Luminescence from Bi2+-activated alkali earth borophosphates for white LEDs

Visible and Near-Infrared Emission in Ba3Sc4O9:Bi Phosphor: An Investigation on Bismuth Valence Modification

Bismuth (Bi)-activated luminescence materials have attracted much attention for their tunable broad emissions ranging from a visible to near-infrared (NIR) region. However, it remains a challenge to regulate the Bi valence state and achieve NIR emission via a facile way. Here, we report the design and preparation of Ba₃Sc₄O₉:Bi phosphors, which emit visible and NIR emissions simultaneously even prepared in the air condition. The self-reduction mechanism of Bi³⁺ species in Ba₃Sc₄O₉ with a rigid crystal structure is illustrated based on the charge compensation model, and the coexistence of different Bi-active centers, Bi³⁺ for visible emission, while Bi⁺ and Bi⁰ for NIR emission, is confirmed by the spectroscopic data and X-ray photoelectron spectroscopy (XPS) analysis. The enhanced NIR emission was further achieved through controlled reducing treatment and the related mechanism has also been clarified. This work paves a new way to control bismuth valence and tune the emission of Bi-based luminescence materials for emerging photonics applications.

SrBPO5: Ce3+, Dy3+ - A cold white-light emitting phosphor

A single-component white-light emitting phosphor SrBPO₅: Ce³⁺, Dy³⁺ with high color purity, good quantum efficiency and high thermal stability was prepared through the conventional high temperature solid state reaction. PXRD studies confirmed its phase purity. The suitability of Strontium borophosphate as a host for phosphor was confirmed through DFT calculations. The presence of Ce³⁺ along with Dy³⁺ in this host resulted in efficient energy transfer from Ce³⁺ to Dy³⁺ through a non-radiative multipole-multipole mechanism leading to the enhancement of Dy³⁺ luminescence towards white light emission. Lifetime decay and time-resolved emission studies confirmed the energy transfer along with multisite occupancy of Ce³⁺. In addition to energy transfer, the thermal stability of the phosphor was confirmed through temperature-dependent photoluminescence studies and the particle shape, size, uniformity of dopants of the phosphor was studied using the SEM and EDX spectroscopy.

The riddle of orange-red luminescence in Bismuth-doped silica glasses

For over the past two decades it has been believed that the intense orange-red photoluminescence in Bismuth-doped materials originates from Bi[Formula: see text] ions. Based on the results from magnetic circular polarization experiments, we demonstrate that this hypothesis fails for Bismuth-doped silica glasses. Our findings contradict the generally accepted statement that the orange-red luminescence arises from [Formula: see text] [Formula: see text] [Formula: see text] transition in a divalent Bismuth ion. The degree of magnetic circular polarization of this luminescence exhibits non-monotonic temperature and field dependencies, as well as sign reversal. This complex behaviour cannot be explained under the assumption of a single Bi[Formula: see text] ion. The detailed analysis enables us to construct a consistent diagram of energy levels involved in the magneto-optical experiments and propose a new interpretation of the nature of orange-red luminescence in Bismuth-doped silica glass. A centre responsible for this notorious photoluminescence must be an even-electron system with an integer total spin, presumably a dimer of Bismuth ions or a complex consisting of Bi[Formula: see text] and an oxygen vacancy.

Trap Energy Upconversion-Like Near-Infrared to Near-Infrared Light Rejuvenateable Persistent Luminescence

Persistent-luminescence phosphors (PLPs) have a wide variety of applications in the fields of photonics and biophotonics due to their ultralong afterglow lifetime. However, the existing PLPs are charged and recharged with short-wavelength high-energy photons or inconvenient and potentially risky X-ray beams. To date, deep tissue penetrable NIR light has mainly been used for photostimulated afterglow emission, which continues to decay and weaken after each cycle, Herein, a new paradigm of trap energy upconversion-like near-infrared (NIR) to near-infrared light rejuvenateable persistent luminescence in bismuth-doped calcium stannate phosphors and nanoparticles is reported. In contrast to the existing PLPs and persistent-luminescence nanoparticles, the materials enable the occurrence of a reversed transition of the carriers from a deep-level energy trap to a shallow-level trap upon excitation by low-energy NIR photons. Thus these new materials can be charged circularly via deep-tissue penetrable NIR photons, which is unable to be done for existing PLPs, and emit afterglow signals. This conceptual work will lay the foundation to design new categories of NIR-absorptive-NIR-emissive PLPs and nanoparticles featuring physically harmless and deep tissue penetrable NIR light renewability and sets the stage for numerous biological applications, which have been limited by current materials.

