8 research outputs found

    Controlled Electron–Hole Trapping and Detrapping Process in GdAlO<sub>3</sub> by Valence Band Engineering

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    Two different trapping and detrapping processes of charge carriers have been investigated in GdAlO<sub>3</sub>:Ce<sup>3+</sup>,Ln<sup>3+</sup> (Ln = Pr, Er, Nd, Ho, Dy, Tm, Eu, and Yb) and GdAlO<sub>3</sub>:Ln<sup>3+</sup>,RE<sup>3+</sup> (Ln = Sm, Eu, and Yb; RE = Ce, Pr, and Tb). Cerium is the recombination center and lanthanide codopants act as electron-trapping centers in GdAlO<sub>3</sub>:Ce<sup>3+</sup>,Ln<sup>3+</sup>. Different lanthanide codopants generate different trap depths. The captured electrons released from the lanthanide recombine at cerium via the conduction band, eventually producing the broad 5d–4f emission centered at ∼360 nm from Ce<sup>3+</sup>. On the other hand, Sm<sup>3+</sup>, Eu<sup>3+</sup>, and Yb<sup>3+</sup> act as recombination centers, while Ce<sup>3+</sup>, Pr<sup>3+</sup>, and Tb<sup>3+</sup> act as hole-trapping centers in GdAlO<sub>3</sub>: Ln<sup>3+</sup>,RE<sup>3+</sup>. In this situation, we find evidence that recombination is by means of hole release instead of the more commonly reported electron release. The trapped holes are released from Pr<sup>4+</sup> or Tb<sup>4+</sup> and recombine with the trapped electrons on Sm<sup>2+</sup>, Eu<sup>2+</sup>, or Yb<sup>2+</sup> and yield characteristic trivalent emission from Sm<sup>3+</sup>, Eu<sup>3+</sup>, or Yb<sup>3+</sup> at ∼600, ∼617, or ∼980 nm, respectively. Lanthanum was introduced to engineer the valence band energy and change the trap depth in Gd<sub>1–<i>x</i></sub>La<sub><i>x</i></sub>AlO<sub>3</sub>:Eu<sup>3+</sup>,Pr<sup>3+</sup> and Gd<sub>1–<i>x</i></sub>La<sub><i>x</i></sub>AlO<sub>3</sub>:Eu<sup>3+</sup>,Tb<sup>3+</sup>. The results show that the valence band moves upward and the trap depth related to Pr<sup>3+</sup> or Tb<sup>3+</sup> decreases

    Electronic Structure and Site Occupancy of Lanthanide-Doped (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> Garnets: A Spectroscopic and First-Principles Study

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    Photoluminescence excitation (PLE) and emission spectra (PL) of undoped (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> as well as Eu<sup>3+</sup>- and Ce<sup>3+</sup>-doped samples have been investigated. The PL spectra show that Eu<sup>3+</sup> enters into both dodecahedral (Ca, Sr) and octahedral (Y, Lu) sites. Ce<sup>3+</sup> gives two broad excitation bands in the range of 200–450 nm. First-principle calculations for Ce<sup>3+</sup> on both dodecahedral and octahedral sites provide sets of 5d excited level energies that are consistent with the experimental results. Then the vacuum referred binding energy diagrams for (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> have been constructed with the lanthanide dopant energy levels by utilizing spectroscopic data. The Ce<sup>3+</sup> 5d excited states are calculated by first-principles calculations. Thermoluminescence (TL) glow curves of (Ce<sup>3+</sup>, Sm<sup>3+</sup>)-codoped (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub> samples show a good agreement with the prediction of lanthanide trapping depths derived from the energy level diagram. Finally, the energy level diagram is used to explain the low thermal quenching temperature of Ce<sup>3+</sup> and the absence of afterglow in (Sr, Ca)<sub>3</sub>(Y, Lu)<sub>2</sub>Ge<sub>3</sub>O<sub>12</sub>

    Luminescence and Energy Transfer between Ce<sup>3+</sup> and Pr<sup>3+</sup> in BaY<sub>2</sub>Si<sub>3</sub>O<sub>10</sub> under VUV–vis and X‑ray Excitation

