Luminescent Afterglow Behavior in the M₂Si₅N₈: Eu Family (M = Ca, Sr, Ba)

Color-Tunable Persistent Luminescence of Ca10M(PO4)7:Eu2+ (M = Li, Na, and K) with a β-Ca3(PO4)2-Type Structure

Eu²⁺-activated Ca₁₀M(PO₄)₇ (M = Li, Na, and K) phosphates have been explored extensively because of their tunable emission wavelengths and excellent luminescence performances. Herein, the persistent luminescence (PersL) properties of Eu²⁺-doped Ca₁₀M(PO₄)₇ phosphors with a β-Ca₃(PO₄)₂-type structure are reported. With the variation of alkali metal M from Li to Na and to K, the PersL color can be adjusted sequentially from yellow to white and to blue, and the persistent durations are prolonged significantly from about ∼61 s for Ca_9.997Li(PO₄)₇:0.003Eu²⁺ to ∼1950 s for Ca_9.999Na(PO₄)₇:0.001Eu²⁺ and to ∼7440 s for Ca_9.9995K(PO₄)₇:0.0005Eu²⁺ at the threshold value (0.32 mcd/m²) after 254 nm irradiation. The trap depths are estimated according to the thermoluminescence glow curves with various heating rates. Comparing the thermoluminescence excitation and photoluminescence excitation spectra, it can be verified that there are two sources of ionized electrons in the charging process: one is excited from the valence band to the conduction band (CB) and the other is excited from the 4f ground state of Eu²⁺ to the higher 5d levels or directly to the CB. Finally, the PersL mechanism is proposed. This work is expected to motivate more research of Eu²⁺-doped phosphate-based PersL materials, as well as contributes to the understanding of the PersL mechanism of Eu²⁺-doped phosphors.

Origin of Fluorescence from Boranils in the Crystalline Phase

A small series of boranil complexes has been studied by fluorescence spectroscopy. Weakly fluorescent in most organic solvents at room temperature, the target compounds display bright emission in the crystalline phase. X-ray diffraction patterns obtained for single crystals indicate a distorted tetrahedral geometry around the O-B-N center with the boron atom being displaced from the plane of the heterobicyclic ring. Consideration of the various bond lengths in comparison with those of reference compounds indicates that the ancillary phenyl ring, bearing different para-substituents, does not make a prominent contribution to the molecular dipole moment in the solid state. Absorption and fluorescence spectra recorded for the crystals remain remarkably similar to those for liquid solutions and display large Stokes shifts. Proximity broadening is observed in one case. The nitrophenyl derivative exhibits additional absorption and emission bands unique to the solid state and could be indicative of an intermolecular charge-transfer transition. The optical properties are discussed in terms of the crystal packing diagrams.

Phase stabilization of red-emitting olivine-type NaMgPO4:Eu2+ phosphors via molten-phase quenching

Eu²⁺-activated red-emitting olivine-type NaMgPO₄:Eu²⁺ phosphor was successfully synthesized by a rapid quenching process, and its luminescence properties were investigated in detail.

Abnormal co-doping effect on the red persistent luminescence SrS:Eu2+,RE3+ materials

Persistent luminescence materials are a reality in several applications. However, there is still a lack of efficient red-emitting materials. SrS:Eu2+ phosphor is a potential candidate since its strong nephelauxetic effect shifts Eu2+4f65d1 → 4f7 to red, and its weak bond between strontium and sulphide, due to the soft base-hard acid character, generates a high number of intrinsic defects. SrS:Eu2+,RE3+ materials were efficiently prepared by two rounds of 22 min microwave-assisted solid-state synthesis. The highly crystalline purity and the material organization at the micro-scale were observed with X-ray powder diffraction and scanning electron microscopy, respectively. X-ray absorption spectroscopy revealed a low amount of Eu2+ compared to Eu3+ due to the efficient Eu2+ photo-oxidation by X-ray irradiation in the high storage capability SrS host matrix. Electron paramagnetic resonance spectra confirmed that at least 50% of Eu2+ ions in the material are photo-oxidized during excitation, reinforcing the previously established mechanisms. The RE2+ energy level positioned very close to or into the conduction band led to an abnormal co-doping effect, with similar effects independent of the co-dopant. The high concentration of intrinsic defects in SrS indicates that the soft-hard pair host is an excellent approach to develop efficient persistent luminescence materials.

