Near-Unity Photoluminescence Quantum Yield in Blue-Emitting

Letter Cite This: ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

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Near-Unity Photoluminescence Quantum Yield in Blue-Emitting Cs3Cu2Br5−xIx (0 ≤ x ≤ 5) Rachel Roccanova,† Aymen Yangui,† Hariharan Nhalil,‡ Hongliang Shi,§ Mao-Hua Du,∥ and Bayrammurad Saparov*,†

ACS Appl. Electron. Mater. Downloaded from pubs.acs.org by STEPHEN F AUSTIN STATE UNIV on 03/16/19. For personal use only.

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Department of Chemistry and Biochemistry, University of Oklahoma, 101 Stephenson Parkway, Norman, Oklahoma 73019, United States ‡ Department of Physics, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel § Key Laboratory of Micro-Nano Measurement-Manipulation and Physics (Ministry of Education), Department of Physics, Beihang University, Beijing 100191, China ∥ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: Recently, interest in developing efficient, low-cost, nontoxic, and stable metal halide emitters that can be incorporated into solid-state lighting technologies has taken hold. Here we report nontoxic, stable, and highly efficient blue-light-emitting Cs3Cu2Br5−xIx (0 ≤ x ≤ 5). Room-temperature photoluminescence measurements show bright blue emission in the 456 to 443 nm range with near-unity quantum yield for Cs3Cu2I5. Density functional theory calculations and power-dependent PL measurements suggest that the emission results from self-trapped excitons induced by strong charge localization within the zero-dimensional cluster structure of Cs3Cu2Br5−xIx. KEYWORDS: 0D all-inorganic metal halides, Pb-free, high blue photoluminescence quantum yield, thermal and air stability, density functional theory Polycrystalline powders of Cs3Cu2Br5−xIx (0 ≤ x ≤ 5) were prepared by reacting stoichiometric amounts of CsX and CuX (X = Br, I). To ensure the homogeneity of all compounds, the respective reactants were ground multiple times under an inert atmosphere, pelletized to ensure no loss of reactants, and then sealed in evacuated quartz ampules. Reaction mixtures were then annealed at 400 °C for 12 h and slowly cooled to room temperature for 21 h. While the bulk compounds can be prepared using both solution7 and solid-state synthesis techniques, a higher phase purity based on powder X-ray diffraction (PXRD) measurements is achieved via solid-state technique. In addition to phase purity, shown in Figure 1(a,b) and Table S2, the measured PXRD patterns confirm the solid solution behavior in Cs3Cu2Br5−xIx. The unit cell parameters and volume follow Vegard’s Law as a function of iodide concentration (Figure 1(c,d)). The 0D Cs3Cu2X5 crystal structures8,9 contain unique [Cu2X5]3− dimers made of a trigonal planar CuX3 sharing an edge with a tetrahedral CuX4 unit, all surrounded by Cs+ (Figure S1). To the best of our knowledge, the dimeric [Cu2X5]3− cluster has not been reported in any other copper halide compounds. Instead,

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istorically, the development of solid-state blue emitters has significantly lagged behind that of red and green emitters, which severely hampered the development of whitelight-emitting diodes (W-LEDs).1 Recently, some hybrid organic−inorganic lead halide perovskites, such as (C6H5CH2NH3)2PbBr4,2 demonstrated efficient blue-light emission. While significant progress has been made in the development of efficient blue-emitting lead halides, these materials continue to suffer from lead toxicity and poor thermal and air stability.3 Other means of producing environmentally-friendly and efficient blue emitters could be done through guided material design of nontoxic halides with zero-dimensional (0D) crystal structures.4 Lowering the dimensionality causes the increase of the exciton binding energy; for instance, the 3D CsPbBr3 has an exciton binding energy of 18 meV,5 whereas the 0D Cs3Sb2I9 exhibits a 1 order of magnitude higher exciton binding energy of 175 meV.6 In this context, copper possesses excellent attributes such as low toxicity and cost, and it is earth-abundant, making it an ideal candidate for the ubiquitous WLEDs. Here, we report a nontoxic and air- and thermally stable Cs3Cu2Br5−xIx (0 ≤ x ≤ 5) solid solution that demonstrates high photoluminescence quantum yield (PLQY) blue emission ranging from 50.1% for Cs3Cu2Br5 to 98.7% for Cs3Cu2I5 (Table S1), which represents a major step toward bringing the next-generation solid-state lighting technologies to marketplace. © XXXX American Chemical Society

