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.

John McLeod

John McLeod

Current Affiliation:
Associate Professor, College of Nano Science and Technology, Soochow University 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou Jiangsu 215123, P. R. China

[john.mcleod@usask.ca]

Education

  • University of Toronto
    Toronto, Ontario
    B.A. Sc. (Engineering Physics)
    May 2007
  • University of Saskatchewan
    Saskatoon, Saskatchewan
    M. Sc. (Physics)
    May 2010
  • University of Saskatchewan
    Saskatoon, Saskatchewan
    Ph.D. Physics
    August 2013

Ph.D. Thesis

X-ray Transitions in Broad Band Materials

Abstract

The general application of soft X-ray spectroscopy in the study of the electronic structure of materials is discussed, with particular emphasis on broad band materials. Several materials are studied using both soft X-ray spectroscopy and density functional theory to provide experimental and theoretical electronic structures, respectively. In particular, bonding, cation hybridization, and band gaps for several binary oxides (the alkali oxides: BeO, MgO, CaO, SrO, BaO; the post-transition metal oxides: ZnO, CdO, HgO; and the period 5 oxides In2O3, SnO, SnO2, Sb2O3, Sb2O5, and TeO2) are studied. The technique of using the peaks in the second derivatives of an X-ray emission and an X-ray absorption spectrum to estimate the band gap of a material is critically analyzed, and a more accurate ``semi-empirical'' method that involves both measured spectra and theoretical calculations is proposed.

The techniques used in the study of binary oxides are then applied to a more interesting (and industrially relevant) group of ternary oxides based on TiO2 (PbTiO3, Sn2TiO4, Bi2Ti4O11, Bi4Ti3O12, and ZnTiO3), and a general rule for the band gaps of these materials is suggested based on empirical data. This research may help direct efforts in synthesizing a hydrogen-producing photocatalyst with a band gap that can efficiently harness the bulk of the solar spectrum.

Finally, several layered pnictide superconductors and related compounds (CaFe2As2, Co-, Ni- and Cu-doped BaFe2As2, LiFeAs, LiMnAs, CaCu1.7As2, SrCu2As2, SrCu2(As0.84Sb0.16)2, SrCu2Sb2, and BaCu2Sb2) are studied. The X-ray spectra provide rather strong evidence that these materials lack strong on-site Hubbard-like correlations, and that their electronic structures are almost entirely like those of a broad band metal. In particular, it is shown that the notion that the transition metals are all divalent is completely wrong for copper in a layered pnictide, and that at best in these systems the copper is monovalent.

M.Sc. Thesis

Structural Characterization of Arsenic-Doped Amorphous Selenium

Abstract

Amorphous selenium has recently found commercial application in modern direct conversion X-ray imaging detectors (DCXID) used in medical facilities. Amorphous selenium is used to directly convert an X-ray signal into an electrical signal - similar to the process in a digital camera. These DCXIDs have faster imaging capabilities and lower costs then traditional X-ray imaging devices. Additionally, there is theoretical justification for extremely high resolution X-ray images.

Unfortunately, amorphous selenium is unstable, and it spontaneously crystallizes. Even partially crystallized selenium is useless for DCXIDs. Doping with arsenic can retard rate of crystallization, but the exact mechanism for this process is unknown. Unfortunately adding arsenic increases charge-trapping, which reduces the resolution of the DCXID.

Using synchrotron-excited X-ray spectroscopy I have studied the structural and electronic properties of arsenic-doped amorphous selenium. I have identified spectral features which coincide with increasing crystallinity, and increasing arsenic concentration. Further study of this system is on-going.

This research is supported by an NSERC Strategic Grant.

Other Projects

Advanced Oxides

  • Multiferroics
  • Pyroxenes
  • Binary metal-oxides

Oxygen is a key element in many correlated materials, and is an excellent element for soft X-ray spectroscopy. Many metal-oxide systems exhibit interesting properties such as (anti)ferromagnetism, (anti)ferroelectricity, and ferroelasticity.

I am using synchrotron-excited soft X-ray spectroscopy (SXS) and density functional theory (DFT) calculations to study the properties of many multiferroics, pyroxenes, and binary metal-oxides.

Iron Pnictide Superconductors

A new class of superconductors was recently discovered based on Fe-As layers. I am using SXS and DFT to study the electronic structure of these materials, helping to identify electron correlation effects, the influence of transition metal dopants, and the physical structure of these materials.

Aromatic Adsorption in Porous Silicon

Porous silicon can trap airborne aromatic molecules in nano-scale pores. These molecules change the surface electronic properties of the silicon. I am using SXS to study the impact of different nitroamide aromatics on the surface electronic structure of porous silicon. This research is aimed at developing solid-state detectors for explosives.

