Multi-Site Cation Control of Ultra-Broadband Near-Infrared Phosphors for Application in Light-Emitting Diodes

Recent Advances in Multi-Site Luminescent Materials: Design, Identification and Regulation

The development of novel phosphor materials with excellent performance and modification of their photoluminescence to meet the higher requirements for applications are the essential research subjects for luminescent materials. Multi-site luminescent materials with crystallographic sites for the activator ions that broaden the tunable range of luminescent spectra and even enhance the luminescent performance have attracted significant attention in the pursuit of high-quality luminescence for white light-emitting diodes. Here, we summarize multi-site luminescence characteristics based on the different kinds of host and activator ions, introduce the identifications of multi-site activator ions via optical analysis, provide a structural analysis and theoretical calculation methods, and introduce the regulation strategies and advance applications of multi-site phosphors. The review reveals the relationship between crystal structure and luminescent properties and discusses future opportunities for multi-site phosphors. This will provide guidance for the design and development of luminescent materials or other materials science.

Multiple Coordination of Chromium Ion Luminescence: A Strategy for Designing Ultra-broadband NIR Long Persistent Luminescent Materials

Near-infrared (NIR) long persistent luminescent (LPL) materials have attracted the interest of many researchers as they have potential applications in many aspects. However, majority of studies on Cr³⁺ ion-doped LPL materials have focused on Cr³⁺ in an octahedral site, and the luminescence is limited to the short-wavelength NIR-I region (700-900 nm), which is detrimental to fully explore Cr³⁺ ion-doped LPL materials with potential applications. In this work, a novel ultra-broadband NIR LPL material, Na₂CaGe₆O₁₄ (NCGO):x%Cr³⁺, was successfully designed and synthesized, covering the luminescence range of 600-1200 nm and having the best afterglow duration of more than 10 h. Combining the luminescence lifetime with the low-temperature spectrum, it was concluded that the luminescence of NCGO:Cr³⁺ consists of the co-emission of Cr³⁺ in octahedra and tetrahedra. And it was confirmed by electron paramagnetic resonance (EPR) spectrum and X-ray absorption near-edge spectrum (XANES). The application prospects of NCGO:x%Cr³⁺ in many aspects were investigated in detail. This work could not only give a reference for researchers to study Cr³⁺ luminescence in multiple coordination but also provide a new strategy for obtaining new ultra-broadband NIR LPL materials.

Molten Salt Synthesis of Broad-Band Near-Infrared InBO3:Cr3+ Submicron Phosphor and Its Luminescent Enhancement by Lanthanide Ion Codoping

Phosphor materials with small particle sizes and high luminescent efficiency are desired for the fabrication of phosphor-converted light-emitting diodes (pc-LEDs). Near-infrared (NIR) pc-LED light sources have great application potential in the food industry and medical fields, which stimulate the extensive exploration of NIR phosphors. In this work, broad-band NIR-emitting InBO₃:Cr³⁺ phosphors with submicron size and spherical morphology are successfully synthesized via the molten salt method. The InBO₃:Cr³⁺ phosphor exhibits a broad emission band covering 700-1000 nm and peaking at ∼820 nm. The maximum emission intensity is obtained for InBO₃:0.02Cr³⁺ with an internal quantum yield (IQY) of ∼62%, which is higher than that of microsized counterparts derived from solid-state reaction. Furthermore, the absorption and emission enhancements are achieved by codoping lanthanide ions into InBO₃:Cr³⁺ submicron phosphors. The codoping of inert La³⁺ ions can increase the absorption efficiency of InBO₃:Cr³⁺, due to the increased octahedral distortion of Cr³⁺ sites. The codoping of active Yb³⁺ ions can significantly enhance the NIR emissions of InBO₃:Cr³⁺ between 950 and 1100 nm. Meanwhile, the increased IQY of ∼73% is achieved for InBO₃:0.02Cr³⁺,0.005Yb³⁺ simultaneously with suppressed thermal quenching, originating from the effective energy transfer from Cr³⁺ to Yb³⁺ ions.

Highly efficient Fe3+-doped A2BB'O6 (A = Sr2+, Ca2+; B, B' = In3+, Sb5+, Sn4+) broadband near-infrared-emitting phosphors for spectroscopic analysis

Near-infrared (NIR)-emitting phosphor-converted light-emitting diodes have attracted widespread attention in various applications based on NIR spectroscopy. Except for typical Cr³⁺-activated NIR-emitting phosphors, next-generation Cr³⁺-free NIR-emitting phosphors with high efficiency and tunable optical properties are highly desired to enrich the types of NIR luminescent materials for different application fields. Here, we report the Fe³⁺-activated Sr_2-yCa_y(InSb)_1-zSn_2zO₆ phosphors that exhibit unprecedented long-wavelength NIR emission. The overall emission tuning from 885 to 1005 nm with broadened full-width at half maximum from 108 to 146 nm was realized through a crystallographic site engineering strategy. The NIR emission was significantly enhanced after complete Ca²⁺ incorporation owing to the substitution-induced lower symmetry of the Fe³⁺ sites. The Ca₂InSbO₆:Fe³⁺ phosphor peaking at 935 nm showed an ultra-high internal quantum efficiency of 87%. The as-synthesized emission-tunable phosphors demonstrated great potential for NIR spectroscopy detection. This work initiates the development of efficient Fe³⁺-activated broadband NIR-emitting phosphors and opens up a new avenue for designing NIR-emitting phosphor materials.

