Figure 1

TEM image of drop-cast ZnO nanocrystals. The crystal shape is recognized to be almost spherical, with average diameter $~$3 nm.Reuse & Permissions

Figure 2

Absorption (dashed line) and photoluminescence (solid line) spectra for the sample with average size of 3.2 nm measured at room temperature. The Stokes shift is estimated to be 135 meV as the difference between the lower edges of the two spectra.Reuse & Permissions

Figure 3

Time-resolved luminescence at 20 K detected by the streak camera for the sample with average size of 3.2 nm. Two components with different peak energies and different lifetimes are recognized. The high-energy component with a very long lifetime has a signal intensity at the negative time region, which is a result of the superposition of the long tails from the previous pulse events onto the measurement time window.Reuse & Permissions

Figure 4

The result of the spectrum decomposition at 20, 100, and 160 K. All spectra are successfully decomposed into two components. At all temperatures, the low-energy component fades away before 950 ps and the spectra become dominated by the high-energy component. Note that the spectrum at 950 ps is mostly composed of the superposed tail signals.Reuse & Permissions

Figure 5

The crystal size dependence of the peak energies of the two components. The horizontal axis shows absorption-edge energy at room temperature, which varies with the particle size as a reflection of the quantum confinement effect. Both of the peak energies are on the slope of unity, indicating that both components are related to the band-edge state of ZnO nanocrystals.Reuse & Permissions

Figure 6

The time profiles of the two components obtained from the spectrum decomposition at each time point. (a) The low-energy component decays almost exponentially. The temperature dependence of the lifetime is explained by the thermal quenching model as shown in the inset. (b) The high-energy component shows a steep decrease just after the excitation and then approaches the constant value above the background level (dashed line) composed of the superposed tail signals. The inset shows a schematic illustration of the model used to analyze the time profile including superposed signals.Reuse & Permissions

Figure 7

(a) Temperature dependence of the decay rate of the high-energy component obtained in the analysis of the time profile. The inset shows the temperature dependence of the amplitude of the component. (b) Temperature dependence of the peak energies of low- and high-energy components. The dashed lines show the temperature dependence of the band-gap of the bulk ZnO, shifted in energy for the coincidence at low temperatures. The deviation of the high-energy component from the bulk band gap is accounted for by the thermal equilibrium model of dark and bright states.Reuse & Permissions