Structural, photoluminescent properties and Judd-Ofelt analysis of Eu3+-activated CaF2 nanocubes

Eu3+-doped CaF2 nanocubes with variable europium concentration, [Eu3+] = 0, 0.6, 1.3, 1.7, 2.2 and 5.4 mol%, have been synthesized by a direct precipitation route. It has been found that, within this concentration range, the nanoparticles present the fluoride-type crystalline structure and the characteristic cubic shape of CaF2 crystals. The nanoparticle size follows a log-normal distribution with a mean value decreasing with the Eu3+ content. Rietveld refinement has been performed to calculate the lattice parameter and crystallite size. Eu3+ concentration affects both parameters giving rise to an increase in the lattice parameter and a reduction of crystallite size. The luminescent properties of Eu3+ ions in these nanostructures have been investigated under CW and pulsed excitation. A Judd-Ofelt analysis, as function of the Eu3+ content, has been performed to determine the transition probabilities, radiative lifetimes and branching ratios of the 5D0 emitting level. It was found that and Judd-Ofelt intensity parameters are dependent on the doping level, showing an evolution that indicates a decrease in the Eu3+ site local symmetry with increasing Eu3+ concentration. Finally, it has been observed that the characteristic luminescence decay time of the 5D0 manifold is reduced with increasing Eu3+ concentration. This effect is partially due to an increase of radiative transition probability, associated with a reduction in the local symmetry of the lanthanide ions, and also to the occurrence of concentration quenching effectsThis work has been partially supported by Ministerio de Economía y Competitividad and Ministerio de Ciencia, Innovación y Universidades under projects MAT2016-75716-2-2-R and RTI2018- 101020-B-I00

Negative-epsilon conditions in the dispersive LiNbO3-Ag nanoparticles composites

The silver (Ag)-embedded lithium niobate (LiNbO3) composites are theoretically analyzed under the effective medium Maxwell-Garnett approximation to account on the optimal conditions through which such composites present negative epsilon conditions. The dielectric function of Ag nanoparticles (NPs) is described by Drude theory with an additional Lorentz oscillator term to take into account the interband electronic transitions which typically occur in noble metals. The LiNbO3 dielectric function is evaluated through the Sellmeier equations. Once the effective dielectric function (epsilon(eff)) is evaluated, we investigate the negative epsilon condition (epsilon'(eff) < 0) as a function of the frequency. The results showed that, for given volumen fraction values, the negative epsilon (NE) condition is satisfied for critical sizes of Ag NPs. This condition defines an interval of energies, called NE range. That NE range enlarges for increasing radius and becames narrower for decreasing volume fractions. Furthermore, the calculated Frohlich frequency is nearly close to the lower-energy limit of NE range. In addition, the calculated extinction spectra of the composite are analyzed in terms of the radius of Ag NPs.This work is partially supported by Spanish MINECO under grants MAT2013-46649-C4-4-P, MAT2015 71070 REDC and TECHNOFUSION (II)-Comunidad de Madrid: CM (52013/MAE-2745

Optical Temperature Sensor Capabilities of the Green Upconverted Luminescence of Er3+ in La3NbO7 Ceramic Powders

We present a study of the Er3+ upconverted luminescence in erbium doped Lanthanum Niobium Oxide, La3NbO7, ceramic powder, prepared by solid state reaction. This study focuses on the analysis of the feasibility of this system as a temperature sensor. Efficient UC luminescence was observed under the 975 nm excitation showing intense green, red and NIR (850 nm) emission bands. The NIR luminescence centred at about 850 nm and lying on the first biological window is mainly insensitive to the temperature. In contrast, the upconverted green bands, associated with the (2H11/2,4S3/2) !4I15/2 transitions, showed a high sensibility to temperature. Their temperature dependence was studied from RT up to 525 K, paying special attention to the physiological range of temperature (303–318 K). The high thermal sensitivities obtained, in comparison with other Er3+ and Er3+-Yb3+ based optical temperature sensors in such ranges, suggest the potential application of this phosphor in thermal sensing, suitable for both biological systems and other industrial applications requiring higher temperatures.This research has been partially supported by the Spanish Ministerio de Ciencia, Innovación y Universidades (MICINN) through the Spanish projects RTI2018-101020-B-100 and PID2019- 106383GB-C44, by the Madrid Government (Comunidad de Madrid-Spain) under the Multiannual Agreement with UC3Min the line of Excellence of University Professors (EPUC3MXX) and in the context of the V PRICIT (Regional Programme of Research and Technological Innovation) by the Agencia Canaria de Investigación, Innovación y Sociedad de la Información (ACIISI) (ProID2017010078). M.A.H.-R. thanks MINECO for the FPI grant (BES-2014-068666). U.R.R.-M. thanks Gobierno de Canarias and EU-FEDER for grant ProID2020010067

