Critical Nucleus Size Research Articles

Impact of MgO-ZnO substitution on nucleation of aluminosilicate glasses

The nucleation mechanisms is of technically interest and important in glass-ceramics production. Here we investigate the nucleation behaviors of NiO-doped MgO-Al2O3-SiO2 and ZnO-Al2O3-SiO2 glasses. It is found that crystallization tends to takes place on the surface and the interior of glasses for the former and the latter, respectively. The origin of different nucleation behavior is studied by comparing aluminum coordination, configurational entropy and rheological properties of the glasses. The substitution of MgO by ZnO leads to increase of tetrahedral aluminum concentration, and decrease of configurational entropy above glass transition temperature, in particular, the latter is closely related to volume nucleation. In addition, the qualitative comparison of thermodynamic and kinetic barriers of steady-state nucleation rate of two glasses indicates that ZnO-bearing aluminosilicate glass possesses higher diffusivity and lower critical nucleus size, which favoring volume nucleation.

Atomistic Insights into the Stabilization of TDP-43 Protofibrils by ATP.

The aberrant accumulation of the transactive response deoxyribonucleic acid (DNA)-binding protein of 43 kDa (TDP-43) aggregates in the cytoplasm of motor neurons is the main pathological hallmark of amyotrophic lateral sclerosis (ALS). Previous experiments reported that adenosine triphosphate (ATP), the universal energy currency for all living cells, could induce aggregation and enhance the folding of TDP-43 fibrillar aggregates. However, the significance of ATP on TDP-43 fibrillation and the mechanism behind it remain elusive. In this work, we conducted multiple atomistic molecular dynamics (MD) simulations totaling 20 μs to search the critical nucleus size of TDP-43282-360 and investigate the impact of ATP molecules on preformed protofibrils. The results reveal that the trimer is the critical nucleus for TDP-43282-360 fibril formation and the tetramer is the minimal stable nucleus. When ATP molecules bind to the TDP-43282-360 trimer and tetramer, they can consolidate the TDP-43282-360 protofibrils by increasing the content of the β-sheet structure and promoting the formation of hydrogen bonds (H-bonds). Binding site analyses show that the N-terminus of TDP-43282-360 protofibrils is the main binding site of ATP, and R293 dominates the direct binding of ATP. Further analyses reveal that the π-π, cation-π, salt bridge, and H-bonding interactions together contribute to the binding of ATP to TDP-43282-360 protofibrils. This study decoded the detailed stabilization mechanism of protofibrillar TDP-43282-360 oligomers by ATP, and may provide new avenues for the development of drug design against ALS.

Capture zone scaling in 2D Ge island nucleation on Si(111)-(7 × 7) at elevated temperatures

Recently we have studied the concentration N2D of 2D Ge island nucleated on large-scale Si(111)-(7 × 7) terraces at 550 − 650 °C as function of the substrate temperature T and Ge deposition rate R [J. Cryst. Growth. 531 (2020) 125347], where we have shown that N2D(R) dependence is characterized by a power law with scaling exponent χ = 1 ± 0.1. However, in the frames of classical Venables rate-equation approach, the obtained range of χ values do not allow one to unambiguously determine the critical nucleus size i, since χ = 1 ± 0.1 can be attributed to both diffusion limited (DL) and attachment limited (AL) growth regimes. In order to determine an exact i size and growth kinetics, in this work, we have investigated the scaled capture-zone distribution (CZD) of nucleated 2D Ge islands on the same Ge/Si(111)-(7 × 7) samples and described it by the generalized Wigner distribution Pβ=aβAβexp-bβA2, where Pβ is the probability density of getting into some region of capture zone values, A is normalized Voronoi cell area for 2D island, β=iχ is scaling parameter directly related to the critical nucleus size i. It has been shown that the kinetics of 2D Ge island nucleation on the wide Si(111)-(7 × 7) terraces at elevated temperatures should procced under AL growth kinetics with i = 3–5 particles.

