Density Functional Tight-binding Approach Research Articles

Growth patterns, shapes, and electronic properties of mixed SimGen clusters with [formula omitted

A global optimization method in combination with the density-functional tight-binding approach is used to determine all putative global minimum energy structures for mixed silicon-germanium clusters SimGen with n+m⩽30. All obtained structures are re-optimized with first-principles density-functional calculations and the influence of the composition on the shapes is studied. Several extraordinary stable structures are identified and it is shown that the stability correlates with the ‘prolateness’ of the cluster. However, it was not possible to relate the electronic properties to a single structural property like shape, composition, or local atomic environment.

Disentangling the Photophysics of DNA-Stabilized Silver Nanocluster Emitters

It has been already established that DNA–silver fluorophores present two intense absorption bands, one in the visible and the other in the UV spectral region, and that the excitation of both bands produces a common fluorescence signal. Nevertheless, the detailed mechanism responsible for this coupling is still elusive. In this context, our work is a significant leap forward in this goal by providing a dynamic picture of the excitation and the processes that take place upon light absorption in these systems using a time-dependent density functional tight-binding approach. In the present work, we explore the electronic coupling mechanism in a cluster composed of a double string of deoxypolycytosine with six bases and a rod of six Ag atoms.

Molecular dynamics simulations of defect formation in thin graphite films using the density functional tight-binding method

Point defect formation within graphene and ultra-thin graphite films is considered by means of molecular dynamics simulations in the framework of the density functional tight-binding approach. The barrier energy for vacancy formation is estimated and two types of defect formation are revealed.

Tin(II) Sulfide (SnS) Nanosheets by Liquid-Phase Exfoliation of Herzenbergite: IV-VI Main Group Two-Dimensional Atomic Crystals.

The liquid-phase exfoliation of tin(II) sulfide to produce SnS nanosheets in N-methyl-2-pyrrolidone is reported. The material is characterized by Raman spectroscopy, atomic force microscopy, lattice-resolution scanning transmission electron microscope imaging, and energy dispersive X-ray spectrum imaging. Quantum chemical calculations on the optoelectronic characteristics of bulk and 10-layer down to monolayer SnS have been performed using a quantum chemical density functional tight-binding approach. The optical properties of the SnS and centrifugally fractionated SnS nanosheet dispersions were compared to that predicted by theory. Through centrifugation, bilayer SnS nanosheets can be produced size-selectively. The scalable solution processing of semiconductor SnS nanosheets is the key to their commercial exploitation and is potentially an important step toward the realization of a future electronics industry based on two-dimensional materials.

Density Functional Tight Binding Study on Solid Electrolyte Interface of Lithium Sulfur Battery

One of the most promising cathode materials for lithium batteries is sulfur, abundant in nature and an undesirable impurity in petroleum. It is thus very cheap. Lithium sulfur batteries, i.e. lithium batteries with a sulfur cathode, have potentially high electric capacity but so far only very poor cyclability. For practical use of lithium sulfur batteries, the poor cyclability should be overcome. However, chemical reactions at lithium and sulfur interface are barely known thus hampering the progress in this field. In particular, theoretical and simulation studies on lithium-sulfur solid electrolyte interface (SEI) at the atomic scale are highly demanding in order to accurately understand interactions between lithium and sulfur. Density functional tight binding (DFTB) approach is based on density functional theory but much more efficient thus enabling simulations of large-scale molecular systems and their long-time dynamics. One of the most severe drawbacks of DFTB approaches is lack of available parameters for lithium batteries. Here we have developed DFTB parameters for SEI of lithium sulfur batteries including electrolytes and performed long-time molecular dynamics simulations to understand kinetics of lithium insertion into sulfur electrodes. Chemical interactions of lithium electrolytes and sulfur electrodes are demonstrated and further compared with more accurate DFT calculations at a smaller system.

Electron quantization in arbitrarily shaped gold islands on MgO thin films

Low-temperature scanning tunneling microscopy has been employed to analyze the formation of quantum well states (QWS) in two-dimensional gold islands, containing between 50 and 200 atoms, on MgO thin films. The energy position and symmetry of the eigenstates are revealed from conductance spectroscopy and imaging. The majority of the QWS originates from overlapping Au 6p orbitals in the individual atoms and is unoccupied. Their characteristic is already reproduced with simple particle-in-a-box models that account for the symmetry of the islands (rectangular, triangular, or linear). However, better agreement is achieved when considering the true atomic structure of the aggregates via a density functional tight-binding approach. Based on a statistically relevant number of single-island data, we have established a correlation between the island geometry and the gap between the highest-occupied and the lowest-unoccupied molecular orbital in the finite-sized islands. The linear eccentricity is identified as a suitable descriptor for this relationship, as it combines information on both island size and island shape. Finally, the depth of the confinement potential is determined from the spatial extension of QWS beyond the physical boundaries of the Au islands. Our paper demonstrates how electron quantization effects can be analyzed in detail in metal nanostructures. The results may help elucidating the interplay between electronic and chemical properties of oxide-supported clusters as used in heterogeneous catalysis.

