Person: Maestre Varea, David
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First Name
David
Last Name
Maestre Varea
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Física de Materiales
Area
Ciencia de los Materiales e Ingeniería Metalúrgica
Identifiers
36 results
Search Results
Now showing 1 - 10 of 36
Item Photochromic mechanism in oxygen-containing yttrium hydride thin films: An optical perspective(Physical review B, 2017) Montero, J.; Martinsen, F. A.; García Tecedor, M.; Karazhanov, S Zh.; Maestre Varea, David; Hauback, B.; Marstein, E. S.Oxygen-containing yttrium hydride thin films exhibit photochromic behavior: Transparent thin films reversibly switch from a transparent state to a photodarkened state after being illuminated with UV or blue light. From optical spectrophotometry and ellipsometry measurements of the transparent state and photodarkened state, it is concluded that the photochromic effect can be explained by the gradual growth, under illumination, of metallic domains within the initial wide-band-gap semiconducting lattice. This conclusion is supported by Raman measurements.Item Micro- and nanopyramids of Manganese-Doped Indium Oxide(Journal of Physical Chemistry C, 2010) Maestre Varea, David; Martínez Velasco, I.; Cremades Rodríguez, Ana Isabel; Amati, M.; Piqueras De Noriega, Francisco JavierMn-doped In(2)O(3) nanopyramids have been grown by a catalyst-free thermal process at 700 degrees C using InN and Mn(2)O(3) powders as precursors. Energy dispersive spectroscopy, as well as X-ray photoelectron spectroscopy, demonstrate the presence of Mn in the pyramids in a content below 1 at. %. In addition to pyramids, nanowires with diameters of about 100 nm grow during treatments at 800 degrees C. Luminescence has been studied by cathodoluminescence in the scanning electron microscope, showing emissions at 1.9, 2.65, and 3.3 eV. Dopant incorporation into the nanostructures and their oxidation states, as well as the effect on the electronic structure, have been measured and discussed.Item Understanding the effects of Cr doping in rutile TiO₂ by DFT calculations and X-ray spectroscopy(Scientific reports, 2018) Vásquez, G. C.; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Ramírez Castellanos, Julio; Magnano, Elena; Nappini, Silvia; Karazhanov, Smagul Zh.The effects of Cr on local environment and electronic structure of rutile TiO₂ are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional Cr impurities Cr_(Ti)(0)* and Cr_(Ti)(-1)* as well as Cr-oxygen vacancy complex 2Cr_(Ti) + V₀ are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO₂ at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the t_(2g) states of the Cr ions in order to reach the stable oxidation state of Cr(3+)*. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO₂ cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO₂ are very sensitive to the concentration of Cr.- Project number: 198
Item Aula de juegos para el fomento del aprendizaje autónomo y la creatividad(2016) Fernández Sánchez, Paloma; López Orozco, José Antonio; Maestre Varea, David; Hidalgo Alcalde, Pedro; Besada Portas, Eva; Sotillo Buzarra, Belén; Neira Gómez, Andrea Paulette; Pereyra Ocampo, Laura Victoria; de las Heras de la Campa, PabloEl juego como modo de aprendizaje es algo inherente no sólo al ser humano sino, en general, al reino animal. Para cualquier mamífero el juego constituye la forma de aprendizaje fundamental. A través del juego se aprende a luchar, a defenderse y las normas básicas de convivencia en la manada. Sin embargo en el ser humano, juego y aprendizaje se han ido desligando progresivamente, excepto en las etapas iniciales de crecimiento, en las que los niños siguen aprendiendo los comportamientos más básicos a través de juegos. A medida que vamos avanzando en la escuela, se va abandonando el juego, contraponiendo las actividades lúdicas a las estrictamente relacionadas con el trabajo, con un aprendizaje más costoso. De esta forma al llegar a la etapa universitaria, el juego se ha abandonado por completo como forma de aprendizaje. No es fácil definir lo que es el juego o cuáles son sus características. Tiene una fuerte componente cultural, actividades que unas culturas pueden considerar eminentemente lúdicas, no lo serán en contextos culturales distintos. No obstante, una vez admitida la importancia del juego en el desarrollo de la personalidad, sí podemos establecer algunas de las funciones básicas que el juego desempeña en