Optical and Morphological Characterization of Tb0.01Zr0.99O2

Spectroscopic Signatures of Sub-Second Laser-Calcined Dy-Doped Oxide Precursors for Use in ex Situ Thermal Impulse Sensors. Benjamin R Anderson , Ray ...
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Optical and Morphological Characterization of Tb0.01Zr0.99O2/ (Precursor Eu0.02Y1.98O3) Core/Shell Nanoparticles as Temperature Sensors in Fast Heating Events Ray Gunawidjaja, Thandar Myint, and Hergen Eilers* Applied Sciences Laboratory, Institute for Shock Physics, Washington State University, Spokane, Washington 99210-1495, United States ABSTRACT: We characterize luminescent Tb0.01Zr0.99O2/(precursor Eu0.02Y1.98O3) core/shell nanoparticles as potential temperature sensors in fast heating events. The core of the nanoparticles consists of crystalline Tb0.01Zr0.99O2, while the shell is an amorphous Eu0.02Y1.98O3 precursor. When subjected to a brief thermal event, the crystalline zirconia core does not undergo any significant phase changes, whereas the yttria precursor shell undergoes an amorphous to crystalline phase change. Using this configuration, the emission intensity due to the Tb3+ ions in the ZrO2 core can be used as a reference. The emission intensity of the 5D0 → 7F2 transition of the Eu3+ ion in the initially amorphous yttria precursor increases with increasing temperature, providing an indicator for temperature. In addition, it is found that the emission intensity of the 5D0 → 7 F1 transition of the Eu3+ ion does not depend on temperature and can also be used as a reference to determine temperature.

1. INTRODUCTION Several recent reports have focused on the development of rareearth (RE)-doped nanophase precursors as temperature indicators in fast heating events.1−7 The need for such temperature sensors is inspired by the development of explosive means for the neutralization of biological agents and the need to measure the temperature inside explosive fireballs. During heating events, the nanophase precursors undergo irreversible phase changes that lead to structural changes, including changes to the local site symmetries and thus changes in the optical spectra of these materials. The optical spectra, such as fluorescence and absorption spectra, of Eu3+ -doped metal oxides can be used to determine the local site symmetry of these ions.8−12 We have previously reported on Eu3+-doped Y2O3 and Eu3+doped ZrO2 as temperature sensor materials for fast heating events.2−7 Eu3+-doped Y2O3 is a well-known red-emitting phosphor. Due to its high quantum yield, it is widely used in fluorescence lamps and projection television tubes.13,14 Although less studied than Y2O3, ZrO2 is also a promising host for rare-earth dopants, and visible emission from REdoped ZrO2 (RE = Tm3+, Tb3+, Eu3+, Pr3+) has been reported.15,16 For Eu3+-doped Y2O3, we used changes in peak position and full-width-at-half-maximum (FWHM) of the excitation peak, while for Eu3+-doped ZrO2 we used changes in peak position and relative peak intensities of the fluorescence spectra as temperature indicators. Here we report on (crystalline Tb0.01Zr0.99O2)/(amorphous precursor Eu0.02Y1.98O3) core/shell nanoparticles as temperature sensors. The core component of these temperature sensors, Tb0.01Zr0.99O2, is crystallized during a high-temperature/long-time heating process prior to forming the shell. As © 2014 American Chemical Society

such, it is relatively unaffected by subsequent short-time temperature exposures. However, the amorphous Y 2 O 3 precursor of the shell undergoes irreversible phase changes during short-time temperature exposures. We use the emission intensity of the crystalline core as a reference and the emission intensity of the shell as an indicator for the temperature to which the sensors are subjected. Such metal oxide core/shell nanoparticles with well-defined shapes and sizes can be prepared via wet chemical techniques in gram quantities with minimal dispersity.17−21

2. EXPERIMENTAL SECTION Synthesis of Precursor Tb 0.01Zr 0.99O 2, Precursor Eu0.02Y1.98O3, and Tb0.01Zr0.99O2/(Precursor Eu0.02Y1.98O3) Core/Shell Nanoparticles. All nanoparticles are synthesized according to previously reported procedures.5 The assynthesized nanoparticles are washed multiple times with deionized water and finally with acetone. For the deposition of three sequential layers, the coated particles are collected, rinsed multiple times with deionized water, and coated again. Subsequently, they are dried in vacuum followed by drying at 150 °C for 12 h. In the following, we will use “p” to denote precursors of uncalcined oxide nanoparticles. p-Tb0.01Zr0.99O2 nanoparticles are synthesized by forced hydrolysis. Two millimolar Tb(NO3)3·6H2O (99.999%, M.V. Laboratories), 0.198 M ZrOCl2·8H2O (99.9% Alfa-Aesar), and 1.5 g/L hydroxypropyl cellulose, HPC (Mw = 80,000, SigmaReceived: December 10, 2013 Revised: February 17, 2014 Published: February 18, 2014 5563

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Figure 1. SEM micrographs of p-Tb0.01Zr0.99O2 (left) and p-Eu0.02Y1.98O3 (right). Insets are TEM micrographs.

