Mark A. Rodriguez

Intrinsic broad-band white-light emission by a tuned, corrugated metal-organic framework.

Herein we report on the broad-band direct white-light originating from a single component emitter, namely a novel three-periodic metal-organic framework (MOF). This material features an unprecedented topology with (3,4)-connected nodes. The structure-function relationship in this system is driven by two complementary unique structural features: corrugation and interpenetration. Good correlation between simulated and experimental emission spectra has been attained, resulting in optimized color properties that approach requirements for solid-state lighting (SSL). Guided by the optimized calculated spectra, the tunability of the assembly was proven by the successful in-framework co-doping of Eu(3+). This resulted in significantly improved color properties, opening new paths for the rational design of alternative materials for SSL applications.

Capture of Volatile Iodine, a Gaseous Fission Product, by Zeolitic Imidazolate Framework-8

Here we present detailed structural evidence of captured molecular iodine (I(2)), a volatile gaseous fission product, within the metal-organic framework ZIF-8 [zeolitic imidazolate framework-8 or Zn(2-methylimidazolate)(2)]. There is worldwide interest in the effective capture and storage of radioiodine, as it is both produced from nuclear fuel reprocessing and also commonly released in nuclear reactor accidents. Insights from multiple complementary experimental and computational probes were combined to locate I(2) molecules crystallographically inside the sodalite cages of ZIF-8 and to understand the capture of I(2) via bonding with the framework. These structural tools included high-resolution synchrotron powder X-ray diffraction, pair distribution function analysis, and molecular modeling simulations. Additional tests indicated that extruded ZIF-8 pellets perform on par with ZIF-8 powder and are industrially suitable for I(2) capture.

Formation of SrBi 2 Ta 2 O 9 : Part I. Synthesis and characterization of a novel “sol-gel” solution for production of ferroelectric SrBi 2 Ta 2 O 9 thin films

We have developed a simple and rapid method for the synthesis of a precursor solution used in the production of SBT powders and thin films of the layered-perovskite phase SrBi{sub 2}Ta{sub 2}O{sub 9} (SBT). Precursor solution preparation takes less than 30 min and involves the generation of two solutions: (a) Bi(O{sub 2}CMe){sub 3} dissolved in pyridine and (b) Ta(OCH{sub 2}Me){sub 5} added to Sr(O{sub 2}CMe){sub 2} and then solubilized by HO{sub 2}CMe. After stirring separately for 10 min, these solutions are combined, stirred for an additional 10 min, and used without any further modifications. The individual solutions and ternary mixture were studied using a variety of analytical techniques. Films of the layered-perovskite phase were formed at temperatures as low as 700{degree}C. Ferroelectric testing of SBT films, fired at 750{degree}C, reveals standard hysteresis loops with no fatigure for up to 4{times}10{sup 9} cycles. {copyright} {ital 1996 Materials Research Society.}

FORMATION OF SRBI2TA2O9 : PART II. EVIDENCE OF A BISMUTH-DEFICIENT PYROCHLORE PHASE

A bismuth-deficient pyrochlore phase has been observed in both powder and film samples fired at 775{degree}C. The estimated stoichiometry of this pyrochlore (based on calculated diffraction patterns) was Sr{sub 0.2}(Sr{sub 0.5}Bi{sub 0.7})Ta{sub 2}O{sub 6.75}. This bismuth-deficient pyrochlore phase may be considered deleterious to the formation of the SrBi{sub 2}Ta{sub 2}O{sub 9} {open_quote}{open_quote}SBT{close_quote}{close_quote} ferroelectric compound since a significant presence of this pyrochlore compound implies a large deviation from the desired cation ratios. Additionally, films prepared on platinized silicon substrates indicate the SBT phase formation may be encouraged by the substrate; there appears to be some 00{ital l} preferential orientation for stoichiometric SBT thin films. {copyright} {ital 1996 Materials Research Society.}

Growth and morphology of cadmium chalcogenides: the synthesis of nanorods, tetrapods, and spheres from CdO and Cd(O2CCH3)2

In this work, we investigated the controlled growth of nanocrystalline CdE (E = S, Se, and Te) via the pyrolysis of CdO and Cd(O2CCH3)2 precursors, at the specific Cd to E mole ratio of 0.67 to 1. The experimental results reveal that while the growth of CdS produces only a spherical morphology, CdSe and CdTe exhibit rod-like and tetrapod-like morphologies of temporally controllable aspect ratios. Over a 7200 s time period, CdS spheres grew from 4 nm (15 s aliquot) to 5 nm, CdSe nanorods grew from dimensions of 10.8 × 3.6 nm (15 s aliquot) to 25.7 × 11.2 nm, and CdTe tetrapods with arms 15 × 3.5 nm (15 s aliquot) grew into a polydisperse mixture of spheres, rods, and tetrapods on the order of 20 to 80 nm. Interestingly, long tracks of self-assembled CdSe nanorods (3.5 × 24 nm) of over one micron in length were observed. The temporal growth for each nanocrystalline material was monitored by UV-VIS spectroscopy, transmission electron spectroscopy, and further characterized by powder X-ray diffraction. This study has elucidated the vastly different morphologies available for CdS, CdSe, and CdTe during the first 7200 s after injection of the desired chalcogenide.

