Material Properties of RE- Doped Ln (Ln= Y, La) oxides and oxysulfides phosphors for red-emitting devices

[摘要] Structural and optical properties of Eu3+-doped Ln (Ln=Y, La) oxide and oxysulfide nanocrystals synthesized by sol-combustion method were analysed as a function of host to fuelratio. Structural characterization shows crystallite nanosized particles and the hexagonalphase as the dominant structure. The red emission of Eu3+ doped Y 20 2S, La20 2S and Y 20 3nanocrystals appearing near 624 nm was assigned to the 5Do-7F2 transition of Eu3+. Due toinsufficient quantities of thiourea at the higher Ln/S mole ratio, the bright red emission hasbeen quenched. Fourier-transform infrared spectrometry analysis showed that there was anegligible difference in the absorbed impurities with various molar ratios. The Ln/Sconcentration also affects the decay time of the red emission of the Eu3+ ions from 140 μs forLn/S=I to 76 μs for the higher concentrations. Structural and optical properties ofLa20 2S:Eu3+micro crystals synthesized by sol-combustion method were analyzed as afunction of La/S concentration. Structural characterization shows a crystallite size of about178 nm and the hexagonal phase as the dominant crystalline structure. The red emission ofEu3+ doped La20 2S microcrystals appearing near 624 nm was assigned to the 5D0-7F2transition of Eu3+. Due to insufficient quantities of thiourea at the higher La/S mole ratio, thebright red emission has been quenched. Fourier-transform infrared spectrometry analysisshowed that there was a negligible difference in the absorbed impurities with various molarratios. The La/S concentration also affects the decay time of the red emission of the Eu3+ ionsfrom 140 μs for La/S= I to 7 6 μs for the higher concentrations.To investigate the effect of co-doping a series of red-emitting phosphors Y 20 3:Eu3+:Ho3+were prepared by the solution combustion method. X-ray diffraction (XRD) patterns indicatethat the Eu3+ and Ho3+ doping do not show obvious effect on the cubic Y 20 3 crystal. Theircrystall ite size estimated by x-ray diffractometry and scanning electron microscopy wasabout 8 nm. Under UV 325 nm excitation, emission wavelengths at 626 nm was quenched athigher mole percent of Ho3+ and energy was transferred from Eu3 ~ to Ho3+. Y20 3:Eu3+: Ho3+phosphor shows a red-emitting afterglow phenomenon, and the Eu3' ions are the luminescentcenter during the decay process. The bright red emission near 626 nm has been noticeabledue to the 5D0-7F2 transition of Eu3... . The intensity of the luminescence has decreased with anincrease of concentration of Ho3+. In sufficient quantities of Eu3+ to Ho3+, the bright redemission near 626 nm has been predominant due to 5D0-7F2 transition of Eu3+. The decay characteristic of Y 20 3:Eu3+: Ho3+ phosphor is according with the double exponentialequation.The as-prepared powder Y 20 2S:Eu3+ was deposited on Si ( 100) substrates by using a pulsedlaser deposition technique. The thin films grown under different oxygen deposition pressureconditions have been characterized using structural and luminescent measurements. The Xraydiffraction patterns showed mixed phases of cubic and hexagonal crystal structures. Asthe oxygen partial pressure increased, the crystallinity of the films improved. Further increaseof the 0 2 pressure to 140 mtorr reduced the crystallinity of the fi lm. Similarly, both scanningelectron microscopy and atomic force microscopy confirmed that an increase in 0 2 pressureaffected the morphology of the films. The average band gap of the films calculated fromdiffuse reflectance spectra using the Kubeika-Munk function was about 4.75 eV. Thephotoluminescence measurements indicated red emission of Y20 2S:Eu3+ thin films with themost intense peak appearing at 619 nm, whi ch is assigned to the 5Do-7F2 transition of Eu3+.This most intense peak was totally quenched at higher 0 2 pressures. X-ray photoelectron(XPS) indicated that Y20 3 thin films are formed on the surfaces of the Y20 2S: Eu3+ thin filmsduring prolonged electron bombardment. The films grown in a lower 0 2 ambient consist ofsmaller but more densely packet particles relative to the films grown at a higher 0 2 ambient.In order to study the effect of annealing temperature on the films, four samples were annealedat various temperatures while one was kept unannealed. X-ray diffraction measurementsshow that the un-annealed thin film was amorphous, while those annealed were crystalline.At lower annealing temperature of 600 °c to 700 °c cubic bixbyite Y20 3:Eu3+ was formed .As the annealing temperatures were increased to 800 °c, hexagonal phase emerged. Theaverage crystall ite size of the fi lm was 64 nm. Photoluminescence (PL) measurementindicates intense red emission around 612 nm due to the 5 00~ 7F2 transition. Scanningelectron microscopy (SEM) indicated that agglomerates of non-crystalline particles withspherical shapes were present for the un-annealed film s. After annealing at high temperature,finer morphology was revealed. Atomic fo rce microscopy (AFM) further confirmed theformation of new morphology at the higher annealing temperatures. UV-vis measurementindicated a band gap in the range of 4.6 to 4.8 eY. It was concluded that the annealingtemperature played an important role in the luminescence intensity and crystallinity of thesefilms.To investigate the effect of different species of gases Y20 2S:Eu3+ thin films have been grownon Si ( I 00) substrates by using a pulsed laser deposition technique. The thin films grownunder different species of gases have been characterized using structural and luminescentmeasurements. The X-ray diffraction patterns showed mixed phases of cubic and hexagonalcrystal structures. The crystallinity of the film deposited in vacuum is poor, but improvedsignificantly in argon and oxygen atmosphere. Similarly, both scanning electron microscopyand atomic force microscopy confirmed that different species of gases affected themorphology of the fi lms. The average band gap of the films calculated from diffusereflectance spectra using the Kubeika-Munk function was about 4.69 eV. Thephotoluminescence measurements indicated red emission of Y20 2S:Eu3+ thin films with themost intense peak appearing at 6 12 nm, which is assigned to the 5D0-7F2 transition of Eu3+.The intensities of this most intense peak greatly depend on the species of gas with argonhaving the highest peak. This phosphor has applications in the flat panel displays.

[发布机构] University of the Free State