Local Bond Breaking Research Articles

Computational indentation in highly cross-linked polymer networks

Indentation is a common experimental technique to study the mechanics of polymeric materials. The main advantage of using indentation is this provides a direct correlation between the microstructure and the small-scale mechanical response, which is otherwise difficult within the standard tensile testing. The majority of studies have investigated hydrogels, microgels, elastomers, and even soft biomaterials. However, a less investigated system is the indentation in highly cross-linked polymer (HCP) networks, where the complex network structure plays a key role in dictating their physical properties. In this work, we investigate the structure-property relationship in HCP networks using the computational indentation of a generic model. We establish a correlation between the local bond breaking, network rearrangement, and small-scale mechanics. The results are compared with the elastic-plastic deformation model. HCPs harden upon indentation.

Atomistic simulation of the uniaxial compression of black phosphorene nanotubes

We study through molecular dynamics finite element method with Stillinger-Weber potential the uniaxial compression of (0, 24) armchair and (31, 0) zigzag black phosphorene nanotubes with approximately equal diameters. Young's modulus, critical stress and critical strain are estimated with various tube lengths. It is found that under uniaxial compression the (0, 24) armchair black phosphorene nanotube buckles, whereas the failure of the (31, 0) zigzag one is caused by local bond breaking near the boundary.

Application of the Real-Time Time-Dependent Density Functional Theory to Excited-State Dynamics of Molecules and 2D Materials

We review our recent developments in the ab initio simulation of excited-state dynamics within the framework of time-dependent density functional theory (TDDFT). Our targets range from molecules to 2D materials, although the methods are general and can be applied to any other finite and periodic systems. We discuss examples of excited-state dynamics obtained by real-time TDDFT coupled with molecular dynamics (MD) and the Ehrenfest approximation, including photoisomerization in molecules, photoenhancement of the weak interatomic attraction of noble gas atoms, photoenhancement of the weak interlayer interaction of 2D materials, pulse-laser-induced local bond breaking of adsorbed atoms on 2D sheets, modulation of UV light intensity by graphene nanoribbons at terahertz frequencies, and collision of high-speed ions with the 2D material to simulate the images taken by He ion microscopy. We illustrate how the real-time TDDFT approach is useful for predicting and understanding non-equilibrium dynamics in condense...

High-Density Hydrogen Storage in a 2D-Matrix from Graphene Nanoblisters: A Prospective Nanomaterial for Environmentally Friendly Technologies

In this paper, the atomic structure and mechanical stability of a new structural graphene modification—a 2D matrix of nanoscale cells in the form of a few-layer graphene substrate and nanoblister of a graphene monolayer—were studied for the first time. It is shown that such matrices are mechanically stable and are promising for environmentally friendly technologies. The calculated local atomic stress fields demonstrate that the atomic framework is not destroyed, even in the presence of defects in the atomic network of graphene nanoblister (Stone-Wales defect, double vacancies defect, ad-dimmer defect, and their combination). However, it was established that the presence of one or more SW defects leads to the appearance of critical stresses. These critical stresses can induce local bond breaking in the atomic network with an increase in temperature or external pressure. It was found that graphene nanoblister can store molecular hydrogen with a maximum density of 6.6 wt % for 1158 m2/g at 77 K under normal pressure.

Compressive buckling of black phosphorene nanotubes: an atomistic study

We investigate through molecular dynamics finite element method with Stillinger-Weber potential the uniaxial compression of armchair and zigzag black phosphorene nanotubes. We focus especially on the effects of the tube’s diameter with fixed length-diameter ratio, effects of the tube’s length for a pair of armchair and zigzag tubes of equal diameters, and effects of the tube’s diameter with fixed lengths. Their Young’s modulus, critical compressive stress and critical compressive strain are studied and discussed for these 3 case studies. Compressive buckling was clearly observed in the armchair nanotubes. Local bond breaking near the boundary occurred in the zigzag ones under compression.

Reactive molecular dynamics simulations of the mechanical properties of various phosphorene allotropes

