Charge Density Functional Tight Binding Research Articles

Investigating Si (100) surface patterns as well as electrical states through integrating molecular dynamics simulations with density functional tight binding at atomic level

The present work performs molecular dynamics simulations within model potential’s framework to investigate surface atoms’ rearrangements, loading states, and stress distribution in Si (100) surface on heating, and then single point energy calculations for the obtained atomic packing structures of Si substrates are used to obtain electrical information through self-consistent charge density functional tight binding approach. From analysis of Lode-Nadai parameters, atomic stress, visually packing images, Mulliken charges of atoms, Mulliken electron populations on atomic orbitals, differential charge densities, and chemical potentials for substrates, the knowledge of surface patterns, micro-loading states on the atoms and electrical states are obtained. The simulation results show the positions of vacancies have a significant impact on the rearrangements of the surface atoms. The appropriate selection of vacancy positions could be helpful in designing patterns of Si (100) surface. The vacancies in the subsurface or deeper atomic layers present migrations, where the migrations are more likely to occur at higher temperatures. There are stress concentration area and apparent charge transfers for the atoms forming dimers and neighboring atoms around the vacancies, and the differential charge densities in the surface atoms also present significant differences. These substrates with different surface patterns have different chemical potentials.

Variation in properties of double-walled zigzag and armchair silicon nanotubes depending on SW defects and applied electric fields by SCC-DFTB method

The geometric structure and electrical properties of the zigzag DWSiNT (8,0)@(12,0) and armchair DWSiNT (5,5)@(7,7) perfect tubes with the same diameter are simulated by self-consistent charge density functional tight binding method (SCC-DFTB). We considered the defective tubes by introducing three types of Stone-Wales (SW) defects, and further investigated the impact of both strong and weak electric fields on these defective tubes. Calculations demonstrate that the atomic arrangement regularity, degree of buckling, stability, energy gap, and charge distribution strongly depend on the type of perfect tubes. Following the introduction of the SW defects, the symmetry of the tubes disappeared, and there is a obvious atomic aggregation occurring at the defects. The armchair tubes demonstrate a higher stability compared to zigzag tubes and all exhibit metallic properties. SWⅡ and SWⅢ defects in zigzag tubes result in a transition from semiconductor to semi-metallic properties. The number of charge transfer increases, and the degree of atomic aggregation is more dispersed. The effect of applied electric field strength on perfect zigzag tubes is more obvious. The weak field has minimally impacted on the stability, and the strong field makes the stability improved to some extent. The presence of a strong field caused the energy gap values of (8,0)@(12,0) tube decrease sharply, and all tubes are transformed into semi-metal or metal. The weak electric field has a more pronounced regulation of band gap within the range of 0 V/nm - 0.3 V/nm on the inner tube. The consequences of this investigation can certainly be helpful in future experimental studies.

Beyond the Status Quo: Density Functional Tight Binding and Neural Network Potentials as a Versatile Simulation Strategy to Characterize Host-Guest Interactions in Metal- and Covalent Organic Frameworks.

In recent years, research focused on synthesis, characterization, and application of metal-organic frameworks (MOFs) has attracted increased interest, from both an experimental as well as a theoretical perspective. Self-consistent charge density functional tight binding (SCC DFTB) in conjunction with a suitable constrained molecular dynamics (MD) simulation protocol provides a versatile and flexible platform for the study of pristine MOFs as well as guest@MOF systems. Although being a semi-empirical quantum mechanical method, SCC DFTB inherently accounts for polarization and many-body contributions, which may become a limiting factor in purely force field-based simulation studies. A number of examples such as CO2, indigo, and drug molecules embedded in various MOF hosts are discussed to highlight the capabilities of the presented simulation approach. Furthermore, a promising extension of the outlined simulation strategy toward the treatment of covalent organic frameworks utilizing state-of-the-art neural network potentials providing a description at DFT accuracy and force field cost is outlined.

Structural Properties of Metal-Organic Frameworks at Elevated Thermal Conditions via a Combined Density Functional Tight Binding Molecular Dynamics (DFTB MD) Approach.

