Timothy Gutu

A key challenge facing fabrication of nanomaterials is the ability to precisely control the surface morphology of the nanostructures through exploitation of process parameters. In this study, palladium thin films with carefully controlled surface morphologies are obtained through the careful choice of the reducing agent used during the pre‐plating activation step in the three‐step autocatalytic electroless plating consisting of 1) surface functionalization; 2) surface activation; and 3) deposition of the film onto alumina substrates. It is important to note that the reducing agents that influenced the surface morphology of the palladium thin films are employed in the pre‐plating activation step instead of the widely utilized electroless deposition step. It is revealed in the high‐resolution scanning electron microscope results that the hydrogen reducing agent yields a dense film with 3D spheroidal morphologies with an average cluster size of ≈415 nm while the hydrazine reducing agent produces a very smooth uniform surface morphology consisting of extremely small grains. The obtained results can be exploited in controlling and tailoring the surface‐dependent properties of the palladium thin film for applications in gas sensors, detectors, or palladium‐based membranes.

The study seeks to determine the most significant factors affecting arsenic and chromium enrichment using novel P-ZrO2CeO2ZnO nanoparticles/alginate beads in order to minimize the total number of runs needed to successfully run the experiment. The effects of interactions between factors were also evaluated so that the optimum conditions which are not affected by the other factors are chosen for the experiments. The most significant factors on arsenic and chromium enrichment were screened for by using a half-factorial design, followed by the optimization of significant factors using the full-factorial design, and the interaction between factors was determined using ANOVA and interaction plots. The most significant factors for chromium recovery were sample volume, eluent flow rate, and sorbent dosage. For both chromium and arsenic recovery, interactions occurred between sample volume, dosage, and pH. The optimum conditions chosen for the experiment that gave favourable results for both metal ions were sample volume 5 mL, dosage 40 mg, pH = 7 and eluent flow rate 1 mL/min. This study showed that a preliminary screening step for the most significant factors for arsenic and chromium enrichment helps to reduce the number of total runs, and for the same experiment interactions between factors were present; hence, it is necessary to take this into account during the experimental design.

Boron carbide nanowires, a promising class of high temperature thermoelectric nanomaterials, are synthesized by co-pyrolysis of diborane and methane in a low pressure chemical vapor deposition system via the vapor–liquid–solid growth mechanism. Nickel and iron are effective catalytic materials. The synthesis is realized at relatively lower temperatures, with 879 °C as the lowest one. Electron microscopy analysis shows that the as-synthesized nanowires have diameters between 15 and 90 nm and lengths up to 10 μm. The nanowires have single crystalline boron carbide cores and thin amorphous oxide sheaths. Both transverse faults and axial faults with fault planes as {101}_h-type are observed, which could provide additional measures to tune the nanowire transport properties for better thermoelectric performance. Measurement of individual boron carbide nanowires reveals that the thermal conductivity is diameter-dependent, which indicates that boundary scattering still provides an effective approach to reduce the wire thermal conductivity for enhanced thermoelectric performance.

Understanding thermal transport in nanostructured materials is important for the development of energy conversion applications^1,2,3,4 and the thermal management of microelectronic and optoelectronic devices⁵. Most nanostructures interact through van der Waals interactions⁶, and these interactions typically lead to a reduction in thermal transport^7,8,9,10. Here, we show that the thermal conductivity of a bundle of boron nanoribbons can be significantly higher than that of a single free-standing nanoribbon. Moreover, the thermal conductivity of the bundle can be switched between the enhanced values and that of a single nanoribbon by wetting the van der Waals interface between the nanoribbons with various solutions.

