Ce-doped YAG Research Articles

(Invited) A Statistical Model of Thermal Ionization of Excited Ce Ions in YAG

In this talk we present a statistical model for quantifying the thermal ionization process of an excited impurity ion in a level just below the conduction band of the host while the impurity ion is under constant excitation. YAG:Ce is used as an example. We assume that under constant pumping of the Ce3+ ion into the 5d1 with a blue LED, a condition of thermal and diffusive equilibrium will exist between the conduction electrons and the electrons in the excited 5d1 level of Ce3+ ion. Using the Gibbs sum, we calculate the probability for an excited Ce3+ ion to be thermally ionized, leading to an electron in the conduction band. The density of conduction electrons is then calculated over a wide range of temperatures and excitation densities. The observed quenching of emission at high temperature in Ce-doped YAG with low Ce concentrations is also investigated within this model. It is shown that the experimentally-observed nonradiative losses in this phosphor can be reasonably simulated assuming losses occur via nonradiative decay from the conduction band to the Ce4+ ground state.

High Performance Ce-Doped YAG Nanophosphors Prepared by Precipitation Method

High Performance Ce-Doped YAG Nanophosphors Prepared by Precipitation Method

Hard X-ray nanoprobe to study the emission properties of Ce-doped YAG wafer by using XEOL and TR-XEOL

In this study, the valent state, emission properties, and decay lifetime of Ce-doped yttrium aluminum garnet (Y3Al5O12, YAG) wafer have been probed using X-ray absorption (XAS), X-ray excited optical luminescence (XEOL), and time-resolved XEOL (TR-XEOL). The XAS spectrum demonstrates that Ce ions are in the trivalent state, and the XEOL spectrum exhibits typical 5 d1-4f1(2F5/2) and 5 d1-4f1(2F7/2) transitions of Ce3+ ions, with emission peaks at ∼528 nm and 582 nm, respectively. From the TR-XEOL measurements, the decay lifetimes of 5 d1-4f1(2F5/2) and 5 d1-4f1(2F7/2) transitions are about ∼90 ns and ∼100 ns, respectively. We found that the reabsorption effects will not only make the 5 d1-4f1(2F7/2) transitions have longer decay lifetime, but also the higher emission intensity. By comparison with photoluminescence (PL) and time-resolved PL using a pulsed laser at 375 nm, we found that the emission spectra and decay lifetime of Ce-doped YAG (111) wafer are different when using X-ray irradiation. The longer decay lifetime measured by TR-XEOL compared TR-PL are caused by high X-ray dose irradiation. In addition, high X-ray doses provided by the nano-focus X-ray beam will be influenced the emission behaviors of Ce-doped YAG (111) wafer. Which phenomena have been demonstrated by using time-dependent XEOL with irradiation for 10 h. These peculiar emission behaviors observed with a hard X-ray nanoprobe may be able to provide important information on quantum technologies and scintillators used in hard X-ray detectors.

Influence of secondary phases on the emission behavior of YAG-based luminescent materials prepared using single and double precipitants

The emission behavior of single and double precipitants-derived Ce-doped YAG involving secondary phases is explored. Single precipitant-derived samples indicate major YAG with secondary YAP, YAM, and Al2O3, whereas double precipitant-derived samples demonstrate YAG with secondary YBO3 and Al2O3 at 1000–1400 °C. The single precipitant-derived samples did not show tunable emissions with calcination due to the contribution of yellow-green. However, double-precipitants-derived samples revealed blue and yellow-green contributions due to the involvement of YBO3 and YAG, respectively. The contribution of blue and yellow-green emissions of YAG-based materials derived from double precipitants developed tunable hues that shift from near-white to light blue on calcination.

