To study thermodynamic properties of materials, lattice model simulation such as lattice Monte Carlo (MC) simulation is one of the simple and fast method. One advantage of the method is that it can treat larger systems both in time scale... more
To study thermodynamic properties of materials, lattice model simulation such as lattice Monte Carlo (MC) simulation is one of the simple and fast method. One advantage of the method is that it can treat larger systems both in time scale and in spatial size compared with atomic-scale molecular dynamics (MD) simulations so that it can treat thermodynamic equilibrium or diffusion phase transition phenomena. However, it has limitation in the description of disordered or liquid phases because displacement of atoms from regular lattice points that may be important at high temperatures could not be considered. That is, lattice models neglect the vibration entropy as well as the elastic energy. The shortcomings lead to overestimation of the phase transition temperatures and underestimation of the width of single-phase fields.
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We investigated impact of type of crystal defects in multicrystalline Si (mc-Si) on electrical properties and their change after gettering process of impurities. A bundle of dislocations gives negative impact on the gettering process,... more
We investigated impact of type of crystal defects in multicrystalline Si (mc-Si) on electrical properties and their change after gettering process of impurities. A bundle of dislocations gives negative impact on the gettering process, while Sigma3 grain boundaries does not affect at all. In addition, we categorized random grain boundaries in mc-Si by the contact angle between adjacent dendrite crystals to form the grain boundary. Change in the contrast of photoluminescence intensity around the grain boundary was found to systematically vary by the contact angle, which showed good correlation with calculated interface energy of the grain boundary. Grain boundaries with low interface energy are concluded to be preferable to weaken recombination activity by the gettering process and improvement of solar cell performance based on mc-Si.
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We here identify by ab initio calculations a new type of three-dimensional (3D) carbon allotropes that consist of phenyl rings connected by linear acetylenic chains in sp+sp(2) bonding networks. These structures are constructed by... more
We here identify by ab initio calculations a new type of three-dimensional (3D) carbon allotropes that consist of phenyl rings connected by linear acetylenic chains in sp+sp(2) bonding networks. These structures are constructed by inserting acetylenic or diacetylenic bonds into an all sp(2)-hybridized rhombohedral polybenzene lattice, and the resulting 3D phenylacetylene and phenyldiacetylene nets comprise a 12-atom and 18-atom rhombohedral primitive unit cells in the symmetry, which are characterized as the 3D chiral crystalline modification of 2D graphyne and graphdiyne, respectively. Simulated phonon spectra reveal that these structures are dynamically stable. Electronic band calculations indicate that phenylacetylene is metallic, while phenyldiacetylene is a semiconductor with an indirect band gap of 0.58 eV. The present results establish a new type of carbon phases and offer insights into their outstanding structural and electronic properties.
The critical behavior of percolation model does not depend on the detail of the embedding lattice. This fact can be a hard obstacle when one attempt to modulate and control the characteristics of the composite materials because the limit... more
The critical behavior of percolation model does not depend on the detail of the embedding lattice. This fact can be a hard obstacle when one attempt to modulate and control the characteristics of the composite materials because the limit of modulation is limited by the percolation threshold, as in the case of substitution of expensive conductor materials by inexpensive insulator materials. Many attempts to solve this problem by changing the sizes and aspect ratios of conductor particles, expecting their effect in enhancing conduction as a ``bridge'' is not working well. We report our attempt to realize the same goal by introducing size differences in the insulator particles, not conductor particles. The effective transition point observed is actually lowered to 0.52 by this modulation from about 0.59 of conventional site percolation model (2D). The statistical nature of this novel model, in particular the optimum design of insulator particle size distribution, is a completely new and interesting theoretical problem. Moreover, this is considered to be a promising technique to reduce the amount of expensive conductor, for example the Indium in typical transparent conductor film.
