Initial Growth Step Research Articles

Effect of an iodine-containing additive on the composition, structure, and morphology of chemically deposited lead selenide films

The effect of an ammonium iodide additive on the elemental and phase compositions, structural parameters, and surface morphology of lead selenide films synthesized by chemical deposition from aqueous solutions has been studied using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis. It has been established that the obtained PbSe films have a multiphase structure. The iodine content of the films is directly proportional to the NH4I concentration in the reaction mixture and increases linearly with an increase in this concentration to 0.25 mol/L. No individual iodine-containing phases have been detected in the film structure. However, the introduction of iodine leads to an increase in the PbSe phase lattice parameter from ∼6.11 to ∼6.16 A and to a decrease in the crystal grain size to ∼ 20 nm. It has been found that there is a correlation between the grain size, lattice parameter, and ammonium iodide concentration in the reaction mixture, which can be explained by changes in the film growth mechanism at the initial growth steps.

The crystallization of sub-cooled water: Measuring the front velocity and mushy zone composition via thermal imaging

Water is one of the most abundant resources on Earth and the study of its phase changes holds many practical implications to both science and technology. In the past, several investigations have researched aspects of the solidification process that occurs when water freezes to ice. One of these facets is its associated linear growth velocity. Traditionally, this has been measured either through free growth or capillary tube techniques. The current study uses a methodology where measuring this velocity is accomplished through the use of infrared imaging. A conventional velocity vs. sub-cooling plot was generated and the data was fitted to a modified Noyes and Whitney growth model. Furthermore, with the use of the thermal images, a novel model was developed to calculate the percentage of water that was frozen after the initial growth step. From this, a conversion vs. sub-cooling plot was produced and the results showed that for the primary growth step, less ice forms under higher sub-coolings.

Viscous Fingering and Dendritic Growth of Surface Crystallized Sr2TiSi2O8 Fresnoite

During the quenching of a melt with the composition 2SrO·TiO2·2.75SiO2, cubic SrTiO3- and tetragonal Sr2TiSi2O8-crystals are formed at the surface. Subsequent crystal growth leads to dendritic fresnoite structures which become increasingly finer until the mechanism changes to viscous fingering during further cooling. In the final stages of this initial growth step, the crystal orientations of these dendrites systematically change. Due to a complete absence of bulk nucleation in this system, crystal growth is resumed upon reheating to 970°C and fractal growth with the c-axis tilted by about 45° from the main growth direction is observed. The results are interpreted to confirm the link between viscous fingering and dendritic growth in the case of a true crystallization process.

Identification of multiple and distinct defects in prostaglandin biosynthetic pathways in eutopic and ectopic endometrium of women with endometriosis

To investigate prostaglandin (PG) biosynthesis and catabolism pathways in eutopic and ectopic endometrium of women with endometriosis. Retrospective study. Human reproduction research laboratory. Forty-five women with endometriosis and 29 normal controls. Endometrial and endometriotic tissue samples were obtained during laparoscopic surgery. Cyclo-oxygenases (Coxs 1 and 2), PGE2 synthases (microsomal [m] PGES 1 and 2 and cytosolic [c] PGES), PGF2α synthases (aldoketoreductase [AKR]-1C3 and AKR-1B1), and the PG catabolic enzyme 15-hydroxyprostaglandin dehydrogenase messenger RNA expression by quantitative real-time polymerase chain reaction and protein localization by immunohistochemistry. This study showed a marked increase in the key PG biosynthesis enzymes Cox-2, mPGES-1, mPGES-2, cPGES, and AKR-1C3 in ectopic endometrial tissue of women with endometriosis, particularly in the earliest and most active stages of the disease, without a noticeable change in the expression of the PG catabolic enzyme 15-hydroxyprostaglandin dehydrogenase. Meanwhile, the significant increase in rate-limiting Cox-2 expression upstream was correlated downstream by a significant stage- and cycle phase-dependent decrease in the terminal specific synthase mPGES-2, thereby revealing the presence of counter-regulatory mechanisms, which operate in the eutopic endometrium of women with endometrium but seem to be lacking in the ectopic implantation sites. This study reveals for the first time multiple defects in PG biosynthesis pathways, which differ between eutopic intrauterine and ectopic endometrial tissues and may, owing to the wide spectrum of PG properties, contribute to the initial steps of endometrial tissue growth and development and have an important role to play in the pathogenesis and symptoms of this disease.

