Tight-binding Molecular Dynamics Study Research Articles

Tight-binding molecular dynamics study of the role of defects on carbon nanotube moduli and failure

We performed tight-binding molecular dynamics on single-walled carbon nanotubes with and without a variety of defects to study their effect on the nanotube modulus and failure through bond rupture. For a pristine (5,5) nanotube, Young's modulus was calculated to be approximately 1.1 TPa, and brittle rupture occurred at a strain of 17% under quasistatic loading. The predicted modulus is consistent with values from experimentally derived thermal vibration and pull test measurements. The defects studied consist of moving or removing one or two carbon atoms, and correspond to a 1.4% defect density. The occurrence of a Stone-Wales defect does not significantly affect Young's modulus, but failure occurs at 15% strain. The occurrence of a pair of separated vacancy defects lowers Young's modulus by approximately 160 GPa and the critical or rupture strain to 13%. These defects apparently act independently, since one of these defects alone was independently determined to lower Young's modulus by approximately 90 GPa, also with a critical strain of 13%. When the pair of vacancy defects adjacent, however, Young's modulus is lowered by only approximately 100 GPa, but with a lower critical strain of 11%. In all cases, there is noticeable strain softening, for instance, leading to an approximately 250 GPa drop in the apparent secant modulus at 10% strain. When a chiral (10,5) nanotube with a vacancy defect was subjected to tensile strain, failure occurred through a continuous spiral-tearing mechanism that maintained a high level of stress (2.5 GPa) even as the nanotube unraveled. Since the statistical likelihood of defects occurring near each other increases with nanotube length, these studies may have important implications for interpreting the experimental distribution of moduli and critical strains.

Self-consistent tight binding molecular dynamics study of TiO2 nanoclusters in water

Self-consistent tight binding molecular dynamics studies of TiO2 anatase and rutile nanoclusters in dissociable water are reported. It is found that the structure of the particle expands as a result of interaction between the particle’s surface and water. Water molecules dissociate at the nanoparticle surface during simulation.

Vibrational modes and diffusion of self-interstitial atoms in body-centered-cubic transition metals: A tight-binding molecular-dynamics study

Using a tight-binding molecular-dynamics method, we have calculated the formation energies, diffusivity, and localized vibrational frequencies of self-interstitial atoms (SIA's) in body-centered-cubic (bcc) transition metals: vanadium, niobium, molybdenum, and tantalum. As a test of our methods, we compare to experiment for the perfect bcc phonon spectra and we compare to previous ab initio SIA formation energies. In addition, we present vibrational spectra calculated from molecular dynamics via the velocity autocorrelation method. For all of the systems studied, we find that the localized vibration frequency of a SIA dumbbell pair is roughly twice the frequency of the bcc phonon-density-of-states peak. We also find an Arrhenius temperature dependence for SIA hopping, with frequency prefactors ranging between the cutoff of the ideal bcc lattice and the highest frequencies of the SIA dumbbell. In all cases, we find that the energy barrier to SIA diffusion is approximately $0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

Nickel-related defect in diamond: A tight-binding molecular-dynamics study

Structural, electronic, and magnetic properties of isolated Ni impurities at point defects and a dislocation core in diamond are investigated using tight-binding molecular-dynamics simulations. The results show the structural stabilization associated by lowering local symmetry in the cases of point defects. The segregation energies of Ni impurity for a substitutional site and for an interstitial site in the dislocation core are estimated to be in the same order within 0.2eV. The local electronic density-of-states reveals that the gap states appeared by the insertion of Ni impurity are strongly localized around the Ni sites. Magnetic moments on neighboring C atoms are induced so as to screen the moment on the Ni atom except for the case of interstitial Ni impurity in which the total magnetic moment remains non-zero. Analyses indicate that localized atomic d states on the Ni atom and the p–d coupling between Ni and neighboring C atoms are responsible for the residual magnetic moment in the system with an interstitial Ni defect. In the other systems investigated, on the other hand, the bonding states between Ni impurity and its neighboring C atoms are dominated by the s–p coupling.

