Recent progress on discovery of novel phosphors for solid state lighting

Journal Pre-proof Recent progress on discovery of novel phosphors for solid state lighting Xuefang Luo, Rong-Jun Xie PII:

S1002-0721(19)30975-5

DOI:

https://doi.org/10.1016/j.jre.2020.01.016

Reference:

JRE 696

To appear in:

Journal of Rare Earths

Received Date: 5 December 2019 Revised Date:

6 January 2020

Accepted Date: 8 January 2020

Please cite this article as: Luo X, Xie RJ, Recent progress on discovery of novel phosphors for solid state lighting, Journal of Rare Earths, https://doi.org/10.1016/j.jre.2020.01.016. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

Recent progress on discovery of novel phosphors for solid state lighting Xuefang Luo, Rong-Jun Xie* College of Materials, Xiamen University, Xiamen 361005, China Email: [email protected] Abstract Luminescent materials play indispensable roles in many application areas such as lighting, advanced displays, bio-imaging, medical treatments, sensing and detection. To pursue new luminescent materials with improved or desired properties is an endless mission, driven by increasing demands of advances in technologies and applications. The traditional trial-and-error method, usually based on intensive experiments, cannot search for new materials in a fast and efficient way, therefore alternative model- or theory-based approaches for materials discovery need to be developed. In this work we overviewed several promising methods for screening and discovering novel luminescent materials, including solid state combinatorial chemistry, chemical unit substitution, single particle diagnosis and high-throughput calculations. These methods, having their own merits and demerits, enable to search for new phosphor materials with interesting properties for white light-emitting diodes (wLEDs) rapidly and efficiently. Finally, data-driven discovery of materials is emphasized as a state-of-art approach. Keywords: phosphor; solid-state lighting; high throughput photoluminescence; combinatorial chemistry; single crystal

calculations;

1. Introduction Rare-earth or transition metal activated inorganic luminescent materials (i.e., phosphors) play an important or even indispensable role in a large number of applications, such as lighting, display, photovoltaic, bioimaging, medical care, anti-counterfeiting, sensing, and etc.[1-11] For different applications, the requirements for luminescent properties of phosphors are varied greatly, which thus leads to a vast number of phosphors with diverse compositions. In addition, everlasting advances in technologies catalyze the development of new types of phosphors. For example, ultraviolet (UV) phosphors are required for water disinfection or skin disease treatments;[12-13] while near-infrared (NIR) phosphors are needed to develop NIR LEDs for use in portable NIR spectrometers, machine visualization and face recognition.[14] Regarded as the 4th generation solid state lighting, white light-emitting diodes (wLEDs) have been extensively used for general lighting and backlighting, because they promise energy saving, high efficiency, high brightness, compactness, long lifetime, environmental friendliness, and ease spectral modulation.[15] In this technology, phosphors down-convert the emission of near-UV or blue LED chips into other visible light, and the mixture of emissions from LEDs and phosphors appears white. These phosphors are also called down-conversion luminescent materials or color converters. Currently, lots of phosphors have been reported as color converters for wLEDs, including orthosilicates, aluminates, phosphates, (oxy)sulfides, fluorides, (oxy)nitrides, and etc.[2,3,16] Although most of investigated phosphors have interesting photoluminescence spectra, they cannot be used practically due to either the low

conversion efficiency, large thermal quenching/degradation, chemical instability, or low competition. Fortunately, a few of them have been commercialized, such as Ln3(Al, Ga)5O12:Ce3+ (Ln = Y, Lu, Gd), M2SiO4:Eu2+ (M = Ca, Sr, Ba), α-sialon:Eu2+, β-sialon:Eu2+, CaAlSiN3:Eu2+, Sr2Si5N8:Eu2+ and K2SiF6:Mn4+.[2,3,16-23] These phosphors play important roles in enhancing the color rendition, color gamut, brightness, luminous efficiency, and/or lifetime of wLEDs. On the other hand, to produce more comfortable, healthy and brilliant light, optical qualities of wLEDs still need to be further improved. For instance, ultra-high color rendering indices (Ra > 95, R9 > 95) and high luminous efficiency are required for general illumination. This means that some specific phosphors must be developed, e.g., (i) blue-green phosphors with λem = 480–510 nm; (ii) red phosphors with less emission above 650 nm; (iii) red phosphors with less absorption of green light; and (iv) broadband phosphors. As to wLED backlights, super-wide color gamut together with high brightness are highly required to realize vivid and bright pictures that are comparable to their quantum dot (QD) and organic light-emitting diode (OLED) counterparts. To fulfill these needs, narrow-band green and red phosphors with full-width at half maximum (FWHM) smaller than 25 nm and desired emission maxima should be pursued.[24] Except for these requirements in photoluminescence spectra, high conversion efficiency and small thermal quenching/degradation are a must for practical applications. To discover new phosphor materials is an endless mission for material researchers and chemists. Since there are no reliable theories to correlate the composition with the emission wavelength and efficiency of phosphors. The traditional trial-and error method has been used to find a variety of phosphors, in which the phosphor host and activator are randomly chosen. The method seems more like that phosphors are discovered accidently more than by design. It is time-consuming, labor intensive and inefficient, therefore cannot keep up with the fast development of lighting technology. In this sense, innovative approaches for materials discovery should be urgently developed. In this paper, we will overview several interesting methods to discover novel phosphors, including solid state combinatorial chemistry, chemical unit substitution, single particle diagnosis, high throughput calculations. These methods offer rapid and efficient ways to screen and search for phosphors with new crystal structures or new compositions. 2. Methodologies for discovering new phosphors 2.1 Solid state combinatorial chemistry The combinatorial chemistry method enables to greatly accelerate the synthesis and investigation of new compositions or materials. This approach promises a substantial improvement in our ability to identify, synthesize and optimize target materials with enhanced or desired properties in large landscapes of diverse compositions and structures. By using a thin film process, a combinatorial library containing thousands or several ten thousand of samples can be prepared, then high throughput screening is carried out to find phosphors with highest luminescence properties from very large numbers of candidates in a short time.[25-27] Danielson et al. found a new and highly efficient red phosphor, Y0.845Al0.070La0.060Eu0.025VO4, for flat panel displays and fluorescence lamps.[25] A phosphor library was designed to cover chemical compositions composed of cations from groups IIA and IIIA and an anion group of VOm−n (m = 3,4; n = 1,3) from group IIIB and IVB elements. The libraries were prepared by depositing layered thin films of

