Spectroscopic Properties of Nanophase Eu-Doped ZrO2 and Its

Sep 10, 2012 - Spectroscopic Properties of Nanophase Eu-Doped ZrO2 and Its Potential Application for Fast Temperature Sensing Under Extreme Conditions...
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Spectroscopic Properties of Nanophase Eu-Doped ZrO2 and Its Potential Application for Fast Temperature Sensing Under Extreme Conditions Thandar Myint, Ray Gunawidjaja, and Hergen Eilers* Applied Sciences Laboratory, Institute for Shock Physics, Washington State University, Spokane, Washington 99210-1495, United States ABSTRACT: Luminescent nanophase europium-doped ZrO2 precursors are subjected to pyroprobe heating with nominal heating rates of 20 000 °C/s for 1 and 10 s to various temperatures. When heated above the crystallization temperature, the material forms tetragonal and monoclinic polymorphs. Optical spectroscopy is used to measure the excitation spectra, fluorescence spectra, and fluorescence lifetimes of the heated samples. These optical signatures are evaluated for their potential as temperature indicators for short heating events. The fluorescence lifetimes and the intensity ratio of two fluorescence peaks appear to be promising temperature indicators. The fluorescence intensity ratio of two peaks at about 591 and 592 nm of the tetragonal polymorph decreases linearly with temperature and the peak position increases linearly with temperature between about 800 and 1300 K. The fluorescence lifetimes of both polymorphs increase with temperature. As the temperature reaches about 1000 K for the tetragonal phase and 1200 K for the monoclinic phase, the fluorescence lifetime starts to decrease again.

1. INTRODUCTION Temperature sensors have been developed for a large number of applications. Many of these sensors use the luminescent properties of the material to indicate temperature.1−6 Such an optical approach is particularly useful for noncontact applications. In thermometry applications, the fluorescence lifetime or fluorescence intensity is often monitored to determine the temperature. Our own work on temperature sensors focuses on rare-earth doped materials such as Eu-doped Y2O3 and Eu-doped ZrO2.2,4,6 The optical properties of Eu dopants have been characterized in a large number of host materials.7−15 Because of the hypersensitivity of the 5D0 → 7F2 transition, Eu ions are particularly well suited to monitor morphological changes in a host material which are induced by external stimuli.16−21 Extreme conditions such as those occurring in explosions in which high pressure and high heat change at the μs time scale are some of the most challenging environments for measuring temperature. Specific explosive charges are being developed to destroy biological agents. Knowledge of the temperature profile in an explosive fireball is required to ensure the complete destruction of biological hazards. Due to the nature of an explosive fireball, including its optical opaqueness, it is not possible to measure the temperature inside a fireball in situ; only temperatures from near the surface can be determined via spectroscopic or pyrometric means. We have been developing, characterizing, and testing luminescent nanoparticles for seeding into an explosive fireball and subsequent collection and evaluation.2,4,6 In particular, we have been testing Eu-doped Y2O3 for this application. The © XXXX American Chemical Society

synthesis and characterization of this material has been described elsewhere. 2,4 Briefly, amorphous Eu-doped Y2(CO3)3 is synthesized via coprecipitation. When subjected to fast heating events, the material converts via multiple stages into crystalline Eu-doped Y2O3. Several of the intermediate stages are also amorphous or can be either amorphous or crystalline. Various spectral signatures of these compounds were evaluated, and it was found that the peak-width of the excitation spectrum measured at 611 nm is the most suitable indicator for temperature. As the amorphous Y2(CO3)3 matrix is converted into amorphous Y2O(CO3)2 and further into amorphous Y2O2(CO3), the excitation peak width increases with temperature. This process continues until the material is converted into crystalline cubic Y2O3 in which the amorphous peak no longer exists. As such, this material is limited to measuring temperatures up to about 930 K. Finding luminescent materials for measuring even higher temperatures under extreme conditions is thus the next challenge. Besides Eu-doped Y2O3, we recently also presented some initial results for Eu-doped ZrO2.6 Heating of this material leads to the formation of monoclinic and tetragonal polymorphs. In particular, we reported on how fast heating events of Eu-doped Zr(OH)4 can lead to the stabilization of the tetragonal phase. In this work, we report on the spectroscopic characterization of Eu-doped ZrO2 and evaluate its potential use as a temperature sensor for temperatures above 930 K. Received: July 17, 2012 Revised: September 7, 2012

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Figure 1. Experimental setup for pyroprobe heating (left) and optical characterization of the samples (right).

