Dual-Emission Origins in Bi3+-Doped M2O3 Sesquioxides (M = Sc, Y, Gd and Lu): A First-Principles Study.

Bi3+-doped sesquioxides exhibit dual emissions, marked by distinct Stokes shift and bandwidth, meaning unraveling their underlying origins is particularly intriguing. In this study, we employ first-principles calculations to investigate the luminescence mechanisms within the M2O3:Bi3+ (M = Sc, Y, Gd, Lu) series, with the goal of addressing the posed inquiry. Our investigation commences with the analysis of the site occupancy and charge state of bismuth ions in the two cationic sites through formation energy calculations. Additionally, we examine the local coordination environments for various excited states of Bi3+ dopants, including the 3P0,1 state and two types of charge transfer states, by evaluating their equilibrium geometric structures. The utilization of the hybrid functional enables us to obtain results of electronic structures and optical properties comparable with experiments. Importantly, the calculated energies for the 6s-6p transitions of Bi3+ dopants in the M2O3 series align well with the observed dual-emission energies. This alignment challenges the conventional spectroscopic sense that emission bands with large Stokes shifts can be exclusively ascribed to charge transfer transitions. Consequently, the integration of experimental and theoretical approaches emerges as the optimal strategy for designing novel Bi3+-doped phosphors.

Effect of Covalence and Degree of Cation Order on the Luminous Efficacy of Mn4+ Luminescence in the Double Perovskites, Ba2BTaO6 (B = Y, Lu, Sc).

The spectroscopic properties of the Mn4+ ion are investigated in the series of isostructural double perovskite compounds, Ba2BTaO6 (B = Y, Lu, Sc). A comparison of these properties highlights the influence of covalent bonding within the perovskite framework and the degree of order between the B3+-Ta cations on the energy and intensity of the Mn4+2E → 4A2 emission transition (R-line). These two parameters of the emission spectrum are of importance for practical application since they determine the phosphor luminous efficacy. The influence of covalent bonding within the corner shared BO6/2 and TaO6/2 perovskite framework on the energy of the R-line energy is investigated. From the spectroscopic data, we have derived information on the influence of the degree of order between the B3+ and Ta5+ cations on the intensity of the R-line. The lowest energy and the highest intensity of the R-line are found in the double perovskite, Ba2ScTaO6. The purpose of this work is to propose for first time an explanation of these effects in the considered double perovskites. The obtained results are useful guidelines for practical improvement and tuning of key parameters of phosphors to the desired values.

Unraveling Broadband Near-Infrared Luminescence in Cr3+-Doped Ca3Y2Ge3O12 Garnets: Insights from First-Principles Analysis.

In this study, we conducted an extensive investigation into broadband near-infrared luminescence of Cr3+-doped Ca3Y2Ge3O12 garnet, employing first-principles calculations within the density functional theory framework. Our initial focus involved determining the site occupancy of Cr3+ activator ions, which revealed a pronounced preference for the Y3+ sites over the Ca2+ and Ge4+ sites, as evidenced by the formation energy calculations. Subsequently, the geometric structures of the excited states 2E and 4T2, along with their optical transition energies relative to the ground state 4A2 in Ca3Y2Ge3O12:Cr3+, were successfully modeled using the ΔSCF method. Calculation convergence challenges were effectively addressed through the proposed fractional particle occupancy schemes. The constructed host-referred binding energy diagram provided a clear description of the luminescence kinetics process in the garnet, which explained the high quantum efficiency of emission. Furthermore, the accurate prediction of thermal excitation energy yielded insights into the thermal stability of the compound, as illustrated in the calculated configuration coordinate diagram. More importantly, all calculated data were consistently aligned with the experimental results. This research not only advances our understanding of the intricate interplay between geometric and electronic structures, optical properties, and thermal behavior in Cr3+-doped garnets but also lays the groundwork for future breakthroughs in the high-throughput design and optimization of luminescent performance and thermal stability in Cr3+-doped phosphors.

Deep-blue narrow-band emissive cesium europium bromide perovskite nanocrystals with record high emission efficiency for wide-color-gamut backlight displays.

