High Structural Uniformity Research Articles

Patterned Silver Nanorod Array Substrates for Surface-Enhanced Raman Scattering

A novel method for batch fabrication of substrates for surface-enhanced Raman scattering (SERS) has been developed. A modified platen that fits in a commercial electron beam evaporator enables the simultaneous deposition of Ag nanorod arrays onto six microscope slides by glancing angle deposition. Following removal of substrates from the evaporator, patterned wells are formed by contact printing of a polymer onto the surface. Well dimensions are defined by penetration of the polymer into the nanorod array and subsequent photochemical curing. Inherent advantages of this method include: (1) simultaneous production of several nanorod array substrates with high structural uniformity, (2) physical isolation of nanorod arrays from one another to minimize cross contamination during sample loading, (3) dimensional compatibility of the patterned array with existing SERS microscope, (4) large SERS enhancement afforded by the nanorod array format, (5) small fluid volumes, and (6) ease of use for manual delivery of fluids to each element in the patterned array. In this article, the well-to-well, slide-to-slide, and batch-to-batch variability in physical characteristics and SERS response of substrates prepared via this method is critically examined.

Microfibrous Entrapped Catalysts for Low Temperature CO Oxidation

AbstractLow temperature CO oxidation is characterized by slow reaction kinetics and CO self-poisoning of the catalyst, which compete with each other. A platinum catalyst with ceria as a promoter supported on a high surface area silica support has been developed. The catalyst has a significant activity for CO oxidation even in the presence of moisture. The catalyst showed a significant increase in activity with decreasing particle size (η<0.95, even at 100μm), indicating a clear transport limitation.In case of larger catalyst particles the diffusional resistances affect the reaction kinetics, leading to greater rates of CO self-poisoning causing deactivation of the catalysts. However, conventional packed bed of smaller particles poses problems such as high pressure drop, packing issues, bed channeling, flow maldistributions, and dead zones in the reactor bed, which result in poor inter-phase heat and mass transfer rates. Most of these problems are related to poor packaging of smaller particles, strongly suggesting the need for immobilization of small particles. A new class of micro-structured materials consisting of sorbents/catalysts entrapped in metal, ceramic, or polymer microfibers (MFES/MFEC) has been developed at Auburn University. These materials immobilize the small particles by sinter-locking them in fiber mesh. The immobilization of particles results in better heterogeneous contacting efficiency. Additional studies have shown that the high voidages and structural uniformity of the MFES lowered the flow maldistributions, eliminated the peaking flows between particles, and helped achieve better radial dispersion. Thus MFEC/MFES improved inter-phase transport rates.The Pt-CeO2/SiO2 catalyst entrapped in metal microfibers demonstrated a significant improvement in CO conversion compared to a conventional packed bed configuration while maintaining a lower pressure drop. Further catalyst entrapment in metal microfibers minimized cold spots in the reactor bed due to better intra-phase heat transfer rate. Higher effective thermal conductivity of MFEC improved the activity of the catalyst, which was particularly visible at high CO concentration wherein the CO inhibition kinetics takes over. For example, MFEC with 1/3 rd of catalyst loading as that of packed bed of same particle size, outperformed packed bed configuration.The catalysts were prepared by incipient wet impregnation. Surface characterization of catalysts was carried out using CO, H2, O2 chemisorption, O2 – H2 titration, N2 physisorption, powder XRD, and was correlated with catalytic activity.This microfibrous entrapped catalyst demonstrated a great potential for low temperature CO oxidation. These MFEC can be used in air purification, emergency escape products, and as a cathode air filter for PEM fuel cells.

Effect of indium-flush method on the control of photoluminescence energy of highly uniform self-assembled InAs quantum dots by slow molecular beam epitaxy growth

The control of the emission energy from self-assembled InAs quantum dots has been demonstrated by using indium flush. The low-temperature indium-flush method was found to control the emission energy preserving the high structural uniformity attributed to the slow dot growth. In the standard indium-flush method, where the substrate temperature was raised up from the dot-growth temperature, blueshift larger than the shift by the low-temperature indium flush was observed and was explained reasonably by the enhanced In/Ga-interdiffusion. Also, the effect of AlGaAs capping layer before the indium-flush step was studied.

6 Studies of spin-coated polymer films

Spin-coating is widely employed for the highly reproducible fabrication of thin film coatings over large areas with high structural uniformity. Research in recent years has extended the scope of spin-coating by chemically engineering the interface of support and solution to obtain specific structural order in the resulting supported thin films. This review will discuss both the fundamental physical and chemical processes governing the conventional spin-coating process and describe methodologies for the preparation of spin-coated polymer thin films. Furthermore, a range of advanced applications and recent developments within this field will be reviewed with focus on engineering chemical and topological structure during the coating process.

Synthesis of amylose acetates and amylose sulfates with high structural uniformity

Amylose triacetate (ATA) dissolved in DMSO was partially deacetylated by 1,6-hexamethylendiamine, 1,8-octamethylendiamine, 1,12-dodecylmethylendiamine and 1,2-cyclohexyldiamine (mixture of cis and trans isomers) at 80 °C. The reaction kinetics of the deacetylation were studied. Differences were found in the course of the reaction depending on the type of alkylene diamine (linear or cyclic). The isolated amylose acetates were dissolved in DMF and subsequently sulfated with sulfamic acid. In the course of the sulfation, the acetyl groups acted as protective groups and were completely cleaved after reaction. The amylose acetates and sulfates obtained were studied by 13C NMR spectroscopy and elemental analysis. It could be shown, that the deacetylation of ATA with the described alkylene diamines as well as the subsequent sulfation are highly regioselective. By proceeding this reaction scheme it is possible to synthesize 6-amyloseacetate, 2,6-di-amyloseacetate and 2-amylosesulfate with a high structural uniformity.

