Density-functional study of structural and electronic properties of GanN (n=1-19) clusters

The lowest-energy geometries and electronic-structure properties have been obtained for GanN (n=1-19) clusters within the density-functional theory using the generalized gradient approximation for the exchange-correlation potential. The resulting geometries show that the nitrogen atom tends to occupy an inside position for n <= 10, but prefers a peripheral position with a bulklike coordination beyond n=10. The stability has been investigated by analyzing the binding energy per atom and the second difference in energy. The results indicate that Ga3N, Ga7N, and Ga15N exhibit particularly higher stability. The bonding property has been analyzed by calculating the Mulliken charges and Ga-N distances. The results show that the N in GanN clusters is less ionic than that in bulk GaN (wurtzite phase). The calculated energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, the vertical ionization potential, and the vertical electron affinity form an even-odd alternating pattern with increasing cluster size. In general, the vertical ionization potential tends to lower as the cluster size increases, while the vertical electron affinity tends to increase as the cluster size increases.