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Koichi Yamashita

Publications and source records attributed to Koichi Yamashita.

13 recordsLinked to original sources
arXiv2023

Definition of the Tetrel Bond

This paper proposes a definition of the term "tetrel bond" based on the evidence documented in the current literature. It lists the donors, acceptors, as well as related characteristic features of tetrel bonds that are commonly observed in the crystalline phase and/or that emerge from first principles calculations in the solid-state and gas phases. These may be used to identify, char-acterize and classify the unique subset of inter- and intramolecular interactions formed by the elements of Group 14 of the Periodic Table that possess an electrophilic site in a molecular entity.

cond-mat.mtrl-sci↗
arXiv2022

Definition of the Pnictogen Bond: A Perspective

This article proposes a definition for the term pnictogen bond and lists its donors, acceptors, and characteristic features. These may be invoked to identify this specific subset of the inter- and intra-molecular interactions formed by elements of Group 15 which possess an electrophilic site in a molecular entity.

physics.chem-ph↗
arXiv2018

Role of Quantum-Confinement in Anatase Nanosheets

Despite most of the applications of anatase nanostructures rely on photoexcited charge processes, yet profound theoretical understanding of fundamental related properties is lacking. Here, by means of ab initio ground and excited-state calculations, we reveal, in an unambiguous way, the role of quantum confinement effect and of the surface orientation, on the electronic and optical properties of anatase nanosheets (NSs). The presence of bound excitons extremely localized along the (001) direction, whose existence has been recently proven also in anatase bulk, explains the different optical behavior found for the two orientations (001) and (101) when the NS thickness increases. We suggest also that the almost two-dimensional nature of these excitons can be related to the improved photoconversion efficiency observed when a high percentage of (001) facet is present in anatase nanocrystals.

cond-mat.mtrl-sci↗
arXiv2018

Influence of the aggregate state on band structure and optical properties of C60 computed with different methods

C60 and C60 based molecules are efficient acceptor and electron transport layers for planar perovskite solar cells. While properties of these molecules are well studied by ab initiomethods, those of solid C60, specifically its optical absorption properties, are not. We present a combined Density Functional Theory - Density Functional Tight Binding study of the effect of solid state packing on bandstructure and optical absorption of C60. The valence and conduction band edge energies of solid C60 differ on the order of 0.1 eV from single molecule frontier orbital energies. We show that calculations of optical properties using linear response TD-DFT(B) or the imaginary part of the dielectric constant (dipole approximation) can result in unrealistically large redshift in the presence of intermolecular interactions compared to available experimental data. We show that optical spectra computed from the frequency-dependent real polarizability better reproduce the effect of C60 aggregation on optical absorption and may be more suited to study effects of molecular aggregation.

cond-mat.mtrl-sci↗
arXiv2018

Halogen in Materials Design: Revealing the Nature of Hydrogen Bonding and Other Non-Covalent Interactions in the Polymorphic Transformations of Methylammonium Lead Tribromide Perovskite

Methylammonium lead tribromide perovskite (CH3NH3PbBr3, or MAPbBr3) as a photovoltaic material has attracted a great deal of recent interest. Factors that are important in their application in optoelectronic devices include their fractional contribution of the composition of the materials as well as their microscopic arrangement that is responsible for the formation of well-defined macroscopic structures. CH3NH3PbBr3 assumes different polymorphs (orthorhombic, tetragonal and cubic) depending on the evolution temperature of the bulk material. Density functional theory calculations have been performed on polymorphs of CH3NH3PbBr3 to demonstrate that the H atoms on C of the methyl group in MA entrapped within a MAPbBr3 perovskite cage are not electronically innocent, as is often contended. We show here that these H atoms are involved in attractive interactions with the surrounding bromides of corner-sharing octahedra of the CH3NH3PbBr3 cage to form Br...H(-C) hydrogen bonding interactions. This is analogous to the way the H atoms on N of the ammonium group in MA form Br...H(-N) hydrogen bonding interactions to stabilize the structure of CH3NH3PbBr3. Both these hydrogen bonding interactions are shown to persist regardless of the nature of the three polymorphic forms of CH3NH3PbBr3. These, together with the Br...C(-N) carbon bonding, the Br...N(-C) pnictogen bonding, and the Br...Br lump-hole type intermolecular non-covalent interactions identified for the first time in this study, are shown to be collectively responsible for the eventual emergence of the orthorhombic geometry of the CH3NH3PbBr3 system. These conclusions are arrived at from a systematic analysis of the results obtained from combined DFT, Quantum Theory of Atoms in Molecules, and Reduced Density Gradient Non-Covalent Interaction calculations carried out on the three temperature-dependent polymorphic geometries of CH3NH3PbBr3.

