Weijie Zhou

Site Occupancies, Luminescence, and Thermometric Properties of LiY9(SiO4)6O2:Ce3+ Phosphors

In this work, we report the tunable emission properties of Ce3+ in an apatite-type LiY9(SiO4)6O2 compound via adjusting the doping concentration or temperature. The occupancies of Ce3+ ions at two different sites (Wyckoff 6h and 4f sites) in LiY9(SiO4)6O2 have been determined by Rietveld refinements. Two kinds of Ce3+ f-d transitions have been studied in detail and then assigned to certain sites. The effects of temperature and doping concentration on Ce3+ luminescence properties have been systematically investigated. It is found that the Ce3+ ions prefer occupying Wyckoff 6h sites and the energy transfer between Ce3+ at two sites becomes more efficient with an increase in doping concentration. In addition, the charge-transfer vibronic exciton (CTVE) induced by the existence of free oxygen ion plays an important role in the thermal quenching of Ce3+ at 6h sites. Because of the tunable emissions from cyan to blue with increasing temperature, the phosphors LiY9(SiO4)6O2:Ce3+ are endowed with possible thermometric applications.

VUV-vis photoluminescence, X-ray radioluminescence and energy transfer dynamics of Ce3+ and Pr3+ doped LiCaBO3

A series of LiCaBO3:Ce3+/Pr3+ phosphors were prepared by a high-temperature solid state reaction method. A structure refinement for the LiCaBO3 compound was performed based on powder X-ray diffraction (XRD) data. The energies of the crystal field split excited 5d states of Ce3+ and Pr3+ in LiCaBO3 were determined from synchrotron radiation VUV-UV excitation spectra. Furthermore, the influence of the doping concentration and temperature on the emission properties of Ce3+ was investigated, and the vacuum referred binding energy (VRBE) scheme for all lanthanide 4f and 5d states in LiCaBO3 was constructed to estimate the thermal activation energy of Ce3+ luminescence quenching. The Pr3+ to Ce3+ energy transfer (ET) and its influence on luminescence decays of Pr3+ and Ce3+ were studied in detail. Finally, the X-ray excited luminescence spectra were measured to evaluate the possible X-ray detection applications.

Spectral Properties and Energy Transfer between Ce3+ and Yb3+ in the Ca3Sc2Si3O12 Host: Is It an Electron Transfer Mechanism?

The downshifting from Ce(3+) blue emission to Yb(3+) near-infrared emission has been studied in the garnet host Ca2.8-2xCe0.1YbxNa0.1+xSc2Si3O12 (x = 0-0.36). The downshifting does not involve quantum cutting, but one incident blue photon is transferred from Ce(3+) to Yb(3+) with an energy transfer efficiency up to 90% when x = 0.36 for the Yb(3+) dopant ion. For x ≤ 0.15, a multiphonon-assisted electric dipole-electric quadrupole mechanism of energy transfer dominates, while for the highest concentration of Yb(3+) employed, the electron transfer mechanism is confirmed. A temperature-dependent increase of the Ce(3+) → Yb(3+) energy transfer rate does not exclusively indicate the electron transfer mechanism. The application of the material to solar energy conversion is indicated.

Luminescence and X-ray absorption studies on 0.5% Ce3+ doped BaCa2MgSi2O8 phosphor

0.5% Ce(3+) doped BaCa2MgSi2O8 phosphor was prepared by a conventional solid state reaction method. Luminescence spectra as well as fluorescence decay were monitored in the VUV-UV range. Ce(3+) emissions are assigned to cerium ions on a Ba(2+) site, and the five 4f-5d excitation bands of Ce(3+) were determined at low temperature. The light yield is estimated to be around 10,600 ph MeV(-1) under X-ray excitation. X-ray absorption near-edge structure (XANES) was explored to study the energy transfer efficiency to optical centers from each element in the phosphor; the results show that the contributions to luminescence are not identical for each element.

Concentration-Driven Selectivity of Energy Transfer Channels and Color Tunability in Ba3La(PO4)3:Tb3+, Sm3+ for Warm White LEDs

Here, we report the large-scale emission color tunability in Ba3La(PO4)3:Tb3+, Sm3+ (BLPO:TS) system based on the detailed discussion on the concentration-driven selectivity of energy transfer (ET) channels from Tb3+ to Sm3+. It is induced by the concentration-dependent 5D3 and 5D4 emissions of Tb3+ and the different interaction mechanisms of ET from Tb3+ to Sm3+ via 5D3 and 5D4 channels. In the diluted Tb3+ scenario, the red emission of Sm3+ is efficiently sensitized via the 5D3 channel, while in the concentrated Tb3+ case, the contribution of 5D4 channel is dominant. Therefore, by simply adjusting the doping concentrations of Tb3+ and Sm3+, the emission color of the phosphors can be tuned from green to red. In view of the phosphors with red emissions are critical to the warm white light-emitting diodes (WLEDs), an orange-red Tb3+, Sm3+ coactivated phosphor Ba3La0.90Tb0.05Sm0.05(PO4)3 (BLPO:5T5S) with good thermal and chromaticity stability and internal quantum efficiency ∼67% is developed in the system. Then, a near-UV WLED (CCT ≈ 4500 K, Ra ≈ 81) is fabricated using this phosphor. These findings not only indicate that the orange-red phosphor BLPO:5T5S is available for near-UV warm white LEDs but also deliver new insights into the ET processes in Tb3+ and Sm3+ activated phosphors.

