The energy transfer processes in LaMgB5O10:Pr, Mn

Pr³⁺, Mn²⁺ singly doped and co-doped LaMgB₅O₁₀ samples were prepared by solid-state reaction and their spectroscopic properties were investigated by synchrotron radiation VUV light. Significant spectra overlap between the Mn²⁺⁶A_1g→(⁴E_g, ⁴A_1g) excitation (centered at 412 nm) and the Pr³⁺¹S₀→(¹I₆, ³P_J) emission (410 nm) provided the possibility of energy transfer from Pr³⁺ to Mn²⁺. In the LaMgB₅O₁₀:Pr³⁺, Mn²⁺ samples investigated, the expected energy transfer process was observed as comparing the emission spectra of LaMgB₅O₁₀:Pr³⁺, Mn²⁺ samples with that of the LaMgB₅O₁₀:Mn²⁺. The shorter decay time of the ¹S₀→(¹I₆, ³P_J) transition in the co-doped samples was also an evidence of energy transfer from Pr³⁺ to Mn²⁺. By analyzing the energy transfer process, it was found that the energy transfer process in LaMgB₅O₁₀:Pr³⁺, Mn²⁺ was likely of resonant energy transfer and the re-absorption process can be excluded. The critical distances of energy transfer based on the electric dipole-dipole interaction and electric dipole-quadrupole interaction were calculated to be 4.78 and 9.46 Å in LaMgB₅O₁₀:Pr³⁺, Mn²⁺, respectively, which are smaller than the mean distance of Pr³⁺ and Mn²⁺ (17 Å) in the highest concentration-doped sample. The near neighboring PrMn clusters formed in the LaMgB₅O₁₀ host is responsible for the energy transfer process.     

Mn, Cr-co-doped MgAl2O4 phosphors for white LEDs

The Mn-, Cr-doped and Mn, Cr-co-doped MgAl₂O₄ powders have been synthesized via a gel-solid reaction method. Energy transfer from Mn²⁺ to Cr³⁺ has been observed for the first time in the co-doped MgAl₂O₄ phosphors. When excited with blue light with a wavelength of 450 nm at room temperature, both green emission from Mn²⁺ around 520 nm and red emission from Cr³⁺ around 675and 693 nm were generated. Moreover, the color of the emission can be modified by controlling the doping concentrations of Mn²⁺ and Cr³⁺. Therefore, MgAl₂O₄: Mn²⁺, Cr³⁺ could be used as a single-phased phosphor for white LED with a blue LED chip. The energy transfer in terms of Mn²⁺ to Cr³⁺ is determined by means of radiation and reabsorption.     

Luminescence properties of red-emitting praseodymium-activated BaTi4O9 phosphor

The photoluminescence and low-voltage cathodoluminescence characteristics of BaTi₄O₉:Pr³⁺ were investigated. The excitation band of intervalence charge transfer (IVCT) of BaTi₄O₉:Pr³⁺ emerged distinctly at 330 nm. The resultant emissions appeared at 606-643 nm corresponding to the ¹D₂→³H₄ transition. In BaTi₄O₉:Pr³⁺, the emission of ³P₀→³H₄ transition at 490 nm was not observed. The results were in a pure red color emission.     

Down-conversion near infrared emission in Pr, Yb co-doped Y2O3 transparent ceramics

Pr³⁺, Yb³⁺ co-doped Y₂O₃ transparent ceramics have been prepared by the solid state reaction and vacuum sintering. Down-conversion near infrared emission has been demonstrated upon a 482 nm excitation. The energy of the 482 nm blue photon was first absorbed by Pr³⁺ and then delivered to Yb³⁺. Possible energy transfer mechanisms from Pr³⁺ to Yb³⁺ have been discussed. Under the 482 nm excitation, the Pr⁴⁺-Yb²⁺ charge transfer state would not seriously influence the energy transfer process. The dominant one should be either the cooperative down-conversion or the two-step photon emission. The efficient down-conversion near infrared emission has potential application in enhancing the conversion efficiency of crystalline silicon solar cells.     