La2O3, Y2O3, MgO, SrCO3, SnO2, V, Al2O3, Eu2O3, Tb4O7, Tm2O3 and CeO2 by electron beam evaporation in vacuum on unheated silicon wafer substrates (Fig. 1(a)), and followed by heating the thin films at certain temperature. A primary mask, containing 230 µm square elements, was used to separate individual library elements. An automated high-throughput screening of the combinatorial libraries was conducted by mapping the emission colors of up to 25000 different samples with a CCD camera when they were excited at 254 nm. The discovered Y0.845Al0.070La0.060Eu0.025VO4 phosphor, isostructural with YVO4, had superior red chromaticity and a comparable efficiency to the standard commercial phosphor Y1.95Eu0.05O3.

Fig. 1 (a) Deposition map of the composition libraries. Thicknesses of target materials deposited on the silicon substrate are given; (b) Masks for producing the quaternary phosphor libraries. Ai, Bi, Ci, Di, and Ei represent a deposition step with mask X rotated counterclockwise by (i−1) × 90°. Reprinted with permission from Refs. 25 and 26.

Wang et al. identified an interesting blue-emitting Gd3Ga5O12/SiO2 composite material in combinatorial libraries containing 1024 different compositions.[26] The libraries were established by depositing thin film precursors, using RF/DC sputtering and pulsed-laser deposition (PLD) methods, at controlled positions of a substrate with aids of shadow masks. A quaternary combinatorial masking scheme was designed for the phosphor libraries that consists of a series of n different masks, then the mask further divided the substrate into a series of self-similar patterns of quadrants (Fig. 1(b)). 4n different compositions can be generated with 4n deposition steps, allowing to investigate materials containing elemental components of up to n. The deposited thin film precursors were then annealed at appropriate temperatures to ensure uniform diffusion and subsequent phase formation. As shown in Fig. 2, the emission color of each composition of the library can be recorded under 254 nm excitation. A scanning spectrophotometer was then applied to measure the photoluminescence spectra of each luminescent film in the libraries, and the quantum yields of 100 selected red, green and blue sites from the library were obtained by comparing them with a standard green phosphor of Zn2SiO4:Mn2+. Finally, an efficient blue luminescent material was discovered for potential applications in flat-panel displays and X-ray imaging systems. In addition, Xiang et al. used a scanning inkjet delivery system to create microscale libraries of 1000 elements for large landscapes of diverse compositions in the system of (LamGd1-m)AInOx:Euy3+.[27] Sohn et al. used a solution combinatorial chemistry to search

for an efficient red PDP (plasma display panel) phosphor (Y0.9(P0.92V0.03Nb0.05)O4:Eu3+) in designed quaternary and ternary combinatorial libraries.[29] The traditional high throughput combinatorial chemistry, however, still has a quite low efficiency to examine all compositions in high-order material systems if the optimization process is not available for generating composition libraries. Sohn et al. firstly proposed to use a computational evolutionary optimization method, combining a genetic algorithm with combinatorial chemistry, to search for highly efficient red phosphors in Eu3+-doped alkali earth borosilicate system (a seven-element system). A multinary composition of Eu0.14Mg0.18Ca0.07Ba0.12B0.17Si0.32Oδ was discovered as a promising red phosphor for wLEDs.

Fig. 2 Luminescent photograph of the processed quaternary library under irradiation from a multiband emission UV lamp at short wavelength (centered around 254 nm). Reprinted with permission from Ref. 26.

