Elastic and inelastic strain in submicron-thick ZnO epilayers grown on r-sapphire substrates by metal-organic vapour phase deposition.

A significant part of the present and future of optoelectronic devices lies on thin multilayer heterostructures. Their optical properties depend strongly on strain, being essential to the knowledge of the stress level to optimize the growth process. Here the structural and microstructural characteristics of sub-micron a-ZnO epilayers (12 to 770 nm) grown on r-sapphire by metal-organic chemical vapour deposition are studied. Morphological and structural studies have been made using scanning electron microscopy and high-resolution X-ray diffraction. Plastic unit-cell distortion and corresponding strain have been determined as a function of film thickness. A critical thickness has been observed as separating the non-elastic/elastic states with an experimental value of 150-200 nm. This behaviour has been confirmed from ultraviolet photoelectron spectroscopy, X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy measurements. An equation that gives the balance of strains is proposed as an interesting method to experimentally determine this critical thickness. It is concluded that in the thinnest films an elongation of the Zn-O bond takes place and that the plastic strained ZnO films relax through nucleation of misfit dislocations, which is a consequence of three-dimensional surface morphology.

White light emission from lead-free mixed-cation doped Cs2SnCl6 nanocrystals.

We have designed a synthesis procedure to obtain Cs2SnCl6 nanocrystals (NCs) doped with metal ion(s) to emit visible light. Cs2SnCl6 NCs doped with Bi3+, Te4+ and Sb3+ ions emitted blue, yellow and red light, respectively. In addition, NCs simultaneously doped with Bi3+ and Te4+ ions were synthesized in a single run. Combination of both dopant ions together gives rise to the white emission. The photoluminescence quantum yields of the blue, yellow and white emissions are up to 26.5, 28, and 16.6%, respectively under excitation at 350, 390, and 370 nm. Pure white-light emission with CIE chromaticity coordinates of (0.32, 0.33) and (0.32, 0.32) at 340 and 370 nm excitation wavelength, respectively, was obtained. The as-prepared NCs were found to demonstrate a long-time stability, resistance to humidity, and an ability to be well-dispersed in polar solvents without property degradation due to their hydrophilicity, which could be of significant interest for wide application purposes.

Testing a Model for the Monitoring of Worked-out Algebra-Problem Examples: From Behaviours to Outcomes on a Math Task / La validación de un modelo para el control de la comprensión de problemas de álgebra resueltos: de los comportamientos a los resultados en una tarea de matemáticas

This study aimed at testing an extension of a theoretical model for the metacognitive monitoring mechanism implied in the detection of inconsistencies when the information provided includes abstract symbols in addition to plain text. Ninety-four postgraduates of STEM specialities were asked to read a worked-out algebra-problem example and to report any incoherence, inconsistency, or error detected in the statement or in the solving procedure. A set of model-inspired indexes was defined to describe participants’ behaviour along the task. The Read & Answer software was used to record online individual processing data and participants’ reports. Results supported model predictions. Indexes correctly predicted participants’ outcomes in the task with high accuracy. Specific students’ behaviours could be associated to observed task outcomes with sufficient reliability within the limitations of the study. In addition, algebra processing was compared with plain text processing.

Este estudio tiene como objetivo probar una extensión de un modelo teórico para el mecanismo de control metacognitivo que sirve para la detección de inconsistencias cuando la información proporcionada incluye símbolos abstractos además de texto natural. Se pidió a 94 postgraduados de especialidades STEM que leyeran un ejemplo resuelto de un problema de álgebra y que informaran sobre cualquier incoherencia, inconsistencia o error detectado en el enunciado o en el procedimiento de resolución. A partir de un modelo teórico se definió un conjunto de índices para describir el comportamiento de los participantes a lo largo de la tarea. Se utilizó el software Read & Answer para registrar online los datos de procesamiento individual y los informes de los participantes. Los resultados confirman las predicciones del modelo. Los índices predicen correctamente los resultados de los participantes en la tarea con gran precisión. Los comportamientos específicos de los alumnos podrían asociarse a los resultados observados de la tarea con suficiente confiabilidad dentro de las limitaciones del estudio. Además se ha comparado el procesamiento del álgebra con el procesamiento del texto natural.

Unravelling the Photocatalytic Behavior of All-Inorganic Mixed Halide Perovskites: The Role of Surface Chemical States.

