Examinando por Autor "Cremades, Ana"

Mostrando 1 - 5 de 5

Ethanol gas sensing mechanisms of p-type NiO at room temperature
(Elsevier, 2022-03-30) Bartolomé, Javier; Taeño, María; Martínez-Casado, Ruth; Maestre, David; Cremades, Ana
Conductometric 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 p- type 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.
Grain selective Cu oxidation and anomalous shift of graphene 2D Raman peak in the graphene–Cu system
(IOPscience, 2018-12-04) Bartolomé, Javier; Álvarez-Fraga, Leo; Aguilar-Pujol, Montserrat X.; Cortijo, Sandra; Cremades, Ana; Prieto, Carlos; de Andrés, Alicia
Understanding the interaction between graphene and its supporting substrate is of paramount importance for the development of graphene based applications. In this work the interplay of the technologically relevant graphene–Cu system is investigated in detail as a function of substrate grain orientation in Cu polycrystalline foils. While (1 0 0) and (1 1 1) Cu grains show the well-known graphene-enhanced oxidation, (1 1 0) grains present a superior oxidation resistance compared to uncovered Cu and an anomalous shift of its graphene 2D Raman band which cannot be explained by the known effects of strain and doping. These results are interpreted in terms of a weak graphene–Cu coupling at the (1 1 0) grains, and show that graphene can actually be used as anticorrosion coating, contrary to previously reported. The anomalous shift is suggested to be the result of an enhanced outer Raman scattering process which surpasses the usually dominant inner process. Since Raman spectroscopy is widely used as first and main characterization tool of graphene, the existence of an anomalous shift on its 2D band not only challenges the current theory of Raman scattering in graphene, but also has profound implications from an experimental point of view.
Influence of Cation Substitution on the Complex Structure andLuminescent Properties of the ZnkIn2Ok+3System
(ACS Publications, 2020-06-25) García-Fernández, Javier; Torres-Pardo, Almudena; Bartolomé, Javier; Martínez-Casado, Ruth; Zhang, Qing; Ramírez-Castellanos, Julio; Terasaki, Osamu; Cremades, Ana; González-Calbet, Jose María
The effect of In3+ substitution by Ga3+ or Al3+ on the structure and luminescent properties of Zn7In2–xMxO10 (M = Ga or Al; 0 ≤ x ≤ 1) oxides has been investigated by means of high spatial resolution X-ray spectroscopy and high-angle annular dark-field images, combined with magic angle spinning nuclear magnetic resonance spectroscopy. Local structural variations have been identified for the Al- and Ga-doped samples through the analysis of atomically resolved chemical maps and the identification of their structural environment within the wurtzite lattice. In3+ is distributed in a zig-zag modulation, while Al3+ and Ga3+ are located in a flat distribution at the center of the wurtzite block. Density functional theory calculations provide unambiguous evidence for the preferential flat location of Ga3+ and Al3+ associated with the different strains introduced in the structure as a result of their ionic radii. The characterization of the photoluminescence response reveals the appearance of new radiative recombination pathways for the doped materials because of the presence of new defect levels in the band gap of the Zn7In2O10 structure.
Li2SnO3 branched nano- and microstructures with intense and broadband white-light emission
(Springer, 2018-11-14) García-Tecedor, Miguel; Bartolomé, Javier; Maestre, David; Trampert, Achim; Cremades, Ana
Exploiting the synergy between microstructure, morphology and dimensions by suitable nanomaterial engineering, can effectively upgrade the physical properties and material performances. Li2SnO3 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 °C using metallic Sn and Li2CO3 as precursors. The achievement of these new morphologies can face challenging applications for Li2SnO3, 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 °C correspond to monoclinic Li2SnO3, while at 900 °C complex Li2SnO3/SnO2 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 Li2SnO3. 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 Li2SnO3 structures.
Self-Organized NiO Microcavity Arrays Fabricated by Thermal Treatments
(ACS Publications, 2020-05-05) Taeño, María; Bartolomé, Javier; Gregoratti, Luca; Modrzynski, Pawel; Maestre, David; Cremades, Ana
The potential use of NiO in low-dimensional devices requires the upgrade of appropriate synthesis methods as well as the achievement of a deeper comprehension of the growth mechanisms and the properties of this p-type oxide at the micro- and nanoscale. In this work, arrays of NiO microcavities with potential use in catalysis, sensing, and high temperature templates have been achieved following a single step process based on the use a controlled argon flow during oxidation of metallic Ni. Fabrication of these self-organized cavities can widen the applicability of NiO in new fields of research avoiding large cost post-fabrication treatments and enhancing NiO properties of interest. Formation of these hole arrays at 1000–1200 °C with unique geometrical morphology and preferential (111) texturing has been discussed based on Ni oxidation mechanisms involving Ni lattice diffusion and strain release phenomena at the NiO surface. Moreover, NiO samples have been fabricated in the range of temperatures 800–1500 °C and characterized by a complete group of techniques in order to shed light on physical aspects related to NiO which still remain unambiguous, such as the bandgap or the understanding of the Raman signal. XRD measurements confirm the presence of strain related phenomena and lattice distortions during thermal treatments. Besides, cathodoluminescence analysis shows a wide signal from near-IR to UV commonly dominated by an emission at 2.5 eV related to Ni deficiency. An increase in the Ni3+/Ni2+ ratio together with a higher p-type character, as demonstrated by surface-sensitive X-ray photoelectron spectroscopy, was promoted in the regions with microcavities as well as in the samples sintered at high temperature.