Evolution of Properties in Prolate (GaAs)n Clusters

Abstract

The structure and properties of prolate (GaAs)n clusters corresponding to the (2, 2) and (3, 3) armchair and (6, 0) zigzag capped single-wall tubes are studied using density functional theory with generalized gradient approximation (DFT-GGA). The largest number of atoms is 120 in the (2, 2) and (3, 3) series and 116 in the (6, 0) series. It is found that the band gap in all three series does not converge to the GaAs bulk value when the cluster length increases. The (2, 2) species has the smallest gaps, which are nearly 2 times smaller than the GaAs bulk gap at larger n. Cohesive energies per atom are found to be nearly independent of the cluster diameter and correspond to 75−78% of the bulk cohesive energy per atom. Special attention is paid to the static electric dipole polarizability and hyperpolarizability because conventional DFT-GGA methods provide satisfactory results only for clusters composed of less than ∼40 atoms. For larger clusters, conventional DFT polarizabilities and especially hyperpolarizabilities exhibit the divergent behavior. An inclusion of long-range corrections drastically changes this behavior and brings the corrected values close to the values obtained in the MP2 computations with the same basis sets. The CAM-B3LYP method recently devised to account for the long-range corrections was tested as well. Finally, we estimated asymptotic values for the (hyper)polarizabilies per unit length.