Orthotropic mechanical properties of chemically treated bovine pericardium.

To facilitate bioprosthetic heart valve design, especially in the use of novel antimineralization chemical technologies, a thorough understanding of the multiaxial mechanical properties of chemically treated bovine pericardium (BP) is needed. In this study, we utilized a small angle light scattering based tissue pre-sorting procedure to select BP specimens with a high degree of structural uniformity. Both conventional glutaraldehyde (GL) and photo-oxidation (PO) chemical treatment groups were studied, with untreated tissue used as the control group. A second set of GL and PO groups was prepared by prestretching them along the preferred fiber direction during the chemical treatment. An extensive biaxial test protocol was used and the resulting stress-strain data fitted to an exponential strain energy function. The high structural uniformity resulted in both a consistent mechanical response and low variability in the material constants. For free fixed tissues, the strain energy per unit volume for GL treated BP was approximately 2.8 times that of PO treated BP at an equibiaxial Green's strain level of 0.16. Pre-stretched tissues exhibited a profound increase in both stiffness and the degree of anisotropy, with the GL treatment demonstrating a greater effect. Thus, structural control leads to an improved understanding of chemically treated BP mechanical properties. Judicious use of this knowledge can facilitate the design and enhanced long-term performance of bioprosthetic heart valves.

Effect of constrained growth on defect structures in microgravity grown CdZnTe boules

In a microgravity environment obtainable in an orbiting space shuttle, it is possible to virtually eliminate gravity related effects such as buoyancy driven convection and hydrostatic forces thus providing an ideal environment for diffusion-controlled, containerless crystal growth processes. Under such conditions, it is possible to investigate the effects of gravity independent growth parameters on crystal growth. Studies of CdZnTe boules grown on space shuttle mission USML-1 revealed that regions of the boules grown with wall contact were associated with a higher defect density than regions grown with partial or no wall contact. Defect densities in certain regions grown without wall contact were as low as 5 × 102/cm2 to 1.2 × 103/cm2. More detailed studies on the effects of wall contact were sought in the USML-2 mission. Two CdZnTe boules (GCRC-1 and GCRC-2) were grown by the seeded Bridgman-Stockbarger method. Boule GCRC-1 was grown under constrained conditions to force full wall contact while boule GCRC-2 had a tapered geometry designed to minimize wall contact. Defect distributions in the boules were investigated by synchrotron white beam x-ray topography. The sample GCRC-1 was characterized by the presence of large inhomogeneous strains, numerous grains and twins, all of which are caused by effects related to wall contact. On the other hand, a part of the boule GCRC-2 that grew free from wall contact revealed minimum surface strains, the absence of twins and a very high structural uniformity. Results clearly verify that ampoule wall contact plays an important role in determining the incidence of crystal imperfections.

Very-Low Surface Roughness in Laser Crystallized Polycrystalline Silicon

ABSTRACTExcimer laser (ECL) crystallization of silicon films on low temperature substrates is one of the most promising technique for large-area polycrystalline silicon films. Crystallization techniques using pulsed-ECL were extensively studied. They are characterized by films with high structural and electrical properties but low uniformity. In this way, the technology using single shot ECL with very large excimer laser (VEL) may be very promising. It was used for the crystallization of amorphous undoped films deposited by PECVD or by LPCVD The LPCVD technique is the most commonly used deposition technique of silicon. It presents numerous advantages and the hydrogen content in the films, known to constitute a drawback in the crystallization process leading to an increased surface roughness, is negligible (∼1 at.%). However, even with these low hydrogen content LPCVD films, high surface roughness is observed after the laser crystallization. Hence surface roughness appears as one of the major problems to be solved in the high performance TFT's realization from laser crystallized films.In this work, the reduction of the surface roughness, determined from Atomic Force Microscopy observations, is presented. This reduction originated from the use of a surface oxidation and an etch treatment to remove the oxide. The mean height of the roughness is then reduced by about 50%.Using these low surface roughness polycrystalline films, TFT's are then realized.

Study of the microstructure of a Ni(100) single crystal grown by electron beam floating zone melting

The electron beam floating zone melting technique was used to grow oriented single crystals of high-purity nickel. Procedures for the preparation of monocrystalline seeds as well as for the growth of Ni single crystals were developed. Studies of the chemical composition of (100)-oriented single crystals were made. Electrochemical and metallographical methods coupled with microstructural observations by scanning electron microscopy showed the high structural uniformity of the grown single crystals.

Atomspheric pressure organometallic vapor-phase epitaxial growth and characterization of Ga0.4In0.6P/(Al0.4Ga0.6)0.5In0.5P strained quantum wells

Single and multiple Ga0.4In0.6P/(Al0.4Ga0.6)0.5In0.5P quantum wells have been grown using atmospheric pressure organometallic vapor phase epitaxy. The Ga0.4In0.6P well layers are coherently strained to match the lattice parameter of the GaAs substrate. Transmission electron microscopic results showed that the quantum-well layers are very uniform in thickness and the interface is abrupt and free of misfit dislocations. The photoluminescence peak energy increases as the well width decreases, due to carrier confinement in the quantum well. Growth interruptions do not change the photoluminescence peak energy of the quantum well. However, the photoluminescence intensity is drastically reduced for longer growth interruption times. Higher-order x-ray diffraction satellite peaks and a narrow photoluminescence halfwidth are observed in a 20-layer multiple-quantum-well sample, indicative of high structural uniformity and precise control of the composition and thickness during the growth process. Considering the effect of strain on the heterojunction band offsets, the photoluminescence peak energy of the strained quantum well can be described by a simple theory as a function of the well width.