EPR, thermo and photoluminescence properties of ZnO nanopowders

Nanocrystalline ZnO powders have been synthesized by a low temperature solution combustion method. The photoluminescence (PL) spectrum of as-formed and heat treated ZnO shows strong violet (402, 421, 437, 485 nm) and weak green (520 nm) emission peaks respectively. The PL intensities of defect related emission bands decrease with calcinations temperature indicating the decrease of Zn i and V o + caused by the chemisorptions of oxygen. The results are correlated with the electron paramagnetic resonance (EPR) studies. Thermoluminescence (TL) glow curves of gamma irradiated ZnO nanoparticles exhibit a single broad glow peak at �343 °C. This can be attributed to the recombination of charge carriers released from the surface states associated with oxygen defects, mainly interstitial oxygen ion centers. The trapping parameters of ZnO irradiated with various γ-doses are calculated using peak shape method. It is observed that the glow peak intensity increases with increase of gamma dose without changing glow curve shape. These two characteristic properties such as TL intensity increases with gamma dose and simple glow curve structure is an indication that the synthesized ZnO nanoparticles might be used as good TL dosimeter for high temperature application. © 2011 Elsevier B.V. All Rights Reserved

Luminescence studies and EPR investigation of solution combustion derived Eu doped ZnO

ZnO:Eu (0.1 mol) nanopowders have been synthesized by auto ignition based low temperature solution combustion method. Powder X-ray diffraction (PXRD) patterns confirm the nanosized particles which exhibit hexagonal wurtzite structure. The crystallite size estimated from Scherrer's formula was found to be in the range 35-39 nm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal particles are agglomerated with quasi-hexagonal morphology. A blue shift of absorption edge with increase in band gap is observed for Eu doped ZnO samples. Upon 254 nm excitation, ZnO:Eu nanopowders show peaks in regions blue (420-484 nm), green (528 nm) and red (600 nm) which corresponds to both Eu2+ and Eu3+ ions. The electron paramagnetic resonance (EPR) spectrum exhibits a broad resonance signal at g = 4.195 which is attributed to Eu2+ ions. Further, EPR and thermoluminescence (TL) studies reveal presence of native defects in this phosphor. Using TL glow peaks the trap parameters have been evaluated and discussed. © 2014 Elsevier B.V. All rights reserved

Influence of reaction time and synthesis temperature on the physical properties of ZnO nanoparticles synthesized by the hydrothermal method

Influence of synthesis temperature and reaction time on the structural and optical properties of ZnO nanoparticles synthesized by the hydrothermal method was investigated using X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray, Fourier transform infra-red spectroscopy, and UV–visible and fluorescence spectroscopy. The XRD pattern and HR-TEM images confirmed the presence of crystalline hexagonal wurtzite ZnO nanoparticles with average crystallite size in the range 30–40 nm. Their energy gap determined by fluorescence was found to depend on the synthesis temperature and reaction time with values in the range 2.90–3.78 eV. Thermal analysis, thermogravimetric and the differential scanning calorimetry were used to study the thermal reactions and weight loss with heat of the prepared ZnO nanoparticles

Influence of K/Na ions on structure and luminescence properties of red phosphors K0.5Na0.5Gd(WO4)2: Eu3+

In this paper, we demonstrated the influence of K/Na alkali ions on the structural and photo-cathodoluminescence properties of novel K0.5Na0.5Gd(WO4)2:Eu3+ red phosphor synthesized by the solid-state method. The purity of phosphors is determined from the X-ray diffraction. Rietveld refinement studies confirms that the compounds both K0.5Na0.5Gd(WO4)2:Eu3+ and NaGd(WO4)2:Eu3+ were crystallized in the tetragonal structure with I41/a space group and KGd(WO4)2:Eu3+ was crystallized in monoclinic structure with C2/c space group. The FTIR results indicate that the presence of characteristic functional groups in synthesized samples. The optical bandgap determined from the UV-Visible reflectance spectra in the order K0.5Na0.5Gd(WO4)2:Eu3+ Ω4 of JO parameters indicates the asymmetric environment around the activator suggesting the co-valency of Eu-O bonds with respect to host matrix. Radiative lifetime, branching ratio, electronic dipole transition probabilities were calculated based on the Judd-Ofelt formalism. Cathodoluminescence studies shows the characteristic emission of the Eu3+ ions for low current and voltage which further confirms the pure red color emission of phosphors. All the synthesized phosphors can emit the red emissions with correlated color temperature range from 1500 to 2000 K. For K0.5Na0.5Gd(WO4)2:Eu3+ sample the chromaticity coordinate (x = 0.667, y = 0.329) are closer to the NTSC trichromatic coordinated (x = 0.67,y = 0.33) than that of Y2O2S:Eu3+ suggesting that K0.5Na0.5Gd(WO4)2:Eu3+ can be a potential red phosphor material for warm white LED's applications

Red luminescence from ZnO: Cr3+ nanophosphors under visible excitation

ZnO: Cr3+ (1 mol) nanophosphor is synthesized by the wet chemical solution combustion method at the temperature of 400°C. Powder X-ray diffraction results confirmed that Cr3+-doped and undoped ZnO nanophosphors exhibit hexagonal wurtzite structure. The average crystallite size calculated from Scherrer-fs method is 25 nm for undoped and 14 nm for Cr3+-doped ZnO. The UV.visible absorption spectra shows red shift in Cr3+-doped ZnO. Photoluminescence studies of undoped ZnO show violet emission peak at 400 nm and blue emission peak at 447 nm. Cr3+-doped ZnO shows red emission peaks at 642, 694 and 746 nm, which are mainly attributed to spin forbidden transitions of 2Eg 4A2g of Cr3+ ion in ZnO: Cr3+ nanophosphor. Thermoluminescence (TL) studies recorded at a heating rate of 6°C s.1 show two well-resolved glow peaks at 124 and 284°C. It is found that the TL intensity increases with the gamma irradiation dose (500 Gy. 10 kGy). © 2015 Indian Academy of Sciences

Structural, optical and EPR studies on ZnO:Cu nanopowders prepared via low temperature solution combustion synthesis

Cu (0.1 mol%) doped ZnO nanopowders have been successfully synthesized by a wet chemical method at a relatively low temperature (300 degrees C). Powder X-ray diffraction (PXRD) analysis, scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Fourier transformed infrared (FTIR) spectroscopy, UV-Visible spectroscopy, Photoluminescence (PL) and Electron Paramagnetic Resonance (EPR) measurements were used for characterization. PXRD results confirm that the nanopowders exhibit hexagonal wurtzite structure of ZnO without any secondary phase. The particle size of as-formed product has been calculated by Williamson-Hall (W-H) plots and Scherrer's formula is found to be in the range of similar to 40 nm. TEM image confirms the nano size crystalline nature of Cu doped ZnO. SEM micrographs of undoped and Cu doped ZnO show highly porous with large voids. UV-Vis spectrum showed a red shift in the absorption edge in Cu doped ZnO. PL spectra show prominent peaks corresponding to near band edge UV emission and defect related green emission in the visible region at room temperature and their possible mechanisms have been discussed. The EPR spectrum exhibits a broad resonance signal at g similar to 2.049, and two narrow resonances one at g similar to 1.990 and other at g similar to 1.950. The broad resonance signal at g similar to 2.049 is a characteristic of Cu2+ ion whereas the signal at g similar to 1.990 and g similar to 1.950 can be attributed to ionized oxygen vacancies and shallow donors respectively. The spin concentration (N) and paramagnetic susceptibility (X) have been evaluated and discussed. (C) 2011 Elsevier B. V. All rights reserved