Ke Li,Tingting Chen,Huibing Mao,Ye Chen,Jiqing Wang
The inverse opal ${\rm LaPO}_4 {:}{{\rm Eu}^{3 + }}$ was successfully fabricated by the sol-gel method, and characterized by the scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), and photoluminescence investigations. The inverse opal ${\rm LaPO}_4 {:}{{\rm Eu}^{3 + }}$ sample has a different emission in comparison with the ordinary luminescence emission bands of ${{\rm Eu}^{3 + }}$ ion at 590, 615, and 695 nm. In the inverse opal structure, the strongest emission is the transition ${^5{{\rm D}_0}{ \to {^7}{\rm F}_4} }$ at about 695 nm, which may be due to the fact that the emission in the plane decreases because of the photonic band structure and the corresponding emission in the vertical direction increases. For the emissions at 590 and 615 nm of the ${{\rm Eu}^{3 + }}$, the results show that the emission ratio of 590 to 615 nm increases from 0.266 to 0.639 for the inverse opal sample in comparison with the ordinary sample, which means that the ratio increases about 2.4 times for the inverse opal sample. An appropriate reason is that in the inverse opal sample the magnetic dipole emission ${^5{{\rm D}_0}{ \to }{^7{\rm F}_1} }$ will increase in comparison with the electric dipole emission ${^5{{\rm D}_0}{ \to }{^7{\rm F}_2} }$.
The behavior of nanoparticles in media of different refractive index has received considerable theoretical and experimental investigation. However, the behavior of larger particles, of micron size, has not been established. We have chosen Eu 3+ as a marker of environmental changes. Dilution of this ion into LaPO 4 gives a simple, fingerprint emission spectrum which has been studied via its luminescent lifetime, the R 2 (( 5 D 0 → 7 F 2 )/( 5 D 0 → 7 F 1 ) intensity ratio) and the electric dipole:magnetic dipole intensity ratio, (ED:MD), of spectral features. Measurements of micron-sized particles of LaPO 4 :Eu 3+ synthesized by several methods have been made for dispersions in alcohols, in compressed glass discs, in melted glass discs, in phosphor in glass (PiG) and in melted PiG. The experimental results are discussed and rationalized. The Eu 3+ 5 D 0 emission lifetime is reasonably constant in the bulk powders, powders dispersed in alcohols or compressed discs but is smaller in PiGs and melted glass discs. In the PiGs the structural integrity of LaPO 4 :Eu 3+ is maintained until they are melted at higher temperature. The change in lifetime, when varying the composition of the PiG, follows the Virtual Cavity Model. For dispersal of the particles in solvents there is a cut-off particle size below 1 μm where the electronic spectra of the moiety become insensitive to the environment and the properties are determined by refractive index. The remarkable change in R 2 ratio in the different systems considered – by a factor of nearly 600 % - and the corresponding decrease in 5 D 0 lifetime – by a factor of nearly 60 % - illustrates the versatility of the lanthanide ion.
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