Insight into the Relationship between Crystal Structure and Crystal-Field Splitting of Ce³⁺ Doped Garnet Compounds

The common understanding of the negative relationship between bond lengths and crystal-field splitting (CFS) is renewed by Ce³⁺ doped garnets in this work. We represent the contradictory relationship between structure data and spectroscopic crystal-field splitting in detail. A satisfactory explanation is given by expressing crystal-field splitting in terms of crystal-field parameters, on the basis of structural data. The results show that not only the bond length, but also the geometrical configuration have influence on the magnitude of crystal-field splitting. Also it is found that the ligand oxygen behaves differently with regard to multiple site substitution in garnet structure

Host Dependency of Boundary between Strong and Weak Crystal Field Strength of Cr³⁺ Luminescence

Cr3+ doped near-infrared phosphors hold significant applications and generate considerable research interest. The critical parameter for assessing the strength of the crystal field for Cr3+ in the Tanabe–Sugano diagram is the boundary value of Dq/B, representing the ratio of crystal field splitting to the Racah parameter B. Nevertheless, there are conflicting values for this parameter, as reported in various studies, such as 2.1, 2.2, and 2.3 for C/B = 4.5–4.8. Moreover, some Cr3+ doped phosphors with wide-band emissions exhibit a Dq/B value that falls within the region of a contradictory strong field. In this study, we numerically determine the boundary value of Dq/B, which distinguishes between strong and weak fields. The results then demonstrate a dependence on the host material and are correlated with the values of Racah parameters B and C. This work resolves the inconsistency between the boundary values of Dq/B and the emission profile of Cr3+, providing researchers with a more profound comprehension of Cr3+ luminescence

Insight into the Controlled Synthesis of Cu₂Zn(Ge,Sn)S₄ Nanoparticles with Selective Grain Size

Controlled synthesis of absorber materials Cu₂ZnGeS₄ (CZGS) has been performed using different Ge precursors, including GeCl₄ and the self-synthesized Ge complexes with Ge-glycolic acid (denoted as Ge-Gly), Ge-tartaric acid (denoted as Ge-Tar), and Ge-citric acid (denoted as Ge-Cit). The grain size of as-prepared CZGS nanocrystals (NCs) is dependent on the Ge precursors. All four Ge precursors enabled the wurtzstannite CZGS phase formation. The Ge-Cit precursor led to the formation of monodispersed NCs owing to the fact that the undissolved metal-Cit complex in OLA absorbed the small CZSG NCs and avoided the irregular crystalline behavior. The other three precursors induced two different sizes, and the corresponding reaction mechanism has been proposed. Moreover, the Cu₂ZnGe_1–<i>x</i>Sn_<i>x</i>S₄ NCs with different Ge/Sn ratios were prepared using the Ge-Cit precursor, verifying the general effect on the phase formation and selective grain sizes. The compositional effect on the band gap variation and morphologies of Cu₂ZnGe_1–<i>x</i>Sn_<i>x</i>S₄ was also studied

Controllable Synthesis and Optical Properties of ZnS:Mn²⁺/ZnS/ZnS:Cu²⁺/ZnS Core/Multishell Quantum Dots toward Efficient White Light Emission

The ability to control dopants and defects, as well as the core/shell structures, of quantum dots (QDs) is an essential nanotechnology to modify and optimize their photoluminescence properties. Herein, the optimized ZnS:Mn²⁺/ZnS/ZnS:Cu²⁺/ZnS core/multishell QDs have been prepared, and their luminescence properties depending on the ratios of the starting materials and the injection temperature of an extra sulfur source were discussed; finally the white light can be possibly obtained by mixing the blue light (emission peak at 450 nm originating from Cu²⁺ dopants or emission peaks at 405 and 430 nm corresponding to a defect emission center) and orange light (emission peak at 585 nm from Mn²⁺ dopants). As a controlled synthesis comparison, the optimum core/shell structures and key synthesis parameters have been determined, and the quantum yield (QY) of the as-obtained ZnS:Mn²⁺/ZnS/ZnS:Cu²⁺/ZnS core/multishell white light emitting QDs without defect emission was determined to be 38%. The practical white light device prototype has been also fabricated and the CIE color coordinate of (0.32, 0.34) with a warm white light has been realized upon the excitation of the commercial 370 nm UV LED chip, which demonstrated potential application for micro/nano optical functional devices

