Rapid Microwave Preparation of Highly Efficient Ce3+-Substituted

Article pubs.acs.org/cm

Rapid Microwave Preparation of Highly Efficient Ce3+-Substituted Garnet Phosphors for Solid State White Lighting Alexander Birkel,†,‡ Kristin A. Denault,§,‡ Nathan C. George,∥,‡ Courtney E. Doll,‡,▽ Bathylle Héry,‡,○ Alexander A. Mikhailovsky,⊥ Christina S. Birkel,⊥,‡ Byung-Chul Hong,# and Ram Seshadri*,§,⊥,‡ †

Mitsubishi Chemical Center for Advanced Materials, ‡Materials Research Laboratory, §Materials Department, ∥Department of Chemical Engineering, and ⊥Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States # Mitsubishi Chemical Group Science and Technology Research Center, 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan S Supporting Information *

ABSTRACT: Ce3+-substituted aluminum garnet compounds of yttrium (Y3Al5O12) and lutetium (Lu3Al5O12)both important compounds in the generation of (In,Ga)N-based solid state white lightinghave been prepared using a simple microwave heating technique involving the use of a microwave susceptor to provide the initial heat source. Carbon used as the susceptor additionally creates a reducing atmosphere around the sample that helps stabilize the desired luminescent compound. High quality, phase-pure materials are prepared within a fraction of the time and using a fraction of the energy required in a conventional ceramic preparation; the microwave technique allows for a reduction of about 95% in preparation time, making it possible to obtain phase pure, Ce3+-substituted garnet compounds in under 20 min of reaction time. It is estimated that the overall reduction in energy compared with ceramic routes as practised in the lab is close to 99%. Conventionally prepared material is compared with material prepared using microwave heating in terms of structure, morphology, and optical properties, including quantum yield and thermal quenching of luminescence. Finally, the microwaveprepared compounds have been incorporated into light-emitting diode “caps” to test their performance characteristics in a real device, in terms of their photon efficiency and color coordinates. KEYWORDS: inorganic materials, microwave preparation, rare-earth phosphors

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INTRODUCTION In order to replace low-efficiency incandescent lighting and mercury-containing fluorescent lighting, great effort has been expended on phosphor-converted light-emitting diodes (LEDs) for white, general-purpose lighting1 with the additional promise of robust, longer lasting light sources.2−6 Several different classes of luminescent materials for phosphor conversion have been examined, including oxides,7−9 oxynitrides,10,11 oxyflourides,12 and nitrides.13−16 However, the most prominent and perhaps most widely used material remains ceriumsubstituted yttrium aluminum garnet, Y3Al5O12:Ce3+ (abbreviated YAG:Ce), which has been known since 196717,18 and has become the canonical phosphor material in solid state white lighting since the mid-1990s.19,20 YAG:Ce combines good chemical stability and high efficiency, although the thermal quenching of luminescence remains an issue.21 Its somewhat high refractive index ensures strong scattering by the phosphor particles, and this is advantageous in terms of mixing the exciting blue light from the semiconductor device with the emitted yellow from the phosphor particles. The Lu analogue Lu3Al5O12:Ce3+ has been investigated to a lesser extent.22−24 We find no prior, comprehensive study comparing the Ce3+doped Y and Lu garnets in the literature, especially in the light © 2012 American Chemical Society

of various preparative pathways, so we also provide here, a careful examination of the similarities and differences between these two hosts. Despite the usefulness of garnet phosphors, there are few suggestions of alternate methods of preparation, especially for the Lu garnet, other than conventional high-temperature heating, which is carried out typically at temperatures above 1500 °C, in atmospheres whose low O2 partial pressures ensure that Ce retains its trivalent state. A possible way to significantly reduce energy consumption (and thus the cost associated with production) is through the use of microwave ovens. Microwave-assisted solid state preparations that rely on dielectric heating25 have attracted the attention of materials scientists since the 1980s.26,27 Potentially offering unmatched reaction speeds and the opportunity to use inexpensive domestic microwave ovens, it has been demonstrated that microwave preparations of a variety of materials are possible. This includes metal compounds,28,29 ceramics,30−33 and thermoelectric compounds34 as well as Received: January 3, 2012 Revised: February 4, 2012 Published: February 8, 2012 1198

