| Light emitting diode(LED)is widely applied for lighting,display and indica-tion due to energy saving,environment friendly and high efficiency.In recent years,closed greenhouses have been used for planting off-season flowers,fruits and vegeta-bles.Because of the short sunshine time in winter and early spring,slow growth of plants and insufficient illumination of vegetable and fruit planting in overseason,LED has been used for plant lighting.The 400-500 nm blue light,620-690 n-m red light and 700-740 nm far red light play important roles in photosynthesis,phototropism and photomorphogenesis,so the LED should be matched with these wavelengths.Far red emitting phosphor Li6(La2Ca)Nb2O12:Mn4+and yellow phosphor Li6(L-a2Ca)Nb2O12:Dy3+are prepared successfully by solid-state reaction method in air.Powder X-ray diffractometer was used to confirm the crystal structure and phase purity of the obtained samples.According to the XRD patterns of phosphors,doping Mn4+or Dy3+ions has no influence on crystal structure of Lie(La2Ca)Nb2O12.The excitation spectrum of monitored at 700 nm shows two absorption bands centered at 327 and 494 nm,respectively.The former one is originated as the overlap of Mn4+-O2-charge transfer band(CTB)and the 4A2g?4T1g transition of the Mn4+ion.The later one centered at 494 nm is attributed to the 4A2g ? 4T2g transition of Mn4+ions.With the excitation wavelength at 327 nm,the far red emission peak is observed at 700 nm,which is attributed to the spin and parity-forbidden 2E?4A2g transition of the Mn4+ions.The transition energy closely related to the Mn4+ligand hybridization.The resistance of luminescence against thenxlal impact is also investigated by emission spectroscopy at heating temperatures from 300 to 500 K.The calculated activation energy ?E(?0.437 eV)demonstrates thermal stability of the phosphors.The luminescence properties indicate that Li6(La2Ca)Nb2O12:Mn4+phosphor has a potential application for plant cultivation.For the Li6(La2Ca)Nb2O12:Dy3+phosphor,the excitation spectrum of Li6(La2—Ca)Nb2O12:Dy3+ exhibits two parts under the monitoring wavelength of 580 n-m:one is a broad absorbtion band at 200?290 nm centered at 269 nm.which is assigned the CTB of Nb-O and Dy-O;the second is a series of sharp absorp-tion peaks between 310 nm and 500 nm.These excitation peaks located at 326,351,366,384,425,453 and 472 nm belong to the f-f transition of 6H15/2 to 4P3/2?6P7/2?6P5/2?4M21/2?4G11/2?4I15/2 and 4F9/2 of Dy3+ion respectively.It indicates that the sample could be excited by near-ultraviolet or blue LEDs.The emission spectrum of the sample mainly has two main emission peaks at 493 nm and 580 nm,which are corresponded to the green transition 4F9/2?6H15/2 and the yellow transition 4F9/2?6H15/2 of Dy3+,respectively.The intensity of yellow emission is higher than that of green one.The color coordinate of the emission spectrum is(0.4703,0.4927),which is located in the yellow region.It was found that the luminescence intensities of the samples are increased with the increase of content of Dy3+ till x=10 mol%.When the concentration of Dy3+is over 10 mol%,the lumi-nescence intensities are decreased due to the concentration quenching mechanism. |
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