Density Functional Tight Binding Study Research Articles

Towards graphene-based asymmetric diodes: a density functional tight-binding study.

Self-consistent charge density functional tight-binding (DFTB) calculations have been performed to investigate the electrical properties and transport behavior of asymmetric graphene devices (AGDs). Three different nanodevices constructed of different necks of 8 nm, 6 nm and 4 nm, named Graphene-N8, Graphene-N6 and Graphene-N4, respectively, have been proposed. All devices have been tested under two conditions of zero gate voltage and an applied gate voltage of +20 V using a dielectric medium of 3.9 epsilon interposed between the graphene and the metallic gate. As expected, the results of AGD diodes exhibited strong asymmetric I(V) characteristic curves in good agreement with the available experimental data. Our predictions implied that Graphene-N4 would achieve great asymmetry (A) of 1.40 at |VDS| = 0.2 V with maximum transmittance (T) of 6.72 in the energy range 1.30 eV. More importantly, while the A of Graphene-N4 was slightly changed by applying the gate voltage, Graphene-N6/Graphene-N8 showed a significant effect with their A increased from 1.20/1.03 under no gate voltage (NGV) to 1.30/1.16 under gate voltage (WGV) conditions. Our results open up unprecedented numerical prospects for designing tailored geometric diodes.

Topological theory of inversion-breaking charge-density-wave monolayer 1T-TiSe2

A charge density wave (CDW) of a nonzero ordering vector q couple electronic states at k and k + q statically, giving rise to a reduced Brillouin zone due to the band folding effect. Its structure, referred to an irreducible representation of the little group of q, would change the symmetry of the system and electronic structure accompanying possible change of band inversion, offering a chance of the topological phase transition. Monolayer 1T-TiSe2 is investigated for it shows an unconventional CDW phase having a triple-q structure. Moreover, the coupling between the triple-q component of the CDW will inevitably produces a small CDW. The CDW yields a band inversion in 1T-TiSe2 but different types of CDW can affect the electronic structure and system topology differently. The impact of CDW of different types was studied by utilizing a symmetrization–antisymmetrization technique to extract the and CDW contributions in the density functional theory-based tight-binding model and study their effects. The results are consistent with the parity consideration, improving understanding of topology for a CDW system with and without parity.

Caffeine adsorption on hybrid hBN/graphene hexagonal nanoflakes: A density functional tight binding study

The adsorption properties of caffeine molecule on nanoflakes are investigated using density functional tight binding model introduced in DFTB+ package. Sensing ability of hybrid graphene/hBN nanoflake with both nitrogen and boron interfaces and different sizes are compared with graphene and hBN nanoflakes for detecting caffeine molecule. Our findings reveal that proposed hybrid structures are better sensors for caffeine adsorption. In hybrid structures, the final position of the molecule is dependent on the dipole-dipole interaction between the molecule and nanoflake surface. Also, It is worth pointing that growing flake size doesn’t significantly influence on the caffeine adsorption.

Structure and ionic conduction study on Li3PO4 and LiPON (Lithium phosphorous oxynitride) with the Density-Functional Tight-Binding (DFTB) method

In this work we studied the structural and dynamical properties of crystalline and amorphous Li3PO4/LiPON (four different materials) using computational approaches based on the Density-Functional Tight-Binding (DFTB) method. To the best of our knowledge, this is the first DFTB study of Li3PO4/LiPON. We observed both apical (Na) and double-bridging (Nd), but not triple-bridging N (Nt) in crystalline and amorphous LiPON structures we generated, consistent with some literatures. For the dynamic properties, we calculated self-diffusivities of all species and ionic conductivities of all four different materials and results are in good agreement with other computational and experimental studies. We also performed bonding analysis to gain mechanistic understanding of factors affecting Li diffusion and conduction and we discussed our perspectives on the effect of N doping on the dynamics of all species (Li, P, O, N), not just Li, in LiPON.

