| Upconversion (UC) nanocrystals doped with trivalent rare-earth ions have recently attracted much attention due to their potential applications in biology, therapeutics, optical imaging probes, solid state lasers and photo-catalysis. However, weak emission intensity of UC nanocrystals still constitutes the main limitation for their practical applications due to very small absorption cross section of rare-earth ions caused by the parity forbidden character of 4f-4f transitions. Recently, various methods have been developed to increase the UC luminescence intensity, such as crystal surface coating, core/shell synthesis, photonic band effect and metal nanoparticle plasmons, etc. but most of these are not enough to enhance the UC intensity. Therefore, the UC emission enhancement of nanocrystals is still a challenging topic. By coupling of metal nanoparticle surface plasmons with photonic crystal effect, the emission properties of materials may be considerably improved.The Yb3+/Tm3+or Yb3+/Er3+co-doped NaYF4 nanoparticles were prepared via the hydrothermal method, and the UC emission nanoparticles deposited on the surface of photonic crystal films were investigated. The UC emission of nanoparticles deposited on PCs surface is obviously enhanced when UC emission wavelengths overlap with the photonic band gaps of selected photonic crystals, due to effective and selective Bragg reflection of photonic crystals. This enhancement of photonic crystals could be extended to other light-emitting materials. A promising application of the strategy could lead to a new generation of UC lighting devices.The inverse opal films decorated with NaYF4: Yb3+, Er3+nanoparticles were prepared, and their UC emission property was investigated. Because of the efficient reflection of the inverse opals surface, upconversion luminescence of NaYF4:Yb3+, Er3+nanoparticles has been enhanced drastically as the blue and red emission wavelengths of the NaYF4:Yb3+, Er3+anoparticles are in the range of the photonic band gaps. This result demonstrates that the inverse opals will be more efficient and selective reflection mirrors when UC emission wavelengths overlap with the photonic band gaps of selected inverse opals.The NaYF4:Yb3+, Er3+, Tm3+ nanocrystal/opal photonic crystal composites with white UC emission were prepared. The influence of photonic band gap on UC emissions was observed in composites. The UC emission enhancement or suppression of NaYF4:Yb3+, Er3+, Tm3+on the photonic crystal surface is attributed to the Bragg reflection of the photonic crystal, which depends strongly on the optical circuit of measurement. White UC emissions of NaYF4:Yb3+, Er3+, Tm3+ nanocrystal/opal photonic crystal composites may have potential applications, such as solid-state multicolor three-dimensional display and back light.The Yb3+/Er3+co-doped NaYF4 nanoparticles were prepared via the hydrothermal method, and the UC emission of Yb3+/Er3+co-doped NaYF4 nanoparticles deposited on the surface of the opal/Ag structure was investigated. It is found that the UC emission of NaYF4:Yb3+, Er3+was effectively improved on the basis of coupling surface plasmons of metal particles with photonic band gap effects. The selective enhancement of red or green UC emission of NaYF4:Yb3+, Er3+ nanoparticles on the opal/Ag hybrid substrates is contributed to coupling effect of surface plasmons of metal nanoparticles and Bragg reflection of photonic band gap. In addition, when the excitation light wavelength overlapped with photonic band gap, the largest enhancement factor of UC emission of NaYF4:Yb3+, Er3+nanoparticles on the opal/Ag hybrid substrate was obtained, which comes from the excitation enhancement effect. We believe that the present opal/Ag hybrid substrate could be used as a platform for UC emission enhancement of nanoparticles, which have great potential applications in biology, therapeutics, optical imaging probes and solid state lasers. |
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