Figure 2. Fluorescence spectra of Eu0.01Zr0.99(OH)4 heated for 10 s to 676 and 1183 K, respectively (left), heated for 3 h to 673 and 1173 K, respectively (center), and heated for 6 h to 673 and 1173 K, respectively (right).

2. EXPERIMENTAL SECTION The synthesis, morphological characterization, and pyroprobe heating of the Eu-doped Zr(OH)4 nanoparticles is described elsewhere.6 Briefly, an aqueous solution of ZrOCl2·8H2O and Eu(NO 3 ) 3 ·5H 2 O is added to an aqueous solution of ammonium hydroxide to reach a pH of 10. For most of the results shown, the white gel that forms is aged for 2 h and is then several times centrifuged and redispersed in deionized water. Subsequently, the material is rinsed in acetone, dried overnight in a vacuum, and then dried for 12 h at 80 °C. The cake is then crushed into a powder which is subsequently sieved (100 mesh). Using this approach, Eu0.01Zr0.99O2 was synthesized, and three sets of samples from the same batch of material were heated and characterized. Pyroprobe heating is performed using a Pyroprobe-1000 (CDS Analytical, Inc.) using a nominal ramp rate of 20 000 °C/ s. The sample material is placed inside thin quartz tubes, together with an Omega CHAL-005 thermocouple; see Figure 1 (left). Quartz wool is placed into the other end of the quartz tubes to prevent material from spilling out. The quartz tubes are then sequentially placed inside the heater coil of the pyroprobe heater. The pyroprobe heater is then set for the desired heating time of 1 or 10 s, and the desired set temperature. The measured temperatures are always lower than the set temperatures. Only the measured temperatures are reported. Transmission electron microscopy (TEM) images of the as-prepared and heated nanoparticles have been published previously, demonstrating the effect of fast heating on crystalline growth.6 The optical spectroscopy measurements are performed using a Continuum Nd:YAG laser operating at 10 Hz in the third harmonic at 355 nm. This laser pumps a Continuum Panther

Optical Parametric Oscillator (OPO). Using several mirrors, laser light of about 10 ns pulse length of the desired wavelength is directed onto the sample material which is placed in a vial; see Figure 1 (right). The emitted fluorescence is collected using a 2” diameter lens with a focal length of 15 cm. Using a 2” diameter lens with a focal length of 50 cm, the fluorescence is then focused onto the entrance slit of an Acton 2750 monochromator/spectrometer with PMT and CCD array attached. The excitation spectra are acquired using a SpectraHub (Acton) interface and in-house Labview software. The fluorescence spectra are acquired using the CCD camera and WinSpec software (Princeton Instruments). The lifetimes are measured using a PMT and an oscilloscope (Tektronix TDS3052B). The data is further processed using Origin Pro 8.5.

3. RESULTS AND DISCUSSION We determined previously that the Eu0.01Zr0.99(OH)4 nanoparticles crystallize into tetragonal and monoclinic phases upon heating.6 Figure 2 shows the fluorescence spectra of samples heated for various times. The spectra shown in Figure 2 (left) are of Eu0.01Zr0.99(OH)4 samples heated for 10 s in the pyroprobe to 676 and 1183 K, respectively. The spectral features of the sample heated to 676 K are relatively broad, indicating that most of the sample is still amorphous. The crystalline spectral features become apparent somewhere between 770 and 840 K.6 The spectrum of the sample heated to 1183 K shows some of the sharp crystalline features corresponding mostly to Eu3+ ions in the tetragonal phase. The spectra in Figure 2 (center) show fluorescence spectra of Eu0.01Zr0.99(OH)4 nanoparticles that were heated for 3 h to 673 and 1173 K, respectively. These spectra are dominated by Eu3+ B

