Metallization of colloidal crystals

Abstract

Colloidal crystals formed by size-asymmetric binary particles coassemble into a wide variety of colloidal compounds with lattices akin to ionic crystals. Recently, a transition from a compound phase with a sublattice of small particles to a metal-like phase in which the small particles are delocalized has been predicted computationally and observed experimentally. In this colloidal metallic phase, the small particles roam the crystal maintaining the integrity of the lattice of large particles, as electrons do in metals. A similar transition also occurs in superionic crystals, termed sublattice melting. Here, we use energetic principles and a generalized molecular dynamics model of a binary system of functionalized nanoparticles to analyze the transition to sublattice delocalization in different coassembled crystal phases as a function of temperature ($T$), number of grafted chains on the small particles, and number ratio between the small and large particles ${n}_{s}:{n}_{l}$. We find that ${n}_{s}:{n}_{l}$ is the primary determinant of crystal type due to energetic interactions and interstitial site filling, while the number of grafted chains per small particle determines the stability of these crystals. We observe first-order sublattice delocalization transitions as $T$ increases, in which the host lattice transforms from low- to high-symmetry crystal structures, including A20 $\ensuremath{\rightarrow}$ bct $\ensuremath{\rightarrow}$ bcc, ${\mathrm{A}}_{\mathrm{d}}$ $\ensuremath{\rightarrow}$ bct $\ensuremath{\rightarrow}$ bcc, and bcc $\ensuremath{\rightarrow}$ bcc/fcc $\ensuremath{\rightarrow}$ fcc transitions and lattices. Analogous sublattice transitions driven primarily by lattice vibrations have been seen in some atomic materials exhibiting an insulator-metal transition also referred to as metallization. We also find minima in the lattice vibrations and diffusion coefficient of small particles as a function of ${n}_{s}:{n}_{l}$, indicating enhanced stability of certain crystal structures for ${n}_{s}:{n}_{l}$ values that form compounds.