Simultaneous enhancement of photoluminescence and afterglow luminescence through Bi3+ co-doping in the Sr3Al2O5Cl2:Eu2+ phosphor

In this work, the Sr3Al2O5Cl2:Eu2+ and Sr3Al2O5Cl2:Eu2+,Bi3+ phosphors are synthesized by high temperature solid state reactions. Various characterization techniques, such as X-ray diffraction (XRD), Rietveld refinement, photoluminescence (PL) spectroscopy, afterglow spectroscopy, decay curves and thermoluminescence (TL) spectroscopy, are used to examine the phase purity and PL properties of all samples. The XRD results show that all samples belong to the targeted orthorhombic Sr3Al2O5Cl2 phase with the space group of P212121. Upon excitation with UV light, Eu2+-related reddish photoemission and afterglow luminescence are observed in the Sr3Al2O5Cl2:Eu2+ samples. More remarkably, we find that co-doping with Bi3+ ions can enhance the Eu2+-related photoemission and afterglow intensity as well the afterglow duration. For the optimal Sr3Al2O5Cl2:Eu2+,Bi3+ sample, the afterglow luminescence can continue for nearly 550 min in the dark, which is almost 3-fold the duration of the afterglow luminescence of the optimal Sr3Al2O5Cl2:Eu2+ sample. The TL spectra reveal that co-doping with Bi3+ ions can enhance the defect population that corresponds to trap depths at 63 °C, 75 °C and 150 °C, of which the former two trap depths may help to improve the Eu2+-related luminescence in addition to the afterglow property. Due to an increase in the trap concentration, there is an increase in the re-trapping possibility for the released carriers. This work not only achieves enhanced afterglow luminescence of the Sr3Al2O5Cl2:Eu2+ phosphor by co-doping with the non-rare earth (RE) Bi3+ ions, but also provides new insights into the design of RE and non-RE related enhanced afterglow photonic materials for the future.

Superbroad visible to NIR photoluminescence from Bi+ evidenced in Ba2B5O9Cl: Bi crystal

The nature of bismuth NIR luminescence is essential to develop the bismuth doped laser materials with high efficiency and desirable emission wavelength, and it, thereby, receives rising interests. Our previous work reported the Bi(0) luminescence from Ba2B5O9Cl: Bi with a lifetime of ~30 μs and the conversion of Bi(2+) to Bi(0). This work found indeed the conversion could be enabled in the compound by an in situ reduction technique and it, however, happens via an intermediate state of Bi(+). Once the ion of Bi(+) is stabilized and built into the compound, it can luminesce in a super broad spectral range from 600 to 1200 nm with a lifetime longer than 1 ms, due to the cascade transitions from (3)P2 and (3)P1 to (3)P0. This is completely different from Bi(0) and Bi(2+) in the compound, and it has never been noticed before. We believe this work can help us better understand the complex nature of bismuth luminescence.

A novel dichromic self-referencing optical probe SrO:Bi3+,Eu3+ for temperature spatially and temporally imaging

A dichromic temperature sensitive probe was synthesized to construct an effective luminescence temperature sensor and to realize real-time monitoring of surface temperature transients from room temperature to 200 °C.

A novel NIR long phosphorescent phosphor:SrSnO3:Bi2+

A novel phosphor with near-infrared (NIR) long persistent luminescence, SrSnO₃:Bi²⁺ was successfully synthesized by traditional solid-state reaction.

Ultra-broadband luminescent from a Bi doped CaO matrix

CaO:Bi phosphor powders were successfully synthesized by the sol–gel combustion method. Both blue and orange emission were obtained with CL excitation, Bi²⁺ in the BiO clusters was responsible to the orange emission under electron beam excitation.

Origin of structural relaxation dependent spectroscopic features of bismuth-activated glasses

For the first time, we studied the effect of structural relaxation on the NIR spectroscopic properties of bismuth-activated germanium glasses below glass transition temperature. Interestingly, distinct change behavior of NIR luminescence is observed at two different heat-treatment temperature ranges corresponding to two different relaxation behavior of glass structure. Besides, when structural modified by partly substituting B(2)O(3) for GeO(2), a narrower and more thermal sensitive luminescence is observed, which is inexplicable by "inhomogeneous broadening" and we tentatively attribute it to a defect-involved reason. Fundamentally the results here not only provide us a deeper insight into the optical property of bismuth-activated materials but also increase our understanding of the glassy state, and practically it delivers some valuable guidance in designing bismuth-activated glasses with superior NIR optical properties.