dx.doi.org/10.1021/cm3000238 | Chem. Mater. 2012, 24, 1198−1204

Chemistry of Materials

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Oxford Inca X-ray system attached for chemical analysis. SEM samples were mounted on aluminum stubs using double-sided conductive carbon tape. The images have been recorded with an acceleration voltage of 5 kV. The particle size distribution was measured on at least 100 particles using the ImageJ software.40 Photoluminescence (PL) spectra were obtained on a Perkin-Elmer LS55 spectrophotometer, scanning a wavelength range from 325 to 825 nm. The samples were thoroughly mixed and finely ground with KBr (≥99%, FT-IR grade, Sigma-Aldrich) and subsequently pressed into a pellet (diameter = 13 mm, final KBr/sample ratio approximately 10:1 by mass). Photoluminescence quantum yield (PLQY) was measured with 457 nm excitation using an argon laser and an experimental protocol as described by Greenham et al.41 Further details of PLQY measurements as well as the procedure to determine the temperature-dependence of the photoluminescence properties are to be found in the Supporting Information. The performance of the garnet compounds in an actual device geometry with a blue light-emitting diode was studied using a “capping” technique that has been described previously by some of us.42,43 Briefly, the powdered phosphors (5 wt % to 8 wt %) were mixed with a silicone resin (GE Silicones, RTV-615) and then molded as caps to fit on top of a clear silicone-capped (In,Ga)N LED (diameter 0.8 cm). The emission spectra were recorded using an integrating sphere with a forward bias current of 20 mA.

several phosphor materials, for example, oxidic compounds doped with Eu3+ and Eu2+.35−37 Since most ceramic materials interact only very weakly with microwave radiation at room temperature27 (i.e., their dielectric loss tangents are very small), the microwave heating of the ceramic can be initiated with use of susceptor materials such as SiC,30 Fe3O4, or carbon,38 which efficiently convert microwave radiation into heat, even at room temperature. The susceptor couples to the microwave and heats the reactants, either by being placed in direct contact with the reactants or by being proximal to them. Once the sample itself has reached a certain temperature, the dielectric loss tangent increases sufficiently such that the microwave heating is generated within the sample as well. Direct heating of the reactants by the microwaves is believed to lead to a more homogeneous heat distribution and potentially rapid and defect-free grain growth of the product due to the very efficient transfer of thermal energy. Economical aspects play a role as well; hybrid microwave solid-state methods have reduced the reaction time needed to prepare high-temperature ceramics by more than 90%.35,36

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EXPERIMENTAL METHODS

RESULTS AND DISCUSSION As a first step, the phase purity of all products has been investigated. Figure 1 displays synchrotron X-ray diffraction

Preparation. Ce-substituted Y3Al5O12 (YAG:Ce) and Lu3Al5O12 (LuAG:Ce) were prepared by thoroughly mixing and grinding stoichiometric amounts of the starting materials Y2O3 (99.99%, Sigma-Aldrich), Lu2O3 (99.9%, Materion), Al2O3 (99.99%, SigmaAldrich), and CeO2 (99.99%, Sigma-Aldrich). The cerium substitution amount was fixed at 2 atom % for all samples, meaning that the intended composition was A2.94Ce0.06Al5O12, A = Y or Lu. BaF2 (99%, Materion) and NH4F (≥99.99%, Sigma-Aldrich) were added in small quantities (5 wt % and 0.5 wt %, respectively) as a flux. For the control samples prepared by conventional ceramic methods, the starting powders were thoroughly ground and pelletized and were fired in dense alumina crucibles in a reducing atmosphere (5%H2/N2) at 1500 °C for 5 h, with a heating ramp of 12 h duration, starting from room temperature. Finally, the sample was cooled to RT over 12 h. The microwave heating procedure followed the prior work of Ramesh et al.30 In a typical synthesis, 3.6 g of granular carbon (Darco 12 mesh to 20 mesh from Sigma-Aldrich) and 300 mg to 400 mg of powdered carbon (activated charcoal, Mallinckrodt) were mixed together and used as the microwave susceptor. The carbon mixture was placed in an alumina crucible (20 mL). A second, smaller alumina crucible (5 mL) was then used to hold the sample powder. This inner reaction crucible was pushed into the carbon mixture in order to surround the sample with a sufficient amount of susceptor, and then both crucibles were covered with an alumina disk. The mixing of the powdered and the granular carbon facilitated the insertion of the reaction crucible into the susceptor. The two crucibles were then placed into a cavity cut into a block of high temperature alumina insulation foam. Finally, the materials were heated in a domestic microwave oven (Panasonic NN-SN667B, 1300 W or LG, 850 W, both operating at 2.45 GHz) for 18 min (Panasonic) or 25 min (LG) at a power level between 750 and 850 W. The duration of heating, the amounts of the susceptor and reactant materials, and the power levels were optimized following a number of trials and were generally found to be highly reproducible (also when using other similar domestic microwave ovens). A typical batch size for a microwave preparation was between 0.5 and 1 g, dictated by the volume of the inner crucible. In order to minimize the amount of ambient oxygen, the Panasonic microwave was also placed into a Nitrogen filled glovebag (Sigma Aldrich, AtmosBag L). Characterization. High resolution synchrotron powder diffraction data were collected at beamline 11-BM at the Advanced Photon Source (APS), Argonne National Laboratory, using an average wavelength of 0.413447 Å. Rietveld analyses were carried out using the XND Rietveld code.39 Field-emission scanning electron microscopy was performed on a FEI XL40 Sirion FEG microscope with an