VUV spectroscopic properties of Ce and Pr-doped AREP2O7-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu)

Spectroscopic properties of Ce³⁺ and Pr³⁺-doped AREP₂O₇-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu) have been investigated using VUV spectroscopy technique. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 325-450 nm. The strong host absorption band starting at around 160 nm indicates that the optical band gap of AREP₂O₇ hosts is at least 7.7 eV, and the host→Ce³⁺ energy transfer process is rather efficient. However, AREP₂O₇:Pr³⁺ samples show less efficient host→Pr³⁺ energy transfer. The direct Pr³⁺ 4f²→4f¹5d¹ excitation, which are 12160±640 cm⁻¹ higher respect to that of Ce³⁺, leads to strong 4f¹5d¹→4f² emission bands in the range of 230-325 nm but no obvious 4f²→4f² emission lines.     

Emission analysis of Dy and Pr:Bi2O3-ZnF2-B2O3-Li2O-Na2O glasses

In this paper, we present the spectral results of Dy³⁺ and Pr³⁺ (1.0 mol%) ions doped Bi₂O₃-ZnF₂-B₂O₃-Li₂O-Na₂O glasses. Measurements of X-ray diffraction (XRD), differential scanning calorimetry (DSC) profiles of these rare-earth ions doped glasses have been carried out. From the DSC thermograms, glass transition (T_g), crystallization (T_c) and melting (T_m) temperatures have been evaluated. The direct and indirect optical band gaps have been calculated based on the glasses UV absorption spectra. The emission spectrum of Dy³⁺:glass has shown two emission transitions ⁴F_7/2→⁶H_15/2 (482 nm) and ⁴F_7/2→⁶H_13/2 (576 nm) with an excitation at 390 nm wavelength and Pr³⁺:glass has shown a strong emission transition ¹D₂→³H₄ (610 nm) with an excitation at 445 nm. Upon exposure to UV radiation, Dy³⁺ and Pr³⁺ glasses have shown bright yellow and reddish colors, respectively, from their surfaces.     

Emission analysis of Pr and Tm: Ca2Gd2W3O14 phosphors

5 mol% of Pr³⁺ and Tm³⁺ ions activated calcium gadolinium tungstate (Ca₂Gd₂W₃O₁₄₎ phosphors were synthesized by traditional solid state reaction method. Crystalline phase structure was identified from the X-ray diffraction (XRD) profiles. From the scanning electron microscopy (SEM) images, we have observed the agglomeration of the particles, and average grain size is around 40-300 nm. Using the energy dispersive X-ray analysis (EDAX) and Fourier transform infrared (FTIR) spectra, identified the elements and functional groups present in the prepared phosphors. The emission spectrum of Pr³⁺: Ca₂Gd₂W₃O₁₄ powder phosphors have shown an intense red emission at 615 nm with the excitation wavelength λ_exci=450 nm and thus these red color emitting powder phosphors are used as one of the components in the preparation of WLEDs. The excitation spectrum of Tm³⁺: Ca₂Gd₂W₃O₁₄ powder phosphor has shown a ligand to metal charge transfer (W-O) band (LMCT) within the WO₄²⁻ group. Emission spectrum of Tm³⁺: Ca₂Gd₂W₃O₁₄ phosphors have shown blue emissions at 453 nm (¹D₂→³F₄).     

Effects of Zn impurities on the electronic properties of Pr doped CaTiO3

Microcosmic investigations of weak red-emitting materials are crucial for their further development and application. In this work, we have focused on the band structures and electronic properties of Pr mono- and (Zn, Pr) co-doped CaTiO₃ using density functional theory. Zn substitution for Ca or Ti tends to form clusters energetically with Pr substituting for Ca in CaTiO₃. In Pr mono-doped CaTiO₃, the O_2p→Ti_3d transition in CaTiO₃ host corresponds to the centered 330 nm excitation spectra. The gap states above the valence band of ∼1.30 eV and ∼2.06 eV are hybridized by Pr_4f, O_2p and Ti_3d orbitals. They are mainly due to Pr_4f orbitals in CaTiO₃:Pr. The former gap level is related to red emission at 614 nm due to ¹D₂→³H₄ transition of Pr³⁺ activator. The latter is related to the excitation spectra centered at 380 nm due to the low-lying Pr-to-mental intervalence charge transfer transitions (Pr³⁺-O²⁻-Ti⁴⁺?Pr⁴⁺-O²⁻-Ti³⁺). The band structures of (Zn, Pr) co-doped CaTiO₃ keep the similar gap levels to those in Pr mono-doped CaTiO₃. The incorporation of Zn brings out the two stronger localized gap states, which are hybridized by Pr_4f, O_2p and Ti_3d orbitals, in comparison with those in Pr mono-doped CaTiO₃. Therefore, when Zn impurities are added into Pr doped CaTiO₃, the present calculations visualize the two enhanced levels and the distorted structures around Pr.     

