Injection Of Precursor Research Articles

A Combined Mitigation/Geoengineering Approach to Climate Stabilization

Projected anthropogenic warming and increases in CO2 concentration present a twofold threat, both from climate changes and from CO2 directly through increasing the acidity of the oceans. Future climate change may be reduced through mitigation (reductions in greenhouse gas emissions) or through geoengineering. Most geoengineering approaches, however, do not address the problem of increasing ocean acidity. A combined mitigation/geoengineering strategy could remove this deficiency. Here we consider the deliberate injection of sulfate aerosol precursors into the stratosphere. This action could substantially offset future warming and provide additional time to reduce human dependence on fossil fuels and stabilize CO2 concentrations cost-effectively at an acceptable level.

Sub‐kilogram‐Scale One‐Pot Synthesis of Highly Luminescent and Monodisperse Core/Shell Quantum Dots by the Successive Injection of Precursors

AbstractThe one‐pot synthesis of core/shell quantum dots (QDs) represents an attractive alternative to conventional synthesis techniques, where the core CdSe QDs are first purified and then an epitaxial shell of the desired thickness is obtained by the slow addition of shell precursors to a solution of the purified QDs at high temperature. We have developed a one‐pot synthesis procedure involving the successive injection of deliberately selected core‐ and shell‐forming reagents at appropriate temperatures. Sub‐kilogram quantities of highly luminescent and monodisperse core/shell QDs with desirable optical properties (full width at half maximum of photoluminescence (PL) band is ca. 30 nm) have been produced by the sequential growth of the core and shell in a controlled manner. This one‐pot method has also been extended to form water‐soluble core/double‐shell CdSe/ZnSe/ZnS QDs exhibiting high PL efficiency and stability.

Synthesis of silver nanoparticles using the polyol process and the influence of precursorinjection

Spherical silver nanoparticles with various sizes and standard deviations weresynthesized by the polyol process. Two different synthesis methods were compared inorder to investigate the influence of reaction parameters on the resulting particlesize and its distribution. In the precursor heating method, wherein a solutioncontaining silver nitrate was heated to the reaction temperature, the ramping rate wasdetermined to be a critical parameter affecting the particle size. In contrast, inthe precursor injection method, in which a silver nitrate aqueous solution wasinjected into hot ethylene glycol, because of rapid nucleation, the injection rateand the reaction temperature were important factors in terms of reducing theparticle size and attaining monodispersity. Silver nanoparticles with a size of17 ± 2 nm were obtained at an injection rate of2.5 ml s−1 and a reactiontemperature of 100 °C.

Solvothermal synthesis and characterization of anatase TiO 2 nanocrystals with ultrahigh surface area

Phase-pure, ultrafine nanocrystalline anatase with high specific surface area (up to 250 m 2 g −1) was obtained upon injection of a titanium alkoxide precursor into ethanol with designed volume of water under mild solvothermal conditions (<200 °C, 2 h). Primary particle sizes were tuned by adjusting various reaction parameters, with the smallest grain sizes occurring at low temperatures (140–150 °C), low initial alkoxide concentrations, and intermediate hydrolysis ratios ( r ≡ [H 2O]/[Ti(OR) 4] = 5–10). Additionally, variations in the reaction temperature result in changes in particle morphology and distribution, with high-temperature samples exhibiting bimodal distributions of small spherical and larger cubic particles that suggest grain growth via Ostwald ripening. A crystalline product with high thermal stability and specific surface area up to 5 times that of commercial nano-titania can be obtained at a relatively low temperature of 150 °C. The physical properties of the titania samples obtained in this study suggest they might be well suited for catalytic applications.

ZnO nanocrystals grown from zinc ketoacidoximate single molecular precursor

ZnO nanoparticles with mean size of 3.9–7 nm were grown from zinc ketoacidoximate by injection of this single source precursor into solvent at different temperatures. The nanocrystal growth mechanism and properties were studied. Nanocrystal size increases with an increase in growth temperature, but increases very slowly with growth time increasing. Ostwald ripening occurs during nanocrystal growth. Morphology of nanocrystals is almost spherical with a hexagonal structure. Photoluminescence of the nanocrystals consists of exciton luminescence and luminescence related to deep level defects. Luminescence of excitons is dominant in all nanocrystals except those grown at 110 °C. With the increase of growth temperature and growth time, the relative intensity of exciton emission increases. Dependence of the lowest exciton state energy on nanocrystal size in this region can be described by Kayanuma mode.

