Back to Table of Contents

42249-B10
Fluorescence from Sol-Gel Materials Doped with Rare Earth Impurity Ions

Ann J. Silversmith, Hamilton College

Fluorescence Yield in Sol-gel Materials doped with Rare Earth Impurity Ions

Ann Silversmith, Hamilton College

Daniel Boye, Davidson College

The primary objective of this grant was to use a variety of optical techniques to investigate sol-gel glasses doped with optically active rare earth (RE) ions.  We have successfully developed several optical tests, supported by thermal measurements, which have increased our knowledge of mechanisms that influence fluorescence yield. Our new understanding has pointed us toward ways to reduce fluorescence loss effects and we intend to pursue these promising avenues in the final year of the grant.

We have continued to concentrate our studies on two main areas, both important for understanding fluorescence yield in silicate glasses synthesized with the sol-gel process. The first issue is the low fluorescence yield due to an abundance of hydroxyl and water-associated complexes that are effective energy loss channels for the excited RE ions through vibrational relaxation. Sol-gel glasses are generally characterized as an open network with interconnecting pores large enough to allow the passage of water molecules. We have studied the quenching of rare earth (RE) emission when water from the atmosphere enters sol-gel glasses after they have been annealed. We call this study Rehydration.

Our second line of inquiry focuses on interactions among RE ions that lead to fluorescence quenching. Such interactions are strong functions of inter-ion distance and have a serious impact to fluorescence yield when RE ions are clustered together. We have focused on glasses with aluminum co-doping because we (and others) have observed that Al enhances fluorescence intensity. We have learned that RE ions are only fully dispersed in the glasses when the Al:RE ratio is 10 or higher. This is in agreement with a recently published theoretical paper, but is in conflict with generally accepted ideas. We call this study Energy Transfer.

I. Rehydration

This year our group published a paper on the use of  Tb³⁺ to probe this effect. The intensity of the violet ⁵D₃ emission lines is extremely sensitive to the presence of hydroxyl complexes. Annealing the glass at high temperatures (>800°C) reduces the number of residual hydroxyl complexes and, consequently, the violet emission becomes stronger. However, upon exposure to ambient, humid air, the violet emission declines dramatically within a few hours.  We have measured material density as a function of annealing temperature, and find that our glass samples are considerably less dense than conventional silicate glass. Significant densification of the sol-gel samples occurs for T_ann>950C, but due to a wide distribution of pore sizes and internal stresses, such heating of the monolithic samples causes them to foam and crumble.

We have recently discovered that use of a drying control chemical additive allows the samples to survive significantly higher annealing temperatures. The high temperatures fully collapse the pore structure and produce a sol-gel glass with density equal to that of a melt glass.  Preliminary observations indicate no rehydration effects in samples made with this new synthesis protocol.  Our focus in the next year will be on exploring the potential of this modified synthesis.

II. Energy Transfer

The Tb³⁺ ion is also ideal for studying RE clustering because of a well-known energy transfer mechanism between a Tb³⁺ ion in the ⁵D₃ state and a neighboring Tb³⁺ ion in the ⁷F₆ ground state.  This cross relaxation is strongly dependent upon the distance between the two ions.  We have used emission spectra of Tb³⁺-doped samples to study the effect of aluminum co-doping by measuring the ratio of the peak emission intensities between the ⁵D₃ and ⁵D₄ levels.  Increasing aluminum concentration strongly enhances this ratio.  For many years, the accepted explanation for this enhancement has been that aluminum causes RE ions to be uniformly dispersed in the glass.  However, recent computer simulations by others have called this interpretation into question.  To examine this effect, we monitored fluorescence decay of the ⁵D₃ emission under pulsed laser excitation.  Our results indicate that aluminum co-doping does not enhance the violet fluorescence of Tb³⁺ in sol-gel glasses by dispersing the RE ions in the glass.  It is likely that aluminum co-doping results in sol-gel glasses with regions of clustered RE ions where fluorescence is largely quenched and regions of isolated ions that emit strongly. 

We have analyzed the time evolution of the ⁵D₃ emission and measured the effective local concentration of emitting Tb³⁺ centers in a series of samples with 2%Al co-doping and Tb³⁺ concentrations ranging from 0.02% to 2%. When the Al:Tb ratio is ³ 10, the effective concentrations are well matched to the actual concentrations. In other words, RE ions are fully dispersed in this regime. However, in samples with more Tb³⁺, the effective concentration near emitting ions is lower than the sample concentration. Only isolated Tb³⁺ ions emit light, and clusters of ions experience complete quenching. Further investigation will repeat these measurements for fully densified samples  (prepared with our new synthesis).

Back to top