Properties of Self-recoverable Mechanoluminescence Phosphor Ca_(5)Ga_(6)O_(14)∶Eu^(3+) and Its Information Encryption Application

A novel self-recoverable mechanoluminescent phosphor Ca_(5)Ga_(6)O_(14)∶Eu^(3+) was developed by the high-tem-perature solid-state reaction method,and its luminescence properties were investigated.Ca_(5)Ga_(6)O_(14)∶Eu^(3+)can produce red mechanoluminescence,and importantly,it shows good repeatability.The mechanoluminescence of Ca_(5)Ga_(6)O_(14)∶Eu^(3+) results from the piezoelectric field generated inside the material under stress,rather than the charge carriers stored in the traps,which can be confirmed by the multiple cycles of mechanoluminescence tests and heat treatment tests.The mechanoluminescence color can be turned from red to green by co-doping varied concentrations of Tb^(3+),which may be meaningful for encrypted letter writing.The encryption scheme for secure communication was devised by harnessing mechanoluminescence patterns in diverse shapes and ASCII codes,which shows good encryption performance.The results suggest that the mechanoluminescence phosphor Ca_(5)Ga_(6)O_(14)∶Eu^(3+),Tb^(3+)may be applied to the optical information encryption.

Thermally enhanced photoluminescence and temperature sensing properties of Sc2W3O12:Eu3+ phosphors

Recently, lanthanide-ion-doped luminescent materials have been extensively used as optical thermometry probes due to their fast responses, non-contact, and high sensitivity properties. Based on different responses of two emissions to temperature, the fluorescence intensity ratio(FIR) technique can be used to estimate the sensitivities for assessing the optical thermometry performances. In this study, we introduce different doping concentrations of Eu^(3+) ions into negative thermal expansion material Sc2W3O12to increase the thermal-enhanced luminescence from 373 K to 548 K, and investigate the temperature sensing properties in detail. All samples can exhibit their good luminescence behaviors thermally enhanced.The emission intensity of Sc2W3O12:6-mol% Eu3+phosphor reaches 147.8% of initial intensity at 473 K. As the Eu3+doping concentration increases, the resistance of the sample to thermal quenching decreases. The FIR technique based on each of the transitions 5D→7F_(1)(592 nm) and 5D→7F_(2)(613 nm) of Eu3+ions demonstrates a maximum relative temperature sensitivity of 3.063% K-1at 298 K for Sc_(2)W_(3)O_(12):6-mol% Eu3+phosphor. The sensitivity of sample decreases with the increase of Eu3+concentration. Benefiting from the thermal-enhanced luminescence performance and good temperature sensing properties, the Sc_(2)W_(3)O_(12):Eu^(3+)phosphors can be used as optical thermometers.

Luminescence and Defect Properties of Novel Bluish-Green Phosphor β-Zn_3(PO_4)_2∶Zr^(4+)

Luminescence and defect properties of novel phosphor β-Zn 3(PO 4) 2:Zr 4+ were systematically investigated. Corresponding to its lowest optical absorption transition at 240 nm, phosphor emits a bluish-green light at 485 nm, which yields the Stokes shift about 20000 cm -1. The unusual optical properties of Zr 4+ ion are ascribed to its uncommon coordination environment. In addition it shows intensive bluish-green long lasting phosphorescence (LLP) due to the existence of electron trap, which is generated by aliovalent substitution of Zr 4+ ion for the cation site in the matrix as shown in thermoluminescence (TL) spectrum.

