Assembly of molecular clusters and nanoparticles in solution is now recognized as an important mechanism of crystal growth in many materials, yet the assembly process and attachment mechanisms are poorly understood.  To achieve this understanding we are investigating nucleation and assembly of iron oxide, titanium oxide and calcium carbonate nanoprticles.  TEM analysis shows that the TiO₂ nanowire branching occurs through attachment of anatase nanoparticles to rutile wires on a specific crystallographic plane for which the anatase-to-rutile transformation leads to creation of a twin plane.  Using an in-situ heating stage on an aberration corrected microscope, we follow the transformation of anatase nanoparticles into single crystal rutile both through heating and by attachment to larger anatase particles.  We document the atomic reorganization and appearance of intermediate structures that lead to formation of a final co-aligned single crystal.  In-situ TEM using a custom-designed holder and fluid cell to obtain sub-nanometer resolution shows that, in the iron oxide system, primary particles of ferrihydrite interact with one another through translational and rotational diffusion until a near-perfect lattice match is obtained either with true crystallographic alignment or across a twin plane.  Oriented attachment (OA) then occurs through a sudden jump-to-contact, after which the interface expands through ion-by-ion attachment at a curvature-dependent rate.  Analysis of the acceleration during the attachment process indicates that OA it is driven by an electrostatic attraction with about one unit of charge on each particle driving the event.

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