IN SITU ATOMIC FORCE MICROSCOPY OF GROWING CRYSTALS REVEALS FUNDAMENTAL MECHANISMS OF CRYSTAL GROWTH AND INCORPORATION OF ADDITIVES

Abstract

Calcite, the most thermodynamically stable form of calcium carbonate (CaCO3), is commonly found in nature and functions as a structural component for a variety of organisms including mollusks, sea urchins, and algae. In particular, the organisms often utilize single crystals that have significantly increased hardness, modulus, and toughness when compared to a geologic sample of calcite. The increased mechanical properties are of evolutionary benefit to the organism and arise due to additives which control the crystal formation and are incorporated within the single crystalline structures. These additives include magnesium substitutions, small molecules and amino acids, and nanometer scale globules of protein. Though the smaller scale additives are now relatively well understood, the interaction mechanisms between the nanoparticle scale organic and the crystal remain unknown. A more complete understanding of such particle-crystal interactions could lead to “design rules” which can optimize the incorporation of nanoparticles into single crystals. This work uses in situ AFM performed on growing calcite in the presence of nanoparticles with tunable surface chemistry and reveals three types of nanoparticle-crystal interactions: attachment-detachment, attachment-incorporation, and attachment-hovering, where the nanoparticle hovers on the surface as growth proceeds unaffected. Additionally, the particle surface chemistry determines whether the interactions are driven by the charge corona on the particle (a particle driven regime) or by local behavior at the crystal surface (a surface driven regime). Further, this work demonstrates that the distribution of particles in an ensemble is divided between the three types of interactions in an equilibrium which can be affected by both surface chemistry and the growth conditions. Together, we now have a more complete picture of how nanoparticles can interact with a growing crystal surface.