Nonorthogonal Tight-binding Model Research Articles

Thermal stability and decomposition mechanisms of hexatetracarbon: Tight-binding molecular dynamics and density functional theory study

In this work, we carry out molecular dynamics and ab initio modeling to determine the thermal decomposition channels and thermal stability of the recently proposed 2D carbon allotrope, hexatetracarbon (HTC). To take into account the role of edges in the initialization of decay, we considered finite size cluster models of HTC passivated by hydrogen. Four models were selected for the study: [Formula: see text], [Formula: see text] [Formula: see text] and [Formula: see text]. Molecular dynamics and hyperdynamics was carried out using the NTBM non-orthogonal tight-binding model. For ab initio calculations, we used the electron density functional theory with the B3LYP three-parameter hybrid functional and the 6-311G[Formula: see text] electronic basis set. Prismane [Formula: see text] demonstrated the highest stability due to the high energy barrier of 1.5 eV preserving its decomposition. Larger clusters possessed lower barriers in the 0.65–0.9 eV range. We concluded that the HTC edges are unstable at room temperatures. However, the destruction of some interlayer bonds can result in strain relaxation and increase of stability. We believe that HTC could exist at room temperatures in the form of nanosized quantum dots that appeared from bilayer graphene under high pressure.

Bilayer Graphene–Stone–Wales Graphene: Structure, Stability, and Interlayer Thermal Conductivity

The interlayer thermal conductivity of two asymmetric bilayer carbon structures has been studied within the nonorthogonal tight binding model. One layer of the first structure proposed in this work for the first time is graphene and the second layer is Stone–Wales graphene, which is recently proposed carbon allotrope. The second asymmetric structure is bilayer graphene, where one layer consists of 12C isotope and the second layer consists of rarer 13C isotope. It has been shown that the interlayer thermal conductivity of asymmetric structures is more than an order of magnitude lower than that for their symmetric analogs, bilayer graphene and Stone–Wales bilayer graphene, with the same isotope composition. A high interlayer thermal conductivity of symmetric structures compared to asymmetric ones is due to the resonant interaction of phonon subsystems of individual layers (phonon spectra of individual layers in symmetric structures coincide, whereas these spectra in asymmetric structures are different). It has been shown that the graphene layer in the unstrained graphene–Stone–Wales graphene structure is flat, whereas the Stone–Wales graphene layer is corrugated. Both layers of this structure biaxially stretched by 5% become flat. The interlayer attraction energy, interlayer distance, activation energy of parallel shear of the layers, and the elastic modulus under vertical compression have been determined for unstrained and deformed structures.

Comparative study of the electron–electron interactions on optoelectronic properties of carbon nanotube within orthogonal and nonorthogonal tight-binding model

We investigate the electron–electron (e–e) interactions effect on the optoelectronic properties of single-walled carbon nanotubes (SWCNTs). We study theoretically the effect of e–e interactions on the optoelectronic properties of zigzag carbon nanotubes and obtain some useful expressions for the band structure and optical absorption of nanotubes for the first nearest-neighbor (1NN) and second nearest-neighbor (2NN) approximation in tight-binding (TB) model. Our computational approach is based on combination of the orthogonal (o-TB) and nonorthogonal (n-TB) tight-binding method with Hubbard model for zigzag SWCNTs. In nonorthogonal tight-binding model we consider the effect of orbital overlap on optoelectronic properties of nanotubes. We also obtain the optical absorption spectrum of zigzag SWCNTs under e–e interactions effect with considering orthogonal and nonorthogonal tight-binding method for 1NN and 2NN approximation. We investigate optical absorption for the incident light polarized parallel to the tube axis and obtain all allowed optical transitions. Our results show some variations in the band structure and optical absorption spectrum of nanotubes under e–e interactions effect when we use orthogonal tight-binding model compare with nonorthogonal tight-binding model.

