Highly Efficient and Thermally Stable K3AlF6:Mn4+ as a Red Phosphor for Ultra-High-Performance Warm White Light-Emitting Diodes

Following pioneering work, solution-processable Mn⁴⁺-activated fluoride pigments, such as A₂BF₆ (A = Na, K, Rb, Cs; A₂ = Ba, Zn; B = Si, Ge, Ti, Zr, Sn), have attracted considerable attention as highly promising red phosphors for warm white light-emitting diodes (W-LEDs). To date, these fluoride pigments have been synthesized via traditional chemical routes with HF solution. However, in addition to the possible dangers of hypertoxic HF, the uncontrolled precipitation of fluorides and the extensive processing steps produce large morphological variations, resulting in a wide variation in the LED performance of the resulting devices, which hampers their prospects for practical applications. Here, we demonstrate a prototype W-LED with K₃AlF₆:Mn⁴⁺ as the red light component via an efficient and water-processable cation-exchange green route. The prototype already shows an efficient luminous efficacy (LE) beyond 190 lm/W, along with an excellent color rendering index (Ra = 84) and a lower correlated color temperature (CCT = 3665 K). We find that the Mn⁴⁺ ions at the distorted octahedral sites in K₃AlF₆:Mn⁴⁺ can produce a high photoluminescence thermal and color stability, and higher quantum efficiency (QE) (internal QE (IQE) of 88% and external QE (EQE) of 50.6%.) that are in turn responsible for the realization of a high LE by the warm W-LEDs. Our findings indicate that the water-processed K₃AlF₆ may be a highly suitable candidate for fabricating high-performance warm W-LEDs.

Towards ultrastrong glasses

The development of new glassy materials is key for addressing major global challenges in energy, medicine, and advanced communications systems. For example, thin, flexible, and large-area glass substrates will play an enabling role in the development of flexible displays, roll-to-roll processing of solar cells, next-generation touch-screen devices, and encapsulation of organic semiconductors. The main drawback of glass and its limitation for these applications is its brittle fracture behavior, especially in the presence of surface flaws, which can significantly reduce the practical strength of a glass product. Hence, the design of new ultrastrong glassy materials and strengthening techniques is of crucial importance. The main issues regarding glass strength are discussed, with an emphasis on the underlying microscopic mechanisms that are responsible for mechanical properties. The relationship among elastic properties and fracture behavior is also addressed, focusing on both oxide and metallic glasses. From a theoretical perspective, atomistic modeling of mechanical properties of glassy materials is considered. The topological origin of these properties is also discussed, including its relation to structural and chemical heterogeneities. Finally, comments are given on several toughening strategies for increasing the damage resistance of glass products.

Faraday rotation and photoluminescence in heavily Tb(3+)-doped GeO2-B2O3-Al2O3-Ga2O3 glasses for fiber-integrated magneto-optics

We report on the magneto-optical (MO) properties of heavily Tb³⁺-doped GeO₂-B₂O₃-Al₂O₃-Ga₂O₃ glasses towards fiber-integrated paramagnetic MO devices. For a Tb³⁺ ion concentration of up to 9.7 × 10²¹ cm⁻³, the reported glass exhibits an absolute negative Faraday rotation of ~120 rad/T/m at 632.8 nm. The optimum spectral ratio between Verdet constant and light transmittance over the spectral window of 400–1500 nm is found for a Tb³⁺ concentration of ~6.5 × 10²¹ cm⁻³. For this glass, the crystallization stability, expressed as the difference between glass transition temperature and onset temperature of melt crystallization exceeds 100 K, which is a prerequisite for fiber drawing. In addition, a high activation energy of crystallization is achieved at this composition. Optical absorption occurs in the NUV and blue spectral region, accompanied by Tb³⁺ photoluminescence. In the heavily doped materials, a UV/blue-to-green photo-conversion gain of ~43% is achieved. The lifetime of photoluminescence is ~2.2 ms at a stimulated emission cross-section σ_em of ~1.1 × 10⁻²¹ cm² for ~ 5.0 × 10²¹ cm⁻³ Tb³⁺. This results in an optical gain parameter σ_em*τ of ~2.5 × 10⁻²⁴ cm²s, what could be of interest for implementation of a Tb³⁺ fiber laser.

