Electric Dipole-dipole Interaction Research Articles

Tuning and high temperature fluorescence properties of LaPO4:Sm3+ nanophosphors

A series of LaPO4:x%Sm3+ nanophosphors were successfully synthesized through a one-step hydrothermal method, where the acidity of the reaction system was meticulously modulated without the addition of any organic compounds. The influence of pH values and Sm3+ doping concentration on the structure, morphology, and luminescent properties of LaPO4:x%Sm3+ was systematically investigated. The composition, structure, morphology, and luminescent properties of the products were characterized and analyzed using advanced techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM, EDS), transmission electron microscopy (TEM), and fluorescence spectrophotometry (FL). The results indicate that the acidity of the reaction system effectively influences the morphology and size of the nanophosphors, facilitating the controlled synthesis of nanorod-shaped, nano-spherical, and ellipsoidal phosphors. When the Sm3+ doping concentration is 2 %, the phosphors exhibit maximum luminescent intensity and fluorescence lifetime. The energy transfer between Sm3+ ions in LaPO4:x%Sm3+ is identified as electric dipole-electric dipole interaction, with a critical distance for energy transfer established at 1.593 nm. LaPO4:2%Sm3+ exhibits good thermal stability, characterized by a thermal quenching activation energy of 0.1660 eV. The main outcome of this research, LaPO4:2%Sm3+ phosphor, exhibits significant potential for use in optical sensors, as well as promising applications in solar cells and biological fields.

Cooperative effects in dense cold atomic gases including magnetic dipole interactions

We theoretically investigate cooperative effects in cold atomic gases exhibiting both electric and magnetic dipole-dipole interactions, such as occurs, for example, in clouds of dysprosium atoms. After introducing a general framework capturing both the quantum degenerate and nondenegerate cases, we focus on the emergence of tailorable spin models in the quantum nondegenerate regime. In the low-excitation limit, we provide analytical and numerical results detailing the effect of magnetic interactions on the directionality of scattered light and characterize sub and superradiant effects. Published by the American Physical Society 2024

Er3+-activated Na0.5La0.5MoO4 multifunctional molybdate phosphors for optical thermometers and white LEDs

In this report, we have successfully prepared a series of Er3+-activated Na0.5La0.5MoO4 green molybdate phosphors by solid-state reaction. The X-Ray diffraction patterns confirmed that all diffraction peaks of Na0.5La0.5MoO4:Er3+ phosphors matched well with the standard data card with orthorhombic phase and the Rietveld refinement results indicated that the space group of the crystal structure was I41/a. Under 379 nm excitation, Na0.5La0.5MoO4:3%Er3+ phosphor exhibited strong green emission, with two strong emission peaks at 530 nm, 551 nm and a weak one centered at 656 nm, corresponding to the transitions of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. The experimental results showed that the optimal doping concentration of Er3+ was x = 3%, and electric dipole-dipole interaction was the major mechanism for the concentration quenching according to Dexter theory. Surprisingly, the optimal Na0.5La0.5MoO4:3%Er3+ phosphor presented thermal enhancement to the ratio of 2H11/2 → 4I15/2 transition to 4S3/2 → 4I15/2 transition in the elevated temperature range of 300–475 K. The maximum Sa and Sr were 1.79 × 10−2 K−1 and 1.094% K−1 respectively, which were applied to optical thermometers based on fluorescence-intensity-ratio technique. Furthermore, Na0.5La0.5MoO4:3%Er3+ phosphor had excellent thermal quenching performance (I425K/I300K = 93.47%) and high activation energy (Ea = 0.366 eV). Finally, the prepared Na0.5La0.5MoO4:3%Er3+ green phosphor, commercial BaMgAl10O17:Eu2+ blue phosphor and CaAlSiN3:Eu2+ red phosphor were combined with the 365 nm near-UV chip to produce a warm white LED device with high color index (Ra = 86.4) and low correlation color temperature (CCT = 4328 K). The above results suggested that the Er3+-activated Na0.5La0.5MoO4 multifunctional phosphors had considerable potential in the field of optical temperature measurement and phosphor-converted white light emitting diodes.

Entanglement Generation of Polar Molecules via Deep Reinforcement Learning.

