Author: Néstor Perea López

Propiedades físicas y químicas del Sr2CeO4 y del Sr2-xEuxCeO4 sintetizados por el método de combustión y su aplicación en dispositivos electroluminiscentes

Physical and chemical properties of Sr2CeO4 and Sr2−xEuxCeO4 sinthesized by the combustion method and their aplication in

electroluminiescent devices

Néstor Perea López (2006)

Se estudiaron las propiedades estructurales y de luminiscencia de los óxidos luminiscentes Sr<sub>2</sub>CeO<sub>4</sub> y Sr<sub>2-x</sub>Eu<sub>x</sub>CeO<sub>4</sub>. Por medio del método de síntesis por combustión seobtuvieron polvos de ambos compuestos. Dichos polvos se caracterizaron por medio delas técnicas de difracción de rayos-X, microscopía electrónica de barrido, microscopíaelectrónica de transmisión. Además, se analizaron sus propiedes ópticas por medio deespectrofotometría de fluorescencia y absorción óptica en el ultravioleta-visible.A partir de los polvos se logró fabricar cerámicos que sirvieron como blancospara hacer depósitos de películas delgadas de ambos compuestos. Los depósitos sehicieron por medio de la técnica de depósito por laser pulsado (PLD, por sus siglasen inglés) y se logró con éxito la fabricación de películas delgadas de Sr<sub>2</sub>CeO<sub>4</sub> sobresustratos de silicio(111). Las películas delgadas obtenidas emiten eficientemente labanda característica blanca-azul del compuesto.Una vez conocidos los parámetros de crecimiento de películas delgadas de estoscompuestos por PLD, se fabricaron dispositivos electroluminiscentes en película delgada(TFEL, por sus siglas en inglés). Los dispositivos TFEL se construyeron sobre una hojacerámica de BaTiO<sub>3</sub>, que a la vez sirve como capa aislante y sustrato que soporta eldispositivo. Se fabricaron dispositivos de ambos compuestos. Sin embargo, solo eldispositivo TFEL construido con capa activa de Sr<sub>2-x</sub>Eu<sub>x</sub>CeO<sub>4</sub> emite luz (roja). Estedispositivo tiene voltaje de umbral de 150V y emite con coordenadas de cromaticidadde la CIE (Comission Internationale de’lEclariage) x = 0.639 y y = 0.338. Su eficienciamáxima es de 16×10<sup>-3</sup> lm/W, lo cual ocurre a 300V donde emite 28×10<sup>-3</sup> cd/m<sup>2</sup>. Elanálisis de la distribución del potencial en cada capa del TFEL revela que el campoeléctrico en la capa de Sr<sub>2-x</sub>Eu<sub>x</sub>CeO<sub>4</sub> alcanza 1.11 MV/cm cuando el TFEL se alimentacon un voltaje de 400V. Dicho campo eléctrico es suficiente para acelerar electrones yexcitar por impacto a los iones de Eu<sup>3+</sup> que finalmente emiten la luz roja que sale deldispositivo. Sin embargo, dicho campo no es suficente para estimular la luminiscenciapor transferencia de carga en el Sr<sub>2</sub>CeO<sub>4</sub>; que requiere ~4.1 eV para que se produzca.

The structural and luminescent properties of the luminescent oxides Sr<sub>2</sub>CeO<sub>4</sub> andSr<sub>2-x</sub>Eu<sub>x</sub>CeO<sub>4</sub> have been studied. Fine powders of both compounds were obtainedby means of the combustion synthesis method. Characterization of their structuralproperties was performed by means of X–Ray diffraction, Scanning electron microscopyand transmission electron microscopy. On the other hand, luminescent properties werestudied by fluorescence spectrophotometry and optical absorption in the ultravioletvisible region.These powders were used to fabricate ceramic pellets, which then where usedas ablation targets to grow thin films of both compounds. Thin film deposition wascarried out by means of the pulsed laser deposition technique. Successful depositionof Sr<sub>2</sub>CeO<sub>4</sub> on silicon(111) substrates was achieved. These films emitted efficiently thecharacteristic white-blue emission band of this compound similar to bulk or powder.Once we controlled all the PLD deposition parameters we proceeded to fabricatethin film electroluminescent devices (TFEL). These TFEL’s were built on BaTiO<sub>3</sub> ceramic sheets, which serve as insulating layer and substrate to support the device. EvenTFEL devices of both compounds were constructed, only that built whit Sr<sub>2-x</sub>Eu<sub>x</sub>CeO<sub>4</sub>active layer emitted light (red). These devices have threshold voltage of 150V and emitin the CIE coordinates (0.639,0.338). The maximum efficiency achived was 16×10<sup>-3</sup>lm/W at 300V where luminance is 28×10<sup>-3</sup> cd/m<sup>2</sup>. The calculation of the potentialdistribution over the TFEL layers reveals that the electric field in the phosphor layerof Sr<sub>2-x</sub>Eu<sub>x</sub>CeO<sub>4</sub> reaches 1.11 MV/cm when the applied voltage is 400V. At this veryhigh electric field, electrons are accelerated and “heated”, acquiring in this way enoughenergy to impact–excite the Eu<sup>3+</sup> ions. From these ions, red light is emmited, whichfinally comes out from the device window. Nevertheless, this electric field is not highenough to stimulate the charge transfer that is required to produce luminescence inSr<sub>2</sub>CeO<sub>4</sub> (~4.1 eV ).

