Nucleation Delay Research Articles

Inhibition of Struvite Crystallization by Tetrasodium Pyrophosphate in Artificial Urine: Chemical and Physical Aspects of Nucleation and Growth

The effect of tetrasodium pyrophosphate on struvite nucleation and growth in the context of infectious urinary stone formation is studied. The crystallization processes occurred in artificial urine, while urease activity was simulated in a chemical way. The spectrophotometric results show that the presence of tetrasodium pyrophosphate causes a delay of struvite nucleation and decrease in growth efficiency. These results are explained on the basis of theoretical chemical speciation analysis of different complexes formed in artificial urine. The obtained results clearly show that the formation of the MgP2O72– complex plays the most important role in inhibition of struvite nucleation. It has been also found that tetrasodium pyrophosphate causes changes in struvite morphology and habit.

Plasma-Enhanced Atomic Layer Deposition of Iron Phosphate as a Positive Electrode for 3D Lithium-Ion Microbatteries

A new plasma-enhanced atomic layer deposition process was developed to deposit iron phosphate by using a sequence of trimethyl phosphate (TMP, Me3PO4) plasma, O2 plasma, and tert-butylferrocene (TBF, Fe(C5H5)(C5H4C(CH3)3)) exposures. Using in situ spectroscopic ellipsometry and ex situ X-ray reflectometry, the growth linearity, growth per cycle (GPC), and density of the resulting thin films was investigated as a function of the pulse times and the substrate temperature. At a substrate temperature of 300 °C and using saturated pulse times, an exceptionally high GPC of 1.1 nm/cycle without nucleation delay was achieved, resulting in amorphous films with an empirical stoichiometry of FeP1.5O4.7 with 0.9% hydrogen and no detectable carbon residue. Trigonal FePO4 (Berlinite) was formed upon annealing in air. Remarkably, annealing in helium resulted in the formation of elemental phosphorus. The as-deposited, amorphous material became active as a Li-ion cathode after an initial irreversible electrochemical lithi...

Exploring the effect of poly(acrylic acid) on pre- and post-nucleation BaSO4species: new insights into the mechanisms of crystallization control by polyelectrolytes

Barium sulphate (BaSO4) precipitation has been suggested to occur by non-classical pathways that include the formation of a dense liquid precursor phase, nucleation of primary nanoparticles and two levels of oriented aggregation resulting in micron-sized barite single crystals. In this study we build from this previous knowledge and explore how Ba2+ and SO42− ions associate in solution prior to nucleation of a solid phase and the effects of poly(acrylic acid) (PAA) in these pre-nucleation and post-nucleation stages. With this aim, in situ potentiometric experiments and transmission electron microscopy (TEM) observations of time-resolved quenched samples were carried out. Additional bulk precipitation experiments in which supersaturation was achieved by rapid mixing of Ba- and SO4-bearing solutions were performed. The resultant precipitates were characterized by scanning electron microscopy (FESEM and ESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analyses. Our study provides evidence that barium sulphate precipitation occurs via the formation of ion associates in solution (ion pairs and/or clusters), that are significantly destabilized in the presence of PAA. This is associated with a noticeable delay in nucleation in the presence of PAA. Thus, our results provide indirect evidence that suggests that prenucleation ion associates must form prior to solid BaSO4 nucleation. Alternatively, BaSO4 mineralization in the presence of PAA seems to occur by a different route that consists in the formation of Ba-PAA globules in solution followed by an amorphous hydrated BaSO4 phase that transform into crystalline barite nanoparticles. PAA seems to stabilize this amorphous phase, which nevertheless also forms in the absence of PAA. Finally, single micron-sized crystals are formed by the oriented attachment of distinguishable smaller subunits, thus forming mesocrystals in the presence and absence of PAA.

Facet-Selective Nucleation and Conformal Epitaxy of Ge Shells on Si Nanowires.