Figure 1. Synchrotron X-ray powder diffraction data (λ = 0.413447 Å) including the Rietveld fits for the garnet samples: (a) YAG:Ce, conventional ceramic, (b) YAG:Ce MW, (c) LuAG:Ce, conventional, ceramic, and (d) LuAG:Ce MW. The open circles represent data, and solid lines the calculated fit. Difference profiles are displayed as well.

patterns of the different garnets, including the calculated Rietveld profiles. It is evident that the powders prepared using ceramic synthesis, as well as the products of microwave heating, show the same degree of phase purity. All refinements converge to very good fits, yielding RBragg values below 3% for all four data sets. The cell parameters (see Table 1) that have been obtained from the Rietveld refinement agree well with the expected increase due to the incorporation of the larger Ce3+ ion [reff = 1.143 Å (CN = 8)] into the lattice in place of the smaller Y3+ [reff = 1.019 Å (CN = 8)] and Lu3+ [reff = 0.977 Å (CN = 8)].44 1199



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Table 1. Cubic Cell Parameters and Full-Width at HalfMaximum of the Most Intense Peaks for the Different Garnets Obtained from Rietveld Refinement of Synchrotron X-ray Scattering Data and Particle Size Estimates Obtained from Scanning Electron Microscopya sample YAG:Ce YAG:Ce (MW) LuAG:Ce LuAG:Ce (MW) a

a (Å)

fwhm (Å−1)

mean size (μm)

median size (μm)

12.01241(1) 12.01230(1)

1.5 × 10−3 1.7 × 10−3

2.9 ± 2.6 1.9 ± 1.5

2.2 1.5

11.91713(1) 11.91833(1)

2.4 × 10−3 2.5 × 10−3

1.4 ± 1.0 1.1 ± 0.6

1.1 0.9

Space group Ia3d̅ (No. 230).

The Scherrer formula for crystalline correlation lengths is not reliably applied when the lengths are of the order of micrometers, so instead, we simply report the full width at half-maximum (fwhm) values for the most intense diffraction peak in Q-space of the four samples; see Table 1. The fwhm values for the YAG:Ce samples are smaller than those for the LuAG:Ce samples, and the fwhm values for the conventionally prepared samples are smaller than those for the microwave prepared ones. This indicates greater crystallinity in the YAG:Ce samples (if compared to the LuAG:Ce samples) and of the conventionally prepared samples, if compared to ones prepared using microwaves. Scanning electron microscopy was employed to compare the particle morphology of the garnet samples prepared conventionally and using microwaves. Representative micrographs are displayed in Figure S1 in the Supporting Information. The microwave reaction leads to small (less than 2 μm in diameter), nearly spherically shaped particles with well-defined surfaces, in addition to larger ill-defined agglomerates. In contrast, the samples prepared using conventional ceramic heating reveal illdefined powders, with somewhat larger particles, and the particle surfaces not being as well-defined. Morphologies exhibited by samples prepared via both methods compare well with previous reports, for example, from Pan et al.45 However, two results are noteworthy (see also Figure S2 in the Supporting Information), that (a) the particles prepared via the microwave route are on average smaller than those prepared via more traditional ceramic routes. This is most likely due to the drastically reduced reaction time which limits grain and crystallite growth45 and (b) that the LuAG:Ce particles are on average smaller than YAG:Ce particles. A possible explanation for this is the higher melting point of Lu2O3 and Lu3Al5O12, if compared to the yttrium analogues. The trends in size observed in the SEM images are in agreement with the trend found in the diffraction patterns. The elemental composition of the particles was determined using energy-dispersive X-ray spectroscopy, and the data are presented in Figure 2. Only yttrium (or lutetium, respectively), aluminum, and oxygen as well as cerium are present, proving that there is no significant amount of flux or carbon (originating from the hybrid reaction system) present within the sample. Due to the rather small amount of cerium that was substituted into the host lattices, the signal originating from the Ce3+ ions is weak and makes quantification difficult. As we mentioned in the Introduction, both garnets find extensive use in optical applications especially as downconverting phosphors, and therefore it is of great interest to investigate the spectroscopic properties of the microwave-