The intrinsic thermal conductivity of an individual carbon nanotube and its contact thermal resistance with the heat source/sink can be extracted simultaneously through multiple measurements with different lengths of the tube between the heat source and the heat sink. Experimental results on a 66‐nm‐diameter multiwalled carbon nanotube show that above 100 K, contact thermal resistance can contribute up to 50% of the total measured thermal resistance; therefore, the intrinsic thermal conductivity of the nanotube can be significantly higher than the effective thermal conductivity derived from a single measurement without eliminating the contact thermal resistance. At 300 K, the contact thermal resistance between the tube and the substrate for a unit area is 2.2 × 10⁻⁸ m² K W⁻¹, which is on the lower end among several published data. Results also indicate that for nanotubes of relatively high thermal conductance, electron‐beam‐induced gold deposition at the tube–substrate contacts may not reduce the contact thermal resistance to a negligible level. These results provide insights into the long‐lasting issue of the contact thermal resistance in nanotube/nanowire thermal conductity measurements and have important implications for further understanding thermal transport through carbon nanotubes and using carbon nanotube arrays as thermal interface materials.

There is significant interest in the fabrication of germanium (Ge) doped silica for optoelectronic device applications. In this study, highly photoluminescent Ge centers, metabolically doped into diatom biosilica, are activated by thermal annealing in air. Diatoms are single celled photosynthetic algae that make silica shells called frustules. These frustules possess intricate features and patterns on the nano- and micro-scale. A two-stage photobioreactor cultivation strategy is used to biologically fabricate diatom biosilica doped with Ge, ranging from 0.24 to 0.96 weight percent Ge. X-Ray photoelectron spectroscopy (XPS) and electron diffraction show that a mixture of amorphous germanium dioxide (GeO₂) and germanium oxide (GeO) is doped into the frustule structure. Annealing in air thermally converts the amorphous GeO₂ to GeO, commensurate with an enhancement in the photoluminescence. Thermal gravimetric analysis (TGA) and photoluminescence of annealed biosilica with and without Ge confirm that the photoluminescence originates from GeO photoluminescent centers, and not from the inherent photoluminescence of the biosilica. This is the first study to thermally activate and characterize highly photoluminescent Ge centers metabolically doped into diatom biosilica.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Diatoms are single-celled photosynthetic algae that make silica shells or "frustules" with intricate features patterned at the nano and microscales. In this study, antibody-functionalized diatom biosilica frustules serve as a biosensor platform for selective and label free antibody-antigen immunocomplex formation by enhanced photoluminescence. Biosilica frustules of 10 micron diameter were isolated from cells of the centric marine diatom Cyclotella sp. They were then mounted on glass and covalently functionalized with the model antibody Rabbit Immunoglobulin G (IgG) to yield a uniform nanostructured surface that selectively binds to its complimentary antigen, Goat anti-Rabbit IgG. Diatom frustules possess an intrinsic capacity to emit blue light when excited with a UV laser light source, a property called photoluminescence. Binding the antibody-functionalized diatom frustule with its complimentary antigen selectively enhanced the intrinsic photoluminescence intensity of the diatom frustule by a factor of three, whereas challenging the antibody-functionalized diatom frustule with a non-complimentary antigen, Goat anti-human IgG did not change the intrinsic photoluminescence intensity. The nucleophilic immunocomplex increases the photoluminescence by donating electrons to non-radiative sites on the photoluminescent diatom biosilica, thereby decreasing non-radiative electron decay and increasing radiative emission. The intensified photoluminescence intensity is correlated to the antigen, goat anti-rabbit IgG concentration, with a binding constant of 2.8 ± 0.7x10^-7 M.

Nearly monodisperse lanthanide-doped magnetite nanoparticles were obtained by thermally decomposing a mixture of Fe(acac)₃ and Ln(acac)₃ (acac = acetylacetonate; Ln = Sm, Eu, Gd) in the presence of passivating surfactants. Magnetic studies revealed room-temperature ferromagnetic behaviors of these doped nanoparticles, distinctly different from those of the undoped parent magnetite or the doped nanoparticles prepared by a coprecipitation method.

Individual shells of the diatom Coscinodiscus were self-assembled into a rectangular array on a glass surface that possessed a polyelectrolyte multilayer patterned through inkjet printing. This patterned thin film possessed hierarchical order with nanostructure provided by the diatom biosilica. The process used two polyelectrolytes with opposite electric potentials to control the surface charge of the substrate. The fine features of the diatom frustules were perfectly preserved as a result of the mild conditions of the deposition process. This technique has the potential to enable large-scale device applications that harness the unique properties of functionalized diatom biosilica.