Luminescent properties of trivalent praseodymium-doped lanthanum aluminum germanate LaAlGe2O7

The luminescent characteristics of Pr³⁺-activated LaAlGe₂O₇ were investigated. In response to excitement using 448 nm blue light, the emission spectra involved most of the ³P₀→³H_J transitions. The dominant emission came from the ³P₀→³H₄ transition at 487 nm. ¹D₂ fluorescence quenching was observed in highly doped samples and is related to the cross-relaxation processes among neighboring Pr³⁺ ions. In contrast with conventional Pr³⁺-activated phosphors, the extraordinary excitation spectra showed only intense f-f transition of Pr³⁺ ions, while the 4f-5d transition was eliminated. This is ascribed to photoionization. By analyzing absorption and excitation spectra, it is recognized that no efficient energy transfer occurs between Pr³⁺ and the host lattice in LaAlGe₂O₇.     

Dyand Mn emission in KMgSO4Cl phosphor

In the present paper, KMgSO₄Cl:Ce³⁺, KMgSO₄Cl:Ce³⁺,Dy³⁺, and KMgSO₄Cl:Ce³⁺,Mn²⁺, new halosulphate phosphors were synthesized by wet chemical method. X-ray powder diffraction (XRD) and photoluminescence (PL) characterization of phosphors have been reported in this paper. The effects of Dy³⁺ co-doping on the PL characteristics of KMgSO₄Cl:Ce phosphor were studied. Energy transfer from Ce³⁺→Dy³⁺and Ce³⁺→Mn²⁺ results in increase in PL peak intensity suggesting that Ce³⁺ plays an important role in PL emission in the present matrix. The PL emission spectra have two peaks (482 and 571 nm) and a single peak (564 nm), which could be attributed to the Ce³⁺→Dy³⁺and Ce³⁺→Mn²⁺ emissions, respectively.     

Emission properties of Eu, Mn in MAl2Si2O8 (M=Sr, Ba)

The emission properties of Eu²⁺ and Mn²⁺ in monoclinic SrAl₂Si₂O₈ (M-SAS) and hexagonal BaAl₂Si₂O₈ (H-BAS), both of which have only one alkaline-earth site, were studied. The emission peaks of both Eu²⁺ (405 nm) and Mn²⁺ (564 nm) in SrAl₂Si₂O₈, are located at longer wavelengths, compared with those in H-BAS (373 nm for Eu²⁺ and 518 nm for Mn²⁺), because of the stronger crystal field strength at the Sr site. EPR spectra showed that the g values of Mn²⁺ are 4.5065 in M-SAS:Mn and 2.0247 in H-BAS:Mn. Magnetic measurements proved that Mn²⁺ was at high-spin state in both hosts. The large g value of Mn²⁺ in M-SAS was ascribed to the mixing of the first excitation state to the ground state, both of which have lower d orbital degeneracy due to the lower symmetry of Mn²⁺ site. The transfer efficiency from Eu²⁺ to Mn²⁺was about 10% in M-SAS, higher than that in H-BAS (5%). This was probably because Eu²⁺ emission overlaps the relatively low excitation level of Mn²⁺ in M-SAS. In order to obtain high transfer efficiency, it was necessary for the Eu²⁺ emission to overlap the lowest excitation level of Mn²⁺. The results obtained in this work may be helpful to design the new white or red phosphors for white-light emitting diode (w-LED) applications.     

