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Yttrium aluminum garnet (YAG), Y3Al5O12 is one of the most important optical materials with many applications such as optical windows, laser host materials, detectors and phosphors. Nano YAG could offer many advantageous over bulk materials and large grain size phosphors. In this work Ce doped YAG (Ce:YAG) nanophosphors (NP) were synthesized using simple chemical methods and crystalized by annealing at 800, 900 and 1100°C. Luminescence was recorded in the range of 200 to 800 nm using X-ray induced luminescence technique to detect all emission centers in the sample and evaluate their relative intensities. The effect of annealing temperature and the use of different complexing and polymerization agents on the particle morphology and luminescence were investigated. Trapping phenomena were studied in Ce:YAG NP and bulk ceramics by thermally stimulated luminescence spectroscopy and a comparison was made between them and Ce:YAG single crystals. Measurements concluded that trapping is dominated by crystal defects in single crystals and by trapping sites at the grain boundaries in ceramics. Ce:YAG NPs -on the other hand- are free of traps, which seems to be characteristic of their small grain structure. This study illustrates the effect of chemical agents and annealing temperatures on the structural and optical properties of Ce:YAG nanophosphors and shed light on the nature and characteristics of traps in YAG, which greatly affect its performance in a wide range of applications. Furthermore it reveals that different trapping mechanisms take place among single crystals, bulk ceramics and NPs which could have impact on understanding the optical and scintillation properties of various luminescent materials.
© 2016 Optical Society of America
1. Introduction
Yttrium aluminum garnets (YAG: Y3Al5O12) doped with rare-earth elements represent one of the most important class of photonic materials. They have interesting optical properties that have been successfully exploited for solid state lasers and phosphors [1–7]. There is current interest in developing advanced YAG Nano-phosphors (NP) for solid-state lighting (SSL), display and scintillation applications [8–14]. As a yellow phosphor, Ce-doped YAG (Ce:YAG) has unique properties, including strong absorption at the blue LED wavelength, broad-band yellow emission and high quantum efficiency [9,10]. Ce:YAG NP could offer more advantages [10, 16–19] as the use of nanoparticles in LED lighting with diameters of less than the wavelength of light should significantly reduce photon scattering and improve its overall performance [12]. It is also possible to tune emission spectra from Ce:YAG NP and build a desired color index for the emitted light by band gap or ligand field engineering through codoping and alloying. Wet chemical methods which are being used for the synthesis of Ce:YAG NP would facilitate the incorporation of dopants in the matrix and alloying and codoping processes. Furthermore, europium or terbium doped YAG can be used as phosphors for high-resolution display devices which require ultra-fine grains [19–21]. The interest in advancing nano YAG synthesis is also driven by the interest in fabricating nanoparticles based transparent ceramics such Nd doped YAG transparent ceramics for laser and several other optical applications [E.g 22–25].
Commercial Ce:YAG phosphors are commonly synthesized by solid state reaction between Y2O3, Al2O3 and CeO2 [26]. This method requires prolonged mechanical mixing and high temperatures and results in inhomogeneity and large grain sizes of several tens of microns. It is difficult to obtain high pure phase by this method at relatively low temperatures. When the reactants are heated below 1600°C, a single phase cannot be formed. These high temperatures required to achieve the YAG phase in solid state reaction are not compatible with maintaining individual nanoparticles. They also induce a high level of defects that may influence the optical properties as well as the formation of color centers [28]. As an alternative to solid state reaction, low cost wet chemical methods [29] can be used to synthesize single-phase YAG at low temperatures. These methods include sol-gel, hydrothermal treatment, and spray pyrolysis and combustion synthesis [e.g 30, 31].
This work reports on the synthesis of Ce:YAG NPs by the sol-gel method [20] and studies their structural and luminescence properties and trapping phenomena. The sol-gel method is a simple low-temperature synthesis scheme that has the advantage of better control of the doping level of activators. Luminescence was studied in this work mainly by X-ray induced luminescence (XRIL) spectroscopy [32], which allows one to detect all luminescence centers in the sample in single scan through the use of X-ray as an excitation source. The emission spectra were then compared with photoluminescence measurements. The results presented here reveal how the particles morphology and luminescence can be altered through the use of different solvents and stabilizers in the synthesis process. They also reveal the dependence of luminescence on particle size and annealing temperature.
To study trapping phenomena and exciton dynamics, we applied high- and low- temperature thermally stimulated luminescence spectroscopy [33,34] to identify both deep and shallow traps in Ce:YAG NPs and bulk ceramics and compared them with traps in Ce:YAG single crystals. The measurements revealed that trapping is dominated by lattice defects in single crystals and grain boundary trapping sites in ceramics, while it is absent in nanophosphors due to the small sized grains. It is interesting to observe a large difference in trapping mechanisms and TSL glow curve characteristics among Ce:YAG single crystals, bulk ceramics and NPs. This finding is significant; it is expected to be vaild for various luminescent and scintillation materials and may have far reaching impact on understanding their exciton dynamics and optical properties.
