Junga Ryou

Origins of dihydrogen binding to metal-inserted porphyrins: electric polarization and Kubas interaction

Using density functional theory calculations, we have investigated the interactions between hydrogen molecules and metalloporphyrins. A metal atom, such as Ca or Ti, is introduced for incorporation in the central N(4) cavity. Within local density approximation (generalized gradient approximation), we find that the average binding energy of H(2) to the Ca atom is about 0.25 (0.1) eV/H(2) up to four H(2) molecules, whereas that to the Ti atom is about 0.6 (0.3) eV per H(2) up to two H(2) molecules. Our analysis of orbital hybridization between the inserted metal atom and molecular hydrogen shows that H(2) binds weakly to Ca-porphyrin through a weak electric polarization in dihydrogen, but is strongly hybridized with Ti-porphyrin through the Kubas interaction. The presence of d orbitals in Ti may explain the difference in the interaction types.

Investigations of Vacancy Structures Related to Their Growth in h -BN Sheet

The atomic, electronic, and magnetic properties of vacancy structures with triangular shape related to their growth in single hexagonal boron nitride (h-BN) sheet are investigated using density functional theory calculations. We find that the optimized structures of triangular vacancies depend on the vacancy sizes with N-terminated zigzag edge. Then, vacancy structures obtained during the vacancy evolution in h-BN sheet are considered by removing a boron-nitrogen pair (BN pair) from edges of triangular vacancies. The magnetic properties of those vacancy structures are investigated by local density of states and spin densities. It is found that the stability of the optimized structures with a BN missing pair depends on the BN-pair missing position: the most stable structure is a BN-pair missing structure at the edge face region with the smallest magnetic moment.

Ab initio Investigations of Carbon Atoms Adsorbed on α-Al2O3 Surfaces in Relation to Graphene Growth

Adsorption behaviors of carbon atoms on Al2O3(0001) are investigated to understand the graphene growth in its initial stage. For the most stable structure of carbon on sapphire, C–O binding is analyzed by examining projected density of states. Increased carbon atoms are found to form cluster structures such as chain-like or distorted graphene-like one. Due to strong C–O binding, at least one carbon atom of the cluster structures binds an oxygen atom of the sapphire surface. Combined with C–C interaction, this result may explain that carbon atoms on sapphire form nanocrystalline graphitic structures within a limited area, observed in experiment.

Edge-functionalization of armchair graphene nanoribbons with pentagonal-hexagonal edge structures

Using density functional theory calculations, we have studied the edge-functionalization of armchair graphene nanoribbons (AGNRs) with pentagonal-hexagonal edge structures. While the AGNRs with pentagonal-hexagonal edge structures (labeled (5,6)-AGNRs) are metallic, the edge-functionalized (5,6)-AGNRs with substitutional atoms opens a band gap. We find that the band structures of edge-functionalized (5,6)-N-AGNRs by substitution resemble those of defect-free (N-1)-AGNR at the Γ point, whereas those at the X point show the original ones of the defect-free N-AGNR. The overall electronic structures of edge-functionalized (5,6)-AGNRs depend on the number of electrons, supplied by substitutional atoms, at the edges of functionalized (5,6)-AGNRs.

Growth Mechanisms of Carbon Nanotubes and Graphene

To understand the initial stage in the growth of carbon nanotubes and graphene, we investigate the adsorption and diffusion of carbon atoms on the surfaces of metals such as nickel and copper. Adsorption geometries and binding energies of carbon atoms on the low‐index metal surfaces and in their subsurfaces are considered. Then, the surface and subsurface diffusion barriers of a single carbon atom are calculated. On the basis of the results obtained, it is found that the carbon diffusion in nickel and copper plays an important role in the growth of these carbon‐based materials.