cond-mat.mtrl-sci↗
arXiv2017

Unraveling a Structure-Property Relationship for Methylammonium Lead/Tin Trihalide Organic-Inorganic Hybrid Perovskite Solar Cells

The esoteric importance of intermolecular hydrogen bonding interaction to design novel methylammonium trihalide organic-inorganic hybrid perovskite solar cell materials is uncovered, establishing the unified structure-property relationship between the calculated Y...H intermolecular hydrogen bonding distance and the experimentally reported onset of optical absorption (bandgap) for the bulk geometries of the ten-membered methylammonium lead trihalide (CH3NH3PbY3) perovskite solar cell series, where Y = X (X = Cl, Br, I) and the mixed halogen derivatives. The same relationship is also revealed for the ten-membered methylammonium tin trihalide (CH3NH3SnY3) perovskite solar cell series. The relationship unequivocally demonstrates that the intermolecular hydrogen bonding interaction does not only enforce the aforesaid materials to become functional for optoelectronic application, but also serve as an asset to partially address the often debated question what is the role played by the CH3NH3+ organic cation.

cond-mat.mtrl-sci↗
arXiv2017

Why Do Eight Units of Methylammonium Enclose PbI6 Octahedron in Large-Scale Crystals of Methylammonium Lead Iodide Perovskite Solar Cell? An Answer from First-Principles Study

Methylammonium lead triiodide (CH3NH3PbI3) perovskite solar cell is a gem in the list of photovoltaic semiconductors. Although there are numerous fundamental and technological questions yet to be addressed covering various aspects of this system for its commercialization, this study has employed first-principles DFT to model the [PbI6(CH3NH3)n]m zero-dimensional nanoclusters. Using the calculated binding energy landscapes, it has answered the question: why the corner-sharing PbI6 octahedron is surrounded by eight units of the organic cation in the large-scale supramolecular structures of the CH3NH3PbI3 system in 3D? The synergistic effect of the methylammonium, as well as the consequence of positive and negative cooperative effects associated with intermolecular hydrogen bonding on the supramolecular evolution of the CH3NH3PbI3 crystals is briefly outlined.

cond-mat.mtrl-sci↗
arXiv2017

Organic-Inorganic Hybrid CH3NH3PbI3 Perovskite Solar Cell Nanoclusters: Revealing Ultra-Strong Hydrogen Bonding and Mulliken Inner Complexes and Their Implication in Materials Design

Methylammonium lead iodide (CH3NH3PbI3) perovskite solar cell has produced a remarkable breakthrough in the photovoltaic history of solar cell technology because of its outstanding device based performance as a light-harvesting semiconductor. Whereas the experimental and theoretical studies of this system in the solid state have been numerously reported in the last 4 years, its fundamental cluster physics is yet to be exploited. To this end, this study has performed theoretical investigations using DFT-M06-2X/ADZP to examine the principal geometrical, electronic, topological, and orbital properties of the CH3NH3PbI3 nanocluster blocks. These clusters are found to be unusually strongly bound, with binding energies lying between 93.53 and 125.11 kcal mol-1 (beyond the covalent limit, 40 kcal mol-1), enabling us to characterize the underlying interactions as ultra-strong type. Based on this, together with the unusually high charge transfers, strong hyperconjugative interactions, sophisticated topologies of the charge density, and short intermolecular distances uncovered, we have characterized the CH3NH3PbI3 as Mulliken inner complexes. Additionally, the consequences of these, as well as of the ultra-strong interactions, in designing novel functional nanomaterials are briefly discussed. The various new results obtained in this study are not in perfect agreement with those already reported experimentally (Nat. Commun. 2015, 6, 7124), and computationally (Chem. Commun., 2015, 51, 6434; Sci. Rep. 2016, doi:10.1038/srep21687; Chem. Mater. 2016, 28, 4259; J. Mat. Chem A 2016, 4, 4728; J. Phys. Chem. Lett. 2016, 7, 1596).