2. Experimental details
2.1 Synthesis
CeYAG NPs doped with 5% Ce were prepared by the sol-gel method using two different chemical routes. In the first route, yttrium nitrate, aluminum nitrate and Ce nitrate were mixed in stoichiometric amounts and dissolved in distilled water. The acetic acid was used as a complexing agent and the solution was heated with stirring at 60°C for 30 minutes. Then, ethylene glycol which acts as a polymerization agent [35] was added and the solution was heated again. To obtain nanopowders, the solution was further heated until it became a gel, then it was dried at about 100°C for one day and calcined at 600°C for a few hours. The powders were then annealed at variable temperatures ranging from 600 to 900°C, each for 12 hours.
In the second route, yttrium nitrate, aluminum nitrate and cerium nitrate were dissolved in distilled water. Urea and poly vinyl alcohol were used as complexing and polymerization agents instead of acetic acid and ethylene glycol. They were added to the solution and heated for 2 hours at 150°C. Then, several heating procedures were carried out, first up to 250°C where the gel formed, then up to 600, 800, 900°C and 1100°C where the sample was annealed for 12 hours at each temperature.
2.2 Structural and luminescence characterization
Samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD pattern was obtained by measuring X-ray intensity versus 2 theta using Siemens D3000 x-ray diffractometer. The average particle size was then determined from the measurements by applying Sherrer equation. SEM measurements were carried out to gain information about the particle size distributions and their agglomeration and morphology. Luminescence was studied using our newly developed X-ray induced luminescence spectrometer [32]. The spectrometer uses an AEG FK 60-04 Cu X–ray tube to generate X–rays that pass through a monochromator and collimator to provide focused mono-energetic X–ray beams. The light emitted from the sample is collected by a lens and transmitted through an optical fiber to an Ocean Optics USB2000 + spectrofluorometer that covers a spectral range of 200–800 nm, with a 1 nm resolution. Photoluminescence measurements were carried out using light emitting diodes (LEDs) coupled to a monochromator to eliminate any tail in LEDs output. The emission from the sample was collected using the same USB2000 + spectrofluorometer used in XRIL.
2.3 Thermoluminesence measurements
TSL measurements were made using a special spectrometer that was designed and constructed in-house and that enables the direct recording of the thermo-luminescence as a function of wavelength and temperature [36–38]. The samples were irradiated in the dark for 30 minutes using the full power of a pulsed Xenon lamp. After that, the lamp was turned off and the light emission over the range of 200 to 800 nm was recorded using a charge-coupled device detector during the linear-ramp heating of the sample. For the identification of shallow traps, the irradiation was carried out at −190°C and the sample was heated up from this temperature to room temperature. For deep trap measurements, the irradiation was carried out at room temperature with ramping up to 400°C. The heating rate for the high- and low-temperature TSL measurements was 60°C/min. The measurements were carried out on Ce:YAG nanophosphors, ceramics and single crystals. They were all irradiated and measured at the same conditions where the light from the Xenon lamp was focused to a small area in the center of the samples. This ensures the measurement of equal areas from single crystals, ceramics and NPs. Ce:YAG single crystals were grown by the Czachralski method under an atmosphere of 40% hydrogen in argon. Ce:YAG ceramics were obtained from Baikowski/ Konoshima Inc.
Special precautions were taken to ensure that all samples were measured under the same conditions in XRIL, PL and TSL experiments and the amount of materials was identical for all Ce:YAG nanophosphor samples.
3. Results and discussion
3.1 Structural characterization
Figure 1 represents an example of the XRD pattern of the samples synthesized as described above and annealed at 900°C. It shows X-ray intensity versus 2-Theta; All the peaks belong to the YAG structure confirming the absence of secondary phases. In fact, XRD patterns for all the prepared samples confirmed a single pure phase of YAG. A detailed XRD study for the effect of annealing temperature on the formation of YAG phase was carried out. Figure 2 represents the XRD patterns of Ce:YAG nanophosphors after each anneal from 600 to 1100°C. The YAG phase begins to form after annealing at 900°C. The average particle size was determined from the XRD line width and was found to strongly depend on the annealing temperature. It was found to be 40 nm after annealing at 900°C and 85 nm after annealing at 1100°C for the sample grown with acetic acid and ethylene glycol. For the sample grown with urea and poly vinyl alcohol, the size was 40 and 62 nm after annealing at 800°C 1100°C respectively. We anticipate that complexing and polymerization agents affect the particle morphology, which can significantly influence the agglomeration of particles during annealing. This is behind the difference in the final particle size of samples grown by different chemical methods. The effect of complexing and polymerization agents on the particle morphology will be discussed below in more details.