Polar molecules are a promising platform for achieving scalable quantum information processing because of their long-range electric dipole-dipole interactions. Here, we take the coupled ultracold CaF molecules in an external electric field with gradient as qubits and concentrate on the creation of intermolecular entanglement with the method of deep reinforcement learning (RL). After sufficient training episodes, the educated RL agents can discover optimal time-dependent control fields that steer the molecular systems from separate states to two-qubit and three-qubit entangled states with high fidelities. We analyze the fidelities and the negativities (characterizing entanglement) of the generated states as a function of training episodes. Moreover, we present the population dynamics of the molecular systems under the influence of control fields discovered by the agents. Compared with the schemes for creating molecular entangled states based on optimal control theory, some conditions (e.g., molecular spacing and electric field gradient) adopted in this work are more feasible in the experiment. Our results demonstrate the potential of machine learning to effectively solve quantum control problems in polar molecular systems.

Observation of millihertz-level cooperative Lamb shifts in an optical atomic clock.

Collective couplings of atomic dipoles to a shared electromagnetic environment produce a wide range of many-body phenomena. We report on the direct observation of resonant electric dipole-dipole interactions in a cubic array of atoms in the many-excitation limit. The interactions produce spatially dependent cooperative Lamb shifts when spectroscopically interrogating the millihertz-wide optical clock transition in strontium-87. We show that the ensemble-averaged shifts can be suppressed below the level of evaluated systematic uncertainties for optical atomic clocks. Additionally, we demonstrate that excitation of the atomic dipoles near a Bragg angle can enhance these effects by nearly an order of magnitude compared with nonresonant geometries. Our work demonstrates a platform for precise studies of the quantum many-body physics of spins with long-range interactions mediated by propagating photons.

Luminescence properties and Judd-Ofelt analysis of Tb3+ doped Sr2YTaO6 double perovskite phosphors for white LED applications.

A series of Tb3+-doped Sr2YTaO6 double perovskite phosphors (SYT:Tb3+) were synthesized using a conventional solid-state reaction method. A strong green emission was observed in the SYT:Tb3+ phosphors, and the optimal doping concentration of Tb3+ was confirmed to be 5 mol%. The electric dipole-dipole interaction was ascribed to be the main mechanism for the luminescence concentration quenching. Analysis of the concentration-dependent fluorescence decay confirmed that the self-generated quenching model holds for the dynamic process of Tb3+ decays in SYT. Furthermore, the internal quantum efficiencies, non-radiative transition rates, and energy transfer rates of the 5D4 level for the SYT:Tb3+ samples were estimated, respectively. The luminescence thermal stability of the sample was also evaluated based on the Arrhenius model. The chromaticity shift of the SYT:5 mol% Tb3+ phosphor was examined to be 0.013 when the sample temperature was increased from 303 to 483 K, thus indicating excellent chromaticity shifting resistance under high temperature conditions. Moreover, the Judd-Ofelt parameters were calculated from the emission spectra of SYT:Tb3+ to be Ω2 = 0.29 × 10-20, Ω4 = 0.45 × 10-20, and Ω6 = 0.72 × 10-20 cm2, respectively. The fluorescence branching ratios and radiative transition rates for the 5D4 level were calculated based on the obtained Judd-Ofelt parameters. Finally, a white light-emitting diode (LED) prototype was assembled using a 310 nm LED chip combined with a prepared green SYT:Tb3+ phosphor and two other commercial blue and red phosphors. The obtained warm white light exhibits good chromaticity coordinates (0.32, 0.32) and a high color rendering index of 96.1. Based on the above results, it can be known that the prepared SYT:Tb3+ phosphors have a potential application as green emitting phosphors in white LEDs.

Synthesis and photoluminescence properties of high-quality reddish-orange emitting Ca4Nb2O9:Eu3+ phosphors for WLEDs and anti-counterfeiting.

