Effect of reaction mechanism on precursor exposure time in atomic layer deposition of silicon oxide and silicon nitride

Atomic layer deposition (ALD) of highly conformal, silicon-based dielectric thin films has become necessary because of the continuing decrease in feature size in microelectronic devices. The ALD of oxides and nitrides is usually thought to be mechanistically similar, but plasma-enhanced ALD of silicon nitride is found to be problematic, while that of silicon oxide is straightforward. To find why, the ALD of silicon nitride and silicon oxide dielectric films was studied by applying ab initio methods to theoretical models for proposed surface reaction mechanisms. The thermodynamic energies for the elimination of functional groups from different silicon precursors reacting with simple model molecules were calculated using density functional theory (DFT), explaining the lower reactivity of precursors toward the deposition of silicon nitride relative to silicon oxide seen in experiments, but not explaining the trends between precursors. Using more realistic cluster models of amine and hydroxyl covered surfaces, the structures and energies were calculated of reaction pathways for chemisorption of different silicon precursors via functional group elimination, with more success. DFT calculations identified the initial physisorption step as crucial toward deposition and this step was thus used to predict the ALD reactivity of a range of amino-silane precursors, yielding good agreement with experiment. The retention of hydrogen within silicon nitride films but not in silicon oxide observed in FTIR spectra was accounted for by the theoretical calculations and helped verify the application of the model.

Revisiting the CO oxidation reaction on various Au/TiO2catalysts: Roles of the surface OH groups and the reaction mechanism

This work aims to understand the influence of TiO2 surface structure in Au/TiO2 catalysts on CO oxidation. Au nanoparticles (3 wt%) in the range of 4 to 8 nm were loaded onto four kinds of TiO2 surfaces, which had different surface structures and were synthesized by calcining hydrogen titanate nanotubes at various temperatures and in different atmospheres. The Au catalyst supported on anatase nanorods exhibited the highest activity in CO oxidation at 30 °C among all the five Au/TiO2 catalysts including the reference catalyst of Au/TiO2-P25. X-ray photoelectron spectroscopy (XPS) and infrared emission spectra (IES) results indicate that the anatase nanorods have the most active surface on which water molecules can be strongly adsorbed and OH groups can be formed readily. Theoretical calculation indicates that the surface OH can facilitate the O2 adsorption on the anatase surface. Such active surface features are conducive to the O2 activation and CO oxidation

On the complexity of the water-gas shift reaction mechanism over a Pt/CeO2 catalyst: Effect of the temperature on the reactivity of formate surface species studied by operando DRIFT during isotopic transient at chemical steady-state

The present report investigates the role of formate species as potential reaction intermediates for the WGS reaction (CO + H2O -> CO2 + H-2) over a Pt-CeO2 catalyst. A combination of operando techniques, i.e., in situ diffuse reflectance FT-IR (DRIFT) spectroscopy and mass spectrometry (MS) during steady-state isotopic transient kinetic analysis (SSITKA), was used to relate the exchange of the reaction product CO2 to that of surface formate species. The data presented here suggest that a switchover from a non-formate to a formate-based mechanism could take place over a very narrow temperature range (as low as 60 K) over our Pt-CeO2 catalyst. This observation clearly stresses the need to avoid extrapolating conclusions to the case of results obtained under even slightly different experimental conditions. The occurrence of a low-temperature mechanism, possibly redox or Mars van Krevelen-like, that deactivates above 473 K because of ceria over-reduction is suggested as a possible explanation for the switchover, similarly to the case of the CO-NO reaction over Cu, I'd and Rh-CeZrOx (see Kaspar and co-workers [1-3]). (c) 2006 Elsevier B.V. All rights reserved.

THE CATALYTIC BEHAVIOR AND REACTION-MECHANISM OF THE BI-CE-O SYSTEM CATALYSTS IN THE OXIDATIVE DEHYDROAROMATIZATION OF PROPYLENE

Oxidative dehydroaromatization of propylene was investigated by the pulse technique over two kinds of single oxide catalysts. With the Bi2O3 catalyst, the main dimer product was 1,5-hexadiene, and the dimerization activity was stable to pulse number even if the catalyst was partly reduced to the bulk. With the CeO2 catalyst, benzene was mainly formed instead of 1,5-hexadiene, but the activity decreased rapidly with increasing pulse number, indicating that only the lattice oxygen near the catalyst surface could be used for oxidative dimerization and the further aromatization. The Bi-Ce-O system catalyst was found in this study to give higher aromatization activity and showed better stability, compared to the Bi-Sn-O catalyst. Although the Bi-Ce-O catalyst was only a mixture of the two component oxides from X-ray diffraction analysis, there was a significant combination effect on the selectivity to benzene. The highest and the most stable selectivity of benzene was obtained at Bi/Ce = 1. In the TPD spectrum of Bi-Ce-O catalyst, there are not only the lattice oxygen (beta-oxygen) over 620-degrees-C due to the reduction of Bi2O3, but also a great deal of the alpha-oxygen desorbed about 400-degrees-C, which is considered the absorbed oxygen in the bulk. This absorbed oxygen could probably be a compensation of the lattice oxygen through the route of gaseous --> absorbed --> lattice oxygen in the binary catalyst system. By the kinetic study on the Bi-Ce-O catalyst, the dimer formation rate was the first-order with respect to the partial pressure of propylene and zero-order of oxygen. Although detail investigation would be made further, it was considered that the complete oxidation of propylene would mainly take place parallelly on some different sites, and the rate-determining step of propylene dimerization occurred probably between an adosrbed propylene and a gaseous one by an Eley-Rideal type mechanism.

