Enhanced upconverted photoluminescence in Er3+ and Yb3+ codoped ZnO nanocrystals with and without Li+ ions

The upconversion luminescence spectral intensity of Er3+ in Er3+ and Yb3+ codoped ZnO nanocrystals with and without Li+ are investigated. Yb3+ ions as a tradition sensibilizer have efficient energy transfer processes from Yb3+ (2F5/2) to Er3+ (4I13/2, 4I11/2, 4F9/2), which lead to the increment of upconversion luminescence intensity. Following by adding Li+ to the Er3+ and Yb3+ codoped ZnO nanocrystals, the upconversion intensity emitted by Er3+ ions is found greatly enhanced. The enhancement is attributed to the distortion of the local field symmetry of Er3+ ions, so increases various intra-4f transitions of Er3+ ions. Both Yb3+ and Li+ can disperse Er3+ ions in specimen, so reduced the interaction between neighboring Er3+ ions.