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RISS 인기검색어
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- 저자 : Cholakh, S. O. (검색결과 4 건)
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Atomic and electronic structure of a copper/graphene interface as prepared and 1.5 years after
Boukhvalov, D.W.,Bazylewski, P.F.,Kukharenko, A.I.,Zhidkov, I.S.,Ponosov, Yu. S.,Kurmaev, E.Z.,Cholakh, S.O.,Lee, Y.H.,Chang, G.S. Elsevier 2017 APPLIED SURFACE SCIENCE - Vol.426 No.-
<P><B>Abstract</B></P> <P>We report the results of X-ray spectroscopy and Raman measurements of as-prepared graphene on a high quality copper surface and the same materials after 1.5 years under different conditions (ambient and low humidity). The obtained results were compared with density functional theory calculations of the formation energies and electronic structures of various structural defects in graphene/Cu interfaces. For evaluation of the stability of the carbon cover, we propose a two-step model. The first step is oxidation of the graphene, and the second is perforation of graphene with the removal of carbon atoms as part of the carbon dioxide molecule. Results of the modeling and experimental measurements provide evidence that graphene grown on high-quality copper substrate becomes robust and stable in time (1.5 years). However, the stability of this interface depends on the quality of the graphene and the number of native defects in the graphene and substrate. The effect of the presence of a metallic substrate with defects on the stability and electronic structure of graphene is also discussed</P> <P><B>Highlights</B></P> <P> <UL> <LI> Proposed model of step by step perforation of graphene on metals during process of oxidation. </LI> <LI> Modeling and measurements demonstrate important role of substrate quality for protective properties of graphene. </LI> <LI> Effect of imperfectness of carbon sheet and substrate to electronic structure of graphene are shown. </LI> <LI> Key conditions for long-term anti-corrosion protection are discussed. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
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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.
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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>
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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>
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