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Green, R.J.,Hunt, A.,Zatsepin, D.A.,Boukhvalov, D.W.,McLeod, J.A.,Kurmaev, E.Z.,Skorikov, N.A.,Gavrilov, N.V.,Moewes, A. North-Holland 2012 Journal of non-crystalline solids Vol.358 No.23
The electronic structures of Sn and Pb implanted SiO<SUB>2</SUB> are studied using soft X-ray absorption (XAS) and emission (XES) spectroscopy. We show, using reference compounds and ab initio calculations, that the presence of Pb?O and Sn?O interactions can be detected in the pre-edge region of the oxygen K-edge XAS. Via analysis of this interaction-sensitive pre-edge region, we find that Pb implantation results primarily in the clustering of Pb atoms. Conversely, with Sn implantation using identical conditions, strong Sn?O interactions are present, showing that Sn is coordinated with oxygen. The varying results between the two ion types are explained using both ballistic considerations and density functional theory calculations. We find that the substitution of Pb into Si sites in SiO<SUB>2</SUB> requires much more energy than substituting Sn in these same sites, primarily due to the larger size of the Pb ions. From these calculated formation energies it is evident that Pb requires far higher temperatures than Sn to be soluble in SiO<SUB>2</SUB>. These results help explain the complex processes which take place upon implantation and determine the final products.
Leedahl, B.,Zatsepin, D. A.,Boukhvalov, D. W.,Kurmaev, E. Z.,Green, R. J.,Zhidkov, I. S.,Kim, S. S.,Cui, L.,Gavrilov, N. V.,Cholakh, S. O.,Moewes, A. American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.48
<P>Herein we systematically study a range of dopants (Cr, Fe, Ni, Cu, and an MnCo alloy) in ZnO and TiO<SUB>2</SUB> using several X-ray spectroscopic techniques. We identify the dopant’s local environment and interaction with the host lattice by employing crystal field multiplet calculations and hence clarify their potential applicability for spintronic technologies. Our density functional theory (DFT) calculations predict a decreasing probability of direct cation (Zn/Ti) substitution by dopant atoms as atomic number increases, as well as a much greater likelihood of metallic clustering in TiO<SUB>2</SUB>. Our spectroscopic measurements confirm that in all cases, except Mn, metallic clusters of dopant atoms form in the TiO<SUB>2</SUB> crystal lattice, thus making it unfit for spintronic capabilities. On the other hand, in ZnO, the dopants substitute directly into zinc sites, which is promising for spintronic technologies.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-48/jp509761c/production/images/medium/jp-2014-09761c_0011.gif'></P>
Local Structure of Fe Impurity Atoms in ZnO: Bulk versus Surface
McLeod, J. A.,Boukhvalov, D. W.,Zatsepin, D. A.,Green, R. J.,Leedahl, B.,Cui, L.,Kurmaev, E. Z.,Zhidkov, I. S.,Finkelstein, L. D.,Gavrilov, N. V.,Cholakh, S. O.,Moewes, A. American Chemical Society 2014 The Journal of Physical Chemistry Part C Vol.118 No.10
<P>By studying Fe-doped ZnO pellets and thin films with various X-ray spectroscopic techniques, and complementing this with density functional theory calculations, we find that Fe-doping in bulk ZnO induces isovalent (and isostructural) cation substitution (Fe<SUP>2+</SUP> → Zn<SUP>2+</SUP>). In contrast to this, Fe-doping near the surface produces both isovalent and heterovalent substitution (Fe<SUP>3+</SUP> → Zn<SUP>2+</SUP>). The calculations performed herein suggest that the most likely defect structure is the single or double substitution of Zn with Fe, although, if additional oxygen is available, then Fe substitution with interstitial oxygen is even more energetically favorable. Furthermore, it is found that ferromagnetic states are energetically unfavorable, and ferromagnetic ordering is likely to be realized only through the formation of a secondary phase (i.e., ZnFe<SUB>2</SUB>O<SUB>4</SUB>), or codoping with Cu.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2014/jpccck.2014.118.issue-10/jp411219z/production/images/medium/jp-2013-11219z_0010.gif'></P>
Structural defects induced by Fe-ion implantation in TiO<sub>2</sub>
Leedahl, B.,Zatsepin, D. A.,Boukhvalov, D. W.,Green, R. J.,McLeod, J. A.,Kim, S. S.,Kurmaev, E. Z.,Zhidkov, I. S.,Gavrilov, N. V.,Cholakh, S. O.,Moewes, A. American Institute of Physics 2014 Journal of Applied Physics Vol.115 No.5
X-ray photoelectron spectroscopy and resonant x-ray emission spectroscopy measurements of pellet and thin film forms of TiO2 with implanted Fe ions are presented and discussed. The findings indicate that Fe-implantation in a TiO2 pellet sample induces heterovalent cation substitution (Fe2+ -> Ti4+) beneath the surface region. But in thin film samples, the clustering of Fe atoms is primarily detected. In addition to this, significant amounts of secondary phases of Fe3+ are detected on the surface of all doped samples due to oxygen exposure. These experimental findings are compared with density functional theory calculations of formation energies for different configurations of structural defects in the implanted TiO2:Fe system. According to our calculations, the clustering of Fe-atoms in TiO2:Fe thin films can be attributed to the formation of combined substitutional and interstitial defects. Further, the differences due to Fe doping in pellet and thin film samples can ultimately be attributed to different surface to volume ratios. (C) 2014 AIP Publishing LLC.