Thermal Stability of Hydrogen Clusters at Graphene and Stone—Wales Graphene Surfaces

In the framework of the nonorthogonal tight-binding model, the possibility to form different thermally stable elements of the hydrogen pattern at graphene and Stone—Wales graphene surfaces is studied. The latter material is the recently predicted allotrope of graphene. The migration of a hydrogen atom adsorbed on these structures is numerically analyzed. The activation energies of migration of a hydrogen atom on the surfaces of graphene and Stone—Wales graphene are equal to 0.52 and 0.84 eV, respectively. The thermal stability of hydrogen clusters having the form of hexatomic rings and located on the surface of graphene, as well as the stability of the pent-, hex-, and heptatomic rings on the surface of Stone—Wales graphene, is estimated. The corresponding activation energies (1.61, 1.25, 1.36, and 1.27 eV, respectively), as well as the frequency-dependent factors in the Arrhenius formula characterizing the thermal decay, are determined. The lifetimes of these clusters at freezing and boiling temperatures of water are estimated.

Molecular Hyperdynamics Coupled with the Nonorthogonal Tight-Binding Approach: Implementation and Validation.

We present the molecular hyperdynamics algorithm and its implementation to the nonorthogonal tight-binding model NTBM and the corresponding software. Due to its multiscale structure, the proposed approach provides the long time scale simulations (more than 1 s), unavailable for conventional molecular dynamics. No preliminary information about the system's potential landscape is needed for the use of this technique. The optimal interatomic potential modification is automatically derived from the previous simulation steps. The average time between adjusted potential energy fluctuations provides an accurate evaluation of physical time during the hyperdynamics simulation. The main application of the presented hyperdynamics method is the study of thermal-induced defects arising in the middle-sized or relatively large atomic systems at low temperatures. To validate the presented method, we apply it to the C60 cage and its derivative C60NH2. Hyperdynamics leads to the same results as a conventional molecular dynamics, but the former possesses much higher performance and accuracy due to the wider temperature region. The coefficient of acceleration achieves 107 and more.

Two-phonon Raman bands of single-walled carbon nanotubes: A case study

It has been long accepted that the second-order Raman bands in carbon nanotubes are enhanced through the double-resonance mechanism. Although separate aspects of this mechanism have been studied for a few second-order Raman bands, including the most intense defect-induced D band and the two-phonon 2D band, a complete computational approach to the second-order bands is still lacking. Here, we propose such an approach, entirely based on a symmetry-adapted nonorthogonal tight-binding model with ab initio derived parameters. As a case study, we consider a nanotube $(6,5)$, for which we calculate the two-phonon spectrum. We investigate in detail the 2D band and identify three contributions to it: a nondispersive one and two dispersive ones, which are found to depend on the electron and phonon dispersion of the nanotube, and on the laser excitation. We also predict two-phonon bands, which are not allowed in the parent structure graphene. The obtained two-phonon bands are in very good agreement with the available experimental data. The symmetry-adapted formalism makes feasible the calculation of the two-phonon Raman bands of any observable nanotube.

On the Thermal Stability of Pentagraphene

In the framework of the nonorthogonal tight-binding model, the thermal stability of pentagraphene is studied by numerical simulation using the molecular dynamics technique. Pentagraphene is the recently predicted two-dimensional allotropic modification of carbon, in which the С–С bonds form only pentagons, whereas the hexagons characteristic of carbon nanostructures are absent. It is found that the thermally activated rotation of one С–С bond by an angle of 45° initiates the formation of а defect region, which does not remain localized and rapidly propagates over the whole sample, leading eventually to the total destruction of the sample structure. Nevertheless, the probability of such rotation turns out to be so small that, even at room temperature, pentagraphene can preserve its structure for a fairly long time. This time decreases with the growth of the sample size.

Effect of the Dehydrogenation of Graphane on Its Mechanical and Electronic Properties

The effect of the desorption of hydrogen on the mechanical characteristics and electronic structure of the armchair conformation of graphane is studied in the context of the nonorthogonal tight-binding model. It is shown that the mechanical stiffness and the Poisson ratio nonmonotonically depend on the hydrogen content and take minimum values at a hydrogen vacancy content of ~50% and ~30%, respectively. As hydrogen is desorbed, the characteristic peaks of the phonon density of states are rapidly reduced. In the initial stage of desorption, local energy levels are formed in the band gap. As the number of hydrogen vacancies is increased, these levels form an impurity band, in which the Fermi level is located.

