Exciton luminescence of boron nitride nanotubes and nano‐arches

Abstract

AbstractWe 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 hypo‐ thesis. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)