Reduction of conductivity and ferromagnetism induced by Ag doping in ZnO:Co
Bieber, H.,Colis, S.,Schmerber, G.,Pierron-Bohnes, V.,Boukhvalov, D.W.,Kurmaev, E.Z.,Finkelstein, L.D.,Bazylewski, P.,Moewes, A.,Chang, G.S.,Dinia, A. Elsevier 2013 THIN SOLID FILMS - Vol.545 No.-
<P><B>Abstract</B></P> <P>Cobalt and silver co-doping has been undertaken in ZnO thin films grown by pulsed laser deposition in order to investigate the ferromagnetic properties in ZnO-based diluted magnetic materials and to understand the eventual relation between ferromagnetism and charge carriers. Hall transport measurements reveal that Ag doping up to 5% leads to a progressive compensation of the native <I>n</I>-type carriers. The magnetization curves show ferromagnetic contributions for all samples at both 5K and room temperature, decreasing with increasing the Ag concentration. First principles modeling of the possible configurations of Co–Ag defects suggests the formation of nano-clusters around interstitial Co impurity as the origin of the ferromagnetism. The Ag co-doping results in a decrease of the total spin of these clusters and of the Curie temperature.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We fabricate ZnO:Co films co-doped by Ag. </LI> <LI> Reduction of the number of charge carriers in co-doped samples is observed. </LI> <LI> Decrease of magnetic moment per Co in Ag-rich samples is observed. </LI> <LI> Theoretical calculations support suppression of ferromagnetism in Ag-doped ZnO:Co. </LI> </UL> </P>
Local electronic structure of Mn dopants in ZnO probed by resonant inelastic x-ray scattering
Chang, G S,Kurmaev, E Z,Jung, S W,Kim, H-J,Yi, G-C,Lee, S-I,Yablonskikh, M V,Pedersen, T M,Moewes, A,Finkelstein, L D IOP Pub 2007 Journal of physics, an Institute of Physics journa Vol.19 No.27
<P>The electronic structure of Mn dopants in ZnO epitaxial thin films synthesized at different temperatures has been investigated using resonant inelastic x-ray scattering. The resulting Mn L<SUB>2,3</SUB> x-ray emission spectra of Zn<SUB>0.8</SUB>Mn<SUB>0.2</SUB>O (resonantly excited at L<SUB>2</SUB> and L<SUB>3</SUB> absorption edges) reveal different spectral features depending on the growth temperature of the films. The relative integral intensity ratio of Mn L<SUB>2</SUB> to Mn L<SUB>3</SUB> emission lines is greatly suppressed in the case of nonmagnetic Zn<SUB>0.8</SUB>Mn<SUB>0.2</SUB>O grown at 700 °C due to L<SUB>2</SUB>L<SUB>3</SUB>M<SUB>4,5</SUB> Coster–Kronig transitions. The ferromagnetic sample grown at 600 °C exhibits a normal oxide structure. The results suggest that a high growth temperature causes direct Mn–Mn bonds from the segregation of Mn atoms in ZnO. Therefore the disappearance of ferromagnetism in Mn-doped ZnO can be attributed to antiferromagnetic Mn–Mn exchange interactions due to the inhomogeneous local environment around the Mn impurities.</P>