Received: January 8, 2019 Accepted: March 11, 2019 Published: March 11, 2019 A

DOI: 10.1021/acsaelm.9b00015 ACS Appl. Electron. Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Electronic Materials

Figure 1. Room-temperature experimental PXRD patterns (black lines) of (a) Cs3Cu2Br5 and (b) Cs3Cu2I5 with Pawley fits (red lines) and resulting difference maps (blue lines). (c) The refined lattice parameters and (d) unit cell volume are shown as a function of I content.

Figure 2. Tandem thermal gravimetric analysis (blue) and differential scanning calorimetry (red) TGA/DSC plots for (a) Cs3Cu2Br5 and (b) Cs3Cu2I5.

significant weight loss below 300 °C.13 Moreover, DSC scans suggest that both Cs3Cu2Br5 and Cs3Cu2I5 undergo two distinct thermal events at T1 and T2 (Figure 2). According to the reported CuI−CsI phase diagrams,14,15 Cs3Cu2I5 exhibits narrowly incongruent melting behavior at 390 °C, which is corroborated by the T2 peak. The T1 feature is close to the eutectic isotherm at 350 °C visible in CuI-rich compositions in the reported phase diagram; however, no CuI impurity could be detected in our PXRD measurements. In contrast, Cs3Cu2Br5 was not present in the reported binary phase diagram, but the DSC scan provided in Figure 2(a) is consistent with what is reported. 15 Thus, Cs 3 Cu 2 Br 5 decomposes peritectically at 318 °C into liquid and CsCu2Br3 phases and further melts at 364 °C, producing CsBr. Indeed, small impurities of CsX appear in the PXRD after multiple heating and cooling cycles (Figure S5), especially for Cs3Cu2Br5, supporting the presence of incongruent melting behavior. Under UV excitation, Cs3Cu2Br5−xIx shows intense blue emission with a maximum emission ranging from 443 to 455 nm and a fwhm increasing linearly from 75 nm for Cs3Cu2Br5 to 99 nm for Cs3Cu2I5 (Figure 3 and Table S3), which are

other known 0D copper halides feature clusters such as the edge-sharing ditrigonal planar units found in [N(C2H5)4]2Cu2Br410 and the edge-sharing ditetrahedral units in (C8H14N2)2Cu2Br6.11 Of practical importance are the thermal and air stability of materials for optoelectronic applications. All-inorganic compositions are expected to exhibit higher stability than hybrid compositions because of the lack of volatile organic molecules and numerous phase transitions. Thus, we monitored the ambient stability of Cs3Cu2Br5 and Cs3Cu2I5 through PXRD over 1 month. For Cs3Cu2Br5, we found no significant degradation (Figure S4(a)). However, as seen in Figure S4(b), a few noticeable changes occur in the Cs3Cu2I5 phase, as it begins losing intensity in the lower-angle region, and the small peaks at 7 and 10° disappear after 2 weeks of ambient exposure. The higher stability of the bromide analogue compared to that of the iodide is consistent with literature reports of decreasing stability of halides going down the halogen group.12 TGA results suggest little to no mass loss up to 475 °C, demonstrating the high thermal stability of these phases compared to Sn-based halides such as Cs2SnI6, which show B


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Figure 3. Photoluminescence excitation (PLE, blue lines) and emission (PL, red lines) of (a) Cs3Cu2Br5, (b) Cs3Cu2Br3.75I1.25, (c) Cs3Cu2Br2.5I2.5, (d) Cs3Cu2Br1.25I3.75, and (e) Cs3Cu2I5 compounds. (f) CIE 1931 chromaticity plot of all compounds.

