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Abstract The design and implementation of dopant-based silicon nanoscale devices rely heavily on knowing precisely the locations of phosphorous dopants in their host crystal. One potential solution combines scanning tunneling microscopy (STM) imaging with atomistic tight-binding simulations to reverse-engineer dopant coordinates. This work shows that such an approach may not be straightforwardly extended to double-dopant systems. We find that the ground (quasi-molecular) state of a pair of coupled phosphorous dopants often cannot be fully explained by the linear combination of single-dopant ground states. Although the contributions from excited single-dopant states are relatively small, they can lead to ambiguity in determining individual dopant positions from a multi-dopant STM image. To overcome that, we exploit knowledge about dopant-pair wave functions and propose a simple yet effective scheme for finding double-dopant positions based on STM images.

An extension of the tight‐binding approximation model using the discrete fractional Fourier transform is proposed. The quantum state between localized and delocalized states is formulated, where the intermediate state is continuously parameterized. The mixed features of the localized molecular‐like state and delocalized wave‐like state are confirmed when the wavefunction and band diagram of the intermediate state are represented. The proposed model is expected to be used to represent the quantum state with localized/delocalized features in, for example, organic semiconductors. [ABSTRACT FROM AUTHOR]

The design and implementation of dopant-based silicon nanoscale devices rely heavily on knowing precisely the locations of phosphorous dopants in their host crystal. One potential solution combines scanning tunneling microscopy (STM) imaging with atomistic tight-binding simulations to reverse-engineer dopant coordinates. This work shows that such an approach may not be straightforwardly extended to double-dopant systems. We find that the ground (quasi-molecular) state of a pair of coupled phosphorous dopants often cannot be fully explained by the linear combination of single-dopant ground states. Although the contributions from excited single-dopant states are relatively small, they can lead to ambiguity in determining individual dopant positions from a multi-dopant STM image. To overcome that, we exploit knowledge about dopant-pair wave functions and propose a simple yet effective scheme for finding double-dopant positions based on STM images. [ABSTRACT FROM AUTHOR]

The superconductivity of CaC6 as a function of pressure and Ca isotopic composition was revisited using DFT calculations on a 2c–double hexagonal superlattice. The introduction of superlattices was motivated by previous synchrotron absorption and Raman spectroscopy results on other superconductors that showed evidence of superlattice vibrations at low (THz) frequencies. For CaC6, superlattices have previously been invoked to explain the ARPES data. A superlattice along the hexagonal c-axis direction is also illustrative of atomic orbital symmetry and periodicity, including bonding and antibonding s-orbital character implied by cosine-modulated electronic bands. Inspection of the cosine band revealed that the cosine function has a small (meV) energy difference between the bonding and antibonding regions, relative to a midpoint non-bonding energy. Fermi surface nesting was apparent in an appropriately folded Fermi surface using a superlattice construct. Nesting relationships identified phonon vectors for the conservation of energy and for phase coherency between coupled electrons at opposite sides of the Fermi surface. A detailed analysis of this Fermi surface nesting provided accurate estimates of the superconducting gaps for CaC6 with the change in applied pressure. The recognition of superlattices within a rhombohedral or hexagonal representation provides consistent mechanistic insight on superconductivity and electron−phonon coupling in CaC6. [ABSTRACT FROM AUTHOR]

The electron conduction mechanism in indium oxide (In2O3) and its implications for amorphous transport have been investigated from an orbital overlap perspective. Combined density functional theory and empirical tight binding modeling reveal that the electron transport is facilitated by the neighboring metal atomic s orbital overlap "without" oxygen's p-orbital involvement. In other words, the electron transport pathway in oxides is only due to the metal-metal medium range connection. This electron conduction mechanism is extended to amorphous In2O3 which unveils that the amorphous disorder influences the electron transport through impacting the metal-metal medium range order including metal-metal coordination number and metal-metal separation. Our results provide an insight into the current theoretical understanding of electron transport in amorphous oxide semiconductors. [ABSTRACT FROM AUTHOR]

In this work, computer simulation of the processes of formation of the internal structure of plasmonic Ag-Cu nanoparticles was carried out. Method of molecular dynamics based on the tight binding potential was used. The crystallization of Ag-Cu nanoparticles with a diameter of 2,0-8,0 nm with an atomic copper content ranging from 10 to 50% was simulated. To remove thermal energy, an Andersen thermostat was used with three different cooling rates ΔT/Δt = 30∙1011, 12∙1011, 5∙1011 K/s. The possible structural transitions arising in this case were determined using the radial distribution function and the change in potential energy. To determine the most stable cluster structure, an ensemble of nanoparticles of the same size was taken. Then, using the visualizers OVITO and xmakemol, the real appearance and structure of the studied nanoparticles were found. In the course of the simulation, it was found that at low levels of copper doping in Ag-Cu nanoparticles, the formation of five-particle symmetry is possible. The conditions for the occurrence of this effect were fixed. It was also determined that D = 8,0 nm for Ag-Cu nanoparticles is the size limit, starting from which the macroscopic effect of stabilizing the fcc structure of the eutectic alloy appears under the condition of very fast sample quenching.

