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Download Author Version (PDF) Journal of Materials Chemistry C Accepted Manuscript This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains. www.rsc.org/materialsC Page 1 of 8 Journal of Materials Chemistry C Journal of Materials Chemistry C RSC Publishing ARTICLE Chemical trends of electronic and optical properties of ns 2 ions in halides Cite this: DOI: 10.1039/x0xx00000x a M. H. Du Manuscript Received 00th January 2014, Accepted Heavy 6p and 5p ions in groups IIIB, IVB, and VB (Tl, Pb, Be, In, Sn, Sb) are multivalent ions, which act as electron and hole traps and radiative recombination centers in many wide band gap materials. In DOI: 10.1039/x0xx00000x this paper, Tl + as a prototypical ns 2 ion (ns 2 ions here refer to 6p and 5p ions with outer electronic www.rsc.org/ configuration of ns 2) is studied as luminescent centers in alkali halides. Density functional calculations reveal the chemical trend that determines the luminescence mechanism in ns 2-ion activated alkali halides. The activator-halogen hybridization strength and the ionicity of the host material strongly affect the positions of the activator levels relative to the valence and conduction band edges. This determines whether the radiative recombination occurs within the activator ion or involves the hole polaron, or the Vk center. Strategies for exploring different combinations of host materials and activators for desired Accepted luminescence mechanisms are discussed. The insight obtained in this work will help the search and the design of more efficient scintillators and phosphors. C I. Introduction performance scintillators that have been widely used for radiation detection. 2 The 6p 6s transition is responsible for the Tl emission + 18,19,20 + Many inorganic and organic semiconducting and insulating in NaI:Tl . The emission in CsI:Tl appear to be related to both materials can emit photons under external excitation (e.g., the hole polaron ( Vk center) and the electron trapped at Tl, or Tl- 2,28,29 electromagnetic waves, chemical reactions, heat, etc.). bound exciton. The optical transitions between the 6p states of 2+ Luminescence of materials when excited by photons or ionizing Bi have also been reported in various materials, leading to radiation is the foundation for numerous technologies, such as emission wavelength in the range of 600-700 nm, which is useful for 21,22,23,24 energy efficient lighting (fluorescent lamps and white LEDs), laser, red phosphors in fluorescent lamps and white LEDs, Many + medical imaging, and nuclear materials detection. 1, 2, 3 6p and 5p ions at low valence states (such as Bi ) can also produce Chemistry Efficient luminescence in inorganic semiconductors and broad-band near IR emission, which is useful for wideband optical 25,26,27 insulators usually relies on the localization of excited electrons and amplification and lasers. The complex luminescence properties holes at certain impurities, which act as luminescence centers. Such of the 6p and 5p dopants in various host materials require detailed impurity is the so-called activator, which can trap electrons and understanding of dopant-induced gap states and their interaction with holes for efficient radiative recombination and is the essential the host states. 2 component of a phosphor or scintillator material. The commonly The optical transitions between the ns and np states of a ns ion 30 used activators are typically multivalent ions, which can insert are usually interpreted and modeled by using Seitz model. The 4 5 multiple electronic states inside the band gap of the host material. Seitz model is based on a two-electron picture, in which the ground 2 1 These gap states trap electrons and holes, leading to radiative state (ns ) is a singlet S0 state and the excited states (nsnp) consist 3 3 3 1 recombination. Good examples of the multivalent ions that can act as of P0, P1, P2 triplet states and a P1 singlet state (in the ascending 3+ 2+ 6 7 8 9 10 11 luminescent centers are rare-earth (e.g., Ce , Eu ) and order in energy). The often observed A, B, C bands in the optical Materials 3+ 4+ 12, 13, 14, 15, 16, 17 1 3 3 1 transition metal ions (e.g., Cr , Mn ). absorption spectra are usually interpreted as the S0 P1, P2, P1 19 Besides rare-earth and transition-metal ions, a number of heavy