Temperature-driven n–p conduction type switching without structural transition in a Cu-rich chalcogenide, NaCu5S3

Chem. Commun.
, 2020,56, 4882-4885. https://doi.org/10.1039/D0CC01429J

We report for the first time the discovery of reversible n–p conduction type switching in a chalcogenide, NaCu5S3, without structural transition. AC impedance and first-principles simulations of the ionic migration confirmed the local melting trends of the hexagonal copper lattice at high temperatures, which could result in superionic conductivity within NaCu5S3.

Crystal structure and luminescence properties of lead-free metal halides (C6H5CH2NH3)3MBr6 (M = Bi and Sb)

J. Mater. Chem. C, 2020,8, 7322-7329. https://doi.org/10.1039/D0TC00562B

Organic–inorganic hybrid metal halides have received extensive attention owing to their versatile structures and optoelectronic properties. Herein, we report two lead-free metal halides, (PMA)3BiBr6 and (PMA)3SbBr6 [PMA+: (C6H5CH2NH3)+, as the abbreviation of phenylmethylammonium], which possess iso-structural zero-dimensional structures and crystallize in the monoclinic space group P21/c. (PMA)3BiBr6 and (PMA)3SbBr6 exhibit optical band gaps of ∼3.50 and 3.40 eV, respectively, and density functional theory calculations reveal their indirect bandgap behaviors. Upon 350 and 425 nm excitation, (PMA)3BiBr6 and (PMA)3SbBr6 exhibit broadband emission peaking at 510 nm and 625 nm with wide full-widths at half-maximum of ∼153 and 175 nm, respectively. The emission mechanism of the metal halides is attributed to self-trapped exciton emission. The relationship between the crystal structure and luminescence intensity is also discussed. Finally, both metal halides have high decomposition temperatures and are stable for long-term storage under ambient conditions, demonstrating their potential for optoelectronic applications.

Effect of nitrogen substitution on luminescence tuning in garnets

Phys. Chem. Chem. Phys., 2020,22, 9513-9517. https://doi.org/10.1039/D0CP00586J

Anionic substitution is attracting research interest as a property modulation strategy. Although the effect of nitrogen incorporation on luminescence tuning has been widely reported, the correlation between dodecahedral expansion on Si4+–N3− co-substitution and crystal-field splitting of Ce3+ in garnets is rarely discussed. This work is devoted to unraveling the structure–property relationship between anionic substitution and spectroscopy tuning. Ligand movement patterns of a dodecahedron and an octahedron are investigated for tetragonal distortion and inter-facial distance, both of which indicate an energy level shift originating from the crystal-field effect. The quantitative crystal-field calculation is performed on the basis of ligand coordinates to derive the analytical expression for further confirmation. This work complements the substitution effects of both cationic and anionic chemical species on spectral tuning in garnets, and will be helpful in material design and property modulation of garnet-based luminescent materials.

Understanding the abnormal lack of spectral shift with cation substitution in highly efficient phosphor La3Si6N11:Ce3+

Phys. Chem. Chem. Phys., 2020,22, 14162-14168. https://doi.org/10.1039/D0CP01445A

Cation substitution is a common strategy to tune the luminescence by modulating the cell parameter, polyhedral volume and bond length in solid-solution-type phosphors. Generally a close correlation between their cationic composition and spectral peak shifts can be observed. In certain compounds, however, luminescence tuning by cationic modification is almost invalid. This work is devoted to providing a reasonable explanation for the anomaly in Ce3+ doped La3Si6N11, which demonstrates unshifted excitation peaks with various cation substitutions. By simplifying the local coordination polyhedron that accommodates Ce3+ to a truncated square pyramid model, the quantitative crystal-field calculations are conducted to demonstrate the influences of the coordination environment on energy levels. The results show that the crystal-field levels become insensitive to this special type of ligand environment, leading to imperceptible peak shifts. Therefore, the relationship between the cationic composition and luminescence is determined not only by the ionic radii but also by the type of coordination polyhedron. This work shows that studying the coordination environment is helpful for achieving effective luminescence tuning.

