Ce(3+)- and Eu(3+)-doped YPO(4) nanorods have been prepared at relatively low temperature (120 degrees C). A detailed investigation of the role of Ce(3+) concentration up to 10 atom % on the luminescence intensity of Eu(3+) in Ce(3+)- and Eu(3+)-doped YPO(4) has been carried out. Phase transformation from a tetragonal to a hexagonal structure occurs with increasing Ce(3+) concentrations, and water molecules are also associated during phase transformation. Thermal study shows that water can be retained up to 800 degrees C in the hexagonal structure. Interestingly, the hexagonal structure returns to the tetragonal structure on annealing above 900 degrees C. As-prepared and 500 degrees C heated samples show uniform sized nanorods, whereas a 900 degrees C heated sample shows distorted nanorods in which pores are present. Initially, the luminescence intensity decreases sharply with increasing Ce(3+) concentrations, even for 2 atom %. This is related to the enhanced nonradiative rate as compared to the radiative rate, since multiphonon relaxation to surrounding water molecules increases. This is not due to the possible oxidation-reduction process between Eu(3+) and Ce(3+) to give Eu(2+) and Ce(4+), as confirmed by X-ray photoelectron spectroscopy and luminescence studies. Then, a significant enhancement of luminescence intensity occurs on annealing above 900 degrees C. This can be ascribed to the loss of water molecules during a phase transformation from the hydrated hexagonal to the dehydrated tetragonal phase. To the authors' knowledge, we for the first time performed a luminescent study with a change of solvent from H(2)O to D(2)O, and significant enhancement in luminescence is found.
YPO(4) nanoparticles codoped with Eu(3+) (5 at. %) and Bi(3+) (2-10 at. %) have been synthesized by a simple coprecipitation method using a polyethylene glycol-glycerol mixture, which acts as capping agent. It has been found that the incorporation of Bi(3+) ions into the YPO(4):Eu(3+) lattice induces a phase transformation from tetragonal to hexagonal, and also a significant decrease in Eu(3+) luminescence intensity was observed. This is related to the association of the water molecules in the hexagonal phase of YPO(4) in which the nonradiative process from the surrounding water molecules around Eu(3+) is dominating over the radiative process. On annealing above 800 °C, luminescence intensity recovers due to significant removal of water. 900 °C annealed Bi(3+) codoped YPO(4):Eu(3+) shows enhanced luminescence (2-3 times) as compared to that of YPO(4):Eu(3+). When sample was prepared in D(2)O (instead of H(2)O), 4-fold enhancement in luminescence was observed, suggesting the extent of reduction of multiphonon relaxation in D(2)O. This study illustrates the stability of water molecules even at a very high temperature up to 800 °C in Eu(3+) and Bi(3+) codoped YPO(4) nanoparticles.
CaF 2 nanoparticles doped with 1 at. % Eu 3+ and codoped with Mn 2+ ions (1, 2, and 3 at. %) were synthesized at 180 °C by using ethylene glycol as a capping agent and solvent medium. The emission spectra of these nanomaterials show two main peaks at 590 and 612 nm after excitation at 240 or 394 nm. Enhancement in luminescence is found upon codoping of Mn 2+ . Parts of Eu 3+ ions are substituted into Ca 2+ sites (having a symmetry environment), which is reflected in the higher probability of magnetic dipole transition over that of electric dipole transition. The polymer film of polyvinyl alcohol incorporated with CaF 2 :1Eu shows the red emission. The prepared nanomaterials are highly water dispersible. The hybrid of CaF 2 :1Eu and Fe 3 O 4 shows the heating ability up to 42 °C under an AC magnetic field. This is suitable for cancer therapy through hyperthermia. A very high specific absorption rate (SAR) of 283 W/g is observed. This can be ascribed to the increased magnetocrystalline anisotropy and Brownian relaxation. Biocompatibility of hybrid nanoparticles up to 75% in HeLa cells is observed. Also, this hybrid shows the red emission, and thus, it will be useful in tracing of magnetic nanoparticles through in vivo and in vitro optical imaging applications. Interestingly, the new exciton band at 300 nm is found in such a hybrid material.
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