Design and tuning Cr3+-doped near-infrared phosphors for multifunctional applications via crystal field engineering

Nowadays, near-infrared (NIR)-emitting luminescence materials with broad application prospects have drawn great attention. SrGa₁₂O₁₉:Cr³⁺ is a new type of solid light source material that emits NIR light with wide application prospects. However, the narrow full width at half maximum (FWHM) restricts its further multifunctional applications. Therefore, we propose a novel methodology to increase the FWHM by artificially adjusting the strength of the crystal field by doping Sc³⁺ ions. By employing Rietveld refinement results, parameters evolution and Raman spectra, Sc³⁺ ions are proved to successfully occupy the Ga³⁺ crystallographic sites. Combining the spectroscopy characteristics, it was confirmed that the FWHM was increased from 78 nm (127 977 cm^-1) to 104 nm (96 739 cm^-1) with the optimum quantum efficiency of 96.55%. In addition, the excellent thermal stability and unprecedented suitability to a 450 nm blue-chip indicate that it is a potential luminescent material candidate for fluorescence conversion NIR light-emitting diode (LED). Finally, the successful implementation of the night vision on vegetation, biological imaging of human tissue and food inspection for different pork portions illustrate the strong commonality of the method.

Multisite Cation Regulation of Broadband Cyan-Emitting (Ba1-xSrx)9Lu2Si6O24/Eu2+ Phosphors for Full-Spectrum wLEDs

Developing broadband cyan-emitting phosphors is an essential issue to achieve high-quality full-spectrum phosphor-converted white light-emitting diodes. Multisite cation regulation to modify the photoluminescence spectrum is a valid way to achieve broadband emission for phosphors. The Ba₉Lu₂Si₆O₂₄ lattice with various cation sites for activator ions is a preferred host for broadband emitting phosphors. The preferential crystallographic sites of Eu²⁺ in the Ba₉Lu₂Si₆O₂₄ lattice are identified based on the crystal field theory, crystal structure, and bond indices (such as NAC and SBOs) of the cations. Sr substitution in Ba₉Lu₂Si₆O₂₄/Eu²⁺ phosphor affects the location of Eu²⁺ activator ions, which is investigated via the first-principles density functional theory calculations, Rietveld refinement, and luminescence decay curves, and results in the modification of luminescence properties and thermal stability. The Sr-substituted (Ba_0.8Sr_0.2)₉Lu₂Si₆O₂₄/Eu²⁺ sample exhibits a broadband emission spectrum peaked at 471 and 518 nm with a large full width half maximum of 139 nm, covering blue-cyan-green regions, which can be an excellent candidate as broadband cyan-emitting phosphors for high-quality full-spectrum wLEDs.

Broadband, Enhanced, and Antithermally Quenched Near-Infrared Phosphors via a Cosubstitution Approach

Wearable biosensing and food safety inspection devices with high thermal stability, high brightness, and broad near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) could accelerate the next-generation NIR light applications. In this work, NIR La_3-xGd_xGa₅GeO₁₄:Cr³⁺ (x = 0 to 1.5) phosphors were successfully fabricated by a high-temperature solid-state method. Here, by doping Gd³⁺ ions into the La³⁺ sites in the La₃Ga₅GeO₁₄ matrix, a 7.9-fold increase in the photoluminescence (PL) intensity of the Cr³⁺ ions, as well as a remarkably broadened full width at half-maximum (FWHM) of the corresponding PL spectra, is achieved. The enhancements in the PL, PLE intensity, and FWHM are attributed to the suppression of the nonradiative transition process of Cr³⁺ when Gd³⁺ ions are doped into the host, which can be demonstrated by the decay curves. Moreover, the La_1.5Gd_1.5Ga₅GeO₁₄:Cr³⁺ phosphor displays an abnormally negative thermal phenomenon that the integral PL intensity reaches 131% of the initial intensity when the ambient temperature increases to 160 °C. Finally, the broadband NIR pc-LED was fabricated based on the as-explored La_1.5Gd_1.5Ga₅GeO₁₄:Cr³⁺ phosphors combined with a 460 nm chip, and the potential applications for the broadband NIR pc-LEDs were discussed in detail.

