Chemical trends of electronic and optical properties of ns<sup>2</sup> ions in halides

Du, Mao‐Hua

doi:10.1039/c4tc00485j

Cited by 24 publications

(30 citation statements)

References 63 publications

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“…In (b), the hybridization between the ns 2 and the halogen ion is relatively weak while the hybridization between the halogen-p states is relatively strong (Ref. 57). …”

Section: +mentioning

confidence: 99%

“…DFT calculations have also been performed to study Tl + in alkali halides. 57 DFT as a single-particle ground-state method cannot reproduce the transitions between multielectronic states in optical spectra. However, the calculated absorption and emission energies are in good agreement with the experimental results between the ground state and the lowest excited state.…”

Section: Ns 2 Ions As Activatorsmentioning

confidence: 99%

“…However, the calculated absorption and emission energies are in good agreement with the experimental results between the ground state and the lowest excited state. 57,58 The advantage of DFT calculations is that it gives good description of the host band structure and provides accurate structural relaxation. Whether the specific emission between the nsnp and ns 2 states as described by the Seitz model can occur depends on the positions of the ns and np states relative the host band edges.…”

Section: Ns 2 Ions As Activatorsmentioning

confidence: 99%

“…The interplay of the Tl-halogen and halogen-halogen hybridization determines the positions of the Tl-6s * and −6p * levels relative to the band edges. 57 A combination of the strong Tl-halogen and weak halogen-halogen hybridization could result in the scenario depicted in Fig. 12a, where both Tl-6s * and −6p * levels are inside the bandgap and act as hole and electron traps, respectively, leading to radiative recombination.…”

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confidence: 99%

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Using DFT Methods to Study Activators in Optical Materials

2015

ECS J. Solid State Sci. Technol.

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Density functional theory (DFT) calculations of various activators (ranging from transition metal ions, rare-earth ions, ns 2 ions, to self-trapped and dopant-bound excitons) in phosphors and scintillators are reviewed. As a single-particle ground-state theory, DFT calculations cannot reproduce the experimentally observed optical spectra, which involve transitions between multi-electronic states. However, DFT calculations can generally provide sufficiently accurate structural relaxation and distinguish different hybridization strengths between an activator and its ligands in different host compounds. This is important because the activator-ligand interaction often governs the trends in luminescence properties in phosphors and scintillators, which can be used to search for new materials. DFT calculations of the electronic structure of the host compound and the positions of the activator levels relative to the host band edges in scintillators are also important for finding optimal host-activator combinations for high light yields and fast scintillation response. Mn 4+ Luminescence of materials when excited by photons or ionizing radiation is the foundation for numerous technologies, such as energy efficient lighting (fluorescent lamps and white LEDs), laser, medical imaging, and nuclear materials detection.1-3 Efficient luminescence in semiconductors and insulators usually relies on the localization of excited electrons and holes at certain impurities, which act as luminescence centers (or activators). An activator can trap electrons and holes for efficient radiative recombination and is the essential component of a phosphor or scintillator material. The commonly used activators are typically multivalent ions, which can insert multiple electronic states inside the bandgap of the host material. 45 Good examples of the multivalent ions that can act as luminescent centers are rare-earth ions (e.g., Ce 3+ , Eu 2+ ), 6-11 transition metal ions (e.g., Cr 3+ , Mn 4+ ), [12][13][14][15][16][17] and ns 2 ions (ions with outer electron configuration of ns 2 , such as Tl + , Sn 2+ ). 18-20Energy efficiency is critically important for phosphors used for lighting. To suppress the energy loss through nonradiative recombination, a phosphor is excited by directly exciting activators, not the host (see Fig. 1a). The excitation energy is less than the bandgap energy of the host but is large enough to excite the activator. Since the excitation occurs locally at the activator, the chance for the excited activator to interact with nonradiative recombination centers, such as deep defects, is small. This is important for achieving high quantum efficiencies for phosphors. In scintillators used for detecting ionizing radiation, the radiation creates charge carriers in the valence and conduction bands of the host, which are subsequently trapped by activators, leading to radiative recombination (see Fig. 1b). In scintillators, a portion of the charge carriers need to travel a certain distance before being trapped by the activators, which ...

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“…In (b), the hybridization between the ns 2 and the halogen ion is relatively weak while the hybridization between the halogen-p states is relatively strong (Ref. 57). …”

Section: +mentioning

confidence: 99%

Section: Ns 2 Ions As Activatorsmentioning

confidence: 99%

Section: Ns 2 Ions As Activatorsmentioning

confidence: 99%

mentioning

confidence: 99%

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Using DFT Methods to Study Activators in Optical Materials

2015

ECS J. Solid State Sci. Technol.