Knowledge of nanoscale heteroepitaxy is continually evolving as advances in material synthesis reveal new mechanisms that have not been theoretically predicted and are different than what is known about planar structures. In addition to a wide range of potential applications, core/shell nanowire structures offer a useful template to investigate heteroepitaxy at the atomistic scale. We show that the growth of a Ge shell on a Si core can be tuned from the theoretically predicted island growth mode to a conformal, crystalline, and smooth shell by careful adjustment of growth parameters in a narrow growth window that has not been explored before. In the latter growth mode, Ge adatoms preferentially nucleate islands on the {113} facets of the Si core, which outgrow over the {220} facets. Islands on the low-energy {111} facets appear to have a nucleation delay compared to the {113} islands; however, they eventually coalesce to form a crystalline conformal shell. Synthesis of epitaxial and conformal Si/Ge/Si core/multishell structures enables us to fabricate unique cylindrical ring nanowire field-effect transistors, which we demonstrate to have steeper on/off characteristics than conventional core/shell nanowire transistors.

Reducing ice accumulation and adhesion by using a flexible micro-rod film

Ice accumulation and adhesion on solid surfaces can cause various problems in aircrafts, power lines, roads, ships, wind turbines, oil platforms, and telecommunication systems. A feasible strategy to combat icing is to cover the surface by using a flexible hydrophobic film. In this study, ice accumulation and adhesion were reduced by using a flexible PDMS micro-rod film. A simulative closed-loop ice wind tunnel and an ice adhesion measurement apparatus were custom built to investigate the relationship between PMDS micro-rod geometry and ice accumulation and adhesion. Our experiments show that decreasing the micro-rod space on the flexible PDMS film will increase the contact angle and decrease the contact angle hysteresis of the surface, thus reducing ice accumulation and adhesion. Wettability change is interpreted by a speculative and qualitative model of intruding depth variation. The reduction in ice accumulation is attributed to ice nucleation delay because of the increased energy barrier in the heterogeneous nucleation. The reduction in ice adhesion is explained by the reduced ice–solid contact area and mechanical interlock. Results indicate that the flexible micro-rod film has great potential in future anti-icing applications.

The Mechanical Behavior of ALD‐Polymer Hybrid Films Under Tensile Strain

Atomic layer deposition (ALD) is increasingly becoming commonplace to deposit high density, conformal films, as novel deposition techniques allow improved deposition rates on various substrates. Diffusion and miscibility of ALD precursors with polymers are known to affect film nucleation. To assess the influence of polymeric underlayer on the growth and mechanical behavior of ALD films, we deposited aluminum oxide (Al2O3) and titanium dioxide (TiO2) films on three different polymers with known reactivity to the ALD precursors and compared them to films on polyethylene terephthalate (PET). We present the trends in nucleation and penetration of the ALD films on the various polymers as well as the in situ mechanical behavior under tensile strain. X‐ray photoelectron spectroscopy (XPS) revealed that Cytop, a fluorinated polymer and polyethyleneimine ethoxylated (PEIE) delay the nucleation of Al2O3 and TiO2 films. Time‐of‐flight mass spectrometry (ToFSIMS) shows the extent of diffusion of the ALD precursors into the polymer underlayer, delaying nucleation. Tensile tests show that while the crack onset strain remained unchanged despite the various polymer underlayers, the crack saturation density decreased for ALD films on PEIE and polyhydroxystyrene (PHOST). Films on Cytop showed no crack saturation limit until the nucleation delay was overcome indicating that judicious choice of the ALD film and polymer underlayer will allow tailoring of the length of the diffusion zone and crack saturation, but not the crack onset strain, which will be addressed in future studies.