A comparative study of density functional and density functional tight binding calculations of defects in graphene

AbstractThe density functional tight binding approach (DFTB) is well adapted for the study of point and line defects in graphene based systems. After briefly reviewing the use of DFTB in this area, we present a comparative study of defect structures, energies, and dynamics between DFTB results obtained using the dftb+ code, and density functional results using the localized Gaussian orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a range of point defect structures such as vacancies and Stone–Wales defects in graphene, as well as various unfunctionalized and hydroxylated graphene sheet edges. Migration barriers for the vacancy and Stone–Wales defect formation barriers are accurately reproduced using a nudged elastic band approach. Finally we explore the potential for dynamic defect simulations using DFTB, taking as an example electron irradiation damage in graphene.magnified image DFTB‐MD derived sputtering energy threshold map for a carbon atom in a graphene plane.

Two-dimensional graphene superlattice made with partial hydrogenation

Electronic properties of two-dimensional 2D graphene superlattice made with partial hydrogenation were thoroughly studied via density functional tight binding approach which incorporates the tight-binding method into the density functional formalism. The 2D pattern of hydrogen atoms on graphene was found to have great effects on electronic structures of graphene superlattice. In particular, the edges of the 2D pattern, armchair or zigzag, are essential for the energy band gap opening, and the energy band gap sensitively depends on the shape, size, and the 2D periodicity of the pattern. Based on these findings, we suggested that the 2D graphene superlattice could be used in fabricating graphene quantum dots or heterojunctions without the need for cutting or etching.

Large-scale first principles and tight-binding density functional theory calculations onhydrogen-passivated silicon nanorods

We present a computational study by density functional theory (DFT) of entire siliconnanorods with up to 1648 atoms without any periodicity or symmetry imposed. Thenanorods have been selected to have varying aspect ratios and levels of surface passivationwith hydrogen. The structures of the nanorods have been optimized using a densityfunctional tight-binding approach, while energies and electronic properties have beencomputed using linear-scaling DFT with plane-wave accuracy with the ONETEP (Skylariset al 2005 J. Chem. Phys. 122 084119) program. The aspect ratio and surface passivation (1 × 1 and2 × 1 reconstructions) along with the size of the nanorods which leads to quantum confinementalong all three dimensions, significantly affect their electronic properties. The structures ofthe nanorods also show interesting behaviour as, depending on their characteristics, theycan in certain areas retain the structure of bulk silicon while in other parts significantlydeviate from it.

Torsional potentials and full-dimensional simulation of electronic absorption and fluorescence spectra of para-phenylene oligomers using the semiempirical self-consistent charge density-functional tight binding approach

A systematic study on the structural properties of para-phenylene oligomers based on the self-consistent charge density-functional tight binding approach (SCC-DFTB) and its time-dependent (TD) version is presented. Our goal is to investigate the applicability of DFTB for the present class of compounds and to use its computational efficiency for on-the-fly dynamics calculations and to perform in this way simulations of absorption and fluorescence spectra. For this purpose geometry optimizations have been performed for the ground state and for the electronically lowest excited state of oligomers containing two to seven aromatic rings. The torsional potential curves have been computed for para-biphenyl and para-terphenyl in the ground and lowest excited state. Agreement with previously computed DFT results is quite encouraging and DFTB seems to be well suited for the treatment of the class of conjugated pi systems investigated in this work. The intrachain vibrational broadening of absorption and emission spectra computed from dynamics simulations are presented and compared with experimental spectra.

Nonempirical calculations of the electronic properties of new boron nitride graphyne-like nanotubes

The nonempirical density-functional tight-binding approach to band structure calculations was used to study the structural and electronic properties of new boron nitride graphyne-like nanotubes (α-BN-NT) constructed on the basis of the atomic motif of α-graphyne. All α-BN tubes were semiconductors with gap widths of 3.8–4.2 eV. The electron energy characteristics of zigzag and armchair α-BN-NTs are discussed in comparison with the known boron nitride graphite-like nanotubes. In addition, the special features of interatomic interactions in α-BN are analyzed using the nonempirical discrete variational method of density functional theory.

