L. E. McNeil

Enhanced saturation lithium composition in ball-milled single-walled carbon nanotubes

Abstract The effects of processing on the structure and morphology of single-walled carbon nanotubes (SWNT) and their electrochemical intercalation with lithium were investigated. Purified SWNTs were processed by impact ball-milling and were electrochemically intercalated with lithium. The reversible saturation Li composition increased from Li 1.7 C 6 in purified SWNTs to Li 2.7 C 6 after 10 min of milling. The irreversible capacity decreased from Li 3.2 C 6 to Li 1.3 C 6 . Electron microscopy, Raman and X-ray diffraction measurements indicated that ball-milling induced disorder within the bundles and fractured the nanotubes.

Charge-transfer complexes: new perspectives on an old class of compounds

The discovery of the organic metal TTF–TCNQ in 1973 led to an explosion of research conducted on organic charge-transfer complexes. While these materials have been studied intensely for several decades, the research was mostly aimed at the discovery of materials with high room-temperature conductivity or high-temperature superconductivity. Recently, attention has turned to technologically-relevant properties of charge-transfer complexes, such as ambipolar transport, metallicity, photoconductivity, ferroelectricity or magnetoresistance. This manuscript reviews the growth, structure and properties of charge-transfer complexes and underlines recent progress in their application in organic devices. Their prospects in future applications are discussed, as well as the challenges yet to be overcome to understand the fundamental parameters governing their operation.

Visible light emission from thin films containing Si, O, N, and H

We report the fabrication, chemical, optical, and photoluminescence characterization of amorphous silicon‐rich oxynitride (SiOxNy:H) thin films by plasma‐enhanced chemical‐vapor deposition. The film compositions were followed by changes in the refractive index. X‐ray photoelectron and Fourier transform infrared spectroscopy indicate that the chemical composition is dominated by silicon suboxide bonding with N present as a significant impurity. A broad tunable photoluminescence (PL) emission is visible at room temperature with a quantum efficiency of 0.011% at peak energies to 3.15 eV. The radiative lifetimes are less than 10 ns, and there is nearly no temperature dependence of the PL intensity down to 80 K. Ex situ annealing at temperatures above 850 °C results in an increase in PL efficiency by nearly three orders of magnitude, and the PL intensity is independent of the annealing ambient. The PL results are remarkably similar to literature results in oxidized porous silicon and oxidized nanocrystalline S...

Elastic moduli of muscovite mica

The authors have used Brillouin scattering to measure the thirteen independent elastic moduli of muscovite mica. The moduli reflect the monoclinic symmetry of the crystal, and demonstrate the anisotropy of the interlayer and intralayer bonding. The decrease in the acoustic velocity with increasing temperature is dominated by the decrease in the moduli that depend on interlayer bonding, giving an estimate of the temperature dependence of those moduli.

Raman study of thin films of amorphous-to-microcrystalline silicon prepared by hot-wire chemical vapor deposition

The structure changes of thin films of amorphous (a) to microcrystalline (μc) silicon are studied by Raman scattering in terms of three deposition parameters: the silane flow rate, the hydrogen flow rate, and the total gas pressure in hot-wire chemical vapor deposition. The Raman transverse optical (TO) mode is deconvoluted into two Gaussian functions for a-Si:H and intermediate components and one Lorenzian function for the c-Si component. We found that (a) in general, the change in structure is a function of the ratio of hydrogen to silane gas flow, R, but also depends on the SiH4 flow rate and total gas pressure; (b) there is a narrow structural transition region in which the short-range order of the a-Si:H network improves, i.e., the variation in bond angle of the a-Si network decreases from ∼10° to ∼8° once the c-Si grains start to grow; and (c) when the films were deposited using a high SiH4 flow rate of 22 sccm, the narrow TO mode with low peak frequency could be related to the column-like structures.

Nanoforest Nb2O5 Photoanodes for Dye-Sensitized Solar Cells by Pulsed Laser Deposition

Vertically aligned bundles of Nb(2)O(5) nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye-sensitized solar cells (DSSC). They were characterized using scanning and transmission electron microscopies, optical absorption spectroscopy (UV-vis), and incident-photon-to-current efficiency (IPCE) experiments. The background gas composition and the thickness of the films were varied to determine the influence of those parameters in the photoanode behavior. An optimal background pressure of oxygen during deposition was found to produce a photoanode structure that both achieves high dye loading and enhanced photoelectrochemical performance. For optimal structures, IPCE values up to 40% and APCE values around 90% were obtained with the N(3) dye and I(3)(-)/I(-) couple in acetonitrile with open circuit voltage of 0.71 V and 2.41% power conversion efficiency.

