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
10 results
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Now showing 1 - 10 of 10
Item Two-dimensional Zn_k In_2O_(k+3) nanostructures: synthesis, growth mechanism, self-assembly, and luminescence(Journal of Nanoparticle Research, 2013) Bartolomé Vílchez, Javier; Maestre Varea, David; Amati, Mateo; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, JavierIndium-zinc oxide nanostructures, such as nanosheets, nanobelts, and wires formed by oriented stacks of nanoplates have been grown by a controlled thermal evaporation method without the use of a foreign catalyst. Surface features in the stacked hexagonal nanoplates suggest a dislocation-driven growth mechanism for these structures. A growth model for these stacks is proposed based on changes in velocity growth rate between the outer and the inner part of the plates. Zn incorporation has been investigated by means of energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, and selected area electron diffraction. The formation of Zn_k In_2O_(k+3) ternary compounds has been demonstrated. Cathodoluminescence emission and its correlation with the morphology of the structures and Zn content have been studied.Item Indium sulfide and ternary In-S-O nanowires for optoelectronic applications(Microscopy and Microanalysis, 2012) Bartolomé Vílchez, Javier; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, JavierItem Synthesis of In_2S_3 and In_6S_7 microcolumns and nanowires by a vapor-solid method(Physica status solidi A-Applications and materials science, 2018) Bartolomé Vílchez, Javier; Maestre Varea, David; Cremades Rodríguez, Ana IsabelIndium sulfide (In_2S_3) is a promising candidate for the replacement of CdS buffer layers in solar cell devices, while hexaindium heptasulfide (In_6S_7) presents interesting properties for its use as absorber material. In this work the fabrication of In_2S_3 microcolumns as well as novel In_6S_7 nanowires with diameters of about 70-120nm is reported. The structures are grown following a thermal evaporation-deposition method at temperatures between 900 and 1000º C. Control of the phase and morphology of the structures is achieved through both the evaporation and deposition temperatures, which can be tuned separately. Energy dispersive spectroscopy shows no traces of residual oxygen, while X-ray photoelectron spectroscopy indicates the presence of small amounts of oxygen incorporated at the surface of the structures. The In_6S_7 nanowires are found to be degenerated n-type semiconductors, with the Fermi level above the conduction band minimum. The origin of this n-type degeneracy is discussed in terms of S vacancies.Item Composition-dependent electronic properties of indium-zinc-oxide elongated microstructures(Acta Materialia, 2013) Bartolomé Vílchez, Javier; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Amatti, M.; Piqueras de Noriega, JavierMicrorods and hierarchical structures of indium-zinc-oxide (IZO) with different compositions were grown by thermal treatments of mixtures of InN and ZnO powders. In long rods, an increase in Zn content along the growth axis is revealed by energy dispersive spectroscopy. The structures obtained range from Zn-doped indium oxide with a few atomic per cent of Zn, to IZO compounds of the type Zn_kIn_2O_k+3. X-ray photoelectron spectroscopy measurements with spatial resolution show that IZO microstructures degenerate at room temperature, with carrier concentration of the order of 10^20 cm^-3. Electron accumulation has been found for undoped (1 0 0) and (1 1 1) surfaces, whereas depletion of carriers at the surface is observed in IZO samples. The Fermi level position correlates with the Zn concentration at the surface which, taking into account the surface dependence of the ionization potentials, work functions and band gaps, could lead to tunable material properties for device applications. Cathodoluminescence emission intensity is enhanced by the presence of Zn, which induces spectral changes and broadening of the emission band compared with undoped material. The results are discussed in terms of the charge neutrality level and the band structure of the material.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, 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 Ethanol gas sensing mechanisms of p-type NiO at room temperature(Applied Surface Science, 2022) Bartolomé Vílchez, Javier; Taeño González, María; Martínez Casado, Ruth; Maestre Varea, David; Cremades Rodríguez, Ana IsabelConductometric gas sensors based on metal oxide semiconductors (MOS) usually require high temperature operation, increasing their energy consumption and limiting their applicability. However, room temperature operation with these devices still remains a challenge in many sensor-analyte systems due in part to the low or null response and recovery speeds obtained at this temperature. In this work, the conductometric response of ptype NiO ceramic samples to ethanol is studied under room temperature operation. An anomalous response consisting in an unexpected resistance decrease upon ethanol exposure is observed depending on sample texturing, which is tuned by changing the temperature at which the samples are synthesized. This anomalous response is characterized by fast response and recovery times. A model based on two competing mechanisms, consisting in either an electron transfer from NiO to the ethanol molecule or the catalytic decomposition of adsorbed ethanol, is proposed to explain the observed anomalous response. Extending this model to other MOS could pave the way for fast sensors operating at room