Unlocking the key to persistent luminescence with X-ray absorption spectroscopy: a local structure investigation of Cr-substituted spinel-type phosphors

Developing new persistent luminescent phosphors, a unique class of inorganic materials that can produce a visible light emission lasting minutes to hours requires improving our understanding of their fundamental structure-property relationships. Research has shown that one of the most critical components governing persistent luminescence is the existence of lattice defects in a material. Specifically, vacancies and anti-site defects that coincide with substitution of the luminescent center, e.g., Eu2+ or Cr3+, are generally considered essential to generate the ultra-long luminescent lifetimes. This research solidifies the connection between defects and the remarkable optical properties. The persistent luminescent compound Zn(Ga1-xAlx)2O4 (x = 0-1), which adopts a spinel-type structure, is investigated by examining the X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) at the Cr K and Zn K edges. This investigation reveals a structural distortion of the octahedrally coordinated main group metal site concurrent with increasing Al3+ content. Moreover, these results suggest there is a dependence between the local crystallographic distortions, the presence of defects, and a material's persistent luminescence. In combination, this work provides an avenue to understand the connection between the structure-defect-property relationships that govern the properties of many functional inorganic materials.

Colour optimization of phosphor-converted flexible nitride nanowire white light emitting diodes

We demonstrate flexible nanowire white light-emitting-diodes (LEDs) with an optimized colour quality. The devices consist of flexible InGaN/GaN nanowire LEDs acting as pumps, capped with removable phosphor-doped polydimethylsiloxane membranes. Five different phosphors with tens of microns in grain size emitting from green to orange are investigated using both violet-blue and a blue-green nanowire-based LED pumps. In addition, a flexible nanowire white LED with a warm white emission is demonstrated using two layers of different phosphors. Compared to the previous realizations of flexible nanowire white LEDs, these novel LEDs improve the colour rendering index from 54 to 86 and show a colour tuneable from a bluish cool white colour to natural white and finally to warm white. The flexibility tests show that the LEDs can be bent down to 1.5 cm curvature radius without significant degradation. Therefore, the replacement of the nano-phosphors used in the previous realization by relatively inexpensive micro-phosphors does not degrade the good mechanical flexibility of the white nanowire LEDs.

A stochastic analysis based on a one-dimensional random walk model of the persistent phosphorescence of Mn2+ ions doped in zinc magnesium phosphate

The persistent phosphorescence of β-Zn₃(PO₄)₂:Mn²⁺ and γ-(Zn²⁺,Mg²⁺)₃(PO₄)₂:Mn²⁺ systems showing long-tailed non-exponential decay is simulated via stochastic analysis using electron hopping between traps.

Predicting the afterglow duration in persistent phosphors: a validated approach to derive trap depth distributions

Persistent phosphors are increasingly investigated due to their potential applications in various fields, such as safety signage, dosimetry and in vivo imaging. These materials act as optical batteries that store and gradually release energy supplied during optical charging. As the energy is stored, or 'trapped', at specific defect sites in the host lattice, a clear understanding of the defects and trapping mechanisms in these materials is important for systematic improvement of their properties. Here, the thermoluminescence and afterglow properties of the near-infrared (NIR) emitting persistent phosphor LiGa5O8:Cr3+ (LGO:Cr) are studied. This phosphor is used as a model system for illustrating a more general approach to reliably derive trap depth distributions in persistent luminescent and storage materials. The combination of the Tstop-Tmax method with initial rise analysis is used to experimentally determine the presence of a broad distribution of trapping states. Computerized glow curve fitting is subsequently used to extract the relevant trapping parameters of the system in a rigorous, consistent manner, by fitting all the experimentally recorded data simultaneously. The resulting, single set of model parameters is able to describe all measured thermoluminescence and afterglow data and hence can be used to predict afterglow and storage properties of the phosphor under various conditions. The methods to analyze and describe the trap structure of the persistent phosphor LGO:Cr are straightforwardly applicable for other persistent and storage phosphors and result in a reliable determination of the relevant trapping parameters of a given material.

Synthesis of Sr2Si5N8:Ce3+ phosphors for white LEDs via an efficient chemical vapor deposition [corrected]

Novel chemical vapor deposition (CVD) process was successfully developed for the growth of Sr₂Si₅N₈:Ce³⁺ phosphors with elevated luminescent properties. Metallic strontium was used as a vapor source for producing Sr₃N₂ vapor to react with Si₃N₄ powder via a homogeneous gas-solid reaction. The phosphors prepared via the CVD process showed high crystallinity, homogeneous particle size ranging from 8 to 10 μm, and high luminescence properties. In contrast, the phosphors prepared via the conventional solid-state reaction process exhibited relative low crystallinity, non-uniform particle size in the range of 0.5–5 μm and relatively lower luminescent properties than the phosphors synthesized via the CVD process. Upon the blue light excitation, Sr_2−xCe_xSi₅N₈ phosphors exhibited a broad yellow band. A red shift of the emission band from 535 to 556 nm was observed with the increment in the doping amount of Ce³⁺ ions from x = 0.02 to x = 0.10. The maximum emission was observed at x = 0.06, and the external and internal quantum efficiencies were calculated to be 51% and 71%, respectively. Furthermore, the CVD derived optimum Sr_1.94Ce_0.06Si₅N₈ phosphor exhibited sufficient thermal stability for blue-LEDs and the activation energy was calculated to be 0.33 eV. The results demonstrate a potential synthesis process for nitride phosphors suitable for light emitting diodes.