Atomic Layer Deposition of Aluminum Phosphate Based on the Plasma Polymerization of Trimethyl Phosphate

Aluminum phosphate thin films were deposited by plasma-assisted atomic layer deposition (ALD) using a sequence of trimethyl phosphate (TMP, Me3PO4) plasma, O2 plasma, and trimethylaluminum (TMA, Me3Al) exposures. In situ characterization was performed, including spectroscopic ellipsometry, optical emission spectroscopy, mass spectrometry and FTIR. In the investigated temperature region between 50 and 320 °C, nucleation delays were absent and linear growth was observed, with the growth per cycle (GPC) being strongly dependent on temperature. The plasma polymerization of TMP was found to play an important role in this process, resulting in CVD-like behavior at low temperatures and ALD-like behavior at high temperatures. Films grown at 320 °C had a GPC value of 0.37 nm/cycle and consisted of amorphous aluminum pyrophosphate (Al4P6O21). They could be crystallized to triclinic AlPO4 (tridymite) by annealing to 900 °C, as evidenced by high-temperature XRD measurements. The use of a TMP plasma might open up the possibility of depositing many other metal phosphates by combining it with appropriate organometallic precursors.

Nanoscale order and crystallization in nitrogen-alloyed amorphous GeTe

The nanoscale order in amorphous GeTe thin films is measured using fluctuation transmission electron microscopy (FTEM). The order increases upon annealing at 145 °C, which indicates a coarsening of subcritical nuclei. This correlates with a reduction in the nucleation delay time in laser crystallization. A shift in the FTEM peak positions may indicate a transformation in local bonding. In samples alloyed with 12 at. % nitrogen, the order does not change upon annealing, the peak does not shift, and the nucleation time is longer. The FTEM data indicate that nitrogen suppresses the structural evolution necessary for the nucleation process and increases the thermal stability of the material.

Growth of highly conformal ruthenium-oxide thin films with enhanced nucleation by atomic layer deposition

Highly conformal and conductive RuO2 thin films were deposited without nucleation delay using atomic layer deposition (ALD) by zero-valent metallorganic precursor, (ethylbenzyl)(1,3-cyclohexadienyl)Ru(0) (EBCHDRu, C14H18Ru) and molecular oxygen (O2) as a precursor and reactant, respectively. RuO2 thin films could be successfully prepared by controlling the process parameters, such as a reactant flow rate, a reactant pulsing time, a precursor pulsing time, and a deposition temperature. X-ray diffractometry, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry analysis revealed that the formation of a RuO2 phase became favorable with increasing both the reactant flow rate and the pulsing time and with decreasing the precursor pulsing time and the deposition temperature. With the optimized pulsing conditions, the RuO2 film deposited at 225°C had a tetragonal structure and exhibited excellent properties such as the low resistivity of 118μΩ-cm, the high density of 6.85g/cm3 close to the bulk value, and the negligible roughness of 0.33nm. The growth rate of ALD-RuO2 was as high as 0.186nm/cycle on the SiO2 substrate and the number of incubation cycles was negligible as 2. The film showed excellent step coverage of ∼100% onto 25-nm-width trench structures with an aspect ratio of 4.5. The ALD-RuO2 was highly stable up to annealing at 700°C in both O2 and N2 ambient. Finally, the ALD-RuO2 film was evaluated as a bottom electrode of a metal–insulator-metal capacitor with a high-k (dielectric constant) ALD-TiO2 dielectric. The dielectric constant of ALD-TiO2 was confirmed to be as high as ∼68. This extremely high dielectric constant was attributed to the formation of a rutile-structured TiO2 film on top of the ALD-RuO2 bottom electrode, as evidenced by high-resolution transmission electron microscopy analysis.

In situ XAS and XRF study of nanoparticle nucleation during O3-based Pt deposition

