Tight-binding Molecular Dynamics Method Research Articles

Achieving Boron-Carbon-Nitrogen Heterostructures by Collision Fusion of Carbon Nanotubes and Boron Nitride Nanotubes.

Heterostructures may exhibit completely new physical properties that may be otherwise absent in their individual component materials. However, how to precisely grow or assemble desired complex heterostructures is still a significant challenge. In this work, the collision dynamics of a carbon nanotube and a boron nitride nanotube under different collision modes were investigated using the self-consistent-charge density-functional tight-binding molecular dynamics method. The energetic stability and electronic structures of the heterostructure after collision were calculated using the first-principles calculations. Five main collision outcomes are observed, that is, two nanotubes can (1) bounce back, (2) connect, (3) fuse into a defect-free BCN heteronanotube with a larger diameter, (4) form a heteronanoribbon of graphene and hexagonal boron nitride and (5) create serious damage after collision. It was found that both the BCN single-wall nanotube and the heteronanoribbon created by collision are the direct band-gap semiconductors with the band gaps of 0.808 eV and 0.544 eV, respectively. These results indicate that collision fusion is a viable method to create various complex heterostructures with new physical properties.

Tribochemical reactions and graphitization of diamond-like carbon against alumina give volcano-type temperature dependence of friction coefficients: A tight-binding quantum chemical molecular dynamics simulation

Diamond-like carbon (DLC) is a promising solid lubricant used as a protective coating to reduce friction against alumina. Friction properties of DLC/alumina are strongly affected by temperature. To improve the friction performance of DLC, we investigate the friction behaviors of DLC/alumina at various temperatures and reveal the mechanisms by using our tight-binding quantum chemical molecular dynamics method. We observe an interesting volcano-type temperature dependence of friction coefficients in our friction simulations. Friction coefficients of DLC/alumina are low and show little change at 300–600 K because no tribochemical reactions occur at the interface. However, as the temperature increases, friction coefficients increase at 600–800 K and subsequently decrease at 800–1000 K. At 600–800 K, interfacial C-O and C-Al bonds between two substrates are formed during friction, leading to a high friction coefficient. Interestingly, further increment of temperature to 800–1000 K induces the graphitization of DLC. The graphite-like surface suppresses the interfacial bond formation, reducing the friction coefficient. We reveal that the volcano-type temperature dependence of friction coefficients is due to the tribochemical reactions generating interfacial bonds at 600–800 K and the graphitization of DLC reducing the number of interfacial bonds at 800–1000 K.

Tight-Binding Quantum Chemical Molecular Dynamics Study on the Friction and Wear Processes of Diamond-Like Carbon Coatings: Effect of Tensile Stress.

Diamond-like carbon (DLC) coatings have attracted much attention as an excellent solid lubricant due to their low-friction properties. However, wear is still a problem for the durability of DLC coatings. Tensile stress on the surface of DLC coatings has an important effect on the wear behavior during friction. To improve the tribological properties of DLC coatings, we investigate the friction process and wear mechanism under various tensile stresses by using our tight-binding quantum chemical molecular dynamics method. We observe the formation of C-C bonds between two DLC substrates under high tensile stress during friction, leading to a high friction coefficient. Furthermore, under high tensile stress, C-C bond dissociation in the DLC substrates is observed during friction, indicating the atomic-level wear. These dissociations of C-C bonds are caused by the transfer of surface hydrogen atoms during friction. This work provides atomic-scale insights into the friction process and the wear mechanism of DLC coatings during friction under tensile stress.

Collision-induced fusion of two single-walled carbon nanotubes: A quantitative study

The coalescence processes of two (6,0) single-walled carbon nanotubes are investigated via coaxial collision based on the self-consistent-charge density-functional tight-binding molecular dynamics method. According to the structure characteristics of the nanotubes, five impact cases are studied to explore the coalescence processes of the nanotubes. The simulation shows that various kinds of carbon nanomaterials, such as graphene sheets, graphene nanoribbons, and single-walled carbon nanotubes with larger diameters, are created after collision. Moreover, some defects formed in the carbon nanomaterials can be eliminated, and even the final configurations which are originally fragmented can almost become intact structures by properly quenching and annealing.

