Electronic excitations in single-walled GaN nanotubes from first principles: Dark excitons and unconventional diameter dependences

We present the first ab initio predictions of electronic and excitonic states in gallium nitride nanotubes. Electron-hole interactions dramatically affect optical properties. Low-energy excitons are dark (dipole forbidden) in all nanotubes with key ramifications for applications. We describe an unusual decrease of energy gaps with decreasing nanotube diameter, opposing expectations from quantum confinement. This stems from a combination of nanoscale curvature and gallium chemistry, should apply to other systems, and furnishes an interesting way to reduce excitation energies with decreasing dimension.