7F2 Transition Research Articles

(Invited) Nanocrystalline Oxide-Based Luminescent Nanophotonic Structures

Light nanoemiters are the cornerstone of photonic integrated applications including displays for consumer products such as phones and smartwatches, and for advanced application such as quantum emitters. In this context it is appealing to have access to a single material platform suitable for tunable wavelength emission, and if possible, that can provide broadband white light emission. Solid state light emitting nanostructures based on Europium ions are a promising solution. When imbedded in a solid they can have two oxidation states Eu3+ or Eu2+ which upon excitation show a either a high purity red emission (associated to the Eu3+ dipole-forbidden intra-4f shell 5D0® 7F2 transition), or a broad emission band in the 450-600 nm spectral region (related to the Eu2+ dipole-allowed 4f6 5d -> 4f7 transitions). Here I will discuss the strategy that our group is following based on europium oxides EuOx, 1/3<x<1, as this approach allows to have a high density of emitters. Moreover, their electric and optical properties can be tuned from the sesquioxide Eu2O3 that is a wide bandgap dielectric that shows the Eu3+ ion related narrow band red emission, to the monoxide EuO that is a semiconductor that shows a wide band white light emission related to the Eu2+ ion. For the nanoemitters we have prepared EuOx nanocrystalline films with high transparency, and in which the coexistence of both oxidation Eu states enables wavelength tunability [1.2]. Finally, recently we have designed 2D hybrid nanoemitters by integrating the EuOx films with metallic metasurfaces in order to induce an enhancement of the emission intensity by selective plasmonic excitation [3].[1] A. Mariscal, A. Quesada, A. Tarazaga Martín-Luengo, Miguel A. García, A. Bonanni, J.F. Fernández and R. Serna, Appl. Surf. Science 456,980 (2018).[2] A. Mariscal-Jiménez, A. Tarazaga Martín-Luengo, B. Galiana, C. Ballesteros, A. Bonanni, J. Martín-Sánchez, R. Serna, J. Phys. Chem. C. 124, 15434–15439 (2020).[3] P. Gomez-Rodriguez, E. Soria, Y. Jin, A. Caño, I. Llorente, A. Cuadrado, A. Mariscal-Jiménez, A. K. Petford-Long, R. Serna and J. Gonzalo, Nanophotonics. 10, 3995–4007 (2021).

(Invited, Digital Presentation) Single-Atom Quantum Magnetism in 2D Materials

With the advent of 2D materials, the playground to study spins in dilute and non-dilute phases has expanded. This is appealing for utilizing the additional degrees of freedom of electron systems such as spin and valley and, from the fundamental point of view, to better understand atomic scale magnetic phenomena in low dimensional materials. Dilute magnetism in 2D materials can lead to complex magnetic phenomena (e.g., Kondo effect, RKKY-interactions, quantum relaxation and coherence), with potential for applications in spintronics (e.g., spin FETs) and quantum technologies (e.g., single-atom quantum memories). We are investigating how to selectively incorporate substitutional magnetic atoms (3d transition metals and 4f rare earths) in 2D materials, using ultra low energy ion implantation, and we characterize their structural, electronic, and magnetic properties [1]. Ultra-low energy (ULE) ion implantation allows us to precisely tune the kinetic energy of the ions, providing control over the form of incorporation and concentration while preserving the structural and electronic properties of graphene. Our approach is based on a wide range of characterization techniques (structural and electronic), including scanning tunneling microscopy and spectroscopy (STM/STS), Raman spectroscopy, synchrotron-based X-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), X-ray magnetic circular dichroism (XMCD), among others. These experimental studies are complemented by density functional theory (DFT) and molecular dynamics (MD) simulations. The new insights provided by our work establish a framework for the controlled incorporation of magnetic dopants in 2D materials, using ULE ion implantation.[1] P. C. Lin et al., ACS Nano 15(3), 5449-5458 (2021).