Doctoral thesis

Electroluminiscencia,Oxidos luminiscentes, Películas delgadas CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA OTRAS ESPECIALIDADES FÍSICAS

Near UV-blue excitable green-emitting nanocrystalline oxide

CARLOS EDUARDO RODRIGUEZ GARCIA NESTOR PEREA LOPEZ GUSTAVO ALONSO HIRATA FLORES (2011)

"Green-emitting Eu-activated powders were produced by a two-stage method consisting of pressure-assisted combustion synthesis and postannealing in ammonia. The as-synthesized powders exhibited a red photoluminescence (PL) peak located at  nm when excited with  nm UV. This emission peak corresponds to the 5D0→7F2 transition in Eu3+. After annealing in ammonia, the PL emission changed to an intense broad-band peak centered at  nm, most likely produced by 4f65d1→4f7 electronic transitions in Eu2+. This green-emitting phosphor has excitation band in the near UV-blue region (–450 nm). X-ray diffraction analysis reveals mainly the orthorhombic EuAlO3 and Al2O3 phases. Transmission electron microscopy observations showed that the grains are formed by faceted nanocrystals (~4 nm) of polygonal shape. The excellent excitation and emission properties make these powders very promising to be used as phosphors in UV solid-state diodes coupled to activate white-emitting lamps."

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ANDRES RAFAEL BOTELLO MENDEZ NESTOR PEREA LOPEZ ANA LAURA ELIAS ARRIAGA Vincent Crespi FLORENTINO LOPEZ URIAS HUMBERTO TERRONES MALDONADO MAURICIO TERRONES MALDONADO (2013)

"The Raman scattering of single-and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A(1g)(C) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single-and few-layered WS2 films."

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INGENIERÍA Y TECNOLOGÍA CIENCIAS TECNOLÓGICAS

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"Nitrogen-doped multiwalled carbon nanotube (CNT) bundles exhibiting pine-tree-like morphologies were synthesized on silicon-silicon oxide (Si/SiO2) substrates using a pressure-controlled chemical vapor deposition process. Electron field emission (FE) measurements showed a notable emission improvement at low turn-on voltages for the CNT pine-like morphologies (e.g., 0.59 V/-m) in comparison with standard aligned N-doped CNTs (>1.5 V/-m). We envisage that these pine-tree-like structures could be potentially useful in the fabrication of efficient FE and photonic devices."

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"The optical damage associated with high intensity laser excitation of silver nanoparticles (NPs) was studied. In order to investigate the mechanisms of optical nonlinearity of a nanocomposite and their relation with its ablation threshold, a high-purity silica sample implanted with Ag ions was exposed to different nanosecond and picosecond laser irradiations. The magnitude and sign of picosecond refractive and absorptive nonlinearities were measured near and far from the surface plasmon resonance (SPR) of the Ag NPs with a self-diffraction technique. Saturable optical absorption and electronic polarization related to self-focusing were identified. Linear absorption is the main process involved in nanosecond laser ablation, but nonlinearities are important for ultrashort picosecond pulses when the absorptive process become significantly dependent on the irradiance. We estimated that near the resonance, picosecond intraband transitions allow an expanded distribution of energy among the NPs, in comparison to the energy distribution resulting in a case of far from resonance, when the most important absorption takes place in silica. We measured important differences in the ablation threshold and we estimated that the high selectiveness of the SPR of Ag NPs as well as their corresponding optical nonlinearities can be strongly significant for laser-induced controlled explosions, with potential applications for biomedical photothermal processes."

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"Co/Au bilayer thin films were deposited on Si/SiOx substrates using the magnetron sputtering method and used as a catalytic support to grow forests of aligned nitrogen-doped multiwalled carbon nanotubes (N-MWCNT) via chemical vapor deposition (CVD) at 850 °C, using benzylamine (C6H5CH2NH2) as a carbon and nitrogen source. Interestingly, the resulting N-MWCNT contains Co@Au core-shell nanoparticles located at their tips. We found that the metal particle cores consist of cobalt coated by an Au shell of few nanometers. Magnetic measurements revealed a ferromagnetic behavior of the system composed of Co@Au nanoparticles encapsulated inside N-MWCNT. The results are compared with pristine N-MWNT containing only Co nanoparticles encapsulated in their cores."

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