transitions, respectively. The A and C bands are the two strong 1 1 6p (Tl, Pb, Bi) and 5p (In, Sn, Sb) ions are also important absorption bands. The C-band ( S0 P1) transition is spin-allowed. of 1 3 multivalent ions, which act as luminescent centers in many The A-band ( S0 P1) transition is spin-forbidden but is still strong, materials. These ions in their ground states all have the outer especially for heavy 6p ions, due to the spin-orbit coupling between 2 2 3 1 electronic configuration of ns and are therefore called ns ion. The the P1 and the P1 states. Emission is more complicated involving hybridization between the ns and np states of these ions and the host lattice polarization in the emitting states. More details on the optical 2 states can create ns- and np-derived electronic states inside the band transitions in ns ions based on the Seitz model can be found in 18,19,20 gap of the host material. These gap states give rise to many sub- several review articles. 31, band-gap optical transitions, which can be between the ns and the np The extensions of the Seitz model based on pure ionic model 18 19 20 32 33, 34 levels, between np levels (due to the spin-orbit splitting of the and molecular orbit (MO) theory have been used to 21, 22, 23, 24, 25, 26, 27 np levels), or between the np level and the native quantitatively calculate various properties related to the optical Journal defects. 2, 28, 29 Tl + doped NaI and CsI are two important high- transitions (e.g., absorption and emission energies, line shapes, This journal is © The Royal Society of Chemistry 2014 J. Mater. Chem. C , 2014, 00 , 1-3 | 1 Journal of Materials Chemistry C Page 2 of 8 ARTICLE Journal Name intensity ratio, etc.). The MO calculations showed reasonable results Brillouin zone for defect and DOS calculations, respectively. All the for some systems when compared with the spectroscopic data. 33, 34 atoms were relaxed to minimize the Feynman-Hellmann forces to However, these calculations, which treat only one ns 2 ion and six below 0.05 eV/Å. nearest-neighbor halogen ions quantum mechanically, lack the description of the host band structure and the accurate structural Table I. The fractions of Hartree-Fock exchange (α) used in the relaxation. As discussed above, many optical transitions between ns, PBE0 calculations and the calculated band gaps, which are compared np, and native defect levels can occur. Whether the specific emission to the experimental band gaps 2 shown in parentheses. Both simple between the nsnp and ns 2 states as described by the Seitz model can cubic (ground state) and rocksalt CsI are considered. occur depends on the positions of the ns and np states relative the NaI KI RbI CsI (RS) CsI (SC) host band edges. The knowledge of the electronic structure of both α 0.37 0.39 0.40 0.37 0.37 the host and the activator is needed to understand the different Eg (eV) 5.81 (5.9) 6.23 (6.3) 6.23 (6.3) 6.28 6.05 (6.1) scintillation mechanisms observed in different host materials (e.g., NaI:Tl vs. CsI:Tl). Sufficiently accurate structural relaxation is important for incorporating correct hybridization strength between The charge transition level ε (q q ') , induced by activators or the activator and its ligands in the calculations. The different polarons, is determined by the Fermi level ( ε ) at which the hybridization strengths in different hosts determine the chemical f Manuscript trend of the activator-induced gap levels relative to the host bands. formation energies of the activator or the defect with charge states q This is important for the understanding of the luminescence and q’ are equal to each other. ε q q ' can be calculated using mechanisms in different hosts and the search of new host-activator ( ) combinations with desirable scintillation properties. EDq, '− E Dq , In this paper, density functional calculations based on density ε ()q q ' = , (1) functional theory (DFT) are used to study the electronic and optical q− q ' properties of Tl + in alkali halides. Tl + ion is used as a prototypical ns 2 ion. The objective of this work is to understand how the where ED, q ( ED, q ' ) is the total energy of the supercell that activator-ligand hybridization and the ionicity of the host material contains the relaxed structure of a defect at charge state q (q’ ).
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