A new synthetic method was developed to produce robust, highly crystalline, organic-monolayer passivated silicon (Si) nanocrystals in a supercritical fluid. By thermally degrading the Si precursor, diphenylsilane, in the presence of octanol at 500 degrees C and 345 bar, relatively size-monodisperse sterically stabilized Si nanocrystals ranging from 15 to 40 A in diameter could be obtained in significant quantities. Octanol binds to the Si nanocrystal surface through an alkoxide linkage and provides steric stabilization through the hydrocarbon chain. The absorbance and photoluminescence excitation (PLE) spectra of the nanocrystals exhibit a significant blue shift in optical properties from the bulk band gap energy of 1.2 eV due to quantum confinement effects. The stable Si clusters show efficient blue (15 A) or green (25-40 A) band-edge photoemission with luminescence quantum yields up to 23% at room temperature, and electronic structure characteristic of a predominantly indirect transition, despite the extremely small particle size. The smallest nanocrystals, 15 A in diameter, exhibit discrete optical transitions, characteristic of quantum confinement effects for crystalline nanocrystals with a narrow size distribution.

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The isobaric rates of excited-state deprotonations of 2-naphthol by acetate and borate anions exhibit only modest deviations from Arrhenius-like behavior from ambient temperature to nearly the critical temperature of water (T c = 374 °C). In contrast, the rates of deprotonation by ammonia and water exhibit marked deviations from Arrhenius-like behavior and go through a maximum at high temperatures. These observations establish a fundamental difference in how the rates of charge-generating reactions, such as proton transfers to neutral molecules like ammonia and water, and those in which ionicity is unchanged, such as proton transfers to acetate and borate anions, depend on temperature. The loss of local water structure and changes in dielectric constant with temperature have a much more profound influence on the charge-generating reactions. These results are interpreted using transition state theory and compared with several molecular dynamics−free energy perturbation simulations. At temperatures above 250 °C, contact ion pair formation further inhibits deprotonation. The formation of contact ion pairs is evident in both the time-resolved fluorescence and steady-state fluorescence spectra. Near the critical point, where solvent properties vary widely with pressure, the bimolecular rate constant for 2-naphthol deprotonation by ammonia increases by nearly an order of magnitude over the pressure range from 3000 to 5000 psia. This effect is caused by the large changes in solvent density induced by pressure changes and leads to electrostriction about the polar transition state.