Polymorphism in Self-Assembled AB6 Binary Nanocrystal Superlattices

We report the formation and systematic structural characterization of a new AB(6) polymorph with the bodycentered cubic (bcc) symmetry in binary nanocrystal superlattices (BNSLs). The bcc-AB(6) phase, lacking any atomic analogue, is isomorphic to certain alkali-metal intercalation compounds of fullerene C-60 (e.g., K6C60). On the basis of the space-filling principle, we further tailor the relative phase stability of the two AB(6) polymorphs-CaB6 and bcc-AB(6)- from coexistence to phase-pure bcc-AB(6), highlighting the entropic effect as the main driving-force of the self-organization of BNSLs. We also discuss the implication of surface topology studies and the observation of twinning and preferential orientation in bcc-AB(6) on the growth mechanism of BNSLs. Furthermore, the connection between the bcc-AB6 phase and the (3(2).4.3.4) Archimedean tiling shows the promise of further exploration on the structural diversity (both periodic and aperiodic) in this emerging class of metamaterials. The identification and the ability to tune the relative phase stability of polymorphic structures provide a unique opportunity to engineer the interparticle coupling through controlled clustering and/or interconnectivity of sublattice in BNSLs with identical stoichiometry.