  The strong excitonic emission at 5.75 eV of hexagonal boron nitride (h-BN)
makes this material one of the most promising candidate for light emitting
devices in the far ultraviolet (UV). However, single excitons occur only in
perfect monocrystals that are extremely hard to synthesize, while regular h-BN
samples present a complex emission spectrum with several additional peaks. The
microscopic origin of these additional emissions has not yet been understood.
In this work we address this problem using an experimental and theoretical
approach that combines nanometric resolved cathodoluminescence, high resolution
transmission electron microscopy and state of the art theoretical spectroscopy
methods. We demonstrate that emission spectra are strongly inhomogeneus within
individual flakes and that additional excitons occur at structural
deformations, such as faceted plane folds, that lead to local changes of the
h-BN stacking order.
