Chirality Effects and Semiconductor versus Metallic Nature in Halide Nanotubes

A density functional theory study of the electronic structure of nanostructures based on the hexagonal layers of LuI₃ is reported. Both bulk and slabs with one to three layers exhibit large and indirect bandgaps. Different families of nanotubes can be generated from these layers. Semiconducting nanotubes of two different chiralities have been studied. The direct or indirect nature of the optical gaps depends on the chirality, and a simple rationalization of this observation based on band folding arguments is provided. Remarkably, a metastable form of the armchair LuI₃ nanotubes can be obtained under a structural rearrangement such that some iodine atoms are segregated toward the center of the nanotube forming chains of dimerized iodines. These nanotubes having an Lu₂ I₅ backbone are predicted to be metallic and should be immune toward a Peierls distortion. The iodine chains in the inner part of the nanotubes are weakly bound to the backbone so that it should be possible to remove these chains to generate a new series of neutral Lu₂ I₅ nanotubes which could exhibit interesting magnetic behavior. Because the LuI₃ structure occurs for a large number of lanthanide and actinide trihalides, a tuning of the optical, transport, and probably magnetic properties of these new families of nanotubes can be a challenging prospect for future experimental studies.