Javier Baca

Probing Nucleation Mechanism of Self-Catalyzed InN Nanostructures

The nucleation and evolution of InN nanowires in a self-catalyzed growth process have been investigated to probe the microscopic growth mechanism of the self-catalysis and a model is proposed for high pressure growth window at ~760 Torr. In the initial stage of the growth, amorphous InNx microparticles of cone shape in liquid phase form with assistance of an InNx wetting layer on the substrate. InN crystallites form inside the cone and serve as the seeds for one-dimensional growth along the favorable [0001] orientation, resulting in single-crystalline InN nanowire bundles protruding out from the cones. An amorphous InNx sheath around the faucet tip serves as the interface between growing InN nanowires and the incoming vapors of indium and nitrogen and supports continuous growth of InN nanowires in a similar way to the oxide sheath in the oxide-assisted growth of other semiconductor nanowires. Other InN 1D nanostructures, such as belts and tubes, can be obtained by varying the InN crystallites nucleation and initiation process.

In situ switch of boron nanowire growth mode from vapor-liquid-solid to oxide-assisted growth

A two-step process was developed to explore ways for massive production of inorganic nanowires (NWs) with well-defined morphology. The first-step vapor-liquid-solid (VLS) growth defined the lateral dimension of boron NWs using Au–B catalytic tips, which was terminated with a quench to solidify the Au–B tip. The second-step oxide-assisted (OA) growth continued the NW growth from the BOx sheath formed around the tip at growth rates more than an order of magnitude higher than in the first step. This two-step process combines the merits of the VLS and OA growths and is promising for massive production of boron and other inorganic NW.

The effect of annealing on the photoconductivity of carbon nanofiber/TiO2 core-shell nanowires for use in dye-sensitized solar cells

Electrical transport properties and photoresponse of individual TiO2-coated carbon nanofibers were studied in an attempt to elucidate the limiting factors of core-shell nanowire-based dye-sensitized solar cells (DSSC). The role of the semiconductor shell microstructure was investigated by comparing as grown and thermally annealed samples. Steady state I-V and transient photoconductivity measurements suggest that improving the microstructure leads to reduced resistivity and contact resistance, a decrease in charge traps, improved surface stoichiometry for dye adsorption, and reduced absorption of visible light by the semiconductor, all of which may improve nanowire-based DSSC performance.

Fabrication and characterization of two-pole X-band HgBa2CaCu2O6+δ microstrip filters

HgBa2CaCu2O6+δ (Hg-1212) two-pole X-band microstrip filters of 5% 3dB bandwidth have been fabricated and evaluated. The Hg-1212 filters exhibited a 449±3MHz bandwidth and a center frequency of 10.86±0.02GHz below the superconducting temperature Tc of 121K with low insertion loss of 0.70±0.04dB at 110K. This performance represents the best so far achieved in superconductor filters at above 100K. The better performance of the Hg-1212 filter at higher temperatures, as compared to its YBa2Cu3O7 and Cu counterparts at 77K, was attributed mainly to its higher Tc, which makes Hg-1212 a promising alternative material for passive microwave devices.

Probing Microscopic Strain Interplay Due to Impurity Doping and Vicinal Growth and Its Effect on Pinning Landscape in YBCO Films

Vortex pinning by insertion of non-superconducting defects like BZO or BSO nanorods into the YBCO matrix is an effective means to enhance pinning since they self-assemble into columnar structures that provide strong pinning along the length of the flux-line. However, only limited control of their geometry is possible by current growth methods. To meet the requirements of applications that operate in magnetic fields of varying intensity or orientation, this work studies strain-mediated self-assembly of 3D pinning landscape through theoretical modeling as well as experimental exploration to achieve controllable growth BZO or BSO nanostructures in YBCO matrix films. The microstructure of BZO- and BSO-doped YBCO thin films was studied using transmission electron microscopy and the findings indicate that it is possible to produce a controllable defect landscape and improved critical current density with respect to different orientation of the magnetic field by manipulation of the strain relationships using vicinal substrates.

Nanorod Self-Assembly in High Jc YBa2Cu3O7−x Films with Ru-Based Double Perovskites

Many second phase additions to YBa2Cu3O7−x (YBCO) films, in particular those that self-assemble into aligned nanorod and nanoparticle structures, enhance performance in self and applied fields. Of particular interest for additions are Ba-containing perovskites that are compatible with YBCO. In this report, we discuss the addition of Ba2YRuO6 to bulk and thick-film YBCO. Sub-micron, randomly oriented particles of this phase were found to form around grain boundaries and within YBCO grains in bulk sintered pellets. Within the limits of EDS, no Ru substitution into the YBCO was observed. Thick YBCO films were grown by pulsed laser deposition from a target consisting of YBa2Cu3Oy with 5 and 2.5 mole percent additions of Ba2YRuO6 and Y2O3, respectively. Films with enhanced in-field performance contained aligned, self-assembled Ba2YRuO6 nanorods and strained Y2O3 nanoparticle layers. A 0.9 µm thick film was found to have a self-field critical current density (Jc) of 5.1 MA/cm2 with minimum Jc(Θ, H=1T) of 0.75 MA/cm2. Conversely, Jc characteristics were similar to YBCO films without additions when these secondary phases formed as large, disordered phases within the film. A 2.3 µm thick film with such a distribution of secondary phases was found to have reduced self-field Jc values of 3.4 MA/cm2 at 75.5 K and Jc(min, Θ, 1T) of 0.4 MA/cm2.

Gold/boron core–shell nanocables synthesized from gold–boron eutectic droplets

Metal/semiconductor core-shell coaxial nanocables are promising building blocks for nanoelectronic devices while in situ growth of these nanocables remains challenging due to the distinctly different synthesis temperature ranges required for metals and semiconductors. To overcome this difficulty, we have developed a vapor-liquid-solid and oxide-assisted bimodal competition growth strategy for in situ metal/semiconductor core-shell nanocable growth. Using this process, gold/boron core-shell nanocables were obtained. A core-shell Au-B/BO(x) eutectic droplet formed via hydrogen gas-assisted rapid cooling was found critical for initiation of the nanocable growth. In addition, the large difference in the boron nanowire growth rates in the vapor-liquid-solid and oxide-assisted mechanisms facilitates the layered growth in the nanocables. The compatibility of this method with the vapor-liquid-solid process applied widely for semiconductor nanowire growth allows in situ connection of metal/semiconductor nanocables with semiconductor nanowires.