Fluorine Passivation Inhibits "Particle Talking" Behaviors under Thermal and Electrical Conditions of Pure Blue Mixed Halide Perovskite Nanocrystals

Owing to outstanding optoelectronic properties, lead halide perovskite nanocrystals (PNCs) are considered promising emitters for next-generation displays. However, the development of pure blue (460-470 nm) perovskite nanocrystal light-emitting diodes (PNC-LEDs), which correspond to the requirements of Rec. 2020 standard, lag far behind that of their green and red counterparts. Here, pure blue CsPb(Br/Cl)3 nanocrystals with remarkable optical performance are demonstrated by a facile fluorine passivation strategy. Prominently, the fluorine passivation on halide vacancies and strong bonding of Pb-F intensely enhance crystal structure stability and inhibit "particle talking" behaviors under both thermal and electrical conditions. Fluorine-based PNCs with high resistance of luminescence thermal quenching retain 70% of photoluminescent intensity when heated to 343 K, which can be attributed to the elevated activation energy for carrier trapping and unchanged grain size. Fluorine-based PNC-LEDs also exhibit stable pure blue electroluminescence (EL) emission with sevenfold promoted luminance and external quantum efficiencies (EQEs), where the suppression of ion migration is further evidenced by a lateral structure device with applied polarizing potential.

Temperature-Dependent Phonon Scattering and Photoluminescence in Vertical MoS2/WSe2 Heterostructures

Transition metal dichalcogenide (TMD) monolayers and their heterostructures have attracted considerable attention due to their distinct properties. In this work, we performed a systematic investigation of MoS2/WSe2 heterostructures, focusing on their temperature-dependent Raman and photoluminescence (PL) characteristics in the range of 79 to 473 K. Our Raman analysis revealed that both the longitudinal and transverse modes of the heterostructure exhibit linear shifts towards low frequencies with increasing temperatures. The peak position and intensity of PL spectra also showed pronounced temperature dependency. The activation energy of thermal-quenching-induced PL emissions was estimated as 61.5 meV and 82.6 meV for WSe2 and MoS2, respectively. Additionally, we observed that the spectral full width at half maximum (FWHM) of Raman and PL peaks increases as the temperature increases, and these broadenings can be attributed to the phonon interaction and the expansion of the heterostructure's thermal coefficients. This work provides valuable insights into the interlayer coupling of van der Waals heterostructures, which is essential for understanding their potential applications in extreme temperatures.

Novel Red-Emitting Eu3+-Doped Y2(WxMo1-xO4)3 Phosphor with High Conversion Efficiency for Lighting and Display Applications

In this study, a series of trivalent europium-doped tungstate and molybdate samples were synthesized using an improved sol-gel and high-temperature solid-state reaction method. The samples had different W/Mo ratios and were calcined at various temperatures ranging from 800 to 1000 °C. The effects of these variables on the crystal structure and photoluminescence characteristics of the samples were investigated. It was found that a doping concentration of 50% for europium yielded the best quantum efficiency based on previous research. The crystal structures were found to be dependent on the W/Mo ratio and calcination temperature. Samples with x ≤ 0.5 had a monoclinic lattice structure that did not change with calcination temperature. Samples with x > 0.75 had a tetragonal structure that remained unchanged with calcination temperature. However, samples with x = 0.75 had their crystal structure solely dependent on the calcination temperature. At 800-900 °C, the crystal structure was tetragonal, while at 1000 °C, it was monoclinic. Photoluminescence behavior was found to correlate with crystal structure and grain size. The tetragonal structure had significantly higher internal quantum efficiency than the monoclinic structure, and smaller grain size had higher internal quantum efficiency than larger grain size. External quantum efficiency initially increased with increasing grain size and then decreased. The highest external quantum efficiency was observed at a calcination temperature of 900 °C. These findings provide insight into the factors affecting the crystal structure and photoluminescence behavior in trivalent europium-doped tungstate and molybdate systems.

Nano-Coprecipitation Route Synthesis of Highly-Efficient Submicron (Sr,Ba)2SiO4:Eu2+ Phosphors with Enhanced Thermal Stability for MicroLED Color Conversion