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Table 1. Fitted Emission Lines of Eu3+ in Mostly Tetragonal and Mostly Monoclinic ZrO2

emission peaks in the tetragonal phase but also show emission peaks from Eu3+ in the monoclinic phase. While the spectra are normalized with respect to each other, it should be kept in mind that the emission intensity of the sample heated to 1173 K is significantly stronger than the emission intensity of the sample heated to 673 K. These two spectra show some of the main features of Eu ions embedded in tetragonal and monoclinic ZrO2. The peaks at about 592 nm and at about 606 nm correspond to the 5D0 → 7F1 and 5D0 → 7F2 transitions mostly from Eu3+ ions (D2d site symmetry) in tetragonal ZrO2 (P42/n m c), respectively.22 The peaks at about 597 and 610 nm through 635 nm correspond to the 5D0 → 7F1 and 5D0 → 7 F2 transitions mostly from Eu3+ ions (C1 site symmetry) in monoclinic ZrO2 (P1 21/c 1).22 The spectra shown in Figure 2 (right) are for Eu0.01Zr0.99(OH)4 nanoparticles that are heated for 6 h to 673 and 1173 K, respectively. Heating to 673 K for 3 h vs 6 h does not show any significant changes. However, heating to 1173 K for 6 h leads to significantly stronger emission from Eu3+ ions in the monoclinic phase, compared to 3 h heating. This observation is consistent with the fact that longer heating at high temperatures leads to the conversion of material in the tetragonal phase to the monoclinic phase. Figure 3 shows the corresponding excitation spectra of two

mostly tetragonal phase

Figure 3. Excitation spectra of Eu ions in tetragonal ZrO2 (bottom) and monoclinic ZrO2 (top). Insets show enlargements of the spectral structures.

Eu0.01Zr0.99(OH)4 samples heated for 3 h to 1773 K (top) and 3 h to 673 K (bottom), respectively. These samples are monitored at 614 and 606 nm, respectively. The spectrum in the top is mostly due to emission from Eu ions in the monoclinic phase, while the spectrum in the bottom is mostly due to Eu ions in the tetragonal phase. Tables 1 and 2 summarize the observed emission and excitation lines. These peak positions are fitted using the Peak Analyzer function in Origin Pro 8.5. In the tetragonal phase, the Eu3+ ions occupy mostly sites of D2d symmetry. In this case, the 7 F1 and 7F2 levels split into two and four components, respectively. In the monoclinic phase, the Eu3+ ions occupy mostly sites of C1 symmetry. In this case, the 7F1 and 7F2 levels completely split into three and five components, respectively.22 The number of observed emission peaks indicates that the tetragonal phase must have at least two emission sites, and the monoclinic phase at least three emission sites. This observation is similar to what has been reported for Eu-doped Y2O3.7 The number of observed excitation lines indicates even more sites.

mostly monoclinic phase

transitions

(cm−1)

(nm)

(cm−1)

(nm)

5

D 0 → F0

17231.5 17216.3

580.3 580.9

5

D 0 → 7 F1

16914.8 16907.4 16888.2 16880.1

591.2 591.5 592.1 592.4

5

D 0 → 7 F2

16633.3 16493.8 16241.7 15872.3 15743.4 15711.6

601.2 606.3 615.7 630.0 635.2 636.5

5

D 0 → 7 F3

15601.4 15355.8 15319.7 15152.2 14846.0

641.0 651.2 652.8 660.0 673.6

5

D 0 → 7 F4

14261.2 14255.9 14234.9 14006.5 14003.9 13982.1 13863.7 13759.6

701.2 701.5 702.5 714.0 714.1 715.2 721.3 726.8

17193.1 17180.0 17155.7 16963.5 16908.5 16903.6 16770.8 16760.9 16725.3 16705.9 16679.9 16327.2 16297.3 16269.4 16245.0 16210.7 16121.9 16049.8 16019.8 15997.1 15833.3 15383.7 15353.6 15316.5 15266.7 15241.8 15177.2 15097.1 15118.6 15064.3 14579.4 14386.7 14340.6 14297.2 14261.9 14241.6 14193.4 14116.0 14076.8 14038.9 14013.1

581.6 582.1 582.9 589.5 591.4 591.6 596.3 596.6 597.9 598.6 599.5 612.5 613.6 614.7 615.6 616.9 620.3 623.1 624.2 625.1 631.6 650.0 651.3 652.9 655.0 656.1 658.9 662.4 661.4 663.8 685.9 695.1 697.3 699.4 701.2 702.2 704.6 708.4 710.4 712.3 713.6

