[Study on Synthesis and Luminescence Mechanism of Novel Green Sr3Y(PO4)3:Ce3+, Tb3+ Phosphors].

A novel green light-emitting phosphor Sr3Y(PO4)3Ce(3+), Tb(3+) was synthesized by the traditional high temperature solid state reaction method. Luminescence mechanism and crystal structure were investigated by X-Ray Diffraction (XRD) and photoluminescence spectra (PL). The XRD patterns demonstrate that the samples belong to the single phase of Sr3Y(PO4)3 in experimental doping concentrations range. Obviously, the excitation band of Sr3Y(PO4)3:Tb(3+) and the emission of Sr3Y(PO4)3:Ce(3+) have a significant spectral overlap in the wavelength range of 330~380 nm, which implies the great possibility of an efficient ET from Ce(3+) to Tb(3+). Under the 315 nm ultraviolet excitation, a blue emission(320~420 nm) from Ce(3+) and a yellowish-green emission (480~500, 530~560 nm) from Tb(3+) were obtained from Sr3Y(PO4)3:Ce(3+), Tb(3+). When the Ce(3+) concentration was 7%, the emission could be adjusted from blue to green region by tuning the Tb(3+) doping concentrations from 1% to 50% through an energy transfer process. This text plot the schematic energy levels of Ce(3+), and Tb(3+) with electronic transitions and energy transfer processes in Sr3Y(PO4)3:Ce(3+), Tb(3+), which disclose the electron motion processes of Sr3Y(PO4)3:Ce(3+), Tb(3+). From the dependence of relative emission intensity of Ce(3+), Tb(3+) ((5)D4 --> (7)Fj) and ET efficiency from Ce(3+) to Tb(3+) on the concentrations of Tb(3+), It can be seen that the relative intensity of Tb(3+) and the values of ηET increase gradually with the increasing of Tb(3+) as well as the relative intensity of Ce(3+) decreases remarkably. The largest energy transfer efficiency reaches as high as 80% when the concentration of Tb(3+) was 50%, demonstrating the efficient energy transfer from Ce(3+) to Tb(3+). The CIE chromaticity coordinate positions are plotted, as can be seen the emitting color of Ce(3+) and Tb(3+) singly doped Sr3Y(PO4)3:Ce(3+), Tb(3+) phosphor are blue and yellowish green, respectively. The emitting color of samples Sr3Y(PO4)3:Ce(3+), Tb(3+) changes from blue region to green region with the rising doping contents of Tb(3+). Sr3Y(PO4)3:Ce(3+) and Tb(3+) phosphor can be used as a green light-emitting phosphor in white LED devices and LCD backlights.

[Crystal structure and upconversion emission of Yb3+/Er(3+) -co-doped NaYF4 nanocrystals].

Yb3+/EP(3+) -co-doped cubic NaYF4 and Yb3+/Er3+/Gd(3+) -tri-doped hexagonal NaYF4 nanocrystals were synthesized by a modified coprecipitation method with ethylenediamine tetraacetic acid (EDTA) as chelating agent. The samples' morphology, crystal phase and upconversion emission were measured with transmission electron microscope (TEM), X-ray diffraction patterns (XRD) and upconversion luminescence spectrum. TEM and XRD results showed that the phase transition from cubic to hexagonal was promoted through Gd3+ doping. It has been reported that the upconversion efficiency of hexagonal NaYF4 is higher than that of cubic NaYF4, however, the effect of crystal phase on upconversion luminescence has not been well understood. This work focuses analysis of measurement results to compare the effect of, crystal phase on the crystal field energy splitting and upconversion emission intensity as well as emission color, and a mechanism of luminescence enhancement and color tunability are revealed. Strong visible upconversion luminescence can be seen clearly by the naked eyes in both cubic phase and hexagonal phase samples upon excitation by a 980 nm laser diode with power of 10 mW, consisting of green emissions centered at around 525/550 nm originating from the transitions of 2H11/2/4 S3/2 --> 4 I15/2 and red emission at about 657 nm from 4F9/2 to 4 I15/2 of Er3+ ions respectively. In comparison to cubic sample, the hexagonal phase sample presented much stronger and sharper upconversion luminescence, whose emission efficiency was enhanced 10 times with an additional transition of 2 H9/2 --> 4I13/2 at 557 nm, furthermore, the intensity ratio of red to green emission increased from 2 :1 to 3 : 1. Doping NaYF4 nanocrystals with Gd3+ ions induced the hexagonal-to-cubic phase transition and thus decreased the crystal symmetry, consequently increased absorption cross-section and 4f-4f transition probabilities by relaxing forbidden selection rules, resulting in stronger emission. In the mean time, the decreasing unit-cell volume of the hexagonal phase increased the crystal field strength around the dopant ions and consequently led to that hexagonal phase samples present much sharper emission compared to cubic counterparts. It demonstrates that phase transition can tune crystal field energy splitting, luminescence intensity and emission color.