Transition and post-transition metal ions in borate glasses: Borate ligand speciation, cluster formation, and their effect on glass transition and mechanical properties

A series of transition and post-transition metal ion (Mn, Cu, Zn, Pb, Bi) binary borate glasses was studied with special consideration of the cations impact on the borate structure, the cations cross-linking capacity, and more generally, structure-property correlations. Infrared (IR) and Raman spectroscopies were used for the structural characterization. These complementary techniques are sensitive to the short-range order as in the differentiation of tetrahedral and trigonal borate units or regarding the number of non-bridging oxygen ions per unit. Moreover, vibrational spectroscopy is also sensitive to the intermediate-range order and to the presence of superstructural units, such as rings and chains, or the combination of rings. In order to clarify band assignments for the various borate entities, examples are given from pure vitreous B₂O₃ to meta-, pyro-, ortho-, and even overmodified borate glass compositions. For binary metaborate glasses, the impact of the modifier cation on the borate speciation is shown. High field strength cations such as Zn²⁺ enhance the disproportionation of metaborate to polyborate and pyroborate units. Pb²⁺ and Bi³⁺ induce cluster formation, resulting in PbO_n- and BiO_n-pseudophases. Both lead and bismuth borate glasses show also a tendency to stabilize very large superstructural units in the form of diborate polyanions. Far-IR spectra reflect on the bonding states of modifier cations in glasses. The frequency of the measured cation-site vibration band was used to obtain the average force constant for the metal-oxygen bonding, F_M-O. A linear correlation between glass transition temperature (T_g) and F_M-O was shown for the metaborate glass series. The mechanical properties of the glasses also correlate with the force constant F_M-O, though for cations of similar force constant the fraction of tetrahedral borate units (N₄) strongly affects the thermal and mechanical properties. For paramagnetic Cu- and Mn-borate glasses, N₄ was determined from the IR spectra after deducing the relative absorption coefficient of boron tetrahedral versus boron trigonal units, α = α₄/α₃, using NMR literature data of the diamagnetic glasses.

Origin of broad NIR photoluminescence in bismuthate glass and Bi-doped glasses at room temperature

Bi-doped glasses with broadband photoluminescence in the near-infrared (NIR) spectral range are presently receiving significant consideration for potential applications in telecommunications, widely tunable fiber lasers and spectral converters. However, the origin of NIR emission remains disputed. Here, we report on NIR absorption and emission properties of bismuthate glass and their dependence on the melting temperature. Results clarify that NIR emission occurs from the same centers as it does in Bi-doped glasses. The dependence of absorption and NIR emission of bismuthate glasses on the melting temperature is interpreted as thermal dissociation of Bi(2)O(3) into elementary Bi. Darkening of bismuthate glass melted at 1300 °C is due to the agglomeration of Bi atoms. The presence of Bi nanoparticles is confirmed by transmission electron microscopy, high-resolution energy dispersive x-ray spectroscopy and element distribution mapping. By adding antimony oxide as an oxidation agent to the glass, NIR emission centers can be eliminated and Bi(3+) is formed. By comparing with atomic spectral data, absorption bands at ∼320 , ∼500 , 700 , 800 and 1000 nm observed in Bi-doped glasses are assigned to Bi(0) transitions [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text], respectively, and broadband NIR emission is assigned to the transition [Formula: see text].

Tailored Near-Infrared Photoemission in Fluoride Perovskites through Activator Aggregation and Super-Exchange between Divalent Manganese Ions

Biomedical imaging and labeling through luminescence microscopy requires materials that are active in the near-infrared spectral range, i.e., within the transparency window of biological tissue. For this purpose, tailoring of Mn²⁺-Mn²⁺ activator aggregation is demonstrated within the ABF₃ fluoride perovskites. Such tailoring promotes distinct near-infrared photoluminescence through antiferromagnetic super-exchange across effective dimers. The crossover dopant concentrations for the occurrence of Mn²⁺ interaction within the first and second coordination shells comply well with experimental observations of concentration quenching of photoluminescence from isolated Mn²⁺ and from Mn²⁺-Mn²⁺ effective dimers, respectively. Tailoring of this procedure is achieved via adjusting the Mn-F-Mn angle and the Mn-F distance through substitution of the A⁺ and/or the B²⁺ species in the ABF₃ compound. Computational simulation and X-ray absorption spectroscopy are employed to confirm this. The principle is applied to produce pure anti-Stokes near-infrared emission within the spectral range of ≈760-830 nm from codoped ABF₃:Yb³⁺,Mn²⁺ upon excitation with a 976 nm laser diode, challenging the classical viewpoint where Mn²⁺ is used only for visible photoluminescence: in the present case, intense and tunable near-infrared emission is generated. This approach is highly promising for future applications in biomedical imaging and labeling.

