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 11/2 ) → 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.