The performance of different density functional tight binding (DFTB) methods for the description of six increasingly complex metal-organic framework (MOF) compounds have been assessed. In particular the self-consistent charge density functional tight binding (SCC DFTB) approach utilizing the 3ob and matsci parameter sets have been considered for a set of four Zn-based and two Al-based MOF systems. Moreover, the extended tight binding for geometries, frequencies, and noncovalent interactions (GFN2-xTB) approach has been considered as well. In addition to the application of energy minimizations of the respective unit cells, molecular dynamics (MD) simulations at constant temperature and pressure conditions (298.15 K, 1.013 bar) have been carried out to assess the performance of the different DFTB methods at nonzero thermal conditions. In order to obtain the XRD patterns from the MD simulations, a flexible workflow to obtain time-averaged XRD patterns from (in this study 5000) individual snapshots taken at regular intervals over the simulation trajectory has been applied. In addition, the comparison of pair-distribution functions (PDFs) directly accessible from the simulation data shows very good agreement with experimental reference data obtained via measurements employing synchrotron radiation in case of MOF-5. The comparison of the lattice constants and the associated X-ray diffraction (XRD) patterns with the experimental reference data demonstrate, that the SCC DFTB approach provides a highly efficient and accurate description of the target systems.

DFTB investigations of structures and electron states for a decahedron Ti7 cluster on graphene on electrical level

We performed self-consistent charge density functional tight binding simulations to investigate the interactions between a decahedron Ti 7 cluster and graphene. Different adsorption behaviors of the cluster on perfect and defective graphene are characterized by adsorption energy, chemical potential, HOMO-LUMO energy gap, charge density differences, Mulliken populations on atoms, and molecular orbitals on HOMO and LUMO energy levels. Simulation results show that relatively contacting orientations of the cluster and graphene as well as adsorption positions greatly affect adsorption's packing configurations. The defective graphene improves high possibility for strongly binding the metal cluster. Apparent charge transfer occurs from the cluster to graphene for the adsorption on defective graphene. The molecular orbitals on the HOMO and LUMO energy levels present differences for different adsorption configurations. • A decahedron Ti 7 cluster and graphene • Different adsorption behaviors of the cluster on perfect and defective graphene • Adsorption energy, chemical potential, HOMO-LUMO energy gap • Charge density differences, Mulliken populations on atoms, and molecular orbitals on HOMO and LUMO energy levels

Molecular dynamics study of crystal lattice effect on mechanical properties of SiO2: A density functional tight binding investigation

In this study, molecular dynamics simulations (MD) based on quantum mechanical method in which the interactions were expressed by self-consistent charge density functional tight binding (SCC-DFTB) to investigate the mechanical properties of four different SiO2 structures. The Young's, shear and bulk modulus of systems are calculated and the results compared to experimental and other computational findings. We have observed that the hexagonal SiO2 exhibits more different mechanical behavior than other SiO2 structures because of its negative bulk modulus value. While the length of a-lattice parameters for four SiO2 crystal structures increase with the change of MD time, the length of the b and c-lattice vectors shortens. Moreover, it is analyzed the stress-strain behavior and yield points are determined from this curve of SiO2 structures. It is revealed that the plastic deformation occurs at higher stress for hexagonal SiO2 than other SiO2 structures because of its rigid solid property. These results can provide a reference for understanding the mechanical properties of various SiO2 crystal structures.

Effects of chiral index, vacancy defects and external electric field on the structures and electronic properties of DWSiNTs by SCC-DFTB calculations

