Person: Fernández Sánchez, Paloma
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First Name
Paloma
Last Name
Fernández Sánchez
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Física de Materiales
Area
Ciencia de los Materiales e Ingeniería Metalúrgica
Identifiers
114 results
Search Results
Now showing 1 - 10 of 114
Item Voids, nanochannels and formation of nanotubes with mobile Sn fillings in Sn doped ZnO nanorods(Nanotechnology, 2010) Ortega Villafuerte, Yanicet; Dieker, Ch; Jaeger, W.; Piqueras de Noriega, Javier; Fernández Sánchez, PalomaZnO nanorods containing different hollow structures have been grown by a thermal evaporation-deposition method with a mixture of ZnS and SnO(2) powders as precursor. Transmission electron microscopy shows rods with rows of voids as well as rods with empty channels along the growth axis. The presence of Sn nanoprecipitates associated with the empty regions indicates, in addition, that these are generated by diffusion processes during growth, probably due to an inhomogeneous distribution of Sn. The mechanism of forming voids and precipitates appears to be based on diffusion processes similar to the Kirkendall effect, which can lead to void formation at interfaces of bulk materials or in core-shell nanostructures. In some cases the nanorods are ZnO tubes partially filled with Sn that has been found to melt and expand by heating the nanotubes under the microscope electron beam. Such metal-semiconductor nanostructures have potential applications as thermal nanosensors or as electrical nanocomponents.Item Recombination processes in Te-doped ZnO microstructures(Physica Status Solidi B-Basic Solid State Physics, 2014) Iribarren, A.; Fernández Sánchez, Paloma; Piqueras de Noriega, JavierCathodoluminescence (CL) of Te doped elongated ZnO microstructures obtained by a vapour-solid (V-S) technique has been investigated. The CL intensity changes along the microstructures axis are related to the gradient of Te content, which influences the intensity of the ZnO deep-level (DL) emission band. The main defects are vacancy complexes which are partially passivated by the isoelectronic Te doping. The weight on the total luminescence of the radiative processes related to defects has been estimated. A straightforward method based on the measurement of the band-to-band luminescence intensity under constant excitation and experimental conditions has been used. The recombination lifetimes for transition through defect levels were also estimated. [GRAPHICS] SEM image and cathodoluminescence emission of needle- and pencil-like TeO2-doped ZnO microstructures.Item Luminescence and Raman study of Zn_4In_2O_7 nanobelts and plates(Superlattices and Microstructures, 2013) Alemán Llorente, Belén; Piqueras de Noriega, Javier; Fernández Sánchez, Paloma; García Martínez, José ÁngelLuminescence of Zn_4In_2O_7 nanobelts and microplates synthesized by a thermal evaporation-deposition method, has been investigated by photoluminescence (PL) and cathodoluminescence (CL) in the scanning electron microscope (SEM). Time resolved and temperature dependent CL measurements show that an emission at 2.37 eV appears to be characteristic of the Indium-Zinc-Oxide (IZO) compound investigated. PL and waveguiding behavior of the microplates is described Raman spectra of the structures as a function of In content has been studied.Item Study of structure and luminescence of CdSe Nanocrystals obtained by ball milling(Journal of Applied Physics, 2004) Urbieta Quiroga, Ana Irene; Fernández Sánchez, Paloma; Piqueras de Noriega, JavierThe effect of mechanical milling on the structure and luminescent properties of CdSe powder has been investigated by means of cathodoluminescence (CL) in the scanning electron microscope, transmission electron microscopy (TEM) and x-ray diffraction. The starting powders were ball milled for times between 5 and 20h. TEM of the milled samples reveals the presence of nanocrystals with a wide range of sizes including nanocrystals of 10 nm and smaller. The CL spectra of the untreated samples show the characteristic band edge emission centered at 1.8 eV. In the milled samples a band centered at about 2.5 eV is observed which appears to be associated to the presence of