Microscopic topography of heterocrystal interfaces

Real-time in situ atomic force microscopy (AFM) of the (010) and (001) planes of isomorphous [1,4-bis(amidinium)benzene] ₂M(CN) ₆·8H ₂O (M = Fe, Ru) crystals, known to exhibit growth of one of the metalate compounds on the other to produce "core - shroud" heterocrystals, reveals high-fidelity epitaxial growth during heterocrystal formation or roughened surfaces, depending on the crystal face and crystallization conditions. The roughened crystal surfaces that define the interface between heteroepitaxial layers under typical growth conditions are consistent with the interface structure observed by electron dispersive spectrometry, which indicated intermixing of the two materials throughout a 0.7 μm thick interfacial region, but could not distinguish between various possible mechanisms for the intermixing. The crystal topography and roughness under various conditions reveal that the intermixing zone observed during heterocrystal formation may be a consequence of a rough growth surface on the inner "core" crystal upon evaporation of solvent prior to immersion in the growth medium used for epitaxial crystallization of the second compound. These observations suggest that the roughness of the growth interface can be regulated using specific growth protocols that minimize the intermixing of the two compounds.