Awards

  • NSERC CGS D3 (September 2010)
  • NSERC Summer in Taiwan Scholarship (July 2010)

Publications

  • "Local structure of Fe impurity atoms in ZnO: bulk versus surface"
    J. A. McLeod, D. W. Boukhvalov, D. A. Zatsepin, R. J. Green, B. Leedahl, L. Cui, E. Z. Kurmaev, I. S. Zhidkov, L. Finkelstein, N. V. Gavrilov, S. O. Cholakh, and A. Moewes J. Phys. Chem. C, 118 (2014) 5336-5345.
    doi: 10.1021/jp411219z

  • "Structural defects induced by Fe-ion implantation in TiO2"
    B. Leedahl, D. A. Zatsepin, D. W. Boukhvalov, R. J. Green, J. A. McLeod, S. S. Kim, E. Z. Kurmaev, I. S. Zhidkov, N. V. Gavrilov, S. O. Cholakh, and A. Moewes J. Appl. Phys. 115 (2014) 053711.
    doi: 10.1063/1.4864748

  • "X-ray Spectroscopic Study of the Conduction Band of K3:Anthracene and K3:Phenanthrene"
    A. L. Pitman, J. A. McLeod, E. Khozeimeh Sarbisheh, E. Kurmaev, J. Muller, and A. Moewes J. Phys. Chem. C 117 (2013) 19616-19621.
    doi: 10.1021/jp406166z

  • "Electronic structure of copper pnictides: Influence of different cations and pnictogens"
    J. A. McLeod, E. Z. Kurmaev, I. Perez, V. K. Anand, P. Kanchana Perera, D. C. Johnston, and A. Moewes Phys. Rev. B 88 (2013) 014508.
    doi: 10.1103/PhysRevB.88.014508

  • "Predicting the band gap of ternary oxides containing 3d10 and 3d0 metals"
    J. A. McLeod, A. Moewes, D. A. Zatsepin, E. Z. Kurmaev, A. Wypych, I. Bobowska, A. Opasinska, and S. O. Cholakh Phys. Rev. B 86 (2012) 195207.
    doi: 10.1103/PhysRevB.86.195207

  • "Chemical bonding and hybridization in 5p binary oxides"
    J. A. McLeod, N. A. Skorikov, L. D. Finkelstein, E. Z. Kurmaev, and A. Moewes, J. Phys. Chem. C 116:45 (2012) 24248-24254.
    doi: 10.1021/jp3077134

  • "Interplay of Ballistic and Chemical Effects in the Formation of Structural Defects for Sn and Pb Implanted Silica"
    R. J. Green, A. Hunt, D. A. Zatsepin, D. W. Boukhvalov, J. A. McLeod, E. Z. Kurmaev, N. A. Skorikov, N. V. Gavrilov, and A. Moewes, J. Non-Cryst. Solids 358 (2012) 3187-3192.
    doi:10.1016/j.jnoncrysol.2012.09.014

  • "Band-gap engineering in TiO2-based ternary oxides"
    J. A. McLeod, R. J. Green, E. Z. Kurmaev, N. Kumada, A. A. Belik, and A. Moewes, Phys. Rev. B, 85 (2012) 195201
    doi:10.1103/PhysRevB.85.195201

  • "Oxygen-vacancy-induced ferromagnetism in undoped SnO2 thin films"
    G. S. Chang, J. Forrest, E. Z. Kurmaev, A. N. Morozovska, M. D. Glinchuk, J. A. McLeod, A. Moewes, T. P. Surkova, and N. H. Hong, Phys. Rev. B, 85 (2012) 165319
    doi:10.1103/PhysRevB.85.165319

  • "Effect of 3d-doping on the electronic structure of BaFe2As2"
    J. A. McLeod, A. Buling, R. J. Green, T. D. Boyko, N. A. Skorikov, E. Z. Kurmaev, M. Neumann, L. D. Finkelstein, N. Ni, A. Thaler, S. L. Bud'ko, P. C. Canfield, and A. Moewes J. Phys.: Condens. Matter, 24 (2012) 215501
    doi:10.1088/0953-8984/24/21/215501

  • "Selective Response of Mesoporous Silicon to Adsorbants with Nitro Groups"
    J. A. McLeod, E. Z. Kurmaev, P. V. Sushko, T. D. Boyko, I. A. Levitsky, and A. Moewes Chem. Eur. J., 18:10 (2012) 2912
    doi:10.1002/chem.201102084

  • "Spectroscopic characterization of a multiband complex oxide: Insulating and conducting cement 12CaO7Al2O3"
    J. A. McLeod, A. Buling, E. Z. Kurmaev, P. V. Sushko, M. Neumann, L. D. Finkelstein, S.-W. Kim, H. Hosono, and A. Moewes, Phys. Rev. B 85 (2012) 045204 (This manuscript was an Editors' Suggestion as a paper of particular importance, interest, and/or clarity.)
    doi:10.1103/PhysRevB.85.045204

