Huajun Wu

Czochralski growth of (La,Sr)(Al,Ta)O3 single crystal

Abstract Twin- and crack-free single crystals of (La,Sr)(Al,Ta)O3 with mixed-perovskite structure have been grown using the Czochralski method. These new crystals with a typical size of 55 mm in diameter and 50 mm in length are potential substrate candidates for growing large size and epitaxial HTS and GaN films. Their dielectric constant and dielectric loss at room temperature are 23 and 1×10−4, respectively.

Er3+/Yb3+ codoped phosphor Ba3Y4O9 with intense red upconversion emission and optical temperature sensing behavior

The Ba3Y4O9 host matrix with a low cutoff phonon energy of 585 cm−1 is first applied to upconversion (UC) luminescence (UCL) by codoping Er3+/Yb3+. The new phosphor shows an intense red UC emission which is 6.8-fold and 5.9-fold stronger than that of Y2O3 and β-NaYF4, respectively, under 980 nm GaAs laser diode (LD) excitation at a low density. A broad absorption band centered at 976 nm is observed, meaning a high adaptability to the GaAs LD required in actual applications. The optical thermometry behaviors based on the temperature dependent fluorescence intensity ratios of thermally coupled green UC bands 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 as well as the thermally coupled red UC emission bands originating from the Stark sublevels of 4F9/2 manifold have been explored. The results show that green emissions are suitable for temperatures above 350 K with the maximum sensitivity of 0.00248 K−1 at 563 K and the red emissions are appropriate for temperatures below 350 K with the maximum sensitivity of 0.00371 K−1 at 143 K in our experimental range, indicating their complementary temperature sensing ranges. Moreover, a new method is proposed for evaluating the radiative lifetime of the Er3+ 4F9/2 state based on the analysis of photoluminescence (PL) spectra and fluorescence decay curves. The radiative lifetime of 879 μs for the red emitting level in Ba3Y4O9 is achieved. Thereby, the emission efficiency of 4F9/2 in the new UCP is a little higher than that in Y2O3. Our results imply that Ba3Y4O9:Er3+/Yb3+ is a promising UCP, which could be applied to wide scope optical thermometry using a dual-color scheme.

A high efficiency broad-band near-infrared Ca2LuZr2Al3O12:Cr3+ garnet phosphor for blue LED chips

The garnet Ca2LuZr2Al3O12 (CLZA) is a promising broad-band NIR phosphor for blue LED chips when it is doped with Cr3+. The photoelectric efficiency of the pc-LED fabricated from CLZA:Cr3+ and a 460 nm LED chip, in the 750–820 nm spectral range, was 4.1%, which was superior to the efficiency of a tungsten lamp (2.9%). In CLZA’s structure, Cr3+ occupied Ca2+/Lu3+ and Zr4+ sites and showed two luminescence centers. The crystal strength parameters of Ce3+ and Cr3+ were calculated to show the coordination environment of the dodecahedral and octahedral sites in CLZA. Low absorbance of Cr3+ was the main constraint on quantum efficiency. Ce3+ was thus introduced as a sensitizer to improve the absorbance. An efficient energy transfer process can be observed between Ce3+ and Cr3+ in CLZA. The temperature dependent properties of CLZA:Cr3+ were also studied. Two thermal processes (thermal quenching and thermal ionization) were observed and discussed in detail.

An efficient green phosphor of Ce3+ and Tb3+-codoped Ba2Lu5B5O17 and a model for elucidating the high thermal stability of the green emission

