Unambiguous identification of recombination lines in single zinc-blende ZnSe nanowires in direct relation to their microstructure

We present results on the low-temperature photoluminescence characterization of individual ZnSe nanowires, whose crystal structure was determined to be zinc-blende by transmission electron microscopy on the same individual nanowires as were studied optically. The photoluminescence response from single ZnSe nanowires was found to be dominated by excitonic emission due to native point defects, while no emission peaks related to the unintentional impurities were detected. Two strong photoluminescence lines were observed at 2.785 eV and 2.780 eV, assigned to the excitons bound to deep neutral acceptors related to the vacancies of Zn (V-Zn) and complexes of V-Zn, respectively. Another recombination peak at 2.800 eV related to the free exciton emission in ZnSe was also observed. Longitudinal optical-phonon replicas of up to three orders were seen for both lines, and the average number of emitted phonons was also determined. The excitonic emission linewidths of 1.5 meV were observed from individual nanowires, which are the narrowest excitonic linewidths reported so far for ZnSe nanowires. The optical response from a single nanowire was also compared to that from a bundle of nanowires, and it was found that the linewidths of excitonic emission from the bundle of nanowires were slightly larger than those from single nanowires, due to the effects of ensemble broadening. It is also suggested that in the case of a bundle of nanowires, the broadening is limited by the nanowire which exhibits the largest excitonic linewidth.