Eu3+ ions doped zinc borotellurite glass system for white light laser application: Structural, physical, optical properties, and Judd-Ofelt theory

The melt-quenching method was used to fabricate zinc borotellurite glasses of composition [{(TeO2)0.7 (B2O3)0.3}0.7 (ZnO)0.3](1-y) (Eu2O3)y, with 0.01 = y ≤ 0.05 M fraction. The calculated density and molar volume values lies between 4.74 and 5.15 g/cm3, and 24.58 and 25.61 cm3/mol. The optical properties were analysed and discussed. The estimated optical band gap (direct and indirect) energies decrease from 3.49 to 3.05 eV and 3.11 to 2.81 eV, whereas the Urbach energy and reflective index increase from 0.262 to 0.334 eV and 2.368 to 2.450 with Eu2O3 content. The evaluated Racah parameters (B and C) ranges from 92.60 to 119.86 cm−1 and from 626.64 to 643.35cm−1, respectively. The calculated Judd-Ofelt Ω2 parameter increases from (2.78 to 3.93)×10−20cm2. Furthermore, the gain bandwidth and optical gain value were found between (14.295 to20.340)×10−28cm2 and (12.576 to 15.693)×10−25cm2, respectively, for the 5D2 → 7F2 transition. In addition, stimulated emission cross-section and branching ratio values of 14.029×10−22cm2 and 66.54% were achieved from TBZEu0.04 prepared glass, indicating a promising potential for white-light lasing application.

Local strain-dependent Zeeman splitting in GaN:Eu

The electronic spins of rare-earth materials are attractive candidates for spin qubits and quantum memories. To access individual spins, tuning of the g-factor is desirable. Here, we report on local strain-dependent g-factors of the 5D0–7F2 transitions of Eu3+ centers in GaN:Eu thin films. We have found a clear correlation between the effective g-factor and the emission energy shift induced by the local strain. The combination of micro-photoluminescence and scanning electron microscope/electron backscattering diffraction measurements has revealed that the compressive strain of 0.2%–0.4%, relative to a surrounding reference point, induces an energy shift of about 3 meV. The strain decreases the g-factor of the emission at 1.991 eV from 2.5 to 1.5, while the strain increases the g-factor of the emission at 1.994 eV from 1.1 to 1.7. The result suggests that the g-factor can be tuned by the local strain. On the basis of the strain-induced energy shift and the g-factor, we have identified the optical sites. The 5D0–7F2 transitions observed in this study consist of three optical sites with C3v symmetry and one site with C1h symmetry.

Enhanced orange persistence in Sr3Al2O5Cl2:Eu2+, Dy3+ phosphor by Mg2+ substitution in Sr2+lattice site

The initial intensity and afterglow duration of existing long-wavelength persistent phosphors are insufficient for practical applications. Herein, the afterglow of orange-emitting Sr 3 Al 2 O 5 Cl 2 :Eu 2+ , Dy 3+ phosphors are enhanced by co-doping with Mg 2+ using the conventional solid-state reaction. Rietveld refinement for the prepared phosphor revealed that all Eu 2+ cations were placed on the Sr 2+ sites, whereas Dy 3+ occupancy in the lattice was improved by incorporating Mg 2+ . All phosphor samples with different Mg 2+ concentrations emit 609-nm-centred orange luminescence caused by the 5d→4f transition of Eu 2+ . The photoluminescence was significantly enhanced with the increase in the Mg 2+ doping due to the increased amount of divalent Eu 2+ in the samples, as confirmed by the X-ray photoelectron spectroscopy investigation. Incorporating Mg 2+ also enhanced the afterglow performance by creating suitable electron traps in the phosphor. The afterglow profiles were simulated by a double exponential decay model, and the results revealed that significant improvement in the afterglow lifetime was achieved for the 0.3 Mg 2+ -doped phosphors. Also, Mg 2+ introduction resulted in a deeper trap level (∼0.72 eV) and an increased trap concentration. Consequently, the Mg 2+ -doped exhibits brighter initial photoluminescence and longer afterglow. • Sr 3−x Mg x O 5 Cl 2 :Eu 2+ , Dy 3+ phosphor exhibits orange emission at 609 nm. • The photoluminescence is improved due to the increased Eu 2+ content after Mg 2+ doping. • Enhanced afterglow is observed after the incorporation of Mg 2+ cations. • The improved afterglow is caused by the increased trap concentration and modified trap depth.

