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Developing efficient photocatalysts that respond to near-infrared (NIR) light holds significant scientific importance for improving the utilization of solar energy. In this study, the photocatalyst β-NaLuF4:Yb3+/Tm3+@TiO2 that simultaneously responds to both ultraviolet (UV) and NIR light is successfully synthesized. Instead of the conventional use of β-NaYF4:Yb3+/Tm3+, β-NaLuF4:Yb3+/Tm3+ upconversion (UC) nanoparticles (NPs) are employed as the NIR-UV light converter to achieve stronger UV UC luminescence. The data indicate that the photocatalytic degradation efficiency and rate of β-NaLuF4:Yb3+/Tm3+@TiO2 synthesized in this work are superior to those of NIR-driven photocatalysts based on β-NaYF4:Yb3+/Tm3+ under both NIR and sunlight irradiation. Further analysis of the UC emission spectra and the decay curves of the samples reveals an effective fluorescence resonance energy transfer (FRET) process between β-NaLuF4:Yb3+/Tm3+ and TiO2 after the combination, which makes the NIR-driven photocatalytic reactions happen. All results confirm that the present β-NaLuF4:Yb3+/Tm3+@TiO2 is an exceptionally efficient photocatalyst, owing to its responsiveness to both UV and NIR light. [ABSTRACT FROM AUTHOR]

High-efficiency upconversion (UC) luminescence has been a hot topic in the development of optical materials. In this study, an extremely strong green UC is successfully achieved in the BaWO4:Yb3+/Er3+ phosphor through engineering the local environment around the luminescent centers, realized by Ca2+ doping. The green UC intensity of Ca2+ doped BaWO4:Yb3+/Er3+ is 6.5 and 2.3 times stronger than that of BaWO4:Yb3+/Er3+ and CaWO4:Yb3+/Er3+, respectively. Eu3+ ions are employed as the structure probe to detect the evolution of the crystal field symmetry with the increase of Ca2+ doping concentration, the results of which match well with the corresponding variation of the UC spectra. Meanwhile, the BaWO4:Yb3+/Er3+/Ca2+ exhibits excellent temperature sensing performance based on the fluorescence intensity ratio (FIR) between the thermally coupled 2H11/2 and 4S3/2 states of Er3+ ions. Its absolute sensitivity and relative sensitivity for optical thermometry can reach 1.21% K−1 and 1.31% K−1, outperforming the majority of the same type optical thermometers. The temperature resolution of the present thermometer remains lower than 0.1 K within the temperature range of 298 K to 573 K. The intense green UC luminescence along with the outstanding thermal properties makes the BaWO4:Yb3+/Er3+ phosphor a promising candidate for optical thermometry. [ABSTRACT FROM AUTHOR]

Studies on the up-conversion charging of persistent phosphors are of great academic and practical significance. To date, up-conversion charging designs have been achieved in Cr3+, Mn2+ and Pr3+-activated phosphors. We herein report, for the first time, an up-conversion charging process in the Tb3+-activated garnet phosphor, Gd3Ga5O12:Tb3+, using a 488 nm laser as the excitation source. The delocalized 4f75d state of Tb3+ is excited by absorbing two excitation photons, followed by the charging of the phosphor. Moreover, spectroscopic investigations reveal that an energy-transfer mechanism dominates the up-conversion excitation. This work provides an up-conversion strategy for charging Tb3+-activated phosphors, paving the way toward expanding the scope of persistent phosphors with up-conversion chargeability. [ABSTRACT FROM AUTHOR]

As phosphor-converted white light emitting diodes have emerged as new-generation light sources, developing phosphors with both high quantum efficiency (QE) and thermal stability is urgently required. One of the effective strategies is exploiting the nonradiative energy transfer to simultaneously improve the absorption of the activator and tune the emission of the sensitizer. In this work, we report a series of color-tunable phosphors by constructing an energy transfer system by co-doping Ce3+ and Tb3+ into Ca3Gd2Si6O18 (CGSO). We show that CGSO:Ce3+,Tb3+ not only exhibits color-tunable emission from violet (395 nm) to green (543 nm), but also possesses high internal QE (90.1%). Furthermore, we demonstrate that thermal stability can be significantly improved via efficient energy transfer from Ce3+ to Tb3+, which is mainly attributed to the fast energy transfer within the nearest Ce3+–Tb3+ pairs. Our results indicate that energy transfer from Ce3+ to Tb3+ provides a promising way to develop new green phosphors with high QE and superior thermal stability. [ABSTRACT FROM AUTHOR]

