Controlled Synthesis, Formation Mechanism, and Great Enhancement of Red Upconversion Luminescence of NaYF₄:Yb³⁺, Er³⁺ Nanocrystals/Submicroplates at Low Doping Level

Abstract

Strong red upconversion luminescence of rare-earth ions doped in nanocrystals is desirable for the biological/biomedical applications. In this paper, we describe the great enhancement of red upconversion emission (4F9/2 --> I15/2 transition of Er3+ ion) in NaYF4:Yb3+, Er3+ nanocrystals at low doping level, which is ascribed to the effectiveness of the multiphonon relaxation process due to the existence of citrate as a chelator and cross relaxation between Er3+ ions. The dissolution-recrystallization transformation, governing both the intrinsic crystalline phase (cubic and/or hexagonal phase) and the growth regime (thermodynamic vs kinetic), is responsible for the phase control of the NaYF4 crystals. The possible formation mechanism of the NaYF4 crystals and the role of trisodium citrate which acts as a chelating agent and shape modifier are discussed in detail. It is also found that the alpha --> beta phase transition is favored by the high molar ratio of fluoride to lanthanide and high hydrothermal temperature as well as long hydrothermal time.

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]…”

Europium-doped NaGd(WO4)2 nanophosphors: synthesis, luminescence and their coating with fluorescein for pH sensing

“…29 In our case, we selected for such purpose a Ln 3+-citrate complex formed in solution as a first step of the synthesis process, which has been shown to be successful for the synthesis of other lanthanide based nanostructures. [30][31][32] It is has been also well stablished that the formation of uniform particles by using this approach requires a rather precise reaction kinetic, which can be found through the proper adjustment of the reaction parameters. These include reagents concentrations, temperature, aging time and also the nature of solvents, whose properties (viscosity and dielectric constant) affect the diffusion process involved in the solid phase formation and the possible aggregation behavior of the initially formed particles.…”

“…Some additional bands in the 3100-2800 and the 1600-1400 cm -1 regions were also detected, which must be attributed to vibration modes of C-H and carboxylate groups, respectively, coming from citrate anions adsorbed on the nanoparticles surface. 30 These anions may act as capping agents controlling particle growth and might also contribute to the higher degree of dispersion of our nanoparticles due to their well-known dispersing ability.…”

Preparation of Upconversion Nanocrystals NaYF4:Yb/Tm with Hydro(solvo)thermal Methods

“…In contrast with citric acid, OA is considered as a more powerful capping reagent which can eliminate the surface defects of the nanocrystals efficiently, leading the nanocrystals to exhibiting a higher upconversion luminescence [31]. The carboxyl group of the citric acid molecule is a high energy vibration group with a high energy vibration mode, which can cause a quench of upconversion luminescence of the Ln 3+ through a multi-phonon relaxation process [32,33]. The reaction temperature in solvethermal method is higher than that of hydrothermal method.…”