Thermal bridging of graphene nanosheets via covalent molecular junctions: A non-equilibrium Green’s functions–density functional tight-binding study

Despite the uniquely high thermal conductivity of graphene is well known, the exploitation of graphene into thermally conductive nanomaterials and devices is limited by the inefficiency of thermal contacts between the individual nanosheets. A fascinating yet experimentally challenging route to enhance thermal conductance at contacts between graphene nanosheets is through molecular junctions, allowing covalently connecting nanosheets otherwise interacting only via weak Van der Waals forces. Beside the bare existence of covalent connections, the choice of molecular structures to be used as thermal junctions should be guided by their vibrational properties, in terms of phonon transfer through the molecular junction. In this paper, density functional tight-binding combined with Green functions formalism was applied for the calculation of thermal conductance and phonon spectra of several different aliphatic and aromatic molecular junctions between graphene nanosheets. Effect of molecular junction length, conformation, and aromaticity were studied in detail and correlated with phonon tunnelling spectra. The theoretical insight provided by this work can guide future experimental studies to select suitable molecular junctions in order to enhance the thermal transport by suppressing the interfacial thermal resistances, particularly attractive for various systems, including graphene nanopapers and graphene polymer nanocomposites, as well as related devices. In a broader view, the possibility to design molecular junctions to control phonon transport currently appears as an efficient way to produce phononic devices and controlling heat management in nanostructures.

Density-Functional Tight-Binding Study on the Effects of Interfacial Water in the Adhesion Force between Epoxy Resin and Alumina Surface.

Adhesion is one of the most interesting subjects in interface phenomena from the viewpoint of wide-range applications as well as basic science. Interfacial water has significant effects on coatings, adhesives, and fiber-reinforced polymer composites, often causing adhesion loss. The way of thinking based on quantum mechanics is essential for a better understanding of physical and chemical properties of adhesive interfaces. In this work, the molecular mechanism of the adhesion interaction between epoxy resin and hydroxylated alumina surface in the presence of interfacial water molecules is investigated by using density-functional tight-binding calculations. Periodic slab model calculations demonstrate that hydrogen bond is an important factor at the adhesion interface. Effects of interfacial water molecules located between epoxy resin and hydroxylated alumina surface are assessed by using a dry model without interfacial water and wet models with water layers of 3, 6, and 9 Å thicknesses. Interesting first- and second-layer structures are observed in the distribution of interfacial water molecules in the tight space between the adhesive and adherend. Energy plots with respect to the displacement of epoxy resin from the alumina surface are nicely approximated by the Morse potential. The adhesion force and stress are theoretically obtained by differentiating the potential curve with respect to the displacement of epoxy resin. Computational results show that the adhesion force and stress are significantly weakened with an increase in the thickness of interfacial water layer. Thus, interfacial water molecules have a clue as to the role of water in the loss of adhesion.

Mechanical properties of pristine and defective carbon–phosphide monolayers: a density functional tight-binding study

Using density functional tight-binding theory, we investigated the elastic properties and deformation and failure behaviors of pristine and defective carbon–phosphide (CP) monolayers subjected to uniform uniaxial tensile strain along arm-chair (AC) and zig-zag (ZZ) directions. Two variants of CP (α-CP and β-CP) were studied and two types of carbon and phosphorous vacancies (single and double) were considered. It was found that carbon monovacancies have the lowest formation energy, while phosphorous divacancies have the highest one in both CP allotropes. A strong mechanical anisotropy for CP was found with the Young’s modulus and the failure stress along ZZ direction being an order of magnitude larger than those along AC direction. In both allotropes, the Young’s modulus, failure stress and strain are considerably affected by vacancies, especially along AC direction. Fracture of pristine CP monolayer occurred via the rupture of phosphorous–phosphorous bonds when CP monolayer is stretched along AC direction, while via the rupture of carbon–phosphorous bonds when stretched along ZZ direction. Defective α-CP and β-CP monolayers both undergo a brittle-like failure initiated around the hosted vacancies at a lower critical strain. The failure strain and stress along the AC direction are affected only by phosphorous vacancies, while along the ZZ direction, they are almost equally affected by both phosphorous and carbon vacancies. These understandings may provide useful guidelines for potential applications of CP monolayers in nanoelectromechanical systems.

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

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

Polyaniline and CN-functionalized polyaniline as organic cathodes for lithium and sodium ion batteries: a combined molecular dynamics and density functional tight binding study in solid state.

