Study And Mechanism Analysis Of Multiply Upconversion Luminescence Of Rare-earth Doped NaYF₄

Infrared upconversion luminescence materials has been paid high attention to by anti-counterfeiting workers recently, because they can be excited by infrared light(700-1500nm) to emit visible light,resulting in visual effects. It is known that red green and blue (RGB) are called three primary colors, and most of the other colors can be obtained by the mixture of red, green, and blue according to different proportion. Based on this, the study of RGB multiply upconversion luminescence properties are carried out in the thesis. At the same time, new excitation band was explored besides 980nm band to increase efforts of up-conversion anti-counterfeiting technology and improve the security of anti-counterfeiting technology. Based on the discussion mentioned above, in the thesis, the main works are described in brief as follows:1. Yb³⁺/Er³⁺-codoped hexagonal-NaYF₄ samples have been prepared by solid-state reaction method. Strong red upconversion emission is obtained under 980nm laser excitation. The dependences of the intensities for the red and emissions on Yb³⁺concentration and pump power are discussed. And its possible mechanisms are further discussed. Interestingly, it is found that the luminescent color of samples can be modulated by changing pump power or Yb³⁺ concentration.2. Yb³⁺/Tm³⁺-codoped hexagonal-NaYF₄ samples have been obtained via solid-state reaction method. With changing Tm³⁺ concentration, the intensities of the blue emissions (450 and 478nm) of Tm³⁺ ion firstly increase and then decrease. The optimal doping concentration of Tm³⁺ is 1mol%. And the evolution of emission intensity of Tm³⁺ on Er³⁺ concentration is also discussed. It is found that the introduction of Er³⁺ could enhance the emission intensity of Tm³⁺ in certain concentration range because of the energy transfer from Er³⁺ to Tm³⁺. Moreover, the intensity of blue emission (455nm) is increased by about 8.3 times.3. Er³⁺-doped hexagonal-NaYF₄ samples have been synthesized by solid-state reaction method. Under the 808nm excitation, Green upconversion emission at 542nm from hexagonal-NaYF₄: Er³⁺ phosphors can be directly observed with the naked eye. And other abundant upconversion bands of Er³⁺ are also presented. The effects of Er³⁺ concentration on upconversion properties are investigated and its mechanism has been discussed. In the meantime, we also study the effects of the Yb³⁺ concentration on upconversion intensity of Er³⁺ in NaYF₄ host. It is found that the evolution of emission intensity of Er³⁺ on Yb³⁺ concentration at different concentration of Er³⁺ content is different. A competitive energy transfer mechanism between Yb³⁺â†'Er³⁺ and Er³⁺â†'Yb³⁺ is proposed to explain the Yb³⁺-codoping effect on Er³⁺ upconversion. More interestingly, it is suggested that the Energy Back Transfer from Yb³⁺ to Er³⁺ at high concentration of Er³⁺ can enhance the upconversion emission intensity of Er³⁺. The investigation offers a new possible way to enhance the near-infrared (808nm) to green (542nm) and red (655nm) upconversion of Er³⁺ ions.