Mesoporous silica composites containing multiple regions with distinct pore size and complex pore organization.

Porous ceramics are of great interest for filtration, catalysis, and reactive separation processes. Performance in these applications is highly dependent on features such as pore size distribution and connectivity and wall composition. Here, we describe a method allowing the rational design and synthesis of mesoporous silica composites with controlled heterogeneous pore architectures and demonstrate its validity by producing structures with predetermined placement of regions having different pore size and pore organization.

Directed assembly of controlled-misorientation bicrystals.

Grain boundaries play a vital role in determining materials behaviour, and the nature of these intercrystalline interfaces is dictated by chemical composition, processing history, and geometry (misorientation and inclination). The interrelation among these variables and material properties may be systematically studied in bicrystals. Conventional bicrystal fabrication offers control over these variables, but its ability to mimic grain boundaries in polycrystalline materials is ambiguous. Here we describe a novel solid-state process for rapidly generating intercrystalline interfaces with controlled geometry and chemistry, applicable to a broad range of materials. A fine-grained polycrystalline layer, contacted by two appropriately misoriented single-crystal seeds, is consumed by an epitaxial solid-state transformation until the directed growth fronts impinge. The seed misorientations establish the geometry of the resulting intercrystalline boundaries, and the composition of the sacrificial polycrystalline layer establishes the chemistry of the boundaries and their adjacent grains. Results from a challenging model system, titanium-doped sapphire, illustrate the viability of the directed assembly technique for preparing high-quality bicrystals in both twist and tilt configurations.

Laser-driven formation of a high-pressure phase in amorphous silica.

Because of its simple composition, vast availability in pure form and ease of processing, vitreous silica is often used as a model to study the physics of amorphous solids. Research in amorphous silica is also motivated by its ubiquity in modern technology, a prominent example being as bulk material in transmissive and diffractive optics for high-power laser applications such as inertial confinement fusion (ICF). In these applications, stability under high-fluence laser irradiation is a key requirement, with optical breakdown occurring when the fluence of the beam is higher than the laser-induced damage threshold (LIDT) of the material. The optical strength of polished fused silica transmissive optics is limited by their surface LIDT. Surface optical breakdown is accompanied by densification, formation of point defects, cratering, material ejection, melting and cracking. Through a combination of electron diffraction and infrared reflectance measurements we show here that synthetic vitreous silica transforms partially into a defective form of the high-pressure stishovite phase under high-intensity (GW cm(-2)) laser irradiation. This phase transformation offers one suitable mechanism by which laser-induced damage grows catastrophically once initiated, thereby dramatically shortening the service lifetime of optics used for high-power photonics.

HRTEM image simulations for the study of ultrathin gate oxides.

We have performed high resolution transmission electron microscope (HRTEM) image simulations to qualitatively assess the visibility of various structural defects in ultrathin gate oxides of MOSFET devices, and to quantitatively examine the accuracy of HRTEM in performing gate oxide metrology. Structural models contained crystalline defects embedded in an amorphous 16-A-thick gate oxide. Simulated images were calculated for structures viewed in cross section. Defect visibility was assessed as a function of specimen thickness and defect morphology, composition, size, and orientation. Defect morphologies included asperities lying on the substrate surface, as well as "bridging" defects connecting the substrate to the gate electrode. Measurements of gate oxide thickness extracted from simulated images were compared to actual dimensions in the model structure to assess TEM accuracy for metrology. The effects of specimen tilt, specimen thickness, objective lens defocus, and coefficient of spherical aberration (Cs) on measurement accuracy were explored for nominal 10-A gate oxide thickness. Results from this work suggest that accurate metrology of ultrathin gate oxides (i.e., limited to several percent error) is feasible on a consistent basis only by using a Cs-corrected microscope. However, fundamental limitations remain for characterizing defects in gate oxides using HRTEM, even with the new generation of Cs-corrected microscopes.