Synthesis and photoluminescence properties of high-quality reddish-orange emitting Ca4Nb2O9:Eu3+ phosphors for WLEDs and anti-counterfeiting

In this report, we successfully synthesized a novel trivalent europium (Eu3+)-activated Ca4Nb2O9 phosphor emitting reddish-orange light via its 5D0 → 7F1 and 5D0 → 7F2 transitions. In the Ca4Nb2O9 host, Eu3+ ions exhibited optimal doping at a concentration of 15 mol%, with the concentration-quenching mechanism predominantly driven by electric dipole-dipole interactions. In addition, the Ca4Nb2O9:Eu3+ phosphor exhibited excellent thermal stability with a photoluminescence (PL) intensity of 71.6% at a working temperature of 423 K. Interestingly, the internal PL quantum yield (PLQY) of the optimal sample was obtained to be 87.43%, and the external PLQY was determined to be 47.81%. The fabricated white light-emitting diode that employed this optimized phosphor alongside commercial phosphors, via a novel silica epoxy gel (parts A and B)-based method, exhibited good color rendering index (color rendering index = 80.65), excellent warm-correlated color temperature (correlated color temperature = 3753 K), and Commission International de l'Eclairage chromaticity coordinate (0.3922, 0.3845). Moreover, the optimal phosphor was introduced into the polyvinyl alcohol (PVA) polymer film, creating a translucent film. These films were then fabricated on glass, plastic, and card, which showed a satisfying emission under ultraviolet radiation. Consequently, the proposed Eu3+-activated Ca4Nb2O9 phosphors can be used as light sources and the Ca4Nb2O9:Eu3+-PVA film is proposed for anti-counterfeiting applications.

Construction of thermally stable Tb3+-activated green-emitting phosphors: dual driving strategy of doping concentration and excitation wavelength

The realization of thermally stable Tb3+-doped green emission at high temperatures in solid-state lighting is still a crucial challenge. Nevertheless, the study on modulating the thermally stable luminescence at high temperatures is seldom reported. The position of the intervalence charge transfer (IVCT) energy level is used to systematically investigate the thermal quenching performance of Tb3+-activated green-emitting phosphors with varying concentration gradients of Gd1-xTaO4:xTb3+ (x = 0.1%, 0.5%, and 2%) in this study. The IVCT energy levels were determined according to the empirical formula to show a decreasing trend, consistent with the position of the IVCT energy levels measured in the excitation and diffuse reflectance spectra. Moreover, the thermal quenching performance of different wavelength excitation positions (host absorption, 4f-5d of Tb3+, and Tb3+-Ta5+ IVCT band) is quite different. The modulation of thermal quenching performance among distinct phosphors when subjected to host excitation or IVCT excitation can be elucidated through optimal positions within the energy levels associated with IVCT. The diverse concentration gradient samples exhibit varying degrees of thermal quenching performance in the variable-temperature spectra. The fluorescence lifetimes of the samples are generally comparable but slightly lower. The quantum efficiency rapidly improves as the Tb concentration increases. The underlying mechanism governing this phenomenon is elucidated by constructing a model that encapsulates the interplay between the compensating and quenching channels, in addition to the energy conversion of Tb3+ into Gd3+. The abovementioned results indicate that the dual driving scheme of the doping concentration and excitation wavelength is an effective means to regulate the thermal quenching performance of Tb-activated green-emitting tantalate phosphors.

Multi-mode anti-counterfeiting guarantees from a single material CaCd2Ga2Ge3O12:Tb3+,Yb3+ - two stimuli-responsive and four-state emission

Luminescent anti-counterfeiting materials have drawn much attention in anti-counterfeiting applications due to their photochemical stability and emission patterns. However, conventional materials majorly use single-mode luminescence, leaving a growing demand for new materials to prevent counterfeiting. In this work, multi-mode anti-counterfeiting is guaranteed from a single luminescent material CaCd₂Ga₂Ge₃O₁₂:Tb³⁺,Yb³⁺via a high-temperature solid-state reaction. The experimental result showed that this single material features green luminescence with excellent photoluminescence, afterglow, thermoluminescence, and up-conversion luminescence, which are ascribed to Tb³⁺ transitions. Upon co-doping with Yb³⁺ as a sensitiser, the photo-stimuli responsiveness was achieved at 254 and 980 nm excitation sources, respectively, and the thermo-stimuli responsiveness was realised after exposure to UV of 254 nm for 10 s and heating at 45 °C, respectively. The band structure calculation, trap distribution, and effective trap depths were used to explain the luminescence mechanism. Based on the two-stimuli responsiveness and four-state emission performance, we prepared images of optical devices using silk screen printing technology. It was found that the images displayed green emission under different luminescence modes. The results prove that we successfully constructed an advanced luminescence anti-counterfeiting material.

Multimodal and Multicolor Anti-counterfeiting Realized in CaCd2Ga2Ge3O12 with a Single Activator of Mn2

The continuously growing significance of information security and authentication has put forward many new requirements and challenges for modern luminescent materials and anti-counterfeiting technologies. Recently, luminescent materials have attracted much attention in this field owing to their legibility, repeatability, multicolor, and multiple stimuli-responsive nature. In this work, the efficient multicolor and multimodal luminescence material CaCd₂Ga₂Ge₃O₁₂:Mn²⁺ was successfully designed and synthesized using the strategy of single-doped Mn²⁺ in a single matrix. Also, we combined the morphology, crystal structure, energy band calculation, luminescence properties, and trap analysis to study the optical data storage capacity of CaCd₂Ga₂Ge₃O₁₂:Mn²⁺. Interestingly, in the presence of the 254 nm UV lamp, the sample can exhibit a tunable emission color from bule to cyan to yellow by increasing the dopant concentration of Mn²⁺. Also, under the afterglow and thermoluminescence luminescence modes, it presented strong yellow emission centered at 558 nm. Based on the advantage of multiple tunable luminescence, samples were made into anti-counterfeiting ink and were used to print four optical devices through the screen printing technology. The results show that the material has excellent multicolor anti-counterfeiting properties under the three luminescence modes, which has contributed to the development of many kinds of luminescent anti-counterfeiting materials for security purposes.