An accurate total energy calculation is essential in materials computation. To date, many tight-binding (TB) approaches based on parameterized hopping can produce electronic structures comparable to those obtained using first-principles calculations. However, TB approaches still have limited applicability for determining material properties derived from the total energy. That is, the predictive power of the TB total energy is impaired by an inaccurate evaluation of the repulsive energy. The complexity associated with the parametrization of TB repulsive potentials is the weak link in this evaluation. In this study, we propose a new method for obtaining the pairwise TB repulsive potential for crystalline materials by employing the Chen-Möbius inversion theorem. We show that the TB-based phonon dispersions, calculated using the resulting repulsive potential, compare well with those obtained by first-principles calculations for various systems, including covalent and ionic bulk materials and two-dimensional materials. The present approach only requires the first-principles total energy and TB electronic band energy as input and does not involve any parameters. This striking feature enables us to generate repulsive potentials programmatically. [ABSTRACT FROM AUTHOR]

The superconductivity of CaC6 as a function of pressure and Ca isotopic composition was revisited using DFT calculations on a 2c–double hexagonal superlattice. The introduction of superlattices was motivated by previous synchrotron absorption and Raman spectroscopy results on other superconductors that showed evidence of superlattice vibrations at low (THz) frequencies. For CaC6, superlattices have previously been invoked to explain the ARPES data. A superlattice along the hexagonal c-axis direction is also illustrative of atomic orbital symmetry and periodicity, including bonding and antibonding s-orbital character implied by cosine-modulated electronic bands. Inspection of the cosine band revealed that the cosine function has a small (meV) energy difference between the bonding and antibonding regions, relative to a midpoint non-bonding energy. Fermi surface nesting was apparent in an appropriately folded Fermi surface using a superlattice construct. Nesting relationships identified phonon vectors for the conservation of energy and for phase coherency between coupled electrons at opposite sides of the Fermi surface. A detailed analysis of this Fermi surface nesting provided accurate estimates of the superconducting gaps for CaC6 with the change in applied pressure. The recognition of superlattices within a rhombohedral or hexagonal representation provides consistent mechanistic insight on superconductivity and electron−phonon coupling in CaC6.

In modelling electrochemical interfaces it is important to treat electrode and electrolyte at the same level of theory. Density functional theory, which is usually the method of choice, suffers from a distinct disadvantage: The inner potential is calculated as the average of the total electrostatic potential. This includes the highly localized potential generated from the nuclei. The resulting inner potential is far too high, of the order of 3.5 V, and not relevant for electrochemistry. In the density functional based tight binding (DFTB) method the electrostatic potential is much smoother, as it stems from atomic charge fluctuations with respect to neutral reference atoms. The resulting values for the electrochemical inner potential are much lower and compare well with those obtained by other, elaborate methods. Thus DFTB recommends itself as a method for treating the electrochemical interface including the inner potential. [ABSTRACT FROM AUTHOR]

Scanning tunneling microscopy measurements of height profiles, along the chains of Si atoms on the terrace edges of a perfectly ordered Si(553)-Au surface, reveal an STM bias-dependent mixed periodicity with periods of one, two and one and a half lattice constants. The simple linear chain model usually observed with STM cannot explain the unexpected fractional periodicity in the height profile. It was found that the edge Si chain stands for, in fact, a zigzag structure, which is composed of two neighboring rows of Si atoms and was detected in the STM experiments. Tight-binding calculations of the local density of states and charge occupancy along the chain explain the voltage-dependent modulations of the STM profiles and show that oscillation periods are determined mainly by the surface and STM tip Fermi energies. [ABSTRACT FROM AUTHOR]

Silver nanoclusters with diameter of 3,0 to 7,0 nm were studied by the molecular dynamics method using the tight binding potential TB-SMA (second moment approximation of tight-binding potential). A search was made for the stability limits of structural modifications of these nanoclusters for determination of the size limit of the thermally induced structural transition from the initial amorphous morphology to the fcc phase. The new data were compared with the results of previous studies for Ag nanoparticles up to 2,0 nm in size with initial fcc and amorphous structures. It is shown that the studied nanoclusters can be conditionally divided into three categories. The first one (N < 100 atoms) is characterized by partial preservation of the original morphology. For the second one (d < 4,0 nm), there is competition between the icosahedral and decahedral structures. And for the thirds (d > 4,0 nm), the mixed fcc/hcp phase predominates. In this case, the size limit of the transition from the initial amorphous morphology to the structure characteristic for the bulk matter is a diameter of about 7,0 nm.