comparable to those of advanced blue organic16 and inorganic phosphors.2 The observed photoluminescence (PL) spectra range from light blue with Commission Internationale de l’Eclairage (CIE) Color Coordinates (x, y) of (0.15, 0.067) for Cs3Cu2Br5 to deep blue (0.15, 0.048) for Cs3Cu2I5 (Figure 3(f)). The PL peaks also display a linear blue-shift going from 455 nm for Cs3Cu2Br5 to 443 nm for Cs3Cu2I5, contrary to the expectations based on electronegativity arguments assuming band-to-band emission. The blue-shift of PL emission going from bromide to iodide is also observed for binary copper halides CuBr and CuI with the observed excitonic blue emission at 420 nm for CuBr17 and 410 nm for CuI.18 Bright excitonic emission in binary copper halides is typically observed at low temperatures, with CuCl being one of the first semiconductors found to exhibit biexcitons in both nanocrystal and bulk forms19 as well as being a model system for strong exciton−photon coupling.20 The proximity of PL peak maximums of Cs3Cu2Br5−xIx to those of the binary halides experimentally suggests that the emission originates from the copper halide clusters themselves and that the isolation of [Cu2X5]3− molecular dimers leads to greater charge localization and stronger excitonic effects, ultimately providing

highly efficient room-temperature emission in Cs3Cu2Br5−xIx. In accordance with the intense room-temperature blue emission, the measured PLQY values for Cs3Cu2Br5−xIx (Table S1) are among the highest observed for organic and inorganic phosphors.2 Thus, PLQY values increase linearly from 50.1% for Cs3Cu2Br5 to 98.7% for Cs3Cu2I5. The nearunity PLQY of Cs3Cu2I5 is unprecedented, as most of the literature reports efficient luminescent metal halides as either bromides or chlorides. Here, it is worth noting that very recently, Jun et al. reported bright blue emission with PLQY values of 90 and 60% from single crystals and thin film of Cs3Cu2I5.7 The small difference between our measured value (98%) and the reported value (90%) is most likely the result of the use of a different synthesis method and thus the purity of the resulting compounds. They also reported a Cs3Cu2I5-based blue LED that shows low electroluminescence efficiency, which they attributed to unfavorable energy level alignment despite the successful creation of white light through mixing Cs3Cu2I5 with a yellow phosphor. In addition to the remarkable increase in charge localization and emission efficiency, lowering the dimensionality of hybrid organic−inorganic metal halides produces large Stokes shifts C


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Figure 4. Electronic band structure and density-of-states (DOS) of Cs3Cu2Br5 (a,b) and Cs3Cu2I5 (c,d).

such as the case of (C4N2H14Br)4SnBr6.21 In agreement with these reports, Cs3Cu2Br5−xIx shows very large Stokes shifts ranging from 135 to 164 nm. Typically, PL spectra with such large Stokes shifts are attributed to self-trapped excitons (STEs) as a result of strong exciton-phonon coupling .22 Indeed, Jun et al. have also recently studied the optical properties of Cs3Cu2I5, attributing the observed large Stokeshifted emission to an excited-state structural reorganization mechanism caused by the Jahn−Teller distorted Cu tetrahedral site.7 However, defect-induced emission can be significant in copper halides as shown by the PL spectra documented for CuI, which exhibits a maximum emission at 410 nm that can red-shift to 428 nm depending on defect concentration.23 To confirm the nature of the PL emission peaks in Cs3Cu2Br5−xIx, we measured the PL as a function of excitation intensity (Figure S6). Because the concentration and recombination lifetime of permanent defects are finite, therefore, their PL should be saturated at high excitation power.24 However, the PL intensity for Cs3Cu2Br5−xIx presents a linear dependence with excitation power; thus, the absence of saturation suggests that the emission likely originates from STEs rather than permanent defects.25−29 Figure 4 shows the electronic band structures and the density-of-states (DOS) of Cs3Cu2Br5−xIx. The valence and the conduction bands are mainly made up of Cu 3d and Cu 4s states, respectively. Both compounds have a direct band gap at the Γ point. Because both the valence and the conduction bands are dominated by the Cu states, the band gaps of Cs3Cu2Br5 (4.51 eV) and Cs3Cu2I5 (4.47 eV) are very close to each other. This finding explains the measured small differences in PL emission peaks (Table S3) between Cs3Cu2Br5, Cs3Cu2I5 and mixed compositions. For both compounds, the Cu-3d-dominated valence band is flat,

