Oriented growth of single-crystalline Bi2S3 nanowire arrays.
Explore the source record for details and available documents.
Journal Article↗
Biomedical subjects
Justin D Holmes
Publications and source records attributed to Justin D Holmes.
Preparation of oriented mesoporous carbon nano-filaments within the pores of anodic alumina membranes.
Oriented mesoporous carbon nano-filaments were prepared as replicas of Fe-containing silica mesostructures deposited inside the channels of anodic alumina membranes (AAMs). Due to the confinement imposed by the channels of the AAMs, rather unusual mesophase structures showing "circular" or "columnar" mesoscopic channels were formed that were replicated as pure carbon analogues after removal of the templating hosts. These structures are assembled in the forms of macroscopic membranes and are hierarchically ordered first due to the arrangement of the channels of the AAMs and second due to the mesophase counterparts.
Journal Article↗
High density germanium nanowire assemblies: contact challenges and electrical characterization.
The conductive properties of vertically aligned germanium nanowires, with mean diameters of 50 and 100 nm, within anodized aluminum oxide (AAO) templates have been characterized by conductive atomic force microscopy (C-AFM) and macrocontact measurements. C-AFM was used to determine the electrical transport properties of individual nanowires within the arrays, while macrocontacts were used to measure the mean current-voltage characteristics of groups of nanowires. Contact resistance between the nanowires and metal macrocontacts was minimized by polishing and gradual etching of the AAO surface, to expose the nanowires, prior to deposition of the contacts. Impedance measurements were used to analyze the importance of defects on the charge transport properties of the germanium nanowire arrays. Conductivity data from C-AFM and macrocontact measurements were found to be comparable suggesting that both methods are inherently suitable for evaluating the electrical transport properties of encapsulated nanowires within a matrix. These results are significant as the ability to make good ohmic contacts to nanowires, within well-defined arrays, is key for the future "bottom-up" fabrication of multilayered device architectures for future electronic and optoelectronic devices.
Journal Article↗
Synthesis and characterization of highly ordered cobalt-magnetite nanocable arrays.
Magnetically tunable, high-density arrays of coaxial nanocables within anodic aluminum oxide (AAO) membranes have been synthesized. The nanocables consist of magnetite nanowires surrounded by cobalt nanotube sheaths and cobalt nanowires surrounded by magnetite nanotube sheaths. These materials are a combination of separate hard (Co) and soft (Fe3O4) magnetic materials in a single nanocable structure. The combination of two or more magnetic materials in such a radial structure is seen as a very powerful tool for the future fabrication of magnetoresistive, spin-valve and ultrafast spin-injection devices with nonplanar geometries. The nanocable arrays were prepared using a supercritical-fluid inclusion process, whereby the nanotube was first deposited onto the pore walls of the nanoporous membranes and subsequently filled with core material to form coaxial nanocables. In essence, this paper describes a technique for placing novel magnetic technologies into well-defined building blocks that may ultimately lead to new multifunctional devices, such as spin valves and high-density magnetic storage devices.
Aluminum Oxide↗
Ultimate-strength germanium nanowires.
Semiconducting nanowires (NWs) are important "building blocks" for potential electrical and electromechanical devices. Here, we report on the mechanical properties of supercritical fluid-grown Ge NWs with radii between 20 and 80 nm. An analysis of the bending and tensile stresses during deformation and failure reveals that while the NWs have a Young's modulus comparable to the bulk value, they have an ultimate strength of 15 GPa, which is the maximum theoretical strength of these materials. This exceptional strength is the highest reported for any conventional semiconductor material and demonstrates that these NWs are without defect or flaws that compromise the mechanical properties.
Journal Article↗
Pore size engineering in mesoporous silicas using supercritical CO2.
In this paper we investigate the use of supercritical carbon dioxide (sc-CO(2)) for synthesizing calcined mesoporous silicas with tunable pore sizes, wall thickness, and d spacings. Small angle neutron scattering was used to probe the controlled swelling of the triblock copolymer surfactant templating agents, P123 (PEO(20)PPO(69)PEO(20)), P85 (PEO(26)PPO(39)PEO(26)), and F127 (PEO(106)PPO(70)PEO(106)), as a function of CO(2) pressure. The transition from the liquid crystal phase to the calcined mesoporous silicas, formed upon condensation and drying, was also studied in detail. Powder X-ray diffraction, transmission electron microscopy, and nitrogen adsorption techniques were used to establish pore diameters, silica wall widths, and the hexagonal packing of the pores within the calcined silicas. Using a direct templating method, the diameters of mesopores and the spacing between the pores could be tuned with a high level of precision. The swelling process was observed to have no detrimental effects on the quality of silica formed, a distinct advantage over conventional swelling techniques, and all of the silicas synthesized in this study were highly ordered over distances of at least 2000 A.
Adsorption↗
Gold nanoparticle-modified etched capillaries for open-tubular capillary electrochromatography.