Abstract In modelling electrochemical interfaces it is important to treat electrode and electrolyte at the same level of theory. Density functional theory, which is usually the method of choice, suffers from a distinct disadvantage: The inner potential is calculated as the average of the total electrostatic potential. This includes the highly localized potential generated from the nuclei. The resulting inner potential is far too high, of the order of 3.5 V, and not relevant for electrochemistry. In the density functional based tight binding (DFTB) method the electrostatic potential is much smoother, as it stems from atomic charge fluctuations with respect to neutral reference atoms. The resulting values for the electrochemical inner potential are much lower and compare well with those obtained by other, elaborate methods. Thus DFTB recommends itself as a method for treating the electrochemical interface including the inner potential.

The dynamics of a bilayer of graphene containing one mono-vacancy in the top layer has been investigated in the framework of DFTB in the absence and in the presence of water. Due to the speed of the code, we can describe details of the behavior, which are not directly accessible experimentally and cannot be treated by DFT or classical molecular dynamics. The presence of water enhances the displacement of carbon atoms in the perpendicular direction to the surface. Our results explain very well a variety of experimental findings. In particular, the stabilization of the Jahn–Teller distortion by hydrogenation of one of the carbon atoms at the edge of a mono-vacancy has been elucidated. This work is the first analysis of the behavior of a graphene vacancy at room temperature in contact with water based on a quantum mechanical molecular dynamics method, where both graphene and solvent are treated at the same level. [ABSTRACT FROM AUTHOR]

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step toward impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS2) grown by chemical vapor deposition using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (CrW) and molybdenum (MoW) at a tungsten site, oxygen at sulfur sites in both top and bottom layers (OS top/bottom), and two negatively charged defects (CD type I and CD type II). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. CrW forms three deep unoccupied defect states, two of which arise from spin-orbit splitting. The formation of such localized trap states for CrW differs from the MoW case and can be explained by their different d shell energetics and local strain, which we directly measured. Utilizing a tight-binding model the electronic spectra of the isolectronic substitutions OS and CrW are mimicked in the limit of a zero hopping term and infinite on-site energy at a S and W site, respectively. The abundant CDs are negatively charged, which leads to a significant band bending around the defect and a local increase of the contact potential difference. In addition, CD-rich domains larger than 100 nm are observed, causing a work function increase of 1.1 V. While most defects are electronically isolated, we also observed hybrid states formed between CrW dimers. The important role of charge localization, spin-orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS2 as reported here will guide future efforts of targeted defect engineering and doping of TMDs.

Abstract A practical method is presented to detect, diagnose, and engineer higher order Van Hove singularities in multiband systems, with no restrictions on the number of bands or hopping terms. The method allows us to directly compute the Taylor expansion of the dispersion of any band at arbitrary points in momentum space, using a generalized extension of the Feynman–Hellmann theorem, which is stated and proved. Being fairly, in general scope, it also allows to incorporate and analyze the effect of tuning parameters on the low energy dispersions, which can greatly aid the engineering of higher order Van Hove singularities. A certain class of degenerate bands can be handled within this framework. The use of this method is demonstrated, by applying it to the Haldane model.

We employ the Tight Binding Fishbone-Wire Model to study the electronic structure and coherent transfer of a hole (the absence of an electron created by oxidation) in all possible ideal B-DNA dimers as well as in homopolymers (one base pair repeated along the whole sequence with purine on purine). The sites considered are the base pairs and the deoxyriboses, with no backbone disorder. For the time-independent problem, we calculate the eigenspectra and the density of states. For the time-dependent problem after oxidation (i.e., the creation of a hole either at a base pair or at a deoxyribose), we calculate the mean-over-time probabilities to find the hole at each site and establish the frequency content of coherent carrier transfer by computing the Weighted Mean Frequency at each site and the Total Weighted Mean Frequency of a dimer or polymer. We also evaluate the main oscillation frequencies of the dipole moment along the macromolecule axis and the relevant amplitudes. Finally, we focus on the mean transfer rates from an initial site to all others. We study the dependence of these quantities on the number of monomers that are used to construct the polymer. Since the value of the interaction integral between base pairs and deoxyriboses is not well-established, we treat it as a variable and examine its influence on the calculated quantities. [ABSTRACT FROM AUTHOR]

