Atomistic Study Research Articles

Atomistic study of two-dimensional discrete breathers in hcp titanium

Atomistic study of two-dimensional discrete breathers in hcp titanium

Insights on the cellulose pretreatment at room temperature by choline-chloride-based deep eutectic solvents: an atomistic study

The pretreatment or disruption of a cellulose I \(\beta \) crystallite by four deep eutectic solvents (DES): choline-chloride ethylene glycol, choline-chloride oxalic acid, choline-chloride urea, and choline-chloride levulinic acid, was described from the atomistic interactions observed in molecular dynamics and ab-initio (NCI-AIM) studies. The cellulose I \(\beta \) disruption was studied considering plausible correlations between the Kamlet-Taft (α and β) solvent parameters, and a series of thermodynamic, structural, and energetic properties. It was found that the Kamlet-Taft parameters correlated with the thermodynamic properties of the DES, as well as their variations after the addition of the cellulose crystallite. Structural analysis revealed that the weaker the interactions within the molecules of the solvent, the stronger the interactions between the hydroxyl group from cellulose with the chloride anion and with the hydrogen bond donor. Further analysis indicated that the \(R-CO-R\) moieties in the hydrogen bond donor within the solvent, displayed best interplaying with the cellulose. The hydrogen bond occupancies within the cellulose crystallite, evidenced that the main \(O6-H6\cdots O2/O3\) and \(O2-H2\cdots O6\) interchain hydrogen bonds in the glucan located at the edge of the solute, were replaced by weak \(O6-H6\cdots O4\) hydrogen bonds in all solvents. This effect was related to the \({O/Cl}_{DES}\cdots H-donor\) and \({O}_{DES}-{H}_{DES}\cdots acceptor\) HBs between cellulose and DES molecules, and it was confirmed by the non-covalent interactions obtained through DFT calculations. The energetic interactions and the atomistic degree of disruption of the cellulose crystallite, were not completely described by the Kamlet-Taft β or α parameters when considered separately. Surprisingly, by using the net basicity (\(\beta -\alpha \)) definition such correlations were improved, suggesting that both parameters must be considered together to develop new, green, and sustainable solvents for cellulose pretreatment process.Graphic abstract

Atomistic study of coreshell and functionally graded nanospheres under compressive loading

Functionally grading and coreshell are two interesting engineering modifications to nanomaterials structure for tailored applications in electronics, energy conversion devices and so on. Understanding the mechanical response of these nanostructures are of particular importance to ensure the reliability of these devices under service conditions. In this paper, functionally graded and coreshell Silver–Gold (Ag–Au) nanospheres are studied under compression load using molecular dynamics simulation. The fracture and deformation mechanism along with the incipient plasticity through dislocation nucleation and propagation has been studied for both functionally graded and coreshell structure for different percentages of Ag and Au of the nanosphere. Our results indicate that plastic deformation is dictated by the partial dislocation nucleation and propagation from the contact surface which varies as the alloying percentages of the coreshell and functionally graded nanosphere. An inverse size effect is observed for the mechanical properties which also affects the deformation mechanism of the nanosphere by forming stacking fault tetrahedra for both the nanostructures. For a range of Ag percentages in Au, the coreshell nanospheres showed higher compressive strength compared to functionally graded nanospheres. As coreshell and functionally grading are two promising nanoscale materials design, current work will inspire developing new metal nanospheres to harness the materials potential for different engineering applications.

Perturbed Angular Correlation Technique at ISOLDE/CERN Applied for Studies of Hydrogenated Titanium Dioxide (TiO2): Observation of Cd-H Pairs

Profound understanding of the local electronic and defect structure in semiconductors always plays a vital role in the further developing of applications of such materials. In the present work an investigation of the electronic structure in hydrogenated TiO2 (rutile) thin films is conducted by virtue of Time-Differential γ-γ Perturbed Angular Correlation spectroscopy (TDPAC or PAC) with 111mCd/Cd isotope, produced and implanted at ISOLDE/CERN. The measurements were performed at 581 K as a function of the temperature of the samples during hydrogenation. Despite the fact, that rutile single crystals usually show the presence of two local environments, when are studies with Cd/In isotopes, the current pristine thin films sample had a single electric field gradient. Upon various degrees of hydrogenation, Cd probe atoms showed underwent alterations, resulting in up to 3 different local surroundings, generally with high electric field gradients. Broad EFG distributions are likely due to randomly distributed point defects in the neighbourhood of Cd acceptors. Observed results suggest that hydrogenations performed at RT and 423 K are not able to promote unique defect configurations, while in the range of 473-573 K the formation of such configurations is observed. Therefore, one may assume that the formation of Cd-defect complexes (Cd-H pairs) is temperature enhanced. At higher levels of hydrogenation (663 K), the samples become partly amorphous that further hinders any atomistic studies with strong damped PAC spectra. Cd-H complexes seem to be stable up to annealing up to 581 K annealing. The obtained results give a deep insight into complex hydrogen defects, their interactions and bond formations with Cd acceptor.