As the size of MicroLED chips shrinks below 50 μm, the emergence of quantum dots (QDs)-based color conversion with narrow-band emission and nanoscale size properties has become one of the powerful full-color solutions for MicroLED displays. However, the stability and toxicity of quantum dots limit their application in full-color MicroLEDs. The phosphor-based conversion has the prominent features of high thermal and chemical stability relative to those of QD-based conversion. Nevertheless, the particle size of phosphor prepared by a traditional high-temperature solid-state reaction (SSR) is equivalent to or even larger than that of the MicroLED chip. In this work, we propose a strategy to prepare (Sr,Ba)₂SiO₄:0.03Eu²⁺ (SBSO:0.03Eu²⁺) submicron phosphors via a nano-coprecipitation method (NCP) using nanoSi₃N₄ as the Si source materials, which enables the particle size to be reduced while maintaining the luminescence efficiency. The optimized SBSO:0.03Eu²⁺ has an average size of less than 2 μm, showing a narrow band green emission centered at 522 nm, with a full width at half-maximum of 60 nm and an external quantum efficiency of 40.2%. At 150 °C, its thermal stability is greatly enhanced to 80.2% of the emission at room temperature. Further, the mechanism for defect compensation thermal stability is investigated. By employing it as a green emitter, we fabricate a high-performance white LED device (WLED) with a wide color gamut of 86.7% NTSC. This work for the preparation of high-brightness and thermal stability enhancement SBSO:0.03Eu²⁺ phosphor not only provides a facile method but also helps to provide an alternative green fluorescent material for the realization of full color MicroLED.

Improved Photoluminescence Performance of Eu3+-Doped Y2(MoO4)3 Red-Emitting Phosphor via Orderly Arrangement of the Crystal Lattice

In this study, we developed a technology for broadening the 465 nm and 535 nm excitation peaks of Eu³⁺:Y₂(MoO₄)₃ via crystal lattice orderly arrangement. This was achieved by powder particle aggregation and diffusion at a high temperature to form a ceramic structure. The powdered Eu³⁺:Y₂(MoO₄)₃ was synthesized using the combination of a sol-gel process and the high-temperature solid-state reaction method, and it then became ceramic via a sintering process. Compared with the Eu³⁺:Y₂(MoO₄)₃ powder, the full width at half maximum (FWHM) of the excitation peak of the ceramic was broadened by two- to three-fold. In addition, the absorption efficiency of the ceramic was increased from 15% to 70%, while the internal quantum efficiency reduced slightly from 95% to 90%, and the external quantum efficiency was enhanced from 20% to 61%. More interestingly, the Eu³⁺:Y₂(MoO₄)₃ ceramic material showed little thermal quenching below a temperature of 473 K, making it useful for high-lumen output operating at a high temperature.

One- and Two-Photon Excited Photoluminescence and Suppression of Thermal Quenching of CsSnBr3 Microsquare and Micropyramid

Thermal photoluminescence (PL) quenching is fundamentally important for perovskite optoelectronic applications. Herein, we investigated PL characteristics of CsSnBr₃ microsquares and micropyramids synthesized by chemical vapor deposition (CVD) and their PL quenching behavior at high temperature. These microstructures have favorable PL performances in ambient atmosphere. Under two-photon excitation, we observed whispering gallery modes (WGMs) in microsquares and amplified spontaneous emission (ASE) in micropyramids. Reversible PL losses due to thermal effect were observed for both samples. Monotonic blue shifts in PL emission upon temperature increase suggest a band gap widening associated with an emphanisis effect. Temperature-dependent spectral line width analysis reveals that a line width broadening is attributed to the dominant electron-longitudinal optical phonon interaction. The estimated activation energy of thermally assisted nonradiative recombination for CsSnBr₃ microsquares and micropyramids is over 310 meV by the Arrhenius equation, which is higher than CsPbBr₃. These results prove that CsSnBr₃ exhibits better thermal stability than Pb-based perovskites.

N-Rich Porous Polymer with Isolated Tb3+ -Ions Displays Unique Temperature Dependent Behavior through the Absence of Thermal Quenching

The challenge of measuring fast moving or small scale samples is based on the absence of contact between sample and sensor. Grafting lanthanides onto hybrid materials arises as one of the most promising accurate techniques to obtain noninvasive thermometers. In this work, a novel bipyridine based porous organic polymer (bpyDAT POP) was investigated as temperature sensor after grafting with Eu(acac)₃ and Tb(acac)₃ complexes. The bpyDAT POP successfully showed temperature-dependent behavior in the 10-310 K range, proving the potential of amorphous, porous organic frameworks. We observed unique temperature dependent behavior. More intriguingly, instead of the standard observed change in emission as a result of a change in temperature for both Eu³⁺ and Tb³⁺ , the emission spectrum of Tb³⁺ remained constant. This work provides framework- and energy-based explanations for the observed phenomenon. The conjugation in the bpyDAT POP framework is interrupted, creating energetically isolated Tb³⁺ environments. Energy transfer from Tb³⁺ to Eu³⁺ is therefore absent, nor energy back transfer from Tb³⁺ to bpyDAT POP ligand (i.e. no thermal quenching) is detected.