Tunable Emission and Energy Transfer of the Novel KY_(1-x)(MoO_(4))_(2-y)(WO_(4))y:xLn^(3+)(Ln^(3+)=Dy^(3+),Eu^(3+),and Tm^(3+))Single-phase White Luminescence Phosphor for White LEDs

The phosphors of KY_(1-x)(MoO_(4))_(2-y)(WO_(4))y:xLn^(3+)(Ln^(3+)=Tm^(3+),Dy^(3+),Eu^(3+))were synthesized by using a sol-gel method.Then,the crystal structure,luminescence properties,energy transfer,and white emission of the prepared materials were researched.The molar ratio of the anion group on the photoluminescence(PL)emission and excitation intensity were investigated,revealing that the optimum intensity could be obtained by using=3:1.The optimal Dy^(3+) doping concentration of KY(MoO_(4))1.5(WO4)0.5was obtained.In addition,the color-tunable emissions of Dy^(3+)/Eu^(3+)-codoped KY(MoO_(4))1.5(WO4)0.5phosphors were observed because of the effective energy transfer(ET)from Dy^(3+)to Eu^(3+)ions.Finally,by doping appropriate concentrations of Tm^(3+),Dy^(3+),and Eu^(3+)and different concentrations of(WO_(4))^(2-),white light emitting phosphors KY_(0.92)(WO_(4))2:0.01Tm^(3+),0.06Dy^(3+),0.01Eu^(3+)with excellent color-rending properties were obtained.The chromaticity coordinate was calculated as(x=0.3238,y=0.3173),closing to the artificial daylight(D65,x=0.313,y=0.329)illuminant,and which indicates the potential application of near ultraviolet White light-emitting diodes(WLEDs).

Effect of Yb^(3+) concentration on the structures and upconversion luminescence properties of Y_2O_3:Er^(3+) ultrafine phosphors

Y2O3：Er^3＋ ultrafine phosphors with a varying Yb^3＋ ion concentration were prepared by a urea homogeneous precipitation method. The results of XRD show that all the samples are of a pure cubic structure and the average crystallite sizes can be calculated as 45, 34, and 28 nm for Y2O3：Er^3＋ ultrafine phosphors with Yb^3＋ ion concentrations of 0, 10%, and 20%, respectively. The lattice constant and cell volume of the ultrafine phosphors decrease with enhancing Yb^3＋ ion concentration. The upconversion luminescence spectra of all the samples were studied under 980 nm laser excitation. The strong green and red upconversion emission were observed, and attributed to the ^2H11/2→^4I15/2, ^4S3/2 → ^4I15/2 and ^4F9/2 →^4I15/2 transitions of Er^3＋, respectively. The intensity of red emission increases with increasing Yb^3＋ ion concentration. The effect of Yb^3＋ ion concentration on the structures and upconversion luminescence mechanism were discussed.

Luminescence and Preparation of LED Phosphor Ca_8Mg(SiO_4)_4Cl_2∶Eu^(2+)

Calcium magnesium chlorosilicate doped by europium, Ca8Mg（SiO4）4Cl2： Eu^2＋, was prepared by the solid state reaction at high temperature. The compound obtained is pure Ca8Mg（SiO4）4Cl2 phase with cubic structure. Its average particle size is 5 μm, and it has good dispersity and morphological form. The excitation spectrum of Ca8Mg（SiO4）4Cl2： Eu^2＋ is a wide band, which covers from 270 to 480 nm. The emission spectrum is also a wide band peaked at 510 nm. The luminescent intensity reaches to the maximum when the concentration of Eu^2 ＋ is 2%. The wavelength of emission and excitation of the phosphor with various Eu^2 ＋ contents keeps constant. This spectrum range matches violet and blue LED chips very well, and its strong luminescence intensity is suitable for a green phosphor of tricolor phosphor of white light LED.

Photo-luminescence properties and thermo-luminescence curve analysis of a new white long-lasting phosphor: Ca_2MgSi_2O_7:Dy^(3+)

A white long-lasting phosphor Ca2MgSi2O7:Dy3+ was prepared by the solid-state reaction. A strong band peaked at 260 nm was shown in the excitation spectrum of 578 nm emission, which might be attributed to the oxygen deficiency of the host. After irradiated with 254 nm for 4 min, the white afterglow of the sample could be seen for 3 h. Moreover, the depths and frequency factors of trap centers were calculated from the thermo-luminescence curve of the sample, which indicated that the trap centers corresponding to the 414 K band were more helpful to the long-lasting phosphorescence.