Fig. 1 X-ray diffraction patterns of YAG NPs grown with acetic acid and ethylene glycol and annealed at 900°C. No impurity phase was detected in the sample.
Fig. 2 X-ray diffraction patterns of Ce:YAG NPs after each anneal. The dotted lines represent fits for the measurements.
After annealing, Ce:YAG NPs became bright yellow because of the formation of YAG phase and the incorporation of Ce in the matrix. The morphology and shape of the particles were studied by SEM and presented in Fig. 3. The images show the effect of solvents and polymerization agents on the particle morphology and size.
Fig. 3 SEM micrographs showing particle morphologies for the two Ce:YAG NPs: (a) grown using acetic acid and ethylene glycol and annealed at 800°C, (b) and (c) grown using urea and vinyl alcohol and annealed at 900°C with different magnification.
All samples presented in the images were annealed at 800 or 900°C. Samples that were prepared using urea and vinyl alcohol have different size and morphology as can be seen in Figs. 3(b) and 3(c). The role of complexing agent is to act as a precipitating agent and it is well known that both urea and acetic acid are widely used as excellent precipitating reagents [39–41]. Various studies suggested that these precipitating agents work as an adsorbing agent on the surface of crystals [42]. On the other hand, vinyl alcohol and ethylene glycol belong to wetting polymers (non-ionic surfactants) category widely used as surfactants which induce micelle formation in solutions and are driven by van der Waals attraction by hydrophobic groups [39,43]. This determines the size and morphology of the synthesized particles. Though till date a detailed understanding that explains the mechanism has yet to be revealed.
3.2 Luminescence properties
Luminescence properties were measured by our newly developed XRIL spectrometer [32], which has many advantages over standard photoluminescence spectrometers. It provides a useful tool to simultaneously monitor most sample emissions from 200 to 800 nm and acquire information about the relative intensities of luminescence peaks. The mechanism of XRIL can be explained as follows. The absorption of X-rays in the matrix leads to the generation of a number of free and bound electrons and holes, which may recombine or transfer their energy to luminescence centers thereby inducing luminescence. Fig. 4 shows the XRIL spectra for Ce:YAG nanophosphors grown by two chemical routes and annealed to different temperatures. The 550 nm emission represents the well-known emission from the 5d-4f transition of Ce3+ ion in YAG structure. It is only emitted from samples after annealing them up at 800°C or above confirming that the YAG phase does not fully crystallize below this temperature. Figure 4(a) displays the XRIL spectra of Ce:YAG NPs synthesized with acetic acid and ethylene glycol and annealed at 900°C while Fig. 4(b) shows the XRIL luminescence of Ce:YAG synthesized with urea and vinyl alcohol and annealed to 800°C. Figures 4(c) and 4(d) illustrate the effect of chemical routes and annealing temperatures on XRIL emission. It is clear that the green emission significantly increases with the increase in annealing temperature. The red emissions between 600 and 800 nm are associated with impurities in YAG [44–47]. Our previous measurements on annealed YAG single crystals [47] showed that a small percentage of Fe impurities produce similar emission at 800 nm. However increasing the level of Fe impurities quenches the luminescence, which explains the lack of more data on this emission in the literature. The emissions at 650 to 750 nm were observed in Cr and Ce codoped YAG samples [46], this range is consistent with the typical emission spectra of the Cr3+ ion (4T–2A transition) in the YAG matrix [44]. We believe that the observed red luminesce bands in Fig. 4 are combination of Fe and Cr emission in YAG. Their relative intensity compared to Ce emission significantly decreases after annealing the samples at higher temperatures as seen in Fig. 4(d). The red emission from the sample grown with urea and vinyl alcohol seems to be more intense and broad.
Fig. 4 XRIL of Ce:YAG NPs: (a) grown using acetic acid and Ethylene glycol and annealed at 900°C, (b) grown using urea and vinyl alcohol and annealed at 800°C, (c) Effect of chemical routes on XRIL emission. (d) Effect of annealing temperature on XRIL emission from the sample prepared by Urea and vinyl alcohol. No emission was detected from samples annealed below 800°C.
Figure 5 displays the PL emission spectra of Ce:YAG NPs under 455 nm excitation. (a) and (b) show the effect of annealing temperature and chemical routes on the 550 nm emission peak. A large increase in PL intensity can be observed after 1100°C anneal. The dependence of the intensity of the 550 nm peak on annealing temperature and chemical routes is consistent with XRIL spectra, however the red emission was not observed under 455 nm excitation.
Fig. 5 PL emission spectra of Ce:YAG NPs under 455 nm excitation light. (a) grown with urea and vinyl alcohol and annealed at 800 and 1100°C. (b) Effect of chemical routes after annealing at 800 and 900°C. No emission was detected from samples annealed below 800°C.
