Nucleation and growth of lead sulfide nano- and microcrystallites in supramolecular polymer assemblies

Multilayer thin film assemblies fabricated by the sequential adsorption of polyelectrolytes on a quartz substrate were used as a supramolecular reaction template to study the in situ nucleation and growth of PbS nano- and microparticles. Chemical reaction within the polymer film was initiated by absorbing pb(2+) from an aqueous solution of Pb(NO3)(2) followed by exposing the film to H2S gas. Electron microscopic examination of the films revealed that while nanoparticles are formed in films that were subject to one or two reaction cycles, large crystallites were formed when these films were exposed to a large number (10) of reaction cycles. In the latter case, a broad distribution of particle sizes is observed and may be attributed to Ostwald ripening. Detailed studies show the nucleation and growth of the PbS particles into crystallites of different shapes. UV-vis absorption studies reveal that the absorption spectral profiles of the films are dependent on the size of the PbS crystallites. The broadened absorption spectral profile observed for films subject to a large number of reaction cycles may be attributed to the superposition of the spectral profiles of the small clusters that tend to be blueshifted due to quantum confinement effects and the large clusters that are redshifted.