Cation-defect-induced self-reduction towards efficient mechanoluminescence in Mn2+-activated perovskites

High-Stability Near-Infrared Luminescent Glass Ceramic and Its Applications

Near-infrared (NIR) light, valuable for its biological penetration and invisibility to the human eye, is a crucial tool in biomedicine, environmental monitoring, anticounterfeiting, and information encryption, yet traditional NIR luminescent materials are often unstable in humid conditions. Here, a highly stable MgGeO3:Mn2+ glass ceramic (GC) with NIR luminescence was successfully synthesized. As-obtained GC700 boasts exceptional luminescent capabilities and possesses abundant trap structures, enabling data inscription with a 405 nm laser and retrieval via laser/thermal excitation. Moreover, the emission peak of Mn2+ can be manipulated from 630 to 691 nm by increasing the annealing treatment temperature. With the harnessing of the effective NIR emission, stable carrier characteristics, and numerous trap structures, there is potential for application in information encryption. Accordingly, we explored the application of MgGeO3:Mn2+ GC (GC700 and GC800) samples in precious three-dimensional (3D) information storage and NIR mechanoluminescence (ML) for biological tissue imaging. These applications demonstrate the potential and versatility of electron-capturing NIR luminescent materials in a range of cutting-edge fields.

Near-Infrared Mechanoluminescence from Cr3+-Doped Spinel Nanoparticles for Potential Oral Diseases Detection

Mechanoluminescence (ML) phosphors have found various promising utilizations such as in non-destructive stress sensing, anti-counterfeiting, and bio stress imaging. However, the reported NIR MLs have predominantly been limited to bulky particle size and weak ML intensity, hindering the further practical applications. For this regard, a nano-sized ZnGa2O4: Cr3+ NIR ML phosphor is synthesized by hydrothermal method. By improving the synthesis method and regulating the chemical composition, the NIR ML (600-1000 nm) intensity of such nano-materials has been further enhanced about four times. The reasons for the ML performance difference between micro-/nano- sized phosphors also have been preliminarily analyzed. Additionally, this work probes into the ML mechanism deeply in traps' aspect from band structure and defect formation energy, which can supply significant references for a new approach to develop efficient NIR ML nanoparticles. Finally, due to excellent tissue penetration capability, nano-sized ZnGa2O4:Cr3+ NIR ML phosphor shows great potential applications in biomedical fields such as for the detection of clinical oral diseases.

Realizing Near Infrared Mechanoluminescence Switch in LAGO:Cr Based on Oxygen Vacancy

Mechanoluminescence (ML) materials are featured with the characteristic of "force to light" in response to external stimuli, which have made great progress in artificial intelligence and optical sensing. However, how to effectively enable ML in the material is a daunting challenge. Here, a Lu3Al2Ga3O12:Cr3+ (LAGO: Cr3+) near infrared (NIR) ML material peaked at 706 nm is reported, which successfully realizes the key to unlock ML by the lattice-engineering strategy Ga3+ substitution for Al3+ to "grow" oxygen vacancy (Ov) defects. Combined with thermoluminescence measurements, the observed ML is due to the formation of defect levels and the ML intensity is proportional to it. It is confirmed by X-ray photoelectron spectroscopy and electron paramagnetic resonance that such a process is dominated by Ov, which plays a crucial role in turning on ML in this compound. In addition, potential ML emissions from 4T2 and 2E level transitions are discussed from both experimental and theoretical aspects. This study reveals the mechanism of the change in ML behavior after cation substitution, and it may have important implications for the practical application of Ov defect-regulated turn-on of ML.

Cation Occupation Engineering Realizes Valence Modulation of Manganese-Activated ZnGa2O4

Herein, a series of Mn-activated ZnGa2O4 (ZGO) phosphors have been developed for multifunctional applications. The characteristic green and red emission at 503 and 668 nm of Mn-activated ZGO phosphors can be observed under excitation of 247 and 375 nm, respectively, attributed to the partial oxidation of Mn2+ ions resulting in the coexistence of Mn2+ and Mn4+ ions in the host lattice. The valence modulation of Mn content not only realizes the adjustment of red and green luminescence intensity but also achieves the management of persistent luminescence time and thermo-luminescence time. Further, the codoping of Mg2+ could transform the position occupancy preference of Mn and effectively facilitate the conversion of Mn2+ to Mn4+, leading to the regulation of the valence state of manganese ions. Surprisingly, the existence of Mg2+ ions broadens the emission band of Mn4+ and enhances the photoluminescence intensity to 3.8 times, which can be ascribed to the weakened crystal field leading to the downward shift of the 4T2 energy level and the increase of Mn4+ concentration. For this valence modulation behavior, two different hypotheses about the occupancy of Mg2+ have been proposed to explain the corresponding phenomenon. Finally, the potential applications of the synthesized phosphors have been explored in advanced anticounterfeiting strategies, information storage, and plant lighting field.