Synthesis and characterization of BaAl2O4:Eu co-doped with different rare earth ions

Combustion method was used in this study to prepare BaAl₂O₄:Eu²⁺ phosphors co-doped with different trivalent rare-earths (Re³⁺=Dy³⁺, Nd³⁺, Gd³⁺, Sm³⁺, Ce³⁺, Er³⁺, Pr³⁺ and Tb³⁺) ions at an initiating temperature of 600 °C. The phosphors were annealed at 1000 °C for 3 h. As confirmed from the X-ray diffraction (XRD) data, both as prepared and post annealed samples crystallized in the well known hexagonal structure of BaAl₂O₄. All samples exhibited bluish-green emission associated with the 4f⁶5d¹→4f⁷ transitions of Eu²⁺ at ∼500 nm. Although the highest intensity was observed from Er³⁺ co-doping, the longest afterglow (due to trapping and detrapping of charge carriers) was observed from Nd³⁺ followed by Dy³⁺ co-doping. The traps responsible for the long afterglow were studied using thermoluminescence (TL) spectroscopy.     

Spectral analysis of Pr-, Sm- and Dy-doped transparent GeO2-BaO-TiO2 glass ceramics

In this paper, we present the photoluminescence properties of Pr³⁺-, Sm³⁺- and Dy³⁺-doped germanate glasses and glass ceramics. From the X-ray diffraction measurement, the host glass structure was determined. These glasses have shown strong absorption bands in the near-infrared (NIR) region. Compared to Pr³⁺-, Sm³⁺- and Dy³⁺-doped glasses, their respective glass ceramics have shown stronger emissions due to the Ba₂TiGe₂O₈ crystalline phase. For Pr³⁺-doped glass and glass ceramic, emission bands centered at 530 nm (³P₀→³H₅), 614 nm (³P₀→³H₆), 647 nm (³P₀→³F₂) and 686 nm (³P₀→³F₃) have been observed with 485 nm (³H₄→³P₀) excitation wavelength. Of them, 647 nm (³P₀→³F₂) has shown bright red emission. Emission bands of ⁴G_5/2→⁶H_5/2 (565 nm), ⁴G_5/2→⁶H_7/2 (602 nm) and ⁴G_5/2→⁶H_9/2 (648 nm) for the Sm³⁺:glass and glass ceramic, with excitation at ⁶H_5/2→⁴F_7/2 (405 nm) have been recorded. Of them, ⁴G_5/2→⁶H_7/2 (602 nm) has shown a bright orange emission. With regard to the Dy³⁺:glass and glass ceramic, a bright fluorescent yellow emission at 577 nm (⁴F_9/2→⁶H_13/2) has been observed, apart from ⁴F_9/2→⁶H_11/2 (667 nm) emission transition with an excitation at 454 nm (⁶H_15/2→⁴I_15/2) wavelength. The stimulated emission cross-sections of all the emission bands of Pr³⁺, Sm³⁺ and Dy³⁺:glasses and glass ceramics have been computed based on their measured full-width at half-maxima (FWHM, Δλ) and lifetimes (τ_m).     

Efficient blue upconversion emission due to confined radiative energy transfer in Tm-Nd co-doped Ta2O5 waveguides under infrared-laser excitation

Intense blue upconversion emission at 480 nm has been obtained at room temperature in Tm³⁺-Nd³⁺ co-doped Ta₂O₅ channel waveguides fabricated on a Si substrate, when the sample is excited with an infrared laser at 793 nm. The upconversion mechanism is based on the radiative relaxation of the Nd³⁺ ions (⁴F_3/2 → ⁴I_11/2) at about 1064 nm followed by the absorption of the emitted photons by Tm³⁺ ions in the ³H₄ excited state. A coefficient of energy transfer rate as high as 3 × 10⁻¹⁶ cm³/s has been deduced using a rate equation analysis, which is the highest reported for Tm-Nd co-doped systems. The confinement of the 1064 nm emitted radiation in the waveguide structure is the main reason of the high energy transfer probability between Nd³⁺ and Tm³⁺ ions.     

