Photoluminescence (PL) spectroscopy was employed to evaluate
the optical properties of the ZnO nanocrystals fabricated at the experimental
conditions, Steps II and III, as shown in Fig. 10 (a) and (b), respectively.
The room-temperature PL spectrum was carried out with
an excitation wavelength of 350 nm. Fig. 10 (b) is very similar to the
typical PL spectrumof ZnO nanocrystals prepared at roomtemperature [17,29]. The PL spectra of the ZnO nanorods have a weak band ultraviolet
(UV) PL band at about 380 nm, attributed to an exciton-related
activity [30] and a relatively strong visible (green/yellow) emission
band at around 550 nm, due to the point defects, such as oxygen vacancies
or Zn interstitials which act as donors at levels located below the
conduction band edge [29,31–33]. It is interesting to note that the PL
spectra (a and b) are very different from each other. The PL spectrum
(a) of the ZnO seed films exhibits a strong blue emission at around
400 nm and a weak green emission at around 510 nm. The presence
of a broad PL emission at around 380 nm and a shoulder PL peak at
~500 nm has been observed previously for the ZnO film grown at
pH 6[21]. The green emission, also known as a deep-level emission, is
caused by the impurities and structural defects in the ZnO nanorods.
The strong PL peak in the blue emission region from the ZnO seed layers
is reduced and green emission is enhanced during the hydrothermal
growth of ZnO nanorods. Therefore, it is suggested that the initially
grown ZnO seed layers having less defect density and hence smaller
defect-related emission may become high defect-density ZnO nanorods
in the hydrothermal growth process. Moreover, the contributions of
the green emission from ZnO nanoflowers grown on top of the ZnO
nanorods during the hydrothermal growth process cannot be ruled out.