Reversible Insertion of a Trivalent Cation onto MnO2 Leading to Enhanced Capacitance

Supercapacitor properties of MnO2 are studied generally in aqueous alkali metal salt solutions, often in a Na salt solution. During electrochemical discharge-charge processes, Na+ ions from the electrolyte get reversibly inserted/deinserted on the surface of MnO2 particles, which leads to redox reaction between MnOONa and MnO2. In the present study, it has been shown that MnO2 exhibits enhanced capacitance behaviour in a rare earth metal salt solution, namely, La(NO3)(3) solution in comparison with NaNO3 and Mg(NO3)(2) aqueous solutions. The specific capacitance increases with an increase in charge on the solution cation (Na+, Mg2+ and La3+). It is proposed that the number of surface sites for adsorption of cations remains unaltered in all solutions. The surface insertion of cation facilitates the reduction of Mn4+ in MnO2 to Mn3+ equivalent to the charge present on the cation. As the specific capacitance is related to the quantity of charge that is exchanged between the solid MnO2 and the aqueous solution, the trivalent cation (La3+) provides greater specific capacitance than in Mg(NO3)(2) and NaNO3 electrolytes. Accordingly, the number of Mn(IV)/Mn(III) redox pairs involved in the neighbourhood of the adsorption site is one, two and three when Na+, Mg2+ and La3+ ions, respectively, are adsorbed. (C) 2011 The Electrochemical Society. DOI: 10.1149/1.3565177] All rights reserved.

A thermodynamic criterion for selection of gas compositions for diamond deposition

Isoactivity lines for carbon with respect to diamond as the standard state have been calculated in the ternary system C-H-O at 1223 K to identify the diamond deposition domain. The gas composition is calculated by suppressing the formation of all condensed forms of carbon using the SOLGASMIX free-energy minimization program. Thirty six gas species were included in the calculation. From the gas composition, isoactivity lines are computed using recent data on the Gibbs energy of diamond. Except for activities less than 0.1, the isoactivity lines are almost linear on the C-H-O ternary diagram. Gas compositions which generate activity of diamond ranging from 1 to 100 at 1223 K fall inside a narrow wedge originating from the point representing CO. This wedge is very similar to the revised lens-shaped diamond growth domain identified by Bachman et al., using inputs from experiment. The small difference between the calculated and observed domains may be attributed to variation in the supersaturation required for diamond deposition with gas composition. The diamond solubility in the gas phase along the isoactivity line for a(di)=100 and P=6.7 kPa exhibits a minimum at 1280 K, which is close to the optimum temperature found experimentally. At higher supersaturations, non-diamond forms of carbon, including amorphous varieties, are expected. The results suggest that thermodynamic calculations can be useful for locating diamond growth domains in more complex CVD systems containing halogens, for which very little experimental data is available.

Thermally Controlled Cyclic Insertion/Ejection of Dopant Ions and Reversible Zinc Blende/Wurtzite Phase Changes in ZnS Nanostructures

We report a reversible phase transformation of platelet-shaped ZnS nanostructures between wurtzite (WZ) and zinc blende (ZB) phases by reversible insertion/ ejection of dopant Mn(II) ions induced by a thermocyclic process. In a reaction flask loaded with WZ ZnS platelets and Mn molecular precursors, during heating Mn ions are incorporated and change the phase of the host nanostructures to ZB; during cooling Mn ions are spontaneously ejected, returning the host nanoplatelets to the original WZ phase. These reversible changes are monitored for several cycles with PL, EPR, XRD, and HRTEM. Interestingly, the (0001) WZ platelets transform to (110) ZB following a nucleation and growth process triggered by a local increase/depletion of the Mn2+ concentration in the nanocrystals.

Thermally Controlled Cyclic Insertion/Ejection of Dopant Ions and Reversible Zinc Blende/Wurtzite Phase Changes in ZnS Nanostructures

We report a reversible phase transformation of platelet-shaped ZnS nanostructures between wurtzite (WZ) and zinc blende (ZB) phases by reversible insertion/ ejection of dopant Mn(II) ions induced by a thermocyclic process. In a reaction flask loaded with WZ ZnS platelets and Mn molecular precursors, during heating Mn ions are incorporated and change the phase of the host nanostructures to ZB; during cooling Mn ions are spontaneously ejected, returning the host nanoplatelets to the original WZ phase. These reversible changes are monitored for several cycles with PL, EPR, XRD, and HRTEM. Interestingly, the (0001) WZ platelets transform to (110) ZB following a nucleation and growth process triggered by a local increase/depletion of the Mn2+ concentration in the nanocrystals.