physics.chem-ph↗
arXiv2014

Semiclassical quantization of nonadiabatic systems with hopping periodic orbits

We present a semiclassical quantization condition, i.e., quantum-classical correspondence, for steady states of nonadiabatic systems consisting of fast and slow degrees of freedom (DOFs) by extending Gutzwiller's trace formula to a nonadiabatic form. The quantum-classical correspondence indicates that a set of primitive hopping periodic orbits, which are invariant under time evolution in the phase space of the slow DOF, should be quantized. The semiclassical quantization is then applied to a simple nonadiabatic model and accurately reproduces exact quantum energy levels. In addition to the semiclassical quantization condition, we also discuss chaotic dynamics involved in the classical limit of nonadiabatic dynamics.

physics.chem-ph↗
arXiv2014

The penetration barrier of water through graphynes' pores: first-principles predictions and force field optimization

Graphynes are novel two-dimensional carbon-based materials that -due to their nanoweb-like structure- have been proposed as molecular filters, especially for water purification technologies. In this work we carry out first principles electronic structure calculations at the MP2C level of theory to assess the interaction between water and graphyne, graphdiyne and graphtriyne pores. The computed penetration barriers suggest that water transport is unfeasible through graphyne while being unimpeded for graphtriyne. Nevertheless, for graphdiyne, which presents a pore size almost matching that of water, a low barrier is found which in turn disappears if an active hydrogen bond with an additional water molecule on the opposite side of the opening is taken into account. These results support the possibility of using graphtriyne as an efficient membrane for water filtration but, in contrast with previous determinations, they do not exclude graphdiyne. In fact, the related first principles penetration barrier leads to water permeation probabilities which are at least two order of magnitude larger than those estimated by employing commonly used force fields. A new pair potential for the water--carbon non-covalent component of the interaction has been built and it is recommended for molecular dynamics simulation involving graphdiyne and water.

physics.chem-ph↗
arXiv2013

Exact Electronic Potentials in Coupled Electron-Ion Dynamics

We develop a novel approach to the coupled motion of electrons and ions that focuses on the dynamics of the electronic subsystem. Usually the description of electron dynamics involves an electronic Schrödinger equation where the nuclear degrees of freedom appear as parameters or as classical trajectories. Here we derive the exact Schrödinger equation for the subsystem of electrons, staying within a full quantum treatment of the nuclei. This exact Schrödinger equation features a time-dependent potential energy surface for electrons (e-TDPES). We demonstrate that this exact e-TDPES differs significantly from the electrostatic potential produced by classical or quantum nuclei.

physics.chem-ph↗
arXiv2013

Electron-electron and electron-phonon correlation effects on the finite temperature electronic and optical properties of zb-GaN

We combine the effect of the electron-electron and electron-phonon interactions to study the electronic and optical properties of zb-GaN. We show that only by treating the two effects at the same time it is possible to obtain an unprecedented agreement of the zero and finite-temperature electronic gaps and absorption spectra with the experimental results. Compared to the state-of-the-art results our calculations predict a large effect on the main absorption peak position and width as well as on the overall absorption lineshape. These important modifications are traced back to the combined electron-phonon damping mechanism and non uniform GW level corrections. Our results demonstrate the importance of treating on equal footing the electron and phonon mediated correlation effects to obtain an accurate description of the III-nitrides group physical properties.

cond-mat.mtrl-sci↗