Van der Waals density-functional theory study for bulk solids with BCC, FCC, and diamond structures

Proper inclusion of van der Waals (vdW) interactions in theoretical simulations based on standard density functional theory (DFT) is crucial to describe the physics and chemistry of systems such as organic and layered materials. Many encouraging approaches have been proposed to combine vdW interactions with standard approximate DFT calculations. Despite many vdW studies, there is no consensus on the reliability of vdW methods. To help further development of vdW methods, we have assessed various vdW functionals through the calculation of structural prop- erties at equilibrium, such as lattice constants, bulk moduli, and cohesive energies, for bulk solids, including alkali, alkali-earth, and transition metals, with BCC, FCC, and diamond structures as the ground state structure. These results provide important information for the vdW-related materials research, which is essential for designing and optimizing materials systems for desired physical and chemical properties.Comment: 10 pages, 6 Figures, 3 Table

Ultrastructure of Cryptosporidium parvum Found in the Small Intestine of Immunosuppressed Mice

The ultrastructure of various stages of Cryptosporidium parvum was observed by transmission electron microscopy. C. parvum infection was activated in the small intestine of Korean laboratory mice (fCR) by immunosuppression with prednisolone for 7 weeks. The oocyst discharge was confirmed by modified Kinyon's acid fast stain of fecal specimens. Various endogenous stages of parasites, i. e., trophozoites, meronts, merozoites, and macrogametocytes, were observed in the middle part of the small intestine, as an extracytoplasmic but intracellular parasite of host mucosal epithelial cells. In trophozoites, a large nucleus with a prominent nucleolus was seen, and as they developed into meronts, endoplasmic reticulum appeared prominently in the cytoplasm. Two kinds of meronts, type I and type II, with eight and four merozoites respectively, were found. New merozoites were produced by nuclear division and external budding of the residual body of the meronts. The merozoites were lined with two unit membranes, unlike C. muris that has three membranes, and a nucleus was located near the posterior end. Mature merozoites had conoids, rhoptries and numerous micronemes; the characteristic structures of coccidian parasites. Macrogametocytes were largely vacuolated and "wall-forming body I" was recognized. Other sexual stages were difficult to recognize from our specimens. The present study confirmed that the Cryptosporidium found in the small intestine of Korean laboratory mice has a characteristic ultrastructure consistent with C. parvum

Anisotropy of Growth of the Close-Packed Surfaces of Silver

The growth morphology of clean silver exhibits a profound anisotropy: The growing surface of Ag(111) is typically very rough while that of Ag(100) is smooth and flat. This serious and important difference is unexpected, not understood, and hitherto not observed for any other metal. Using density functional theory calculations of self-diffusion on flat and stepped Ag(100) we find, for example, that at flat regions a hopping mechanism is favored, while across step edges diffusion proceeds by an exchange process. The calculated microscopic parameters explain the experimentally reported growth properties.Comment: RevTeX, 4 pages, 3 figures in uufiles form, to appear in Phys. Rev. Let

Ab initio study of step formation and self-diffusion on Ag(100)

Using the plane wave pseudopotential method we performed density functional theory calculations on the stability of steps and self-diffusion processes on Ag(100). Our calculated step formation energies show that the {111}-faceted step is more stable than the {110}-faceted step. In accordance with experimental observations we find that the equilibrium island shape should be octagonal very close to a square with predominately {111}-faceted steps. For the (100) surface of fcc metals atomic migration proceeds by a hopping or an exchange process. For Ag(100) we find that adatoms diffuse across flat surfaces preferentially by hopping. Adatoms approaching the close-packed {111}-faceted step edges descend from the upper terrace to the lower level by an atomic exchange with an energy barrier almost identical to the diffusion barrier on flat surface regions. Thus, within our numerical accuracy (approx +- 0.05 eV) there is no additional step-edge barrier to descent. This provides a natural explanation for the experimental observations of the smooth two-dimensional growth in homoepitaxy of Ag(100). Inspection of experimental results of other fcc crystal surfaces indicates that our result holds quite generally.Comment: 10 pages, 9 figures. Submitted to Phys. Rev B (October 31, 1996

