Oriented Growth of inorganic crystals at organic templates: synchrotron X-ray scattering studies

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Living organisms grow precisely controlled assemblies of inorganic crystals using organic substrates. This observation has inspired the strategy of using synthetic organic templates for the growth of tailored inorganic thin films. It has been previously shown that monomolecular organic layers floating on supersaturated aqueous subphases (Langmuir monolayers) select the structure (where more than one is possible) and the orientation of the inorganic crystals nucleating under them. However, the mechanisms governing such selective crystal nucleation process remained unclear. This project attempts to understand the roles played by geometric influences such as structural match between the interfacial lattices and the interactions between monolayer headgroups and aqueous ions in determining the orientation and structure of the inorganic nucleate. To perform such studies we have monitored the organic-inorganic interface during the nucleation process using grazing incidence X-ray diffraction (GID). Scanning electron microscopy (SEM) was used to perform morphological studies on grown crystals. Our studies show that different mechanisms govern the early and late stages of crystal growth. In the early stages interplay between the monolayer headgroup - aqueous ion interactions and ion specific effects determine the inorganic species that nucleates. During crystal growth of barium fluoride and barium fluoride chloride under a fatty acid monolayer, we found that both the inorganic forms nucleate in an oriented manner. However, when the monolayer is in a deprotonated state, only barium fluoride nucleation was observed. In nearly all the cases of oriented crystal growth we found a lattice match between the interfacial structures. During barium fluoride and barium fluoride chloride crystal growth under a fatty acid monolayer, the interfacial lattices demonstrated sufficient flexibility; to achieve an epitaxial match. A variant was observed during hydrocerussite (2PbCO3. Pb(OH)2) nucleation under a fatty acid monolayer, a surface reconstructed lattice mediated between the unstrained crystal surface and the monolayer lattice such that all the neighboring lattices were commensurate. Different energetic processes are shown to govern the late crystal growth stages. The average alignment of preformed nucleate at the interface is enhanced via spontaneous aggregation of crystals into oriented chains. Two examples of enhanced alignment of crystals due to oriented attachment are discussed

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  • 08/07/2018
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