Initial Steps of Rubicene Film Growth on Silicon Dioxide.

The film growth of the conjugated organic molecule rubicene on silicon dioxide was studied in detail. Since no structural data of the condensed material were available, we first produced high quality single crystals from solution and determined the crystal structure. This high purity material was used to prepare ultrathin films under ultrahigh vacuum conditions, by physical vapor deposition. Thermal desorption spectroscopy (TDS) was applied to delineate the adsorption and desorption kinetics. It could be shown that the initial sticking coefficient is only 0.2 ± 0.05, but the sticking coefficient increases with increasing coverage. TDS further revealed that first a closed, weakly bound bilayer develops (wetting layer), which dewets after further deposition of rubicene, leading to an island-like layer. These islands are crystalline and exhibit the same structure as the solution grown crystals. The orientation of the crystallites is with the (001) plane parallel to the substrate. A dewetting of the closed bilayer was also observed when the film was exposed to air. Furthermore, Ostwald ripening of the island-like film takes place under ambient conditions, leading to films composed of few, large crystallites. From TDS, we determined the heat of evaporation from the multilayer islands to be 1.47 eV, whereas the desorption energy from the first layer is only 1.25 eV.

Gas phase formation of extremely oxidized pinene reaction products in chamber and ambient air

Abstract. High molecular weight (300–650 Da) naturally charged negative ions have previously been observed at a boreal forest site in Hyytiälä, Finland. The long-term measurements conducted in this work showed that these ions are observed practically every night between spring and autumn in Hyytiälä. The ambient mass spectral patterns could be reproduced in striking detail during additional measurements of α-pinene (C10H16) oxidation at low-OH conditions in the Jülich Plant Atmosphere Chamber (JPAC). The ions were identified as clusters of the nitrate ion (NO3−) and α-pinene oxidation products reaching oxygen to carbon ratios of 0.7–1.3, while retaining most of the initial ten carbon atoms. Attributing the ions to clusters instead of single molecules was based on additional observations of the same extremely oxidized organics in clusters with HSO4− (Hyytiälä) and C3F5O2− (JPAC). The most abundant products in the ion spectra were identified as C10H14O7, C10H14O9, C10H16O9, and C10H14O11. The mechanism responsible for forming these molecules is still not clear, but the initial reaction is most likely ozone attack at the double bond, as the ions are mainly observed under dark conditions. β-pinene also formed highly oxidized products under the same conditions, but less efficiently, and mainly C9 compounds which were not observed in Hyytiälä, where β-pinene on average is 4–5 times less abundant than α-pinene. Further, to explain the high O/C together with the relatively high H/C, we propose that geminal diols and/or hydroperoxide groups may be important. We estimate that the night-time concentration of the sum of the neutral extremely oxidized products is on the order of 0.1–1 ppt (~106–107 molec cm−3). This is in a similar range as the amount of gaseous H2SO4 in Hyytiälä during day-time. As these highly oxidized organics are roughly 3 times heavier, likely with extremely low vapor pressures, their role in the initial steps of new aerosol particle formation and growth may be important and needs to be explored in more detail in the future.

