Structure and optical properties of Cd substituted ZnO (Zn1-xCdxO) nanostructures synthesized by high pressure solution route

We report synthesis of Cd substituted ZnO nanostructures (Zn1-xCdxO with x upto \approx .09) by high pressure solution growth method. The synthesized nanostructures comprise of nanocrystals that are both particles (~ 10-15 nm) and rods which grow along (002) direction as established by Transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. Rietveld analysis of the XRD data shows monotonous increase of the unit cell volume with the increase of Cd concentration. The optical absorption as well as the photoluminescence (PL) shows red shift on Cd substitution. The line width of the PL spectrum is related to the strain inhomogenity and it peaks in the region where the CdO phase separates from the Zn1-xCdxO nanostructures. The time resolved photoemission showed a long lived (~10ns) component. We propose that the PL behavior of the Zn1-xCdxO is dominated by strain in the sample with the redshift of the PL linked to the expansion of the unit cell volume on Cd substitution

Ground state properties of the bond alternating spin-12\frac{1}{2} anisotropic Heisenberg chain

Ground state properties, dispersion relations and scaling behaviour of spin gap of a bond alternating spin-$\frac{1}{2}$ anisotropic Heisenberg chain have been studied where the exchange interactions on alternate bonds are ferromagnetic (FM) and antiferromagnetic (AFM) in two separate cases. The resulting models separately represent nearest neighbour (NN) AFM-AFM and AFM-FM bond alternating chains. Ground state energy has been estimated analytically by using both bond operator and Jordan-Wigner representations and numerically by using exact diagonalization. Dispersion relations, spin gap and several ground state orders have been obtained. Dimer order and string orders are found to coexist in the ground state. Spin gap is found to develop as soon as the non-uniformity in alternating bond strength is introduced in the AFM-AFM chain which further remains non-zero for the AFM-FM chain. This spin gap along with the string orders attribute to the Haldane phase. The Haldane phase is found to exist in most of the anisotropic region similar to the isotropic point.Comment: 16 pages, 6 figures, 1 tabl

Electric field induced reversible control of visible photoluminescence from ZnO nanoparticles

Reversible control of the photoluminescence of ZnO occurring in the visible range, has been achieved by application of a few volts (< 5V) to a device consisting of nanostructured ZnO film sandwiched between Indium Tin Oxide electrode and polyethylene oxide-lithium perchlorate, a solid polymer electrolyte. The photoluminescence intensity shows nearly 100% modulation with a response time less than 30 seconds, when the bias is applied at the electrolyte-electrode. A model is proposed for the observed effect that is based on defect states of ZnO and the band bending at the ZnO-electrolyte interface that can be changed by the applied bias

Structural and Optical properties of Zn(1-x)MgxO nanocrystals obtained by low temperature method

In this paper we report structural and optical properties of Magnesium substituted Zinc Oxide (Zn1-xMgxO) nanocrystals (~10-12nm) synthesized by low temperature route. In the low temperature synthesis route it was possible to reach x = 0.17 without segregation of Mg rich phase. The exact chemical composition has been established by quantitative analysis. Rietveld analysis of the XRD data confirms the Wurzite structure and a continuous compaction of the lattice (in particular the c-axis parameter) as x increases. There is an enhancement of the strain in the lattice as the Mg is substituted. The bandgap also gets enhanced as x is increased and reaches a value of 4eV for x = 0.17. From the TEM and the XRD data it has been concluded that when there is a phase segregation for x > 0.17, there is a shell of Mg(OH)2 on the ZnO. The absorption also shows persistence of the excitoinc absorption on Mg substitution. The nanocrystals show near band edge photo luminescence (PL) at room temperature which shows blue shift on Mg incorporation. In addition to the near band edge emission the ZnO and Zn1-xMg xO alloy nanocrystals show considerable emission in the blue-green region at wavelength of ~550 nm. We find that the relative intensity of the green emission increases with the Mg concentration for very low x (upto x = 0.05) and on further increase of the Mg concentration there is a sharp decrease of relative intensity of the green emission eventually leading to a complete quenching of blue emission. It is concluded that due to phase segregation (for x \geq 0.20), the formation of the shell of Mg(OH)2 on the ZnO leads to quenching of the green emission .However, this shell formation does not have much effect on the near band edge PL

Voltage bias induced modification of all oxide Pr0.5Ca0.5MnO3/SrTi0.95Nb.05O3 junctions

In this paper we report what happens to a pristine oxide junction Pr0.5Ca0.5MnO3/SrTi0.95Nb.05O3 (PCMO/Nb:STO), when it is subjected to cycling of voltage bias of moderate value ({\pm}4V). It is found that the initial cycling leads to formation of a permanent state of lower resistance where the lower resistance arises predominantly due to development of a shunt across the device film (PCMO). On successive voltage cycling with increasing magnitude, this state transforms into states of successive lower resistance that can be transformed back to the initial stable state on cycling to below a certain bias. A simple model based on p-n junction with shunt has been used to obtain information on the change of the junction on voltage cycling. It has been shown that the observation can be explained if the voltage cycling leads to lowering of barrier at the interface and also reduction in series resistance. It is suggested that this lowering can be related to the migration of oxygen ions and vacancies at the junction region. Cross-sectional imaging of the junction shows formation of permanent filamentary bridges across the thickness of the PCMO after the pristine p-n junction is first taken through a voltage cycle, which would explain appearance of a finite shunt across the p-n junction.Comment: 12 pages,7figure

Three Dimensional Gauge Theory with Topological and Non-topological Mass: Hamiltonian and Lagrangian Analysis

Three dimensional (abelian) gauged massive Thirring model is bosonized in the large fermion mass limit. A further integration of the gauge field results in a non-local theory. A truncated version of that is the Maxwell Chern Simons (MCS) theory with a conventional mass term or MCS Proca theory. This gauge invariant theory is completely solved in the Hamiltonian and Lagrangian formalism, with the spectra of the modes determined. Since the vector field constituting the model is identified (via bosonization) to the fermion current, the charge current algebra, including the Schwinger term is also computed in the MCS Proca model.Comment: Eight pages, Latex, No figures

Electrode erosion in low power arcjet thrusters

Low power arcjet thrusters (0.5 to 3 kW) are currently being considered for North-South station keeping of geosynchronous communication satellites. The erosion mechanisms of cathodes in these thrusters are not well understood. The experimental set-up to produce a steady state electric arc in a gas flow is described and some preliminary theoretical results on cathode erosion are presented