CaWO4 Host Research Articles

Photoluminescence properties of manganese activated calcium tungstate phosphors

This study presents the novel Mn doped CaWO4 nanophosphors as an excellent alternatives of rare earth free materials for display application. The Mn doped CaWO4 phosphor were characterized by various techniques, such as UV–Vis-DRS, Raman, PL analysis. Scheelite type tetragonal structure with space group I41/a has been confirmed. Rietveld analyses confirms the formation of single-phase solid solution. Lattice parameters for Mn free and Mn doped samples were calculated and observed that cell volume decreases after Mn incorporation. FTIR and Raman studies confirm the involvement of functional group and vibrational modes of vibration in the compound. Band gap values are estimated to be in the range of 4.2 to 4.33 eV with Mn doping. Photoluminescence study confirms the strong green emission at 450 and 515 nm (d-d transition in Mn2+) after Mn doping. Also, it was observed that strong emission peak appears ~422 nm is mainly due to the electronic transition, 1 T2 → 1 A1 in [WO4] 2- tetrahedron of CaWO4 host matrices. CIE study confirms that prepared nanophosphor exhibits strong blue colour after Mn incorporation. It can be employed as a potential material for blue phosphors in LEDs applications.

Synthesis and Luminescence Properties of Ho3+ and Er3+-Doped CaWO4 Nanocrystalline Powders Prepared by Self-Propagating Combustion Method.

Nanocrystalline Er3+-doped CaWO4 (CaWO4:Er3+) and Ho3+-doped CaWO4 (CaWO4:Ho3+) powders were fabricated by a facile sol-gel self-propagating combustion method using glycin as reductant. The components, microstructure and luminescence properties of samples were studied in detail. The as-prepared CaWO4:Er3+ and CaWO4:Ho3+ samples presented meshy and porous structure, which can be attributed to the instantaneous releasing of a lot of gases during the combustion reaction. The emission spectra of CaWO4:Ho3+ presented two emission peaks around 415 and 542nm, corresponding to the intrinsic emission of [WO4]2- complex and the characteristic emission of Ho3+ from excited state 5F4 to ground state 5I8 transition. For the CaWO4:Er3+ sample, three emission peaks centered at 420, 525 and 550nm can be observed. The mechanism of emission process and energy transfer process in CaWO4:Er3+ and CaWO4:Ho3+ was explained in detail. This work shows that the multiple luminescence emission can be realized by doping a small amount of Er3+ or Ho3+ into CaWO4 host materials, which is significant for developing luminescent materials of tungstate systems.

Experimental insight into the thermal quenching of the Tb3+ ion’s green photoluminescence in CaWO4 host

By directly exciting the Tb3+ ion’s 7F6-5D4 absorption line, the 5D4-7F5 emission underwent a serious thermal quenching from 333 to 813 K in CaWO4 host. Through steady-state rate equation, it was shown in addition to the 5D4 state’s lifetime, the 7F6 state’s population also affected this thermal quenching. When using a diode laser at 405 nm to excite the Tb3+ ion’s second lowest 7F5 state, the 5D4-7F5 emission increased dramatically from 333 to 813 K. It reveals the thermal population from the lowest 7F6 ground state to the higher 7F5 state is one of the possible thermal quenching channels for the 5D4-7F5 emission.

The effect of Mg2+ ion on the optical characteristics of rare-earth ion in CaWO4 polycrystal

The MgxCa(1-x)WO4 polycrystals codoped with Er/Yb and Tm/Yb were prepared by the high temperature solid-state reaction method. The structural properties of MgxCa(1-x)WO4 polycrystals codoped with Er3+ and Yb3+ ions were confirmed by the X-ray diffraction patterns. The effect of Mg2+ ion on the optical characteristics of Er3+ and Tm3+ ions in CaWO4 polycrystals under 980 nm excitation was investigated. The strong blue and green upconversion emissions were observed for CaWO4 polycrystal doped with 30 mol% Mg2+ ions. The Mg2+ ions affected the optical characteristics of Er3+ and Tm3+ ions in CaWO4 host material through tuning the rate of cross relaxation process. Studies on the upconversion emission spectra and power pump dependence of Er/Yb:Mg0.3Ca0.7WO4 polycrstals indicated that the inefficient cross relaxation processes of 4S3/2 + 4I15/2 → 4I9/2 + 4I13/2 and 4S3/2 + 4I15/2 → 4I13/2 + 4I9/2 between Er3+ ions were contribution to the increased green upconversion emission under 980 nm excitation.