Green phosphors codoped with Ce3+ and Tb3+ have been studied extensively for application in UV-based white LEDs, but only few of them show both high luminescence efficiency and thermal stability. This study reports a new green phosphor of Ce3+ and Tb3+-codoped Ba2Lu5B5O17 prepared by a solid-state reaction. Emission color tuning from blue to green was studied as a function of Tb3+ concentration under UV excitation and well-described based on energy transfer from Ce3+ to Tb3+. The Tb3+ emission endowed the green phosphor with a high quantum yield of 86%, and a high thermal stability over the 303–483 K range was achieved. A model has been proposed to explain a general phenomenon observed in Ce3+ and Tb3+-codoped phosphors that Tb3+ emission is thermally more stable than the Ce3+ emission. A relationship between the temperature dependencies of Tb3+ and Ce3+ emissions was obtained in connection with room temperature energy transfer efficiency. The thermally stable Tb3+ emission was well-predicated using the obtained relationship. The new phosphor was employed for fabricating a UV-based white LED. A high color-rendering index of 91.4 at a low correlated color temperature of 3809 K is obtained, indicating promising application of Ba2Lu5B5O17:Ce3+,Tb3+ in solid-state lighting.

Enhanced ∼2 μm Emission of Tm3+ in Lu2O3 by Addition of a Trace Amount of Er3+

Er3+-induced intensity enhancement of ∼2 μm emission is observed in 2 atom % Tm3+ doped Lu2O3 under 782 nm excitation. The maximum enhancement reaches 41.9% with only 0.05 atom % Er3+. Er3+ introduces a new quantum cutting process which is proved to be a Tm3+ → Er3+ → Tm3+ forward-backward energy transfer (FBET) system. The FBET system is observed to work efficiently even at very low Er3+ concentration. Thus, energy loss due to energy migration among Tm3+ ions is suggested to be suppressed by the FBET process. The Tm3+ → Er3+ → Tm3+ FBET system may be a new route to improve the performance of Tm3+ lasers.

Efficient Blue-emitting Phosphor SrLu 2 O 4 :Ce 3+ with High Thermal Stability for Near Ultraviolet (~400 nm) LED-Chip based White LEDs

Blue-emitting phosphors for near ultraviolet (NUV) based tri-color RGB phosphor blend converted white light emitting diodes (LEDs) have been extensively investigated in the past few years. LED chip peaked near 400u2009nm is the most efficient among the NUV chips currently. However, most of blue phosphors show inefficient excitation around 400u2009nm. Herein, a novel blue phosphor SrLu2O4:Ce3+ matching well with near 400u2009nm chip and showing high thermal stability has been developed. The photoluminescence spectrum presents a broad emission band peaking at 460u2009nm with a bandwidth of nearly 90u2009nm. By optimizing the Ce3+ concentration, an internal quantum efficiency (IQE) as high as 76% was achieved. Furthermore, 86% of the room-temperature emission intensity is still maintained at 150u2009°C, indicating a good thermal stability and practicality. A series of white LEDs were fabricated based on 405u2009nm chips coated with a blend of the new blue phosphor with the commercial yellow and red phosphors. High color rendering indexes (≥90) were achieved while the correlated color temperature was tuneable in the range of 3094 to 8990u2009K. These results suggest that SrLu2O4:Ce3+ can be utilized as a blue-emitting phosphor in NUV based white LEDs.

Near-infrared quantum cutting and energy transfer mechanism in Lu2O3: Tm3+/Yb3+ phosphor for high-efficiency photovoltaics

Near-infrared downconversion phenomenon has been demonstrated in Lu2O3: Tm3+/Yb3+ phosphor upon direct excitation of Tm3+:1G4 level at 463xa0nm. The efficient energy transfer from Tm3+: 1G4u2009→u2009Yb: 2F5/2 has been elucidated by the excitation spectra, the visible and NIR spectra as well as the decay curves of Tm: 1G4 state. The mechanism of downconversion in Lu2O3:Tm3+/Yb3+ has been discussed in detail. According to analysis of the dependence of the initial transfer rate over Yb3+ ion concentration, it could be included that energy transfer (ET) from Tm3+ to Yb3+ is a single-step ET process instead of a cooperative one. By varying the Yb3+ concentrations, we obtain the Lu2O3: 0.2%Tm3+/30%Yb3+ sample with theoretical quantum efficiency as high as 148.2%. Because the excited state of Yb3+ just above the band edge of crystalline silicon, it suggested that Lu2O3: Tm3+/Yb3+ sample will be beneficial to improve the conversion efficiency of c-Si solar cells.