Initial Nucleation Stage Research Articles

Morphological study of branched Sn structure formed under selected electrochemical conditions

A controlled electrodeposition process of branched micrometric and nanometric metallic tin structures was developed. Selected potentiostatic and galvanostatic techniques were explored with the aim of forming hierarchical shaped Sn on carbon porous electrodes by a simple template-free synthesis. We have studied the influence of continuous potential steps ranging from −0.9 to −4 V versus Ag/AgCl which show a classical nucleation growth mechanism. Under high overpotentials above −1.5 V, the reaction is governed by mass transport, which enables the development of vertically aligned dendrites. Upon reaching a dendrite particle size of 2–5 µm, Sn2+ reduction is facilitated on branches extending at an angle of about 45° from the main stem due to enhanced spherical diffusion to these newly evolving sites. A competing reaction of hydrogen evolution plays a significant role during initial nucleation stages and throughout particle evolution by reducing the overall columbic efficiency. Further study of means to affect the mass transport and morphology has led us to investigate the influence of pulse deposition duty cycle as well as use of anionic (SDS—sodium dodecyl sulfate) and cationic (HDTAB—hexadecyltrimethylammonium bromide) surfactants. While short pulses and long rest time promote the formation of high surface density of small nuclei, surfactants directly influence the tin ions (SDS) or adsorbed on the negatively charged electrode (HDTBA). Finally, the study of an electrodeposition method using strong acid additives was developed. It is shown from SEM and EQCM studies that careful selection of the acid type and concentration gives rise to the formation of a much more advanced network structure promoted by selective etching and co-reduction of dissolved ions. Highly interesting two-dimensional tin films formed in this process are also reported.

Droplet formation and growth inside a polymer network: A molecular dynamics simulation study.

We present a molecular dynamics simulation study that focuses on the formation and growth of nanoscale droplets inside polymer networks. Droplet formation and growth are investigated by the liquid-vapor phase separation of a dilute Lennard-Jones (LJ) fluid inside regularly crosslinked, polymer networks with varying mesh sizes. In a polymer network with small mesh sizes, droplet formation can be suppressed, the extent of which is dependent on the attraction strength between the LJ particles. When droplets form in a polymer network with intermediate mesh sizes, subsequent growth is significantly slower when compared with that in bulk without a polymer network. Interestingly, droplet growth beyond the initial nucleation stage occurs by different mechanisms depending on the mesh size: droplets grow mainly by diffusion and coalescence inside polymer networks with large mesh sizes (as observed in bulk), whereas Ostwald ripening becomes a more dominant mechanism for droplet growth for small mesh sizes. The analysis of droplet trajectories clearly reveals the obstruction effect of the polymer network on the movement of growing droplets, which leads to Ostwald ripening of droplets. This study suggests how polymer networks can be used to control the growth of nanoscale droplets.

How does shear zone nucleate? An example from the Suretta nappe (Swiss Eastern Alps)

In order to address the question of the processes involved during shear zone nucleation, we present a petro-structural analysis of millimetre-scale shear zones within the Roffna rhyolite (Suretta nappe, Eastern central Alps). Field and microscopic evidences show that ductile deformation is localized along discrete fractures that represent the initial stage of shear zone nucleation. During incipient brittle deformation, a syn-kinematic metamorphic assemblage of white mica + biotite + epidote + quartz precipitated at ca. 8.5 ± 1 kbar and 480 ± 50 °C that represent the metamorphic peak conditions of the nappe stacking in the continental accretionary wedge during Tertiary Alpine subduction. The brittle to ductile transition is characterized by the formation of two types of small quartz grains. The Qtz-IIa type is produced by sub-grain rotation. The Qtz-IIb type has a distinct CPO such that the orientation of c-axis is perpendicular to the shear fracture and basal and rhombhoedric slip systems are activated. These Qtz-IIb grains can either be formed by recrystallization of Qtz-IIa or by precipitation from a fluid phase. The shear zone widening stage is characterized by a switch to diffusion creep and grain boundary sliding deformation mechanisms. During the progressive evolution from brittle nucleation to ductile widening of the shear zone, fluid–rock interactions play a critical role, through chemical mass-transfer, metasomatic reactions and switch in deformation mechanisms.

Statistical analysis of formation and relaxation of atomic clusters based on data of molecular-dynamic modeling of gas-phase nucleation of metallic nanoparticles

Statistical analysis of the results of molecular-dynamic modeling for nucleation of metal (Cu) vapors in inert gas atmosphere (Ar) is carried out. Features of the initial stage of nucleation are considered for estimation of the probability of the growth of small atomic clusters within a supersaturated nonequilibrium medium and the efficiency of heat removal from them during collisions with inert gas atoms and other metal atoms.