Photoluminescence of Long Lasting Phosphors Ca_2MgSi_2O_7∶Eu^(2+),Dy^(3+)

The Eu (2+) and Dy (3+) codoped silicates,Ca_2MgSi_2O_7∶Eu (2+),Dy (3+),was synthesized,and its photoluminescence properties were studied with synchrotron radiation. Under synchrotron radiation excitation at 305 nm,only the characteristic emission from Eu (2+) was excited. But the characteristic emission from Dy (3+) was also found under 175 nm excitation,besides the characteristic emission of Eu (2+). The characteristic emission from Dy (3+) was detected in the Eu (2+) and Dy (3+) codoped long lasting phosphorescence materials,which indicates that Dy (3+) is still existed in the phosphors under continue excitation at 175 nm.

Photoluminescence degradation mechanism of BaMgAl_(10)O_(17):Eu^(2+) phosphor by vacuum ultraviolet irradiation

A real high power vacuum ultraviolet light source is applied to the investigation on the vacuum ultraviolet irra- diation degradation of BaMgAl10O17：Eu2＋ phosphor. The degradations of emission intensity and color quality of the sample are clearly observed after irradiation. It reveals that the oxidation of Eu2＋ during irradiation is partly respon- sible for the degradations. The excitation and absorption spectra show that some traps generated during irradiation have negative influence on the luminescence of sample and these traps have been identified as positively charged oxygen vacancies by positron annihilation. The investigations on host emission and decay curve further confirm that these oxygen vacancies are involved in the perturbation of energy transfer from the host to Eu2＋ and finally result in the degradation.

Synthesis and Luminescence Properties of Gd_2O_3∶Eu^(3+) Phosphors

Gd 2O 3∶Eu 3+ phosphors were prepared by urea homogeneous precipitation with different surfactant and sol-gel method. XRD patterns show that all the obtained samples are in cubic Gd 2O 3, and the results of FTIR and fluorescent spectra conformed that OP is a good surfactant for preparing the Gd 2O 3∶Eu 3+ phosphors. The SEM photographs show that the particles prepared by urea homogeneous precipitation method are all spherical and well-dispersed, and grain morphology can be controlled by different surfactant. XRD and SEM indicate that the particle sizes prepared by sol-gel method are in the range of 5～30 nm, and the grain sizes increase with increasing of heated temperatures. Luminescence spectra indicat that the main emission peaks of all samples are at 610 nm, the intensities are different from samples prepared with different surfactant and the luminescence intensities increase with increasing of annealed temperatures.

Concentration effect of the near-infrared quantum cutting of 1788-nm luminescence of Tm^(3+) ion in(Y_(1-x)Tm_x)_3Al_5O_(12) powder phosphor

In the present paper, the concentration effect of near-infrared quantum cutting of Tm3＋ ion in （Y1-xTmx）3Al5Ol2 powder phosphor is studied by means of experiments and calculations. In addition, the absorption spectra, visible-to-near- infrared excitation and emission spectra, and fluorescence lifetimes are measured. It is found that （Y1-xTmx）3Al5O12 powder phosphor has a strong four-photon near-infrared quantum cutting luminescence of 1788.0-nm 3F4 →3H6 fluores- cence of Tm3＋ ion, when excited by 357.0-nm light. It is also found that the up-limit of the four-photon near-infrared quantum cutting luminescence efficiency of （Yo.700Tmo.300）3Al5 O12 powder phosphor is approximately 302.19%. To the knowledge of the authors, this is the first time that a near-infrared quantum cutting efficiency up-limit exceeding 300% has been reported. The results of this manuscript are valuable in aiding the probing of the new generation Ge solar cell.