nanocrystals. The evolution of defect structure with milling time has been monitored by recording infrared cathodoluminescence spectra. A partial phase transformation from wurtzite to zinc blende structure also has been observed.Item In-depth structural and optical analysis of ce-modified zno nanopowders with enhanced photocatalytic activity prepared by microwave-assisted hydrothermal method(Catalysts, 2020) Bazta, Otman; Urbieta Quiroga, Ana Irene; Trasobares, Susana; Piqueras de Noriega, Javier; Fernández Sánchez, Paloma; Addou, Mohammed; Calvino, Jose Juan; Hungría, Ana BelénPure and Ce-modified ZnO nanosheet-like polycrystalline samples were successfully synthesized by a simple and fast microwave-based process and tested as photocatalytic materials in environmental remediation processes. In an attempt to clarify the actual relationships between functionality and atomic scale structure, an in-depth characterization study of these materials using a battery of complementary techniques was performed. X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-Ray spectroscopy-scanning transmission electron microscopy (STEM-XEDS), photoluminescence spectroscopy (PL) and UV-Visible absorption spectroscopy were used to evaluate the effect of Ce ions on the structural, morphological, optical and photocatalytic properties of the prepared ZnO nanostructures. The XRD results showed that the obtained photocatalysts were composed of hexagonal, wurtzite type crystallites in the 34-44 nm size range. The SEM and TEM showed nanosheet-shaped crystallites, a significant fraction of them in contact with bundles of randomly oriented and much smaller nanoparticles of a mixed cerium-zinc phase with a composition close to Ce_(0.68)Zn_(0.32)O_x. Importantly, in clear contrast to the prevailing proposals regarding this type of materials, the STEM-XEDS characterization of the photocatalyst samples revealed that Ce did not incorporate into the ZnO crystal lattice as a dopant but that a heterojunction formed between the ZnO nanosheets and the Ce-Zn mixed oxide phase nanoparticles instead. These two relevant compositional features could in fact be established thanks to the particular morphology obtained by the use of the microwave-assisted hydrothermal synthesis. The optical study revealed that in the ZnO:Ce samples optical band gap was found to decrease to 3.17 eV in the samples with the highest Ce content. It was also found that the ZnO:Ce (2 at.%) sample exhibited the highest photocatalytic activity for the degradation of methylene blue (MB), when compared to both the pure ZnO and commercial TiO_2-P25 under simulated sunlight irradiation. The kinetics of MB photodegradation in the presence of the different photocatalysts could be properly described using a Langmuir-Hinshelwood (LH) model, for which the ZnO:Ce (2 at.%) sample exhibited the highest value of effective kinetic constant.Item Cathodoluminescence study of laser recrystallized CdTe layers(Applied Physics Letters, 1997) Fernández Sánchez, Paloma; Piqueras de Noriega, Javier; Sochinskii, N. V.; Muñoz, V.; Bernardi, S.CdTe(100)/GaAs(100) and CdTe(lll)/CdTe(lll) layers grown by metalorganic vapor phase epitaxy (MOVPE) were investigated The layers were recrystallized to improve their morphology by scanning the surface with a 100 mu m diameter spot from an Ar ion laser beam (lambda=514.4 nm). Cathodoluminescence spectra from both as-grown and recrystallized CdTe MOVPE layers are used to monitor the effect of the recrystallization procedure. The laser recrystallization results in important changes on the spectral distribution of luminescence. Deep-level bands associated to different defects are shown to be very sensitive to the laser recrystallization procedure. The effect of the different substrates on the defect structure of the layers is also related to the changes observed in the cathodoluminescence spectra.Item LTA zeolite particles functionalized with nanomagnetite for effective recovery of dysprosium from liquid solutions(Microporous and Mesoporous Materials, 2024) Alcaraz, Lorena; Sotillo Buzarra, Belén; Iglesias, Carlos; López, Félix A.; Fernández Sánchez, Paloma; Belviso, Claudia; Urbieta Quiroga, Ana IreneRare earth elements (REEs) as Dysprosium (Dy) are critical elements for the fabrication of components in many green energy technologies, from electric vehicles to wind turbines. Consequently, there is an increasing interest in creating sustainable and effective materials for the recovery and recycling of these elements. Zeolite materials have demonstrated a high affinity and selectivity for REEs. Thus, this paper aims to study the use of a synthetic LTA zeolite functionalized with nanomagnetite for Dy absorption, including a complete characterization of the synthetic zeolite, the kinetics and the factors affecting the adsorption efficiency. The maximum adsorption capacity reaches a value around 35 mg of Dy per gram of zeolite. The results from the adsorption isotherms and kinetic study revealed a good agreement with both Langmuir and Temkin models and pseudo-second-order kinetics. Furthermore, the thermodynamic analysis suggests that the adsorption of Dy onto the zeolite is a spontaneous and favorable process. The findings from this work could provide insights into the design and optimization of zeolite-based processes for REE recovery and recycling, contributing to the development of a more sustainable and circular economy.Item Effect of ion beam milling on the defect structure of CdTe(Semiconductor Science and Technology, 1996) Panin, G. N.; Fernández Sánchez, Paloma; Piqueras de Noriega, JavierThe effect of ion milling on the defect structure of CdTe crystals has been investigated in the scanning electron microscope by cathodoluminescence. Enhancement in the luminescence intensity is observed after ion treatment. Luminescence spectra of treated and untreated zones of the samples indicate that ion milling causes generation of tellurium vacancies and filling of cadmium vacancies in a subsurface layer. In addition, enhancement of the concentration of cadmium vacancy related defects in the region extending up to 20 mu m from the layer is revealed. This effect is discussed in connection with models of p- to n-type conversion of CdTe during ion milling.Item Study of electronic deep levels in CdTe and CdTe:V by cathodoluminescence microscopy(Electron Microscopy 1994, Vols 2a and 2b: Applications in Materials Sciences, 1994) Pal, U.; Piqueras de Noriega, Javier; Fernández Sánchez, Paloma; Serrano, M. D.; Diéguez, E.Item ZnO nanoparticles with controllable Ce content for efficient photocatalytic degradation of MB synthesized by the Polyol method(Catalysts, 2021) Flores Carrasco, Gregorio; Rodríguez Peña, Micaela; Urbieta Quiroga, Ana Irene; Fernández Sánchez, Paloma; Rabanal, María EugeniaThis paper reports on the synthesis of Ce-doped ZnO (CZO) nanoparticles (NPs) by an alternative polyol method at low temperature. The method, facile and rapid, uses acetate-based precursors, ethylene glycol as solvent, and polyvinylpyrrolidone as capping agent. The effects of the Ce-doping concentration (ranging from 0 to 8.24 atomic%) on the structural, morphological, compositional, optical, luminescence, and photocatalytic properties of the NPs were investigated by several techniques. The structural findings confirmed that the CZO NPs have a typical hexagonal wurtzite-type structure with a preferred orientation along the (101) plane. The results obtained by Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM) revealed that the NPs size decreased (from similar to 30 to similar to 16 nm) with an increase in the Ce-doping concentration. Energy Dispersive X-Ray Spectroscopy (EDS) and High Resolution Transmission Microscopy (HRTEM) results confirmed the incorporation of Ce ions into the ZnO lattice. Ce-doping influences the photoluminescence (PL) emission compared to that of pure ZnO. The PL emission is related to the presence of different kinds of defects, which could take part in charge transfer and/or trapping mechanisms, hence playing an essential role in the photocatalytic activity (PCA). In fact, in this work we report an enhancement of PCA as a consequence of Ce-doping. In this sense, the best results were obtained for samples doped with 3.24 atomic%, that exhibited a photocatalytic degradation efficiency close to 99% after 60 min ultraviolet (UV) illumination, thus confirming the viability of Ce-doping for environmental applications.