Figure 2. Energy dispersive X-ray (EDX) spectroscopy of the microwave-prepared garnet compounds, (a) YAG:Ce and (b) LuAG:Ce. The insets show regions of the spectra in higher magnification. For acquiring the data, a large area with more than 50 particles was scanned.

prepared materials and compare them with samples prepared more traditionally. Figure 3 shows the room-temperature

Figure 3. Photoluminescence data of (a) Y3Al5O12:Ce3+ and (b) Lu3Al5O12:Ce3+. The excitation spectra were recorded with an emission wavelength of 539 nm (LuAG:Ce) and 548 nm (YAG:Ce). The emission spectra were recorded with λex. = 457 nm for both samples. All measurements were performed at room temperature.

excitation and emission spectra of conventionally and microwave-prepared Y3Al5O12:Ce3+ and Lu3Al5O12:Ce3+, with the key results summarized in Table 2. YAG:Ce and LuAG:Ce show two intense bands within the measured range of the excitation spectra. These can be attributed to the transition from the ground state of the Ce3+ ion (2F5/2) to the two lowest-lying 5d states, dx2−y2 and dz2 following crystal field trends present on the distorted cubic environments of garnets.46 Absorption from the 2 F7/2 state can be neglected because the spin−orbit coupling is much greater (≈2000 cm−1) compared to the thermal energy kBT at room temperature (207 cm−1). Another interesting feature of the photoluminescence spectra is that the maximum of the Ce3+-emission in the yttrium garnet is shifted to longer wavelengths (into the red) as compared to 1200



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oven), thus enhancing homogeneous grain growth of the materials. The most efficient products have subsequently been used for all further optical characterizations. The temperature dependence of the emission properties are presented in Figures 4 and 5. At low temperatures, for both

Table 2. Spectroscopic Properties and Quantum Yield of the Garnet Compounds, Recorded at Room Temperaturea sample YAG:Ce YAG:Ce (MW) LuAG:Ce LuAG:Ce (MW)

λex. (nm) 340, 341, 347, 347,

464 463 451 450

λem. (nm)

δ1 (cm−1)

ΔS (cm−1)

QY (%)

548 546 540 538

7860 7727 6646 6596

2474 2559 2286 2246

87 88 84 84

δ1 is the energy difference between the two lowest lying d-states of the garnets; ΔS is the Stokes shift, calculated as the difference between the maximum of the excitation band and the first emission, derived from the measurements at 77 K. Multiple measurements of the quantum yield suggest a reliability of ±2 on the percentage values presented.

a

the lutetium garnet, contradictory to conventional crystal field theory. The crystal field splitting (Dq or Δ) can be expressed by the following equation:47

Dq =

1 2 r4 Ze 5 6 R

(1)

