Synthesis, Characterization, Mechanism And Surface Dynamics Of Rare-Earth Doped Upconverting Nanomaterials

Rare-earth-doped luminescent materials have wide applications, including phosphors, display monitor, X-ray imaging, scintillators, lasers, and amplifies for fiber-optic communications. An upconverting phosphor is one which takes multiple photons of lower energy and converts them to one photon of higher energy. Rare earth doped upconversion luminescence nanocrystals used as fluorescent labels has drawn great attention due to its low background light, nonfading, and no significant influence of environment under near infrared radiation (NIR). Another attractive advantage of these materials is that it can be effectively excited with NIR light, which is outside the luminescent absorption spectra of biomolecules, thus minimizing the loss of excitation energy to the surrounding materials compared with excitation with UV light. Recently,rare earth-doped up-converting phosphors have been reported to be used for the detection of nucleic acid and immunoassay. However, applications in biotechnology require size-controlled, monodisperse, bright nanoparticles that can be specifically conjugated to biological macromolecules, which asks for improvement of the novel synthetic routes. Upconverting transfer efficiency lies on phonon energy. Phonons are attice vibrations in a material that can provide nonradiative decay pathways to suppress upconversion luminescence. To overcome the phonon decay problem it is necessary to choose a lattice that has much lower phonon energy. To obtain the nanocrystal with small size and good luminescence, it is necessary to find new synthesis route. The understanding of the luminescence mechanisms is important because of synthesis. In particular, the quenching mechanism of the luminescence in nanosize is significant. In this paper, we synthesize different upconverting nanomaterials by different methods, and characterized the properties. The quenching mechanisms of luminescence and surface dynamics were investigated.(1) Synthesis and optical properties of YVO₄: Er³⁺(Yb³⁺, Er³⁺) nanocrystals. Strong visible emissions of Er³⁺ resulting from two-photon absorption by excitation 980 nm wavelength and energy transfer from the host YVO₄ by UV (λ= 280 nm) excitation were observed in nanocrystalline Er³⁺-doped YVO₄, which was prepared by a hydrothermal method using citrate/yttrium/vanadate complex as precursor. The highly crystalline YVO₄: Er³⁺ nanoparticles with an average diameter of 30 nm have well dispersion. From the PL and FT-IR spectra, it is concluded that the multiphonon relaxations, which originate from the organic group on the surface of the nanoparticles, quench the upconversion luminvescence. Upconversion emission of different nanocrystalline YVO₄:Er³⁺, Yb³⁺, synthesized by a hydrothermal process at low temperature, is studied under excitation of 980 nm where green [(²H_11/2, ⁴S_3/2)→⁴I_15/2] and red [⁴F_9/2→⁴I_15/2] emissions demonstrate sensitivity on Er³⁺ local environments. Small particles size, high Yb³⁺ concentration or high temperature favors the emission of the ²H_11/2→⁴I_15/2 transition. Both XRD patterns and Raman spectra have confirmed that distortion of crystal lattice of YVO₄:Er³⁺, Yb³⁺ nanocrystals is more serious when the nanoparticle size is getting smaller or Yb³⁺ concentration is getting higher.(2) Morphology impact on the upconverted luminescence of ZnO:Er³⁺ nanocrystals was studied with controllable morphology of nanorod, prickly spherelike, columnlike, branch rod, prism-, and grainlike, prepared via the cetyltrimethylammonium bromide (CTAB) assisted hydrothermal process. Under UV direct excitation, where exciton and defect emissions of ZnO appear, morphology sensitivity is discussed in terms of surface-to-volume ratios. The upconversion emission of Er³⁺ with 980 nm excitation demonstrates morphology sensitivity which is related with the local environments of Er³⁺ in ZnO and doping efficiency.(3) NaYF4:Yb, Er nanocrystals with controllable size and morphology are synthesized using different ligands by hydrothermal process. The spherical nanoparticles have uniform size, well dispersion and strong upconversion luminescence, which can be used in the bioapplication. Ligand effect on nanocrystal formation is investigated through size and morphology controllable synthesis of cubic and hexagonal Yb³⁺, Er³⁺ codoped NaYF4 nanocrystals, employing various ligands, such as EDTA and citrate. Spherical particle size was found determined by the nucleation rate, which depends on reactant concentration, molar ratio and ligand. The phase transformation from cubic to hexagonal is sensitive to reaction time and reactant concentration. The morphology dependent upconversion photoluminescence of the nanocrystals is used to characterize the crystalline quality and structure of the nanoparticles.(4)Well-dispersed and uniformly distributed nanocrystalline NaYF4 (YVO₄):Yb,Er/NaYF₄(YVO₄) and NaYF4(YVO₄):Yb,Er/SiO₂ core/shell structures were synthesized directly by reverse microemulsion method via hydrothermal process. For the YVO₄:Yb,Er Enhancement of upconversion luminescence was only observed in homogeneous coated nanocrystals, owing to the reduction of the luminescence quenching centers on the surface of the nanocrystals. The ratio of the green to the red emissions was enlarged for homogeneous and inhomogeneous coating, indicating that the coating leads to a less efficient multiphonon relaxation possibly due to separation effect. The underline mechanism was discussed with the help of luminescence lifetime results.