Initial Nucleation Stage Research Articles

Unveiling the kinetics of interphase and random quaternary Nb[sbnd]Ti precipitations, and strengthening mechanism of HSLA steel

Traditional methods for analyzing multi-element precipitation are often time-consuming, labor-intensive, and expensive. Particularly, the initial nucleation stages of such precipitations remain poorly understood. A revised precipitation theory was employed to elucidate the nucleation and growth kinetic mechanisms of quaternary (Ti, Nb)(C, N) precipitation with two modes: interphase precipitation (IP) along the migrating α/γ interface and random precipitation (RP) along dislocations. The findings indicated that TiN preferentially nucleates at the ferrite side compared to that at the austenite side. The nucleation occurring at α/γ interface leads to a precipitation particle angle of approximately 5.3° within a single IP sheet. The interface provided more favorable nucleation sites than dislocations, significantly reducing the relative nucleation time. Additionally, the initial yielding behavior and strengthening mechanism were analyzed based on the two factors of the hot-rolling microstructure and precipitation. A favorable slip system was along the RD-ND plane as the texture index of α-{110} approximately 3.74, compared to 2.20 along the TD-ND plane, resulting in the lower upper yield strength. Once the number density of mobile dislocations increased during plastic deformation, the tensile stress decreased, leading to the local plastic instability. Finally, (Ti, Nb)(C, N) contributed to a strengthening increment of about 154 MPa. This study provides critical insights into the precipitation behavior in HSLA steels, offering significant implications for the design and development of advanced materials with enhanced mechanical properties.

Surface structure of MOVPE-prepared As-modified Si(100) substrates

In the pursuit of high-efficiency tandem devices for solar energy conversion based on III-V-semiconductors, low-defect III-V nucleation on Si(100) substrates is essential. Here, hydrogen and arsenic are key ingredients in all growth processes with respect to industrially scalable metalorganic vapor phase epitaxy. Our study provides insight into Si(100) surface preparation for the initial stage of III-V nucleation. The samples investigated, prepared on substrates with different offcut angles, show single domain surfaces consisting of rows of preferentially buckled dimers. Low energy electron diffraction and reflection anisotropy spectroscopy confirm well-defined (1 × 2)/(2 × 1) majority domains. Fourier-transform infrared spectroscopy revealed hydrogen bonding to the surface dimers, while no impurities were found by XPS. Density functional theory calculations support the experimental results and reveal a novel surface motif of H-passivated Si-As mixed dimers.

Liquefaction efficiency study of heterogeneous condensation of methane-ethane binary gas mixtures with different component contents

Natural gas is a mixture of hydrocarbons. During the liquefaction of natural gas, the mixed gas undergoes heterogeneous condensation on the surface of the heat exchanger. Currently, the heterogeneous condensation mechanism of mixed gases is not fully understood, and the influence of component content on the liquefaction efficiency is not clear. Therefore, the molecular dynamics simulation was employed to investigate the heterogeneous condensation process of methane-ethane binary mixed gas on a solid surface. The characteristics of condensation nucleation cluster growth was explored and the variations in temperature, density, heat flux, and liquefaction rate were analyzed. The impact of ethane on methane condensation was also discussed. The results indicate that increasing wettability enhances surface nucleation quantity and shortens nucleation time, resulting in a 3.6- times increase in heat flux. The addition of ethane gas in the system promotes condensation nucleation, which increases the liquefaction rate by 60 % when x = 20 % compared to the pure methane system. However, for methane gas, ethane only benefits the liquefaction of methane molecules in the initial nucleation stage. As the mixture in the system enters the cluster growth stage, component migration in the mixed gas leads to a lower liquefaction rate compared to pure methane gas.

