Manipulating the ferroelectric polarization state of BaTiO 3 thin films

International audienceControlling the ferroelectric polarization at macroscopic or microscopic levels is crucial in the framework of the development of ferroelectric materials used in yet challenging photo-electrochemical (PEC) cells and spintronic applications. We report here on polarization methods allowing to electrically polarize prototypical samples of BaTiO 3 (001) films. Epitaxial single crystalline layers were grown up to a thickness of 25 nm by atomic oxygen assisted molecular beam epitaxy on 1 at.% Nb doped SrTiO 3 (001) single crystals. The samples were both microscopically and macroscopically polarized using Piezoresponse Force Microscopy and electrochemical poling in an electrolyte respectively. In addition we demonstrate the possibility to retrieve a quasi-native mixed ferroelectric polarization state after annealing. These polarization methods may be applied to many other ferroelectric thin films

Local electronic structure and photoelectrochemical activity of partial chemically etched Ti-doped hematite

International audienceThe direct conversion of solar light into chemical energy or fuel through photoelectrochemical water splitting is promising as a clean hydrogen production solution. Ti-doped hematite (Ti:α-Fe 2 O 3) is a potential key photoanode material, which despite its optimal band gap, excellent chemical stability, abundance, non-toxicity and low cost, still has to be improved. Here we give evidence of a drastic improvement of the water splitting performances of Ti-doped hematite photoanodes upon a HCl wet-etching. In addition to the topography investigation by atomic force microscopy, a detailed determination of the local electronic structure has been carried out in order to understand the phenomenon and to provide new insights in the understanding of solar water splitting. Using synchrotron radiation based spectromicroscopy (X-PEEM), we investigated the X-ray absorption spectral features at the L 3 Fe edge of the as grown surface and of the wet-etched surface on the very same sample thanks to patterning. We show that HCl wet etching leads to substantial surface modifications of the oxide layer including increased roughness and chemical reduction (presence of Fe 2+) without changing the band gap. We demonstrate that these changes are profitable and correlated to the drastic changes of the photocatalytic activity

Determination of the cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron spectroscopy

International audienceThe properties of CoFe2O4/BaTiO3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe2O4 layers grown on BaTiO3, with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor

Profiles of learner readers and their early literacy skills and environmental predictors: a large-scale longitudinal study from preschool to grade 1

As part of the French national cohort ELFE, a two-year longitudinal study was conducted and children’s literacy skills were assessed twice (N = 2,767), namely at the end of preschool (t1) and at the end of Grade 1 (t2). At t1, letter-name knowledge (LNK), phonological skills (PhoSk) and vocabulary were assessed and at t2, phonemic segmentation, word reading, listening and reading comprehension. Latent profile analyses conducted at t2 resulted in five profiles, ranging from good readers (48.8%), to average readers above the mean (16.2%), readers with very low phonological awareness skill (8.2%), poor readers with low reading comprehension skill (19.4%), and poor readers with general difficulties (7.4%). Two early literacy skills (LNK and PhoSk) and two environmental variables (parents’ educational level and household income) constituted good predictors of membership of a profile. However, home literacy practices also predicted membership of the good reader/poor reader profiles, albeit at a lower level

Évaluation des connaissances précoces prédictives de l’apprentissage de la lecture en grande section de maternelle

Le premier objectif de ce travail, mené auprès de plus de 3000 élèves de grande section de maternelle, était de valider une batterie de tests évaluant les compétences prédictives de l’acquisition de la lecture dans ses deux composantes, identification de mots écrits et compréhension. Pour cela, les qualités métrologiques des épreuves expérimentales en reconnaissance de lettres, en habiletés phonologiques, en vocabulaire et en compréhension orale ont fait l’objet d’un examen. Le second objectif était d’identifier des profils d’élèves et les difficultés des plus faibles (par l’analyse des erreurs). La conclusion souligne l’intérêt des sessions précoces d’évaluation afin de prévenir les difficultés avec des aides ciblées à l’apprentissage de la lecture en CP.The first goal of this study, conducted among more than 3000 kindergarten children, was to validate a series of tests which assess predictive skills necessary to master reading in two main components, word recognition and language comprehension. For that purpose, measurements from experimental tasks in letter recognition, phonological skills, vocabulary and comprehension were examined. The second aim was to specify pupils’ profiles and the difficulties of the poorest pupils (through error analysis). The conclusion emphasized the benefit of early assessment sessions, in order to prevent difficulties with targeted support for learning to read

Effet d’entraînements bimodaux à la connaissance des lettres. Étude transversale chez des enfants de trois et cinq ans

International audienceThe aim was to test the effect of two-mode trainings on letter knowledge, pseudowords spelling and reading with 3-year- and 5-year-old children. A classical pretest/training/post-design was used. Trainings differed on the letter sensory exploration (auditory/visual [V], auditory/ haptic [H] and auditory/graphomotor [G]). Results indicated that performance on letter-sound knowledge and spelling was globally better with H training with 3-year old. Five-year-old children improved in all tasks after all trainings. In reading, G group obtained better performance. These results suggest that tactilo-kinesthesic informations in H and G forms contribute to the elaboration of visual and phonological representations of the letter.L’objectif est d’évaluer l’effet d’entraînements bimodaux sur la connaissance des lettres, l’écriture et la lecture des pseudomots auprès des enfants de trois et cinq ans. Un paradigme classique prétest – entraînement – post-test est utilisé. Les entraînements diffèrent sur l’exploration sensorielle des lettres (auditif/visuel [V] ; auditif/haptique [H] et auditif/graphomoteur [G]). Les résultats indiquent que la connaissance des sons et l’écriture des lettres des enfants de trois ans sont tendanciellement meilleures avec l’entraînement H. Les enfants de cinq ans progrèssent dans toutes les tâches. En lecture, le groupe G obtient de meilleures performances. L’ensemble de ces résultats suggère que les informations tactilo-kinesthésiques sous formats H et G contribuent à la construction des représentations visuelle et phonologique de la lettre

Effet d'une exploration "multi-sensorielle séquentielle orientée" sur le développement de la compréhension du principe alphabétique chez les enfants de 5 ans faibles connaisseurs de lettres

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Effet d'entraînements bi-modaux à la connaissance des lettres : approche développementale

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Effet du feedback visuel sur les principaux indices dynamiques du geste graphique et les habiletés de transcription chez les enfants de 5 ans

Effect of visual feedback on main dynamic cues in handwriting and transcription skills with kindergarten children . Word writing requires alphabetic code acquisition and motor gesture mastery. The aim of this study is to evaluate with 5 y. o. children (1) the effect of the visual feedback in a motor experience of letters on letter highlighting; (2) the effect of motor processes development on pseudo-word transcription. A pre-test / training (with or without visual feedback) / post-tests design is used. The main dynamic indicators that reflect the motor processes are identified and analysed.L’écriture de mots nécessite l’acquisition du code alphabétique et la maîtrise du geste graphique. L’objectif de cette étude est d’évaluer avec des enfants de 5 ans : (1) l’effet du feedback visuel dans l’exploration motrice des lettres sur le tracé de lettres ; (2) l’effet du développement des processus moteurs sur la transcription de pseudo-mots. Un design pré-test / entraînement (avec ou sans feedback visuel) / post-tests est utilisé. Les principaux indices dynamiques qui reflètent les processus moteurs sont identifiés et analysés

Effet d'entraînements bi-modaux à la connaissance des lettres : approche développementale

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