David N. McIlroy

Principles and mechanisms of gas sensing by GaN nanowires functionalized with gold nanoparticles

Electrical properties of a chemical sensor constructed from mats of GaN nanowires decorated with gold nanoparticles as a function of exposure to Ar, N2, and methane are presented. The Au nanoparticle decorated nanowires exhibited chemically selective electrical responses. The sensor exhibits a nominal response to Ar and slightly greater response for N2. Upon exposure to methane the conductivity is suppressed by 50% relative to vacuum. The effect is fully reversible and is independent of exposure history. We offer a model by which the change in the current is caused by a change in the depletion depth of the nanowires, the change in the depletion depth being due to an adsorbate induced change in the potential on the gold nanoparticles on the surface of the nanowires.

On the importance of boundary conditions on nanomechanical bending behavior and elastic modulus determination of silver nanowires

Nanomechanical bending behavior and elastic modulus of silver nanowires (65–140nm∅) suspended across silicon microchannels were investigated using digital pulsed force mode (DPFM) atomic force microscopy through coincident imaging and force profiling. Deflection profiles analyzed off-line demonstrate the role of bending nanowire shape and symmetry in experimentally determining boundary conditions, eliminating the need to rely on isolated midpoint bending measurements and the usual assumptions for supported-end behavior. Elastic moduli for as-prepared silver nanowires ranged from 80.4±5.3to96.4±12.8GPa, which met or exceeded the literature values for bulk silver. The calculated moduli were based on classic modeling, both with one-dimensional analytical solutions and three-dimensional finite element analysis. Modeling results indicate that the classic models are accurate as long as the boundary conditions are not arbitrarily assumed but directly confirmed by data analysis. DPFM also facilitated the experime...

Fabrication of n-type nickel doped B5C1+δ homojunction and heterojunction diodes

We have successfully nickel doped a boron carbide (B5C) alloy film. The nickel doped boron-carbide (Ni-B5C1+δ) thin films were fabricated from a single source carborane cage molecule and nickelocene [Ni(C5H5)2] using plasma enhanced chemical vapor deposition. Nickel doping transforms the highly resistive undoped film from a p-type material to an n-type material. This has been verified from the characteristics of diodes constructed of Ni-B5C1+δ on both n-type silicon and p-type B5C. The homojunction diodes exhibit excellent rectifying properties over a wide range of temperatures.

Nanospring formation—unexpected catalyst mediated growth

Nanosprings are a new form of nanowires that have potential applications in nanoelectronics, nanomechanics, and nanoelectromechanical systems. In this review we will examine the growth mechanism of these novel nanostructures. The synthesis of nanowires by the vapour–liquid–solid growth mechanism, first proposed by Wagner and Ellis, will be explored and then extended to the development of a model to explain the formation of nanosprings.

High yield synthesis and lithography of silica-based nanospring mats

In this study we report a novel technology for synthesizing silica-based nanosprings with a yield higher than 90%, and with 100% repeatability. The nanospring mats are grown via the liquid–vapour–solid mechanism using a gold catalyst, where the deposition temperature can be as low as 350 ◦ C. XPS analysis shows that the as-grown nanosprings have components of silicon and oxygen with an atomic ratio close to silica. Both SEM and TEM images illustrate that the helical structure of the nanosprings is extremely uniform. Tw ot ypes of nanosprings are observed using TEM. The first type of silica nanospring is formed from a single nanowire, whereas the second type consists of multiple intertwined nanowires. Patterned deposition of nanosprings has been achieved using this technology.

Controlled growth of gold nanoparticles on silica nanowires

Production of gold nanoparticles with the specific goal of particle size control has been investigated by systematic variation of chamber pressure and substrate temperature. Gold nanoparticles have been synthesized on SiO 2 nanowires by plasma-enhanced chemical vapor deposition. Determination of particle size and particle size distribution was done using transmission electron microscopy. Average nanoparticle diameters were between 4 and 12 nm, with particle size increasing as substrate temperature increased from 573 to 873 K. A bimodal size distribution was observed at temperatures ≥723 K indicating Ostwald ripening dominated by surface diffusion. The activation energy for surface diffusion of gold on SiO 2 was determined to be 10.4 kJ/mol. Particle sizes were found to go through a maximum with increases in chamber pressure. Competition between diffusion within the vapor and dissociation of the precursor caused the pressure effect.

