Jeffrey S. Lannin

Disorder induced Raman scattering of nanocrystalline carbon

Weak, low frequency disorder induced Raman scattering is observed in nanocrystalline, ‘‘glassy’’ carbon having an in‐plane correlation length La≂30 A. Similar, relatively more intense low frequency scattering is observed after the annealing of amorphous carbon films to 600 °C, suggesting the formation of quite small nanocrystallites of diameters estimated to be in the range of 10–15 A. A comparison with the glassy carbon inelastic neutron scattering spectrum demonstrates that the Raman spectra represent a weighted phonon density of states of low frequency modes. The results confirm theoretically predicted low frequency, disorder induced scattering from nanocrystalline solids.

Bilayer interference enhanced Raman spectroscopy

A combination of the interference of incident and reflected coherent beams from a metal surface with a dielectric overlayer is calculated to yield an enhancement of the Raman signal of ultrathin adsorbed layers. The thickness of the dielectric layer is determined by optimizing the interference effect of the incident and reflected beams to enhance the electric field. In the case of SiO2/Al an interference enhancement factor of 27 is found. An experimental confirmation of this enhancement is obtained by comparing the in situ multichannel Raman signals of one monolayer C60 on an Al substrate and on a SiO2/Al bilayer. The use of the same bilayer substrate for a range of ultrathin films makes the metal‐dielectric bilayer a versatile tool to investigate a number of nanoscale systems.

Structural order and dynamics of amorphous Si and Ge

Abstract The role of variable short range order on the physical properties of a-Si, a-Ge and their H alloys are discussed. Emphasis is placed on the influence of bond angle distribution modifications with the conditions of film formation that result in changes in the vibrational and electronic states. Raman and inelastic neutron scattering, optical absorption, and differential scanning calorimetry in variable ordered systems are discussed.

Vibrational states of tetrahedral amorphous carbon

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.

Inelastic light scattering from liquid and amorphous selenium

Abstract Raman scattering measurements are reported for supercooled liquid Se at 180 C and compared with amorphous Se spectra at 25 C. The liquid spectra indicate an appreciable broadening of the high frequency optic-like band upon melting with little change in peak frequency. The phonon gap of a-Se is narrowed in l-Se with evolution toward a pseudogap with increasing temperature. Observation of a weak polarized low frequency mode near that of a-Se suggests the direct observation of rings in the liquid. The weak intensity of this mode suggests the dominant scattering by chain configurations in the Raman spectra of a-Se and l-Se.

Neutron diffraction and structural models of RbC60 phases

Abstract 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 RbC 60 . The PDF exhibits small differences, primarily due to the shortened inter-fullerene distance in the bco phase. The bco-RbC 60 system is well fit to a model derived from first-principles quantum molecular dynamics in which the C-C interfullerene distance is 1.57 A. While the geometry of the interfullerene linkages is similar to a previously proposed model, the present results imply less distortion of the C 60 molecules and larger interfullerene distances. The PDF analysis also indicates significant orientational disorder between chains of linked molecules.

Electronic and vibrational properties of laser modified C60

Abstract 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.

Raman scattering studies in dilute and concentrated a-Si1-xHx alloys

Abstract Raman scattering and optical absorption measurements have been used to study the structural and electronic properties of a-Si l-x H x films made by homogeneous chemical vapor deposition. The Raman spectra show that the hydrogenated films are more ordered than pure a-Si films. For x ≲0.04 the optical gap E 0 correlates with Δ VH −1 , the structural order parameter. At higher hydrogen concentrations, however, Δ VH −1 is relatively constant and E 0 increases due to alloy rather than structural ordering effects.

Chapter 6 Raman Scattering of Amorphous Si, Ge, and Their Alloys

Publisher Summary This chapter emphasizes Raman scattering as a sensitive probe of changes in the phonon states in a-Si, a-Ge, and their alloys, particularly those arising from small changes in short-range order. These changes, which are difficult to measure accurately in diffraction studies, have theoretically been shown to modify substantially the phonon density of states and thus the Raman spectra. As such, the Raman spectra provide a means of studying amorphous solids prepared under a wide range of deposition conditions to assess local structural order and its variation. The observation of a correspondence between changes in the depolarized VH Raman spectra in a-Si and a-Ge and the optical gap further emphasizes the importance of small changes in short-range order on near-band-edge electronic states. However, further theoretical studies of the relation between changes in phonon and electron states due to modifications of short-range order are required. In addition, the influence of variations in the Raman coupling parameters with short-range order requires further theoretical analysis. Experimental studies that determine more accurate changes in the radial distribution function, particularly in a-Si, as well as the phonon density of states, are of fundamental importance. Such studies will allow a more quantitative measure of structural disorder variations to be determined in future Raman scattering studies.

Raman scattering and electron-phonon coupling in A3C60

Abstract Raman scattering measurements performed in situ in UHV on thin and ultrathin films of doped C60 are reported for variations in alkali-metal type, composition and temperature. The results emphasize the important role of disorder on Raman processes and the probing of electron-phonon coupling relevant to superconductivity. Detailed comparisons of free carrier phonon damping effects on selected radial and tangential intramolecular phonons are discussed in Rb3C60 and K3C60, using nonmetallic Na3Ca60 as a reference. Elevated temperature studies of Na3C60 suggest a transition to a more metallic state. Intennolecular Raman scattering associated with alkali-metal motion, weakly coupled to C60, clusters is reported. The effects of alkali size, lattice thermal expansion and disorder are discussed.