Person: Díaz-Guerra Viejo, Carlos
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First Name
Carlos
Last Name
Díaz-Guerra Viejo
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Física de Materiales
Area
Física Aplicada
Identifiers
12 results
Search Results
Now showing 1 - 10 of 12
Item Formation of beta-Bi₂O₃ and delta-Bi₂O₃ during laser irradiation of Bi films studied in-situ by spatially resolved Raman spectroscopy(Journal of alloys and compounds, 2017) Díaz-Guerra Viejo, Carlos; Almodóvar, P.; Camacho López, M.; Camacho López, S.; Piqueras De Noriega, Francisco JavierThe formation of different phases of Bi₂O₃ induced by laser irradiation of Bi films has been assessed in situ by micro-Raman spectroscopy as a function of laser wavelength, power density and irradiation time. Raman mapping of the irradiated samples enabled a spatially-resolved study of the distribution of the formed Bi₂O₃ phases. Red laser (633 nm) irradiation was found to induce the appearance of ß- Bi₂O₃, within a certain range of power densities, by diffusion- controlled processes. In contrast, ultraviolet (UV, 325 nm) laser irradiation, above a certain power density threshold, initially induces the formation of both beta and δ- Bi₂O₃ phases. The amount of the produced δ- Bi₂O₃ phase increases by increasing the irradiation time, while that of the beta phase follows the opposite trend. UV laser irradiation seems to be a suitable method to produce room temperature stable δ- Bi₂O₃ patterns on Bi films.Item α-Bi_2O_3 microcrystals and microrods: thermal synthesis, structural and luminescence properties(Journal of Alloys and Compounds, 2013) Piqueras De Noriega, Francisco Javier; Díaz-Guerra Viejo, Carlos; Vila Santos, Maríaα-Bi_2O-3 microcrystals and microrods with pyramidal tips have been grown by a vapour–solid method using pure Bi as starting material. The morphology of the obtained microstructures depends on the annealing temperature and time. At 650 °C and for short annealing times, microcrystals with well defined facets and sizes of tens of microns, as well as clusters of microcrystals, are formed. Microrods were grown at 800 °C, mainly from nucleation sites in the clusters of microcrystals, and reach lengths of hundreds of microns. Micro-Raman spectroscopy measurements reveal the homogeneity of the obtained microstructures regarding the crystalline α-phase. The luminescence properties of the microrods have been investigated by cathodoluminescence in the scanning electron microscope and by photoluminescence in a confocal microscope. The obtained results were correlated with X-ray photoelectron spectroscopy measurements.Item Influence of chromium content on the optical and electrical properties of Li1+xCrxTi2-x(PO4)3(Solid State Ionics, 2013) Pérez Estébanez, Marta; Isasi Marín, Josefa; Díaz-Guerra Viejo, Carlos; Rivera Calzada, Alberto Carlos; León Yebra, Carlos; Santamaría Sánchez-Barriga, JacoboOptical and electrical properties of samples with Li1 + xCrxTi2 − x(PO4)3 composition (x = 0, 0.05 and 0.1) prepared by a low temperature sol-gel method have been investigated. XRD and Raman spectroscopy measurements respectively reveal the rhombohedral-Nasicon structure of the grown samples and a disorder of Li+ ions with increasing chromium content. Electrical properties were studied by impedance spectroscopy and both dc and grain boundary ionic conductivity increase with Cr content. A high ionic conductivity value of 1.4·10−4 S cm−1 at room temperature and an activation energy of 0.31 eV have been obtained for x = 0.1, which are comparable to those measured in the best ionic conductors reported to date. Photoluminescence and cathodoluminescence spectra indicate that self-trapped excitons and oxygen defects related to the TiO6 octahedra structural units are responsible for the observed luminescence in the visible spectral range, explaining the possible electronic origin of a residual conductivity measured in all the samples. Cr incorporation gives rise to infrared emission bands, attributed to different Cr3+ and Ti3+ intraionic transitions, that may be related to the better