Controlled Synthesis Of Small Size NaGdF₄ Upconversion Nanocrystal And Great Enhancement Of Upconversion Luminescence

Rare earth（RE）-doped upconversion materials have gained considerable attentions because of their applications ranging from all solid state lasers, volumetric displays, high contrast bioimaging and infrared solar cells. Sodium gadolium fluoride（Na Gd F₄） is known not only to be one of the the most efficient upconversion materials, but also to have excellent paramagnetic properties because of the seven unpaired electrons contained in the trivalent gadolinium ion. As a result, RE-doped Na Gd F₄ nanocrystals hold promises for both optical and magnetic resonance（MR） dual modal bioimaging. However, the size of reported Na Gd F₄ nanocrystals are generally large（>20 nm）, which prevents them from clearance from the body, raising a significant concerns on their long-term toxicity. This work aims to prepare small size yet highly efficient Na Gd F₄ nanocrystals（<6 nm） using solution-based chemistry, empowering them to be efficiently cleared from the b ody through a renal route while still being bright enough for optical bioimaging.First, we used high temperature thermolysis method, involving the use of oleic acid（OA） as chelating agent, and octadecene（ODE） as high temperature solvent, to prepare RE-doped Na Gd F₄ nanocrystals. It is found that the nucleation and growth process of Na Gd F₄ nanocrystals in solution can be effectively controlled through variations of the content of doped-Yb, reaction temperature, and reaction time, to yield size-tunable RE-doped Na Gd F₄ nanocrystals. The x-ray diffraction（XRD） and transmission electron microscopy（TEM） have been performed to characterize the crystal phase, the size and morphology of the resulting nanocrystals. It is revealed that low reaction temperature, low Yb dopant concentration, and the appropriate reaction time, favors the growth of small size nanoparticles through limitation of the nanocrystal growth process. We are able to produce nanocrystal with size as low as just 3 nm through selecting appropriate reaction parameters. Moreover, multicolor upconversion luminescence undersingle wavelength excitation from small size nanoparticles can be achieved through variation of lanthanide activators such as Ho for green upconversion emission, and Tm for blue up conversion emission. Unfortunately, the small size nanocrsytals have exceptionally high surface area to volume ratio that results upconversion luminescence to be significantly quenched.To overcome this problem, we then construct acore/shell structure nanocrystals to suppress the surface quenching effect of the core nanocrystal. Through a seed mediated growth in solution, we are able to grow a thin NaGd F₄ layer on top of 3 nm sized Na Gd F₄:30%Yb/2%Er core nanocrystals, leading the overall size less than the critical size of 6 nm that entails particle to be efficien tly get cleared from the body.We show that the core/shell Na Gd F₄: 30%Yb /2% Er @Na Gd F₄ can be up to 1000 times more efficient than the corresponding core Na Gd F₄:30%Yb/2%Er nanocrystlas due to the effective suppression of surface-related quenching effects for the core. The observation of prolonged PL lifetime for the core/shell（ 68 ms） than for the core（29 ms） nanocrystals demonstrates the role of the inert shell layer for the protection of the core, substantiating that the core/shell structure indeed can effectively suppress surface-releated quenching effects. It is also demonstrated in this work that the core/shell strategy can also be extended to enhance upconversion luminescence from 3 nm Na Gd F₄ core nanocrystals doped with Yb/Ho or Yb/Tm. Pertinent luminescence enhancement mechanisms have been discussed.Despite achieving significant enhancement through the core/shell strutructure, the low and narrow absorption problem of rare earth ions due to the nature of 4f electrons still persists. The absorption cross section of rare earth ion can be about 2-3 orders of magnitude lower than that of the organic dye molecule, which thus tightly limits the brightness of upconversion from the core/shell nanocrystals. To overcome this, we build an organic-inorganic hybrid system of IR-783-sensitized NaGd F₄:Yb/Er @Na Gd F₄:Nd/Yb core/shell nanocrystals. The sulfate-containing IR-783 molecules are used to replace the oleic acid molecules on the surface of the nanocrystals through a ligand exchange protocol. Through optimization of the IR 783 dye concentration versus the nanoparticlethe,Nd concentration in the shell layer, and the Yb concentration in the shell layer, we are able to enhance the luminescence of the core/shell nanocrystals 46 times through the dye sensitization effect.