Quantum Chemical Molecular Dynamics Method Research Articles

Orientation of Laurdan in Phospholipid Bilayers Influences Its Fluorescence: Quantum Mechanics and Classical Molecular Dynamics Study.

Fluidity of lipid membranes is known to play an important role in the functioning of living organisms. The fluorescent probe Laurdan embedded in a lipid membrane is typically used to assess the fluidity state of lipid bilayers by utilizing the sensitivity of Laurdan emission to the properties of its lipid environment. In particular, Laurdan fluorescence is sensitive to gel vs liquid–crystalline phases of lipids, which is demonstrated in different emission of the dye in these two phases. Still, the exact mechanism of the environment effects on Laurdan emission is not understood. Herein, we utilize dipalmitoylphosphatidylcholine (DPPC) and dioleoylphosphatidylcholine (DOPC) lipid bilayers, which at room temperature represent gel and liquid–crystalline phases, respectively. We simulate absorption and emission spectra of Laurdan in both DOPC and DPPC bilayers with quantum chemical and classical molecular dynamics methods. We demonstrate that Laurdan is incorporated in heterogeneous fashion in both DOPC and DPPC bilayers, and that its fluorescence depends on the details of this embedding.

Thermal Stability and Flexibility of Hydrogen Terminated Phosphorene Nanoflakes

Phosphorene nanoflakes are emerging candidates of black phosphorus nanostructures due to their unique, tunable properties like size-dependent band gap, effective synthesis methods, and possible widespread applications. While it is apparent that the stability and flexibility of phosphorene nanoflakes are crucial for several applications, there is little information available. In this article, we investigate the stability and flexibility of phosphorene nanoflakes in the gas and liquid phases using quantum chemical and ab initio molecular dynamics methods as well as the External Force is Explicitely Included (EFEI) method. Our results show that phosphorene nanoflakes are energetically more stable than white phosphorus, while in the gas phase free energies show the opposite trend. However, in liquid phase, the formation of phosphorene nanoflakes is preferred over white phosphorus. Molecular dynamics simulations show that phosphorene nanoflakes have flat shapes and are stable, while their flexibility depends o...

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.

(Invited) Artificial Intelligence Integrated Multiscale, Multiphysics Computational Chemistry Methods Based on Ultra-Accelerated Quantum Chemical Molecular Dynamics for Luminescence Materials

Our previous research work using different computational methods for the design of phosphor/luminescence materials, show our multiscale, multiphysics computational chemistry methods based on the ultra-accelerated quantum chemical molecular dynamics(UA-QCMD) method which can perform quantum chemical molecular dynamics calculation around 10,000,000 times faster than the conventional first-principles molecular dynamics. In the present study we have demonstrated that the UA-QCMD has high accuracy in comparison with density functional theory(DFT) method and thermodynamic data for a variety of compounds including metal, metal oxides, nitrides, and sulfides which are important as luminescence materials and devices. A variety of applications of the artificial intelligence integrated multiscale, multiphysics computational chemistry methods based on the UA-QCMD are also presented in the meeting.

Synergistic inhibition effect of 5-aminotetrazole and 4,6-dihydroxypyrimidine on the corrosion of cold rolled steel in H3PO4 solution

The synergistic inhibition effect of 5-aminotetrazole (AT) and 4,6-dihydroxypyrimidine (DHP) on the corrosion of cold rolled steel (CRS) in H3PO4 solution was studied by weight loss, potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), quantum chemical calculation and molecular dynamics (MD) methods. The results show that AT exhibits a moderate inhibitive effect, and DHP has a poor effect. However, incorporation AT with DHP significantly improves the inhibitive performance, and yields synergism. The adsorption of AT in the absence and presence of DHP obeys Langmuir adsorption isotherm. AT/DHP mixture acts as a mixed-type inhibitor. The combined AT and DHP molecules co-adsorb on the Fe (001) surface in the nearly flat manner, and the adsorption energy is larger than individual AT or DHP.

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.

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...

Chemical Reaction Mechanism of Polytetrafluoroethylene on Aluminum Surface under Friction Condition

To develop a novel shearing resin material, it is necessary to understand the mechanism of friction-induced chemistry during the friction process. For this purpose, the chemical reaction of the polytetrafluoroethylene (PTFE) resin on an aluminum surface during friction was first focused on and investigated by a quantum chemical molecular dynamics method. From our simulation, an aluminum atom on a native oxide of aluminum surface led to a tribochemical reaction, which included defluorination of PTFE and aluminum fluoride formation. It was inferred that the aluminum surface acted as a catalytic Lewis acid in which the fluorine atom was removed from the PTFE polymer chain. The tribological performance of the investigated system was reduced by the forming of aluminum fluoride since a self-lubrication by PTFE was inhibited by an interfacial electrostatic repulsion. On the basis of our study, it was suggested that the key to increase tribological performance was a chemical reaction between reactive defluorinate...

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.

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

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.

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.

量子分子動力学法に基づくマルチフィジックスシミュレータの開発と低炭素化機械システムへの応用

量子分子動力学法に基づくマルチフィジックスシミュレータの開発と低炭素化機械システムへの応用

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.

