Stephen H. Garofalini

Empirical three‐body potential for vitreous silica

A three‐body potential suitable for molecular dynamics (MD) simulations has been developed for vitreous silica by adding three‐body interactions to the Born–Mayer–Huggins (BMH) pair potential. Previous MD simulations with the BMH potential have formed glassy SiO2 through the melt‐quench method with some success. Though bond lengths were found to be in fair agreement with experiment, the distribution of tetrahedral angles was too broad and the model glass contained 6%–8% bond defects. This is indicative of a lack of the local order that is present in the laboratory glass. The nature of the short range order is expected to play an important role when investigating defect formation, surface reconstruction, or surface reactivities. An attempt has been made to increase the local order in the simulated glass by including a directional dependent term in the effective potential to model the partial covalency of the Si–O bond. The vitreous state obtained through MD simulation with this modified BMH potential shows an increase in the short range order with a narrow O–Si–O angle distribution peaked about the tetrahedral angle and a low concentration of bond defects, typically ∼1%–2%. The static structure factor S(q) is calculated and found to be in good agreement with neutron scattering results. Intermediate range order is also discussed in reference to the distribution of ring sizes.

Molecular dynamics simulations of α-alumina and γ-alumina surfaces

Molecular dynamics simulations of crystalline aluminum oxide were performed for α-Al2O3 and γ-Al2O3 phases. Both bulk crystals and surfaces of each phase were studied. For each of the surfaces, several possible atomic terminations were examined and surface energies, density profiles, and atom configurations have been calculated. It was found that due to processes of surface relaxation and reconstruction some terminations of the α-alumina surfaces become more likely to appear. For γ-alumina, the occurrence of cation vacancies in the crystal structure has a significant influence on surface morphology. On the surfaces, additional active sites were observed which are not predicted by idealized models which omit vacancies.

Molecular dynamics computer simulations of silica surface structure and adsorption of water molecules

Molecular dynamics computer simulations have been used to study the structure of vitreous silica surfaces and adsorption of water molecules onto the silica surface. Results of simulations of bulk vitreous silica using pair potentials and multibody potentials, as well as simulations of the interaction between H4SiO4H2O molecules are presented as the background to the application of these surfaces. The simulations of water adsorption follow the expected trends of dissociation of adsorbed water molecules, silanol formation, siloxane bond rupture, and preferential association of adsorbed water molecules. Visualization of these surface reactions via dynamic graphics enabled observation of the specific mechanisms of adsorption and bond rupture. Most importantly, this visualization indicates the significant complexity involved in this adsortion process.

Topological and bonding defects in vitreous silica surfaces

A model structure for an annealed silica surface was obtained through the molecular dynamics simulation technique employing three‐body interaction potentials. Nonbridging oxygen and edge‐sharing tetrahedra were found to form on the oxygen‐terminated surface with three‐coordinated silicon, three‐coordinated oxygen, and three‐membered rings just below the outermost atoms. Four‐membered rings were also created in relatively large concentrations during the surface relaxation. When considering the effect of removing periodic boundary conditions along the z direction, the concentration of larger rings, relative to the bulk, remained unchanged throughout the ∼11 A simulated surface region. A strong physical association between three‐membered rings and three‐coordinated oxygen was found which may account for the D2 defect peak observed in Raman scattering.

Water‐induced relaxation of the vitreous silica surface

The formation of a vitreous silica surface in the presence of water vapor is investigated through the molecular dynamics simulation technique. Three‐body potentials are employed to describe the interatomic interactions. The structure of the reconstructed surface is analyzed with respect to the concentration and type of defects. Comparison between surfaces created in the presence of water and those created in a vacuum indicate that H2O‐surface reactions substantially reduce the number of topological (two‐ , three‐ , and four‐membered rings) and bonding defects (under‐ and overcoordinated species) incurred during the relaxation process. Due to the dissociation of water molecules, the wet surface contains excess oxygen with a concentration of 3.1±0.6 silanols per 100 A2, involving approximately 13% geminal sites.