Chemical bonding states and atomic distribution within Zn(S,O) film prepared on CIGS/Mo/glass substrates by chemical bath deposition

Zn(S,O) thin films fabricated on CIGS/Mo/glass substrates by using chemical bath deposition (CBD) in acidic and basic solutions were studied. The Zn(S,O) thin films prepared in acidic solution [A-Zn(S,O) thin film] showed better crystallinity and a more compact surface morphology with larger grains than those prepared in basic solution [B-Zn(S,O) thin film] did. From the analysis of the chemical bonding states, at the initial growth step, the concentration ratio of Zn–O/Zn–S bonds in A-Zn(S,O) thin films was found to be approximately zero, while that in B-Zn(S,O) thin films was approximately equal to 1. The elemental distribution according to depth, determined by secondary ion mass spectroscopy (SIMS), was shown to be uniform throughout both the A- and B-Zn(S,O) thin films. To reduce the number of Zn–O bonds in the B-Zn(S,O) thin films, the samples were post-annealed at up to 300 °C under vacuum, after which the concentration ratio of Zn–O/Zn–S bonds decreased by about 71% without any change in the crystallinity or surface morphology.

Growth and interfacial properties of epitaxial oxides on semiconductors: ab initio insights

Crystalline metal oxides display a large number of physical functionalities such as ferroelectricity, magnetism, superconductivity, and Mott transitions. High quality heterostructures involving metal oxides and workhorse semiconductors such as silicon have the potential to open new directions in electronic device design that harness these degrees of freedom for computation or information storage. This review describes how first-principles theoretical modeling has informed current understanding of the growth mechanisms and resulting interfacial structures of crystalline, coherent, and epitaxial metal oxide thin films on semiconductors. Two overarching themes in this general area are addressed. First, the initial steps of oxide growth involve careful preparation of the semiconductor surface to guard against amorphous oxide formation and to create an ordered template for epitaxy. The methods by which this is achieved are reviewed, and possibilities for improving present processes to enable the epitaxial growth of a wider set of oxides are discussed. Second, once a heterointerface is created, the precise interfacial chemical composition and atomic structure is difficult to determine unambiguously from experiment or theory alone. The current understanding of the structure and properties of complex oxide/semiconductor heterostructures is reviewed, and the main challenges to prediction—namely, (i) are these heterostructures in thermodynamic equilibrium or kinetically trapped, and (ii) how do the interfaces modify or couple to the degrees of freedom in the oxide?—are explored in detail for two metal oxide thin films on silicon. Finally, an outlook of where theoretical efforts in this field may be headed in the near future is provided.

Kinetics and mechanism of metal–organic framework thin film growth: systematic investigation of HKUST-1 deposition on QCM electrodes

We describe a systematic investigation of the factors controlling step-by-step growth of the metal–organic framework (MOF) [Cu3(btc)2(H2O)3]·xH2O (also known as HKUST-1), using quartz crystal microbalance (QCM) electrodes as an in situ probe of the reaction kinetics and mechanism. Electrodes coated with silica, alumina and gold functionalized with OH– and COOH–terminated self-assembled monolayers (SAMs) were employed to determine the effects of surface properties on nucleation. Deposition rates were measured using the high sensitivity available from QCM-D (D = dissipation) techniques to determine rate constants in the early stage of the process. Films were characterized using grazing incidence XRD, SEM, AFM, profilometry and reflection–absorption IR spectroscopy. The effects of reaction time, concentration, temperature and substrate on the deposition rates, film crystallinity and surface morphology were evaluated. The initial growth step, in which the surface is exposed to copper ions (in the form of an ethanolic solution of copper(II) acetate) is fast and independent of temperature, after which all subsequent steps are thermally activated over the temperature range 22–62 °C. Using these data, we propose a kinetic model for the Cu3(btc)2 growth on surfaces that includes rate constants for the individual steps. The magnitude of the activation energies, in particular the large entropy decrease, suggests an associative reaction with a tight transition state. The measured activation energies for the step-by-step MOF growth are an order of magnitude lower than the value previously reported for bulk Cu3(btc)2 crystals. Finally, the results of this investigation demonstrate that the QCM method is a powerful tool for quantitative, in situ monitoring of MOF growth in real time.