Investigation of energy transfer mechanisms between Bi(2+) and Tm(3+) by time-resolved spectrum

Here, we report for the first time the optical properties of Bi(2+) and Tm(3+) co-doped germanate glasses and elucidate the potential of this material as substrates to improve the performance of CdTe solar cell. A strong emission peak at 800nm is observed under the excitation of 450-700nm in this material. The energy transfer processes from the transitions of Bi(2+) [(2)P3/2(1)→(2)P1/2]: Tm(3+) [(3)H6→(3)H4] are investigated by time-resolved luminescence spectroscopy. A cover glass exhibiting an ultra-broadband response spectrum covering the entire solar visible wavelength region is suggested to enhance the conversion efficiency of CdTe solar cells significantly.

Temperature dependent red luminescence from a distorted Mn4+ site in CaAl4O7:Mn4+

Thermal luminescence quenching behavior of a phosphor is essential for application in phosphor converted white light emitting diodes (pc-WLEDs) because the phosphor layer can be heated up to 473K in a working high power WLEDs. Here, we have confirmed indeed a red luminescence of Mn(4+) substituting for calcium sites rather than tetrahedral aluminum sites in CaAl(4)O(7):Mn which can be synthesized in pure phase even with boron acid as flux, and examined the low and high temperature luminescent properties in the range of 10 to 500K. We have revealed as well as thermal quenching mechanism that distorted octahedral Mn(4+) sites suffer severe thermal quenching. This work, thus, hints a strategy to find a new Mn(4+) phosphor with better resistance to thermal impact in the future.

Yellow-to-orange emission from B2+-doped RF2 (R = Ca and Sr) phosphors

RF2:Bi (R = Ca and Sr) phosphors were synthesized by solid state reaction method in air and their luminescence properties were investigated. Broad yellow-to-orange emissions peaking at ~550 nm (CaF2:Bi) and ~600 nm (SrF2:Bi) were observed under ~260 nm excitation. The emission centers inRF2:Bi (R = Ca and Sr) phosphors are Bi2+ ions, and the excitation and emission bands of RF2:Bi (R = Ca and Sr) phosphors can be attributed to 2P 1/2 → 2S 1/2 and 2P 3/2(1) → 2P 1/2 transitions of Bi2+ ions, respectively. The phosphors are promising for application in lighting due to broad yellow-to-orange emission.

Superbroad near to mid infrared luminescence from closo-deltahedral Bi5(3+) cluster in Bi5(GaCl4)3

Closo-deltahedral Bi(5)(3+) cluster in Bi(5)(GaCl(4))(3), which can be synthesized in benzene by oxidizing bismuth metal either with BiCl(3) or GaCl(3), respectively, can absorb ultraviolet, visible and infrared lights, and luminesce superbroadly in near to mid infrared (NMIR) spectral range from 1 to 3μm at room temperature. Slight geometry change of the cluster can lead to the redshift of emission peak. These observations may initialize the development of Bi-based NMIR light sources with superbroad emission spectrum, where Bi(5)(3+) or similar polycationic species act as activators. Disputable crystal structure of Bi(5)(GaCl(4))(3) was redefined by classic Rietveld refining analysis. Consistent with crystallographic data, excitation, emission, temporal decay and time-resolved infrared emission spectra all reveal only one type of luminescent centers, viz. Bi(5)(3+), in the compound. And a new absorption of Bi(5)(3+) was found at ~1100nm.

A new study on bismuth doped oxide glasses

Spectroscopic properties of bismuth doped borate, silicate and phosphate glasses have been reinvestigated in this work. It shows the typical decay time of Bi(3+) is around 500ns rather than 2.7-to-3.9 μs reported by Parke and Webb at room temperature. Introduction of higher content either alkali or alkali earth into borate glasses favors the Bi(3+) emission. As the contents increase excitation peak shifts regularly red while emission peak shows reverse trend. This, as revealed by Huang-Rhys factor, is due to the weakening of coupling between bismuth and glass host, and it can be interpreted within the frame of configurational coordinate diagrams. Differently, as bismuth concentration increases, both the excitation and emission shift red. The unknown origin of red emission from bismuth doped calcium or magnesium phosphate glass has been identified as Bi(2+) species on the basis of excitation spectrum and emission lifetime particularly after comparing with Bi(2+) doped materials. No near infrared (NIR) emission can be detected in these glasses within instrument limit.

Superbroad near-to-mid-infrared luminescence from Bi5(3+) in Bi5(AlCl4)3

Superbroad near-to-mid infrared (NIR-MIR) photoluminescence was observed from Bi5(AlCl4)3 at room temperature, spanning the spectral range of about 1000 to 4000 nm. On the basis of structural considerations and dynamic analyses, Bi5(3+) clusters were identified as the optically active species, inherently differing from the species which is typically believed to be active in NIR-emitting Bi-doped glasses. In comparison to most other NIR-luminescent Bi-doped materials, the MIR-part of the luminescence spectrum is still present at room temperature. Emission intensity and excited state lifetime were found to exhibit abnormal temperature dependence, where the former increases with temperature up to a critical value of about 150 K. This behavior is related to a temperature-dependent overlap between ground state and excited states. The observed stabilization of MIR photoemission at room temperature may be a starting point for the development of Bi-based NIR-MIR light sources with superbroad emission spectrum, where Bi5(3+) or similar polycationic species act as optical gain medium.