As a new candidate material, silicon nanotubes (SiNTs) have been widely studied. However, various types of structural defects and external electric field are usually inescapability introduced in the experimental process of SiNTs synthesis. Structures, stability and electronic properties of the materials especially for double-walled silicon nanotubes (DWSiNTs) require to be considered. In this work, effects of vacancy defects and external electric field on properties of the geometrically structural optimization and electronic properties of DWSiNTs zigzag (3,0)@(7,0) and armchair (3,3)@(7,7) are studied via the self-consistent charge density functional tight binding (SCC-DFTB) method. The results of us show that the regular arrangement of the atoms, quantum molecular descriptors, distribution of charge, the degree of warping, stability, energy gap and Fermi energy levels intensely hinge on the chirality index of the tube and impacts of vacancy defects and electric field. Especially, the appearance of monovacancy and the change of vacancy density have the most significant effect on the band gap. For the pristine tubes, the charges transfer from the inside to the outside along the radial direction of the tube exhibiting an axisymmetric distribution. With the introduction of defects, the range of the charge transfer is broadened dramatically. The direction of charge transfer in the applied electric field is always opposite to the field direction. Our work may provide a new route to tune the electronic properties of DWSiNTs and opens a wider field of its application in nano-electronic devices.

C60 adsorption on Si (100) 2×1 reconstructed surface with single vacancy in sub-surface from DFTB algorithm

The adsorption geometries and electronic information of C60 on the Si (100) 2 × 1 reconstructed surface having single vacancy in the sub outermost surface layer was investigated by self-consistent charge density functional tight binding (SCC-DFTB) calculations. The contacting orientations of the C60, the atomic arrangement of the Si surfaces, and the adsorption positions being related to Si dimers on the surfaces, greatly affect the stably packing configurations of C60/Si (100) systems. When the 6:6 ring bonds of the C60 is in contact with the Si surface, the adsorption configurations have high stability for the perfect Si substrates and defective substrates having the vacancy. For these configurations, the defective substrates significantly improve adsorption ability. Charge transfers among those bonded C and Si atoms become significant due to stronger C–Si bonding compared with that in the intact cases. In addition to strong C–Si covalent bonds, the weak ionicity between the atoms also appeared in the C60 adsorption on the defect-system.

Stable nanotube construction conditions and electronic properties of possible Si double-walled nanotubes (nin,min)@(6,mout) (nin =3, 4) by SCC-DFTB calculations

The geometric structural optimization and electronic properties of double-walled silicon nanotubes (DWSiNTs) (nin,min)@(6,mout) (nin = 3, 4, min = 0 to nin, mout = 0 to 6) are studied in terms of the self-consistent charge density functional tight binding (SCC-DFTB) method. A suitable spacing range of initial model that can form a stable DWSiNT is explored. Most of the nanotubes present circular-like shapes of their cross-sectional configurations. When a DWSiNT has the same chiral angle, 0 or 30 in degree, in its inner and outer walls, a regular periodic structure develops. The bond length, inner and outer wall diameters, and wall spacing are also significantly affected by changes in the chiral index. The stability of the nanotube is affected by three factors: the configuration, the diameter, and the regularity of the atomic arrangements. The electrons in the inner wall are transferred to the outer wall, and the electron distribution of the atoms in the outer wall tube has the regional distribution of the interval between the electron-obtaining and the electron-losing regions. Among the seven tubes with fully commensurate inner and outer walls, DWSiNTs (3,2)@(6,4) exhibit semi-metallic characteristics; the others exhibit semiconductor-like properties with a narrow band gaps. The band gap of DWSiNTs is changed by tuning the chirality index, resulting in the transition between direct and indirect band gaps of nanotubes, which can be adjusted to match the performance needs of different devices, inducing a metal-semiconductor transition and direct-indirect band gap transition. These theoretical studies provide references for the preparation, and future application, of DWSiNTs.

Study on the structures and electronic properties of double-walled silicon nanotubes (4,min)@(8,mout) under external electric field by SCC-DFTB calculations

The geometric structural optimization and electronic properties of double-walled silicon nanotubes (DWSiNTs) (4,min)@(8,mout) (min = 0 to 4, mout = 0 to 8) are studied in terms of the self-consistent charge density functional tight binding (SCC-DFTB) method. Calculations demonstrate that the regularity of the atomic arrangement, the diameter of the inner and outer walls, the degree of buckling, stability, energy gap, Fermi energy level, quantum molecular descriptors, and charge distribution strongly depend on the chiral index of the tube. In particular, the possibility of tuning the band structure by changing the chiral index has been demonstrated to help for the performance needs of different devices, inducing a metal–semiconductor transition and direct–indirect band gap transition. In addition, the influence of adding an external electric field on electronic properties of the zigzag DWSiNT (4,0)@(8,0) has been investigated. The tube applied electric field transforms from semiconductor to semi-metallic property. The tube becomes metallic upon the critical electric field strength of 0.7 V/nm and 1.0 V/nm. As increasing the external field strengths, the stability of the tube is reduced and the EF level is increased. The direction of charge transfer is always in the reverse of the field direction, which is different from the case of absent field.