In this study, the performance of armchair phosphorene nanoribbons (APNRs) tunnel field-effect transistors (TFETs) is compared to that of conventional Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) based on the self-consistent solution of the Poisson and Schrödinger equation within the non-equilibrium Green's Function formalism and a tight-binding Hamiltonian. As channel length decreases, undesirable consequences such as increased OFF-current and sub-threshold swing can affect MOSFETs' performance. The study thoroughly investigates various aspects of TFET performance, including the impact of channel length, gate length, and doping on parameters like ON-current, OFF-current, the ON-/OFF-current ratio, and sub-threshold swing. An important finding of this research relates to the influence of source and drain doping. We demonstrate that fine-tuning impurity levels directly affects phosphorene nanoribbon TFET (PTFET) performance. The article also investigates the impact of gate length on PTFET performance. New transistor configurations with different gate lengths are proposed in this research. The study shows that optimizing gate length can significantly reduce OFF-current. Furthermore, the combined impact of gate length and doping concentration on PTFET performance is investigated. Through the strategic extension of the gate length towards the drain side and precise adjustments in doping levels, notable improvements in subthreshold swings, ON-current, and the ON-/OFF-current ratio can be realized. • Performance of APNRs TFETs is compared to that of conventional MOSFETs. • Real-space Non-Equilibrium Green Function (NEGF) formalism is used. • The simultaneous investigation of the gate length and doping effect is examined. • The proposed transistor structure (DGDPTFET) shows better performance compared to the conventional MOSFETs and TFETs. [ABSTRACT FROM AUTHOR]

In order to accurately determine the electrostatics of Ultra-Thin-Body (UTB) devices, the semi-empirical tight-binding (TB) approach is widely used for calculating the channel thickness dependent band structure of any material at those temperatures where TB parameters are available (generally defined at 0 K and 300 K). In this work, we analyze the variation of band structure for Si, Ge, and GaAs over different channel thicknesses at 0 K and 300 K, and show that the band curvature at the band minima remains unchanged with temperature, while the band gap changes significantly and affects the channel electrostatics. Based on this finding, we propose an approach to simulate the electrostatics of UTB devices, at any temperature between 0 K and 300 K, using the band structure obtained at 0 K, along with a suitable channel thickness and temperature-dependent band gap correction. From the results obtained for the channel charge density, we show good agreement with band structure based simulation results, at 300 K, over a wide range of channel thicknesses, for Si, Ge, and GaAs, while also showing good agreement with TCAD simulation results, at a typical intermediate temperature of 150 K, thus highlighting the accuracy, simplicity and wide applicability of the proposed approach. [ABSTRACT FROM AUTHOR]

Consider the tight binding model of graphene, sharply terminated along an edge 1 parallel to a direction of translational symmetry of the underlying period lattice. We classify such edges 1 into those of “zigzag type” and those of “armchair type,” generalizing the classical zigzag and armchair edges. We prove that zero-energy/flat-band edge states arise for edges of zigzag type, but never for those of armchair type. We exhibit explicit formulae for flat-band edge states when they exist. We produce strong evidence for the existence of dispersive (nonflat) edge state curves of nonzero energy for most 1. [ABSTRACT FROM AUTHOR]

Hydrogen adsorbed on a graphene electrode in contact with an aqueous solution is in a metastable state. Here, its desorption induced by a small positive charge pulse was studied by molecular dynamics with a tight binding method based on density functional theory. The efficiency of the code allowed to follow the trajectory of the desorbed particle in detail. Desorption was triggered by an oxygen atom of water, which was attracted to the surface and approached the adsorbed hydrogen. Charge transfer occured, and the hydrogen was attached to the water molecule to form a hydronium ion, which drifted into the solution, changed its solvation and underwent Grotthus steps. The reverse process, adsorption of a proton from the solution, required a pulse with a high negative charge. The reaction was governed by the field right in front of the electrode, and not by the electrode potential. The time that the proton transfer takes is much shorter than the relaxation time of the double layer, so that simulations performed with a constant electrode potential may not be realistic. [ABSTRACT FROM AUTHOR]

The dynamics of a bilayer of graphene containing one mono-vacancy in the top layer has been investigated in the framework of DFTB in the absence and in the presence of water. Due to the speed of the code, we can describe details of the behavior, which are not directly accessible experimentally and cannot be treated by DFT or classical molecular dynamics. The presence of water enhances the displacement of carbon atoms in the perpendicular direction to the surface. Our results explain very well a variety of experimental findings. In particular, the stabilization of the Jahn–Teller distortion by hydrogenation of one of the carbon atoms at the edge of a mono-vacancy has been elucidated. This work is the first analysis of the behavior of a graphene vacancy at room temperature in contact with water based on a quantum mechanical molecular dynamics method, where both graphene and solvent are treated at the same level.