Multiple scattering from rutile TiO2 particles

The physical properties of rutile titania (TiO 2) have led to its wide use as a white pigment in many applications. The success of these applications depends not only on the optical properties of the bulk material, but also on subtle aspects of the scattering of light from collections of small TiO 2 particles embedded in a transparent medium. We consider here the problem of multiple scattering in dense systems containing particles of a size comparable to the wavelength. Our method of analysis allowed us to understand the effects that the number density of particles and the particle size distribution have on the measured diffuse reflectance of such films. We apply this analysis to films of TiO 2 particles in a transparent medium. We present the results of diffuse reflectance and transmittance measurements in the visible range of films with concentrations that span the range in which the radiation fields of adjacent particles begin to interact. We find that for l,400 nm the spectra are predominantly determined by the scattering properties of the individual single particles. At longer wavelengths, where multiple-scattering effects become important, the particles behave as independent scatterers for volume concentrations of less than approximately 1%. At higher concentrations, where the interparticle spacing becomes less than the wavelength of light in the medium, the interaction of the radiation fields of adjacent particles lowers the backscattering fraction of the multiple-scattering function. This reduction in backscattering is significant for many of the applications of films containing TiO 2, such as coatings and paper, which rely upon multiple scattering from large numbers of particles to provide the desired opacity.

Analysis of boron strain compensation in silicon-germanium alloys by Raman spectroscopy

The impact of heavy boron doping on the biaxial compressive strain in Si1−xGex layers grown on Si has been investigated using Raman spectroscopy and theoretical calculations. It is shown that one boron atom is sufficient to compensate the strain due to approximately 6.9 Ge atoms. This effect is appreciably large for boron concentrations as low as 1%, typical for applications, which employ heavily boron doped layers. Using strain compensation, the Ge content can be substantially increased without increasing the stored strain energy. This phenomenon can be useful in applications, which require low-resistivity p-type strained Si1−xGex layers with high Ge content.

Orientation dependence in near-field scattering from TiO 2 particles

This scattering of light by small particles embedded in a continuous transparent medium is influenced not only by the bulk optical properties of the particles and the medium but also by the size, shape, and spatial arrangement of the particles-that is, by the microstructure. If the particles are close together, as in agglomerated coatings or stereolithographic suspensions, interactions between the radiation fields of adjacent particles can lead to variations in the magnitude and spatial arrangement of the scattered light in the near and the far field, which can affect the color and hiding power of a coating, the cure depth and homogeneity in stereolithography, and the threshold intensity for stimulated emission in random lasers. Our calculations of the near- and the far-field scattering distribution for 200-nm TiO(2) spheres in pairs of various orientations and in an ordered array of five particles show that, depending on the orientation of the particles with respect to the incident light, these interactions can either increase or decrease the scattering efficiency, the isotropy of the scattering, and the magnitude of the electric field strength within the matrix and the particles. In the mid-visible range, two particles in line increase the backscattering fraction by 28% and the scattering strength by 38% over that of a single particle, whereas if the particles are in the diagonal configuration the backscattering fraction and scattering strength are actually reduced by addition of the second particle. At shorter or longer wavelengths the backscattering fraction is reduced regardless of the location of the second particle, by as much as 60% when five particles are arranged in the zigzag configuration. These results are surprising in that it is generally assumed that multiple scattering enhances backscattering. Simple models of multiple scattering or scattering of two particles as a single, larger particle are inadequate to explain these results.

Raman studies of Ge-promoted stress modulation in 3C–SiC grown on Si(111)

We present a study of the stress state in cubic silicon carbide (3C–SiC) thin films (120 and 300 nm) grown by solid-source molecular-beam epitaxy (SSMBE) on Si(111) substrates modified by the deposition of germanium prior to the carbonization of Si. μ-Raman measurements were used to determine the residual stress existing in the 3C–SiC layers. The stress is found to decrease linearly with increasing Ge quantity but with different strength depending on the 3C–SiC thickness deposited after the introduction of Ge. Based on secondary ions mass spectroscopy (SIMS) and transmission electron microscopy (TEM) analyses it is suggested that the Ge introduced prior to the carbonization step remains in the near-interface region and reduces the Si outdiffusion, which further reduces the stress state of the 3C–SiC layers.