In this report, we successfully synthesized a novel trivalent europium (Eu3+)-activated Ca4Nb2O9 phosphor emitting reddish-orange light via its 5D0 → 7F1 and 5D0 → 7F2 transitions. In the Ca4Nb2O9 host, Eu3+ ions exhibited optimal doping at a concentration of 15 mol%, with the concentration-quenching mechanism predominantly driven by electric dipole-dipole interactions. In addition, the Ca4Nb2O9:Eu3+ phosphor exhibited excellent thermal stability with a photoluminescence (PL) intensity of 71.6% at a working temperature of 423 K. Interestingly, the internal PL quantum yield (PLQY) of the optimal sample was obtained to be 87.43%, and the external PLQY was determined to be 47.81%. The fabricated white light-emitting diode that employed this optimized phosphor alongside commercial phosphors, via a novel silica epoxy gel (parts A and B)-based method, exhibited good color rendering index (color rendering index = 80.65), excellent warm-correlated color temperature (correlated color temperature = 3753 K), and Commission International de l'Eclairage chromaticity coordinate (0.3922, 0.3845). Moreover, the optimal phosphor was introduced into the polyvinyl alcohol (PVA) polymer film, creating a translucent film. These films were then fabricated on glass, plastic, and card, which showed a satisfying emission under ultraviolet radiation. Consequently, the proposed Eu3+-activated Ca4Nb2O9 phosphors can be used as light sources and the Ca4Nb2O9:Eu3+-PVA film is proposed for anti-counterfeiting applications.

Thermally stable Dy2O3/Sm2O3 co-doped glass ceramics containing La2Sn2O7 crystal with tunable luminescence

By using the melt crystallization process, Dy2O3/Sm2O3 co-doped glass ceramics (GCs) containing La2Sn2O7(LSO) crystal were successfully prepared. By using X-ray diffraction (XRD), the sample's phase was determined, and scanning electron microscopy (SEM), the sample's morphology was examined. The optimal heat treatment condition of glass ceramics was determined to be 650 °C/1.5 h. Under 362 nm excitation, the energy transfer of Dy3+→Sm3+ was observed in Dy2O3/Sm2O3 co-doped glass ceramic samples from the emission spectrum. Electric dipole-electric dipole interaction is identified as the mechanism of energy transfer from Dy3+ to Sm3+ according to the Dexter formula for energy transfer. The emission spectra of 0.5%Dy2O3-0.4%Sm2O3 co-doped glass ceramic samples in a range from 298 K to 458 K show that the samples have high thermal stability, and the thermal quenching activation energy at 575 nm has been estimated to be 0.265 eV. According to chromaticity coordinates and correlated color temperature, warm white light emission may be achieved by co-doping Dy2O3/Sm2O3 glass ceramics. The findings demonstrate Dy2O3/Sm2O3 co-doped glass ceramics containing La2Sn2O7 crystal have a potential application prospect in warm white light-emitting diodes (w-LEDs).

The effect of Pr3+ concentration on the microstructure, luminescence and scintillation properties of Pr:LuAG single crystals

High-quality lutetium aluminum garnet doped with various Pr3+ concentrations ((Lu1-xPrx)3Al5O12, abbreviated to Pr:LuAG) single crystals were grown by the optical floating zone method. The concentration of Pr3+ dependence on microstructure, luminescence and scintillation properties were systematically studied. Based on the results of Rietveld refinement, the doping of Pr3+ leads to lattice distortion and lowers its symmetry. The symmetry of the LuO8 dodecahedron center is lowest at 0.010, as confirmed by the Raman spectra. When excited at 450 nm and X-rays, the primary emission peak at 614 and 300 nm, respectively, induced by Pr3+ 4f2-4f2 and 4f5d-4f2 transitions. The optimal doping concentration is 0.010. The electric dipole-electric dipole interaction between adjacent Pr3+ was responsible for the concentration quenching in the Pr:LuAG single crystals. (Lu0.990Pr0.010)3Al5O12 shows the shortest average scintillation decay time (22.55 ns) and highest light yield (23,842 ± 2300 ph./MeV) under 662 KeV 137Cs gamma-ray irradiation. In terms of overall luminescence and scintillation performance, 0.010 doping was the best for Pr3+ doped LuAG single crystals.