Visible-to-Near-Infrared Mechanoluminescence in Bi-Activated Spinel Compounds for Multiple Information Anticounterfeiting

Mechanoluminescence (ML) is the nonthermal luminescence generated in the process of force-to-light conversion, which has broad prospects in stress sensing, wearable devices, biomechanics, and multiple information anticounterfeiting. Multivalence emitter ions utilize their own self-reduction process to realize multiband ML without introducing another dopant, such as Eu3+/Eu2+, Sm3+/Sm2+, and Mn4+/Mn2+. However, self-reduction-induced ML in bismuth-activated materials has rarely been reported so far. In this work, a novel visible-to-near-infrared (vis-NIR) ML induced by the self-reduction of Bi3+ to Bi2+ in the spinel-type compound (MgGa2O4) is reported. The photoluminescence (PL) spectra, PL excitation (PLE) spectra, and PL lifetime curves demonstrate that Bi3+/Bi2+ ions are the main luminescence centers. Notably, the possible self-reduction model is proposed, where a magnesium vacancy (VMg″) is considered as the driving force for the self-reduction of Bi3+ to Bi2+. Furthermore, an oxygen vacancy (VO••) is confirmed by electron paramagnetic resonance (EPR) spectroscopy. Combined with thermoluminescence (TL) glow curves and ML spectra, a plausible trap-controlled ML mechanism is illustrated, where electron-hole (VO••/VMg″) pairs play a significant role in capturing electrons and holes. It is worth noting that the proof-of-concept dual-mode electronic signature application is implemented based on the flexible ML film, which improves the capabilities of signature anticounterfeiting for high-level security applications. Besides, multistimulus-responsive luminescence behaviors of the ML film are realized under the excitation of a 254 nm UV lamp, thermal disturbance, 980 nm laser, and mechanical stimuli. In general, this study provides new insights into designing vis-NIR ML materials toward wider application possibilities.

The luminescent properties of metal halides are determined by the inorganic framework and solvent molecules

The luminescent properties of metal halides are usually considered to be determined by the inorganic framework. In this work, we propose that the luminescent properties of metal halides are determined by both the inorganic framework and the solvent [Denoted as (inorganic framework + n·solvent molecules), n = 0, 1, 2…] through the abundant solvatochromic or thermochromic effect of tetrabutylammonium lead bromides [TPB, T = TBA (tetrabutylammonium), P = Pb (lead), B = Br (bromide)] containing water (H2O) and ethanol (EtOH). One-dimensional (1D) TPB can form ligands of [[Pb5Br18]8- + 2H2O(H)], [[Pb5Br18]8- + 2H2O(H) + 2H2O] and [[Pb5Br18]8- + 2EtOH] by solvent or heat treatment has completely different luminescent properties resulting from different solvents. They exhibit broad spectral emission due to strong electron-phonon coupling, as do other 1D metal halides. However, the 1D TPB containing only [[Pb5Br18]8- achieves extremely rare narrow-band green emission, with full width at half maximum (FWHM) of 21 nm at room temperature and 8 nm at low temperature, color gamut covers 95 % of the International Telecommunication Union recommendation 2020 standard. This work provides new guidance for the modulation of photophysical properties of metal halides, as well as new materials for the display and smart materials fields.

Pressure-Induced Remarkable Spectral Red-Shift in Mn2+ -Activated NaY9 (SiO4 )6 O2 Red-Emitting Phosphors for High-Sensitive Optical Manometry

To settle the low sensitivity of luminescent manometers, the Mn2+ -activated NaY9 (SiO4 )6 O2 red-emitting phosphors with splendid pressure sensing performances are developed. Excited by 408 nm, the resulting products emit bright red emission originating from 4 T1 (4 G) → 6 A1 transition of Mn2+ , in which the optimal concentration of the activator ion is ≈1 mol%. Moreover, the admirable thermal stability of the developed phosphors is studied and confirmed by the temperature-dependent emission spectra, based on which the activation energy is derived to be 0.275 eV. By analyzing the pressure-dependent Raman spectra, the structural stability of the synthesized compounds at extreme conditions is verified. Furthermore, the designed phosphors exhibit remarkable spectral red-shift at elevated pressure. Especially, as pressure increases from 0.75 to 7.16 GPa, the emission band centroid shifts from 617.2 to 663.4 nm, resulting in a high sensitivity (dλ/dP) of 7.00 nm GPa-1 , whereas the full width at half maximum (FWHM) increases from 83.0 to 110.6 nm, leading to the ultra-high sensitivity (dFWHM/dP) of 10.13 nm GPa-1 . These achievements manifest that the designed red-emitting phosphors are appropriate for ultrasensitive optical manometry. More importantly, the developed manometer is a current global leader in sensitivity, when operating in the band-width mode, that is, FWHM.

Optical enhancement of highly efficient organic-inorganic oxyfluoride red phosphors via the cation co-doping strategy

Herein, a new organic cationic matrix [N(CH3)4]3MoO3F3 suitable for Mn4+ doping was constructed. Due to the large steric hindrance of N[CH3]4+ (TMA), charge compensation defects can be effectively prevented in the heterovalent Mn4+-doping process, and a high IQE (91.05%) was obtained. Through the cation co-doping strategy, Mg2+/Zn2+/Li+ cations were introduced into the Mo6+ cationic site, which improved the crystallinity of the matrix and reduced energy losses, so as to improve luminescence intensity, QE, thermal stability, water stability and other spectral properties. Meanwhile, [N(CH3)4]2TiF6:Mn4+ phosphors with the same TMA organic cation and equivalent Mn4+ doping were synthesized for comparison, and the effects of the Mg2+ cation co-doping strategy on the spectral properties of phosphors with different matrix types (fluoride/oxyfluoride) and substitution types (equivalent/non-equivalent) were analyzed. These findings provide the basis for the preparation of new luminescent materials. Furthermore, according to the optical properties exhibited by these phosphors, they are packaged into WLED devices with excellent photoelectric properties, which are suitable for indoor lighting and display fields.