Atomistic analysis of Auger recombination in c -plane (In,Ga)N/GaN quantum wells: Temperature-dependent competition between radiative and nonradiative recombination

We present an atomistic theoretical study of the temperature dependence of the competition between Auger and radiative recombination in $c$-plane (In,Ga)N/GaN quantum wells with indium (In) contents of 10%, 15%, and 25%. The model accounts for random alloy fluctuations and the connected fluctuations in strain and built-in field. Our investigations reveal that the total Auger recombination rate exhibits a weak temperature dependence; at a temperature of 300 K and a carrier density of ${n}_{3D}=3.8\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, we find total Auger coefficients in the range of $\ensuremath{\approx}6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}30}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{6}/\mathrm{s}$ (10% In) to $\ensuremath{\approx}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}31}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{6}/\mathrm{s}$ (25% In), thus large enough to significantly impact the efficiency in (In,Ga)N systems. Our calculations show that the hole-hole-electron Auger rate dominates the total rate for the three In contents studied; however, the relative difference between the hole-hole-electron and electron-electron-hole contributions decreases as the In content is increased to 25%. Our studies provide further insight into the origin of the ``thermal droop'' (i.e., the decrease in internal quantum efficiency with increasing temperature at a fixed carrier density) in (In,Ga)N-based light-emitting diodes. We find that the ratio of radiative to nonradiative (Auger) recombination increases in the temperature range relevant to the thermal droop ($\ensuremath{\ge}300$ K), suggesting that the competition between these processes is not driving this droop effect in $c$-plane (In,Ga)N/GaN quantum wells. This finding is in line with recent experimental studies.

An atomistic study of self-accommodation martensite morphologies and microstructure evolution during forward and reverse martensitic transformations in single crystal and bicrystal NiTi alloys

The forward and reverse martensitic transformations in NiTi alloys with different crystallography conditions, in terms of [100]-oriented and [110]-oriented single crystal models as well as a bicrystal model possessing twist grain boundary, are studied deeply using molecular dynamics simulations. An atomic tracing method is proposed to identify the specific numbers of B19′ martensite variants, hence the self-accommodation martensite morphologies and microstructure evolution during cooling and heating are clearly demonstrated at the atomic scale. The triangular self-accommodation morphology consisted of three correspondence variants (CVs) possessing {1¯1¯1}M type I twin relationships is formed in the NiTi single crystals at the beginning of the martensitic transformation, and stays settled till the end of the transformation. Whereas three types of self-accommodation morphologies including triangular, “herring-bone” and mixed morphologies are formed in the bicrystal, due to the geometrical constraint induced by the grain boundary. The mixed self-accommodation morphology is unstable, which occurs briefly during martensitic transformation and thus evolves into “herring-bone” morphology ultimately. During reverse transformation, B2 austenite phase nucleates preferentially at the triple-junctions of the triangular self-accommodation morphology in NiTi single crystals, and propagates along the junction plane rather than the (001)M compound twin boundaries in the bicrystal alloy.

Transport in Lithium Garnet Oxides as Revealed by Atomistic Simulations

Lithium garnet oxides are a family of fast-ion conductors with appreciable lithium ionic conductivity in the solid state, making them promising candidates as electrolytes for all-solid-state batteries. In their structures, lithium is partially (along with vacancy) distributed among more than one crystallographically distinct sites, just as with other fast-ion conductors. This disorder has a great influence on lithium's transport properties such as diffusivity and ionic conductivity. We review atomistic simulation studies in conjunction with complementary experimental investigations, which offer atomic-scale visualization of and insight into lithium transport phenomena in lithium garnet oxides.

Atomistic and machine learning studies of solute segregation in metastable grain boundaries

The interaction of alloying elements with grain boundaries (GBs) influences many phenomena, such as microstructural evolution and transport. While GB solute segregation has been the subject of active research in recent years, most studies focus on ground-state GB structures, i.e., lowest energy GBs. The impact of GB metastability on solute segregation remains poorly understood. Herein, we leverage atomistic simulations to generate metastable structures for a series of [001] and [110] symmetric tilt GBs in a model Al–Mg system and quantify Mg segregation to individual sites within these boundaries. Our results show large variations in the atomic Voronoi volume due to GB metastability, which are found to influence the segregation energy. The atomistic data are then used to train a Gaussian Process machine learning model, which provides a probabilistic description of the GB segregation energy in terms of the local atomic environment. In broad terms, our approach extends existing GB segregation models by accounting for variability due to GB metastability, where the segregation energy is treated as a distribution rather than a single-valued quantity.

