Multiple source quantum well model of porous silicon light emission

Based on photoluminescence, Fourier transform infrared spectroscopy, and atomic force microscopy results, a new light emitting model for porous silicon (multiple source quantum well model) is proposed.

Dynamique de recombinaison radiative dans les nanofils InGaN/GaN : étude détaillée de la photoluminescence

Mesures effectuées dans le laboratoire de caractérisation optique des semi-conducteurs du Prof. Richard Leonelli du département de physique de l'université de Montréal. Les nanofils d'InGaN/GaN ont été fournis par le groupe du Prof. Zetian Mi du département de génie électrique et informatique de l'université McGill.

Photoluminescence studies of spray pyrolytically grown nanostructured tin oxide semiconductor thin films on glass substrates

SnO2 nanocrystalline thin films were deposited on glass substrates by the spray pyrolysis technique in air atmosphere at 375, 400, 425, 450 and 500 ◦C substrate temperatures. The obtained films were characterized by using XRD. The room temperature photoluminescence (PL) spectra of these films have near band edge (NBE) and deep level emission under the excitation of 325 nm radiation. NBE PL peak intensity decreased consistently with temperatures for samples prepared at 400, 450 and 500 ◦C, while a sudden reduction in intensity is observed for the sample prepared at 425 ◦C. A similar effect was observed for the optical transmittance spectra. These effects can be explained on the basis of the change in population of oxygen vacancies as indicated by the change in a values

Enhanced UV emission from ZnO nanoflowers synthesized by the hydrothermal process

ZnO nanoflowers were synthesized by the hydrothermal process at an optimized growth temperature of 200 ◦C and a growth/reaction time of 3 h. As-prepared ZnO nanoflowers were characterized by x-ray diffraction, scanning electron microscopy, UV–visible and Raman spectroscopy. X-ray diffraction and Raman studies reveal that the as-synthesized flower-like ZnO nanostructures are highly crystalline with a hexagonal wurtzite phase preferentially oriented along the (1 0 1 1) plane. The average length (234–347 nm) and diameter (77–106 nm) of the nanorods constituting the flower-like structure are estimated using scanning electron microscopy studies. The band gap of ZnO nanoflowers is estimated as 3.23 eV, the lowering of band gap is attributed to the flower-like surface morphology and microstructure of ZnO. Room temperature photoluminescence spectrum shows a strong UV emission peak at 392 nm, with a suppressed visible emission related to the defect states, indicating the defect free formation of ZnO nanoflowers that can be potentially used for UV light-emitting devices. The suppressed Raman bands at 541 and 583 cm−1 related to defect states in ZnO confirms that the ZnO nanoflowers here obtained have a reduced presence of defects

Photoluminescence Studies Of Microwave Assisted Synthesized Zno Micro Structures

ZnO micro particles in the range 0.4-0.6 μm were synthesized by microwave irradiation method. The XRD analysis reveals that the sample is in the wurtzite phase with orientation along the (101) plane. SAED pattern of the sample reveals the single crystalline nature of the micro grains. TEM images show the formation of cylindrical shaped ZnO micro structures with hexagonal faces. The optical phonon modes were slightly shifted in the Raman spectrum,attributed to the presence of various crystalline defects and laser induced local heating at the grain boundaries. A broad transmission profile was observed in the FTIR spectrum from 1550-3400 cm-1 which falls in the atmospheric transparency window region. PL spectrum centered at 500 nm with a broad band in the region 420-570 nm comprised of different emission peaks attributed to transition between defect levels. Various emission levels in the sample were expliained with a band diagram

Reliable and Damage-Free Estimation of Resistivity of ZnO Thin Films for Photovoltaic Applications Using Photoluminescence Technique

This work projects photoluminescence (PL) as an alternative technique to estimate the order of resistivity of zinc oxide (ZnO) thin films. ZnO thin films, deposited using chemical spray pyrolysis (CSP) by varying the deposition parameters like solvent, spray rate, pH of precursor, and so forth, have been used for this study. Variation in the deposition conditions has tremendous impact on the luminescence properties as well as resistivity. Two emissions could be recorded for all samples—the near band edge emission (NBE) at 380 nm and the deep level emission (DLE) at ∼500 nm which are competing in nature. It is observed that the ratio of intensities of DLE to NBE ( DLE/ NBE) can be reduced by controlling oxygen incorporation in the sample. - measurements indicate that restricting oxygen incorporation reduces resistivity considerably. Variation of DLE/ NBE and resistivity for samples prepared under different deposition conditions is similar in nature. DLE/ NBE was always less than resistivity by an order for all samples.Thus from PL measurements alone, the order of resistivity of the samples can be estimated.