Periodic density functional and tight-binding quantum chemical molecular dynamics study of catalytic properties on γ-Al 2O 3 supported Pt catalysts

We investigated the interactions of platinum metal clusters on the γ-alumina using a periodic density functional theory method. Results showed that electron transfer occurs between platinum metal clusters and γ-Al 2O 3. We also investigated NO adsorption properties (NO–Pt 4/γ-Al 2O 3) in order to clarify the support effect on NO activation in terms of the elongation of N–O bond length and NO molecular charge. In our results, the support increases NO activation for three-fold sites but decreases it for on-top and bridge sites. Moreover, the support changes the site preference of NO adsorption. In addition, in order to clarify the support effect at finite temperature, we studied the dynamic behaviors of supported precious metal catalysts by using a tight-binding quantum chemical molecular dynamics program named ‘Colors’.

Tight-Binding Quantum Chemical Molecular Dynamics Study on First Proton Transfer Process of ORR Catalyzed by Cobalt-Porphyrin Complex

We investigated oxygen reduction reaction (ORR) dynamics catalyzed by cobalt-porphyrin using our original novel tight-binding quantum chemical molecular dynamics method. First, we determined parameters for tight-binding calculation based on first-principles parameterization. We have successfully simulated a first proton transfer process of the oxygen reduction reaction with the explicit consideration of surrounding water molecules. The transferred proton formed hydrogen bonding with a water molecule and conformational change of the OOH species was observed. It was proved that our tight-binding quantum chemical molecular dynamics method is effective for the investigation of oxygen reduction reaction dynamics in large complex system.

Growth of tetrahedral amorphous carbon film: Tight-binding molecular dynamics study

Molecular dynamics simulation using tight-binding potential has been performed to examine the growth and performance of tetrahedral amorphous carbon during ion deposition. The sp 3 hybrid atom content, density, and compressive stress of the tetrahedral amorphous carbon film depend on the growing conditions such as substrate temperature, ion energy, ion dose, and annealing temperature. The critical temperature for sp 3 transition to sp 2 decreases with ion energy (40, 80, and 120 eV). At low temperatures (<300 K) and low ion energies, the sp 3 fraction increases up to 82%. At the annealing temperature less than 1200 K or with a few ions (<20) implanted into the film, its sp 3 content and density have only slight changes while the compressive stress has a large reduction with the annealing temperature and the number of implanted ions. This large reduction in the compressive stress is due to a structural relaxation.

Periodic density functional and tight-binding quantum chemical molecular dynamics study of surface hydroxyl groups on ZrO 2(111)-supported Pt catalyst

Periodic density functional and tight-binding quantum chemical molecular dynamics calculations were employed to clarify the characteristics and dynamics of surface hydroxyl groups on Pt/ZrO 2 catalysts. We applied the periodic density functional theory calculations to the investigations on the effect of hydroxyl groups on the adsorption characteristics of Pt atom on ZrO 2(111) support and hydroxyl groups were found to decrease the adsorption energy of Pt atom on the ZrO 2(111) support. Furthermore, we applied our original tight-binding quantum chemical molecular dynamics method to the clarification of the dynamic behaviors of the hydroxyl group on the Pt/ZrO 2(111) and ZrO 2(111) surface. We found that the Pt atom triggers the migration and hopping of the H atom of the hydroxyl group. The migration of H atoms on the Pt/ZrO 2 catalyst was discussed in the sight of their catalytic activity.

Photocatalytic oxidation dynamics of acetone on TiO 2: tight-binding quantum chemical molecular dynamics study

The clarification of the excited states dynamics on TiO 2 surface is important subject for the design of the highly active photocatalysts. In the present study, we applied our novel tight-binding quantum chemical molecular dynamics method to the investigation on the photocatalytic oxidation dynamics of acetone by photogenerated OH radicals on the hydrated anatase TiO 2 surface. The elucidated photocatalytic reaction mechanism strongly supports the previous experimental proposal and finally the effectiveness of our new approach for the clarification of the photocatalytic reaction dynamics employing the large simulation model was confirmed.

Physical trends in amorphous carbon: A tight-binding molecular-dynamics study

Tight-binding molecular dynamics simulations reveal interesting physical trends in amorphous carbon networks. The variation of $s{p}^{3}$ fraction, or mean coordination, is found to be linear over the whole possible range of densities. The density at the floppy transition is $\ensuremath{\sim}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{g}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, while the density of ``amorphous diamond'' is $\ensuremath{\sim}3.3\phantom{\rule{0.3em}{0ex}}\mathrm{g}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The bulk modulus vanishes at the floppy transition, having a critical coordination near 2.4, and its variation with the mean coordination has a scaling exponent of 1.5, confirming the constraint-counting model of Phillips and Thorpe. A hypothetical fully tetrahedral network has a bulk modulus of $360\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, about 15% lower than diamond's. The bulk modulus is also found to vary with the average bond length $\overline{d}$ as ${(\overline{d})}^{3.5}$. The homopolar gap of ``amorphous diamond'' is $\ensuremath{\sim}11.5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, compared to $\ensuremath{\sim}14\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for diamond.

