Temperature Dependent Fluorescence of Nanocrystalline Ce-doped Garnets For Use as Thermographic Phosphors
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Temperature Dependent Fluorescence of Nanocrystalline Ce-doped Garnets For Use as Thermographic Phosphors Rachael Hansel1, Steve Allison2, and Greg Walker3 1 Interdisciplinary Graduate Program in Materials Science, Vanderbilt University, Nashville, TN, 37212 2 Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6054 3 Department of Mechanical Engineering, Vanderbilt University, Nashville, TN, 37212 ABSTRACT Four samples of (Y1-xCex)3(Al 1-y Gay)5O12 (where x=0.01, 0.02 and y=0, 0.5) were synthesized via the simple, efficient combustion synthesis method in order to determine the effect of substituting Ga3+ for Al3+ on the temperature-dependent fluorescent lifetime. X-ray diffraction shows that the Ga-doped samples have longer lattice constants and transmission electron microscopy data show that each sample consists of nanocrystallites which have agglomerated in micron-sized particles. Photoluminescence data reveal that the addition of gallium into the YAG:Ce matrix induces a red shift in the absorption spectra and a blue-shift in the emission spectra. The laserinduced fluorescent lifetime was determined as a function of temperature over the range of 0-125°C using two different emission filters. Increasing the amount of dopant ultimately results in a decrease of the fluorescent lifetime. The quenching temperatures for the Ga-doped samples were lower than the samples without gallium. The results of this work show that combustion synthesis is viable method for making highly luminescent, nanocrystalline TGPs. In addition, these results show that the quenching temperature of YAG:Ce can be altered by substituting ions which alter the location of the charge transfer state and by changing the morphology of the sample. INTRODUCTION Cerium-doped yttrium aluminum garnet (Y3Al5O12:Ce, YAG:Ce) is used in several applications such as solid state lighting, displays, and scintillators [18], [3], [16]. The Ce3+ ion is responsible for nanosecond decay time and an intense visible emission. Cerium-doped garnets are also being considered for use as a thermographic phosphor. Thermographic phosphors are a special class of materials commonly used as non-contact thermometers because the fluorescent decay lifetime is temperature dependent [3]. The luminescent properties of doped nanocrsytals are attracting great interest now because the fluorescent characteristics of nanocrsystalline phosphor materials change with particle size. For example, the decay time of bulk YAG:Ce particles is temperature dependent between 150-300°C. However, Allison et al. [4] showed that nanocrystalline YAG:Ce exhibits temperature dependency between 7-77°C. The shift in temperature dependency is observed as a function of size and has also been observed in Y2O3:Eu nanocrystals. The size effect is likely due to increasing lattice distortions in smaller particles [12]. Similarly, the quenching temperature of bulk yttrium aluminum garnet, where Ga3+ is substituted for Al3+ and Ce3+ is substituted for Y3+ (YAGG:Ce), is approximately 100°C and shows microsecon
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