Lead halide perovskite nanocrystals (NCs) are highly promising for backlighting display applications due to their high photoluminescence quantum yields (PLQYs) and wide color gamut values. However, the practical applications of blue emitters are limited due to the toxicity of lead, unstable structure, and unsatisfactory PLQY. Herein, we report the successful synthesis of divalent europium-based perovskite CsEuBr3 NCs using a modified hot injection method. By optimizing the reaction conditions, the CsEuBr3 NCs display a deep-blue emission at 443 nm with a full width at half maximum (FWHM) of 28.5 nm, a color purity of 99.61%, and a record high PLQY of 93.51% for deep-blue narrow-band emissive lead-free perovskite NCs as far as we know. The emission mechanism of CsEuBr3 NCs is proved through first-principles calculations and spectral analysis. Notably, the CsEuBr3 NCs exhibit remarkable stability when exposed to high temperature, UV irradiation, and long-term sealed storage. The incorporation of CsEuBr3 NCs into polydimethylsiloxane (PDMS) serving as a converter is utilized for white light-emitting devices (WLEDs). WLEDs for backlight displays achieves a wide color gamut of 127.1% of the National Television System Committee standard (NTSC), 94.9% coverage of the ITU-R Recommendation BT.2020 (Rec.2020), and their half-lifetime is up to 1677 h, providing a promising pathway for highly efficient, environment-friendly and practical liquid crystal display backlights.

Aqueous-Based Inorganic Colloidal Halide Perovskites Customizing Liquid Scintillators.

Compared to solid scintillators and organic liquid scintillators, aqueous-based liquid scintillators (AbLS) have more superiority in highly flexible scalability, yet are now limited by their low light yield (≈100 photons MeV-1 ). Here, aqueous-based inorganic colloidal halide perovskites with high photoluminescence quantum yield (PLQY) of three primary color luminescence up to 88.1% (red), 96% (green), and 81.8% (blue) are respectively synthesized, and a new generation of colloidal perovskite-mediated AbLS (PAbLS) with light yield increased in comparison with the commercial scintillator AbLS is fabricated. This paper exhibits that the excellent PLQY and colloidal dispersion of halide perovskites benefit from poly(ethylene glycol) modification and this modification ensures the vacancy inhibition and formation of defect-free surfaces in an aqueous solution. Moreover, their high luminescent emission can be maintained for 100 days at low temperatures, and such modification also promises the heat-to-cold customization of operating temperature even in ice below 0 °C. Finally, depending on the light yield of around 3058 and 8037 photons MeV-1 at room temperature and low temperature, PAbLS with shape/size scalability exhibit their robust radiation hardness (dose rate as high as 23 mGy s-1 ) and conceptual application potential in high-energy ray radiation detection from every angle of 360°.

Intervalence charge transfer of Cr3+-Cr3+ aggregation for NIR-II luminescence.

The increasing demand for high-contrast biological imaging, non-destructive testing, and infrared night vision can be addressed by the development of high-performance NIR light-emitting materials. Unlike lanthanide (Ln3+) with sharp-line multiplets and isolated Cr3+ with NIR-I emission, this study reports the first-ever NIR-II broadband luminescence based on the intervalence charge transfer (IVCT) of Cr3+-Cr3+ aggregation in gallate magentoplumbite. In particular, LaMgGa11O19:0.7Cr3+ exhibits dual-emission (NIR-I, 890 nm and NIR-II, 1200 nm) with a full width at half maximum (FWHM) of 626 nm under 450 nm blue LED excitation. Moreover, this dual-emission exhibits anti-thermal quenching behavior (432% @ 290 K), attributed to the energy transfer among multiple Cr3+ centers. Cryogen absorption spectra, lifetimes decay (2.3 ms), and electron paramagnetic experiments reveal the NIR-II luminescence of the Cr3+-Cr3+ → Cr2+-Cr4+ IVCT transition. The application of LaMgGa11O19:0.7Cr3+ in NIR-II biological imaging as an optical contrast agent, non-destructive testing, and night vision is demonstrated. This work provides new insights into broadband NIR-II luminescence under UV-NIR excitation based on the IVCT of Cr3+-Cr3+ aggregation.

Mechanistic insights for regulating the site occupancy, valence states and optical transitions of Mn ions in yttrium-aluminum garnets via codoping.