Efficient and Tunable Luminescence in Ga2-xInxO3:Cr3+ for Near-Infrared Imaging

Broadband near-infrared (NIR) emitting materials are in great demand as next-generation smart NIR light sources. In this work, a Cr³⁺-substituted phosphor capable of efficiently converting visible to NIR light is developed through the solid solution, Ga_2-xIn_xO₃:Cr³⁺ (0 ≤ x ≤ 0.5). The compounds were prepared using high-temperature solid-state synthesis, and the crystal and electronic structure, morphology, site preference, and photoluminescence properties are studied. The photoluminescence results demonstrate a high quantum yield (88%) and impressive absorption efficiency (50%) when x = 0.4. The NIR emission is tunable across a wide range (713-820 nm) depending on the value of x. Moreover, fabricating a prototype of a phosphor-converted NIR light-emitting diode (LED) device using 450 nm LED and the [(Ga_1.57Cr_0.03)In_0.4]O₃ phosphor showed an output power that reached 40.4 mW with a photoelectric conversion efficiency of 25% driven by a current of 60 mA, while the resulting device was able to identify damaged produce that was undetectable using visible light. These results demonstrate the outstanding potential of this phosphor for NIR LED imaging applications.

A novel Cr3+-doped Lu2CaMg2Si3O12 garnet phosphor with broadband emission for near-infrared applications

Near-infrared (NIR) phosphor-converted light emitting diode (pc-LED) light sources have broad application prospects in environmental science, biomedical and plant growth fields. However, NIR phosphors still suffer from narrowband emission and low thermal stability. Here, we prepared a novel Lu2CaMg2-xSi3O12:xCr3+ (LCMS:xCr3+, x = 0.005-0.06) phosphor with broadband emission by a high-temperature solid state reaction. Under excitation at 450 nm, the emission spectrum of the LCMS:0.05Cr3+ phosphor shows a broadband emission in the range of 650-1000 nm with a large full width at half maximum (FWHM) of 125 nm and an R-line emission of 692 nm. In addition, the LCMS:0.05Cr3+ phosphor has good thermal stability and can maintain 70% emission intensity at 150 °C relative to that at room temperature. The LCMS:0.05Cr3+ phosphor exhibits a high internal quantum efficiency (IQE) of ∼76%. Using this phosphor, a NIR pc-LED with photoelectric efficiencies of 14.8% at 100 mA and 6.0% at 320 mA and NIR output powers of 59.5 mW at 100 mA and 181.0 mW at 320 mA was obtained. The broadband LCMS:0.05Cr3+ phosphor with a NIR emission peaked at 750 nm can serve as a light source for plant cultivation and has superior application prospects in the agriculture field.

Trivalent Chromium Ions Doped Fluorides with Both Broad Emission Bandwidth and Excellent Luminescence Thermal Stability

Recently, trivalent chromium ion doped phosphors have exhibited significant application potential in broadband near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs). However, developing an NIR phosphor with both broad emission bandwidth and excellent luminescence thermal stability is still a great challenge. Here, we demonstrate an NIR phosphor, ScF₃:Cr³⁺, which can fulfill both conditions simultaneously. The prepared phosphors show broadband emission in the range of 700 to 1100 nm, with a full width at half-maximum (FWHM) of 140 nm peaking at 853 nm. These phosphors also demonstrate an excellent luminescence thermal stability (the emission intensity of ScF₃:Cr³⁺ keeps 85.5% at 150 °C compared with the value at room temperature). An NIR pc-LED based on blue LED chips was fabricated and tested. The results show that the NIR pc-LED can yield strong broadband NIR emission. This work not only provides a promising phosphor for the application of NIR pc-LEDs but also has important guiding significance for effect of synthesis conditions on the luminescence properties of Cr³⁺-doped fluorides.

Disorder-Induced Broadband Near-Infrared Persistent and Photostimulated Luminescence in Mg2SnO4:Cr3

Materials with near-infrared (NIR) persistent luminescence (PersL) and NIR-to-NIR photostimulated luminescence (PSL) properties are attractive platforms for photonic energy harvesting and release. In this work, we develop Mg₂SnO₄:Cr as a broadband NIR PersL and NIR-to-NIR PSL material (luminescence maxima at ∼800 nm) and reveal the origin of the PersL and PSL properties. The material has an inverse spinel structure with the Mg²⁺ and Sn⁴⁺ disorder at the Wyckoff 16d site based on the Rietveld refinement. Cr K-edge X-ray absorption near-edge structure (XANES) spectra uncover that the doped Cr ions have a +3 valence state and occupy the disordered (Mg,Sn) site with octahedral coordination. The disorder results in multiple Cr³⁺ centers, and the broadband luminescence originates from the ⁴T₂(⁴F) → ⁴A₂ transition of Cr³⁺ at sites with intermediate crystal field strength. The distribution of trap depths is continuous according to the analysis of thermoluminescence (TL) spectra using the initial rising method, which relates to the random distribution of Mg²⁺ and Sn⁴⁺ at the second coordination sphere of the Cr³⁺ centers rather than the oxygen-related defects. Stimulating the material with a NIR laser, the NIR PersL gets significantly enhanced due to a PSL process. The broadband PersL and PSL are detectable beyond 100 h and have good tissue penetrability and therefore the developed Mg₂SnO₄:Cr³⁺ has potential in applications of optical information storage/reading and autofluorescence-free bioimaging. Finally, three crystal and electronic structure factors are proposed for screening new Cr³⁺-activated PersL and PSL materials.