Doping of ZnO inorganic-organic nanohybrids with metal elements.

We present a general and in-depth study of the effect of dopants in hybrid inorganic/organic ZnO/PAA (polyacrylic acid) nanocomposites. These dopants vary as much by their ionic size, as by their electronic valence and some of them have been used in ZnO due to their known magnetic and/or optical properties. The chemical nature of the dopants controls their ability to incorporate into ZnO crystal lattice. Three concentrations (0.1%, 1% and 5%) of dopants were studied in order to compare the effect of the concentration with the results obtained previously in the literature. Our results confirm in the first place the trend observed in the literature, that increase in dopant concentration leads to quenching of visible luminescence for ZnO nanocrystals obtained by very different processes. However, the degradation of photoluminescence quantum yield (PL QY) is not inevitable in our nanocomposites. At low doping concentration for some dopants with a small or comparable ionic radius than Zn2+, PL QY can be maintained or even improved, making it possible to tune the visible emission spectrum between 2.17 eV and 2.46 eV. This opens up the prospect of synthesizing phosphors without rare earth for white LEDs, whose spectrum can be tuned to render warm or cold white light, by a chemical synthesis process with a low environmental impact.

Tunable mid IR plasmon in GZO nanocrystals.

Degenerate metal oxide nanoparticles are promising systems to expand the significant achievements of plasmonics into the infrared (IR) range. Among the possible candidates, Ga-doped ZnO nanocrystals are particularly suited for mid IR, considering their wide range of possible doping levels and thus of plasmon tuning. In the present work, we report on the tunable mid IR plasmon induced in degenerate Ga-doped ZnO nanocrystals. The nanocrystals are produced by a plasma expansion and exhibit unprotected surfaces. Tuning the Ga concentration allows tuning the localized surface plasmon resonance. Moreover, the plasmon resonance is characterized by a large damping. By comparing the plasmon of nanocrystal assemblies to that of nanoparticles dispersed in an alumina matrix, we investigate the possible origins of such damping. We demonstrate that it partially results from the self-organization of the naked particles and also from intrinsic inhomogeneity of dopants.

Drop evaporation on superhydrophobic PTFE surfaces driven by contact line dynamics.

In the present study, we experimentally study the evaporation modes and kinetics of sessile drops of water on highly hydrophobic surfaces (contact angle ∼160°), heated to temperatures ranging between 40° and 70 °C. These surfaces were initially constructed by means of controlled tailoring of polytetrafluoroethylene (PTFE) substrates. The evaporation of droplets was observed to occur in three distinct phases, which were the same for the different substrate temperatures. The drops started to evaporate in the constant contact radius (CCR) mode, then switched to a more complex mode characterized by a set of stick-slip events accompanied by a decrease in contact angle, and finally shifted to a mixed mode in which the contact radius and contact angle decreased simultaneously until the drops had completely evaporated. It is shown that in the case of superhydrophobic surfaces, the energy barriers (per unit length) associated with the stick-slip motion of a drop ranges in the nJ m(-1) scale. Furthermore, analysis of the evaporation rates, determined from experimental data show that, even in the CCR mode, a linear relationship between V(2/3) and the evaporation time is verified. The values of the evaporation rate constants are found to be higher in the pinned contact line regime (the CCR mode) than in the moving contact line regime. This behavior is attributed to the drop's higher surface to volume ratio in the CCR mode.

YAG:Ce nanoparticle lightsources.

We investigate the luminescence properties of 10 nm yttrium aluminum garnet (YAG) nanoparticles doped with Ce ions at 0.2%, 4% and 13% that are designed as active probes for scanning near-field optical microscopy. They are produced by a physical method without any subsequent treatment, which is imposed by the desired application. The structural analysis reveals the amorphous nature of the particles, which we relate to some compositional defects as indicated by the elemental analysis. The optimum emission is obtained with a doping level of 4%. The emission of the YAG nanoparticles doped at 0.2% is strongly perturbed by the crystalline disorder whereas the 13% doped particles hardly exhibit any luminescence. In the latter case, the presence of Ce(4+) ions is confirmed, indicating that the Ce concentration is too high to be incorporated efficiently in YAG nanoparticles in the trivalent state. By a unique procedure combining cathodoluminescence and Rutherford backscattering spectrometry, we demonstrate that the enhancement of the particle luminescence yield is not proportional to the doping concentration, the emission enhancement being larger than the Ce concentration increase. Time-resolved photoluminescence reveals the presence of quenching centres likely related to the crystalline disorder as well as the presence of two distinct Ce ion populations. Eventually, nano-cathodoluminescence indicates that the emission and therefore the distribution of the doping Ce ions and of the defects are homogeneous.

InGaAs quantum dots grown by molecular beam epitaxy for light emission on Si substrates.

The aim of this study is to achieve homogeneous, high density and dislocation free InGaAs quantum dots grown by molecular beam epitaxy for light emission on silicon substrates. This work is part of a project which aims at overcoming the severe limitation suffered by silicon regarding its optoelectronic applications, especially efficient light emission device. For this study, one of the key points is to overcome the expected type II InGaAs/Si interface by inserting the InGaAs quantum dots inside a thin silicon quantum well in SiO2 fabricated on a SOI substrate. Confinement effects of the Si/SiO2 quantum well are expected to heighten the indirect silicon bandgap and then give rise to a type I interface with the InGaAs quantum dots. Band structure and optical properties are modeled within the tight binding approximation: direct energy bandgap is demonstrated in SiO2/Si/InAs/Si/SiO2 heterostructures for very thin Si layers and absorption coefficient is calculated. Thinned SOI substrates are successfully prepared using successive etching process resulting in a 2 nm-thick Si layer on top of silica. Another key point to get light emission from InGaAs quantum dots is to avoid any dislocations or defects in the quantum dots. We investigate the quantum dot size distribution, density and structural quality at different V/III beam equivalent pressure ratios, different growth temperatures and as a function of the amount of deposited material. This study was performed for InGaAs quantum dots grown on Si(001) substrates. The capping of InGaAs quantum dots by a silicon epilayer is performed in order to get efficient photoluminescence emission from quantum dots. Scanning transmission electronic microscopy images are used to study the structural quality of the quantum dots. Dislocation free In50Ga50As QDs are successfully obtained on a (001) silicon substrate. The analysis of QDs capped with silicon by Rutherford Backscattering Spectrometry in a channeling geometry is also presented.

Ion beam nanopatterning in graphite: characterization of single extended defects.

The morphology and the electronic structure of a single focused ion-beam-induced artificial extended defect is probed by several methods including micro-Raman spectroscopy, atomic force and scanning tunneling microscopies and Monte Carlo and/or semi-analytical simulation within standard codes. The efficiency of the artificial defect for deposited metallic cluster pinning is also investigated. We show a correlation between the ion dose, morphology, electronic structure and cluster trapping efficiency. At room temperature, cluster pinning is efficient when the displacement per atom is one or more. Well-ordered patterned cluster networks are considered for potential applications.