Michael W. Fay

Structural and electrical characterization of AuTiAlTi/AlGaN/GaN ohmic contacts

AuTiAlTi/AlGaN/GaN ohmic contact structures rapid thermal annealed at 650, 750, 850, and 950 °C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. The relationship between annealing temperature, interfacial microstructure, and contact resistance is examined. Annealing temperatures of 750 °C or higher are required to produce an ohmic contact. Contacts annealed at 750 and 850 °C show a planar interface between contact and the AlGaN layer, with minimal consumption of the AlGaN and the formation of a thin TiN interfacial layer. Annealing at 950 °C produces the lowest contact resistance, with a structure showing inclusions through the AlGaN/GaN layer. These inclusions are also shown to be a Ti-nitride, having an Al/Au-rich metallurgical barrier layer surrounding them. However, this metallurgical layer does not produce an electrical barrier.

Assembly, Growth, and Catalytic Activity of Gold Nanoparticles in Hollow Carbon Nanofibers

Graphitized carbon nanofibers (GNFs) act as efficient templates for the growth of gold nanoparticles (AuNPs) adsorbed on the interior (and exterior) of the tubular nanostructures. Encapsulated AuNPs are stabilized by interactions with the step-edges of the individual graphitic nanocones, of which GNFs are composed, and their size is limited to approximately 6 nm, while AuNPs adsorbed on the atomically flat graphitic surfaces of the GNF exterior continue their growth to 13 nm and beyond under the same heat treatment conditions. The corrugated structure of the GNF interior imposes a significant barrier for the migration of AuNPs, so that their growth mechanism is restricted to Ostwald ripening. Conversely, nanoparticles adsorbed on smooth GNF exterior surfaces are more likely to migrate and coalesce into larger nanoparticles, as revealed by in situ transmission electron microscopy imaging. The presence of alkyl thiol surfactant within the GNF channels changes the dynamics of the AuNP transformations, as surfactant molecules adsorbed on the surface of the AuNPs diminished the stabilization effect of the step-edges, thus allowing nanoparticles to grow until their diameters reach the internal diameter of the host nanofiber. Nanoparticles thermally evolved within the GNF channel exhibit alignment, perpendicular to the GNF axis due to interactions with the step-edges and parallel to the axis because of graphitic facets of the nanocones. Despite their small size, AuNPs in GNF possess high stability and remain unchanged at temperatures up to 300 °C in ambient atmosphere. Nanoparticles immobilized at the step-edges within GNF are shown to act as effective catalysts promoting the transformation of dimethylphenylsilane to bis(dimethylphenyl)disiloxane with a greater than 10-fold enhancement of selectivity as compared to free-standing or surface-adsorbed nanoparticles.

Biomaterial modification of urinary catheters with antimicrobials to give long-term broadspectrum antibiofilm activity

Catheter-associated urinary tract infection (CAUTI) is the commonest hospital-acquired infection, accounting for over 100,000 hospital admissions within the USA annually. Biomaterials and processes intended to reduce the risk of bacterial colonization of the catheters for long-term users have not been successful, mainly because of the need for long duration of activity in flow conditions. Here we report the results of impregnation of urinary catheters with a combination of rifampicin, sparfloxacin and triclosan. In flow experiments, the antimicrobial catheters were able to prevent colonization by common uropathogens Proteus mirabilis, Staphylococcus aureus and Escherichia coli for 7 to 12weeks in vitro compared with 1-3days for other, commercially available antimicrobial catheters currently used clinically. Resistance development was minimized by careful choice of antimicrobial combinations. Drug release profiles and distribution in the polymer, and surface analysis were also carried out and the process had no deleterious effect on the mechanical performance of the catheter or its balloon. The antimicrobial catheter therefore offers for the first time a means of reducing infection and its complications in long-term urinary catheter users.

Structural and electrical characterization of AuPdAlTi ohmic contacts to AlGaN∕GaN with varying Ti content

AuPdAlTi∕AlGaN∕GaN ohmic contact structures with varying Ti:Al ratios have been investigated. The relationship between Ti:Al ratio, interfacial microstructure, and contact resistance is examined. Rapid thermal annealing temperatures of 850°C or higher are required to produce an ohmic contact with annealing at 950°C producing the lowest contact resistance in the majority of samples. Samples annealed at 950°C have been analyzed using complementary transmission electron microscopy and electrical characterization techniques. A thin Ti-nitride region is found to form at the contact/semiconductor interface in all samples. Ti-nitride inclusions through the AlGaN∕GaN layer are also observed, surrounded by an Al∕Au rich metallurgical barrier layer, with the size of the inclusions increasing with Ti content. The size of these inclusions does not have any clear effect on the electrical characteristics of the contacts at room temperature, but samples with fewer inclusions show superior electrical characteristics at h...

