High-Quality Sodium Rare-Earth Fluoride Nanocrystals:  Controlled Synthesis and Optical Properties

Abstract

We report a general synthesis of high-quality cubic (alpha-phase) and hexagonal (beta-phase) NaREF4 (RE: Pr to Lu, Y) nanocrystals (nanopolyhedra, nanorods, nanoplates, and nanospheres) and NaYF(4):Yb,Er/Tm nanocrystals (nanopolyhedra and nanoplates) via the co-thermolysis of Na(CF3COO) and RE(CF3COO)3 in oleic acid/oleylamine/1-octadecene. By tuning the ratio of Na/RE, solvent composition, reaction temperature and time, we can manipulate phase, shape, and size of the nanocrystals. On the basis of its alpha --> beta phase transition behavior, along the rare-earth series, NaREF4 can be divided into three groups (I: Pr and Nd; II: Sm to Tb; III: Dy to Lu, Y). The whole controlled-synthesis mechanism can be explained from the point of view of free energy. Photoluminescent measurements indicate that the value of I610/I590 and the overall emission intensity of the NaEuF4 nanocrystals are highly correlative with the symmetries of the Eu3+ ions in both the lattice and the surface.

Multifunctional upconversion–nanoparticles–trismethylpyridylporphyrin–fullerene nanocomposite: a near-infrared light-triggered theranostic platform for imaging-guided photodynamic therapy

“…The synthesis of the core/shell UCNPs was enabled with a modified thermolysis method (Supplementary Scheme S1, step 2). 39,40 As given in Supplementary Figure S1a, the core and core/shell UCNPs were sphere-like, with high uniformity in size and morphology. After epitaxial growth of the nanoshells (Figure 2a), the mean particle size increased by~20 nm.…”

Synthesis of β-Phase NaYF4:Yb,Er Upconversion Nanocrystals and Nanorods by Hot-Injection of Small Particles of the α-Phase

“…During the last years, a large number of research papers was pub-lished on the synthesis of NaREF 4 nanocrystals (RE = rare-earth), since NaREF 4 and LiREF 4 are attractive host lattices for a variety of functional optical materials such as upconversion phosphors, [6][7][8][9][10][11][12][13][14] quantum-cutting phosphors, [15][16][17][18][19] laser materials, [20,21] and materials for solar cell applications [22][23][24][25][26]. A variety of synthetic methods has been described for producing solvent-dispersible sodium rare-earth tetrafluoride nanocrystals with controlled size, shape and capping ligand, including thermal decomposition, [7,[27][28][29][30][31][32] co-precipitation, [6,11,[33][34][35][36][37][38][39][40] or solvothermal methods [41][42][43][44][45][46][47]…”

“…Ostwald-ripening is also observed for particles of the cubic -phase, but if heating is continued for sufficiently long time small seeds of the hexagonal -phase nucleate in solution. After nucleation of these seeds, the -phase particles rapidly dissolve whereas the -phase seeds rapidly increase in size [27,29,[53][54][55]. The fast release of monomers during the rapid dissolution of the -phase results in a monomer concentration so high that the particles of the less soluble -phase grow under the condition of monomer supersaturation.…”

“…Nearly monodisperse -NaREF 4 nanocrystals are often synthesized by the thermal decomposition of the metal trifluoroacetates in hot solvent mixtures containing oleic acid [7,[27][28][29][30][31][32]. It has been reported, however, that even in this case very small NaYF 4 :Yb,Er nanocrystals of the cubic -phase are formed as intermediate and that the final -NaYF 4 :Yb,Er nanocrystals with the hexagonal phase grow from these particles at elevated temperatures [27].…”

Morphologically controlled synthesis of colloidal upconversion nanophosphors and their shape-directed self-assembly

“…However coprecipitation methods usually necessitate postsynthesis treatments to improve crystallinity of the products, and hydrothermal approaches typically involve long reaction times (ranging from a few hours up to several days) in pressurized reactors (e.g., autoclaves). Importantly, Yan et al pioneered the synthesis of lanthanide fluoride NCs via the thermal decomposition of metal trifluoroacetate precursors (31,32). Preparations of β-NaYF 4 -based UCNPs through decomposition of mixed trifluoroacetates (33,34) or through a two-step ripening process using the premade α-NaYF 4 NCs as precursors (35) have subsequently been reported.…”