Spatial and Morphological Control of Cyanine Dye Thin Films

Cyanine dyes are organic semiconductor compounds with light absorption and emission properties useful for emerging technologies such as solar cells and light-emitting devices. The characteristics of these materials in the solid state depend on their organization of the constituting building blocks. This thesis focuses on controlling the morphology of cyanine dye thin films at different length scales and clarifying the resulting properties. When microstructures present features whose size matches visible light wavelengths, new properties may arise from light-matter interactions. Here the properties resulting from the light-matter interactions of cyanine droplet films cast from solution are studied. Based on experimental evidence, it is shown that dye droplet ensembles scatter light with different efficiencies and wavelength ranges depending on their dimensions. FDTD simulations are used to show that this effect results fromscattering enhancement at the absorption edge of the dye where the refractive index varies considerably. Simulations also provide a better understanding of individual dropletsâ interaction with light. While earlier work had hypothesized that the observed scattering phenomenon were due to crystalline clusters within the droplets, this work highlights the contribution of the dye filmmorphology. Cyanines also form single crystals whose fabrication induces molecular-scale order in the material. Previous work demonstrated that thin single crystals could be grown by solvent vapor annealing of dye droplets. Here it is shown that in uncontrolled conditions, cyanine single crystals destabilize to formdendritic crystals. In-situ microscopy observations highlight the solute reservoir role of the droplet distribution surrounding a growing crystal; when the distance between droplets and the crystal front is large, the solute supply is diffusion-limited. Moreover variations in local pressure equilibrium between the droplets and crystal front lead to advection fluxes which perturb the crystal growth. These observations help design configurations to either prevent crystal destabilization or take advantage of the dendritic growth in a controlled manner. In addition, the patterning of crystals on a substrate is relevant for their application in devices. A practical challenge is to induce single crystal growth at specific locations. Here, surfaces patterned with SAMs of hydrophilic and hydrophobic thiols are used to create dye droplet arrays from which crystals can be grown. This method is shown to yield local crystallization of the dye and to prevent crystal destabilization through better control of the droplet distribution. By varying the dye solution concentration, partial control over crystal density is achieved, however it proved difficult to control the number of nuclei per droplet. A more controlled evaporation and solvent vapor annealing system might be necessary to master the nucleation process. Finally the structure and optical properties of cyanine single crystals are addressed. The crystal structure was determined by X-ray diffraction. Structural aspects are shown to lead to excitonic couplings, which are evidenced by orientation-dependent spectroscopic measurements of single crystals. Although further investigation of the absorption band structure is necessary, the results are promising for photovoltaic devices as they might improve exciton transport compared to amorphous layers

Le réel dans le texte

Leclaire. Serge. Le réel dans le texte. In: Littérature, n°3, 1971. Littérature. Octobre 1971. pp. 30-32

« Psy-show » : un certain regard sur l'inconscient (entretien de Serge Leclaire par Y. Geoffroy)

Leclaire Serge, Geffroy Yannick. « Psy-show » : un certain regard sur l'inconscient (entretien de Serge Leclaire par Y. Geoffroy). In: Bulletin de psychologie, tome 43 n°395, 1990. Audiovisuel et psychologie clinique. pp. 418-425