Taper-Free and Vertically Oriented Ge Nanowires on Ge/Si Substrates Grown by a Two-Temperature Process

Taper-free and vertically oriented Ge nanowires were grown on Si (111) substrates by chemical vapor deposition with Au nanoparticle catalysts. To achieve vertical nanowire growth on the highly lattice mismatched Si substrate, a thin Ge buffer layer was first deposited, and to achieve taper-free nanowire growth, a two-temperature process was employed. The two-temperature process consisted of a brief initial base growth step at high temperature followed by prolonged growth at lower temperature. Taper-free and defect-free Ge nanowires grew successfully even at 270 °C, which is 90 °C lower than the bulk eutectic temperature. The yield of vertical and taper-free nanowires is over 90%, comparable to that of vertical but tapered nanowires grown by the conventional one-temperature process. This method is of practical importance and can be reliably used to develop novel nanowire-based devices on relatively cheap Si substrates. Additionally, we observed that the activation energy of Ge nanowire growth by the two-te...

Role of Interfacial Reaction in Atomic Layer Deposition of TiO2 Thin Films Using Ti(O-iPr)2(tmhd)2 on Ru or RuO2 Substrates

The role of the interfacial reactions in the atomic layer deposition of TiO2 films was examined using titanium diisopropoxide bis(tetramethylheptadionate) (Ti(O-iPr)2(tmhd)2) as the titanium precursor, and H2O or O3 as the oxygen sources when the films were grown on ruthenium (Ru) or ruthenium dioxide (RuO2) substrate at a growth temperature of 370 °C. Anatase-TiO2 films with a dielectric constant of 32 were grown on ruthenium substrates when H2O was used, whereas rutile-TiO2 films with a much higher dielectric constant (89) were grown when O3 was used as the oxygen source. The rutile-TiO2 film was grown with the aid of an in-situ-formed thin RuO2 layer on the surface by the strong oxidation power of O3, which has structural similarity to the rutile-TiO2 film, as has been reported previously with different precursor and growth temperature [S. K. Kim et al. Appl. Phys. Lett. 2004, 85, 4112]. The initial and steady-state growth rates of the TiO2 films were strongly dependent on the oxygen source. O3 induced a substrate-enhanced growth mode at the initial growth step (<25 cycles), whereas H2O resulted in a linear growth mode, regardless of the number of cycles, and a lower growth rate at the steady regime, because of its lower reactivity toward tmhd ligands. A substrate-enhanced growth mode was also observed on the RuO2 substrate, regardless of the oxygen source. Interestingly, RuO2 was reduced completely to metallic Ru during TiO2 film growth. Oxygen atoms that come from the reduction of the RuO2 substrate induced substrate-enhanced growth, and the reduction process was limited kinetically by the growth temperature. Reduction of the RuO2 substrate occurred at the early stages of TiO2 film growth. Therefore, it played a key role in determining the phase of the growing TiO2 films. Reoxidation of the reduced Ru substrate during the O3 pulse step promoted the formation of rutile TiO2.

Atmospheric nucleation and initial steps of particle growth: Numerical comparison of different theories and hypotheses

New particle formation in boreal forest environment in Hyytiälä, Finland, was studied in an aerosol dynamical model. Basing on the concepts of activation or kinetic collision of two pre-existing clusters at 1.5nm, several semi-empirical nucleation rate formulae were parameterized. The mechanisms had linear or squared dependence on the concentration of sulphuric acid, or a low volatile organic vapor, or both, and they all showed good agreement with field measurements. A new method for examining the power dependence of apparent formation rate (at 2nm or 3nm) on sulphuric acid concentration was developed. The new method produced exponents 1.6–2.1 for cluster activation, and exponents 2.4–3.1 for kinetic collision, which suggests that the activation scenarios are in better agreement with experimental observations that imply exponents around 1–2. However, it was found that if low volatile organic vapors with concentrations exceeding that of sulphuric acid are present, they have a major role in shaping the temporal behavior of the apparent formation rates, causing error in the exponent analyses. Finally, a sensitivity study showed that the analyzed exponents grew even further, if the size of the critical cluster was assumed smaller that 1.5nm.