  • "Nature of the electronic states involved in the chemical bonding and superconductivity at high pressure in SnO"
    J. A. McLeod, A. V. Lukoyanov, E. Z. Kurmaev, L. D. Finkelstein, and A. Moewes, JETP Letters, 94:2 (2011) 146
    doi:10.1134/S0021364011140098

  • "The electronic structure of lithium metagallate"
    N. W. Johnson, J. A. McLeod, and A. Moewes J. Phys.: Condens. Matter, 23 (2011) 445501
    doi:10.1088/0953-8984/23/44/445501

  • "Valence Structure of Alkaline and Post-Transition Metal Oxides"
    J. A. McLeod, R. J. Green, N. A. Skorikov, L. D. Finkelstein, M. Abu-Samak, E. Z. Kurmaev, and A. Moewes,Proc. SPIE 7940, 79400R (2011) Invited Talk.
    doi:10.1117/12.881181

  • "Band gaps and electronic structure of alkaline-earth and post-transition-metal oxides"
    J. A. McLeod, R. G. Wilks, N. A. Skorikov, L. D. Finkelstein, M. Abu-Samak, E. Z. Kurmaev, and A. Moewes, Phys. Rev. B 81 (2010) 245123
    doi:10.1103/PhysRevB.81.245123

  • "Electronic structure of BiMO3 multiferroics and related oxides"
    J. A. McLeod, Z. V. Pchelkina, L. D. Finkelstein, E. Z. Kurmaev, R. G. Wilks, A. Moewes, I. V. Solovyve, A. A. Belik, and E. Takayama-Muromachi, Phys. Rev. B 81 (2010) 144103
    doi:10.1103/PhysRevB.81.144103

  • "Correlation effects in Ni 3d states of LaNiPO"
    A. V. Lukoyanov, S. L. Skornyakov, J. A. McLeod, M. Abu-Samak, R. G. Wilks, E. Z. Kurmaev, A. Moewes, N. A. Skorikov, Yu. A. Izyumov, L. D. Finkelstein, V. I. Anisimov, and D. Johrendt, Phys. Rev. B 81 (2010) 235121
    doi:10.1103/PhysRevB.81.235121

  • "Valence Band Structure and X-ray Spectra of Oxygen-Deficient Ferrites SrFeOx"
    V. R. Galakhov, E. Z. Kurmaev, K. Kuepper, M. Neumann, J. A. McLeod, A. Moewes, I. A. Leonidov, and V. L. Kozhevnikov, J. Phys. Chem. C 114:11 (2010) 5154
    doi:10.1021/jp909091s

  • "Metal-insulator transition in NiS2−xSex"
    J. Kuneš, L. Baldassarre, B. Schächner, K. Rabia, C. A. Kuntscher, Dm. M. Korotin, V. I. Anisimov, J. A. McLeod, E. Z. Kurmaev, and A. Moewes, Phys. Rev. B 81 (2010) 035122
    doi:10.1103/PhysRevB.81.035122

  • "Electronic properties of pyroxenes NaCrSi2O6 and NaFeSi2O6"
    S. V. Streltsov, J. McLeod, A. Moewes, G. J. Redhammer, and E. Z. Kurmaev, Phys. Rev. B 81 (2010) 045118
    doi:10.1103/PhysRevB.81.045118

  • "Contribution of Fe 3d states to the Fermi level of CaFe2As2"
    E. Z. Kurmaev, J. A. McLeod, A. Buling, N. A. Skorikov, A. Moewes, M. Neumann, M. A. Korotin, Yu. A. Izyumov, N. Ni, and P. C. Canfield, Phys. Rev. B 80 80 (2009) 054508
    doi:10.1103/PhysRevB.80.054508

  • "Structural models of FeSex"
    E. Z. Kurmaev, J. A. McLeod, N. A. Skorikov, L. D. Finkelstein, A. Moewes, M. A. Korotin, Yu. A. Izyumov, Y. L. Xie, G. Wu and X. H. Chen, J. Phys.: Condens. Matter 21 (2009) 435702
    doi:10.1088/0953-8984/21/43/435702

  • "Identifying valence structure in LiFeAs and NaFeAs with core-level spectroscopy"
    E. Z. Kurmaev, J. A. McLeod, N. A. Skorikov, L. D. Finkelstein, A. Moewes, Yu. A. Izyumov and S. Clarke, J. Phys.: Condens. Matter 21 (2009) 345701
    doi:10.1088/0953-8984/21/34/345701