Alkali Metal Ions Regulate The Upconversion Luminescence Of Rare Earth-doped Y₂O₃ Thin Films

In recent years,up-conversion luminescent materials have been widely used in many fields,such as electronic display technology,communication,information engineering,LED lighting,solar cells and biological sciences.In order to further research the mechanism and explore the role of IA ions in up conversion process,we prepared the Li⁺,Na⁺,K⁺,Rb⁺-Yb³⁺/Er³⁺and Yb³⁺/Ho³⁺co-doped Y₂O₃ up-conversion films with sol-gel and spin-coating.The key of the researches and are shown as follows:（1）.For Li⁺-Yb³⁺/Er³⁺ co-doped Y₂O₃ up-conversion luminescence films,excited with a 980-nm diode laser,all the films had purple,green and red up-conversion emission bands.Li⁺ doping decreased the lattice symmetry of the matrix and improved energy transfer,and both green and red emissions were enhanced by more than one hundred times.Moreover,the ⁴S_3/2 energy level tended to be populated to increase the 556 nm emission with increasing Li⁺ ion concentration,which could change the matrix phonon energy and depress the fluorescence quenching.When the ratio of the rare earth ions to Li⁺was over 1:2,the purple emission shifted from 436 nm to 409 nm.This was because the energy level transition changed from ⁴F_3/2-⁴I_15/2 to ²H_9/2-⁴I_15/2.Here,Li⁺ played a switching role in the conversion of these two emission channels,which could be called a photo-switch effect.The mechanism was that Li⁺doping up to a critical concentration value changed the matrix phonon energy,thus improving the match of the phonon energy with the energy gap between ²G_7/2--H_9/2 while decreasing that with the energy gap between ²G_7/2-⁴F_3/2.Both of these two energy matches were necessary for the purple emission,a three-photon up conversion.（2）.For Na⁺-Yb³⁺/Er³⁺ co-doped Y₂O₃ up-conversion luminescence films,All the samples had green emission at 535 nm and red emission at 660 nm when exited with a 980-nm laser.For lower doping levels,Na⁺ion doping suppressed the green emission with the intensity reduced to 1/8 of the original,while it reduced the red emission only a little.As the doping concentration further increased,the red emission improved and then declined again.In other words,Na⁺ ion doping regulated the up-conversion luminescence spectra to some extent.The Na⁺ ions were a positive controller in 535 nm emission and effectively shielded the green emission by its own polarity around 535 nm.The Na⁺ ions also impacted the process of the energy transfer via simultaneously changing the phonon energy,the lattice symmetry and the surface defects.（3）.For K⁺or Rb⁺-Yb³⁺/Er³⁺co-doped Y₂O₃ up-conversion luminescence films,K⁺ion doping enhanced the 556 nm green emission for 2.92 times of original,and also enhanced the 660 nm red emission for 2.46 times of original respectively;Rb⁺ ion doping enhanced the 556 nm green emission for 3.93 times of original,and also enhanced the 660 nm red emission for 2.38 times of original respectively.The reason of K⁺&Rb⁺ions’the enhancement of the up conversion was weaker than Li⁺ion’s was that the Li⁺ion had two advantages:first,Li⁺ion can increase the crystallinity.Second,Li⁺ ion can enter the gap of the lattice to build a middle energy level,and then improved energy transfer efficiency,finally,enhanced the luminescence.（4）.For IA group ion（Li⁺,Na⁺,K⁺）-Yb³⁺/Ho³⁺ co-doped Y₂O₃ up-conversion luminescence films,Li⁺,Na⁺,K⁺ ion played a enhancer role,they didn’t change the mode of the up-conversion process.The order of enhancement from strong to weak was Li⁺>K⁺>Na⁺.There were two reasons to make the Li⁺to be the leader.First,only Li⁺ion can increase the crystallinity;second,Li⁺ion can enter the gap of the lattice to build a middle energy level.The polarity of the Na⁺ion wasted some energy leaded the Na⁺ ion’s luminescence enhancement was the weakest.Compared with the Na⁺-Yb³⁺/Er³⁺co-doped Y₂O₃ up-conversion luminescence films,Na⁺ion can’t adjust the emission of Ho³⁺/Yb³⁺ co-doped Y₂O₃ up-conversion luminescence films.The reason was that the Na⁺ ion can’t absorb much energy of the 5F4/5S2 of Ho³⁺ ions directly.（5）.After Rb⁺ ion doping,the luminescence of Yb³⁺/Ho³⁺ co-doped Y₂O₃ films decreased obviously.The decrease of ET1 process and the increase of lattice phonon energy were the main and secondary reasons for the decrease of luminescence efficiency,respectively.（6）.For all the Samples,all the films were composed of nanoparticles and had high transmittance.IA group ion decreased transmittance slightly.After IA group ion doping,the Y₂O₃ lattice maintained its original basic structure,while the IA group ions slightly magnified the lattice,reducing the lattice symmetry and creating surface defects.