The site-dependent photoluminescence of activators can be regulated by the sintering atmosphere, coexistence conditions, and especially cation codoping, which have been intensively studied for design and optimization of optical functional materials. Here, first-principles calculations are performed to determine the regulation of the site occupancy, valence states and optical transitions of Mn activators via codoping in yttrium aluminum garnets (YAGs), which contain three different cation sites. Without any codopants, Mnoct3+ dominates in defect concentration and photoluminescence, which can hardly be tuned by the sintering atmosphere or coexistence conditions of YAGs with other competing compounds. With the low formation energy of Ca2+, Be2+, Mg2+, and Sr2+ codopants and in an oxidation sintering atmosphere, the Fermi energy is lowered and the concentration and luminescence of Mnoct4+ are enhanced. Na+ and Li+ codopants with relatively high formation energy have little influence on tuning the Fermi energy. Then with the low formation energy of Ti4+, Si4+ codopants and in a reducing sintering atmosphere, the Fermi energy is lifted and the luminescence of Mndod2+ and Mnoct2+ is enhanced as a result of increased concentrations. The proposed first-principles scheme, with general applicability and encouraging predictive power, provides an effective approach for elucidating the effects of codoping impurities on the design and optimization of optical materials.

Tb3+ Ion Optical and Magneto-Optical Properties in the Cubic Crystals KTb3F10.

The optical and magneto-optical characteristics of KTb3F10 crystals in the transition region of 5D4 → 7F6 4f8 configurations of the Tb3+ ion at temperatures of 90 and 300 K were studied. The schemes of the optical transitions in the KTb3F10 crystals were constructed, and the energies of most of the Stark sublevels of the ground 7F6 and excited 5D4 multiplets of the Tb3+ ion split by the C4v symmetry crystal environment were determined. The presence of three- and two-doublet states in the energy spectra of the Tb3+ion multiplets 7F6 and 5D4, respectively, was established, which is in good agreement with theoretical predictions. The use of the wavefunctions of the Stark sublevels of multiplets split by a tetragonal crystal field and combining in the studied optical transition made it possible to explain some of the magnetic and magneto-optical features observed in the KTb3F10 single crystals.

Mn5+-activated Ca6Ba(PO4)4O near-infrared phosphor and its application in luminescence thermometry.

The near-infrared luminescence of Ca6Ba(PO4)4O:Mn5+ is demonstrated and explained. When excited into the broad and strong absorption band that spans the 500-1000 nm spectral range, this phosphor provides an ultranarrow (FWHM = 5 nm) emission centered at 1140 nm that originates from a spin-forbidden 1E → 3A2 transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 µs. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe-Sugano diagram for this phosphor. We found that 1E emission quenches due to the thermally-assisted cross-over with the 3T2 state and that the relatively high Debye temperature of 783 K of Ca6Ba(PO4)4O facilitates efficient emission. Since Ca6Ba(PO4)4O also provides efficient yellow emission of the Eu2+ dopant, we calculated and explained its electronic band structure, the partial and total density of states, effective Mulliken charges of all ions, elastic constants, Debye temperature, and vibrational spectra. Finally, we demonstrated the application of phosphor in a luminescence intensity ratio thermometry and obtained a relative sensitivity of 1.92%K-1 and a temperature resolution of 0.2 K in the range of physiological temperatures.

Angular Jahn-Teller Effect and Photoluminescence of the Tetrahedral Coordinated Mn2+ Activators in Solids-A First-Principles Study.

First-principles calculations based on density functional theory have been performed to investigate the electronic structure, excited-state Jahn-Teller distortion, and photoluminescence of the multielectron d5 system of the strongly covalent tetrahedral coordinated Mn2+ activator in solids. The electronic structure of the 4T1 and 4A1/4E excited states is analyzed, and Slater's transition-state method and occupation matrix control methodology are applied to deal with the spin contamination in the lower-spin excited states, which is due to the mixing of the ground state of the same spin projection number. In a series of covalent tetrahedral coordinations, the 6A1 → 4T1 and 4A1/4E excitations and the 4T1 → 6A1 emission energies are obtained and compared to the reported experimental results. The nephelauxetic effect follows O2- < S2- ≈ Se2- < N3-, and the larger nephelauxetic effect and crystal field strength lead to the red-shifted emission of nitride phosphors. The Jahn-Teller distortion of the 4T1 states is dominated by the e-type angular distortion of the [MnL4] moiety (L being the ligand), which accounts for the small Stokes shift of tetrahedral coordinated Mn2+. The results show that the ground- and excited-state electronic and geometric structures and the luminescent property of tetrahedral coordinated Mn2+ can be reliably predicted. The method can be further explored to interpret and discriminate the luminescent properties of materials containing a variety of different Mn2+ sites and complexes and even other transition metals.

