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The time evolution of the stimulated emission in ZnSe/Zn(S,Se) superlattices has been studied by picosecond photoluminescence spectroscopy. ?The threshold of the stimulated emission is characterised by a sharp decrease of the transition lifetime from 150 ps to less than 30 ps at a carrier density of about 10(12) cm(-2). While the experimental results are consistent with a stimulated emission process due to excitons, they show no evidence of electron-hole plasma recombination.

Highly stretchable polyvinyl alcohol (PVA) films with a strain at break of around 700% were obtained from solutions in trifluoroacetic acid (TFA). Structural and chemical analysis by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (ATR-FTIR) showed that TFA is retained by PVA via hydrogen bonds between the carboxylic acid groups and the hydroxyl groups of the polymers causing a strong plasticizing effect. Additionally, composites of PVA with cellulose could be developed using TFA as common solvent. The morphological and mechanical properties of the polymer composites could be accurately tuned by modifying the relative concentrations of the two polymers. Data from water adsorption isotherms and wetting measurements indicated that the presence of trifluoromethyl groups in PVA render the composite films relatively hydrophobic. © 2015 Elsevier B.V.

A method for enhancing the surface wettability of PDMS microchannels used in microfluidic devices, is presented. Colloidal TiO2 nanorods are mixed in a PDMS solution that was used subsequently to realize parallel microchannels, through a replica molding procedure. TiO2 has the intrinsic capability of increasing dramatically its hydrophilicity upon UV irradiation. Due to this property, incorporating TiO2 nanofillers into the PDMS microchannels we manage to induce their hydrophobic-to-hydrophilic conversion upon UV irradiation. This conversion is essential for water to fill the microfluidic channels, in contrast to what happens for the non-irradiated nanocomposite or pure PDMS channels, which are characterized by high hydrophobicity and do not allow water to penetrate. The reversibility of the wettability changes permits to the microchannels to recover their original hydrophobicity.

This chapter illustrates the relationships between structural and optical properties in oligothiophenes. In particular, it highlights the proper functionalization of the thienyl units with oxygen atoms; it is possible to increase the photoluminescence efficiency up to values comparable and even better than those of the best organic materials commonly used as active compounds in light emitting devices. Such a characteristic, together with the high chemical stability and the wide color tunability, makes oligothiophene-5,5'-dioxides excellent candidates not only for applications to electronic devices, but also to optoelectronic devices, such as multi-color LEDs and lasers. Recent achievement of high-performances thiophene-based devices is discussed.

Self-assembly of molecular units into complex and functional superstructures is ubiquitous in biology. The number of superstructures realized by self-assembly of man-made nanoscale units is also growing. However, assemblies of colloidal inorganic nanocrystals 1‐3 are still at an elementary level, not only because of the simplicity of the shape of the nanocrystal building blocks and their interactions, but also because of the poor control over these parameters in the fabrication of more elaborate nanocrystals. Here, we show how monodisperse colloidal octapod-shaped nanocrystals self-assemble, in a suitable solution environment, on two sequential levels. First, linear chains of interlocked octapods are formed, and subsequently the chains spontaneously self-assemble into threedimensional superstructures. Remarkably, all the instructions for the hierarchical self-assembly are encoded in the octapod shape. The mechanical strength of these superstructures is improvedbyweldingtheconstituentnanocrystalstogether. The organization of colloidal nanocrystals into ordered structures is a necessary step towards the fabrication of artificial solids and new devices. Superstructures can be built either by self-assembly directly in solution, or on a substrate following solvent evaporation or de-wetting 4 6 . A variety of forces can be involved in their formation: van der Waals (vdW) attractions between the particles, steric repulsions between the hydrophobic tails of the surfactants (often coating the nanocrystal surface), capillary forces during solvent evaporation, attractive depletion forces, Coulomb forces between surface charges or electric dipoles, and magnetic forces 1,3,5,7 12 . The assembly of many ordered threedimensional (3D) superstructures, for example, simple, binary, or ternary assemblies of spherical nanoparticles 13 17 , and smectic-like multilayers of hexagonally packed nanorods 18 , as well as liquid crystalline phases, is found to be solely driven by entropy 19 21 . More elaborate assemblies could be achieved from such simple building blocks by encoding information for the self-assembly in the surface pattern of the nanoparticles, for instance by DNA functionalization to modify the strength and directionality of particleparticle

We report on the design of highly reflecting two-dimensional photonic crystal (2D-PC) mirrors, which can be integrated in a short InGaAs QWs laser source, operating around 970 nm. These mirrors consist of a triangular lattice of air holes into the GaAs slab, whose geometrical parameters have been calculated by the effective index method and 2D plane wave expansion method. A 2D finite-difference time-domain (2D-FDTD) model has also been used in order to evaluate the transmission and reflection characteristics of 2D-PC front and back mirrors of the in-plane laser. Finally, a detailed description of the nanotechnology processes necessary for the fabrication of ultra-compact in-plane laser is shown.

Band-gap renormalization due to the presence of a dense electron-hole plasma in GaAs quantum wells is studied by time-resolved luminescence and magnetoluminescence spectroscopy. We show that heterostructures with long carrier lifetimes exhibit saturation of the band-gap renormalization in a magnetic field, in accord with previous results. However, the usual carrier-density-dependent gap shrinkage in a magnetic field is observed in multiple quantum wells with short carrier lifetimes. The strong dependence of the magneto-optical properties on the carrier lifetimes indicates that two different phases can be formed in the plasma at high magnetic field: a highly correlated phase, analogous to the condensed electronic or excitonic state at the equilibrium density, in quantum wells with sufficiently long carrier lifetimes; and a free-carrier gas in heterostructures with short lifetimes. In the former state, the band gap is found to renormalize by an amount equal to twice the exciton binding energy, independent of the actual photogeneration rate. On the other hand, condensation processes are prevented in the short-lived phase, thus resulting in the expected density-dependent band-gap renormalization.