Controllable Synthesis,spectral Tuning And Application Of Lithium-based Rare Earth Fluoride Core-shell Nanocrystals

Rare earth（RE）doped upconversion nanocrystals are a special class of luminescent nanomaterials that convert long-wavelength low-energy photons（e.g.near-infrared light）into short-wavelength high-energy photons（e.g.ultraviolet or visible light）to produce tunable upconversion emissions.Upconversion nanocrystals have large anti-Stokes shift,sharp emission bandwidth,longer excited-state lifetime,good photochemical stability and high resistance to optical blinking and photobleaching,showing great promise in diverse frontier application,such as security,multicolor display,optical storage,biological imaging and therapeutics.However,most of the upconversion materials only focus on the sodium rare earth fluoride（Na REF₄）host matrix,and the lithium rare earth fluoride（Li REF₄）with similar composition is rarely studied.In contrast to sodium,lithium-based sublattice not only provides comparable emission intensity,but also has many unique optical properties,such as more preferable ultraviolet-blue upconversion,obvious spectra Stark-splitting feature.Therefore,it is fundamentally significant to systematically study the controllable synthesis,spectral tuning and application of lithium-based core-shell nanocrystals for in-depth understanding the optical properties and the physical mechanism of upconversion emission.Optimizing various synthetic experiment parameters to obtain general synthetic strategy of lithium-based nanocrystals and introducing rational core-shell structure design to regulate interionic interactions,the efficient photon upconversion and down-shifting emission from a series of RE³⁺in lithium-based sublattice should be achieved.It is of great importance to broaden the upconversion host matrixes,enrich control means of optical performance,and explore more potential applications.In this thesis,lithium-based core-shell nanocrystals were selected as the main research object to conduct a systematic study in many aspects,including the controllable synthesis of lithium rare earth fluoride core-shell structure,visible multicolor regulation in migratory sublattice,optimization and enhancement of multiphoton ultraviolet（UV）upconversion emission in sensitizing sublattice,near-infrared（NIR）emission regulation,spectral splitting and optical performance evaluation.This thesis is divided into six chapters.In chapter 1,the synthesis and characterization of core-shell nanocrystals are briefly reviewed and the research progress of photon upconversion manipulation using core-shell structure are summarized.By outlining the research status and existing problems in the controllable synthesis and spectral regulation of lithium-based nanocrystals,the research subject of this thesis is proposed.The synthesis and characterization of the samples are presented in chapter 2.For chapters 3-5,the visible multicolor regulation,the optimization and enhancement of UV upconversion and NIR down-shifting emission of RE³⁺in the lithium-based sublattice core-shell structure are systematically studied.According to optical characteristics of lithium-based systems,the potential applications of lithium-based core-shell nanocrystals in multiple anti-counterfeiting,photocatalytic degradation and temperature sensing are also explored.The final chapter 6summarizes the whole works and proposes a prospect.The main achievements of this thesis are as follows:（1）The controllable synthesis of lithium migratory sublattice Li Gd F₄ core-shell nanostructures were realized by using functionalized（sensitive and emissive）seed-induced epitaxial growth strategy.By means of core-shell structure design,the Gd³⁺-mediated interfacial energy transfer（IET）pathway was constructed to realize both photon upconversion and down-shifting emissions from a series of RE³⁺ions(e.g.,Eu³⁺,Tb³⁺,Dy³⁺,Sm³⁺,Nd³⁺).Compared with the sodium-based counterparts,it was found that the lithium-based not only provides comparable emission intensity,but also exhibits higher IET energy transfer efficiency and lanthanides acceptor emission.In addition,the lithium-based sublattice have lower local crystal field structural symmetry leading to obvious spectra Stark-splitting feature.Finally,with the help of a simple multilayer core-shell structural design,the tri-channel photon emission of lanthanide emitters under dual-excitations（980/254 nm）was further realized by effectively integrating upconversion and down-shifting emission in a single nanoparticle.Combined with screen printing and time-gating technique,the multiple（patterns,excitation wavelengths,lifetime,emission colors and intensity）anti-counterfeiting identification was further realized.（2）A novel core-shell mechanistic strategy was successfully proposed to enhance the multiphoton UV upconversion emission of RE³⁺(e.g.,Tm³⁺,Er³⁺,Gd³⁺)by selectively controlling the interionic interactions in a modified core-shell nanostructure through spatial confinement of lanthanide emitters inside a lithium sensitizing sublattice（Li Yb F₄）.This design ensures the entire surrounding of emitters located in the inner region by the sensitizers,which is able to increase the localized excitation energy density and further accelerate the population of intermediate energy levels as well as the higher-lying UV emitting level by sequential energy transfer upconversion processes,thus to realize efficient UV upconversion.980/808 nm dual-wavelength responsive UV upconversion emission has also been enabled by incorporating Nd³⁺into inert shells.More importantly,the UV upconversion is easily available upon excitation by a commercial 940 nm NIR LED,which can be used to expand the spectral response towards improved solar photocatalysis reaction activity through fabricating a nanocomposite photocatalyst combined with g-C₃N₄ nanosheets with UV absorption properties.（3）The NIR emission properties of a series of RE³⁺(e.g.,Er³⁺,Tm³⁺,Ho³⁺,Pr³⁺,Nd³⁺,Yb³⁺)in lithium-based core-shell structure were systematically investigated.The concentration dependence on NIR emission in Er³⁺activated system was mainly analysed in detail to obtain efficient NIR-II luminescence.Furthermore,by introducing the cross-relaxation process between Ce³⁺-Er³⁺and controlling the surface quenching effect,the NIR-II emission of Er³⁺was further regulated and enhanced.In addition,980/808 nm dual-wavelength responsive efficient NIR-II emission at the single-particle level has also been enabled through rational core-shell structure design.In constrast to the sodium-based counterparts,lithium-based provides more efficient down-shifting NIR emission and shows obvious spectra Stark-splitting feature.Combining with low-temperature high-resolution NIR emission spectra and temperature-dependent emission spectra,the Stark-splitting of Er³⁺NIR-II emission and the assignment of intricate energy sub-levels were studied in detail.The NIR-II splitting emission peak with thermal-coupling properties was founded and further to analysis its temperature sensing performance by adopting luminescence intensity ratio（LIR）technique.Finally,NIR-II temperature probe with the maximum relative sensitivity of 0.248%K^-1 was designed,and it shows excellent thermal crycling stability.