[Structure and luminescence properties of MgGa2O4 : Cr3+ with Zn substituted for Mg].

A series of red long afterglow phosphors with composition Zn(x) Mg(1-2) Ga2 O4 : Cr3+ (x = 0, 0.2, 0.6, 0.8, 1.0) were synthesized by a high temperature solid-state reaction method. The X-ray diffraction studies show that the phase of the phosphors is face-centered cubic structure. Photoluminescence spectra show that the red emission of Cr3+ originated from the transition of 2E-4A2. Due to the large overlap between absorption band of Cr3+ and emission band of the host. Cr3+ could obtain the excitation energy from the host via the effective energy transfer. The afterglow decay characteristics show that the phosphor samples with different Zn contents have different afterglow time and the afterglow time also changes with the value of x. The measurement of thermoluminescence reveals that the trap depth of the phosphor samples with different Zn contents is different. The samples with deeper traps have longer afterglow time.

[Structure and luminescence properties of Ga2O3 : Cr3+ by Al doping].

The Al doping gallate phosphor (Ga(1-x)Al(x))2O3 : Cr3+ (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) was synthesized by a high temperature solid-state reaction method. The X-ray diffractions show that the phase of the phosphors remains to be Ga2 O3 structure with increase in the contents of Al3+ ion. Beside, the fact that the X-ray diffraction peak shifts towards big angles with increasing Al3+ ions content shows that Al3+ ions entered the Ga2 O3 lattice. The peaks of the excitation spectra located at 258, 300, 410 and 550 nm are attributed to the band to band transition of the matrix, charge transfer band transition, and 4A2 --> 4T1 and 4A2 --> 4T2 transition of Cr3+ ions, respectively. Those excitation spectrum peak positions show different degrees of blue shift with the increase in the Al3+ ions content. The blue shift of the first two peaks are due to the band gap energy of substrate and the electronegativity between Cr3+ ions and ligands increasing, respectively. The blue shift of the energy level transition of Cr3+ ion is attributed to crystal field strength increasing. The Cr3+ ion luminescence changes from a broadband emission to a narrow-band emission with Al3+ doping, because the emission of Cr3+ ion changed from 4 T2 --> 4A2 to 2E --> 4A2 transition with the crystal field change after Al3+ ions doping. The Al3+ ions doping improved the long afterglow luminescence properties of samples, and the sample showed a longer visible near infrared when Al3+ ions content reaches 0.5. The thermoluminescence curve shows the sample with suitable trap energy level, and this is also the cause of the long afterglow luminescence materials.

[Luminescence properties of Eu, Dy doped BaAl12 O19 long afterglow phosphors].

