Effective Screening Medium Method Research Articles

Geometric structure and electronic properties of bilayer graphene with a Moire superlattice by interlayer asymmetric tensile strain

Using the density functional theory combined with the effective screening medium method, we investigated the energetics and electronic properties of bilayer graphene, comprising graphene layers without and with tensile strain. An interlayer interaction does not affect the structural reconstruction of each graphene layer despite the bilayer structure possessing a Moire superlattice. The structural asymmetry between the graphene layers causes a potential difference across the layers, leading to electron and hole doping in the layers without and with the tensile strain. Accumulated carriers show unique lateral distribution in each graphene layer, which depends on the interlayer atomic arrangements.

Bias-dependent surface stress by density functional theory combined with the effective screening medium method

Bias-dependent surface stress by density functional theory combined with the effective screening medium method

Effect of interlayer stacking arrangement on the dielectric properties of hexagonal boron nitride thin films

Electrostatic properties of hexagonal boron nitride (hBN) thin films with different stacking arrangements were investigated using density functional theory combined with the effective screening medium method. Our calculations showed that the dielectric properties across layers of hBN thin films are sensitive to both the interlayer stacking arrangement and the number of layers. The polarization of bilayer hBN gradually decreases with increasing lateral displacement from AB stacking, and polarity inversion occurs for particular stacking arrangements. The polarity of bilayer hBN is sensitive to twisting displacement. The polarity monotonically increases with increasing the number of layers in hBN films with rhombohedral stacking arrangement.

Energetics and electronic structure of Janus WSSe formation by continuous chalcogen substitutions

We investigated the energetics and the electronic structure of Janus WSSe when formed by continuous sulfurization and selenization of WSe2 and WS2, respectively, using density functional theory combined with the effective screening medium method. The total energy of WS2−x Se x is sensitive to the stoichiometry. The total energy increases monotonically as the substitutional surface selenization of WS2 increases. The sulfurization of WSe2 causes complex energetics with respect to the number of S atoms involved. Competition between the energy cost of polarization and the energy gain through S–W bond formation produces the compound WS0.445Se1.445, which is a metastable structure that gives the local minimum in the energy landscape. The electronic structures of the partially substituted structures are interpolated smoothly from those of Janus WS2 and WSe2.

Energetics and electronic structure of bilayer Janus WSSe

Employing density functional theory along with the effective screening medium method, we investigated the energetics and electronic structure of bilayer Janus WSSe in terms of their interlayer stacking arrangement. Through the orbital hybridization between chalcogen atoms at interfaces, the energetics are sensitive to the interlayer stacking orientation and interface atomic arrangements. This interface atomic arrangement creates the unique electronic structure of bilayer Janus WSSe determined by the dipole moment arrangement of the constituent WSSe layers. The net polarity of thin films of Janus transition-metal dichalcogenides is a simple superposition of the dipole moments of the constituent layers.

Electron Transport through Nanoconfined Ferrocene Solution: Density Functional Theory─Nonequilibrium Green Function Approach

We investigate the transport of electrons in a ferrocene aqueous solution nanoconfined between two Pt electrodes using density functional theory and a nonequilibrium Green function method. The system consists of three characteristic phases: metal electrodes, the electrode–solution interface, and the nanoconfined solution phase. By performing the geometry optimization of such systems, it is found that the molecular configuration of water molecules at the Pt surface is adjusted due to the Pt–water interaction, and the charges at the Pt surface are redistributed. Next, by applying the external bias potential via the effective screening medium method during ab initio molecular dynamics simulations, it is revealed that water molecules are packed at the Pt–water interfacial region, and the electrostatic potential profile in the water phase is significantly changed due to aligned water layers. From the analysis of the electron transport characteristics, it is discovered that the ferrocene molecule generates a strong transmission function near the Fermi level and high electron density of states spatially connecting both electrodes through the water phase. These features explain the drastic enhancement in electrical current–voltage characteristics. Therefore, it is concluded that ferrocene enhances electron transport through nanoconfined solutions, indicating that it can be used as a signaling molecule in nanoscale systems for electrochemical sensor applications.

First-Principles Investigation of Charged Germagraphene as a Cathode Material for Dual-Carbon Batteries.