Synthesis and Photoluminescence Properties of Novel Red Phosphors for White Light Emitting Diode Applications

Several series of Eu3+ based red emitting phosphor materials were synthesized using solid state reaction route and their properties were characterized. The present studies primarily investigated the photoluminescence properties of Eu3+ in a family of closely related host structure with a general formula Ln3MO7. The results presented in the previous chapters throws light to a basic understanding of the structure, phase formation and the photoluminescence properties of these compounds and their co-relations. The variation in the Eu3+ luminescence properties with different M cations was studied in Gd3-xMO7 (M = Nb, Sb, Ta) system.More ordering in the host lattice and more uniform distribution of Eu3+ ions resulting in the increased emission properties were observed in tantalate system.Influence of various lanthanide ion (Lu, Y, Gd, La) substitutions on the Eu3+ photoluminescence properties in Ln3MO7 host structures was also studied. The difference in emission profiles with different Ln ions demonstrated the influence of long range ordering, coordination of cations and ligand polarizability in the emission probabilities, intensity and quantum efficiency of these phosphor materials. Better luminescence of almost equally competing intensities from all the 4f transitions of Eu3+ was noticed for La3TaO7 system. Photoluminescence properties were further improved in La3TaO7 : Eu3+ phosphors by the incorporation of Ba2+ ions in La3+ site. New red phosphor materials Gd2-xGaTaO7 : xEu3+ exhibiting intense red emissions under UV excitation were prepared. Optimum doping level of Eu3+ in these different host lattices were experimentally determined. Some of the prepared samples exhibited higher emission intensities than the standard Y2O3 : Eu3+ red phosphors. In the present studies, Eu3+ acts as a structural probe determining the coordination and symmetry of the atoms in the host lattice. Results from the photoluminescence studies combined with the powder XRD and Raman spectroscopy investigations helped in the determination of the correct crystal structures and phase formation of the prepared compounds. Thus the controversy regarding the space groups of these compounds could be solved to a great extent. The variation in the space groups with different cation substitutions were discussed. There was only limited understanding regarding the various influential parameters of the photoluminescence properties of phosphor materials. From the given studies, the dependence of photoluminescence properties on the crystal structure and ordering of the host lattice, site symmetries, polarizability of the ions, distortions around the activator ion, uniformity in the activator distribution, concentration of the activator ion etc. were explained. Although the presented work does not directly evidence any application, the materials developed in the studies can be used for lighting applications together with other components for LED lighting. All the prepared samples were well excitable under near UV radiation. La3TaO7 : 0.15Eu3+ phosphor with high efficiency and intense orange red emissions can be used as a potential red component for the realization of white light with better color rendering properties. Gd2GaTaO7 : Eu3+, Bi2+ red phosphors give good color purity matching to NTSC standards of red. Some of these compounds exhibited higher emission intensities than the standard Y2O3 : Eu3+ red phosphors. However thermal stability and electrical output using these compounds should be studied further before applications. Based on the studies in the closely related Ln3MO7 structures, some ideas on selecting better host lattice for improved luminescence properties could be drawn. Analyzing the CTB position and the number of emission splits, a general understanding on the doping sites can be obtained. These results could be helpful for phosphor designs in other host systems also, for enhanced emission intensity and efficiency.

Blackbody emission under laser excitation of silicon nanopowder produced by plasma-enhanced chemical-vapor deposition

Previous results concerning radiative emission under laser irradiation of silicon nanopowder are reinterpreted in terms of thermal emission. A model is developed that considers the particles in the powder as independent, so under vacuum the only dissipation mechanism is thermal radiation. The supralinear dependence observed between the intensity of the emitted radiation and laser power is predicted by the model, as is the exponential quenching when the gas pressure around the sample increases. The analysis allows us to determine the sample temperature. The local heating of the sample has been assessed independently by the position of the transverse optical Raman mode. Finally, it is suggested that the photoluminescence observed in porous silicon and similar materials could, in some cases, be blackbody radiation