Ethylenediamine-Assisted Hydrothermal Synthesis of NaCaSiO₃OH: Controlled Morphology, Mechanism, and Luminescence Properties by Doping Eu³⁺/Tb³⁺

This paper demonstrates a facile hydrothermal method using ethylenediamine (EDA) as a “shape modifier” for the controlled synthesis of rod bunch, decanedron, spindle, flakiness, and flowerlike NaCaSiO₃OH microarchitectures. The set of experimental conditions is important to obtain adjustable shape and size of NaCaSiO₃OH particles, as the change in either the amount of EDA/H₂O or reaction time, or the amount of NaOH. Accordingly, the crystal growth mechanism during the synthesis process is proposed, and it is found that the EDA, acting as the chelating agent and shape modifier, plays a crucial role in fine-tuning the NaCaSiO₃OH morphology. Morphology evolution process of flowerlike NaCaSiO₃OH as a function of NaOH is also explained in detail. Eu³⁺/Tb³⁺ doped NaCaSiO₃OH samples exhibit strong red and green emission under ultraviolet excitation, corresponding to the characteristic electronic transitions of Eu³⁺ and Tb³⁺. These results imply that the morphology-tunable NaCaSiO₃OH:Eu³⁺/Tb³⁺ microarchitectures with tunable luminescence properties are expected to have promising applications for micro/nano optical functional devices

Luminescence Tuning, Thermal Quenching, and Electronic Structure of Narrow-Band Red-Emitting Nitride Phosphors

Exploring high-performance narrow-band red-emitting phosphor is an important challenge for improving white light LEDs. Here, on the basis of three interesting nitride phosphors with similar vierer rings framework structure, two phosphor series, Eu²⁺-doped Sr­(LiAl)_1–<i>x</i>Mg_2<i>x</i>Al₂N₄ and Sr­(LiAl₃)_1–<i>y</i>(Mg₃Si)_<i>y</i>N₄ (<i>x</i>, <i>y</i> = 0–1), are successfully synthesized by a solid state reaction. They show narrow-band red emission with tunable emission peaks from 614 to 658 nm and 607 to 663 nm. The varying luminescence behaviors with composition and structure are discussed based on centroid shift, crystal field splitting and Stokes shift. On the basis of experimental data, we construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of divalent/trivalent lanthanide-doped end-member compounds, and further give thermal quenching mechanism of these series phosphors

Structural Phase Transformation and Luminescent Properties of Ca_2–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ Orthosilicate Phosphors

The orthosilicate phosphors demonstrate great potential in the field of solid-state lighting, and the understanding of the structure–property relationships depending on their versatile polymorphs and chemical compositions is highly desirable. Here we report the structural phase transformation of Ca_2–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ phosphor by Sr²⁺ substituting for Ca²⁺ within 0 ≤ <i>x</i> < 2. The crystal structures of Ca_2–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ are divided into two groups, namely, β phase (0 ≤ <i>x</i> < 0.15) and α′ phase (0.18 ≤ <i>x</i> < 2), and the phase transition (β → α′) mechanism originated from the controlled chemical compositions is revealed. Our findings verified that the phase transition <i>Pnma</i> (α′-phase) ↔ <i>P</i>2₁/<i>n</i> (β-phase) can be ascribed to the second-order type, and Sr²⁺ ions in Ca_2–<i>x</i>Sr_<i>x</i>SiO₄ preferentially occupy the seven-coordinated Ca²⁺ sites rather than the eight-coordinated sites with increasing Sr²⁺ content, which was reflected from the Rietveld refinements and further clarified through the difference of the Ca–O bond length in the two polymorphs of Ca₂SiO₄. The emission peaks of Ce³⁺ shift from 417 to 433 nm in the composition range of 0 ≤ <i>x</i> ≤ 0.8, and the difference in the decay curves can also verify the phase transformation process. Thermal quenching properties of selected Ca_2–<i>x</i>Sr_<i>x</i>SiO₄:Ce³⁺ samples were evaluated, and the results show that the integral emission intensities at 200 °C maintain >90% of that at room temperature suggesting superior properties for the application as white light-emitting diodes (w-LEDs) phosphors