General insight into CO oxidation: A density functional theory study of the reaction mechanism on platinum oxides

CO oxidation on PtO2(110) has been studied using density functional theory calculations. Four possible reaction mechanisms were investigated and the most feasible one is the following: (i) the O at the bridge site of PtO2(110) reacts with CO on the coordinatively unsaturated site (CUS) with a negligible barrier; (ii) O-2 adsorbs on the bridge site and then interacts with CO on the CUS to form an OO-CO complex; (iii) the bond of O-OCO breaks to produce CO2 with a small barrier (0.01 eV). The CO oxidation mechanisms on metals and metal oxides are rationalized by a simple model: The O-surface bonding determines the reactivity on surfaces; it also determines whether the atomic or molecular mechanism is preferred. The reactivity on metal oxides is further found to be related to the 3rd ionization energy of the metal atom.

Oxygen reduction reaction mechanism on nitrogen-doped graphene:A density functional theory study

Nitrogen-doped graphene (N-graphene) was reported to exhibit a good activity experimentally as an electrocatalyst of oxygen reduction reaction (ORR) on the cathode of fuel cells under the condition of electropotential of similar to 0.04 V (vs. NNE) and pH of 14. This material is promising to replace or partially replace the conventionally used Pt. In order to understand the experimental results. ORR catalyzed by N-graphene is studied using density functional theory (DFT) calculations under experimental conditions taking the solvent, surface adsorbates, and coverages into consideration. Two mechanisms, i.e., dissociative and associative mechanisms, over different N-doping configurations are investigated. The results show that N-graphene surface is covered by O with 1/6 monolayer, which is used for reactions in this work. The transition state of each elementary step was identified using four different approaches, which give rise to a similar chemistry. A full energy profile including all the reaction barriers shows that the associative mechanism is more energetically favored than the dissociative one and the removal of O species from the surface is the rate-determining step. (C) 2011 Elsevier Inc. All rights reserved.

Nanoparticle Platform to Modulate Reaction Mechanism of Phenothiazine Photosensitizers

Herein, we report on the synthesis of photosensitizing nanoparticles in which the generation of different oxidizing species, i.e., singlet oxygen ((1)O(2)) or radicals, was modulated. Sol gel and surface chemistry were used to obtain nanoparticles with specific ratios of dimer to monomer species of phenothiazine photosensitizers (PSs). Due to competition between the reactions involving electron transfer within dimer species and energy transfer from monomer triplets to oxygen, the efficiency of (1)O(2) generation could be controlled. Nanoparticles with an excess of dimer have an (1)O(2) generation efficiency (S(Delta)) of 0.01 while those without dimer have a S, value of 0.4. Furthermore, we demonstrate that the PS properties of the nanoparticles are not subjected to interference from the external medium as is commonly the case for free PSs, i.e., PS ground and triplet states are not reduced by NADH and ascorbate, respectively, and singlet excited states are less suppressed by bromide. The modulated (1)O(2) generation and the PS protection from external interferences make this nanoparticle platform a promising tool to aid in performing mechanistic studies in biological systems. Also, it offers potential application in technological areas in which photo-induced processes take place.

Active site topology and reaction mechanism of GTP cyclohydrolase I.

GTP cyclohydrolase I of Escherichia coli is a torus-shaped homodecamer with D5 symmetry and catalyzes a complex ring expansion reaction conducive to the formation of dihydroneopterin triphosphate from GTP. The x-ray structure of a complex of the enzyme with the substrate analog, dGTP, bound at the active site was determined at a resolution of 3 A. In the decamer, 10 equivalent active sites are present, each of which contains a 10-A deep pocket formed by surface areas of 3 adjacent subunits. The substrate forms a complex hydrogen bond network with the protein. Active site residues were modified by site-directed mutagenesis, and enzyme activities of the mutant proteins were measured. On this basis, a mechanism of the enzyme-catalyzed reaction is proposed. Cleavage of the imidazole ring is initiated by protonation of N7 by His-179 followed by the attack of water at C8 of the purine system. Cystine Cys-110 Cys-181 may be involved in this reaction step. Opening of the imidazole ring may be in concert with cleavage of the furanose ring to generate a Schiff's base from the glycoside. The gamma-phosphate of GTP may be involved in the subsequent Amadori rearrangement of the carbohydrate side chain by activating the hydroxyl group of Ser-135.

