Impurity energy levels

It is important to emphasize that the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light (X < 380 nm) retained the same photocatalytic efficiency as the unimplanted original pure titanium oxides under the same UV light irradiation conditions. When metal ions were chemically doped into the titanium oxide photocatalyst, the photocatalytic efficiency decreased dramatically under UV irradiation due to the effective recombination of the photo-formed electrons and holes through the impurity energy levels formed by the doped metal ions within the band gap of the photocatalyst (in the case of Fig. 6)... 

Our results clearly show that modification of the electronic state of titanium oxide by metal ion implantation is closely associated with the strong and longdistance interaction which arises between the titanium oxide and the metal ions implanted, as shown in Fig. 13, and not by the formation of impurity energy levels within the band gap of the titanium oxides resulting from the formation of impurity oxide clusters which are often observed in the chemical doping of metal ions, as shown in Figs. 6 and 13. 

In semiconductor phosphors the energy band structure of the host crystal plays a central role. Some semiconductor luminescence arises from decay of exciton states, other emission arises from decay of donor states generated by impurity or defect centers. It is not the magnitude of the band gap itself that separates insulator from semiconductor phosphors it is a question of whether the spectrum is characteristic of impurity energy levels as perturbed by the local crystal structure or whether the spectrum is characteristic of the band structure as modified by impurities. 

Figure 24.2 Impurity energy levels in the gap for (Zn29056X)56- with the impurity at the O substitutional site.

Figure 24.3 Impurity energy levels in the gap for (Zn56 Y096)78 with the impurity Y at the Zn substitutional site. The meanings of the symbols used in this figure, —, , and are the same in Figure 24.2.

In conclusion, we briefly summarize the chief points made in this report. We calculated the electronic structures of ZnO with impurity atoms from Li to Bi without radioactive elements to study impurity energy levels. Atomic cluster models used in the calculations were based on the (Zn29056X)56- cluster with an X atom located at its O site and on the (Zn56Y096)78 cluster with a Y atom... 

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