Atomistic Mechanisms of Chemical Mechanical Polishing of a Cu Surface in Aqueous H2O2: Tight-Binding Quantum Chemical Molecular Dynamics Simulations.

We applied our original chemical mechanical polishing (CMP) simulator based on the tight-binding quantum chemical molecular dynamics (TB-QCMD) method to clarify the atomistic mechanism of CMP processes on a Cu(111) surface polished with a SiO2 abrasive grain in aqueous H2O2. We reveal that the oxidation of the Cu(111) surface mechanically induced at the friction interface is a key process in CMP. In aqueous H2O2, in the first step, OH groups and O atoms adsorbed on a nascent Cu surface are generated by the chemical reactions of H2O2 molecules. In the second step, at the friction interface between the Cu surface and the abrasive grain, the surface-adsorbed O atom intrudes into the Cu bulk and dissociates the Cu-Cu bonds. The dissociation of the Cu-Cu back-bonds raises a Cu atom from the surface that is mechanically sheared by the abrasive grain. In the third step, the raised Cu atom bound to the surface-adsorbed OH groups is removed from the surface by the generation and desorption of a Cu(OH)2 molecule. In contrast, in pure water, there are no geometrical changes in the Cu surface because the H2O molecules do not react with the Cu surface, and the abrasive grain slides smoothly on the planar Cu surface. The comparison between the CMP simulations in aqueous H2O2 and pure water indicates that the intrusion of a surface-adsorbed O atom into the Cu bulk is the most important process for the efficient polishing of the Cu surface because it induces the dissociation of the Cu-Cu bonds and generates raised Cu atoms that are sheared off by the abrasive grain. Furthermore, density functional theory calculations show that the intrusion of the surface-adsorbed O atoms into the Cu bulk has a high activation energy of 28.2 kcal/mol, which is difficult to overcome at 300 K. Thus, we suggest that the intrusion of surface-adsorbed O atoms into the Cu bulk induced by abrasive grains at the friction interface is a rate-determining step in the Cu CMP process.

Giga- and terahertz-range nanoemitter based on peapod structure

We propose a theoretical model of a nanoemitter for giga- and terahertz-range waves. The model is based on a peapod structure comprising a carbon nanotube with chiral indices (10, 10). Three encapsulated and partially polymerized fullerene C60 molecules and a positively charged C60 fullerene are trapped inside the nanotube. The motion of the charged fullerene and the radiation frequency were controlled using an external electric field. Stable terahertz radiation at a frequency of 0.36 THz was produced at 300 K with an external electrical field of 10 V/μm. Stable radiation in the gigahertz range was observed at 50 K with an electric field below 10 V/μm. A theoretical simulation was performed using the tight-binding molecular dynamics method with a description of the van der Waals interaction by the Morse potential. The system described by the theoretical model was experimentally observed.

Tight-Binding Quantum Chemical Molecular Dynamics Simulations of Mechanisms of SiO2 Etching Processes for CF2 and CF3 Radicals

The plasma etching of SiO2 by CF2 and CF3 radicals is investigated by using our etching simulator based on tight-binding quantum chemical molecular dynamics method. During etching simulations, C–F and Si–O bonds dissociate and C–O and Si–F bonds are generated. Moreover, CO, CO2, COF, and COF2 molecules form, which is consistent with previous experimental studies. We also examine the dependence of the etching mechanism of CF2 and CF3 radicals on the kinetic energy of irradiation. At a low kinetic energy of 10 eV, a CF2 radical dissociates more Si–O bonds than a CF3 radical does. This is because the high chemical reactivity of the CF2 diradical accelerates the etching process. At a high kinetic energy of 150 eV, a CF3 radical dissociates more Si–O bonds than a CF2 radical does. This is because a CF3 radical generates a greater number of reactive F atoms than a CF2 radical does and thus forms more Si–F bonds. Thus, we conclude that our etching simulator modeled the different SiO2 etching mechanisms of CF2 an...