A molecular dynamics simulation of the vitreous silica surface

The molecular dynamics (MD) computer simulation technique was used to simulate the vitreous silica (v‐SiO2) surface. A modified Born–Mayer–Huggins potential function was used in the simulation; periodic boundary conditions in two dimensions (x and y) only were used to create a thin slab with free surfaces in the z directions. Radial distribution functions and bond angle distributions for interior regions and for surface regions (within several angstroms of the surface) were evaluated. In accordance with our understanding of the v‐SiO2 surface, the MD simulation generated a surface in which the oxygen atoms rather than the silicon atoms predominated at the outer surface. Nonbridging oxygen (NBO) as well as bridging oxygen were found at the surface. The NBO–Si interatomic spacing was found to be about 0.08 A less than the normal bridging O–Si spacing and is in accordance with calculations made from spectroscopic data of defects in v‐SiO2. Also an additional peak at 110° was observed in the Si–O–Si bond angl...

Molecular dynamics simulation of the frequency spectrum of amorphous silica

The dynamic behavior of atoms in bulk amorphous silica SiO2 has been investigated by using the molecular dynamics computer simulation technique to generate the frequency spectrum. A modified Born–Mayer–Huggins equation was used as the interatomic potential function. Due to the covalency of the Si–O bond, the ability of using a central‐force model to reproduce the short time atomic motion in SiO2 was evaluated. The frequency spectrum was generated from the Fourier transform of the velocity autocorrelation function and was compared with the experimentally obtained spectrum presented in the literature. Results show that the frequency spectrum generated here has the three major peaks which are characteristic of silica— i.e., peaks at ∼400, ∼800, and ∼1100 cm−1. Changes in the Si–Si or O–O repulsive parameters in the potential function can be used to alter the frequency spectrum. The 800 cm−1 peak, due to oxygen bending and Si motion, and the 150 cm−1 correlated motion peak are the most affected by the alterat...

Surface studies of TiO2-SiO2 glasses by X-ray photoelectron spectroscopy

Abstract The near-surface chemistry of titania-doped silicate glasses using X-ray photoelectron spectroscopy (XPS) has been investigated. It has been found that the Ti(2p) photoelectrons from these glasses have distinctly higher binding energy than those from pure titania (rutile) even though Ti has the same oxidation state in both solids. This difference has been attributed to differing oxygen co-ordinations, as previously shown by X-ray, infrared and Raman spectroscopic studies. This finding will be useful for future analysis of TiO systems. The oxygen 1s is peak has been successfully resolved into two components: one from SiOSi as in fused silica and the other from SiOTi, based on a four-coordinated Ti model. Silicon was found to be preferentially etched by ion bombardment, leaving a Ti-rich surface. High temperature annealing of both ion-etched and freshly cut samples led to substantial depletion of Ti from the near-surface region indicating that at the surface where the network is disrupted, occupancy by Ti is energetically less favored than by Si. This surface segregation phenomenon may have a major effect on the surface properties of these glasses.

Modeling of hydrophilic wafer bonding by molecular dynamics simulations

The role of moisture in hydrophilic wafer bonding was modeled using molecular dynamics computer simulations of interface formation between amorphous silica surfaces. Three different surface treatments were used in order to determine the effect of moisture on the formation of siloxane (Si–O–Si) bridges across the interface at two temperatures. The three surface conditions that were studied were: (a) wet interfaces containing 1 monolayer of water adsorbed at the interface (based on the room temperature bulk density of water), (b) hydroxylated interfaces with concentrations of 3–5 silanols/nm2 on each surface and no excess water molecules initially in the system, and (c) pristine interfaces that had only Si and O and no water or H present. The surfaces were slowly brought together and siloxane bond formation was monitored. In the pristine interfaces, siloxane bridges formed across the interface by the coalescence of various defect species in each surface. A bimodal distribution of siloxane bond angles formed...

Microstructural evaluation of simulated sodium silicate glasses

Abstract A molecular dynamics study of sodium trisilicate and sodium disilicate glasses has been performed in order to evaluate structural details of these systems. The total potential energy is a function of both two-body and three-body interactions in order to account for the partially covalent nature of silica bonding. The simulations reproduce the bulk structural features observed in XRD and EXAFS experiments. The simulations show that the connectivity of the silica network, as in the classical picture, although the distribution of non-bridging oxygen is not as uniform as generally assumed. This feature is supported byu recent NMR studies in which Q-species indicate a significant degree of disorder in the arrangement of non-bridging oxygen. Channels are observed in the simulated glasses at both compositions and are associated with lower order Q n species.