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The concept of percolation plays an important role in explaining various important physical phenomena, including transport, mechanical, and electromagnetic properties of disordered systems. To date, many percolation models have been... more
The concept of percolation plays an important role in explaining various important physical phenomena, including transport, mechanical, and electromagnetic properties of disordered systems. To date, many percolation models have been developed. Contrary to the ordinary site percolation models with homogeneous particles, systems have a certain particle-size distribution. Such a distribution may affect the properties of the system in certain ways. In the present study, site-percolation models with two different sizes of particles are systematically introduced on a square lattice to understand the effect of nonhomogeneity of the particles in the system. To estimate the critical phenomena with high accuracy, a finite-size scaling analysis is performed with a Monte Carlo simulation. The critical coverage at the percolation threshold is examined as a function of the size distribution of elements in the system. Fractal dimension and the critical exponentials are also estimated.
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The increase in threats from global warming due to the consumption of fossil fuels requires our planet to adopt new strategies to harness the inexhaustible sources of energy. Hydrogen is an energy carrier which holds tremendous promise as... more
The increase in threats from global warming due to the consumption of fossil fuels requires our planet to adopt new strategies to harness the inexhaustible sources of energy. Hydrogen is an energy carrier which holds tremendous promise as a new renewable and clean energy option. Hydrogen is a convenient, safe, versatile fuel source that can be easily converted to a desired form of energy without releasing harmful emissions. However, no materials was found satisfy the desired goals and hence there is hunt for new materials that can store hydrogen reversibly at ambient conditions. In this chapter, we discuss and compare various nanofullerene materials proposed theoretically as storage medium for hydrogen. Doping of transition elements leads to clustering which reduces the gravimetric density of hydrogen, while doping of alkali and alkali-earth metals on the nanocage materials, such as carborides, boronitride, and boron cages, were stabilized by the charger transfer from the dopant to the nanocage. Further, the alkali or alkali-earth elements exist with a charge, which are found to be responsible for the higher uptake of hydrogen, through a dipole- dipole and change-induced dipole interaction. The binding energies of hydrogen on these systems were found to be in the range of 0.1 eV to 0.2 eV, which are ideal for the practical applications in a reversible system.
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Synthesis of pristine MXene sheets from MAX phase is one of the foremost challenges in getting a complete understanding of the properties of this new technologically important 2D-material. Efforts to exfoliate Nb4AlC3 MAX phase always... more
Synthesis of pristine MXene sheets from MAX phase is one of the foremost challenges in getting a complete understanding of the properties of this new technologically important 2D-material. Efforts to exfoliate Nb4AlC3 MAX phase always lead to Nb4C3 MXene sheets, which are functionalized and have several Al atoms attached. Using the first-principles calculations, we perform an intensive study on the chemical transformation of MAX phase into MXene sheets by inserting HF, alkali atoms and LiF in Nb4AlC3 MAX phase. Calculated bond-dissociation energy (BDE) shows that the presence of HF in MAX phase always results in functionalized MXene, as the binding of H with MXene is quite strong while that with F is weak. Insertion of alkali atoms does not facilitate pristine MXene isolation due to the presence of chemical bonds of almost equal strength. In contrast, weak Li-MXene and strong Li-F bonding in Nb4AlC3 with LiF ensured strong anisotropy in BDE, which will result in the dissociation of the Li-MXene bond. Ab initio molecular dynamics calculations capture these features and show that at 500-650 K, the Li-MXene bond indeed breaks leaving a pristine MXene sheet behind. The approach and insights developed here for chemical exfoliation of layered materials bonded by chemical bonds instead of van der Waals can promote their experimental realization.