whereas the Cu-4s-derived conduction band is much more dispersive. However, despite the dispersive conduction band, the excitons in the two compounds are strongly localized due to the strong Coulomb binding between the electron and hole and the strong local structural distortion of the Cu2X53+ cluster upon excitation as discussed below. The calculated binding energies of the unrelaxed excitons in Cs3Cu2Br5 and Cs3Cu2I5 are 0.40 and 0.33 eV, respectively, which are already significant due to the strong Coulomb binding; further exciton structural relaxation increases the binding energies to 1.24 and 1.10 eV, respectively, as shown in Table S4. The strong exciton localization and binding should lead to exciton emission. The structural distortion due to the exciton localization is significant in both compounds, as shown in Figure S7(a,b) for the case of Cs3Cu2Br5. The structural relaxation breaks two Cu−halogen bonds and moves the two Cu atoms closer to each other; the Cu−Cu distance decreases from 2.51 to 2.32 Å in Cs3Cu2Br5 and from 2.49 to 2.35 Å in Cs3Cu2I5. The partial density contours of the electron and the hole of the exciton in Cs3Cu2Br5 are shown in Figure S7(c,d). The hole has Cu 3d character and is localized on the two Cu atoms in the Cu2Br5 cluster, whereas the electron is localized mostly in the bond-center position between the two Cu atoms. The spatial distribution of the electron and hole of the exciton in Cs3Cu2I5 is similar to that in Cs3Cu2Br5. The calculated exciton excitation and emission energies in both compounds (Table S4) are in good agreement with the experimental results, which suggest that the calculated electron excitation and excited-state structural relaxation are reliable. The large excited-state structural relaxation in Cs3Cu2X5 as revealed in the calculation explains the observed large Stokes shifts and the absence of the significant spectral overlap between the excitation and emission (Figure 3); these are important for D