Luminescence properties of CaWO4 and CaWO4:Eu3+ nanostructures prepared at low temperature

The synthesis of luminescence nanostructures is always related with complicated procedures, and it is still challenging to control their luminescence properties by a facile method. In this work, CaWO4:Eu3+ nanostructures with different morphology, structures and sizes were prepared at low temperatures (2 °C), and the luminescence properties of the prepared nanostructures were studied. The nanostructure growth can be controlled by pH values of the precursor, reaction temperatures and Eu3+ doping concentrations. Low temperature (2 °C) synthesis is believed to control the nanocrystal growth easily due to slow growth process caused by the low temperature. The pH values of the precursor have a significant impact on the structure of CaWO4, leading to the formation of self-assembled microstructures and isolated nanoparticles. CaWO4:Eu3+ nanowires were produced as the Eu3+ doping concentration is 10%. The luminescence properties show that the reaction temperatures and pH values play an important role in the luminescence of CaWO4 host and the energy transfer from the host to Eu3+ leading to the tunable luminescence color from CaWO4:Eu3+. The method developed in this paper is believed to be beneficial for large-scale synthesis of luminescence nanostructures toward white light emitting diode phosphors.

Using the upconversion luminescence of the CaWO4:Yb3+–X3+ (X = Er/Ho/Tm) phosphors for ratiometric thermal sensing

Based on the luminescence ratiometric method, here we systematically investigate the relative sensitivity (Sr) of three pairs of typical thermally coupled energy levels (TCELs), that are, the 2H11/2/4S3/2 levels of Er3+, the two sub-Stark 5F5 levels of Ho3+, and the 3F3/3H4 levels of Tm3+, embedded in the CaWO4 host. On the basis of the emission spectra of the CaWO4:Yb3+–X3+ (X = Er/Ho/Tm) phosphors, the gaps that separate the 2H11/2/4S3/2, the two sub-Stark 5F5, and the 3F3/3H4 TCELs are calculated to be 719, 402, and 1785 cm−1, respectively. According to the expression Sr = ΔE/kT2 (where ΔE is the energy gap, k is the Boltzmann constant and T is the absolute temperature), the maximum relative sensitivities of these three pairs of TCELs are expected to be 1.12, 0.63, and 2.79% K−1 at room temperature 303 K. However, the practical relative sensitivities 1.03, 0.23, and 0.22% K−1 are achieved at 303 K for these three pairs of TCELs accordingly. It is then demonstrated that not only at 303 K but over the whole experimental temperature range, the relative sensitivities of these three pairs of TCELs are confirmed to be overestimated to some degree. The work not only presents the practical relative sensitivities of these three pairs of TCELs, but also probably discloses a quite common phenomenon that the real relative sensitivity of TCELs is often overestimated. Therefore, it is expected to provide guidance for the practical applications of the luminescence ratiometric thermometry in the future.

Color selection and red fluorescence enhancement through the controllable energy transfer in NaxCa1-2xWO4:Eux3+ phosphor for UV converted LEDs

Phosphor-converted warm white light-emitting diodes (pc-wLEDs) have played a key role in our daily lighting life, but they are difficult to realize without adding red-emitting phosphors. Conventional Eu3+ doped red phosphors, however, suffer greatly from the intrinsic shortcomings such as low quantum efficiency, limited absorption of UV LED light and chemical and thermal stability. In this work, we design the Eu3+ doped NaxCa1-2xWO4:Eux3+ (0 ≤ x ≤ 0.20) phosphors, which can exhibit enhanced Eu3+ red fluorescence and the tunable color at the same time. Phase purity analysis, which was checked by X-ray powder diffraction (XRD), shows all samples crystallize in a scheelite structure with the space group of I41/a (No.88). Our findings reveal that increasing the Eu3+ content gives rise to moving the XRD position to low angle direction and, in the meanwhile, decreasing the charge transfer (CT) intensity of samples. Spectral results show that controlling the charge compensation content and energy transfer efficiency from host to Eu3+ enables us to achieve the enhanced Eu3+ red fluorescence and simultaneously the tunable color. Unlike the decrease change in the CT band intensity, the Eu3+ emission upon excitation at 393 nm and 464 nm exhibits a continuous increase in the emission intensity. Due to the inefficiency energy transfer between host and Eu3+ dopant, controlling the x value enables tuning the relative photoemission intensity between host and Eu3+ dopant and therefore the color tuning from blue to red. Our time-resolved PL results indicate that the energy transfer between the CaWO4 host and Eu3+ dopant begins to occur at t = 5 μs. A feasible profile, which bases on the energy transfer, has been also established in the work. This work not only demonstrates simultaneous enhanced Eu3+ fluorescence and color tuning in CaWO4 matrix through charge compensation method, but also can provide new insight into improving the PL properties in other phosphors for UV converted pc-wLEDs.

