Efficient Luminescence Enhancement of Mg2TiO4:Mn4+ Red Phosphor by Incorporating Plasmonic Ag@SiO2 Nanoparticles

Adjusting luminescence properties of ZnAl2O4:Mn2+(Mn4+), Li+ phosphors through cation substitution

ZnAl2O4 with a typical spinel structure is highly expected to be a novel rare-earth-free ion-activated oxide phosphor with red emission, which holds high actual meaning for advancing phosphor-converted light-emitting diode (pc-LED) lighting. Among the rare-earth-free activators, Mn4+ ions have emerged as one of the most promising activators. Considering the price advantage of MnCO3 generating Mn2+ ions and the charge compensation effect potentially obtaining Mn4+ ions from Mn2+ ions, this research delves into a collection of ZnAl2O4:Mn2+(Mn4+), x Li+ (x = 0%-40%) phosphors with Li+ as co-dopant and MnCO3 as Mn2+ dopant source prepared by a high temperature solid-state reaction method. The lattice structure was investigated using X-ray diffraction (XRD), photoluminescence (PL), and photoluminescence excitation (PLE) spectroscopy. Results suggest a relatively high probability of Li+ ions occupying Zn2+ lattice sites. Furthermore, Li+ ion doping was assuredly found to facilitate the oxidization of Mn2+ to Mn4+, leading to a shift of luminescence peak from 516 to 656 nm. An intriguing phenomenon that the emission color changed with the Li+ doping content was also observed. Meanwhile, the luminescence intensity and quantum yield (QY) at different temperatures, as well as the relevant thermal quenching mechanism, were determined and elucidated detailedly.

Enriching the Deep-Red Emission in (Mg, Ba)3M2GeO8: Mn4+ (M = Al, Ga) Compositions for Light-Emitting Diodes

Red emission from Mn⁴⁺-containing oxides inspired the development of high color rendering and cost-effective white-light-emitting diodes (WLEDs). Aiming at this fact, a series of new crystallographic site modified (Mg, Ba)₃M₂GeO₈: Mn⁴⁺ (M = Al, Ga) compositions were developed with strong deep-red emission in the reaction to UV and blue lights. The Mg₃Al₂GeO₈ host is composed of three phases: orthorhombic-Mg₃Ga₂GeO₈, orthorhombic-Mg₂GeO₄, and cubic-MgAl₂O₄. However, Mg₃Ga₂GeO₈ secured an orthorhombic crystal structure. Interestingly, Mg₃Al₂GeO₈: Mn⁴⁺ showed a 13-fold more intense emission than Mg₃Ga₂GeO₈: Mn⁴⁺ since Mn⁴⁺ occupancy was preferable to [AlO₆] sites compared to [GaO₆]. The coexisting phases of MgAl₂O₄ and Mg₂GeO₄ in Mg₃Al₂GeO₈: Mn⁴⁺ contributed to Mn⁴⁺ luminescence by providing additional [AlO₆] and [MgO₆] octahedrons for Mn⁴⁺ occupancy. Further, these sites reduced the natural reduction probability of Mn⁴⁺ to Mn²⁺ in [AlO₄] tetrahedrons, which was confirmed using cathodoluminescence analysis for the first time. A cationic substitution strategy was employed on Mg₃M₂GeO₈: Mn⁴⁺ to improve the luminescence, and Mg_3-xBa_xM₂GeO₈: Mn⁴⁺ (M = Al, Ga) phosphors were synthesized. Partial substitution of larger Ba²⁺ ions in Mg²⁺ sites caused structural distortions and generated a new Ba impurity phase, which improved the photoluminescence. Compositionally tuned Mg_2.73Ba_0.27Al_1.993GeO₈: 0.005Mn⁴⁺ exhibited a 35-fold higher emission than that of Mg₃Ga_1.993GeO₈: 0.005Mn⁴⁺. Additionally, this could retain 70% of its ambient emission intensity at 453 K. A warm WLED with a correlated color temperature (CCT) of 3730 K and a CRI of 89 was fabricated by combining the optimized red component with Y₃Al₅O₁₂: Ce³⁺ and 410 nm blue LED. By tuning the ratio of blue (BaMgAl₁₀O₁₇: Eu²⁺), green (Ce_0.63Tb_0.37MgAl₁₁O₁₉), and red (Mg_2.73Ba_0.27Al₂GeO₈: 0.005Mn⁴⁺) phosphors, another WLED was developed using a 280 nm UV-LED chip. This showed natural white emission with a CRI of 79 and a CCT of 5306 K. Meanwhile, three red LEDs were also fabricated using the Mg_2.73Ba_0.27Al_1.993GeO₈: 0.005Mn⁴⁺ phosphor with commercial sources. These could be potential pc-LEDs for plant growth applications.

Up-to-Five-Photon Upconversion from Near-Infrared to Ultraviolet Luminescence Coupled to Aluminum Plasmonic Lattices

The incorporation of upconversion luminescence (UCL) materials into various plasmonic structures promotes light-matter interactions in nanophotonic systems. It has been experimentally demonstrated that UCL enhancement entailing two photons exhibits a quadratic dependence on the excitation intensity. However, in the field of plasmonics, there have not been sufficient studies on high-order multi-photon upconversion processes. We report up-to-five-photon UCL, wherein λ = 1550 nm near-infrared light is converted to 382 nm ultraviolet light, from core-inert shell nanoparticles coupled to aluminum plasmonic lattices. The five-photon UCL intensity of nanoparticles on the plasmonic lattice is over 800 times stronger than that on the flat glass. We demonstrate that the enhancement of UCL scales with the nth power of the local field enhancement for n-photon process. These findings give a strategy to obtain high-order multi-photon UPL with aluminum plasmonic nanostructures and can contribute to anti-counterfeiting application.