grain connectivity achieved by Cr doping.Item Optical and magnetic properties of CuO nanowires grown by thermal oxidation(Journal of Physics D: Applied Physics, 2010) Piqueras De Noriega, Francisco Javier; Vila Santos, María; Díaz-Guerra Viejo, CarlosCuO nanostructures with different morphologies, such as single-crystal nanowires, nanoribbons and nanorods, have been grown by thermal oxidation of copper in the 380–900 °C temperature range. Cathodoluminescence spectra of the nanostructures show a band peaked at 1.31 eV which is associated with near band gap transitions of CuO. Two additional bands centred at about 1.23 and 1.11 eV, suggested to be due to defects, are observed for nanostructures grown at high temperatures. The magnetic behaviour of nanowires with lengths in the range of several micrometres and diameters of 50–120 nm has been investigated. Hysteresis loops of the nanowires show ferromagnetic behaviour from 5 K to room temperature.Item Structural and cathodoluminiscent properties of Zr0.95Ce0.05O2 nanopowders prepared by sol–gel and template methods(Journal of Luminescence, 2011) Fernández Castillo, Jesús; Isasi Marín, Josefa; Pérez Estébanez, Marta; Aldama, Iván; Arévalo, Pablo; Díaz-Guerra Viejo, CarlosNanopowders of Zr0.95Ce0.05O2 composition have been prepared by a standard Pechini-type sol–gel process and by means of a colloidal crystal template approach. In the latter method, inverse opal Zr0.95Ce0.05O2 powders were fabricated employing poly(methylmethacrylate) (PMMA) colloidal crystals as a template.The effects of the two different synthesis routes on the structure and microstructural characteristics of the prepared nanopowders were evaluated by X-ray diffraction and scanning electron microscopy. For both preparation routes, the X-ray diffraction analysis has shown that a tetragonal fluorite structure is formed with a crystallite size of 35–40 nm. The scanning electron microscopy measurements indicate that the powder obtained by the sol–gel Pechini-type process is comprised of nanoparticles that are arranged in agglomerates with shape and size relatively uniform whereas the inverse opal Zr0.95Ce0.05O2 nanopowders exhibit the formation of macropores with a mean size of 100 nm. The cathodoluminescence spectra of the prepared Zr0.95Ce0.05O2 nanomaterials have been measured in the 300–800 nm wavelength range. The powder prepared by sol–gel method yields a broad emission band centered at 482nm whereas the emission from the inverse opal preparation is considerably less intense.Item Synthesis and cathodoluminescence of undoped and Cr^(3+)-doped Sodium Titanate nanotubes and nanoribbons(Journal of Physical Chemistry C, 2010) Díaz-Guerra Viejo, Carlos; Umek, Paloma; Gloter, Alexandre; Piqueras De Noriega, Francisco JavierWe report on the synthesis of Cr^(3+)-doped sodium titanate nanotubes and nanoribbons by a hydrothermal method. The presence of dopant ions in these nanostructures was confirmed by high angle annular dark field scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements. Luminescence properties of undoped and Cr^(3+)-doped sodium titanate nanotubes and nanoribbons were investigated by cathodoluminescence in the scanning electron microscope. A broad visible band in the range 1.7-2.7 eV is observed in these nanostructures. Such emission is similar to that observed in bulk anatase TiO_2 and titanate powders, and is related to TiO_6 octahedra, which is a common feature to all the samples investigated. Near-infrared emission, sometimes attributed to Ti^(3+) interstitials, is observed in bulk powders but is absent in the titanate nanotubes and nanoribbons. Incorporation of Cr^(3+) between the titanate layers of the nanostructures is revealed by the characteristic intraionic emission line at 1.791 eV. Sodium titanate nanoribbons appear to be an effective host for optically active Cr^(3+) ions, as compared with nanotubes or bulk powder.Item Exchange bias in single-crystalline CuO nanowires(Applied physics Letters, 2010) Piqueras De Noriega, Francisco Javier; Díaz-Guerra Viejo, Carlos; Vila Santos, MaríaExchange anisotropy has been observed and investigated in single-crystalline CuO nanowires grown by thermal oxidation of Cu. The exchange bias field decreases by increasing temperature and can be tuned by the strength of the cooling field. A training effect has also been observed. The obtained results can be understood in terms of a phenomenological core-shell model, where the core of the CuO nanowire shows antiferromagnetic behavior and the surrounding shell behaves as a spin glass-like system due to uncompensated surface spins.- Project number: 126