Nickel doping of boron carbide grown by plasma enhanced chemical vapor deposition

We have nickel doped boron carbide grown by plasma enhanced chemical vapor deposition. The source gas closo‐1,2‐dicarbadodecaborane (ortho‐carborane) was used to grow the boron carbide, while nickelocene [Ni(C5H5)2] was used to introduce nickel into the growing film. The doping of nickel transformed a p‐type, B5C material, relative to lightly doped n‐type silicon, to an n‐type material. Both n‐n heterojunction diodes and n‐p heterojunction diodes were constructed, using as substrates n‐ and p‐type Si(111), respectively. With sufficient partial pressures of nickelocene in the plasma reactor, diodes with characteristic tunnel diode behavior can be successfully fabricated.

A novel enzymatic microreactor with Aspergillus oryzae β-galactosidase immobilized on silicon dioxide nanosprings.

The use of silicon dioxide (SiO2) nanosprings as supports for immobilized enzymes in a continuous microreactor is described. A nanospring mat (2.2 cm2 × 60 μm thick) was functionalized with γ‐aminopropyltriethoxysilane, then treated with N‐succinimidyl‐3‐(2‐pyridyldithio)‐propionate (SPDP) and dithiothreitol (DTT) to produce surface thiol (SH) groups. SPDP‐modified β‐galactosidase from Aspergillus oryzae was immobilized on the thiolated nanosprings by reversible disulfide linkages. The enzyme‐coated nanospring mat was placed into a 175‐μm high microchannel, with the mat partially occluding the channel. The kinetics and steady‐state conversion of hydrolysis of o‐nitrophenyl β‐D‐galactosylpyranoside at various substrate flow rates and concentrations were measured. Substantial flow was observed through the nanosprings, for which the Darcy permeability κ ≈ 3 × 10−6 cm2. A simple, one‐parameter numerical model coupling Navier‐Stokes and Darcy flow with a pseudo‐first‐order reaction was used to fit the experimental data. Simulated reactor performance was sensitive to changes in κ and the height of the nanospring mat. Permeabilities lower than 10−8 cm2 practically eliminated convective flow through the nanosprings, and substantially decreased conversion. Increasing the height of the mat increased conversion in simulations, but requires more enzymes and could cause sealing issues if grown above channel walls. Preliminary results indicate that in situ regeneration by reduction with DTT and incubation with SPDP‐modified β‐galactosidase is possible. Nanosprings provide high solvent‐accessible surface area with good permeability and mechanical stability, can be patterned into existing microdevices, and are amenable to immobilization of biomolecules. Nanosprings offer a novel and useful support for enzymatic microreactors, biosensors, and lab‐on‐chip devices.

Differential cytotoxicity exhibited by silica nanowires and nanoparticles

Silica nanowires are one-dimensional nanomaterials that are being developed for use in biological systems. Unfortunately, little is known regarding the cytotoxic potential of this type of nanomaterial. Here, using two different human epithelial cell lines we have examined the cytotoxicity of silica nanowires over a broad concentration range. The results indicate that silica nanowires are nontoxic at concentrations below 190 µg/ml but exhibit considerable cytotoxicity at higher concentrations. Examination of the mechanisms responsible for nanowire-induced cytotoxicity indicates that apoptotic pathways are not activated. Instead, cytotoxicity appears to be primarily due to increased necrosis in cells exposed to high concentrations of nanowires. In contrast to what was seen with silica nanowires, analysis of silica nanoparticles revealed very little cytotoxicity even at the highest concentrations tested. These results indicate that structural differences between silica nanomaterials can have dramatic effects on interaction of these nanomaterials with cells.

The electronic structure of gadolinium grown on Mo(112)

The electronic structure of strained ultra-thin and thin films of Gd grown on a corrugated Mo(112) surface are described. Gadolinium overlayers order at a coverage of monolayers forming a LEED pattern. At this coverage an interface state of symmetry is formed at a binding energy of at the surface Brillouin zone centre . The effective mass of this interface state was determined to be , in both orthogonal directions along the nearly square reduced Brillouin zone. For thicker Gd films of approximately 3 to 10 ML thickness, the corresponding gadolinium state switches symmetry to the representation (Gd or 6s character) and has a much larger effective mass. The overlayer forms a rectangular surface Brillouin zone resembling the hcp surface. Gadolinium films thicker than approximately 10 ML form strained hexagonal ordered films also with substantial misfit dislocations. The strain of the thin hexagonal ordered Gd films is reflected by a reduced Brillouin zone size along by approximately 4% with respect to the less strained Gd overlayers on W(110) and Gd(0001) single crystals. The induced strain severely alters the band structure of the Gd 5d/6s bulk bands, which disperse in the opposite direction relative to the corresponding bands of the relaxed Gd(0001) structure. The surface of the strained hexagonal fcc (111) or hcp (0001) Gd films forms a localized state of symmetry (Gd or 6s character) at approximately 0.7 eV binding energy. There is little observed strain relief within the Gd films up to approximately 150 A film thickness.