Defective to fully coordinated crossover in complex directionally bonded nanoclusters

Abstract

Defect-free fully coordinated (FC) structures are well known to be highly stable for a number of materials exhibiting three-coordinated bonding at the nanoscale (e.g., ${\text{C}}_{60}$). For topologically more complex nanosystems with higher bonding connectivities the structures and stabilities of the lowest energy FC structures with respect to low-energy defective isomers are unknown. Herein, we describe a general method to thoroughly search through the low-energy geometries of only those nanoclusters that possess FC atomic connectivities. As a pertinent example of our approach we investigate four-connected ${\text{SiO}}_{2}$, a fundamentally important network-forming material used in many applications at the nanoscale. Using our method we predict that a structurally complex stability crossover from defective to FC nanoclusters occurs in ${\text{SiO}}_{2}$ at a size of $\ensuremath{\sim}100$ atoms. At variance with previous works, based on constructing FC ${\text{SiO}}_{2}$ cagelike nanoclusters by hand, we also show that cagelike clusters are only favored for smaller cluster sizes with dense FC topologies becoming energetically favored with increasing system size.