Nucleation mechanism in the absence and presence of an electric field

The objective of this article is to investigate the mechanism of the ice nucleation process theoretically by assaying the thermodynamics of the ice nucleus as a small system, and experimentally verified by studying the effect of the static electric field on the ice nucleation process. Using the thermodynamic of small system related to Hill, we predicted that the equilibrium temperature of water and ice nucleus decreases with the reduction of nucleus size. By applying an electrostatic field, the Gibbs free energy of nucleus formation decreases due to this fact that the electrostatic work on the ice nucleus is lower than that for bulk water which it causes a decrease in the critical size of the ice nucleus. The experimental results showed that the nucleation temperature of water increases by applying the electric field with the voltage of 4.5 kV (internal strength of 2.2 × 104 V/m) while the size of ice crystals decreases in the frozen agar gel at the presence of electric field.

On-Surface Synthesis of 2D Polymers at the Liquid-Solid Interface

Unraveling the structure and dynamics of the formation of covalent and non-covalent organic-based 2D crystalline materials is key to controlling the quality of these materials, such as the defect density and their size. Understanding these characteristics is important for controlling their properties. This contribution highlights our efforts in using scanning probe microscopy, particularly scanning tunneling microscopy (STM), to visualize the structure and dynamics of substrate-supported metal-organic frameworks (sMOFs) and substrate-supported covalent organic frameworks (sCOFs) at the liquid-solid interface.We will focus on the growth of covalent organic frameworks, primarily based on boroxine chemistry, and reveal both qualitative and quantitative details of the nucleation-elongation processes in real-time and under ambient conditions. Sequential data analysis enables the observation of the amorphous-to-crystalline transition, the time-dependent evolution of nuclei, the existence of 'non-classical' crystallization pathways, and, importantly, the experimental determination of essential crystallization parameters with excellent accuracy, including critical nucleus size, nucleation rate, and growth rate. Other aspects that will be addressed include sCOFs and chirality, as well as the formation of multilayered sCOFs. Additionally, we demonstrate that in specific cases, the electric field between the STM tip and the substrate can induce, on demand, the polymerization or depolymerization process.

Determination of γ/γ' interface free energy for solid state precipitation in Ni-Al alloys from molecular dynamics simulation.

Interface free energy is a fundamental material parameter needed to predict the nucleation and growth of new phases. The high cost of experimentally determining this parameter makes it an ideal target for calculation through a physically informed simulation. Direct determination of interface free energy has many challenges, especially for solid-solid transformations. Indirect determination of the interface free energy from the nucleation data has been done in the case of solidification. However, a slow on molecular dynamics (MD) simulation time scale atomic diffusion makes this method not applicable to the case of nucleation from the solid phase when precipitate composition is different from that in matrix. To address this challenge, we outline the development of a new technique for determining the critical nucleus size from an MD simulation using a recently developed method to accelerate solid-state diffusion. The accuracy of our approach for the Ni-Al system for Ni3Al (γ') precipitates in a Ni-Al (γ) matrix is demonstrated well within experimental accuracy and greatly improves upon previous computational methods [Herrnring et al., Acta Mater. 215(8), 117053 (2021)].

A General Nucleation Model for Semiconductor Nanocrystals.

We introduce a nonclassical model for nanocrystal nucleation in solution which centers on the dynamic interplay of chemical bond breakage and formation coupled with the desolvation of precursor molecules, which we term the molecular chemistry (MC) model. Departing from classical theory, our model employs the bond count as the key variable rather than particle size, thereby redefining the role of supersaturation and its role in determining the so-called critical nucleus size. We apply the model to CdSe nanocrystal formation in nonpolar solvents and showcase its efficacy in predicting solvent dynamics, precursor characteristics, crystal phase, stoichiometry, "magic number" behavior, and transition states. While the coupled-cluster method is used to determine the bond energy, we show that it is possible to derive reaction pathways by reducing the calculations to algebraic approximations for the nucleation energy. This singular set of bond energy parameters allows nanocrystal nucleation and growth to be conceptualized as a straightforward chemical reaction.

