Xiantao Wei

Efficient near-infrared quantum cutting in NaYF 4 : Ho 3+ , Yb 3+ for solar photovoltaics

Quantum cutting converting a ultraviolet photon into two near-infrared photons has been demonstrated by spectroscopic measurements in NaYF4:Ho3+,Yb3+ synthesized by hydrothermal method. Evidence is provided to confirm the occurrence of quantum cutting. Upon excitation of Ho3+ 5G4 level, near-infrared quantum cutting could occur through a two-step resonance energy transfer from Ho3+ to Yb3+ by cross relaxation, with a maximum quantum efficiency of 155.2%. This result reveals the possibility of violet to near-infrared quantum cutting with a quantum efficiency larger than 100% in Ho3+/Yb3+ codoped fluorides, suggesting the possible application in modifying the solar spectrum to enhance the efficiency of silicon solar cells.

Cooperative energy transfer in Eu3+, Yb3+ codoped Y2O3 phosphor

Abstract Under 980 nm laser excitation, red emission (5D0–7FJ (J=0, 1, 2)) of Eu3+ was observed in cubic Y2O3 codoped with Eu3+ and Yb3+. The dependence of the upconverted emission on doping concentration and laser power was studied. Yb3+ emission around 1000 nm (2F5/2–2F7/2) was reported upon excitation of Eu3+ ions. The decay curves of 5DJ (J=0, 2) emission of Eu3+ under excitation of 266 nm pulse laser were examined to investigate the Eu3+ → Yb3+ energy transfer process. Cooperative energy transfer process was discussed as the possible mechanism for the visible up-conversion luminescence of Eu3+ and near-infrared down-conversion emission of Yb3+.

Energy transfer in Tb3+,Yb3+ codoped Lu2O3 near-infrared downconversion nanophosphors

Tb3+ and Yb3+ codoped Lu2O3 nanophosphors were synthesized by the reverse-strike co-precipitation method.The obtained Lu2O3:Tb3+,Yb3+ nanophosphors were characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectra.The XRD results showed that all the prepared nanophosphors could be readily indexed to pure cubic phase of Lu2O3 and indicated good crystallinity.The Tb3+→Yb3+ energy transfer mechanisms in the UV-blue region in Lu2O3 nanophosphors were investigated.The experimental results showed that the strong visible emission around 543 nm from Tb3+ (5D4→7F5) and near-infrared (NIR) emission around 973 nm from Yb3+ (2F5/2→2F7/2) of LuzO3:Tb3+,Yb3+ nanophosphors were observed under ultraviolet light excitation,respectively.Tb3+ could be effectively excited up to its 4f75d1 state and relaxed down to the 5D4 level,from which the energy was transferred cooperatively to two neighboring Yb3+.The Yb3+concentration dependent luminescent properties and lifetimes of both the visible and NIR emissions were also studied.The lifetime of the visible emission decreased with the increase of Yb3+ concentration,verifying the efficient energy transfer from the Tb3+to the Yb3+.Cooperative energy transfer (CET) from Tb3+ to Yb3+ was discussed as a possible mechanism for the near-infrared emission.When doped concentrations were 1 mol.％ Tb3+ and 2 mol.％ Yb3+,the intensity of NIR emission was the strongest.

Synthesis and luminescent properties of Pr-doped Lu3Al5O12 translucent ceramic

Abstract Lutetium aluminum garnet (LuAG) precursors doped with different Pr3+ concentration (0.25at.%, 0.5at.%, 1.0at.%, 3.0at.%, 5.0at.%) were synthesized via a co-precipitation method using ammonium hydrogen carbonate as precipitant. The phase evolution and morphology of the precursor were characterized with X-ray diffractometer (XRD) and transmission electron microscopy (TEM). The resultant LuAG: Pr3+ powder was sintered into translucent ceramic without any additives in vacuum at 1150 °C and then in nitrogen atmosphere at 1700 °C. Photoluminescence spectra of LuAG:Pr3+ powder and ceramic were measured at room temperature in vacuum ultraviolet (VUV) and ultraviolet (VU) region. For the 5d-4f transition of Pr3+ ions, dominant emission of ceramic samples peaking round 311 nm had higher luminescence intensity. And the host absorption in ceramic samples was not as intensive as that in powder samples. The luminescent intensity of LuAG:Pr3+ varied with the Pr3+ contents and the quenching concentration was about 1.0at.% for ceramic and 3.0at.% for powder, which was much higher than 0.24at.% for LuAG:Pr3+ single crystals. This phenomenon showed that the ceramic had some superiority over single crystals.

Photoluminescence studies of Eu2+-Yb3+ co-doped BaMgAl10O17 phosphor synthesized by the combustion method

Abstract BaMgAl 10 O 17 :Eu 2+ ,Yb 3+ was investigated as a possible quantum cutting system to enhance solar cells efficiency. Phosphors were synthesized by combustion method and composed of nanorods. Photoluminescence spectra showed that Eu in the sample was reduced to bivalence while Yb remained trivalence. Through a cooperative energy transfer process, the obtained powders exhibited both blue emission of Eu 2+ (around 450 nm) and near infrared emission of Yb 3+ (around 1020 nm) under broad band excitation (250–410 nm) originating from 4f→5d transition of Eu 2+ . Energy transfer phenomenon between the sensitizer Eu 2+ and the activator Yb 3+ was investigated via the luminescent spectra and the decay curves of Eu 2+ with different Yb 3+ concentrations. Results indicated that energy transfer efficiency from Eu 2+ to Yb 3+ was not high. The poor efficiency can be explained by the long distance between rare earth ions.

