Visible PL phenomenon at room temperature in disordered structure of SrWO4 powder

The SrWO4 (SWO) powders were synthesized by the polymeric precursor method and annealed at different temperatures. The SWO structure was obtained by X-ray diffraction and the corresponding photoluminescence (PL) spectra was measured. The PL results reveal that the structural order-disorder degree in the SWO lattice influences in the PL emission intensity. Only the structurally order-disordered samples present broad and intense PL band in the visible range. To understand the origin of this phenomenon, we performed quantum-mechanical calculations with crystalline and order-disordered SWO periodic models. Their electronic structures were analyzed in terms of band structure. The appearance of localized levels in the band gap of the order-disordered structure was evidenced and is a favorable condition for the intense PL to occur.

Synthesis of (Ca,Nd)TiO3 powders by complex polymerization, Rietveld refinement and optical properties

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Surface morphology and optical characterization of OC1 OC6-PPV films

We report optical and morphological properties of poly(2-methoxy-5-hexyloxy-p-phenylenevinylene) (OC1OC6-PPV) films processed by casting, spin-coating (SC) and Langmuir-Blodgett (LB) techniques. The absorption spectra are practically the same, with an absorption maximum at approximately at 500 nm. For the photoluminescence (PL) spectra at low temperature (T=10K), a small but significant difference was noted in the cast film, in comparison with the LB and SC films. The zero-phonon transition shifted from 609 nm for the LB film to 615 and 621 nm for the SC and cast films, respectively. At room temperature, the PL spectra are similar for all films, and blue shifted by ca. 25 nm in comparison with the spectra at low temperature due to thermal disorder. Using atomic force microscopy (AFM) we inferred that the distinctive behavior of the cast film, probably associated with structural defects, is related to the large thickness of this film. The surface roughness, which was surprisingly higher for the LB film, apparently played no role in the emission properties of OC1OC6-PPV films.

Photoluminescence in amorphous zirconium titanate

Photoluminescence at room temperature was observed in amorphous zirconium titanate obtained from the polymeric precursor method. This was the first time in which PL was noticed in an amorphous compound made of two network formers. The PL spectra could be deconvoluted into two bands, whose center 1 was located from 540 nm to 552 nm and center 2 from 625 nm to 641 nm. The co-existence of fivefold and sixfold oxygen coordination of titanium and/or zirconium could be the origin of the radiative recombination of electron-hole pairs in amorphous ZT, which may be responsible for the emission of the photoluminescence.

Photoluminescence at room temperature in disordered Ba(0.50)Sr(0.)5(0)(Ti0.80Sn0.20)O-3 thin films

Photoluminescence (PL) properties at room temperature of disordered Ba0.50Sr0.50(Ti0.80Sn0.20)O-3 (BST:Sn) thin films were obtained by the polymeric precursor method. X-ray diffraction data and corresponding PL properties have been measured using the 488 nm line of an argon ion laser. The PL spectra of the film annealed at 350 degrees C for 21 h are stronger than those of the film annealed at 350 degrees C for 28 h, indicating a disorganized structure. The energy band gaps of the crystalline and amorphous BST:Sn thin films were 3.35 and 2.25 eV, respectively. The doped BST thin films also tend to a cubic structure, resulting from TiO6 deformations. (c) 2006 American Institute of Physics.

Spectral redistribution of waveguided emission in BEH-PPV films

We report on spectral redistribution of the photoluminescence (PL) emission from the edge of thin-film planar waveguides of the conjugated polymer BEH-PPV [Poly(2,5-bis(2'-ethyl-hexyl)-1,4-phenylenevinylene] induced by self-absorption in the polymer film. The PL spectra present drastic changes and displace to longer wavelengths with increasing self-absorption. We observe an enhancement of the absolute PL intensity at longer wavelengths, which was interpreted as due to re-emission of self-absorbed photons. The significant efficiency for the PL re-emission suggests the use of self-absorption as a mechanism for tuning the emission into the near infrared.

Conditions giving rise to intense visible room temperature photoluminescence in SrWO4 thin films: the role of disorder

The nature of intense visible photoluminescence at room temperature of SrWO4 (SWO) non-crystalline thin films is discussed in the light of experimental results and theoretical calculations. The SWO thin films were synthesized by the polymeric precursors method. Their structural properties have been obtained by X-ray diffraction data and the corresponding photoluminescence (PL) spectra have been measured. The UV-vis optical spectra measurements suggest the creation of localized states in the disordered structure. The photoluminescence measurements reveal that the PL changes with the degree of disorder in the SWO thin film. To understand the origin of visible PL at room temperature in disordered SWO, we performed quantum-mechanical calculations on crystalline and disordered SWO periodic models. Their electronic structures are analyzed in terms of DOS, hand dispersion and charge densities. We used DFT method with the hybrid non-local B3LYP approximation. The polarization induced by the symmetry break and the existence of localized levels favors the creation of trapped holes and electrons, giving origin to the room temperature photoluminescence phenomenon in the SWO thin films. (c) 2004 Elsevier B.V. All rights reserved.