• Electronic heat conductivity at high electronic temperatures is calculated. • XTANT-3: combined tight-binding and linear response theory for conductivity. • Wide variety of materials studied: 39 metals, 14 semiconductors, semimetal. Heat transport in solids is governed by two fundamental contributions, atomic and electronic. The electronic energy transport in transient excited states is a defining factor in the problem of ultrafast material irradiation. Here, we calculate the electronic heat conductivity at elevated electron temperatures up to 40,000 K. We apply the novel combined method of tight binding formalism to calculate the electron-phonon contribution to the electronic heat conductivity, and the linear response theory (in the single-pole Ritchie-Howie loss function approximation) for its electron-electron counterpart, implemented in the hybrid code XTANT-3. It allows us to evaluate the electronic heat conductivity in a wide range of materials – fcc metals: Al, Ca, Ni, Cu, Sr, Y, Zr, Rh, Pd, Ag, Ir, Pt, Au, and Pb; hcp metals: Mg, Sc, Ti, Co, Zn, Tc, Ru, Cd, Hf, Re, and Os; bcc metals: V, Cr, Fe, Nb, Mo, Ba, Ta, and W; other metals: Sn, Ga, In, Mn, Te, and Se; semimetal graphite; semiconductors – group IV: Si, Ge, and SiC; group III-V: AlAs, AlP, GaP, GaAs, and GaSb; oxides: ZnO, TiO 2 , and Cu 2 O; and others: PbI 2 , ZnS, and B 4 C. [ABSTRACT FROM AUTHOR]

A study of 250 commercial drugs to act as corrosion inhibitors on steel has been developed by applying the quantitative structure-activity relationship (QSAR) paradigm. Hard-soft acid-base (HSAB) descriptors were used to establish a mathematical model to predict the corrosion inhibition efficiency (IE%) of several commercial drugs on steel surfaces. These descriptors were calculated through third-order density-functional tight binding (DFTB) methods. The mathematical modeling was carried out through autoregressive with exogenous inputs (ARX) framework and tested by fivefold cross-validation. Another set of drugs was used as an external validation, obtaining SD, RMSE, and MSE, obtaining 6.76%, 3.89%, 7.03%, and 49.47%, respectively. With a predicted value of IE% = 87.51%, lidocaine was selected to perform a final comparison with experimental results. By the first time, this drug obtained a maximum IE%, determined experimentally by electrochemical impedance spectroscopy measurements at 100 ppm concentration, of about 92.5%, which stands within limits of 1 SD from the predicted ARX model value. From the qualitative perspective, several potential trends have emerged from the estimated values. Among them, macrolides, alkaloids from Rauwolfia species, cephalosporin, and rifamycin antibiotics are expected to exhibit high IE% on steel surfaces. Additionally, IE% increases as the energy of HOMO decreases. The highest efficiency is obtained in case of the molecules with the highest ω and ΔN values. The most efficient drugs are found with pKa ranging from 1.70 to 9.46. The drugs recurrently exhibit aromatic rings, carbonyl, and hydroxyl groups with the highest IE% values. [ABSTRACT FROM AUTHOR]

The recently developed efficient protocol combining implicit and explicit, accurate quantum‐mechanical modeling of the condensed state (Katsyuba et al., J. Chem. Phys. 155, 024507 [2021]) is used to describe the IR spectra of liquid ethanol and its solutions in CCl4. The relative abundance of the anti and gauche conformers of ethanol is shown to increase from ~40:60 in the gas phase to ~55:45 in the liquid phase. In spite of a moderate impact of media effects on the conformational composition of the liquid, the solvent strongly influences vibrational frequencies, IR intensities, and normal modes of each conformer, producing qualitatively different spectra compared to the gas phase and CCl4 solution. Further, these solvent effects affecting IR frequencies and intensities depend not only on the conformation of the solvated molecule but also on the solvating species. Nevertheless, vibrational frequencies of anti and gauche conformers of liquid ethanol and its several isotopomers practically coincide with each other. Convenient liquid‐state conformational markers in the fingerprint region of IR spectra are revealed for the hydroxyl‐deuterated species: CH3CH2OD, CH3CHDOD, CH3CD2OD, and CD3CD2OD. [ABSTRACT FROM AUTHOR]

This Standard Operating Protocol (SOP) describes the key steps of experimental setup for an inhibition assay of enzymatic activity. The protocol begins with the design of an experiment, including the choice of a catalytic reaction, optimal conditions, fraction and concentration of the active enzyme, substrate and inhibitor concentrations and the positive and negative controls. The protocol ends with the data analysis followed by a typical example of an experiment. Despite an apparently standard procedure, the assay has a number of possible pitfalls such as low fraction of the active enzyme or errors in the analysis such as application of an improper model or incorrect determination of the inhibition constant while not recognizing the dependence on enzyme concentration. The protocol provides examples of necessary steps and controls to avoid these problems and obtain highly reliable results. [ABSTRACT FROM AUTHOR]

