Crystalline-structure-dependent green and red upconversion emissions of Er3+–Yb3+–Li+ codoped TiO2

Crystalline structures and infrared-to-visible upconversion luminescence spectra have been investigated in 1 mol% Er3+, 10 mol% Yb3+ and 0–20 mol% Li+ codoped TiO2 [1Er10Yb(0–20)Li:TiO2] nanocrystals. The crystalline structures of 1Er10Yb(0–20)Li:TiO2 were divided into three parts by the addition of Yb3+ and Li+. Both green and red upconversion emissions were observed from the 2H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+ in Er3+–Yb3+–Li+ codoped TiO2, respectively. The green and red upconversion emissions of 1Er:TiO2 were enhanced significantly by Yb3+ and Li+ codoping, in which the intensities of green and red emissions and the intensity ratio of green to red emissions (Igreen/Ired) were highly dependent on the crystalline structures. The significant enhanced upconversion emissions resulted from the energy migration between Er3+ and Yb3+ as well as the distortion of crystal field symmetry of Er3+ caused by the dissolving of Li+ at lower Li+ codoping concentration and the phase transformation at higher Li+ concentration. It is concluded that codoping with ions of smaller ionic radius like Li+ can efficiently improve the upconversion emissions of Er3+ or other rare-earth ions doped luminsecence materials.

Research Highlights
► Crystalline structures of Er:TiO2 are divided into three parts by Yb and Li codoping.
► Green and red UC emissions are strongly dependent on the crystalline structures.
► Change of Er symmetry and phase structure by Li codoping enhanced UC emissions.