Here, Z is the valence of the coordinating anion, e is the elemental charge, r is the radius of the d-wave function, and R is the bond length between the activator ion and the coordinating anion. The formula suggests that with shorter Lu−O (and therefore Ce−O) bond distances (due to the smaller size of the Lu3+ compared to Y3+), one would expect an increased crystal field splitting (i.e., a larger Dq or Δ) and thus a further red shift in the lutetium garnet. As we see, this is not the case. The large, unexpected, but empirically found48,49 blue shift in emission of the lutetium garnet (λem. = 539 nm) can be explained from the distortion of the dodecahedral geometry in which the Ce3+ ion sits, as demonstrated, for example, by Wu et al. who have documented the effects on the 5d orbitals of Ce3+, of distortions from the cubic environment around Ce3+ in the garnet structure.50 Increased distortion around Ce3+ leads to a larger splitting (δ) of the dx2−y2 and dz2 levels which can be seen by observing the distance between the two excitation bands (see Table 2). In the YAG structure, the larger yttrium ion causes a greater distortion in comparison to the smaller lutetium ion. This leads to a greater δ for YAG:Ce (λem. = 548 nm) and a greater red shift in the emission maximum. This trend agrees well with previous results. As we find no significant difference in the spectral properties in terms of position of the bands (as well as in the structural parameters), we conclude that the microwave-prepared materials as well as the conventionally prepared materials, respectively, possess a similar coordination environment for the activator ion. In general, for the conditions used here, the garnet compounds prepared in the Panasonic microwave oven exhibited better optical properties (such as a slightly higher quantum yield) than the materials prepared in the LG microwave oven. This is most likely due to two reasons: First, the surrounding atmosphere contains less oxygen due to use of the glovebag, therefore providing a more reducing environment and eventually leading to a more efficient reduction of the Ce4+ ions, which are optically silent in the region of interest, to their trivalent state. And second, the Panasonic microwave oven is advertised to employ so-called inverter technology, which allows the microwave power to be modulated to lower power levels, rather than simply cycling the magnetron between on and off modes (as in the LG microwave

Figure 4. Emission spectra of the different garnets, collected at 77 K. The solid lines represent the measured spectra, while the dotted lines show the deconvolution of the two underlying transitions. (a) Represents YAG:Ce, (b) LuAG:Ce, (c) YAG:Ce MW, and (d) LuAG:Ce MW.

Figure 5. Temperature dependence of the emission spectra (457 nm excitation) measured from 250 to 500 K for (a) YAG:Ce, (b) YAG:Ce MW, (c) LuAG:Ce, and (d) LuAG:Ce MW. The trace above each plot displays quantum efficiency, obtained from the relative intensity normalized to the quantum yield value at at room temperature as a function of increasing temperature. 1201



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garnets, the broad emission band centered around 548 nm (YAG:Ce) and 539 nm (LuAG:Ce) splits into two distinct bands. These arise from the transitions of the lowest lying 5dstates (2D3/2) to the 2F5/2 and 2F7/2 states of the cerium ion. A peak-deconvolution of the emission spectra obtained at 77 K yields two bands centered at around 19100 cm−1 (524 nm) and 17400 cm−1 (575 nm) for the yttrium garnet as well as 19950 cm−1 (502 nm) and 18200 cm−1 (549 nm) for the lutetium garnet. There is no significant difference in these positions between the microwave- and conventionally prepared materials, again pointing to the spectroscopic properties being comparable. With an increase in temperature, these two bands broaden and eventually overlap to “form” the broad emission band that is observed at room temperature and elevated temperatures; see Figure 5. The difference between the maximum of the two resolved emission bands yields the splitting of the lowest-lying term 2FJ of the Ce3+ ion, which is due to the spin−orbit coupling. The obtained values are ≈1700 cm−1 for the YAG:Ce and ≈1750 cm−1 for the LuAG:Ce (conventional and MW) samples which is in good agreement with the expected value (≈2000 cm−1).17 It has been shown by Dorenbos51 that the Stokes shift ΔS for all 14 lanthanide ions is almost the same once they are introduced into a host lattice, meaning that it can be regarded as a feature of the host. One can estimate this shift by simply measuring the distance between the maximum of the excitation band with the lowest energy and the center of the emission band with the highest energy. The calculated values for ΔS in our samples are ≈2500 cm−1 for the yttrium garnets (furnace and microwave) and ≈2270 cm−1 for the lutetium garnets. This is comparable to the values found in the literature.21,52 In order to test the materials for their potential applications, the quantum efficiency of each sample was determined. Although the samples prepared at very high temperatures in the furnace require a much longer reaction time, the microwave-prepared samples are equally efficient. This is especially remarkable since many different synthesis routes have been tried to prepare garnets, usually yielding rather inferior products although they require a comparable amount of energy and time.2 The quality of all prepared samples was confirmed by comparing them versus a commercial YAG:Ce sample, which showed a quantum efficiency of 91%. We believe the slightly higher quantum efficiency for the commercial sample is a reflection of it having been subject potentially to sieving, for the particles to be uniformly sized, as well as some surface treatment to remove excess flux and so forth. One important feature of the performance of phosphor materials is the temperature stability of the luminescence. Figure 5 shows the evolution of the quantum yield in all samples with increasing temperature. As the temperature increases from room temperature, we see a decrease in the quantum yield (and emission intensity) due to a higher probability of nonradiative transitions, as expected. However, it becomes evident that the microwave-prepared materials exhibit the same thermal stability as the investigated Ce-substituted phosphors that have been prepared via the solid-state route, proving again the high quality of the microwave-prepared materials. As a last step of the characterization of the phosphors, the performance of microwave-prepared garnets in a device was compared to the samples prepared more conventionally. The results are presented in Figure 6. The spectra consist of the