Herein, the spin‐dependent transport properties of a zigzag graphene nanoribbon (zGNR) with edges decorated with a fluoranthene group are studied. Atomically perfect zigzag edge, including phenyl‐edge functionalization, was synthesized by Ruffieux et al. in a bottom‐up chemical technique. By performing nearest‐neighbor tight‐binding model in combination with Landauer formalism and Green's function approach, as well as considering Coulomb electronic interaction computed both with density functional calculations and mean‐field approximation of the Hubbard model, the magnetic and spin‐resolved transmissions are studied. Both the model Hamiltonian and density functional theory (DFT) descriptions yield very similar results. Importantly, by generating a special supercell, three different antiferromagnetic alignments for phenyl‐edge zGNR have been calculated, which was not developed in previous studies. The results show that by adding pentagon rings on both sides of the zGNR, the conductance loses its step‐like behavior. Our structure of interest is a semiconductor with a transport gap of 0.56 eV. This kind of defective structure will enable the spin‐feature characteristic, such as spin filtering, and add the spin degree of freedom to graphene‐based logic devices. [ABSTRACT FROM AUTHOR]

Thermoelectric properties of AA-stacked bilayer SiC in presence of a magnetic field and a bias voltage were studied using the tight binding model. Green function method was applied to calculate thermal conductivity and Seebeck coefficient under the influence of bias voltage and external magnetic field within the linear response theory. We obtained that the effect of bias voltage is to increase energy gap of the bilayer SiC, unlike the magnetic field effect. Thermal conductivity and Seebeck coefficient of bilayer SiC depend on magnetic field and bias voltage. We obtained that the Seebeck coefficient sign is positive, which means the charge carriers are holes, in the whole range of temperature for different values of applied bias voltage and applied magnetic field. Also, the peak appeared in temperature dependence of thermal conductivity, Seebeck coefficient, and figure of merit ZT decreased with bias voltage. Finally, we investigated the effects of applying magnetic field on Seebeck coefficient and thermal conductivity of AA-stacked bilayer SiC in details. [ABSTRACT FROM AUTHOR]

Phase‐change material (PCMs) store data using the contrast (electrical or optical) between two phases: a conductive crystalline phase and a weakly conductive amorphous phase. Most PCMs have a distorted octahedral structure. The contrast comes mainly from the electronic structure. In PCMs, a spontaneous symmetry breaking mechanism, the Peierls distortion, transforms the metallic crystalline structure into a lower‐density semiconducting structure. In a simple tight‐binding model of the covalent bond, the parameters that control this distortion, characterized by a parameter η, are analyzed. The effective interatomic potential E(η) is developed in a Landau‐type series in η: E(η) = E0 + E2η2 + E4η4. The PCMs with the largest contrast are those for which the effective potential E(η) of the crystalline phase has a disappearing harmonic contribution (E2=0) and a vanishing electronic gap. This is called as an "incipient Peierls distortion." It coincides with the so‐called "incipient metal". The hardness of the repulsive potential and the number of electrons per atom play an important role. The vibrational properties and the anomalous Grüneisen parameter, specific to PCMs, are also studied. [ABSTRACT FROM AUTHOR]

Guanine quadruplex recognition has gained increasing attention, inspired by the growing awareness of the key roles played by these non‐canonical nucleic acid architectures in cellular regulatory processes. We report here the solution and solid‐state studies of a novel planar platinum(II) complex that is easily assembled from a simple ligand, and exhibits notable binding affinity for guanine quadruplex structures, while maintaining good selectivity for guanine quadruplex over duplex structures. A crystal structure of this ligand complexed with a telomeric quadruplex confirms double end‐capping, with dimerization at the 5′ interface. [ABSTRACT FROM AUTHOR]

Resumo Neste trabalho realizamos a montagem de uma matriz hamiltoniana para um gás de elétrons bidimensional não interagente. Estes sistemas podem ser formados na interface de heteroestruturas do GaAs-AlGaAs e, na presença de potenciais de confinamento, serem usados como transistores quânticos. Partindo da equação de Schrödinger na aproximação da massa efetiva, deduzimos os Hamiltonianos 1D e 2D na base de sítios Tight Binding. Estes Hamiltonianos foram obtidos através do procedimento de discretização da equação de Schrödinger. Para o caso unidimensional, o resultado encontrado foi a conhecida matriz tridiagonal e, para o caso bidimensional, uma matriz de blocos tridiagonais. A discretização realizada permitiu a dedução dos valores das energias de sítio e de hopping do sistema estudado. Estes resultados demonstraram a ligação direta entre a equação de Schrödinger e o método Tight Binding, e, tais resultados, são muito úteis na realização de métodos numéricos, os quais não são abordados na literatura básica de Física do Estado Sólido.

