  Hexagonal boron nitride (h-BN) and graphite are structurally similar but with
very different properties. Their combination in graphene-based devices meets
now a huge research focus, and it becomes particularly important to evaluate
the role played by crystalline defects in them. In this work, the
cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are
reported and compared to those of nanosheets mechanically exfoliated from them.
First the link between the presence of structural defects and the recombination
intensity of bound-excitons, the so-called D series, is confirmed. Low
defective h-BN regions are further evidenced by CL spectral mapping
(hyperspectral imaging), allowing us to observe new features in the
near-band-edge region, tentatively attributed to phonon replica of exciton
recombinations. Second the h-BN thickness was reduced down to six atomic
layers, using mechanical exfoliation, as evidenced by atomic force microscopy.
Even at these low thicknesses, the luminescence remains intense and exciton
recombination energies are not strongly modified with respect to the bulk, as
expected from theoretical calculations indicating extremely compact excitons in
h-BN.