Electron-beam induced deposition of a nanotip within a nano-aperture structure

This work details the electron-beam induced deposition of a nanotip with apical radius <10 nm within the nano-aperture structure present in the nanopentode vacuum nanoelectronic device. Simple vapour priming of the substrate is used to provide the precursor for the deposition, enabling the consistent fabrication of nanotips using conventional electron beam systems without gas precursor injection apparatus. When these tips are grown through nano-apertures with radii ∼50 nm, SEM observations and electrical characterization show that material is deposited on the apertures, which can make contact with the centrally positioned nanotip. While this degrades the electrical isolation between the electrodes in nanopentode devices, it permits the estimation of the deposited material resistivity to be of the order 1 Ωm. Isotropic plasma etching is suggested as a method to improve the inter-electrode electrical isolation.

Liquid spray vs. gaseous precursor injection — Its influence on the performance of particle coating by CVD in the fluidized bed

Aluminum coatings were created onto glass beads by chemical vapor deposition in a fluidized bed reactor at different temperatures. Two different routes were examined. First, tri-isobutyl-aluminum (TIBA) vapor was enriched in nitrogen and thermally decomposed inside the fluidized bed to deposit elemental aluminum. On the other hand, liquid injection of TIBA via a two-fluid nozzle directly into the fluidized bed was tested. To ensure homogeneous coating on the bed material, the fluidizing conditions necessary to avoid agglomeration were investigated for a broad range of temperatures. Also, different glass types and pretreatments of the substrate surface were investigated to elucidate the influence of the surface chemistry on the growth and morphology of the layers deposited.

Effects of reactor pressure and residence time on GaN MOVPE growth efficiency

Experimental and modeling study of GaN MOVPE is carried out for a single-wafer horizontal reactor with a premixed precursor injection and for the multiwafer production-scale Planetary Reactor Aix2000HT where III- and V-group precursors are injected separately. In the Planetary Reactor the growth rate is found to strongly decrease with residence time at a constant pressure and to noticeably increase with pressure for a given residence time. At the 400 mbar pressure the former effect is mainly caused by the parasitic deposit growth over the reactor ceiling, while the latter effect is due to the mass-transport redistributing the growth rate on the substrate. At the 800 mbar pressure a sublinear character of the growth rate variation with the TMGa precursor flow rate evidences for a generation of nanoparticles in the reactor chamber. In some regimes both trends counteract each other, giving a weak variation of the growth rate on the reactor pressure. In the single-wafer horizontal reactor only a quite negligible (≈10%) variation of the growth rate with pressure and residence time has been found. This indicates almost no losses in this reactor, which should result from the short residence times of the reacting mixture in the hot zone and premixed gas injection into the reactor.

A New Class of Capping Ligands for CdSe Nanocrystal Synthesis

A new method is described for the synthesis of high-quality CdSe nanocrystals (NCs) that eliminates the need for tri-octylphosphine oxide (TOPO). This new method is an adaptation of previously reported methods that use Cd salts as precursors for the CdSe NCs. High-boiling esters and ketones such as hexadecyl hexadecanoate (HH) and benzophenone (BP) appear to be excellent replacements for TOPO in this reaction. Compared with TOPO, HH offers a significant synthetic advantage as it slows the initial growth of the NC nuclei formed after injection of the Se precursor. This allows for superior control over particle size in the initial stages of the reaction, and thus permits facile production of high-quality CdSe NCs with very small diameters (<4 nm). Based upon the results presented here, a possible role is identified for TOPO in the CdSe NC synthesis. Understanding of the role of TOPO and other reaction components is crucial for continued progress in the synthesis of high-quality NCs.

Synthesis of CdSe and CdTe Nanocrystals without Precursor Injection

With high quality and in high yield: The synthesis presented here affords CdSe and CdTe semiconductor nanocrystals without the need of a precursor injection. It allows the detailed control of the size and shape of the nanocrystals, as can be seen from the tetrahedral CdSe nanocrystals that have been prepared (TEM image). The method is suitable for industrial-scale preparations.