A dose rate independent 2D Ce-doped YAG scintillating dosimetry system for time resolved beam monitoring in ultra-high dose rate electron “FLASH” radiation therapy

Preclinical studies have shown that radiotherapy treatments benefit from ultra-high dose rate (UHDR) irradiations. These trigger the FLASH-effect, resulting in a strong decrease in normal tissue toxicity with conserved tumor control probability, compared with conventional irradiations. However, the beam parameters to trigger the FLASH-effect and the radiobiological mechanisms behind it remain to be elucidated. A limiting factor in many studies is the lack of accurate real time dosimetry. In this work an innovative solution for 2D real time dosimetry in UHDR electron beams is presented and characterized.The in-house developed ImageDosis system consists of a scientific camera, with high temporal resolution, and a coating, containing 12% of Y3Al5O12:Ce3+ as scintillating material. Reference dosimetry was performed by means of radiochromic film, and a (C38H34P2)MnBr4 point scintillator was used to validate the pulse discrimination properties of the ImageDosis system. Irradiations were performed in two centers (Antwerp and Orsay), with an ElectronFlash accelerator. The ImageDosis system was tested with varying number of pulses, pulse length, pulse repetition frequency (PRF) and energy. In addition, its temporal resolution and 2D properties were investigated.The ImageDosis system showed negligible dose, dose rate and energy dependence for doses up to 13 Gy and average dose rates up to 140 Gy/s, with a dose per pulse up to 2 Gy and a PRF up to 300 Hz. The system is capable of discriminating and measuring the dose of individual pulses and shows promising 2D characteristics that need further optimization.

A statistical model of thermal ionization of excited Ce ions in YAG

A statistical model based on the condition of thermal and diffusive equilibrium between the conduction electrons and the electrons in the excited 5d1 level of Ce3+ is presented. Using this model, we find the probability for an excited Ce3+ ion to be thermally ionized leading to an electron in the conduction band. The density of conduction electrons is then calculated over a wide range of temperatures and excitation densities. The observed quenching of emission at high temperature in Ce-doped YAG with low Ce concentrations is also investigated within this model. It is shown that the experimentally-observed nonradiative losses in this phosphor can be reasonably simulated assuming losses occur via nonradiative decay from the conduction band to the Ce3+ ground state.

Ce-doped YAG single-crystals prepared by continuous wave (CW)–CO2 laser combustion technique with attractive characteristics and moderate white LED performance

In this work, the influence of continuous wave –CO2 laser-induced solution combustion as a new and effective technique to prepare YAG:Ce phosphor was investigated. The as-prepared ceramic displayed a direct pure YAG phase within the laser exposure time of 1.5 min with an attractive characteristics and moderate white light-emitting diode (WLED) performance, suggesting that laser synthesis has a role to play in accelerating the diffusion of the cations and an increase in reaction rate. However, the thermal treatment step plays a key role to adjust efficient luminescence processes. In addition, the luminescence properties were strongly dependent on the sintering strategies due to the expulsion of defects (e.g. CeO2 and other impurity phases) from matrix. We showed experimentally that increasing the luminescence intensity has an effect observed on the CIE chromaticity parameters and the theoretically calculated luminous efficiency of radiation (LER). This work presents the effects of thermal treatment properties on the YAG host lattice and explains the mechanism of YAG formation under CO2 laser beam.

Blue laser diode-pumped Ce:YAG phosphor-coated cylindrical rod-based extended white light source with uniform illumination

We report the development of an extended white light source with uniform illumination using a cylindrical rod coated with Ce-doped YAG phosphor and excited by a blue diode laser. A long cylindrical rod made of transparent acrylic was first ground in order to diffuse laser light uniformly throughout the rod. Ce:YAG phosphor was then coated on the cylindrical rod and a blue color laser beam of 1 W power pumped into it. The laser beam propagates through the entire length of the rod via total internal reflection. This is possible because the laser light is highly directional and coherent with high brightness. To achieve a uniform scattering distribution of blue laser light throughout the length of the cylindrical rod, variations in grinding as well as making a hollow core inside the tube are reported. While propagating through the rod, the blue photons are also scattered out due to the rough scattering surface of the acrylic rod. Some of the blue photons are absorbed by yellow phosphor thus generating yellow color photons and the remaining blue photons are mixed with the yellow light generating white light. Such an extended light source acts as a conventional tubelight of 30 cm length. Due to this extended light source, the illumination is uniform and produces no glare as compared to high brightness laser-based point sources as well as LED-based white light sources. Since the phosphor is coated on the outside of the acrylic rod and laser power is distributed uniformly, the burning of phosphor is completely avoided. The light source was characterized by measuring Commission International de l’Eclairage coordinates and correlated color temperature. This kind of light source is highly useful for general purpose illumination.

