Derek C. Johnson

Cu2Se Nanoparticles with Tunable Electronic Properties Due to a Controlled Solid-State Phase Transition Driven by Copper Oxidation and Cationic Conduction

Stoichiometric copper(I) selenide nanoparticles have been synthesized using the hot injection method. The effects of air exposure on the surface composition, crystal structure, and electronic properties were monitored using X-ray photoelectron spectroscopy, X-ray diffraction, and conductivity measurements. The current-voltage response changes from semiconducting to ohmic, and within a week a 3000-fold increase in conductivity is observed under ambient conditions. The enhanced electronic properties can be explained by the oxidation of Cu(+) and Se(2-) on the nanoparticle surface, ultimately leading to a solid-state conversion of the core from monoclinic Cu(2)Se to cubic Cu(1.8)Se. This behavior is a result of the facile solid-state ionic conductivity of cationic Cu within the crystal and the high susceptibility of the nanoparticle surface to oxidation. This regulated transformation is appealing as one could envision using layers of Cu(2)Se nanoparticles as both semiconducting and conducting domains in optoelectronic devices simply by tuning the electronic properties for each layer through controlled oxidation.

Synthesis of copper silicide nanocrystallites embedded in silicon nanowires for enhanced transport properties

Here we report the in situ doping of Si nanowires with Cu, which results in nanowires containing nanocrystalline inclusions of Cu3Si and significantly enhanced electrical conductivity. These nanowires are of interest for use in secondary Li batteries as well as nanowire arrays that can be directly sensitized for photovoltaic applications. This synthesis route is based on controlling the vapour-phase flux of precursor materials into the catalyst tip whereby the flux of the Cu is much less than that of Si. A compositional study utilizing SEM–EDS, XRD, and TEM–EDS techniques of vapour–liquid–solid (VLS) grown Si nanowires in the presence of Cu vapour confirms that the bulk nanowire matrix is Si doped with crystalline Cu3Si and low concentrations of Cu. The electronic transport measurements conducted on single nanowires indicate that the electronic resistivity of the doped nanowires is several orders of magnitude lower than undoped Si, thereby making them more conductive. Based on the data collected from the nanowire growth in conjunction with the in situ VLS doping mechanism, the doping density can be controlled by varying the gas-phase concentration of the dopant or the thermodynamic conditions of the nanowire growth. Both approaches will result in a change in the relative fluxes from the gas phase into the VLS catalyst as well as the kinetics for Cu3Si formation. This is advantageous because dopant density can be used to tune both the electronic and the optical properties of the nanowires.

Three-dimensional lithium-ion batteries with interdigitated electrodes

Secondary lithium-ion batteries have found multiple applications in portable electronics where high charge and discharge rates are not required to improve performance. However, lithium-ion batteries are currently being sought for high power applications that require long cycle life, such as those encountered in the transportation sector. To meet these performance requirements, the shortcomings that have relegated the use of conventional lithium-ion batteries to low-power applications need to be addressed. In an attempt to fabricate batteries with high power densities, current technology is moving toward electrode materials with irregular surfaces resulting in high interfacial surface areas and short characteristic lithium-ion diffusion lengths. The use of three-dimensional (3D) architectures with interdigitated electrodes with the above described electrode characteristics have been proposed to alleviate these shortcomings because it allows a significant decoupling of the inversely proportional relationship between energy and power density. This conference proceeding manuscript is focused on the idealized calculations of both nanowire and foam 3D architectures utilizing electrode and electrolyte components that are currently being developed. A brief discussion of the use of electrodeposition as the main synthetic technique towards realizing a truly 3D solid-state lithium-ion cell is also presented.

Evidence of Induced Underpotential Deposition of Crystalline Copper Antimonide via Instantaneous Nucleation

Cu2Sb was electrodeposited onto transmission electron microscopy TEM grids to investigate changes in morphology, composition, and crystal structure during the early stages of nucleation and growth. Multiple transitions were observed within the first second of the deposition, leading to the formation of crystalline Cu2Sb. These transitions were analyzed using TEM, scanning electron microscopy, selected area electron diffraction, and energy-dispersive X-ray spectroscopy. The nucleation sites are initially polycrystalline antimony with amorphous copper, which then transition through a polycrystalline copper intermediate containing some antimony before forming crystalline Cu2Sb. These analyses provide direct evidence that Cu2Sb does not deposit directly from solution but deposits by induced underpotential deposition. This is indicative of the electrodeposition of a typical alloy initially, but what is unusual is that the deposit at longer time scales is a highly crystalline intermetallic. This investigation is unique because TEM grids allow the interface between the deposited material and the substrate to be investigated. This is possible because the composite carbon film on the TEM grid behaves as a transparent substrate. This approach can be extended to other systems, allowing the development of a comprehensive understanding of the electrodeposition of intermetallic compounds.