Realizing superior white LEDs with both high R9 and luminous efficacy by using dual red phosphors

An unique spectral configuration was designed to improve both luminous efficacy and color rendering by using dual red phosphors.

Chemically engineered persistent luminescence nanoprobes for bioimaging

Imaging nanoprobes are a group of nanosized agents developed for providing improved contrast for bioimaging. Among various imaging probes, optical sensors capable of following biological events or progresses at the cellular and molecular levels are actually actively developed for early detection, accurate diagnosis, and monitoring of the treatment of diseases. The optical activities of nanoprobes can be tuned on demand by chemists by engineering their composition, size and surface nature. This review will focus on researches devoted to the conception of nanoprobes with particular optical properties, called persistent luminescence, and their use as new powerful bioimaging agents in preclinical assays.

Homogeneity of doping with paramagnetic ions by NMR

In NMR, paramagnetic dopants change the relaxation behavior and the chemical shift of the nuclei in their immediate environment. Based on the concept that the "immediate environment" in a diamagnetic host material can be described as a sphere with radius r0, we developed a function for the fraction of unperturbed nuclei (the fraction of nuclei outside the sphere) which gives a link between the effective radius and the doping concentration. In the case of a homogeneous doping scenario a characteristic dependence is observed in both theory and experiment. We validated the model on a sample series where paramagnetic Eu(II) ions are doped into crystalline SrH2. The fraction of unperturbed nuclei was determined from the (1)H NMR signal and follows the predicted curve for a homogeneous doping scenario where the radius r0 is 17 Å.

Synthesis, persistent luminescence, and thermoluminescence properties of yellow Sr3SiO5:Eu2+,RE3+ (RE=Ce, Nd, Dy, Ho, Er, Tm, Yb) and orange-red Sr(3-x)Ba(x)SiO5:Eu2+, Dy3+ phosphor

Sunlight-excitable orange or red persistent oxide phosphors with excellent performance are still in great need. Herein, an intense orange-red Sr3-xBaxSiO5:Eu(2+),Dy(3+) persistent luminescence phosphor was successfully developed by a two-step design strategy. The XRD patterns, photoluminescence excitation and emission spectra, and the thermoluminescence spectra were investigated in detail. By adding non-equivalent trivalent rare earth co-dopants to introduce foreign trapping centers, the persistent luminescence performance of Eu(2+) in Sr3SiO5 was significantly modified. The yellow persistent emission intensity of Eu(2+) was greatly enhanced by a factor of 4.5 in Sr3SiO5:Eu(2+),Nd(3+) compared with the previously reported Sr3SiO5:Eu(2+), Dy(3+). Furthermore, Sr ions were replaced with equivalent Ba to give Sr3-xBaxSiO5 :Eu(2+),Dy(3+) phosphor, which shows yellow-to-orange-red tunable persistent emissions from λ=570 to 591 nm as x is increased from 0 to 0.6. Additionally, the persistent emission intensity of Eu(2+) is significantly improved by a factor of 2.7 in Sr3-xBaxSiO5 :Eu(2+),Dy(3+) (x=0.2) compared with Sr3SiO5 :Eu(2+),Dy(3+). A possible mechanism for enhanced and tunable persistent luminescence behavior of Eu(2+) in Sr3-xBaxSiO5:Eu(2+),RE(3+) (RE=rare earth) is also proposed and discussed.

Time evolution of luminescence of Sr₂SiO₄:Eu²⁺

In this contribution, the photoluminescence, time-resolved luminescence and luminescence kinetics of α'-Sr2SiO4:Eu(2+) are studied. The luminescence of Sr2SiO4:Eu(2+) consists of two broad bands, peaked at 490 nm (blue-green) and 570 nm (yellow-orange), which originate from two luminescence centers, related to Eu(2+) in ten-coordinated SI and nine-coordinated SII sites, respectively. Based on spectroscopic data the energetic structure of Sr2SiO4:Eu(2+) has been developed, which includes the bands edges, energies of Eu(2+) in the SI and SII sites and energies of strontium and oxygen vacancies. To investigate the long-lasting luminescence phenomenon in Sr2SiO4:Eu(2+) the temperature influence on the time evolution of luminescence was analyzed. It has been found that the long-lasting luminescence is related to the Eu(2+) in SII site. The shallowest traps responsible for emission decaying within a few seconds are tentatively attributed to the [Eu(3+)(SII)-[Formula: see text]] centers. The depth of traps responsible for the long-lasting luminescence observed at room temperature has been estimated as equal 0.73 eV.