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By combining empirical potential approach with first-principles calculations, we investigate the atomic and electronic structures of grain boundary in silicon to estimate the deleterious effect on photovoltaic properties. Optimized... more
By combining empirical potential approach with first-principles calculations, we investigate the atomic and electronic structures of grain boundary in silicon to estimate the deleterious effect on photovoltaic properties. Optimized geometries of several boundary structures are obtained by using a Tersoff potential. Moreover, the electronic structures of boundary have been examined using the density-functional theory with the plane-wave pseudopotential method. Calculations show that the electronic properties depend strongly on the atomistic structures, their properties are corresponding to efficiency of photovoltaic cell. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO)
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We report ab initio identification of initial dissociation pathways for Sb4 and Bi4 tetramer precursors on Si(001). We reveal a two-stage double piecewise rotation mechanism for the tetramer to ad-dimer conversion involving two distinct... more
We report ab initio identification of initial dissociation pathways for Sb4 and Bi4 tetramer precursors on Si(001). We reveal a two-stage double piecewise rotation mechanism for the tetramer to ad-dimer conversion involving two distinct pathways: one along the surface dimer row via a rhombus intermediate state and the other across the surface dimer row via a rotated rhombus intermediate state. These two-stage double piecewise rotation processes play a key role in lowering the kinetic barrier by establishing and maintaining energetically favorable bonding between adatoms and substrate atoms. These results provide an excellent account for experimental observations and elucidate their underlying atomistic origin that may offer useful insights for other surface reaction processes.
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Although an understanding of the mechanical behavior such as elastic properties and hardness of hydrides is important for their applications, theoretical studies have received little attention and only recently some progress has been... more
Although an understanding of the mechanical behavior such as elastic properties and hardness of hydrides is important for their applications, theoretical studies have received little attention and only recently some progress has been made. In the present study, first-principles calculations have been performed on hydrides of Ti, Zr, and Hf. The elastic properties are estimated as a function of hydrogen concentration. Equilibrium lattice constants and the bulk moduli are estimated using Murnaghan EOS. While, the elastic constants, shear moduli, and Young's moduli are estimated introducing the strain tensor. The origin of these properties is explained in terms of the changes in the bonding characters as well as cohesive energy. A semi-empirical relationship between the bulk modulus/shear modulus and the Vickers hardness is introduced to predict hardness of these materials from the present first-principles calculation results.
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An ab initio structure optimization technique is newly developed to determine the valley line on a total-energy surface for zone-center distortions of ferroelectric perovskite oxides and is applied to barium titanate BaTiO3 and lead... more
An ab initio structure optimization technique is newly developed to determine the valley line on a total-energy surface for zone-center distortions of ferroelectric perovskite oxides and is applied to barium titanate BaTiO3 and lead titanate PbTiO3. The proposed technique is an improvement over King-Smith and Vanderbilt's scheme [Phys. Rev. B 49, p.5828 (1994)] of evaluating total energy as a function of the amplitude of atomic displacements. The results of numerical calculations show that total energy can be expressed as a fourth-order function of the amplitude of atomic displacements in BaTiO3 but not in PbTiO3.
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... more
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 Møller-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 π-π 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.
Research Interests: Engineering, Density-functional theory, Chemical Physics, Physical sciences, Aromatic amino acid, and 12 moreElectron Density, CHEMICAL SCIENCES, Electrons, Spectrum, Second Order, Binding Energy, Linear combination of atomic orbitals, Amino Acid Profile, Molecular Conformation, Intermolecular Interaction, Nanotubes Carbon, and Perturbation Theory
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An extraordinarily large GRID environment has been established over Japan by using SuperSINET based on ITBL connecting 4 supercomputer facilities. This new supercomputing environment has been used for a large scale numerical simulations... more
An extraordinarily large GRID environment has been established over Japan by using SuperSINET based on ITBL connecting 4 supercomputer facilities. This new supercomputing environment has been used for a large scale numerical simulations using original ab initio code TOMBO and several remarkable results have already been obtained to proof that this newly built computer environment is actually useful to accelerate
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Using an approach different from the conventional vapor doping methods, Cs positive ions in a magnetized-plasma column are irradiated upon a negatively biased substrate which is covered with dispersed single-walled carbon nanotubes... more
Using an approach different from the conventional vapor doping methods, Cs positive ions in a magnetized-plasma column are irradiated upon a negatively biased substrate which is covered with dispersed single-walled carbon nanotubes (SWNTs). The Cs ions are ...