Broadband Photoluminescence in 2D Organic–Inorganic Hybrid Perovskites: (C7H18N2)PbBr4 and (C9H22N2)PbBr4

J. Phys. Chem. Lett. 2020, 11, 8, 2934–2940. https://pubs.acs.org/doi/10.1021/acs.jpclett.0c00578.

Organic–inorganic hybrid perovskites have aroused intense research interest because of their excellent physical performance and potential for use in optoelectronic field. Herein, we report two new 2D hybrid lead bromides, (C7H18N2)PbBr4 [C7H18N2 is 1,7-diaminoheptane] and (C9H22N2)PbBr4 [C9H22N2 is 1,9-diaminononane], both of which possess ⟨100⟩-oriented inorganic layers consisting of corner-sharing octahedra. The optical bandgaps are experimentally determined to be 2.76 eV for (C7H18N2)PbBr4 and 2.78 eV for (C9H22N2)PbBr4. Upon 390 nm excitation, (C7H18N2)PbBr4 exhibits white-light emission centered at 600 nm, and (C9H22N2)PbBr4 exhibits red-light emission centered at 620 nm. These broad photoluminescent spectra originate from the synergistic emission of free excitons (FEs) and self-trapped excitons (STEs). This work provides a strategy for realizing single-component white-light emission and efficient red-light emission in two-dimensional perovskites, demonstrating the vast application prospects of 2D perovskites in photoelectric devices.

Yellow persistent luminescence and electronic structure of Ca-α-Sialon: Eu2+

J. Alloys Compd., 2020, 821, 153482. https://doi.org/10.1016/j.jallcom.2019.153482

Ca-α-Sialon: Eu2+, a well-known yellow phosphor, has been widely studied due to its broad UV-blue excitation with high quantum efficiency. Herein, we report the yellow persistent luminescence (PersL) of a series of Ca-α-Sialon: Eu2+ compounds with chemical formula CaSi10-nAl2+nOnN16-nxEu2+ (m = 2, n = 0∼1, x = 0.1%∼8%) prepared by high-temperature solid-state method. Upon 254 nm ultraviolet excitation, Ca-α-Sialon: Eu2+ shows yellow PersL, and the persistent time is strongly dependent on the Eu2+ and oxygen concentrations. The best persistent time is measured to be about 60 min for the CaSi10Al2N16: 0.5% Eu2+ sample. A very broad trap depth distribution, i.e. 0.6–1.4 eV, originating from two categories, are obtained by analyzing preheating thermoluminescence (TL) spectra using initial rise method. Comparing thermoluminescence excitation spectra (TLEs) with photoluminescence excitation spectra (PLEs), we verify that electrons as charge carriers are excited from 4f ground state of Eu2+ to conduction band (CB) directly in the charging process for PersL. By functional theory (DFT) calculations, we verify that the trap levels responsible for PersL are impurity defects including VO and VN in Ca-α-Sialon. Furthermore, the PersL mechanism is given on the basis of constructing the host referred binding energy (HRBE) diagram. By virtue of NIR photo-stimulating PersL spectrum, we demonstrate that Ca-α-Sialon has a potential application in anti-counterfeit and information storage. This work would encourage more exploration of Eu2+-doped nitride phosphor for persistent or long-persistent luminescence.