Non-destructive depth compositional profiles by XPS peak-shape analysis

The measured peak shape and intensity of the photoemitted signal in X-ray photoelectron spectroscopy (XPS) experiments (elastic and inelastic parts included) are strongly correlated, through electron-transport theory, with the depth distribution of photoelectron emitters within the analyzed surface. This is the basis of so-called XPS peak-shape analysis (also known as the Tougaard method) for non-destructive determination of compositional in-depth (up to 6-8 nm) profiles. This review describes the theoretical basis and reliability of this procedure for quantifying amounts and distributions of material within a surface. The possibilities of this kind of analysis are illustrated with several case examples related to the study of the initial steps of thin-film growth and the modifications induced in polymer surfaces after plasma treatments.

Quantification of initial steps of nucleation and growth of silica nanoparticles: An in-situ SAXS and DLS study

The initial steps of silica polymerization and silica nanoparticle formation have been studied in-situ and in real-time. The experiments were carried out in near neutral pH (7–8) solutions with initial silica concentrations of 640 and 1600 ppm ([SiO 2]) and ionic strengths (IS) of 0.02, 0.05, 0.11 and 0.22 M. The polymerization reactions were induced by neutralizing a high pH silica solution (from pH 12 to 7) and monitored by the time-dependent depletion in monosilicic acid concentration over time. The accompanied nucleation and growth of silica nanoparticles (i.e., change in particle size over time) was followed in-situ using time-resolved synchrotron-based Small Angle X-ray Scattering (SAXS) and conventional Dynamic Light Scattering (DLS) combined with scanning and (cryo)-transmission electron microscopy (SEM/cryo-TEM). The critical nucleus diameter was quantified (1.4–2 nm) and results from SAXS and DLS showed that over 3 h the particle diameter increased to a final size of ∼8 nm. SEM and TEM photomicrographs verified the SAXS and DLS data and confirmed the spherical and hydrous structure of the forming silica nanoparticles. Furthermore, fractal analysis (i.e., fractal dimension, D m ∼ 2.2) indicated that the formed particles consisted of open, polymeric, low-density structures. For the nucleation and growth of silica nanoparticles a 3-stage growth process is proposed: (1) homogeneous and instantaneous nucleation of silica nanoparticles, (2) 3-D, surface-controlled particle growth following 1st order reaction kinetics and (3) Ostwald ripening and particle aggregation.

Organization of the vitelline envelope in ovarian follicles of Torpedo marmorata Risso, 1810 (Elasmobranchii: Torpediniformes)

In Torpedo marmorata, the vitelline envelope (VE), an extracellular envelope surrounding the growing oocyte, consists of fibrils and amorphous materials that are deposited in the perivitelline space starting from the initial steps of oocyte growth. SDS-PAGE analysis of the isolated and purified VE reveals that it consists of different glycoproteins. Furthermore, our investigations showed that the 120 and 66 kDa glycoproteins are positive to an antibody directed against gp69/64 of the Xenopus laevis VE and are synthesized under the control of 17beta-estradiol in the liver, that, together follicle cells and the oocyte, is the biosynthetic site of VE components.