Figure 2. (a,b) SEM micrographs of Tb0.01Zr0.99O2 (left) and Eu0.02Y1.98O3 (right) calcined at 1000 °C/30 min. Insets are TEM micrographs. (c,d) Corresponding size distributions.

Baker) is maintained in a thermostatted oil bath at 90 °C for 3 h. Homogeneous precipitation of the p-Eu0.02Y1.98O3 shell material is performed in the presence of crystalline Tb0.01Zr0.99O2 nanoparticles (5 mg/mL concentration) to synthesize the Tb0.01Zr0.99O2/p-Eu0.02Y1.98O3 core/shell nanoparticles. This process uses lower concentrations of metallic salts than those used for the p-Eu0.02Y1.98O3 nanoparticle synthesis. The total metallic salt concentration is 5 mM, and the urea concentration is 0.2 M. The p-Eu0.02Y1.98O3 shell is grown as one or three sequential layers. We refer to the shell as “p-nEu0.02Y1.98O3” to indicate the number of layers (i.e., the notations for one and three layers are “p-1-Eu0.02Y1.98O3” and “p-3-Eu0.02Y1.98O3”, respectively).

Aldrich) are dissolved in a 5:1 isopropanol:water mixture. The mixture is quickly heated in a 700 W microwave (Haier MWM0701TB, Haier America Trading, LLC). For example, a 60 mL solution requires 20 s of heating using the maximum power setting. The hot solution is then transferred into a thermostatted oil bath, which is maintained at a temperature of 75−80 °C for 30 min without stirring. After 30 min, 4.5 M NH4OH solution (J.T. Baker) is added dropwise to neutralize the solution pH. p-Eu0.02Y1.98O3 nanoparticles are synthesized by homogeneous precipitation. An aqueous solution containing 0.0198 M Y(NO3)3·6H2O (99.9%, Alfa-Aesar), 0.2 mM Eu(NO3)3·5H2O (99.999%, M.V. Laboratories), and 0.8 M urea (99.7%, J.T. 5564

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Figure 3. X-ray diffraction patterns of Tb0.01Zr0.99O2 (left) and Eu0.02Y1.98O3 (right) calcined at 1000 °C/30 min. Histograms correspond to diffraction peaks of t-ZrO2 (dark green), m-ZrO2 (red), and c-Y2O3 (blue). Dots correspond to diffraction peaks of t-ZrO2 (light green), m-ZrO2 (orange), and c-Y2O3 (cyan), resulting from the W-Lα line of the XRD source. Additional smaller peaks can be assigned to the Cu−Kβ line, but are not pointed out here for clarity reasons.

Figure 4. Fluorescence spectra, excited at λex = 488 nm, of Tb0.01Zr0.99O2 (left) and Eu0.02Y1.98O3 (right), calcined at 1000 °C/30 min.

Pyroprobe Heating and Sample Characterizations. A Pyroprobe 1000 (CDS analytical, Inc.) is used for rapid heattreatments of the nanopowders. The sample powder is placed inside a quartz tube, which is placed inside a heating coil. The sample temperature is recorded using a CHAL-005 type K thermocouple and an RD8250 series paperless recorder (Omega Engineering, Inc.) during the heating process. A Lindberg/Blue M 1700 °C box furnace (Kendro Laboratory Products, Inc.) is used for long duration heat-treatments. The nanopowder morphologies are characterized using a scanning electron microscope (SEM), NOVA nanoSEM 230 (FEI), and a transmission electron microscope (TEM), Tecnai G2 20 Twin TEM (FEI). The SEM is equipped with X-Max Silicon Drift Detector (SDD) with an 80 mm2 active area (Oxford Instruments, PLC) for energy dispersive spectroscopy (EDS) measurements. A PANalytical X′Pert Pro diffractometer (PANalytical B.V.), using Cu−Kα radiation (λ=1.5418 Å) and operated at 45 kV and 40 mA, is used to measure X-ray diffraction patterns. Diffraction pattern were measured using a PIXcel3D detector (PANalytical B.V.). The X-ray beam was collimated using a fixed divergence slit (FDS) with 0.04 rad Soller slits, 0.5° divergence slit, and a 10 mm mask. The X-ray beam was monochromatized using a X′Celerator monochro-

