Seeding and controlling colloidal self-assembly through focused ion beam deposition

To obtain high yields of a desired structure from self-assembly, one often needs a seed: a template that favors formation of the structure. The physical mechanisms of seeding have been studied in detail in micrometer-scale colloidal systems, but much of this prior work has focused on quasi-planar substrates. Our overarching aim is to seed colloidal self-assembly on curved surfaces, for which the orientation of a two-dimensional (2D) crystal nucleus---typically unimportant for assembly on a flat surface---determines whether defects form. Because existing methods lack the spatial control to seed colloidal crystals on curved surfaces, we develop a new method that can be applied to both flat and curved surfaces. Our method leverages the spatial precision of focused ion beam (FIB) deposition. We deposit perhaps the simplest nanostructures that can seed the growth of 2D crystals: triangular configurations of three wells. We then use confocal microscopy to monitor the dynamics of depletion-mediated colloidal self-assembly in the presence and absence of the FIB nanostructures. On flat surfaces we find that nucleation can be either directly or indirectly controlled by the seeding nanostructure, depending on the supersaturation and the interaction strength, and we observe growth occurring by both particle attachment and oriented attachment. Based on these results, we extend our seeding method to control the crystal orientation on a highly-curved \SI{5}{\micro\meter} glass fiber. Our FIB-deposition approach to seeding might be useful not only for controlling assembly on geometrically frustrated systems, but also for modifying functional devices such as optical fibers and neural probes.