Structure and electronic properties of Ge_n (n=2–25) clusters from density-functional theory

The geometrical and electronic structures of the germanium clusters with up to 25 atoms are studied by using density-functional theory with the generalized gradient approximation. The Ge_n clusters follow a prolate growth pattern with n>~13. For medium-sized clusters, we find two kinds of competing structures, stacked layered structures and compact structures. The stacked layered structures with capped tetrahedron Ge₉ cluster are more stable than compact structures and other stacked structures. The size dependence of cluster binding energies, highest-occupied and lowest-unoccupied molecular orbital gap, and ionization potentials are discussed and compared with experiments.