Study Of Fine-tuning Luminescence Properties Of Ho³⁺,Yb³⁺ Ions Highly Doped Core-multi-shell Upconversion Nanoparticles

Rare-earth doped upconvertion nanoparticles（UCNPs）have surpassed traditional quantum dots and organic dyes with tremendous advantages,including large anti-Stokes shift,narrow emission band,high physical and chemical stability,low toxicity,etc.,which guarantees a wide application from bioimaging,anti-counterfeiting to phototherapy.Among these UCNPs,sensitizer or activator highly doped ones,provided with unique upconvertion luminescence（UCL）properties such as exceptional brightness,single-band emission and lifetime tuning,have aroused great attention.Many effective strategies,principally comprises inert or active shell coating,high excitation power density,homogeneous doping and unit cell optimization,have emerged to enhance their UCL performance.Due to short electronic relaxation,high effective magnetic moment and attenuation,NaHoF₄ material is an ideal choice for CT imaging and MRI contrast agents.However,research on its UCL properties has been tripped by severe cross-relaxation process and Ho³⁺’s difficulty in tuning single band red emission in Na REF₄ host lattice.In this thesis,methods of finetuning Ho-based red UCL of UCNPs containing NaHoF₄ have been presented,and the complex energy mechanism underneath capricious UCL regulars is well uncovered.Major contents are as follows:First,by high temperature pyrolysis,various NaHoF₄:Yb³⁺core-only and NaHoF₄@Na Yb F₄（Ho@Yb）featured core-multi-shell UCNPs were synthesized using oleate chemicals.X-ray diffraction（XRD）,transmission electron microscopy（TEM）scans and mapping,X-ray photoelectron spectroscopy（XPS）,UCL spectrum,UCL lifetime,and pump power dependency were used to characterize the samples.CT and MR imaging,1,3-diphenylisobenzofuran（DPBF）absorbance curves,in vitro cytotoxicity of Chlorin e6（Ce6）modified UCNPs were examined.By using the method of simple structure alteration of core size and shell thickness,dominant emission shift from green to red was successfully achieved for highly doped NaHoF₄@Na Yb F₄ UCNPs.Meanwhile,high-quality red-upconversion luminescence with low G/R（green/red）ratio and high intensity from Ho³⁺ions were obtained.An innovative theory,serving as a revised version of traditional concentration quenching and size effect,was introduced and proved that a large core and thick shell would both enhance UCL intensity,but acted conversely in terms of G/R ratio.An extra active or inert shell would effectively increase the UCL intensity under low-power laser excitation and enable UCL color to vary from green to yellow to red.This study may broaden the application domain for NaHoF₄ material from MR imaging to photo-induced therapy.Second,featuring Ho@Yb structures,sub‐30 nm core-multi-shell UCNPs were synthesized with a small NaHoF₄ core and varied Gd³⁺/Yb³⁺coexisting shells.XRD,TEM scan,UCL spectrum and lifetime were adhibited for characterization.Compared with the work above,except for a smaller total size,tunable emission in color from red to yellow to green,and intensity from low to stronger than that of traditional UCNPs was achieved for～10nm NaHoF₄ core size by means of changing number of layers and Gd³⁺/Yb³⁺concentration ratios in different layers.Besides,simultaneously doping Ho³⁺into the shells would result in lowered UCL intensity and lifted G/R ratio.Surface energy loss and sensitizing energy supply,which can be modulated with inert shielding of Gd³⁺and sensitization of Yb³⁺,were proved to be the essential determinant.More UCL properties of these peculiar Ho@Yb UCNPs were uncovered and detailedly summarized,and the findings can help to expand the application scope of NaHoF₄ into photo-induced therapy.Third,Na Gd F₄:Yb³⁺,Ce³⁺,Ho³⁺,Zn²⁺UCNPs doped with varying concentrations of Zn²⁺were synthesized in one step by high temperature pyrolysis using oleate chemicals.XRD,TEM scan,XPS,ICP-MS（inductively coupled plasma-mass spectrometry）,UCL spectrum and UCL lifetimes were used to study the samples.It was found that as the concentration of Zn²⁺increased from 0 to 15 mol%,the UCL intensities of both red and green emissions first increased and then decreased,with the maximum value of Zn²⁺doping at 10 mol%.We concluded that Zn²⁺dopant and F^-vacancies accounted for the intensity changes.This research can be beneficial for the structural design and applications of RE-doped UCNPs,and also offers theoretical foundation for follow up study of Ho@Yb featured UCNPs.Fourth,by means of oleate route,Ho@Yb featured core-multi-shell UCNPs were synthesized with equivalent diameter of～18 nm and varied Zn²⁺concentration distributions.XRD,TEM,UCL spectrum,UCL lifetime are adopted for characterization.When Zn²⁺is doped with Ho³⁺or Yb³⁺alone,UCL reacted differently in pace with Zn²⁺concentration.After further shell coating,more variation of Zn²⁺and Yb³⁺concentration distributions were allowed,among which the structure with Zn²⁺internally located with Ho³⁺and Yb³⁺concentration high inside but low outside was most admiring,exhibiting strong and pure red UCL even better than traditional recipes.Besides,similar changes could be found with～10 nm NaHoF₄core as well.We conjectured factors below all greatly mattered in UCL tuning:the lowered crystal field symmetry around RE³⁺,the energy mechanism difference between red and green emission processes,the peculiarity of Ho@Yb structures and the energy loss through inside defects and surface quenchers.Introducing Zn²⁺into Ho@Yb featured core-multi-shell structures makes it more flexible and efficient to finetune Ho-based UCL.