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RE3+ FLUORESCENCE ENHANCEMENT BY 3+ 3+ CHELATION OF Eu AND Tb IONS IN SOL GELS RE3+ A. J. Silversmitha A. P. Magyara, K.S. Brewera, and D.M. Boyeb aPhysics Department, Hamilton College, Clinton, NY 13323 USA bPhysics Department, Davidson College, Davidson, NC 28036 USA Sol-gel Glass versus Melt Glass Abstract Advantages • Easy to vary recipe •Versatile form • Monoliths of good size and clarity • Lower temperature processing •Higher concentration of RE’s Chelation of rare earth (RE) ions has been used for many years as a way of enhancing the optical excitation of the ions in solution. The chelating molecules, which absorb strongly in the near uv, bind to the RE ion. Optical excitation of the chelate followed by efficient energy transfer to the RE results in visible fluorescence. In this work we incorporated the chelate-RE complex into sol-gels made with the organic precursor tetramethoxysilane (TMOS). Two chelating agents - 2,6-pyridine-dicarboxylic acid (PDC) and 3pyridinepropionic acid (PPA) - and two different synthesis techniques are used. Optical properties of the dried gels (heated to 90˚C) and annealed SiO2 doped glasses (heated to 900˚C) were studied to determine firstly, whether the chelate/RE complex remained intact after incorporation into the gel and secondly, whether the optical properties of the annealed glasses differed from those of glasses synthesized without chelation. In addition to studying energy transfer between the chelate molecule and the RE, we investigated whether incorporation of the chelate reduced fluorescence quenching due to residual OH- in the glass – a common problem in RE doped sol-gel glasses. Pr,Nd,Er,Eu-doped sol-gel glasses: Sol-Gel Recipes Europium Results Eu(PDC) 5D 2 20 10 5 0 7F 2 7F 1 7F 0 585 595 Fluorescnece/absorbance (arb) 5D 0 15 x20 575 5D 1 Eu(PDC) xtal 605 615 625 560 Eu(PDC) gel excitation (monitoring 5D07F2) PDC absorption 7F 200 300 400 0 5 D J 500 Eu/Al gel 580 600 200 640 Eu(PPA) gel excitation (monitoring 5D07F2) 7F 300 400 0 5 D J 500 All syntheses form optically clear gels PDC gels crack and turn powdery after several weeks, but storage with a dessicant helps PDC gels dissintegrate when annealing PPA gels retain good optical clarity upon annealing 600 Terbium results wavelength(nm) 7F Fluorescence Decays 0 2 of chelated Eu3+ dry gels Tb3+ x103cm-1 Fluorescence Decays of Annealed glasses 25 0 5D 3 -3 t=0.18ms -4 Eu2O3 powder Eu(PDC) glass 15 Eu/Al glass 10 -5 7F 0.5 1 1.5 2 560 t [ms] 580 600 620 Tb(PPA)3 gel TbPDC)3 gel Tb(PDC) crystal 5 640 Wavelength (nm) 0 7F 5 7F 6 450 500 550 600 650 5D 7F Fluorescence 4 5 Excitation spectra (monitoring 5D47F5 ) Fluorescence (arb units) Discussion Strong Eu3+ excitation band that correlates with the PDC absorption indicates that the Eu(PDC) association remains complete – after incorporation into the gel. Long fluorescence lifetime of Eu(PDC) gel offers further evidence that the chelation is complete. Eu(PDC) samples degrade and are partially opaque after annealing. The fluorescence spectrum has a peak at 611nm, which coincides with the strongest 5D07F2 line in Eu2O3. Fluorescence decay time in Eu(PPA) gels is longer than in Eu/Al gels, indicating partial association of the chelate and Eu3+. The absence of the excitation band for wavelengths below 300nm implies little energy transfer from chelate to Eu3+. The bi-dentate PPA is short and may not be able to bond at two sites. The decay time from Eu(PPA)6 is longer than from