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    A detailed investigation on photoluminescence properties and energy transfer (ET) dynamics of Ce<sup>3+</sup>, Pr<sup>3+</sup>-doped BaY<sub>2</sub>Si<sub>3</sub>O<sub>10</sub> is provided along with the potential X-ray excited luminescence application. The luminescence properties of Pr<sup>3+</sup> are studied in VUV–UV–vis spectral range at low temperature, and the spectral profiles of Pr<sup>3+</sup> <sup>3</sup>P<sub>0</sub> and <sup>1</sup>D<sub>2</sub> emission lines are determined using time-resolved emission spectra. Upon 230 nm excitation, the electron population from Pr<sup>3+</sup> 4f5d state to its 4f<sup>2</sup> excited state is discussed in detail. As Pr<sup>3+</sup> concentration rises, Pr<sup>3+</sup> <sup>3</sup>P<sub>0</sub> and <sup>1</sup>D<sub>2</sub> luminescence possess different concentration-related properties. The incorporation of Ce<sup>3+</sup> in the codoped sample produces the strong Ce<sup>3+</sup> luminescence under 230 nm excitation, which is the combined result of Pr<sup>3+</sup> 4f5d → Ce<sup>3+</sup> 5d ET and Ce<sup>3+</sup> intrinsic excitation. On the other hand, the increasingly strong ET of Ce<sup>3+</sup> 5d → Pr<sup>3+</sup> 4f<sup>2</sup> results in the decrease of Ce<sup>3+</sup> emission intensity and the gradual deviation of Ce<sup>3+</sup> luminescence decay from the single exponential in the system. By employing the Inokuti–Hirayama model, the dipole–dipole interaction is confirmed as the predominant multipolar effect in controlling this ET process, and the value of <i>C</i><sub><i>DA</i></sub> is determined to be 9.97 × 10<sup>–47</sup> m<sup>6</sup>·s<sup>–1</sup>. Finally, the relatively low scintillation light yield of Ce<sup>3+</sup>-doped BaY<sub>2</sub>Si<sub>3</sub>O<sub>10</sub> material impedes its application potential in the scintillator field, and the cosubstitution of Pr<sup>3+</sup> results in the observable decline of scintillation performance

    Electronic Properties of Ce<sup>3+</sup>-Doped Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub>: A Combined Spectroscopic and Theoretical Study

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    Photoluminescence properties of Ce-doped Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub> crystals prepared by a solid-state reaction method are first investigated with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range. Six bands are observed in the excitation spectrum of the Ce<sup>3+</sup> 5d → 4f emission at 15 K. The highest energy band is attributed to the host excitonic absorption, from which the band gap energy of the host is estimated to be around 7.2 eV. The four lowest energy bands are assigned to the 4f<sub>1</sub> → 5d<sub>1–4</sub> transitions of Ce<sup>3+</sup> located on the three distinct Sr<sup>2+</sup> sites in Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub> with almost equal probability, based on a comparison between excitation band maxima energies and 4f → 5d transition energies obtained from wave-function-based CASSCF/CASPT2 calculations with spin–orbit coupling on Ce-centered embedded clusters. The 4f<sub>1</sub> → 5d<sub>5</sub> transition, not observed in the low-temperature excitation spectrum, is found to be overshadowed by a nearby defect-related excitonic absorption. On the basis of present experimental and calculated results for Ce-doped Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub>, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions on the Sr<sup>2+</sup> sites of Sr<sub>3</sub>Al<sub>2</sub>O<sub>5</sub>Cl<sub>2</sub> is constructed and discussed in association with experimental findings

    Vacuum Referred Binding Energy Scheme, Electron–Vibrational Interaction, and Energy Transfer Dynamics in BaMg<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>:Ln (Ce<sup>3+</sup>, Eu<sup>2+</sup>) Phosphors

