THE recent synthesis of silica-based mesoporous materials by the cooperative assembly of periodic inorganic and surfactant-based structures has attracted great interest because it extends the range of molecular-sieve materials into the very-large-pore regime. If the synthetic approach can be generalized to transition-metal oxide mesostructures, the resulting nanocomposite materials might find applications in electrochromic or solid-electrolyte devices, as high-surface-area redox catalysts and as substrates for biochemical separations. We have proposed recently6 that the matching of charge density at the surfactant/inorganic interfaces governs the assembly process; such co-organization of organic and inorganic phases is thought to be a key aspect of biomineralization. Here we report a generalized approach to the synthesis of periodic mesophases of metal oxides and cationic or anionic surfactants under a range of pH conditions. We suggest that the assembly process is controlled by electrostatic complementarity between the inorganic ions in solution, the charged surfactant head groups and—when these charges both have the same sign—inorganic counterions. We identify a number of different general strategies for obtaining a variety of ordered composite materials

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 (P6(3)/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.

Large energy shifts in the luminescence emission from strained InGaAs quantum dots are observed as a result of postgrowth annealing and also when raising the upper cladding layer growth temperatures. These blueshifts occur concurrently with narrowing (from 61 to 24 meV) of the full width at half-maxima for the emission from the quantum dot ensemble. These energy shifts can be explained by interdiffusion or intermixing of the interfaces rather than strain effects due to variations in capping layer thickness. Temperature behavior of the luminescence in annealed and nonannealed samples indicates a change in the shape and depth of the quantum dot confining potential. Quenching of the wetting layer luminescence after interdiffusion is also observed.

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

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We report significant differences in the temperature-dependent and time-resolved photoluminescence ͑PL͒ from low and high surface density In x Ga 1Ϫx As/GaAs quantum dots ͑QD's͒. QD's in high densities are found to exhibit an Arrhenius dependence of the PL intensity, while low-density ͑isolated͒ QD's display more complex temperature-dependent behavior. The PL temperature dependence of high density QD samples is attributed to carrier thermal emission and recapture into neighboring QD's. Conversely, in low density QD samples, thermal transfer of carriers between neighboring QD's plays no significant role in the PL temperature dependence. The efficiency of carrier transfer into isolated dots is found to be limited by the rate of carrier transport in the In x Ga 1Ϫx As wetting layer. These interpretations are consistent with time-resolved PL measurements of carrier transfer times in low and high density QD's. ͓S0163-1829͑99͒04748-7͔

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.