Reliable treatment of electrostatics in combined QM/MM simulation of macromolecules

A robust approach for dealing with electrostatic interactions for spherical boundary conditions has been implemented in the QM/MM framework. The development was based on the generalized solvent boundary potential (GSBP) method proposed by Im et al. [J. Chem. Phys. 114, 2924 (2001)], and the specific implementation was applied to the self-consistent-charge density-functional tight-binding approach as the quantum mechanics (QM) level, although extension to other QM methods is straightforward. Compared to the popular stochastic boundary-condition scheme, the new protocol offers a balanced treatment between quantum mechanics/molecular mechanics (QM/MM) and MM/MM interactions; it also includes the effect of the bulk solvent and macromolecule atoms outside of the microscopic region at the Poisson-Boltzmann level. The new method was illustrated with application to the enzyme human carbonic anhydrase II and compared to stochastic boundary-condition simulations using different electrostatic treatments. The GSBP-based QM/MM simulations were most consistent with available experimental data, while conventional stochastic boundary simulations yielded various artifacts depending on different electrostatic models. The results highlight the importance of carefully treating electrostatics in QM/MM simulations of biomolecules and suggest that the commonly used truncation schemes should be avoided in QM/MM simulations, especially in simulations that involve extensive conformational samplings. The development of the GSBP-based QM/MM protocol has opened up the exciting possibility of studying chemical events in very complex biomolecular systems in a multiscale framework.

The Franck–Condon effect in 1 1B 1u state of 3,4,9,10-perylenetetracarboxylic-dianhydride

The high-resolution laser-induced fluorescence (LIF) electronic spectrum associated with the 1 1A g→1 1B 1u transition in the perylenetetracarboxylic-dianhydride (PTCDA) molecule is discussed in terms of time-dependent density functional theory at TD-B3LYP/4-31G and TD-B3LYP/cc-pVDZ levels. It is shown that the simple vibronic model based on the displaced harmonic oscillators leads to quite reasonable agreement with the LIF experiment, even if some discrepancies occur. The results of TD-B3LYP/cc-pVDZ computations are compared to those obtained earlier within density-functional tight-binding (DFTB) approximation. We argue that the DFTB approach is inadequate to account correctly for the observed intensity distributions in the absorption spectra associated with the 1 1A g→1 1B 1u transition in the PTCDA molecule.

Organic and Inorganic Nanostructures: An Atomistic Point of View

Empirical and density-functional tight-binding approaches as well as pseudopotential approaches are applied to the study of organic and inorganic semiconductor nanostructures. We will show how these microscopic methods relax all limitations of simplified approaches based on envelope function approximations, maintaining, at the same time, the computational cost low. Typical calculations for nanostructured devices are shown for both inorganic and organic materials.

Tight-binding methods for transport and optical properties in realistic nanostructures

Empirical and density-functional tight-binding approaches are applied to the study of organic and inorganic semiconductor nanostructures. We will show how tight binding relaxes all limitations of simplified approaches based on envelope function approximations, maintaining, at the same time, the computational cost low. We will present typical calculations for semiconductor nanostructures ranging from single quantum well up to nanostructured device in the presence of an electron current flow.

Electronic structures of GaN edge dislocations

We investigate atomic and electronic structures of the threading edge dislocations of GaN using self-consistent-charge density-functional tight-binding approaches. Full-core, open-core, Ga-vacancy, and N-vacancy edge dislocations are fully relaxed in our total-energy scheme. The Ga-vacancy dislocation is the most stable in a wide range of Ga chemical potentials, whereas full-core and open-core dislocations are more stable than others in the Ga-rich region. Partial dehybridization takes place during the lattice relaxation near the dislocation in all cases. The dangling bonds at Ga atoms mostly contribute to the deep-gap states, whereas those at N atoms contribute to the valence-band tails. All the edge dislocations can act as deep trap centers, except the Ga-vacancy dislocation, which may act as an origin of yellow luminescence.

A density functional tight-binding approach for modelling Ge and GeH structures

We present here details of a density-functional-based non-orthogonal tight-binding approach for germanium. This is carried out within the framework of the LCAO ansatz, using a two-centre approximation for the evaluation of the Hamiltonian matrix elements. The Hamiltonian and overlap matrix elements for Ge - Ge, Ge - H are derived using a localized atomic basis set and existing parameters for H - H are utilized. These are tested against results from existing experimental and theoretical data for bulk germanium and germanium surface reconstructions, both with and without hydrogen. In addition we generate the first fully self-consistent ab initio LDA results for small Ge clusters and Ge - H molecules using the AIMPRO program and compare DF-TB results against these. Finally we describe our study of H point defects in bulk germanium. These are the first calculations of this type done on Ge using large fully optimized supercells and confirm self-consistent ab initio calculations done on small clusters. We consequently demonstrate that, despite the extreme simplicity of the approach, it is accurate and highly transferable across a broad range of structural systems ranging from clusters to the bulk phase.