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“…A decrease in the host band gap energy generally leads to an increase in the number of electron-hole pairs produced but the host gap has to be sufficiently large to accommodate the energy levels of the dopants in the ground and excited states. In a recent publication, Du [18] hypothesized that an electron trapped at a ns 2 dopant could associate with a localized hole in the valence band to allow for an efficient np n -V k transition. This alternative recombination mechanism could be used to exploit low-band gap insulators.…”

Section: Introductionmentioning

confidence: 99%

First-principles search for efficient activators for LaI3

Prange

Gao

et al. 2016

Journal of Luminescence

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a b s t r a c tFirst-principles calculations were performed using density functional theory with Hubbard corrections or hybrid exchange-correlation functionals, as well as the GW approximation, to predict dopants that could serve as efficient activators for LaI 3 , a potentially very bright scintillator for which an appropriate activator has not been identified yet. The dopants considered in this work included a series of lanthanide ions (Ce, Pr, Nd, Eu, Gd, and Tb) and several ns 2 ions (Tl, Pb, Bi, and Sb). Based on both ground-state calculations and constrained DFT calculations to simulate excited states, the trivalent lanthanide dopants were shown not to constitute an improvement over Ce 3 þ , for which experimental data exist, as they showed occupied 4f states below the valence band maximum (VBM) and 5d states above the conduction band maximum (CBM). In contrast, the only divalent lanthanide considered, Eu 2 þ , displayed occupied 4f states within the band gap of the host, but its 5d states were calculated to be above the CBM. Eu 2 þ could nonetheless be exploited as an activator by increasing the band gap energy slightly through substitution of iodide ions by bromide ions. Similar results were obtained for Tl þ . Finally, Bi 3 þ and Sb 3 þ were predicted to be efficient activators in LaI 3 by virtue of having unoccupied p states below the CBM, which can serve as electron traps and can combine with holes at the VBM to form localized luminescence centers. Therefore, Bi-and Sb-doped LaI 3 should be grown and tested experimentally. Comparison with empirical relationships of the relative positions of the energy levels of rare-earth dopants and implications for efficient doping schemes and scintillation mechanisms in LaI 3 are also discussed.

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Recent R&D Trends in Inorganic Single‐Crystal Scintillator Materials for Radiation Detection

Nikl

Yoshikawa

2015

Advanced Optical Materials

663

374

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In this review, the major achievements and research and development (R&D) trends from the last decade in the field of single crystal scintillator materials are described. Two material families are included, namely, those of halide and oxide compounds. In most cases, the host crystals are doped with Ce3+, Pr3+ or Eu2+ rare earth ions. Their spin‐ and parity‐allowed 5d–4f transitions enable a rapid scintillation response, on the order of tens to hundreds of nanoseconds. Technological recipes, extended characterization by means of optical and magnetic spectroscopies, and theoretical studies are described. The latter provide further support to experimental results and provide a better understanding of the host electronic band structure, energy levels of specific defects, and the emission centers themselves. Applications in medical imaging and dosimetry, security measures, high‐energy physics and the high‐tech industry, in which X(γ)‐rays or particle beams are used and monitored, are recognized as the main driving factor for R&D activities in this field.

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Chemical trends of electronic and optical properties of ns² ions in halides

Abstract: Density functional calculations demonstrate how the hybridization between the ns2 ion (e.g., Tl+) and its ligand and the ionicity of the host material affect electronic structure and optical transitions in ns2 ion-activated luminescent materials.

Cited by 24 publications

References 63 publications

Using DFT Methods to Study Activators in Optical Materials

Using DFT Methods to Study Activators in Optical Materials

First-principles search for efficient activators for LaI3

Recent R&D Trends in Inorganic Single‐Crystal Scintillator Materials for Radiation Detection

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Chemical trends of electronic and optical properties of ns2 ions in halides

Abstract: Density functional calculations demonstrate how the hybridization between the ns2 ion (e.g., Tl+) and its ligand and the ionicity of the host material affect electronic structure and optical transitions in ns2 ion-activated luminescent materials.

Cited by 24 publications

References 63 publications

Using DFT Methods to Study Activators in Optical Materials

Using DFT Methods to Study Activators in Optical Materials

First-principles search for efficient activators for LaI3

Recent R&D Trends in Inorganic Single‐Crystal Scintillator Materials for Radiation Detection

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Chemical trends of electronic and optical properties of ns² ions in halides