Large-scale atomistic studies of sliding friction in polycrystalline aluminum interfaces

We discuss large-scale non-equilibrium molecular dynamics (NEMD) simulations of ductile metal sliding comprising up to 1.8 × 109 atoms over time scales of 100 ns. The results of these simulations have identified a variety of physical mechanisms that are important in determining the steady-state frictional force for a wide range of velocities at compressed metal–metal interfaces. These include grain growth and refinement, the evolution of large plastic strains and strain rates, material mixing, and melting. These phenomena can be included in a strain, strain rate, and grain size model that gives good agreement with the NEMD simulations and can be applied to macroscopic continua.

Template-Induced Precursor Formation in Heterogeneous Nucleation: Controlling Polymorph Selection and Nucleation Efficiency.

We present an atomistic study of heterogeneous nucleation in Ni employing transition path sampling, which reveals a template precursor-mediated mechanism of crystallization. Most notably, we find that the ability of tiny templates to modify the structural features of the liquid and promote the formation of precursor regions with enhanced bond-orientational order is key to determining their nucleation efficiency and the polymorphs that crystallize. Our results reveal an intrinsic link between structural liquid heterogeneity and the nucleating ability of templates, which significantly advances our understanding toward the control of nucleation efficiency and polymorph selection.

Contribution of atomistic study to better understand water saturation effect on mechanical behavior of clayey rocks in triaxial compression

The mechanical behavior of clayey rocks is generally sensitive to water saturation degree, mainly due to the high content of swelling clay minerals. In this study, the mechanical behavior of Montmorillonite (MMT) crystal with different water content at the atomistic scale is investigated by using molecular dynamics (MD) simulation. In accordance with macroscopic laboratory tests, we consider here triaxial compression with constant mean stress respectively for the dry and hydrated MMT systems containing one, two and three water layers. While whole stress–strain responses are calculated, the emphasis is put on the key mechanisms which are related to water saturation effect on both failure and deformation properties. It is found that the mechanical behavior of a hydrated MMT system is not only dependent on the number but also the arrangement of interlayer water molecules. In particular, the water sensitivity of mechanical behavior is driven by both the clay–solution (water and Na cation mixture) and the solution–solution interactions. It seems that the MMT system with the two-layer hydrated state provides a more stable arrangement of water molecules and has a higher mechanical strength than the other two ones. The results issued from this study brings a microscopic insight onto the water sensitivity of macroscopic deformation and failure properties of clayey rocks.

Atomistic Study on the Sintering Process and the Strengthening Mechanism of Al-Graphene System.

The powder metallurgy process of the Al–graphene system is conducted by molecular dynamics (MD) simulations to investigate the role of graphene. During the sintering process, graphene is considered to reduce the pore size and metal grain size based on the volume change and atomic configuration of the Al parts in the composite. Compared with the pure Al system, the space occupied by the same number of Al atoms in the sintered composite is 15–20 nm3 smaller, and the sintered composite has about 5000 fewer arranged atoms. Because these models are carefully designed to avoid a serious deformation of graphene in the tension of sandwich-like composite models, the strengthening mechanism close to the experimental theory where graphene just serves to transfer a load can be studied dynamically. The boundary comprising of two phases is confirmed to hinder the motion of dislocations, while the crack grows along the interface beside graphene, forming a fracture surface of orderly arranged Al atoms. The results indicate that single-layer graphene (SLG) gives rise to an increase of 1.2 or 0.4 GPa in tensile strength when stretched in in-plane or normal direction, while bilayer graphene (BLG) brings a clear rise of 1.2–1.3 GPa in both directions. In both in-plane and normal stretching directions, the mechanical properties of the composite can be improved clearly by graphene giving rise to a strong boundary, new crack path, and more dense structure.

Revisiting the Rate-Dependent Mechanical Response of Typical Silicon Structures via Molecular Dynamics

Strain rate is a critical parameter in the mechanical application of nano-devices. A comparative atomistic study on both perfect monocrystalline silicon crystal and silicon nanowire was performed to investigate how the strain rate affects the mechanical response of these silicon structures. Using a rate response model, the strain rate sensitivity and the critical strain rate of two structures were given. The rate-dependent dislocation activities in the fracture process were also discussed, from which the dislocation nucleation and motion were found to play an important role in the low strain rate deformations. Finally, through the comparison of five equivalent stresses, the von Mises stress was verified as a robust yield criterion of the two silicon structures under the strain rate effects.