Item Análisis de las actitudes de los estudiantes hacia el aprendizaje colaborativo y en línea(2019) Fernández Sánchez, Paloma; Díaz-Guerra Viejo, Carlos; López García, María Luisa; Urbieta Quiroga, Ana Irene; Nogales Díaz, Emilio; Arias Velasco, Marta; Romero Izquierdo, Carlos; Solís Fernández, Fernando; Sotillo Buzarra, BelénEl presente proyecto aborda dos aspectos muy importantes y estrechamente ligados a la adquisición de competencias básicas de cualquier titulación de grado actual y altamente demandadas por los empleadores: la adquisición de habilidades para desarrollar trabajo colaborativo y la capacidad de desarrollar actividades de aprendizaje en línea. Estas últimas tienen una doble función. Por una parte, deben preparar a los estudiantes para ser capaces de gestionar su trabajo, de manera autónoma, con vistas al posible desarrollo de trabajo no presencial, cada vez más importante en muchos entornos laborales. Por otra parte, deber permitirles adquirir las habilidades necesarias para continuar se formación a lo largo de su vida (LLL, Long Life Learning) a través de entornos más informales, pero no por ello menos efectivos, como puede ser el caso de los Massive Online Open Courses (MOOC), ofertados cada vez por más instituciones, con cursos de calidad, impartidos por grandes expertos y con posibilidades de certificación del trabajo realizado. Item Laser irradiation-induced α to δ phase transformation in Bi_2O_3 ceramics and nanowires.(Applied Physics Letters, 2012) Piqueras De Noriega, Francisco Javier; Vila Santos, María; Díaz-Guerra Viejo, CarlosThe α-Bi_2O_3 to δ-Bi_2O_3phase transformation has been locally induced by laser irradiation in ceramic samples and single-crystal nanowires of this oxide. The threshold power densities necessary to induce this transformation, as well as the corresponding transformation kinetics and its temporal stability, have been investigated in both kinds of samples by micro-Raman spectroscopy. The appearance of the δ phase was also monitored by spatially resolved photoluminescence spectroscopy. An emission band peaked near 1.67 eV, not observed in α-Bi_2O_3, is tentatively attributed to δ-Bi_2O_3 near band gap transitions.Item Synthesis and Cathodoluminescence of Undoped and Cr^3^+-Doped Sodium Titanate Nanotubes and Nanoribbons(Journal of Physical Chemistry C, 2010) Piqueras De Noriega, Francisco Javier; Díaz-Guerra Viejo, Carlos; Umek, Polona; Gloter, AlexandreWe report on the synthesis of Cr^3^+-doped sodium titanate nanotubes and nanoribbons by a hydrothermal method. The presence of dopant ions in these nanostructures was confirmed by high angle annular dark field scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements. Luminescence properties of undoped and Cr^3^+-doped sodium titanate nanotubes and nanoribbons were investigated by cathodoluminescence in the scanning electron microscope. A broad visible band in the range 1.7−2.7 eV is observed in these nanostructures. Such emission is similar to that observed in bulk anatase TiO_2 and titanate powders, and is related to TiO_6 octahedra, which is a common feature to all the samples investigated. Near-infrared emission, sometimes attributed to Ti^3^+ interstitials, is observed in bulk powders but is absent in the titanate nanotubes and nanoribbons. Incorporation of Cr^3^+ between the titanate layers of the nanostructures is revealed by the characteristic intraionic emission line at 1.791 eV. Sodium titanate nanoribbons appear to be an effective host for optically active Cr^3^+ ions, as compared with nanotubes or bulk powder.