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Hydrogen-free defects in hydrogenated black TiO<sub>2</sub>
Choi, Heechae,Moon, Seong-I,Song, Teaseup,Kim, Seungchul The Royal Society of Chemistry 2018 Physical chemistry chemical physics Vol.20 No.30
<P>Black anatase TiO2 has surprisingly enhanced solar energy harvesting efficiency and electrical conductivity, which makes it a promising material in a wide range of energy and environmental applications. Several experimental and theoretical studies have successfully revealed the mechanisms of band gap reduction by surface hydrogenation of anatase TiO2. However, recent experimental evidence suggests the existence of bulk point defects that yield infrared (∼1.0 eV) photoabsorption and high conductivity of black anatase TiO2. In the current study, using a combination of <I>ab initio</I> molecular dynamics simulations and electronic structure calculations, we successfully explain the physical properties, metallicity, and infrared/microwave absorption (<I>i.e.</I>, black color) of highly reduced anatase TiO2 crystal in a hydrogenated state with a newly found pair defect (Tii-VO)<SUP>4+</SUP>. Hydrogen atoms in the bulk are unnecessary to understand the observed properties.</P>
Choi, Heechae,Song, Jin Dong,Lee, Kwang-Ryeol,Kim, Seungchul American Chemical Society 2015 Inorganic Chemistry Vol.54 No.8
<P>The visible-light absorption and luminescence of wide band gap (3.25 eV) strontium titanate (SrTiO3) are well-known, in many cases, to originate from the existence of natural oxygen deficiency in the material. In this study based on density functional theory (DFT) calculations, we provide, to the best of our knowledge, the first report indicating that oxygen vacancies in the bulk and on the surfaces of SrTiO3 (STO) play different roles in the optical and magnetic properties. We found that the doubly charged state of oxygen vacancy (V-O(2+)) is dominant in bulk SrTiO3 and does not contribute to the sub-band gap photoexcitation or intrinsic magnetism of STO. Neutral oxygen vacancies (V-O(0)) on (001) surfaces terminated with both TiO2 and SrO layers induce magnetic moments, which are dependent on the charged state of VO. The calculated absorption spectra for the (001) surfaces exhibit mid-infrared absorption (<0.5 eV) and sub-band gap absorption (2.5-3.1 eV) due to oxygen vacancies. In particular, V-O(0) on the TiO2-terminated surface has a relatively low formation energy and magnetic moments, which can explain the recently observed spin-dependent photon absorptions of STO in a magnetic circular dichroism measurement [Rice, W. D.; et al. Nat. Mater.13, 481, 2014].</P>
Interface-Dependent Spin-Reorientation Energy Barrier in Fe/MgO(001) Thin Film
Heechae Choi,Eung-Kwan Lee,Sung Beom Cho,Dong Su Yoo,Yong-Chae Chung IEEE 2011 IEEE electron device letters Vol.32 No.9
<P>Using the density-functional-theory-based atomic modeling, the stable interface structure and the resultant magnetocrystalline anisotropy (MCA) of the Fe/MgO(001) for magnetic random access memory have been studied. The most stable surface structure of Fe/MgO(001) thin-film system was found to be either defect free or possessing oxygen vacancies in a c(2 ×1) periodicity. The formation of the oxygen vacancies in c(2 ×1) periodicity on MgO(001) surface reduced the MCA of Fe layer from 1.38 to 0.31 meV/atom. The reduced MCA is originated from the filling of the minority states of the Fe orbital below Fermi level.</P>
Magnetic Properties of Strained L1<SUB>0</SUB>-ordered FePt and CoPt
Heechae Choi 한국진공학회(ASCT) 2014 Applied Science and Convergence Technology Vol.23 No.5
Using ab initio calculations, the effects of uniaxial, biaxial, and hydrostatic strains on the magnetocrystalline anisotropy of L10-orderd FePt and CoPt alloys were systematically investigated. Interestingly, the rates and the signs of magnetocrystalline anisotropy changes of FePt and CoPt were determined by the directions and dimensions of strains. The calculation results are consistent with the previous experimental observations and are expected to provide directions to tailor magnetic properties of various types of L10-ordered FePt and CoPt systems.
Magnetic Properties of Strained L1<sub>0</sub>-ordered FePt and CoPt: An ab initio Study
Choi, Heechae The Korean Vacuum Society 2014 Applied Science and Convergence Technology Vol.23 No.5
Using ab initio calculations, the effects of uniaxial, biaxial, and hydrostatic strains on the magnetocrystalline anisotropy of $L1_0$-orderd FePt and CoPt alloys were systematically investigated. Interestingly, the rates and the signs of magnetocrystalline anisotropy changes of FePt and CoPt were determined by the directions and dimensions of strains. The calculation results are consistent with the previous experimental observations and are expected to provide directions to tailor magnetic properties of various types of $L1_0$-ordered FePt and CoPt systems.
Detecting gas molecules <i>via</i> atomic magnetization
Choi, Heechae,Lee, Minho,Kim, Seungchul,Lee, Kwang-Ryeol,Chung, Yong-Chae The Royal Society of Chemistry 2014 Dalton Transactions Vol.43 No.34
<P>Adsorptions of gas molecules were found to alter the directions and magnitudes of magnetic moments of transition metal (Co, Fe) atoms adsorbed on graphene. Using first-principles calculations, we demonstrated that magnetism of surface atoms can be used to identify the kind of existing gas molecules <I>via</I> spin-reorientation and/or demagnetizations caused by the reconfigurations of 3d electron energy levels of Co and Fe. We suggest for the first time that magnetic properties of transition metal-embedded nanostructures can be used in highly selective gas-sensing applications.</P> <P>Graphic Abstract</P><P>Adsorptions of gas molecules were found to alter the directions and magnitudes of magnetic moments of transition metal (Co, Fe) atoms adsorbed on graphene. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4dt01401d'> </P>