Fluorochromic Properties of Undoped and Ln3+-Doped CaWO4 Phosphor Particles

Undoped and Ln3+-doped (Ln3+ = Eu3+, Tb3+) CaWO4 phosphor particles were synthesized by a hydrothermal method, aiming at generating fluorochromic properties in response to redox reactions. The phosphor particles obtained were first treated with an aqueous L(+)-ascorbic acid solution as a reductant at room temperature, which resulted in a considerable decrease in photoluminescence (PL) intensity of both CaWO4 (the host-lattice blue emission) and CaWO4:Ln3+ (the 4f-4f red or green emission). The quenched PL could then be recovered by treating the particles with a sodium hypochlorite solution as an oxidant also at room temperature. This PL switching behavior was predominantly caused by redox reactions between the solid particles and the redox species in the solutions based on a change of W6+/W5+ valance states of the CaWO4 host crystal. More interestingly, CaWO4:Eu3+ exhibited a change of emission colors, depending on the excitation wavelength, due to different sensitivity of the host emission and the Eu3+ emission to the redox treatment. Such a changeable fluorochromism is regarded as a newly developed function of the phosphor materials for use in facile optical detectors and sensors.

Red-emitting enhancement by inducing lower crystal field symmetry of Eu3+ site in CaWO4:Eu3+ phosphor for n-UV w-LEDs

A series of enhanced red-emitting phosphors Ca1-x-yMgxSryWO4:zEu3+ (x = 0.00–0.30, y = 0.00–0.12, z = 0.00–0.16) were successfully synthesized via a high temperature solid-state reaction method. The phase structure, photoluminescence properties and thermal stability were investigated detailedly. All as-prepared samples, especially Ca0.65Mg0.20Sr0.05WO4:0.10Eu3+ phosphors exhibit an extremely enhanced red emission peaking at 616 nm originating from 5D0―7F2 transition of Eu3+ under the 392 nm n-UV excitation. Mg2+ and Sr2+ were introduced into CaWO4:0.10Eu3+ to induce crystallographic parameters and crystal symmetry changes of Eu3+ ions in CaWO4 host. Enhanced red emission, broadband excitation spectra (from 200 to 600 nm), strong ultraviolet absorption matched well with n-UV LEDs chips (350–420 nm) and high color purity (the R/O value is up to 7.52) indicate that as-prepared Ca0.65Mg0.20Sr0.05WO4:0.10Eu3+ sample can be a potential candidate red phosphor for n-UV white light-emitting diodes.

Synthesis, electronic structure and luminescence properties of color-controllable Dy3+/Eu3+-codoped CaWO4 phosphors

Color-tunable CaWO4:0.05Dy3+/xEu3+ phosphors were synthesized via a solid-state reaction method and the electronic properties of CaWO4 host lattice were analyzed through density function theory calculation. Under the excitations of 250 and 351nm, characteristic emissions corresponding to 5D0→4FJ (J=0, 1, 2) transitions of Eu3+ ions and characteristic blue and yellow emissions corresponding to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+ ions, respectively, were observed in all the Dy3+/Eu3+ ions codoped CaWO4 phosphors. The photoluminescence spectra and decay curves revealed that there existed an energy transfer from Dy3+ to Eu3+ ions and this was certified to be a resonant type via a dipole–dipole interaction. Furthermore, color-tunable emissions were observed by adjusting the relative content between Dy3+ and Eu3+ ions and the excitation wavelength. In addition, the Dy3+/Eu3+ ions codoped CaWO4 phosphors exhibited strong cathodoluminescence properties and their emission intensity increased with the increment of accelerating voltage from 1 to 5kV and filament current from 34 to 55μA.