Enhancing classical gold nanoparticle simulations with electronic corrections and machine learning

Classical simulations of materials and nanoparticles have the advantage of speed and scalability but at the cost of precision and electronic properties, while electronic structure simulations have the advantage of accuracy and transferability but are typically limited to small and simple systems due to the increased computational complexity. Machine learning can be used to bridge this gap by providing correction terms that deliver electronic structure results based on classical simulations, to retain the best of both worlds. In this study we train an artificial neural network (ANN) as a general ansatz to predict a correction of the total energy of arbitrary gold nanoparticles based on general (material agnostic) features, and a limited set of structures simulated with an embedded atom potential and the self-consistent charge density functional tight binding method. We find that an accurate model with an overall precision of 14 eV or 8.6% can be found using a diverse range of particles and a large number of manually generated features which were then reduced using automatic data-driven approach to reduce evaluation bias. We found the ANN reduces to a linear relationship if a suitable subset of important features are identified prior to training, and that the prediction can be improved by classifying the nanoparticles into kinetically limited and thermodynamically limited subsets based prior to training the ANN corrections. The results demonstrate the potential for machine learning to enhance classical molecular dynamics simulations without adding significant computational complexity, and provides methodology that could be used to predict other electronic properties which cannot be calculated solely using classical simulations.

Effects of chiral indices on the atomic arrangements and electronic properties of Si double-walled nanotubes (6,min)@(9,mout) (min = 0 to 6, mout = 0 to 9) by SCC-DFTB calculations

The geometric structural optimization and electronic properties of double-walled silicon nanotubes (DWSiNTs) (6, min)@(9, mout) (min= 0 to 6, mout= 0 to 9) are studied in terms of the self-consistent charge density functional tight binding (SCC-DFTB) method. In particular, a cross-section configuration transformation of the DWSiNTs affected by the different inner and outer wall chiral indices is explored. It seems that six different shapes including circular-like, ellipse-like, trilateral-like, quadrilateral-like, pentagon-like, and hexagon-like shapes appear. The interactions between the atoms in the inner and outer walls possess most of the atoms in sp3 hybridization, followed in sp2 hybridization, with very few in high-order hypervalent states. The chiral indices of the inner and outer walls as well as the cross-section configuration of the tube have obvious effects on the stability of the DWSiNTs. The DWSiNTs (6,5)@(9,7), (6,6)@(9,8), and (6,6)@(9,9) show metal characteristic: others exhibit semiconductor properties with a narrow band gap. Furthermore, the possibility of tuning the band structure by changing the chiral indices is demonstrated to help for the performance needs of different devices, inducing a metal-semiconductor transition and direct-indirect band gap transition. These theoretical studies provide references for the preparation, and future application, of DWSiNTs.

Reduced graphene oxide/NH2-MIL-125(Ti) composite: Selective CO2 photoreduction to methanol under visible light and computational insights into charge separation

Abstract The development of visible-light active photocatalysts is highly desirable for CO2 reduction to hydrocarbons and alcohols using sunlight. Here, we report the metal-organic frameworks (MOF) of amino-benzene dicarboxylate with titanium oxocluster center (NH2-MIL-125(Ti)) and modified with reduced graphene oxide (RGO), RGO-NH2-MIL-125(Ti), ideal for the visible-light-driven photocatalytic reduction of CO2 to hydrocarbons and methanol. The catalyst provides high quantum efficiency and selectivity for methanol. The cluster model and self-consistent charge density functional tight binding methods were used to investigate the photogenerated charge separation for NH2-MIL-125(Ti). The quantum modelling suggests that holes were accumulated in the central ring Ti8O8(OH)4, where strongly adsorbed electron donor, triethanolamine, undergoes photooxidation while electrons were located in the organic ligand of MOF including the NH2 group. The binding affinity of NH2 reaction sites to CO2 possibly work to improve the photocatalytic reduction of CO2 to methanol. The RGO also play an important role for charge separation and better photocatalytic reduction with RGO-NH2-MIL-125(Ti).

