P.P. Newcomer

Synthesis and optical properties of MoS2 and isomorphous nanoclusters in the quantum confinement regime

Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude lar...

Wet oxidation of AlxGa1−xAs: Temporal evolution of composition and microstructure and the implications for metal-insulator-semiconductor applications

Three important processes dominate the wet thermal oxidation of AlxGa1−xAs on GaAs: (1) oxidation of Al and Ga in the AlxGa1−xAs alloy to form an amorphous oxide, (2) formation and elimination of crystalline and amorphous elemental As and of amorphous As2O3, and (3) crystallization of the amorphous oxide film. Residual As can lead to strong Fermi-level pinning at the oxidized AlGaAs/GaAs interface, up to a 100-fold increase in leakage current, and a 30% increase in the dielectric constant of the oxide layer. Thermodynamically favored interfacial As may impose a fundamental limitation on the use of AlGaAs wet oxidation in metal-insulatorsemiconductor devices in the GaAs material system.

Strong quantum confinement effects in semiconductors: FeS2 nanoclusters

High quality, size-selected, nano-size clusters of FeS2 have been successfully grown inside inverse micellar cages and their optical absorption studied. Remarkable structure in the absorption spectrum is observed and large blue shifts in the absorption edge and other spectral features are observed upon crossing from the weak to strong quantum confinement regimes with decreasing cluster size. The bandgap appears to remain indirect down to the smallest cluster size studied (2 nm).

Monolithic periodic mesoporous silica gels

We have synthesized monolithic, surfactant-templated particulate gels that have pore volumes and surface areas comparable to silica xerogels and aerogels. The gels have a complex microstructure with micro-, meso- and macroscopic features that emerge over five orders of magnitude in length (1) amorphous silica walls (characterized by a broad distribution of Si-Si spacings of ≈0.4 nm), (2) periodic hexagonal arrays of 1-d channels within each particle (≈3 nm channel diameter), (3) a feature in the gas adsorption measurements that indicates a second class of ≈10–50 nm diameter mesopores, (4) particles that are ≈ 150 to ≈500 nm in diameter, (5) interparticle pores that are on the order of the particle size, and (6) fractal domains larger than the particle size (>10 000nm). The microstructure can be controlled by the varying the initial silica content, the template size, the drying conditions, or the calcination conditions. The wet gel monoliths exhibit calculated densities as low as ≈0.02 g/cm3; the dried and calcined gels have bulk densities that range from 0.3 to 0.5 g/cm3. The materials possess large interparticle (1.1–2.2 cm3/g) and intraparticle (0.3–0.6 cm3/g) porosities.

Annealing studies on Hg-containing materials

Abstract The superconductors, HgBa 2 Ca n Cu n +1 O 4+2 n +δ , with n =1,2 and 3 were prepared and annealed under a variety of conditions including high-pressure oxygen. Superconducting transition temperatures T c are determined as a function of oxygen annealing pressure. These results show that the n = 1 and 2 materials will overdope when annealed at pressures in excess of 1 bar while n =3 requires tens of bars.

Superconducting properties of strontium copper oxyfluoride, Sr2CuO2F2+δ

Abstract Superconducting properties of Sr2CuO2F2+δ have been measured as a function of synthesis parameters. Fluorination initiates above 160°C, appears optimum near 180°C and is difficult to control above 250°C in forming superconducting samples. Optimum Tc values are nominally 47 K; however, Meissner data show fluctuations extending to 60 K.

Importance of thermodynamics and kinetics in the growth of thin film TlBaCaCuO superconductors

The importance of near-equilibrium thermodynamics in the growth of TlBaCaCuO materials is well-documented; i.e. the independent control of sample temperature, Tl-oxide partial pressure, and oxygen partial pressure. Here we demonstrate how this is accomplished using a simple version of the crucible process. In addition, we report the relevance of growth kinetics. Superconducting thin films are grown in a two-step process. First, /spl sim/2000 /spl Aring/ thick, amorphous Ba/sub 2/CaCu/sub 2/O/sub x/ and Ba/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ precursors are sputter-deposited onto LaAlO/sub 3/(100) substrates. Films are then thallinated and annealed using the crucible-pellet process. With this method, it is possible to grow essentially single-phase Tl-1212, Tl-1223, Tl-2212, and Tl-2223 thin films. Under similar thermodynamic conditions, we find that different phases dominate the film growth as a function of reaction time. For example, the Tl-x212 phases generally nucleate first, and then, depending on growth conditions, convert to the Tl-1223 or Tl-2223 phase with extended annealing times. We demonstrate that knowledge of both the thermodynamics and the kinetics of growth can yield high-quality films of each of the major Tl-superconducting phases.

Enhancement of the superconducting properties of TlBa2CaCu2O7+δ thin films via postannealing

The superconducting properties of TlBa2CaCu2O7+δ(Tl‐1212) films are greatly enhanced by annealing in unreactive ambients such as nitrogen at temperatures ranging from Ta=250–600 °C. The transition temperature, Tc, of these Tl‐1212 films as‐grown in oxygen is 70 K. Annealing for 1 h at 250 °C elevates Tc above 90 K. Tc further increases and sharpens for Ta=600 °C. In addition, subtle changes occur in the microstructure correlating with improved critical current density. These results indicate that Tl‐1212 films may be of greater relevance for electronics applications than previously believed.

IMPORTANCE OF CONTROLLING THE TL-OXIDE PARTIAL PRESSURE THROUGHOUT THE PROCESSING OF TLBA2CACU2O7 THIN FILMS

TlBa2CaCu2O7 (Tl‐1212) superconducting films 5000–6000 A thick have been grown on LaAlO3 (100) substrates using oxide precursors in a closed two‐zone thallination furnace. Tl‐1212 films can be grown with transition temperatures ∼100 K, and critical current densities measured by magnetization of Jcm(5 K)≳107 A/cm2 and Jcm(77 K)≳105 A/cm2. Processing conditions, substrate temperatures and Tl‐oxide source temperatures are found which result in smooth, nearly phase‐pure Tl‐1212 films. Variations in the respective temperature ramps of the Tl‐oxide zone and the substrate zone can greatly influence resulting film properties such as microstructure, morphology, superconducting transition temperature, and critical current density.

Growth of TlBa/sub 2/CaCu/sub 2/O/sub 7/ thin films using a controlled Tl-oxide source during processing

TlBa/sub 2/CaCu/sub 2/O/sub 7/ superconducting films 5000-6000 /spl Aring/ thick have been grown on LaAlO/sub 3/(100) substrates using oxide precursors in a two-zone thallination furnace. Smooth, nearly phase-pure 1212 films are reported, with transition temperature (T/sub c/)/spl sim/87 K, and low-magnetic field critical current densities J/sub c/(5 K)/spl sim/1/spl times/10/sup 7/ and J/sub c/(40 K)/spl sim/2-10/sup 6/ A/cm/sup 2/.<<ETX>>