Near-Infrared Luminescence and Color Tunable Chromophores Based on Cr³⁺-Doped Mullite-Type Bi₂(Ga,Al)₄O₉ Solid Solutions

Cr³⁺-activated mullite-type Bi₂Ga_{(4‑<i>x</i>)}­Al_<i>x</i>O₉ (<i>x</i> = 0, 1, 2, 3, and 4) solid solutions were prepared by the solid state reaction, and their spectroscopic properties were investigated in conjunction with the structural evolution. Under excitation at 610 nm, Bi₂[Ga_{(4‑<i>y</i>)}Al_<i>y</i>]_3.97­O₉:0.03Cr³⁺ (<i>y</i> = 0, 1, 2, 3, and 4) phosphors exhibited broad-band near-infrared (NIR) emission peaking at ∼710 nm in the range 650–850 nm, and the optimum Cr³⁺ concentrations and concentration quenching mechanism were determined. Except for the interesting NIR emission, the body color changed from white (at <i>x</i> = 0) to green (at <i>x</i> = 0.08) for Bi₂Ga_4–<i>x</i>­O₉:<i>x</i>Cr³⁺, and from light yellow (at <i>x</i> = 0) to deep brown (at <i>x</i> = 0.08) for Bi₂Al_4–<i>x</i>­O₉:<i>x</i>Cr³⁺, respectively. Moreover, as a result of variable Al/Ga ratio, the observed body color for Bi₂[Ga_{(4‑<i>y</i>)}Al_<i>y</i>]_3.97­O₉:0.03Cr³⁺ (<i>y</i> = 0, 1, 2, 3, and 4) varied from deep brown to green. The relationship between the observed colors and their diffuse reflectance spectra were also studied for the understanding of the different absorption bands. The results indicated that Cr³⁺-doped Bi₂Ga_{(4‑<i>x</i>)}­Al_<i>x</i>O₉ solid solutions appeared as the bifunctional materials with NIR phosphors and color-tunable pigments

Near-Infrared Luminescence and Color Tunable Chromophores Based on Cr³⁺-Doped Mullite-Type Bi₂(Ga,Al)₄O₉ Solid Solutions

Cr³⁺-activated mullite-type Bi₂Ga_{(4‑<i>x</i>)}­Al_<i>x</i>O₉ (<i>x</i> = 0, 1, 2, 3, and 4) solid solutions were prepared by the solid state reaction, and their spectroscopic properties were investigated in conjunction with the structural evolution. Under excitation at 610 nm, Bi₂[Ga_{(4‑<i>y</i>)}Al_<i>y</i>]_3.97­O₉:0.03Cr³⁺ (<i>y</i> = 0, 1, 2, 3, and 4) phosphors exhibited broad-band near-infrared (NIR) emission peaking at ∼710 nm in the range 650–850 nm, and the optimum Cr³⁺ concentrations and concentration quenching mechanism were determined. Except for the interesting NIR emission, the body color changed from white (at <i>x</i> = 0) to green (at <i>x</i> = 0.08) for Bi₂Ga_4–<i>x</i>­O₉:<i>x</i>Cr³⁺, and from light yellow (at <i>x</i> = 0) to deep brown (at <i>x</i> = 0.08) for Bi₂Al_4–<i>x</i>­O₉:<i>x</i>Cr³⁺, respectively. Moreover, as a result of variable Al/Ga ratio, the observed body color for Bi₂[Ga_{(4‑<i>y</i>)}Al_<i>y</i>]_3.97­O₉:0.03Cr³⁺ (<i>y</i> = 0, 1, 2, 3, and 4) varied from deep brown to green. The relationship between the observed colors and their diffuse reflectance spectra were also studied for the understanding of the different absorption bands. The results indicated that Cr³⁺-doped Bi₂Ga_{(4‑<i>x</i>)}­Al_<i>x</i>O₉ solid solutions appeared as the bifunctional materials with NIR phosphors and color-tunable pigments