Les descendants d’immigrés à l’école

Les débats sur les origines des inégalités scolaires entre les élèves descendants d’immigrés et les « natifs » restent vifs. Pour certains, les politiques de démocratisation de l’enseignement secondaire, telles qu’elles se sont développées dans les pays européens dès le milieu du XXe siècle, ont suffi à éliminer la spécificité des destins scolaires des descendants d’immigrés. Dans cette perspective, les inégalités résiduelles ne seraient que la résultante des conditions sociales d’existence de ces élèves, souvent plus défavorisés au plan économique et culturel. Pour d’autres, ces interprétations en termes de discontinuité culturelle ne suffisent plus à rendre compte de la réalité dès lors que, selon les indicateurs choisis et les données mobilisées, le handicap des descendants d’immigrés persiste à milieu social égal. L’origine des inégalités serait alors à rechercher du côté du système éducatif lui-même, de la nature de l’offre de formation adressée aux différents types d’élèves, de la ségrégation entre établissements scolaires. Ces discriminations systémiques relèveraient alors plus des politiques scolaires elles-mêmes que de la distance culturelle et sociale entre familles et école. Ce dossier est une mise en perspective de ce débat au travers de quatre contributions basées sur des données comparatives nationales et internationales. Elles proposent des analyses originales des destins scolaires des élèves issus de l’immigration pour identifier les origines sociales et scolaires des inégalités de réussite. The debate about the roots of educational inequality between immigrant and native students is still going strong. Some believe that the secondary education democratisation policies that have been implemented in European countries since the mid-20th century have done enough to remove the specific features of immigrant students’ academic futures. From this point of view, any remaining inequality is merely due to these students’ living conditions as they often come from economically and culturally disadvantaged backgrounds. Others believe that this view, in terms of cultural discontinuity, is no longer appropriate given the reality as, based on selected indicators and data, educational inequality towards immigrants remains high, even when social background is controlled. The roots of inequality go back to the education system itself, to the curriculum available for different types of students and to segregation between schools. This systemic discrimination is therefore linked to school policies rather than to social and cultural distance between families and school. The four articles in this issue shall shed light on this debate using national and international comparative data. Immigrant students’ academic paths are analysed to identify the social and educational roots of achievement inequality

Interroger l’efficacité des pratiques d’enseignement de la lecture-écriture au cours préparatoire

Qu’est-ce qui fait l’efficacité d’un enseignement du lire-écrire au début de l’école obligatoire (le cours préparatoire ou CP) ? Quelles sont les pratiques favorables aux élèves les plus fragiles ? Les dix contributions à ce numéro de Repères rendent compte de résultats issus de la recherche L’influence des pratiques d’enseignement de la lecture et de l’écriture sur la qualité des apprentissages au cours préparatoire, soutenue par l’IFÉ, la DGESCO du ministère de l’Éducation nationale et plusieurs laboratoires de recherche. Dans le cadre de cette étude, 131 classes françaises de CP ont été observées pendant une année scolaire et leurs 2 507 élèves évalués à trois reprises. Les données ont été analysées avec des méthodes statistiques et qualitatives. Les contributions réunies ici abordent la production des textes, la compréhension en lecture, l’acculturation des élèves à l’univers de l’écrit, les effets des pratiques d’encodage, l’étude de la langue. Elles cherchent notamment à éclairer ce qui caractérise les pratiques enseignantes lorsque les élèves d’origine sociale défavorisée progressent plus qu’ailleurs. Chaque article possède sa propre cohérence, mais la lecture croisée, à laquelle ce numéro invite, permet d’éviter de tirer trop vite d’un résultat une prescription pour l’action. Car si les résultats présentés, expliquant les performances des élèves, leurs progrès et les différences observées d’une classe à l’autre, peuvent inspirer des préconisations, celles-ci doivent embrasser la complexité des facteurs mis en évidence : il existe plusieurs configurations qui conduisent aux mêmes apprentissages. What brings about the effectiveness of the teaching of reading and writing at the beginning of compulsory primary schooling? And which practices favour weaker pupils? This edition of the academic review, “Repères”, puts together or collates ten research papers. These papers are resultant from a research conducted by IFÉ, DGESCO, the Ministry of Education and several other research laboratories on the theme: "The influence of teaching practices of reading and writing on the quality of learning in the primary school". As part of this study, 131 French primary school classes were observed during a particular school year out of which 2507 pupils were evaluated on three different occasions. The data were analyzed using statistical and qualitative methods. The contributions gathered here in this review deal with topics such as: writing tasks, reading comprehension, the pupils’ acculturation to the world of writing, the effects of encoding practices, and the study of language. These articles seek in particular to shed light on what is characteristic of teaching practices when pupils from deprived/disadvantaged social backgrounds begin to progress. Each article is also in itself coherent, but the cross-reading / comparative reading to which this review encourages makes it possible to avoid drawing too quickly conclusions to be enforced as prescriptive measures. This is because if the results presented, explaining the students' performances, their progress and the differences observed from one class to another, can inspire recommendations, then they must embrace the complexity of the factors highlighted which include the several configurations that lead to the same learning