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
4 results
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Item The controlled transition-metal doping of SnO_2 nanoparticles with tunable luminescence(CrystEngComm, 2014) Peche Herrero, M. A.; Maestre Varea, David; Ramirez Castellanos, J.; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, Javier; González Calvet, J. M.SnO_2 nanoparticles doped with transition metals (V, Cr, Mn) have been synthesized by both the hydrothermal method (HDT) in a basic media and the liquid mixed method (LQM) based on the Pechini method. Nanocrystalline particles obtained via a liquid mixed technique show a well-defined chemical composition and an average size of 6 nm, with a high degree of both crystallinity and chemical homogeneity. Nanoparticles prepared via a hydrothermal method exhibit a high dispersion in size as well as agglomeration effects. As the LQM demonstrates advantages with respect to the HDT, a more detailed investigation has been carried out on the SnO_2 nanoparticles doped with V, Cr and Mn grown by this method. The microstructure of the materials was elucidated by means of X-ray Diffraction (XRD), Selected-Area Electron Diffraction (SAED), and High-Resolution Transmission Electron Microscopy (HRTEM). Luminescence from undoped and doped SnO_2 nanoparticles was characterized by cathodoluminescence (CL). The luminescence studies demonstrate a strong dependence of CL signals with transition metal doping, thus inducing red, green or orange emissions when doping with Cr, V or Mn respectively.Item Growth and luminescence of N doped TiO_2 nanowires(Journal of Neuroscience and Nanotechnology, 2008) Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Gregoratti, Luca; Piqueras de Noriega, JavierTitanium oxide nanowires have been grown by thermal treatment of pressed TiN powder under argon or N-2 flow. It has been found that two-step treatments at two different temperatures, 400 degrees C and 800 degrees C, lead to the growth of nanowires all over the sample surface. The nanowires are of single crystalline rutile structure. Energy dispersive X-ray spectroscopy and photoelectron spectroscopy measurements show that the oxide nanostructures contain N from the starting nitride. The mechanism of N incorporation into the oxide and its possible effect on the luminescence are discussed.Item Cr doped titania microtubes and microrods synthesized by a vapor-solid method(CrystEngComm, 2013) Cristian Vasquez, G.; Andrea Peche-Herrero, M.; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Ramirez-Castellanos, Julio; Gonzalez-Calbet, José M.; Piqueras de Noriega, JavierCr doped TiO_2 rutile nanoparticles have been used as precursor of microrods and microtubes grown by a vapor-solid method. The grown microstructures have nearly square cross-sections of a few microns wide and lengths of up to about 100 microns. By longer thermal treatments or higher growth temperatures, the ratio of microrods to microtubes increases. The presence of partially filled openings with growth steps in the internal faces of the tubes indicates that the tubes transform into rods by extended or intense thermal treatments, which enables to control the nature of the microstructures, tubes or rods, by varying the parameters of the thermal treatment. Cr incorporation has been found to be homogeneous along the growth axis, with amounts in the range from 1.2 to 2.8% of cationic fraction, which depend on the Cr content in the precursor and on the growth parameters. Optical activation of the Cr ions has been demonstrated by cathodoluminescence in the scanning electron microscopy, and crystallographic assessment of the structures has been carried out using Raman spectroscopy and electron backscattered diffraction.Item Effect of thermal and mechanical treatments on the cathodoluminescence of tin and titanium oxides(Spatially resolved characterization of local phenomena in materials and nanostructures, 2003) Maestre Varea, David; Plugaru, R; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, JavierThe luminescence of titanium oxide and tin oxide has been investigated by cathodoluminescence in the SEM, as a function of the structural changes induced by thermal treatments. The evolution of the luminescence of TiO2 rutile, anatase and mixture phase with the annealing temperature is related to the process of thermal induced grain texture and to transition of metastables phases to the stable rutile. The emission band is peaked at 480 nm for the composite phase and at 580 nm for anatase and rutile phases respectively. A broad band in the 520-560 nm region is present in the spectra of both anatase and rutile phase. Thermal annealing leads to a red shift of the luminescence emission, with the emission band peaked at around 820 nm. In sintered tin oxide the main emission bands appear centered at about 480 nm and 630 nm. The intensity of these bands increases with annealing temperature up to 1200degreesC, whereas for samples annealed at 1500degreesC these emissions are quenched. Mechanical ball milling has been used to produce nanocrystalline SnO_2 grains to investigate the influence of the presence of nanocrystals on the CL emission.