Photoluminescence behavior of Eu(3+) ion doped into gamma- and alpha-alumina systems prepared by combustion, ceramic and Pechini methods

Al(2)O(3):Eu(3+)(1%) samples were prepared by combustion, ceramic, and Pechini methods annealed from 400 to 1400 degrees C. XRD patterns indicate that samples heated up to 1000 degrees C present disordered character of activated alumina (gamma-Al(2)O(3)). However, alpha-Al(2)O(3) phase showed high crystallinity and thermostability at 1200-1400 degrees C. The sample characterizations were also carried out by means of infrared spectroscopy (IR), scanning electron microscopy (SEM) and specific surface areas analysis (BET method). Excitation spectra of Al(2)O(3):Eu(3+) samples present broaden bands attributed to defects of Al(2)O(3) matrices and to LMCT state of O -> Eu(3+), however, the narrow bands are assigned to (7)F(0) -> (5)D(J),(5)H(J) and (5)L(J) transitions of Eu(3+) ion. Emission spectra of samples calcined up to 1000 degrees C show broaden bands for (5)D(0) -> (7)F(J) transitions of Eu(3+) ion suggesting that the rare earth ion is in different symmetry sites showed by inhomogeneous line broadening of bands, confirming the predominance of the gamma-alumina phase. For all samples heated from 1200 to 1400 degrees C the spectra exhibit narrow (5)D(0) -> (7)F(J) transitions of Eu(3+) ion indicating the conversion of gamma to alpha-Al(2)O(3) phases, a high intensity narrow peak around 695 nm assigned to R lines of Cr(3+) ion is shown. Al(2)O(3):Eu(3+) heated up to 1100 degrees C presents an increase in the Omega(2) intensity parameter with the increase of temperatures enhancing the covalent character of metal-donor interaction. The disordered structural systems present the highest values of emission quantum efficiencies (eta). CIE coordinates of Al(2)O(3):Eu(3+) are also discussed. (C) 2007 Elsevier Inc. All rights reserved.

Characterization, photoluminescence, thermally stimulated luminescence and electron spin resonance studies of Eu(3+) doped LaAlO(3) phosphor

Europium-doped lanthanum aluminate (LaAlO(3)) powder was prepared by using a combustion method. The crystallization, surface morphology, specific surface area and luminescence properties of the samples have been investigated. Photoluminescence studies of Eu doped LaAlO(3) showed orange-reddish emission due to Eu(3+) ions. LaAlO(3):Eu(3+) exhibits one thermally stimulated luminescence (TSL) peak around 400 degrees C. Room temperature electron spin resonance spectrum of irradiated phosphor appears to be a superposition of two centres. One of them (centre I) with principal g-value 2.017 is identified as an O(-) centre while centre II with an isotropic g-value 2.011 is assigned to an F(+) centre (singly ionized oxygen vacancy). An additional defect centre observed during thermal annealing around 300 degrees C grows with the annealing temperature. This centre (assigned to F(+) centre) originates from an F-centre (oxygen vacancy with two electrons) and the F-centre along with the associated F(+) centre appear to correlate with the observed TSL peak in LaAlO(3):Eu(3+) phosphor. The activation energy for this peak has been determined to be 1.54 eV from TSL data. (C) 2010 Elsevier Masson SAS. All rights reserved.

Infrared emission and defect centres in Er and Yb codoped Y(3)Al(5)O(12) phosphors

YAG phosphor powders doped/codoped with Er(3+)/(Er(3+) + Yb(3+)) have been synthesised by using the solution combustion method. The effect of direct pumping into the (4)I(11/2) level under 980 nm excitation of doped/codoped Er(3+)/Yb(3+)-Er(3+) in Y(3)Al(5)O(12) (YAG) phosphor responsible for an infrared (IR) emission peaking at similar to 1.53 mu m corresponding to the (4)I(13/2)->(4)I(15/2) transition has been studied. YAG exhibits three thermally-stimulated luminescence (TSL) peaks at around 140A degrees C, 210A degrees C and 445A degrees C. Electron spin resonance (ESR) studies were carried out to identify the centres responsible for the TSL peaks. The room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0176 is identified as O(-) ion, while centre II with an isotropic g-factor 2.0020 is assigned to an F(+) centre (singly ionised oxygen vacancy). An additional defect centre is observed during thermal-annealing experiments and this centre (assigned to F(+) centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and these two centres appear to correlate with the observed high-temperature TSL peak in YAG phosphor.