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The percolation threshold shows a universality that may cause a strict limit on the mixture ratio of composite materials. When particles A and B are randomly filling a material and A must form an interconnected cluster (e.g. for... more
The percolation threshold shows a universality that may cause a strict limit on the mixture ratio of composite materials. When particles A and B are randomly filling a material and A must form an interconnected cluster (e.g. for electrical conduction), there is a strict limit on the fraction of A (for example, 0.598 in 2D). A solution to solve this problem is introducing size distribution on B particles (N.Lebovka J.Phys.D (2006) and WJ Kim J.Appl.Phys (1998)). However, theoretical understanding of this phenomenon is still in a quite immature stage despite of its importance in applications. We report the reduction of the percolation threshold observed in square lattices with a number of binary size distributions, as well as our approach toward semi-empirical theoretical method, that is based on an enumeration of local particle configurations generated in a totally random manner. This is a notable advance because most of previous theoretical methods were considering only limited comb...
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ABSTRACT The study on the absorption of toxic gases such as mustard gas by organic host is essential to the development of inexpensive detection and decontamination equipments. Using quantum chemical methods, we propose cucurbituril as an... more
ABSTRACT The study on the absorption of toxic gases such as mustard gas by organic host is essential to the development of inexpensive detection and decontamination equipments. Using quantum chemical methods, we propose cucurbituril as an effective host to capture mustard gas. It was found that stable complexes are formed with the inclusion of the toxic gas molecules inside the cucurbituril cavity, compared with the lateral and exterior interactions. Oxygen mustard has a comparable binding energy with sulfur mustard and hence can be used during experimental investigation. Additionally, during the inclusion complex formation, the presence of heteroatoms helps the guest molecules to undergo a larger structural reorganization to get accommodated inside the cucurbituril macromolecule. Atoms-in-molecules analysis shows the existence of strong intermolecular CH…O bonding between the guest molecules and cucurbituril macromolecule. The presence of an intramolecular CH…Cl type of bonding accounts for the higher stabilization of sulfur mustard inside the cucurbituril macromolecule.
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The vibrational spectra and far-infrared (IR) absorption of DAST crystal have been investigated by first-principles calculations. The calculated vibrational frequencies have been overestimated and frequency interval have been divided on... more
The vibrational spectra and far-infrared (IR) absorption of DAST crystal have been investigated by first-principles calculations. The calculated vibrational frequencies have been overestimated and frequency interval have been divided on two-parts: the low-frequency region where lattice phonon modes are the most important and the high-frequency region with main intramolecular contribution in IR absorption intensity.
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Li adsorption on isoreticular MOFs with metal Fe, Cu, Co, Ni and Zn was studied using density function theory. Li functionalization shows a considerable structural change associated with a volume change in isoreticular MOF-5 except for... more
Li adsorption on isoreticular MOFs with metal Fe, Cu, Co, Ni and Zn was studied using density function theory. Li functionalization shows a considerable structural change associated with a volume change in isoreticular MOF-5 except for the Zn metal center. Hydrogen binding energies on Li functionalized MOFs are seen to be in the range of 0.2 eV, which is the desired value for an ideal reversible storage system. This study has clearly shown that Li doping is possible only in Zn-based MOF-5, which would be better candidate to reversibly store hydrogen.