From GaN crystallinity to device performance: Nucleation mode vs Surface energy of single-crystalline AlN template

In this paper, the surface state of the single-crystalline AlN template's impact on the epitaxial growth of gallium nitride (GaN) was studied. Subsequently, Schottky barrier devices were fabricated and analyzed. The results indicate that surface states subjected to different treatments exhibit varying epitaxial growth modes at the initial nucleation stage, which have a correlated effect on surface topography, crystalline quality, strain and other material characteristics of the epitaxial GaN. Higher surface energy leads to a reduction in screw dislocation of GaN material, but has less influence on edge dislocation. Single-crystalline AlN templates with higher surface energy are more likely to exhibit Stranski-Krastanow and Frank-van der Merwe growth model and achieve high-quality epitaxial materials. Schottky devices prepared using GaN material with lower screw dislocation density exhibit lower reverse leakage current. First-principles simulation analysis revealed that the migration barrier of gallium and nitrogen atoms on the surface can be overcome with the larger surface energy of single-crystalline AlN template. This facilitates their migration on the surface, resulting in higher quality material epitaxy. The conclusions of this work provide insights for quality control of epitaxial GaN materials on single-crystalline AlN templates, as well as for studying device leakage in the backend of the device fabrication process, or similar homogeneous compound semiconductor heteroepitaxy studies.

Formation of a zirconium oxide crystal nucleus in the initial nucleation stage in aluminosilicate glass investigated by X-ray multiscale analysis

Understanding the nucleation mechanism in glass is crucial for the development of new glass-ceramic materials. Herein, we report the structure of a commercially important glass-ceramic ZrO2-doped lithium aluminosilicate system during its initial nucleation stage. We conducted an X-ray multiscale analysis, and this analysis was used to observe the structure from the atomic to the nanometer scale by using diffraction, small-angle scattering, absorption, and anomalous scattering techniques. The inherent phase separation between the Zr-rich and Zr-poor regions in the pristine glass was enhanced by thermal treatment without changing the spatial geometry at the nanoscale. Element-specific pair distribution function analysis using anomalous X-ray scattering data showed the formation of a liquid ZrO2-like local structural motif and edge sharing between the ZrOx polyhedra and (Si/Al)O4 tetrahedra during the initial nucleation stage. Furthermore, the local structure of the Zr4+ ions resembled a cubic or tetragonal ZrO2 crystalline phase and formed after 2 h of annealing the pristine glass. Therefore, the Zr-centric periodic structure formed in the early stage of nucleation was potentially the initial crystal nucleus for the Zr-doped lithium aluminosilicate glass-ceramic.

Phase-Centric MOCVD Enabled Synthetic Approaches for Wafer-Scale 2D Tin Selenides.

Following an initial nucleation stage at the flake level, atomically thin film growth of a van der Waals material is promoted by ultrafast lateral growth and prohibited vertical growth. To produce these highly anisotropic films, synthetic or post-synthetic modifications are required, or even a combination of both, to ensure large-area, pure-phase, and low-temperature deposition. A set of synthetic strategies is hereby presented to selectively produce wafer-scale tin selenides, SnSex (both x=1 and 2), in the 2D forms. The 2D-SnSe2 films with tuneable thicknesses are directly grown via metal-organic chemical vapor deposition (MOCVD) at 200°C, and they exhibit outstanding crystallinities and phase homogeneities and consistent film thickness across the entire wafer. This is enabled by excellent control of the volatile metal-organic precursors and decoupled dual-temperature regimes for high-temperature ligand cracking and low-temperature growth. In contrast, SnSe, which intrinsically inhibited from 2D growth, is indirectly prepared by a thermally driven phase transition of an as-grown 2D-SnSe2 film with all the benefits of the MOCVD technique. It is accompanied by the electronic n-type to p-type crossover at the wafer scale. These tailor-made synthetic routes will accelerate the low-thermal-budget production of multiphase 2D materials in a reliable and scalable fashion.