DFTB Investigations on Transition Metals Doped TiO2 Quantum Dots

The structural, optical and electronic properties of different-sized pure as well as transition metals doped TiO2 quantum dots were investigated using density functional tight binding (DFTB) methods. The self-consistent charge density functional tight binding theory (SCC-DFTB) was used to model the TiO2 quantum dots (QDs) of increasing size up to 3.03 nm. The size dependence of density of states, position of molecular orbitals, and gap between highest orbital molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of the QDs were studied. It appeared that, when the size of QDs approaches 3.03 nm, the energy gap narrowed down due to quantum confinement effects. The QD Ti27O54 was doped with 3d transition metals to further explore the possibility of tailoring the properties. The dopants introduced 3d impurity gap states which opens the opportunity to modify the band gap or positions of principal bands. The position of a shifted Fermi level was monitored to explore the changes in conductivity and n- or p-type behaviors of the doped QDs. It was observed that TiO2 QDs doped with V and Cr showed the n-type behavior while those doped with Sc, Mn and Fe showed p-type behavior. The findings are helpful to enhance the photocurrent efficiency of dye-sensitized solar cells and use the materials in electronic and optoelectronic properties.

Critical Stable Length in Wrinkles of Two-Dimensional Materials.

The emergent two-dimensional (2D) materials are atomically thin and ultraflexible, promising for a variety of miniaturized, high-performance, and flexible devices in applications. On one hand, the ultrahigh flexibility causes problems: the prevalent wrinkles in 2D materials may undermine the ideal properties and create barriers in fabrication, processing, and quality control of materials. On the other hand, in some cases the wrinkles are used for the architecturing of surface texture and the modulation of physical/chemical properties. Therefore, a thorough understanding of the mechanism and stability of wrinkles is highly needed. Herein, we report a critical length for stabilizing the wrinkles in 2D materials, observed in the wrinkling and wrinkle elimination processes upon thermal annealing as well as by our in situ TEM manipulations on individual wrinkles, which directly capture the evolving wrinkles with variable lengths. The experiments, mechanical modeling, and self-consistent charge density functional tight binding (SCC-DFTB) simulations reveal that a minimum critical length is required for stabilizing the wrinkles in 2D materials. Wrinkles with lengths below a critical value are unstable and removable by thermal annealing, while wrinkles with lengths above a critical value are self-stabilized by van der Waals interactions. It additionally confirms the pronounced frictional effects in wrinkles with lengths above critical value during dynamical movement or sliding.

SCC-DFTB-PIMD Method To Evaluate a Multidimensional Quantum Free-Energy Surface for a Proton-Transfer Reaction.

The self-consistent charge density functional tight binding method was combined with the path-integral molecular dynamics method for the first time to evaluate the two-dimensional free-energy surface including nuclear quantum effects of a proton-transfer reaction in a 2,4-dichlorophenol-trimethylamine complex. A statistically converged two-dimensional quantum free-energy surface was evaluated by the multidimensional blue moon ensemble method. The accuracy was guaranteed by optimizing the repulsive potential between the sp3-hybridized nitrogen and hydrogen atoms in a SCC-DFTB3 parameter set for the system to reproduce high-level quantum chemical calculations. The present study illustrates the usefulness of this new approach to investigate nuclear quantum effects in various realistic proton-transfer reactions.