Significant THz absorption in CH3NH2 molecular defect-incorporated organic-inorganic hybrid perovskite thin film

The valid strong THz absorption at 1.58 THz was probed in the organic-inorganic hybrid perovskite thin film, CH3NH3PbI3, fabricated by sequential vacuum evaporation method. In usual solution-based methods such as 2-step solution and antisolvent, we observed the relatively weak two main absorption peaks at 0.95 and 1.87 THz. The measured absorption spectrum is analyzed by density-functional theory calculations. The modes at 0.95 and 1.87 THz are assigned to the Pb-I vibrations of the inorganic components in the tetragonal phase. By contrast, the origin of the 1.58 THz absorption is due to the structural deformation of Pb-I bonding at the grain boundary incorporated with a CH3NH2 molecular defect

Ab-initio study of physical properties of MgO/FeOx/Fe(001) interfaces

The effects of partially oxidized interfacial FeO layers on the physical properties of MgO/FeO/Fe(001) (x = 0, 0.25, 0.5, 0.75, 1) were studied using ab-initio calculations. The free energy calculations as a function of the O chemical potential ΔμO showed that the formation of the interfaces occurred in the order of p(1×1) MgO/Fe(001) (in the Mg-rich limit) → p(2×2) MgO/FeO/Fe(001) → c(2×2) MgO/FeO/Fe(001) → p(2×2) MgO/FeO/Fe(001) → p(1×1) MgO/FeO/Fe(001) (in the O-rich limit) with increasing ΔμO. The electronic structures, the work functions, and the electron rearrangements for MgO/FeO/Fe(001) interfaces were also investigated. These results indicate that the O contents x of FeOx at the interfaces between MgO and Fe can significantly affect the physical properties of the MgO/FeO/Fe(001) interfaces.This work was supported by the Incheon National University Research Grant in 2014 (Y-R. Jang).Peer Reviewe

Importance of Interfacial Structures in the Catalytic Effect of Transition Metals on Diamond Growth

Here, using ab initio calculations, we investigated the interaction between transition metals (M) and diamond C(111) surfaces. As a physical parameter describing the catalytic effect of a transition metal on diamond growth, we considered interfacial energy difference, Delta E-int, between 1 x 1 and 2 x 1 models of M/C(111). The results showed that the transition-metal elements in the middle of the periodic table (groups 4-10) favor a 1 x 1 M/C(111) structure with diamond bulk-like interfaces, while the elements at the sides of the periodic table (groups 3, 11, and 12) favor a 2 x 1 M/C(111) structure with the 2 x 1 Pandey chain structure of C(111) underneath M. In addition, calculations of MC carbide formation for early transition metals (groups 3-6) showed that they have a tendency to form MC rather than M/C(111), which explains their low efficiency as catalysts for diamond growth. Further analysis suggests that Delta E-int, could serve as another parameter (catalytic descriptor) for describing catalytic diamond growth in addition to the conventional parameter of the melting temperature of M.N

Ab-initio study of the effects of charging on the adsorption and diffusion of Au2 on MgO(100)

By using ab-initio calculations, the adsorption and diffusion properties of charged Au dimers (Au2+ and Au2−) on MgO(100) were investigated and compared with those of the charge-neutral dimer (Au20). The most favored adsorption structure of Au2+ on MgO(100) is the upright dimer on a surface O atom in agreement with that of Au20. The surface diffusion of Au2+ on MgO(100) occurs by the leapfrog process with a barrier height of 0.38 eV lower than that of Au20 (0.66 eV). The most stable adsorption structure of Au2− on MgO(100) is the flat-lying dimer on two surface Mg atoms. The surface diffusion of Au2− on MgO(100) can proceed by the hopping and walking processes with a barrier height in the range of 0.05–0.11 eV, which is much lower than the diffusion barriers of Au2+ and Au20. Furthermore, detailed information on the electronic properties of Au20, Au2+, and Au2− are also presented. © 2021 Korean Physical SocietyN