Nonthermal Plasma Synthesis of Metallic Ti Nanocrystals

Nanoscale metallic titanium (Ti) offers unique energetic and biocompatible characteristics for the aerospace and biomedical industries. A rapid and sustainable method to form purified Ti nanocrystals is still in demand due to their high oxygen affinity. Herein, we report the production of highly purified Ti nanoparticles with a nonequilibrium face center cubic (FCC) structure from titanium tetrachloride (TiCl4) via a capacitively coupled plasma (CCP) route. Furthermore, we demonstrate a secondary H2 treatment plasma as an effective strategy to improve the air stability of a thin layer of nanoparticles by further removal of chlorine from the particle surface. Hexagonal and cubic-shaped Ti nanocrystals of high purity were maintained in the air after the secondary H2 plasma treatment. The FCC phase potentially originates from small-sized grains in the initial stage of nucleation inside the plasma environment, which is revealed by a size evolution study with variations of plasma power input.

Investigation of the CdZnTe (2 1 1) and (1 3 3) films grown on GaAs (2 1 1) controlled by temperature: Experiment and first-principles calculations

The origin and evolution of dual epitaxy in CdZnTe epilayer grown on GaAs substrates by close spaced sublimation has been studied, both (211) and (133) films are suitable for HgCdTe growth. And the effect of growth temperature on the orientation and crystalline quality of CdZnTe films has been investigated, more uniform orientation of adjoining growth islands in the growth of (133) film leads to a smoother surface with less threading dislocations and roughness. The interfacial properties of CdZnTe films were studied by the first-principles calculations combined with scanning transmission electron microscopy observations. By calculation the work of adhesion between the CdTe films and GaAs (211), obtain the CdTe (211)/GaAs (211) interface is the more stable interface structure at the initial nucleation stage. Combining to the results of experiment, at higher growth temperature, only (133) nucleus can reach the critical size and thus stabilize the growth. More importantly, the results on the dual epitaxy of CdZnTe films are expected to play a crucial role in guiding the dual epitaxy growth of other zincblende-type crystal.

Unidirectional alignment and orientation pinning mechanism of h-BN nucleation on Ir(111) via reactive probe atomic force microscopy

The epitaxial growth of wafer-scale single-crystalline two-dimensional materials requires precise control over the crystallographic orientation and morphology of clusters formed during the initial stages of nucleation. However, there is limited knowledge about the critical nucleus and its growth mechanism for h-BN on high-symmetry surfaces of transition metals. In this study, we provide atomic insights into h-BN nucleation on Ir(111) using scanning tunneling microscopy and noncontact atomic force microscopy, corraborated by density functional theory (DFT) calculations. The atomic-resolved structural characterization reveals that the smallest h-BN cluster, exhibiting a non-1:1 stoichiometric ratio of boron and nitrogen atoms, maintains a triangular shape with zigzag-type edges. Through force spectroscopy, individual B and N atoms within the cluster, as well as the N-termination edge, are clearly identified, indicating the atomic-scale elemental sensitivity of the chemical-reactive metallic tip. While DFT calculations suggest that the NtopBfcc configuration is thermodynamically favored for larger clusters, a predominant crystalline orientation is observed for nuclei of various sizes, unequivocally attributed to the NtopBhcp registry. This orientation pinning behavior presents opportunities for large-scale growth of binary and ternary materials by impeding rotational alignment of nuclei in the early stages of nucleation.

Fabrication of ultrafine cerium oxide nanoparticles as an aqueous colloidal solution with single-molecular dispersant via shear agitation reactor or one-pot hydrolysis

Herein, ultrafine CeO2 nanoparticles are synthesized using monomolecular amino acids as dispersants. Monomolecular amino acids are suitable dispersants for the dispersion of nanoparticles with a diameter of 1 nm. Batch-type room-temperature hydrolysis (RTH) synthetic method, which takes advantage of the low decomposition pH of Ce (IV) nitrate, and a shear agitation (SA) reactor, which is highly versatile for multicomponent systems of colloidal synthesis, are used. After adding glycine, ultrafine CeO2 colloids of sizes 1.1 and 1.2 nm are obtained via the batch-type RTH synthesis method and SA reactor, respectively. The smallest CeO2 colloid size is 0.7 nm, which earned at the close concentration of the saturation of glycine using the batch-type RTH method. Because RTH can slowly hydrolyze Ce (IV) nitrate over several hours, we were able to observe the initial stage of nucleation (0.7 nm), which passes instantaneously in the SA reactor. Furthermore, glycine was found to be particularly effective in stabilizing the surface of cerium oxide due to its high adsorption capacity. Therefore, cerium oxide, which is amorphous in the synthesis stage, remained amorphous up to 100 °C due to glycine adsorption, neutralized the colloidal dispersion to form precipitates, and crystallized at room temperature after glycine was removed by washing treatment, resulting in a fluorite structure with a crystallite diameter of 1.5 nm. This crystallite diameter is maintained after heat treatment at 200 °C for 30 min.