Strictly speaking, the phosphors discovered by the traditional combinatorial chemistry cannot be regarded as real new luminescent materials but new compositions, because their crystal structures are already known. Without new discovery strategies, to explore phosphors with new crystal structures rapidly is still a challenge. Fortunately, Sohn et al. applied the heuristics optimization strategy to find new (oxy)nitride phosphors with promising properties.[30-34] The strategy is to couple the high throughput experimentation (HTE) with the non-dominated-sorting genetic algorithm (NSGA) and particle swarm optimization (PSO). The proposed strategy for discovering new phosphors includes three sequential steps (Fig. 3(a)). The first step is to choose a composition space in randomly selected ternary or multinary systems. The second one is to select a new phosphor by applying an NSGA- and PSO-assisted combinatorial materials search, followed by the parameterization of the material novelty. The last step is to determine the crystal structure and composition of the newly identified phosphor. As the heuristics optimization process is not only a computation but also an experiment-oriented process, the size of the composition space should be as small as possible to reduce the experimental burdens and cost. As seen in Fig. 3b, based on the knowledge of phase diagram and the practical experience, the composition library is confined to the area close to the Si3N4 corner in the system of

AEO-Al2O3-AlN-Si3N4-Eu2O3 (AE = Mg, Ca, Sr, Ba), and the number of the synthesized samples will be reduced to several hundreds.[34] Using this approach, Sohn et al. found a yellow-green phosphor of La4– 2+ (λem = 565 nm) in the xCaxSi12O3+xN18−x:Eu SrO-CaO-BaO-La2O3-Y2O3-Si3N4-Eu2O3 search space.[27] Similarly, a yellow-emitting phosphor, Ce4–xCaxSi12O3+xN18−x:Eu2+ (λem = 581 nm), was reported in the system of SrO-CaO-BaO-CeO2-Y2O3-Si3N4-Eu2O3.[28] A red phosphor Ca15Si20N30O10:Eu2+ (λem = 641 nm) was identified in the CaO–Si3N4 binary system.[29] In the SrO-BaO-CaO-Si3N4 quaternary composition search space, a yellow phosphor Ba1.5Ca0.5Si5N6O3:Eu2+ (λem = 585 nm) was discovered.[30] Further, Sohn et al. improved the discovery strategy by incorporating an elitism-involved NSGA (NSGA-II) to reduce the complexity, leading to the discovery of an interesting blue phosphor Ba(Si,Al)5(O,N)8:Eu2+ in the ternary BaCO3−Al2O3−Si3N4 composition space.[34]

Fig. 3 (a) Overall description of the proposed discovery process for new luminescent materials; (b) Design of decision parameter space (phosphor composition search space) for NSGACMS. Reprinted with permission from Refs. 30 and 34.

2.2 Chemical unit substitution Screening and searching for suitable phosphor hosts from the ICSD database have led to the discovery of new interesting phosphors for wLEDs, such as Ca-α-sialon:Eu2+, β-sialon:Eu2+, CaAlSiN3:Eu2+, M2Si5N8:Eu2+ (M = Ca, Sr, Ba), MSi2O2N2:Eu2+ (M = Ca, Sr, Ba), JEM:Ce3+, etc.[19-22, 35, 36] In addition, with partial or complete cation substitution or cation/anion double substitution, new phosphor compositions with tunable emission color can be generated. For example, replacing Ca by Sr in a deep-red CaAlSiN3:Eu2+ (λem = 660 nm) phosphor yields the formation of a red SrAlSiN3:Eu2+ (λem = 620 nm) phosphor with the same crystal structure.[37] In Ca-α-sialon:Eu2+ (Cam/2Si12-m-nAlm+nOnSi16-n), partial substitution of Si-N with Al-O and Al-N results in a wide composition range, and thus a tunable emission color from orange-red to yellow-green can be obtained.[38]

Minerals are stable nature materials, and form a large family of material systems. Kahihana et al. firstly proposed the mineral-inspired approach to search for new phosphors, in combination with a solution parallel synthesis (SPS) method.[39] A large numbers of phosphor samples were synthesized simultaneously at the same firing conditions by applying the “polymerizable complex method” or the “amorphous metal complex method,” allowing for the rapid screening of phosphor candidates.[39-43] Fig. 4(a) shows the scheme of the SPS method for discovering new phosphors in AxByAlzSiqOw:Eu2+ (A = Li, Na, K; B = Sr, Ba) systems.[42] The glycol-modified silane, water-dispersible silicon compound, was used as a silicon source to prepare silicon-containing phosphors. Using minerals as model materials, a composition library of artificial compositions can be established (Table 1). A new green-yellow phosphor NaAlSiO4:Eu2+ (Fig. 4(b)), was discovered from the silicate mineral of nepheline,[42] after carrying out a series of experimental steps, such as the synthesis of a mixed solution according to the composition table, chemical reactions, thermal processing, visual inspection of light emissions and the identification of new phosphors. Similarly, a novel blue-green phosphor BaZrSi3O9:Eu2+ was found in benitoite.[43]