Photoluminescence behavior of Eu3+ doped XAl2O4 (X = Mg, Ca, Sr and Ba) phosphors: a comparative study

In this work, the Eu3+ doped stuffed tridymite type structure of alkaline earths aluminate i.e. XAl2O4 (X = Mg, Ca, Sr and Ba) phosphor materials have been synthesized by conventional high temperature solid state reaction method at 1623 K. The Samples were structurally and morphologically characterized by x-ray diffraction (XRD) and Scanning electron microscope (SEM) measurements. The vibrational behavior of the phosphor samples were investigated by Fourier transform infrared (FTIR) measurements. The phosphor samples emit intense red emission in 610–615 nm range due to 5D0 → 7F2 transition of Eu3+ ion on excitation with charge transfer band (CTB) wavelength arising due to Eu3+-O2- and also by the discrete bands of Eu3+ ions .The decay time of 5D0 level of Eu3+ ion were recorded on excitation with 393 nm and by the CTB wavelength for all the four samples. The optimized 1 mol% Eu3+ doped CaAl2O4 phosphor exhibits optimum emission intensity and color purity under the excitation with 393 nm than others. The decay time is also found to be larger in the case of Eu3+ doped CaAl2O4 phosphor sample. Therefore, Eu3+ doped CaAl2O4 phosphor may be promising material for red color light emitting applications and white light generation.

Scintillation and photoluminescence investigations of Gd2MoB2O9: CeF3 phosphors

The CeF3 doped Gd2-xMoB2O9 phosphors (where x = 0.00, 0.25, 0.50 and 0.75 mol%) were synthesized by the conventional high-temperature solid-state reaction method at 900 °C for 12 h in air. DSC and TGA analysis executed to study the optimum temperature for sintering. The structural, photoluminescence and X-ray luminescence properties were investigated. The structure of the samples was analyzed by using X-ray diffractometer (XRD). The absorption spectra were analyzed and found that absorption edge increased with the increasing concentration of CeF3. The photoluminescence of phosphors shows the overlapping of excitation and emission peaks in the range of 300 nm–450 nm. These emission peak correspond to the 5d – 4f transitions of Ce3+ ion. The optimum concentration of CeF3 in these phosphors is 0.50 mol% corresponding to maximum emission intensity. The X-ray induced optical luminescence spectra showed a strong emission peak at 411 nm the concentration quenching at 0.75 mol%, which is as efficient as 54.4% compared with bismuth germanium oxide (BGO) crystal. The luminescence decay increased with the increment of CeF3 concentration. From the optical properties investigated, it was found that the phosphors performed a strong and fast luminescence signal which is appropriate for the development of scintillators in X-ray detector devices.

Exploration of the Crystal Structure and Thermal and Spectroscopic Properties of Monoclinic Praseodymium Sulfate Pr2(SO4)3.

Praseodymium sulfate was obtained by the precipitation method and the crystal structure was determined by Rietveld analysis. Pr2(SO4)3 is crystallized in the monoclinic structure, space group C2/c, with cell parameters a = 21.6052 (4), b = 6.7237 (1) and c = 6.9777 (1) Å, β = 107.9148 (7)°, Z = 4, V = 964.48 (3) Å3 (T = 150 °C). The thermal expansion of Pr2(SO4)3 is strongly anisotropic. As was obtained by XRD measurements, all cell parameters are increased on heating. However, due to a strong increase of the monoclinic angle β, there is a direction of negative thermal expansion. In the argon atmosphere, Pr2(SO4)3 is stable in the temperature range of T = 30–870 °C. The kinetics of the thermal decomposition process of praseodymium sulfate octahydrate Pr2(SO4)3·8H2O was studied as well. The vibrational properties of Pr2(SO4)3 were examined by Raman and Fourier-transform infrared absorption spectroscopy methods. The band gap structure of Pr2(SO4)3 was evaluated by ab initio calculations, and it was found that the valence band top is dominated by the p electrons of oxygen ions, while the conduction band bottom is formed by the d electrons of Pr3+ ions. The exact position of ZPL is determined via PL and PLE spectra at 77 K to be at 481 nm, and that enabled a correct assignment of luminescent bands. The maximum luminescent band in Pr2(SO4)3 belongs to the 3P0 → 3F2 transition at 640 nm.