Irradiation effects in single-walled carbon nanotubes: Density-functional theory based treatments

The collision process of a low-energy carbon ion impinging single-walled carbon nanotubes is studied by using a self-consistent-charge density-functional tight-binding molecular dynamics method. The simulation shows that the outcome of the collision highly depends on the incident kinetic energy and the impact location in the nanotube. There are five types of processes observed: adsorption, reflection, substitution, penetration and damage. The adsorption process becomes dominant at energies lower than 20eV. Defect formation events are observed at energies larger than 20eV. For this process, 5-1DB, 5-8-5, Stone–Wales, 7-4-5-9-5 and 5-7-7-6-5 defects are obtained. The formation processes of the typical defects are described in detail. Moreover, the energy exchange and the charge transfer between the incident carbon ion and the nanotube have also been quantitatively studied.

Energetics of graphene flakes

Based on the idea that the binding energy of a graphene flake is a sum of atomic energy contributions, which depend on the local atomic environment of each atom of the flake, we propose a model for graphene flake energetics, which can very accurately predict the cohesive energy of graphene flakes. In our study, we calculate the cohesive energy of hexagonal graphene flakes with up to ≈1000 atoms using the tight binding molecular dynamics method and we show that the calculated values fit extremely accurately to the expression derived from the proposed model. For a further validation, we show that the proposed expression can very accurately predict the calculated values of other graphene flakes with random shapes. Based on that model, we show that the graphene flake stability obeys the following rules: (1) Between isomers the most stable are those with the larger number of bonds (or equivalently, with the smaller number of edge atoms) and (2) between isomers with the same number of bonds (or with the same number of edge atoms), the most stable are those with the smaller number of zig-zag atoms.

Early Stage Oxidation Initiation at Different Grain Boundaries of fcc Fe–Cr Binary Alloy: A Computational Chemistry Study

Tight-binding quantum chemical molecular dynamics method has been applied in order to study the Σ3 (111), Σ5 (100) and random grain boundaries oxidation initiation mechanism of fcc Fe–Cr binary alloy in a boiling water reactor environment. The metal–water interaction at high temperatures causes diffusion of environmental species and segregation of metallic atoms. Water molecules favorably permeate through the random grain boundary (GB) to find the space generated by atomic rearrangement, although it is difficult to diffuse in the Σ3 (111) and Σ5 (100) grain boundaries. Moreover, applied strain creates extra spaces in the lattice that can facilitate the absorption of environmental species. The highly positively charged chromium and the negatively charged oxygen atoms or OH remain along the GB by forming bonds. The GB atoms selectively lose their valence electrons when dissociated atoms adsorb, indicating that the oxidation process is a possible mechanism of intergranular cracking initiation.

J061054 量子分子動力学法に基づくリチウムイオン電池の劣化プロセスシミュレーション(〔J061-05〕燃料電池・二次電池におけるナノ・マイクロ現象とマクロ性能(5):各種電池)

To clarify the degradation mechanism of lithium-ion battery cathode, we studied the degradation process of the cathode caused by a chemical reaction with ethylene carbonate (EC) via our tight-binding quantum chemical molecular dynamics (TB-QCMD) method. We simulated the dynamics of the chemical reaction of LiCoO_2(010) surface and EC. In addition, our calculated atomic bond population revealed that a Co-O bond of the LiCoO_2(010) surface weakened when an O atom bound with a C atom of EC. This weakening of the Co-O bond indicates that the O atom of the surface is easily removed from the surface and the Co atom is reduced. It is known that the structural change of the cathode after the reduction of the Co atom causes the degradation of the lithium-ion battery cathode. Thus, we suggested that the adsorption of EC resulting in the reduction of Co atoms was the initial process of the degradation of the lithium-ion battery cathode.