Roles of Eu^(2+), Dy^(3+) Ions in Persistent Luminescence of Strontium Aluminates Phosphors

The polycrystalline Eu^2＋ and Dy ^3＋ co-doped strontium aluminates SrAl2O4： Eu^2＋, Dy^3＋ with different compositions were prepared by solid state reactions. The UV-excited photoluminescence, persistent luminescence and thermo-luminescence were studied and compared. Results show that the doped Eu^2＋ ion in SrAl2O4： Eu^2＋, Dy^3＋ phosphors works as not only the UV-excited luminescent center but also the persistent luminescent center. The doped Dy^3＋ ion can hardly yield any luminescence under UV-excitation, but effectively enhance the persistent luminescence and thermo-luminescence of SrAl2O4： Eu^2＋. Dy^3＋ co-doping can help form electron traps with appropriate depth due to its suitable electro-negativity, and increase the density and depth of electron traps. Based on above observations, a persistent luminescence mechanism, electron transfer model, is proposed and illustrated.

Enhancement of photoluminescence in Sr_2CeO_4 phosphors by doping with non-rare earth impurities

Non-rare earth impurity doped Sr2CeO4：X （X ：Zn, Hg, Al, Ag, Cr） phosphors are prepared by using the combustion method. The structural and photoluminescent properties of the as-prepared phosphors are investigated by X-ray diffraction （XRD） and photoluminescence at room temperature. Experimental results show that zinc addition and firing processing can effectively enhance the photoluminescence of Sr2CeO4 phosphors.

Luminescence Properties of Green-Emitting Phosphor (Ba_(1-x),Sr_x_2SiO_4∶Eu^(2+) for White LEDS

The （Ba1- x, Srx ） 2 SiO4 ： EU^2＋ green-emitting phosphors were synthesized by conventional solid-state reaction in a CO-reductive atmosphere, and their luminescent properties were investigated. The XRD data show that the Ba/Sr ratio not only affects the lattice parameters, but also influences the emission peak. The excitation spectra indicate that this phosphor can be effectively excited by UV light from 370 to 470 nm. The emission band is due to the 4f^65d^1→4f^7 transition of the Eu^2＋ ion. With an increase in x, the emission band shifts to longer wavelength and the reason was discussed. The emission spectra exhibit a satisfactory green performance under different excitation wavelength（380,398,412,420,460 nm）. （Ba1- x, Srx ） 2 SiO4 ： EU^2＋ is a promising phosphor for green white-lighting-emission diode by ultraviolet chip.

Luminescence Characterization of Mg Doped Y_2O_2S:Ti Long Afterglow Phosphor

Y1.94-xMgxO2S:0.06Ti (0≤x≤0.10) phosphors with long afterglow were synthesized by solid state reaction route. The photoluminescence spectra, decay curves, thermoluminescent spectra and chromaticity coordinate curves were investigated. The results show that the luminescence intensity of Y1.94-xMgxO2S :0.06Ti (0≤x≤0. 10) phosphors decrease gradually with increasing Mg2+ ion content, and the shape of luminescence spectra and chromaticity coordinate change as well. Furthermore, two thermoluminescent peaks in single Ti-doped Y2O2S sample are found at 91.8 and 221.5℃, respectively. Nevertheless, significant different spectra were found for the Mg, Ti co-doped Y2O2S samples that three thermoluminescence peaks appear at 52.3, 141.7 and 226.8℃, respectively. These results indicate that the co-doped Mg ion changes the inherent trap depth of single Ti-doped Y2O2S: Ti phosphor, and induces simultaneously a new trap level in the Y1.94-xMgxO2S:0.06Ti phosphor. Based on the analysis of thermoluminescent spectra, photolumi-nescent spectra, decay curve and crystal structure defect, it was proposed that the varied structure defect and introduced new trap level by the doped Mg2+ ions should be responsible for reducing luminescence intensity and varying color in the Y1.94-x Mgx O2S : 0.06Ti phosphor.