Luminescence and scintillation properties of Ce-doped calcium hafnate perovskite single crystals

Hafnium-based materials are attractive for a scintillator owing to their high density and effective atomic number. In this study, single crystals of Ce-doped calcium hafnate perovskite (Ce:CaHfO3) were synthesized from a Ca-rich composition, and photoluminescence (PL) and scintillation properties were investigated. The X-ray diffraction patterns revealed that the synthesized single crystals were a single phase of CaHfO3. The Ce:CaHfO3 exhibited blue luminescence at 430 nm and fast decay due to the 5d–4f transition of Ce3+ in both PL and scintillation. From the measurement of pulse height spectra under 137Cs irradiation, the highest light yield of 7800 photons/MeV was observed in the 3% Ce:CaHfO3.

Growth and properties of Pr3+-doped NaGd(MoO4)2 single crystal: A promising InGaN laser-diode pumped orange-red laser crystal

Trivalent praseodymium (Pr3+) doped materials have received widely attentions due to their promising prospects for InGaN laser-diodes pumped visible lasers. In this work, a 2 at.% Pr3+ doped oxide crystal, Pr:NaGd(MoO4)2 crystal, was grown successfully by Czochralski method. The color of the crystal changes from black to green after annealing in air atmosphere. The structure of the crystal before and after annealing were characterized by powder X-ray diffraction. Then, Rietveld refinement method was performed to the X-ray diffraction data to determine the lattice parameters of the crystal. The structure of the crystal was further confirmed by Raman scattering spectrum. The spectroscopic properties of the crystal, including absorption and fluorescence spectra, were characterized and discussed. In visible range, the crystal has a strongest absorption band with center at 450 nm, which is well matched with the output wavelength of InGaN laser-diodes. The absorption cross-section at this wavelength was 1.07 × 10−20 cm2. Under the excitation at 450 nm, the strongest emission was detected at 652 nm, which corresponding to the 3P0→3F2 transition of Pr3+. The fluorescence decay time for the 3P0→3F2 transition of Pr3+ was fitted to be 4.01 μs. The results indicate that Pr:NaGd(MoO4)2 crystal is a potential candidate for InGaN-based LD pumped orange-red lasers.

Optical and structural properties of composites films based on Ce3+ -doped yttrium aluminium garnet nanoparticles embedded in polystyrene.

In the present work, attempts have been made to prepare scintillating nanoparticle composite films of Ce3+ -doped Y3 Al5 O12 (YAG:Ce) embedded in a polystyrene (PS) polymer. A YAG:Ce phosphor has been previously synthesized using the sol-gel method. YAG:Ce-PS composite films of 250 ± 30 μm thickness were prepared using a solvent casting procedure with different PS/solvent concentration and a different mass ratio between nanoparticles of YAG:Ce and PS. X-ray diffraction analysis confirmed that the YAG:Ce powders were successfully prepared. Using thermogravimetric analyses and differential scanning calorimetry, we found that the glass transition temperature (Tg) and thermal degradation were shifted to higher temperatures for composite films relative to pure PS. Photoluminescence showed the yellow emission of the Ce3+ -doped YAG phosphors, which was attributed to the 5d→4f transition of Ce3+ ion and the intensity of the emissions changed with the mass ratio of the YAG:Ce nanoparticles incorporated in the polymer and with the concentration of the polymer solution.

Synthesis of Eu(HCOO)3 and [formula omitted] crystals and observation of their 5D0→ 7F0 transition for quantum information systems

Two stoichiometric metal-organic frameworks containing Eu3+ cations are probed as candidates for photon-based quantum information storage. Synthesis procedures for growing 0.2 mm, rod-shaped Eu(HCOO)3 and 1–3 mm, rhombohedral Eu(HCOO)3⋅(HCONH2)2 single crystals are presented with visible precipitation as soon as 1 h into heating for Eu(HCOO)3 and 24 h for Eu(HCOO)3⋅(HCONH2)2. Room temperature and 1.4 K photoluminescence measurements of the 5D0→ 7FJ transitions of Eu3+ are analyzed for both compounds. Comparisons of peak width and intensity are discussed along with the notable first report for both of the 5D0→ 7F0 transition, the hyperfine structure of which has potential use in quantum memory applications. The air instability of Eu(HCOO)3⋅(HCONH2)2 and the transformation of its photoluminescence properties are discussed.

