Jon L. Schindler

Phase‐selective route to high Tc superconducting Tl2Ba2Can−1CunO2n+4 films: Combined metalorganic chemical vapor deposition using an improved barium precursor and stoichiometry‐controlled thallium vapor diffusion

Films of the Tl2Ba2Can−1CunO2n+4 high Tc superconductors (n=2 or 3) can be prepared with a high degree of phase selectivity using a combination of metalorganic chemical vapor deposition (MOCVD) and vapor diffusion. Ba‐Ca‐Cu‐O films are first prepared by MOCVD using the volatile metalorganic precursors Ba(hexafluoroacetyl‐ acetonate)2(tetraglyme), Ca(dipivaloylmethanate)2, and Cu(acetylacetonate)2. The ‘‘second‐generation’’ barium precursor exhibits significantly improved thermal stability and volatility over previously used compounds. Thallium is then incorporated into these films by vapor diffusion using a Tl‐Ba‐Ca‐Cu oxide mixture of controlled composition as the source of volatile thallium oxides. Phase control is achieved by a combination of improved stoichiometry of the deposited film, a result of the new Ba source, and annealing with the appropriate oxide mixture. The resultant films consist predominantly of the Tl2Ba2Ca2Cu3Ox or Tl2Ba2CaCu2Ox phase, each having preferential orientation of the cryst...

Nanoscale Composites Formed by Encapsulation of Polymers in MoS2. From Conjugated Polymers to Plastics. Detection of Metal to Insulator Transition

Abstract Polyaniline, poly(ethylene oxide), poly(propylene glycol), poly-(vinylpyrrolidinone), methyl cellulose, polyethylenimine, polyethylene, and Nylon-6, were encapsulated into MoS2. Electrical conductivity and thermoelectric power measurements of pressed pellets of (PEO)0.92MoS2 and (Polyaniline)0.35MoS2 show p-type metallic behavior. Below 14 K and 9 K respectively both show a metal to insulator transition. The thermal stability of these materials is reported.

Electrical transport properties of epitaxial BaTiO3 thin films

Measurements of the temperature dependent transport properties of epitaxial BaTiO3 are reported. Electrical resistivity and thermoelectric power were measured over the temperature range of 77–300 K. Room temperature resistivities of the as‐deposited, undoped films range from 105 to 108 Ω cm, while values as low as 55 Ω cm are obtained for the La‐doped films. The resistivity shows an activated temperature dependence with the measured activation energies ranging between 0.11 and 0.50 eV. The activation energy depends strongly upon the thin film carrier concentration. Thermoelectric power measurements indicate that the films are n‐type. The Seebeck coefficient for La‐doped BaTiO3 exhibits metallike behavior, with its magnitude directly proportional to temperature. Temperature dependent resistivity and thermopower measurements indicate that the carrier mobility is activated. A transport model is proposed whereby conduction occurs in the La‐doped films via hopping between localized states within a pseudogap fo...

Lamellar polymer-LixMoO3 nanocomposites via encapsulative precipitation

With Li x MoO 3 (x=0.31–0.40) as a host material, a new family of polymer–molybdenum bronze nanocomposites has been synthesized using an exfoliation/encapsulation methodology. Nanocomposites with poly(ethylene oxide), poly(ethylene glycol), poly(propylene glycol), poly(vinylpyrrolidinone), methyl cellulose, polyacrylamide, and nylon-6 were prepared and characterized by thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), powder X-ray diffraction, FTIR spectroscopy, UV–VIS spectroscopy, variable-temperature 7 Li and 13 C solid-state NMR spectroscopy and magnetic susceptibility measurements. The electrical conductivity of these materials lies in the range from 10 -2 to 10 -7 S cm -1 , and decreases as the interlayer separation increases. The intercalated polymer imparts both mechanical strength and ease of processing to these materials. The water-soluble polymer–Li x MoO 3 nanocomposites can be cast into films and other shapes, which may provide opportunities for applications. Factors affecting the intercalation reaction and the structure of nanocomposites, such as variations in the preparation procedure, the polymer molecular mass and the annealing behaviour of the products are discussed.

Thin films for superconducting electronics: Precursor performance issues, deposition mechanisms, and superconducting phase formation-processing strategies in the growth of Tl2Ba2CaCu2O8 films by metal-organic chemical vapor deposition