The study of polydimethylsiloxane nanocone distortion in the demolding process using molecular dynamics method

Experimental results show that after demolding process from the polymethylmethacrylate mold, the poly(dimethlysiloxane) (PDMS) nanocone replica yields 150%–160% larger in height as compared to the mold size dimensions before rupture. The observation from the experiment gives the direction for this study to investigate the distortion of PDMS nanocone structures in the demolding soft lithography process using molecular dynamics simulation. The aim of this study is to employ the molecular dynamics simulations and study the stress–strain curve of the nanocone structures that were subjected to uniaxial stress. Two force fields (polymer consistent forcefield and condensed-phase optimized molecular potentials for atomistic simulation studies) were utilized for the modeling. The results from the molecular dynamics simulation show that when the PDMS nanocone is subjected to tensile stress, it shows characteristics of flexible plastic pattern curve with significant yielding. This study also found that ultimate tensile stress for PDMS nanocones is within the value found in the bulk structure of 4.335–6.478 MPa.

An atomistic simulation study of rapid solidification kinetics and crystal defects in dilute Al–Cu alloys

Rapid solidification kinetics of dilute Al–Cu alloys is simulated using a quantum mechanics based bond-order potential (BOP), in free solidification conditions, to determine kinetic and thermodynamic properties of solidification, as well as point defects and chemical ordering of the solidified structures. We measure the anisotropic kinetic coefficient, anisotropic solid–liquid interface energy, as well as solute trapping kinetics in terms of partition coefficient versus velocity and solute drag coefficient. Furthermore, solid–liquid interface free energy and its anisotropy are measured in equilibrium simulations, showing reasonably good agreement with previous studies. We also verified the self-consistency of the MD simulations, by comparing the interfacial temperature vs. velocity to that predicted by the continuous growth model. These solid–liquid interface properties are important for quantitative parametrization of larger scale solidification modeling techniques such as phase field models. We also investigated the point defect content, local chemical ordering, and local crystalline structures in the rapidly solidified samples. We found clustering of solute with vacancies, whereas copper atoms repelled each other in these dilute alloy simulations. In addition to vacancies, a large number of interstitials were found. In solidification velocities approaching the complete solute trapping regime, we found that the vacancies and interstitials formed in conjunction, i.e. as Frenkel pairs. Finally, in addition to FCC, we detected BCC and HCP phases, where the latter two were accompanied by an increase in local copper content. Understanding the formation of point defects and their relationship to chemical ordering is an important step towards controlling the formation of pre-precipitates and precipitates, which are an important strengthening mechanism for aluminum–copper alloys.

Atomistic study of stability and new phase transitions of zinc selenide using FP-LMTO-PLW method

By full potential linear muffin-tin orbitals (FP-LMTO-PLW) method, we have studied the phase transitions of ZnSe under high pressures. The local density approximation (LDA) was used for the exchange and correlation energy. The most important result is the prediction of the possibility of four phase transitions from the cubic zincblende (ZnS-B3) to the B81 (NiAs) structure at 8.00 GPa, the second from B3 to the B32 (NaTi) and Bh (WC) structures at above 45 GPa for each one of them, and from B3 to B2 (CsCl) phase at around 31GPa. The first one (ZnS–NiAs) occurring at a lower pressure than the well knownZnS to NaCl transition (found here to be 11.5 GPa).

Interaction of Cyanine-D112 with Binary Lipid Mixtures: Molecular Dynamics Simulation and Differential Scanning Calorimetry Study.

This comprehensive molecular dynamics (MD) simulation and experimental study investigates the lipid bilayer interactions of dye D112 for potential photodynamic therapy (PDT) applications. PDT involves formation of a reactive oxidant species in the presence of a light sensitive molecule and light, interrupting cellular functions. D112 was developed as a photographic emulsifier, and we hypothesized that its combined cationic and lipophilic nature can render a superior photosensitizing property–crucial in various light therapies. The focus of this study is to elucidate the binding and insertion mechanisms of D112 with mixed lipid bilayers of anionic dipalmitoyl-phosphatidylserine (DPPS) and zwitterionic dipalmitoyl-phosphatidylcholine (DPPC) lipids to resemble cancer cell membranes. Our studies confirm initial electrostatic binding between the positively charged moieties of D112 and negatively charged lipid headgroups. Additionally, MD simulations combined with differential scanning calorimetry (DSC) studies confirm that D112-lipid interactions are governed by enthalpy-driven nonclassical hydrophobic effects in the membrane interior. It was further noted that despite the electrostatic preference of D112 toward the anionic lipids, D112 molecules colocalized on DPPC-rich domains after insertion. Atomistic level MD studies point toward two possible insertion mechanisms for D112: harpoon and flip. Further insights from the simulation showcase the interactions of low and high aggregates of D112 with the bilayer as the concentration of D112 increases in solution. The size of aggregates modulates the orientation and degree of insertion, providing important information for future studies on membrane permeation mechanisms.