Fig. 4 (a) Scheme of solution parallel synthesis scheme for exploration of new phosphors in AxByAlzSiqOw:Eu2+ (A = Li, Na, K; B = Sr, Ba) systems. GMS stands for “glycol-modified silane,” which was used as the silicon source. Visual observation of emission light by illumination with near-UV light is also shown as the image; (b) Excitation and emission spectra of the nepheline origin NaAlSiO4:Eu2+ (1 mol% for Na) discovered by employing the SPS method. Reprinted with permission from Ref. 42. Table 1 Library of artificial compositions inspired by natural minerals of AxByAlzSiqOw (A = Li, Na, K; B = Sr, Ba) systems. Sr substitutions for Na or Ba

Name of Mineral

Original compositions minerals

1

Jadeite

NaAlSi2O6

NaAlSi2O6

2

Albite

NaAlSi3O8

NaAlSi3O8

3

Nepheli ne

NaAlSiO4

NaAlSiO4

No.

of Standard compositions

5% for Na and 20% 10% for Na and 40% for Ba for Ba Na0.95Sr0.05Al1.05Si1.95 O6 Na0.95Sr0.05Al1.05Si2.95 O8 Na0.95Sr0.05Al1.05Si0.95 O4

Na0.9Sr0.1Al1.1Si1.9O6 Na0.9Sr0.1Al1.1Si2.9O8 Na0.9Sr0.1Al1.1Si0.9O4

4

Natrolit e Barrerit e

Na2[Al2Si3O10]·2H2O

Na2Al2Si3O10

Na1.9Sr0.1Al2.1Si2.9O10

Na1.8Sr0.2Al2.2Si2.8O10

Na2[Al2Si7O18]·6H2O

Na2Al2Si7O18

Na1.9Sr0.1Al2.1Si6.9O18

Na1.8Sr0.2Al2.2Si6.8O18

Gobbins Na5[Al5Si11O32]·12H2 Na5Al5Si11O32 ite O Orthocl KAlSi3O8 KAlSi3O8 ase

Na4.75Sr0.25Al5.25Si10.75 O32

Na4.5Sr0.5Al5.5Si10.5O3

-

-

8

Kalsilite KAlSiO4

KAlSiO4

-

-

9

Leucite

KAlSi2O6

KAlSi2O6

-

-

10

Lithosit e

K6Al4Si8O25·21H2O

K6Al4Si8O25

-

-

11

Petalite

LiAlSi4O10

LiAlSi4O10

-

-

5 6 7

12 13

Banalsit e Stronals ite

BaNa2Al4Si4O16 SrNa2Al4Si4O16

BaNa2Al4Si4O Ba0.8Sr0.2Na2Al4Si4O16 16

SrNa2Al4Si4O

-

Ba0.6Sr0.4Na2Al4Si4O16 -

16

Inspired by the discovery strategy of the mineral-inspired approach, Schnick et al. discovered several kinds of new nitride phosphors that are isostructural with UCr4C4 (Fig. 5 and Table 2). With substitutions of CrC4 tetrahedra by AlN4, LiN4, MgN4, SiN4, GaN4, and GeN4, several promising red phosphors have been identified., such as Ca[LiAl3N4], Sr[LiAl3N4], Ba[Mg2Ga2N4], Sr[Mg2Al2N4], Sr[Mg3SiN4], [44-48] Sr[Mg3GeN4]. Interestingly, a very narrow-band red phosphor SrLiAl3N4]:Eu2+ (FWHM = 50 nm) was discovered, which enabled to improve the luminous efficiency by 14% due to the less emission above 700 nm compared to CaAlSiN3:Eu2+.[45] The common structure of these nitride phosphors consists of a highly condensed three-dimensional framework that is built up of edge- and corner-sharing MN4 tetrahedra (M = Si, Al, Li, Mg, Ga, Ge). Vierer ring channels are formed, in which alkaline earth (AE) metals are centered. The AEN8 (AE = Ca and Sr) polyhedra are connected to each other by common faces, forming infinite strands. The degree of condensation (i.e., atomic ratio M:N) of these compounds is 1.0. The highly symmetric cuboid coordination of Eu2+ leads to the narrow-band emission. Xia et al. summarized the above strategy as chemical unit substitution, which means that each polyhedral unit in the structure of a compound can be replaced by others while the structure remains the same.[50]

Fig. 5 New nitride phosphors discovered from UCr4C4 by chemical unit substitution. Table 2 New nitride phosphors with the UCr4C4-structure type

UCr4C4-type

Emission maximum (nm)

FWHM (nm)

Ref.

