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In-situ scanning force microscopy was used to examine how corrosion at the nanometre scale depends on material microstructure. The results reveal an unusual porous corrosion morphology in partially nanocrystalline Al 87 Ni 8.7 Y 4.3 alloys exposed to dilute Cl - solutions at various pH values. This nanometre-scale porosity is absent in the fully amorphous state of the same alloy. This difference is surprising because the macroscopic corrosion behaviours of the two materials are similar. This work shows that the partially nanocrystalline material sustains nanometre-scale corrosion damage in addition to localized corrosion pits. We propose a mechanism for porosity development based on acid-induced depassivation of protective oxides and two key microstructural differences between the fully amorphous and the partially nanocrystalline alloys.

We studied the optical properties of silicon nanocrystals incorporated into colloidal and solgel matrices. The silicon nanocrystals are produced by ultrasonic dispersion of porous silicon layers. We report results on the dependence of the photoluminescence (PL) spectra with excitation intensity. The PL shows a blue peak (at ∼ 415-460 nm.) and a red peak (at ∼ 680 nm). This PL spectrum shows a remarkable dependence on the excitation intensity. As the intensity is increased, the blue peak grows at the expense of the red. A model is suggested for this behavior. We also report on the excitation intensity dependence and the emission wavelength dependence of the PL decay at low (1 kHz) and high (82 MHz) repetition rates of optical excitation. When low repetition rate excitation is used, the PL decay times are all exponential, short (ns), and appear to vary little with emission wavelength. This sharply contrasts with what is observed in porous silicon. With high repetition rate excitation, both red and blue peaks show long (100's ns) and short (ps-ns) lifetime components. We contrast the different optical properties of these silicon nanocrystals with that observed in porous silicon.

Nanometer-sized silicon particles have been produced by ultrasonic dispersion of thin sections of porous silicon (PS) in organic solvents. The particles have been characterized by HRTEM (high-resolution transmission electron microscopy), FTIR (Fourier transform infrared) spectroscopy, and fluorescence spectroscopy. HRTEM shows both aggregates of and monodispersed crystallite particles of Si. The larger aggregates range in size from20 to 50 nm and are made up of small crystallites with diameters of 2-10 nm. Monodispersed crystallites ranged in size from 2-10 nm. All the particles have an amorphous layer of SiO{sub 2}. The photoluminescence (PL) spectrum shows two peaks: a red peak at around 680 nm, which is typical for PS and a blue peak between 415 and 446 nm, which is not typical for as-prepared PS. As a function of increased excitation intensity, the blue peak grows at the expense of the red. This is discussed in light of photoluminescence lifetime data. The red luminescence is attributed to quantum confinement. The blue luminescence is attributed to either extremely small Si crystallites or (Si(II)){sup 0} defects embedded in the oxide matrix. 28 refs., 9 figs., 1 tab.