Within the most mesmerizing materials in the world of optoelectronics, mixed halide perovskites (MHPs) have been distinguished because of the tunability of their optoelectronic properties, balancing both the light-harvesting efficiency and the charge extraction into highly efficient solar devices. This feature has drawn the attention of analogous hot topics as photocatalysis for carrying out more efficiently the degradation of organic compounds. However, the photo-oxidation ability of perovskite depends not only on its excellent light-harvesting properties but also on the surface chemical environment provided during its synthesis. Accordingly, we studied the role of surface chemical states of MHP-based nanocrystals (NCs) synthesized by hot-injection (H-I) and anion-exchange (A-E) approaches on their photocatalytic (PC) activity for the oxidation of ß-naphthol as a model system. We concluded that iodide vacancies are the main surface chemical states that facilitate the formation of superoxide ions, O2●-, which are responsible for the PC activity in A-E-MHP. Conversely, the PC performance of H-I-MHP is related to the appropriate balance between band gap and a highly oxidizing valence band. This work offers new insights on the surface properties of MHP related to their catalytic activity in photochemical applications.

Controlling the Phase Segregation in Mixed Halide Perovskites through Nanocrystal Size.

Mixed halide perovskites are one of the promising candidates in developing solar cells and light-emitting diodes (LEDs), among other applications, because of their tunable optical properties. Nonetheless, photoinduced phase segregation, by formation of segregated Br-rich and I-rich domains, limits the overall applicability. We tracked the phase segregation with increasing crystalline size of CsPbBr3-x I x and their photoluminescence under continuous-wave laser irradiation (405 nm, 10 mW cm-2) and observed the occurrence of the phase segregation from the threshold size of 46 ± 7 nm. These results have an outstanding agreement with the diffusion length (45.8 nm) calculated also experimentally from the emission lifetime and segregation rates. Furthermore, through Kelvin probe force microscopy, we confirmed the correlation between the phase segregation and the reversible halide ion migration among grain centers and boundaries. These results open a way to achieve segregation-free mixed halide perovskites and improve their performances in optoelectronic devices.

Study of the Partial Substitution of Pb by Sn in Cs-Pb-Sn-Br Nanocrystals Owing to Obtaining Stable Nanoparticles with Excellent Optical Properties.

Halide perovskites are revolutionizing the photovoltaic and optoelectronic fields with outstanding performances obtained in a remarkably short time. However, two major challenges remain: the long-term stability and the Pb content, due to its toxicity. Despite the great effort carried out to substitute the Pb by a less hazardous element, lead-free perovskite still remains more unstable than lead-containing perovskites and presents lower performance as well. In this work, we demonstrate the colloidal preparation of Cs-Pb-Sn-Br nanoparticles (NPs) where Sn is incorporated up to 18.8%. Significantly, we have demonstrated that the partial substitution of Pb by Sn does not produce a deleterious effect in their optical performance in terms of photoluminescence quantum yield (PLQY). We observed for the first time a positive effect in terms of enhancement of PLQY when Sn partially substitutes Pb in a considerable amount (i.e., higher than 5%). PLQYs as high as 73.4% have been obtained with a partial Pb replacement of 7% by Sn. We present a systematic study of the synthesis process in terms of different growth parameters (i.e., precursor concentration, time, and temperature of reaction) and how they influence the Sn incorporation and the PLQY. This high performance and long-term stability is based on a significant stabilization of Sn2+ in the NPs for several months, as determined by XPS analysis, and opens an interesting way to obtain less Pb-containing perovskite NPs with excellent optoelectronic properties.

Questioning and reading goals: information-seeking questions asked on scientific texts read under different task conditions.

BACKGROUND: A number of studies report that few questions are asked in classrooms and that many of them are shallow questions. AIMS: This study investigates the way in which reading goals determine questioning on scientific texts. Reading goals were manipulated through two different tasks: reading for understanding versus reading to solve a problem. SAMPLE: A total of 183 university students. METHODS: In the first and third questioning experiments, the participants read two short texts. Students in one condition were instructed to understand the texts, whereas in the alternative condition they had to read texts to solve a problem. Students were instructed to write down any questions they might have about the texts. The questions were categorized according to the type of underlying obstacle: associative, explanatory, or predictive. The second experiment used a think-aloud methodology to identify the mental representations generated by the students. RESULTS AND CONCLUSIONS: The two questioning experiments show that the questions asked depend on the reading goals. Significantly more explanation questions were asked in the understanding condition than in the problem-solving condition. Also, the two conditions were found to have a different influence on the generation of association and explanation questions. Very few prediction questions were asked in either condition. The think-aloud experiment revealed that the mental representations attempted by readers under the two conditions were indeed different. In conclusion, the experiments showed that, given a certain textual input, readers' questions depend on the reading goals associated with tasks.

Variables related to the informal caregivers' burden of dependent senior citizens in Spain.