Low electric field, easily reversible electrical set and reset processes in a Ge15Te83Si2 glass for phase change memory applications

We report here an easily reversible set-reset process in a new Ge15Te83Si2 glass that could be a promising candidate for phase change random access memory applications. The I-V characteristics of the studied sample show a comparatively low threshold electric field (E-th) of 7.3 kV/cm. Distinct differences in the type of switching behavior are achieved by means of controlling the on state current. It enables the observation of a threshold type for less than 0.7 mA beyond memory type (set) switching. The set and reset processes have been achieved with a similar magnitude of 1 mA, and with a triangular current pulse for the set process and a short duration rectangular pulse of 10 msec width for the reset operation. Further, a self-resetting effect is seen in this material upon excitation with a saw-tooth/square pulse, and their response of leading and trailing edges are discussed. About 6.5 x 10(4) set-reset cycles have been undertaken without any damage to the device. (C) 2011 American Institute of Physics. doi: 10.1063/1.3574659]

Electrochemical co-deposition of bimetallic Pt-Ru nanoclusters dispersed on poly(3,4-ethylenedioxythiophene) and electrocatalytic behavior for methanol oxidation

Nanoclusters of bimetallic Pt-Ru are electrochemically deposited on conductive polymer, poly(3,4-ethylenedioxythiophene)(PEDOT), which is also electrochemically deposited on a carbon paper substrate. The bimetallic deposition is carried out in an acidic electrolyte consisting of chloroplatinic acid and ruthenium chloride at 0.0 V versus saturated calomel electrode (SCE) on PEDOT coated carbon paper. A thin layer PEDOT on a carbon paper substrate facilitates the formation of uniform, well-dispersed, nano clusters of Pt-Ru of mean diameter of 123 nm, which consist of nanosize particles. In the absence of PEDOT, the size of the clusters is about 251 nm, which are unevenly distributed on carbon paper substrate. Cyclic voltammetry studies suggest that peak currents of methanol oxidation are several times greater on PtRu-PEDOT electrode than on Pt-Ru electrode in the absence of PEDOT. (C) 2011 Elsevier B.V. All rights reserved.

Simulation and experimental study on compositional evolution of Li-Co in LiCoO(2) thin films during sputter deposition

The compositional evolution in sputter deposited LiCoO(2) thin films is influenced by process parameters involved during deposition. The electrochemical performance of these films strongly depends on their microstructure, preferential orientation and stoichiometry. The transport process of sputtered Li and Co atoms from the LiCoO(2) target to the substrate, through Ar plasma in a planar magnetron configuration, was investigated based on the Monte Carlo technique. The effect of sputtering gas pressure and the substrate-target distance (d(st)) on Li/Co ratio, as well as, energy and angular distribution of sputtered atoms on the substrate were examined. Stable Li/Co ratios have been obtained at 5 Pa pressure and d(st) in the range 5 11 cm. The kinetic energy and incident angular distribution of Li and Co atoms reaching the substrate have been found to be dependent on sputtering pressure. Simulations were extended to predict compositional variations in films prepared at various process conditions. These results were compared with the composition of films determined experimentally using x-ray photoelectron spectroscopy (XPS). Li/Co ratio calculated using XPS was in moderate agreement with that of the simulated value. The measured film thickness followed the same trend as predicted by simulation. These studies are shown to be useful in understanding the complexities in multicomponent sputtering. (C) 2011 American Institute of Physics. doi:10.1063/1.3597829]

Investigation of reversible and irreversible polarizations in thin films of SrBi2(Ta-0.5,Nb-0.5)(2)O-9

The reversible and irreversible components of the total polarization in a thin film of SrBi2(Ta-0.5,Nb-0.5)(2)O-9 were calculated. The C-V loop was integrated to obtain the reversible part of the total polarization. The reversible polarization was only 20% of the total polarization and showed almost no hysteresis. However, the dielectric constant due to the total polarization was almost the same as that for the reversible polarization in the saturation region of the large signal P-E hysteresis loop. The reversible part was subtracted from the total polarization to calculate the irreversible counterpart of it. The irreversible polarization showed a near-square shaped hysteresis loop, while the reversible polarization was obeying the Rayleigh law. The small signal hysteresis was simulated from the parameters obtained from the Rayleigh-curve fit with the experimental curve and then it was compared with the result obtained from direct measurement with small amplitude. (C) 2002 Elsevier Science B.V. All rights reserved.