temperature.Item In-situ scanning electron microscopy and atomic force microscopy Young's modulus determination of indium oxide microrods for micromechanical resonator applications(Applied Physics Letters, 2014) Bartolomé Vílchez, Javier; Hidalgo Alcalde, Pedro; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, JavierElectric field induced mechanical resonances of In2O3 microrods are studied by in-situ measurements in the chamber of a scanning electron microscope. Young's moduli of rods with different cross-sectional shapes are calculated from the resonance frequency, and a range of values between 131 and 152GPa are obtained. A quality factor of 1180-3780 is measured from the amplitude-frequency curves, revealing the suitability of In2O3 microrods as micromechanical resonators. The Young's modulus, E, of one of the rods is also measured from the elastic response in the force-displacement curve recorded in an atomic force microscope. E values obtained by in-situ scanning electron microscopy and by atomic force microscopy are found to differ in about 8%. The results provide data on Young's modulus of In2O3 and confirm the suitability of in-situ scanning electron microscopy mechanical resonance measurements to investigate the elastic behavior of semiconductor microrods.Item Li_2SnO_3 branched nano- and microstructures with intense and broadband white-light emission(Nano research, 2019) García Tecedor, Miguel; Bartolomé Vílchez, Javier; Maestre Varea, David; Cremades Rodríguez, Ana Isabel; Trampert, A.Exploiting the synergy between microstructure, morphology and dimensions by suitable nanomaterial engineering, can effectively upgrade the physical properties and material performances. Li_2SnO_3 elongated nano-and microstructures in form of belts, wires, rods and branched structures have been fabricated by a vapor-solid method at temperatures ranging from 700 to 900 degrees C using metallic Sn and Li_2CO_3 as precursors. The achievement of these new morphologies can face challenging applications for Li_2SnO_3, not only in the field of energy storage, but also as building blocks in optoelectronic devices. The micro-and nanostructures grown at 700 and 800 degrees C correspond to monoclinic Li2SnO3, while at 900 degrees C complex Li_2SnO_3/SnO_2 core-shell microstructures are grown, as confirmed by X-ray diffraction and Raman spectroscopy. Transmission electron microscopy reveals structural disorder related to stacking faults in some of the branched structures, which is associated with the presence of the low-temperature phase of Li_2SnO_3. The luminescent response of these structures is dominated by intense emissions at 2, 2.5 and 3 eV, almost completely covering the whole range of the visible light spectrum. As a result, white-light emission is obtained without the need of phosphors or complex quantum well heterostructures. Enhanced functionality in applications such as in light-emitting devices could be exploited based on the high luminescence intensity observed in some of the analysed Li_2SnO_3 structures.Item High-performance room-temperature conductometric Gas sensors: materials and strategies(Chemosensors, 2022) Vázquez López, Antonio; Bartolomé Vílchez, Javier; Cremades Rodríguez, Ana Isabel; Maestre Varea, DavidChemiresistive sensors have gained increasing interest in recent years due to the necessity of low-cost, effective, high-performance gas sensors to detect volatile organic compounds (VOC) and other harmful pollutants. While most of the gas sensing technologies rely on the use of high operation temperatures, which increase usage cost and decrease efficiency due to high power consumption, a particular subset of gas sensors can operate at room temperature (RT). Current approaches are aimed at the development of high-sensitivity and multiple-selectivity room-temperature sensors, where substantial research efforts have been conducted. However, fewer studies presents the specific mechanism of action on why those particular materials can work at room temperature and how to both enhance and optimize their RT performance. Herein, we present strategies to achieve RT gas sensing for various materials, such as metals and metal oxides (MOs), as well as some of the most promising candidates, such as polymers and hybrid composites. Finally, the future promising outlook on this technology is discussedItem Tubular micro- and nanostructures of TCO materials grown by a vapor-solid method(AIMS Materials Science, 2016) García Tecedor, Miguel; Prado Hurtado, Félix del; Bueno, Carlos; Vásquez, G. Cristian; Bartolomé Vílchez, Javier; Maestre Varea, David; Díaz, Tomás; Cremades Rodríguez, Ana Isabel; Piqueras de Noriega, JavierMicrotubes and rods with nanopipes of transparent conductive oxides (TCO), such as SnO_2, TiO_2, ZnO and In_2O_3, have been fabricated following a vapor-solid method which avoids the use of catalyst or templates. The morphology of the as-grown tubular structures varies as a function of the precursor powder and the parameters employed during the thermal treatments carried out under a controlled argon flow. These materials have been also doped with different elements of technological interest (Cr, Er, Li, Zn, Sn). Energy Dispersive X-ray Spectroscopy (EDS) measurements show that the concentration of the dopants achieved by the vapor-solid method ranges from 0.5 to _3 at.%. Luminescence of the tubes has been analyzed, with special attention paid to the influence of the dopants on their optical properties. In this work, we summarize and discuss some of the processes involved not only in the anisotropic growth of these hollow micro and nanostructures, but also in their doping.