Epitaxial Tl2Ba2CaCu2O8 thin films with excellent electrical transport characteristics are grown in a two-step process involving metal-organic chemical vapor deposition (MOCVD) of a BaCaCuO(F) thin film followed by a postanneal in the presence of Tl2O vapor. Vapor pressure characteristics of the recently developed liquid metal-organic precursors Ba(hfa)2 • mep (hfa › hexafluoroacetylacetonate, mep › methylethylpentaglyme), Ca(hfa)2 • tet (tet › tetraglyme), and the solid precursor Cu(dpm)2 (dpm › dipivaloylmethanate) are characterized by low pressure thermogravimetric analysis. Under typical film growth conditions, transport is shown to be diffusion limited. The transport rate of Ba(hfa) 2 • mep is demonstrated to be stable for over 85 h at typical MOCVD temperatures (120 ‐ C). In contrast, the vapor pressure stability of the commonly used Ba precursor, Ba(dpm)2, deteriorates rapidly at typical growth temperatures, and the decrease in vapor pressure is approximately exponential with a half-life of ,9.4 h. These precursors are employed in a low pressure (5 Torr) horizontal, hot-wall, film growth reactor for growth of BaCaCuO(F) thin films on (110) LaAlO3 substrates. From the dependence of film deposition rate on substrate temperature and precursor partial pressure, the kinetics of deposition are shown to be mass-transport limited over the temperature range 350‐ 650 ‐ C at a 20 nmymin deposition rate. A ligand exchange process which yields volatile Cu(hfa)2 and Cu(hfa) (dpm) is also observed under film growth conditions. The MOCVD-derived BaCaCuO(F) films are postannealed in the presence of bulk Tl2Ba2CaCu2O8 at temperatures of 720‐890 ‐ Ci n flowing atmospheres ranging from 0‐100% O2. The resulting Tl2Ba2CaCu2O8 films are shown to be epitaxial by x-ray diffraction and transmission electron microscopic (TEM) analysis with the c-axis normal to the substrate surface, with in-plane alignment, and with abrupt film-substrate interfaces. The best films exhibit a Tc › 105 K, transport-measured Jc › 1.2 3 10 5 Aycm 2 at 77 K, and surface resistances as low as 0.4 mV

Valence fluctuations and metallic behavior in K6Cu12U2S15, a new quaternary sulfide with a unique three-dimensional cubic framework

Mixed S2-/S1- oxidation states have been discovered in the new quaternary compound K6Cu12U2S15. Synthesized from the reaction of Cu and U in a molten alkali metal/polysulfide flux, the compound crystallizes in the cubic space group Ia3d (no. 230) with a = 18.642(7) angstroms. Its complex structure is built from [US6] octahedra connected into one-dimensional columns with [CuS3] trigonal planar units, which also serve as inter-column connection points for the extended three-dimensional structure. Magnetic susceptibility measurements reveal Curie-Weiss paramagnetic behavior at temperatures above 100 K with a microeff of 2.96 microB per formula. This corresponds to 2.1 microB per U atom, and it is insufficient to distinguish between U4+ and U5+ paramagnetic ions. The conductivity of hot-pressed polycrystalline pellets is approximately 1500 Scm(-1) at room temperature, and increasing conductivity with decreasing temperature is observed, indicating metallic behavior for the material. Small and positive values for the materials thermopower (3-4 microVK(-1) from 100-300 K) also confirm p-type metallic behavior.

Metal‐organic chemical vapor deposition/open flow thallium annealing route to epitaxial Tl2Ba2Ca2Cu3O10 thin films

Phase‐pure epitaxial Tl2Ba2Ca2Cu3O10 thin films have been grown on single crystal (110) LaAlO3 substrates using an improved metal‐organic chemical vapor deposition process. First, Ba‐Ca‐Cu‐Ox precursor films are grown on LaAlO3 (110) substrates using Ba(hfa)2⋅tet, Ca(hfa)2⋅tet, and Cu(hfa)2 (hfa=hexafluoroacetylacetonate; tet=tetraglyme) as volatile metalorganic precursors. Thallium is then incorporated into the films during a post‐anneal in the presence of a Tl2O3, BaO, CaO, CuO powder mixture at 820 °C for 12 h in a flowing 10% O2/Ar atmosphere. The films have a transport‐measured Tc=115 K and Jc=1.5×105 A/cm2 (80 K), while magnetic hysteresis measurements yield Jc=6×105 A/cm2 (77 K). Preliminary surface resistance measurements give Rs=0.35 mΩ at 5 K, 10 GHz.

Inclusion of poly(aniline) into MoO3

Protonated emeraldine [poly(aniline)] has been encapsulated in MoO3 by first inserting aniline into its lattice followed by successful polymerization with (NH4)2S2O8; the new material is characterized by X-ray diffraction, IR spectroscopy, scanning electron microscopy and charge transport measurements.

Liquid source metal–organic chemical-vapor deposition of high-quality YBa2Cu3O7−x films on polycrystalline silver substrates

High-temperature superconducting YBa2Cu3O7−x films were grown by liquid delivery metal–organic chemical-vapor deposition on silver-coated stainless-steel substrates. The films are highly c-axis oriented, have a Tc above 90 K, and exhibit a surface resistance of 110 μΩ at 77 K and 3 GHz.

Encapsulation of polymers into MoS2 and metal to insulator transition in metastable MoS2

A general approach for the encapsulation of a variety of saturated polymers between the layers of MoS2 giving electrically conductive lamellar compounds is reported.