Leupeptin is a naturally occurring inhibitor of various proteases, in particular serine proteases. Following its discovery, the inhibitory properties of several other peptidyl argininals have been studied. The specificity of leupeptin is most likely due to the Leu–Leu–Argininal sequence, and its C‐terminal aldehyde group has been suggested to enhance the binding efficiency and to be essential for function. The terminal aldehyde group makes the structure less vulnerable to carboxypeptidases. Here, we investigated whether the inhibitory function of leupeptin toward serine proteases is retained after oxidation or reduction of the aldehyde group. The oxidized form, which corresponds to the natural precursor, was shown to be superior to the reduced form in terms of inhibitory properties. However, the original leupeptin possessed enhanced inhibitory properties as compared with the oxidized form. Based on these results, new synthetic leupeptin analogues, 6‐aminohexanoic acid (Ahx)–Phe–Leu–Arg–COOH and Ahx–Leu–Leu–Arg–COOH, were prepared by solid‐phase peptide synthesis using the Fmoc strategy. In these analogues, the N‐terminal capping acetyl group was replaced with a 6‐aminohexanoyl group to allow conjugation. The structures of the modified leupeptin and the synthetic peptides were confirmed by mass spectrometry. Determination of the inhibitory properties against trypsin (IEC 3.4.21.4, Chymotrypsin IEC 3.4.21.1) revealed that these further modified tripeptides were tight binding inhibitors to their target enzyme, similar to the naturally occurring leupeptin, with Ki values generally in the micromolar range. The Ahx–Phe–Leu–Arg–COOH analogue was selected for conjugation to inorganic oxide nanoparticles and agarose gel beads. All conjugates exhibited inhibitory activity in the same range as for the free peptides. [ABSTRACT FROM AUTHOR]

We address the role of hydrogen in modifying the behaviour of screw dislocations in α-iron under the influence of the tensile and compressive stresses that are expected to arise in the vicinity of a crack tip. The quantum mechanical tight binding approximation is employed to calculate the variation of the hydrogen/screw dislocation interaction with strain. The locations and binding energies of hydrogen trap-sites surrounding the unstrained core are shown to be of similar accuracy to previous ab-initio results. Bi-axial tension and compression is applied normal to the dislocation line; the binding energies are found to vary linearly with strain. Estimations of the effect of hydrogen in strained regions are made based on the proposition that hydrogen may both enhance the nucleation of kink-pairs by lowering the formation energy, and slow the movement of kinks by a solute drag effect. Our results suggest that these effects will be enhanced in tensile regions. The chance of hardening by the collision of crossed kinks, to form a jog, is estimated by comparing the nucleation and annihilation rate of kinks on a straight screw dislocation for a range of hydrogen concentrations and temperatures, suggesting a greatly increased incidence with tension that is accentuated at lower temperatures. • Validation of the tight binding approximation for dislocation H interactions in iron. • The role of crack tip strain field on dislocation mobility with trapped hydrogen. • Effects of local strain fields on plastic localisation during hydrogen embrittlement. [ABSTRACT FROM AUTHOR]

Metallic nanoparticles are highly reactive objects, often used for their catalytic properties which strongly depend on their shape and morphology. Here we show that controlling the wetting properties on a substrate enables one to control the nanoparticle's shape, prevent their coalescence and maximize their surface reactivity. The highly ordered mesoporous carbon structures (CMK) are ideal to achieve such a control. CMK can be tuned during their synthesis and display convex and concave surfaces capable of modifying the wetting properties and the morphology of the nanoparticles incorporated. On a concave substrate, the nanoparticle tends to spread on the surface of the substrate resulting in a platelet particle shape, while on flat or convex ones, the nanoparticle shows a limited wetting behavior corresponding to a spherical shape. In addition, the carbon enrichment of the metallic nanoparticles in contact with CMK plays a key role in controlling their equilibrium morphology. This atomic scale study allows us to better understand the interaction between metal nanoparticles and CMK in order to master their morphology and improve their reactivity. Image 1 [ABSTRACT FROM AUTHOR]

La solución en mecánica cuántica para sistemas, en la cual el operador de Hamilton no depende del tiempo (estados estacionarios), se centra en la solución de la ecuación de Schrödinger independiente del tiempo. Sin embargo, cuando el sistema se transforma en un sistema con muchas partículas, la solución de la ecuación no se puede abordar por medios analíticos. Por tal motivo, existen varios métodos de solución aproximada, los cuales se enmarcan en dos grandes categorías: los numéricos y los autoconsistentes. La diferencia esencial radica en el proceso que debe seguirse para encontrar las soluciones. Entre todos los métodos, uno que es de gran aplicación en problemas con muchas partículas es el denominado tight binding (enlace fuerte), ya que permite cierta libertad de programación y de seguimiento del algoritmo, y además cuenta con muy buenos márgenes de aproximación. De acuerdo con lo anterior, en el presente trabajo de investigación se establecerán las generalidades del método, así como su aplicación en dos problemas específicos (conjunto de cargas lineales y potencial coulombiano). Adicionalmente, se desarrollarán los dos sistemas mencionados por tres razones principales. La primera se asocia con la simplicidad del proceso y esclarecimiento del método; la segunda se relaciona con la utilidad de aplicación práctica del modelo físico en la ingeniería, los circuitos integrados, alternadores de resistencia y demás sistemas complejos, y la tercera, porque en algunas referencias se realiza una comparación de los métodos alternativos (generalmente autoconsistentes) a problemas muy similares.

