Photonic crystal patterning of luminescent sol-gel films for light extraction.

Structured luminescent thin films are investigated in the context of improved light extraction of phosphors for solid-state-lighting applications. Thin films composed of a sol-gel titania matrix doped with europium chelates are studied as a model system. These films, patterned with a square photonic lattice by soft nanoimprint lithography, are characterized by angle-resolved fluorescence. Modeling of this simple technique is shown to fit well the experimental data, revealing in great detail the guided modes of the film and their extraction parameters. An eightfold extraction enhancement factor of the film emission is measured. To further improve the extraction efficiency, we investigate the role of an additional low-index mesoporous silica underlayer through its influence on the guided modes of different polarizations and their interactions with the photonic crystal. Results obtained on model systems open the way towards the optimization of light-emitting devices, using a strategy of dielectric microstructure engineering using the sol-gel process.

Plasmon-induced modification of fluorescent thin film emission nearby gold nanoparticle monolayers.

When placed in the vicinity of metal nanoparticles, fluorophore molecules can have their fluorescence intensity enhanced. In order to engineer highly fluorescent thin films, surface plasmon enhancement fluorescence was studied on macroscopic systems composed of gold nanoparticles deposited on a substrate and coated by a dye-containing polymer film. We developed a simple method based on surface silanization to get a good dispersion of up to 100 nm gold nanoparticles on a substrate. While controlling the nanoparticle size and the fluorophore concentration, we measured the fluorescence enhancement factors of systems doped with dyes possessing different quantum yields. We evidenced experimentally that a fluorescence enhancement factor of 4 could be reached for a low-quantum yield dye and that the fluorophore quantum yield affects significantly the enhancement factor. We then discussed how our experimental results agree with previously developed models.

Thermal annealing treatment to achieve switchable and reversible oleophobicity on fabrics.

Surfaces that are strongly nonwetting to oil and other low surface tension liquids can be realized by trapping microscopic pockets of air within the asperities of a re-entrant texture and generating a solid-liquid-vapor composite interface. For low surface tension liquids such as hexadecane (gamma(lv) = 27.5 mN/m), this composite interface is metastable as a result of the low value of the equilibrium contact angle. Consequently, pressure perturbations can result in an irreversible transition of the metastable composite interface to the fully wetted interface. In this work, we use a simple dip-coating and thermal annealing procedure to tune the liquid wettability of commercially available polyester fabrics. A mixture of 10% 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) and 90% polyethyl methacrylate (PEMA) is used to uniformly coat the fabric surface topography. Contact angle measurements show that a robust metastable composite interface with high apparent contact angles can be supported for hexadecane (gamma(lv) = 27.5 mN/m) and dodecane (gamma(lv) = 25.3 mN/m). To tune the solid surface energy of the coated surface, we also developed a reversible treatment using thermal annealing of the surface in contact with either dry air or water. The tunability of the solid surface energy along with the inherent re-entrant texture of the polyester fabric result in reversibly switchable oleophobicity between a highly nonwetting state and a fully wetted state for low surface tension liquids such as hexadecane and dodecane. This tunability can be explained within a design parameter framework, which provides a quantitative criterion for the transition between the two states, as well as accurate predictions of the measured values of the apparent contact angle (theta*) for the dip-coated polyester fabrics.

A protected annealing strategy to enhanced light emission and photostability of YAG:Ce nanoparticle-based films.

A significant obstacle in the development of YAG:Ce nanoparticles as light converters in white LEDs and as biological labels is associated with the difficulty of finding preparative conditions that allow simultaneous control of structure, particle size and size distribution, while maintaining the optical properties of bulk samples. Preparation conditions frequently involve high-temperature treatments of precursors (up to 1400 °C), which result in increased particle size and aggregation, and lead to oxidation of Ce(iii) to Ce(iv). We report here a process that we term protected annealing, that allows the thermal treatment of preformed precursor particles at temperatures up to 1000 °C while preserving their small size and state of dispersion. In a first step, pristine nanoparticles are prepared by a glycothermal reaction, leading to a mixture of YAG and boehmite crystalline phases. The preformed nanoparticles are then dispersed in a porous silica. Annealing of the composite material at 1000 °C is followed by dissolution of the amorphous silica by hydrofluoric acid to recover the annealed particles as a colloidal dispersion. This simple process allows completion of YAG crystallization while preserving their small size. The redox state of Ce ions can be controlled through the annealing atmosphere. The obtained particles of YAG:Ce (60 ± 10 nm in size) can be dispersed as nearly transparent aqueous suspensions, with a luminescence quantum yield of 60%. Transparent YAG:Ce nanoparticle-based films of micron thickness can be deposited on glass substrates using aerosol spraying. Films formed from particles prepared by the protected annealing strategy display significantly improved photostability over particles that have not been subject to such annealing.