As part of the concerted effort for development of energy storage technologies, dual-ion batteries (DIBs) or dual-carbon batteries (DCBs) are attracting interest, owing primarily to their eco-friendly active materials. The use of carbon as the active materials of DCBs brings about several challenges involving capacity and stability. This contribution aims to provide an in-depth understanding of the structural and electronic properties of Ge-doped graphene (Germagraphene) as a novel cathode material for DCBs. Density functional theory (DFT) calculations are combined with the effective screening medium (ESM) method for analyzing the electronic and band structure of PF6 - anion-adsorbed Germagraphene under a potential bias. These theoretical investigations indicate that the use of Ge as a dopant for graphene has a positive impact on the adsorption of the anion on the cathode under both neutral and electrically biased conditions.

Development of a dielectrically consistent reference interaction site model combined with the density functional theory for electrochemical interface simulations

The dielectrically consistent reference interaction site model (DRISM) is one of the methods used to solve the well-known drawback of the reference interaction site model (RISM) theory: That it underestimates the dielectric constant of the solutions. Recently, Nishihara and Otani developed the first-principles effective screening medium (ESM) method combined with RISM theory, called ESM-RISM, to simulate physical properties at the electrode/electrolyte interface [Phys. Rev. B 96, 115429 (2017)]. In this study, we combined DRISM with the ESM-RISM framework to increase the accuracy of electrochemical interface simulations, which was applied to the Pt(111)/aqueous water interface as a benchmark calculation. The electrochemical properties at the interface, such as the potential of zero charge, standard hydrogen electrode potential, and double-layer capacitance, all reasonably agree with previous experiments. The thickness of the contact layer was found to correlate well with double-layer capacitance. We believe that the present method is a useful tool to better model the physical properties at electrochemical interfaces.

First-principles calculation of gate-tunable ferromagnetism in magic-angle twisted bilayer graphene under pressure

Magic-angle twisted bilayer graphene (MATBG) is notable as a highly tunable platform for investigating strongly correlated phenomena such as unconventional superconductivity and quantum spin liquids, due to easy control of doping level through gating and sensitive dependence of the magic angle on hydrostatic pressure. Experimental observations of correlated insulating states, unconventional superconductivity and ferromagnetism in MATBG indicate that this system exhibits rich exotic phases. In this work, using density functional theory calculations in conjunction with the effective screening medium method, we find the MATBG under pressure at a twisting angle of 2.88∘ and simulate how its electronic states evolve when doping level and electric field perpendicular to plane are tuned by gating. Our calculations show that, at doping levels between two electrons and four holes per moiré unit cell, a ferromagnetic (FM) solution with spin density localized at AA stacking sites is lower in energy than the nonmagnetic solution. The magnetic moment of this FM state decreases with both electron and hole doping and vanishes at four electrons/holes doped per moiré unit cell. Hybridization between the flat bands at the Fermi level and the surrounding dispersive bands can take place at finite doping. On increasing the out-of-plane electric field at zero doping, a transition from the FM state to the nonmagnetic one is seen. An investigation of impurity effects shows that both absorption of molecules and occurrence of Stone–Wales impurities suppress the FM state, and the mechanisms are understood from our calculations. We also analyze the interlayer bonding character due to flat bands via Wannier functions. Finally, we report trivial band topology of the flat bands in the FM state at a certain doping level.

Electrostatic properties of two-dimensional C60 polymer thin films under an external electric field

Electrostatic properties of different C60 thin films under external electric fields have been investigated from first-principle total-energy calculations. Density functional theory calculations combined with the effective screening medium method reveal that the electrostatic properties of C60 thin films in an electric field strongly depend on the arrangement and conformation of the C60 molecules. The relative permittivity across the thin films exhibits clear a positional dependence resulting from the π electron distribution within the films. An electrostatic polarization is uniformly induced by weak electric fields, typically 0.1 V nm−1, because of the semiconducting electronic structure of the thin films, whereas the polarization is highly concentrated in the outermost C60 layer under strong electric fields of 0.5 V nm−1.