Role of dislocations on martensitic transformation temperatures and microstructure: A molecular dynamics study

Microstructure and defects strongly affect martensitic transformations in metallic alloys. Significant progress has been made in understanding the atomic-level processes that control the role of grain boundaries and precipitates in these solid-to-solid phase transformations. Yet, the role of dislocations and their structures on martensitic transformation temperature and the resulting microstructure remains unclear. Therefore, we used large-scale molecular dynamics simulations to study the forward and reverse transformation of a martensitic material modeled after Ni63Al37 under cyclic thermal loading. The simulations reveal that dislocations in the austenite phase act as one-dimensional seeds for the martensite phase, which is present at temperatures significantly above the martensite start value. We find a reduction in the dislocation density during cyclic thermal loading, which results in the increase in martensite and austenite transition temperatures, in agreement with experiments. Importantly, we extracted a critical martensitic nuclei size for developing stable domains and found that relatively low dislocation densities are needed to grow independent martensitic variants resulting in a multi-domain structure.

Guanidine Hydrochloride-Induced Hepatitis B Virus Capsid Disassembly Hysteresis.

Hepatitis B virus (HBV) displays remarkable self-assembly capabilities that interest the scientific community and biotechnological industries as HBV is leading to an annual mortality of up to 1 million people worldwide (especially in Africa and Southeast Asia). When the ionic strength is increased, hepatitis B virus-like particles (VLPs) can assemble from dimers of the first 149 residues of the HBV capsid protein core assembly domain (Cp149). Using solution small-angle X-ray scattering, we investigated the disassembly of the VLPs by titrating guanidine hydrochloride (GuHCl). Measurements were performed with and without 1 M NaCl, added either before or after titrating GuHCl. Fitting the scattering curves to a linear combination of atomic models of Cp149 dimer (the subunit) and T = 3 and T = 4 icosahedral capsids revealed the mass fraction of the dimer in each structure in all the titration points. Based on the mass fractions, the variation in the dimer-dimer association standard free energy was calculated as a function of added GuHCl, showing a linear relation between the interaction strength and GuHCl concentration. Using the data, we estimated the energy barriers for assembly and disassembly and the critical nucleus size for all of the assembly reactions. Extrapolating the standard free energy to [GuHCl] = 0 showed an evident hysteresis in the assembly process, manifested by differences in the dimer-dimer association standard free energy obtained for the disassembly reactions compared with the equivalent assembly reactions. Similar hysteresis was observed in the energy barriers for assembly and disassembly and the critical nucleus size. The results suggest that above 1.5 M, GuHCl disassembled the capsids by attaching to the protein and adding steric repulsion, thereby weakening the hydrophobic attraction.

Supersaturation and Critical Size of Cloud Condensation Nuclei in Marine Stratus Clouds

AbstractObservations of marine stratus clouds in clean air off the Californian coast reveal a functional relationship between the number of cloud condensation nuclei (CCN) and supersaturation. Satellite‐derived liquid droplet density estimates the number density of CCN. Combining the estimated supersaturation using Köhler theory, global maps of supersaturation and the critical activation size of CCN are estimated. Here, we show that high supersaturation >0.5% persists over the oceans with a critical CCN size of 25–30 nm, which is smaller than the conventional wisdom of 60 nm. Independent support for such high supersaturation in the marine cloud is obtained from CCN measurements provided by the “Atmospheric Tomography Mission.” Higher supersaturation implies smaller activation size for CCN making cloud formation more sensitive to changes in aerosol nucleation.

