W. R. McKinnon

Synthesis conditions and oxygen stoichiometry effects on Li insertion into the spinel LiMn[sub 2]O[sub 4]

Using a new electrolyte composition which is stable against oxidation up to 5 V, the full electrochemical deintercalation of lithium from the spinel LiMn[sub 2]O[sub 4], is studied. The origin of two new reversible oxidation-reduction peaks near 4.5 and 4.9 V are examined. The capacity associated with these peaks depends on both the nominal composition x in Li[sub x]Mn[sub 2]O[sub 4] and the synthesis conditions (annealing temperatures and cooling rates), and thereby can be used as an indicator for electrochemically optimized LiMn[sub 2]O[sub 4] powders. The authors present evidence that these peaks are related to local structural defects. Thermogravimetric measurements (TGA) on Li[sub x]Mn[sub 2]O[sub 4] powders show a reversible loss of oxygen that can reach 5% at 1,000 C. The authors find that some of this weight loss is associated with the conversion of cubic LiMn[sub 2]O[sub 4] to a new tetragonal spinel phase and then to the decomposition of this phase into the orthorhombic LiMnO[sub 2] phase plus other products. This new tetragonal LiMn[sub 2]O[sub 4] spinel is prepared as a single phase, and its electrochemical properties are reported.

Passivation of InGaAs surfaces and InGaAs/InP heterojunction bipolar transistors by sulfur treatment

The surface properties of InGaAs(100) after ex situ treatment with (NH4)2S solution were investigated by photoluminescence (PL) and high-energy resolution x-ray photoelectron spectroscopy. The As 3d, Ga 2p3/2, and In 3d5/2 core level studies show that the surface is free of native oxides and is terminated by S after treatment. A dramatic increase (∼40 times) in the PL efficiency was observed on undoped InGaAs(100) surfaces after sulfur passivation. This S treatment has also been applied to the passivation of the extrinsic base of InGaAs/InP heterojunction bipolar transistors (HBTs). The effectiveness of the sulfur passivation treatment was confirmed by the resulting devices which exhibited dc current gain values of up to 200 at very low collector currents (nA). Further, the sulfur passivated HBTs do not show any dependence on the perimeter-to-area (P/A) ratio of the emitter junction which is of interest for high frequency characteristics while maintaining high current gain.

New non-superconducting layered Bi-oxide phases of formula Bi2M3Co2Oy containing Co instead of Cu

Abstract We report the cationic substitution of Cu by Co within the 85K Bi based high Tc oxides to produce new phases of general formula Bi 2 M 3 Co 2 O y with M = Ca, Sr and Ba. Single crystals of these phases, grown using Co 3 O 4 (excess) flux, were investigated for their structural, magnetic and electrical properties. The subcell appears similar to that of Bi 2 Sr 2 CaCu 2 O y phase but the superstructure is more complicated due to the presence of a more complex incommensurate structural modulation. The substitution of Cu by Co is accompanied by an uptake of additional oxygen. With M = Sr or Ca, the compounds are semiconductors, whereas when M = Ba the compound is metallic above 100K, and semiconducting below 100K. The magnetic susceptibility varies little with temperature, and shows no magnetic transitions. Photoemission measurements show that Co is in a + 3 oxidation state with a low spin configuration.

USE OF DIPOLE DOPING TO SUPPRESS SWITCHING IN INDIUM PHOSPHIDE DOUBLE HETEROJUNCTION BIPOLAR TRANSISTORS

In this article we show the effect on the dc and rf transport characteristics of dipole doping at the collector heterojunction in an InP/InGaAs double heterojunction bipolar transistor. We show how the switching characteristics of devices with abrupt undoped InGaAs/InP collector heterojunctions can almost be eliminated by using the dipole doping at that interface and how the high-frequency performance is also improved.

