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
50 results
Search Results
Now showing 1 - 10 of 50
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 Improved silicon surface passivation by hybrid composites formed by PEDOT:PSS with anatase TiO_2 nanoparticles(Materials letters, 2020) Vázquez López, Antonio; Yaseen, Anisa; Maestre Varea, David; Ramírez Castellanos, Julio; Karazhanov, Smagul Zh; Marstein, Erik S; Cremades Rodríguez, Ana IsabelIn this work, hybrid composites formed by PEDOT:PSS (poly(3,4-ethylene dioxythiophene):-polystyrene sulfonate) and anatase TiO_2 nanoparticles synthesized by hydrolysis were spin-coated on Si substrates in order to evaluate their potential implementation in Si-hybrid solar cells, based on their passivation behaviour. X-ray diffraction (XRD), photoluminescence (PL), Raman spectroscopy, quasi-steady state photoconductance (QSS-PC) based on PL imaging, and Hall effect measurements were carried out for the structural, optical and electrical characterization. Enhanced carrier lifetime values of tau similar to 0.5 ms were obtained by adding a controlled amount of TiO_2 nanoparticles (0.5% wt.) to the PEDOT:PSS matrix.Item Cathodoluminescence of defects in sintered tin oxide(Journal of Applied Physics, 2004) Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras De Noriega, Francisco JavierCathodoluminescence (CL) in the scanning electron microscope (SEM) has been used to investigate the luminescence mechanisms in tin oxide. Sintered material prepared from high purity powder has been found to show a strong dependence of the CL emission on the thermal treatments applied during sample preparation. SEM images show the presence of nano and microcrystalline grains. The correlation of the grain size and morphology with the optical emission is analyzed by CL microscopy and spectroscopy. The main emission bands appear centered at about 2.58, 2.25, and 1.94 eV depending on the sintering treatment. CL images reveal that the 2.25 and the 2.58 eV bands are associated at specific crystal faces. The evolution of the luminescence bands with mechanical milling shows a complex evolution of the 1.94 and 2.58 eV emissions which is explained by formation and recovery of defects during milling.Item Temperature-dependent photoluminescence of anatase Li-doped TiO_(2) nanoparticles(Optical materials express, 2022) Vázquez López, Antonio; Cremades Rodríguez, Ana Isabel; Maestre Varea, DavidTiO_(2) is currently one of the most employed material in photocatalysis and optoelectronic applications. By doping with different elements, tailored luminescent and optoelectronic properties can be obtained, which further enhances its applicability. The achievement of deeper knowledge and control on the recombination processes via light-metal doping engineering promises a wider use in optoelectronic applications. In this work, the luminescent properties of undoped and Li-doped anatase TiO_(2) nanoparticles obtained by hydrolysis process are studied, by means of photoluminescence (PL) measurements from 10 K to room-temperature. TiO_(2) presents a wide emission which covers the visible range, while Li-doping quenches the emission on the low visible region. By terms of time-resolved luminescence (TR-PL), lifetime of the recombination processes can be obtained, in which a decrease on lifetime values can be observed for the doped samples.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.Item Formation and luminescence of nanoterraces and elongated structures in sintered TiO2(Nanotechnology, 2006) Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras De Noriega, Francisco JavierSamples of rutile TiO2 have been prepared by sintering compacted powders under argon flow. Long (above 20 h) sintering times at 1500 degrees C led to the formation of rods with squared cross-sections in a broad range of sizes. A two-step annealing treatment, at two temperatures, was found to favour the growth of low-dimensional elongated structures as well as a terraced structure on the grain surface, producing samples with high surface to volume ratio. The cathodoluminescence (CL) spectrum of the initial powder, shows an emission centred at about 2.40 eV, which can be separated into three Gaussian bands at 2.19, 2.30 and 2.55 eV. The main features of the CL spectra of sintered samples are an infrared band at 1.52 eV and a complex band in the visible range, whose peak position shifts with the annealing temperature. In samples sintered for 30 h an emission at 1.80 eV appears, while the dominant emission at 1.52 eV, due to titanium interstitials, is quenched.Item Fabrication and study of self-assembled NiO surface networks assisted by Sn doping(Journal of alloys and compounds C, 2020) Taeño González, María; Maestre Varea, David; Cremades Rodríguez, Ana IsabelComplex patterning of surfaces commonly requires a significant effort in terms of time, cost, and advanced post-fabrication treatments design, which could be overcome by self-organization mechanisms avoiding damage to the surface materials and enhancing properties of interest. In the present work, grid self-assembled NiO complex surfaces have been fabricated assisted by Sn incorporation during growth, following thermal treatments at 1400 degrees C under a controlled Ar flow. The singular morphologies achieved enable the fabrication of robust high surface to volume ratio surfaces with a wide range of surface properties and functionalities. Based on the research performed by a set of complementary techniques, the mechanisms involved in the formation of these textured surfaces have been discussed and some of the fundamental electronic and optical properties of NiO have been analyzed, both aspects necessary to head up the potential development of applications based on this p-type material which is arousing growing attention. These singular micro- and nanostructures present high luminescence and tunable performance unusual to bulk NiO samples, which can broaden the applicability of this material to light-emitting devices. Moreover, the surface evolution depends on a controllable way on the preferential state of charge of the incorporated Sn, which could be selected through the convenient Sn-based precursor, its ratio to the metallic Ni starting precursor, and the atmosphere and duration of the fabrication treatment. Finally, in order to assess the dependence of some potential NiO-based applications on the insights achieved on the surface characterization, the gas-sensing response to ethanol from the Sn doped NiO samples was also evaluated. The study of the processes and mechanisms involved in the growth of these grid-patterned surfaces can be extended to similar oxide-based systems.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.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 Study of the morphology and luminescence of SnO_2 micro- and nanostructures synthesized by a two step thermal process(Journal of Crystal Growth, 2009) Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras De Noriega, Francisco JavierA study of the variation of structural and optical properties of different SnO2 micro- and nanostructures grown by thermal treatments including a post-growth annealing under argon flow has been carried out. The samples have been characterized by atomic force microscopy (AFM) and cathodoluminescence (CL) in the scanning electron microscope (SEM). It has been found that a post-growth annealing under argon flow at high temperature modifies the appearance of the initial structures as well as their optical properties. In this way, micro- and nanoislands terraced features or microtubes with smooth surfaces have been obtained as a function of the crystallographic orientation of the initial structure used as a substrate. Some other effects such as the increase of the 1.94eV CL emission, related to oxygen vacancies, which involves an electrical conductivity increase, are achieved by the annealing treatment.