Enhancement of rare-earth-material luminescence

The effect of metal nanoparticles (NPs) on the luminescence of materials has been attracting increasing attention.^1–6 Metal NPs can alter both the excitation field and the emission process of rare-earth ions, which mainly depend on the spectral overlap between the localized surface-plasmon resonant (LSPR) spectra of the NPs and the excitation and emission bands of these ions.^4,6

Figure 1.Scanning-electron-microscopy images of nanoparticles (NPs). (a) Silver (Ag) nanospheres. (b) Ag nanoprisms. (c) Ag nanocubes. (d) Gold (Au) nanoprisms.

We have used many kinds of NPs (see Figure 1) with different LSPR bands to modify the luminescence of europium thenoyltrifluoroacetonate—Eu(TTFA)₃—complexes. We chemically synthesized the NPs. Their resonant absorption bands center at different wavelengths (see Figure 2). Eu(TTFA)₃ has an absorption peak at a wavelength of 350nm and its emission is centered at 612nm. Figure 2 shows that the peaks in the LSPR spectra of silver (Ag)-nanosphere and nanoprism NPs overlap with both these excitation and emission wavelengths.

Figure 2.Resonant absorption bands of the four kinds of NPs in arbitrary units (a.u.).

Figure 3 shows the luminescence-enhancement factor at 612nm in the presence of different NPs. Upon addition of NPs, this factor increases to a maximum and then decreases. The maximum enhancement factor is approximately 5.5 and 2.6 for Ag nanoprisms and nanospheres, respectively. For Ag nanocubes and gold (Au) nanoprisms, the enhancement factor is very small (only 1.5 and 1.48, respectively). These results show that luminescence enhancement is indeed strongly affected by the spectral overlap between the LSPR and the excitation/emission bands. However, a small enhancement in luminescence is also obtained even if the overlap between the LSPR spectra and the emission/excitation bands of the samples is not good, as for Ag nanocubes and Au nanoprisms.

Figure 3.Enhancement factor of the luminescence at 612nm in the presence of different NPs.

We measured the variation in Eu(TTFA)₃ lifetimes at 612nm in the presence of each of these four types of NPs. The lifetime increased from 0.147 to 0.258ms upon increasing the concentration of Ag nanoprisms. This was driven by the good spectral overlap between the LSPR spectra and the emission band. For Ag nanospheres, Ag nanocubes, and Au nanoprisms, the lifetimes changed from 0.147 to 0.113, 0.136, and 0.135ms, respectively. These smaller changes in lifetime occur even though the spectral overlap is not so good.

The spontaneous-emission rate from an excited level |i> to a lower level |j> is Γ_ij∝ |M_ij| ²ρ(ν_ij), where M_ij is the matrix element relating the two energy levels and ρ_ij is the optical density, also known as the photon-mode density (PMD). It is determined by the local environment of the rare-earth ions.⁷ In the presence of nanoparticles, the PMD of the samples changes,⁸ which causes changes in the luminescence properties and lifetime of Eu³⁺. When there is a good overlap between the LSPR spectra and the emission band, the values of both M_ij and the PMD may be modified, so that greater changes in the luminescence properties and the lifetime of Eu³⁺ are achieved. Further research aimed at exploring the mechanism behind these effects is in progress.

The authors acknowledge support from the National Natural Science Foundation of China (grant 90923035) and the 973 program (grant 2006CB302904).