Nanocrystalline particles of ZnS:͑Ag, Al͒ semiconductor phosphor, whose sizes are mostly 3-5 nm in diameter, are prepared by the gas-evaporation method with cw CO 2 laser heating. The Raman scattering spectrum as well as the transmission electron microscope observation demonstrates that the crystallization of the nanoparticles was caused successfully through the gas-phase condensation. Under irradiation of ultraviolet light, the nanoparticles exhibit blue luminescence, as in the case of the starting material of ZnS:͑Ag, Al͒ bulk powder. The peak of the luminescence spectrum of the nanoparticles shifts to lower energy with increasing delay time and also with decreasing excitation intensity, showing that the luminescence originates from the donor-acceptor pair recombination. However, it is concluded that the luminescence of the nanoparticles is not ascribed to the blue Ag luminescence mechanism responsible for the luminescence of the bulk powder, by taking into account the spatial confinement of an electron trapped at the donor and a hole at the acceptor. It is argued that the luminescence mechanism of the nanoparticles is the so called self-activated luminescence, which involves zinc vacancies.

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Silver (Ag) was evaporated in argon (Ar) and xenon (Xe) gases at various pressures in the low-gravity environment aboard the space shuttle. Four glass bulbs with filament tips coated with 50 mg of silver were filled with Ar gas of 6.7 Pa (A) or 40 Pa (B) or Xe gas of 0.67 Pa (C) or 1.33 Pa (D) and ignited one by one in the low gravitational field of space. The evaporation temperatures were maintained at 1150°C at which smoke plumes were barely detectable in all cases in the ground experiment. A ball of smoke particles appeared to grow around the evaporation source instead of rising as it would under earth gravity conditions. No smoke was observed in (A), but it was observed in (B) and (C) and bursts of smoke extended in various directions from the smoke ball in the case of (D). The experiment suggested that vapor could be confined locally around the source with high pressure and temperature by the surrounding gas in the low gravity. This suggestion cannot be derived from any conventional model of evaporation in the gas.

Fine zinc particles were produced in an argon gas atmosphere and in a helium gas atmosphere of 1.33 × 10 4 Pa without thermal convection. To investigate the initial process of crystal growth, particles deposited at various places and at different moments during evaporation were collected. The shape and size of the particles were observed using a scanning electron microscope. When the evaporation time increased, the mean size of the deposited particles decreased in the argon gas atmosphere, while it increased in the helium gas atmosphere. These experimental results can be interpreted in terms of the assumption that nucleation and crystal growth are dominated only by the diffusion of vapor from the evaporation source.

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