We present the first atomistic-scale simulation of the discharge process of polymeric cathode materials for electrochemical batteries in solid state. The oxidation of polyaniline (PANI) and of cyano groups (CN) functionalized PANI induced by coordination to the electrolyte anions is computed and voltage curves are estimated. To deal with the large required numbers of atoms and structures, a combination of molecular dynamics and density functional tight binding (DFTB) is used. The DFTB is benchmarked to density functional theory (DFT) calculations using different functionals to confirm its accuracy. The voltages computed with the solid state model are in good agreement with available experimental data and ab initio models based on oligomers. The solid state model also predicts substantially increased voltage with PANI functionalized with cyano groups.

Defects in crystalline PVDF: a density functional theory-density functional tight binding study.

We present a comparative density functional theory (DFT) and density functional tight binding (DFTB) study of structures, energetics, vibrational properties as well as electronic structures of the four crystalline phases of polyvinylidene fluoride (PVDF) with different types of defects. For pure phases, the relative energies of PVDF strands (i.e. absent van der Walls bonding) agree well between DFT (using a GGA or a hybrid functional) and DFTB. For crystals, DFTB needs to be calibrated due to deficiencies in the treatment of vdW interactions. Defect formation energies were computed in large-scale DFTB simulations. For single chain vacancies, they are 0.41, 0.59, 0.08 and 0.40 eV per monomer removed in α, β, γ, and δ PVDF, respectively. The energy required to form double vacancies is 0.38, 0.52, 0.33 and 0.39 eV per monomer removed, respectively, i.e. the effect is nearly additive except in the γ phase. Interstitial defects were found to be unstable and convert into vacancies. The relatively high defect formation energies (vs. kT at room temperature) imply that phase purity is feasible in PVDF. Vibrational contributions affect the relative phase energies by up to 0.1 eV but do not significantly affect the relative phase stability.

Comparative density functional theory-density functional tight binding study of fullerene derivatives: effects due to fullerene size, addends, and crystallinity on band structure, charge transport and optical properties.

We present a systematic comparative density functional theory-density functional tight binding study of multiple derivatives of C60 and C70 with different addends, in molecular as well as solid state. In particular, effects due to fullerene size, type and number of addends, and of crystallinity on band structure, charge transport, and optical properties are investigated. These are important, in particular, for rational selection of fullerene derivatives as acceptor and electron transport layers in organic as well as planar inverted perovskite solar cells. We find that by the choice of type and number of addends, one can modulate the LUMO within 0.4 eV. Changes in the HOMO can reach 0.7 eV. Substituting C70 for C60 results in destabilization of the HOMO by about 0.1 eV for indene and quinodimethane addends and by a less significant amount for PCBM addends. The effect of C70-C60 substitution on the LUMO is of similar magnitude. A more significant change in HOMO-LUMO energy is seen for the aryl addends. On the other hand, all C70 based molecules have strong visible absorption. For most addends, the crystal packing leads to a stabilization of both the LUMO and HOMO by about ∼0.2 and ∼0.1 eV, respectively, vs. single molecules. When using bis-addends, it is also possible to enhance the visible absorption. Electron and hole transport rates are computed to vary vastly depending on the addends chosen; specifically, we compute that indene and quimodimethane addends can enhance charge transport rates while the aryl addend is predicted to result in substantially smaller mobilities of electrons and holes, vs. PC60BM. Furthermore, the -CH2 and bisaddend addition can significantly enhance the charge transfer rates for the PCBM addend.

Mechanical properties of phosphorene nanotubes: a density functional tight-binding study