Relationship between Crystal Structure and Luminescence Properties of (Y_(0.96-x)Ln_xCe_(0.04))_3Al_5O_(12) (Ln=Gd, La, Lu) Phosphors

The doping effects of La^3＋, Gd^3＋ and Lu^3＋ on the crystal structure and luminescence properties of （Yo96-x LnxCe0.04）3Al5O12（Ln = Gd, La, Lu） phosphors were studied. The X-ray diffraction patterns presented that with the inerease of the doping concentrations of La^3＋ and Gd^3＋ ions, the d-value of （Y0.96-xLnxCe0.04）3Al5O12 （Ln = Gd, La） inereased and the larger the doping ion, the stronger the effect would be. The doping amount causing phase transition in （Y0.96-xLnxCe0.04）3Al5O12 decreased with the inerease of the ionic radii of the doping lanthanide ions （La^3＋： 0.106 nm, Gd^3＋： 0. 094 nm, Lu^3＋ ： 0.083 nm）. The bigger doping ion of Gd^3＋ made the emission of （Y0.96-xGdxCe0.04）3Al5O12 move to red spectral region, but the smaller one of Lu^3＋ made it blue.

Preparation and luminescence properties of the red-emitting phosphor(Sr_(1-x)Ca_x)_2Si_5N_8:Eu^(2+) with different Sr/Ca ratios

A series of Eu2＋-doped ternary nitride phosphors, with a formula of（Sr1-xCax）2Si5N8：Eu2＋, were synthesized by high-temperature solid-state method.The structure and luminescence properties were characterized, indicating the potential application as a red phosphor in the phosphor-converted white light-emitting diodes.The X-ray diffraction patterns showed that the Sr2Si5N8 and Ca2Si5N8 phases were generated at each end of（Sr1-xCax）2Si5N8：Eu2＋ and coexisted in the range of 0.5≤x≤0.75.The emission spectra showed broad emission bands originating from the 4f65d1→4f7 transition of Eu2＋ ions.The emission peak changed with the variations in Ca2＋ concentration.

Study on Luminescence Properties and Crystal-Lattice Environment of Eu^(2+) in Sr_(4-x)Mg_x Si_3O_8Cl_4∶Eu^(2+) Phosphor

According to the Van Uitert experimental equation, crystal-lattice environment of Eu^(2+) in the Sr_4Si_3O_8Cl_4 crystal was discussed. By adding Mg^(2+) to the host lattice, Sr_(4- x )Mg_ x Si_3O_8Cl_4∶Eu^(2+) was synthesized and the emission peak shifted from blue-green (488 nm) to blue-violet (411 nm) with the increase of amount of the magnesium which replaced the strontium. By analyzing the spectra of Sr_(4- x )Mg_ x Si_3O_8Cl_4∶Eu^(2+) the two Eu^(2+) emission centers were found because of the change of crystal-lattice environment in the host and the crystal structure was obtained by X-ray diffraction data.

Luminescence properties of YAl_3(BO_3)_4 phosphors doped with Eu^(3+) ions

YAl3 （BO3）4： Eu^3＋ phosphors were prepared by the conventional solid state reaction. The phase structure and morphology were investigated by X-ray diffraction （XRD） and scanning electron microscope （SEM）. Doping YAl3（BO3）4： Eu^3＋ phosphors with concentration of Eu^3＋ ions of 0, 2, 5, 8 and 10 mol% were studied and their luminescent properties at room temperature were discussed. The excitation spectrum of Y0.95Eu0.05Al3（BO3）4 was composed of a broad band centered at about 252 nm and a group of lines in the longer wavelength region. In the emission spectra, the peak wavelength was about 614 nm under a 252 nm UV excitation. The optimal doping concentration of Eu^3＋ ions in YAl3（BO3）4： Eu^3＋ phosphors was 8 mol%.

Luminescence characteristics of Eu^(2+) activated Ca_2SiO_4,Sr_2SiO_4 and Ba_2SiO_4 phosphors for white LEDs

This paper investigates the luminescence characteristics of Eu2＋ activated Ca2SiO4, Sr2SiO4 and Ba2SiO4 phosphors. Two emission bands are assigned to the f-d transitions of Eu2＋ ions doped into two different cation sites in host lattices, and show different emission colour variation caused by substituting M2＋ cations for smaller cations. This behaviour is discussed in terms of two competing factors of the crystal field strength and covalence. These phosphors with maximum excitation of around 370 nm can be applied as a colour-tunable phosphor for light-emitting diodes （LEDs） based on ultraviolet chip/phosphor technology.