Preferential water vapor condensation on a corrugated surface: A molecular dynamics study

In this study, we investigate the nucleation and subsequent growth of water clusters on corrugated surfaces using molecular dynamics simulations, with a specific focus on scenarios where the nanostructure sizes are comparable to the critical nucleus size. The primary objective is to reveal the coupled effects of different surface morphologies, i.e., plane, convex, and concave, on water vapor condensation. Regardless of the nanostructures, the condensation process can be divided into two stages, the nucleation stage and the growth stage. It is shown that in the nucleation stage, the concave areas are more conducive to nucleation compared to plane areas, aligning with classical nucleation theory. The total number of condensed molecules is generally proportion to the nucleation ability, while the average size of condensation clusters is affected by the surface nanostructures, with the combination of concave and plane surfaces exhibiting the strongest capability for both water vapor nucleation and cluster growth. Compared to the plane regions, the convex regions not only inhibit nucleation but also block collision and coalescence between clusters. Additionally, increasing the characteristic width of concave and convex structures leads to condensation behaviors approaching those on the plane surface. This indicates that the regulation of the condensation process is feasible by nanostructures within prescribed structural size limits.

Homogeneous water vapor condensation with a deep neural network potential model.

Molecular-level nucleation has not been clearly understood due to the complexity of multi-body potentials and the stochastic, rare nature of the process. This work utilizes molecular dynamics (MD) simulations, incorporating a first-principles-based deep neural network (DNN) potential model, to investigate homogeneous water vapor condensation. The nucleation rates and critical nucleus sizes predicted by the DNN model are compared against commonly used semi-empirical models, namely extended simple point charge (SPC/E), TIP4P, and OPC, in addition to classical nucleation theory (CNT). The nucleation rates from the DNN model are comparable with those from the OPC model yet surpass the rates from the SPC/E and TIP4P models, a discrepancy that could mainly arise from the overestimated bulk free energy by SPC/E and TIP4P. The surface free energy predicted by CNT is lower than that in MD simulations, while its bulk free energy is higher than that in MD simulations, irrespective of the potential model used. Further analysis of cluster properties with the DNN model unveils pronounced variations of O-H bond length and H-O-H bond angle, along with averaged bond lengths and angles that are enlarged during embryonic cluster formation. Properties such as cluster surface free energy and liquid-to-vapor density transition profiles exhibit significant deviations from CNT assumptions.

Molecular dynamics study on heterogeneous nucleation mechanism: Plausible nanobubble coalescence pathway

The Bubble Nucleation process has been attracted research attention due to its wide practical applications. In literature, researchers commonly accepted vapour trapping in cervices on the heated substrate as the mechanism initialising the incipient of bubble nuclei. However, experimental observations found that the onset of nucleation could occur on ultra-smooth surfaces where vapour trapping is not applicable. Aiming to investigate the nucleation mechanism, molecular dynamics (MD) simulations have been carried out to study nanobubble nucleation behaviour on smooth and grooved substrates. The Lennard-Jones (LJ) 12-6 potential field is adopted to describe the force field of argon atoms which are heated by solid substrates. Instantaneous snapshots of MD results show nanobubbles pinned on the two-grooved surfaces could merge, forming a metastable nucleus via the coalescence event. The coalescence pathways effectively lower the required nucleation energy cost. Energy analyses also clearly show a consistent energy level between the right and left regions of the two grooves. The results confirm that nanobubble coalescence could be an alternative pathway in the nucleation process, which could reduce the critical nucleus size and its associated energy cost.

Insight into the nucleation behavior of tiamulin hydrogen fumarate methanol solvate in methanol-ethyl acetate binary mixtures

To investigate the nucleation behavior of tiamulin hydrogen fumarate methanol solvate (THFMS), the solubility, induction time, and metastable zone width (MSZW) of THFMS in methanol-ethyl acetate binary mixtures were measured. The nucleation potential model, which is based on the classical nucleation theory and correlates the induction time and MSZW, has been employed to deal with the experimental data and estimate the interfacial energy and the critical nucleation potential. The estimated values determined from the nucleation potential model are in accordance with the experimental values. The interfacial energy and the critical nucleation potential of THFMS steadily decrease with the increasing saturation temperature, whereas those of THFMS initially decrease and subsequently increase as the ethyl acetate content increases. The results demonstrate that an increase in saturation temperature leads to enhanced nucleation favorability, while the difficulty of nucleation initially decreases and then increases with increasing ethyl acetate content. Furthermore, the critical nucleation parameters, including the critical Gibbs free energy, the critical nucleus size, and the critical number of molecules in a nucleus, exhibit the same trend in alterations regarding both saturation temperature and ethyl acetate content.