Computational and electrochemical studies on the inhibition of corrosion of mild steel by l-Cysteine and its derivatives

This article presents the investigation of l-Cysteine (CYS) and its derivatives including N-Acetyl-l-Cysteine (NACYS), N-Acetyl-S-Benzyl-l-Cysteine (NASBCYS), and N-Acetyl-S-Hexyl-l-Cysteine (NASHCYS) as green chemical corrosion inhibitors for mild steel in 1 M HCl solutions. Weight loss method and Tafel polarization measurement were performed to determine the corrosion parameters and inhibition efficiencies. Experimental results showed that these compounds suppressed both anodic and cathodic reactions, and the inhibition efficiency of the four inhibitors followed the order NASBCYS > NASHCYS > NACYS > CYS. In order to further study the corrosion mechanisms, quantum chemical calculation and molecular dynamics method were applied. The relationships between quantum chemical parameters and corrosion inhibition efficiency were discussed. Molecular dynamics method was used to simulate the adsorption behavior of each inhibitor in solvent. The results showed that these four inhibitors can adsorb on mild steel surface by donor acceptor interactions between lone-pair electrons of heteroatoms/π-electrons of aromatic ring and vacant d-orbital of iron.

QM/MD Simulation of SWNT Nucleation on Transition-Metal Carbide Nanoparticles

The mechanism and kinetics of single-walled carbon nanotube (SWNT) nucleation from Fe- and Ni-carbide nanoparticle precursors have been investigated using quantum chemical molecular dynamics (QM/MD) methods. The dependence of the nucleation mechanism and its kinetics on environmental factors, including temperature and metal-carbide carbon concentration, has also been elucidated. It was observed that SWNT nucleation occurred via three distinct stages, viz. the precipitation of the carbon from the metal-carbide, the formation of a "surface/subsurface" carbide intermediate species, and finally the formation of a nascent sp(2)-hybidrized carbon structure supported by the metal catalyst. The SWNT cap nucleation mechanism itself was unaffected by carbon concentration and/or temperature. However, the kinetics of SWNT nucleation exhibited distinct dependences on these same factors. In particular, SWNT nucleation from Ni(x)C(y) nanoparticles proceeded more favorably compared to nucleation from Fe(x)C(y) nanoparticles. Although SWNT nucleation from Fe(x)C(y) and Ni(x)C(y) nanoparticle precursors occurred via an identical route, the ultimate outcomes of these processes also differed substantially. Explicitly, the Ni(x)-supported sp(2)-hybridized carbon structures tended to encapsulate the catalyst particle itself, whereas the Fe(x)-supported structures tended to form isolated SWNT cap structures on the catalyst surface. These differences in SWNT nucleation kinetics were attributed directly to the relative strengths of the metal-carbon interaction, which also dictates the precipitation of carbon from the nanoparticle bulk and the longevity of the resultant surface/subsurface carbide species. The stability of the surface/subsurface carbide was also influenced by the phase of the nanoparticle itself. The observations agree well with experimentally available data for SWNT growth on iron and nickel catalyst particles.

Adsorption and dissociation of molecular hydrogen on Pt/CeO 2 catalyst in the hydrogen spillover process: A quantum chemical molecular dynamics study

Ultra accelerated quantum chemical molecular dynamics method (UA-QCMD) was used to study the dynamics of the hydrogen spillover process on Pt/CeO 2 catalyst surface for the first time. The direct observation of dissociative adsorption of hydrogen on Pt/CeO 2 catalyst surface as well as the diffusion of dissociative hydrogen from the Pt/CeO 2 catalyst surface was simulated. The diffusion of the hydrogen atom in the gas phase explains the high reactivity observed in the hydrogen spillover process. Chemical changes, change of adsorption states and structural changes were investigated. It was observed that parallel adsorption of hydrogen facilitates the dissociative adsorption leading to hydrogen desorption. Impact with perpendicular adsorption of hydrogen causes the molecular adsorption on the surface, which decelerates the hydrogen spillover. The present study also indicates that the CeO 2 support has strong interaction with Pt catalyst, which may cause an increase in Pt activity as well as enhancement of the metal catalyst dispersions and hence increasing the rate of hydrogen spillover reaction.

Comparison of single-walled carbon nanotube growth from Fe and Ni nanoparticles using quantum chemical molecular dynamics methods

Metal-catalyzed SWCNT growth has been modeled using quantum chemical molecular dynamics (QM/MD) in conjunction with feeding of carbon atoms to C 40–Fe 55 and C 40–Ni 55 model complexes at 1500 K. The rate of Fe 55-catalyzed SWCNT growth determined in this work was 19% slower than the Fe 38-catalyzed growth rate. Conversely, Ni 55-catalyzed SWCNT growth exhibited a growth rate 69% larger than of Fe 55-catalyzed SWCNT growth, a fact consistent with excellent performance of Ni in laser evaporation and carbon-arc experiments. Ni 55-catalyzed growth was preceded by the formation of extended polyyne chains at the base of the SWCNT, and so differed fundamentally from Fe 55-catalyzed growth. These polyyne chains usually persisted for 10–30 ps. Subsequent polyyne ring condensation resulted in carbon polygon addition at the SWCNT base. The relative stabilities of the C n carbon cluster moieties on the Fe 55 and Ni 55 surfaces were consistent with the relative strengths of the Fe–C, Ni–C and C–C interactions. The presence of smaller carbon moieties on the Fe 55 surface led to the dissemination of surface iron atoms, and subsequent diffusion of short C n units through the subsurface region of the catalyst particle. Conversely, the Ni 55 catalyst particle was observed to be more stable, remaining intact to a greater extent.