One-dimensional nanostructures such as nanowires (NWs) have attracted much attention due to their unique properties. The reduction in size leads to novel electrical, mechanical, chemical, and optical properties. NWs are also expected to be important functional units for optoelectronic nanoscale applications, when being integrated in nanodevices. Zinc oxide (ZnO) is a semiconductor material of great importance for optoelectronics due to its wide direct band-gap of 3.37 eV and its extremely large exciton binding energy of 60 meV. Therefore, ZnO NWs have great potential for an advantageous use in devices. In this respect, it is crucial to have detailed information about the recombination dynamics of the ZnO NWs and their dependence on wire dimensions. In this work, the influence of finite-size on the recombination dynamics of the neutral donor-bound exciton (DX) around 3.365 eV has been investigated for single-crystal ZnO NWs with different diameters grown on SiO2/Si substrates by the vapor transport method using Au as catalyst. We demonstrate that the lifetime of this excitonic transition decreases with increasing the surface-to-volume ratio due to a surface induced recombination process. Furthermore, we have observed two broad transitions around 3.341 eV (S1) and 3.314 eV (S2) whose intensity increases for the smaller NWs diameters [1]. In order to study their origin we have investigated the temperature dependence of their photoluminescence (PL) intensity as well as their thermal activation energy. Comparing their intensities and recombination times to those of the main excitonic recombination around 3.365 eV we conclude that S1 and S2 might originate from surface states. We observed that the diameters as well as the length of the NWs determine the lifetime of the neutral donor bound excitons. Our findings suggest that while the length is mainly responsible for different mode quality factors of the cavity-like NWs, the diameter determines the influence of --surface states as alternative recombination channels for the optical modes trapped in the nanocavity [2]. These results are of great interest for a precise design of ZnO-based nanostructures, since they represent a step toward a deep understanding of its size-dependent recombination dynamics.
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