7

However, a more detailed analysis of these different sites using low-temperature spectroscopy is beyond the scope of this work. Our investigation of heated Eu0.02Y1.98(CO3)3 indicates that the peak width and peak position of excitation spectra are good temperature indicators.2,4 Thus, excitation spectra of heated Eu0.01Zr0.99(OH)4 are measured between 510 and 590 nm for samples heated for 10 s and between 510 and 600 nm for samples heated for 1 s to various temperatures, while monitored at 606 nm (tetragonal phase) and 614 nm (monoclinic phase), respectively; see Figures 4 and 5. The samples monitored at 606 nm show peaks between 520 and 550 nm corresponding to the 7F0 → 5D1 transitions and peaks between 575 and 590 nm corresponding to the 7F0 → 5D0 transitions, respectively. Figure 4 (right), which shows the excitation spectra monitored at 614 nm, shows some of the same peaks as shown in Figure 4 (left) which are monitored at C

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Table 2. Fitted Excitation Lines of Eu3+ in Mostly Tetragonal and Mostly Monoclinic ZrO2 mostly tetragonal phase

samples heated for 10 s and the samples heated to 1 s but monitored at 606 nm. This observation could be explained if the tetragonal phase starts to crystallize before the monoclinic phase. The higher the temperature and the longer the heating time, the more intense the peaks between 520 and 540 nm compared to the peaks between 575 and 590 nm. This observation is more pronounced for the excitation spectra monitored at 606 nm (tetragonal) than the ones monitored at 614 nm (monoclinic), and is consistent with the fact that the 5 D0 → 7F0 transition is forbidden in D2d symmetry (tetragonal phase) but electric dipole allowed in C1 symmetry (monoclinic phase). While the excitation spectra show some sharpening with increasing heating temperature, the overall shape of the excitation peaks make fitting with a single peak function difficult. This spectral shape is in contrast to the spectral shape of heated Eu0.02Y1.98(CO3)3 which allows for easy fitting with a single peak function. As such, the excitation spectra of heated Eu0.01Zr0.99(OH)4 do not appear to be good indicators for temperature. Therefore, the fluorescence lifetime and fluorescence spectra are evaluated as potential temperature indicators. Figure 6 shows the fluorescence lifetimes of Eu-doped ZrO2 heated for 10 s to various temperatures, excited at 536 nm, and monitored at 606 and 614 nm, respectively. This data shows the average and error bars from measurements on three sets of samples. The decay curves of the samples monitored at 614 nm (monoclinic phase) show a very short lifetime component as well as a longer lifetime component. These curves are fitted with a single exponential for all temperatures starting at 0.3 ms. The lifetime data for the samples monitored at 606 nm (tetragonal phase) are fitted with a single exponential for temperatures below 700 K (amorphous phase). Above 700 K, the data are fitted with a double exponential. One of the exponentials is the same as the monoclinic lifetime, indicating that a mixed phase is observed at 606 nm. The second exponential (tetragonal) is the one shown in Figure 6 (left). As expected, at temperatures below about 700 K, when the material is still amorphous, the observed lifetimes are about the same for both observation wavelengths. As the temperature is further increased, the tetragonal phase displays a jump in fluorescence lifetime from about 1 ms to more than 4 ms. Subsequently, the lifetime increases a little more before it drops again to about 3 ms at about 1400 K. The monoclinic phase on the other hand displays a different temperature dependence. The lifetime continues to increase more slowly to a maximum of about 1.6 ms, at which it levels out. Only at temperatures above 1200 K does the lifetime decrease again to about 1.5 ms

mostly monoclinic phase

(cm−1)

(nm)

(cm−1)

(nm)

18957.2 18642.1 18620.7 18247.3

527.5 536.4 537.0 548.0

17227.6 16920.4 16896.4 16891.7 16883.1 16773.7

580.5 591.0 591.8 592.0 592.3 596.2

19132.1 19081.0 18998.1 18965.5 18949.6 18936.1 18917.6 18903.4 18891.6 18876.3 18752.5 18725.5 18680.6 18680.6 18624.8 18543.9 18515.7 18486.0 18091.4 18045.3 17995.4 17755.1 17564.5 17410.3 17370.7 17328.1 17282.0 17192.0 17179.1 16965.2 16905.5 16774.1 16710.3

522.7 524.1 526.4 527.3 527.7 528.1 528.6 529.0 529.3 529.8 533.3 534.0 535.3 535.3 536.9 539.3 540.1 541.0 552.7 554.2 555.7 563.2 569.3 574.4 575.7 577.1 578.6 581.7 582.1 589.4 591.5 596.2 598.4

