On Doping Eu³⁺ in Sr_0.99La_1.01Zn_0.99O_3.495: The Photoluminescence, Population Pathway, De-Excitation Mechanism, and Decay Dynamics

Eu³⁺, with the 4f⁶ electronic configuration, generally exhibits bright red f-f emissions arising from its ⁵D₀ multiplet, and Eu³⁺ doped phosphors have attracted lots of attention for applications in lighting and display fields. However, the electron population mechanisms between relevant Eu³⁺ excited states as well as charge-transfer state (CTS) still need to be further clarified since the puzzles on these issues limit the exploration of new luminescent materials and the improvement of the luminescence efficiency of the potential phosphors. In this work, a series of Sr_0.99[La_{(1–<i>x</i>)}Eu_<i>x</i>]_1.01Zn_0.99O_3.495 phosphors was prepared by a high-temperature solid-state reaction technique and was characterized by X-ray diffraction (XRD) measurements at different temperatures, infrared (IR) spectrum, and diffusion reflectance spectra (DRS) at room temperature (RT). The temperature-, doping concentration-, and excitation wavelength-dependent luminescence properties were systematically studied to clarify the population pathway, de-excitation mechanism, and decay dynamics of Eu³⁺ in this low-phonon-frequency compound. The impacts of cross relaxation (CR) and multiphonon relaxation (MPR) processes on the luminescence and decay spectra were investigated in detail. The special coordination polyhedron around Eu³⁺ played a dominant role in the intense Eu^3+ 5D₀–⁷F₄ emission. The CTS peaks shifted to longer wavelengths with increasing temperatures, which seems to relate to the lattice expansion at higher temperatures

First-Principles Screening and Design of Novel Triphenylamine-Based D−π–A Organic Dyes for Highly Efficient Dye-Sensitized Solar Cells

We screen a series of π-conjugated bridge groups and design a range of metal-free organic donor−π–acceptor (D−π–A) <b>SPL101</b>–<b>SPL108</b> dyes based on the experimentally synthesized <b>C217</b> dye for highly efficient dye-sensitized solar cells (DSSC) using density functional theory (DFT) and time-dependent DFT (TDDFT), and further calculate their physical and electronic properties, including geometrical structures, electronic cloud distribution, molecular orbital energy levels, absorption spectra, light harvesting efficiency (LHE), driving force of injection (Δ<i>G</i>_inj) and regeneration (Δ<i>G</i>_reg), and electron dipole moment (μ_normal). Results reveal that the π-conjugated bridge groups in <b>SPL103</b> and <b>SPL104</b> are promising functional groups for D−π–A organic dyes. In particular,<b> SPL106 </b>and<b> SPL108</b> have not only smaller energy gaps, higher molar extinction coefficients, and 128 and 143 nm redshifts, but also a broader absorption spectrum covering the entire visible range up to the near-IR region of 1200 nm compared to <b>C217</b> dye

Structure Refinement and Two-Center Luminescence of Ca₃La₃(BO₃)₅:Ce³⁺ under VUV–UV Excitation

A series of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed using the powder X-ray diffraction (XRD) data, which shows occupation of Ce³⁺ on both Ca²⁺ and La³⁺ sites with a preferred location on the La³⁺ site over the Ca²⁺ site. The prepared samples contain minor second phase LaBO₃ with contents of ∼0.64–3.27 wt % from the Rietveld analysis. LaBO₃:1%Ce³⁺ was prepared as a single phase material and its excitation and emission bands were determined for identifying the influence of impurity LaBO₃:Ce³⁺ luminescence on the spectra of the Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples. The luminescence properties of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples under vacuum ultraviolet (VUV) and UV excitation were investigated, which exhibited two-center luminescence of Ce³⁺, assigned to the Ce(1)³⁺ center in the La³⁺ site and Ce(2)³⁺ center in the Ca²⁺ site, taking into account the spectroscopic properties and the Rietveld refinement results. The influences of the doping concentration and the excitation wavelength on the luminescence of Ce³⁺ in Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ are discussed together with the decay characteristics

Structure Refinement and Two-Center Luminescence of Ca₃La₃(BO₃)₅:Ce³⁺ under VUV–UV Excitation