Correlation of surface oxidation with xanthate adsorption and pyrite flotation

Various studies have emphasized the importance of surface oxidation of sulfide minerals with regard to xanthate adsorption and their flotation response. However, the correlation between pyrite surface oxidation, xanthate adsorption and its flotation response has not been clearly established. The complexity lies in various intermediate sulfur oxidation products in different solution conditions. Thus, investigation of sulfur oxidation behavior is the key to understand pyrite oxidation. Ion chromatography (IC) and high performance liquid chromatography (HPLC) results showed an increase in the total amounts of occurring sulfur species with increasing slurry pH. Meanwhile, the ratio of S0/sulfur oxyanions was to become lower; X-ray Photoelectron Spectroscopy (XPS) identified the oxidized pyrite surface was rich in SO42− and FeOOH. Both the contact angle results and single mineral flotation data indicated that the more pyrite surface oxidized, the lower hydrophobicity and floatability were. Time-of-flight secondary ion mass spectroscopy (Tof-SIMS) was adopted to analyse xanthate adsorption on the surface of pyrite under conditions of either sufficient oxidation or on freshly polished surface which is assumed little oxidation. A significantly lower intensities of C4H9O and C4H9OCSS was found on the intensely oxidized pyrite surface, indicating xanthate adsorption had strong selectivity on FeS2 rather than FeOOH. The experimental results were further confirmed by theoretical calculation. The self-consistent charge density functional tight binding (SCC-DFTB) calculations indicated that the distances between the xanthate S atoms and surface Fe atoms of FeOOH were larger than the atomic radius of SFe. Essentially, the different electron configurations of their Fe atoms led to the selective adsorption of xanthate on FeOOH and FeS2 surfaces in terms of the coordination chemistry theory. This work provides valuable implications for understanding the correlation of surface oxidation, xanthate adsorption and its floatation behavior to mediate pyrite flotation.

Molecular dynamics simulation of primary detonation process of TATB crystal under shock loading

Molecular dynamics simulations were performed to gain fundamental insights into the mechanisms for the primary detonation process of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) under shock wave loading using self-consistent charge density-functional tight binding(SCC-DFTB) calculations combined with the multiscale shock technique (MSST). The primary process starts with shock loading and ends with the formation of dynamically stable heterocyclic clusters, which could inhibit the reactivity of TATB. The results show that the initial step of shocked TATB decomposition is the N–O bond cleavage; then carbon rings aggregate and connect by N atoms to form clusters; after the carbon rings open, heterocyclic clusters with nitrogen are formed, and persist throughout the simulation. This is a new mechanism for the primary processes of shocked TATB and this initiation mechanism is independent of the initial shock speeds.

Stability and Effects of Subsurface Oxygen in Oxide-Derived Cu Catalyst for CO2 Reduction

Oxide-derived copper (OD-Cu) catalysts are promising candidates for the electrochemical CO2 reduction reaction (CO2RR) due to the enhanced selectivity toward ethylene over methane evolution, which has been linked to the presence of subsurface oxygen (Osb). In this work, Osb is investigated with theoretical methods. Although Osb is unstable in slab models, it becomes stabilized within a “manually” reduced OD-Cu nanocube model which was calculated by self-consistent charge density functional tight binding (SCC-DFTB). The results obtained with SCC-DFTB for the full nanocube were confirmed with subcluster models extracted from the nanocube, calculated with both density functional theory (DFT) and SCC-DFTB. The higher stability of Osb in the nanocube is attributed to the disordered structure and greater flexibility. The adsorption strength of CO on Cu(100) is enhanced by Osb withdrawing electron density from the Cu atom, resulting in reduction of the σ-repulsion. Hence, the coverage of CO may be increased, fac...

Self-consistent charge and dipole density functional tight binding method and application to carbon-based systems

The density functional tight binding (DFTB) method is a fast, semi-empirical, total energy electronic structure method based upon and parameterized to density functional theory (DFT). The standard self-consistent charge (SCC) DFTB approximates the charge fluctuations in a system using a multipole expansion truncated to the monopole term. For systems with asymmetric charge distributions, such as might be induced by an applied external field, higher terms in the multipole expansion are likely to be important. We have extended the formalism to include dipoles (SCCD), have implemented the method computationally, and test it by calculating the response of various carbon nanotubes and fullerenes to an applied electric field. A comparison of polarizabilities with experimental data or more sophisticated DFT calculations indicates a substantial improvement over standard SCC-DFTB. We also discuss the issues surrounding parameterization of the new SCCD-DFTB scheme.