  We analyse the morphological, structural and luminescence properties of
self-assembled ZnO nanowires grown by chemical vapour transport on Si(001). The
examination of nanowire ensembles by scanning electron microscopy reveals that
a non-negligible fraction of nanowires merge together forming coalesced
aggregates during growth. We show that the coalescence degree can be
unambiguously quantified by a statistical analysis of the cross-sectional shape
of the nanowires. The examination of the structural properties by X-ray
diffraction evidences that the nanowires crystallize in the wurtzite phase,
elongate along the c-axis, and are randomly oriented in plane. The luminescence
of the ZnO nanowires, investigated by photoluminescence and cathodoluminescence
spectroscopies, is characterized by two bands, the nearband-edge emission and
the characteristic defect-related green luminescence of ZnO. The
cross-correlation of scanning electron micrographs and monochromatic
cathodoluminescence intensity maps reveals that: (i) coalescence joints act as
a source of non-radiative recombination, and (ii) the luminescence of ZnO
nanowires is inhomogeneously distributed at the single nanowire level.
Specifically, the near-band-edge emission arises from the nanowire cores, while
the defect-related green luminescence originates from the volume close to the
nanowire sidewalls. Two-dimensional simulations of the optical guided modes
supported by ZnO nanowires allow us to exclude waveguiding effects as the
underlying reason for the luminescence inhomogeneities. We thus attribute this
observation to the formation of a core-shell structure in which the shell is
characterized by a high concentration of green-emitting radiative point defects
with respect to the core.