Phase transitions in 2D multistable mechanical metamaterials via collisions of soliton-like pulses

In recent years, mechanical metamaterials have been developed that support the propagation of an intriguing variety of nonlinear waves, including transition waves and vector solitons (solitons with coupling between multiple degrees of freedom). Here we report observations of phase transitions in 2D multistable mechanical metamaterials that are initiated by collisions of soliton-like pulses in the metamaterial. Analogous to first-order phase transitions in crystalline solids, we observe that the multistable metamaterials support phase transitions if the new phase meets or exceeds a critical nucleus size. If this criterion is met, the new phase subsequently propagates in the form of transition waves, converting the rest of the metamaterial to the new phase. More interestingly, we numerically show, using an experimentally validated model, that the critical nucleus can be formed via collisions of soliton-like pulses. Moreover, the rich direction-dependent behavior of the nonlinear pulses enables control of the location of nucleation and the spatio-temporal shape of the growing phase.

Two-dimensional silver nanonetwork on Ag4Ti5O12 film as highly efficient SERS substrate

Surface Enhanced Raman Spectroscopy (SERS) is a powerful technique to study and detect single molecules adsorbed onto a surface. In this work, molar concentrations were varied over three consecutive orders of magnitude (10-3 M, 10-6 M, 10-9 M) of Methylene Blue (MB) to study SERS on different thicknesses of silver (6, 8, 10, 12 nm) coated on Ag4Ti5O12 deposited onto glass substrates as the base material. The Ag4Ti5O12 thin films were prepared by a unique ion-exchange method where Li4Ti5O12 thin films were initially deposited by the sol–gel method followed by the exchange of Li+ ion with Ag+ ion. The varied thicknesses of thermally deposited Ag-thin films were grown with unaltered Ag+ ions nucleation sites atop the Ag4Ti5O12 layer to create the two-dimensional (2D) nanonetwork. The structure and morphology of the as-prepared thin-film samples were characterized using X-ray diffraction, Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) techniques. The SERS enhancement factor (EF), calculated using Raman spectroscopy, showed the systematic changes in the Raman signal enhancement as a function of Ag-thickness. The 10 nm thick as-deposited Ag sample showed the highest signal enhancement in Raman spectra than the other Ag thickness films. The successive signal enhancement was obtained due to forming the critical nuclei size of Ag on top of Ag4Ti5O12, which was further bolstered with film surface roughness and surface energy measurements. The as-prepared Ag-10 nm sample showed the lowest film roughness of ∼2.07 nm, in tune with the maximum enhancement factor (EF). We believe this work could open wide opportunities for developing efficient SERS substrates with sensing capabilities for ultra-low analyte concentration coupled with low cost and high enhancement factors.

Strain-induced precipitation behaviors of 7Mo super-austenitic stainless steel

Strain-induced precipitation (SIP) behaviors of 7Mo super-austenitic stainless steel (SASS) were studied by strain relaxation test at 850 °C. Our results reveal that sigma (σ) phases are the predominant SIP of 7Mo SASS, with the chemical formula of (FeNi) (CrMo). SIP first precipitates in granular shape at HAGBs, such as deformed grain boundaries (SIP-GBs), interfaces between deformation twin layers and matrix (SIP-TW), and HAGBs inside deformed grains (SIP-HAGBs). This is attributed to the ability of HAGBs for providing high deformation stored energy, nucleation sites, and efficient element diffusion channels. When the nucleation sites at HAGBs become saturated with increasing holding time, needle-like SIPs (SIP-NL) are precipitated along closely packed planes of γ-Fe matrix. The specific orientation relationships between SIP-NL and matrix, namely (−111)γ-Fe//(00–1)σ, [0–11]γ-Fe//[-110]σ or (-1-1-1)γ-Fe//(00–1)σ, [0–11]γ-Fe//[-110]σ, and the needle-like shape are contribute to minimizing interfacial energy and elastic strain energy of SIP, respectively, thereby promoting SIP-NL. The absence of SIP on dislocations can be attributed to the large critical nucleus size of σ phase (about 4.87 nm), which poses challenges for SIP nucleation through dislocation distortion energy. Mo is the controlling element for the nucleation and growth of SIP. SIP can promote cDRX through particle-induced nucleation (PSN) mechanism during the deformation process, while residual high-density dislocations around SIP can also promote cSRX during the holding process. During holding process, the boundaries of recrystallized grains can also serve as nucleation sites for SIP, thereby promoting SIP, and finally form a multi-layer structure of “σ phase/Recrystallization/σ phase”.