Extinción de la luminiscencia del Eu2+ en haluros alcalinos inducida por precipitación

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias, Departamento de Física Aplicada. Fecha de lectura: 19-04-199

Design of GaAs nanowires array based photovoltaic solar cells: simulations of optical reflectance

We report the optical response of a periodic square array of GaAs nanowires embedded in epoxy as a candidate for photovoltaic solar cells. The simulated system is a multilayer array constituted by alternating layers of epoxy and an effective medium constituted by GaAs nanowires arrays embedded in epoxy. To discuss the optical response, we investigate the reflectance dependence on the number of bilayers considered in the array and the angle of incident light. The GaAs nanowire dielectric function is described in terms of Webb formalism to take into account the confinement energy of the excitons. The effective dielectric function of GaAs nanowires embedded in epoxy is evaluated within the Maxwell-Garnett theory. We evaluate the reflectance for s- and p-polarized light through the transfer matrix formalism for bilayers. For both s- and p-polarization, we observe an oscillating behavior of the reflectance, similar to that reported in the literature. We have also obtained a feature peaked around 850 nm. While the oscillations can be ascribed to multiple interference from periodic bilayers, the peak at 850 nm can be understood in term of the gap energy in the nanowire dielectric function. Attending to the reflectance dependence on the light incidence angle, we have found that for s-polarized light, the reflectance is higher with increasing angles, in comparison to p-polarized light cases.This work is partially supported by Spanish MICINN under grant RTI 2018-101020-B-I00 and TECHNOFUSION III CM–S2018IEMAT–4437. This work has also been supported by Comunidad de Madrid (Spain) -multiannual agreement with UC3M (Excelencia para el Profesorado Universitario-EPUC3M14)-Fifth regional research plan 2016–2020

Simulations of optical reflectance in vertically aligned GaAs nanowires array: The effect of the geometrical structural parameters

We report the effects of radius-, length- and pitch-sizes on the optical reflectance of a periodic square array of GaAs nanowires embedded in epoxy. The simulated system is a multilayer array constituted by alternating layers of epoxy and an effective medium of GaAs nanowires embedded in epoxy. For both s- and p-polarizations, we observe an oscillating behavior in the reflectance spectra, as a consequence of interferences in periodical systems. We found that the radius- and pitch-sizes significantly affect the reflectance of GaAs nanowires array, while the length-sizes do not present evidence of changes in the optical reflectance. For higher radius, the number of oscillations increases and consequently, the peak-to-peak distance decreases. Besides, there is a red-shift of the reflectance for increasing radius. For higher pitch, the number of oscillations also increases, and a red-shift is observed. We obtain dependence laws for the peak-to-peak distance and red-shift versus radius and versus pitch. These dependences obey approximate quadratic relations. Attending to the reflectance dependence on the light incidence angle, we have found that for s-polarized light, the reflectance is higher with increasing angles, in comparison to p-polarized light cases, independently of the radius and pitch values. For both polarizations, we found that the reflectance is increasing for greater radii and smaller pitches, independently of the incident angle.This work is partially supported by Spanish MICINN under grant RTI 2018-101020-B-I00 and TECHNOFUSION III CM-S2018IEMAT-4437. This work has also been supported by Comunidad de Madrid under the agreement with UC3M in the line of Excellence of University Professors (EPUC3M14)

Development of Luminescent Nd-Doped LaNbO Compound Thin Film Growth by Magnetron Sputtering for the Improvement of Solar Cells

