2, University of California Riverside, Riverside, California, United States
The extraordinary properties of carbon nanotubes have driven a search for new nanotubes with unique properties. Here we present detailed analyses on the electronic properties of two novel nanotube families, phosphorene and porphyrin nanotubes, from large-scale DFT calculations (up to 1476 atoms and 18,432 orbitals). In the phosphorene nanotubes, we uncover a direct-to-indirect bandgap transition with decreasing nanotube diameter, a property which has direct implications for applications that require either (i) fast charge recombination and high light absorption (i.e., a direct bandgap ) or (ii) slow recombination and large diffusion length (i.e., an indirect bandgap). In the porphyrin nanotubes, we find extremely large oscillations in the bandgap as a function of size, in contradiction to quantum confinement effects (i.e., the bandgap increases with size in several of these nanotubes). As a result of these unusual oscillations, we find that both type I and type II p-n heterojunctions are possible in this single nanotube family. We emphasize that each of these families possess properties not present in conventional carbon nanotubes, each offering a wide range of tunability for applications in both light-emitting diodes (LEDs) and solar cells.