Figure 6. Electroluminescence spectra of the different garnets, in combination with a blue light-emitting diode emitting at at 457 nm. The data was collected at room temperature under a forward bias current of 20 mA. Again, (a) represents YAG:Ce, (b) LuAG:Ce, (c) YAG:Ce MW, and (d) LuAG:Ce MW.

rather sharp emission (457 nm) of the blue diode and the broad emission band of the phosphor, encapsulated in the silicone resin. Combined, the device emits a blueish white light in the case of the YAG:Ce and greenish-white light in the LuAG:Ce, respectively, which is also evident in the obtained color coordinates within the CIE (Commission Internationale de l’Éclairage) 1931 chromaticity diagram, as shown in Table 3 and Figure 7. Table 3. Optical Properties of the White LEDs Fabricated Employing the Various Garnet Samples, Y3Al5O12:Ce3+ and Lu3Al5O12:Ce3+, Recorded at Room Temperaturea

a

sample

Ra

CIE x

CIE y

η (%)

efficacy (lm/W)

YAG:Ce YAG:Ce (MW) LuAG:Ce LuAG:Ce (MW)

68 69 67 67

0.35 0.34 0.28 0.28

0.39 0.37 0.36 0.36

60 61 64 68

110 112 113 116

The emission wavelength of the LED used was λ = 457 nm.

Figure 7. Color coordinates of the LEDs employing caps with the different garnet phosphors studied here. 1202



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Indeed, Katz54 and Kiggans et al.55 have pointed out that microwave-assisted preparation of ceramics can be up-scaled. The reduced reaction time allows one to increase the number of samples that can be prepared and investigated if compared to conventional solid state synthesis, especially under laboratory conditions. Assuming that the hot zone of an average tube furnace can accomodate between 6 and 10 crucibles (depending on the diameter of the samples and the tube), that means that over a 24 h duration, a maximum of 10 samples can be screened. The microwave, however, can hold (depending on the size of the cavity) up to three samples at a time. Additionally, with the appropriate insulation material (as described in Experimental Methods), the samples can be removed almost immediately after the microwave radiation treatment has finished and can be allowed to cool down outside the cavity. As a simplification, we estimate a total reaction time (including cooldown) of 1 h. This means that one can screen up to 36 samples within 12 h and up to 72 samples within 24 h, which is a 7-fold increase compared to the classical solid-state reaction. Overall, with all important reaction parameters adjusted, the microwave-assisted solid-state synthesis may be suitable as a high-throughput reaction method.