Structure and stability of copper clusters: A tight-binding molecular dynamics study

In this paper we propose a tight-binding molecular dynamics with parameters fitted to first-principles calculations on the smaller clusters and with an environment correction, to be a powerful technique for studying large transition/noble metal clusters. In particular, the structure and stability of $Cu_n$ clusters for $n=3-55$ are studied by using this technique. The results for small $Cu_n$ clusters ($n=3-9$) show good agreement with {\it ab initio} calculations and available experimental results. In the size range $10\le n \le 55$ most of the clusters adopt icosahedral structure which can be derived from the 13-atom icosahedron, the polyicosahedral 19-, 23-, and 26-atom clusters and the 55-atom icosahedron, by adding or removing atoms. However, a local geometrical change from icosahedral to decahedral structure is observed for $n = 40-44$ and return to the icosahedral growth pattern is found at $n=45$ which continues. Electronic "magic numbers" ($n=2$, 8, 20, 34, 40) in this regime are correctly reproduced. Due to electron pairing in HOMOs, even-odd alternation is found. A sudden loss of even-odd alternation in second difference of cluster binding energy, HOMO-LUMO gap energy and ionization potential is observed in the region $n\sim40$ due to structural change there. Interplay between electronic and geometrical structure is found.

Photo-induced volume changes in selenium. Tight-binding molecular dynamics study

Tight-binding molecular dynamics simulations of photo-excitations in small Se clusters (isolated Se 8 ring and helical Se chain) and glassy Se networks (containing 162 atoms) were carried out in order to analyze the photo-induced instability in amorphous selenium. In the cluster systems after taking an electron from the highest occupied molecular orbital to the lowest unoccupied molecular orbital a bond breaking occurs. In the glassy networks photo-induced volume expansion was observed and at the same time the number of coordination defects changed significantly due to illumination.

Tight-Binding Molecular Dynamics Study on the Structural Change of Amorphous Germanium with the Increase of Density

The structures of low-density and high-density amorphous germanium ( a -Ge) are studied, by using the order- N non-orthogonal tight-binding molecular dynamics method, where N is the number of atoms. The initial amorphous configuration is taken from the results of a glass-transition simulation performed in a separate work, with N =512. From this initial configuration, we gradually increase the density of the system. In low-density a -Ge, the short-range order (SRO) is characterized essentially by the four-fold configuration although, at lowest densities, the coordination number is smaller than four, thus indicating that quite a few dangling bonds exist. The tetrahedral nature of the SRO in a -Ge is in substance the same as the SRO of a diamond structure which is the low-pressure form of crystalline germanium ( c -Ge). On the other hand, the SRO in high-density a -Ge is similar to that of a β-Sn structure, the latter being the high-pressure form of c -Ge.

Degradation of inter-atomic bonds during structural phase change in intermediate Ni-clusters (Ni39–Ni49)

Results based on a symmetry- and spin-unrestricted tight-binding molecular-dynamics study are presented for the ground-state geometries of intermediate Ni(n), n in [39,49], clusters. A structural phase change is found to take place around n=43 during which a structural transition from fcc/hcp structure to icosahedral one is observed. This is in good agreement with recent experimental findings. This structural transition is found to be associated with a degradation of the inter-atomic bond energy which indicates that the inter-atomic bond does not only depend on the coordination number of each atom but also on its point group symmetry.

A Tight-binding Molecular Dynamics Study of the Dissociation of Boron Clusters in c-Si

Boron clusters with silicon self-interstitials have been implicated in the phenomenon of transient enhanced diffusion (TED) following ion implantation of boron and subsequent annealing steps. This paper explores possible dissolution mechanisms for boron-interstitial clusters during the simulation of a typical annealing process. Using tight-binding molecular dynamics (TBMD) and employing a Goodwin-Skinner-Pettifor sp-based TB model, we have been able to observe the complete dissolution of a B 4 I 4 cluster into the surrounding crystalline silicon matrix. Many unsuccessful attempts to observe dissolution are also presented, highlighting the effect of cluster stability, temperature and the role of vacancies in cluster dissolution. Though we can make no unambiguous statements on the definitive dissolution mechanism of boron-defect clusters based on one successful dissolution event, we can hint at key events that appear to be important, such as the diffusion of self-interstitials (presciently predicted by Pelaz et al. ), the "stranding" of boron atoms in their wake, and the importance of mobile boron-self-interstitial (B-I) pairs. The intrinsic diffusivity of boron in a c-Si lattice and its retardation of the diffusivity of Si self-interstitials is also discussed.