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ABSTRACT We report a density functional theory (DFT) study regarding the effects of atomic defects, such as vacancies and adatom adsorption, on the electronic and magnetic properties of phosphorene (a two-dimensional monolayer of black... more
ABSTRACT We report a density functional theory (DFT) study regarding the effects of atomic defects, such as vacancies and adatom adsorption, on the electronic and magnetic properties of phosphorene (a two-dimensional monolayer of black phosphorous). A mono-vacancy in the phosphorene creates an in-gap state in the band gap of pristine phosphorene and induces magnetic moments, even though pristine phosphorene is non-magnetic. In contrast, both planar and staggered di-vacancies do not change the magnetic properties of phosphorene, although a staggered di-vacancy creates states in the gap. Our DFT calculations also show that adsorption of non-metallic elements (C, N, and O) and transition metal elements (Fe, Co, and Ni) can change the magnetic properties of phosphorene with or without vacancies. For example, the non-magnetic pristine phosphorene becomes magnetic after the adsorption of N, Fe, or Co adatoms, and the magnetic phosphorene with a mono-vacancy becomes non-magnetic after the adsorption of C, N, or Co atoms. We also demonstrate that for O- or Fe- adsorbed mono-vacancy structure, the electronic and magnetic properties are tunable via the control of charge on the phosphorene system. These results provide insight for achieving metal-free magnetism and a tunable band gap for various electronic and spintronic devices based on phosphorene.
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ABSTRACT With tremendous progress in computer technologies and applications during the last decade, atomistic-level simulation is rapidly becoming an essential tool in materials science for the study of the physical and chemical... more
ABSTRACT With tremendous progress in computer technologies and applications during the last decade, atomistic-level simulation is rapidly becoming an essential tool in materials science for the study of the physical and chemical properties of various materials. Moreover, in parallel with the experimental efforts, computer-aided materials design is also an important factor in the fabrication of novel materials, to be applied in driving engineering innovations and urgent technological needs for achieving a sustainable society. Here, an original approach has been demonstrated that allows us to construct a p − T phase diagrams of various hydrates with complex gas compositions. In order to evaluate the parameters of weak interactions, a time-dependent density-functional formalism and local density (TDLDA) technique entirely in real space have been implemented for the calculations of frequency-dependent polarizabilities and van der Waals dispersion coefficients for atoms within the all-electron mixed-basis approach (TOMBO code) developed at the Institute for Materials Research, Tohoku University. The combination of both methods enables one to calculate thermodynamic properties of clathrate hydrates without resorting to any empirical parameter fittings. Using the proposed method, it is possible to not only confirm the existing experimental data but also predict the unknown region of thermodynamic stability of clathrate hydrates, and also propose the gas storage ability as well as the gas composition for which high-stability region of clathrate hydrates can be achieved. The proposed method is quite general and can be applied to the various nonstoichiometric inclusion compounds with weak guest-host interactions. From this point of view, the present methodology can support experimental explorations of the novel storage materials.
ABSTRACT A systematic investigation on electronic band structure of a series of isoreticular metal–organic frameworks (IRMOFs) using density functional theory has been carried out. Our results show that halogen atoms can be used as... more
ABSTRACT A systematic investigation on electronic band structure of a series of isoreticular metal–organic frameworks (IRMOFs) using density functional theory has been carried out. Our results show that halogen atoms can be used as functional groups to tune not only the band gap but also the valence band maximum (VBM) in MOFs. Among halogen atoms (F, Cl, Br, I), iodine is the best candidate to reduce the band gap and increase the VBM value. In addition, it has been found that for the antiaromatic linker DHPDC (1,4-dihydropentalene-2,5-dicarboxylic acid) the energy gap is 0.95 eV, which is even lower than those calculated for other aromatic linkers, i.e., FFDC (furo[3,2-b]furan-2,5-dicarboxylic acid) and TTDC (thieno[3,2-b]thiophene-2,5-dicarboxylic acid). By analyzing the lowest unoccupied molecular orbital–highest occupied molecular orbital gaps calculated at the molecular level, we have highlighted the important role of the corresponding organic linkers in the MOF band gap. In particular, the change of C–C–C═O dihedral angle in the organic linker can be used to analyze the difference of band gaps in MOF crystals. It is shown that a deep understanding of chemical bonding within linker molecules from electronic structure calculations plays a crucial role in designing semiconductor properties of MOF materials for engineering applications.
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