The color rendering index (Ra) as well as the luminous efficacy of the microwave phosphors are virtually the same as those of the solid-state prepared materials, demonstrating that the microwave-prepared materials, although rapidly obtained, are as suitable for applications in lighting devices as the material prepared by more conventional high temperature ceramic methods. We have also calculated the overall photon efficiency on the basis of the total number of photons from the LED in the capping geometry employed here, recognizing that if the phosphor cap were 100% efficient, the total number of photons would be conserved. The ratio η = PLED+cap/PLED then provides the efficiency of the cap on the basis of the number of photons, that is, ignoring energy losses due to the Stokes shift. Here, PLED is the emission intensity of the blue diode without phosphor cap (i.e., the original optical output of the diode) and PLED+cap is the emission intensity of the device, that is, the diode with the phosphor cap. This method gives a better estimation of the quantum efficiency of the materials in a device, where, for example, scattering losses and reabsorption play a more important role. We find that the Lu3Al5O12:Ce3+ samples show a slightly higher overall efficiency, reaching up to 68% in the case of the microwave-prepared powders. The significant drop in the device photon conversion efficiency, in contrast to the nearly 90% PLQY, is potentially due to backscattering of photons into the LED semiconductor. As in all characterization steps before, we find the microwave-prepared materials to be similar to the conventionally prepared samples, also evident in the luminous efficacy values; see Table 3. As expected from the calculated efficiencies, the lutetium garnet shows a slightly better behavior than the yttrium garnet. The overall energy balance of materials is considered increasingly important, and in this regard, it is notable that the microwave preparation of the garnet materials as presented here not only requires a fraction of the reaction time but also consumes significantly less energy compared to conventional methods. Traditional furnace preparation requires an average preparation time of 10 h (including heating ramps) carried out at 1500 °C or even higher temperatures. A commonly used tube furnace (e.g., Lindberg Blue M 1700 °C, 10 kW) uses approximately 85 kWh of electrical energy if we estimate an average power consumption as 80% of its maximum (we averaged the power consumption over the complete heating cycle; that is, during the heating ramps, less energy is used than at the maximum temperature). The microwave synthesis, however, uses only ≈0.4 kWh for a reaction that requires 25 min of total heating time at a power consumption of ≈1200 W. This is a reduction in energy demand of more than 99% to obtain materials that are as pure and as efficient as the standard materials. Again, optimizations in the experimental procedure have already shown that more efficient materials are accessible. The savings in energy and time are vast, even if compared with much more energy efficient syntheses; for example, for the solvothermal fabrication of nanosized garnet phosphors,53 the reduction in energy is still around 90% (here, we assume that the shortest reaction time including the heating ramp is 4 h, carried out in an oven that consumes at least 1000 W). The reaction time can again be shortened, in this case by more than 85%. Further cost savings, for example, in an actual industrial process, could be expected due to further optimization of the various parameters, for example, of being able to properly waveguide microwaves onto the samples.

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CONCLUSIONS We have successfully prepared cerium-substituted Y3Al5O12 and Lu3Al5O12 phosphors using a microwave heating method. This preparative method resulted in a drastic reduction of time, about 95%, compared to a classical solid-state synthesis method that relies on convection heating in tube or box furnaces. Additionally, all reactions could be carried out using inexpensive and readily available domestic microwave ovens and inexpensive insulating material. The microwave-prepared garnets are phase pure and show comparable luminescent properties and thermal stability to those prepared using more classical methods. The quantum efficiency (both in a device and of the pristine phosphor) of all materials presented here is virtually the same, again proving the high quality of the microwave-prepared garnets. Further optimization of the microwave synthesis method, for example, optimum flux conditions, Ce3+ concentration, and adjustment of the reducing atmosphere, is expected to lead to phosphors that will be even more competitive and eventually superior to conventionally prepared phosphors.

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ASSOCIATED CONTENT

* Supporting Information S

Details of quantum yield and thermal luminescence quenching measurements, SEM micrograph, and size distribution (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ▽

Department of Chemistry, Ithaca College, Ithaca, New York 14850, United States ○ UFR Structures et Propriétés de la Matière, Université de Rennes 1 - 35042 Rennes Cedex, France Notes

The authors declare no competing financial interest. 1203



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ACKNOWLEDGMENTS We thank Alasdair Morrison for useful discussions and Stuart Brinkley for help with the device characterization. N.C.G. and K.A.D. have been supported by the ConvEne IGERT Program (NSF-DGE 0801627). C.E.D. received funding from the RISE internship program through the UCSB Materials Research Laboratory Award No. DMR05-20415. B.H. acknowledges funding from the General Council of Brittany and thanks the ICMR, supported by IMI Program of the National Science Foundation under Award No. DMR 0843934. C.S.B. is a recipient of the Feodor Lynen Research Fellowship supported by the Alexander von Humboldt Foundation. The research carried out here made extensive use of shared experimental facilities of the Materials Research Laboratory: an NSF MRSEC, supported by NSF DMR 1121053. The MRL is a member of the NSF-supported Materials Research Facilities Network (www.mrfn.org). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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