Insights into the initial stages of lipid-mediated haemozoin nucleation

Lipid-mediated haemozoin nucleation, as probed by molecular dynamics, proceeds via aggregation of ferrihaem π–π dimers at a lipid–aqueous interface.

The energy and stability of helium-related cluster in nickel: A study of molecular dynamics simulation

Using molecular dynamics simulation, we investigated the energy and stability of helium-related cluster in nickel. All the binding energies of the He-related clusters are demonstrated to be positive and increase with the cluster sizes. Due to the pre-existed self-interstitial nickel atom, the trapping capability of vacancy to defects becomes weak. Besides, the minimum energy configurations of He-related clusters exhibit the very high symmetry in the local atomistic environment. And for the HeN and HeNV1SIA1 clusters, the average length of He–He bonds shortens, but it elongates for the HeNV1 clusters with helium cluster sizes. The helium-to-vacancy ratio plays a decisive role on the binding energies of HeNVM cluster. These results can provide some excellent clues to insight the initial stage of helium bubbles nucleation and growth in the Ni-based alloys for the Generation-IV Molten Salt Reactor.

Aggregation behaviour of amphiphilic cyclodextrins: the nucleation stage by atomistic molecular dynamics simulations.

Amphiphilically modified cyclodextrins may form various supramolecular aggregates. Here we report a theoretical study of the aggregation of a few amphiphilic cyclodextrins carrying hydrophobic thioalkyl groups and hydrophilic ethylene glycol moieties at opposite rims, focusing on the initial nucleation stage in an apolar solvent and in water. The study is based on atomistic molecular dynamics methods with a “bottom up” approach that can provide important information about the initial aggregates of few molecules. The focus is on the interaction pattern of amphiphilic cyclodextrin (aCD), which may interact by mutual inclusion of the substituent groups in the hydrophobic cavity of neighbouring molecules or by dispersion interactions at their lateral surface. We suggest that these aggregates can also form the nucleation stage of larger systems as well as the building blocks of micelles, vesicle, membranes, or generally nanoparticles thus opening new perspectives in the design of aggregates correlating their structures with the pharmaceutical properties.

Cross-Seeding Interaction between β-Amyloid and Human Islet Amyloid Polypeptide.

Alzheimer's disease (AD) and type 2 diabetes (T2D) are two common protein misfolding diseases. Increasing evidence suggests that these two diseases may be correlated with each other via cross-sequence interactions between β-amyloid peptide (Aβ) associated with AD and human islet amyloid polypeptide (hIAPP) associated with T2D. However, little is known about how these two peptides work and how they interact with each other to induce amyloidogenesis. In this work, we study the effect of cross-sequence interactions between Aβ and hIAPP peptides on hybrid amyloid structures, conformational changes, and aggregation kinetics using combined experimental and simulation approaches. Experimental results confirm that Aβ and hIAPP can interact with each other to aggregate into hybrid amyloid fibrils containing β-sheet-rich structures morphologically similar to pure Aβ and hIAPP. The cross-seeding of Aβ and hIAPP leads to the coexistence of both a retarded process at the initial nucleation stage and an accelerated process at the fibrillization stage, in conjunction with a conformational transition from random structures to α-helix to β-sheet. Further molecular dynamics simulations reveal that Aβ and hIAPP oligomers can efficiently cross-seed each other via the association of two highly similar U-shaped β-sheet structures; thus, conformational compatibility between Aβ and hIAPP aggregates appears to play a key role in determining barriers to cross-seeding. The cross-seeding effects in this work may provide new insights into the molecular mechanisms of interactions between AD and T2D.