mator (PANalytical B.V.). Due to the age of the X-ray source, it also contained W-Lα radiation that led to small ghost peaks in the XRD spectra. The optical spectroscopy setup has been described previously.6 Briefly, a Continuum Q-switched (∼10 ns pulse length) Nd:YAG laser operating at 10 Hz in the third harmonic at 355 nm pumps a Continuum Panther Optical Parametric Oscillator (OPO). Using multiple mirrors, the OPO output of the desired wavelength is directed onto the sample material, which is contained in a glass vial. Using two 2″ lenses, the sample fluorescence is collected, collimated, and focused onto the entrance slit of an Acton 2750 monochromator/ spectrometer with a PMT and CCD array attached. The fluorescence spectra are acquired using a PMT (Hamamatsu R5108) and SpectraSense software (Princeton Instruments). The data is further processed using Origin Pro 8.6 and 9.0.

3. RESULTS AND DISCUSSION Characterizations of p-Tb 0 . 0 1 Zr 0 . 9 9 O 2 and pEu0.02Y1.98O3. Forced hydrolysis and homogeneous precipitation yields nanoparticles with spherical morphologies (see Figure 1). The average diameters of the nanoparticles are 300 ± 5565

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Figure 5. (a,b) SEM micrographs of Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles with n = 1 (left) and n = 3 (right); insets are TEM micrographs. (c,d) Corresponding size distributions.

Figure 4 shows the fluorescence spectra, excited at λex = 488 nm, of the calcined Tb0.01Zr0.99O2 and Eu0.02Y1.98O3 nanoparticles. According to the assigned PDF of the X-ray diffraction patterns, the Zr4+, and presumably the Tb3+, cations occupy sites of D2d and C1 symmetry in the t-ZrO2 and m-ZrO2 phases, respectively, while the Y3+, and presumably the Eu3+, cations occupy sites of C2 and C3i symmetry in c-Y2O3. The fluorescence from the Tb0.01Zr0.99O2 sample is due to Tb3+ 5D4 →7FJ (J = 3−6) transitions and appears green, while the fluorescence from the Eu0.02Y1.98O3 sample is due to Eu3+ 5 D0→7FJ (J = 0−3) transitions and appears red. Characterization of Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 Core/Shell Nanoparticles. Figure 5a,b show the SEM and TEM micrographs of Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/ shell nanoparticles. The Tb0.01Zr0.99O2 core is polycrystalline, and the p-n-Eu0.02Y1.98O3 shell is amorphous. The combination of forced hydrolysis and homogeneous precipitation for the synthesis of the core and shell, respectively, yields nanoparticles with a well-defined core/shell morphology. These nanoparticles remain dispersible in water, alcohol, and acetone. The shell matrix conforms to the shape of the core, which is no longer spherical in shape (see insets for TEM micrographs). Figure 5c,d show the size distributions. The diameters of the nanoparticles are 280 ± 50 nm and 285 ± 70 nm for the single and triple layer shell, respectively. By comparing these values with the average diameter of the calcined Tb0.01Zr0.99O2 core (Figure 2a), the single-layer shell thickness is estimated to

60 nm and 345 ± 30 nm for p-Tb0.01Zr0.99O2 and pEu0.02Y1.98O3, respectively. After calcination at 1000 °C for 30 min, the Tb0.01Zr0.99O2 and Eu0.02Y1.98O3 nanoparticles remain dispersible in water, alcohol, and acetone. The calcined Tb0.01Zr0.99O2 nanoparticles show crystalline facets and some degree of neck growth, while the calcined Eu0.02Y1.98O3 nanoparticles retain their spherical shape (see Figure 2a,b). This difference is due to the lower activation energy for crystallization for the zirconia phase compared to the yttria phase.22 Due to decomposition of the salt precursors, the diameters of the calcined nanoparticles have decreased for both materials. The size distributions are shown in Figure 2c,d and indicate a normal distribution. The average diameters of the calcined Tb0.01Zr0.99O2 and Eu0.02Y1.98O3 nanoparticles are 255 ± 45 nm and 265 ± 25 nm, respectively. X-ray diffraction patterns of the calcined nanoparticles are shown in Figure 3. The positions of the diffraction peaks of Tb0.01Zr0.99O2 are compared with t-ZrO2 phase (PDF 01-0791771) and m-ZrO2 phase (PDF 01-070-2491), while those of Eu0.02Y1.98O3 are compared with c-Y2O3 phase (PDF 01-0830927). Figure 3 shows that the majority of the calcined Tb0.01Zr0.99O2 nanoparticles form the m-ZrO2 phase and the calcined Eu0.02Y1.98O3 nanoparticles form the c-Y2O3 phase. Peaks marked with dots can be assigned to diffraction due to W-Lα radiation due to an aged X-ray source. Peaks of even smaller intensity are due to Cu−Kβ radiation (not indicated in Figure 3). 5566