Some observable consequences that follow from a fiber bundle description of a tight binding condensed matter system on a lattice are described. The geometrical picture can be extended to describe non-periodic lattice structures, where a single Brillouin zone is predicted. [ABSTRACT FROM AUTHOR]

In this work, we set up a Hamiltonian matrix for a non-interacting two-dimensional electron gas. These systems can be formed at the heterostructures interface of GaAs-AlGaAs and, in the presence of confinement potentials, be used as quantum transistors. Starting from the Schrödinger equation in the approximation of the effective mass, we deduce the Hamiltonians 1D and 2D on the basis of Tight-Binding sites. These Hamiltonians were obtained through the procedure of discretization of the Schrödinger equation. For the one-dimensional case, the result found was the well-known tridiagonal matrix and, for the two-dimensional case, a block tridiagonal matrix. The discretization performed allowed the deduction of the values of the site and hopping energies of the studied system. These results demonstrate the direct link between the Schrödinger equation and the Tight-Binding method, and such results are very useful in the realization of numerical methods, which are not addressed in the basic literature of Solid State Physics. [ABSTRACT FROM AUTHOR]

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Using the tight-binding method based on density functional theory (DFT) calculations, the thermoelectric properties of pure and B/N doped carbon nanotubes (CNTs) are investigated in terms of temperature (T) and in the presence of external electric and magnetic fields. All thermoelectric properties for pure and doped structures increase with temperature and the increasing rate is stronger in the presence of the external fields. The electrical conductivity and heat capacity of pure CNTs is larger than doped structures. For pure and doped structures, the heat capacity increases linearly with T independent to the dopant atoms. In the presence of electric (magnetic) field, the electrical conductivity and heat capacity of pure CNT are smaller (larger) than that other. The thermoelectric functions are strongly dependent on the strength of applied electric field rather than electric field. • The electronic structure of CNTs are dependent on the dopant atoms and external electric and magnetic fields. • Energy gap reduction shows dependence on impurity and external electric (F) and magnetic (Π) fields. • The heat capacity Cv(T) and electrical conductivity σ(T) increase with temperature. • The σ(F = 0,Π = 0,T) and σ(F = 0,Π≠0,T) for pure CNT is larger than doped structures increase with temperature and field. • For pure and doped structures, the Cv increases linearly (parabollicly) with T independent to the dopant atoms. [ABSTRACT FROM AUTHOR]

α-Graphyne nanotubes as quasi-one-dimensional structures could have interesting electronic and optical properties in comparison to graphene. In this work, we have calculated electronic band structure, density of states and optical absorption spectrum of some single-wall zigzag α-graphyne nanotubes by using a simple tight-binding method in a reduced lattice. The results show that two-thirds of nanotubes have semi-conductive behavior accompanied with an oscillatory decreasing band gap, while the rest presented metallic characteristics. The outcome reveals the optical gap fluctuates per different incremental nanotube diameters; in addition, the highest amount of optical absorption occurs when emission energies exceed 4 eV. [ABSTRACT FROM AUTHOR]

The solution in quantum mechanics for systems in which the Hamilton operator is not time-dependent (stationary states) focuses on the solution of the Schrodinger equation independent of time. However, when the system becomes one with many particles, the solution of the equation cannot be addressed by analytical means. Therefore, there are several methods of approximate solution; these fall into two broad categories: the numerical and the self-consistent. The essential difference lies in the process that must be followed to find those solutions. Among these methods, one that is widely applied to problems with many particles is the so-called Tight Binding (Hold Strong) as it allows some freedom of programming and tracking algorithm besides having very good approximation margins. In this article, an overview of the method and its application to three specific problems (set of linear loads and Coulomb potential) will be provided. These two systems will be developed for three main reasons. The first is associated with the simplicity of the process and clarification of the method, the second is related to the utility of practical application of the physical model in engineering such as integrated circuits, resistance alternators and other complex systems, and the third is because in some references a comparison is made with alternative (usually self-consistent) methods to very similar problems. [ABSTRACT FROM AUTHOR]