Preparation and luminescence properties of Ce3+/Sm3+ codoped A-Lu2SiO5@SiO2 core-shell nanospheres

X-ray induced photodynamic therapy (X-PDT) has been applied as an effective strategy against cancer. A novel nanoscintillator A-Lu2SiO5:Ce3+,Sm3+@SiO2 core-shell nanosphere was fabricated and its practicability in X-PDT has been demonstrated in this work. The emission and excitation spectra of the co-doped samples were measured, and the energy transfer (ET) from Ce3+ to Sm3+ was investigated using the Inokuti-Hirayama (I-H) model, revealing that the dominant mechanism of ET in the sample Lu2SiO5:Ce3+,Sm3+@SiO2 nanospheres would be electric dipole-dipole interaction. In particular, the corresponding spectra demonstrated ET between Ce3+ and Sm3+ under X-ray excitation, while light yield has been estimated for a representative co-doped sample. It was concluded that the as-prepared A- Lu2SiO5:Ce3+,Sm3+@SiO2 core-shell nanospheres have potential applications for the simultaneous excitation of blue and red activated photosensitizers in X-PDT as material for nanoscintillators.

Energy-looping and photon-avalanche-like phenomena in NdxY1.00-xAl3(BO3)4 powders excited at 1064 nm

Recently, a photon-avalanche-like (PA-like) process based on trivalent neodymium ions (Nd3+) under an unconventional excitation at 1064 nm was demonstrated using stoichiometric NdAl3(BO3)4 particles. Although the Nd3+ can emit radiation at 1064 nm efficiently, they present very small absorption at this wavelength due to the absence of resonant ground-state transitions. However, phonon-assisted excitation (4I9/2 → 4F3/2 and/or 4I11/2 → 4F3/2) followed by cross-relaxation between one excited ion (4F3/2) with another at the ground state (4I9/2), and subsequent phonon emissions {4I15/2, 4I15/2 → 4I13/2, 4I13/2 → 4I11/2, 4I11/2}, provide two ions at the 4I11/2 state, from which can occur the resonant excited-state absorption to the 4F3/2 level. Reestablishing this sequence of events, the absorption of photons at 1064 nm can be greatly increased. Besides the emission around 880 nm (4F3/2 → 4I9/2), there are thermal excitations to upper-lying states, with subsequent emissions in the visible and near-infrared regions (480–2000 nm). Here, we investigate the role of the Nd3+ content on the PA characteristics in NdxY1.00-xAl3(BO3)4 particles with x ranging from 0.05 to 1.00. It is known that the replacement of Y3+ by Nd3+ into the YAl3(BO3)4 crystalline structure can introduce strong modifications of the lattice properties as well as in the photoluminescence characteristics, such as luminescence concentration quenching and broadening of spectral lines. Despite that, we observed, for low x (≤0.20), an energy-looping preceding the PA-like that ensues for x ≥ 0.40. It is associated to the proximity between the Nd3+ ions, fundamental to the electric dipole-electric dipole interaction responsible for the Nd3+ energy transfer {4F3/2, 4I9/2} → {4I15/2, 4I15/2}. We discuss the present results focusing on emerging technologies with development of ultra-sensitive thermal sensors.

Blue-emitting Ba2Ca(PO4)2:Eu2+, Tb3+ phosphor for full-spectrum w-LEDs applications

Ba2Ca(PO4)2:xEu2+, yTb3+ phosphors with good crystallinity and morphology was prepared by high-temperature solid-state method. The optimal excitation wavelength of Ba2Ca(PO4)2:xEu2+, yTb3+ phosphor is 349 nm, it shows a broad emission band with the highest peak at 453 nm and a narrow emission band at 544 nm after being excited. With the addition of Tb3+, the blue emission peak intensity of Eu2+ increase, the intensity of blue emission(453 nm) in Ba2Ca(PO4)2:0.5%Eu2+, 1%Tb3+ (y = 1%) is more than three folds that of Ba2Ca(PO4)2:0.5%Eu2+(y = 0). The energy transfer from Tb3+ to Eu2+ and the promotion of Eu3+→Eu2+ reaction by co-doping Tb3+ in BCP enhances the blue emission intensity. In Ba2Ca(PO4)2 matrix, the energy transfer mechanism from Tb3+ to Eu2+ is electric dipole-dipole interaction, and the energy transfer efficiency can reach up to 88%. The activation energy of Ba2Ca(PO4)2:0.5%Eu2+, 1%Tb3+ phosphor is 0.229 eV, which indicates that it has good thermal stability. The w-LED made of ultraviolet chip and Ba2Ca(PO4)2:0.5%Eu2+, 1%Tb3+ and Ba2Mg(PO4)2:1%Eu2+ phosphors has ultrahigh color rendering index (Ra = 92.0). Ba2Ca(PO4)2:xEu2+, yTb3+ powder is a new blue-cyan phosphor that can be applied to full-spectrum w-LEDs.