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Figure 6. EDS (left) and X-ray diffraction patterns (right) of Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles.

be about 25 nm, and the triple-layer shell thickness is estimated to be about 30 nm. Figure 6 shows the EDS spectra and X-ray diffraction patterns of Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles. The EDS data confirm the presence of the yttria precursor shell. The Si peak in the EDS signals is due to the substrate. The corresponding X-ray diffraction patterns for the single and triple layer core/shell materials are similar, showing mainly the crystalline m-ZrO2 phase of the core and less of the t-ZrO2 phase, consistent with the assignment for Figure 3a. Pyroprobe Heated Tb 0.01 Zr 0.99O 2/p-n-Eu 0.02Y1.98 O3 Core/Shell Nanoparticles. Using a pyroprobe-based Tjump technique, the core/shell nanoparticles are heated for 10 s to various temperatures between 300 and 900 °C. The measurements are repeated three times for each set temperature. However, the measured temperatures vary slightly. Figure 7 shows typical heating profiles for five different temperatures.

Figure 8. X-ray diffraction patterns of the pyroprobe heated Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles for n = 1 (left) and n = 3 (right). Refer to Figure 7 for the corresponding temperature profiles. Red and blue histograms correspond to diffraction peaks of m-ZrO2 and c-Y2O3, respectively.

shell. This peak is first observed for the 800 °C/10 s and 700 °C/10 s heated samples for n = 1 and n = 3, respectively. The peak width narrows with higher calcinations temperatures. This behavior indicates a nucleation and grain growth mechanism. No other significant changes in the X-ray diffraction characteristics of the Tb0.01Zr0.99O2 phase are observed. The presence of the crystalline domain in the shell can be seen with TEM. Representative micrographs are shown for the 900 °C/10 s pyroprobe-heated samples (see Figure 9). Note that the grainy crystallite domains across the surface of the core/shell nanoparticles are smaller than the crystallite domains of the core (compare insets of Figure 2 with Figure 9). Figure 10 shows the photoluminescence spectra of Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles heated for 10 s to various temperatures. An excitation wavelength of λex = 488 nm is used to simultaneously excite the Tb3+ and Eu3+ ions. The spectra are normalized with respect to the peak intensity of the Tb3+ emission at 543 nm, but are purposely not corrected for temperature-dependent changes to the absorption since this is part of the change that is being recorded. Incidentally, when this normalization is performed, the intensities of the magnetic dipole allowed Eu3+ 5D0 → 7F1 transitions in the 585 − 605 nm range are identical for all samples. The intensities of the 5D0 → 7F1 magnetic dipole transitions are independent of changes to the local site symmetry and are often used as a reference for comparison with the hypersensitive 5D0 → 7F2 electric dipole transition which is very sensitive to changes in the site symmetry.23 Subsequently, the maximum peak intensities of the electric dipole allowed 5D0 → 7F2 transitions of Eu3+ at λ = 611 nm, I611, are plotted as a function of measured temperature (see

Figure 7. Measured temperature profiles for various pyroprobe heating experiments.

Despite a set heating duration of 10 s, the maximum temperature is attained only briefly. Nevertheless, crystallization of the amorphous p-n-Eu0.02Y1.98O3 is achieved. The measured X-ray diffraction patterns of the pyroprobeheated Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles show a gradual amorphous to crystalline phase transition in the p-n-Eu0.02Y1.98O3 shell (see Figure 8). The peak corresponding to the (222) plane of the c-Y2O3 phase at 2θ = 29° gradually appears. The appearance of this peak is more apparent for the nanoparticles with n = 3 and is due to the relatively thicker 5567

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Figure 9. TEM micrographs of 900 °C/10 s pyroprobe-heated Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles with n = 1 (left) and n = 3 (right).