Controllable growth of single-crystal graphene

Chemical vapor deposition (CVD) is one of the popular methods for the controllable synthesis of large-area high-quality graphene. CVD grown graphene over metals can be established over large area and this is important for applications, for example transparent conducting electrodes for solar cells, where a contiguous covering of graphene is required. However, due to the morphology of the catalytic substrates and the kinetic factors in the process of growth, the as-synthesized graphene is polycrystalline film merged by many small domains. The grain boundaries will make the physical and chemical properties far different from those of the intrinsic graphene. Instead, ideal single crystals have no grain boundaries, resulting in excellent properties with are close to the theoretical expectations. Accordingly, much effort has been devoted to the controllable synthesis of single-crystal graphene in terms of its size and morphology, which will have significant influence on its properties. It has been demonstrated that the crucial point for the growth of large single-crystal graphene is to reduce the nucleation densities. To achieve this, many strategies have been employed to optimize the morphology of the catalytic substrates, such as electropolishing, high-pressure annealing and resolidifying and to control the growth environment, such as equilibrating the catalyst vapor, using enclosure-like catalyst structures, maintaining inactive layer during the initial nucleation stage and so on. In spite of the size, the shape of graphene crystals can be modulated by the crystallographic orientation of substrates and the growth conditions. The competition and equilibrium between the intrinsic crystal form of graphene and the crystallographic orientation of substrates lead to the final shape of the graphene crystal. In addition, a deep insight into the mechanism of size and morphology will offer us an access to understand the growth process of graphene crystals. Here, the extreme importance of large-area graphene single crystals for electrical applications when comparing to polycrystalline graphene film have been highlighted and the representative accomplishments of single-crystal graphene using various metal catalyst or insulating substrate by CVD will be reviewed, including the derivations of its size and morphology and its applications in electronic devices. Further, the spatial structure of graphene crystals will be presented, which can offer more information about the growth mechanism of graphene crystals and open a new territory for graphene-based electronic and optical devices. The opportunities and challenges in the controllable growth of graphene single crystal will also be discussed.

Microstructures of InN film on 4H-SiC (0001) substrate grown by RF-MBE**Project supported by the Thailand Center of Excellence in Physics (ThEP) and the King Mongkut's University of Technology Thonburi under The National Research University Project. One of the authors (S. Sanorpim) was supported by the National Research Council of Thailand (NRCT) and the Thai Government Stimulus Package 2 (TKK2555), under

InN film was grown on 4H-SiC (0001) substrate by RF plasma-assisted molecular beam epitaxy (RF-MBE). Prior to the growth of InN film, an InN buffer layer with a thickness of ∼5.5 nm was grown on the substrate. Surface morphology, microstructure and structural quality of InN film were investigated. Micro-structural defects, such as stacking faults and anti-phase domain in InN film were carefully investigated using transmission electron microscopy (TEM). The results show that a high density of line contrasts, parallel to the growth direction (c-axis), was clearly observed in the grown InN film. Dark field TEM images recorded with diffraction vectors and g = 0002 revealed that such line contrasts evolved from a coalescence of the adjacent misoriented islands during the initial stage of the InN nucleation on the substrate surface. This InN nucleation also led to a generation of anti-phase domains.

Electrochemistry of Pb(II)/Pb during preparation of lead wires from PbO in choline chloride—urea deep eutectic solvent

The electrochemistry of Pb(II)/Pb on a stainless steel electrode during the preparation of lead wires from PbO in choline chloride (ChCl)-urea deep eutectic solvent (DES) was investigated by means of cyclic voltammetry, cathodic polarization and chronoamperometry. The experimental results indicated that the reduction of Pb(II) to Pb is a quasi—reversible process controlled by diffusion at temperature varying from 323 to 343 K, and the corresponding apparent activation energy Ea is 52.37 kJ mol-1. The analysis of chronoamperometry measurements suggested that the initial stage of nucleation of lead on stainless steel electrode is a three-dimensional instantaneous nucleation under diffusion control. The effects of reaction time and temperature on the morphology of lead deposits are also examined. The lead wires obtained at 343 K for 120 min have a mean particle size of 30 µ.m in length and 2.5 µ.m in diameter. Based on experimental evidence, the deposition mechanism of sub-micrometer lead wires on stainless steel substrate is proposed by diffusion controlled growth mechanism.

Molecular dynamic simulation for the evaluation of free energy distribution along the reaction coordinates at the initial stage of carbon nanotube nucleation

In order to evaluate the free energy associated with supplying an additional carbon atom to a catalyst cluster, we performed umbrella sampling and used the weighted histogram analysis in our classical molecular dynamic simulation. We assumed a one-dimensional reaction coordinate and derived the free energy. The derived free energy distribution was consistent with the observed phenomena and we found a reaction barrier of 0.6eV near the cluster surface. Beyond the reaction barrier, the free energy increased with increasing density of the carbon atoms, and this indicated that the saturation started from the cluster center.

