R. Leon

Mesostructure design with Gemini surfactants : supercage formation in a three-dimensional hexagonal array

At low temperatures, liquid crystal-like arrays made up of inorganic-cluster and organic molecular units readily undergo reversible lyotropic transformations. Gemini surfactants, with two quaternary ammonium head groups separated by a methylene chain of variable length and with each head group attached to a hydrophobic tail, can be used to control organic charge sitting relative to the bivariable hydrophobic tail configurations. This approach has led to the synthesis of a mesophase (SBA-2) that has three-dimensional hexagonal (P63/mmc) symmetry, regular supercages that can be dimensionally tailored, and a large inner surface area. This mesostructure analog of a zeolite cage structure does not appear to have a lyotropic surfactant or lipid liquid crystal mesophase counterpart. Through the modification of gemini charge separation and each of the two organic tails, these syntheses can be used to optimize templating effects, including the synthesis of MCM-48 at room temperature.

Spatially resolved visible luminescence of self-assembled semiconductor quantum dots.

Ensembles of defect-free InAIAs islands of ultrasmall dimensions embedded in AIGaAs have been grown by molecular beam epitaxy. Cathodoluminescence was used to directly image the spatial distribution of the quantum dots by mapping their luminescence and to spectrally resolve very sharp peaks from small groups of dots, thus providing experimental verification for the discrete density of states in a zero-dimensional quantum structure. Visible luminescence is produced by different nominal compositions of InxAI(1–x)As-AIyGa(1–y)As.

Temperature effects on the radiative recombination in self-assembled quantum dots

Abstract Several ensembles of self-assembled quantum dots (QDs) based on the AlInAsAl/GaAs and InGaAs/GaAs material systems have been investigated using photoluminescence (PL), PL excitation (PLE) and time-resolved PL (TRPL). The influence of the temperature is measured by monitoring sharp spectral features (as narrow as ∼ 90 μeV) obtained when probing the PL of small QD ensembles (few hundreds QDs). Thermionic emission of the photocarriers out of the QD potential is found to be the dominant mechanism leading to the thermal quenching of the PL and temperature-independent linewidths are observed up to the onset of the PL quenching.

Surfactant controlled preparation of mesostructured transition-metal oxide compounds

The use of surfactants as templates allows the synthesis of mesostructured transition-metal oxides by a cooperative, interface controlled condensation process.

Surfactant based synthesis of oxidic catalysts and catalyst supports

The preparation of mesostructured metal oxides based on the surfactant controlledsynthesis of MCM-41 is presented. It can be expected that the new metal oxide materials will exhibit a high surface area of nearly 1000 m 2 /g and could have a possible use as catalysts, especially in partial oxidation reactions and as hosts for quantum sized material, if the surfactant can be removed without destroying the structure.