Preparation and Luminescence Properties of Rare-Earth-Ion-Doped Upconversion Phosphors

Preparation and Luminescence Properties of Rare-Earth-Ion-Doped Upconversion Phosphors
　　Seeking novel efficient upconversion luminescence materials (especially white light upconversion materials) and preparing upconversion nanocrystals with spherical morphology, favorable dispersity, high upconversion efficiency and good water-solubility are the hots of the upconversion luminescence researches. Both of them are investigated in this work.1 Er3+ doped and Er3+/Yb3+ codoped M’ type YTaO4 upconversion phosphors are prepared and the upconversion luminescence properties under 980 nm excitation are investigated in detail. It is found that with increasing the Yb3+ concentration, the intensity of green emission increases and then decreases, while Neodymium magnets those of the red and infrared emissions increase and decrease alternately. We propose that the variation of Yb3+ concentration can change the upconversion mechanism. When the Yb3+ concentration is lower than 12 mol% the two-step energy transfer from Yb3+ to Er3+ is responsible for the upconversion luminescence; when the Yb3+ concentration is equal to 12 mol% the cooperating sensitization is dominated; when the Yb3+ concentration is larger than 12 mol% the energy-back-transfer from Er3+ to Yb3+ takes place.
　　The research results of brightness and color purity indicate that YTaO4:1%Er3+/12%Yb3+ is a favorable green emitting upconversion phosphor.In additinon, according to the upconversion spectra of YTaO4:Er3+/Yb3+, GdTaO4:Er3+/Yb3+ and LaTaO4:Er3+/Yb3+, it is found that under 980 nm LD excitation, YTaO4:Er3+/Yb3+ and GdTaO4:Er3+/Yb3+ show strong green upconversion emissions, but LaTaO4:Er3+/Yb3+ presents different phenomenon, viz. weaker upconversion luminescence intensity, lower Yb3+ quenching concentration and blue shift of the strongest green emission peak.2 An efficient blue emitting upconversion phosphor based on Yb3+/Tm3+ codoped oxyfluoride is prepared via heating the multi-component fluorides at atmosphere. Under the excitation of a single 980 nm diode laser with the power density of 5.56 W/cm2, the upconversion luminescence brightness can reaches to 13053 mcd/m2, which is much stronger than that of commercial Y2O2S:Yb3+,Tm3+ phosphor (8194 mcd/m2). The research of upconversion mechanism indicates that cooperating sensitization dominates the emissions at 479 and 645 nm, while the emission at 454 nm results from the combination of cooperating sensitization and phonon-assisted energy transfer from Yb3+ to Tm3+.Additionally, the bight white upconversion luminescence is obtained by tri-doping of Yb3+, Tm3+ and Er3+.
　　Because the blue (main emission at 479 nm), green and red emissions are two-photon process, the white light is not sensitive to the pumping power. When the power density increases from 2.46 W/cm2 to 6.78 W/cm2, the color coordinate just varies from (0.24,0.32) to (0.23,0.30), and when the pumping power reaches to 1W, the phosphor keeps emitting the white light.3 The white emitting upconversion nanocrystal based on Yb3+/Tm3+/Er3+ tri-doped KY3F10 is designed and synthesized by using the hydrothermal procedure. Under a single 980 nm LD excitation, the nanocrystal show not only high upconversion efficiency but also excellent color balance. When the pumping power increases from 146.7 mW to 742 mW, the color coordinate just varies from (0.339,0.356) to (0.306, 0.363). The research of upconversion mechanism indicates that in addition to the multi-step http://www.chinamagnets.biz/Neodymium/Ball-Neodymium-Magnets.php energy transfer from Yb3+ to Tm3+ and Er3+, respectively, there is new energy transfer process:1G4 (Tm3+)+4I11/2(Er3+)→3H4 (Tm3+)+4S3/2 (Er3+).4 A simple method to prepare the thin layer of silica/aminosilane coated single Y2O3:Yb3+,Ho3+ nanoparticles is established. It is found that the factors playing key roles in the success of coating are the flocculation of nanoparitlces during the silica coating and the aggregation of silica-coated particles during centrifugation. The first problem is solved by adsorbing the PVP to nanoparticles prior to silica coating, and the latter is overcomed by using the one-step ammoniating method. Also, the upcoversion property of the coated particles is investigated.5 The monodisperse Y2O2S:Yb3+/Ho3+-silica/aminosilane core-shell nanoparticles are prepared via a solid-gas method together with PVP assistant coating method. The nanoparticles may well-disperse in ethanol and water to form the stable colloidal solutions, and produce the green light with high luminescence intensity and color purity under the 980 nm LD excitation. In addition, it is found that the silica/aminosilane coating is helpful to enhance the green-emissive intensity and color purity of the Y2O2S:Yb/Ho nanoparticles, the reason for this is that the coating decreases the surface defects,i.e.non-radiative relaxation centers.