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(2) Gong, X.; Voznyy, O.; Jain, A.; Liu, W.; Sabatini, R.; Piontkowski, Z.; Walters, G.; Bappi, G.; Nokhrin, S.; Bushuyev, O.; et al. Electron−phonon interaction in efficient perovskite blue emitters. Nat. Mater. 2018, 17, 550−556. (3) Lyu, M.; Yun, J.-H.; Chen, P.; Hao, M.; Wang, L. Addressing Toxicity of Lead: Progress and Applications of Low-Toxic Metal Halide Perovskites and Their Derivatives. Adv. Energy Mater. 2017, 7 (15), 1602512. (4) Zhou, C.; Lin, H.; Worku, M.; Neu, J.; Zhou, Y.; Tian, Y.; Lee, S.; Djurovich, P.; Siegrist, T.; Ma, B. Blue Emitting Single Crystalline Assembly of Metal Halide Clusters. J. Am. Chem. Soc. 2018, 140, 13181. (5) Yang, H.; Zhang, Y.; Pan, J.; Yin, J.; Bakr, O. M.; Mohammed, O. F. Room-temperature engineering of all-inorganic perovskite nanocrsytals with different dimensionalities. Chem. Mater. 2017, 29 (21), 8978−8982. (6) Correa-Baena, J.-P.; Nienhaus, L.; Kurchin, R. C.; Shin, S. S.; Wieghold, S.; Putri Hartono, N. T.; Layurova, M.; Klein, N. D.; Poindexter, J. R.; Polizzotti, A.; Sun, S.; Bawendi, M. G.; Buonassisi, T. A-Site Cation in Inorganic A3Sb2I9 Perovskite Influences Structural Dimensionality, Exciton Binding Energy, and Solar Cell Performance. Chem. Mater. 2018, 30 (11), 3734−3742. (7) Jun, T.; Sim, K.; Iimura, S.; Sasase, M.; Kamioka, H.; Kim, J.; Hosono, H. Lead-Free Highly Efficient Blue-Emitting Cs3Cu2I5 with 0D Electronic Structure. Adv. Mater. 2018, 30, 1804547. (8) Hull, S.; Berastegui, P. Crystal structures and ionic conductivities of ternary derivatives of the silver and copper monohalidesII: ordered phases within the (AgX)x(MX)1−x and (CuX)x(MX)1−x (M= K, Rb and Cs; X= Cl, Br and I) systems. J. Solid State Chem. 2004, 177 (9), 3156−3173. (9) Bigalke, K.; Hans, A.; Hartl, H. Synthese und Strukturuntersuchungen von Iodocupraten(I)IX. Synthese und Kristallstrukturen von Cs3Cu2I5 und RbCu2I3. Z. Anorg. Allg. Chem. 1988, 563 (1), 96−104. (10) Asplund, M.; Jagner, S. Crystal Structure of bis(tetraethylammonium)di-mu-bromo-dibromodicuprate(I), [N(C2H5)4]2[Cu2Br4]. Acta Chem. Scand. A 1984, 38, 135−139. (11) Haddad, S.; Willett, R. D. Dimeric Cu(I) Bromide Species Consisting of Two Edge-Shared Tetrahedra: Crystal Structure of (C8H14N2)2Cu2Br6. Inorg. Chem. 2001, 40 (4), 809−811. (12) Roccanova, R.; Ming, W.; Whiteside, V. R.; McGuire, M. A.; Sellers, I. R.; Du, M.-H.; Saparov, B. Synthesis, Crystal and Electronic Structures, and Optical Properties of (CH3NH3)2CdX4 (X = Cl, Br, I). Inorg. Chem. 2017, 56 (22), 13878−13888. (13) Saparov, B.; Sun, J.-P.; Meng, W.; Xiao, Z.; Duan, H.-S.; Gunawan, O.; Shin, D.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-film deposition and characterization of a Sn-deficient perovskite derivative Cs2SnI6. Chem. Mater. 2016, 28 (7), 2315−2322. (14) Wojakowska, A.; Gorniak, A.; Kuznetsov, A. Y.; Wojakowski, A.; Josiak, J. Phase diagram of the system copper (I) iodide+ cesium iodide. J. Chem. Eng. Data 2003, 48 (3), 468−471. (15) Wojakowska, A.; Krzyżak, E.; Wojakowski, A. Phase diagram for the CuBr−CsBr system. Thermochim. Acta 2000, 344 (1−2), 55−59. (16) Lee, J.; Chen, H.-F.; Batagoda, T.; Coburn, C.; Djurovich, P. I.; Thompson, M. E.; Forrest, S. R. Deep blue phosphorescent organic light-emitting diodes with very high brightness and efficiency. Nat. Mater. 2016, 15, 92. (17) Goto, T.; Ueta, M. Luminescence of CuBr and Its Correlation to the Exciton. J. Phys. Soc. Jpn. 1967, 22 (2), 488−498. (18) Chen, D.; Wang, Y.; Lin, Z.; Huang, J.; Chen, X.; Pan, D.; Huang, F. Growth Strategy and Physical Properties of the High Mobility P-Type CuI Crystal. Cryst. Growth Des. 2010, 10 (5), 2057− 2060. (19) Valenta, J.; Dian, J.; Gilliot, P.; Hönerlage, B. Photoluminescence and Optical Gain in CuBr Semiconductor Nanocrystals. Phys. Status Solidi B 2001, 224 (1), 313−317. (20) Woggon, U.; Wind, O.; Langbein, W.; Gogolin, O.; Klingshirn, C. Confined biexcitons in CuBr quantum dots. J. Lumin. 1994, 59 (3), 135−145.

hindering the exciton migration (by resonant transfer of the excitation energy) and the subsequent nonradiative recombination at defects. In summary, we report new nontoxic and highly efficient blue-emitting 0D all-inorganic Cs3Cu2Br5−xIx (0 ≤ x ≤ 5), with improved stability compared to that of Pb- and Sn-based halides. DFT calculations and power-dependent PL measurements suggest that the emission results from STEs induced by strong charge localization attributed to the 0D cluster structures. This work paves the way for consideration of Cubased multinary halides as nontoxic, earth-abundant, and lowcost optical and electronic materials. Importantly, the presence of a very high PLQY accompanied by the large Stokes shift could give a high conversion efficiency and a low selfabsorption. With proper energy level matching, Cs3Cu2Br5−xIx could be used as an efficient blue phosphor combined with red and green phosphors for WLEDs.30