Aqueous Plasma Pharmacy: Preparation Methods, Chemistry, and Therapeutic Applications

Plasma pharmacy is a subset of the broader field of plasma medicine. Although not strictly defined, the term aqueous plasma pharmacy (APP) is used to refer to the generation and distribution of reactive plasma-generated species in an aqueous solution followed by subsequent administration for therapeutic benefits. APP attempts to harness the therapeutic effects of plasma-generated oxidant species within aqueous solution in various applications, such as disinfectant solutions, cell proliferation related to wound healing, and cancer treatment. The subsequent use of plasma-generated solutions in the APP approach facilitates the delivery of reactive plasma species to internal locations within the body. Although significant efforts in the field of plasma medicine have concentrated on employing direct plasma plume exposure to cells or tissues, here we focus specifically on plasma discharge in aqueous solution to render the solution biologically active for subsequent application. Methods of plasma discharge in solution are reviewed, along with aqueous plasma chemistry and the applications for APP. The future of the field also is discussed regarding necessary research efforts that will enable commercialization for clinical deployment.

An innovative non-thermal plasma reactor to eliminate microorganisms in water

AbstractThe growing need for scalable systems that can inactivate microbiological contaminants and recycle water in industrial operations has led to the development of a variety of new advanced oxidation process (AOP) technologies. In this paper, we report on the capability and techno-economics of a new AOP method to generate aqueous plasma species for inhibition of microbiological contaminants. The test microorganisms in this work were Acidithiobacillus ferrooxidans (a motile, Gram-negative bacterium that oxidizes sulfides to sulfates and ferrous iron to ferric iron, used as a model biofouling organism) and Legionella gratiana (a Gram-negative bacteria used as a surrogate of the human pathogen Legionella pneumophila, which can be a dangerous contaminant in cooling water systems). The cultured bacteria were dispersed in water and treated within a non-thermal plasma treatment system for varied exposure times, and then the bactericidal effects were measured. The results demonstrated plasma inhibition of A. ...

Effects of transport gradients in a chemical vapor deposition reactor employing vapor-liquid-solid growth of ternary chalcogenide phase-change materials

Chemical vapor deposition (CVD) with vapor-liquid-solid (VLS) growth is employed to synthesize individual Ge(2)Sb(2)Te(5) nanowires with the ultimate goal of synthesizing a large scale nanowire array for universal memory storage. A consistent challenge encountered during the synthesis is a lack of control over the composition and morphology across the growth substrate. To better understand the challenges associated with the CVD synthesis of the ternary chalcogenide, computational fluid dynamics simulations are performed to quantify 3D thermal and momentum transients in the growth conditions. While these gradients are qualitatively known to exist, they have not been adequately quantified in both the axial and radial directions when under pressure and flow conditions indicative of VLS growth. These data are not easily acquired by conventional means for the axial direction under vacuum and are a considerable challenge to accurately measure radially. The simulation data shown here provide 3D insights into the gradients which ultimately dictate the region of controllable stoichiometry and morphology. These results help explain the observed inhomogeneity of the characterized ternary chalcogenide growth products at various growth substrate locations.

A computational fluid dynamics investigation of fluid flow in a dense medium plasma reactor

Computational fluid dynamics are applied to the study of three-dimensional fluid flow in a dense medium plasma reactor (DMPR) under different operating conditions. Reaction mechanisms and rates for the removal of methyl t-butyl ether (MTBE) in a DMPR are developed from experimental data to determine the plasma volume, the rate of interphase mass transfer and the photolysis rate of MTBE via UV emission from the plasma. The simulations utilize the plasma volume determined from the kinetic data to show that the volume of fluid in contact with the plasma in the DMPR only constitutes a maximum of approximately 10% of the fluid intended to be cycled through the plasma tubules. The simulations also predict appreciable pressure gradients on the surface of the pin electrodes, resulting in a small discharge area located away from the region in which the electric field strength is a maximum. This result has been confirmed indirectly through observation in that the pin electrodes sputter metal from an area of similar size and location to the low-pressure region predicted by the simulations. The pressure gradients are shown to be a function of operating conditions as well as pin location, indicating that the plasma discharge conditions are not uniform throughout the reactor.