The Modulation Of Upconversion In NaYF₄:Yb³⁺,Tm³⁺ Inverse Opal Structures

Photonic band gap（PBG）materials are three-dimensional（3D）periodic dielectric structures with the capability to control the propagation of electromagnetic radiation,which can be understood as similar to the modulation of electron propagation in crystalline structures.The noteworthy characteristics of the PBG are that optical electromagnetic state density attenuates in the center of PBG and grows on the edges of PBG.So the spontaneous emission of object ions can be enhanced or suppressed with respect to the relative location between emission band and PBG.Inverse opal photonic crystal structures has made a lot of progress in modulating spontaneous emission of embedded luminescent species,owing to the simple and low cost template method,and precise doped concentration controlling.Fluoride material and oxide material are the mainly two types of upconversion materials,wherein the NaYF₄ is a kind of well-known fluoride host with the highest upconversion efficiency,and lowest phonon energy,which is easily detectable for nanoscale samples,which has been subjected to researches’attention.Inverse opal photonic crystals（IOPCs）ofβ-NaYF₄:Yb³⁺,Tm³⁺with various photonic band gap（PBG）are prepared by the sol-gel method with polystyrene（PS）template technique.Then analysis and discussion the effect of the various photonic band gap on the upconversion luminescence properties.The paper mainly described in the following:（1）The monodisperse polystyrene microspheres were made by the emulsifier-free emulsion polymerization of styrene using potassium persulfate as the initiator.The synthesized spheres with diameters of 554 nm,633 nm and 651 nm were self-assembled on quartz substrates through the float controlled vertical deposition（FCVD）technique.Thereby the opal photonic crystals（OPCs）were obtained on the quartz substrates with various photonic band gaps.The NaYF₄:40%Yb³⁺,0.5%Tm³⁺precursor solution was prepared by sol-gel methods.The precursor solution is filled into the voids among the OPCs by spin-coating method.After heating at 500℃for 1h,the highly ordered replica consisting of NaYF₄:40%Yb³⁺,0.5%Tm³⁺frameworks with air holes are obtained by using opal templates constructed with polystyrene microspheres of 554 nm,633 nm and 651 nm in diameter:denoted as IOPC1,IOPC2 and IOPC3 respectively.Also,a reference sample（denoted as REF）was prepared by mixing three kinds of microspheres together in the same procedure.X-ray powder diffraction（XRD）data of IOPCs and REF sample are obtained on a Tongda TD-3500 automatic X-ray diffractometer system.The upconversion emission measurement of the inverse opals and reference sample are recorded on a Hitachi F-7000 fluorescence spectrometer under 980 nm excitation.The microstructures of the samples are observed by a S-4800 scanning electron microscopy.The transmittance spectra were obtained by a Shimadzu UV-3600 UV-VIS-NIR spectrophotometer.（2）The modification of upconversion emissions is observed in IOPCs,where the near ultraviolet and blue emissions of Tm³⁺ions are alternately suppressed or enhanced with regard to the location of the PBG.These are mainly attributed to the depression effect in the center and the enhancement effect on the edges of PBG,respectively.Also,the branch ratio modified by PBG,the excitation state absorption（ESA）and the cross-relaxation（CR）processes from the initial levels of suppressed transitions,may also play important roles in the change of upconversion emissions.