Ca[LiAl3N4]:Eu2+ Sr[LiAl3N4]:Eu2+ Ba[Mg2Ga2N4]:Eu2+ Sr[Mg2Al2N4]:Eu2+ Sr[Mg3SiN4]:Eu2+ Ba[Mg3SiN4]:Eu2+ Sr4[LiAl11N14]:Eu2+

668 654 649 612 615 670 670

60 50 87 43 88 85

44 45 46 46 47 47 49

Recently, Huppertz et al. and Xia et al. reported a new class of luminescent materials with surprising properties – Eu2+-doped alkali lithosilicates by using the chemical unit substitution strategy (Table 3).[51-55] These phosphors have similarities in structure that is derived from NaLi3SiO4 or KLi3SiO4. Surprisingly, these new phosphors have narrow-band cyan or green emissions or broadband white emissions with a relatively high quantum efficiency and small thermal quenching. The narrow-band emission is originated from Eu2+ occupying the only one crystallographic site of alkali metals, which is coordinated to eight oxygen atoms to form highly symmetric cubic-like polyhedra. While the broadband emission is ascribed to the fact that there are several occupancy sites for Eu2+ in some alkali lithosilicates like NaK7[Li3SiO4]8.[51] The extremely narrow-band green-emitting RbNa[Li3SiO4]2 and RbLi[Li3SiO4]2 phosphors are best suited for use in LCD backlights, enabling to create much wider color gamut when coupled with a narrow-band red phosphor such as K2SiF6:Mn4+ (Fig. 6(a)).[52] On the other hand, alkali lithosilicate phosphors have a serious problem of chemical instability, and need to be solved for practical applications.

Fig. 6 (a) Photoluminescence spectra of RbLi[Li3SiO4]2:Eu2+ and β-sialon:Eu2+; (b) Normalized excitation (grey, for the emission at λmax = 614 nm) and emission spectra (red, excited with λexc = 460 nm) of Sr[Li2Al2O2N2]:Eu2+ in comparison to Sr[LiAl3N4]:Eu2+ (purple) and the human-eye sensitivity curve (black dotted). Reprinted with permission from Refs. 52 and 56. Table 3 New alkali lithosilicate phosphors having structural similarities with NaLi3SiO4

Phosphors NaK7[Li3SiO4]8:Eu2+ RbLi[Li3SiO4]2:Eu2+ RbNa3[Li3SiO4]4:Eu2+ RbNa2K[Li3SiO4]4:Eu2+ CsNa2K[Li3SiO4]4:Eu2+ RbNa[Li3SiO4]2:Eu2+

Emission maximum (nm)

FWHM (nm)

Ref.

515/598 530 471 480/530 485 523

49/138 42 22.4 26/60.4 26 41

51 52 53 54 54 55

With the same chemical unit substitution approach, Huppertz et al. reported a promising and unique red-emitting oxonitridolithoaluminate phosphor (Sr[Li2Al2O2N2]:Eu2+), which possesses a similar structure with UCr4C4.[56] In the structure, there are two types of tetrahedra – [AlON3] and [LiO3N], forming a highly condensed network of vierer rings arranged in three kinds of channels along the [001] axis. One of the channels, built up upon alternating [AlON3] and [LiO3N] tetrahedra, is centered by the strontium cations. Each Sr atom is coordinated by four nitrogen and four oxygen atoms, leading to a cuboid with high symmetry. When Eu2+ is doped in the lattice replacing Sr2+, Sr[Li2Al2O2N2]:Eu2+ shows an emission maximum of 614 nm and a very small FWHM of 48 nm, allowing to prepare warm wLEDs with a superior luminous efficacy over those fabricated by using Sr[LiAl3N4]:Eu2+ as a red component (Fig. 6(b)). Schnick et al. again reported a narrow-band red-emitting CaBa[Li2Al6N8]:Eu2+ (FWHM = 48−57, λem = 636–639 nm ), which is isostructural with RbNaLi6Si2O8.[57] 2.3 Single particle diagnosis To identify a new compound by analyzing its single crystal is simple, which has been practiced by Schnick et al., who recently discovered RE4Ba2[Si12O2N16C3]:Eu2+, Li24Sr12[Si24N47O]F:Eu2+, Lu4Ba2[Si9ON16]O:Eu2+, Ca3Mg[Li2Si2N6]:Eu2+, and [44-49, 58-62] Li38.7RE3.3Ca5.7[Li2Si30N59]O2F. However, it is not easy to grow large-size (> 50 µm) and perfect single crystals required for structure analyses, and the single crystals should be prepared one by one, which cannot promise the rapid discovery of new materials. To speed up the screening and identifying candidate compounds for phosphors, Xie et al. proposed the single-particle diagnosis approach to discover new LED phosphors in a more effective and efficient way (Fig. 7).[63] The method consists of five sequential steps. Step I, a composition library is designed in a multi-nary material system containing randomly selected elements (usually alkaline earth, alkali earth, lanthanide, silicon, aluminum, oxygen, nitrogen). Then, a traditional solid-state reaction method is used to synthesize the compositions in the library, and all samples are fired at the same conditions. Step II, tiny luminescent powder particles or single crystals with a diameter as small as 10 µm are pinpointed from the synthesized powders of a selected composition under the UV or blue light irradiation, followed by the determination of the lattice constants by using the single crystal x-ray diffractometer. If the lattice parameters of a single particle cannot match with those in Inorganic Crystal Structure Database (ICSD), then the substance of the particle may have a new crystal structure. Step III, the crystal structure and chemical composition of the new substance are then determined, finalizing the identification of a new phosphor. Step IV, the photoluminescence properties of a single luminescent particle, such as photoluminescence spectra, quantum