A new processing method of CaZn(2)(OH)(6)center dot 2H(2)O powders: Photoluminescence and growth mechanism

In this communication, we report on the formation of calcium hexahydroxodizincate dehydrate, CaZn(2)(OH)(6)center dot 2H(2)O (CZO) powders under microwave-hydrothermal (MH) conditions. These powders were analyzed by X-ray diffraction (XRD), Field-emission gum scanning electron microscopy (FEG-SEM), ultraviolet-visible (UV-vis) absorption spectroscopy and photoluminescence (PL) measurements. XRD patterns confirmed that the pure CZO phase was obtained after MH processing performed at 130 degrees C for 2 h. FEG-SEM micrographs indicated that the morphological modifications as well as the growth of CZO microparticles are governed by Ostwald-ripening and coalescence mechanisms. UV-vis spectra showed that this material have an indirect optical band gap. The pure CZO powders exhibited an yellow PL emission when excited by 350 nm wavelength at room temperature. (C) 2009 Elsevier Masson SAS. All rights reserved.

Intense violet-blue photoluminescence in BaZrO3 powders: A theoretical and experimental investigation of structural order-disorder

Intense violet-blue photoluminescence (PL) emission at room temperature was verified in BaZrO3 (BZO) powders with structural order-disorder. Ab-initio calculations, ultraviolet-visible absorption spectroscopy and PL were performed. Theoretical results showed that the local disorder in the network-formed Zr clusters present an important role in the formation of hole-electron pair. The experimental data and theoretical results are in agreement, indicating that the PL emission in BZO powders can be related to the structural order-disorder degree in the lattice. (C) 2008 Elsevier B.V. All rights reserved.

Hierarchical Assembly of CaMoO(4) Nano-Octahedrons and Their Photoluminescence Properties

Hierarchical assemblies of CaMoO4 (CM) nano-octahedrons were obtained by microwave-assisted hydrothemial synthesis at 120 degrees C for different times. These structures were structurally, morphologically and optically characterized by X-ray diffraction, micro-Raman spectroscopy, field-emission gun scanning electron microscopy, ultraviolet-visible absorption spectroscopy and photoluminescence measurements. First-principle calculations have been carried out to understand the structural and electronic order-disorder effects as a function of the particle/region size. Supercells of different dimensions were constructed to simulate the geometric distortions along both they and z planes of the scheelite structure. Based on these experimental results and with the help of detailed structural simulations, we were able to model the nature of the order-disorder in this important class of materials and discuss the consequent implications on its physical properties, in particular, the photoluminescence properties of CM nanocrystals.

A Multitechnique Study of Structure and Dynamics of Polyfluorene Cast Films and the Influence on Their Photoluminescence

This article describes the microstructure and dynamics in the solid state of polyfluorene-based polymers, poly(9,)-dioctylfluorenyl-2,7-diyl) (PFO), a semicrystalline polymer, and poly [(9,9-dioctyl- 2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)- 1,4-phenylene vinylene}, a copolymer with mesomorphic phase properties. These Structures were determined by wide-angle X-ray scattering (WAXS) measurements, Assuming a packing model for the copolymer structure, where the planes of the phenyl rings are stacked and separated by an average distance of similar to 4.5 angstrom and laterally spaced by about similar to 16 angstrom, we followed the evolution of these distances as a function of temperature using WAXS and associated the changes observed to the polymer relaxation processes identified by dynamical mechanical thermal analysis. Specific molecular motions were studied by solid-state nuclear magnetic resonance. The onset of the side-chain motion at about 213 K (beta-relaxation) produced a small increase in the lateral spacing and in the stacking distance of the phenyl rings in them aggregated Structures, Besides, at about 383 K (alpha-relaxation) there occurs a significant increase in the amplitude of the torsion motion in the backbone, producing a greater increase in the stacking distance of the phenyl rings. Similar results were observed in the semicrystalline phase of PFO, but in this case the presence of the crystalline structure affects considerably the overall dynamics, which tends to be more hindered. Put together, Our data explain many features of the temperature dependence of the photoluminescence of these two polymers.