Hiroshi Mizuseki

A theoretical study on the interaction of aromatic amino acids with graphene and single walled carbon nanotube

In this study we have investigated the interaction of phenylalanine (Phe), histidine (His), tyrosine (Tyr), and tryptophan (Tryp) molecules with graphene and single walled carbon nanotubes (CNTs) with an aim to understand the effect of curvature on the non-covalent interaction. The calculations are performed using density functional theory and the Moller-Plesset second-order perturbation theory (MP2) within linear combination of atomic orbitals-molecular orbital (LCAO-MO) approach. Using these methods, the equilibrium configurations of these complexes were found to be very similar, i.e., the aromatic rings of the amino acids prefer to orient in parallel with respect to the plane of the substrates, which bears the signature of weak pi-pi interactions. The binding strength follows the trend: His<Phe<Tyr<Tryp. Although the qualitative trend in binding energy is almost similar between the planar graphene and rolled nanotube structure but they differ in terms of the absolute magnitude. For the nanotube, the binding strength of these molecules is found to be weaker than the graphene sheet. To get an insight about the nature of these interactions, we have calculated the polarizability of the aromatic motifs of the amino acids. Remarkably, we find excellent correlation between the polarizability and the strength of the interaction; the higher the polarizability, greater is the binding strength. Moreover, we have analyzed the electronic densities of state spectrum before and after adsorption of the amino acid moieties. The results reveal that the Fermi level of the free CNT is red-shifted by the adsorption of the amino acids and the degree of shift is consistent with the trend in polarizability of these molecules.

Electronic Structure and Optical Properties of the Co-doped Anatase TiO2 Studied from First Principles

The Co-doped anatase TiO2, a recently discovered room-temperature ferromagnetic insulator, has been studied by the first-principles calculations in the pseudo-potential plane-wave formalism within the local-spin-density approximation (LSDA), supplemented by the full-potential linear augmented plane wave (FP-LAPW) method. Emphasis is placed on the dependence of its electronic structures and linear optical properties on the Co-doping concentration and oxygen vacancy in the system in order to pursue the origin of its ferromagnetism. In the case of substitutional doping of Co for Ti, our calculated results are well consistent with the experimental data, showing that Co is in its low spin state. Also, it is shown that the oxygen vacancy enhances the ferromagnetism and has larger effect on both the electronic structure and optical properties than the Co-doping concentration only.

Designing Nanogadgetry for Nanoelectronic Devices with Nitrogen‐Doped Capped Carbon Nanotubes

A systematic analysis of electron transport characteristics for 1D heterojunctions with two nitrogen-doped (N-doped) capped carbon nanotubes (CNTs) facing one another at different conformations is presented considering the chirality of CNTs (armchair(5,5) and zigzag(9,0)) and spatial arrangement of N-dopants. The results show that the modification of the molecular orbitals by the N-dopants generates a conducting channel in the designed CNT junctions, inducing a negative differential resistance (NDR) behavior, which is a characteristic feature of the Esaki-like diode, that is, tunneling diode. The NDR behavior significantly depends on the N-doping site and the facing conformations of the N-doped capped CNT junctions. Furthermore, a clear interpretation is presented for the NDR behavior by a rigid shift model of the HOMO- and LUMO-filtered energy levels in the left and right electrodes under the applied biases. These results give an insight into the design and implementation of various electronic logic functions based on CNTs for applications in the field of nanoelectronics.

Vacancy induced structural and magnetic transition in MnCo1−xGe

The authors report ab initio total energy calculations on the first-order structural transition of the ferromagnetic MnCo1−xGe(0.00⩽x⩽0.25) intermetallic compound. They show that increasing Co vacancies induce a transition from an orthorhombic structure at 0⩽x⩽0.08 to a hexagonal structure at x>0.08. A concomitant high-to-low moment magnetic transition and a large magnetovolume effect occur due to the change of the symmetry and the resulting coupling distance between the magnetic atoms. These results provide an excellent account for the experimental results and reveal the crucial role of the Co vacancies in determining the relative structural stability and the magnetic properties of MnCo1−xGe.

First-principles study of hydrogen storage over Ni and Rh doped BN sheets

Absorption of hydrogen molecules on Nickel and Rhodium-doped hexagonal boron nitride (BN) sheet is investigated by using the first principle method. The most stable site for the Ni atom was the on top side of nitrogen atom, while Rh atoms deservers a hollow site over the hexagonal BN sheet. The first hydrogen molecule was absorbed dissociatively over Rh atom, and molecularly on Ni doped BN sheet. Both Ni and Rh atoms are capable to absorb up to three hydrogen molecules chemically and the metal atom to BN sheet distance increases with the increase in the number of hydrogen molecules. Finally, our calculations offer explanation for the nature of bonding between the metal atom and the hydrogen molecules, which is due to the hybridization of metal d orbital with the hydrogen s orbital. These calculation results can be useful to understand the nature of interaction between the doped metal and the BN sheet, and their interaction with the hydrogen molecules.