Self-assembled ZnO nanoparticles on ZnO microsheet: ultrafast synthesis and tunable photoluminescence properties

We report on the tunable photoluminescence characteristics of porous ZnO microsheets fabricated within 1–5 min of microwave irradiation in the presence of a capping agent such as citric acid, and mixture of citric acid with polyvinylpyrrolidone (PVP). The UV emission intensity reduces to 60% and visible emission increases tenfold when the molar concentration of citric acid is doubled. Further diminution of the intensity of UV emission (25%) is observed when PVP is mixed with citric acid. The addition of nitrogen donor ligands to the parent precursor leads to a red shift in the visible luminescence. The deep level emission covers the entire visible spectrum and gives an impression of white light emission from these ZnO samples. The detailed luminescence mechanism of our ZnO samples is described with the help of a band diagram constructed by using the theoretical models that describe the formation energy of the defect energy levels within the energy band structure. Oxygen vacancies play the key role in the variation of the green luminescence in the ZnO microsheets. Our research findings provide an insight that it is possible to retain the microstructure and simultaneously introduce defects into ZnO. The growth of the ZnO microsheets may be due to the self assembly of the fine sheets formed during the initial stage of nucleation.

IN SITU INFRARED MEASUREMENTS OF FREE-FLYING SILICATE DURING CONDENSATION IN THE LABORATORY

We developed a new experimental system for infrared (IR) measurements on free-flying nucleating nanoparticles in situ and applied it to studies on silicate particles. We monitored the condensation of magnesium-bearing silicate nanoparticles from thermally evaporated magnesium and silicon monoxide vapor under an atmosphere of oxygen and argon. The IR spectrum of newly condensed particles showed a spectral feature for non-crystalline magnesium-bearing silicate that is remarkably consistent with the IR spectrum of astronomically observed non-crystalline silicate around oxygen-rich evolved stars. The silicate crystallized at <500 K and eventually developed a high crystallinity. Because of the size effects of nanoparticles, the silicate would be expected to be like a liquid at least during the initial stages of nucleation and growth. Our experimental results therefore suggest decreasing the possible formation temperature of crystalline silicates in dust formation environments with relatively higher pressure.

Nucleation and initial growth of atomic layer deposited titanium oxide determined by spectroscopic ellipsometry and the effect of pretreatment by surface barrier discharge

This paper reports on the use of spectroscopic ellipsometry to characterise the initial nucleation stage of the atomic layer deposition of the anatase phase of titanium dioxide on silicon substrates. Careful control and analysis of the ellipsometric measurements enables the determination of the evolution of crystallite diameter and surface density in the nucleation stage before a continuous film is formed. This growth behaviour is in line with atomic force microscopy measurements of the crystallite size. The crystallite diameter is a linear function of the number of ALD cycles with a slope of approximately 1.7Å cycle−1 which is equivalent to a layer growth rate of 0.85Å cycle−1 consistent with a ripening process which increases the crystallite size while reducing their density. The crystallite density decreases from ∼3×1017m−3 in the initial nucleation stages to ∼3×1015m−3 before the film becomes continuous. The effect of exposing the substrate to a diffuse coplanar surface barrier discharge in an air atmosphere before deposition was measured and only small differences were found: the plasma treated samples were slightly rougher in the initial stages and required a greater number of cycles to form a continuous film (∼80) compared to the untreated films (∼50). A thicker layer of native oxide was found after plasma treatment.

Probing the early stages of salt nucleation--experimental and theoretical investigations of sodium/potassium thiocyanate cluster anions.

Due to the fast solvent evaporation in electrospray ionization (ESI), the concentration of initially dilute electrolyte solutions rapidly increases to afford the formation of supersaturated droplets and generation of various pristine anhydrous salt clusters in the gas phase. The size, composition, and charge distributions of these clusters, in principle, witness the nucleation evolution in solutions. Herein, we report a microscopic study on the initial stage of nucleation and crystallization of sodium/potassium thiocyanate salt solutions simulated in the ESI process. Singly charged Mx(SCN)x+1(-), doubly charged My(SCN)y+2(2-) (M = Na, K), and triply charged Kz(SCN)z+3(3-) anion clusters (x, y, and z stand for the number of alkali atoms in the singly, doubly, and triply charged clusters, respectively) were produced via electrospray of the corresponding salt solutions and were characterized by negative ion photoelectron spectroscopy (NIPES). The vertical detachment energies (VDEs) of these sodium/potassium thiocyanate cluster anions were obtained, and theoretical calculations were carried out for the sodium thiocyanate clusters in assisting spectral identification. The measured VDEs of singly charged anions Mx(SCN)x+1(-) (M = Na and K) demonstrate that they are superhalogen anions. The existence of doubly charged anions My(SCN)y+2(2-) (y = 2x, x ≥ 4 and 3 for M = Na and K, respectively) and triply charged anions Kz(SCN)z+3(3-) (z = 3x, x ≥ 6) was initially discovered from the photoelectron spectra for those singly charged anions of Mx(SCN)x+1(-) with the same mass-to-charge ratio (m/z), and later independently confirmed by the observation of their distinct mass spectral distributions and by taking their NIPE spectra for those pure multiply charged anions with their m/z different from the singly charged species. For large clusters, multiply charged clusters were found to become preferred, but at higher temperatures, those multiply charged clusters were suppressed. The series of anion clusters investigated here range from molecular-like M1(SCN)2(-) to nano-sized K22(SCN)25(3-), providing a vivid molecular-level growth pattern reflecting the initial salt nucleation process.