Atomistic study on simultaneous achievement of partial crystallization and rejuvenated glassy structure in thermal process of metallic glasses

ABSTRACT The relaxation state and partial crystallization are crucial factors that affect the material properties of metallic glasses. Rejuvenation induces a less relaxed glass state and hence facilitates the control of the relaxation state. The rejuvenation and crystallization during thermal processing are associated closely with the phase changes of metallic glasses upon heating. Most nanocrystalline metallic glasses formed via conventional thermal annealing have a relaxed glassy matrix. In this molecular dynamics study, we investigate the feasibility of thermal processing to simultaneously realise both partial crystallization and rejuvenation using model alloy system. Dynamic mechanical analyses reveal relaxation behaviours and local deformation features of metallic glasses composed of a crystalline phase and a rejuvenated glass matrix. The crystalline phase increases the macroscopic shear stiffness of the composite model over a wide temperature range, whereas it does not significantly affect the internal friction of the rejuvenated glass matrix. In addition, dispersed nanocrystals suppress the development of sharp and concentrated shear localisation in the rejuvenated glass matrix. The simultaneous adjustment of the relaxation state and crystalline phase in the glass matrix is discussed from a viewpoint of the microstructure design of metallic glasses.

An atomistic study on the structural and thermodynamic properties of Al–Fe bimetallic nanoparticles during melting and solidification: The role of size and composition

Multimetallic nanoparticles (NPs) are emerging as critical nanomaterials in different fields, due to their unique physicochemical properties. In particular, bimetallic NPs provide a precise platform for controlling thermophysical properties with high sensitivity and broad versatility. In this work, the importance of size and composition in the structural and thermodynamic properties of Al–Fe bimetallic NPs are studied using molecular dynamics simulations. Relevant quantities are used to describe their melting and solidification behavior. Our results indicate a linear character of the melting temperature as a function of the inverse nanoparticle size for Al, Fe, and Al50Fe50 NPs. These values were estimated using the calorific curve and heat capacity. Furthermore, body-centered cubic, hexagonal close-packed, and local icosahedral structures are observed at room temperature in cooled Al50Fe50 NPs for sizes greater than a thousand atoms. Moreover, the cohesive energy, nanoparticle radius, and surface effect of Al50Fe50 NPs roughly reproduce the scaling law. Composition effects reveal that structural identification and atomic mobility of Al and Fe atoms strongly depend on the composition x in (Al100−xFex)N NPs. Also, it is found that the melting temperature can be tuned with the size and composition. On the other hand, the liquid-to-crystalline phase transition is extensively influenced by the Fe composition for cooled Al100−xFex NPs. Additionally, the cohesive energy and the nanoparticle radius show a quadratic dependence with composition. Finally, we have found several Al100−xFex NPs, which are stable at room temperature. The possibility to control the concentration in these NPs opens up potential applications in catalysis, magnetism, for hydrogen storage, or as an additive for aircraft coatings.

Ductile-brittle transition at edge cracks (001)[110] in BCC iron under different loading rates in mode I: a 3D atomistic study

3D atomistic simulations via molecular dynamics (MD) at temperature of 0 K and 295 K (22 °C) with a high quasi-static loading rate dP/dt of 2.92 kN s−1 show that cleavage fracture is supported by surface emission of oblique dislocations and by their subsequent cross slip to {112} planes, which increases separation of the (001) cleavage planes inside the crystal. Under the slower loading rate by a factor 5, the crack growth is hindered by twin generation on oblique planes {112} and the fracture is ductile. The MD results explain the contribution of the crack itself to the ductile-brittle transition observed in our fracture experiments on Fe-3wt%Si single crystals of the same orientation and geometry, loaded at the same rates dP/dt as in MD. The loading rates are equivalent to the cross head speed of 5 mm min−1 and 1 mm min−1 used in the experiment. The MD results also agree with the stress analysis performed by the anisotropic LFM and comply with experimental observations.