Photoluminescence properties of Eu and Mn-activated BaMgP2O7 as a potential red phosphor for white-emission

A phosphate compound, BaMgP₂O₇ was co-doped with Eu²⁺ and Mn²⁺ for making a red-emitting phosphor. The phosphor was prepared by a solid-state reaction at high temperature. The photoluminescence properties were investigated under ultraviolet (UV) ray excitation. From a powder X-ray diffraction (XRD) analysis, the formation of single-phased BaMgP₂O₇ with a monoclinic structure was confirmed. In the photoluminescence spectra, the BaMgP₂O₇:Eu,Mn phosphor emits two distinctive colors: a blue band centered at 409 nm originating from Eu²⁺ and a red band at 615 nm caused by Mn²⁺. Also, efficient energy transfer from Eu²⁺ to Mn²⁺ in the BaMgP₂O₇:Eu,Mn system was verified by observing that the excitation spectra of BaMgP₂O₇:Eu,Mn emitted at 409 and 615 nm by Eu²⁺ emission and Mn²⁺ emission, respectively, are almost the same as that of BaMgP₂O₇:Eu monitored at 409 nm. The optimum concentration of Eu²⁺ ions in BaMgP₂O₇:0.015Eu excited at 309 nm wavelength is 1.5 mol%. With an increase of Mn²⁺ content up to 17.5 mol%, a systematic decline in the intensity of the excitation spectrum by Eu²⁺ and a gradual growth in the intensity of emission band by Mn²⁺ were observed. Accordingly, the optimum concentration of Mn²⁺ in BaMgP₂O₇:0.015Eu,Mn is 17.5 mol%. The maximum spectral overlap between emission of Eu²⁺ and excitation of Mn²⁺ is achieved in a composition of BaMgP₂O₇:0.015Eu,0.175Mn, resulting in considerable red-emission at 615 nm.     

BaAl12O19:Mn green emitting nanophosphor for PDP application synthesized by solution combustion method and its Vacuum Ultra-Violet Photoluminescence Characteristics

Nanostructured BaAl₁₂O₁₉:Mn²⁺ phosphor particles of nano-rod morphology with diameter 40-100 nm and length up to 200-600 nm has been synthesized by solution combustion method and its photoluminescence characteristics have been studied by Vacuum Ultra-Violet Photoluminescence spectrometer (VUVPL) under 147 nm excitation. The crystallographic phase purity of BaAl₁₂O₁₉:Mn²⁺ nanostructured phosphor particle synthesized by solution combustion approach is confirmed by X-ray diffraction (XRD). The broadening of XRD diffraction peaks indicates nanocrystalline nature of particles present in powder. The emission spectrum of BaAl₁₂O₁₉:Mn²⁺ nanophosphor on 147 nm excitation consists of a wide green band with a peak at about 515 nm, which is due to a 3d⁵ (⁴T_1g)-3d⁵ (⁶A_1g) transition corresponds of Mn²⁺ ions. It is found that the concentration quenching is obtained when Mn²⁺ content (x) is 0.05 in BaAl₁₂O₁₉:xMn²⁺ nanophosphor on 147 nm excitation. The decay time of 3d⁵ (⁴T_1 g)-3d⁵ (⁶A_1 g) transition of Mn²⁺ ions at 147 nm excitation is about 23 ms for BaAl₁₂O₁₉:Mn²⁺ nanophosphor. This nanostructured green emitting BaAl₁₂O₁₉:Mn²⁺ phosphor can find potential application in Plasma Display Panels (PDPs) and mercury-free fluorescent lamps.     

Enhanced blue-green-red up-conversion and 1.3-μm emission of Pr/Yb-codoped oxyhalide tellurite glasses with PbCl2 doping

