Zinc Oxide Nanowire Films: Solution Growth, Defect States and Electrical Conductivity - Advanced Materials for Agriculture, Food and Environmental Safety - page 466

126]. Band edge transition is related to various exciton recombinations.
h e associations of impurities/defects with exciton or exciton impurity
complexes play an important role in the luminescence [127]. Balandin
et al. stated that UV emission in ZnO may be attributed to the coni ned
excitons, transverse optical phonon band of the coni ned excitons, donor-
bound excitons, acceptor-bound excitons, or donor acceptor pairs [128].
h eir group reported that recombination of the acceptor-bound excitons is
mainly responsible for emission at all temperatures, while recombination
of the acceptor-bound excitons is dominant in PL only at low temperature
[128]. In the case of ZnO quantum dots (2 to 6 nm), excitons coni ned in
the quantum dot and excitons bound to an ionized impurity located at the
quantum-dot surface are considered as a source of emission [129]. Polarity
of ZnO facets also af ect the emission from free excitons and it is found that
the O face is more intense than the Zn face [130]. Dif erent shape- and size-
dependent nanostructures show variable peak position of UV PL spectra
and more than 10 nm shit is observed, these shit s in the UV emission peak
are not due to the coni nement (size is larger than Bohr radius) but due to
dif erent concentrations of native defects/surface defects [125].

16.3.2.2 Defect Emission

Figure 16.3a shows PL measurement of a CVD grown high quality ZnO
nanowire i lm, where only one emission peak appears at room tempera-
ture. Figure 16.3b shows a PL spectra of solution grown ZnO nanowire;
emission spectra shows another very intense and broad peak in the range
of 450 to 750 nm and is called defect emission peak. Unstructured broad
emission peak in visible range can deconvolute in many peaks.

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Figure 16.3 Room-temperature PL spectra of (a) high quality CVD grown ZnO
nanowires [124], (b) Defect rich solution grown ZnO nanowires [29].