EPITAXIAL FORMATION OF PBS CRYSTALS UNDER ARACHIDIC ACID MONOLAYERS

Lead sulfide (PbS) particulate films composed of highly oriented, equilateral-triangular crystals have been in situ generated by the exposure of arachidic acid (AA)-monolayer-coated aqueous lead nitrate [Pb(NO3)2] solutiOns to hydrogen sulfide (H2S). The AA-coated PbS particulate films, at different stages in their growth, were transferred to solid substrates and characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), and electron diffraction measurements. Each individual crystal had its [111] axis perpendicular and its [112BAR], [121BAR], and [211BAR] axes parallel (arranged in 3-fold symmetry at 120-degrees angles) to the AA monolayer surface. The rate of H2S infusion influenced the size of crystals grown under the AA monolayer. The mean length of equilateral-triangular PbS crystals decreased from 607 angstrom (sigma = 133 angstrom) to 297 angstrom (sigma = 91 angstrom) upon diminishing the H2S exposure time from 30 to 5 min. The epitaxial growth of PbS crystals has been rationalized in terms of an almost perfect fit between the (111) plane of the cubic crystalline PbS and the (100) plane of the hexagonally close-packed AA monolayer. The presence of a monolayer and the slow encounter of the precursors have been found to be essential requirements for the oriented growth of PbS crystals. Exposure of Pb(NO3)2 solutions to H2S in the absence of monolayer surface coverage furnished only irregular PbS crystals at the air-water interface.