Mid-infrared photoluminescence in small-core fiber of praseodymium-ion doped selenide-based chalcogenide glass

Rare earth (RE)-ion doped chalcogenide glasses are attractive for mid-infrared (MIR) fiber lasers for operation >4 μm. Our prior modeling suggests that praseodymium (Pr) is a suitable RE-ion dopant for realizing a selenide-based, chalcogenide-glass, step index fiber (SIF) MIR fiber laser operating at 4-5 μm wavelength. Fabrication of RE-ion doped chalcogenide glass fiber, especially with a small core, is a demanding process because crystallization must be avoided during the heat treatments required to effect shaping. Here, a 500 ppmw (parts per million parts, by weight) Pr3+-doped Ge-As-Ga-Se glass SIF with a 10 μm or 15 μm diameter core is reported; the cladding glass is Ge-As-Ga-Se-S. The multistage process to produce the fiber is outlined. Thermal and optical properties of the core/clad. glass pair, and the crystalline/amorphous nature and optical behavior of the small core fiber are reported. MIR photoluminescence and lifetime of a RE-ion doped chalcogenide glass small core fiber are reported for the first time.

Tailoring the physical properties of thiol-capped PbS quantum dots by thermal annealing.

We show that the thermal annealing of thiol-capped PbS colloidal quantum dots provides a means of narrowing the nanoparticle size distribution, increasing the size of the quantum dots and facilitating their coalescence preferentially along the 100 crystallographic axes. We exploit these phenomena to tune the photoluminescence emission of an ensemble of dots and to narrow the optical linewidth to values that compare with those reported at room temperature for single PbS quantum dots. We probe the influence of annealing on the electronic properties of the quantum dots by temperature dependent studies of the photoluminescence and magneto-photoluminescence.

An appraisal of ultramicrotomy, FIBSEM and cryogenic FIBSEM techniques for the sectioning of biological cells on titanium substrates for TEM investigation.

Ultramicrotomy, focused ion beam scanning electron microscopy (FIBSEM) and cryogenic FIBSEM (cryo‐FIBSEM) techniques, as developed for the controlled cross‐sectioning of mesenchymal stem cells (MSCs) and human osteoblasts (HObs) on titanium (Ti) substrates for transmission electron microscopy (TEM) investigation, are compared. Conventional ultramicrotomy has been used to section cells on Ti‐foil substrates embedded in resin, but significant problems with cell detachment using this technique restricted its general applicability. Conventional FIBSEM ‘lift‐out’ procedures were found to be effective for the preparation of uniform sections of fixed and dehydrated cell/Ti specimens, but the control of cell staining remains an issue. Cryo‐FIBSEM procedures used with an ‘H‐bar’ sample geometry enabled the sectioning of fixed and hydrated cell/Ti specimens, but issues remain over ion beam‐induced artefacts and control of frost on the sample foils.

Mg/Ti multilayers: Structural and hydrogen absorption properties

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a “spinodal-like” microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer shortrange order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming

Quantum confined acceptors and donors in InSe nanosheets

We report on the radiative recombination of photo-excited carriers bound at native donors and acceptors in exfoliated nanoflakes of nominally undoped rhombohedral γ-polytype InSe. The binding energies of these states are found to increase with the decrease in flake thickness, L. We model their dependence on L using a two-dimensional hydrogenic model for impurities and show that they are strongly sensitive to the position of the impurities within the nanolayer.

Interactions of Gold Nanoparticles with the Interior of Hollow Graphitized Carbon Nanofibers

Interactions of free-standing gold nanoparticles and hollow graphitized nanofibers in colloidal suspension are investigated, revealing the first example of the controlled arrangement of nanoparticles inside nano-containers, as directed by their internal structure. The ordering is highly effective for small gold nanoparticles whose sizes are commensurate with the height of graphitic step-edges in the graphitized carbon nanofibers and is less effective for larger gold nanoparticles. Studies aimed at understanding the role of the organic-solvent surface tension, employed for the filling experiments, demonstrate that gold nanoparticles become preferentially anchored into the hollow graphitized carbon nanofibers under a mixture of pentane/CO(2) in supercritical conditions. It is shown that a three-step cleaning procedure enables effective removal of gold nanoparticles adsorbed on the exterior surface of graphitized carbon nanofibers, while ordered arrays of encapsulated nanoparticles are retained.