Preparation and Spectral Properties of MgAl2O4: Tb3+ Phosphor

A series of Tb3+ doped cubic MgAl2O4 green phosphors were synthesized via hydrothermal method followed by a post-annealing process. The variation regularity of phase structure, morphology, particle size and concentration quenching mechanism were investigated. The results showed that the crystal structure of matrix materials was not affected when Tb3+ ions were introduced. In addition, the micrometer-sized rods and excellent dispersion of the particles were obtained by adding urea. Under excitation at 377 nm, the phosphors exhibited the characteristic of Tb3+ emissions with the peaks located at 488 nm (5D4 → 7F6), 544 nm (5D4 → 7F5) and 588 nm (5D4 → 7F4), respectively. Moreover, based on optical analysis, it can be inferred that electric dipole-dipole (d-d) interaction plays an important role in concentration quenching process of Tb3+ ions. The green light emitting phosphor MgAl2O4: Tb3+ can be realized by adjusting the concentrations of Tb3+, and the optimum concentration of Tb3+ ions in MgAl2O4 was found to be 0.02. Finally, the average fluorescence decay lifetime and the CIE chromaticity coordinates of Mg0.98Al2O4: 0.02Tb3+ phosphor were calculated to be 1.2005 ms and (0.3197 0.4109), respectively. The above results suggest that the developed phosphor could have potential to be used as a single-phase green emission phosphor for near-ultraviolet light-emitting diode (NUV-LED) applications.

Tunable white light photoluminescence of a single phase Tm3+/Tb3+/Eu3+ codoped GdPO4 phosphor

The present study describes the production of triply-doped GdPO4 and its optical characteristics. Structural information of the prepared material has been investigated by using X-ray diffraction and Fourier transform infrared spectroscopy. The material synthesized crystallizes into a single monoclinic crystal of space group P 21/n. The excitation curves of photoluminescence of single doped GdPO4: Tm3+, Tb3+, and Eu3+ are recorded and their emission spectra are analyzed. Triple-doped material emits multi colour light after a single UV stimulation. White light emission was seen in both triply-doped GdPO4 with variable Tm3+ concentration and triply-doped GdPO4 with variable Eu3+ concentration. GdPO4 doped with Tb3+ (7 mol%), Tm3+ (1.5 mol%), and Eu3+ (2 mol%) has colour coordinates (x = 0.3290, y = 0.3291) approximately near the value of typical white colour coordinates (x = 0.333, y = 0.3333). Furthermore, the transfer of energy in Tm3+, Tb3+ and Eu3+ in triple doped phosphor with varying Eu3+ concentrations was explained. Electric dipole-dipole interaction is used to justify and describe the process of energy transfer from Tb3+ to Eu3+. Decay time measurements were performed to explain energy transfer. Colour rendering index (CRI), Corelated colour temperature (CCT), Quantum efficiency and thermal stability of the prepared phosphor was calculated. All these parameters infer that GdPO4 doped with Tm3+, Tb3+ and Eu3+ may have potential application in white LEDs.

Spectroscopy evaluation of Nd3+-activated novel CdO–V2O5–ZnO–B2O3 inverted glasses for near infrared laser applications