X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF) were combined in situ to study the ALD-based synthesis of Pt catalysts. This first time combination of synchrotron-based techniques was applied during the (methylcyclopentadienyl)trimethylplatinum/ozone deposition process executed at 150°C on a silica support. A nucleation delay indicative for nanoparticle formation was observed for Pt loadings below 1 equivalent monolayer. XAS and XRF were recorded simultaneously at different catalyst loadings in this nucleation regime. Analysis of the combined in situ data yielded a quantitative picture of the evolution of the diameter, shape, lattice packing and density of the deposited Pt clusters. Additionally, the degree of oxidation at the cluster surface after the ozone pulse could be monitored. At the early start of the deposition process, Pt adatoms cluster together to form stable nuclei. A strong increase in the density of nuclei is seen below 0.16 equivalent monolayers, after which coalescence gradually occurs. From 0.04 to 0.71 equivalent monolayers, Pt clusters are fcc packed and correspond best to a hemispherical (111)-truncated cuboctahedral shape. By crosslinking the XRF and XAS data, a linear increase in cluster diameter with Pt loading is observed within this range. The surface of the Pt clusters is shown to be oxidized immediately after the ozone exposure. The degree of surface oxidation remains approximately constant for clusters with a 1–3nm diameter. This surface oxygen is shown to be crucial for further growth during deposition. The combined application of in situ XRF and XAS thus allowed for an advanced identification of the ALD-deposited Pt nanoparticles.

A RHEED investigation of self‐assembled GaN nanowire nucleation dynamics on bare Si and on Si covered with a thin AlN buffer layer

Abstractmagnified imageBased on reflection high energy electron diffraction (RHEED) experiments, we show that the nucleation delay of GaN nanowire growth by plasma‐assisted molecular beam epitaxy on nitrided Si(111) and on AlN/Si(111) exhibit similar behaviors in function of the substrate temperature and of the Ga flux. On bare Si(111), a finite delay, estimated to be one minute in our system and under our set of growth conditions, is necessary to form and subsequently amorphize crystalline β‐Si3N4 before GaN can start to form. Although the amorphization time is found to be independent of the substrate temperature, this process adds a supplemental delay within the overall delay in the growth of GaN nanowires on bare Si(111), compared to growth on AlN/Si(111) or on amorphous silicon nitride on Si(111). (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Experimental and computational investigation of chemical vapor deposition of Cu from Cu amidinate

Experiments and computations are performed for the Cu MOCVD from copper(I) N,N′-di-isopropylacetamidinate [Cu(iPr–Me–amd)]2 or [Cu(amd)]2 where amd=CH(CH3)2NC(CH3)NCH(CH3)2. The a priori choice of this precursor is dictated mainly by its oxygen and halogen–free ligands allowing co-deposition with oxophilic elements such as Al and by its ability to provide conformal Cu films in atomic layer deposition processes. The nucleation delay and the deposition rate as a function of deposition temperature and the evolution of the deposition rate along the radius of the substrate holder are experimentally determined with depositions performed at 1333Pa in a vertical, warm wall, MOCVD reactor. With the aim to propose a kinetic scenario for Cu deposition, based on recently published experimental results for the decomposition of [Cu(amd)]2, a predictive 3D model of the process is built, based on the mass, momentum, energy and species transport equations. In agreement with the previously mentioned experimental results, it is demonstrated that a single surface reaction is responsible for the deposition of Cu. Two surface kinetics expressions are implemented depending on the deposition regime; a simple Arrhenius type expression in the reaction limited regime and a Langmuir–Hinshelwood type expression prevailing in the transport limited regime which takes into account the inhibition effects. The two different kinetics designate a modification in the surface reaction mechanism. The results show good agreement between experiments and computations. Complementary computations are performed, in order to compare the deposition rates of the Cu films deposited via the [Cu(amd)]2 and the (hfac)Cu(VTMS) and Cu(hfac)2 so as to determine relative advantages and disadvantages of Cu MOCVD from [Cu(amd)]2.

Influence of Oxygen Exposure on the Nucleation of Platinum Atomic Layer Deposition: Consequences for Film Growth, Nanopatterning, and Nanoparticle Synthesis

Control of the nucleation behavior during atomic layer deposition (ALD) of metals is of great importance for the deposition of metallic thin films and nanoparticles, and for nanopatterning applications. In this work it is established for Pt ALD, that the exposure to O2 during the O2 pulse of the ALD process is the key parameter controlling the nucleation behavior. The O2 dependence of the Pt nucleation is explained by the enhanced diffusion of Pt species in the presence of oxygen, and the resulting faster aggregation of Pt atoms in metal clusters that catalyze the surface reactions of ALD growth. Moreover, it is demonstrated that the O2 exposure can be used as the parameter to tune the nucleation to enable (i) deposition of ultrathin films with minimal nucleation delay, (ii) preparation of single element or core/shell nanoparticles, and (iii) nanopatterning of metallic structures based on area-selective deposition.