Monoclinic ZrO2 (m-ZrO2) is an attractive material for photonic applications due to its low site symmetry and phonon energy. In this paper, we present detailed studies on the luminescence properties of Ti4+ and Eu3+ in m-ZrO2. In view of the continuing controversy over the origin of "intrinsic" white-blue luminescence in pristine m-ZrO2, more spectroscopic information obtained by characterizing m-ZrO2 intentionally doped with Ti4+, Ti3+ and other metal ions and extensive discussions on the general characteristics of different luminescence processes were provided to evidence the luminescence as the Ti3+ → O− charge transfer transition in the [Ti(IV)O7]10− complex due to the trace amount of Ti4+ impurity in ZrO2 reagent. Competitive absorption was observed between host exciton and oxygen–metal charge transfer processes (O2−–Ti4+, O2−–Eu3+), and even between the two types of charge transfer process because of the sharing of electrons at the top of the valence band, which greatly influenced the absorption and excitation processes. The spectral components comprising the broad excitation band of O2−–Ti4+ white-blue or Eu3+ red emissions in different phosphors were identified, and their evolutions with doping concentration were explained. The energy state locations of oxygen defects produced by intrinsic charge compensation in m-ZrO2:Eu3+ were proposed to be ∼2.41 eV above the top of the valence band by using Eu3+ for the luminescent probe. Efficient energy transfer from the [Ti(IV)O7]10− complex to Eu3+ was first observed in m-ZrO2:Ti4+,Eu3+ phosphors, which were demonstrated to be high performance ratiometric self-referencing optical thermometric materials based on the dual-emitting combination strategy, with the relative sensitivity amounting to ∼3.84% K−1. Optical thermometry not only covering the physiological temperature range but also with high relative sensitivity could be achieved upon appropriate doping. [ABSTRACT FROM AUTHOR]

Na doped SnSe shows excellent electronic properties and ultralow thermal conductivity, leading to a record high ZT value in its single crystal. Owing to weak orientation, polycrystalline SnSe shows a lower electrical conductivity and power factor. In this work, additional Te is introduced in the process of sample preparation to increase the orientation of polycrystalline Sn0.97Na0.03Se. The orientation obviously improves with the increase of Te amount, leading to a great improvement of the electronic properties, especially in the low temperature region. An enhanced averaged power factor (∼6.01 μW cm−1 K−2) and ZT (∼0.45) from 300 to 830 K are achieved. A maximum ZT value of 0.8 at 830 K is obtained in Sn 0.97Na0.03Se with 5 wt% additional Te. [ABSTRACT FROM AUTHOR]

Traditional phosphor-converted white LEDs suffer from the so-called blue-green cavity in their emission spectra. Hence, satisfactory phosphors with sufficient emission around 490 nm are still lacking. Herein, we report a newly designed garnet-type phosphor, Lu2SrAl4SiO12:Ce3+ (LSAS:Ce3+), through chemical unit cosubstitution, whose emission can well fill the blue-green cavity. Importantly, LSAS:Ce3+ exhibits nearly zero thermal quenching even at 473 K (200 °C) as well as a high internal quantum efficiency of 93.3%. Finally, the as-fabricated warm white LED device shows a high color rendering index of 97.6 with R9 = 94, indicating its great potential for white LEDs. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Developing phosphors with high quantum efficiency and superior thermal stability is still a challenge for phosphor-converted white light-emitting diodes (w-LEDs). Energy transfer between ions is usually utilized to tune the emission wavelength. In this study, we demonstrate that in addition to the tunable emission color, an improved thermal stability can be achieved by energy transfer from Ce3+ to Tb3+ in Ce3+ and Tb3+ codoped Ba2Y3(SiO4)3F (BYSF:Ce3+,Tb3+) phosphors without quantum efficiency loss, which is ascribed to the combined effect of fast energy transfer within the nearest Ce3+–Tb3+ pairs via electric dipole–quadrupole interactions, and the following energy diffusion among the Tb3+ ions. An efficient energy transfer from Ce3+ to Tb3+ results in the novel green phosphor BYSF:2%Ce3+,40%Tb3+ with an internal quantum efficiency of as high as 83.12%. In addition, a w-LED lamp was fabricated to explore its possible application in w-LEDs based on near UV LEDs. Our results indicate that fast energy transfer to Tb3+ may provide an alternative way of improving the thermal stability of phosphors. [ABSTRACT FROM AUTHOR]