All solution-chemistry approach for YBa2Cu3O7−δ-coated conductors

A need exists for low-cost coated-conductor fabrication methods for applications in magnet and electric-power technologies. We demonstrate high-critical current density (Jc) YBa2Cu3O7−δ (YBCO) films grown on Nb-doped SrTiO3 (Nb:STO) buffered Ni(100) tapes. All buffer and superconductor layers are deposited using solution chemistry. A 50 nm thick Nb:STO seed layer on Ni(100) acts as a template for the growth of subsequent thicker layers of Nb:STO. Nb doping improves the electrical conductivity and oxygen diffusion barrier properties of STO. YBCO grows heteroepitaxially directly on this buffer layer, resulting in a transport Jc(77 K)=1.3 MA/cm2.

Highly ordered multilayered 3D graphene decorated with metal nanoparticles

Highly ordered multi-layered three-dimensional (3D) graphene structures decorated with Pd, Pt and Au metal nanoparticles are prepared and characterized. The ability to control the morphology, distribution and size of the metal nanoparticles on the 3D graphene support upon changing the electro- and electroless-deposition conditions is demonstrated. Tailor-made Pt nanostructures, with nanospike and nanoparticle shapes, are prepared using electroless deposition techniques. Au nanoflowers and nanoparticle structures and Pd nanocubes are obtained following electrodeposition onto the 3D graphene support. The deposition patterns and trends are characterized. The greatly enhanced electrocatalytic activity of the metal-NP–graphene surfaces has been illustrated in connection to voltammetric measurements of ORR and hydrogen peroxide at 3D-graphene coated with Pt and Pd nanoparticles, respectively. Such metal nanoparticles decorated multi-layer 3D graphene allows for high mass transport access and catalytic activity for a diverse range of applications, including sensor and fuel-cell technologies.

Highly Versatile Rare Earth Tantalate Pyrochlore Nanophosphors

Rare earth tantalate materials are of considerable interest in energy and environmentally related applications including photocatalytic H(2) generation or contaminant decomposition, ion conductivity for batteries and fuel cells, and phosphors for light-emitting diodes (LEDs). These Eu-doped rare earth tantalate pyrochlore nanoparticles, K(1-2x)LnTa(2)O(7-x):Eu(3+) (Ln = Lu, Y, Gd; x = (1)/(3) for Gd, x = 0 for Lu and Y), have quantum yields up to 78% when excited with blue light (464 nm), which is remarkable for nanoparticle forms that can suffer efficiency loss by surface effects or poor crystallinity, and are furthermore quite suitable for LED applications. The Gd analogue with its framework distortions has particularly high quantum yields. The blue excitation peak matches the emission of the GaN LED. The combination of the high quantum yield under blue excitation, low thermal quenching, and chemical stability renders these new materials promising red phosphors for blue-excitation white LEDs for solid-state lighting. In addition, the pyrochlore lattice is very accommodating to dopants and vacancies and will incorporate virtually any metal and coordination environment ranging from four-coordinate to eight-coordinate. Thus, there are virtually unlimited possibilities for tailoring and optimizing photoluminescent properties, as demonstrated by these scoping studies.

Soluble Heteropolyniobates from the Bottom of Group IA

Surprising solubility: While it is already well known that [Nb(6)O(19)](8-) salts exhibit an unusual solubility trend, that is, Cs>Rb>K>Na>Li, the heteropolyniobates of Cs and Rb had not yet been crystallized. These very soluble entities have now been obtained from solution by a simple and universal process. New polyoxoniobate geometries are thus unveiled, and the [SiNb(12)O(40)](16-) Keggin ion is characterized in solution for the first time.

Investigation of PZT//LSCO/IPT//Aerogel Thin Film Composites for Uncooled Pyroelectric Ir Detectors

A monolithic thin film structure for uncooled pyroelectric infrared detector arrays was developed to integrate thermally isolated Pb(Zr,Ti)O 3 -based thin films on silicon, with a low thermal conductivity, aerogel thin film interlayer. Aerogel thin films of 0.4-1.2μm thickness and 70-85% porosity were deposited on silicon substrates by dip coating to form a thermal isolation layer. 200-400nm thick Pb(Zr 0.4 Ti 0.6 )O 3 films deposited on (La,Sr)CoO 3 [LSCO]//Pt electrodes atop these aerogel films displayed a remanent polarization P r = 28μ C/cm 2 , pyroelectric coefficient p = 30 nC/cm 2 K, and calculated noise equivalent temperature difference NETD = 0.07°C. Processing of Pb(Zr,Ti)O 3 thin films atop the aerogel interlayer structure required modifications to deposition and heat treatment methods to minimize tensile stress accumulation. Effects of thermal processing and thin film stress state on ferroelectric properties are also reported.