Metal-organic chemical vapour deposition of thin films of cobalt on different substrates: study of microstructure

We have investigated the microstructure of thin films grown by metal-organic chemical vapour deposition using a beta-diketonate complex of cobalt, namely cobalt (11) acetylacetonate. Films were deposited on three different substrates: Si(100), thermally oxidised silicon [SiO2/Si(100)] and glass at the same time. As-grown films were characterised by X-ray diffraction, scanning electron microscopy, scanning tunnelling microscopy, atomic force microscopy and secondary ion mass spectrometry. Electrical resistivity was measured for all the films as a function of temperature. We found that films have very fine grains, resulting in high electrical resistivity Further, film microstructure has a strong dependence on the nature of the substrate and there is diffusion of silicon and oxygen into cobalt from the substrate. (C) 2002 Elsevier Science B.V. All rights reserved.

Phase transformation and semiconductor-metal transition in thin films of VO2 deposited by low-pressure metalorganic chemical vapor deposition

Thin films of the semiconducting, monoclinic vanadium dioxide, VO2(M) have been prepared on ordinary glass by two methods: directly by low-pressure metalorganic chemical vapor deposition (MOCVD), and by argon-annealing films of the VO2(B) phase deposited by MOCVD. The composition and microstructure of the films have been examined by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Films made predominantly of either the B or the M phase, as deposited, can only be obtained over a narrow range of deposition temperatures. At the lower end of this temperature range, the as-deposited films are strongly oriented, although the substrate is glass. This can be understood from the drive to minimize surface energy. Films of the B phase have a platelet morphology, which leads to an unusual microstructure at the lower-deposition temperatures. Those grown at similar to370 degreesC convert to the metallic, rutile (R) phase when annealed at 550 degreesC, whereas those deposited at 420 degreesC transform to the R phase only at 580 degreesC. (When cooled to room temperature, the annealed films convert reversibly from the R phase to the M phase.) Electron microscopy shows that annealing leads to disintegration of the single crystalline VO2(B) platelets into small crystallites of VO2(R), although the platelet morphology is retained. When the annealing temperature is relatively low, these crystallites are nanometer sized. At a higher-annealing temperature, the transformation leads to well-connected and similarly oriented large grains of VO2(R), enveloped in the original platelet. The semiconductor-metal transition near 68 degreesC leads to a large jump in resistivity in all the VO2(M) films, nearly as large as in epitaxial films on single-crystal substrates. When the annealed films contain well-connected large grains, the transition is very sharp. Even when preferred orientation is present, the transition is not as sharp in as-deposited VO2(M), because the crystallites are not densely packed as in annealed VO2(B). However, the high degree of orientation in these films leads to a narrow temperature hysteresis. (C) 2002 American Institute of Physics.

Reversible and irreversible switching processes in pure and lanthanum modified lead zirconate thin films

The mechanism of field induced phase switching in antiferroelectric lead zirconate and La-modified lead zirconate thin films has been analysed in terms of reversible and irreversible switching process under weak fields as a function of donor concentration. Extension of Rayleigh law of ferromagnetic materials to the present antiferroelectric and modified antiferroelectric compositions have clearly showed that origin of small signal dielectric permittivity is due to reversible domain wall motion. Rayleigh's constant, a measure of irreversible switching process, exhibited a slight increase with lower La3+ concentrations and followed by a gradual fall for higher concentration. This clearly illustrates that donor addition to antiferroelectric thin films controls the domain switching even under weak fields. (C) 2002 Elsevier Science B.V. All rights reserved.

Cobalt oxide thin films prepared by metalorganic chemical vapor deposition from cobalt acetylacetonate

Thin films of cobalt oxide have been deposited on various substrates, such as glass, Si(100), SrTiO3(100), and LaAlO3(100), by low pressure metalorganic chemical vapor deposition (MOCVD) using cobalt(IL), acetylacetonate as the precursor. Films obtained in the temperature range 400-600 degreesC were uniform and highly crystalline having Co3O4 phase as revealed by x-ray diffraction. Under similar conditions of growth, highly oriented thin films of cobalt oxide grow on SrTiO3(100) and LaAlO3(100). The microstructure and the surface morphology of cobalt oxide films on glass, Si(100) and single crystalline substrates, SrTiO3(100) and LaAlO3(100) were studied by scanning electron microscopy. Optical properties of the films were studied by uv-visible-near IR spectrophotometry.