Browsing by Department "Korea Advanced Institute of Science and Technology"

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Density-functional calculations of defect formation energies using supercell methods
(2005-01) Shim, Jihye; Lee, Eok Kyun; Lee, Y. -J.; Nieminen, Risto
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Density-functional theory combined with periodic boundary conditions is used to systematically study the dependence of defect formation energy on supercell size for diamond containing vacancy and self-interstitial defects. We investigate the effect of the electrostatic energy due to the neutralization of charged supercells and the effect of the alignment of the valence band maximum (VBM) on the formation energy. For negatively charged vacancies and positively charged interstitials, the formation energies show a clear dependence on supercell size, and the electrostatic corrections agree with the trend given by the Makov-Payne scheme (Ref. 28). For positively charged vacancies and negatively charged interstitials, the size dependence and the electrostatic corrections are quite weak. An analysis of the spatial charge density distributions reveals that these large variations in electrostatic terms with defect type originate from differences in the screening of the defect-localized charge, as explained by using a simple electron-gas model. Several VBM alignment schemes are also tested. The best agreement between the calculated and asymptotically exact ionization levels is obtained when the levels are based on the formation energies referenced to the VBM of the defect-containing supercell.
Rapid Access to Ordered Mesoporous Carbons for Chemical Hydrogen Storage
(2021-10-04) Jeong, Uiseok; Kim, Hyeon Ji; Ramesh, Sreerangappa; Dogan, Nesibe A.; Wongwilawan, Sirinapa; Kang, Sungsu; Park, Jungwon; Cho, Eun Seon; Yavuz, Cafer T.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Ordered mesoporous carbon materials offer robust network of organized pores for energy storage and catalysis applications, but suffer from time-consuming and intricate preparations hindering their widespread use. Here we report a new and rapid synthetic route for a N-doped ordered mesoporous carbon structure through a preferential heating of iron oxide nanoparticles by microwaves. A nanoporous covalent organic polymer is first formed in situ covering the hard templates of assembled nanoparticles, paving the way for a long-range order in a carbonaceous nanocomposite precursor. Upon removal of the template, a well-defined cubic mesoporous carbon structure was revealed. The ordered mesoporous carbon was used in solid state hydrogen storage as a host scaffold for NaAlH4, where remarkable improvement in hydrogen desorption kinetics was observed. The state-of-the-art lowest activation energy of dehydrogenation as a single step was attributed to their ordered pore structure and N-doping effect.
Real rank geometry of ternary forms
(2017-06) Michałek, Mateusz; Moon, Hyunsuk; Sturmfels, Bernd; Ventura, Emanuele
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We study real ternary forms whose real rank equals the generic complex rank, and we characterize the semialgebraic set of sums of powers representations with that rank. Complete results are obtained for quadrics and cubics. For quintics, we determine the real rank boundary: It is a hypersurface of degree 168. For quartics, sextics and septics, we identify some of the components of the real rank boundary. The real varieties of sums of powers are stratified by discriminants that are derived from hyperdeterminants.