Long afterglow phosphors BaAl12 O19:Eu2+/Eu3+, Dy3+ were synthesized by high temperature solid state method under different atmosphere. X-ray powder diffraction (XRD) shows that pure BaAl12 O19 phase structure was obtained and the do ping ions Eu2+/Eu3+, Dy3+ didn't change the phase structure. By comparison, the authors found that the doping ions Eu2+/ Eu3+, Dy3+ caused the XRD diffraction peaks moving to the high angle slightly which displayed that the inter-planar spacing was changed via Eu and Dy replacing Ba lattice in BaAl12 O19. Emission spectra show that all the samples prepared under different conditions exhibit the 4f6 5d1 --> 4J7 broadband transition which is the features emission of Eu2+ and the existence of the features emission of Eu2+ in the sample synthesized in air indicates that Eu3+ ions can be reduced to divalent state in air. The doping ions Dy3+ can not only enhance the luminous intensity of samples but also make the samples to obtain long afterglow characteristics. The afterglow decay and thermoluminescence studies of the Eu, Dy co-doped sample synthesized under reducing atmosphere reveal that the sample has good long afterglow properties at room temperature and high temperature.

[Thermoluminescence study on Sr2SiO4 long afterglow material doped with Eu2+].

Sr2SiO4:Eu0.03(2+) phosphors were synthesized through the solid-state reaction technique. The X-ray diffraction shows that the phase of the phosphors is orthorhombic alpha'-Sr2SiO4. The produced phosphors show one intense emission band located at 490 nm. The phosphor shows a long afterglow properties excited by the sunlight. The decay characteristics show that the phosphors consist of a quick decay process and a slow decay process. The experimental results demonstrate that the thermoluminescence (TL) curves of the samples containing four peaks, located at 346, 420, 457 and 552 K, respectively. Meanwhile, the different peaks show the different decay characteristics, and the electron transfer between the trap levels was measured.

[Luminescence properties of Eu3+/Dy3+ coating ZnO nanocrystals].

Some kinds of phosphors were synthesized with Eu/Dy coating ZnO nanocrystals by co-precipitation approach derived from Zn(AC)2 2H2O, NaOH as precursors. The crystal structure and size were characterized by the X-ray diffraction Photoluminescence(PL) measurements show an intense red luminescence at 612 nm caused by transition of Eu3+ ions in ZnO:Eu3+ and intense luminescence band at 484 and 575 nm caused by transition of Dy3+ ions in ZnO:Dy3+. The results show that the energy transfer was realized from the host to the rare earth ions. The mechanism of the energy transfer was discussed. Meanwhile Eu/Dy co-coating nano ZnO can be achieved white light emission by energy transfer.

[Luminescence investigation of Na(z)Ca(1-x-2y-z)Bi(y)MoO4 : Eu(x+y)3+, red phosphors].

A series of red phosphors with the composition Na(z)Ca(1-x-2y-z), Bi(y) MoO4 : Eu(x+y)3+ (y, z = 0, x = 0.24, 0.26, 0.30, 0.34, 0.38; x = 0.30, y = 0.01, 0.02, 0.03, 0.03, 0.05, 0.06, 0.07; x = 0.30, y = 0.04, z = 0.38) were prepared via traditional solid-state method. The crystal structures of the obtained phosphors were identified by X-ray powder diffraction (XRD) method. The photoluminescence properties of the samples were characterized by fluorescence spectrophotometer. The results indicated that the concentration of Eu3+ single doped Ca(1-x) MoO4 : Eu3+ with the maximum luminescence intensity was found to be 0.30 (namely, Ca0.70 MoO4 : Eu(0.30)3+); the photoluminescence properties with different ratio of Bi3+/Eu3+ codoped Ca0.70-2y Bi(y) MoO4 : Eu(0.30+y)3+, were also investigated, and the results showed that the charge band (CTB) reached the maximum value when the y value was equal to 0.03; for the characteristic excitation peaks of Eu3+, however, the intensity of the excitation spectral line locating at 393 nm was stronger than that at 464 nm when y < 0.03, while the intensity at 464 nm was greater than that at 393 nm when y > or = 0.03; the intensity of excitation peaks locating at 393 and 464 nm respectively both reached the maximum intensity when the y value was 0.04. The relative intensity of the excitation and emission of the above phosphor was enhanced greatly when Na2CO3 acting as charge compensation was added. The above results showed that the relative intensity between 393 and 464 nm could be changed by adjusting the ratio of Bi3+ /Eu3+ codoping concentrations.