A series of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ phosphors were prepared by a high-temperature solid-state reaction technique. Rietveld refinement was performed using the powder X-ray diffraction (XRD) data, which shows occupation of Ce³⁺ on both Ca²⁺ and La³⁺ sites with a preferred location on the La³⁺ site over the Ca²⁺ site. The prepared samples contain minor second phase LaBO₃ with contents of ∼0.64–3.27 wt % from the Rietveld analysis. LaBO₃:1%Ce³⁺ was prepared as a single phase material and its excitation and emission bands were determined for identifying the influence of impurity LaBO₃:Ce³⁺ luminescence on the spectra of the Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples. The luminescence properties of Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ samples under vacuum ultraviolet (VUV) and UV excitation were investigated, which exhibited two-center luminescence of Ce³⁺, assigned to the Ce(1)³⁺ center in the La³⁺ site and Ce(2)³⁺ center in the Ca²⁺ site, taking into account the spectroscopic properties and the Rietveld refinement results. The influences of the doping concentration and the excitation wavelength on the luminescence of Ce³⁺ in Ca₃La_{3(1–<i>x</i>)}Ce_3<i>x</i>(BO₃)₅ are discussed together with the decay characteristics

Rational Design of Dithienopicenocarbazole-Based Dyes and a Prediction of Their Energy-Conversion Efficiency Characteristics for Dye-Sensitized Solar Cells

A series of metal-free organic donor–acceptor (D–A) derivatives (<b>ME01</b>–<b>ME06</b>) of the known dye <b>C281</b> were designed using first-principles calculations in order to evaluate their potential for applications in dye-sensitized solar cells (DSSCs). Their physical and electronic properties were calculated using density functional theory (DFT) and time-dependent density functional theory (TD-DFT). These include molecular properties that are required to assess the feasibility of a dye to function in DSSCs: UV–vis absorption spectra, light-harvesting efficiency (LHE), and driving forces of electron injection (<i>Δ<i>G</i></i>_inj). <b>ME01</b>, <b>ME02</b>, and <b>ME04</b> are predicted to exhibit broad absorption optical spectra that cover the entire visible range, rendering these three dyes promising DSSC prospects. Device-relevant calculations on these three short-listed dyes and the parent dye <b>C281</b> were then performed, whereupon the dye molecules were adsorbed onto anatase TiO₂ surfaces to form the DSSC working electrode. Associated DSSC device characteristics of this dye···TiO₂ interfacial structure were determined. These include the light-harvesting efficiency, the number of injected electrons, the electron-injection lifetime, and the quantum-energy alignment of the adsorbed dye molecule to that of its device components. In turn, these calculated parameters enabled the derivation of the DSSC device performance parameters: short-circuit current density, <i>J</i>_SC, incident photon-to-electron conversion efficiency, IPCE, and open-circuit voltage, <i>V</i>_OC. Thus, we demonstrate a systematic <i>ab initio</i> approach to screen rationally designed D–A dyes with respect to their potential applicability in high-performance DSSC devices

Optical Properties of Ce-Doped Li₄SrCa(SiO₄)₂: A Combined Experimental and Theoretical Study

Investigation of optical properties of Ce³⁺-activated phosphors is not only of practical importance for various applications but also of fundamental importance for providing a basis to understand relevant properties of other lanthanide ions in the same host. We report herein a combined experimental and theoretical study of optical properties of Ce³⁺ in Li₄SrCa­(SiO₄)₂. Photoluminescence properties of the material prepared by a solid-state reaction method are investigated with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range at low temperatures. The band maxima in the excitation spectra are assigned with respect to 4f → 5d transitions of Ce³⁺ at the Sr and Ca sites, from comparison between experimental and <i>ab initio</i> predicted transition energies. As a result of the two-site occupation, the material displays luminescence at 300–500 nm with a high thermal quenching temperature (>500 K), consistent with the calculated large gaps (∼1.40 eV) between the emitting 5d levels and the bottom of the host conduction band. On the basis of experimental and calculated results for Ce³⁺ in Li₄SrCa­(SiO₄)₂, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions at the Sr and Ca sites of the same host is constructed and discussed in association with experimental findings

Vacuum Referred Binding Energy Scheme, Electron–Vibrational Interaction, and Energy Transfer Dynamics in BaMg₂Si₂O₇:Ln (Ce³⁺, Eu²⁺) Phosphors

