ZnₓMg₆₀–ₓO₆₀ Nanoclusters with Tunable Near-Ultraviolet Energy Gaps

(MgO)₆₀ and (ZnO)₆₀ are predicted to be magic-number clusters in the shapes of cuboid (CB) and double-layer cage (DC), respectively. The composite ZnₓMg₆₀–ₓO₆₀ nanoclusters that are isovalent to (MgO)₆₀ and (ZnO)₆₀ are studied using a basin hopping method with our newly developed combinatorial-ion-exchange (CIE) sampling technique and molecular dynamics in conjunction with density functional theory. The most stable ZnₓMg₆₀–ₓO₆₀ is found to be either a CB (at low x) or a DC (at high x) nanocluster depending on the value of x with the crossover point at x = 42 or at Zn/Mg ≈ 2:1. CIE basin hopping calculations reveal that Zn prefers sites with the lowest Zn–O coordination, whereas Mg prefers sites with the highest Mg–O coordination. Aggregation of two small nanoclusters to form a large magic-number ZnO–MgO composite nanocluster is thermodynamically favorable. Born–Oppenheimer molecular dynamics simulations show that the aggregation is dependent on the structural motifs of the small (MgO)ₙ and (ZnO)ₙ reactants. The rocksalt MgO and double-layer cage ZnO nanoclusters are mostly preserved during aggregation, although their surface is reactive, whereas single-layer cage ZnO nanoclusters will deform to actively participate in the formation of larger single-layer ZnO, partial multilayer ZnO, and ZnO–MgO core–shell CB structures. The electronic properties of the CB and DC ZnₓMg₆₀–ₓO₆₀ nanoparticles are investigated. The EHOMO, ELUMO, and highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap for ZnₓMg₆₀–ₓO₆₀ are essentially monotonic functions of x. The form and delocalization of the HOMO and LUMO for ZnₓMg₆₀–ₓO₆₀ are dependent on x. The EHOMO, ELUMO, and HOMO–LUMO gap of DC and CB ZnₓMg₆₀–ₓO₆₀ nanoclusters are highly tunable. The tunable HOMO–LUMO gap range for all of the DC and CB ZnₓMg₆₀–ₓO₆₀ is from 3.1 to 4.8 eV, and the range for the global minima ZnₓMg₆₀–ₓO₆₀ is from 3.2 to 4.2 eV.