Eu(PPA)3 and shorter than Eu(PDC). Further evidence chelation is incomplete in the PPA gels. Incomplete chelation with PPA may be due to the physical size of the molecule - the PPA (bidentate) is a relatively short molecule and may not be long enough to grab on to the RE in two places. The crystalline chelate synthesis ensures that the RE is completely associated; the in-situ technique is a “stirand-hope” approach. Tb/Al “standard” gel 0 Chelate absorption edge 0 Fluorescence (arb units) -2 20 5D 4 Energ y Eu/Al “standard” gel Eu(PPA)3 gel Eu(PPA)6 gel Eu(PDC)3 gel Eu(PDC) crystal Fluorescence (arb units) t=2.3ms -1 ln (fluorescence) 620 In Situ Chelate RE3+-Doped Sol Gel Synthesis dissolve 3-pyridinepropionic acid (PPA) in water add RE nitrate salt for RE3+:PPA 1:3 molar ratio reduce volume of solution by half with gentle boiling (20 minutes) cool solution and adjust pH to 4 using concentrated HCl add TMOS and stir until homogeneous Processing of Sols cast sols into polypropylene test tubes and cap tightly heat until gelled at 40 ˚Cage gels 60 ˚C for 24 h, then remove test tube caps heat at 60 ˚C for an additional 24 h increase temperature to 90 ˚C and age gels for 48 h anneal in air Sample quality wavelength(nm) 5D 600 “Standard” RE3+-Doped Sol Gel Synthesis dissolve europium nitrate and aluminum nitrate in water acidify solution with concentrated HCl add silica precursor, tetramethylorthosilicate (TMOS) mix until homogeneous Crystalline Chelate RE3+-Doped Sol Gel Synthesis suspend pyridinedicarboxylic acid (PDC) in water and heat to boiling add rare earth nitrate salt as a solution in water cool solution and adjust pH to 8 using 2 M sodium hydroxide crystallize the rare earth chelate by slow evaporation dissolve crystals in water adjust pH to 4 using concentrated HCl add TMOS and stir until homogeneous Decays 0 -0.5 Tb/Al gel Tb(PPA) gel ln (fluorescence) 5D 3 Fluorescence (arb units) 25 Eu(PPA) gel fluorescence lex=254nm Fluorescnece/absorbance (arb) x103cm-1 Energy Fluorescence (arb units) Eu3+ Eu(PPA) Eu(PDC) gel fluorescence lex=254nm Disadvantages •Fluorescence quenching due to: Lanthanide clustering OH- vibrations Tb(PDC)3 gel 4f8 4f75d1 -1 t=2.1ms -1.5 -2 -2.5 t=1.3ms t=0.83ms -3 -3.5 230 280 330 380 430 480 530 0 1 2 3 4 5 t [ms] Wavelength [nm] Experimental Setup Discussion Argon Laser Dye Laser aom Hg lamp Very bright green emission from Tb(PDC) dry gels under 254nm excitation Tb(PDC) complex remains intact in sol-gel Terbium behavior mirrors that of Europium, with the addition of the broad 4f8 4f75d1 excitation line in the chelated gels. Conclusions Ammeter Computer with DataLogger or Labview software PMT Monochromator PDC synthesis is effective at isolating the RE within the sol-gel. The synthesis results in enhanced excitation efficiency and reduced fluorescence quenching, resulting in intense red (Eu) or green (Tb) fluorescence under uv excitation. Oscilloscope In-situ synthesis with PPA does not result in fully chelated RE ions in gels. Corresponding author: Dr. Ann Silversmith Physics Department, Hamilton College 198 College Hill Rd. Clinton, NY 13323 [email protected] This work sponsored in part by the Research Corporation through a Cottrell College Science Award. Further Investigation Other chelating agents, in particular a longer bidentate to replace PPA in the in-situ synthesis. Adjustment of annealing conditions to improve quality of PDC annealed glasses. Fabrication of thin films with chelated RE’s. Synthesis with increased RE concentration.