The use of gold nanoparticles in conjunction with etched capillary-based open-tubular capillary electrochromatography (OTCEC) to improve the efficiency of separation and the selectivity between selected solutes is described. The fused-silica capillaries (50-microm i.d.) were etched with ammonium hydrogen difluoride, followed by prederivatization of the new surface with (3-mercaptopropyl)trimethoxysilane (MPTMS) for the immobilization of dodecanethiol gold nanoparticles, for OTCEC. The electrochromatography of a "reversed-phase" test mixture and of selected polycylic aromatic hydrocarbons was investigated, and efficient separations and high theoretical plate numbers per meter were obtained. The electroosmotic flow characteristics of the etched gold nanoparticle capillary, unetched gold nanoparticle capillary, bare capillary, and etched bare capillary were studied by varying the percentage of organic modifier in buffer, buffer pH, and separation voltage. Optical microscopy and scanning electron microscopy were used to examine the process of etching and modification and the surface features of the etched gold nanoparticle capillary. The results confirm that dodecanethiol gold nanoparticles bonded on the etched inner wall of the fused-silica capillary can provide sufficient solute-bonded phase interactions to obtain OTCEC separations with reproducible retention, as well as characteristic reversed-phase behavior, even with the inner diameter of the capillary of 50 microm.
Capillary Electrochromatography↗
Water-in-CO2 emulsions: reaction vessels for the production of tetra-ethyl pyrone.
Water-in-CO2 (W/C) emulsions formed using a nickel Triton X-100 surfactant complex (Ni-surfactant) were used as microreactors for the carbon-carbon coupling reaction between hex-3-yne and CO2 to produce tetra-ethyl pyrone (TEP). The structure of the product, which was isolated in the CO2 phase, was determined using gas chromatography mass spectrometry (GC-MS) and infrared spectroscopy. The Ni-surfactant acted as both an emulsion forming agent and a water soluble catalyst for this carbon-carbon coupling reaction. The optimum yield of TEP was determined to be 69% after 72 h, at a temperature of 70 degrees C and a pressure of 206 bar. The ease with which the emulsion could be collapsed allowed the facile separation of products from the catalyst and any unreacted starting material. Product selectivity in the W/C emulsions was found to be greater than that obtained using conventional organic methodologies.
Journal Article↗
The synthesis of matrices of embedded semiconducting nanowires.
In this work we report how single crystal nanowires can be assembled into regular arrays using mesoporous thin films to define the architecture. Mesoporous thin films were prepared by a sol-gel method. These provide films of very regular structure and dimensions. The films produced in this way have almost single crystal like structures and can also exhibit strong epitaxy to the underlying silicon substrate. The films are subjected to a supercritical fluid (SCF) environment in which a precursor is decomposed to yield nanowires of metals, semiconductors or oxides. Using these SCF conditions, pore filling is complete and the products are nanowires which are single crystals and structurally aligned in one direction. The growth mechanism of the nanowires is described and size effects discussed.
Journal Article↗
Supercritical fluid generated stationary phases for liquid chromatography and capillary electrochromatography.
Chromatographic silica-bonded stationary phases have been prepared using supercritical CO(2) as the reaction medium. (29)Si solid-state NMR spectra of the generated bonded silica phases show unreacted silica species Q(3) and Q(4), alongside important resonances for surface-bound ligands, T(1), T(2), and T(3). Initially, a fluorinated octyl silica (C(8)) phase was produced, by reacting (1)H,(1)H,(2)H,(2)H-perfluorooctyltriethoxysilane with silica particles (3 microm) in sc-CO(2) at 60 degrees C and 450 atm for 3 h. In-house-packed LC columns of this fluorinated sc-C(8) silica phase yielded typical reversed-phase behavior when a standard test mix of benzamide (k' = 1.03), benzophenone (k' = 8.11), and biphenyl (k' = 14.92) was eluted. When packed into fused-silica capillaries for CEC, this fluorinated sc-C(8) silica phase gave linear plots of log k' versus percentage MeOH for benzophenone and biphenyl and, in contrast to octyl or octadecyl silica phases, displayed selectivity for aromatic thioureas when chromatographed among a series of synthetic organic thiourea test solutes. Similarily, an octadecyl silica phase (sc-C(18) silica) was prepared by reaction of n-octadecyltriethoxysilane in sc-CO(2), packed at 9500 psi and examined by LC. The sc-C(18) silica LC column yielded high column efficiency (up to 141 000 N/m (fluorene)) and excellent asymmetry factors (1.06, fluorene) without resource to end-capping. Following a second silylating or end-capping step using hexamethyldisilazane in sc-CO(2), sc-end-capped sc-C(18) silica phases elute N,N-DMA before toluene and the toluidine isomers as a single peak, indicating lowered silanol activity according to the Engelhardt test. A rapid separation of the important pharmaceutical substances, ketoprofen, naproxen, fentoprofen, and ibuprofen, on an sc-end-capped sc-C(18) silica phase is also shown.
Journal Article↗
Alkylthiol gold nanoparticles in open-tubular capillary electrochromatography.