Nd3+-activated CdO–V2O5–ZnO–B2O3 (CVZB) inverted glasses with chemical composition of 80–5.0-2.5–12.5 mol%, respectively, were prepared by melt-quenching to analyze their optical spectroscopy features by applying the Judd-Ofelt (JO) theory. The Nd3+ content was changed from 0.4 up to 4.0 mol%. The structural characterization carried out by X-ray diffraction patterns and Raman spectroscopy, confirmed the amorphous nature for doping contents up to 4.0 mol% of Nd3+. The direct (Egdir) and indirect (Egind) band gap obtained from the absorption spectra, showed values of 2.76 ± 0.12 and 2.50 ± 0.10 eV, respectively, without significative changes by the Nd3+ doping variations. Such band gap energy values are suitable to allow the Nd3+ emissions centered at 878, 1060 and 1332 nm, corresponding to the 4F3/2 → 4I9/2,11/2,13/2 transitions, respectively. The Nd3+ emission intensity reached the optimum at 2.8 mol% of Nd3+. At larger Nd3+ concentrations, the emission is gradually quenched due to cross-relaxation processes. These processes were dominated by electric dipole-dipole interactions within Nd3+-Nd3+ clusters, as suggested by the Inokuti-Hirayama model and critical interaction distance. The experimental branching ratios (βexp) were stables up to 1130 mW of 808 nm laser excitation, with values of (4F3/2 → 4I9/2) βexp = 0.28, (4F3/2 → 4I11/2) βexp = 0.56, and (4F3/2 → 4I13/2) βexp = 0.16. The JO parameters for the optimal emitting glass (2.8 mol% of Nd3+) obtained from least-square fitting between theoretical (fcal) and experimental (fexp) oscillator strengths, were Ω2 = 5.31 × 10−20, Ω4 = 2.81 × 10−20 and Ω6 = 3.34 × 10−20 cm2. The stimulated cross-section peak (σp), obtained from the JO parameters, resulted to be σp = 1.21 × 10−20 and 3.50 × 10−20 cm2 for the 4F3/2 → 4I9/2 and 4F3/2 → 4I11/2 transitions, respectively. The radiative (τR) and experimental (τexp) lifetimes led to quantum yield value of 0.49. Finally, laser parameters for the 4F3/2 → 4I11/2 emission, such as bandwidth (σPEMI × Δλem) and optical gain (σPEMI × τR) were calculated from the emission cross-section peak (σPEMI, 1061 nm), resulting values of 165 × 10−27 cm3 and 37 × 10−25 cm2s, respectively.

The roles of dopant concentration and defect states in the optical properties of Sr2MgSi2O7:Eu2+, Dy3+

The blue-emitting Sr2MgSi2O7:Eu2+,Dy3+ was prepared by sol-gel method. The effects of dopant concentration on crystal structure, photoluminescence and afterglow properties were studied. Based on First-principles calculations, the formation difficulty and electronic structures of intrinsic defects and the external defects caused by doping Eu, Dy were predicted, and the defect-dominated persistent luminescence mechanism was proposed. The results showed that when the content of Eu2+ reached 0.03 mol%, Sr2MgSi2O7:Eu2+,Dy3+ had the best luminescence performance. After exceeding this content, the concentration quenching caused by electric dipole-dipole interaction would occur, leading to a decrease in luminescence intensity. The oxygen vacancy as an intrinsic defect was more easily formed in Sr2MgSi2O7. The impurity levels in the forbidden band, which were introduced by oxygen vacancies and Dy doping defects, can effectively capture and release Eu electrons. It is beneficial to prolong the afterglow duration of Eu 4f65d1→Eu 4f7. This work reveals the electron transition and capture paths in the luminescence of Sr2MgSi2O7:Eu2+,Dy3+, which has reference significance for designing defect structures and optimizing optical properties.

Tunable Interferometric Effects between Single-Molecule Suzuki-Miyaura Cross-Couplings.

Parallel two molecular bridges loaded with a palladium catalyst were integrated into separate pairs of graphene electrodes in the same device. Based on the complete description of the one-palladium catalytic pathway by single-molecule electrical spectroscopy, this setup enables the mapping of the cross-correlation between different catalysts and demonstrates the emergent complexity in the extrapolation from single molecule to ensemble. The anticorrelation behaviors at the time scale of two individual catalysts in sufficiently close proximity were revealed in the Suzuki-Miyaura cross-coupling. Further experimental evidence demonstrates that the long-range electric dipole-dipole interaction induced by solvent leads to the destructive interferometric effect of two catalysts. In contrast, the cooperative coupling of the elementary step between two catalysts affords a local acceleration. This new form of reaction dynamics measurement via focusing on multiple molecules with single-event resolution holds great promise to build a bridge between single molecule and ensemble.

