Jorge Oliva

Influence of surface coating on the upconversion emission properties of LaPO4:Yb/Tm core-shell nanorods

A solution based hydrothermal method is used for the preparation of LaPO4:Yb–Tm doped and LaPO4:Yb/Tm core-shell upconverted nanorods. The morphologies, structure, formation mechanism, and upconversion emission properties of these nanocrystals are investigated in detail. Tensile strain is observed for LaPO4:Yb–Tm doped material but compressive strain is obtained for LaPO4:Yb/Tm core-shell material. Analysis suggests that the lattice strain plays an important role on the upconversion emission. Bright blue (475 nm) and red (650 nm) upconversion emissions associated to G14→H36 and F32→H36 transitions were observed and a significant higher in upconversion emission intensity is observed in core-shell nanorods. The effective decay time calculated are 230 and 110 μs for core-shell and doped nanorods, respectively, indicating the removal of surface defects due to surface coating. It reveals that the core-shell structure shows better upconversion emission efficiency.

Current improvement in hybrid quantum dot sensitized solar cells by increased light-scattering with a polymer layer

We investigate the effect of the incorporation of a material with efficient electron transport into a Hybrid Quantum Dot Sensitized Solar Cell (HyQDSSC). The performance of different quantum dot sensitized solar cells is compared with a cell containing poly [(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) with structure TiO2/PbS/PFN/CdS/ZnS. The photoconversion efficiency, as compared to our reference samples, was enhanced in some cases by approximately 45%, while the short-circuit current increased by up to 100% after the incorporation of PFN. The deposition order of the different components was crucial in achieving these improvements. Diffuse reflectance and impedance spectroscopy were used to study the behaviour of the cells. The observed enhancements are attributed to increased light scattering in the active layer, better charge transport, and a decrease in charge recombination and transport resistance.

Annealing effect on the luminescence properties of BaZrO3:Yb3+ microcrystals

Yb-doped barium zirconate (BaZrO3:Yb3+) was synthesized by a hydrothermal method at 100 °C. It presents a cubic perovskite crystalline phase, which is stable between 100 and 100 °C. Upon increasing the annealing temperature, two major changes are observed. (1) The near infrared (NIR) absorption band of Yb3+ decreases, and an enhancement in both the NIR emission of Yb3+ single ion and visible emission of Yb3+ pairs occurs, which is attributed to the reduction in impurities. (2) The annealing process induces new absorption bands in the ultraviolet-visible (UV-vis) region that increase as annealing temperature increases. These results suggest that reduction in Yb3+ to Yb2+ is taking place. The visible emission is considered to be a cooperative emission from Yb3+ pairs. Under UV and visible excitation, no emission of Yb2+ ions was observed. This suggests that the broadband absorption in the UV-vis region might be related to F color centers, which are responsible for the inhibition of the Yb2+ ion emission.

Enhancing the Photocatalytic Activity of Sr4Al14O25: Eu2+, Dy3+ Persistent Phosphors by Codoping with Bi3+ Ions

The photocatalytic activity of Bismuth‐codoped Sr4Al14O25: Eu2+, Dy3+ persistent phosphors is studied by monitoring the degradation of the blue methylene dye UV light irradiation. Powder phosphors are obtained by a combustion synthesis method and a postannealing process in reductive atmosphere. The XRD patterns show a single orthorhombic phase Sr4Al14O25: Eu2+, Dy3+, Bi3+ phosphors even at high Bismuth dopant concentrations of 12 mol%, suggesting that Bi ions are well incorporated into the host lattice. SEM micrographs show irregular micrograins with sizes in the range of 0.5–20 μm. The samples present an intense greenish‐blue fluorescence and persistent emissions at 495 nm, attributed to the 5d–4f allowed transitions of Eu2+. The fluorescence decreases as Bi concentration increases; that suggest bismuth‐induced traps formation that in turn quench the luminescence. The photocatalytic evaluation of the powders was studied under both 365 nm UV and solar irradiations. Sample with 12 mol% of Bi presented the best MB degradation activity; 310 min of solar irradiation allow 100% MB degradation, whereas only 62.49% MB degradation is achieved under UV irradiation. Our results suggest that codoping the persistent phosphors with Bi3+ can be an alternative to enhance their photocatalytic activity.

