Exciton luminescence of boron nitride nanotubes and nano-arches

We report photoluminescence (PL) and PL-excitation spectroscopy of BN nanotubes (nt-BN) mixed with some residual hexagonal crystalline (h-BN) starting material, and of pure h-BN microcrystalline powder. The nanotube phase exhibits a broad-band PL near 380 nm, in agreement with a published report of cathodoluminescence from a sample comprising >90% nanotubes. This emission is almost 3 eV lower in energy than unrelaxed exciton states found in recent all-electron theories of nt-BN and h-BN and about 1.4 eV lower than the lowest (perturbed dark?) exciton seen in absorption of nt-BN. This may suggest that excitons in nt-BN vibrationally relax to self-trapped states before emitting, a path found in many wide-gap solids, especially in quasi-1-dimensional forms. Exciton emission from bulk single-crystal h-BN has been shown to occur from vibrationally unrelaxed (free-exciton) states. We suggest a hypothesis in which known nano-arch reconstructions on the surface of h-BN may provide the low-dimensional environment to make exciton self-trapping on the surfaces of h-BN likely. This allows consistent interpretaton of the surface-related 380 nm emission from h-BN powder within a half-nanotube self-trapped exciton hypothesis.