Nd3+-doped LaNbO phosphor thin films were prepared by radio-frequency magnetron sputtering on Si substrates. The effects of a 1% Nd-doping concentration, after annealing at 1200 &deg;C for 12 h, on the light-emitting properties of the sputtered thin films were characterized via several experimental techniques and deeply discussed. Photoluminescence characterization showed strong emission peaks typical of Nd3+ centers at 880 nm and 1060 nm when a 325 nm wavelength laser source was applied. Similar responses were detected in Nd3+-doped La3NbO7 powder samples fabricated by the solid-state reaction method. The coexistence of two phases (LaNbO4 and La3NbO7) in the thin films with higher nominal thickness was clearly identified based on different structural analyses. The promising results open the possibility for developing phosphor substrates as a preliminary step for the improvement of solar cells based on photon recycling mechanisms

Development of Luminescent Nd-Doped LaNbO Compound Thin Film Growth by Magnetron Sputtering for the Improvement of Solar Cells

Nd3+-doped LaNbO phosphor thin films were prepared by radio-frequency magnetron sputtering on Si substrates. The effects of a 1% Nd-doping concentration, after annealing at 1200 °C for 12 h, on the light-emitting properties of the sputtered thin films were characterized via several experimental techniques and deeply discussed. Photoluminescence characterization showed strong emission peaks typical of Nd3+ centers at 880 nm and 1060 nm when a 325 nm wavelength laser source was applied. Similar responses were detected in Nd3+-doped La3NbO7 powder samples fabricated by the solid-state reaction method. The coexistence of two phases (LaNbO4 and La3NbO7) in the thin films with higher nominal thickness was clearly identified based on different structural analyses. The promising results open the possibility for developing phosphor substrates as a preliminary step for the improvement of solar cells based on photon recycling mechanisms

Structural, morphology and luminescence study of Er+3-doped garnet-type Li5La3Nb2O12 electrolytes as a potential new phosphor

Erbium doped garnet-type Li5La3Nb2O12 electrolytes were synthesized by conventional solid state reaction in air at 900 degrees C. Formation, microstructure and luminescence properties were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), UV VIS absorption and luminescence techniques. The limits of the Li5La3-xErxNb2O12 solid solution (0 0.05, garnet structure coexists with segregated Er2O3. Rietveld refinement of room temperature powder XRD patterns shows that this solid solution exhibits a cubic garnet structure (a similar to 1179 angstrom and S.G. la-3d) and trivalent Er ions replace La cations in cubic coordination (located in 24c site). The typical green, red and infrared luminescence of Er3+ ions can be efficiently excited under several wavelength lights in the UV VIS NIR region. Optical measurements have been improved dispersing Li5La3-xEr-xNb2O12 powder in KBr pellets. Our results indicate the potentiality of rare earth doped Li5La3Nb2O12 garnets as a new family of phosphors.Projects funded by the Spanish Government (MINECO through MAT2013-46649-C4-4-P, MAT2015-71070-REDC and MAT2016-78362-C4-3R Projects), Regional Government (Comunidad de Madrid through MATERYENER3CM S2013/MIT-2753) and by the EU-FEDER funds

Spectroscopy and Judd-Ofelt analysis of Er3+ ions in Li5La3Nb2O12 garnet-type ceramic powder

The optical spectroscopy of Er3+ activated Lanthanum Lithium Niobium Garnet ceramic powder is presented. The absorption and photoluminescence spectra have been both obtained at room temperature. Emission bands related to transitions starting at I-4(13/2), I-4(11/2), F-4(9/2), (S-4(3/2), H-2(2)(11/2)) and H-2(2)(9/2) manifolds are presented. Efficient excitation of the luminescence bands is obtained under ultraviolet excitation (270 nm) using a Mercury Xenon arc lamp. The Judd-Ofelt intensity parameters of Er3+ in this garnet are calculated using the absorption spectra. The obtained results Omega(2) = 7.67 x 10(20) cm(2); Omega(4) = 0.84 x 10(20) cm(2); Omega(6) = 0.60 x 10(20) cm(2) are used to calculate the radiative lifetimes of the Er3+ luminescence originated up to (4)G(11/2) manifolds and the branching ratios of the radiative transitions on them. The luminescence decays from the main emission lines (H-2(2)(11/2):S-4(3/2)), I-4(11/2), and I-4(13/2) excited levels to the I-4(15/2) ground state of Er3+ have also been obtained. Experimental decay time values are compared with the radiative lifetime values obtained from the Judd-Ofelt analysis.This work is partially supported by the Spanish MINECO Projects MAT2013-46649-C4-4-P and MAT2015-71070-REDC, and by the EU-FEDER Funds