Treatment of Rabbit Bullous Keratopathy with Precursors Derived from Cultured Human Corneal Endothelium

To establish a method for the mass production of human corneal endothelium (HCE) precursors and the therapeutic application of these cells in a rabbit CE-deficiency model. A sphere-forming assay was performed to produce precursors from cultured HCE. Various marker expressions were examined in the sphere colonies, and their progenies by immunocytochemistry and reverse transcription-polymerase chain reaction (RT-PCR). The transport activity of the sphere-derived cell sheet was evaluated by the Ussing chamber system. DiI-labeled precursors obtained from cultured HCE were injected the anterior chamber of the eye in a rabbit CE-deficiency model, and the eye-down position was maintained for 24 hours for attachment to Descemet's membrane (sphere eye-down group). The sphere eye-down and control groups, observed for 28 days after surgery, underwent histologic and fluorescence microscopic examinations. Cultured HCE formed primary and secondary sphere colonies. The spheres expressed alpha-smooth muscle actin and nestin, and progeny expressed alpha-smooth muscle actin, confirmed by RT-PCR. The progeny showed an HCE-like hexagonal shape, were confluent, and had adequate transport activity. Mean corneal thickness in the sphere eye-down group was significantly less than in the other control groups 14, 21, and 28 days (P < 0.006) after surgery. The HCE-like hexagonal cells detected on the Descemet's membrane are DiI-positive in the sphere eye-down group. The findings demonstrate that culture of HCE can promote mass production of HCE precursors, determined by sphere-forming assay. Injection of precursors derived from cultured HCE into the anterior chamber is an effective treatment strategy for CE deficiency in a rabbit model.

General Shape Control of Colloidal CdS, CdSe, CdTe Quantum Rods and Quantum Rod Heterostructures.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Study on the Step Coverage of Metallorganic Chemical Vapor Deposited TiO2 and SrTiO3 Thin Films

The growth behavior and step coverage of thin films deposited by liquid-delivery metallorganic chemical vapor deposition (MOCVD) at temperatures ranging from 410 to 490°C were investigated. The film growth rates were controlled by the gas phase thermal decomposition reaction of the precursors, and the reaction shows an apparent activation energy of . The gas phase thermal decomposition produced several intermediate precursor molecules with different sticking coefficients. The intermediate precursors played a crucial role in the film growth and step coverage change. As the growth rate decreases, the step coverage degrades, which can be explained by Langmuir’s isothermal adsorption model of the precursors with the assumption of the various intermediate molecular structures of the precursors. The bulkier intermediate molecular structure of the precursor increases the steric hindrance between the previously adsorbed molecules and the impinging molecules so that the step coverage improves. The unusual step coverage of an MOCVD film previously observed for a low precursor injection rate was successfully explained using the results derived from the MOCVD and nonsticking property of the Ti-precursor molecules on the SrO surface.

General Shape Control of Colloidal CdS, CdSe, CdTe Quantum Rods and Quantum Rod Heterostructures

We report a general synthetic method for the formation of shape-controlled CdS, CdSe and CdTe nanocrystals and mixed-semiconductor heterostructures. The crystal growth kinetics can be manipulated by changing the injection rate of the chalcogen precursor, allowing the particle shape-spherical or rodlike-to be tuned without changing the underlying chemistry. A single injection of precursor leads to isotropic spherical growth, whereas multiple injections promote epitaxial growth along the length of the c-axis. This method was extended to produce linear type I and type II semiconductor nanocrystal heterostructures.

Bulk growth of high-purity 6H-SiC single crystals by halide chemical-vapor deposition

High-purity 6H-SiC single crystals were grown by the halide chemical-vapor deposition process. Growth was performed in a vertical hot-wall reactor with a separate injection of a silicon precursor (silicon tetrachloride) and a carbon precursor (propane). Typical growth rates were between 100 and 300μm∕h. The crystals contain very low concentrations of residual impurities. The main contaminants, namely, nitrogen and boron, are in the 1014atomscm−3 range. Crystals grown under Si-rich conditions were n type with low room temperature electron concentrations in the 1014–1015atomscm3 range and with room-temperature electron mobilities approaching 400cm2∕Vs. The resistivity of the material increased up to 1010Ωcm with increasing C∕Si ratio. Deep levels spectra show that the electron traps density decreases with increasing C∕Si ratio.

Evaluation of Pb Precursors for Liquid Injection Atomic Layer Deposition of Pb(ZrxTi1-x)O3 Thin Films

AbstractPb(DPM)2 and Pb(TMOD)2 dissolved in ethylcyclohexane were evaluated as precursors for future atomic layer deposition (ALD) of Pb(Zr,Ti)O3 films. PbO films were deposited by a liquid injection atomic layer deposition on Pt-covered Si substrates at different deposition temperature and precursor volume per cycle. Pb(DPM)2 and Pb(TMOD)2 started thermal decomposition at deposition temperature of around 270°C and 320°C, respectively. Against increasing Pb(DPM)2 injection at 240°C, the deposition rate of PbO films saturated at around 1 Å/cycle, but kept increasing at 300°C, which is above the thermal decomposition temperature. The deposition rate of PbO films at 240°C dropped to a constant value with enough purge time after precursor injection and reactant supply. A saturated deposition rate of PbO films was also observed for Pb(TMOD)2 below the thermal decomposition temperature. However, the saturation behavior observed for Pb(TMOD)2 was slower and the saturated growth rate was higher comparing to Pb(DPM)2. In addition, the film thickness of the PbO films had an apparent gradient over the substrates. These results indicate that Pb(DPM)2 shows more reactive and stable chemisorption comparing to Pb(TMOD)2 for the self-limiting growth rate.