Using the density functional tight-binding method, we studied the elastic properties, deformation and failure of armchair (AC) and zigzag (ZZ) phosphorene nanotubes (PNTs) under uniaxial tensile strain. We found that the deformation and failure of PNTs are very much anisotropic. For ZZ PNTs, three deformation phases are recognized: the primary linear elastic phase—which is associated with interactions between neighboring puckers, succeeded by the bond rotation phase—where the puckered configuration of phosphorene is smoothed via bond rotation, and lastly the bond elongation phase—where the P–P bonds are directly stretched up to the maximally allowed limit and failure is initiated by the rupture of the most stretched bonds. For AC PNTs, the applied strain stretches the bonds up to the maximally allowed limit, causing their ultimate failure. For both AC and ZZ PNTs, their failure strain and failure stress are sensitive— while the Young’s modulus, flexural rigidity, radial Poisson’s ratio and thickness Poisson’s ratio are relatively insensitive—to the tube diameter. More specifically, for AC PNTs, the failure strain decreases from 0.40 to 0.25 and the failure stress increases from 13 GPa to 21 GPa when the tube diameter increases from 13.3 Å to 32.8 Å; while for ZZ PNTs, the failure strain decreases from 0.66 to 0.55 and the failure stress increases from 4 GPa to 9 GPa when the tube diameter increases from 13.2 Å to 31.1 Å. The Young’s modulus, flexural rigidity, radial and thickness Poisson ratios are 114.2 GPa, 0.019 eV · nm2, 0.47 and 0.11 for AC PNTs, and 49.2 GPa, 0.071 eV · nm2, 0.07 and 0.21 for ZZ PNTs, respectively. The present findings provide valuable references for the design and application of PNTs as device elements.

Comparative density functional theory and density functional tight binding study of arginine and arginine-rich cell penetrating peptide TAT adsorption on anatase TiO2.

We present a comparative density functional theory (DFT) and density functional tight binding (DFTB) study of geometries and electronic structures of arginine (Arg), arginine adsorbed on the anatase (101) surface of titania in several adsorption configurations, and of an arginine-rich cell penetrating peptide TAT and its adsorption on the surface of TiO2. Two DFTB parameterizations are considered, tiorg-0-1/mio-1-1 and matsci-0-3. While there is good agreement in the structures and relative energies of Arg and peptide conformers between DFT and DFTB, both adsorption geometries and energies are noticeably different for Arg adsorbed on TiO2. The tiorg-0-1/mio-1-1 parameterization performs better than matsci-0-3. We relate this difference to the difference in electronic structures resulting from the two methods (DFT and DFTB) and specifically to the band alignment between the molecule and the oxide. We show that the band alignment of TAT and TiO2 modeled with DFTB is qualitatively correct but that with DFT using the PBE functional is not. This is specific to the modeling of large molecules where the HOMO is close to the conduction band of the oxide. We therefore report a case where the approximate DFT-based method - DFTB (with which the correct band structure can be effectively obtained) - performs better than the DFT itself with a functional approximation feasible for the modeling of large bio-inorganic interfaces, i.e. GGA (as opposed to hybrid functionals which are impractical at such a scale). Our results highlight the utility of the DFTB method for the modeling of bioinorganic interfaces not only from the CPU cost perspective but also from the accuracy point of view.

Comparative density functional theory and density functional tight binding study of 2-anthroic acid on TiO2

A comparative DFTB (density functional tight binding)–DFT (density functional theory) study of several adsorption modes of 2-anthroic acid on titania is presented. Two parameterizations of DFTB previously used for dye–TiO2 interfaces are tested. DFTB predicts adsorption energies which differ from those computed by DFT not only in magnitude (by up to 0.5eV) but also in the order among different configurations. The band alignment computed with DFTB is not consistent with DFT results and with experimental data. The strategy of geometry optimization with DFTB followed by single-point DFT calculations also does not necessarily result in plausible adsorption energies.

A Comparative Density Functional Theory and Density Functional Tight Binding Study of Phases of Nitrogen Including a High Energy Density Material N8

We present a comparative dispersion-corrected Density Functional Theory (DFT) and Density Functional Tight Binding (DFTB-D) study of several phases of nitrogen, including the well-known alpha, beta, and gamma phases as well as recently discovered highly energetic phases: covalently bound cubic gauche (cg) nitrogen and molecular (vdW-bound) N8 crystals. Among several tested parametrizations of N–N interactions for DFTB, we identify only one that is suitable for modeling of all these phases. This work therefore establishes the applicability of DFTB-D to studies of phases, including highly metastable phases, of nitrogen, which will be of great use for modelling of dynamics of reactions involving these phases, which may not be practical with DFT due to large required space and time scales. We also derive a dispersion-corrected DFT (DFT-D) setup (atom-centered basis parameters and Grimme dispersion parameters) tuned for accurate description simultaneously of several nitrogen allotropes including covalently and vdW-bound crystals and including high-energy phases.