Effect of Hydrogen Adsorption on the Stone–Wales Transformation in Small-Diameter Carbon Nanotubes

The effect of hydrogenation of (4, 0) and (3, 0) carbon nanotubes on the Stone–Wales transformation is studied in the framework of the nonorthogonal tight-binding model. It is shown that the atomic hydrogen adsorption can lead to both a decrease and an increase in the barriers for the direct and inverse transformations depending on the orientation of a rotating C–C bond with respect to the nanotube axis. The characteristic times of formation and annealing the Stone–Wales defects have been estimated. The Young’s moduli have been calculated.

Optical third harmonic generation in black phosphorus

We present a calculation of Third Harmonic Generation (THG) for two-band systems using the length gauge that avoids unphysical divergences otherwise present in the evaluation of the third order current density response. The calculation is applied to bulk and monolayer black Phosphorus (bP) using a non-orthogonal tight-binding model. Results show that the low energy response is dominated by mixed inter-intraband processes and estimates of the magnitude of THG susceptibility are comparable to recent experimental reports for bulk bP samples.

Влияние адсорбции водорода на трансформацию Стоуна- Уэльса в углеродных нанотрубках малого диаметра

AbstractThe effect of hydrogenation of (4, 0) and (3, 0) carbon nanotubes on the Stone–Wales transformation is studied in the framework of the nonorthogonal tight-binding model. It is shown that the atomic hydrogen adsorption can lead to both a decrease and an increase in the barriers for the direct and inverse transformations depending on the orientation of a rotating C–C bond with respect to the nanotube axis. The characteristic times of formation and annealing the Stone–Wales defects have been estimated. The Young’s moduli have been calculated.

Влияние дегидрирования графана на его механические и электронные свойства

AbstractThe effect of the desorption of hydrogen on the mechanical characteristics and electronic structure of the armchair conformation of graphane is studied in the context of the nonorthogonal tight-binding model. It is shown that the mechanical stiffness and the Poisson ratio nonmonotonically depend on the hydrogen content and take minimum values at a hydrogen vacancy content of ~50% and ~30%, respectively. As hydrogen is desorbed, the characteristic peaks of the phonon density of states are rapidly reduced. In the initial stage of desorption, local energy levels are formed in the band gap. As the number of hydrogen vacancies is increased, these levels form an impurity band, in which the Fermi level is located.

Thermal Stability of Heptaprismane C14H14

Heptaprismane has been numerically simulated by the method of the nonorthogonal tight-binding model in this paper. Thermokinetic parameters of its decomposition (activation energy and the frequency factor) have been determined. The temperature dependence of the heptaprismane lifetime in the temperature range T = 300 − 1000K has been investigated. Obtained data have been compared well with the earlier reported results concerning the “smaller” polyprismanes.

Chemisorption of hydrogen atoms and hydroxyl groups on stretched graphene: A coupled QM/QM study

Using the density functional theory coupled with the nonorthogonal tight-binding model, we analyze the chemisorption of hydrogen atoms and hydroxyl groups on the unstrained and stretched graphene sheets. Drawback of finite cluster model of graphene for the chemisorption energy calculation in comparison with the QM/QM approach applied is discussed. It is shown that the chemisorption energy for the hydroxyl group is sufficiently lower than for hydrogen at stretching up to 7.5%. The simultaneous paired chemisorption of hydrogen and hydroxyl groups on the same hexagon has also been examined. Adsorption of two radicals in ortho and para positions is found to be more energetically favorable than those in meta position at any stretching considered. In addition the energy difference between adsorbent pairs in ortho and para positions decreases as the stretching rises. It could be concluded that the graphene stretching leads to the loss of preferred mutual arrangement of two radicals on its surface.