3.3 TSL of Ce:YAG
TSL can provide information about exciton dynamics and trapping in optical materials [32,33]. Here we compare trapping phenomena in Ce:YAG NPs, bulk ceramics and single crystals. Figure 6 shows the contour plots of high-temperature TSL emission in bulk ceramics and single crystals as a function of temperature and wavelength. The glow curves - which represent the luminescence intensity versus temperature - were constructed from the contour plots by integrating the luminescence intensity over the entire range of wavelengths at every temperature. They are displayed in Fig. 7 for the Ce:YAG bulk ceramics, nanophosphors and single crystals.
Fig. 6 Contour plots of TSL in Ce:YAG single crystals and bulk ceramics from room temperature to 400 °C.
Figures 8 and 9 present the contour plots and glow curves of low-temperature TSL respectively. High- and low- temperature TSL emission from Ce:YAG NPs are extremely weak indicating the absence of both shallow and deep traps. This was the case for all Ce:YAG NP samples grown using both chemical routes. It is probably because the grain boundaries of these small nanoparticles do not form effective traps. Their contour plot and glow curves are presented in Fig. 10. It is important to note that PL measurements showed stronger emission from Ce:YAG NPs than ceramics and single crystals under the same excitation conditions probably because of higher Ce concentrations. This confirms that the absence of TSL emission in Ce:YAG NP is associated with the absence of traps and is not due to less light absorption or absence of luminescence centers. On the contrary to Ce:YAG NP, there is a strong TSL emission from Ce:YAG ceramics (Figs. 7 and 9) indicating high density of deep and shallow traps. It can be seen that the characteristics of high-temperature TSL curve for ceramics are significantly different from single crystals. Distinct individual peaks in the spectra of single crystals represent three deep traps, mostly associated with oxygen and Al vacancies, which we have previously detected in YAG crystals [48, 49]. On the other hand, high-temperature TSL emission from ceramic samples features a broad peak over a wide range of temperatures, which can be interpreted due to the presence of a series of deep traps. This is an indication that the grain boundaries in ceramics generate a large array of traps with different size and energy level in the band gap. The size of these grains is in the order of micrometers, much larger than the grain size of the Ce:YAG nanophosphors synthesized in this work. However, there is some analog between the features of low-temperature TSL of ceramics and single crystals. Their contour plots in Fig. 8 show four individual peaks. It can be seen from their glow curves in Fig. 9 that the position of all peaks was shifted to higher temperature for ceramic samples indicating deeper energy levels. The peak at −40 °C in the glow curve of the single crystal was shifted to −20 °C and its intensity was significantly reduced in the glow curve of the ceramic sample. We have earlier reported on the association of this peak with hydrogen impurities in Ce:YAG [34]. It is very strong in the glow curve of single crystal because the growth process took place in reducing atmosphere of hydrogen and argon, which is necessary to incorporate Ce in + 3 charge state in single crystals.
Fig. 10 (a): Contour plot and (b) glow curve of high- and low-temperature TSL in Ce:YAG nanophosphors. The measurements exhibit high level of noise because of the very weak emission.
4. Summary and conclusion
Nanophosphors of Ce:YAG were prepared by the sol-gel method at low temperatures using different chemical agents and annealed at variable temperatures from 600 to 1100°C. The YAG phase was only formed by annealing the nanocrystals at 800 or 900°C. As expected, annealing at high temperature lead to the agglomeration of the particles and the formation of larger sized grains. The measurements showed that the type of complexing and polymerization agents have influenced the particles size and morphology and their luminescence. The strong green emission from Ce3+ transitions in Ce:YAG was monitored by XRIL and PL spectroscopies. XRIL Spectra also revealed red emission associated with impurities in YAG. Trapping was studied in Ce:YAG nanophosphors, ceramics and single crystals. No shallow or deep traps were detected in the synthesized Ce:YAG nanophosphors which was attributed to their small grain size. Trapping in ceramics is significantly different from single crystals; it exhibits a broad intense emission associated with the presence of an array of deep and shallow traps. Some shallow traps that are associated with point defects are also present in ceramics; this was demonstrated through the comparison with TSL measurements in Ce:YAG single crystals.
This study highlights the significant role of complexing and polymerization agents in the sol-gel synthesis of nanophosphors and reveals different trapping mechanisms between NPs, ceramics and single crystals of luminescent materials which would be reflected in their exciton dynamics and luminescent and scintillation properties. The work also shows the benefits of employing two uncommon techniques- XRIL and TSL- in the study of phosphors. XRIL reveals all luminescence centers in the sample while TSL allows the identification of shallow and deep traps and provides insight to exciton dynamics in the matrix.
Acknowledgement
We acknowledge receiving funds from the National Science Foundation under DMR 1359523 grant (Charles Ying).
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