Understanding the Reaction Mechanism of Nanocomposite Thermites

Nanocomposite energetics are a relatively new class of materials that combine nanoscale fuels and oxidizers to allow for the rapid release of large amounts of energy. In thermite systems (metal fuel with metal oxide oxidizer), the use of nanomaterials has been illustrated to increase reactivity by multiple orders of magnitude as a result of the higher specific surface area and smaller diffusion length scales. However, the highly dynamic and nanoscale processes intrinsic to these materials, as well as heating rate dependencies, have limited our understanding of the underlying processes that control reaction and propagation. For my dissertation, I have employed a variety of experimental approaches that have allowed me to probe these processes at heating rates representative of free combustion with the goal of understanding the fundamental mechanisms. Dynamic transmission electron microscopy (DTEM) was used to study the in situ morphological change that occurs in nanocomposite thermite materials subjected to rapid (10^11 K/s) heating. Aluminum nanoparticle (Al-NP) aggregates were found to lose their nanostructure through coalescence in as little as 10 ns, which is much faster than any other timescale of combustion. Further study of nanoscale reaction with CuO determined that a condensed phase interfacial reaction could occur within 0.5-5 µs in a manner consistent with bulk reaction, which supports that this mechanism plays a dominant role in the overall reaction process. Ta nanocomposites were also studied to determine if a high melting point (3280 K) affects the loss of nanostructure and rate of reaction. The condensed phase reaction pathway was further explored using reactive multilayers sputter deposited onto thin Pt wires to allow for temperature jump (T-Jump) heating at rates of ~5x10^5 K/s. High speed video and a time of flight mass spectrometry (TOFMS) were used to observe ignition temperature and speciation as a function of bilayer thickness. The ignition process was modeled and a low activation energy for effective diffusivity was determined. T-Jump TOFMS along with constant volume combustion cell studies were also used to determine the effect of gas release in nanoparticle systems by comparing the reaction properties of CuO and Cu2O.

Material processing and surface reaction studies in free piston driven shock tube

Recently established moderate size free piston driven hypersonic shock tunnel HST3 along with its calibration is described here. The extreme thermodynamic conditions prevalent behind the reflected shock wave have been utilized to study the catalytic and non-catalytic reactions of shock heated test gases like Ar, N2 or O2 with different material like C60 carbon, zirconia and ceria substituted zirconia. The exposed test samples are investigated using different experimental methods. These studies show the formation of carbon nitride due to the non-catalytic interaction of shock heated nitrogen gas with C60 carbon film. On the other hand, the ZrO2 undergoes only phase transformation from cubic to monoclinic structure and Ce0.5Zr0.5O2 in fluorite cubic phase changes to pyrochlore (Ce2Zr2O7±δ) phase by releasing oxygen from the lattice due to heterogeneous catalytic surface reaction.

Mass transfer with surface reaction in arrays of cylinders at low reynolds and high peclet numbers

Creeping flow hydrodynamics combined with diffusion boundary layer equation are solved in conjunction with free-surface cell model to obtain a solution of the problem of convective transfer with surface reaction for flow parallel to an array of cylindrical pellets at high Peclet numbers and under fast and intermediate kinetics regimes. Expressions are derived for surface concentration, boundary layer thickness, mass flux and Sherwood number in terms of Damkoehler number, Peclet number and void fraction of the array. The theoretical results are evaluated numerically.

Effect of Bottom Bubbling Conditions on Surface Reaction Rate in Oxygen-Water System

In secondary steelmaking, the enhancement of the reaction rate in the low carbon period during the decarburization of steel is considered the most effective method to produce ultralow carbon steel. In a previous study, it was revealed that the surface reaction is dominant during the final stage of the actual refining process. In order to improve the surface reaction rate, it is necessary to enlarge the reaction region, which is usually achieved by increasing the plume eye area. In this study, water model experiments were carried out to estimate the influence of bottom stirring conditions on the gas-liquid reaction rate; for this purpose, the deoxidation rate during the bottom bubbling process was measured. Five types of nozzle configurations were used to study the effect of the plume eye area on the reaction rate at various gas flow rates. The results reveal that the surface reaction rate is influenced by the gas flow rate and the plume eye area. An empirical correlation was developed for the reaction rate and the plume eye area. This correlation was applied to estimate the gas-liquid reaction rate mat the bath surface.