We are studying the electronic structures of new and advanced materials. We are using synchrotron radiation to perform soft x-ray emission and absorption spectroscopy of systems like biomaterials, superconductors and transition metal compounds.

Sam Leitch

sam.jpg

[sam.l@usask.ca]

Education

  • University of Saskatchewan
    Saskatoon, Saskatchewan
    B.E. in Engineering Physics
    April 2002
  • M.Sc. in Physics
    June 2005

Current Employment

  • Development Team, Shaw Cable. (Calgary, Alberta)

M.Sc. Thesis

Spectroscopic Analysis of Selected Silicon Ceramics

Abstract

Silicon ceramics are popular in both commercial applications and material research. The purpose of this thesis is to present measurements and analysis of four different silicon ceramics: alpha, beta and gamma phases of silicon nitride and silicon oxynitride using soft x-ray spectroscopy, which analyses the electronic structure of materials by measuring the absorption and emission of x-ray radiation. Absorption and emission spectra of these materials are presented, many of which have not be previously documented. The results are compared to model spectra and together they provide information about the electronic structure of the material. Assignments of emission features to element, orbital, and site symmetry are performed for each material. Combinations of silicon and nitrogen emission spectra provide insight into the strained bonding structure of nitrogen. It is concluded that p-dpi interaction plays a role in the bonding arrangement of nitrogen and oxygen sites within these structures. The emission features of non-equivalent silicon sites within gamma-Si3N4 are identified, which represents some of the first analysis of same element, non-equivalent sites in a material. Silicon absorption and emission spectra were plotted on the same energy scale to facilitate measurement of the band gap. Since previously measured band gaps are not well represented in literature, the measured band gaps were compared to values predicted using DFT calculations. The band gap values are in reasonable agreement to calculated values, but do not vary as widely as predicted.

Project

Many materials contain atoms of the same element in different bonding geometries. These differences in bonding structure cause the electronic densities of states around the respective atomic sites to differ. My research involves developing a method for probing the properties of these non-equivalent sites using soft x-ray absorption and emission spectroscopies. Perfection of these methods will allow us to gain insight into otherwise inaccessible properties of new materials.

Research Example: gamma-Si3N4

Structure of gamma-Si3N4 showing octahedral and tetrahedral silicon sites

Gamma-Si3N4 is a ceramic that is configured in a spinel phase. It is the first nitride ceramic to be synthesized in this spinel phase, produced in an environment of 13GPa and 1800K. The spinel phase of this ultra-hard ceramic, when first synthesized, was the third hardest known material, next to diamond and h-BN.

Gamma-Si3N4 has Si atoms in both octahedral and tetrahedral geometries. The measured XAS and XES spectra contain contributions from each of these two non-equivalent sites. Although methods do exist for determining the properties of non-equivalent sites based on their geometry, these methods require a single crystal sample of known orientation. All synthesized gamma-Si3N4 to date are polycrystalline, and therefore cannot be analysed using traditional methods.

The local partial density of states (LPDOS) of the non-equivalent sites can be determined by exploiting site-specific absorption probabilities of the material.. Resonant excitation of electrons from one non-equivalent site of silicon produce emission from only that site. The emission spectrum will consequently exhibit features unique to that non-equivalent site. The challenge is to efficiently excite core electrons from one non-equivalent site, while minimizing the excitation of electrons from the other sites.

Measured Si absorption spectrum of gamma-Si3N4 (a), along with calculated total (b), tetrahedral (c), and octahedral (d) Si spectra.

Figure 2 shows an absorption spectrum for gamma-Si3N4, as well as calculated x-ray absorption near-edge spectra (XANES) for comparison. The bottom two XANES spectra show the contributions to the absorption spectrum from each of the non-equivalent sites. Above them is the total weighted average of the two, based on a 2:1 octahedral-to-tetrahedral site density. The two sharp, low-energy features in the measured spectrum are excitonic peaks, which are a complex phenomenon that cannot be predicted by our LPDOS theory. Discounting these peaks, the experimental data closely agree with the calculated Si spectrum. Any disagreement between the two spectra is largely a result of spectral broadening. Calculated data use a standard 1 eV Gaussian broadening. Conversely, the experimental spectrum has 0.1eV Gaussian broadening due to instrumentation, but is dominated by Lorentzian broadening, which is the result of the finite lifetime of the excited states.

The arrows in Figure 2 mark the features of the XAS spectrum corresponding to the excitation energies used for XES measurements. These energies were chosen based on their high absorption probability, as well as their correspondence to significant features in the calculated spectrum. Excitation energies corresponding to features which are prominent in only one of the site-specific spectra will produce an emission spectrum dominated by that particular non-equivalent site. Tall, narrow features, which indicate low levels of lifetime broadening, allow excitation energies to be determined to within a small energy range, which further adds to the probability of site-specific emission. The broad features found in the Si L2,3 emission spectrum make determining appropriate excitation energies difficult. However, the excitonic peaks are very narrow and each is due to a different non-equivalent site, and therefore provide ideal indicators for site-selective excitation.

Publications

  • S. Leitch, A. Moewes, L. Ouyang, W. Y. Ching, and T. Sekine,
    "Properties of non-equivalent sites and band gap of spinel-phase silicon nitride." J. Phys.: Condens. Matter, 16, (2004) 6469.
  • P.F. Karimov, N.A. Skorikov, E.Z. Kurmaev, L.D. Finkelstein, S. Leitch, J. MacNaughton, A. Moewes, T. Mori,
    "Resonant inelastic soft x-ray scattering and electronic structure of LiBC." J. Phys.: Condens. Matter, 16, (2004) 5137.
  • A.V. Sokolov, E.Z. Kurmaev, S. Leitch, A. Moewes, J. Kortus, L.D. Finkelstein, N.A. Skorikov, C. Xiao, and A. Hirose,
    "Band dispersion of MgB2, graphite and diamond from resonant inelastic scattering", J. Phys.: Condens. Matter 15, 1-9, (2003).