Phase State, Surface Tension, Water Activity, and Accommodation Coefficient of Water-Organic Clusters Near the Critical Size for Atmospheric New Particle Formation.

Interactions between water and organic molecules in sub-4 nm clusters play a significant role in the formation and growth of secondary organic aerosol (SOA) particles. However, a complete understanding of the relevant water microphysics has not yet been achieved due to challenges in the experimental characterization of soft nuclei. Here, we use molecular dynamics simulations to study the phase-mixing states, surface tension, water activity, and water accommodation coefficient of organic-water clusters representative of freshly nucleated SOA particles. Our results reveal large deviations from the behavior expected based on continuum theories. In particular, the phase-mixing state has a strong dependence on cluster size; surface tension displays a minimum at a specific organic-water mass ratio (morg/mw ∼ 4.5 in this study) corresponding to a minimum inhibition of droplet nucleation associated with the Kelvin effect; and the water accommodation coefficient increases by a factor of 2 with nanocluster hygroscopic growth, in agreement with recent experimental studies. Overall, our results yield parametric relations for water microphysical properties in sub-4 nm clusters and provide insight into the role of water in the initial stages of SOA nucleation and growth.

Membrane flux response technology for early warning of initial surface scaling in membrane distillation

Membrane distillation (MD) is a promising green separation technology for the treatment of high salinity wastewater attributed to excellent salt rejection, which is significant in relevant environmental and chemical engineering application. However, membrane scaling is an inevitable issue during long-term membrane distillation process. Thus, we developed a membrane flux response technology with data processing method to in-situ monitor the membrane scaling according to the reduction of permeate flux. By real-time analyzing the derivative of deviation between theoretical and experimental fluxes, early warning of initial membrane surface scaling was observed at initial nucleation stage. Besides, such timely detection was also achieved under different temperatures and flow velocities of feed solution, even for the poor light transmittance aqueous solution (such as NiSO4 and CoSO4 solution), which exhibited wide application potential on diverse high salinity wastewater systems. In addition, the metastable zone width of salt solution within MD system was obtained for the efficient operation design. To sum up, membrane flux response technology provides an early warning at imminent membrane scaling, which will maintain the great separation effect for a fresh feed solution after a facile cleaning by pure water, indicating the extension of device lifetime.

Nucleation of Co and Ru Precursors on Silicon with Different Surface Terminations: Impact on Nucleation Delay.

Early transition metals ruthenium (Ru) and cobalt (Co) are of high interest as replacements for Cu in next-generation interconnects. Plasma-enhanced atomic layer deposition (PE-ALD) is used to deposit metal thin films in high-aspect-ratio structures of vias and trenches in nanoelectronic devices. At the initial stage of deposition, the surface reactions between the precursors and the starting substrate are vital to understand the nucleation of the film and optimize the deposition process by minimizing the so-called nucleation delay in which film growth is only observed after tens to hundreds of ALD cycles. The reported nucleation delay of Ru ranges from 10 ALD cycles to 500 ALD cycles, and the growth-per-cycle (GPC) varies from report to report. No systematic studies on nucleation delay of Co PE-ALD are found in the literature. In this study, we use first principles density functional theory (DFT) simulations to investigate the reactions between precursors RuCp2 and CoCp2 with Si substrates that have different surface terminations to reveal the atomic-scale reaction mechanism at the initial stages of metal nucleation. The substrates include (1) H:Si(100), (2) NHx-terminated Si(100), and (3) H:SiNx/Si(100). The ligand exchange reaction via H transfer to form CpH on H:Si(100), NHx-terminated Si(100), and H:SiNx/Si(100) surfaces is simulated and shows that pretreatment with N2/H2 plasma to yield an NHx-terminated Si surface from H:Si(100) can promote the ligand exchange reaction to eliminate the Cp ligand for CoCp2. Our DFT results show that the surface reactivity of CoCp2 is highly dependent on substrate surface terminations, which explains why the reported nucleation delay and GPC vary from report to report. This difference in reactivity at different surface terminations may be useful for selective deposition. For Ru deposition, RuCp2 is not a useful precursor, showing highly endothermic ligand elimination reactions on all studied terminations.