el ser humano, en relación con el perfeccionamiento y adquisición de habilidades tanto cognitivas como sociales o conductuales. El juego facilita la integración de experiencias en la conducta, contribuye a inhibir conductas no admitidas socialmente y a reforzar aquéllas con una mayor aceptación dentro del marco cultural de referencia. Mejora considerablemente la interacción social y la adquisición de las habilidades básicas necesarias para que se produzca dicha interacción de modo satisfactorio. En el caso de juegos competitivos, enseña a manejar situaciones desfavorables, a soportar y superar la frustración. Tradicionalmente, los juegos se han usado en los niveles iniciales de enseñanza, sin embargo son una poderosa herramienta también en el nivel universitario, especialmente para promover el aprendizaje activo y la adquisición de variadas competencias profesionales. En este proyecto se plantea la elaboración de una herramienta para la creación de simuladores de juegos de mesa con fines didácticos. Item Growth and characterization of mn doped SnO_2 nanowires, nanobelts, and microplates(Journal of Physical Chemistry C, 2013) Herrera, Manuel; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras De Noriega, Francisco JavierUndoped and Mn doped SnO_2 nanowires, nanobelts, and microplates have been grown by a thermal evaporation method that enables the morphology and the Mn content in the structures to be controlled. The structural and morphological characterization was carried out by scanning and transmission electron microscopy (SEM and TEM) and electron backscattered diffraction (EBSD). A crystallographic model has been proposed to describe the SnO2:Mn microplates. X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) demonstrated the incorporation of Mn into the SnO2 lattice in concentrations up to 1.6 at % depending on the thermal treatment employed for the growth of the structures. Variations in the luminescence of the doped nanostructures as a function of the Mn content have been studied. A correlation between facets of the SnO_2:Mn microplates, identified by EBSD, with higher Mn content, and the increase of the luminescence emissions associated to, oxygen vacancies related defects was demonstrated.- Project number: 312
Item Nuevas herramientas de software libre para la corrección automática de ejercicios complejos(2016) Díaz García, Elena; Cabrera Granado, Eduardo; Gómez Calderón, Óscar; Maestre Varea, David; Domínguez-Adame Acosta, FranciscoEn este trabajo presentamos diferentes entornos online de gran utilidad para el proceso enseñanza-aprendizaje y la autoevaluación o evaluación automática de competencias científicas en el cálculo numérico a nivel universitario. Destacamos las ventajas más importantes comunes de este tipo de plataformas y comparamos tres de ellas en concreto: SageMath Cloud, Wakari y JupyterHub + Nbgrader. Presentamos además una valoración inicial de profesores y estudiantes que han aprendido y utilizado algunas de estas herramientas, en concreto Jupyter Notebooks y SageMath Cloud. Item Nanopipes in In_2 O_3 nanorods grown by a thermal treatment(Crystal Growth & Design, 2011) Maestre Varea, David; Haeussler, Dietrich; Cremades Rodríguez, Ana Isabel; Jaeger, Wolfgang; Piqueras De Noriega, Francisco JavierIn_2O_3 nanorods have been grown by a catalyst free evaporation-deposition method. Transmission electron microscopy (TEM) investigations reveal that the rods contain tubular cavities, herein referred to as "nanopipes", along the total length of the nanorods. The nanopipe diameters are constant along the nanorod axis and appear to be independent of the nanorod thickness. In most of the investigated In_2O_3 nanorods, these nanopipes are centered within the nanorod. An average nanopipe diameter of (18.5 +/- 0.7) nm has been determined from the TEM observations. Extended thermal treatments lead to nanorods with more complex morphologies, roughened interfaces, and formation of voids. The possibility that the nanopipes are related with a growth mechanism involving a dislocation along the growth axis is discussed.Item Effects of transition metal doping on the growth and properties of Rutile TiO_2 nanoparticles(Journal of Physical Chemistry C, 2013) Cristian Vasquez, G.; Andrea Peche-Herrero, M.; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Ramirez-Castellanos, Julio; María Gonzalez-Calbe, José; Piqueras De Noriega, Francisco JavierRutile TiO_2 nanoparticles doped with V, Cr, or