Thermally Induced Luminescence Enhancement Of Small-Sized Upconversion Nanocrystals And Its Anticounterfeiting Applications

Lanthanide doped upconversion materials can absorb near-infrared photons and emit visible photons.This unique optical characteristic makes them have great application prospects in life science,photovoltaic,sensor,anticounterfeiting and other fields,which become a research hotspot in rare earth luminescent materials.Especially with the development of nano preparation technology,compared with organic dye molecules or semiconductor quantum dots,upconversion materials for biomedical applications have significant advantages,such as low toxicity,good stability,large detection depth,low radiation damage,high signal-to noise ratio and no fluorescence blinking.Lanthanide doped hexagonal(?)NaYF₄ nanocrystals are considered to be the most efficient upconversion system at present.Aiming at their applications in biological imaging,detection,disease treatment and so on,scholars at home and abroad have carried out a lot of excellent research works.However,the luminescent efficiency of upconversion nanocrystals is still low.Even if a certain thick inert-shell is coated on core nanocrystals,their luminescent efficiency is still 1-2 orders of magnitude lower than that of the corresponding bulk materials.The involved energy loss mechanism is still in dispute,so it is necessary to reexamine the energy transfer characteristic and loss path between the luminescent core and the surface quenching source,and then explore the improvement method of upconversion luminescence(UCL)efficiency from a new perspective.On the other hand,upconversion materials usually suffer from thermal quenching at high temperature,but we find that small-sized upconversion nanocrystals exhibit anomalous thermally induced UCL enhancement(30-150 ?).The key determinants of this anomalous UCL thermal behavior are still unclear.Further research is needed to understand physical mechanisms of the internal energy migration and the UCL more deeply.In addition,upconversion nanocrystals with multicolor emission are the current research hotspot for anticounterfeiting application.However,the existing identification methods are expensive and complex,which limits the commercial application of high-level upconversion anticounterfeiting materials.Therefore,it is necessary to develop novel upconversion anticounterfeiting materials with high-level security and convenient recognition method.In this thesis,the above problems are systematically studied,and the specific contents are as follows:?.Mechanism of thermally induced UCL enhancement in small-sized nanocrystalsTaking various-sized NaGdF₄:Yb/Ln and NaYF₄:Yb/Ln(Ln=Ho,Er,or Tm)nanocrystals as well as core/shell nanocrystals as research objects,we study the temperature dependent luminescence and lifetime in air as well as the UCL thermal behavior in Ar,Ar/H₂O,or Ar/D₂O atmosphere.The phenomenon of thermally induced UCL enhancement does exist in upconversion nanocrystals.It is size dependent,and the smaller the nanocrystal is,the more significant the enhancement is.Moreover,it is only exists in Ar/H₂O atmosphere and active-shell nanocrystals.On the contrary,thermal quenching is observed in Ar,or Ar/D2O atmosphere and inert-shell nanocrystals.In addition,the downconversion luminescence(DCL)lifetimes of Yb³⁺²F_5/2 in small-sized nanocrystals increase at elevated temperature,whereas that in large-sized microcrystals show temperature dependent decrease.The UCL lifetimes of activators in all samples decrease with increasing temperature.Based on above results,we propose that the key external factor of enhanced UCL is H₂O molecules in air,while the main internal factor is Yb³⁺ions in nanocrystals,and H₂O molecules mainly quench Yb³⁺²F_5/2 excited state energy.Due to the large surface/volume ratios of small-sized nanocrystals,they can absorb a large number of H₂O molecules.At room temperature,due to the Yb³⁺²F_5/2 deexcitation caused by H₂O molecules,the energy transfer from Yb³⁺to activator is reduced,and the UCL intensity is low.At high temperature,owing to the desorption of H₂O molecules,Yb³⁺transfers more energy to activator,and the UCL is enhanced.Therefore,thermally induced UCL enhancement is attributed to the gradually attenuated surface H₂O quenching effect,which can perfectly explain all above experimental phenomenon.?.Energy loss mechanism of upconversion core/shell nanocrystalsTaking NaGdF₄:Yb/Er@NaGdF₄ nanocrystals with various thick inert-shell(core,5.7 nm;shell,0-17.7 nm)as the research objects,we study the UCL intensities in various environments,both the DCL spectrum of Yb³⁺²F_5/2 and the absorption spectrum of water or toluene,the shell thickness dependent lifetimes and the lifetime equation.Investigation results show that surface quenching becomes subordinate at shell thickness of around 6nm,where the UCL is the strongest,while the 11nm thick inert-shell is needed to completely suppress the surface quenching effect.Besides,the absorption spectrum of water or toluene has partial overlap with the DCL spectrum of Yb³⁺²F_5/2.Moreover,the luminescence lifetimes tend to be saturated at 11 nm,and the derived lifetime equation can describe precisely the decay times as a function of shell thickness.From above results,we may conclude that surface quenching effect is essentially an overtone energy transfer that Yb³⁺²F_5/2 excited state energy is absorbed by the overtone band of surface molecular group vibrations.The overtone energy transfer occurs through the interaction between the rare earth ion dipole and the molecular group atom dipole,which is also proved by the lifetime equation,and the effective coupling distance is 0-11 nm.Based on the energy loss mechanism,we designed ultrathick inert-shell nanocrystals to achieve stronger UCL by increasing the effective luminescent volume or the optically active Ln³⁺(Yb/Er)concentration.?.Anticounterfeiting applicationsInert-shell nanocrystals(core,7nm;shell,3.5 nm)suffer from thermal quenching at high temperature,while active-shell nanocrystals(shell,NaGdF₄:20Yb%)exhibit temperature dependent UCL enhancement.Based on the opposite luminescent temperature dependence,we design various core/shell nanocrystal hybrids with temperature responsive multicolor emissions.Under 975 nm excitation,when increasing temperature from 30-150 ?,these hybrids exhibit multicolor emissions along various paths,such as from white to green/red/blue,blue to green,green to red,red to blue,etc.,and the color shift value is 0.11-0.23.Moreover,multicolor emissions of these hybrids can also be realized by the simple heating treatment,or by increasing the laser power and prolonging the irradiation time.The results indicate the great potential of these core/shell nanocrystal hybrids for anticounterfeiting applications with high-level security and convenient authentication methods.In addition,based on the opposite UCL thermal behavior between the large-sized microcrystals and the small-sized active-shell nanocrystals(active shell,Na Yb F₄),we also designed micro/nanocrystal hybrids.Upon 975 excitation,emission colors of these hybrids can change significantly along various paths(such as green?blue or red?green)at elevated temperature,and the color shift value is between 0.21 and 0.33.Importantly,due to the excellent photo thermal conversion ability of Na Yb F₄ shell,these hybrids exhibit obvious color change within 30 s under 975nm excitation at a low power density of 3.6 W/cm².The sensitivity of multicolor emission to temperature and laser power is further improved,indicating that the practicability of these high level anticounterfeiting materials is further enhanced.