Auto-combustion synthesis of Sm3+-doped NaYF4 phosphors: Concentration quenching, optical transition and luminescent properties

A series of NaYF4:Sm3+ phosphors with various Sm3+ concentrations were prepared via an auto-combustion-assisted fluorination method. By the use of X-ray diffraction (XRD), we investigated the phosphors' crystal structure, and pure phase NaYF4:Sm3+ were confirmed. Irregular and agglomerated hexagonal-like particles were observed in FE-SEM images of the phosphors. Moreover, higher magnification TEM images have been collected. Based on fluorescence spectra, the concentration quenching and luminescence thermal stability of NaYF4:Sm3+ phosphors were studied. On the basis of Van Uitert's model, the electric dipole-dipole interaction was proven to be the mechanism for energy transfer between Sm3+ ions. Furthermore, the temperature-dependent fluorescence was investigated in depth, and it was shown that the Arrhenius model-based crossover process can adequately describe the temperature-dependent thermal quenching behavior of 4G5/2 level for Sm3+ in NaYF4 powders. Finally, based on the diffuse-diffraction spectrum and the fluorescence decay curve, the optical transition properties of Sm3+ in the examined NaYF4 phosphors were analyzed in the framework of Judd-Ofelt theory.

Color-tunable emissions realized by Tb3+ to Eu3+ energy transfer in ZnGdB5O10 under near-UV excitation.

Photoluminescent (PL) energy transfer (ET) between two typical rare earth activators Tb3+ and Eu3+ is utilized to achieve color-tunable emission and the color range is apparently dependent on the ET efficiency. In the target host ZnGdB5O10 (ZGBO), the relatively low symmetric coordination environment of the rare earth cation not only suppresses the parity-forbidden law of the 4f-4f transitions of Tb3+ in the near-UV region, but also enhances the internal quantum efficiency (IQE), where the optimal IQE is 65.61% for ZGBO:0.8Tb3+. Moreover, its ET to Eu3+ is highly efficient, i.e. 94.71% in ZGBO:0.8Tb3+,0.10Eu3+, which eventually leads to a wide range of color-tunable emissions from green (0.2915, 0.5915) to red (0.6207, 0.3731). The systematic PL spectral study on Tb3+/Eu3+ singly doped and co-doped phosphors suggests that the ET mechanism takes place through the electric dipole-dipole interaction according to the Inokuti-Hirayama (I-H) model. Additionally, the in situ high temperature PL spectra indicate the very high thermal stability of ZnGd0.19Tb0.8Eu0.01B5O10, indicating that it can be a potential candidate for near-UV light emitting diode-pumped phosphors.

Thermally stable and color-tunable bi-activated (Dy3+/Eu3+) alkaline earth metasilicate phosphor for luminescent devices.

A solid-state reaction methodology has been adopted to synthesize Dy3+ (dysprosium)/Eu3+ (europium) co-activated Na4Ca4Si6O18 (NCMS) phosphors. The structural, morphological, and luminescence characteristics of the prepared materials have been investigated. The phase purity of the material was confirmed by X-ray diffraction (XRD) by comparing the diffraction peaks with the JCPDS standard pattern (JCPDS card no. 75-1687). The photoluminescence (PL) spectra of NCMS phosphors activated with Dy3+ and co-activated with Dy3+ (sensitizer)/Eu3+ (activator) ions were investigated. The as-prepared NCMS phosphors co-activated with Dy3+/Eu3+ ions were excited with near-ultraviolet light (λex = 348 nm) and showed the utmost energy transfer of up to 97.80% from sensitizer to activator. Dexter and Reisfeld's approximation specifically confirms that the energy transfer from sensitizer to activator was through electric dipole-dipole interactions. The Dy3+-activated NCMS phosphor showed an illumination shift from yellow to red by varying the Eu3+ ion concentration and colour tunability is also observed by altering the excitation energy. The emission intensity was sustained up to 92.21% at 423 K (∼150 °C), indicating an excellent thermal stability of the bi-activated NCMS phosphor. The Dy3+/Eu3+ co-doped NCMS phosphors display excellent thermal stability with flexible color tunability to emerge as promising contenders in the field of lighting and display technologies.