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Electronic Structure Descriptor for the Discovery of Narrow-Band Red

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Electronic Structure Descriptor for the Discovery of Narrow-Band Red Article pubs.acs.org/cm Electronic Structure Descriptor for the Discovery of Narrow-Band Red-Emitting Phosphors † † ‡ † Zhenbin Wang, Iek-Heng Chu, Fei Zhou, and Shyue Ping Ong*, † Department of NanoEngineering, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093-0448, United States ‡ Physics Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, United States *S Supporting Information ABSTRACT: Narrow-band red-emitting phosphors are a critical component of phosphor-converted light-emitting diodes for highly efficient illumination-grade lighting. In this work, we report the discovery of a quantitative descriptor for narrow-band Eu2+-activated emission identified through a comparison of the electronic structures of known narrow- band and broad-band phosphors. We find that a narrow emission bandwidth is characterized by a large splitting of more than 0.1 eV between the two highest Eu2+ 4f7 bands. By incorporating this descriptor in a high-throughput first- principles screening of 2259 nitride compounds, we identify five promising new nitride hosts for Eu2+-activated red-emitting phosphors that are predicted to exhibit good chemical stability, thermal quenching resistance, and quantum efficiency, as well as narrow-band emission. Our findings provide important insights into the emission characteristics of rare-earth activators in phosphor hosts and a general strategy to the discovery of phosphors with a desired emission peak and bandwidth. ■ INTRODUCTION activator concentration is found to have little influence on the 2+ Phosphor-converted light-emitting diodes (pc-LEDs) are an emission band position. The chemically similar CaLiAl3N4:Eu energy efficient and environmentally friendly lighting source for narrow-band red-emitting phosphor also has an emission peak 1−7 λ ∼ 6 solid-state lighting. For illumination-grade lighting, a warm that is too deep red ( em 668 nm) for high-powered LEDs. 2+ white light LED with a high luminous efficacy and color Another narrow-band red-emitting phosphor, SrMg3SiN4:Eu , rendering index is desirable. A common strategy to improve the has an ideal peak position at ∼615 nm and a narrow bandwidth color rendering index is to add a red-emitting component into of ∼43 nm.7 However, the thermal quenching of 2+ 2+ an LED. In recent years, Eu -activated nitride phosphors have SrMg3SiN4:Eu is too severe for practical applications. Despite emerged as highly promising red-emitting phosphors for pc- these discoveries, the overall number of known narrow-band LEDs due to their high chemical and thermal stabilities, small red-emitting phosphors remains very small. The development − thermal quenching, and high quantum efficiency (QE).8 10 of highly efficient narrow-band red-emitting phosphors with an λ ∼ Unfortunately, the broad bandwidth and/or deep red maximum optimal spectral peak position ( em 615 nm) and a narrow of the emission spectra of commercial red-emitting phosphors width (fwhm <50 nm) is therefore a critical materials challenge lead to a substantial portion of the emitted energy being outside for warm white illumination-grade lighting.11,12 the range of human vision, severely reducing the luminous In this work, we report the discovery of a quantitative efficacy of the LED. For example, although the widely used 2+ 2+ descriptor for narrow-band Eu -activated emission that commercial CaAlSiN3:Eu phosphor has an excellent QE > fi ° 1 λ ∼ 2 emerged from a comparison of the rst-principles electronic 95% at 150 C, its emission peak ( em 650 nm) is too red- structures of nine well-known phosphors. We then identified shifted and its bandwidth (characterized by the full width at fi ̅ ∼ ve new narrow-band red-emitting phosphors, CaLiAl3N4 (P1), half-maximum, or fwhm, 90 nm) is too broad. ̅ ̅ Recently, Pust et al.5 reported the discovery of a highly SrLiAl3N4 (I41/a), SrLiAl3N4 (P1), SrMg3SiN4 (P1), and 2+ BaLiAl3N4 (P1),̅ for high-power pc-LED applications through a promising narrow-band red-emitting phosphor, SrLiAl3N4:Eu , with an emission peak position at ∼650 nm and a fwhm of ∼50 high-throughput screening of ternary and quaternary nitride fi nm. A 14% improvement in luminous efficacy was achieved compounds. These ve new phosphors are predicted to satisfy a 2+ with SrLiAl3N4:Eu as a red-emitting component in a pc-LED compared with that of a commercial LED device.5 The Received: April 14, 2016 luminous efficacy would be further enhanced if the emission Revised: May 9, 2016 2+ maximum of SrLiAl3N4:Eu could be blue-shifted, but the Published: May 9, 2016 © 2016 American Chemical Society 4024 DOI: 10.1021/acs.chemmater.6b01496 Chem. Mater. 