Solar-Driven Overproduction of Biofuels in Microorganisms.

Microbial cell factories reinvigorate current industries by producing complex fine chemicals at low costs. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) is the main reducing power to drive the biosynthetic pathways in microorganisms. However, insufficient intrinsic NADPH limits the productivity of microorganisms. Here, we report that supplying microorganisms with long-lived electrons from persistent phosphor mesoporous Al2 O3 (meso-Al2 O3 ) can elevate the NADPH level to facilitate efficient fine chemical production. The defects in meso-Al2 O3 were demonstrated to be highly efficient in prolonging electrons' lifetime. The long-lived electrons in meso-Al2 O3 can pass the material-microorganism interface and power the biosynthetic pathways of E. coli to produce jet fuel farnesene. This work represents a reliable strategy to design photo-biosynthesis systems to improve the productivity of microorganisms with solar energy.

Efficient Broadband Near-Infrared Emission from Lead-Free Halide Double Perovskite Single Crystal.

Ultra-broadband near-infrared (NIR) luminescent materials are the most important component of NIR light-emitting devices (LED) and are crucial for their performance in sensing applications. A major challenge is to design novel NIR luminescent materials to replace the traditional Cr3+ -doped systems. We report an all-inorganic bismuth halide perovskite Cs2 AgBiCl6 single crystal that achieves efficient broadband NIR emission by introducing Na ions. Experiments and density functional theory (DFT) calculations show that the NIR emission originates from self-trapped excitons (STE) emission, which can be enhanced by weakening the strong coupling between electrons and phonons. The high photoluminescence quantum efficiency (PLQY) of 51 %, the extensive full width at half maximum (FWHM) of 270 nm and the stability provide advantages as a NIR luminescent material. The single-crystal-based NIR LED demonstrated its potential applications in NIR spectral detection as well as night vision.

The photoluminescence of isolated and paired Bi3+ ions in layered LnOCl crystals: a first-principles study.

Luminescent ns2 centers have shown great potential for applications as phosphors and scintillators. First-principles calculations based on density functional theory are performed to systematically analyze the luminescent centers of isolated and paired Bi3+(6s2) ions in layered LnOCl (Ln = Y, Gd, La) crystals. The spin-orbit coupling and orbital hybridization both show important effects on the luminescence properties. The luminescence of the isolated Bi ion is confirmed as the interconfigurational transition of 3P0,1 → 1S0. For the Bi pair, the adiabatic potential energy surfaces are calculated and the charge transfer excited state is the most stable, which accounts for the visible emission of a large Stokes shift. Furthermore, the electron-hole pair separation, absorption, excitonic state and emission of the material with fully-concentrated Bi3+, BiOCl, are discussed. This study shows that the first-principles calculations can serve as an effective tool for the photoluminescence analysis and engineering of materials activated with isolated, paired and even fully-concentrated ns2 ions.

Experimental and Theoretical Studies of the Site Occupancy and Luminescence of Ce3+ in LiSr4(BO3)3 for Potential X-ray Detecting Applications.