Efficient Ce3+ → Tb3+ energy transfer pairs with thermal stability and internal quantum efficiency close to unity

The incorporation of Ce3+–Tb3+ pairs has been reported in the Ca3Lu2Si6O18 (CLSO) host for identifying a novel green-emitting material with extremely high internal quantum efficiency (close to unity) and excellent thermal stability.

Tailored Luminescence Output of Bi3+-Doped BaGa2O4 Phosphors with the Assistance of the Introduction of Sr2+ Ions as Secondary Cations

In this work, a tunable luminescence color from yellow to orange of photoluminescence (PL), long persistent luminescence (LPL), and photostimulated luminescence (PSL) is successfully achieved in BaGa₂O₄:Bi³⁺ phosphors with the introduction of Sr²⁺ ions as secondary cations. It is confirmed that broad-band emissions located at 500 and 600 nm originate from the occupation of Bi³⁺ ions at different lattice sites in the BaGa₂O₄ host matrix. The replacement of Sr²⁺ for Ba²⁺ ions makes the emission red-shift from 600 to 650 nm; moreover, two additional emissions appeare at 743 and 810 nm due to the occupational preference of Bi³⁺ ions at Ga³⁺ sites. Furthermore, the doped Sr²⁺ ions promote the reconstruction of the trapping centers, which conduces to the fundamental improvement of the optical storage capacity behavior of Bi³⁺-doped phosphors. Our results clarify the dependence of the luminescence performance on the crystal sites of Bi³⁺ ions with fascinating broad-band emissions in the BaGa₂O₄:0.01Bi³⁺ host matrix and will benefit the design and exploration of Bi³⁺-doped solid solutions for optical storage applications.

Design of highly efficient deep-red emission in the Mn4+ doped new-type structure CaMgAl10O17 for plant growth LED light

The auxiliary light equipment for plant growth requires phosphor-converted light-emitting-diodes (pc-LEDs) with high luminous efficiency and a stable structure, and the properties of phosphors highly determine the performance of the pc-LEDs. This work reports a deep-red emitting phosphor with an ultra-wide response range which is regarded as CaMgAl₁₀O₁₇:Mn⁴⁺. The absorption range spans the ultraviolet, near-ultraviolet, blue, and green light regions from 250 to 550 nm. Under the excitation of the best excitation position at 343 nm, deep-red light at 654 nm is emitted, and the quantum efficiency is as high as 86.7%. The luminous efficiency of the two pc-LED devices prepared based on CaMgAl₁₀O₁₇:Mn⁴⁺ with 395 and 460 nm chips reached 54.3 and 59.6 lm W^-1, respectively. The spectra of the two pc-LEDs exhibit high resemblance to the absorption spectra of chlorophyll A and B in plant growth photosynthesis. These indicate that the CaMgAl₁₀O₁₇:Mn⁴⁺ phosphor can be an excellent candidate for plant growth LED light.

Utilizing energy transfer strategy to produce efficient green luminescence in Ca2LuHf2Al3O12:Ce3+,Tb3+ garnet phosphors for high-quality near-UV-pumped warm-white LEDs

White light-emitting diodes (LEDs) are widely used in lighting and display devices, and the exploration of phosphors with excellent luminescence performance and good stability is the key to the development of white LEDs. In this work, we reported novel near-UV-excited green-emitting Ca₂LuHf₂(AlO₄)₃:Ce³⁺,Tb³⁺ garnet phosphors with efficient Ce³⁺ → Tb³⁺ energy transfer. The CLHA:Ce³⁺,Tb³⁺ green phosphors were successfully synthesized by a high-temperature solid-state method, and the phosphors were characterized by X-ray diffraction, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, elemental mapping, photoluminescence excitation and photoluminescence spectra, the Commission International de I'Eclairage (CIE) chromaticity coordinates, quantum efficiency and temperature-dependent emission spectra. Due to the spin-allowed 4f → 5d transition of Ce³⁺ ions, the CLHA:Ce³⁺,Tb³⁺ phosphors exhibited a strong broad excitation band in the 300-470 nm near-ultraviolet (near-UV) range. Under 408 nm excitation, the CLHA:Ce³⁺,Tb³⁺ phosphors showed strong green light with emission center at 543 nm. The mechanism of energy transfer from Ce³⁺ to Tb³⁺ ions was attributed to quadruple-quadruple interaction. Impressively, the internal quantum efficiency and external quantum efficiency of the optimal CLHA:0.02Ce³⁺,0.5Tb³⁺ green phosphors were measured to be 77.1% and 55.8%, respectively. Finally, using the CLHA:0.02Ce³⁺,0.5Tb³⁺ phosphors as green-emitting color converter, a near-UV-pumped white LED device was fabricated, and under 80 mA driving current the LED device demonstrated bright warm-white light with high color rendering index (93.7), low correlated color temperature (3574 K), CIE chromaticity coordinates (0.3922, 0.3633), and high luminous efficacy (29.35 lm/W).