efficiency, absorption, decay time and thermal quenching, are measured by using the customized fluorescence spectrometers. Step V, phase-pure phosphor powders are finally synthesized by controlling the firing conditions and starting materials. In fact, this method combines the simplified combinatorial chemistry (to prepare composition libraries) with the single crystal analysis (to determine the crystal structure and photoluminescence), without the preparation of phase pure powders in Step I and the growth of single crystals in Step III. Differing from others, the composition space or the number of elements can be freely selected in this method, and the new phase really exists stably as it is identified from the synthesized powder mixture.

Fig. 7 Schematics of single-particle diagnosis approach showing the sequential experimental steps. Reprinted with permission from Ref. 63.

Several (oxy)nitride phosphors with new crystal structures and compositions have been discovered by using the single particle diagnosis approach (Table 4).[63-69] These phosphors cover a broad emission color from deep-blue to orange, among these the narrow-band green-emitting Ba2LiSi7AlN12:Eu2+ can be potentially applied in wLED backlights.[64] Starting from the stoichiometric composition of a single crystal, Wang et al. obtained phase pure Sr3Si8-xAlxO7+xN8-x:Eu2+ and Ca1.62Eu0.38Si5O3N6:Eu2+ powders by using gas-pressure sintering.[65,66] But in some cases, synthesis of single-phase phosphor powders is not easy, and needs special techniques such as starting from nonstoichimetric compositions, use of sealed crucibles, unusual firing schemes, and etc. Till now, about 50 new (oxy)nitride phosphor compounds have been found by the single particle diagnosis method, but it is still an experiment-based discovery strategy that depends on the designed composition space, number of samples, and synthesis conditions. Table 4 Nitride and oxynitride phosphors discovered by the single-particle diagnosis approach Crystal structure FWHM λem New phosphors Ref. (nm) (nm) Ba9Si11Al7N25:Eu2+ BaSi4Al3N9:Eu2+ Ba2LiSi7AlN12:Eu2+ Sr3Si8-xAlxO7+xN8-x:Eu2+ Ca1.62Eu0.38Si5O3N6:Eu2+ Sr2B2-2xSi2+3xAl2-xN8+x:Eu2+ La2.5Ca1.5Si12O4.5N16.5:Eu2+ Sr3.61LiSi14.27Al5.61O6.19N23.25:Eu2+ La2.85Sr0.76LiSi14.86Al4.93O2.89N26.51:Eu2+ Sr3.61LiSi14.27Al5.61O6.19N23.25:Ce3+

Orthorhombic, Pnnm Monoclinic, P21/C Orthorhombic, Pnnm Monoclinic, C2/c Monoclinic, Cm Hexagonal, P62c Monoclinic, C2 Trigonal, P3m1 Trigonal, P3m1 Trigonal, P3m1

568 500 515 465 600 596 495 475 470 432

98 67 61 70 102 147 73 90 82 84

63 63 64 65 66 67 68 69 69 69

2.4 High throughput calculations There exist a vast number of unexplored chemical compositions or spaces that may yield luminescent materials with useful properties, but little apriori theoretical understanding to navigate the searching and screening of specific candidates.[70] The materials discovery methods introduced above are basically experiment-oriented approach, requiring intensive labor burdens, cost and time. To keep up with rapid advances in solid-state lighting technologies and applications, there is an urgent need to find phosphors with desired properties as soon as possible. To fulfill the needs of wide color gamut LED backlights, Schnick et al. designed and discovered several narrow-band red phosphors by constructing crystal structures of nitride compounds from UCr4C4 via chemical unit substitution.[44-48] The selection of this model material is stemmed from its highly symmetric crystallographic site where Eu2+ occupies, validating the necessity of understanding the structure-property relation for materials design. Although some empirical equations could roughly predict the emission or thermal quenching of phosphors from their crystal structure or band structure, there are no theoretic models to build a new material with the properties required. On the other hand, big data of phosphors discovered so far allow one to summarize or find rules that control the emission, quantum efficiency or thermal stability of phosphors, and then to use these rules to screen or design new luminescent materials with aids of high throughput DFT (density function theory) calculations or machine learning or the combination of both.[71-73] These rules are also named descriptors that connect one of photoluminescence properties with a qualifiable parameter.[71-74] For example, Wang et al. proposed an electronic structure descriptor for screening narrow-band red phosphors from 2259 nitride compounds by using the high-throughput first-principles calculation.[71] They proposed that a large splitting of more than 0.1 eV (∆ES > 0.1 eV) between the two highest Eu2+ 4f7 bands could be used as a structure descriptor for discovering phosphors with narrow-band emissions (Fig. 8(a)). Fig. 8(b) gives a flowchart of screening narrow-band red phosphors based on high-throughput computations.[71] It consists of five steps. Step I is to make a full list of all nitride candidates from Materials Project database, followed by increasing the data set of nitride compounds with the general chemical formula of AxByCzNn (A = Ca/Sr/Ba, B = Li/Mg, C = Al/Si) by using the data-mined ionic substitution algorithm. Thus the initial candidates are generated. Step II is to pick up materials with Ehull < 50 meV (energy above hull), implying they are phase stable. Step II is to calculate the host band gaps (Eg) using the Perdew-Burke-Ernzerhof (PBE) functional, and those materials with 2.42 eV < Eg < 3.58 eV are selected. Step IV is to screen the candidates with ∆ES > 0.1 eV. Finally, the band gap using the Heyd-Scuseria-Ernzerhof (HSE) functional (HSE Eg) and Debey temperature (ΘD) of all remained candidates are calculated for evaluating their quantum efficiency and thermal quenching. From initial 2259 candidates, 8 nitride compounds with narrow-band emissions have been screened by the high throughput calculations and 5 of them are totally new.

