Upconversion Mechanisms Of Er³⁺/Yb³⁺/Li⁺ Doped Oxide Nanocrystals And Their Applications

Rare-earth ions, especially Er³⁺, are with the characteristics of abundant energy levels and long lifetime of excited states. Therefore it is suitable for the photoluminescence processes. Infrared-to-visible upconversion of Er³⁺ has been studied extensively for their potential applications in solid-state lasers, information processing, color display, ?uorescence labeling, and so on.Discussions of the mechanisms in Er³⁺ doped various matrixes are mostly qualitative evaluation, which only list some possible processes and hence worth elaborating. In view of this situation, this work investigated the dependence of upconversion intensity on the pump power and sample temperature, combined the decay signal of upconversion under square-wave excitation. Based on the rate eqution model, this work gave the estimated value of Er³⁺ excited state absorption cross section and cross relaxation rate. In addition, the influence of sample temperature on the excited state absorption cross section was proposed.It should be noted that a key consideration of upconversion applications is the upconversion photoluminescence efficiency since every upconversion application need high signal-noise ratio. However, at the current stage, efficiency of upconversion emission is still fairly low, which greatly restricts the applications of upconversion. Recently, our team has reported that by introducing Li⁺ ions in the Er³⁺ doped oxide nanocrystals, upconversion intensity could be greatly enhanced, and its achieve mechanisms is still worth studying. This work investigated upconversion fluorescence in alkali metal ions and Er³⁺ codoped oxide nanocrystals. Based on the structural data, it was found that the influence of Li⁺ on the matrix structure is stronger than that of Na+ and K+, and no obvious change on the surface defect was observed, thus only Li⁺ can greatly enhanced the upconversion efficiency of Er³⁺ which was attributed the decrease of local symmetry around Er³⁺ in the host aroused by doping Li⁺. To study the effects of Li⁺ in more detail, this work designed a new simple antimony silicate host doped with Er³⁺ and Li⁺. On the basis of J-O theory, the enhancement mechanisms of doping Li⁺ were discussed and consistent with the previous analysis in the nanocrystals.Phosphor with pure color has obvious advantages for their application such as biolabel and color display. This work further realized efficient color-tunable phosphor in Er³⁺/Yb³⁺/Li⁺:ZrO₂ nanocrystals, specifically, single green emission from Er³⁺/Li⁺ doped ZrO₂ nanocrystals and single red emission from Er³⁺/Yb³⁺/Li⁺ doped ZrO₂ nanocrystals. In addition, upconversion mechamisms of above samples were disscussed on the basis of rate equation model.In recent years, ?uorescent temperature sensing techniques based on the optical methods have been studied widely. Among the ?uorescent temperature sensing techniques, ?uorescence intensity ratio technique using upconversion emission has attracted much attention since it can improve the sensitivity and reduce the dependence of measurement conditions, such as ?uorescence loss and ?uctuations of pumping intensity. In this work, to the best of our knowledge, we for the first time synthesize Er³⁺/Yb³⁺/Li⁺:ZrO₂ nanocrystals with intense red emission under 976 nm excitation and use red emission of Er³⁺ in fluorescence intensity ratio technology. In addition, a simple way to increase the sensitivity of fluorescence intensity ratio technology is proposed.