Doctor of Philosophy
Prof. Lyudmila Goncharova
Doping and structural modification affects such important characteristics as conductivity, catalytic activity, luminescent and magnetic properties of modern materials. Ion beam implantation is a conventional doping method which combines a low processing temperature with convenient control of concentration and distribution of dopant and irradiation damage. In this study, the ion beam implantation method was used to modify strontium titanate (STO) and highly oriented pyrolytic graphite (HOPG). Both materials have immense potential for applications in different areas of modern technology, including gas sensing, catalysis, electronics and spintronics. Fe-implanted STO and N- and O-implanted HOPG were examined with complementary experimental techniques, including Particle Induced X-ray Emission (PIXE), Rutherford Backscattering (RBS), X-ray Absorption Near Edge Structure (XANES) and X-ray Photoemission (XPS). Magnetic properties were analyzed with Superconducting quantum interference device (SQUID) magnetometry.
Irradiation with ion beams modifies structure and increases the surface reactivity of STO and HOPG. XPS reveals an increase of O and C content on STO surface due to reactions with gases from the ambient atmosphere with the surface defects. XANES analysis detects the formation of carbonyl and other functional groups as well as amorphization with formation of sp3 carbon species on the ion irradiated HOPG surfaces. Iron irradiation and post-implantation annealing in O2 at 350°C cause unexpected loss of Sr at the surface area of STO due to formation of lower density SrCO3 and Sr(OH)2 phases and possible SrO desorption.
The STO single crystals exhibit weak ferromagnetic moments prior to implantation. The maximum saturation moment is obtained after our highest implantation dose of 2x1016 Fe atom/cm2, which could be correlated with the metallic Fe0 phases in addition to the presence of O/Ti vacancies. The annealing in oxygen atmosphere partially heals implantation damages and changes the oxidation state of the implanted iron from metallic Fe0 to Fe2+/Fe3+ oxide, accompanied by a loss of the ferromagnetic response. Iron oxide phases with Fe2+ and Fe3+ states corresponding to this regime are identified and their structures are confirmed by calculations using the Real Space Multiple Scattering program (FEFF9). Magnetic moments of the N-and O-implanted HOPG samples are correlated well with transition metal impurities.
Lobacheva, Olga, "Ion beam modification of strontium titanate and highly oriented pyrolytic graphite" (2015). Electronic Thesis and Dissertation Repository. 2903.