Fig. 8 (a) Descriptor for narrow-band red emissions. ∆ES means the energy splitting between the two highest Eu2+ 4f7 bands. When ∆ES <0.1 eV, a broadband is created, whereas a narrow-band is formed when ∆ES >0.1 eV; (b) Flowchart showing high-throughput screening procedure for narrow-band red-emitting phosphor hosts. Reprinted with permission from Ref. 71.

With the similar high throughput screening procedure, Wang et al. discovered a novel Sr2LiAlO4 phosphor host, which is the first compound reported in the quaternary Sr-Li-Al-O system.[72] As seen in Fig. 9(a), a solid-state lighting periodic table was created, in which the elements in the green zone are commonly used to form phosphor hosts, whereas those in the gray zone are never used. With this table, candidate phosphor hosts are searched or screened in ternary M-X-O (M = Ba/Sr/Ca, X = P/Si/Al/B) and quaternary M-L-X-O (M = Ba/Sr/Ca, L = Li/Mg/Y, X = P/Si/Al/B) oxides. In the phase diagram SrO-Li2O-Al2O3 calculated at 0 K, a thermodynamically stable new phase Sr2LiAlO4 was discovered (Fig. 9(b)). Sr2LiAlO4 is derived from Ba2LiReN4 (ICSD No. 411453) via a multi-species substitution of Ba2+ with Sr2+, Re7+ with Al3+, and N3− with O2−.[72] Doped with Eu2+, Sr2LiAlO4:Eu2+ shows a broad and asymmetric emission band, which has a FWHM of 73.6 nm, an emission maximum of 512 nm and a shoulder peak of 559 nm (Fig. 9(c)). The excitation spectrum exhibits a broad band with two dominant peaks at 310 and 394 nm when monitored at 512 nm. Under 394 nm excitation, the internal quantum efficiency is 25%. Xie et al. recently discovered a super-broad white-emitting phosphor Sr2AlSi2O6N:Eu2+ in system SrO-SiO2-Si3N4-Al2O3 by using the data-driven strategy based on DFT high throughput calculations.[73] Sr2AlSi2O6N is derived from Ba2ZnGe2S6O (ICSD No. 14174) via a multi-species substitution of Ba2+ with Sr2+, Zn2+ with Al3+, Ge4+ with Si4+, S2− with O2−, and O2− with N3−. The emission spectrum of Sr2AlSi2O6N:Eu2+ covers a broad range of visible light and shows a world-record super-wide FWHM of 230 nm, showing a white

emission color under UV excitation.

Fig. 9 (a) Solid-state lighting periodic table showing the mostly used elements in compounds having the word “phosphor” in the publication title in the 2017 version of ICSD. Only non-rare-earth elements are shown. Elements with frequency 0 are shaded in gray; (b) Calculated SrO-Li2O-Al2O3 phase diagram at 0 K. Blue circles stand for stable phases in the Materials Project database; the red square shows a new stable quaternary phase, Sr2LiAlO4; (c) Photoluminescence spectra of Sr2LiAlO4:Eu2+. Reprinted with permission from ref. 72.

Brgoch et al. used Debye temperature and band gap as material descriptors to search for new inorganic phosphors with high thermal stability and quantum efficiency.[74] The Debye temperature was predicted by machine learning, and the band gap was evaluated by high throughput DFT calculations. The calculated Debye temperatures of 2071 compounds (including borates, silicates, aluminates, phosphates, (oxy)nitrides, fluorides, sulfides) were plotted against the calculated band gaps (Fig. 10(a)), establishing a sorting diagram that enables to screen vast phase space to identify new phosphors. As seen from the diagram, one can find that borates tend to possess large band gaps and high Debye temperatures simultaneously (Fig. 10(b-e)). An outstanding borate phosphor NaBaB9O15:Eu2+ was then distinguished from the sorting diagram, which has a high Debye temperature of 729 K and a wide band gap of 5.5 eV.[74] The high structural stiffness is ascribed to a unique corner-sharing [B3O7]5-polyanionic backbone. Under 315 nm excitation, the borate phosphor shows a violet emission maximum at 416 nm and a narrow FWHM of 34.5 nm (Fig. 10(f)). Furthermore, NaBaB9O15:Eu2+ has an internal quantum efficiency as high as 95% and extremely small thermal quenching (Fig. 10(g-h)). This work validates that machine learning is a powerful and indispensable tool to navigate the screening of new types of phosphors.

