Modeling the Aggregation of MgO Clusters on Highly Oriented Graphite

Abstract

Nanoscale assemblies hold great potential for the design of novel materials with tunable properties. The main factors driving the assembly are the interactions between the building blocks together or with their environment, as well as their intrinsic properties, starting with their size. In the present work, we have modeled the aggregation between magnesium oxide clusters (MgO)n soft-landed on highly oriented pyrolytic graphite (HOPG), including atomistic details. Our approach includes a many-body polarizable potential for magnesium and oxide ions that takes into account the polarizability and the corrugation of the graphite substrate, and a simplified version of this potential for simulating larger self-assemblies. The simulations show that (MgO)n clusters aggregate into fractal-like islands in the case of the magic size n = 32, coalescence being driven by rotation and reorientation of individual cubic clusters. Rocksalt order is locally preserved, although oriented aggregation is hindered by the presence of voids due to geometric frustration in the ramified arrangement. In contrast, the self-assembly of (MgO)13 clusters is much more globular, as a result of a broader variety of conformers exposing very different faces to one another. Our predictions highlight the importance of geometry on substrate-mediated cluster interactions, and are supported by experiments on chemically and structurally analogous PbS clusters.