Study On The Growth Phase Diagram And Upconversion Luminescence Of NaLuF₄ Nanocrystals

Rare earth?RE?ions are widely used in fluorescent materials because of their unique electronic structure and abundant energy levels.With the rise of“nano science and technology”at the end of last century,RE-doped nanomaterials have gradually aroused the interest of scientific researchers.In recent years,thermal decomposition method has been widely used in the preparation of nanomaterials.The advantages of this method are that shorter preparation cycle and small nanoparticles are easier to obtain than other preparation methods.In addition,the matrix has great influence on the luminescent properties of materials.In order to reduce the energy loss caused by non-radiative transition and maximize the radiative energy transition in RE-doped luminescent materials,the matrix material should have low phonon energy.Fluorides are suitable as matrix materials because of the higher chemical stability and lower phonon energy.Among them,NaREF₄ is considered to be the most important category.The physical and chemical properties of nanomaterials are closely related to the crystalline phase,size and shape of nanoparticles.Doping appropriate atoms or ions into the matrix material is an effective way to improve or even change the electrical,magnetic,and optical properties of nanomaterials.For several decades,great endeavors have been devoted to investigate the impact of nanocrystalline structure,size and shape on the behaviors of dopants.On the contrary,few works concerned the impurity doping-induced control over the growth and phase structure of functional NCs.In fact,impurity doping was recently found to have significant influence on the nucleation and growth of many functional nanomaterials,and provide a fundamental approach to modify the crystallographic phase,size,shape and electronic configuration of NCs.In this article,we studied the nucleation,phase transition and Oswald ripening process of excellent Na Lu F₄ nanocrystals.Furthermore,the UV and deep UV upconversion luminescence of Tm³⁺and Gd³⁺were also studied.On the other hand,good results have been obtained in the research on upconversion white light materials.The main contents are outlined as follows:1.Based on the thermal decomposition method,a series of Gd³⁺variable NaLuF₄:Yb³⁺/Tm³⁺nanomaterials were prepared by the self-developed automatic nanomaterial synthesizer.In order to ensure the accuracy of the experimental results,we used a syringe to extract samples at different times during the same experiment.Based on a large number of experimental data,the T-T-P-S phase diagram of NaLuF₄ nanocrystals with temperature,time,crystal phase and size is plotted.This phase diagram can guide us to synthesize samples with definite purposes in order to obtain desired NaLuF₄ nanomaterials.In addition,the influence of Gd³⁺concentrations on upconversion luminescence of NaLu F₄:Yb³⁺/Tm³⁺/Gd³⁺nanocrystals was investigated.These results indicated that 10%Gd³⁺was the best doping concentration for obtaining the NaLu F₄ NCs with relatively small size and high brightness.Further increasing Gd³⁺concentration induced a drastic decrease in UCL intensity.2.We studied the luminescence properties of Yb³⁺,Tm³⁺,Gd³⁺co-doped NaLuF₄ nanomaterials.The UV upconversion luminescence of Gd³⁺was measured at vacuum of 5?10^-4 Pa and low temperature of 10 K.7 mol%Gd³⁺doped nanocrystals?size<30nm?exhibited the strongest UV upconversion luminescence properties.The emission peaks are observed at 201.4 nm,201.6 nm,202 nm,203.2nm,and 203.5 nm could be attributed to the transitions ⁶G_j?j=5/2,11/2,9/2,7/2??⁸S_7/2,of Gd³⁺ions respectively.It is found that the high order upconversion process may be a seven photon process.It is the first time that seven photon upconversion luminescence of Gd³⁺was observed in nanoparticles of less than 20 nm.3.We prepared a series of NaLu F₄:Yb³⁺/Tm³⁺nanocrystals by using a automatic nanomaterial synthesizer which can precisely control the experimental parameters.The nucleation and phase transformation of nanocrystals at 285,295 and305^°C were studied respectively.The morphology and phase analysis showed that the synthesized?-phase nanocrystals have larger size at higher temperature,but less crystallinity obtained.We further discussed the influence of the size and crystallinity of nanocrystals on the upconversion luminescence intensity of these samples.The results indicated that?-NaLuF₄:Yb³⁺/Tm³⁺nanomaterials with small crystal size and strong luminescence can be synthesized at relatively low temperature of 295^°C.4.White upconversion luminescence was achieved under 980 nm excitation in the CaF₂:Yb³⁺/Eu³⁺material using Y³⁺to adjust the luminescence performance.In this luminescent system,Yb³⁺not only plays the role of a sensitizer of Eu³⁺,but also generates green fluorescence from Yb³⁺dimers(2-Yb³⁺)by cooperative transitions in the CaF₂ matrix.One of the primary colors of green corresponds to the 2-Yb³⁺cooperative emission exactly.Eu³⁺acts as an activator for emitting red and blue fluorescence simultaneously.Interestingly,the color of the UCL can be controlled by adjusting the doping concentration of Y³⁺ions,and white UCL was realized when the concentration of Y³⁺was 1%.