Ce-doped YAG phosphor powder synthesized via microwave combustion and its application for white LED

Cerium-doped yttrium aluminum garnet (YAG : Ce3 + ) powder phosphor have been extensively studied as phosphors for blue to white light conversion. Y(3 − x)Al5O12 : x Ce3 + (x = 2.0 at. %) were successfully synthesized by microwave-assisted combustion (MW) using the organic fuel urea. A direct conversion from the amorphous phase to the cubic one was obtained at 1050°C for 5 h, together with an increase in the particle size into the range of 50 to 60 nm and decrease in the specific surface area. The as-prepared precursors and powder sintered at 1050°C were characterized for their structure, particle size, morphology, electroluminescence properties, and chromaticity by x-ray diffraction, Brunauer–Emmett–Teller method, Fourier transform infrared spectroscopy, field emission-scanning electron microscopy, transmission electron microscopy, electroluminescence, and standard CIE 1931 chromaticity analysis (CIE) chromaticity diagram, respectively. The results show that the obtained sintered YAG : Ce3 + phosphor powder has spherical-shaped particles and strong yellow emission compared to as-prepared phosphor powder. White light emitting diodes with proper color rendering index, and tunable correlated color temperature properties can be produced by controlling the injection currents and coating thickness of the sample, offering daylight white and neutral white LEDs.

Fabrication and performance assessment of coprecipitation-based YAG:Ce nanopowders for white LEDs

The quality of YAG:Ce phosphors will influence the luminescent performance of white LEDs devices. Therefore, in this study, we attempted to synthesis high quality YAG:Ce phosphors using coprecipitation method. YAG:Ce precursors were synthesized through coprecipitation at different rotational speeds of an electromagnetic stirrer and then heated at different calcination temperatures ranging from 900 to 1300 °C. X-ray diffraction analysis revealed that phase-pure phosphors could be obtained from the precursors synthesized at 400 rpm and then calcined at 1300 °C. The emission intensity of Ce-doped YAG phosphor increased with the calcination temperature, owing to the higher crystallinity of the YAG phase. Moreover, at the same calcination temperature from 900 to 1300 °C, the maximum fluorescence intensity at approximately 540 nm of the YAG:Ce nanopowders synthesized at 400 rpm increased three to five times that of the YAG:Ce nanopowders synthesized at 200 rpm. The optimal luminescence performance of the YAG:Ce nanopowders was obtained at 400 rpm and at a calcination temperature of 1300 °C. The results of luminescence performance indicated YAG:Ce nanopowders has a great potential for development of white LEDs devices.

Synthesis of high quality Ce:YAG nanopowders by graphene oxide nanosheet-assisted co-precipitation method

(Ce0.04Y2.96)Al5O12 phosphor nanoparticles were prepared by a modified co-precipitation method with graphene oxide (GO) nanosheets used as dispersing agent. The GO concentration is controlled at 0, 0.005, 0.01, 0.02, and 0.03 g/L. The addition of lamellar GO nanosheets in the precipitant solution possibly enhances both the dispersity of precursor particles and the crystallinity of phosphor nanoparticles. Pure Ce-doped YAG phase is obtained by calcining the precipitate at 1000 °C for 3 h. The (Ce0.04Y2.96)Al5O12 phosphor nanoparticles have an average size of 64 nm and there is no significant change on particle size with increase of the GO concentration in precipitant solution. The luminescence property of (Ce0.04Y2.96)Al5O12 phosphor nanoparticles varies with different concentrations of GO. The photoluminescence emission intensity of the optimum sample with 0.02 g/L GO is about 1.6 times higher than the sample without using GO.