TemperatureDependent Luminescence of RedEmitting Ba2Y5B5O17: Eu3+ Phosphors with Efficiencies Close to Unity for NearUV LEDs

Solid state white light sources based on a near-UV LED chip are gaining more and more attention. This is due to the increasing efficiency of near-UV-emitting LED chips and wider phosphors selection if compared to devices based on blue LED chips. Here, a brief overview is given of the concepts of generating white light employing near-UV LED and some optical properties of the available phosphors are discussed. Finally, the synthesis and optical properties of very efficient red-emitting Ba₂Y₅B₅O₁₇:Eu³⁺ phosphor powder and ceramics is reported and discussed in terms of possible application as a red component in near-UV LED-based white light sources.

Evaluating Thermal Quenching Temperature in Eu3+-Substituted Oxide Phosphors via Machine Learning

One of society's grand challenges is to reduce energy usage in ways that are cost-effective, sustainable, and environmentally benign. Replacing incandescent and compact fluorescent light bulbs with energy-efficient, solid-state white lighting is one of the easiest and most promising solutions. Eu³⁺-substituted inorganic oxide phosphors are one class of materials that can serve as the red component in these new light bulbs, allowing the creation of warm white light. Unfortunately, the emission intensity in most of these materials cannot be reliably maintained at elevated temperatures. There is therefore a need to discover entirely novel phosphor materials that are thermally robust; however, this is generally a prolonged and expensive process requiring extensive synthetic effort. In this work, we develop a machine-learning regression algorithm based on 134 experimentally measured temperature-dependent Eu³⁺ emission data points to rapidly estimate the thermal quenching temperature (T₅₀), which is defined as the temperature when the emission intensity is half of the initial value. The T₅₀ was then predicted for more than 1000 potential oxide Eu³⁺ phosphor hosts using this model. Five compounds with predicted thermal quenching temperatures >423 K were subsequently selected and synthesized for validation of this approach. The phosphors, Sr₂ScO₃F, Cs₂MgSi₅O₁₂, Ba₂P₂O₇, LiBaB₉O₁₅, and Y₃Al₅O₁₂, all exhibit good thermal stability when substituted with Eu³⁺, suggesting the success of our methodology.

Optical Properties of Red-Emitting Rb2Bi(PO4)(MoO4):Eu3+ Powders and Ceramics with High Quantum Efficiency for White LEDs

There are several key requirements that a very good LED phosphor should meet, i.e., strong absorption, high quantum efficiency, high colour purity, and high luminescence quenching temperature. The reported Rb2Bi(PO4)(MoO4):Eu3+ phosphors have all these properties. The Rb2Bi(PO4)(MoO4):Eu3+ phosphors emit bright red light if excited with near-UV radiation. The calculated colour coordinates show good stability in the 77-500 K temperature range. Moreover, sample doped with 50% Eu3+ possesses quantum efficiency close to unity. Besides the powder samples, ceramic disks of Rb2Eu(PO4)(MoO4) specimen were also prepared, and the red light sources from these disks in combination with near-UV emitting LED were fabricated. The obtained results indicated that ceramic disks efficiently absorb the emission of 375 and 400 nm LED and could be applied as a red component in phosphor-converted white LEDs.

Thermoluminescence as a Research Tool to Investigate Luminescence Mechanisms

Thermally stimulated luminescence (TSL) is known as a technique used in radiation dosimetry and dating. However, since the luminescence is very sensitive to the defects in a solid, it can also be used in material research. In this review, it is shown how TSL can be used as a research tool to investigate luminescent characteristics and underlying luminescent mechanisms. First, some basic characteristics and a theoretical background of the phenomenon are given. Next, methods and difficulties in extracting trapping parameters are addressed. Then, the instrumentation needed to measure the luminescence, both as a function of temperature and wavelength, is described. Finally, a series of very diverse examples is given to illustrate how TSL has been used in the determination of energy levels of defects, in the research of persistent luminescence phosphors, and in phenomena like band gap engineering, tunnelling, photosynthesis, and thermal quenching. It is concluded that in the field of luminescence spectroscopy, thermally stimulated luminescence has proven to be an experimental technique with unique properties to study defects in solids.

