Controlled Synthesis And Great Enhancement Of Upconversion Luminescence Of Rare-earth-doped Fluoride Micro/nanocrystals

Lanthanide-doped upconverting nanocrystals （UCNCs） have been developed as anew class of luminescent optical labels that have become promising and advantageousalternative to conventional fluorescent labels used in short-wavelength lasers, displaysand many biomedical applications, etc. Despite favorable physico-chemical properties,there are number of serious synthesis and spectrocscopic problems to be solved,such as the controlled growth and organization of UCNCs, optimization ofcomposition and structure, upconversion enhancement and multicolor tune usingcore-shell structure to make these NCs more sensitive than they currently are, aswell as finding novel upconversion hosts. Therefore, based on rare-earth dopedfluoride nano-materials, we performed systematic investigations on the growthmechanism, upconversion enhancement, multicolor tune with higher sensitivity, aswell as on applying designed UCNPs in biomedicine.We performed a systematic study on the synthetic mechanism of hexagonalNaYF₄:Yb³⁺/Tm³⁺(Er³⁺) micro-crystal using oleic acid as the ligand at relativelylow temperature （180℃）. The special dissolution-recrystallization transformationmechanism, governing both the intrinsic crystalline phase （cubic and hexagonalphase） and the growth regime, is responsible for the phase control of the finalproducts. The surface energy controlled by selective absorption of oleic acidresulted in anisotropic morphology evolution of the rare-earth doped NaYF₄micro-crystals. Importantly, the systematic study on the photoluminescence ofrare-earth doped hexagonal NaYF₄with different shapes shows that theupconversion （UC） properties of these products strongly depend on surface/volumeratio due to surface quenching.The rare-earth doped NaYF₄nanocrystals with well-controlled morphologies,sizes, compositions and core/shell structure were prepared by using the developedthermal decomposition method. The sizes （20-150nm） and strong blueupconversion emission were realized in NaYF₄: Yb³⁺/Pr³⁺nanocrystals by changingthe dopant concentration of Yb³⁺ions. Their growth and spectra-tunablemechanisms were explained in detail. Moreover, the homogeneous core/shellstructures with upconversion enhancement was developed in NaYF₄nanocrystalswith the hope that the surface, hence the quenching centers, would be separated inspace from the emission centers. Furthermore, we successfully demonstrated a novelmulticolor bioimaging for in vivo use by employing the as-synthesized core/shellstructured nanocrystals as probes. The animal experimentals demonstrated that ourdesigned core/shell structured nanocrystals can be successfully applied to autofluorescence-free multicolor imaging.Upconversion enhancement and multicolor output were realized bymanipulating nanoscale interactions between core and judiciously selected shells.The unique heterogeneous core-shell CaF₂:2Ho³⁺/20Yb³⁺@NaGdF₄nanocrystalwith small size （only17nm） and high efficiency have been designed for biomedicalimaging. The increase of39times at⁵40nm was obtained, which is attributed tothe prominent effect of the NaGdF₄shell on the ultrasmall core （only4nm）, bysignificantly suppressing the surface quenching. Additionally, the PL decay ofheterogeneous core-shell structured CaF₂:2Ho³⁺/20Yb³⁺@NaGdF₄NCs indicates alonger lifetime, which also demonstrates the role of isolating the ions located on thesurface of the core from interaction with solvent. Furthermore, compared with thehigh UC efficiency NCs (NaYF₄:2Ho³⁺/20Yb³⁺), our green-emitting heterogeneouscore-shell NCs exhibits even stronger fluorescence. Multicolor core-shellnanocrystals with high efficiency were produced by coating different shells on thesurface, and strong multicolor upconversion fluorescence is tuned based on energytransfer between the cores and varying shells. The multicolor tune mechanisms wereproposed in details.Single-band UC emissions were designed in new upconversion hosts. Singlered colors were obtained by adopting KMnF₃micro-crystals as host. It is noted thatonly single-band UC emissions can be produced from Yb³⁺/Er³⁺, Yb³⁺/Ho³⁺, andYb³⁺/Tm³⁺dopants in KMnF₃micro-crystals, respectively. Importantly, we observethat the single band feature is independent of dopant concentration and pump power.In addition, multicolor upconversion emission is tuned by adjusting Gd³⁺concentration in KMnF₃micro-crystals. Yb³⁺/Er³⁺/Gd³⁺doped KMnF₃micro-crystals have shown substantially change of R/G emission ratios because ofthe energy transfer between Er³⁺, Mn²⁺and Gd³⁺. The pure monoclinic Na₃ScF₆canbe also easily synthesized. More significantly, the small radius of Sc³⁺results in anew nanophosphor exhibiting efficient green UC emission in1%Er³⁺/2%Yb³⁺dopedNa₃ScF₆micro-crystals, which further complements the exploration ofrare-earth-based micro/nano-materials for a wide variety of applications includingdisplay, solar energy and bioimaging.