Gallium nitride nanowires and microwires with exceptional length grown by metal organic chemical vapor deposition via titanium film

We present a new approach for synthesis of GaN nanowires and microwires by metal organic chemical vapor deposition via a thin titanium film evaporated onto sapphire substrate prior to growth. Titanium etches a two-dimensional GaN layer deposited at the initial stage and GaN nanowires subsequently emerge at the boundaries of the etched grains. These wires grow at an exceptional elongation rate of 18 μm/min and extend radially at a rate of 0.14 μm/min. The GaN layer between the wires grows at a rate of 0.1 μm/min. High material quality of these structures is confirmed by micro-photoluminescence spectroscopy. We investigate the initial nucleation stage, the time evolution of the wire length and diameter, the length and diameter distributions and speculate about a mechanism that yields the observed growth behavior.

Surface Morphology Evolution of Cu Thin Films Electrodeposited Directly on Ti Diffusion Barrier in Citric Acid

The formation of Cu thin film directly on Ti diffusion barrier by electrodeposition is important for next generation Cu interconnect in Si-based microelectronic devices. It depends on the surface morphology evolution of Cu deposit at the initial stage of nucleation and growth. In this study, we investigated the surface morphology evolution of Cu thin film on Ti in citric acid solution, introducing several simple ways to determine the stages of nucleation, Cu nuclei overlap, and Cu film formation with deposition time. During potentiostatic Cu deposition, studies on morphology change using the chronoamperometry, the change of surface roughness, and the change of sheet resistance and sheet resistivity are carried out along with microscopic observation. Resultant surface morphology of Cu deposit on Ti is greatly influenced by nucleation behavior, and its dependence on deposition potential is also studied in relation with the calculation of critical Cu nucleus size. According to the suggested ways, it is available to indirectly observe the transition of Cu nuclei to Cu thin film on Ti diffusion barrier during electrodeposition.

Ni cluster nucleation and growth on the anatase TiO2(101) surface: a density functional theory study

The Ni–TiO2 interaction is the main driving force at the initial stage of Ni nucleation. The Ni–Ni interaction begins to control the growth process as the cluster size gets larger.

Electrochemical analyses of diffusion behaviors and nucleation mechanisms for neodymium complexes in [DEME][TFSA] ionic liquid

ABSTRACT The electrochemical and nucleation behavior of Nd(III) in the ammonium-based ionic liquid (IL), N,N -diethyl- N -methyl- N -(2-methoxyethyl) ammonium bis(trifluoromethyl-sulfonyl) amide, [DEME] [TFSA], were investigated in this study. The cathodic reaction of Nd(III) [Nd(III) + 3e − → Nd(0)] was observed at −3.30 V vs. Ag/Ag(I) using cyclic voltammetry at 353 K. The diffusion coefficient of Nd(III) was estimated to be 1.35 ± 0.10 × 10 −13 m 2 s −1 at 353 K using semi-integral and semi-differential analyses. The initial process of Nd electrodeposition was also evaluated by chronoamperometry, indicating that the initial nucleation and growth of Nd on the Pt electrode occurred via instantaneous nucleation at −3.40 V. As the applied potential became more negative, the mechanism changed from instantaneous to progressive nucleation. The number density of Nd nuclei in the initial stage of nucleation decreased as the overpotential increased. Furthermore, the electrodeposition of Nd was carried out under the conditions of −3.40 V and −3.60 V at 353 K. SEM observations of the electrodeposits were consistent with the series of results obtained by chronoamperometry. The electrodeposits consisted mainly of Nd metal and oxide mixtures, whereas bonding with the light elements (C, F, and S) of the IL was suppressed, as demonstrated by EDX and XPS. The results suggested that sufficient dehydration and control of the water content of the electrolyte are important factors for obtaining metallic Nd with high purity.