New insights into the adsorption of Fe on Ti(CxN1−x) surface: First-principles study

The present study systematically investigates the adsorption behavior of Fe atoms on the titanium carbonitride (Ti(CxN1−x)) surface during the initial stage of austenite nucleation. The results indicate that the formation energy of TiN is lower than that of TiC, suggesting that TiN is more likely to form. However, it is observed that the TiN content in the steel is so little. The C-Top site of TiC exhibits the highest adsorption capacity for Fe atoms. For the various stacking models evaluated for Ti(CxN1−x), the CNNC model exhibits the most favorable adsorption performance for Fe atoms in Ti(C0.5N0.5). Furthermore, TiC exhibits superior adsorption performance as compared to TiN, which is attributed to its shorter bond length and higher electron orbital overlap. When the Fe atoms adsorb on the TiC(001) surface, covalent bonds are formed among C, Ti, and Fe atoms, revealing the presence of metal bonds.

Controlling crystallization: what liquid structure and dynamics reveal about crystal nucleation mechanisms.

Over recent years, molecular simulations have provided invaluable insights into the microscopic processes governing the initial stages of crystal nucleation and growth. A key aspect that has been observed in many different systems is the formation of precursors in the supercooled liquid that precedes the emergence of crystalline nuclei. The structural and dynamical properties of these precursors determine to a large extent the nucleation probability as well as the formation of specific polymorphs. This novel microscopic view on nucleation mechanisms has further implications for our understanding of the nucleating ability and polymorph selectivity of nucleating agents, as these appear to be strongly linked to their ability in modifying structural and dynamical characteristics of the supercooled liquid, namely liquid heterogeneity. In this perspective, we highlight recent progress in exploring the connection between liquid heterogeneity and crystallization, including the effects of templates, and the potential impact for controlling crystallization processes. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Investigation of the impact of amorphous silicon layers deposited by PECVD and HDP-CVD on oxide precipitation in silicon

The effect of deposited a-Si layers with different layer stress on oxide precipitation was investigated in order to find out if intrinsic point defects affecting oxide precipitation are generated at the interface a-Si/Si and if possibly hydrogen affects the oxide precipitation. A thermal cycle of nucleation at 650 °C for 4 h or 8 h followed by stabilization at 780 °C for 3 h, and growth at 1000 °C for 16 h was applied. It was found that there are no signs for the injection of intrinsic point defects from the interface a-Si/Si into the Si substrate during the applied thermal treatment. However if a-Si is deposited on 1000 nm silicon oxide, deposited previously from TEOS in a plasma process, silicon self-interstitials seem to be injected from the interface silicon oxide/Si into the silicon substrate retarding oxide precipitation in the initial stage of nucleation annealing at 650 °C. There are also no signs of any impact of the layer stress on oxide precipitation or self-interstitial injection. The concentration of hydrogen in the layers can be controlled via the RF bias power. The hydrogen concentration is reduced markedly already during annealing at 650 °C. Part of the hydrogen diffuses into the silicon substrate and enhances oxide precipitation if its initial concentration in the layers is higher than 1.5 × 1022 cm−3. For a-Si deposited on 1000 nm silicon oxide, the enhancement effect appears for hydrogen concentrations in the layer higher than approximately 2.8 × 1022 cm−3.