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    The host structure and the synchrotron radiation VUV–UV luminescence properties of samples BaMg<sub>2</sub>Si<sub>2</sub>O<sub>7</sub> (BMSO):Ln (Ce<sup>3+</sup>, Eu<sup>2+</sup>) at different doping levels and different temperatures were investigated in detail. Three important aspects are studied to elucidate the luminescence properties of samples: (1) the vacuum referred binding energy (VRBE) scheme is constructed with the electron binding in the BMSO host bands and in the Ce<sup>3+</sup> and Eu<sup>2+</sup> impurity levels with the aim to explain the different thermal stabilities of Ce<sup>3+</sup> and Eu<sup>2+</sup> emissions; (2) the electron–vibrational interaction analysis on the narrow Eu<sup>2+</sup> emission indicates a weak electron–phonon interaction in the current case; (3) by using three models (Inokuti–Hirayama, Yokota–Tanimoto, and Burshteĭn models) at different conditions, the energy transfer dynamics between Ce<sup>3+</sup> and Eu<sup>2+</sup> was analyzed. It reveals that the energy transfer from Ce<sup>3+</sup> to Eu<sup>2+</sup> via electric dipole–dipole (EDD) interaction is dominant while energy migration between Ce<sup>3+</sup> is negligible. Finally, the X-ray excited luminescence spectra of samples BMSO:Ce<sup>3+</sup>/Eu<sup>2+</sup> are collected to evaluate their possible scintillator applications

    The Effect of Sr<sup>2+</sup> on Luminescence of Ce<sup>3+</sup>-Doped (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>

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    A series of Ce<sup>3+</sup>-doped (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub> phosphors with different Ce<sup>3+</sup> and Ca<sup>2+</sup>/Sr<sup>2+</sup> concentrations were prepared by a high temperature solid-state reaction technique. To get insight into the structure–luminescence relationship, the impact of incorporation of Sr<sup>2+</sup> on structure of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub> was first investigated via Rietveld refinement of high quality X-ray diffraction (XRD) data, and then the VUV–UV excitation and UV–vis emission spectra of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup> were collected at low temperature. The results reveal that the crystal structure evolution of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup> has influences on band gaps and Ce<sup>3+</sup> luminescence properties including 4f–5d<sub><i>i</i></sub> (<i>i</i> = 1–5) transition energies, radiative lifetime, emission intensity, quantum efficiency, and thermal stability. Moreover, the influence of Sr<sup>2+</sup> content on the energy of Eu<sup>3+</sup>–O<sup>2–</sup> charge-transfer states (CTS) in (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Eu<sup>3+</sup> was studied in order to construct vacuum referred binding energy (VRBE) schemes with the aim to further understand the luminescence properties of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup>. Finally, X-ray excited luminescence (XEL) spectra were measured to evaluate the possibility of (Ca,Sr)<sub>2</sub>Al<sub>2</sub>SiO<sub>7</sub>:Ce<sup>3+</sup> as a scintillation material

    High Light Yield of Sr<sub>8</sub>(Si<sub>4</sub>O<sub>12</sub>)Cl<sub>8</sub>:Eu<sup>2+</sup> under X‑ray Excitation and Its Temperature-Dependent Luminescence Characteristics

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    In this work, we first investigate the relationship between temperature and lattice parameters by means of Rietveld refinement and then demonstrate its impact on the luminescence peak position of Eu<sup>2+</sup> in Sr<sub>8</sub>(Si<sub>4</sub>O<sub>12</sub>)­Cl<sub>8</sub>. It is found that with increases in temperature, lattice expansion takes place without significant distortion of the coordination around Eu<sup>2+</sup>. As a result, the crystal field splitting of the Eu<sup>2+</sup> 5d state decreases. At the same time, with the experimental data of the full width at half-maximum of Eu<sup>2+</sup> emission at different temperatures and the infrared spectrum, the effective phonon frequency is evaluated and the main vibration motions are determined using first-principles calculation. Due to the high light yield under X-ray excitation and the excellent thermal stability of luminescence intensity and decay, a further optimized sample Sr<sub>7.7</sub>Eu<sub>0.3</sub>(Si<sub>4</sub>O<sub>12</sub>)­Cl<sub>8</sub> could be a potential scintillation material
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