Mn ions have been synthesized via a modified Pechini method using polymeric precursors. The final particle sizes range between 20 and 500 nm depending on the selected dopant. The TiO_2 rutile phase has been stabilized in the doped nanoparticles at 650 degrees C. Microstructural analysis shows a good crystallinity and cationic homogeneity of the doped nanoparticles. The cathodoluminescence study of the doped and undoped nanoparticles shows a luminescence signal related to the structural defects of the samples and the presence of dopants. In particular, an intense 1.52 eV emission associated with Ti^3+ interstitials dominates the luminescence of undoped nanoparticles, which also exhibit less intense emissions extending from 2 to 3.4 eV. The presence of V, Cr, or Mn in the rutile TiO_2 nanoparticles induces variations in the associated cathodoluminescence signal which would be useful in order to achieve a deeper understanding of the doping process and spread future optical applications. X-ray photoelectron spectroscopy (XPS) confirmed the presence of Ti^3+ in the near-surface region of the nanoparticles, the concentration of which decreases when doping. The presence of Ti^3+ interstitials related states in the band gap is discussed.Item Indium Zinc Oxide pyramids with pinholes and nanopipes(Journal of Physical Chemistry C, 2011) Bartolomé Vílchez, Javier; Maestre Varea, David; Amati, Mateo; Cremades Rodríguez, Ana Isabel; Piqueras De Noriega, Francisco JavierMicropyramids of zinc-doped indium oxide have been grown by thermal treatments of compacted InN and ZnO powders at temperatures between 700:and 900 degrees C Under argon flow. X-ray diffraction (XRD) measurements and energy-dispersive X-ray (EDS) mappings as well as local EDS spectra enable the identification of rough surfaces of the pyramids with the nucleation of a shell of nanocrystallites with high Zn/In ratio because of the formation of Zn(k)In(2)O(k+3). Some of the pyramids have a truncated tip with pinholes with regular crystalline facets. The apexes of these pinhole's present a hollow core or nanopipe The possible relation of the nanopipes with a dislocation driven growth is discussed. A growth model is proposed from the morphology evolution of the pyramids during the formation of the In(2)O(3)-ZnO (IZO) compound X-ray photoelectron spectroscopy and microscopy (XPS-ESCA) Measurements are used to discuss the Zn incorporation as a dopant and the formation of Zn(k)In(2)O(k+3) ternaries. Cathodoluminescence (CL) in the scanning electron microscopy (SEM) shows a dependence of the luminescence of the microstructures on the Zn concentration and the growth temperature.Item Fe-TiO_2 nanoparticles synthesized by green chemistry for potential application in waste water photocatalytic treatment(Journal of nanatechnology, 2019) Solano, Ricardo A.; Herrera, Adriana P.; Maestre Varea, David; Cremades Rodríguez, Ana IsabelAnatase TiO_2 nanoparticles doped with iron ions have been synthesized via the green chemistry method using aqueous extract of lemongrass (Cymbopogon citratus) obtained from Soxhlet extraction and doped by wet impregnation. The TiO_2 anatase phase has been doped with Fe^3+ (0.05, 0.075, and 0.1 Fe^3+ : Ti molar ratio) at 550 degrees C and 350 degrees C, respectively. The scanning electron microscopy with energy-dispersive X-ray (SEM-EDS) shows nanoparticle clusters and efficiencies of impregnations between 66.5 and 58.4% depending on the theoretical dopant amount. The electron transmission microscopy (TEM) reveals final particle sizes ranging between 7 and 26 nm depending on the presence or not of the dopant. The cathodoluminescence (CL) and photoluminescence (PL) studies of the doped and undoped nanoparticles show a luminescence signal attributed to surface oxygen vacancies (visible CL emission 380-700 nm and PL emission 350-800 nm); additionally, a decrease in emission intensity is observed due the inhibition of the recombination of the photogenerated electron-holes pairs; moreover, nanopowders were analyzed by UV-Vis spectrophotometry of diffuse reflectance, and the absorption edge of the Fe-TiO_2 in comparison to undoped TiO_2 is extended greatly toward the visible light. The six bands (A_1g + 2B_1g + 3E_g) found by Raman spectroscopy and the x-ray diffraction pattern (XRD) confirm that synthesized TiO_2 is only anatase phase, which is commonly used as a catalyst in waste water treatment, specifically in heterogeneous photocatalytic processes.