Although various phosphorene allotropes have been theoretically predicted to be stable at 0 K, the mechanical properties and fracture mechanism at room temperature remain unclear for many of them. We investigate through reactive molecular dynamics simulations at room temperature the mechanical properties of phosphorene allotropes including: five sheets with hexagonal structures (β-, γ-, δ-, θ-, and α-phosphorene), one sheet with 4-8 membered rings (4-8-P), and two sheets with 5-7 membered rings. High, moderate and slight anisotropies in their mechanical properties are observed, depending on their crystal structures. Their Young’s moduli and tensile strength are approximately in the range from 7.3% through 25%, and from 8.6% through 22% of those of graphene, respectively. At the early stage of fracture, eye-shaped cracks are formed by local bond breaking and perpendicular to the tensile direction in hexagonal and 4-8-P sheets. Complete fractures take place with straight cracks in these hexagonal sheets under tension along the zigzag direction and under tension along the square edge direction in the 4-8-P sheet. Crack meandering and branching are observed during the tension of α-, β-, and γ-phosphorene along the armchair direction; and along the square diagonal direction in the 4-8-P sheet. Under uniaxial tension of two phosphorene sheets with 5-7 atom rings, 12 and 10 membered rings are formed by merging two neighbor heptagons, and a heptagon and its neighbor pentagon, respectively. These 12 and 10 membered rings coalesce subsequently, causing the failure of these two sheets. The results are of great importance in the design of these novel phosphorene allotropes.

Bending of boron nitride nanotubes: An atomistic study

ABSTRACTThe present work investigates through molecular dynamics finite element method with Tersoff potential the buckling behavior of boron nitride (BN) armchair and zigzag nanotubes under bending. The critical bending buckling angle, moment, and curvature are studied and discussed with respect to the tube length-diameter ratios from 10 to 50. Effects of a Stone-Wales defect in the middle tube on the bending behavior are also examined. The bending buckling mechanism is governed by the local instabilities and bond breaking in the compressive zone of the tubes. Results are useful for the design of nanocomposites and nano-devices with BN nanotubes.

Pressure-induced silica quartz amorphization studied by iterative stochastic surface walking reaction sampling.

The crystal to amorphous transformation is a common phenomenon in Nature and has important impacts on material properties. Our current knowledge on such complex solid transformation processes is, however, limited because of their slow kinetics and the lack of long-range ordering in amorphous structures. To reveal the kinetics in the amorphization of solids, this work, by developing iterative reaction sampling based on the stochastic surface walking global optimization method, investigates the well-known crystal to amorphous transformation of silica (SiO2) under external pressures, the mechanism of which has long been debated for its non-equilibrium, pressure-sensitive kinetics and complex product components. Here we report for the first time the global potential energy surface (PES) and the lowest energy pathways for α-quartz amorphization from first principles. We show that the pressurization at 15 GPa, the reaction condition, can lift the quartz phase energetically close to the amorphous zone, which thermodynamically initializes the amorphization. More importantly, the large flexibility of Si cation coordination (including four, five and six coordination) results in many kinetically competing routes to more stable dense forms, including the known MI, stishovite, newly-identified MII and TI phases. All these pathways have high barriers due to the local Si-O bond breaking and are mediated by amorphous structures with five-fold Si. This causes simultaneous crystal-to-crystal and crystal-to-amorphous transitions. The high barrier and the reconstructive nature of the phase transition are the key kinetics origin for silica amorphization under pressures.

Fracture of perfect and defective graphene at the nanometer scale: Is graphene the strongest material?

The basic relationships between the linear elastic and nonlinear fracture properties given by a local bond-breaking model and Griffith's criterion are used to characterize the two-dimensional (2D) mechanical properties of an intrinsic and a defective graphene, respectively. The analytical 2D models describe the localized bond-breaking in perfect monolayers and the failure of defective graphene by the global energy balance concept. From the experimental data, density functional theory calculations, molecular dynamics simulations, and continuum 2D models, a consistent set of 2D mechanical properties consisting of Young's modulus, fracture strength, fracture toughness, line (edge) energy, and critical strain energy release rate can be obtained. The critical fracture stress shows a linear dependence on the square root of the effective defect length from the subnanometer to the micrometer scale. The lower limit of fracture toughness and strain energy release rate is essentially independent of the defect size for vacancies, slits, and pre-cracks in the nanometer range. In the subnanometer range, the direct bond breaking and Griffith models deliver a consistent description of mode I fracture by a uniaxial tension. The promising results suggest an extension of the continuum models to other fracture modes such as the failure by shear load.

