| In this paper,a series of near-infrared phosphors,Mg3Y2Ge3O12:x Cr3+,Mg3Y2Ge3O12:0.03Cr3+,y Bi3+ and(Mg1-z Lnz)3Y2Ge3O12:0.03Cr3+(Ln=Zn,Ca),are synthesized by a high-temperature solid-phase reaction method.The optical properties of the samples are improved by varying the activator concentration,introducing the sensitizer,and regulating the host component.The properties of the samples are studied by means of X-ray diffraction pattern,transient/steady-state fluorescence spectra,thermoluminescence spectra and afterglow decay curve.The results are as follows:(1)A series of near-infrared broadband emission phosphors with Cr3+ ions as activators and Mg3Y2Ge3O12 as host are synthesized by the high-temperature solid-state reaction method.The structure and related properties of the host Mg3Y2Ge3O12 are systematically studied.The crystal structure,luminescent properties,occupancy,crystallinity,particle size analysis and the change of crystal field environment of the samples Mg3Y2Ge3O12:x Cr3+ are studied.Firstly,the valence of Cr in the host is positive trivalent proved by XPS.Then,taking Mg3Y2Ge3O12 as the initial structure,the occupancy of Cr3+ ions in the host Mg3Y2Ge3O12 is studied in detail by Rietveld structure refinement.Mg3Y2Ge3O12:x Cr3+ can emit the near infrared spectra(4T2→4A2)ranging 650-1200 nm under excitation of 436 nm light.With the increase of the Cr3+ ions concentration,the spectra red shift continuously.Meanwhile,the full width at half maxima(FWHM)is broadened continuously.Based on the Cr3+ ion lattice analysis,the crystal field environment is regulated by increasing the concentration of Cr3+ ions to improve the luminescent properties.By increasing the Cr3+ concentration,a new luminescent center is constructed by cation inversion,and the crystal field intensity around Cr3+ ions is reduced,which cause the red shift of the spectra,the broadening of FWHM and the enhancement of the emission intensity.From the experimental results,it can be seen that with the increase of Cr3+ concentration,the luminescence intensity of Cr3+ ions increases twice,and the main peak of the spectrum moves to 811 nm,and FWHM broadens to 226 nm.Through structural analysis,it is attributed to the cation reversal between Mg2+ and Ge4+ which cause the emergence of new luminescent centers.Finally,the near infrared LED is obtained by combining the Ga N blue LED chip and the sample Mg3Y2Ge3O12:0.40Cr3+ with the FWHM of 260 nm.The photoelectric efficiency and optical power are about 6.6% and 5.9 m W,respectively.(2)A series of phosphors of Mg3Y2Ge3O12:yBi3+ and Mg3Y2Ge3O12:0.03Cr3+,yBi3+ are synthesized.Firstly,the occupancy and luminescence properties of Bi3+ ions in Mg3Y2Ge3O12:y Bi3+ are analyzed by XRD and fluorescence spectra.The electron transitions of Bi3+ ions are calculated and analyzed from 1S0→3P1,1S0→1P1 and MMCT,respectively.Bi3+ ion is introduced as sensitizer in Mg3Y2Ge3O12:0.03Cr3+,y Bi3+.The luminescence characteristics of the sample were analyzed by XRD,fluorescence spectra,lifetime,thermoluminescence spectra and afterglow characteristics.With the increase of Bi3+ concentration,there is energy transfer from Bi3+ ions to Cr3+ ions,which improves the luminescence intensity of Cr3+ ion.With the introduction of Bi3+ ions,there are the trap of Bi Mg˙ and Bi2+(Bi3++e-),which can store electrons under the irradiation of ultraviolet lamp(254 nm),and then stop irradiation,resulting in afterglow phenomenon.The afterglow time can reach 5400 s.Therefore,co-doped sensitizer Bi3+ can effectively improve near infrared luminescence intensity and obtain afterglow properties.(3)The luminescent properties of the samples are improved by adjusting the host composition and the phosphors(Mg1-x Lnz)3Y2Ge3O12:0.03Cr3+(Ln=Zn,Ca)are synthesized.The improvement of the luminescent properties of Cr3+ ions by introducing Zn/Ca is analyzed by Rietveld structure refinement,fluorescence spectra,lifetime,defects and afterglow.With the increase of the content of Zn,the emission intensity of Cr3+ ions increases,the spectra red shift(776 nm→830 nm),FWHM broaden(189 nm→227 nm)and the afterglow time of the samples can reach 4000 s.With the increase of the content of Ca,the emission intensity of Cr3+ increases obviously,which is larger than that of Zn,the spectra blue shift(776 nm→753 nm),FWHM is narrowing(189 nm→136 nm),and the afterglow time can reach 4 hours.The trap depth in the sample(Mg1-z Znz)3Y2Ge3O12:0.03Cr3+ is 0.23 e V,and that in the sample(Mg1-z Caz)3Y2Ge3O12:0.03Cr3+ is 0.67 e V. |
