1. The Effect of Annealing Conditions and Concentration on 5 D 3  7 F J Emission in Terbium-doped Sol-gel Glasses * Colleen Gillespie and Dan Boye, Davidson College, Davidson, NC Ann Silversmith, Hamilton College, Clinton, NY Abstract Sol-gel glasses have optical properties similar to those of traditional melt glasses, but are appealing because they can hold a higher concentration of dopants due to their lower processing temperatures. In silicate sol-gel doped with trivalent terbium, the intensity of fluorescence from the 5 D 3 level to the 7 F J (J = 0…6) ground state manifold levels is highly dependent on both terbium concentration and annealing conditions. 5 D 3 emission is observed in glasses annealed at 750  C and increases in intensity with increasing annealing time and with higher annealing temperature. The relative intensity of emission from the 5 D 3 state decreases with increasing Tb 3+ concentration. A cross-relaxation process involving two nearby Tb 3+ ions depopulates the 5 D 3 level and causes this concentration quenching. Project supported by a grant from the NSF-NRI program 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Energy (1000cm -1 ) 7F67F6 5 4 3 7F07F0 1 2 5D45D4 5D35D3 414nm 436nm 460nm 490nm 545nm 590nm 620nm 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 Energy (1000cm -1 ) 7F67F6 5 4 3 7F07F0 1 2 5D45D4 5D35D3 Conclusions Terbium Energy Levels Results: Annealing Conditions Theory: Concentration Dependence Results: Concentration Dependence  Dissolve 11.8 mg of terbium nitrate (Tb(NO 3 ) 3 *5H 2 O) and 20.3 mg of aluminum nitrate (Al(NO 3 ) 3 *9H 2 O) in 7.8 mL deionized water.  Add 20 μL concentrated nitric acid and 4.00 mL TMOS (tetramethylorthosilicate, 99%) to solution and stir for 10 minutes  Put sol into 4 tightly capped polystyrene disposable test tubes  Gel at room temperature for 72 hours  Ramp at 5ºC/hr to 60ºC, then sit for 48 hours  Ramp at 5ºC/hr to 90ºC, then sit for 48 hours  Ramp at 2ºC/hr to 110ºC, then sit for 48 hours  Let cool to room temperature This is a partial energy level diagram of trivalent terbium, with labeled transitions corresponding to observed fluorescence lines. The samples were excited with 240 nm excitation light. The 5 D 4  7 F 5 transition dominates the emission spectrum and produces a green color. A cross-relaxation process involving two Tb 3+ ions depopulates the 5 D 3 level and reduces 5 D 3 emission intensity. At higher terbium concentrations, the terbium atoms are closer together, so there is stronger cross-relaxation. Therefore, we predict that as terbium concentration increases, the intensity of the 5 D 3 peak will decrease. After the sample is taken out of the oven, it begins to reabsorb hydroxyl groups, and the 5 D 3 peak begins to shrink. Here is a close up of the 5 D 3 peaks at three consecutive times. This decay is enhanced by increasing the hydroxyl concentration by putting the sample in water for a few minutes. We determined that heating the samples to 900ºC with a 12 hour dwell gives optimal optical properties without damaging the sample. We confirmed that the 5 D 3 emission intensity decreases with increasing terbium concentration Now that we have determined the best protocol for processing these samples, we can use our knowledge to begin working with erbium-doped sol-gels. Motivation: Sol-gel glasses Sol-gel synthesis is a low temperature was to prepare optically transparent materials. Because the sol-gel is prepared at room temperature, it is easy to include metallic, organic, and inorganic additives. The optical properties of a sol-gel that has been heated to ~1000ºC are similar to those of traditional melt glasses, but since this temperature is below the melting point of the material, the sol-gels can hold more dopants than melt glasses. Sol-gels with rare earth ions as dopants are used in phosphors, solid-state lasers, and amplifiers. We can use the optical properties of these materials to characterize the interactions between the dopants and the surrounding material. Theory: Annealing conditions Hydroxyl groups are present in the sol-gel materials. They have an absorption band between 2000 and 4000 cm -1. This allows electrons in the 5 D 3 level to relax to the 5 D 4 level by losing energy to another hydroxyl group combined with a lattice vibrational mode. This means that there is virtually no fluorescence from the 5 D 3 level. However, the number of hydroxyl grounds in the material can be significantly reduced by annealing the samples at a high temperature. Consequently, the 5 D 3 emission becomes comparable to 5 D 4 emission. When the samples are taken to 750ºC, the 5 D 3 peak becomes detectable. Taking the samples to higher temperatures and for longer dwell times at those temperatures, the intensity of the 5 D 3 peak becomes stronger. However, if the samples are annealed at too high a temperature, they can start to crystallize or crack. Therefore, we wanted to find an ideal annealing temperature and time to have a high 5 D 3 emission without any damage to the sample. I heated the sample to 800, 900, and 1000ºC for various dwell times. I then plotted the ratio of the 5 D 3  7 F 4 transition to the 5 D 4  7 F 6 transition. At higher temperatures and for higher dwell times, the ratio increases. The sample shattered after being at 1000ºC for 4 hours. Picture of Samples Which sample looks bluer? So, which sample has the lower concentration? The sample on the left has a concentration of 0.5%, and the one on the right is 0.1%. The 0.5% sample has very little emission from the 5 D 3 level (blue light) while the 0.1% sample has a substantial amount of emission from this level, and therefore looks blue-ish. Plotting spectra of four different concentrations, it is clear that 5 D 3 emission decreases as terbium concentration increases. Exposure to Humidity Sol-gel Recipe