| Inorganic nano/micro-rare earth doped upconversion (UC) and downconversion (DC) materials have potential applications prospect, such as solid-state laser, laser anti-counterfeiting brand, fluorescence probe, three dimensional display, light-emitting diode black light and solar cells, etc. It is common sense that the properties of inorganic nano/micro-materials are dependent on their composition, size, shape and crystalline structure. Investigation on the ralationships among these features, and then designing and assembling the structure with unique properties have great significance. Researchers have invested substantive manpower, material resources and funds, but the study suggests that the low efficiency of UC would limit their practical application. Therefore, the selection of new luminescence mechanism, new preparation method and new matrix in order to obtain high luminescence efficiency is a hot spot in this field. In contrary to UC process, DC process is defined as that absorbing one high-energy photon and emitting two or more low-energy photons. In recent years, how to enhance the energy conversion efficiency of solar cells has become an important issue of concern. Near infrared quantum cutting (NIR QC) DC emission process, converting one high-energy photon (UV/VIS) into two or more low-energy infrared photons which can be better absorbed by silicon crystal, is a promising approach to enhance the energy conversion efficiency of Si solar cells. The energy loss due to the thermalization in c-Si solar cells could be minimized. However, intensive studies on the NIR QC materials are focused on high temperature solid phase synthesis of bulk materials and the quantum efficiency should be further enhanced. We combined UC and DC in one multi, prepared and investigated effects of different morphologies, sizes and dopants on the luminescence properties of Y2SiO5GdF3/BaGdF5/ErF3and Lu6O5F8. The main points are listed below:(1) Considering oxy-compound possesses high chemical stability, we select Y2Si05as the host of UC emission firstly.Under excitation at980nm, the UC emission processes of Y1.98-2xYb2x Er0.02Si05(0.00≤x≤0.15) were studied. The concentration quenching of the red light emission is higher than that of green, due to the back energy transfer between Yb3+and Er3+ions (4F7/2(Er3+)+2F7/2(Yb)-â†'4I11/2(Er)+2F5/2(Yb)); Furthermore, Yb3+/Tm3+and Yb3+/Ho3+codoped Y2Si05samples emit strong blue and yellow light, respectively. Bright white luminescence upon980nm near-infrared excitation was obtained in Y2SiO5:0.4%Tm3+,5%Yb3+,0.1%Ho3+by tuning the doping concentrations and pump power.(2) Fluoride has very low vibrational energy, and its nonradiative loss could be suppressed and a high quantum efficiency of the desired luminescence could be obtained. So we select fluoride as not only UC luminescence but also NIR QC host at the same time.It is well known that Gd3+ions have a4f7electronic configuration, so we choose GdF3and BaGdF5as matrices, which are expected to be efficient magnetic-optical multifunctional materials. The band structures and densities of state of GdF3were studied with the help of first principles calculations, and the direct band gap of GdF3was estimated to be7.443eV wide. A series of GdF3:Yb3+/Ho3+/Tm3+nanorods were prepared by a simple and green hydrothermal method. After heat treatment, the emission color coordinates of GdF3:0.15Yb3+/0.002Ho3+/0.008/Tm3+moved from red region to the central white region of the chromaticity diagram. In addition, NIR QC mechanisms and quantum efficiencies of GdF3:Yb3+/Ln3+(Ln=Ho, Tm, Er, Pr, Tb) nanorods were investigated. The results indicated that the quantum efficiency of GdF3:10%Yb3+,0.5Er3+and GdF3:10%Yb3+/0.5Pr3+are150%and166%, respectively. However, there is not any NIR QC phenomenon on the GdF3:x%Yb3+/0.5Ho3+(Tm3+,Tb3+) samples. Therefore, GdF3:10%Yb3+/0.5Er3+and GdF3:10%Yb3+/0.5Pr3+nanomaterials are convenient for practical applications in the coating of solar cells.For BaGd0.9-x%Yb0.1Hox%F5(0≤x≤1.1) samples, the structure information of BaGdF5solid-solution is obtained as the BaF2cubic system with Gd3+ions occupying Ba2+sites with the same possibility. Nanoparticles, capsule-like and peanut-like BaGdF5:Yb3+/Ho3+nano/microcrystals were obtained by hydrothermal method, and possible mechanism for the formation of capsule-like and peanut-like morphologies was regarded as self-assemble process. Under excitation at980nm, BaGd0.891Yb0.1Ho0.008F5with the peanut morphology has the highest UC emission intensity, and the energy transfer efficiency of Yb3+â†'Ho3+was calculated to be0.348%. In addition, the measured field dependence of magnetization of the BaGdFs nanoparticles shows excellent paramagnetism. At last, NIR QC properties of BaGdF5:Yb3+/Ho3+were investigated, and the back energy transfer Yb3+â†'Ho3+process was proposed to analyze the characteristic of the NIR emission spectra. The optimal quantum efficiency is192%.ErF3samples with different morphologies (flakes, truncated octahedral, flower-like and rice-like submicrocrystals) were prepared by aqueous-based hydrothermal and co-precipitation route, respectively. ErF3shows almost bright red UC emission under excitation at980nm, because cross-relaxation process of Er3+ions is the main one populating the4F9/2level of Er3+. It is worthwhile to note that UC and cathodoluminescence (CL) intensities are enhanced via further doping with Li+. As a conclusion, ErF3has potential applications in the biological and field emission display devices and so forth.(3) Combining the merits of oxide and fluoride, oxyfluoride was also chosen as the UC and DC luminescence host.Lu6O5F8, as a novel oxyfluoride matrix, has good chemical stability and low vibrational energy advantages. However, Lu6O5F8has not been investigated up to now on the synthetic method, detailed crystal structure as well as luminescence properties. The result indicated that Lu6O5F8belongs to orthorhombic system, and seventy-six atoms are presented in one unit cell, and the indirect band gap of Lu6O5F8is estimated to be4.13eV wide.Li+-doped Lu6O5F8:20%Yb3+,1%Er3+(Tm3+) samples were prepared by coprecipitaion method and fabricated on the basis of the luminescence efficiency and intensity increment by Li+doping, and their UC, DC and CL properties in one compound were studied. It was found that doping of Li+could enhance all the intensities of UC, DC and CL. The differences between PL and CL spectra are due to the different excitation mechanisms. It is worthwhile to point out that according to the effects of Li+on emission intensity ratio, white UC emission was achieved in the Lu6O5F8:6%Yb3+,0.3%Er3+,0.4%Tm3+5%Li+compared to Li+free sample with the same activator concentration. The integrated UC emission intensity of Lu6O5F8:20%Yb3+,1%Er3+,3%Li+is5times as strong as that of commercial UC phosphor (NaYF4:20%Yb3+,2%Er3+).In addition, Lu6O5F8:Eu3+/Tb3+/Ce3+/Dy3+with diverse controlled morphologies were prepared by additive-assisted hydrothennal method。 Furthermore, excellent luminescence properties of Eu3+/Tb3+/Ce3+/Dy3+single doped NH4Lu2F7/Lu(OH)1.57F1.43precursors and final product Lu6O5F8under excitation at VUV/UV/electron beam were investigated. In a word, combining UC, DC, CL and magnetism in one multi, would find potential applications in solid-state laser, solar cell, field emission display and biological imaging. |
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