Highly crystalline nanoclusters of hexagonal (2H polytype) MoS2 and several of its isomorphous Mo and W chalcogenides have been synthesized with excellent control over cluster size down to ∼2 nm. These clusters exhibit highly structured, bandlike optical absorption and photoluminescence spectra which can be understood in terms of the band-structures for the bulk crystals. Key results of this work include: (1) strong quantum confinement effects with blue shifts in some of the absorption features relative to bulk crystals as large as 4 eV for clusters ∼2.5 nm in size, thereby allowing great tailorability of the optical properties; (2) the quasiparticle (or excitonic) nature of the optical response is preserved down to clusters ≲2.5 nm in size which are only two unit cells thick; (3) the demonstration of the strong influence of dimensionality on the magnitude of the quantum confinement. Specifically, three-dimensional confinement of the carriers produces energy shifts which are over an order of magnitude larger than those due to one-dimensional (perpendicular to the layer planes) confinement emphasizing the two-dimensional nature of the structure and bonding; (4) the observation of large increases in the spin-orbit splittings at the top of the valence band at the K and M points of the Brillouin zone with decreasing cluster size, a feature that reflects quantum confinement as well as possible changes in the degree of hybridization of the electronic orbitals which make up the states at these points; and (5) the observation of photoluminescence due to both direct and surface recombination. Several of these features bode well for the potential of these materials for solar photocatalysis.

Silica/surfactant mesophases have been synthesized in 14 water:cosolvent mixtures by combining tetramethoxysilane with a basic 2 wt % CTAB solution. The effects of the water-to-cosolvent ratio on the formation of supramolecular surfactant templates and ultimately silica/surfactant mesophases is reported for:  diethyl ether, ethyl acetate, tetrahydrofuran, tetraglyme, methylene chloride, 2-propanol, acetone, ethanol, methanol, ethylene glycol, acetonitrile, glycerol, formamide, and N-methylformamide. X-ray diffraction (XRD), dynamic and static light scattering (DLS/SLS), scanning and transmission electron microscopies (SEM/TEM), and nitrogen sorption techniques are used to characterize the mesophases. Generally, polar cosolvents decrease the extent of aggregation of CTAB and lead to an evolution from ordered (o-H) hexagonally packed silica (HPS) to disordered (d-H) HPS as the cosolvent concentration is increased. Polar cosolvents allow the unit cell size of the mesophase to be tuned continuously over ∼5 Å:  protic solvents decrease the cell size; aprotic solvents increase the cell size. Highly polar protic solvents, such as formamide and ethylene glycol, support substantially nonaqeous synthesis of o-H and d-H mesophases with water:silica ratio less than 4.0. Low dielectric constant cosolvents lead to expanded o-H mesophases at low concentrations, and cubic and lamellar phases at higher concentrations. Cosolvents can be used to synthesize mixed-metal framework structures from homogeneous solutions by premixing molecular inorganic precursors in a compatible nonaqueous solvent and then controllably hydrolyzing the precursors. Cosolvents also influence microstructure, leading to smaller, more curved primary particles than in pure water.

We use light-scattering techniques to study the effects of methanol concentration on alkaline, cetyltrimethylammonium bromide (CTAB) water:methanol micellar solutions. We use X-ray diffraction, SEM, TEM, 29Si NMR, and gas sorption measurements to study the structure, microstructure, and porosity of surfactant-templated silica (STS), synthesized by adding tetramethoxysilane (TMOS) to the above micellar solutions. The measured critical micelle concentration (cmc) for CTAB at 25 °C in a 0.22 M NaOH (pH 13.2) solvent increases from ∼1.3 × 10-3 M for r = 0% to ∼5.5 × 10-2 M for r = 60% (where r is the wt % methanol in the mixture) as the concentration of methanol increases. In turn, the long-range order of STS decreases as the methanol concentration increases. Ordered STS forms for 0 ≤ r < 60%, where the concentration of CTAB, c, is greater than cmc in the precursor solution; disordered STS (resembling wormlike micelles) forms for 60 ≤ r ≤ 90%, where c < cmc. For r > 90% transparent, amorphous chemical gels form. The presence of methanol leads to a uniform submicron microstructure as compared to faceted 1−10-μm particles with pure water. After template removal, apparent BET surface areas for STS can exceed 950 m2/g, and the void volume can exceed 0.6 cm3/g. Initially, there is a high fraction of uncondensed silica in the as-made product (Q 3/Q 4 ≈ 2.1), but after calcination a strong, bonded siloxane framework forms (Q 3/Q 4 ≈ 0.40).

We have synthesized periodic mesoporous silica thin films (PMSTF) from homogeneous solutions. To synthesize the films a thin layer of a pH = 7 micellar coating solution that contains TMOS is dip-or spin-coated onto silicon wafers, borosilicate glass, or quartz substrates. Ammonia gas is diffused into the solution and causes rapid hydrolysis and condensation of the TMOS and the formation of periodic mesoporous thin films within -10 seconds. The combination of homogeneous solutions and rapid product formation maximizes the concentration of desired product and provides a controlled, predictable microstructure. The films have been made continuous and crack-free by optimizing initial silica concentration and film thickness.

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Three important processes dominate the wet thermal oxidation of AlxGa1−xAs on GaAs: (1) oxidation of Al and Ga in the AlxGa1−xAs alloy to form an amorphous oxide, (2) formation and elimination of crystalline and amorphous elemental As and of amorphous As2O3, and (3) crystallization of the amorphous oxide film. Residual As can lead to strong Fermi-level pinning at the oxidized AlGaAs/GaAs interface, up to a 100-fold increase in leakage current, and a 30% increase in the dielectric constant of the oxide layer. Thermodynamically favored interfacial As may impose a fundamental limitation on the use of AlGaAs wet oxidation in metal-insulatorsemiconductor devices in the GaAs material system.

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