Tight-binding molecular-dynamics study of melting behaviour of small silicon clu sters

The Tight_binding molecular_dynamics(TBMD) has been used to study the melting behaviour of small silicon clusters Sin（n=5—10）. We report the calcu lated res ults of the latent heat of fusion Δμls and the melting temperature Tm as a function of cluster size. Δμls and Tm exhibit a s trong dependen ce on cluster size. The melting mechanisms of silicon clusters are distinguished from metal clusters. The melting of metal clusters can be described as a tran sition from a low_energy solid_like structure at low temperatures to a higher_en ergy liquid_like structure at high temperatures. At the transition temperature, metal clusters undergo a dynamics coexistence of the two states, while the silic on clusters undergo an intermediate state which is neither liquid_like structure nor solid_like structure. The melting points obtained by using different calcul ation methods and definitions also have been compared in this work.

Tight-binding quantum chemical molecular dynamics study of cathode materials for lithium secondary battery

Our tight-binding quantum chemical molecular dynamics (TBMD) program was applied to the investigation of the structural and electronic properties of Li x Mn 2O 4. The calculated lattice constants of the LiMn 2O 4 and λ-MnO 2 were 8.17 and 8.08 Å, respectively, which are in good agreement with experimental results. The electronic analysis showed that the manganese atoms in the LiMn 2O 4 were separated into two groups based on their electronic states, which correspond to Mn 3+ and Mn 4+. We calculated the average potential for the reaction of Li+Mn 2O 4→LiMn 2O 4 and obtained the average potential of 4.2 V. The experimental value of the potential is approximately 4.0 V, indicating that our program is a powerful tool to predict the potential of various materials, as well as their atomistic and electronic structures.

Time development during growth and relaxation of amorphous carbon films. Tight-binding molecular dynamics study

The growth of amorphous carbon (a-C) thin film on a [1 1 1] diamond surface has been studied by a tight-binding (TB) molecular dynamics (MD) technique. Six different three-dimensional networks were constructed with periodic boundary conditions in two dimensions. Time-dependent non-equilibrium growth was simulated with atom-by-atom deposition and it was described as in real experiments without an artificial model of energy dissipation. An additional seventh structure was constructed by a melt quenching procedure which is widely used in computer generations of amorphous networks. The final structures consist of over 100 atoms. Densities, radial distribution functions (RDFs), coordination numbers, bond angle distributions and ring statistics were analyzed. During relaxation the temperature in the amorphous film decreases with stretched-exponential function. The time dependence of bond length and bond angle deviations were also investigated.

Neck formation processes of nanocrystalline silicon carbide: A tight-binding molecular dynamics study

Atomistic and electronic structures in the process of intergranular neck formation of nanocrystalline silicon carbide (SiC) have been investigated by a tight-binding molecular dynamics method. An order-N parallel algorithm is employed for efficient calculations of electronic energy and forces. We find that a defect-free neck is formed between SiC nanocrystals aligned along [112] axis at T

Dynamical and optical properties of warm dense hydrogen

We report dynamical and optical properties of hydrogen (deuterium) around the region of the principal Hugoniot that starts from the cryogenically cooled molecular liquid. These properties were determined by finite-temperature density functional (FTDF) molecular dynamics (MD) simulations within the local density and generalized gradient approximations. The principal Hugoniot, calculated from the FTDF-MD equation of state, agrees with gas-gun and recent laser-shock experiments for pressures up to 50 GPa. However, the maximum compression of the FTDF-MD Hugoniot (4.6 at 52 GPa) significantly differs from that of the laser shock experiment (6 at about 150 GPa). On the other hand, the optical reflectivities show reasonable agreement with the laser measurements for pressures up to 70 GPa. The system smoothly dissociates along the Hugoniot with the electrical conductivity reaching a value of 4000 ${\ensuremath{\Omega}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}1}$ at maximum compression. We also obtain good agreement with previous tight-binding and ab initio molecular dynamics studies.