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We analyse the morphological, structural and luminescence properties ofself-assembled ZnO nanowires grown by chemical vapour transport on Si(001). Theexamination of nanowire ensembles by scanning electron microscopy reveals thata non-negligible fraction of nanowires merge together forming coalescedaggregates during growth. We show that the coalescence degree can beunambiguously quantified by a statistical analysis of the cross-sectional shapeof the nanowires. The examination of the structural properties by X-raydiffraction evidences that the nanowires crystallize in the wurtzite phase,elongate along the c-axis, and are randomly oriented in plane. The luminescenceof the ZnO nanowires, investigated by photoluminescence and cathodoluminescencespectroscopies, is characterized by two bands, the nearband-edge emission andthe characteristic defect-related green luminescence of ZnO. Thecross-correlation of scanning electron micrographs and monochromaticcathodoluminescence intensity maps reveals that: (i) coalescence joints act asa source of non-radiative recombination, and (ii) the luminescence of ZnOnanowires is inhomogeneously distributed at the single nanowire level.Specifically, the near-band-edge emission arises from the nanowire cores, whilethe defect-related green luminescence originates from the volume close to thenanowire sidewalls. Two-dimensional simulations of the optical guided modessupported by ZnO nanowires allow us to exclude waveguiding effects as theunderlying reason for the luminescence inhomogeneities. We thus attribute thisobservation to the formation of a core-shell structure in which the shell ischaracterized by a high concentration of green-emitting radiative point defectswith respect to the core.

Coalescence, crystallographic orientation and luminescence of ZnO nanowires grown on Si(001) by chemical vapour transport