ZnS nanoclusters in LB multilayers

Zinc arachidate (ZnA) LB multilayers have been exposed to H2S to form ZnS nanoclusters within the layered matrix of arachidic acid (AA). The as-deposited ZnA multilayer consists of arachidate molecules packed in a hexagonal layer cell, with the alkyl chain tilted at about 32° away from the layer normal. FTIR and UV-vis studies show that the conversion of arachidate salt into arachidic acid and the formation of ZnS is completed in 2-3 h. Blue shift in absorption onset and excitonic peak ∼290 nm indicates nanocrystalline nature of ZnS. TEM studies show that ZnS nanoclusters of size ∼5 nm are formed. X-ray reflection and grazing angle diffraction studies and changes in the nature of CH2 scissoring mode peak of ZnS-AA composite multilayer show that the molecular packing corresponds to a closely packed structure based on orthorhombic subcell in which the chains are tilted by ∼27° towards nearest neighbours. This is in contrast to a loose 'rotator phase like' packing of molecules in the precursor ZnA multilayer.