The Morphology Control And Luminescence Modulation Of Rare Earth Doped NaREF₄ Up-conversion Nanoparticles

Trivalent lanthanide ions(Yb³⁺ and Er³⁺/Ho³⁺/Tm³⁺)doped sodium rare earth fluoride（NaREF₄）upconversion nanoparticles（UCNPs）exhibit an anti-Stokes phenomenon with the process of absorbing two or more low energy photons and generating one single high energy photon.Due to their size of nano-dimension and color-tunable light emitting,they have been applied in medical and biotechnology fields.Most UCNPs are single crystals producted with epitaxial growth process.However,their small specific surface area and low luminous efficiency have restricted their in drug delivery and bioimaging fields.In this paper,hierarchical polycrystalline UCNPs and the modulation of up-conversion emission by photonic crystal have been designed and investigated.Through a core-shell approach,“sea chestnut” and “Jasminum sambac” hierarchical nanostructure UCNPs have been synthesized via thermal decomposition method.The amount of core precursor,reaction time and heating rate influence shape and size of core-shell UCNPs.The sizes range from 15-100 nm with morphologies of sphere,hexagonal plane,tetrahedron or flower-like aggregates.When the size of core is 6 nm,core/shell atomic ratio of 0.25,and the speed of heat rate of 10 oC/min,“sea chestnut” NaGdF4@NaGdF4:Yb³⁺,Tm³⁺ UCNPs aggregates has been formed.The selected area electron diffraction（SAED）pattern showed a ring-like pattern,indicating polycrystal and multi-layer structure.In addition,when the transition ion has been introduced in the core region,flower-like NaGdF4:Cr³⁺@NaGdF4:Yb³⁺,Tm³⁺ UCNPs have been synthesized and then discomposed to small particles and recrystallized into bigger “Jasminum Sambac Flower” structured multicrystals（100 nm）,within which every petal is a single crystal.The prepared “sea chestnut” and “Jasminum sambac” hierarchical polycrystal UCNPs have larger specific surface area,and it’s promising to exhibit great properties on drug delivery and bio-imaging fields.The emission intensity of up-conversion materials with nanoscale is not as high as that of bulk materials.In order to improve the luminous efficiency of UCNPs by strong photonic crystal effect,especially manipulating a certain wavelength of light.We have used CdS spheres with high refractive index as building blocks,to fabricate the CdS/UCNPs composite PCs in one step.We obervoused the greatly enhancement of the red emission intensity and color obviously changed from green to yellow,luminescent dynamics of UCNPs was strongly modified with lifetimes dramatically shortened to 172 μs,compared with the 292 μs of reference sample,and the decrease of lifetime became to be 41.1%.Meanwhile,the lifetime increased to 370 μs with a growth of 26.7%,when the peak of emission located at the centre of photonic crystal band gap.The results agree well with Fermi’s golden rule and manipulating the certain emission of up-conversion achived by photonic crystal effect.We have made use of the same method to synthesized polysryrene（PS）PS/UCNPs composites with low refractive index contrast,little or no obvious change of lifetime nethier alternative regulation can be observed,which strongly proves that the large enough refractive index contrast is necessary for PCs to have a strong ability to realize strong photonic crystal effect,thus affecting the emission intensity and lifetime of a certain wavelength of light significantly.