3D Islands on 2D Materials as Individual Reactors for Measuring Nanoscale Growth Processes and Phase Transformations

When metals such as Au, Ag, Hf or Ti are deposited on a van der Waals material, the result is not a thin film but instead an array of nanoscale islands, a consequence of the difficulty of nucleation and the fast adatom diffusion on the clean and inert surface. These spontaneously formed, crystalline islands can be aligned with the substrate through the process of quasi-van der Waals epitaxy and can show regular shapes and surface facets. The resulting 2D/3D hybrid structures are of interest for their applications in catalysis or in electronic or plasmonic devices. In this presentation, however, we emphasize a different aspect of these structures: the exciting opportunities they offer to probe fundamental questions in crystal growth and transformations in nanoscale volumes.<br/><br/>Our experiments take place in an ultrahigh vacuum electron microscope equipped with gas delivery and evaporation capabilities. The samples consist of flakes of graphene, boron nitride or a transition metal dichalcogenide suspended over holes in a thick silicon nitride membrane on Si. After a cleaning step, the island array is grown at low flux and with the substrate at a temperature of up to several hundred degrees. The islands are then covered by a second metal that is deposited while maintaining vacuum, or exposed to reactive gases such as disilane, digermane or oxygen, and imaged <i>in situ</i> or post-growth. We find that individual islands on the weakly interacting substrate act as essentially isolated systems in which growth phenomena or phase transformations can be resolved. With an entire island within the field of view during an experiment, nucleation sites, defects and the spatial extent of phase transformations can not hide from the view of the microscopist. Analysis of the ensemble of islands with their range of sizes helps determine nucleation statistics, mechanisms and rate-limiting steps. We will illustrate such measurements with several examples. These include strain relaxation through island curvature or through formation of a dislocation array in specific regions of the islands; stability of strained bimetallic islands; lateral growth to form core-shell structures; nucleation of Ge at island corners; silicide formation, and transformations between strain-related crystal structures. Phase transitions in nanoscale volumes are not constrained to follow the rules of the bulk phase diagram by virtue of the surface and interface energy terms and the kinetic hindrance arising from scarce nucleation sites. Microscopy shows the ways in which nanoscale systems behave differently from larger ones, and opens an intriguing window into materials reactions.