We report on the blue-green-red up-conversion spectroscopic properties of Pr³⁺/Yb³⁺-codoped oxyhalide tellurite glasses upon excitation of a conventional 980 nm laser diode (LD). Significant enhancement of the blue-green-red up-conversion emission intensity has been observed with increasing PbCl₂ doping. The up-conversion intensity has a quadratic dependence on incident pump laser power, indicating a two-photon process. The population of the Pr³⁺ upper ³P₀ emitting level was accomplished through a combination of a ground state absorption, energy transfer and excitated state absorption. 1.3-μm emission in the second telecom window originated from Pr³⁺:¹G₄→³H₅ transition has also been investigated upon excitation at 980 nm LD. The measured peak wavelength and full width at half-maximum of the fluorescent are 1335 nm and ∼100 nm, respectively. An enhanced 1.3μm emission with increasing PbCl₂ doping has also been observed. Codoping of Yb³⁺ significantly enhance both the blue-green-red up-conversion emission and 1.3-μm emission intensity by way of a nonradiative Yb³⁺:²F₅→Pr³⁺:¹G₄ energy transfer.     

Eu/Tb ions co-doped white light luminescence Y2O3 phosphors

Y₂O₃:Eu³⁺, Tb³⁺ phosphors with white emission are prepared with different doping concentration of Eu³⁺ and Tb³⁺ ions and synthesizing temperatures from 750 to 950 °C by the co-precipitation method. The resulted phosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. The results of XRD indicate that the crystallinity of the synthesized samples increases with enhancing the firing temperature. The photoluminescence spectra indicate the Eu³⁺ and Tb³⁺ co-doped Y₂O₃ phosphors show five main emission peaks: three at 590, 611 and 629 nm originate from Eu³⁺ and two at 481 and 541 nm originate from Tb³⁺, under excitation of 250-320 nm irradition. The white light luminescence color could be changed by varying the excitation wavelength. Different concentrations of Eu³⁺ and Tb³⁺ ions were induced into the Y₂O₃ lattice and the energy transfer from Tb³⁺→Eu³⁺ ions in these phosphors was found. The Commission International de l’Eclairage (CIE) chromaticity shows that the Y₂O₃:Eu³⁺, Tb³⁺ phosphors can obtain an intense white emission.     

The influence of Pr co-doping on the luminescent properties of Lu2O3:5 mol% Eu films

Uniform and crack free polycrystalline lutetium oxide (Lu₂O₃:(Eu,Pr)) films were fabricated by Pechini sol-gel method combined with the spin-coating technique. X-ray diffraction (XRD) and atomic force microscope (AFM) characterizations indicated that the obtained film was composed of polycrystalline cubic Lu₂O₃ phase with an average grain size around 30 nm. The photoluminescence(PL) spectra and decay performances of the Lu₂O₃:5 mol% Eu films co-doped by 0-0.5 mol% Pr³⁺ with different concentrations were characterized. It was found that the afterglow was reduced obviously due to the introduction of 0-0.5 mol% Pr³⁺ in the Lu₂O₃:5 mol% Eu films coupled by decrease in the emission intensity at 612 nm. The mechanism of afterglow diminishing was discussed based on the thermoluminescence measurements.     

Luminescence characterization and electron beam induced chemical changes on the surface of ZnAl2O4:Mn nanocrystalline phosphor

Luminescence characteristics and surface chemical changes of nanocrystalline Mn²⁺ doped ZnAl₂O₄ powder phosphors are presented. Stable green cathodoluminescence (CL) or photoluminescence (PL) with a maximum at ∼512 nm was observed when the powders were irradiated with a beam of high energy electrons or a monochromatic xenon lamp at room temperature. This green emission can be attributed to the ⁴T₁ → ⁶A₁ transitions of the Mn²⁺ ion. Deconvoluted CL spectra resulted in two additional emission peaks at 539 and 573 nm that may be attributed to vibronic sideband and Mn⁴⁺ emission, respectively. The luminescence decay of the Mn²⁺ 512 nm emission under 457 nm excitation is single exponential with a lifetime of 5.20 ± 0.11 ms. Chemical changes on the surface of the ZnAl₂O₄:Mn²⁺ phosphor during prolonged electron beam exposure were monitored using Auger electron spectroscopy. The X-ray photoelectron spectroscopy (XPS) was used to determine the chemical composition of the possible compounds formed on the surface as a result of the prolonged electron beam exposure. The XPS data suggest that the thermodynamically stable Al₂O₃ layer was formed on the surface and is possibly contributing to the CL stability of ZnAl₂O₄:Mn phosphor.