Broadband near-infrared luminescence and energy transfer of Cr3+, Ce3+ co-doped Ca2LuHf2Al3O12 phosphors☆
Graphical abstract
The emission intensity of CLHA:0.03Cr3+,0.03Ce3+ increases nearly three times compared with that of sample without Ce3+ because of the energy transfer from Ce3+ to Cr3+. And the broadband NIR emission peaked at 775 nm is favorable to applications in a low-cost phosphor dominating nondestructive quality-control analysis systems for food.
Introduction
The near-infrared (NIR) light (700–2500 nm) is essential in daily life due to its extensive applications in in vivo imaging, surveillance, photovoltaics and nondestructive measurement for food.1, 2, 3, 4, 5, 6, 7 Recently, the application of food testing has promoted the demand for the broadband NIR light. However, the commercial NIR light is generally achieved by NIR LED chips, which is limited by the high price and narrowband emission. In order to overcome above limitations, the cheaper broadband NIR phosphor is an alternative to promote the application of NIR light.
Lanthanide ions are widely studied as luminescence centers to obtain NIR light with different full width at half maximum (FWHM).8, 9, 10, 11, 12, 13, 14 For example, Liang et al. reported NIR luminescence of Pr3+ in MgGeO3 with two sharp peaks at 900 and 1085 nm, which show a long-lasting persistent luminescence over 120 h.9 Shahi et al. researched NIR quantum cutting emission of Nd3+ doped GdNbO4 phosphors, there are four emission peaks locating in NIR range and the luminescence intensity of Nd3+ in NIR range is enhanced two times by energy transfer from Bi3+.11 Xia et al. firstly reported the NIR emission of Eu2+ doped K3LuSi2O7 phosphors with an internal quantum efficiency about 15%, which can be excited by 460 nm and have a FWHM about 160 nm.14 However, it is very difficult for most of rare earth ions to achieve a broadband emission, which is hard to satisfy the application for a broadband emission. NIR luminescence of bismuth and transition metals were also investigated because of the broadband luminescence, such as SrSnO3:Bi2+ (808 nm, FWHM = 75 nm),15 LiGa5O8:Fe3+ (1115–1400 nm),16 La3Ga5GeO14:Cr3+ (980 nm, FWHM = 250 nm),17 and Zn2SnO4:Cr3+ (650–1200 nm).18 Among them, the luminescence of Cr3+ can be excited efficiently by light range from 400 to 600 nm.
Garnet phosphors have attracted great interest in recent years since they feature good chemical and thermal stability. The garnet structure can be summarized as the formula A3X2C3O12, where A site coordinates with eight O anions, X site coordinates with six O anions, and C site coordinates with four O anions.26 The abundant different coordination environments make it possible to create a weak crystal strength for Cr3+ by a substitution based on a simple composition. Various emissions from far-red region to NIR region depending on the crystal field of the host materials can be achieved by Cr3+ doped garnet phosphors.19, 20, 21, 22, 23, 24, 25 For example, Katayama et al. observed a sharp emission band peak at 690 nm in YAGG:Cr3+ originating from 2E →4A2 transition and explained the persistent luminescence of Cr3+ based on electron trapping model.21 Malysa et al. reported tuning spectra in X3Sc2Ga3O12:Cr3+ (X = Lu, Y, Gd, La) phosphors, the emission peaks change from 710 to 800 nm as the X site substituted from Lu to La.22 Xu et al. reported the tunable PL spectra of Cr3+ from sharp R-line emission to broadband emission in garnet structure when dodecahedral site and tetrahedral site are substituted by larger cations.23 Wang et al. reported Ca2LuHf2Al3O12:Ce3+ phosphors, which show good thermal stability.27 The centroid shift of Ca2LuHf2Al3O12 is close to that of CaSiN2,24 which shows a weak crystal field. This means that Ca2LuHf2Al3O12 owns a weak crystal field strength and it may provide a suitable crystal field environment for a broadband emission of Cr3+. However, the phosphor is still in a low efficiency for practical applications. To enhance the emission intensity of activators ions, energy transfer is a frequently used strategy in many reports, it can be well accommodated in numerous inorganic matrices. Sensitizer ions like Bi3+, Tb3+, Pr3+, and Ce3+ are commonly used to create energy transfer in Cr3+ doped garnet phosphor,17,19,25 and Ce3+ has been widely used in many matrices since the 4f→5d transition and superior absorption ranging from 250 to 450 nm.
In this article, we prepared broadband NIR Ca2LuHf2Al3O12:Cr3+ (CLHA:Cr3+) and Ca2LuHf2Al3O12:Cr3+,Ce3+ (CLHA:Cr3+,Ce3+) phosphors by the high temperature solid state method. The luminescence intensity of CLHA:Cr3+,Ce3+ sample can be enhanced almost three times compared with CLHA:Cr3+ sample by energy transfer from Ce3+ to Cr3+. The energy transfer mechanism and energy transfer efficiency were investigated in detail by decay curves and Dexter's energy transfer formula.
Section snippets
Synthesis
A series of CLHA:xCe3+,yCr3+ (0 ≤ x ≤ 0.11, 0 ≤ y ≤ 0.11) phosphors were synthesized by traditional high-temperature solid state method. Analytical reagent CaCO3, Lu2O3, HfO2, Al2O3, Cr2O3 and CeO2 were used as raw materials, which were mixed homogeneously according to the nominal compositions by an agate for 30 min, and then sintered in a muffle furnace at 1500 °C for 3 h in a reductive atmosphere.
Characterization
Crystal structure and phase purity of samples were identified by X-ray ponder diffraction (XRD)
Phase identification
Fig. 1 shows the XRD patterns of CLHA:xCe3+,yCr3+ (0 ≤ x ≤ 0.11, 0 ≤ y ≤ 0.11). An impurity phase of HfO2 appears in samples, which belongs to Pbcm space group with JCPDF card No. 21–0904. To know the amount of HfO2 and the structural parameters of CLHA host, the Rietveld refinement was performed with Ca2GdZr2(AlO4)3 as the original structure model by the GSAS program,28,29 and HfO2 is considered as the second phase in the Rietveld refinement. Fig. 2 presents the experimental, calculated, and
Conclusions
In conclusion, the photoluminescence properties of CLHA:Cr3+ and energy transfer between Ce3+ and Cr3+ were investigated in detail. The critical distance between Cr3+ ions is 2.47683 nm and the concentration quenching mechanism of CLHA:Cr3+ belongs to dipole-dipole interaction. The red shift of CLHA:yCr3+ (0.01 ≤ y ≤0.11) may be ascribed to radiation reabsorption and non-radiative process. The emission intensity measured at 423 K of CLHA:0.03Cr3+ maintains 80.29% of the intensity measured at
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Foundation item: Project supported by the National Key Research and Development Program of China (2016YFB400605).