A typical result of TEM measurements for undoped ZnO nanocrystals. The nanocrystals are almost spherical and their size is about 3 nm.
XRD results for (a) bulk ZnO (powder) and nanocrystals with (b) and (c) . The analysis of the linewidth using modified Scherrer’s formula gives 3.0 nm as an average diameters for the nanocrystals.
Room temperature (a) absorption and (b) PL spectra for nanocrystals with = 0, 0.001, 0.002, 0.005, and 0.01 dispersed in ethanol. The absorption spectra are measured with a 1 mm thick optical cell. The inset in (a) shows absorption increase relative to the undoped sample measured in the mid-gap region. The inset in (b) shows normalized spectra of the band-edge PL. In all figures the cobalt concentration increases from black to blue.
Streak camera signal for (a) undoped and (c) x = 0.005 samples measured at T = 20 K. (b) and (d) are time resolved spectra extracted from the (a) and (c), respectively. The undoped sample shows a signal at negative time region because of its long lifetime longer than the laser repetition period. This component is interpreted as dark exciton PL and almost disappeared in the doped sample. The faster one seen at lower energy is a PL perturbed by a charge at the surface or internal trap state, which become faster in doped sample. (e) shows schematic diagram of the states. (f) shows mole fraction of cobalt atoms estimated under the assumption that the dark exciton PL comes only from the nanocrystals without any cobalt inclusion.
Decay profiles for the two components of the band-edge PL evaluated by numerical decomposition of the time resolved spectrum. Figure (a) shows an example of the decomposition. (b) and (c) shows obtained decay profiles of the two components. (d) shows cobalt mole fractions estimated from the curve fitting of the decay profile for the low energy component under the assumption of Poisson distribution of cobalt atoms.
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