2410 第一原理分子動力学法とTight-Binding量子分子動力学法によるトライボケミカル反応ダイナミクスシミュレーション

2410 第一原理分子動力学法とTight-Binding量子分子動力学法によるトライボケミカル反応ダイナミクスシミュレーション

Influence of geometrical deformation and electric field on transport characteristics through carbon nanotubes

We study computationally the electronic transport properties through mechanically squashed zigzag carbon nanotubes (CNTs) under the uniform electric field perpendicular to the tube axis, based on the tight-binding molecular dynamics method for the structural analysis and the Landauer-Büttiker's formalism for the transport analysis. Our simulations show that the band gaps of the zigzag carbon nanotubes exhibit nonlinear decrease as increasing the deformation ratio in the presence of the external perpendicular electric field, in contrast to the case of zero electric field, where the band gap decreases linearly as increasing the deformation ratio. Such properties allow us to tune the sensitivity of the electromechanical response in CNT devices by applying the external electric field.

Development of Crystal Growth Simulator Based on Tight-Binding Quantum Chemical Molecular Dynamics Method and Its Application to Silicon Chemical Vapor Deposition Processes

We have developed a crystal growth simulator based on tight-binding quantum chemical molecular dynamics (TB-QCMD) method and applied it to plasma-enhanced chemical vapor deposition (PECVD) processes for silicon thin-film growth via SiH3 radicals on hydrogen-terminated Si(001). We successfully simulated the abstraction of a surface hydrogen atom by irradiated SiH3 radical and the formation of a dangling bond on the hydrogen-terminated Si(001) surface. SiH3 radical was subsequently adsorbed on this dangling bond. When these processes were repeated, the thin film grew. Thus, a detailed mechanism was successfully found for the chemical reaction and electron transfer dynamics of silicon thin film growth by PECVD.

Fate of methanol molecule sandwiched between hydrogen-terminated diamond-like carbon films by tribochemical reactions: tight-binding quantum chemical molecular dynamics study

Recently, much attention has been given to diamond-like carbon (DLC) as a solid-state lubricant, because it exhibits high resistance to wear, low friction and low abrasion. Experimentally it is reported that gas environments are very important for improving the tribological characteristics of DLC films. Recently one of the authors in the present paper, J.-M. Martin, experimentally observed that the low friction of DLC films is realized under alcohol environments. In the present paper, we aim to clarify the low-friction mechanism of the DLC films under methanol environments by using our tight-binding quantum chemical molecular dynamics method. We constructed the simulation model in which one methanol molecule is sandwiched between two hydrogen-terminated DLC films. Then, we performed sliding simulations of the DLC films. We observed the chemical reaction of the methanol molecule under sliding conditions. The methanol molecule decomposed and then OH-termination of the DLC was realized and the CH3 species was incorporated into the DLC film. We already reported that the OH-terminated DLC film is very effective to achieve good low-friction properties under high pressure conditions, compared to H-terminated DLC films. Here, we suggest that methanol environments are very effective to realize the OH-termination of DLC films which leads to the good low-friction properties.

Tribochemical Reaction Dynamics Simulation of Hydrogen on a Diamond-like Carbon Surface Based on Tight-Binding Quantum Chemical Molecular Dynamics

Diamond-like carbon (DLC) has recently attracted much attention as a solid-state lubricant, because of its resistance to wear, low friction, and low abrasion. Several factors, such as the hydrogen atoms in DLC and transfer film formation are important for improving the tribological characteristics of DLC. In this paper, we discuss the low-friction mechanism of DLC by using our tight-binding quantum chemical molecular dynamics method. The method employs a DLC film sliding simulation in order to explore the effect of hydrogen atoms on the carbon-based transfer film. The formation of C–C bonds between DLC films increases friction, while surface hydrogen atoms suppress C–C bond formation, which results in the low-friction state. Moreover, the steric effect of hydrogen molecule generation was found to remove the load from the substrate, inhibiting C–C bond formation. In addition, we determined that surface hydrogen atoms play a key role in the cleavage of C–C bonds formed during sliding of DLC films.