Figure 10. Fluorescence spectra (λex = 488 nm) of Tb0.01Zr0.99O2/p-1-Eu0.02Y1.98O3 (left) and Tb0.01Zr0.99O2/p-3-Eu0.02Y1.98O3 (right) core/shell nanoparticles heated to various temperatures. Refer to Figure 7 for the corresponding temperature profiles.

Figure 11. Plot of maximum emission intensity, I611, of pyroprobe-heated Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles for n = 1 (left) and n = 3 (right) for λex = 488 nm as a function of temperature.

Figure 11). The plot shows a continuous increase in emission intensity with temperature, reflecting the continuous decomposition and amorphous to crystalline phase change in the p-nEu0.02Y1.98O3 shell. The trend in the Eu3+ photoluminescence spectra is consistent with an amorphous to crystalline nucleation and grain growth phase transition of the host matrix.5,24 While X-ray diffraction measurements reveal the formation of the Y2O3 phase only at calcination temperature ≥700 °C/10 s, the photoluminescence spectra of Tb3+ and Eu3+

ions reveal continuous phase changes across the whole temperature range, uncalcined to 900 °C/10 s. A more complete kinetic experiment involving time and temperature sweep is currently underway.

4. CONCLUSIONS Dispersible core/shell nanoparticles, consisting of crystalline Tb0.01Zr0.99O2 cores and amorphous p-Eu0.02Y1.98O3 shells, are 5568

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(11) Tiseanu, C.; Parvulescu, V. I.; Sanchez-Dominguez, M.; Boutonnet, M. Spectrally and Temporarily Resolved Luminescence Study of Short-Range Order in Nanostructured Amorphous ZrO2. J. Appl. Phys. 2011, 110, 103521. (12) Binnemans, K.; Gorller Walrand, C. Application of the Eu3+ Ion for Site Symmetry Determination. J. Rare Earth 1996, 14, 173−180. (13) Justel, T.; Nikol, H.; Ronda, C. New Developments in the Field of Luminescent Materials for Lighting and Displays. Angew. Chem., Int. Ed. 1998, 37, 3085−3103. (14) Yamamoto, H.; Urabe, K. Host Sensitization Mechanism of Eu3+ Luminescence in (Y,In)2O3. J. Electrochem. Soc. 1982, 129, 2069−2074. (15) Romero, V. H.; De la Rosa, E.; Lopez-Luke, T.; Salas, P.; Angeles-Chavez, C. Brilliant Blue, Green and Orange-Red Emission Band on Tm3+-, Tb3+- and Eu3+-Doped ZrO2 Nanocrystals. J. Phys. D: Appl. Phys. 2010, 43, (16) Ramos-Brito, F.; Alejo-Armenta, C.; Garcia-Hipolito, M.; Camarillo, E.; Hernandez, J.; Murrieta, H.; Falcony, C. Photoluminescent Emission of Pr3+ Ions in Different Zirconia Crystalline Forms. Opt. Mater. 2008, 30, 1840−1847. (17) Matijevic, E.; Hsu, W. P. Preparation and Properties of Monodispersed Colloidal Particles of Lanthanide Compounds 0.1. Gadolinium, Europium, Terbium, Samarium, and Cerium(III). J. Colloid Interface Sci. 1987, 118, 506−523. (18) Titirici, M. M.; Antonietti, M.; Thomas, A. A Generalized Synthesis of Metal Oxide Hollow Spheres Using a Hydrothermal Approach. Chem. Mater. 2006, 18, 3808−3812. (19) Chaudhuri, R. G.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373−2433. (20) Gunawidjaja, R.; Myint, T.; Eilers, H. Synthesis of Silver/SiO2/ Eu:Lu2O3 Core−Shell Nanoparticles and Their Polymer Nanocomposites. Powder Technol. 2011, 210, 157−166. (21) Djerdj, I.; Arcon, D.; Jaglicic, Z.; Niederberger, M. Nonaqueous Synthesis of Metal Oxide Nanoparticles: Short Review and Doped Titanium Dioxide as Case Study for the Preparation of Transition Metal-Doped Oxide Nanoparticles. J. Solid State Chem. 2008, 181, 1571−1581. (22) Ghosh, A.; Upadhyaya, D. D.; Prasad, R. Primary Crystallization Behavior of ZrO2−Y2O3 Powders: In Situ Hot-Stage XRD Technique. J. Am. Ceram. Soc. 2002, 85, 2399−2403. (23) Myint, T.; Gunawidjaja, R.; Eilers, H. Light-Induced Structural Changes in Eu-Doped (Pb,La)(Zr,Ti)O3 Ceramics. Appl. Phys. Lett. 2011, 98, 171906. (24) Yermolayeva, Y. V.; Tolmachev, A. V.; Korshikova, T. I.; Yavetskiy, R. P.; Dobrotvorskaya, M. V.; Danylenko, N. I.; Sofronov, D. S. Spherical Core−Shell Structured Nanophosphors on the Basis of Europium-Doped Lutetium Compounds. Nanotechnology 2009, 20, 325601.

