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            In this project, our specific goal was to synthesize, fabricate, and investigate the spectroscopic and laser properties of various trivalent rare earth-based nanoparticles embedded in polymeric plastic host 2-hydroxyethyl methacrylate (HEMA).

Nd³⁺:Y₂O₃ nanoparticles embedded in poly-hydroxyethyl methacrylate, often referred to as p-HEMA, was obtained. Aggregation of nanoparticles in the HEMA matrix was prevented through sonication of the HEMA-nanoparticle solution for an adequate amount of time prior to the polymerization process. Polymerization occurs before the nanoparticles have time to fall out of the solution, thereby forming a uniform distribution of nanoparticles in the polymer matrix. The Judd-Ofelt theoretical model was applied to the room temperature absorption intensities of rare earth ions such as Nd³⁺_­(4f ³) in HEMA, to determine the three phenomenological intensity parameters: W₂, W₄, and W₆. Values are used to determine the spectroscopic quality factor for Er³⁺ in HEMA and are compared to those for Nd³⁺ ions in crystalline hosts. The intensity parameters are subsequently used to determine the spectroscopic quality factor, radiative decay rates, branching ratios and radiative lifetimes from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds ^2S+1L_J of Er³⁺_­(4f¹¹) in HEMA. Radiative lifetimes of the excited states were calculated and found to be on the same order of magnitude of those obtained for the Nd³⁺:Y₂O₃ ceramic. However, measured lifetimes were found to be longer due to re-absorption mechanisms of the Nd³⁺ ions on the surface of the particles and potential energy transfer processes between the polymer and the nanoparticles. Our results indicate that the system is capable of storing energy which can lead to low power diode and waveguide applications.

            More recently we have focused on the influence of polymeric hosts on the optical properties of near infrared emitters.  Holmium has been demonstrated as an important near infrared emitter, yet there has been a lack of detailed analysis on the optical properties of Ho³⁺ in Y₂O₃. We report the first experimentally determined values intensity (Judd-Ofelt) parameters for the Ho³⁺:Y₂O₃ system. These parameters are important to characterizing the photonic applicability of rare earth based materials. In this work we show that embedding Ho³⁺:Y₂O₃ in polymers such as epoxy exhibit similar optical properties to the nanoparticles free of the polymer environment from the visible to near infrared. Additionally, fluorescence lifetimes, emission cross sections and quantum efficiencies for the various intermanifold transitions indicate that these materials have important photonic applications including the development of ceramics and biosensors where photostability plays an important role.

            We have also synthesized and characterized trivalent erbium-doped yttrium-oxide, Er³⁺:Y₂O­₃ nanocrystals. These particles were synthesized by the precipitation from a homogeneous solution. The SEM picture shows the particle size is approximately 200 nm. The room temperature optical absorption and emission spectra show that the trivalent erbium ions in Er³⁺:Y₂O­₃ nanocrystals possess sharp absorption lines and strong emission in near infrared region that are characteristic of Er³⁺:Y₂O­₃ grown as large single crystals. Also, a detailed spectroscopic analysis of the aggregates indicates that the material has optical properties similar to those reported for single crystals grown by a flame fusion method. Photostability is maintained in the nanocrystalline aggregates. The ability of these aggregates to attach to different surfaces by either chemical or physical means renders them useful in numerous technologies.

The support from the Petroleum Research Fund (PRF # 43862-B6) has been acknowledged in all papers and presentations.

Dhiraj K. Sardar, P.I., Grant Award No.: PRF # 43862-B6

Papers Published in Refereed Journals from this Award:

1.         Sardar, D. K., S. R. Chandrasekharan*, K. L. Nash*, and J. B. Gruber, “Optical Intensity Analyses of Er³⁺:YAlO_3,” Journal of Applied Physics, Vol. 104, 023102: 1-8 (2008).

2.         Gruber, J. B., D. K. Sardar, S. R. Chandrasekharan*, U. V. Valiev, N. I. Juraeva, G. W. Burdick, “Modeling Optical Spectra and Van Vleck Paramagnetism in Er³⁺:YAlO₃,” Journal of Applied Physics, Vol. 104, 023112: 1-13 (2009).

3.         Nash*, K. L., R. C. Dennis*, J. B. Gruber, and D. K. Sardar, “Intensity Analysis and Energy-Level Modeling of Nd³⁺:Y₂O₃ Nanocrystals in Polymeric Hosts,” Journal of Applied Physics, Journal of Applied Physics, Vol. 105, 033102: 1-6 (2009).

4.         Sardar, D. K., S. R. Chandrasekharan*, and J. B. Gruber, “Preparation and Spectroscopic Characterization of Nd³⁺:Y₂O₃ Nanocrystals Suspended in Poly-Methyl Methacrylate (PMMA)_,” Journal of Applied Physics, Vol. 105, 093105: 1-8 (2009).

5.         Sardar, D. K., K. L. Nash*, R. C. Dennis*, N. J. Ray*, and J. B. Gruber, “Absorption Intensities, Emission Cross Sections and Crystal Field Splitting of Selected Intermanifold Transitions of Ho³⁺ in Ho³⁺:Y₂O₃ Nanocrystals,” Journal of Applied Physics, (2009).

6.         Nash*, K. L., R. C. Dennis*, J. B. Gruber, and D. K. Sardar, “Intensity Analysis and Energy-Level Modeling of Nd³⁺:Y₂O₃ Nanocrystals in Polymeric Hosts,” Journal of Applied Physics, Journal of Applied Physics, Vol. 105, 033102: 1-6 (2009).

7.         Nash, K. L.*, R. C. Dennis*, N. J. Ray*, J. B. Gruber, and D. K. Sardar, “Absorption Intensities, Emission Cross Sections and Crystal Field Splitting of Selected Intermanifold Transitions of Ho³⁺ in Ho³⁺:Y₂O₃ Nanocrystals,” Journal of Applied Physics, Vol. 106, 063117: 1-8 (2009).

8.         Gruber, J. B., G. Burdick, U. Valiev, K. L. Nash*, S. Rakhimov, and D. K. Sardar, “Energy Levels and Symmetry Assignments for Stark Components of Ho³⁺(4f¹⁰)in Yttrium Gallium Garnet (Y₃Ga₅O₁₂),” J. Appl. Phys., 106, 113110: 1-12 (2009).

^*Student author; The ACS-PRF support has been acknowledged in all papers published