Enhanced persistent luminescence via Si4+ co-doping in Y3Al2Ga3O12:Ce3+, Yb3+, B3+

J. Lumin., 2020, 222, 117190. https://doi.org/10.1016/j.jlumin.2020.117190.

Persistent luminescence phosphors with long duration and high emitting intensity have attracted considerable attention for applications in safety signage and energy storage. Herein, we successfully introduce non-equivalent ions Si4+ into Al3+ sites in the garnet phosphor Y3Al2Ga3O12:Ce3+,Yb3+,B3+ by conventional solid-state reaction. The persistent luminescence duration has been dramatically enhanced over 40 h at the 0.32 mcd/m2 threshold value after visible light radiation, almost twice longer than the sample without Si4+. Moreover, the afterglow emission intensity of the persistent luminescence is also improved. We confirm that the synthesized phosphors possess not only deeper trap depth but also wider trap distribution and higher trap density after the cooperation of Si4+. The initial rise approach is used by performing a series of thermoluminescence analyses at various temperatures after 432 nm excitation, which demonstrates the exact trap distribution from 0.47 to 1.11 eV. At the end, the mechanism of the persistent luminescence is depicted using a schematic energy diagram of the vacuum referred binding energy of Y3Al2Ga3O12.

Broadband deep‐red‐to‐near‐infrared emission from Mn2+ in strong crystal‐field of nitride MgAlSiN3

 J Am Ceram Soc., 2020; 103: 6793– 6800. https://doi.org/10.1111/jace.17304

Broadband near‐infrared (NIR) phosphors have received increasing attention for fabricating phosphor‐converted light‐emitting diodes (pc‐LEDs) as NIR light source. Most of the reported broadband NIR phosphors originate from Cr3+ in weak crystal field environments. Herein, we report a luminescent material, MgAlSiN3:Mn2+ with CaAlSiN3‐type structure, demonstrating that broadband deep‐red‐to‐NIR emission can be achieved via doping Mn2+ into crystallographic sites with strong crystal field in inorganic solids. This phosphor is synthesized via easy‐handle solid‐state reaction, and the optimized sample, (Mg0.93Mn0.07) AlSiN3 shows an emission band with peak at ~754 nm, FWHM of 150 nm, and internal quantum efficiency of 70.1%. The photoluminescence intensity can further be enhanced by co‐doping Eu2+ as sensitizer. This work provides a new strategy for discovering new broadband NIR phosphors using Mn2+ in strong crystal field as luminescence center.

Crystal structure and photoluminescence properties of (Y1−xCex)4Si2O7N2

Opt. Mater., 2010, 33, 1, 91. https://doi.org/10.1016/j.optmat.2010.08.011

Oxonitridosilicate compounds with compositions of (Y1−xCex)4Si2O7N2 (x = 0–0.1) compounds have been synthesized. The relations of structure, photoluminescence properties and Ce content have been studied. These compounds have a broad band excitation covering from 350 to 450 nm and a broad band emission covering from 450 to 575 nm. There exist four different types of photoluminescence centers, which correspond to four different crystallographic sites accommodating the Ce3+ dopants in the lattice. The energy transfer among Ce3+ ions in different sites enhance with the increasing Ce content, leading to red-shift of the emission peaks and concentration quenching. The relationship between crystallographic sites of Ce3+ ions and luminescence properties is established. (Y1−xCex)4Si2O7N2 is a promising phosphor for use in white light-emitting diodes.

Stability of divalent/trivalent oxidation state of europium in some Sr-based inorganic compounds

J. Lumin., 2012, 132, 7, 1768. https://doi.org/10.1016/j.jlumin.2012.02.013

This work investigates the stability of Eu2+ and Eu3+ in some Sr-based inorganic compounds. Generally reducing condition is adopted in order to obtain Eu2+, however, the Eu doped SrAl2O4/SrLaAlO4 case indicates that for some compounds Eu3+ is stabilized even in reducing atmosphere. Bond valence method is applied to explain this phenomenon and it reveals that crystal structure also determines the valence state of europium cations along with reducing/oxidizing condition. An analysis of other Eu doped Sr-based materials is performed which shows the relationship between Eu2+/Eu3+ stability and the Global Instability Index (GII). This research provides a guideline for synthesizing specific novel Eu2+/Eu3+ phosphors.