Optical Transitions in Artificial Few-Electron Atoms Strongly Confined inside ZnO Nanocrystals

We have studied the optical transitions in artificial atoms consisting of one to ten electrons occupying the conduction levels in ZnO nanocrystals. We analyzed near IR absorption spectra of assemblies of weakly coupled ZnO nanocrystals for a gradually increasing electron number and found four allowed dipole transitions with oscillator strengths in quantitative agreement with tight-binding theory. Furthermore, this spectroscopy provides the single-particle energy separation between the conduction levels of the ZnO quantum dots.

Figure 1

The increase in the absorption (blue lines) due to charging of a quantum dot assembly (average diameter is 4.2 nm) with on average 0.8 (the lower spectrum) and 3.7 excess electrons per quantum dot. The black lines show the increase in absorption of a colloidal solution of the same ZnO quantum dots, due to excitation with UV light ($\lambda =300\mathrm{n}\mathrm{m}$). The latter spectra are isomorphic to those obtained with ZnO quantum dot assemblies. The spikes in the spectra at 2500 and $3000{\mathrm{c}\mathrm{m}}^{-1}$ are due to photoinduced absorption by the solvent molecules. Inset: time-dependence of the IR absorption peak of a ZnO quantum dot dispersion upon switching the UV illumination on and off.

Figure 2

Absorption spectra (blue) and corresponding fits (black) for a ZnO quantum dot assembly (average diameter is 4.2 nm) at low and higher electron occupation $⟨N⟩$. The spectrum for $⟨N⟩=0.8$ can be fitted with the $S\mathrm{\text{−}}P$ ($+$) and $P\mathrm{\text{−}}D$ (□) transition. The spectrum for $⟨N⟩=4.9$ can be fitted with the $S\mathrm{\text{−}}P$ ($+$), $P\mathrm{\text{−}}D$ (□), $P\mathrm{\text{−}}{S}^{\prime }$ ($\times$), and $D\mathrm{\text{−}}F$ (○) transition. Inset: size distribution of quantum dots with an average diameter of 4.2 nm.

Figure 3

The separations between the single-electron energy levels obtained from analysis of the IR absorption spectra (symbols) and calculated with tight-binding theory (lines) as a function of the diameter of the ZnO nanocrystals. The dashed line shows the $S\mathrm{\text{−}}P$ separation calculated with an effective mass approximation.