Magnetic coupling in dilute magnetic semiconductors: A new perspective

AbstractThe ferromagnetic coupling (FMC) among magnetic impurities in the dilute magnetic semiconductors (DMSs) ZnO and GaN is revisited within a molecular orbital (MO) approximation. In particular, based on computational results obtained by both ab initio and tight binding molecular dynamics methods, we demonstrate that the FMC appears to be the coupling between MOs consisting of MO‐parts confined in the tetrahedra centered at the cationic impurities. This new interpretation links the defect‐induced magnetism in DMSs and TMOs (transition metal oxides) to the previously proposed theories of magnetic organic salts and carbon‐based materials.

Theoretical investigation of encapsulation processes of C 60 into single-wall carbon nanotubes

The encapsulation of C 60 into single-wall carbon nanotubes is systematically studied by tight-binding molecular dynamics method with an appropriate long-range Lennard-Jones potentials. For a ( 16 , 0 ) nanotube, it is impossible to encapsulate C 60 into the tube because of a much larger energy barrier with the relaxation of the open end.

Thermal annealing of SiC nanoparticles induces SWNT nucleation: evidence for a catalyst-independent VSS mechanism

Density-functional tight-binding molecular dynamics (DFTB/MD) methods were employed to demonstrate single-walled carbon nanotube (SWNT) nucleation resulting from thermal annealing of SiC nanoparticles. SWNT nucleation in this case is preceded by a change of the SiC structure from a crystalline one, to one in which silicon and carbon are segregated. This structural transformation ultimately resulted in the formation of extended polyyne chains on the SiC nanoparticle surface. These polyyne chains subsequently coalesced, forming an extended sp(2)-hybridized carbon cap on the SiC nanoparticle. The kinetics of this process were enhanced significantly at higher temperatures (2500 K), compared to lower temperatures (1200 K) and so directly correlated to the surface premelting behavior of the nanoparticle structure. Analysis of the SiC nanoparticle Lindemann index between 1000 and 3000 K indicated that SWNT nucleation at temperatures below 2600 K occurred in the solid, or quasi-solid, phase. Thus, the traditional vapor-liquid-solid mechanism of SWNT growth does not apply in the case of SiC nanoparticles. Instead, we propose that this example of SWNT nucleation constitutes evidence of a vapor-solid-solid process. This conclusion complements our recent observations regarding SWNT nucleation on SiO(2) nanoparticles (A. J. Page, K. R. S. Chandrakumar, S. Irle and K. Morokuma, J. Am. Chem. Soc., 2011, 133, 621-628). In addition, similarities between the atomistic SWNT nucleation mechanisms on SiC and SiO(2) catalysts provide the first evidence of a catalyst-independent SWNT nucleation mechanism with respect to 'non-traditional' SWNT catalyst species.

Pressure-induced structural phase transition of small-diameter carbon nanotubes

We study pressure-induced structural phase transition of carbon nanotube solids using the constant-pressure tight-binding molecular-dynamics method. We find that the bundle with armchair nanotube transforms into a new phase consisting of so-called zigzag graphene nanoribbons and sp 3 carbon atoms by applying the pressure of 10 GPa. Under higher pressure of 30 GPa, anisotropic sp 3 phases possessing high hardness comparable to cubic diamond are obtained from armchair nanotube solid, whereas amorphous diamond phases are obtained from the bundle with chiral nanotubes. Using the density-functional theory, we also study electronic structure of a new sp 3 phase obtained in the molecular-dynamics simulation.