Electrical conductivity study of NOx trap materials BaCO3 and K2CO3/La-Al2O3 during NOx exposure

Alkaline (earth-) carbonates are applied in NOx storage and reduction catalysts, given their ability to accumulate NO2 in excess oxygen environments, and releasing it under rich-burn conditions. The dependency of their conductivity on the NOx loading state makes them suitable as NO2 sensitive layers and enables the in situ characterization of the status of such catalyst systems. The conductivity of the NOx trap components BaCO3 and K2CO3, with different morphologies, is analyzed in various NOx containing atmospheres by impedance spectroscopy. While the stepwise uptake of NO2 can be detected electrically on both carbonates, only a weak corresponding influence of NO is detected. The increase in the conductivity of the carbonates during NO2 accumulation can be described as a diffusion controlled process with a characteristic nucleation delay in the case of BaCO3. Arrhenius-like plots comparing the conductivities of the carbonates in the unloaded and NO2 saturated states from 250 to 600°C reveal that the conductivity of BaCO3 is more sensitive to NO2 than those of K2CO3/La-Al2O3, but the latter exhibiting about five orders of magnitude higher conductivity.

Wetting transitions of polymers via thermal and plasma enhanced atomic layer depositions

Polypropylene (PP) and a blend of poly(ethylene terephthalate)/cotton (PET/cotton) nonwoven fiber felts were modified using aluminum and zinc oxides via thermal and plasma enhanced atomic layer deposition (ALD and PEALD). Deposition of aluminum or zinc oxide on PP induced a wetting transition from the hydrophobic behavior characteristic of the untreated PP to a hydrophilic behavior for ALD coated PP. PEALD needed a fewer number of cycles to induce hydrophilic effect, compared to thermal ALD. This correlated with the observation of an initial nucleation delay for thermal ALD on PP, while PEALD resulted in deposition of oxide material from the initial cycles. The hydrophilic effect could be achieved for a wider range of deposition temperatures for PEALD as compared to thermal ALD. PEALD of aluminum and zinc oxides did not alter the surface properties of the PET/cotton felts, as they stayed hydrophilic even after 400 cycles of deposition, while thermal ALD of zinc oxide transformed hydrophilic PET/cotton felts to hydrophobic.

In situ study of self-assembled GaN nanowires nucleation on Si(111) by plasma-assisted molecular beam epitaxy

Nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy is studied through a combination of two in situ tools: grazing incidence x-ray diffraction and reflection high energy electron diffraction. Growth on bare Si(111) and on AlN/Si(111) is compared. A significantly larger delay at nucleation is observed for nanowires grown on bare Si(111). The difference in the nucleation delay is correlated to a dissimilarity of chemical reactivity between Al and Ga with nitrided Si(111).

Probing the Room Temperature Deuterium Absorption Kinetics in Nanoscale Magnesium Based Hydrogen Storage Multilayers Using Neutron Reflectometry, X-ray Diffraction, and Atomic Force Microscopy

Magnesium hydride has high storage capacity (7.6 wt % H) but very slow sorption kinetics. Addition of catalytic phases on the surface as well as alloying with transition metals is known to improve the properties. In this study, the sorption kinetics of a 50-nm Mg layer and Mg-10%Cr-10%V layer, capped with a CrV/Pd bilayer catalyst, are compared using a combination of neutron reflectometry (NR), X-ray diffraction (XRD), and atomic force microscopy to elucidate the effects of alloying on the hydrogen storage properties of Mg at room temperature. From NR it is found that the Cr–V alloyed layer shows both a delay in expansion in the first absorption cycle and a delay in contraction in the first desorption, which indicates a delay in nucleation of MgD2 and formation of substoichiometric MgD2-δ, respectively. Compared to pure Mg, the kinetics are strongly improved as no blocking MgD2 layer is formed. XRD showed a strong reduction in the Mg grain size for the Cr–V alloyed layer after one cycle. For pure Mg, the grain size is almost unchanged although the film becomes nanocrystalline in the hydrided state. NR is shown to be highly sensitive to both the deuterium distribution as well as the layer thickness, which makes it a valuable tool for studying reaction mechanisms and quantification of the expansion of both crystalline and amorphous energy storage materials.