Improvement of photoluminescence intensity of Ce-doped Y3Al5O12 phosphor by Si3N4 addition

Yttrium aluminum garnet (Y3Al5O12, YAG) has been widely used as a host for luminescent ions. The present paper describes the effects of Si3N4 addition on the formation and photoluminescence properties of the Ce-doped YAG yellow phosphors. Phosphor powders with the nominal compositions of Y2.95Ce0.05Al5-mSimO12-mNm (m = 0–0.6) were prepared by calcining the mixed raw materials at 1500 °C in nitrogen atmosphere. X-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscope, and transmission electron microscopy equipped with an energy dispersive x-ray spectrometer were used to characterize the structure of the calcined powders. The photoluminescence properties were measured with fluorescence spectrophotometry. It was found that in the range of m = 0–0.27, single phase YAG solid solution (s.s.) in which the Y, Al, and O sites are partially occupied by Ce, Si, and N ions, respectively. The nitrogen ions do not distribute homogeneously over the YAG lattice. The tendency to bond with nitrogen ion for the cations is (Y, Ce) > Si > Al. With the increase in the Si3N4 content, the increase in both the Ce3+/(Ce3+ + Ce4+) ratio and the CeN bonds improve the intensity of the photoluminescent emission. At m = 0.27, the emission intensity reaches a maximum which is about 2.5 and 1.6 times of that for the Si3N4-free composition (m = 0) calcined in air and nitrogen, respectively. When the Si3N4 content (m) is higher than 0.27, the emission intensity decreases due to the existence of residual Si3N4 phase.

Improvement of dopant distribution in radial direction of single crystals grown by micro-pulling-down method

Two iridium crucibles with one and five capillaries at a nozzle were developed for the growth of Ce-doped Y3Al5O12 [Ce:YAG] single crystals by the micro-pulling-down [μ-PD] method. The purpose of the study was to examine the effect of the capillary number on the Ce distribution in the radial direction of the crystals. The crystals grown from 2mol% and 5mol% Ce-doped YAG melts were then cut perpendicular to the growth direction and polished to produce specimens suitable for the measurement of Ce distribution. The Ce:YAG crystals grown using the crucible with one capillary [Ce:YAG(1C)] had greater Ce content in the central portion of the crystal when compared to its peripheral parts. On the other hand, in the case of the Ce:YAG crystal grown using the crucible with five capillaries [Ce:YAG(5C)], the increased Ce concentration was detected in five specimen sections positioned just below the capillaries. The results indicated that increase of the number of capillaries is effective practice that results in an improvement of the radial homogeneity of the single crystals grown by the μ-PD method.

Growth and characterization of Ce-doped YAG and LuAG fibers

Undoped and Ce-doped Lu3Al5O12 (LuAG) and Y3Al5O12 (YAG) single crystal fibers were grown by the micro-pulling down technique (μ-PD) with a purpose to fit the design of new dual-readout calorimeter planned to operate in future experiences of high energy physics. Fibers up to 20 cm in length and 1 mm in diameter were grown along [111] direction. Based on the measurements of the attenuation length along the fibers, the growth conditions to improve the fibers quality were selected. Our results showed that the grown fibers have the capability to be used for future detectors.

Low-temperature solution synthesis and characterization of Ce-doped YAG nanoparticles

Monophasic Ce3+-doped yttrium aluminum garnet (Ce:YAG) nanoparticles with high crystallinity and tunable grain size ranging from ∼19–30 nm were prepared by a modified co-precipitation process with a follow-up calcination treatment. For the synthesis, aluminum, yttrium, and cerium nitrates were used as starting materials, ammonium sulfate as dispersant, and a combination of ammonium bicarbonate and ammonia as precipitating agent. Influence of precipitation temperature, the pH value of precipitant solutions, aging period, calcination conditions, and Ce-doping level were investigated for controlling the purity, particle size, and photoluminescence performance of the Ce:YAG nanoparticles. High-purity YAG nanoparticles were prepared at pH=10.50–11.00 and calcination temperatures of 850–1100 °C with a calcination time of 3 h. With increasing Ce3+ concentration, the peak in the emission spectra of the obtained nanopowders shifted from 529 nm for the 0.67 wt.%-Ce:YAG to 544 nm for the 3.4 wt.%-Ce:YAG, while the strongest photoluminescence intensity was observed for the 1.3 wt.%-Ce:YAG nanoparticles.