The study aims at analyzing the psychosocial variables associated with the informal caregivers' burden of dependent older people. A sample of 296 dependent people and their informal caregivers (n=153) was randomly selected among users and non-users of the Spanish public In-Home Help Service (HHS) in an autonomous Spanish region (Comunidad Valenciana). Diverse variables in reference to the care context and the caregiver as well as the care recipient show major associations with the burden: those associated to disease and the social situation of the dependent person, the greater frequency and intensity of care, and the low frequency in which the caregiver receives help from others. The obtained data makes possible to establish guidelines based on the psychological and educational interventions which relieve the informal caregivers' burden of dependent senior citizens, which must be combined with respite services, in order to promote the permanence of this population group in the community environment.

The informal caregiver's burden of dependent people: theory and empirical review.

This paper reviews the main theories and results of the existing research to date about the concept of the informal caregiver's burden. The explanation of the burden concept, the theoretical approaches which attempt to explain it, the variables which have emerged in the investigation, the predictors of its appearance, as well as the intervention programs developed to relieve burden, allow us to approach the appropriate solutions to deal with the current social and political reality of this problem. In this sense, the psycho-educational intervention programs framed within the respite services jointly with the knowledge of the determining variables of the burden can comprise the first optimal approach in order to effectively deal with the burden problem of the informal caregivers of dependent senior citizens.

Formation and rupture of Schottky nanocontacts on ZnO nanocolumns.

In this paper, the electrical transport and mechanical properties of Pt/ZnO Schottky nanocontacts have been studied simultaneously during the formation and rupture of the nanocontacts. By combining multidimensional conducting scanning force spectroscopy with appropriated data processing, the physical relevant parameters (the ideality factor, the Schottky barrier height, and the rupture voltage) are obtained. It has been found that the transport curves strongly depend on the loading force. For loading forces higher than a threshold value, the transport characteristics are similar to those of large-area Schottky contact, while below this threshold deviations from strictly thermionic emission are detected. Above the threshold, stable and reproducible Pt/ZnO nanocontacts with ideality factors of about 2 and Schottky barrier heights of around 0.45 eV have been obtained.

Nanogoniometry with scanning force microscopy: a model study of CdTe thin films.

In this paper scanning force microscopy is combined with simple but powerful data processing to determine quantitatively, on a sub-micrometer scale, the orientation of surface facets present on crystalline materials. A high-quality scanning force topography image is used to determine an angular histogram of the surface normal at each image point. In addition to the known method for the assignment of Miller indices to the facets appearing on the surface, a quantitative analysis is presented that allows the characterization of the relative population and morphological quality of each of these facets. Two different CdTe thin films are used as model systems to probe the capabilities of this method, which enables further information to be obtained about the thermodynamic stability of particular crystallographic facets. The method, which is referred to as nanogoniometry, will be a powerful tool to study in detail the surface of crystalline materials, particularly thin films, with sub-micrometer resolution.

Suitability of the health and social care resources for persons requiring long-term care in Spain: an empirical approach.

The objective of this paper is to examine the suitability of specific facilities for dependent persons for meeting users' needs. A total of 1265 users of social and health facilities for dependent persons were interviewed in a study carried out in a typical southern European region with a Mediterranean welfare system: the Valencia Autonomous Region in Spain. Data were obtained on users' socio-demographic profile, health, functional dependence, cognition, social support and housing suitability. Based on these data and the institutional definitions of the specific facilities for dependent persons, the suitability index was drawn up for each facility and suitability was evaluated using discriminant analysis. The results give a high suitability index for most of the facilities (between 0.661 for Units for Home Hospitalisation and 1.000 for Units for Psychiatric Hospitalisation). But a significant percentage of patients (17% in Hospitals for Chronically Ill and Long-stay Patients) could be cared for in different facilities to the ones they actually use.

Polarity effects on ZnO films grown along the nonpolar [1120] direction.

The surface electrical properties of ZnO thin films grown along the nonpolar [1120] direction have been investigated by Kelvin probe microscopy on a nanometer scale. Two different charge domains, with a 75 meV work function difference, coexist within the ZnO surface, which is covered by rhombohedral pyramids whose sidewalls are shown to be {1011}-type planes. The presence and relative orientation of the two kinds of charge domains are explained in terms of the atomic arrangement at the {1011} polar surfaces.

Towards a new welfare state: the social sustainability principle and health care strategies.

In this paper we propose a social and health care model that offers alternatives to three problems arising in converging European welfare states, particularly in the southern nations: the rise in demand for services and features linked to the ageing process, the increase in dependency and the crisis of informal support. Development of the principles of social sustainability implies re-formulation of the regulatory, care, economic, administrative, cultural, and axiological framework enabling a response to the needs of long term care without compromising the welfare of future generations. Together with this principle, quality of life elevated to a subjective right directs attention towards the sphere closest to citizens, eliminating all barriers, which hamper exercise of this right. All of the above produces economic and social costs which must be accepted from a viewpoint of social co-responsibility, which brings with it the supply of welfare individually, without detriment to the exercise of state responsibility in guaranteeing a social protection system of a universal nature.