Thermally stimulated luminescence properties of magnesium orthosilicate single crystals doped with Dy3+ ions

In this study, we grew the Dy-doped Mg2SiO4 single crystals by the floating zone method to investigate the photoluminescence (PL) and thermally stimulated luminescence (TSL) properties. All the Dy-doped Mg2SiO4 showed the emission peaks at 480, 580, and 670 nm in PL and TSL spectra, and these peak wavelengths were typical for the 4f–4f transitions of Dy3+ ions. In the TSL glow curves, all the samples had the glow peaks at 50 °C, 200 °C, 280 °C, and 380 °C. The TSL dose response function of the 1% Dy-doped Mg2SiO4 single crystal showed a good linearity across 1–10000 mGy.

Scintillation characteristics of Eu2O3-doped WO3–Al2O3–TeO2 glasses

Eu2O3-doped tellurite glasses (15WO3–5Al2O3–80TeO2) were fabricated and their scintillation characteristics were investigated. Typical emissions originating from the 4f→4f transitions of Eu3+ were observed at 580, 592, 614, 653, and 702 nm when irradiated with visible light. The quantum yield of the Eu2O3-doped glass increased as the Eu2O3 concentration increased, and the 5.0% Eu2O3-doped glass exhibited the largest quantum yield of 76%. Further, scintillation originating from the 4f→4f transition of Eu3+ was also detected under X-ray irradiation, and the integrated scintillation intensity depended on the quantum yield. The afterglow levels of the glasses decreased with increasing the Eu2O3 concentration, and the level of the 5.0% Eu2O3-doped glass was 193 ppm, which was close to that of the Tl-doped CsI.

Red luminous ternary europium complexes: Optoelectronic and photophysical analysis

Four luminescent ternary europium complexes of general formula Eu(PBD)3L were synthesized and characterized. Here, L represents auxiliary unit (2,2′-bipyridine and its derivatives) and PBD (1-phenyl-1,3-butanedione) is an organic chromophoric ligand. Elemental composition and complexation has been estimated by elemental and spectroscopic studies. Photoluminescence emission spectra clearly indicate the chromophoric nature of diketone ligand that has effectively sensitized the energized level of Eu3+ ion through antenna effect. The detailed photophysical parameters of the complexes were evaluated theoretically and experimentally. Theoretical parameters were estimated via LUMPAC software and their outcomes were found to be in good concurrence with experimentally derived data. Luminescence decay curve were recorded by monitoring the wavelength corresponding to hypersensitive 5D0→7F2 transition. Geometry of the complexes was resolved via Avogadro software. Colorimetric and photoluminescence spectral analyses suggest the red luminous nature of ternary europium complexes. These red emissive materials might be utilized in designing display devices.

Emission and EPR studies on green and red color emitting gallate phosphor containing manganese/chromium ions

The ZnGa2O4:Mn2+/Cr3+ ceramic phosphor is synthesized via high temperature solid state reaction method and then phosphor is intensively studied for green/red color emission. Prepared samples are structurally characterized through powder X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM) and it is found that the samples possess single phase cubic spinel structure. The X-ray photoelectron spectroscopy (XPS) is performed for the identification of chemical states of different elements present in the samples. The Zn0.9Ga2O4:0.1%Mn2+ sample has shown broad emission band (475–575 nm) having peak at 505 nm (green) due to the electronic transition of Mn2+ ion. The ZnGa1.9O4:0.1%Cr3+ sample has shown intense emission at 696 nm (deep red) due to the 2E (2G) →4A2(4F) transition of Cr3+ ions. The co-doped Zn0.9Ga1.9O4:Mn2+/Cr3+ sample on the other hand can be excited with different excitations to give green and red emissions. Electron paramagnetic resonance (EPR) spectroscopy is done to inspect defects caused by the substitution of Ga3+ and Zn2+ by Cr3+ and Mn2+ ions, respectively. Emission decay measurements are done to decipher afterglow and photoluminescence decay.