Photoluminescence of Sr(2)P(2)O(7):Bi(2+) as a red phosphor for additive light generation

We report on photoluminescence of Sr(2)P(2)O(7):Bi(2+) as a potential red-emitting phosphor for multichromatic light sources. If excited with blue light, photoluminescence of this compound spans the spectral range of about 600 to 760nm. Static and dynamic spectral data reveal the presence of two distinct emission centers. Based on bond covalency and emission lifetime, luminescence can clearly be assigned to Bi(2+) ions on Sr(1) and Sr(2) lattice sites, respectively. Energy transfer is observed from Bi(1) to Bi(2). Transfer efficiency, estimated from the lifetimes of the excited states, increases with increasing dopant concentration.

Broadband NIR photoluminescence from Bi-doped Ba2P2O7 crystals: insights into the nature of NIR-emitting Bismuth centers

We report on a novel type of Bi-doped crystal that exhibits ultrabroadband photoluminescence in the near infrared (NIR). Emission centers can be generated and degenerated reversibly by annealing the material in CO atmosphere and air, respectively, indicating that emission is related to the presence of Bi-species in low valence states. Correlating static and dynamic excitation and emission data with the size and charge of available lattice sites suggests that two types of Bi(0)-species, each located on one of the two available Ba(2+) lattice sites, are responsible for NIR photoemission. This is further confirmed by the absence of NIR emission in polycrystalline Ca(2)P(2)O(7):Bi and Sr(2)P(2)O(7):Bi. Excitation is assigned to transitions between the doubly degenerated ground state (4)S(3/2) and the degenerated excited levels (2)D(3/2), (2)D(5/2) and (2)P(1/2), respectively. NIR emission is attributed to (2)D(3/2)?(4)S(3/2). The NIR emission center can coexist with Bi(2+) species. Then, also Bi(2+) is accommodated on one of the two Ba(2+)-sites. Energy transfer between Bi(2+) ions occurs within a critical distance of 25.9 A.

Synthesis and luminescent properties of CaTiO3: Pr3+ microfibers prepared by electrospinning method

One-dimensional Pr(3+)-doped CaTiO(3) microfibers were fabricated by a simple and cost-effective electronspinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential analysis (TG-DTA), scanning electron microscopy (SEM), energy-dispersive X-ray spectrum (EDS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), quantum efficiency (QE), and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. Under ultraviolet excitation and low-voltage electron beams (1-3 kV) excitation, the CaTiO(3):x Pr(3+) samples show the red emission at 612 nm, corresponding to (1)D(2)-(3)H(4) transition of Pr(3+). The luminescence intensity, quantum efficiency, and the lifetime have been studied as a function of the doping concentration of Pr(3+) in the CaTiO(3) samples.

Intense red photoluminescence from Mn2+-doped (Na+; Zn2+) sulfophosphate glasses and glass ceramics as LED converters

We report on intense red fluorescence from Mn(2+)-doped sulfophosphate glasses and glass ceramics of the type ZnO-Na(2)O-SO(3)-P(2)O(5). As a hypothesis, controlled internal crystallization of as-melted glasses is achieved on the basis of thermally-induced bimodal separation of an SO(3)-rich phase. Crystal formation is then confined to the relict structure of phase separation. The whole synthesis procedure is performed in air at <or= 800 degrees C. Electron spin resonance and Raman spectroscopy indicate that Mn(2+) species are incorporated on Zn(2+) sites with increasingly ionic character for increasing concentration. Correspondingly, in the glasses, increasing MnO content results in decreasing network polymerization. Stable glasses and continuously increasing emission intensity are observed for relatively high dopant concentration of up to 3 mol.%. Recrystallization of the glass results in strongly increasing emission intensity. Dynamic emission spectroscopy reveals only on type of emission centers in the glassy material, whereas three different centers are observed in the glass ceramic. These are attributed to octahedrally coordinated Mn(2+) in the residual glass phase and in crystalline phosphate and sulfate lattices, respectively. Relatively low crystal field strength results in almost ideal red emission, peaking around 625 nm. Excitation bands lie in the blue-to-green spectral range and exhibit strong overlap. The optimum excitation range matches the emission properties of GaN- and InGaN-based light emitting devices.