Continuous Fermi level tuning of Nb-doped WSe2 under an external electric field

The possibility of continuous Fermi level tuning of Nb-doped WSe2 under an external electric field is investigated, using the density functional theory combined with the effective screening medium method. The Fermi level monotonically increases and decreases as the carrier concentration increases and decreases, respectively, by controlling the external electric field. The electronic structure of Nb-doped WSe2 is insensitive to the Nb concentration and arrangement. Furthermore, it was demonstrated that the electric field simply shifts the Fermi level of Nb-doped WSe2, resulting in the constant quantum capacitance through the gate voltage, irrespective of the Nb concentration and arrangement.

Geometric structure and piezoelectric polarization of MoS2 nanoribbons under uniaxial strain

Using density functional theory with the generalized gradient approximation and effective screening medium method, we investigated the geometric and piezoelectric properties of MoS2 nanoribbons under uniaxial strain in terms of their edge shapes. The strain ranged from 6% compressive to 6% tensile. Ribbons with zigzag and armchair edges are stiffer than those with chiral edges under compressive strain. Substantial edge reconstruction occurs at the chiral edges where S atomic sites are dominant under 6% compressive strain. Relative piezoelectric polarity between edges decreases and increases under compressive and tensile strains, respectively, when a ribbon possesses the zigzag edges, indicating possible application to piezoelectric devices. Polarity across a ribbon with armchair and chiral edges is insensitive to uniaxial strain except when there is edge reconstruction under a large compressive strain. The electronic structure of a ribbon slightly depends on uniaxial strain.

Dynamics of a charged Ne atom near graphene edges under a positive static electric field

Using the density functional theory combined with the effective screening medium method, the dynamical properties of a Ne atom near graphene edges under an external electric field are investigated. The field-induced motions of a Ne atom near the graphene edges are sensitive to the initial position of Ne and the edge morphology of graphene. The Ne atom is attracted to the edge atomic sites of graphene when it is initially located above the edges. The Ne atom moves to the counter negative electrode when it is located alongside the armchair edge. The Ne atom approximately maintains its initial position when it is located alongside the zigzag edge owing to the polarity of the Ne atom induced by the edge morphology.

Breaking the Controversy of the Electropolymerization of Pyrrole Mechanisms by the Effective Screening Medium Quantum Charged Model Interface.

Several mechanisms for the electropolymerization of pyrrole have been proposed since the first report 40 years ago. However, none of them were consensual despite a range of assumptions. We simulated and explained the preliminary steps governing the electropolymerization of pyrrole in a charged model interface using first-principles molecular dynamics calculations to solve the problem. We have shown under these conditions that adjacent pyrrole molecules in water can react together, causing their electropolymerization at the interface with a biased platinum electrode in anodic oxidation. In this work, the effective screening medium method that prevents energy divergence of the system was applied to different configurations of pyrrole, water, and electrolyte molecules to best screen the phase space. Furthermore, we worked on a Pt(100) electrode surface in an aqueous electrolyte to be as close as possible to the experimental conditions, MD taking the average of their different orientations.

Quantifying Uncertainties in Solvation Procedures for Modeling Aqueous Phase Reaction Mechanisms.

Computational quantum chemistry provides fundamental chemical and physical insights into solvated reaction mechanisms across many areas of chemistry, especially in homogeneous and heterogeneous renewable energy catalysis. Such reactions may depend on explicit interactions with ions and solvent molecules that are nontrivial to characterize. Rigorously modeling explicit solvent effects with molecular dynamics usually brings steep computational costs while the performance of continuum solvent models such as polarizable continuum model (PCM), charge-asymmetric nonlocally determined local-electric (CANDLE), conductor-like screening model for real solvents (COSMO-RS), and effective screening medium method with the reference interaction site model (ESM-RISM) are less well understood for reaction mechanisms. Here, we revisit a fundamental aqueous hydride transfer reaction-carbon dioxide (CO2) reduction by sodium borohydride (NaBH4)-as a test case to evaluate how different solvent models perform in aqueous phase charge migrations that would be relevant to renewable energy catalysis mechanisms. For this system, quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations almost exactly reproduced energy profiles from QM simulations, and the Na+ counterion in the QM/MM simulations plays an insignificant role over ensemble averaged trajectories that describe the reaction pathway. However, solvent models used on static calculations gave much more variability in data depending on whether the system was modeled using explicit solvent shells and/or the counterion. We pinpoint this variability due to unphysical descriptions of charge-separated states in the gas phase (i.e., self-interaction errors), and we show that using more accurate hybrid functionals and/or explicit solvent shells lessens these errors. This work closes with recommended procedures for treating solvation in future computational efforts in studying renewable energy catalysis mechanisms.

