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Oxides and derivatives materials

Pigments and UV absorbers

Stéphane Jobic (DR), Martine Bujoli-Doeuff (MC), Mayte Caldes (CR), Philippe Deniard (DR), Camille Latouche (MCF).

A pigment absorbs light in the visible range (400 nm <λ <800 nm), a UV absorber beyond (λ <400 nm). Both are the seat of electronic transitions which, depending on their positioning in energy and their intensity, will give rise to colored materials with varying degrees of coloring power, their colorimetric characteristics being obviously strongly affected by their granulometry, morphology and Surrounding environment.
In addition to the synthesis of new materials, which is the first priority of our research activity, determining the nature of these transitions is essential in order to be able to model them and thus to reasonably synthesize the material with the desired properties. To date, we are always looking for new pigments for the replacement of materials based on heavy elements (e.g. lead, mercury) or proven toxicity (e.g., antimony, arsenic). In this context, the synthesis of inorganic materials having a red coloring similar to Ferrari red is a Grail.

P-type conductive transparent materials

Stéphane Jobic (DR), Laurent Cario(DR) (Equipe PMN), Martine Bujoli-Doeuff (MC), Congcong Shang.

A transparent conducting material (TCM) is a solid that does not absorb visible light (optical gap greater than 3 eV) and exhibits a good electrical conductivity. Such materials are used in flat panel displays, low emissivity and electrochromic glasses, antistatic shields, defrosting windows, light emitting diodes, etc. At the laboratory, our field of interest concerns more specifically the synthesis of TCM as nanoparticles for applications in p-type semiconductor (SC) based dye-sensitized solar cells (DSSCs), i.e. cells where holes are injected from the photo-excited dye into the valence band (VB) of the p-type SC (inverse principle to that of the Grätzel cells). In that context, we are looking for materials with large specific surfaces (to promote the adsorption of dyes and therefore high photo-generated currents) and a deep valence band (to increase the open circuit voltage, Voc).

Thermochromism and Tribochromism

Stéphane Jobic (DR), Hélène Serier-Brault (MC), Philippe Deniard (DR), Emmanuel Fritsch (Pr), Martine Bujoli-Doeuff (MC), Laurence Ourry (Postdoc).

An X-chromic material is a material which color changes under the action of an external stimulus such as temperature, pressure or light excitation. Thermochomism and tribochromism (or piezochromism) often go hand in hand. If the temperature is raised up, the material will expand, if the material undergoes a pressure, it will contract. In both cases, the two stimuli will modify the interatomic distances and thus trigger a reorganization of the electronic cloud within the material. Then, a continuous color change is expected in relation with a regular modification of the electronic transitions in energy. Temperature and pressure can also lead to a so-called first order transition with the appearance of a new structural type for a given value of the stimulus. The color change can then be abrupt. Namely, our goal is to understand the origin of the color in the synthesized materials at the laboratory and to predict as much as possible the color characteristics of solid materials in a given temperature or pressure range. Thermochromic effects may also be observed in composites not described herein.

Gem materials

Emmanuel FRITSCH (PR-IMN), Benjamin RONDEAU (MC-LPGN) Olivier SEGURA (PhD student), Aurélien DELAUNAY (PhD student)

Gem materials are known for their luster (diamond), color (ruby, emerald) and high clarity. In general, we strive to understand the properties of these exceptional optical materials, with high added value, and to find the difference between the authentic and the various shades of "fake". We have a keen interest in the origin of the color, which often carries the value, and may be "enhanced" in the laboratory. There are even some gem materials that are naturally thermochromic (chameleon diamonds) or photochromic (sodalite variety hackmanite). Luminescence is sensitive to very low concentrations of defects. It helps detect laboratory treatments sometimes very close to natural processes, especially for colorless diamond. Optical spectroscopies and observation, both non destructive, are our favourite investigation methods.

Luminescent materials

S. Jobic (DR), P. Deniard (DR), E. Faulques (DR), H. Brault (MC), C. Latouche (MC), R. Gautier (CR), F. Massuyeau (IE), Romain Génois (Student).

A luminescent material emits light after its excitation by any source other than heat (e.g. UV radiation, electron beam, pressure, etc). In that context, we can distinguish fluorescent materials with lifetimes of the excited state ranging approximately between 10-9 and 10-3 s and materials with long persistent luminescence (also called phosphorescence) that have the propensity to emit light for several minutes or hours after cessation of the excitation. In both cases, we are interested in the characterization of the physical phenomenon, the determination of its origin and its adaptability to the target application. In a non-exhaustive manner, the materials synthesized at the laboratory are oxides, silicates, aluminosilicates or sulphides, in the form of micrometric powders or nanoparticles. Besides luminescence in the visible, we focus more and more on materials with emission in the infra-red domain.

Materials for photocatalysis

Stéphane Jobic (DR), Xavier Rocquefelte (MCF, then Pr. à l'Univ. de Rennes 1 since Sept. 2014)

Our expertise in the field of materials with photoinduced properties (luminescence, photovoltaic, photochromes) naturally led us to have a look on semiconductors that can exhibit photocatalytic properties. Our objective was to define the ingredients necessary to obtain a semiconductor with good performance for photocatalysis in the visible field. To this end, we first studied the properties of excitons (electron-hole pairs) via a Bethe Salpeter approach for two allotropic varieties of BiVO4, the scheelite variety presenting a photocatalytic activity much greater than the dreyerite one. We then developed an approach to determine ab initio the energy positions, on an absolute scale, of the valence band and conduction band to predict the capability of a solid material to oxidize or reduce specific species. Indeed, to be a good candidate for photocatalysis in the visible domain, a solid material must absorb the visible light, but also have an appropriate position in energy of its valence and conduction bands. This activity goes hand in hand with the development of new plasmon photocatalysts described in the section "Photoactivable hybrid materials".

Zinc oxide

Stéphane Jobic (DR), Laurent Cario(DR) (Equipe PMN), Martine Bujoli-Doeuff (MC), Romain Gautier (CR), Camille Latouche (MC), Philippe Deniard (DR), Eric Faulques (DR), Congcong Shang (Postdoctorant, 2014-2017).

Undoped or unintentionally doped zinc oxide is naturally a n-type semiconductor that is widely used in the industry for its pigmental, photocatalytic, piezoelectric, antibacterial and varistor properties. Moreover ZnO exhibits a strong emission in the near ultraviolet (UV) at room temperature. This material therefore has a real interest for devices such as light-emitting diodes and laser diodes emitting in the UV region. Unfortunately, this very promising luminescence is not exploited so far due to the lack of p-type zinc oxide (p-ZnO) necessary for the setting-up of transparent p-n homojunctions.

Magnetic materials

1. Low-dimensional magnetism, frustration and quantum effects

Catherine Guillot-Deudon (IR), Christophe Payen (PR), Mélanie Viaud (IE)

Some crystalline materials have unusual magnetic properties because they do not exhibit a magnetic transition although the strength of the magnetic interactions and the size of the crystallites favor the magnetic order at low temperature. The magnetic state observed at low-temperature can be a "spin liquid" due to the low dimension of the magnetic network, to frustration, or to quantum effects. In this context, we study materials that contain transition metals distributed in the crystal structure on a particular network (spin chains or networks made of triangles, for example). Spin liquids are of considerable interest in condensed matter physics and offer perspectives in the field of quantum information.

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