Ostwald Ripening And Surface Quenching Mechanism Of NaREF₄ Nanocrystals

Luminescent nanomaterials exhibit unique chemical and physical characteristics,thereby playing a pivotal role across numerous fields.Notably,rare earth doped luminescent materials,renowned for their high luminous efficiency,are increasingly indispensable in biomedicine,communication engineering,and other fields.However,the synthesis of luminescent materials is often hampered by Oswald ripening,an unavoidable thermodynamic process that typically results in irregular growth of the material morphology.Additionally,the characteristics of surface effect and volume effect also cause the fluorescence properties of nanomaterials to be easily affected and limited by surface defects and surrounding ligands.To address the exigencies of rapidly advancing science and technology,luminescent nanomaterials necessitate specific properties such as uniform morphology,suitable particle size,and strong fluorescence emission.In addition,the luminescent intensity of materials tends to diminish with decreasing particle size.Small particle size and strong luminescence are two contradictory requirements and difficult to achieve simultaneously.Consequently,enhancing the luminescence intensity of nanomaterials within a limited particle size range is a key scientific problem in this research.Systematic design of nanomaterials encompassing size,morphology,composition,structure,and surface properties offers a solution to the aforementioned challenges.On the one hand,meticulous control of the Oswald ripening process during material preparation facilitates precise adjustments to material shape and size.On the other hand,shell coating on the surface of nanomaterials represents an effective strategy to enhance their fluorescence properties.This study delves into the surface quenching mechanism of core-shell materials,optimizing the optical properties of nanomaterials through the modulation of shell thickness and the design of core-shell composition.Based on the above content,the excellent matrix material NaREF₄ was taken as the research object.By controlling the Ostwald ripening process and studying the surface quenching mechanism and optimum shell thickness in core-shell coating,the key technology to improve the optical properties of nanomaterials was systematically studied.The main contents are as follows:1.A series of NaYF₄ nanocrystals under varying reaction conditions were synthesized using an automatic nanosynthesizer（ANS-02）.The study established correlations between reaction time,morphology,crystal phase,particle size,and ion desorption and adsorption models during the reaction of nanocrystals at different growth stages.The occurrence and progression of Ostwald ripening during material synthesis were comprehensively explored.By investigating reactant concentration and reaction rate,the critical concentration and time windows for control of Ostwald ripening were defined,and proved to be the key to control Ostwald ripening process.Ostwald ripening was divided into four ripening types:Type Ⅰ（the same substrate with the same crystalline phases）,Type Ⅱ（the same substrate with the different crystalline phases）,Type Ⅲ（the different substrate with the same crystalline phases）,and Type Ⅳ（the different substrate with the different crystalline phases）.This classification has a good universality across different Ostwald ripening conditions.2.A method for controlling Ostwald ripening during material synthesis through precursor solution injection was proposed and validated.By adjusting the concentration of precursor solution or the amount of injection,the process of Ostwald ripening can be adjusted more carefully.Reaction temperature can also be used as a factor to regulate the Ostwald ripening process.Lowering the reaction temperature effectively suppressed or delayed the Ostwald ripening process.Based on the control of above parameters,pureβ-NaYF₄ nanocrystals with uniform morphology,excellent monodispersity,high crystallinity,and outstanding luminescence performance were synthesized across a broad size range.In addition,a methodology for preparing core-shell structured nanomaterials by controlling Ostwald ripening during synthesis was proposed and realized.Manipulation of shell precursor solution composition facilitated the synthesis of homogeneous/heterogeneous core-shell materials.The growth direction,morphology and shell thickness of core-shell materials were controlled by adjusting the precursor solution concentration,injection amount and injection times.This method can ensure the isotropic growth of the material.Notably,compared to the nuclear injection method,NaYF₄:Yb,Tm@NaYF₄ synthesized using this approach demonstrated greater flexibility in adjusting growth direction,transitioning from long rod shapes to uniform spherical shapes,and shows greater advantages in blue-purple luminescence.3.A theoretical model incorporating multiple factors was developed to elucidate the regularity of shell coating inhibiting surface quenching and the effect of shell thickness on the fluorescence properties in core-shell materials.By comprehensively analyzing the light transport process in core-shell materials,factors including energy transfer,surface quenching,light scattering of particles,light absorption of materials,and ion diffusion were considered.The theoretical results show that the light scattering of particles,light absorption and ion diffusion of materials can reduce the luminescence properties of materials,while modification of surface quenching via shell coating substantially enhances luminescence properties.The competition between these positive and negative factors theoretically results in an optimal shell thickness for the material to achieve the strongest luminescence.4.Five sets of NaYF₄:Yb,Tm@NaLuF₄ core-shell materials were precisely synthesized using ANS-02,and the spectral experimental data were consistent with the fitting results of theoretical model.The experimental results show that with the increasing of shell thickness,the UCL intensity of core-shell material first increases and then decreases,and the optimal shell thickness is about 3.0 nm.The fitting results show that the inert shell can effectively inhibit the fluorescence quenching caused by surface defects.Electron exchange energy transfer（DET）is the main physical mechanism that causes surface quenching.Surface quenching,light scattering of nanoparticles,and ion diffusion of luminescence centers into the shell emerged as principal factors influencing core-shell material luminescence properties.