T. van Buuren

Nanodispersed silicon in pregraphitic carbons

Using chemical‐vapor deposition nanodispersed silicon has been prepared in carbon at temperatures between 850 and 1050u2009°C. Samples with up to 11% atomic silicon in carbon show the same pregraphitic x‐ray‐diffraction pattern as those without silicon. X‐ray‐absorption spectroscopy shows that the silicon is bonded mostly to carbon neighbors and that large clusters of silicon are not found. It is believed that silicon atoms, or small clusters of a few silicon atoms, are located in regions of ‘‘unorganized carbon’’ which separate small regions of organized graphene layers. These materials may have application as electrode materials in advanced rechargeable lithium batteries.

Oxide thickness effect and surface roughening in the desorption of the oxide from GaAs

The temperature for thermal desorption of the gallium oxide from GaAs is shown to increase linearly with oxide thickness. In addition, we show by diffuse light scattering that highly polished GaAs substrates roughen during the oxide desorption. These results are interpreted in terms of a model in which the oxide evaporates inhomogeneously.

Behavior of nitrogen-substituted carbon (N[sub z]C[sub 1[minus]z]) in Li/Li(N[sub z]C[sub 1[minus]z])[sub 6] cells

Nitrogen-containing carbons N[sub z]C[sup 1[minus]z] have been made from different precursors at temperature between 850 and 1,050 C. Their composition and structure have been studies by chemical analysis, powder X-ray diffraction, X-ray absorption spectroscopy, and Auger electron spectroscopy. These techniques show that some nitrogen has been incorporated substitutionally for carbon. Nitrogen affects the behavior of Li/Li (N[sub z]C[sub 1[minus]z])[sub 6] electrochemical cells in two ways. First, the irreversible capacity observed during the first electrochemical reaction of Li with N[sub z]C[sup 1[minus]z] (during the first discharge) increases with the nitrogen content of the samples. Second, the incorporated nitrogen causes a shift of the cell capacity to lower voltages compared to pure carbon electrodes. The first effect can be understood qualitatively using a model where Li reacts irreversibly with nitrogen-containing species (the authors call these nitrogen atoms chemical nitrogen) to form lithium-nitrogen-organic compounds. The second effect is caused by the nitrogen which has been substituted for carbon in the lattice (so-called lattice nitrogen). Such nitrogen-containing carbons are not considered useful as anodes for Li-ion cells.

Photoelectron spectroscopy measurements of the band gap in porous silicon

Photoemission and x‐ray absorption spectroscopy show that both the conduction and valence bands of porous silicon are shifted relative to the bands for bulk silicon, as expected in the quantum confinement model for the optical properties of porous silicon. The shift in the valence band is larger than the shift in the conduction band and proportional to it, with a proportionality constant that is consistent with effective mass theory. No oxygen is detected in the as‐prepared porous silicon.

Evidence for quantum confinement in porous silicon from soft x-ray absorption

Measurements of x‐ray absorption in the vicinity of the silicon L edge in porous silicon show a blueshift and a broadening of the conduction band edge, consistent with a distribution of quantum confinement energies. The absorption spectrum for porous silicon can be fit by a broadened and energy‐shifted version of the crystalline silicon absorption spectrum. The average quantum shift and broadening used in the fit to the absorption spectrum are in reasonable agreement with the corresponding parameters derived from the photoluminescence spectrum.

X‐ray diffraction and x‐ray absorption studies of porous silicon, siloxene, heat‐treated siloxene, and layered polysilane

Several porous silicon, siloxene (Si6H6O3), heat‐treated siloxene, and layered polysilane (Si6H6) samples have been studied with K‐ and L‐edge x‐ray photoabsorption, photoemission, and powder x‐ray diffraction. The x‐ray absorption of layered polysilane and porous‐Si are found to be remarkably similar. In particular, the K absorption edges of these samples shift by about 0.4–0.6 eV to higher energy relative to crystalline silicon. Siloxene samples heated to 400 °C in inert gas are best described as a mixture of SiO2 and amorphous‐Si. When heat‐treated siloxene is studied by photoelectron spectroscopy (surface sensitive) it resembles SiO2, when it is studied by x‐ray absorption (bulk and surface) features from both SiO2 and amorphous‐Si are observed and when it is studied by x‐ray diffraction (bulk measurement) it resembles amorphous‐Si. The SiO2 is therefore predominantly at the surface and heat‐treated siloxene is very small amorphous‐Si particles coated with SiO2. The Si L edge of heat‐treated siloxene ...

Quantum confinement effects in the soft X-ray fluorescence spectra of porous silicon nanostructures

Abstract The electronic structure of porous Si is investigated using soft x-ray emission and absorption spectroscopies. Porous Si prepared under different wavelengths of illumination was studied along with porous Si that was purposely oxidized. In the x-ray fluorescence measurements we are able to selectively probe the silicon core of oxidized nanostructures by suitable choice of the excitation wavelength. The observed changes in the valence and conduction bands are consistent with electronic structure changes associated with quantum confinement in silicon nanostructures and are similar for the oxidized and unoxidized porous silicon.

Comparison of the Band Gap of Porous Silicon as Measured by Photoelectron Spectroscopy and Photoluminescence

The peak energy of the room temperature photoluminescence of porous silicon is compared with the bandgap determined from photoelectron spectroscopy measurements for a series of porous silicon samples prepared under different conditions. The photoluminescence bandgap is found to be smaller than the photoelectron spectroscopy bandgap, but exhibits the same trend with preparation conditions. The width of both the photoluminescence spectrum and the L-absorption edge increases when the current density during the preparation is increased or the sample is allowed to soak in HF after preparation.

Soft X-ray emission of porous silicon nanostructures

Abstract The electronic structure of Si nanostructures in porous Si and purposely oxidized porous Si is investigated using soft x-ray emission spectroscopy. Significant changes as compared to bulk Si are observed. We interpret the spectral changes as due to an altered electronic structure in the Si nanostructures. By imposing standing wave boundary conditions to the valence band wavefunctions we calculate the emission spectrum for thin Si sheets, in good agreement with the experimental spectra.

Single-State Electronic Structure Measurements Using Time-Resolved X-Ray Laser Induced Photoelectron Spectroscopy

We demonstrate single-shot x-ray laser induced time-of-flight photoelectron spectroscopy on semiconductor and metal surfaces with picosecond time resolution. The LLNL COMET compact tabletop x-ray laser source provides the necessary high photon flux (>10{sup 12}/pulse), monochromaticity, picosecond pulse duration, and coherence for probing ultrafast changes in the city, chemical and electronic structure of these materials. Static valence band and shallow core-level photoemission spectra are presented for ambient temperature Ge(100) and polycrystalline Cu foils. Surface contamination was removed by UV ozone cleaning prior to analysis. In addition, the ultrafast nature of this technique lends itself to true single-state measurements of shocked and heated materials. Time-resolved electron time-of-flight photoemission results for ultra-thin Cu will be presented.