Superconductivity at 27 K in modulation-free Bi2−xPbxSr2−yLayCuO6 phases with x≅y+0.2

Abstract Structural and physical properties are reported for compounds made by simultaneous substitution of Bi by Pb and of Sr by La in the 10 K superconducting Bi cuprate Bi 2− x Pb x Sr 2− y La y CuO z . The phase diagram in the x − y plane was was established. It contains two phases separated by a region of two-phase coexistence: the O(1) phase of the starting material ( x = y =0), which is pseudotetragonal, and the O(2) phase found at higher Pb concentrations, which is orthorhombic. In agreement with TEM observations, which show the disappearance of the modulation in the O(2) phase, a single crystal of Bi 1.2 Pb 0.8 Sr 1.5 La 0.5 CuO 6 showed orthorhombic symmetry and no modulation by X-ray diffraction, with the space group Pnan and cell parameters a =5.328(1) A, b =5.415(1) A and c =24.383(3) A. The structure shows ribbons of Bi-O-Bi bonds as in BiPbSr 2 MnO 6 , with three short (2.1 A) mutually perpendicular bonds at Bi. Measurements of resistivity, AC susceptibility and Meissner effect show that superconductivity persists in the unmodulated O(2) phase, with critical temperatures as high as 27 K and a Meissner fraction greater than 20%. This is the first rationalize demonstration of superconductivity in a non-modulated bismuth cuprate.

EPR spectra in YBaCu oxides

Abstract EPR spectra of samples consisting predominantly of YBa 2 Cu 3 O 7− x (black phase), Y 2 BaCuO 5 (green phase) and YBa 3 Cu 2 O y (brown phase) are examined. The black phase is the celebrated 93 K superconductor, and it shows very weak signals attributable to Cu ++ at temperatures between 300 and 4 K. It is concluded that the Cu ++ signal in the black phase at room temperature is probably due to green or brown phase impurity. It would therefore appear that the superconducting black phase is EPR silent, at least in the g = 2 region. Reasons for the EPR silence of the black phase are briefly discussed.

Sensitivity of field-effect biosensors to charge, pH, and ion concentration in a membrane model

In field-effect transistors used to detect charged biomolecules (BioFETs), the biomolecules form a charged membrane on the transistor surface. In this paper, the one-dimensional Poisson–Boltzmann equation is used to calculate the charge sensitivity (the sensitivity of the BioFET to changes in biomolecule charge), ion sensitivity (to changes in ion concentration of the solution), or pH sensitivity (to changes in pH of the solution), both analytically and numerically, and the results are compared to models where the charged molecules are represented as an infinitely thin plane. Complexation of ions with the oxide surface is shown to have a negligible effect on parameters typical of devices, but the layer used to tether the charged molecules to the surface could modify the sensitivity considerably.

Single electron charging in deterministically positioned InAs/InP quantum dots

We demonstrate precise control of electron charging within a single deterministically positioned InAs/InP quantum dot emitting in the telecommunications band around 1500 nm. Photolumine-scence emission as a function of vertical electric field is used to monitor the electron number within the dot. From Stark shift spectroscopy, we extract strength and orientation for the built-in dipole moment that suggests a uniform InAs dot composition and a configuration in which the electron lies above the hole at zero electric field. The scalable gating technology we employ to electrically contact individual prepositioned quantum dots promises arrays of initialized single spins that can be used for fiber-based quantum information applications.

Sensitivity of a field-effect transistor in detecting DNA hybridization, calculated from the cylindrical Poisson-Boltzmann equation

The sensitivity of a DNA biosensor based on a field-effect transistor is calculated from the numerical solution of the Poisson-Boltzmann equation in a cylindrical cell. The sensitivity is typically about half that found from the one-dimensional Poisson-Boltzmann equation for an ion-permeable DNA membrane with the same average charge density. For the combination of low DNA densities and low concentration of ions in solution, however, the discrepancy between the two calculations can be a factor of 10 or higher. The difference in the two calculations is due to accumulation of screening charge around the DNA cylinder, an effect which is ignored in the membrane model.

ELECTRONIC STRUCTURE STUDY OF THE FORMAL OXIDATION STATE OF BISMUTH IN COPPER-OXIDE SUPERCONDUCTORS CONTAINING Bi-O DOUBLE LAYERS

Abstract Tight-binding band electronic structure calculations were performed on the various structures of the Bi 4 O 8 double layer found in Bi-based copper-oxide superconductors. With respect to the Fermi level, the position of the Bi 6p-block bands of the BiO double layers is strongly affected by distortion around Bi atoms and by insertion of extra oxygen atoms in the BiO double layer. Our results show that the bottom of the Bi 6p-block bands lies more than 1.0 eV above the Fermi level, so that the formal oxidation state appropriate for Bi is +3. The observation of previous band structure studies that the bottom of the Bi 6p-block bands lies below the Fermi level is a direct consequence of using an ideal structure for the BiO double layer.