Host-sensitized luminescence of Dy3+ in LuNbO4 under ultraviolet light and low-voltage electron beam excitation: energy transfer and white emission

A series of LuNbO4:xDy3+ (x = 0–0.20) phosphors was prepared using a high-temperature solid-state reaction technique. X-ray diffraction (XRD) along with Rietveld refinement, field emission scanning electron microscopy (FE-SEM) observations, diffuse reflectance spectra (DRS), UV-vis photoluminescence (PL), fluorescence decays, PL quantum yields (QYs), and low-voltage cathodoluminescence (CL) were employed to characterize the phosphors. Nonradiative relaxation and host sensitization dramatically influence the LuNbO4:Dy3+ luminescence spectra and decay dynamics. It is shown that cross-relaxation arising from electric dipole–dipole interactions between adjacent Dy3+ ions is the leading mechanism of quenching the Dy3+ emission. The host sensitization for Dy3+ emission in LuNbO4 was confirmed and the energy transfer efficiency from the host to Dy3+ increased with increasing Dy3+ doping concentration/temperature. Upon excitation with ultraviolet light (261 nm) and a low-voltage electron beam (2 kV, 127 μA cm−2), the synthesized LuNbO4:Dy3+ phosphors show both the blue broadband emission of the LuNbO4 host and the characteristic emission of Dy3+ (the dominant one is the 4F9/2 → 6H13/2 transition, yellow), and the luminescence colour of the LuNbO4:Dy3+ phosphors can be tuned over a large gamut of colours by varying the Dy3+ doping concentration, and a single-phase intense white-light-emission has been achieved in the LuNbO4:0.015Dy3+ phosphor. On the basis of the good UV-vis PL and CL properties, LuNbO4:Dy3+ phosphors might be promising for applications in UV light-emitting diodes (UV-LEDs) and field emission displays (FEDs).

Unique Spectral Overlap and Resonant Energy Transfer between Europium(II) and Ytterbium(III) Cations: No Quantum Cutting

Samples of the Ca3 Sc2 Si3 O12 (CSS) host singly doped with Eu2+ or Yb3+ , doubly doped with Eu2+ and Yb3+ , and triply doped with Ce3+ , Eu2+ and Yb3+ were synthesized by a sol-gel combustion process under reducing conditions. Unlike previous reports of Eu2+ →Yb3+ energy transfer in other systems, the energy transfer is resonant in the CSS host and the transfer efficiency reaches 100 % for lightly doped samples. The transfer mechanism is multipolar rather than electron transfer for the sample compositions employed herein. The emission intensity of Yb3+ is further enhanced by co-doping with Ce3+ in addition to Eu2+ . The quantum efficiencies of the doped materials range between 9 % and 93 %.

A new insight into the electrochemical growth of Ag nanodendrites without a strong electrolyte

We have developed a simple and green electrochemistry technique to synthesize Ag nanostructures with a highly pure, crystalline and smooth surface, which takes place in a clean and slow reaction environment (just highly pure de-ionized water) without any chemical additives. Here, we report a new insight into the Ag nanodendrite (ND) synthesis via the developed electrochemistry without a strong electrolyte. It is found that Ag2CO3 nanospindles and AgNDs form in water and on a graphite negative electrode, respectively, when an Ag target is immersed in the highly pure de-ionized water under an external voltage, which is totally different from conventional electrochemistry. The reaction processes are investigated in detail both theoretically and experimentally. The results show that CO2 from air plays a crucial role in the formation of AgNDs. CO2 is dissolved into the de-ionized water to form HCO3−, weak acid anions, which results in the formation of Ag2CO3. Then, Ag+ is reduced to metallic AgNDs on the negative graphite electrode. The AgND modified graphite electrode (AgND/GE) shows a significant improvement in the electroreduction of H2O2 compared to the bare graphite electrode. These investigations suggest that introducing a weak electrolyte in the electrochemical cell offers an alternative route to design and fabricate novel nanostructures for practical applications.

Luminescence properties and site occupancy of Ce3+ in Ba2SiO4: a combined experimental and ab initio study

Photoluminescence properties of Ba2−2xCexNaxSiO4 (x = 0.0005) prepared by a solid-state reaction method are first studied with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range at low temperature. Five bands are observed in the excitation spectrum of Ce3+ 5d → 4f emission at 26.5 K. The highest energy band is attributed to the host excitonic absorption, from which the band gap energy of the host is estimated to be around 7.36 eV. The four lower energy bands are assigned to the 4f → 5d transitions of Ce3+ located at one of the two types of Ba sites in Ba2SiO4, based on a comparison of excitation spectra at different monitoring wavelengths. Under UV excitation, the material exhibits bright luminescence at 350–450 nm, with a fast decay time (∼26 ns at 4 K) and a high thermal quenching temperature (>500 K). In view of this, X-ray excited luminescence measurements are then conducted, and the results suggest a potential application of Ba2SiO4:Ce3+ as scintillation phosphors. Hybrid density functional theory (DFT) calculations within the supercell model are carried out to optimize the local structures of Ce3+ at the two Ba sites in Ba2SiO4, on which wave function-based ab initio embedded cluster calculations are performed to derive the 4f1 and 5d1 energy levels of Ce3+. On the basis of the calculated DFT total energies and the comparison between experimental and calculated 4f → 5d transition energies, we find that the luminescence originates predominantly from Ce3+ occupying nine-coordinated Ba2 sites. Furthermore, electronic properties of Ce3+ in Ba2SiO4 are evaluated to provide an understanding of the high thermal stability of the 5d luminescence at the level of electronic structures.