Zhi-Jun Zhang

Energy transfer mechanisms in Tb3+, Yb3+ codoped Y2O3 downconversion phosphor

Tb3+ and Yb3+ co-activated luminescent material that can cut one photon of around 483 nm into two NIR photons of around 1000 nm could be used as a downconversion luminescent convertor in front of crystalline silicon solar cell panels to reduce thermalization loss of the solar cell. The Tb3+ → Yb3+ energy transfer mechanisms in the UV–blue region in Y2O3 phosphor were studied by PL excitation spectra and time-resolved luminescence, from which the charge transfer mechanism and the cooperative transfer mechanism were identified. Tb3+ ions in the 4f75d1 state relax down to the 5D4 level and cooperatively transfer energy to two Yb3+ ions, which is followed by the emission of two photons (λ ~ 1000 nm). It was found in the (Y0.79Tb0.01Yb0.20)2O3 sample that 37% of the Tb3+ ions at the 5D4 level transfer energy to two neighbouring Yb3+ ions by the cooperative energy transfer mechanism Tb3+ (5D4) → 2Yb3+ (2F5/2). Unfortunately, the high Yb3+ concentration leads to severe concentration quenching that significantly reduces the external quantum efficiency. Moreover, the energy of the Tb3+ 4f75d1 state can also be lost non-radiatively or transferred to the Yb3+ 2F5/2 state via the charge transfer state Tb4+–Yb2+. In conclusion, RE3+ (RE = Ce, Pr, Tb) with strong absorption in the UV region is not an appropriate sensitizer of Tb3+ in Tb3+–Yb3+ codoped downconversion phosphor.

Luminescence properties of CaZr(PO4)2:RE (RE = Eu3+, Tb3+, Tm3+) under x-ray and VUV–UV excitation

Novel phosphors of Eu3+, Tb3+ and Tm3+ doped CaZr(PO4)2 were synthesized by solid-state reactions and their x-ray and vacuum ultraviolet–ultraviolet (VUV–UV) spectroscopic properties were investigated. The bands near 147 or 185 nm in the VUV excitation spectra of those doped samples are attributed to the PO4 absorption and O to Zr charge transfer transition in the host. The excitation and emission spectra indicate that the phosphors can be effectively excited by x-ray or 147 nm and 172 nm and exhibit a satisfactory red, green and blue light performance, respectively. Considering the high luminescent intensity, excellent colour purity and chemical stability, CaZr(PO4)2:RE (RE = Eu3+ Tb3+, Tm3+) are attractive red, green and blue emitting x-ray and plasma display panel phosphors.

Preparation of g-C3N4/Graphene Composite for Detecting NO2 at Room Temperature

Graphitic carbon nitride (g-C3N4) nanosheets were exfoliated from bulk g-C3N4 and utilized to improve the sensing performance of a pure graphene sensor for the first time. The role of hydrochloric acid treatment on the exfoliation result was carefully examined. The exfoliated products were characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-Vis spectroscopy. The exfoliated g-C3N4 nanosheets exhibited a uniform thickness of about 3–5 nm and a lateral size of about 1–2 µm. A g-C3N4/graphene nanocomposite was prepared via a self-assembly process and was demonstrated to be a promising sensing material for detecting nitrogen dioxide gas at room temperature. The nanocomposite sensor exhibited better recovery as well as two-times the response compared to pure graphene sensor. The detailed sensing mechanism was then proposed.

VUV spectroscopic properties of rare-earth (RE3+ = Eu, Tb, Tm)-doped AZr2(PO4)3 (A+ = Li, Na, K) type phosphate

The excitation spectra of pure and rare-earth (RE3+ = Eu, Tb, Tm)-doped AZr2(PO4)3 (A+ = Li, Na, K) in the vacuum ultraviolet (VUV) regions are investigated. The results indicate that these samples show strong absorption in the VUV range. The band ranging from 130 to 160 nm is due to the absorption band of host lattice or PO4 groups; the band at 160–200 nm is related to the charge transfer (CT) transition of O–Zr. For Eu3+-doped samples, the weak band in the excitation spectra at about 208 nm is related to the CT band of O2−–Eu3+. As for Tb3+-doped samples, the strong band at 225 nm and the weak band at 261 nm in the excitation spectra are assumed to the spin-allowed and spin-forbidden f–d transitions (4f8→ 4f75 d1) of Tb3+. However, there is no obvious band of CT of O2−–Tm3+ in the Tm3+-doped samples.