Novel synthesis method of YAG: Ce3+ micron-sized powders and its luminescence properties

Micron-sized Ce-doped yttrium aluminum garnet (YAG: Ce3+) powders are synthesized successfully by a new method including three processes including solvothermal treatment, precursor preheated and annealing treatment in a mild condition. The phase, morphology and luminescent properties are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence excitation (PLE) and photoluminescence (PL) spectra respectively. The single-phase sample can be formed after solvothermal treatment at 180 °C for 12 h and annealing at 1200 °C for 4 h. The results obtained by SEM show that the particles with narrow size distribution (∼4 μm) and nice morphology are formed after annealing treatment. This indicates that it has good homogeneity and dispersion. The micron-sized Ce-doped YAG shows broad emission bands in the range of 500–680 nm with the maximum intensity at 577 nm.

Scintillation properties of composite ceramic YAG and its capability on pulse shape descrimination

Transparent ceramic composite of non-doped and Ce-doped YAG was prepared by Konoshima Chemical with the vacuum sintering technique and investigated on pulse shape discrimination capability for functional collimator. Scintillation responses such as X-ray induced radioluminescence and decay time profiles of non-doped and Ce-doped YAG were evaluated. Pulse shape discrimination capability of the composite ceramic was evaluated by 137Cs 662 keV γ-ray irradiation. As a result, clear separation of γ-ray events at non-doped or Ce-doped YAG layers was observed in two dimensional histogram by using signal processing with two different shaping times.

Lumino-magnetic YAG:Ce nanophosphors: novel synthesis routes for efficient luminescence and magnetic properties

Multifunctional lumino-magnetic YAG:Ce nanophosphors for LEDs and spintronics devices.

Ce-Doped YAG Nanophosphor and Red Emitting CuInS2/ZnS Core/Shell Quantum Dots for Warm White Light-Emitting Diode with High Color Rendering Index

In this work, we report the solvothermal synthesis of Ce-doped YAG (YAG:Ce) nanoparticles (NPs) and their association with a free-Cd CuInS2/ZnS (CIS/ZnS) core/shell QDs for application into white light emitting diode (WLED). 1500 °C-annealed YAG:Ce NPs and CIS/ZnS core/shell QDs exhibited intense yellow and red emissions band with maxima at 545 and 667 nm, respectively. Both YAG:Ce nanophosphor and CIS/ZnS QDs showed high photoluminescence quantum yield (PL QY) of about 50% upon 460 nm excitation. YAG:Ce nanophosphor layer and bilayered YAG:Ce nanophosphor-CIS/ZnS QDs were applied on blue InGaN chip as converter wavelength to achieve WLED. While YAG:Ce nanophosphor converter layer showed low color rendering index (CRI) and cold white light, bilayered YAG:Ce nanophosphor-CIS/ZnS QDs displayed higher CRI of about 84 and warm white light with a correlated color temperature (CCT) of 2784 K. WLED characteristics were measured as a function of forward current from 20 to 1200 mA. The white light stability of bilayered nanophosphor-QDs-based WLED operated at 200 mA was also studied as a function of operating time up to 40 h. Interestingly, CRI and CCT of such device tend to remain constant after 7 h of operating time suggesting that layer-by-layer structure of YAG:Ce phosphor and red-emitting CIS/ZnS QDs could be a good solution to achieve stable warm WLED, especially when high current density is applied.

A nanosecond gate-mode-driven silicon avalanche photodiode and its application to measuring fluorescence lifetimes of Ce-doped YAG ceramics

We propose a silicon avalanche photodiode (Si-APD)-based boxcar-integrator, in which the Si-APD is driven in the gate mode. In the gate mode, by setting the direct current reverse-bias voltage Vr applied to the APD to below its breakdown voltage Vb, we superimpose a gate pulse of amplitude Vg on Vr so that the total voltage is nearly equal to Vb, but does not exceed it, i.e. Vr + Vg < Vb. In this case, the instantaneous current multiplication factor Mi of the APD is noticeably enhanced, especially when the duration of the gate pulse tw is reduced to the nanosecond scale. Because the gated Si-APD plays the role of a sampling unit as well as that of a photodetector for the repeatable signal light incident on the APD, in principle, the proposed scheme has an advantage in terms of the measured signal-to-noise ratio (SNR). To demonstrate the usefulness of the scheme, we have measured the fluorescence lifetimes of cerium-doped yttrium–aluminum–garnet (Ce:YAG) ceramics where the concentration of Ce is varied in steps.