Co-precipitation Synthesis and Optical Properties of Mn4+-doped Hexafluoroaluminate w-LED Phosphors

Mn4+-activated hexafluoroaluminates are promising red-emitting phosphors for white light emitting diodes (w-LEDs). Here, we report the synthesis of Na₃AlF₆:Mn4+, K₃AlF₆:Mn4+ and K₂NaAlF₆:Mn4+ phosphors through a simple two-step co-precipitation method. Highly monodisperse large (~20 μm) smoothed-octahedron shaped crystallites are obtained for K₂NaAlF₆:Mn4+. The large size, regular shape and small size distribution are favorable for application in w-LEDs. All Mn4+-doped hexafluoroaluminates show bright red Mn4+ luminescence under blue light excitation. We compare the optical properties of Na₃AlF₆:Mn4+, K₃AlF₆:Mn4+ and K₂NaAlF₆:Mn4+ at room temperature and 4 K. The luminescence measurements reveal that multiple Mn4+ sites exist in M₃AlF₆:Mn4+ (M = Na, K), which is explained by the charge compensation that is required for Mn4+ on Al3+ sites. Thermal cycling experiments show that the site distribution changes after annealing. Finally, we investigate thermal quenching and show that the luminescence quenching temperature is high, around 460-490 K, which makes these Mn4+-doped hexafluoroaluminates interesting red phosphors for w-LEDs. The new insights reported on the synthesis and optical properties of Mn4+ in the chemically and thermally stable hexafluoroaluminates can contribute to the optimization of red-emitting Mn4+ phosphors for w-LEDs.

Composition Screening in Blue-Emitting Li4Sr1+xCa0.97-x(SiO4)2:Ce3+ Phosphors for High Quantum Efficiency and Thermally Stable Photoluminescence

Photoluminescence quantum efficiency (QE) and thermal stability are important for phosphors used in phosphor-converted light-emitting diodes (pc-LEDs). Li₄Sr_1+xCa_0.97-x(SiO₄)₂:0.03Ce³⁺ (-0.7 ≤ x ≤ 1.0) phosphors were designed from the initial model of Li₄SrCa(SiO₄)₂:Ce³⁺, and their single-phased crystal structures were found to be located in the composition range of -0.4 ≤ x ≤ 0.7. Depending on the substitution of Sr²⁺ for Ca²⁺ ions, the absolute QE value of blue-emitting composition-optimized Li₄Sr_1.4Ca_0.57(SiO₄)₂:0.03Ce³⁺ reaches ∼94%, and the emission intensity at 200 °C remains 95% of that at room temperature. Rietveld refinements and Raman spectral analyses suggest the increase of crystal rigidity, increase of force constant in CeO₆, and decrease of vibrational frequency by increasing Sr²⁺ content, which are responsible for the enhanced quantum efficiency and thermal stability. The present study points to a new strategy for future development of the pc-LEDs phosphors based on local structures correlation via composition screening.

Luminescence and Luminescence Quenching of K2Bi(PO4)(MoO4):Eu3+ Phosphors with Efficiencies Close to Unity

A very good light emitting diode (LED) phosphor must have strong absorption, high quantum efficiency, high color purity, and high quenching temperature. Our synthesized K₂Bi(PO₄)(MoO₄):Eu³⁺ phosphors possess all of the mentioned properties. The excitation of these phosphors with the near-UV or blue radiation results in a bright red luminescence dominated by the ⁵D₀ → ⁷F₂ transition at ∼615 nm. Color coordinates are very stable when changing Eu³⁺ concentration or temperature in the range of 77-500 K. Furthermore, samples doped with 50% and 75% Eu³⁺ showed quantum efficiencies close to 100% which is a huge benefit for practical application. Temperature dependent luminescence measurements showed that phosphor performance increases with increasing Eu³⁺ concentration. K₂Eu(PO₄)(MoO₄) sample at 400 K lost only 20% of the initial intensity at 77 K and would lose half of the intensity only at 578 K. Besides, the ceramic disks with thicknesses of 0.33 and 0.89 mm were prepared from K₂Eu(PO₄)(MoO₄) powder, and it turned out that they efficiently converted the radiation of 375 nm LED to the red light. The conversion of 400 nm LED radiation to the red light was not complete; thus, the light sources with various tints of purple color were obtained. The combination of ceramic disks with 455 nm LED yielded the light sources with tints of blue color due to the low absorption of ceramic disk in this spectral range. In addition, these phosphors possess a very unique emission spectra; thus, they could also be applied in luminescent security pigments.

Photoluminescence characteristics of Cd1-xMnxTe single crystals grown by the vertical Bridgman method

In this paper, we report a systematic investigation of band-edge photoluminescence for Cd1-xMnxTe crystals grown by the vertical Bridgman method. The near-band-edge emissions of neutral acceptor-bound excitons (labeled as L1) were systematically investigated as a function of temperature and of alloy composition. The parameters that describe the temperature variation of the energy were evaluated by the semiempirical Varshni relation. From the temperature dependence of the full width at half maximum of the L1 emission line, the broadening factors Γ(T) were determined from the fit to the data. The activation energies of thermal quenching were obtained for the L1 peak from the temperature dependence of the bound exciton peaks and were found to decrease with increasing Mn concentration.