Structures and Stabilities of (CaO)_n Nanoclusters

The global energy minima structures for (CaO)_n for n ≤ 40 were predicted using density functional theory. The cubic structures are found to be the lowest energy isomers for most (CaO)_n, n ≥ 4. A fragment-based structure-energy relationship model gave an excellent fit for the calculated total energy. Based on the fitting results, the bulk limit for the normalized clustering energy for (CaO)_n particles was predicted to be 157.8 kcal/mol for the enthalpy at 298 K, in good agreement with the experimental/computational bulk value of 156.5 kcal/mol. A (CaO)_n nanoparticle with a size of 10 nm is predicted to have a CaO binding energy close to that of the bulk crystal. The infinite chain limit for normalized clustering energy for various one-dimensional (1-D) cubic nanoparticle series was also obtained. The surface energy densities were predicted to be 62 kcal/mol per CaO for the 3-coordinate corner fragment, 30 kcal/mol per CaO for the 4-coordinate edge fragment, and 11 kcal/mol per CaO for the 5-coordinate face fragment. On the basis of the values of the parity sum for the atom counts for the cube’s edges in three dimensions, cubic (CaO)_n nanoclusters can be classified into three types with different geometries. Although no significant difference in stability was found for different types of cubic (CaO)_n, several electronic properties of the cubic (CaO)_n are related to the parity sum at small n. The type-2 (CaO)_n clusters and ultrasmall particles in the shape of the odd × odd × even cube, with a parity sum of 2, exhibit unique electronic properties. The type-2 3 × 3 × m 1-D nanoparticle series has the lowest HOMO–LUMO excitation energy among all of the 1-D nanoparticle series at all particle sizes: 2.51 eV for the 3 × 3 × 4 cube and 0.70 eV for the 3 × 3 × 18 cube. The patterns for the variation of E_g during the 1-D layer-wise growth of (CaO)_n were analyzed.