2,215 research outputs found
Comment on ``Theorem for nonrotating singularity-free universes''
We show that Raychaudhuri's recently proposed theorem on nonrotating
universes cannot be used to rule out realistic singularity-free descriptions of
the universe, as suggested by him in PRL 80, 654 (1998).Comment: 1 page, to appear in Phys.Rev.Let
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
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
Oscillating non-singular relativistic spherical model
A particular choice of the time function in the recently presented spherical
solution by Dadhich [1] leads to a singularity free cosmological model which
oscillates between two regular states. The energy-stress tensor involves
anisotropic pressure and a heat flux term but is consistent with the usual
energy conditions (strong, weak and dominant). By choosing the parameters
suitably one can make the model consistent with observational data. An
interesting feature of the model is that it involves blue shifts as in the
quasi steady state model [2] but without violating general relativity.Comment: 6 pages, REVTEX versio
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
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