606 nm, indicating that the emission at 614 nm may not be purely due to the monoclinic phase but may also contain contribution from the tetragonal phase. Figure 5 shows the excitation spectra of samples heated for 1 s. These spectra are very similar to the spectra of samples heated for 10 s and shown in Figure 4, indicating that even for heating times as short as 1 s some of the material crystallizes. However, the peaks of the samples monitored at 614 nm, see Figure 5 (right), remain relatively broad compared to the

Figure 4. Excitation spectra of Eu0.01Zr0.99(OH)4 heated for 10 s to various temperatures and monitored at 606 nm - tetragonal phase (left) and 614 nm - monoclinic phase (right). D

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Figure 5. Excitation spectra of Eu0.01Zr0.99(OH)4 heated for 1 s to various temperatures and monitored at 606 nm - tetragonal phase (left) and 614 nm - monoclinic phase (right).

Figure 6. Fluorescence lifetime of Eu0.01Zr0.99(OH)4 heated for 10 s, excited at 536 nm, and monitored at 606 nm (left) and 614 nm (right), respectively.

Figure 7 shows a small section with two peaks of the fluorescence spectra of Eu0.01Zr0.99(OH)4 heated for 10 and 1 s

at about 1300 K. The increase and subsequent decrease of the fluorescent lifetime with temperature is not unusual in nanoparticles. A similar behavior is also seen in Eu:Y2O3,23 and the concept of this effect was described by Meltzer et al.24 Equation 1 describes the dependence of the radiative lifetime of an electronic transition: 2

τR =

λ0 1.5 × 104 f (ED) ⎡ 1 (n2 + 2)⎤2 n ⎣3 ⎦

(1)

where f(ED) is the oscillator strength, n is the index of refraction, and λ is the wavelength in vacuum. Because the nanoparticles are much smaller than the wavelength of light, their immediate surrounding environment needs to be considered to determine an effective index of refraction. In our case, the nanoparticles' matrix starts out as Zr(OH)4 and decompose into ZrO2 as they are heated. During the decomposition process, the size, shape, and surrounding of the particles change, resulting in changes in the radiative lifetime. Because the fluorescence lifetime depends on the morphology, extra care was taken to ensure that the material was handled in a manner that prevents changes to the morphology, i.e., no mechanical compression of the sample material occurred during handling. A more detailed description of the above-described effect can be found in the literature.23,24 The fact that the shape of the fluorescence lifetime data for Eu ions in the tetragonal phase is different from the shape of the fluorescence lifetime data of Eu ions in the monoclinic phase indicates that their morphological development must be different during the heating process. Considering the shape of the fluorescence lifetime curves, the data monitored at 614 nm could provide some indication for temperature in the range between about 500 and 1100 K.

Figure 7. Fluorescence spectra of Eu0.01Zr0.99(OH)4 heated for 10 and 1 s to various temperatures.

to various temperatures. Both of these peaks have the same fluorescence lifetime, indicating that they are both resulting from the same level of Eu ions in the tetragonal phase. It is apparent that the shorter wavelength peak decreases in intensity as the heating temperature is increased. Figure 8 (left) shows the ratio of the two emission peak intensities as a function of temperature, and Figure 8 (right) shows the shift in peak position as a function of temperature. As can be seen, a linear relationship exists between peak ratio and temperature as well E

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Figure 8. Ratio of fluorescence intensity (left) and wavelength shift (right) of Eu0.01Zr0.99(OH)4 heated for 10 and 1 s to various temperatures.

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as between peak position and temperature in the range between about 800 and 1300 K. At lower temperatures, the material is still amorphous and the peaks are not well characterized. At temperatures above about 1300 K, the curve flattens out. The changing intensity peak ratio with temperature is most likely due to the continued crystallization process of the tetragonal phase over the observed temperature range. The transition from an amorphous phase to a crystalline phase with higher symmetry leads to the reduction of energy level splittings. This process starts at the crystallization temperature and ends when all material has either been crystallized or when another more stable phase is formed.