Bandgap guidance in hybrid chalcogenide-silica photonic crystal fibers

We report a hybrid chalcogenide-silica photonic crystal fiber made by pressure-assisted melt-filling of molten glass. Photonic bandgap guidance is obtained at a silica core placed centrally in a hexagonal array of continuous centimeters-long chalcogenide strands with diameters of 1.45 μm. In the passbands of the cladding, when the transmission through the silica core is very weak, the chalcogenide strands light up with distinct modal patterns corresponding to Mie resonances. In the spectral regions between these passbands, strong bandgap guidance is observed, where the silica core transmission loss is 60 dB/cm lower. The pressure-assisted fabrication approach opens up new ways of integrating sophisticated glass-based devices into optical fiber circuitry with potential applications in supercontinuum generation, magneto-optics, wavelength selective devices, and rare-earth-doped amplifiers with high gain per unit length.

Quantitative image analysis for evaluating the abrasion resistance of nanoporous silica films on glass

The abrasion resistance of coated glass surfaces is an important parameter for judging lifetime performance, but practical testing procedures remain overly simplistic and do often not allow for direct conclusions on real-world degradation. Here, we combine quantitative two-dimensional image analysis and mechanical abrasion into a facile tool for probing the abrasion resistance of anti-reflective (AR) coatings. We determine variations in the average coated area, during and after controlled abrasion. Through comparison with other experimental techniques, we show that this method provides a practical, rapid and versatile tool for the evaluation of the abrasion resistance of sol-gel-derived thin films on glass. The method yields informative data, which correlates with measurements of diffuse reflectance and is further supported by qualitative investigations through scanning electron microscopy. In particular, the method directly addresses degradation of coating performance, i.e., the gradual areal loss of antireflective functionality. As an exemplary subject, we studied the abrasion resistance of state-of-the-art nanoporous SiO₂ thin films which were derived from 5–6 wt% aqueous solutions of potassium silicates, or from colloidal suspensions of SiO₂ nanoparticles. It is shown how abrasion resistance is governed by coating density and film adhesion, defining the trade-off between optimal AR performance and acceptable mechanical performance.

Optical breathing of nano-porous antireflective coatings through adsorption and desorption of water

We report on the direct consequences of reversible water adsorption on the optical performance of silica-based nanoporous antireflective (AR) coatings as they are applied on glass in photovoltaic and solar thermal energy conversion systems. In situ UV-VIS transmission spectroscopy and path length measurements through high-resolution interferometric microscopy were conducted on model films during exposure to different levels of humidity and temperature. We show that water adsorption in the pores of the film results in a notable increase of the effective refractive index of the coating. As a consequence, the AR effect is strongly reduced. The temperature regime in which the major part of the water can be driven-out rapidly lies in the range of 55°C and 135°C. Such thermal desorption was found to increase the overall transmission of a coated glass by ~ 1%-point. As the activation energy of isothermal desorption, we find a value of about 18 kJ/mol. Within the experimental range of our data, the sorption and desorption process is fully reversible, resulting in optical breathing of the film. Nanoporous AR films with closed pore structure or high hydrophobicity may be of advantage for maintaining AR performance under air exposure.

Metal-organic framework and inorganic glass composites

Metal-organic framework (MOF) glasses have become a subject of interest as a distinct category of melt quenched glass, and have potential applications in areas such as ion transport and sensing. In this paper we show how MOF glasses can be combined with inorganic glasses in order to fabricate a new family of materials composed of both MOF and inorganic glass domains. We use an array of experimental techniques to propose the bonding between inorganic and MOF domains, and show that the composites produced are more mechanically pliant than the inorganic glass itself.