Asymmetric carrier penetration into hexagonal boron nitride in graphene field-effect transistors

The electronic structure of graphene field-effect transistors with a hexagonal boron nitride (h-BN) gate dielectric was studied in terms of the gate electric field and dielectric thickness using the density functional theory combined with the effective screening medium method. The calculation results showed that the carrier penetration into monolayer h-BN depended on the gate voltage that injected electrons or holes into graphene. The critical voltage for hole penetration was lower than that for electron penetration. At a fixed carrier concentration, carrier penetration soon occurred with increasing h-BN thickness for hole doping.

Influence of interlayer stacking arrangements on carrier accumulation in bilayer graphene field effect transistors

The electronic structure of bilayer graphene under an external electric field is studied in terms of the interlayer stacking arrangements using the density functional theory combined with the effective screening medium method. The calculations showed that the accumulated carrier distribution strongly depended on the interlayer stacking arrangement. The carriers were highly concentrated on the topmost layer of twisted bilayer graphene, while carriers were also found in the second layer of bilayer graphene with the AA and AB interlayer arrangements. In addition to the carrier accumulation, ab initio molecular dynamics calculations also revealed that the interlayer Seebeck coefficient and thermal diffusivity in the bilayer graphene depended on the interlayer stacking arrangements.

Toward Engineering of Solution Microenvironments for the CO2 Reduction Reaction: Unraveling pH and Voltage Effects from a Combined Density-Functional-Continuum Theory.

Engineering the electrolyte microenvironment represents an attractive route to tuning the selectivity of electrocatalytic reactions beyond catalyst composition and morphology. However, harnessing the full potential of this approach requires understanding the interplay between voltage, electrolyte composition, and adsorbate binding within the electrical double layer, which is absent from the usual theoretical approaches. In this work, we apply a recently developed density functional theory (DFT)-continuum approach based on the effective screening medium method and reference interaction site model (ESM-RISM) to explore electrolyte effects with an enhanced description of the electrochemical interface. Applying this method to the binding of CO adsorbates in potassium-containing electrolytes on copper, a problem of direct relevance to CO2 electroreduction to value-added products, we show that the interdependence of voltage and pH leads to an unexpected change in adsorption site preference on Cu(001) terraces. Our findings highlight the often-overlooked importance of the electrical double-layer structure for predicting catalyst operation.

Edge morphology effect on field emission properties of graphene thin films

Field emission current density from the edge of graphene thin films is calculated in terms of their edge morphology, using density functional theory combined with the effective screening medium method and the Fowler-Nordheim theory. The field emission current from edges of the bilayer graphene is higher than that from the monolayer graphene edges. Edge displacement of the bilayer graphene causes a higher emission current than the aligned edges for both zigzag and armchair edges. Bilayer graphene with closed edges further enhances the emission current compared with those with open clean edges, owing to the decrease of their work functions and the potential barrier outside them.

From Anodic Oxidation of Aliphatic α-Amino Acids to Polypeptides by Quantum Electrochemistry Approach: Beyond Miller-Urey Experiments.

For years, polypeptide formation has fascinated the scientific world because its understanding could lead to one of the possible explanations for the origin of life. Anodic oxidation of aliphatic α-amino acids in aqueous electrolytes can result either in their decomposition or in their polymerization into polypeptide. This behavior depends experimentally on both amino acid concentration and pH. The elucidation of the involved mechanisms remains a challenge because of the multitude of products which can be obtained. In this context, the electrochemical behavior of glycine and alanine on a biased platinum surface was examined at the nanoscale by quantum electrochemistry via the effective screening medium method. Several electrochemical systems with different concentrations and pH values have been explored. Simulations of the anodic oxidation of the amino acids have not only confirmed their electropolymerization and decomposition at high and low concentrations, respectively, but also have revealed unsuspected mechanisms at the origin of polypeptide formation. This sheds new light on electrochemistry of α-amino acids, on occurrence of polypeptides, and more generally on organic electrochemistry.