VUV spectroscopic properties of rare-earth (RE=Eu, Tb and Dy)-doped A2Zr(PO4)2 (A=Li, Na and K) phosphates

Abstract This study fully investigated the vacuum ultraviolet excitation spectra of pure and rare-earth (RE=Eu, Tb and Dy)-doped A 2 Zr(PO 4 ) 2 (A=Li, Na and K) phosphors. The synthesized Na and Li compounds were characterized by XRD showing two new types of phases after indexation. Although these three pure compounds had different crystal structures, they exhibited similar luminescence properties. For Eu 3+ -activated samples, the broad excitation band centered at 217 nm could be attributed to the CT transition between O 2– ( 2 p 6 ) and Eu 3+ ions. For Tb 3+ -doped samples, two groups of f-d transitions were observed, where a strong broad band at 221 nm was due to the spin-allowed f-d transition. Energy transfer from O 2– to Dy 3+ was not observed in Dy 3+ -doped phosphors, probably because it overlapped considerably with the CT transition from O 2– to Zr 4+ at 187 nm.

Photoluminescence properties and energy levels of RE (RE = Pr, Sm, Er, Tm) in layered-CaZnOS oxysulfide

RE3+ (RE = Pr, Sm, Er, Tm)-activated CaZnOS samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. Doping with RE3+ (RE = Pr, Sm, Er, Tm) into layered-CaZnOS resulted in typical RE3+ (RE = Pr, Sm, Er, Tm) f-f line absorptions and emissions, as well as the charge transfer band of Sm3+ at about 3.3 eV. The energy level scheme containing the position of the 4f and 5d levels of all divalent and trivalent lanthanide ions with respect to the valence and conduction bands of CaZnOS has been constructed based on the new data presented in this work, together with the data from literature on Ce3+ and Eu2+ doping in CaZnOS. The detailed energy level scheme provides a platform for interpreting the optical spectra and could be used to comment on the valence stability of the lanthanide ions in CaZnOS.

High quality thin film phosphors of Y2O3:Eu3+ deposited via chemical bath deposition

High transparency in visible region was required for red-light-emitting Y2O3:Eu3+ thin film phosphors. Such films were obtained via chemical bath deposition on bare SiO2 glass substrates through heterogeneous nucleation with further heat treatment. Thin amorphous yttrium basic carbonate films could be completely transformed to crystalline Y2O3 at 650 °C. X-ray diffraction and field-emission scanning electron microscopy were used to characterize these products. The deposition temperature and the post-annealing conditions were crucial factors for preparing high-quality thin films. The thin films prepared with optimized parameters emitted a bright red light around 612 nm. Meanwhile, these thin films exhibited optical transmittance of 90% to 95% in a wavelength range of 400 to 800 nm depending on the chemical deposition solution. These results clearly indicated that chemical solution deposition is a promising technique for large-scale production of thin film phosphors in application of display technology.

Controlled synthesis and enhanced luminescence of BiOCl:Eu3+ ultrathin nanosheets

BiOCl:Eu3+ ultrathin nanosheets were synthesized through a facile solvothermal method with the assistance of polyvinyl pyrrolidone. The ultrathin nanosheets showed a size ranging from 50–100 nm and a height of 2.9 nm, which is about four [Cl–Bi–O–Bi–Cl] layers. In addition, the luminescence properties and mechanism were investigated in detail, and compared with that of the nanoplate. It was found that the luminescence intensity for the BiOCl:Eu3+ ultrathin nanosheet was 6 times stronger than that of the nanoplate. Importantly, the oxygen vacancies act as a luminescence centers, and there was effective energy transfer from BiOCl ultrathin nanosheets to Eu3+ ions. In BiOCl:Eu3+ ultrathin nanosheets, white-light emission was observed under near-ultraviolet (NUV) excitation. These BiOCl:Eu3+ ultrathin nanosheets exhibit great potential as color-emitting phosphors for white light-emitting diode applications.

Disordered Tm:Ca 9 La(VO 4 ) 7 : a novel crystal with potential for broadband tunable lasing

We report on the crystal growth, structural and spectroscopic investigation of a novel trigonal calcium vanadate crystal, Tm3+:Ca9La(VO4)7. Polarized absorption, stimulated-emission and gain cross-section spectra are determined for this material. The maximum σSE corresponding to the 3F4 → 3H6 transition amounts to 1.28 × 10−20 cm2 at 1854 nm for σ-polarization. The measured lifetime of the Tm3+ ions in the 3F4 state is 1.19 ms. The Judd-Ofelt analysis performed yielded intensity parameters of Ω2 = 4.682, Ω4 = 0.659 and Ω6 = 0.475 [10−20 cm2]. The polarized Raman spectra indicate a strong and broad band centered at 867 cm−1. Owing to the disordered nature of Ca9La(VO4)7, the Tm3+ ions exhibit a broad and smooth gain profile at ~2 μm. The spectroscopic properties of Tm3+:Ca9La(VO4)7 are very promising for broadly tunable and ultrashort pulse lasers near 2 µm.

Fe-doped Bi4O5Br2 Visible Light Photocatalyst: A First Principles Investigation

This study employed first principles calculations to investigate Fe-doped Bi4O5Br2 as a potential photocatalyst with high efficiency. Based on formation energy calculation, the Fe atoms prefer to r...