Ce3+-doped LiSr4(BO3)3 phosphors have been prepared by a high-temperature solid-state reaction method, and structural refinement of the host compound has been performed. The excitation and emission spectra in the vacuum ultraviolet-ultraviolet-visible range at cryogenic temperatures reveal that Ce3+ ions preferentially occupy eight-coordinated Sr2+ sites in LiSr4(BO3)3. Such experimental attribution is well corroborated by the calculated 4f-5d transition energies and defect formation energies of Ce3+ ions at two distinct Sr2+ sites in the first-principles framework. In addition, the doping concentration-dependent luminescence and the temperature-dependent luminescence are systematically investigated by luminescence intensity and lifetime measurements, respectively. This shows that concentration quenching does not occur in the investigated doping range, but inhomogeneous broadening exists in the concentrated samples. With the estimated thermal quenching activation energy, the discussions on the thermal quenching mechanisms suggest that the thermal-ionization process of the 5d electron is a dominant channel for thermal quenching of Ce3+ luminescence, despite the fact that thermally activated concentration quenching cannot be excluded for the highly doped samples. Finally, the X-ray excited luminescence measurement demonstrates the promising applications of the phosphors in X-ray detection.

Influence of Isostatic Pressure on the Elastic and Electronic Properties of K2SiF6:Mn4.

Isostatic pressure effects on the elastic and electronic properties of non-doped and Mn4+-doped K2SiF6 (KSF) have been investigated by first-principles calculations within density functional theory (DFT). Bulk modulus was obtained by the Murnaghan's equation of states (EOS) using the relationship between volume and pressures at pressures between 0 and 40 GPa, and elastic constants were calculated by the stress-strain relationship giving small distortions at each pressure point. The other elastic parameters such as shear modulus, sound velocity and Debye temperature, which can be obtained from the elastic constants, were also estimated. The influence of external isostatic pressure on the electronic properties, such as crystal field strength 10Dq and emission energy of 2E → 4A2 transition (Eem), of KSF:Mn4+ was also studied. The results suggest that 10Dq and Eem linearly increase and decrease, respectively, with increasing pressure.

Self-Activated and Bismuth-Related Photoluminescence in Rare-Earth Vanadate, Niobate, and Tantalate Series-A First-Principles Study.

Rare-earth vanadates, niobates, and tantalates have shown self-activated and Bi3+-activated emissions. Their intrinsic emission has been attributed to self-trapped excitons (STEs), but the detailed information concerning the geometric and electronic structures of the excited states has remained unknown. Regarding the Bi3+ dopants in these hosts, the luminescence has been attributed to two different mechanisms, i.e., Bi3+↔ (V/Nb/Ta)5+ metal-to-metal charge transfer and interconfigurational (3P0,1 → 1S0) transition. Here, first-principles calculations using hybrid functionals are employed to resolve these issues. The STEs are shown to be composed of an electron localized on an individual vanadium, niobium, or tantalum ion and a hole localized on a single nearest-neighbor oxygen ion that is not shared by covalent complexes, and the bond length of the (V/Nb/Ta)-O bond with oxygen accommodating the hole is significantly elongated. The Bi3+-related emission is identified as the recombination of an exciton with a hole and an electron localized correspondingly at Bi3+ and (V/Nb/Ta)5+ ions, while the excitation is dominated by the 6s → 6p transition of Bi3+. Furthermore, Bi3+ has a hole trap level in all of the hosts considered with the trap levels in the vacuum-referred binding energy diagram being nearly flat but has an electron trap level only in rare-earth tantalates. Furthermore, the long-wavelength emission observed in niobates and tantalates is interpreted based on our calculations to be excitons bound to intrinsic defects. The insights gained in this work deepen our understanding of the STEs and form the basis for interpreting similar luminescence phenomena in other ternary closed-shell d0 transition-metal oxides. The clarification of Bi3+-related transitions and the analyses with the vacuum-referred binding energy diagram may find applications for the design and optimization of Bi3+-activated phosphors.

First Principles Calculations of Atomic and Electronic Structure of TiAl3+- and TiAl2+-Doped YAlO3.

In this paper, the density functional theory accompanied with linear combination of atomic orbitals (LCAO) method is applied to study the atomic and electronic structure of the Ti3+ and Ti2+ ions substituted for the host Al atom in orthorhombic Pbnm bulk YAlO3 crystals. The disordered crystalline structure of YAlO3 was modelled in a large supercell containing 160 atoms, allowing simulation of a substitutional dopant with a concentration of about 3%. In the case of the Ti2+-doped YAlO3, compensated F-center (oxygen vacancy with two trapped electrons) is inserted close to the Ti to make the unit cell neutral. Changes of the interatomic distances and angles between the chemical bonds in the defect-containing lattices were analyzed and quantified. The positions of various defect levels in the host band gap were determined.

