Surface Passivation and Supersaturation: Strategies for Regioselective Deposition in Seeded Syntheses.

Crystal growth theory predicts that heterogeneous nucleation will occur preferentially at defect sites, such as the vertices rather than the faces of shape-controlled seeds. Platonic metal solids are generally assumed to have vertices with nearly identical chemical potentials, and also nearly identical faces, leading to the useful generality that heterogeneous nucleation preserves the symmetry of the original seeds in the final product. Herein, we test the limits of this generality in the extreme of low supersaturation, in an effort to expand the methods available for inducing anisotropic overgrowth. We formulate a strategy for favoring localized deposition that differentiates between both different vertices and different edges or faces, i.e., regioselective deposition. Deposition followed a simple kinetic model for nucleation rate, depending on wetting, supersaturation, and temperature. We demonstrate our ability to independently study the effects of varying supersaturation and surface passivation. Regioselective heterogeneous nucleation was achieved at low supersaturation by a kinetic preference for high-energy defect-rich sites over lower-energy sites. This outcome was also achieved by using capping agents to passivate facet sites where deposition was not desired. Collectively, the results presented herein provide a model for breaking the symmetry of seeded growth and for achieving regioselective deposition.