Heterogeneous nucleation behavior of solidified α-Al phase on agglomerated TiB2 particles in aluminum-copper alloy: Study by synchrotron radiation multiscale tomography

The size of the TiB2 particles and the orientation relationship (OR) between the Al-TiB2 particles in Al solidified grains in Al-Cu hypo-eutectic alloy are characterized by a combined method of synchrotron multiscale-CT and SEM/EBSD to identify the true nucleation particle. Not only single TiB2 particles but also TiB2 agglomerates were observed in each of the four analyzed grains (A-D). The OR of {0001}TiB2∥{111}Al was confirmed for the largest TiB2 particle in each grain of A to C, corresponding to the free growth model. Such OR was absent in Grain D and EDS analysis revealed Cu segregation at the basal plane of its largest TiB2 particle. It is possible that the eutectic reaction of Al and Cu affects the nucleation and growth of Al, resulting in the change of OR. This paper evidently demonstrates that the eutectic reaction occurs on the nucleant TiB2 particles at the initial stage of nucleation.

CO2 Aggregation on Monoethanolamine: Observations from Rotational Spectroscopy.

The initial stages of the gas-phase nucleation between CO2 and monoethanolamine were investigated via broadband rotational spectroscopy with the aid of extensive theoretical structure sampling. Sub-nanometer-scale aggregation patterns of monoethanolamine-(CO2 )n , n=1-4, were identified. An interesting competition between the monoethanolamine intramolecular hydrogen bond and the intermolecular interactions between monoethanolamine and CO2 upon cluster growth was discovered, revealing an intriguing CO2 binding priority to the hydroxyl group over the amine group. These findings are in sharp contrast to the general results for aqueous solutions. In the quinary complex, a cap-like CO2 tetramer was observed cooperatively surrounding the monoethanolamine. As the cluster approaches the critical size of new particle formation, the contribution of CO2 self-assembly to the overall stability increases.

Inter-relationship of stress and microstructure in BCC and ‘beta’ tungsten films

In this work, a series of W films are deposited at different deposition rates (0.2, 0.5, and 1.0 nm/s) and pressures (0.27, 0.47, 0.67, and 1.33 Pa). Comparing the residual stresses between different deposition rates, the stress was found to become more tensile at higher depositions rates over the pressure ranges studied. Films deposited at the three highest pressures were tensile in stress, had small grains (~15 to 20 nm), and stabilized the metastable A15 phase often referred to as β-W. At the lowest pressure, 0.27 Pa, the films were compressive in stress, larger grain sizes (~70 to 90 nm), and primarily stabilized the body centered cubic α-W phase. If a W seed layer was grown under either the α-W or β-W growth conditions, the subsequent W layer adopted the phase state of the seed layer, independent of processing conditions and/or grain sizes, suggesting that the phase state is most likely determined in the initial stages of nucleation. The seed layer experiment also suggest that these layers can promote more controlled grain sizes in thicker β-W films, which has not been observed in previous work. The stress measurements are interpreted in terms of a previously developed kinetic model that includes effects of growth kinetics, microstructural evolution, and energetic particle bombardment.

The Role of Structural Defects in the Growth of Two-Dimensional Diamond from Graphene.

The presented work is devoted to the study of the formation of the thinnest diamond film (diamane). We investigate the initial stages of diamond nucleation in imperfect bilayer graphene exposed by the deposition of H atoms (chemically induced phase transition). We show that defects serve as nucleation centers, their hydrogenation is energy favorable and depends on the defect type. Hydrogenation of vacancies facilitates the binding of graphene layers, but the impact wanes already at the second coordination sphere. Defects influence of 5|7 is lower but promotes diamondization. The grain boundary role is similar but can lead to the final formation of a diamond film consisting of chemically connected grains with different surfaces. Interestingly, even hexagonal and cubic two-dimensional diamonds can coexist together in the same film, which suggests the possibility of obtaining a new two-dimensional polycrystal unexplored before.