Defects in alpha and gamma crystalline nylon6: A computational study

We present a comparative Density Functional Tight Binding study of structures, energetics, and vibrational properties of α and γ crystalline phases of nylon6 with different types of defects: single and double chain vacancies and interstitials. The defect formation energies are: for a single vacancy 0.66 and 0.64 kcal/mol per monomer, and for an interstitial strand 1.35 and 2.45 kcal/mol per monomer in the α and γ phases, respectively. The presence of defects does not materially influence the relative stability of the two phases, within the accuracy of the method. The inclusion of phononic contributions has a negligible effect. The calculations show that even if it were possible to synthesize the pure phases of nylon6, the defects will be easily induced at room temperature, because vacancy formation energies in both phases are of the order of kT at room temperature. The formation of interstitial defects, on the contrary, requires the energy equivalent to multiple kT values and is much less likely; it is also much less probable in the γ phase than in α. The vibration spectra do not show significant sensitivity to the presence of these defects.

A density functional tight binding study of acetic acid adsorption on crystalline and amorphous surfaces of titania.

We present a comparative density functional tight binding study of an organic molecule attachment to TiO2 via a carboxylic group, with the example of acetic acid. For the first time, binding to low-energy surfaces of crystalline anatase (101), rutile (110) and (B)-TiO2 (001), as well as to the surface of amorphous (a-) TiO2 is compared with the same computational setup. On all surfaces, bidentate configurations are identified as providing the strongest adsorption energy, Eads = −1.93, −2.49 and −1.09 eV for anatase, rutile and (B)-TiO2, respectively. For monodentate configurations, the strongest Eads = −1.06, −1.11 and −0.86 eV for anatase, rutile and (B)-TiO2, respectively. Multiple monodentate and bidentate configurations are identified on a-TiO2 with a distribution of adsorption energies and with the lowest energy configuration having stronger bonding than that of the crystalline counterparts, with Eads up to −4.92 eV for bidentate and −1.83 eV for monodentate adsorption. Amorphous TiO2 can therefore be used to achieve strong anchoring of organic molecules, such as dyes, that bind via a -COOH group. While the presence of the surface leads to a contraction of the band gap vs. the bulk, molecular adsorption caused no appreciable effect on the band structure around the gap in any of the systems.

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.

Adsorption of Glyoxal (CHOCHO) and Its UV Photolysis Products on the Surface of Atmospheric Ice Nanoparticles. DFT and Density Functional Tight-Binding Study

The structures, energies, harmonic vibrational frequencies, and thermodynamic parameters of the water clusters (H2O)48, (H2O)72, and (H2O)270 were calculated using the standard DFT theory (BLYP/6-31++G(d,p) for small and medium clusters) and the modern tight-binding method SCC-DFTB (DFTBA and DFTB+). The adsorption and embedding of s-cis- and s-trans-glyoxal molecules as well as its sunlight UV photolysis products (molecules CH2O, HCOOH, H2O2, CO, CO2 and radicals CHO, HO, HO2) on nanosized ice clusters of up to 2.5 nm in diameter were studied within the above theoretical models. The structures of adsorption complexes on different sites of ice nanoparticles, the corresponding adsorption energies and thermodynamic parameters were estimated. We found that the DFTB method is a very promising tool for the calculations of structures and energies of ice nanoparticles, when compared to both DFT and semiempirical (PM3) methods. The obtained results are discussed in relation to the possible photolysis pathways, the reaction rates in the gas phase and in the adsorbed state, and the mechanisms of glyoxal photolysis catalyzed by the ice nanoparticles in the Earth’s atmosphere.

Density Functional Based Tight Binding Study on Wurtzite ZnO Prismatic Nanoparticles

We have performed the Density Functional Tight Binding (DFTB) study on the structural properties of Zinc Oxide Nanoparticles (NP), focusing on the effects induced by the surfaces and quantum size effect. Effects of surface relaxation and surface stress which is absent in atomistic model are taken carefully into account. The studying Nanoparticle size range up to 2.3nm. We illustrated the structural properties changes by decreasing NP sizes while the typical length of surface relaxation (about 1nm) remain unchanged and comparable with the particle size. The NP electronic properties, i.e. Density of State (DOS), charge transfer in hetero-bond also showed the deviation from bulk material value.