2016, 28, 4024−4031 Chemistry of Materials Article Figure 1. Electronic structures for selected (oxy)nitride phosphors. (a) Band structure and density of states (DOS; arbitrary units) of a narrow-band 2+ 2+ red-emitting phosphor, SrLiAl3N4:Eu . (b) Band structure and DOS (arbitrary units) of a broad-band red-emitting phosphor, CaAlSi3N4:Eu . (c) Average Eu2+ 4f band levels for five narrow-band and four broad-band phosphors. A numbered suffix (e.g., “-1”) is added where necessary to distinguish between distinct Eu2+ activator sites in the same host structure, with increasing numbers indicating increasing site energy. The highest 4f band, which lies on the Fermi level, is set at 0 eV for ease of comparison. balance of chemical stability, good thermal quenching behavior calculations of the Eu2+-activated phosphors were performed without and quantum efficiency, and narrow-band emission. spin−orbital coupling (SOC) as the difficult convergence of SOC calculations makes them unsuitable for a high-throughput screening ff ■ METHODS e ort. Nevertheless, we performed SOC analyses on the new narrow- band red-emitting phosphors identified and confirmed that the Structure Relaxation and Energy Calculations. Spin-polarized relevant electronic structure feature, i.e., a large splitting in the top density functional theory calculations were performed using the two 4f bands, does not change significantly with the inclusion of SOC. Vienna ab initio simulation package (VASP) within the frozen-core Debye Temperature, Θ . The Debye temperature, Θ , was 13,14 D D projector-augmented wave method. The generalized gradient calculated using the quasi-harmonic model.24 The elastic tensor was 15 approximation Perdew−Burke−Ernzerhof (PBE) functional was calculated with more stringent electronic convergence criterion of 10−6 used for all structural relaxations and energy calculations. A plane eV per formula unit, and the elastic moduli were calculated using the wave energy cutoff of 520 eV was used, and the electronic energy and Voigt−Reuss−Hill approximation.25 atomic forces were converged to within 10−5 eV and 0.01 eV/Å, respectively. The Brillouin zone was integrated with a k-point density ■ RESULTS of 1000 per reciprocal atom. All crystal structure manipulations and data analysis were carried out using the Python Materials Genomics Descriptor for Narrow-Band Emission. The emission 16 package. spectrum of Eu2+-activated phosphors is attributed to the 4f65d1 E Energy above Hull, hull. The phase stability of predicted → 4f7 electronic transition. We therefore begin with a materials was estimated by calculating the energy above the linear hypothesis that narrow- and broad-band emissions can be combination of stable phases in the first-principles phase diagram,17 identified by differences in the electronic band structure. In this also known as the energy above hull, Ehull. For phase diagram fi construction, the energies of all compounds other than those of direct work, we adopt the de nition of having a measured fwhm < 60 interest in this work were obtained from the Materials Project18 using nm as a narrow-band emitter. The calculated electronic band the Materials Application Programming Interface.18,19 structures of two representative red-emitting phosphors, E 2+ 2+ 2+ Host Band Gap g and Band Structure of Eu -Activated SrLiAl3N4:Eu (narrow-band) and CaAlSiN3:Eu (broad- Phosphors. The band gaps for all host materials were calculated using band), are shown in Figure 1, panels a and b, respectively. the PBE functional as well as the more accurate screened hybrid fi 2+ − − 20,21 We nd that the Eu 4f bands are very narrowly distributed in Heyd Scuseria Ernzerhof (HSE) functional. To obtain the band energy and that the energy level of each band may be estimated structure and 4f levels of Eu2+-activated phosphors, supercell models 2+ by averaging across all k-points. were constructed with relatively low Eu doping concentrations 2+ fi (<10%) to mimic experimental doping levels. PBE calculations with a Figure 1c shows the average Eu 4f band levels of ve 22 23 2+ 2+ Hubbard U parameter of 2.5 eV for Eu was used for these relatively narrow-band (SrLiAl3N4:Eu ,SrMg3SiN4:Eu ,CaL- 2+ 2+ β large systems. A Gaussian smearing of 0.05 eV was used for all band iAl3N4:Eu , BaLi2Al2Si2N6:Eu , and Si141Al3ON191( -SiAlO- 2+ 5−7,26,27 2+ structure and density of states calculations. All electronic structure N):Eu ) and four broad-band (CaAlSiN3:Eu , 4025 DOI: 10.1021/acs.chemmater.6b01496 Chem. Mater. 2016, 28, 4024−4031 Chemistry of Materials Article Figure 2. Relationship between emission bandwidth and Eu2+ 4f band levels. When there are multiple Eu2+ 4f levels within 0.1 eV from the highest Δ band (left), overlapping emissions result in a broad bandwidth. Conversely, a large energy splitting, ES, of >0.1 eV between the two highest 4f bands (right) results in narrow-band emission. a Table 1. Calculated Properties of 10 Known Hosts for Red-Emitting Phosphors Eg (eV) Θ materials space group Ehull (meV) PBE HSE exp. D (K) 42 5 SrLiAl3N4 P1̅ 0 2.97 4.47 4.56, 4.70 716 7 SrMg3SiN4 I41/a 0 2.42 3.68 3.90 648 CaLiAl3N4 I41/a 13 3.03 4.45 743 b 3 − 2 CaAlSiN3 Cc 0 3.40 4.76 4.80, 5.0 5.2 787 28 43 Ca2Si5N8 Cc 0 3.34 4.62 4.96, 4.9 788 28 43 Sr2Si5N8 Pmn21 0 3.20 4.40 4.67, 4.5 709 c 28 43 Ba2Si5N8 Pmn21 0 2.88 4.06 4.59, 4.1 661 SrAlSi4N7 Pna21 31 3.58 4.72 745 34 SrSiN2 P21/c 0 2.95 4.18 4.20 375 34 BaSiN2 Cmca 0 2.92 4.03 4.10 360 a The band gap Eg was calculated using the PBE and screened hybrid HSE functionals.
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