and are in qualitative agreement with theoretical calculations for amorphous carbon networks.

The body centered orthorhombic (bco) phase of RbC60 has attracted considerable interest due to the proposal of intermolecular bond formation. In the present work, the structure, vibrational and electronic states of the RbC60 compound have been investigated with a variety of techniques. Raman scattering through the fcc to orthorhombic phase transition exhibits substantial changes attributed to increased intermolecular interactions. Corresponding modifications of the electronic states, measured by valence band photoemission and electron energy loss spectroscopy, are considerably smaller. Comparison of the measurements of bco RbC60 with observations on laser modified C-60, for which there is strong evidence of interfullerene bonding, indicates stronger perturbations of the molecular symmetry in the fatter. A structural study, employing a pair distribution function analysis of neutron diffraction data, suggests a model with a C-C interfullerene distance of 1.57 Angstrom, considerably larger than previously proposed. Improved agreement with the neutron data is achieved by considering a two-ball per cell Jahn-Teller distortion, suggested by the quantum molecular dynamics calculations of Adams and Page.

The use of high resolution electron energy spectroscopy (HREELS) as a new method for studies of subgap absorption in thin films of amorphous semiconductors is demonstrated. For a-Si films, the α(ω) values extracted from the measured loss spectra are in quantitative agreement with previous optical measurements. The method is also applied to both threefold and diamond-like amorphous carbon films, yielding α(ω) down to considerably lower energies (∼50 meV) than previously reported. The HREELS method is shown to be complementary to existing techniques in that it can access the regime of low energies and ultrathin films which is difficult to investigate with the conventional methods.

Electron energy loss spectroscopy (EELS) has been used to investigate the electronic states of isolated, nanocrystalline carbon particles. Small carbon nanocrys-tals were prepared via sputter deposition onto SiO2 substrates, followed by annealing to 700C. The structure and size distribution of the particles have been characterized by Raman scattering, Auger electron spectroscopy and electron microscopy. EELS observations indicate that a semimetal to semiconductor transition occurs for particles smaller than lnm. In addition, hydrogen adsorption is found to significantly affect the electronic states of these particles, indicating that both finite size and dangling bond effects modify the properties of small carbon nanocrystallites.

Substantial changes in the electronic and vibrational properties of crystalline C-60 modified by laser irradiation are reported. Ultraviolet photoemission identifies new states relative to C-60. Electron energy loss spectra indicate broadening of interband transitions with the gap remaining unchanged upon laser modification. Intermolecular Raman scattering exhibits one strong mode at 115 cm(-1) and two previously unreported weaker bands centered at 85 and 140 cm(-1). Comparison of the observed vibrational and electronic states with calculations for dimers and infinite chains indicate the laser modified films consist primarily of dimers.

Intermolecular vibrations in a variety of fullerene systems are reported that provide information on C60-C60 and alkali-metal - C60 interactions and disorder effects. Low frequency Raman scattering in laser polymerized C60, films are shown to exhibit additional modes not reported for bulk materials. This result is consistent with recent dimer calculations. Raman scattering and HREELS measurements are shown to provide complementary information on alkali-metal motions in K3C60. In FCC Na6C60 both octahedral site disorder effects and reduced Na-C bond lengths account for changes in the Raman spectrum relative to Na3C60.