Luminescent Features of Ternary Europium Complexes: Photophysical and Optoelectronic Evaluation.

Trivalent europium complexes exhibit good luminescent characteristics. A series of octacoordinated ternary europium complexes with fluorinated diketone and heteroaromatic auxiliary unit were synthesized. The synthesized europium complexes were characterized by elemental, thermal, electrochemical and spectroscopic analyses. Band gap values lie in range of semiconductors which confirm the conducting behavior of prepared complexes. Photoluminescence spectra were recorded in solid state and DMSO solvent. Emission spectral profiles have displayed most intense peak at ~ 612nm corresponding to hypersensitive 5D0 → 7F2 transition. Colorimetric parameters suggest red luminous nature of europium complexes. The luminescent heteroleptic europium complexes might be utilized as emissive materials for fabricating display.

Photoluminescence properties of Eu3+ doped CaSr(WO4)2 phosphor by Li+ charge compensation

Abstract A series of CaSr(WO4)2: Eu3+ and CaSr(WO4)2: Eu3+, Li+ phosphors with different Eu3+ or Eu3+/Li+ molar contents were successfully synthesized by high-temperature solid-state reaction. Through a series of characterization methods, the composition, structure and luminescence characteristics of the sample are obtained. Upon excitation at 393 nm, the strong red light corresponding to the 5D0 → 7F2 transition of Eu3+ can be observed. The effect of Li+ charge compensation on the photoluminescence of the sample is studied. The strongest red emission is obtained through 1% Eu3+ and 1% Li+ doping. All of the results indicate that Li+ charge compensation can effectively enhance the red emissions intensity of Eu3+-doped CaSr(WO4)2 phosphors.

Investigation of Eu3+ Doped Oxy-Fluoride Phosphate Glass for Red Laser Gain Medium Application

The oxyfluoride phosphate glass doped with europium oxide was synthesized by melt quenching technique. The structural and spectroscopic analysis has been carried out by Raman, absorption, photoluminescence, phonon side band and lifetime analysis. The absorption spectrum revealed the characteristic transitions between the 7F0 ground state and 7F1 first excited state to the upper-lying electronic states. The several J-O parameters: oscillator strength, radiative transition probability, stimulated emission cross-section and branching ratio. The local structure around the Eu3+ ions and the phonon energy of oxyfluoride phosphate glass have been confirmed on the basis of PSB associated with the 7F0→5D2 transition. The highest excitation spectrum showed at 395 nm for transition 7F0→5L6. The emission spectrum showed two major peaks corresponding to 591 nm orang emission (5D0→7F1) and 613 nm red emission (5D0→7F2). This work confirmed that 1 mol% Eu2O3 doped oxyfluoride phosphate glass performs a high potential for using as efficient luminescence materials for solid-state lighting application, especially for reddish-orange light-emitting glass.

Efficient Near‐Infrared Luminescence in Lanthanide‐Doped Vacancy‐Ordered Double Perovskite Cs2ZrCl6 Phosphors via Te4+ Sensitization

All-inorganic lead-free perovskite-derivative metal halides have shown great promise in optoelectronics, however, it remains challenging to realize efficient near-infrared (NIR) luminescence in these materials. Herein, we report a novel strategy based on Te4+ /Ln3+ (Ln=Er, Nd, and Yb) co-doping to achieve efficient NIR luminescence in vacancy-ordered double perovskite Cs2 ZrCl6 phosphors, which are excitable by a low-cost near-ultraviolet light-emitting diode (LED) chip. Through sensitization by the spin-orbital allowed 1 S0 →3 P1 transition of Te4+ , intense and multi-wavelength NIR luminescence originating from the 4f→4f transitions of Er3+ , Nd3+ , and Yb3+ was acquired, with a quantum yield of 6.1 % for the Er3+ emission. These findings provide a general approach to achieve efficient NIR emission in lead-free metal halides through ns2 -metal and lanthanide ion co-doping, thereby opening up a new avenue for exploring NIR-emitting perovskite derivatives towards versatile applications such as NIR-LEDs and bioimaging.