James Adams

Density-functional calculation of CeO2 surfaces and prediction of effects of oxygen partial pressure and temperature on stabilities

We have used density-functional theory to investigate (111), (110), (210), (211), (100), and (310) surfaces of ceria (CeO2). Compared with previous interatomic-potential-based studies, our calculations reported a slightly different relative stability ordering and significantly lower surface energies for the stoichiometric surfaces. Using a defect model, the surface stabilities were evaluated as functions of oxygen partial pressure and temperature. Our investigations were restricted to ideal surface terminations, without considering defect formation on those surfaces. We found that at 300 K, the stoichiometric (111) has the lowest free energy for a wide range of oxygen partial pressures up to 1 atm, and only at ultrahigh vacuum does the Ce-terminated (111) becomes the most stable one. The transition point for the Ce-terminated (111) surfaces moves to higher oxygen partial pressures when temperature increases. To improve the prediction of electron density of states, we used the local-density approximation plus U(J) correction method to correct the on-site Coulomb correlation and exchange interaction due to the strongly localized Ce-4f electrons. The optimal parameter combination of U = 7 eV and J = 0.7 eV was found to improve the O 2p-Ce 4f gap without much degradation of ground-state bulk properties or the O 2p-Ce 5d gap. The bulk and surface electronic structures were then analyzed based on the improved density of states.

Electronic structure and bonding in hexagonal boron nitride

The bonding, cohesive, and electronic properties of hexagonal boron nitride were studied using density functional theory calculations. The properties of this system were calculated using three different exchange–correlation functionals (local density approximation and two forms of the generalized gradient approximation) to determine their relative predictive abilities for this system. In-plane and interplanar bonding was examined using band diagrams, the density of states, and the electron localization function. Different stackings, or arrangements of one basal plane with respect to another, were examined to determine how the bonding and electronic structure changed between different stackings. Calculated band gaps were in the 2.9–4.5 eV range and predominantly indirect, regardless of stacking or the exchange–correlation functional used. The calculated band gaps are in the low range of experimental band gap values, and do not explain the large range of experimental values.

Theoretical study of environmental dependence of oxygen vacancy formation in CeO2

Density functional methods were used to study the environmental dependence of O vacancy formation in CeO2. It was found that an O vacancy in the 2+ charged state has the lowest formation energy for a wide range of Fermi energies (EF) from 0 to ∼1.9eV, while a neutral vacancy becomes the most stable at higher EF values. The O vacancy formation energy can be strongly affected by temperature (T) and the partial pressure of oxygen (PO2). The effect of T and PO2 on the equilibrium compositions of reduced ceria (CeO2−x) was also calculated and showed qualitative agreement with gravimetric experiments.

Cholesteric Films as Optical Filters

A new class of large‐area optical filters based on the special reflection properties of cholesteric liquid crystals is described. Several filtering functions are provided by various combinations of liquid‐crystal films, polarizers, and retardation plates.

Pitch Dependence on Composition in Mixtures of Liquid Crystals

Abstract The linear additive law for pitch dependence in binary mixtures of cholesterics (or combinations of ingredients producing cholesterics) is extended to include first order interactive effects. The resulting expression is shown to effectively treat an apparently widely diverse collection of experimental data. The knowledge of the functional form of any given system is of value in predicting pitch sensitivity to heat, vapor, etc.

Molecular dynamics simulations of mechanical deformation of amorphous silicon dioxide during chemical–mechanical polishing

We present molecular dynamics simulations of the mechanical deformation which occurs during chemical–mechanical polishing (CMP) of amorphous silicon dioxide for different geometries and relative velocities. The simulations clarify asperity shape evolution during the process of shear and reveal temperature distributions as a function of time. It was found that the ratio of radii of a particle and asperity strongly affects the amount of the material removed whereas the relative velocity has a weaker affect on it. During shear, the local temperature inside the deformed asperity is raised to 700 K at 0.5 A/ps and up to 2200 K at 5.0 A/ps. This temperature increase lasts for only a short time, but it can have a major impact on the amount of material removed. It was found that there could be significant deposition of the material from the particle to the slab, which can fill surface trenches and thereby make the surface smoother. An analytic model was developed for describing the amount of material removed as a...

