Upconversion In Rare-Earth Ion-Doped NaYF 4 Crystals and Nanocolloids
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Upconversion In Rare-Earth Ion-Doped NaYF4 Crystals and Nanocolloids Darayas N. Patel, Lauren A. Hardy, Tabatha J. Smith, Eva S. Smith and Donald M. Wright III Oakwood University, Department of Mathematics & Computer Science, 7000 Adventist Blvd. Huntsville, AL 35896, USA. ABSTRACT Nano-colloids and nano-crystals doped with ions of rare-earth elements have recently attracted a lot of attention of scientific community. This attention is due to unique physical, chemical and optical properties attributed to nanometer size of the particles. They have great potential of being used in applications spanning from new types of lasers, especially blue and UV lasers, phosphorous display monitors, optical communications, and fluorescence imaging. In this paper we investigate the infrared-to-visible upconversion luminescence in bulk crystals doped with ytterbium and holmium co-doped and ytterbium and thulium co-doped NaYF4 upconversion phosphors. The phosphors were prepared by using simple co-precipitation synthetic method. The initially prepared phosphor has very weak upconversion fluorescence. The fluorescence significantly increased after the phosphor was annealed at a temperature of 600 0C. Nanocolloids of this phosphor were obtained using methanol as solvents and they were utilized as laser filling medium in photonic crystal fibers. Under 980 nm laser excitation very strong upconversion signals were obtained for ytterbium and holmium co-doped phosphor at 541 nm, 646 nm and 751 nm, and 376 nm, 476 nm, 646 nm, 696 nm and 803 nm for ytterbium and thulium co-doped phosphor. The particle sizes of the nanocolloids were analyzed using Atomic Force Microscope. The reported nanocolloids are good candidates for fluorescent biosensing applications and also as a new laser filling medium in fiber lasers. INTRODUCTION Nano-colloids and nano-crystals doped with ions of rare-earth elements have recently attracted a lot of attention from the scientific community. This attention is due to unique physical, chemical and optical properties attributed to nanometer size of the particles. They have great potential of being used in applications spanning from new types of lasers, especially blue and UV ones, phosphorous display monitors, optical communications, fluorescence imaging and novel approaches in vivo imaging, disease detection and diagnosis1-4. Compared to the conventional fluorophores like dyes, upconverting phosphors have many advantages as fluorescence labeling materials, such as low photobleaching, absence of autofluorescence, low back ground noise, high sensitivity for detection and feasibility of multiple labeling with different emissions under the same laser excitation. Upconversion is a process where low-energy light, usually near-infrared or infrared is converted to high energy light (UV or visible), through multiple photon absorptions or energy transfers. Upconversion emission of rare-earth ion doped nanocrystals has been observed in several different host materials. Up to now, hexagonal phase NaYF4 has been regarded as the most
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