Jennifer L. Giocondi

An atomic force microscopy study of super-dislocation/micropipe complexes on the 6H-SiC(0 0 0 1) growth surface

We have used atomic force microscopy (AFM) to study the (0 0 0 1) growth surface of a 6H-SiC single crystal at the points where micropipes emerge on the growth surface. All of the micropipes examined are origins of spiral steps, indicating that dislocations intersect the surface at these points. The dislocations observed at the surface/micropipe intersections have Burgers vectors of at least 4b0, where b0 is the Burgers vector of a unit screw dislocation aligned along the c-axis (b0 = 15.19A). Single and double unit dislocations were also observed, but they are not associated with micropipes. Micron-scale deposits of a heterogeneous phase were observed in the vicinity of the micropipes. The curvature of growth steps around these heterogeneities indicates that they impeded step motion while the crystal was growing. Based on our observations, we propose a model for the formation of super-dislocation/micropipe complexes that involves the coalescence of unit screw dislocations that are forced towards one another as large steps grow around heterogeneous material on the surface.

Surface engineering along the close-packed direction of SrTiO3

We have investigated the effects of wet etching with a 3 : 1 mixture of HCl : HNO3 and of annealing at 8508C on the surface morphology of [1 1 1]-oriented SrTiO3 single crystals. Atomic force microscopy is used to demonstrate that the surface morphology is a strong function of both etching and annealing time. All surfaces have step heights equal to integral or half-integral multiples of the (1 1 1) interplanar spacing. However, step bunching, non-regular step heights, granularity, inhomogeneous surface morphology, and etch pits are observed on many surfaces. A combination of etching and annealing leads to surfaces that are free of these irregularities and are characterized by well-developed step

Orientation Dependence of Photochemical Reduction Reactions on SrTiO3 Surfaces

Polished and annealed surfaces of randomly oriented crystallites were used to study the orientation dependence of the photochemical activity of SrTiO 3 surfaces. Silver cations reduced from an aqueous solution produce solid silver metal at the reaction site. The amounts of silver produced by a fixed exposure were used as a relative measure of each grains activity. The surface structure of the grains was observed using atomic force microscopy and the surface orientation of each grain was determined by electron backscattered diffraction. Surfaces annealed in air for 6h at 1200° C were bound by some combination of the following three planes: {110}, {111}, and a complex facet inclined approximately 24° from {100}. By correlating the orientations of individual grains to the amount of deposited silver, we conclude that surfaces with the complex {100} facet are the most active.

Photochemical Reduction and Oxidation Reactions on Barium Titanate Surfaces

The influence of the ferroelectric domain structure of BaTiO3 on the photochemical reactions that occur on its surface has been examined using atomic force microscopy. Both the photochemical reduction of aqueous silver cations and the oxidation of steric acid thin films were studied. During reduction, silver selectively deposits on the surface in patterns determined by the ferroelectric domain structure. Based on the analysis of domain polarization in single crystals, we find that the photochemical reduction reaction occurs preferentially on the positive ends of the dipoles. The most likely explanation for this phenomenon is that when the static dipolar field is oriented with the positive end of the dipole on the surface, photogenerated electrons are driven to the solid-liquid interface where they reduce metal cations. The oxidation of steric acid films, on the other hand, is not spatially selective. During oxidation, the films dissipate uniformly as they are converted to CO2 and H2O. In this case, we conclude that the oxidation occurs indirectly. Photogenerated holes create hydroxyl radicals which can migrate on the surface before reacting with the steric acid molecules.

Photochemical Reactivity of Sr 2 Nb 2 O 7 and Sr 2 Ta 2 O 7 as a Function of Surface Orientation

Sr2Nb2O7 and Sr2Ta2O7 have a (110) layered perovskite structure and are efficient photolysis catalysts. Aqueous silver and lead cations were photochemically reduced and oxidized, respectively, on the surfaces of Sr2Nb2O7 and Sr2Ta2O7 crystals with a wide range of orientations. Atomic force microscopy has been used to observe the distribution of photochemically reduced and oxidized products and determine the orientation dependence of the reactivity. On surfaces with the same orientation, reaction products frequently had a non-uniform distribution. The reactivity of both compounds proved to be only weakly anisotropic, with the highest relative reactivity for both oxidation and reduction occurring for surfaces oriented between (010), (110), and (011). These low index orientations have structures similar to the ideal {110} and {100} planes in the perovskite structure, respectively. The relationship of the perovskite structure to the reactivity is discussed.

Orientation Dependence of the Photochemical Reactivity of BaTi4O9

BaTi4O9 is a photocatalyst with a pentagonal prism tunnel structure. It has been hypothesized that the tunnels promote the separation of photogenerated carriers and, therefore, lead to the spatial separation of oxidation and reduction half reactions. This hypothesis has been tested by observing the distribution of reduced and oxidized reaction products on BaTi4O9 surfaces over a wide range of orientations. The surface orientations were determined by electron backscattered diffraction and atomic force microscopy was used to examine the structure of the surface both before and after the deposition of reaction products. Reduction products (Ag) are distributed uniformly. The distribution of oxidation products (PbO2) is also not correlated to the surface orientation or to the orientation of the tunnels with respect to the surface. Based on these observations, we conclude that the tunnels in this structure do not separate photogenerated charge carriers and that this mechanism is not responsible for this compounds relatively high photocatalytic activity.