Strong dependence of upconversion luminescence on doping concentration in holmium and ytterbium co-doped Y2O3 phosphor

Abstract Under the excitation of 980 nm diode laser, intense green emission ( 5 F 4 + 5 S 2 - 5 I 8 ) of Ho 3+ was observed in Ho 3+ and Yb 3+ co-doped cubic Y 2 O 3 . The doping concentration and laser power dependence of the upconverted emission were studied. The decay curves of 5 F 4 + 5 S 2 emission of Ho 3+ under the excitation of 355 nm pulse laser were measured to investigate the energy transfer process between Ho 3+ and Yb 3+ . The results indicated that two-photon process was responsible for the upconversion (UC) emission. The Ho 3+ concentration of 0.04 mol.% and the Yb 3+ concentration of 5 mol.% were determined to be the best value for the strongest Ho 3+ emission under the excitation of 980 nm light. The cross-relaxation between two neighboring Ho 3+ ions and the back energy transfer from Ho 3+ to Yb 3+ were important factors for determining the optimal doping concentration. This material was a promising candidate for the application in biomedical fluorescent labels for the intense green emission upon excitation of near-infrared (NIR) light.

Luminescent properties of Lu2MoO6:Eu3+ red phosphor for solid state lighting

Abstract The Eu 3+ activated Lu 2 MoO 6 phosphors were synthesized by high-temperature solid-state reaction method. The X-ray diffraction (XRD), excitation spectra, emission spectra and decay lifetime of the phosphors were measured to characterize the structure and luminescent properties. The XRD results show that all the prepared phosphors can be assigned to the monoclinic structure. The experimental results indicate efficient absorption of near ultraviolet light from the Mo 6+ –O 2– group followed by intensive emission in the visible spectral range. The optimal content of Eu 3+ is 10% (mole fraction). The critical distance R c and energy transfer mechanism were also discussed in detail. This red emitting material may be applied as a promising red phosphor for the near ultraviolet excited white light emitting diodes.

Near-Infrared Downconversion in LuPO 4 : Tm 3+ , Yb 3+ Phosphors

Tm3+ and Yb3+ codoped LuPO4 phosphors were synthesized by the reverse-strike co-precipitation method. The obtained LuPO4:Tm3+,Yb3+ phosphors were characterized by X-ray diffraction (XRD), diffuse reflectance spectra, photoluminescence (PL) spectra, and decay lifetime to understand the observed near-infrared downconversion (DC) phenomena. The XRD results show that all the prepared phosphors can be readily indexed to the pure tetragonal phase of LuPO4 and exhibit good crystallinity. The experimental results showed that the strong visible emission around 649 nm from Tm3+(1G4 --> 3F4) and near-infrared (NIR) emission around 1003 nm from Yb3+(2F5/2 --> 2F7/2) of LuPO4:Tm3+,Yb3+ phosphors were observed under 468 nm excitation, respectively. The Yb3+ concentration dependence of luminescent properties and lifetimes of both the visible and NIR emissions have also been investigated. The quenching concentration of Yb3+ ions approaches 30 mol%. The DC mechanism is also discussed in detail.

Spectroscopic properties of Yb3+ doped CaNb2O6 phosphor.

The spectroscopic measurements of orthorhombic CaNb2O6 phosphor doped with Yb3+ ions have been performed. Low temperature spectra indicate that Yb3+ ions may occupy three types of nonequivalent sites inside the CaNb2O6 lattice: a main Yb3+ site dominating the near-infrared emission, and two other minor sites of similar behaviors to each other. An attempt to determine the Stark sublevels energies of the 2F7/2 and 2F5/2 manifolds of three Yb3+ nonequivalent ions was carried out. The fluorescence lifetimes associated to the emission lines of Yb3+ ions located at the main site and minor sites are also quite different. Furthermore, there exists energy transfer from the main site to the minor sites.

Synthesis and photoluminescent properties of NaYF4:Eu3+ core and NaYF4:Eu3+/NaYF4 core/shell nanocrystals.

NaYF4:Eu3+ core and NaYF4:Eu3+/NaYF4 core/shell nanocrystals (NCs) were synthesized via a wet chemical method. The transmission electron microscope photographs show that the core and core/shell nanoparticles are monodisperse and uniform NCs with average diameters of 22 and 26 nm respectively. The photoluminescence (PL) properties of the samples, including the PL excitation and emission spectra, and luminescent decay curves, are investigated in detail. The results show that the intensity of 5D2 emission relative to that of 5D0 is stronger in NaYF4:Eu3+/NaYF4 core/shell NCs than that in NaYF4:Eu3+ core NCs, and a longer decay lifetime of 5D2 is observed in core/shell samples. In addition, from the corrected emission spectra of 5D0, the 5D0 radiative lifetimes were calculated. These together with the measured decay lifetime of 5D0 emission give the intrinsic quantum yields of 5D0. The results were well interpreted by considering the surface effects.