Superlattices of Semiconductor Quantum Size Particles in Layered Organic Acids
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Recently, we have followed a synthetic route using topotactic gas-solid reaction performed on three dimensional crystals or crystalline thin films1 0 of organometallic precursors resulting in the formation of superlattices of nanoparticles embedded in the organic crystals or films 1 .The organic matrix serves as a template and a confinig environment for the resulting particles. The organization of the particles in a superlattice may be achieved if the packing arrangement of the precursor is partially preserved after reaction (scheme). This approach has been applied for the preparation of lead sulfide nanoparticles arranged in periodic layers within n-alkanoic acids by exposing the lead salts of the corresponding acids [CH3(CH2)nCO2]2Pb (n= 16-28) to H2S gas11. The lead ions in the salts are arranged in layers that are separated from one another by a distance defined by the dimensions of the organic bilayer. Within this homologous series, direct correlation between the degree of crystalline order of the formed superlattice and the dimensions of the bilayer spacing in the precursor were observed. The larger the bilayer spacing the higher the degree of crystalline order in the resulting superlattice. Thus, the best example was obtained with n=28. Since most salts of long chain carboxylic acids assume layer packing motifs, we extended our studies to systems bearing an aromatic ring along the hydrocarbon chain, such as derivatives of phenyl propionic and benzoic acids, of which few examples shall be described here.
159 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
4
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H2 S
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0 =M+2
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S= nanoparticles
EXPERIMENT Materials Alkyloxy-phenyl propionic acids, w-hydroxyalkyloxy-phenyl propionic acids and alkyloxy benzoic acids were synthesized by o-alkylation of p-hydroxy-phenyl propionic acid and p-hydroxy-benzoic acid with the respective co-hydroxyalkylbromides or alkylbromides. Their corresponding potassium salts were prepared by reacting the acids with potassium hydroxide in methanol. Crystalline powders of lead and cadmium salts were prepared by mixing a methanol solution of potassium carboxylate with an equivalent amount of aqueous solution (10-2 M) of Pb(N03)2 or of CdCl2. H2S gas was prepared by reacting Na2S with dilute (20%) solution of H2SO4 and used without drying. Characterization Methods FT-IR spectra were recorded on a Nicolet-460 spectrometer and powders were measured in KBr pellets. UV-visible absorption spectra were recorded on a Beckman DU7500 spectrometer. After completion of the reactions of cadmium salts, the powders were dissolved in DMSO with 10% 3-mercapto-1,2-propanediol and the spectra were measured. For the lead salts the spectra were measured by smearing a thin layer of the reacted powder onto a glass slide. X-ray powder diffraction spectra were measured on a Rigaku RU-200B rotaflex diffractometer using CuKca X,=1.54A. Transmission electron microscopy images and electron diffraction patterns were taken on a Philips CM12 transmission electron microscope op
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