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00015. Synthesis of Cs3Cu2Br5−xIx (0 ≤ x ≤ 5), experimental details of single crystal and powder XRD, optical measurements, DSC/TGA measurements, and DFT calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aymen Yangui: 0000-0002-6695-972X Mao-Hua Du: 0000-0001-8796-167X Bayrammurad Saparov: 0000-0003-0190-9585 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. R.R. prepared the samples, performed the XRD, TGA/DSC, and optical measurements, and wrote the paper, A.Y. assisted on the photoluminescence measurements and wrote the paper, H.N. assisted on the fabrication and the structural characterization of samples, H.S. and M.-H.D. did the DFT calculations and wrote the paper, and B.S. wrote the paper and supervised the work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support for this work provided by the University of Oklahoma startup funds and by a grant from the Oklahoma Center for the Advancement of Science and Technology (OCAST) under grant AR18-008. M.-H.D. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.

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REFERENCES

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ACS Applied Electronic Materials (21) Zhou, C.; Tian, Y.; Wang, M.; Rose, A.; Besara, T.; Doyle, N. K.; Yuan, Z.; Wang, J. C.; Clark, R.; Hu, Y.; Siegrist, T.; Lin, S.; Ma, B. Low-Dimensional Organic Tin Bromide Perovskites and Their Photoinduced Structural Transformation. Angew. Chem., Int. Ed. 2017, 56 (31), 9018−9022. (22) Zhou, C.; Lin, H.; Shi, H.; Tian, Y.; Pak, C.; Shatruk, M.; Zhou, Y.; Djurovich, P.; Du, M. H.; Ma, B. A Zero-Dimensional Organic Seesaw-Shaped Tin Bromide with Highly Efficient Strongly StokesShifted Deep-Red Emission. Angew. Chem. 2018, 130 (4), 1033− 1036. (23) Gao, P.; Gu, M.; Liu, X.; Zheng, Y. Q.; Shi, E. W. Photoluminescence study of annealing effects on CuI crystals grown by evaporation method. Cryst. Res. Technol. 2012, 47 (7), 707−712. (24) Reshchikov, M. A.; Morkoç, H. Luminescence properties of defects in GaN. J. Appl. Phys. 2005, 97 (6), 061301. (25) Dohner, E. R.; Hoke, E. T.; Karunadasa, H. I. Self-Assembly of Broadband White-Light Emitters. J. Am. Chem. Soc. 2014, 136 (5), 1718−1721. (26) Yangui, A.; Garrot, D.; Lauret, J.-S.; Lusson, A.; Bouchez, G.; Deleporte, E.; Pillet, S.; Bendeif, E.-E.; Castro, M.; Triki, S.; et al. Optical investigation of broadband white-light emission in selfassembled organic−inorganic perovskite (C6H11NH3)2PbBr4. J. Phys. Chem. C 2015, 119 (41), 23638−23647. (27) Hu, T.; Smith, M. D.; Dohner, E. R.; Sher, M.-J.; Wu, X.; Trinh, M. T.; Fisher, A.; Corbett, J.; Zhu, X. Y.; Karunadasa, H. I.; Lindenberg, A. M. Mechanism for Broadband White-Light Emission from Two-Dimensional (110) Hybrid Perovskites. J. Phys. Chem. Lett. 2016, 7 (12), 2258−2263. (28) Yuan, Z.; Zhou, C.; Messier, J.; Tian, Y.; Shu, Y.; Wang, J.; Xin, Y.; Ma, B. A Microscale Perovskite as Single Component Broadband Phosphor for Downconversion White-Light-Emitting Devices. Adv. Opt. Mater. 2016, 4 (12), 2009−2015. (29) Feldmann, J.; Peter, G.; Göbel, E.; Dawson, P.; Moore, K.; Foxon, C.; Elliott, R. Linewidth dependence of radiative exciton lifetimes in quantum wells. Phys. Rev. Lett. 1987, 59 (20), 2337. (30) Xiao, Y.; Hao, Z.; Zhang, L.; Wu, H.; Pan, G.-H.; Zhang, X.; Zhang, J. An efficient blue phosphor Ba2Lu5B5O17:Ce3+ stabilized by La2O3: Photoluminescence properties and potential use in white LEDs. Dyes Pigm. 2018, 154, 121−127.

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Near-Unity Photoluminescence Quantum Yield in Blue-Emitting

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