Crystal binding and metal-semiconductor transition in aluminate nanotube bundles

Abstract

We perform first-principles theoretical calculations to study the bonding and electronic characteristics of AlO2 nanotube bundles. We find that the nanotubes are tightly linked by O-O peroxy or Al-O-Al bridge bonds, forming crystalline bundles, where tubes are arranged in a triangular or a square lattice in the two-dimensional plane perpendicular to their common axes. Intertube interactions mostly occur between the outer O shells of individual tubes, and the metallicity of bundles is strongly affected by intertube distance and doping. The bundles with only the peroxy bonds are semiconducting, while those having purely the bridge bonds or a mixture of two types of intertube bonds are metallic. A semiconductor-metal transition can occur by applying compressive strains perpendicular to the tube axes or by intercalating the Li ions, accompanied with the change of intertube bonds. We also examine the formation of bundles in the form of Al2O3, where tubes are connected by bridge-type bonds. We find that these bundles are energetically more favorable than the AlO2 nanotube bundles and exhibit the semiconducting behavior.