Controlling the synthesis of TaC nanopowders by injecting liquid precursor into RF induction plasma

Thermal plasma processing has been used to synthesize nano-size powders through the condensation of reactant species from a vapor phase. Further development of this synthesis method will require the careful selection of an appropriate precursor and precise control of products species and their particle sizes. Direct introduction of liquid mist into thermal plasma gives us a wider choice of precursors than does vapor-phase precursor injection and lets us inject the precursors in larger amounts. In the present work, nano-size tantalum carbide powder was prepared from a liquid precursor, tantalum ethoxide Ta(OC2H5)5, by using r.f. thermal plasma. The liquid precursor was atomized to generate micron-sized mist droplets, and the mist was introduced into plasma. This atomized precursor evaporated quickly in the high-temperature plasma flame, and nanoparticles were formed as temperature decreased. The process was controlled by changing the hydrogen addition, process pressure, carrier gas flow rate for mist injection, and quenching condition. Adding hydrogen improved the powder quality by removing solid carbon, but excess hydrogen suppressed the formation of tantalum carbide. The quenching conditions gave significant effects on the reduction of particles size by two thirds and yielded average particle sizes as small as 8 nm.

Injectable Scaffolds for Tissue Regeneration

AbstractFor Abstract see ChemInform Abstract in Full Text.

Atomic layer deposition of hafnium silicate films using hafnium tetrachloride and tetra-n-butyl orthosilicate

Atomic layer chemical vapor deposition (ALCVD) of hafnium silicate films with a precursor combination of HfCl4 and TBOS (tetra-n-butyl orthosilicate) was studied for high dielectric gate insulators. The effect of deposition conditions, such as deposition temperature and pulse time for precursor injection on the deposition rate per cycle and composition of the film (fraction of hafnia phase in the silicate film) were studied. The growth rate and composition ratio were saturated with the increase of the injection time of HfCl4 and TBOS and decreased with the increased deposition temperature from 300 to 500 °C. The growth rate was 1.4 Å/cycle and the fraction of hafnium phase in the Hf silicate thin films was 0.19 at the deposition temperature of 500 °C. Impurity content, such as carbon and chlorine was below the detection limit of XPS (x-ray photoelectron spectroscopy) and the impurity level detected by SIMS decreased with increasing deposition temperature. It was found that the incorporation rate of metal from halide source was lower than alkoxide source.

Copper nucleation by chemical vapour deposition on organosilane treated SiO2 surfaces

Self-assembled monolayers of organosilanes deposited onto SiO2/Si substrate surfaces by either vapour-phase or wet chemical methods can act as ultrathin interfacial barriers which effectively prevent the diffusion of copper into the SiO2 dielectric, enhance its adhesion onto it and also offer the possibility of achieving selective copper chemical vapour deposition. The initial stages of copper nucleation by chemical vapour deposition on organosilane treated SiO2 surfaces at different substrate temperatures are investigated in detail by scanning electron and atomic force microscopy. The growth behaviour of copper clusters before coalescence is elucidated and analysed in detail. At temperatures below 190 � C nucleation occurs mostly on already formed copper clusters and coalescence is obtained due to an increase of the size of clusters in three dimensions. Above that temperature, secondary nucleation of smaller size clusters on the gaps among the larger clusters is observed, leading to coalescence almost in two dimensions. The stable clusters density initially increases with precursor injection time, passes through a maximum and then decreases, due to an increase at low temperatures of the size of clusters. An almost substrate temperature independent maximum clusters density which is obtained in the low temperatures region, indicates that nucleation of copper atoms occurs on surface defect sites which are assumed to be Cu ðIÞ atoms. A low value of the apparent activation energy for nucleation, indicates higher affinity for copper chemical vapour deposition of the organosilane treated SiO2 as compared to TiN substrates used previously. At 200 � C, coalescence following the initial nucleation period, results in continuous films with low roughness and low average height, thus leading to the practical realization of a thin yet continuous film. � 2004 Elsevier B.V. All rights reserved.