Synthesis of PbS Semiconductor Microcrystallites In Situ in Reverse Micelles
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ABSTRACT Colloid semiconductor particles of PbS have been prepared in reversed micelles of AOTheptane. The effects of the AOT, water and reactants concentrations have been studied. The initial crystallite size and its growth with the time aging are strongly dependent on the water/AOT ratio and the ionic pool concentration. The absorption spectroscopic characteristics of the PbS particles have been examined. As the particle size decreases the band gap shifts in color from orange-brown to yellow and approaches the transition energy of the first allowed excited state of a PbS molecule. Moreover, the optical nonlinear property of PbS particles was investigated by Degenerate Four-Wave Mixing (DFWM) experiment using a frequency-doubled Nd:YAG laser.
INTRODUCTION Light absorption leads to an electron in the conduction band and a positive hole in the valence band. In small particles they are confined to potential wells of small lateral dimensions, this leads to an increase of the gap energy and a quantization of the conduction and valence bands, which in the bulk material constitute virtually a continuuum in the
conduction and valence bands. The phenomena arise when the size of a colloidal particle becomes comparable to the de Broglie wavelength of a charge carrier. The effective mass of an electron in a crystal lattice is often substantially smaller than that of an electron in the free space; i.e., the de Broglie wavelength is rather long and therefore the size effects can appear already in particles with a few nanometers of diameter. The size quantization effects occur more drastically in low band gap materials when the effective mass of the electron is particularly small. In the past decade systems consisting of semiconductor particles of nanometer size dispersed in transparent matrices such as glasses or organic polymers have received much attention. This was so because it has been found that these materials have large optical nonlinearity and fast response and therefore they were taken as one of the most important candidates for application to optics devices. For the semiconductor-doped glasses, the large nonlinearity is attributed to the quantum confinement effect of electronic states. A variety of techniques including molecular beam epitaxy (MBE), chemical vapor deposition (CVD) and sputter and vapor deposition have been developed for semiconductor superlattice formation and band-gap engineering. In situ generation of semiconductor particles and semiconductor films at the interfaces of organized surfactant assembles presents an alternative approach. Versatility, maneuverability, and relative simplicity are the advantages of this colloid chemical method. Cadmium 253
Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
sulfide have been synthesized in AOT (Sodiun dioctyl sulfosucinate) reverse micelles [1][2J with very high concentration of surfactant and in presence of the small quantities of water. Microdispersed and colloidal semiconductors have been increasingly utilized in photochemical and
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