Double-Shock Compression Pathways from Diamond to BC8 Carbon.

Carbon is one of the most important elements for both industrial applications and fundamental research, including life, physics, chemistry, materials, and even planetary science. Although theoretical predictions on the transition from diamond to the BC8 (Ia3[over ¯]) carbon were made more than thirty years ago, after tremendous experimental efforts, direct evidence for the existence of BC8 carbon is still lacking. In this study, a machine learning potential was developed for high-pressure carbon fitted from first-principles calculations, which exhibited great capabilities in modeling the melting and Hugoniot line. Using the molecular dynamics based on this machine learning potential, we designed a thermodynamic pathway that is achievable for the double shock compression experiment to obtain the elusive BC8 carbon. Diamond was compressed up to 584GPa after the first shock at 20.5 km/s. Subsequently, in the second shock compression at 24.8 or 25.0 km/s, diamond was compressed to a supercooled liquid and then solidified to BC8 in around 1ns. Furthermore, the critical nucleus size and nucleation rate of BC8 were calculated, which are crucial for nano-second x-ray diffraction measurements to observe BC8 carbon during shock compressions. The key to obtaining BC8 carbon lies in the formation of liquid at a sufficient supercooling. Our work provides a feasible pathway by which the long-sought BC8 phase of carbon can be reached in experiments.

Theoretical formulation of the distribution function of critical nuclei under precipitation polymerization conditions

Precipitation Polymerization (Pre-Poly) can be considered a nucleation and growth process in which complex and high molecular weight branched polymers are involved. From an experimental point of view, it is well-known that under Pre-Poly conditions the phase transition (nucleation) occurs in the first minutes of polymerization, and then a long growth stage is observed in which the critical nuclei simultaneously grow until reaching a highly monodisperse distribution of microspheres (characteristic mechanism of binodal decomposition). The high rate at which nucleation and growth processes usually take place when radical polymerization (extremely high polymerization rate) under Pre-Poly conditions is used, makes it very difficult to study experimentally some aspects of these processes, such as the size and concentration of critical nuclei, among others. Based on the thermodynamic principles of Pre-Poly, this analytical paper covers for the first time, to the best of our knowledge, the theoretical formulation of the distribution function of critical nuclei under Pre-Poly conditions. In addition, a simple empirical method to calculate the concentration of critical nuclei was also developed using only three global, physical experimental parameters, and good agreement was found between empirical and theoretical calculations.

In search of the limits of CuO thermal oxidation nanowire growth by combining experiment and theory

In the present study, we report on the temperature limiting conditions for the growth of copper oxide nanowires via thermal oxidation of copper, which have been puzzling scientists for decades and were up until now unknown. We explore the lowest temperature limits for nanowire synthesis and find the lowest nanowire diameter that can be achieved via thermal oxidation. This was achieved by gradually lowering the temperature of the oxidation and observing when the nanowire growth will cease with the use of electron microscopy techniques. The lowest temperature, where the growth of nanowires was observed, corresponded to 187 ± 5 °C, and the lowest nanowire diameter which can be achieved was found to be between 3 and 4 nm. Experimental results are supported by theoretical modeling of the nanowire nucleation processes, providing deterministic aspects. The nanowire limiting diameter is governed by the temperature dependence of the critical nucleus size.