(1) Cui, K.; Zhu, D. D.; Cui, W.; Lu, X. M.; Lu, Q. H. J. Phys. Chem. C 2012, 116, 6077. (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”; APS SCCM 2011, 2011, Chicago, IL. (3) Kose, M. E.; Carroll, B. F.; Schanze, K. S. Langmuir 2005, 21, 9121. (4) Myint, T.; Gunawidjaja, R.; Eilers, H. J. Phys. Chem. C 2012, 116, 1687. (5) Pugh-Thomas, D.; Walsh, B. M.; Gupta, M. C. Nanotechnology 2011, 22, 185503. (6) Gunawidjaja, R.; Myint, T.; Eilers, H. Chem. Phys. Lett. 2011, 515, 122. (7) Bihari, B.; Eilers, H.; Tissue, B. M. J. Lumin. 1997, 75, 1. (8) Debnath, R.; Nayak, A.; Ghosh, A. Chem. Phys. Lett. 2007, 444, 324. (9) Eilers, H.; Tissue, B. M. Chem. Phys. Lett. 1996, 251, 74. (10) Hou, X. R.; Zhou, S. M.; Li, Y. K.; Li, W. J. J. Alloys Compd. 2010, 494, 382. (11) Jia, G. H.; Wang, C. F.; Xu, S. Q. J. Phys. Chem. C 2010, 114, 17905. (12) Jia, M. L.; Zhang, J. H.; Lu, S. Z.; Sun, J. T.; Luo, Y. S.; Ren, X. G.; Song, H. W.; Wang, X. J. Chem. Phys. Lett. 2004, 384, 193. (13) Qu, X. S.; Yang, H. K.; Moon, B. K.; Choi, B. C.; Jeong, J. H.; Kim, K. H. J. Phys. Chem. C 2010, 114, 19891. (14) Shang, C. Y.; Jiang, H. B.; Shang, X. H.; Li, M. C.; Zhao, L. C. J. Phys. Chem. C 2011, 115, 2630. (15) Shang, C. Y.; Shang, X. H.; Qu, Y. Q.; Li, M. C. Chem. Phys. Lett. 2011, 501, 480. (16) Binnemans, K. Chem. Rev. 2009, 109, 4283. (17) Binnemans, K.; Gorllerwalrand, C. Chem. Phys. Lett. 1995, 245, 75. (18) Blasse, G. Chem. Phys. Lett. 1973, 20, 573. (19) Blasse, G.; Bril, A.; Nieuwpoort, W. C. J. Phys. Chem. Solids 1966, 27, 1587. (20) Myint, T.; Gunawidjaja, R.; Eilers, H. Appl. Phys. Lett. 2011, 98, 171906. (21) Reisfeld, R.; Zigansky, E.; Gaft, M. Mol. Phys. 2004, 102, 1319. (22) Speghini, A.; Bettinelli, M.; Riello, P.; Bucella, S.; Benedetti, A. J. Mater. Res. 2005, 20, 2780. (23) Gunawidjaja, R.; Myint, T.; Eilers, H. J. Solid State Chem. 2011, 184, 3280. (24) Meltzer, R. S.; Feofilov, S. P.; Tissue, B.; Yuan, H. B. Phys. Rev. B 1999, 60, 14012.

4. CONCLUSIONS The characterization of the spectroscopic properties of pyroprobe-heated Eu0.01Zr0.99(OH)4 nanoparticles shows that the material crystallizes for heating times as short as 1 s. When the material crystallizes, both tetragonal and monoclinic polymorphs are formed. The emission spectra of these two phases can be differentiated, as the transition energies in the tetragonal phase are shifted to higher energies by about 200 cm−1. Unlike for pyroprobe heated Eu0.02Y1.98(CO3)3 nanoparticles, the excitation spectra of the pyroprobe-heated Eu0.01Zr0.99(OH)4 samples do not provide a readily apparent temperature indication. The fluorescence lifetimes for the two polymorphs increase with increasing temperature and then decrease again after reaching a maximum. This effect can be explained by the changing surrounding of the nanoparticles due to their morphological development. The fluorescence lifetimes provide some indication for temperature over certain temperature ranges. However, the best temperature indicator appears to be the intensity ratio for two peaks and the peak shift observed in fluorescence at about 591.4 nm and about 592.0 nm. Lifetime measurements confirm that both of these peaks belong to the tetragonal phase. The intensity ratio as well as the peak shift shows a linear temperature dependence from about 800 to 1300 K.

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REFERENCES

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. F

dx.doi.org/10.1021/jp307092b | J. Phys. Chem. C XXXX, XXX, XXX−XXX