Effect of solvent on the up- and downconversion emissions of Y 2 O 3 :Yb 3+ −Er 3+ nanofibers synthesized by a hydrothermal method

We report on the structural, morphological, and luminescent properties of Y2O3:Yb3+ (2%)–Er3+ (1%) nanofibers synthesized by a hydrothermal method as a function of the solvent composition ethanol/water. The average length and diameter of the nanofibers ranges from 1.1 to 2.3 μm, and from 50 to 110 nm, respectively. A cubic crystalline structure was obtained, and no effect of the solvent was observed. However, the increment of OHs, because of the increment of water, modifies the quality of the nanofibers. Such impurities improve the emission bands under 940 and 490 nm excitation, especially the red band, via multiphonon relaxation. Relaxation dynamics is explained, based on direct and back energy transfer, multiphonon relaxation, and cross-relaxation. The direct energy transfer coefficients (Cb2, Cb4, and Cb5) calculated by the proposed theoretical model point out the fact that upconverted emissions are notably favored by the increment of OHs. The energy backtransfer (C5b) and cross-relaxation (C51) coefficients depend only on the ion concentration and not on the OH content.

BaZrO 3 :YB NANOPHOSPHOR FOR EFFICIENT UP-CONVERSION LIGHT EMISSION

Strong visible green upconversion emission in nanocrys- talline BaZrO3:Yb 3+ powder, obtained by a hydrothermal process at 100 - C, is reported. The unconverted emission has a quadratic depen- dence on the pump intensity with a lifetime around half that of the NIR lifetime. Results suggest cooperative upconversion as the mechanism responsible for the green ∞uorescence. This e-cient Yb 3+ -based coop- erative up-conversion process allows the development of novel emitting materials in the ultraviolet to visible (UV-VIS) range.

Photocatalytic Activity of LaSr2AlO5:Eu Ceramic Powders

The photocatalytic activity, of undoped and Europium‐doped LaSr2AlO5 powders, has been investigated by degrading methylene blue dye in water solutions. Those powders were fabricated by a combustion method and an annealing treatment in air. All samples showed a tetragonal single phase according to by X‐ray diffraction measurements (XRD). Scanning electron microscopy (SEM) revealed irregular semi‐oval grains with sizes in the range of 3.5–4.27 μm. Photoluminescence spectrum showed sharp emission peaks at 588 nm and at 617 nm which are associated with 7F1,7F2→ 5D0 Eu3+ ion forbidden transitions, respectively, under UV light excitation of 322 nm. The methylene blue (MB) degradation under UV light (254 nm) was studied by monitoring changes in the absorbance peak of MB at 665 nm. Finally, LaSr2AlO5:Eu powders were used three times and the efficiency for the degradation of MB decreased from 100 to 61% after the third cycle of use.

Tunable white light from photo- and electroluminescence of ZnO nanoparticles

Tunable white light emission was obtained from ZnO nanorods with lengths ranging from 125 to 165 nm and quantum dots with an average size of 5 nm, synthesized with dodecylamine (DCA) as surfactant. It was possible to tailor the emission from cool to warm white light by changing the concentration of DCA in the synthesis process. The CIE coordinates for the white light obtained were (0.33, 0.32) in the case of photoluminescence and (0.31, 0.33) for electroluminescence from ZnO-based LEDs with a turn-on voltage of 4 and 6 V and a maximum current consumption of 2.32 mA at 10 V.

Labeling of HeLa cells using ZrO2:Yb3+-Er3+ nanoparticles with upconversion emission

Abstract. This work reports the synthesis, structural characterization, and optical properties of ZrO2:Yb3+-Er3+ (2–1 mol%) nanocrystals. The nanoparticles were coated with 3-aminopropyl triethoxysilane (APTES) and further modified with biomolecules, such as Biotin-Anti-rabbit (mouse IgG) and rabbit antibody-AntiKi-67, through a conjugation method. The conjugation was successfully confirmed by Fourier transform infrared, zeta potential, and dynamic light scattering. The internalization of the conjugated nanoparticles in human cervical cancer (HeLa) cells was followed by two-photon confocal microscopy. The ZrO2:Yb3+-Er3+ nanocrystals exhibited strong red emission under 970-nm excitation. Moreover, the luminescence change due to the addition of APTES molecules and biomolecules on the nanocrystals was also studied. These results demonstrate that ZrO2:Yb3+-Er3+ nanocrystals can be successfully functionalized with biomolecules to develop platforms for biolabeling and bioimaging.

Effect of TEA on the blue emission of ZnO quantum dots with high quantum yield

This work reports the luminescence, morphology and synthesis of ZnO quantum dots using a simple wet chemical method and different concentrations of Triethanolamine (TEA) as surfactant. Those nanoparticles emitted a strong blue emission band centered at 429 nm when they are dispersed in hexane. Spherical quantum dots with sizes ranging from 3 to 7 nm were obtained for concentrations from 0 to 0.7 ml. of TEA, whereas a mixture with oval-like nanoparticles was observed from concentrations above of 1.1 ml of TEA. It was also possible to control the values of the band gap in ZnO quantum dots depending on the content of TEA. Based on the high quantum yield of 81% measured for those ZnO nanoparticles respect to quinine sulfate dye (QS), it is suggested that such nanoparticles could be used for biolabeling and ZnO based LEDs.