Modeling PGM-Free Active Site Structures: Activity and Durability

Two of the largest hurdles facing platinum group metal-free (PGM-free) oxygen reduction reaction (ORR) electrocatalysts for polymer electrolyte fuel cell (PEFC) cathodes are the need for improved activity and increased durability. Quantum chemical modeling (in particular, density functional theory, DFT, coupled with thermochemical models) can be utilized to better understand both activity and durability of these materials at the atomic scale. In this work, we apply quantum chemical modeling approaches to study the activity of N-coordinated Mn, Fe, Co, and Ni containing zig-zag edge sites. Both Metal-N4 as well as Metal2-N5edge structures are considered. Spontaneous *OH ligand modification in which the *OH intermediate is persistently bound to metal centers above a relatively low potential is found to be applicable to both structures but only for certain metal species. Experimentally determined trends in activity as a function of metal species fit well with theoretical predictions based on the spontaneous *OH ligand modification hypothesis. In addition, strain-modification of structures and resultant shifts in activity are explored. It is found that, due to anisotropic changes in binding energies with strain, this approach can lead to minor improvements of the computationally obtained ORR activity descriptor. With regards to the durability hurdle of PGM-free electrocatalysts, a structurally sensitive and computationally accessible descriptor is explored. First-principles molecular dynamics derived knock-on displacement threshold energy may serve as a high-throughput, corrosion-mechanism-agnostic descriptor of active site structure durability that does take into account the kinetics of local bond breaking. Further comparisons between computational predictions and experimental findings are required to ascertain the applicability of this simplified durability descriptor for non-PGM active site structures. Current progress and future directions will be discussed.

Gels under stress: The origins of delayed collapse

Attractive colloidal particles can form a disordered elastic solid or gel when quenched into a two-phase region, if the volume fraction is sufficiently large. When the interactions are comparable to thermal energies the stress-bearing network within the gel restructures over time as individual particle bonds break and reform. Typically, under gravity such weak gels show a prolonged period of either no or very slow settling, followed by a sudden and rapid collapse – a phenomenon known as delayed collapse. The link between local bond breaking events and the macroscopic process of delayed collapse is not well understood. Here we summarize the main features of delayed collapse and discuss the microscopic processes which cause it. We present a plausible model which connects the kinetics of bond breaking to gel collapse and test the model by exploring the effect of an applied external force on the stability of a gel.

Microscopic Picture of Cooperative Processes in Restructuring Gel Networks

Colloidal gel networks are disordered elastic solids that can form even in extremely dilute particle suspensions. With interaction strengths comparable to the thermal energy, their stress-bearing network can locally restructure via breaking and reforming interparticle bonds. This allows for yielding, self-healing, and adaptive mechanics under deformation. Designing such features requires controlling stress transmission through the complex structure of the gel and this is challenging because the link between local restructuring and overall response of the network is still missing. Here, we use a space resolved analysis of dynamical processes and numerical simulations of a model gel to gain insight into this link. We show that consequences of local bond breaking propagate along the gel network over distances larger than the average mesh size. This provides the missing microscopic explanation for why nonlocal constitutive relations are necessary to rationalize the nontrivial mechanical response of colloidal gels.

Molecular Dynamic Simulations of the Collision between Copper Ions, SF6 Molecules and a Polyethylene Surface: A Study of Decomposition Products and an Evaluation of the Self‐Diffusion Coefficients

AbstractMolecular dynamics simulations are used to study the interaction between incident copper ions, SF6 molecules and a polyethylene surface. Average particle velocities from 15 to 21 km · s−1 are tested in steps of 2 km · s−1. The damage to the polyethylene crystal is reviewed in terms of material heating, local molecular disorder and bond breaking accompanied by particle emissions. Two types of emitted particles are distinguished to explain the degradation of the polymer. The first corresponds to free radicals with molecular formula CnH2n+1 and the second takes into account all other possible molecules, especially hydrogen atoms and carbonaceous molecules. The formation of CuF molecules during the interaction is shown and this result is discussed on the basis of other theoretical results concerning the calculations of plasma composition. The self‐diffusion coefficients of the emitted atoms are calculated by using two different methods: the alternative Green‐Kubo expression, based on the integrated velocity autocorrelation function, and the Einstein expression, based on the mean‐square displacement autocorrelation function.magnified image

Network Dynamics and Species Exchange Processes in Aluminophosphate Glasses: An in situ High Temperature Magic Angle Spinning NMR View

In this contribution, we present an in situ high temperature 27Al and 31P magic angle spinning (MAS) NMR study of binary and ternary phosphate glasses at temperatures above the glass transition temperature TG. For binary phosphate glasses, xK2O-(1 - x)P2O5 and ternary aluminophosphate glasses 30K2O-xAl2O3-(70 -x)P2O5 with 7 < x < 15 dynamic exchange processes between the various phosphate species (and aluminatespecies) present in the glasses could be identified in the temperature range between TG and the maximum achievable temperature Tmax of our high temperature MAS NMR setup, TG < T < Tmax. This observation indicates rapid P-O-P and P-O-Al bond formation and bond breaking in the (alumino)phosphate glasses.From a modeling of the temperature dependence of these exchange processes, the activation energy EA for the corresponding process could be determined. These local bond breaking and making processes are ultimately linked to the macroscopic viscous flow and may indeed form the basic microscopic local step of viscous flow.

