42249-B10
Fluorescence from Sol-Gel Materials Doped with Rare Earth Impurity Ions
Fluorescence Yield in Sol-gel Materials doped with Rare Earth Impurity Ions
Ann Silversmith, Hamilton College
Daniel Boye, Davidson College
In this grant we have sought to develop optical techniques to investigate sol-gel glasses doped with optically active rare earth (RE) ions. In previous reports we described several optical tests, supported by thermal measurements, that have greatly increased our understanding of mechanisms that influence fluorescence yield. This year we have focused on ways to reduce fluorescence loss effects by varying the chemical synthesis methods. In particular, we have incorporated a Drying Control Chemical Additive (DCCA) into the precursor solution for the synthesis sol-gel derived silicate glasses. The modification allows such glasses to be treated at significantly higher annealing temperatures without degradation of optical quality. Fluorescence yield improves markedly compared to sol-gel glass made by a traditional method and the denser glasses are not subject to rehydration.
A year ago we reported on studies of rehydration – an effect whereby water from the atmosphere enters sol-gel glasses after they have been annealed through an open network with interconnecting pores. We attributed the difference in behavior to a full densification of the annealed gel that is not possible with the traditional preparation method. We presented this work as a lecture at conference in July 2008 (International Conference on Luminescence) and have submitted an article to the Journal of Luminescence. Other studies came to fruition this year and two papers were published, based on posters that were presented at the Dynamics Processes Conference in 2007. Both publications dealt with the distribution of RE ions in glasses.
This year we have added the drying agent DMF (N,N dimethylformamide) to the sol gel reaction to modify the distribution of water in the formed glass. Water is produced during the condensation reaction of the sol-gel process. Because of its greater affinity to the reaction site than water, a DCCA facilitates drying by pushing the water molecule away from the reaction site and into the pore network where the water can more easily be removed. The expulsion of water from the glass network results in RE impurity ions better isolated from the OH stretches that cause quenching. In addition to creating glass with less residual water, the addition of drying agents also produces glasses more resistant to optical deterioration during annealing. The DCCA reduces surface tension between the liquid and the gel matrix. As a consequence, the average pore size is increased and there is a narrower pore size distribution. These properties permit the glass to densify without the large internal strains that results in "foaming" in glasses formed without drying agents.
For samples made with the traditional preparation, density increases with annealing temperature and the glasses remain optically clear up through 950°C. Above this temperature, the materials continue to densify, but samples foam and become opaque. Samples made with the addition of DMF have lower density than standard samples for a given annealing temperature. However, these materials remain crack free and optically clear at much higher annealing temperatures. Most importantly, the samples with DMF can achieve the density of melt glass, and thus, enable the production of dense silicate glass that are immune to water diffusion but still possess elevated impurity doping concentrations not possible through melt glass fabrication.
Emission spectra from RE dopants are strongly affected by the higher annealing temperature. The emission spectra from DMF samples - containing 0.02%Tb and 0.2%Al show highly enhanced of 5D3 emission. The increased intensity of 5D3 emission is attributed to the more complete removal of OH- for the DMF glass. It is important to note that the enhancement of emission intensity due to DMF is in addition to the enhancement due to Al co-doping (discussed in earlier reports). Al co-doping only affects those Tb ions located in Al rich regions, whereas the DMF can affect all impurity ions throughout the glass. We observed the largest 5D3/5D4 intensity ratio in glasses containing DMF indicating that these samples exhibit the least amount of quenching of any glasses we have fabricated.
Full densification of the DMF samples stabilizes the stronger 5D3 emission obtained by annealing at high temperature. In a study of 5D3/5D4 relative intensity as a function of elapsed time after annealing, we see that a 24hr annealing time produces a 5D3/5D4 relative intensity that remains constant; the material exhibits no rehydration. This fully densified glass has excellent optical clarity and continues to show the same emission spectrum months after these experiments were performed. Much of the early spectroscopy on RE-doped sol-gel systems used annealing temperatures ~900°C and reported very low emission levels. It is now clear that glasses are not stable in behavior unless they have been fully densified by annealing at higher temperature and addition of DMF makes such annealing possible while maintaining high optical quality.