Effect of co-adsorbed dopants on initial diamond growth steps: H abstraction from an adsorbed CH 3

The effect induced by a neighbouring co-adsorbed dopant on H abstraction from an adsorbed CH 3 species on diamond has been investigated by using an ultra-soft pseudo-potential density functional theory (DFT) method under periodic boundary conditions. Both the (100) and (111) diamond surface orientations were considered with various types of dopants in two different hydrogenated forms; AH x ( A = N, B, S, or P; X = 0 or 1 for S; X = 1 or 2 for N, B and P, and X = 2 or 3 for C). The H abstraction by gaseous radical H was found to be energetically favoured by the presence of the dopants in all of their different hydrogenated forms. For NH 2, SH, or PH 2, this effect is induced by a destabilisation of the diamond surface by sterical repulsions between the adsorbed growth species CH 3 and the co-adsorbed dopant. For BH 2 and the dopants in their radical form, the abstraction reaction is favoured due to the formation of a new covalent bond between the dopant and the co-adsorbed CH 2 (product of the abstraction reaction), which strongly stabilises the surface after the abstraction process.

A Theoretical Study of Nitrogen‐Induced Effects on Initial Steps of Diamond CVD Growth

AbstractAn investigation based on an ultra‐soft, pseudo‐potential, density functional theory (DFT) method, using a generalized gradient approximation (GGA) under periodic boundary conditions, is performed in order to investigate the effect of substitutional nitrogen on the CH3 adsorption and the H abstraction from the adsorbed CH3 species. Both (111) and (100)‐2x1 surface are considered with 4 and 6 substitutional N positions, respectively. For the two surface orientations, no strong effect of N is generally observed when positioning it within the first C layer (i.e., surface layer). The only exception is for N in one of three investigated first layer positions within the (100) surface, for which a spontaneous β‐scission occurs as a result of the H abstraction reaction. For N in the second carbon layer, the CH3 adsorption reaction is calculated to be strongly disfavored, whilst the H abstraction reaction is markedly improved.

New approach to the growth of SiO x nanowire bunch using Au catalyst and SiN x film on Si substrate

SiO x nanowire bunches were fabricated on a SiN x film with Au catalytic metal in the presence of an Ar flow of 50 sccm at 1150 °C. The resulting samples were characterized by field-emission scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. A SiN x film serves as a barrier to the diffusion of Si atoms from the Si substrate to the catalytic Au metal, where a substrate is a Si source material for SiO x nanowire (NW) growth. Using this process, we could temporally control the initial growth step of SiO x NWs and easily grow the NW bunch.

Chemical Vapor Deposition of Diamond Films on Patterned GaN Substrates via a Thin Silicon Nitride Protective Layer

Integrating diamond films with GaN-based devices may enhance heat dissipation and thus improve device performance for high power loading. Direct deposition of diamond films on GaN layers has been hampered by GaN degradation in the chemical vapor deposition environment for diamond growth. In this work, three approaches were introduced to grow high-quality diamond films on patterned GaN substrates via a thin silicon nitride protective layer, that is, (i) a two-step process involving an initial rapid growth step, (ii) addition of nitrogen to hydrogen-based plasma to suppress reactions between GaN and hydrogen, and (iii) deposition in argon-based plasma. Continuous, adherent, and high-quality micro- and nanocrystalline diamond films were successfully deposited. All three approaches were effective in reducing plasma-induced GaN decomposition and etching, and in eliminating film cracks and delamination.

Synthesis of nanocrystalline diamond films by DC plasma-assisted argon-rich hot filament chemical vapor deposition

Continuous nanocrystalline diamond (NCD) films were grown in an argon-rich gas atmosphere with relatively high growth rates by sustaining a low power (5 W) DC plasma in a hot filament chemical vapor deposition system (HFCVD). The parameter window for the synthesis of NCD films was studied as a function of argon, methane and hydrogen concentrations, as well as substrate temperature and DC bias. The results are consistent with reports indicating that the DC plasma induces re-nucleation by ion bombardment during the initial growth step and helps to maintain the atomic H and hydrocarbon species near the growing surface. It was found that DC plasma-assisted HFCVD enables high NCD growth rates and expands the parameter window, rendering it unnecessary to heat the filament above 2800 K.