Open-tubular columns for capillary electrochromatography (CEC) were formed by immobilising dodecanethiol gold nanoparticles on prederivatised 3-aminopropyl-trimethoxysilane (APTMS) or 3-mercaptopropyl-trimethoxysilane (MPTMS) fused-silica capillaries. The initial stage of this research involved the synthesis and characterisation of dodecanethiol gold nanoparticles, with tunnelling electron microscopy analysis of the dispersed phase of the gold nanoparticles dispersion in CHCl3, revealing spherical particles. The surface features of an Au-MPTMS coated capillary column were determined using scanning electron microscopy. The electroosmotic flow characteristics of Au-APTMS and Au-MPTMS capillary columns were then determined, by varying the pH and the voltage. The electrochromatographic properties of the gold nanoparticles CEC capillaries were investigated using a "reversed-phase" test mixture of thiourea, benzophenone and biphenyl and selected pyrethroid pesticides. Efficient separations of benzophenone and biphenyl solutes on Au-MPTMS and Au-APTMS capillary columns were obtained, as were linear plots of logarithm capacity factor versus % MeOH. A study of the reproducibility of retention for these solutes on Au-APTMS, Au-MPTMS and on a loosely coated capillary demonstrated the necessity of a coupling agent to prevent the gold nanoparticles from washing-off. These dodecanethiol gold capillary columns are easier to produce and operate than packed capillary columns. The research work confirms the use of gold nanoparticles as a novel phase for open-tubular CEC, demonstrating reproducible retention and characteristic reversed-phase behaviour.
Chromatography, Micellar Electrokinetic Capillary↗
Supercritical fluid synthesis of metal and semiconductor nanomaterials.
In the near future physical and economic constraints are expected to limit the continued miniaturisation of electronic and optical devices using current "top-down" lithography-based methods. Consequently, nonlithographic methods for synthesising and organising materials on the nanometre scale are required. In response to these technological needs a number of research groups are developing new supercritical fluids methodologies to synthesise and self-assemble "building blocks" of nanomaterials, from the "bottom-up", into structurally complex device architectures. This concept paper highlights some of the recent advances in the synthesis of metal and semiconductor nanoparticles and nanowires by using supercritical fluids. In addition, we describe an efficient supercritical fluid approach for constructing ordered arrays of metal and semiconductor nanowires within mesoporous silica templates.
Journal Article↗
Three dimensional architectures of ultra-high density semiconducting nanowires deposited on chip.
We report a "clean" and fast process, utilizing supercritical carbon dioxide, for producing ultrahigh densities, up to 10(12) nanowires per square centimeter, of ordered germanium nanowires on silicon and quartz substrates. Uniform mesoporous thin films were employed as templates for the nucleation and growth of unidirectional nanowire arrays orientated almost perpendicular to a substrate surface. Additionally, these nanocomposite materials display room-temperature photoluminescence (PL), the energy of which is dependent on the diameter of the encased nanowires. The ability to synthesis ultrahigh-density arrays of semiconducting nanowires on-chip is a key step in future "bottom-up" fabrication of multilayered device architectures for nanoelectronic and optoelectronic devices.
Journal Article↗
Cadmium-specific formation of metal sulfide 'Q-particles' by Klebsiella pneumoniae.
Klebsiella pneumoniae overcomes cadmium toxicity through the 'biotrans-formation' of cadmium ions into photoactive, nanometre-sized CdS particles deposited on the cell surface. The kinetics of particle formation during batch culture growth was monitored by electron microscopy (EM), energy-dispersive X-ray analysis and electronic absorption spectroscopy (EAS). During the deceleration phase of bacterial growth, the presence of CdS particles on the outer cell wall of K. pneumoniae (> or = 5 nm in diameter) was detected by EM. The size of these electron-dense particles continued to increase throughout the stationary phase of growth, with some of the particles reaching a diameter > 200 nm. The formation of the extracellular CdS particles contributed to around 3-4% of the total cell biomass. EAS undertaken on these extracellular 'bio-CdS' particles suggested that the large 'superparticles' observed by EM, e.g. 200 nm, were aggregates of smaller particles termed 'Q-particles', approximately 4 nm in diameter. Metal sulfide particles were not formed in batch cultures of K. pneumoniae grown in the presence of lead, zinc, mercury, copper or silver ions. Growth in the presence of lead ions resulted in the formation of extracellular electron-dense particles containing lead but not sulfide or phosphate. Intracellular phosphorus-containing electron-opaque particles were formed during growth in the presence of copper and mercury. Intracellular electron-dense particles were formed in the presence of zinc ions but these did not contain phosphorus. From these results it was thought that metal sulfide formation in K. pneumoniae showed some cadmium-specificity. When cadmium and zinc ions were both added to the growth medium, metal sulfide particles were formed that were predominantly composed of cadmium, e.g. 48.6% cadmium and 0.04% zinc. Similarly, when cadmium and lead ions were both present during growth only CdS particles formed. In both cases analysis of the cells by EAS confirmed the presence of CdS only. These observations suggest that the mechanism of CdS formation is unlikely to occur simply through a cadmium-induced release of hydrogen sulfide by the cells into the external environment. If hydrogen sulfide production was the mechanism of sulfide formation then metal sulfide particles containing lead and zinc ions in addition to cadmium ions should have been produced.
Cadmium↗