Dynamics of Ice Nucleation on Water Repellent Surfaces

Prevention of ice accretion and adhesion on surfaces is relevant to many applications, leading to improved operation safety, increased energy efficiency, and cost reduction. Development of passive nonicing coatings is highly desirable, since current antiicing strategies are energy and cost intensive. Superhydrophobicity has been proposed as a lead passive nonicing strategy, yet the exact mechanism of delayed icing on these surfaces is not clearly understood. In this work, we present an in-depth analysis of ice formation dynamics upon water droplet impact on surfaces with different wettabilities. We experimentally demonstrate that ice nucleation under low-humidity conditions can be delayed through control of surface chemistry and texture. Combining infrared (IR) thermometry and high-speed photography, we observe that the reduction of water-surface contact area on superhydrophobic surfaces plays a dual role in delaying nucleation: first by reducing heat transfer and second by reducing the probability of heterogeneous nucleation at the water-substrate interface. This work also includes an analysis (based on classical nucleation theory) to estimate various homogeneous and heterogeneous nucleation rates in icing situations. The key finding is that ice nucleation delay on superhydrophobic surfaces is more prominent at moderate degrees of supercooling, while closer to the homogeneous nucleation temperature, bulk and air-water interface nucleation effects become equally important. The study presented here offers a comprehensive perspective on the efficacy of textured surfaces for nonicing applications.

Ru Films from Bis(ethylcyclopentadienyl)ruthenium Using Ozone as a Reactant by Atomic Layer Deposition for Capacitor Electrodes

Ru films were produced by atomic layer deposition (ALD) with an alternating supply of bis(ethylcyclopentadienyl)ruthenium (Ru(EtCp)2) and ozone at deposition temperatures of 225–275°C. Ozone acted as an effective reactant for Ru(EtCp)2. The Ru film thicknesses formed during one cycle were saturated at relatively high values of 0.09–0.12 nm/cycle depending on the deposition temperatures, and their resistivities were about 16 μΩ cm. Moreover, a reduced nucleation delay was found for Ru ALD using ozone when compared to Ru ALD using oxygen gas. The amount of oxygen impurity incorporated into the Ru films was less than 1 at%, as determined by Auger electron spectroscopy. The interfacial adhesion property between Ru films prepared via ALD using ozone (ozone-Ru) and ZrO2 was good and 80% step coverage was achieved on a 3-D structure with a very high aspect ratio of 16:1, making them suitable for use as a top electrode material.

Nucleation and Growth of Platinum Films on High-k/Metal Gate Materials by Remote Plasma and Thermal ALD

Platinum films were grown on Si wafers, SiO2, Al2O3 and high-k dielectric HfO2 ALD films on Si substrates by both remote plasma and thermal atomic layer deposition (ALD) at 300°C, using methylcyclopentadienyl_trimethyl platinum (MeCpPtMe3) and O2 as precursors. The ALD Pt-films deposited were homogeneous and resulted in a low resistivity of 12.8 μΩ-cm. AES studies revealed high quality Pt films deposited by both thermal and plasma ALD with carbon impurity less than 1.5% and oxygen found only in the interface. SEM and EDX were used to investigate Pt nucleation and growth in ALD processes. It is remarkable that the nucleation delay of between thermal and plasma ALD Pt was different though they are a similar growth rate of 0.45 Å/cycle. In this work, the Pt ALD nucleation and growth behaviours with precursor dose times, O2 or O2 plasma exposures and substrates are also investigated.