On the Mechanism of Silicon Nitride Chemical Vapor Deposition from Dichlorosilane and Ammonia

Silicon nitride, Si 3N4, films have broad industrial applications, particularly in semiconductors and integrated circuit technology. 1 One common way for depositing the films is the reaction of dichlorosilane (DCS, SiH2Cl2) with ammonia (NH3). Optimization of the film deposition and properties often requires knowledge of gas-phase and surface reaction kinetics. The commonly used approach for deriving a mechanism and kinetics of chemical vapor deposition (CVD) is based on experimental results combined with reasonable assumptions. The reaction rate parameters of the resulting gas-phase reactions and semiempirical deposition reactions are then fit to experimental film growth rate data in reactor simulations. 2-5 Such an approach, although useful, has serious limitations due to the lack of understanding of the reaction mechanism on a molecular level. Fundamental understanding of deposition chemistry can aid in implementation of new technologies and in obtaining films of high quality with respect to their structure and properties. 6 A few kinetic models available in the literature for silicon nitride deposition 2-5,7 are based on different assumptions about the mechanism of gas-phase and surface reactions. Surface reactions have been treated by assuming various forms for the conversion of adsorbed gas-phase reactants and intermediates into the final Si 3N4 film without considering possible elementary reactions. In the model suggested by Peev et al. 3 a single power-law expression, rate 5 k [SiH2Cl2] 0.49 [NH3] 0.46 (Freundlich adsorpbtion isotherm), has been used to fit to experimental data and no gas-phase reactions are considered. Roenigk and Jensen 2 have included gas-phase decomposition of DCS into dichlorosililene and hydrogen into their kinetic models, Eq. 1, in order to explain film nonuniformities and growth rate changes across the wafer during low pressure CVD growth. The formation of aminochlorosilane (ACS, SiH2(Cl)NH2) via direct reaction of DCS with ammonia, Eq. 2, was also considered as an alternative. Under the low pressure conditions studied, inclusion of this bimolecular reaction “gives less quantitative agreement with experimental data” 2 SiH2Cl2 r SiCl2 1 H2

The Relationship Between Pitch Change and Stimulus in Cholesterics

Abstract An expression is derived which relates the change of pitch of a cholesteric liquid crystal to the stimulus which causes the change. The relationship is a measure of the sensitivity of the cholesteric. The expression applies to mixtures to two or more molecular species which produce a cholesteric and to any stimulus which actually or effectively changes the ratios of constituents.

INDUCED ROTARY POWER IN TERNARY MIXTURES OF LIQUID CRYSTALS

We have observed that a ternary mixture comprising one nematic component and a compensated binary cholesteric results in an inversion wavelength (λo) in the near infrared for a broad range of nematic ratios. In particular, the system consisting of N — (p — methoxybenzylidene) — p — butylaniline, cholesteryl nonanoate and cholesteryl chloride was studied over a wide compositional region. Measured pitch values indicate a large deviation from the linear additive law. An interpretation lies in the possibility that both cholesteric species interact with the nematic, but with different strengths. Consequently, the nematic exhibits some net rotary power. A determination of coupling constants will be presented.

Conductivity Differences in the Cholesteric Textures

Both the focal conic and Grandjean textures can be characterized by a conductivity tensor with uniaxial symmetry. We have observed that the diagonal element corresponding to the conductivity in the special direction is ≈ 20% larger in the focal conic case. The film studied was a mixture of 80% N-(p-methoxybenzyli-dene)-p-butylaniline and 20% cholesteryl oleyl carbonate. The transformation to the focal conic texture was produced by the application of a dc voltage and the reverse transformation was induced by an ac voltage. At sufficiently low current levels, current does not perturb structure (at least to the extent that the optical and electrical properties are not altered). At higher levels, the measuring stimulus converts the film to an intermediate state and both textures approach the same current level since they are converted from their original symmetries into the same state. The conductivity differences observed are consistent with conductivity ratio data for the nematic component and the molecular distributions in the two textures. The influence of resistivity and temperature on the focal conic/Grandjean conductivity ratio is shown.