Mechanisms for species-selective oriented crystal growth at organic templates

Langmuir monolayers floating on supersaturated aqueous subphases can act as templates for the growth of oriented inorganic films - a "bioinspired" nucleation process. We have performed in situ grazing incidence x-ray diffraction studies of the selective nucleation of BaClF and BaF₂ under fatty acid monolayers. The arrangement of the fatty acid headgroups, the monolayer charge, and ion-specific effects all play important roles in selecting the inorganic species. When the monolayer is in a neutral state, both BaClF and BaF₂ nucleate at the interface and are well aligned, but when the monolayer headgroup is deprotonated, only oriented BaF₂ grows at the interface. We also observe an enhanced alignment of BaF₂ crystals during growth from highly supersaturated solutions, presumably due to reorganization of preformed crystals at the organic template. These results show that a delicate interplay between multiple factors governs the oriented growth of inorganic films at organic templates.