Richard S. Sorbello

The metal-surface selection rule for infrared spectra of molecules adsorbed on small metal particles

Abstract The metal-surface selection rule predicts that some infrared absorption bands should be absent from the spectra of molecules adsorbed on metal particles. It predicts that those bands corresponding to molecular vibrations with an oscillating dipole moment parallel to the surface should be suppressed. We calculate that this selection rule should apply to adsorption on metal particles larger than about 20 A in diameter; for smaller particles the rule is weakened.

Theory of Electromigration

This chapter discusses the concepts of electromigration. Electromigration is defined as the movement of atoms in a solid subject to an electric field. It is a complex phenomenon in which the locally inhomogeneous electric field accompanying the electron transport process acts as the driving force for atom migration. A general description of the response of electrons and atoms to an applied electric field is contained in the phenomenological equations of irreversible thermodynamics. Theoretical and experimental work has established that the driving force for electromigration can conveniently be separated into contributions arising from the electron-wind force and the direct force. In particular, for electromigration the linear phenomenological equations of transport theory provide the necessary connection between particle currents and applied fields. A quantitative description of electromigration thus requires realistic models of the migrating atom and its local environment, of electron transport in the vicinity of the atom and of the diffusion jump itself.

Coupled harmonic oscillator models of carbon monoxide adsorbed on stepped, platinum surfaces

Abstract Harmonic oscillator models are used to explain recent experimental data on infrared absorption by CO molecules adsorbed on two stepped platinum surfaces. These data reveal only a lower frequency band at low coverage and only a higher frequency band at high coverage. Both bands exist over a range of intermediate coverages. The data are explained by a coupled-dipole model which includes the effects of electronic polarizability, the tilted orientation of CO molecules at step sites, and the electric field enhancement at step sites. The lower-frequency band is associated with CO molecules adsorbed on step sites and the higher-frequency band is associated with two-dimensional islands consisting of both step and terrace CO. The model explains the observed variation of frequency and intensity with coverage for CO adsorption on Pt(533) and Pt(432) surfaces. The model calculations indicate that the wavenumber for a single, linearly bonded CO molecule is about 9 cm−1 higher on a terrace site than on a step site.

Site-specific, coupled-harmonic-oscillator model of carbon monoxide adsorbed on extended, single-crystal surfaces and on small crystals of platinum

Abstract A model describes the vibration of CO molecules adsorbed on extended crystal surfaces and small particles of platinum by treating the molecules as coupled harmonic oscillators. The frequency of vibration of single, adsorbed CO molecule and the forces coupling the vibrations of adsorbed CO molecules depend on the coordination numbers of the platinum atoms on which the CO molecules are adsorbed. The model is applied to CO adsorbed on several extended, single-crystal surfaces of platinum and to CO adsorbed on small nearly spherical particles of platinum. A calculation gives the normal modes and infrared intensities for the idealized case of a ring of molecules adsorbed in a great circle about a small particle. This idealized example helps us understand which of the collective vibration modes contribute to the infrared adsorption spectrum for systems with three-dimensional geometries (non-infinite, non-flat surfaces).

Near-edge optical absorption behavior of sputter deposited hafnium dioxide

The absorption behavior at the onset of the ultraviolet optical edge of sputter deposited hafnium dioxide is reported. X-ray diffraction showed that films grown at room temperature on fused silica were nanocrystalline monoclinic, and that after air annealing at 1273 K for 24 h, they were well-crystallized monoclinic with a strong (11–1) texture. Spectrophotometry was used to measure transmission and reflection in the 190–1100 nm wavelength range. The absorption coefficient, α(E), as a function of energy, E, was determined from spectrophotometric data. Two persistent features were identified. Feature I is the rapid rise in α(E) above 6.24 eV that is unchanged upon annealing. A linear α(E)1/2 vs E dependence associates this feature with indirect interband transitions. Feature II initiates at ∼5.65 eV and saturates at 5.94 eV. It appears as a low energy shoulder on the absorption edge of the as-grown films and develops into a discrete and more intense band in the annealed films, in which the coordination of ...

Variational calculation of the work function for small metal spheres

Abstract A variational calculation based on the density functional formalism is performed for small metal spheres. The electron density is expressed in terms of two variational parameters chosen to minimize the energy. We find that the work function decreases as the size of the particle increases, approximately in agreement with the classical result for radii greater than a few angstroms.

Fundamental optical absorption edge of undoped tetragonal zirconium dioxide

The high-frequency optical absorption edge of pure tetragonal ZrO2, isolated in a ZrO2–Al2O3 nanolaminate film structure, was determined using transmission spectrophotometry. The functional dependence of the absorption coefficient on photon energy shows two interband transitions: an initial indirect transition at 5.22 eV (i.e., the band gap) followed by a direct transition at 5.87 eV. The edge structure is associated with O 2p→Zr 4d electron states and discussed in terms of ab initio calculations reported in the literature.

Force sum-rule and static electronic polarizability of atoms and microstructures

Abstract A sum rule is derived within density functional formalism for the response of an electronic system to a uniform electrostatic field. The sum rule states that the sum of the electrostatic forces on the positive charge background equals zero. The sum rule provides a useful test of electronic polarizability calculations for atoms and microstructures. It also leads to a self-consistency relationship for the induced charge density obtained in image-plane calculations.

Crystallization and (Al,Ti)-oxide growth in annealed TiO2–Al2O3 multilayers

A family of TiO2–Al2O3 multilayers (Λ=2–72nmTiO2∕7nmAl2O3) is sputter deposited on fused silica substrates, sequentially annealed at 973 and 1273K, and analyzed by x-ray diffraction. The goal is to examine crystallization behavior upon annealing at temperatures at which thermodynamically stable mixed-cation phases should not form. The results show: (1) After the 973K anneal, films with Λ=18–72nmTiO2∕7nmAl2O3 weakly crystallize with a preferred (110) rutile orientation. In addition, enhancement of (200) rutile diffraction increases with increasing TiO2 layer thickness. (2) Significant crystallization occurs in films after the 1273K anneal. In films with Λ=36–72nmTiO2∕7nmAl2O3, a metastable pseudobrookite phase, Al0.95Ti2.05O5, crystallizes along with (110)r. However, only rutile TiO2 and α-Al2O3 crystallize in films with thinner TiO2 layers. An architecture-sensitive crystallization model is presented in which the first step common to all architectures is diffusive amorphization of TiO2 by Al2O3 at 973K to...

Crystallization and segregation in vitreous rutile films annealed at high temperature

Vitreous titania films with rutile short-range order were sputter deposited on unheated fused silica substrates, sequentially annealed at 973 and 1273 K, and examined by Raman microscopy, scanning electron microscopy, and x-ray diffraction. A segregated microstructure developed after the 1273 K anneal. This microstructure consists of supermicron-size craters dispersed in a matrix of submicron rutile crystals. Ti–O short-range order in the craters is characteristic of a mixture of two high pressure phases, m-TiO2 (monoclinic P21∕c space group) and α-TiO2 (tetragonal Pbcn space group). We calculated that a high average compressive stress parallel to the substrate must be accommodated in the films at 1273 K, caused by the difference in the thermal expansion coefficients of titania and fused silica. The formation of the segregated microstructure is modeled by considering two processes at work at 1273 K to lower a film’s internal energy: crystallization and nonuniform stress relief. The Gibbs–Thomson relation ...