The effect of Li on the spectrum of Er3+ in Li- and Er-codoped ZnO nanocrystals

Infrared-to-visible upconversion emission intensities are investigated in Li(+) and Er(3+) codoped ZnO nanocrystals. Li(+) ions doped in ZnO/Er(3+) nanocrystals can greatly enhance the upconversion emission intensity of Er(3+) ions. The extended X-ray absorption fine structure spectroscopy data show that both the Er-O bond length and coordination number of the Er-Er bond have been altered by introducing Li(+) ions in the ZnO/Er(3+) nanocrystals. The variation of Er-O bond length leads to the change in the local asymmetry around Er(3+) ions. Meanwhile, the greater coordination number of the Er-Er bond causes stronger interaction between neighboring Er(3+) ions and, hence, strengthens the reaction Er(3+) ((4)F(7/2)) + Er(3+) ((4)I(1/2)) --> 2Er(3+) ((4)F(9/2)). In this process,the (4)F(9/2) state is a metastable state that could be excited to the (2)H(9/2) state by absorbing one photon at high excitation power. Li(+) ions also can reduce the OH groups in specimen, which is the other reason for enhancing the upconversion emission intensities.