The host structure and the synchrotron radiation VUV–UV luminescence properties of samples BaMg₂Si₂O₇ (BMSO):Ln (Ce³⁺, Eu²⁺) at different doping levels and different temperatures were investigated in detail. Three important aspects are studied to elucidate the luminescence properties of samples: (1) the vacuum referred binding energy (VRBE) scheme is constructed with the electron binding in the BMSO host bands and in the Ce³⁺ and Eu²⁺ impurity levels with the aim to explain the different thermal stabilities of Ce³⁺ and Eu²⁺ emissions; (2) the electron–vibrational interaction analysis on the narrow Eu²⁺ emission indicates a weak electron–phonon interaction in the current case; (3) by using three models (Inokuti–Hirayama, Yokota–Tanimoto, and Burshteĭn models) at different conditions, the energy transfer dynamics between Ce³⁺ and Eu²⁺ was analyzed. It reveals that the energy transfer from Ce³⁺ to Eu²⁺ via electric dipole–dipole (EDD) interaction is dominant while energy migration between Ce³⁺ is negligible. Finally, the X-ray excited luminescence spectra of samples BMSO:Ce³⁺/Eu²⁺ are collected to evaluate their possible scintillator applications

Excitation Wavelength Dependent Luminescence of LuNbO₄:Pr³⁺Influences of Intervalence Charge Transfer and Host Sensitization

A series of LuNbO₄:Pr³⁺ phosphors was prepared by a solid-state reaction method at high-temperature. Rietveld refinements were performed based on powder X-ray diffraction (XRD) data. Diffuse reflectance spectra (DRS), UV–vis photoluminescence (PL), time-resolved emission spectra (TRES), and fluorescence decays were utilized to study the luminescence and host sensitization processes of Pr³⁺ in LuNbO₄. Excitation wavelength dependent luminescence of LuNbO₄:Pr³⁺ was investigated and explained in consideration of the processes of nonradiation relaxation via cross-relaxation, multiphonon relaxation, and crossover to the intervalence charge transfer (IVCT) state. Furthermore, the host sensitization of Pr³⁺ emission in LuNbO₄ was confirmed and the energy transfer efficiency from host to Pr³⁺ increased with increasing Pr³⁺ doping concentration/temperature. Because the change of emission intensities for both blue from the host and red from ¹D₂ is sensitive to temperature, a large variation of emission color is observed between RT and 500 K

The Effect of Sr²⁺ on Luminescence of Ce³⁺-Doped (Ca,Sr)₂Al₂SiO₇

A series of Ce³⁺-doped (Ca,Sr)₂Al₂SiO₇ phosphors with different Ce³⁺ and Ca²⁺/Sr²⁺ concentrations were prepared by a high temperature solid-state reaction technique. To get insight into the structure–luminescence relationship, the impact of incorporation of Sr²⁺ on structure of (Ca,Sr)₂Al₂SiO₇ was first investigated via Rietveld refinement of high quality X-ray diffraction (XRD) data, and then the VUV–UV excitation and UV–vis emission spectra of (Ca,Sr)₂Al₂SiO₇:Ce³⁺ were collected at low temperature. The results reveal that the crystal structure evolution of (Ca,Sr)₂Al₂SiO₇:Ce³⁺ has influences on band gaps and Ce³⁺ luminescence properties including 4f–5d_<i>i</i> (<i>i</i> = 1–5) transition energies, radiative lifetime, emission intensity, quantum efficiency, and thermal stability. Moreover, the influence of Sr²⁺ content on the energy of Eu³⁺–O^2– charge-transfer states (CTS) in (Ca,Sr)₂Al₂SiO₇:Eu³⁺ was studied in order to construct vacuum referred binding energy (VRBE) schemes with the aim to further understand the luminescence properties of (Ca,Sr)₂Al₂SiO₇:Ce³⁺. Finally, X-ray excited luminescence (XEL) spectra were measured to evaluate the possibility of (Ca,Sr)₂Al₂SiO₇:Ce³⁺ as a scintillation material

High Light Yield of Sr₈(Si₄O₁₂)Cl₈:Eu²⁺ under X‑ray Excitation and Its Temperature-Dependent Luminescence Characteristics

In this work, we first investigate the relationship between temperature and lattice parameters by means of Rietveld refinement and then demonstrate its impact on the luminescence peak position of Eu²⁺ in Sr₈(Si₄O₁₂)­Cl₈. It is found that with increases in temperature, lattice expansion takes place without significant distortion of the coordination around Eu²⁺. As a result, the crystal field splitting of the Eu²⁺ 5d state decreases. At the same time, with the experimental data of the full width at half-maximum of Eu²⁺ emission at different temperatures and the infrared spectrum, the effective phonon frequency is evaluated and the main vibration motions are determined using first-principles calculation. Due to the high light yield under X-ray excitation and the excellent thermal stability of luminescence intensity and decay, a further optimized sample Sr_7.7Eu_0.3(Si₄O₁₂)­Cl₈ could be a potential scintillation material