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

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

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

Novel Red-Emitting Ba₂Tb(BO₃)₂Cl:Eu Phosphor with Efficient Energy Transfer for Potential Application in White Light-Emitting Diodes

A novel red-emitting Ba₂Tb­(BO₃)₂Cl:Eu phosphor possessing a broad excitation band in the near-ultraviolet (<i>n</i>-UV) region was synthesized by the solid-state reaction. Versatile Ba₂Tb­(BO₃)₂Cl compound has a rigid open framework, which can offer two types of sites for various valence’s cations to occupy, and the coexistence of Eu²⁺/Eu³⁺ and the red-emitting luminescence from Eu³⁺ with the aid of efficient energy transfer of Eu²⁺–Eu³⁺(Tb³⁺) and Tb³⁺–Eu³⁺ have been investigated. Ba₂Tb­(BO₃)₂Cl emits green emission with the main peak around 543 nm, which originates from ⁵D₄<i> → </i>⁷F₅ transition of Tb³⁺. Ba₂Tb­(BO₃)₂Cl:Eu shows bright red emission from Eu³⁺ with peaks around 594, 612, and 624 nm under <i>n</i>-UV excitation (350–420 nm). The existence of Eu²⁺ can be testified by the broad-band excitation spectrum, UV–vis reflectance spectrum, X-ray photoelectron spectrum, and Eu L₃-edge X-ray absorption spectrum. Decay time and time-resolved luminescence measurements indicated that the interesting luminescence behavior should be ascribed to efficient energy transfer of Eu²⁺–Eu³⁺(Tb³⁺) and Tb³⁺–Eu³⁺ in Ba₂Tb­(BO₃)₂Cl:Eu phosphors

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

Two-Step Design of a Single-Doped White Phosphor with High Color Rendering

A strategy to design step by step an inorganic single-doped white phosphor is demonstrated. The method consists in tuning different contributions of the emission by successively controlling the chemical compositions of the solid solution or nanosegregated host matrix and the oxidation states of the single dopant. We use this approach to design a white phosphor Na₄CaMgSc₄Si₁₀O₃₀:Eu with excellent color rendering (CRI > 90) that is similar to common mixed-phosphor light sources but for a single-phase. We show that this methodology can also be extended to other phosphors for use in diverse applications such as biomedicine or telecommunications

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 and Luminescence Properties of Yellow-Emitting NaScSi₂O₆:Eu²⁺ Phosphors: Eu²⁺ Site Preference Analysis and Generation of Red Emission by Codoping Mn²⁺ for White-Light-Emitting Diode Applications

The structural properties of clinopyroxene NaScSi₂O₆ have been investigated using the X-ray powder diffraction refinement, and the luminescence properties of Eu²⁺ and Eu²⁺/Mn²⁺-activated NaScSi₂O₆ have been studied to explore the new materials for phosphor-converted white light ultraviolet light-emitting diodes (UV-LEDs). Eu²⁺ was introduced into the NaScSi₂O₆ host in the reducing atmosphere, and the preferred crystallographic positions of the Eu²⁺ ions were determined based on the different structural models of the NaScSi₂O₆ host. The as-obtained NaScSi₂O₆:Eu²⁺ phosphor shows greenish yellow emission with the broad-band peak at 533 nm, and efficient energy transfer (ET) takes place between Eu²⁺ and Mn²⁺ in NaScSi₂O₆, leading to a series of color-tunable phosphors emitting at 533 and 654 nm for the designed NaScSi₂O₆:Eu²⁺,Mn²⁺ phosphors under excitation at 365 nm. The ET mechanism of Eu²⁺ and Mn²⁺ has also been evaluated. We have demonstrated that NaScSi₂O₆:Eu²⁺ and NaScSi₂O₆:Eu²⁺,Mn²⁺ materials exhibit great potential to act as the effective phosphors for UV-LEDs

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