Nanoantennas Patterned by Colloidal Lithography for Enhanced Nanophosphor Light Emission

Transparent coatings made of rare-earth doped nanocrystals, also known as nanophosphors, feature efficient photoluminescence and excellent thermal and optical stability. Herein, we demonstrate that the optical antennas prepared by colloidal lithography render thin nanophosphor films with a brighter emission. In particular, we fabricate gold nanostructures in the proximity of GdVO₄:Eu³⁺ nanophosphors by metal evaporation using a mask made of a monolayer of polymer beads arranged in a triangular lattice. Optical modes supported by the antennas can be controlled by tuning the diameter of the polymer spheres in the colloidal mask, which determines the shape of the gold nanostructure, as confirmed by numerical simulations. Confocal microscopy reveals that metallic antennas induce brighter photoluminescence at specific spatial regions of the nanophosphor film at targeted frequencies as a result of the coupling between gold nanostructures and nanophosphors. Patterning of nanophosphor thin layers with arrays of metallic antennas offers an inexpensive nanophotonic solution to develop bright emitting coatings of interest for color conversion, labeling, or anti-counterfeiting.

Amplified Fluorescence by Hollow-Porous Plasmonic Assembly: A New Observation and Its Application in Multiwavelength Simultaneous Detection

Surface plasmon coupled emission (SPCE) is a new analytical technique that provides increased and directional radiation based on the near-field interaction between fluorophores and surface plasmons but suffers from the limitation of insufficient sensitivity. The assembly of hollow-porous plasmonic nanoparticles could be the qualified candidate. After the introduction of gold nanocages (AuNCs), fluorescence signal enhancement was realized by factors over 150 and 600 compared with the normal SPCE and free space emission, respectively, with a fluorophore layer thickness of approximately 10 nm; hence, the unique enhancement of SPCE by the AuNCs effectively overcomes the signal quenching induced by resonance energy transfer (in normal SPCE). This enhancement was proven to be triggered by the superior wavelength match, the enhanced electromagnetic field, and new radiation channel and process induced by the AuNC assembly, which provides an opportunity to increase the detection sensitivity and establish an optimal plasmonic enhancement system. The amplified SPCE system was employed for multiwavelength simultaneous enhancement detection through the assembly of mixed hollow nanoparticles (AuNCs and gold nanoshells), which could broaden the application of SPCE in simultaneous sensing and imaging for multianalytes.

Surface Plasmon Enhancement of Eu3+ Emission Intensity in LaPO4/Ag Nanoparticles

A promising way to improve the performance of luminescent materials is to combine them with noble metal nanoparticles. Herein, a set of silver/europium-doped lanthanum orthophosphate (Ag/La_0.95Eu_0.05PO₄) nanostructures with different concentrations of silver nanoparticles were prepared and investigated. The presented overlap between the strongest europium (Eu³⁺) excitation line and the broad silver nanoparticle surface plasmon resonance makes the combination prospective for coupling. X-ray powder diffraction confirmed the monoclinic monazite structure. The transmission electron microscopy revealed particles with a rod-like shape and ~4 aspect ratio. Photoluminescence spectra show characteristic Eu³⁺ ion red emission. One of the requirements for an enhanced luminescence effect is the precise control of the distance between the noble metal nanoparticles and the emitter ion. The distance is indirectly varied throughout the change of Ag nanoparticle concentration in the La_0.95Eu_0.05PO₄ host. The emission intensity increases with the increase in Ag nanoparticles up to 0.6 mol %, after which the luminescence decreases due to the nanoparticles’ close packing and aggregation leading to the displacement of La_0.95Eu_0.05PO₄ from the vicinity of the metal particles and reabsorption of the emitted light. The emission intensity of La_0.95Eu_0.05PO₄ increases more than three times when the Eu³⁺ excitation is supported by the localized surface plasmon resonance in the Ag/La_0.95Eu_0.05PO₄ nanostructures.

Eu3+ , Sm3+ Deep-Red Phosphors as Novel Materials for White Light-Emitting Diodes and Simultaneous Performance Enhancement of Organic-Inorganic Perovskite Solar Cells

The luminous efficiency of inorganic white light-emitting diodes, to be used by the next generation as light initiators, is continuously progressing and is an emerging interest for researchers. However, low color-rendering index (Ra), high correlated color temperature (CCT), and poor stability limit its wider application. Herein, it is reported that Sm³⁺ - and Eu³⁺ -doped calcium scandate (CaSc₂ O₄ (CSO)) are an emerging deep-red-emitting material with promising light absorption, enhanced emission properties, and excellent thermal stability that make it a promising candidate with potential applications in emission display, solid-state white lighting, and the device performance of perovskite solar cells (PSCs). The average crystal structures of Sm³⁺ -doped CSO are studied by synchrotron X-ray data that correspond to an extremely rigid host structure. Samarium ion is incorporated as a sensitizer that enhances the emission intensity up to 30%, with a high color purity of 88.9% with a 6% increment. The impacts of hosting the sensitizer are studied by quantifying the lifetime curves. The CaSc₂ O₄ :0.15Eu³⁺ ,0.03Sm³⁺ phosphor offers significant resistance to thermal quenching. The incorporation of lanthanide ion-doped phosphors CSOE into PSCs is investigated along with their potential applications. The CSOE-coated PSCs devices exhibit a high current density and a high power conversion efficiency (15.96%) when compared to the uncoated control devices.