On the construction of hybrid potential energy functions for the treatment of local anharmonicity in molecular vibrations†

This paper reports a process by which a local, accurate, and intrinsically anharmonic potential function may be joined with a larger harmonic oscillator representation of the remainder of a molecule to build a hybrid potential energy function. Such functions find use in chemical dynamics in which there is local anharmonic bond breaking and forming, but the remainder of the molecular system behaves essentially as a set of harmonic oscillators. In constructing such a potential, for malonaldehyde as an example, it is assumed that the harmonic-limit potential can be found for the system using a given data set of the Cartesian forces and force constants derived with the use of a computational quantum chemistry program. The Pulay transformation is used to establish a set of internal coordinate forces and force constants consistent with the local model potential function. The success (i.e. faithfulness) of the construction of the hybrid potential strongly depends on the choice of internal coordinates. The Pulay transformation together with the vibrational projection analysis of Grafton and Wheeler allows one to see the strengths or weaknesses of particular choices of internal coordinates. †Approved for Public Release. Distribution Unlimited. Dedicated to Péter Pulay on the occasion of the celebration of his scientific achievements: Budapest, 24 May–3 June 2007.

Scanning Probe Lithography of Self-Assembled N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane Monolayers on SiO2 Surface

A new approach is introduced for the selective deposition of colloidal gold nanoparticles (AuNPs) onto the surface of unpatterned self-assembled monolayers (SAMs). The patterning of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AEAPTMS) SAMs is realized by local field-induced bond breaking using scanning probe lithography (SPL) on a thin SiO2 surface. Different tip/sample biases were investigated to determine the bond breaking efficiency of AEAPTMS SAMs. It was found that bond breaking efficiency was limited by tunneling current through the thin SiO2 film so that both the tip bias and the tip scanning speed play important roles. AuNPs with negatively charged citrate surfaces were selectively anchored onto the unpatterned area via electrostatic force between AEAPTMS SAMs and AuNPs. Single-digit numbers of AuNPs anchored onto unpatterned AEAPTMS SAMs were demonstrated.

Local bond breaking on semiconductor surfaces induced by laser beam

Local bond breaking on semiconductor surfaces induced by laser beam

Local bond breaking via STM-induced excitations: the role of temperature

We discuss the influence of temperature on local bond breaking through multiple vibrational excitations induced by inelastic tunneling in the STM. We focus on hydrogen desorption from the HSi(111) and HSi(100) systems, but the results are general. The substrate temperature affects the desorption yield in two important ways: first, lowering the temperature increases the HSi vibrational energy relaxation time, resulting in a higher effective adsorbate temperature and an increased desorption yield. Second, lowering the substrate temperature decreases the dephasing rate of the HSi modes (manifested by a decrease of the infrared absorption linewidth), which then reduces the rate of incoherent (Förster) vibrational energy transfer away from the Stark-shifted HSi mode under the tip. This increases the localization of the vibrational energy and enhances the probability for multiple vibrational excitation and desorption. Finally, we discuss the possible implications of our findings on the mechanism of MOS device degradation by hot electrons.

Silicate species exchange, viscosity, and crystallization in a low-silica melt; in situ high-temperature MAS NMR spectroscopy

High-resolution, magic-angle-spinning (MAS), 29Si NMR spectra have been obtained for a sodium silicate glass and liquid (60 mol% Si02) to temperatures over 600 °C. Crystallization was monitored in situ, and motional averaging of the peaks for distinct silicate anionic species (Q2 and Q3 species) in the liquid was observed. Modeling of the spectra yielded species exchange frequencies. The inverses of these exchange frequencies are slightly longer than shear relaxation times calculated from bulk viscosity using the Maxwell relation; viscosities calculated from the NMR data using an Eyring model are close to, but slightly less than, measured viscosities. These results are similar to those obtained for Q3_ Q4 exchange in K2Si409 liquid, indicating that local Si-O bond breaking occurs on a time scale consistent with it being the primary control on viscous flow even in lower silica liq uids.

The effects of the electric field in the STM on excitation localization. Implications for local bond breaking

We discuss the effects of the electric field present in the STM on the frequency and spatial localization of (vibrational) excitations induced by inelastic electron tunneling. We show that, depending on its direction and size, the field can produce either a weakening or a strengthening of chemical bonds. The resulting vibrational frequency shifts could lead to the localization of the excitation under the STM tip, reducing energy transfer rates to surrounding sites by several orders of magnitude. This localization gives the STM the ability to locally activate surface dynamical processes, and induce atomic-scale modification of materials.