synthesized by wet chemical means. The use of the two different dopants, Tb3+ and Eu3+, ensures that the photoluminescence from the core (centered at about 543 nm) and the shell (centered at about 611 nm) can be easily distinguished. Upon exposure to heat the amorphous shell gradually transforms into the crystalline yttria phase. Since the core material is already fully crystallized, its emission intensity does not change under subsequent short heat exposures and can thus be used as an internal reference. Moreover, we observe that the emission intensity of the 5D0 → 7F1 transition of the Eu3+ ion does not depend on heating temperature and thus can also be used as an internal reference standard. The maximum fluorescence intensity due to the 5D0 → 7F2 transition of the Eu3+ ion increases with increasing temperature, reflecting the amorphous to crystalline phase transition. These results show that the Tb0.01Zr0.99O2/p-n-Eu0.02Y1.98O3 core/shell nanoparticles can be used as temperature sensors in fast heating events.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (509) 358-7681. Fax: (509) 358-7721. E-mail: eilers@ wsu.edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Defense Threat Reduction Agency, Basic Research Award # HDTRA1-10-1-0005 to Washington State University.

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REFERENCES

(1) Doull, B. A.; Oliveira, L. C.; Yukihara, E. G. Effect of Annealing and Fuel Type on the Thermoluminescent Properties of Li2B4O7 Synthesized by Solution Combustion Synthesis. Radiat. Meas. 2013, 56, 167−170. (2) Eilers, H.; Gunawidjaja, R.; Myint, T.; Lightstone, J.; Carney, J. Irreversible Phase Transitions in Doped Metal Oxides for Use As Temperature Sensors in Explosions. In APS SCCM 2011, Proceedings of the 17th APS SCCM Conference, Chicago, IL, 2011. (3) Gunawidjaja, R.; Myint, T.; Eilers, H. Correlation of Optical Properties and Temperature-Induced Irreversible Phase Transitions in Europium-Doped Yttrium Carbonate Nanoparticles. J. Solid State Chem. 2011, 184, 3280−3288. (4) Gunawidjaja, R.; Myint, T.; Eilers, H. Stabilization of Tetragonal Phase in ZrO2:Eu by Rapid Thermal Heating. Chem. Phys. Lett. 2011, 515, 122−126. (5) Gunawidjaja, R.; Myint, T.; Eilers, H. Temperature-Dependent Phase Changes in Multicolored ErxYbyZr1−x−yO2/Eu0.02Y1.98O3 Core/ Shell Nanoparticles. J. Phys. Chem. C 2013, 117, 14427−14434. (6) Myint, T.; Gunawidjaja, R.; Eilers, H. Spectroscopic Properties of Nanophase Eu-Doped ZrO2 and Its Potential Application for Fast Temperature Sensing Under Extreme Conditions. J. Phys. Chem. C 2012, 116, 21629−21634. (7) Myint, T.; Gunawidjaja, R.; Eilers, H. Fast Pyroprobe-HeatingInduced Structural Changes of Y2O3:Eu Precursors and Their Optical Signatures. J. Phys. Chem. C 2012, 116, 1687−1693. (8) Bihari, B.; Eilers, H.; Tissue, B. M. Spectra and Dynamics of Monoclinic Eu2O3 and Eu3+:Y2O3 Nanocrystals. J. Lumin. 1997, 75, 1− 10. (9) Bunzli, J. C. G. Luminescent Probes. In Lanthanide Probes in Life, Chemical and Earth Sciences: Theory and Practice; Bunzli, J.-C. G.; Choppin, G. R., Eds.; Elsevier: Amsterdam, 1989; pp 219−293. (10) Eilers, H.; Tissue, B. M. Laser Spectroscopy of Nanocrystalline Eu2O3 and Eu3+:Y2O3. Chem. Phys. Lett. 1996, 251, 74−78. 5569

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