(363g) Driving Forces for Oriented Aggregation-Based Crystallization and Assembly

Driving forces for oriented aggregation-based crystallization and assembly depend on azimuthal crystallographic alignment between particles. The types of driving forces that can be sensitive to relative orientation include Coulombic, van der Waals (vdW), solvation, and ion correlation forces. The theoretical underpinnings of these anisotropic forces are well established, but techniques that can directly measure them for a given pair of interacting oriented crystal faces have generally been limited to use of macroscopic yet atomically flat single crystals. We report measurement of anisotropic forces between rutile TiO₂ (001) nanocrystals as a function of their azimuthal orientation and surface hydration extent using a combined environmental transmission electron microscopy-atomic force microscopy (AFM) technique. Atomically flat rutile TiO₂ (001) AFM tips and opposing rutile TiO₂ (001) substrates were fabricated by focused ion beam milling to excise nanocrystals from the surface of a single monolith that was pre-oriented, cut, and polished to prepare the (001) face. At tens of nanometers of separation, the attractive forces are weak and show no dependence on azimuthal alignment nor surface hydration. At separations of approximately one hydration layer, attractive forces are strongly dependent on azimuthal alignment and systematically decrease as intervening water density increases. Measured forces closely agree with predictions from Lifshitz theory and show that dispersion forces are capable of generating a torque between particles interacting in solution and between grains in materials.