Band Structures of Perovskite-Like Fluorides for Vacuum-Ultraviolet-Transparent Lens Materials

In the semiconductor industry, perovskite-like fluorides that have wide band gaps are potential candidates for vacuum-ultraviolet-transparent lens materials in optical lithography steppers. KMgF3 and BaLiF3 single crystals have already been grown, but materials with wider band gaps are desired. To find compositions more effective than KMgF3 and BaLiF3, we performed local density approximation (LDA) based ab initio band calculations for perovskite-like fluorides ABF3, where A and B include an exhaustive list of alkali metals, alkaline-earth metals, and other selected elements. We found that LiBeF3, NaBeF3, KBeF3, and RbMgF3 may have wider indirect band gaps than KMgF3 does. We also found that SrLiF3 may have a wider direct band gap than BaLiF3 does.

Structural investigation of thiophene thiol adsorption on Au nanoclusters: Influence of back bonds

The adsorption of thiolate radicals on the Au24 nanocluster truncated from the Au (111) surface is investigated using first principles electronic structure calculations under the density functional theory formalism. Particular emphasis is given to understanding the chemical interactions at the gold-sulfur interface. In order to describe the influence of the back bonds at the thiolate sites we have carried out adsorption studies with thiophene 2-thiolate (-ST) and thiophen–2-yl-methanethiolate (-SCH2T) along with atomic sulfur (S), mercapto (-SH), and methylthiolate (-SCH3). The results suggest that the adsorption geometry at the gold-sulfur interface is strongly dependent on the local environment of the terminal sulfur atom. The interfacial charge transfer is found to be localized along the Au-S bonds and does not influence the molecular structure of the thiophene ring.

Band gap engineering of silicene zigzag nanoribbons with perpendicular electric fields: a theoretical study

The electronic properties of silicene zigzag nanoribbons with the presence of perpendicular fields are studied by using first-principles calculations and the generalized nearest neighboring approximation method. In contrast to the planar graphene, in silicene the Si atoms are not coplanar. As a result, by applying perpendicular fields to the two-dimensional silicene sheet, the on-site energy can be modulated and the band gap at the Dirac point is open. The buckled structure also creates a height difference between the two edges of the silicene zigzag nanoribbons. We find that the external fields can modulate the energies of spin-polarized edge states and their corresponding band gaps. Due to the polarization in the plane, the modulation effect is width dependent and becomes much more significant for narrow ribbons.

Titanium-Doped Nickel Clusters TiNin (n ) 1-12): Geometry, Electronic, Magnetic, and Hydrogen Adsorption Properties

Using the first principles method, we study the growth behavior and electronic and magnetic properties of TiNi(n) (n = 1-12) clusters to clarify the effect of Ti modulation on the nickel nanostructures. Furthermore, chemisorption of H(2) was studied to understand the chemical reactivity of H(2) on the small Ni- and Ti-doped Ni clusters. The calculations are performed using the plane wave pseudopotential approach under the density functional theory and generalized gradient approximation for the exchange and correlation functional. The optimized geometries of TiNi(n-1) clusters indicate that the substitution of Ti brings a substantial structural reconstruction from 3D structure to a layer structure in which Ti atom is found to coordinate with Ni atoms to a maximum extent. This is accompanied by a significant enhancement in binding energies and reduction in chemical reactivity. Furthermore, the magnetic moments of the small Ti-doped Ni clusters are quenched because of the antiferromagnetic alignment of the Ti electrons. The lowest-energy structure of H(2) chemisorbed on Ni clusters shows that hydrogen prefers to adsorb on the edge site with two hydrogen atoms on these clusters in neighboring sites as the preferred arrangement. The incorporation of Ti atom improves the chemisorption energy of Ni clusters. Bader charge analysis indicates that with the formation of metal hydride, the H atoms withdraw charges from the metal centers, making them lose an electron, and carry a positive charge over them. Furthermore, Ti doping is found to enhance the chemical reactivity of Ni clusters.

First-principles study of the electronic structures of icosahedral TiN (N=13,19,43,55) clusters

We have studied the electronic structures of icosahedral Ti(N) clusters (N=13, 19, 43, and 55) by using a real-space first-principles cluster method with generalized gradient approximation for exchange-correlation potential. The hexagonal close-packed and fcc close-packed clusters have been studied additionally for comparisons. It is found that the icosahedral structures are the most stable ones except for Ti(43), where fcc close-packed structure is favorable in energy. We present and discuss the variation of bond length, the features of the highest occupied molecular orbitals and the lowest unoccupied molecular orbital, the evolution of density of states, and the magnetic moment in detail. The results are in good agreement with the predictions from the collision-induced dissociation and size-selected anion photoelectron spectroscopy experiments.