Synthesis and luminescence properties of color-tunable Dy3+-activated CaWO4 phosphors

Color-tunable CaWO4:xDy3+ phosphors were prepared via a simple conventional solid-state reaction and their luminescent properties were investigated as a function of Dy3+ ion concentration under 258 and 353 nm excitations. A gradual enhancement in the emission intensity was observed with the increment of Dy3+ ion concentration, reaching its maximum value when x = 0.05. The main reason for the concentration quenching of Dy3+ emission in CaWO4 host lattice is due to the electric dipole-dipole interaction. The cathodoluminescence spectra, which were measured at different accelerating voltages and filament currents, were in consistent with the photoluminescence spectrum excited at 258 nm. Additionally, the emission color of CaWO4:xDy3+ phosphors can be suitably tuned from blue to green, and finally to yellow by the modulation of excitation wavelength and Dy3+ ion concentration. Ultimately, these color-tunable phosphors may have potential applications in the fields of miniature color displays.

Luminescence of CaWO4:Pr3+ and CaWO4:Tb3+ at ambient and high hydrostatic pressures

Photoluminescence spectra of CaWO4 doped with Pr3+ and Tb3+ obtained at high hydrostatic pressures up to 315 kbar applied in a diamond anvil cell (DAC) are presented. The intensities of the luminescence from the 3P0 state of Pr3+ and from the 5D3 state of Tb3+ decreased with increasing pressure. At pressures greater than 50 kbar, the 1D2 → 3HJ transitions in Pr3+ and the 5D4 → 7FJ transitions in Tb3+ dominated the spectra. At pressures greater than 100 kbar, only emissions from the lower excited states were observed. At pressures greater than 150 kbar, luminescence from the 1D2 and 5D4 states also decreased with increasing pressure, and at a pressure of 315 kbar for CaWO4:Pr3+ and 190 kbar for CaWO4:Tb3+, the emissions related to the Pr3+ and Tb3+ were quenched. These effects were related to the influence of impurity trapped excitons (ITEs) on the efficiency of the f–f emission in the Pr3+ and Tb3+ ions. Analysis of the emission spectra collected at different pressures allowed the energies of the ground states of the Pr3+ and Tb3+ ions with respect to the band edges of the CaWO4 host to be estimated.

Synthesis and luminescence properties of CaMO4:Dy3+ (M = W, Mo) nanoparticles prepared via an ethylene glycol route

CaWO4 and CaMoO4 doped with Dy3+ nanoparticles are obtained via an ethylene glycol route at temperature as low as 130 °C. The XRD results reveal that all the doped samples are well assigned to the scheelite structure of the CaWO4 and the CaMoO4 phase. Upon excitation by ultraviolet radiation, the CaWO4:Dy3+ and CaMoO4:Dy3+ phosphors demonstrate the characteristic 4F9/2–6H13/2–15/2 emission line of Dy3+. Photoluminescence examination revealed yellow phosphorescence for CaWO4:Dy3+ and yellowish white for CaMoO4:Dy3+ nanoparticles. The highest emission is observed when doped with 5 at% Dy3+ for the CaWO4 host and 7 at% Dy3+ for CaMoO4 samples. Doping the host with Dy3+ tunes the emission wavelength from blue-green to white for CaMoO4 and blue to yellow for CaWO4 nanoparticles. This multicolour tuning emission could increase their applicability in fluorescent lamps and display panels.

Tunable white light-emission of a CaW1−x Mo x O4:Tm3+, Tb3+, Eu3+ phosphor prepared by a Pechini sol–gel method

A white light-emitting CaW1−x Mo x O4:Tm3+, Tb3+, Eu3+ phosphor was prepared by a Pechini sol–gel method. The incorporation of Mo6+ into the CaWO4 host matrix can broaden its excitation range and promote tunability to its emission. When the CaW1−x Mo x O4 system is triply-doped with Tm3+, Tb3+, and Eu3+ ions, energy transfer occurs from both WO4 2− and MoO4 2− groups to Tm3+ and Tb3+ ions. A significant red-shift in the excitation of Eu3+ allows the resulting emission to be tunable between cool, natural, and warm white light by varying the excitation wavelength. The undoped and triply-doped CaW1−x Mo x O4 phosphors were characterized by X-ray diffraction, scanning electron microscopy, photoluminescence excitation and emission spectra, and CIE chromaticity (x, y) coordinates.