Theoretical and experimental study of the relation between photoluminescence and structural disorder in barium and strontium titanate thin films

Thin films of barium and strontium titanate (BST), synthesized by the polymeric precursor solution and spin coated on [Pt (140nm)/Ti (10 nM)/SiO2(1000 nm)/Si] substrates were found to be photoluminescent at room temperature when heat treated below 973 K, i.e. before their crystallization. First principles quantum mechanical techniques, based on density functional theory (DFT) were employed to study the electronic structure of two periodic models: one is standing for the crystalline BST thin film and the other one for the structurally disordered thin film. The aim is to compare the photoluminescence (PL) spectra of the crystalline and disordered thin films with their UV-vis spectra and with their computed electronic structures. The calculations show that new localized states are created inside the band gap of the crystalline model, as predicted by the UV-vis spectra. The study of the charge repartition in the structure before and after deformation of the periodic model shows that a charge gradient appears among the titanate clusters. This charge gradient, together with the new localized levels, gives favorable conditions for the trapping of holes and electrons in the structure, and thus to a radiative recombination process. Our models are not only consistent with the experimental data, they also allow to explain the relations between structural disorder and photoluminescence at room temperature. (c) 2005 Elsevier Ltd. All rights reserved.

Spectroscopic properties of polycarbonate and poly(methyl methacrylate) blends doped with Europium(III) acetylacetonate

Spectroscopic properties of blends formed by bisphenol-A polycarbonate (PC) and poly(methyl methacrylate) (PMMA) doped with Europium (III) acetylacetonate [Eu(acac)(3)], have been studied by photoacoustic spectroscopy (PAS) and photoluminescent (PL) spectroscopy. Emission and excitation spectra, excited state decay times, and quantum efficiency have been evaluated as well. PAS studies evidenced chemical interactions between the Europium complex and the PC/PMMA blend, which presented typical percolation threshold behavior regarding the Eu3+ content. PL spectra evidenced the photoluminescence of the Eu3+ incorporated into the blend. Photoluminescence property enhancement was observed for the composite in comparison with the precursor compound. Optimized emission quantum efficiency was observed for the 60/40 blend doped with 2% and 4% Europium (III) acetylacetonate. (c) 2005 Elsevier B.V. All rights reserved.

Effect of temperature on the optical properties of GaAsSbN/GaAs single quantum wells grown by molecular-beam epitaxy

GaAsSbN/GaAs strained-layer single quantum wells grown on a GaAs substrate by molecular-beam epitaxy with different N concentrations were studied using the photoluminescence (PL) technique in the temperature range from 9 to 296 K. A strong redshift in optical transition energies induced by a small increase in N concentration has been observed in the PL spectra. This effect can be explained by the interaction between a narrow resonant band formed by the N-localized states and the conduction band of the host semiconductor. Excitonic transitions in the quantum wells show a successive red/blue/redshift with increasing temperature in the 2-100 K range. The activation energies of nonradiative channels responsible for a strong thermal quenching are deduced from an Arrhenius plot of the integrated PL intensity. (C) 2003 American Institute of Physics.

Interfacial photoluminescence emission properties of core/shell Al2O3/ZrO2

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

New results on bound excitons in quantum wells

The formation of bound excitons (BE) is investigated for a GaAs/GaAlAs multiple quantum well (QW) system. The photoluminescence (PL) spectra are analysed as a function of the excitation energy. It was found that the carriers photogeneration, either in the barrier or directly in the well, do not play an important role on the BE formation. We conclude that defects localized at interfaces are ionized by of capture charges which in turn bound the free exciton (FE).

Magneto-optical studies of the correlation between interface microroughness parameters and the photoluminescence line shape in GaAs/Ga0.7Al0.3As quantum wells

In this work we analyze the relation between the interface microroughness and the full width at half maximum (FWHM) of the photoluminescence (PL) spectra for a GaAs/Ga0.7Al0.3As multiple quantum well (QW) system. We show that, in spite of the complex correlation between the microscopic interface-defects parameters and the QW optical properties, the Singh and Bajaj model [Appl. Phys. Lett. 44, 805 (1984)] provides a good quantitative description of the excitonic PL-FWHM. ©1999 The American Physical Society.

Excitons in undoped AlGaAs/GaAs wide parabolic quantum wells

In this work the electronic structure of undoped AlGaAs/GaAs wide parabolic quantum wells (PQWs) with different well widths (1000 and 3000 ) were investigated by means of photoluminescence (PL) measurements. Due to the particular potential shape, the sample structure confines photocreated carriers with almost three-dimensional characteristics. Our data show that depending on the well width thickness it is possible to observe very narrow structures in the PL spectra, which were ascribed to emissions associated to the recombination of confined 1s-excitons of the parabolic potential wells. From our measurements, the exciton binding energies (of a few meV) were estimated. Besides the exciton emission, we have also observed PL emissions associated to electrons in the excited subbands of the PQWs. © 2010 IOP Publishing Ltd.

Photoluminescence of core-shell nanoparticles made from yttrium stabilized zirconia powder grain coated with alumina

A YSZ@Al2O3 nanocomposite was obtained by Al 2O3 coating on the surface of yttrium stabilized zirconia via a polymeric precursor method. The resulting core-shell structures were characterized by X-ray diffraction, scanning electron microscopy, transmission electronic microscopy and PL spectra. The TEM micrographs clearly show a homogeneous Al2O3 shell around the ZrO2 core. The observed PL is related to surface-interface defects. Such novel technologies can, in principle, explore materials which are not available in the bulk single crystal form but their figure-of-merit is dramatically dependent on the surface-interface defect states. © 2013 This journal isThe Royal Society of Chemistry.