Systematic Analysis of the Crystal Chemistry and Eu3+ Spectroscopy along the Series of Double Perovskites Ca2LnSbO6 (Ln = La, Eu, Gd, Lu, and Y).

Eu3+ (1 mol %)-doped Ca2LnSbO6 (replacing Ln3+; Ln = Lu, Y, Gd, and La) and Ca2EuSbO6 were synthesized and structurally characterized by means of X-ray powder diffraction. The Eu3+ luminescence spectroscopy of the doped samples and of Ca2EuSbO6 has been carefully investigated upon collection of the excitation/emission spectra and luminescence decay curves of the main excited states. Surprisingly, apart from the dominant red emission from 5D0, all the doped samples show an uncommon blue and green emission contribution from 5DJ (J = 1, 2, and 3). This is made possible thanks to both multiphonon and cross-relaxation mechanism inefficiencies. However, the emission from 5D3 is more efficient and the decay kinetics of the 5DJ (J = 0, 1, and 2) levels is slower in the case of Y- and Lu-based doped samples. This evidence can find a possible explanation in the crystal chemistry of this family of double perovskites: our structural investigation suggests an uneven distribution of the Eu3+ dopant ions in Ca2YSbO6 and Ca2LuSbO6 hosts of the general A2BB'O6 formula. The luminescent center is mainly located in the A crystal site, and on average, the Eu-Eu distances are longer than in the case of the Gd- and La-based matrix. These longer distances can further reduce the efficiency of the cross-relaxation mechanism and, consequently, the radiative transitions are more efficient. The slower depopulation of Eu3+ 5D2 and 5D1 levels in Ca2YSbO6 and Ca2LuSbO6 hosts is reflected in the longer rise observed in the 5D1 and 5D0 decay curves, respectively. Finally, in Ca2EuSbO6, the high Eu3+ concentration gives rise to an efficient cross-relaxation within the subset of the lanthanide ions so that no emission from 5DJ (J = 1, 2, and 3) is possible and the 5D0 decay kinetics is faster than for the doped samples.

Efficient Luminescence from CsPbBr3 Nanoparticles Embedded in Cs4PbBr6.

Cs4PbBr6 is regarded as an outstanding luminescent material with good thermal stability and optical performance. However, the mechanism of green emission from Cs4PbBr6 has been controversial. Here we show that isolated CsPbBr3 nanoparticles embedded within a Cs4PbBr6 matrix give rise to a "normal" green luminescence while superfluorescence at longer wavelengths is suppressed. High-resolution transmission electron microscopy shows that the embedded CsPbBr3 nanoparticles are around 3.8 nm in diameter and are well-separated from each other, perhaps by a strain-driven mechanism. This mechanism may enable other efficient luminescent composites to be developed by embedding optically active nanoparticles epitaxially within inert host lattices.

Unraveling the Near-Unity Narrow-Band Green Emission in Zero-Dimensional Mn2+-Based Metal Halides: A Case Study of (C10H16N)2Zn1-xMnxBr4 Solid Solutions.

Zero-dimensional (0D) Mn2+-based metal halides are potential candidates as narrow-band green emitters, and thus it is critical to provide a structural understanding of the photophysical process. Herein, we propose that a sufficiently long Mn-Mn distance in 0D metal halides enables all Mn2+ centers to emit spontaneously, thereby leading to near-unity photoluminescence quantum yield. Taking lead-free (C10H16N)2Zn1-xMnxBr4 (x = 0-1) solid solution as an example, the Zn/Mn alloying inhibits the concentration quenching that is caused by the energy transfer of Mn2+. (C10H16N)2MnBr4 exhibits highly thermal stable luminescence even up to 150 °C with a narrow-band green emission at 518 nm and a full width at half maximum of 46 nm. The fabricated white light-emitting diode device shows a high luminous efficacy of 120 lm/W and a wide color gamut of 104% National Television System Committee standard, suggesting its potential for liquid crystal displays backlighting. These results provide a guidance for designing new narrow-band green emitters in Mn2+-based metal halides.