Toward CL-20 crystalline covalent solids: On the dependence of energy and electronic properties on the effective size of CL-20 chains

One-dimensional CL-20 chains have been constructed using CH2 molecular bridges for the covalent bonding between isolated CL-20 fragments. The energy and electronic properties of the obtained nanostructures have been analyzed by means of density functional theory and nonorthogonal tight-binding model considering Landauer-Büttiker formalism. It has been found that such systems become more thermodynamically stable as the efficient length of the chain increases. Thus, the formation of bulk covalent CL-20 solids may be energetically favorable, and such structures may possess high kinetic stability comparing to the CL-20 molecular crystals. As for electronic properties of pure CL-20 chains, they are wide-bandgap semiconductors with energy gaps equal to several electron volts that makes their use in nanoelectronic applications problematic without any additional modification.

Theoretical Studies of the Stone-Wales Defect in C36 Fullerene Embedded inside Zigzag Carbon Nanotube

We apply density functional theory at PBE/6-311G(d) level as well as nonorthogonal tight-binding model to study the Stone-Wales transformation in C36 fullerene embedded inside the (14,0) zigzag carbon nanotube. We optimize geometries of two different isomers with the D2d and the D6h symmetries and the transition state dividing them. The mechanism of Stone-Wales transformation from D2d to D6h symmetry for the encapsulated C36 is calculated to be the same as for the isolated one. It is found that the outer carbon wall significantly stabilizes the D6h isomer. However, carbon nanotube reduces the activation barrier of Stone-Wales rearrangement by 0.4 eV compared with the corresponding value for the isolated C36.

Various Stone–Wales defects in phagraphene

Various Stone–Wales defects in phagraphene, which is a graphene allotrope, predicted recently are studied in terms of the nonorthogonal tight-binding model. The energies of the defect formation and the heights of energy barriers preventing the formation and annealing of the defects are found. Corresponding frequency factors in the Arrhenius formula are calculated. The evolution of the defect structure is studied in the real-time mode using the molecular dynamics method.

Comparative study of the two-phonon Raman bands of silicene and graphene

We present a computational study of the two-phonon Raman spectra of silicene and graphene within a density-functional non-orthogonal tight-binding model. Due to the presence of linear bands close to the Fermi energy in the electronic structure of both structures, the Raman scattering by phonons is resonant. We find that the Raman spectra exhibit a crossover behavior for laser excitation close to the π-plasmon energy. This phenomenon is explained by the disappearance of certain paths for resonant Raman scattering and the appearance of other paths beyond this energy. Besides that, the electronic joint density of states (DOS) is divergent at this energy, which is reflected on the behavior of the Raman bands of the two structures in a qualitatively different way. Additionally, a number of Raman bands, originating from divergent phonon DOS at the M point and at points, inside the Brillouin zone, is also predicted. The calculated spectra for graphene are in excellent agreement with available experimental data. The obtained Raman bands can be used for structural characterization of silicene and graphene samples by Raman spectroscopy.

Intensity Ratio of Resonant Raman Modes for (n,m) Enriched Semiconducting Carbon Nanotubes

Relative intensities of resonant Raman spectral features, specifically the radial breathing mode (RBM) and G modes, of eleven, chirality-enriched, single-wall carbon nanotube (SWCNT) species were established under second-order optical transition excitation. The results demonstrate an under-recognized complexity in the evaluation of Raman spectra for the assignment of (n,m) population distributions in agreement with theory predicted by a symmetry-adapted nonorthogonal tight-binding model.[1] Strong chiral angle and mod dependencies affect the intensity ratio of the RBM to G modes. Furthermore, we report five additional values for chirality dependent G+ and G- Raman peak positions and intensity ratios extending the available data to cover more of the smaller diameter regime by including the first (5,4) second-order, resonance Raman spectra.[2] Together, the Raman spectral library is demonstrated to be sufficient for decoupling multiple species viaa spectral fitting process, and enables fundamental characterization even in mixed chiral population samples.

Exciton effects in strained armchair graphene nanoribbons

The exciton effects in 1-nm-wide armchair graphene nanoribbons (AGNRs) under the uniaxial strain were studied within the nonorthogonal tight-binding (TB) model, supplemented by the long-range Coulomb interactions. The obtained results show that both the excitation energy and exciton binding energy are modulated by the uniaxial strain. The variation of these energies depends on the ribbon family. In addition, the results show that the variation of the exciton binding energy is much weaker than the variation of excitation energy. Our results provide new guidance for the design of optomechanical systems based on graphene nanoribbons.