Fig. 10 Debye temperature predicted by machine learning as a function of band gap calculated by the density functional theory (DFT); (a) Debye temperature (ΘD,SVR) versus calculated bandgap (Eg,DFT) for 2071 compounds. Groups of typical phosphor hosts are highlighted, including (b) borates and sulfides, (c) nitrides and (oxy)halides, (d) silicates and fluorides, and (e) aluminates and phosphates. Photoluminescence properties of the discovered phosphor NaBaB9O15:Eu2+: (f) emission and excitation spectra, (g) quantum yield against the Eu concentration, and (h) temperature-dependent photoluminescence intensity. Reprinted with permission from Ref. 74.

The revolutionary data-driven method, consisting of high throughput DFT calculations, data mining, machine learning or the combination of them, helps us accelerate the design, screening and discovery of new materials with appropriate properties. In general, it runs in four sequential steps, including (i) database construction of the structure-property relationships of known photoluminescence materials; (ii) determination of quantifiable descriptors of some properties such as emission, FWHM, quantum yield, thermal quenching; (iii) determination of the searching space and candidates; and (iv) identification of target materials with desired properties. Although only several new phosphors are discovered by this method, it is believed that the data-driven discovery of materials will be a mainstreaming approach to speed up the searching and identification of new materials. 3. Perspectives Advances in solid-state lighting technologies have catalyzed the research and development of luminescent materials, and more materials with specific properties and applications need to be explored. In this paper, we have overviewed several promising methods for materials discovery, which pave avenues for rapidly navigating the

searching and screening of new luminescent materials with desired emission colors, narrow or broad band, high quantum yield, or small thermal quenching. Although they have their own advantages, their limitations or shortcomings are also obvious. For example, the heuristics-assisted combinatorial chemistry allows to find new phosphors in a reduced composition library, with aids of computation and experimental processes. It is still required to synthesize hundreds of phosphor powders to identify one target, and it is time-consuming to determine the crystal structure of the new phase from the x-ray powder diffraction patterns. The limited numbers of appropriate model materials are a big issue for the chemical unit substitution strategy. Moreover, it has the risk of being unable to synthesize the compositions designed by the substitution strategy. For the single-particle diagnosis approach, it is hard to select a suitable searching space, and the screening is random. As to the data-driven discovery method, the lack of phosphor database as well as quantifiable descriptors hinders its application in new phosphors searching. In addition, in some cases the materials discovered by those methods still have the problems of (i) difficulties in scale-up powder synthesis; (ii) low chemical stability; (iii) low quantum yield; or (iv) low competition with already used counterparts, which thus make them hard to be practically used. Parallel to these methods, the traditional one is still playing the role in finding new materials. Schnick et al. further expanded the composition space to beryllium-containing compounds, and discovered several interesting oxoberyllate (AELi2[Be4O6]:Eu2+, AE = Sr, Ba), oxonitridoberyllate (Sr[Be6ON4]:Eu2+) and nitridoberyllosilicate (Sr[BeSi2N4]:Eu2+) phosphors.[75-77] Very interestingly, AELi2[Be4O6]:Eu2+, isotypic to BaLi2[(Al2Si2)N6]:Eu2+, shows ultra-narrow band blue emission with a FWHM of 25 nm and an emission maximum of 454–456 nm.[75] Driven by the needs of drug design, genetic/protein engineering, the uses of database are more popular in biotechnology and life science research. It is also essential for materials researchers to use the databases, manipulation tools and models to design materials smartly. In fact, high throughput calculations have shown a strong power to substantially improve our ability to screen large composition spaces and rapidly discover new materials. In addition to calculation tools, there are two key factors required for high throughput calculations: (i) databases of property and structure; and (ii) quantifiable descriptors for screening. The databases can be constructed from the experimental data of phosphor materials that are available from various interesting disciplines, such as journals, books, notes, ICSD, etc. Up to now, databases of phosphors discovered so far are not available yet, which need to be urgently developed. Trends in photoluminescence properties over a large quantity of structures and compositions as well as their relations with crystal structure (especially local structure) and electronic structure could yield important guidance in developing fundamental models/theories for predicting photoluminescence properties or in building quantifiable descriptors for screening materials. It is still a big challenge to find suitable descriptors relating one photoluminescence property (i.e., emission maximum, band width, quantum efficiency, thermal quenching, decay time, etc.) to some crystal or electronic structure features. To this end, investigations on the structure-property relationship of phosphors need to be strengthened. Foundation item:Project supported by the National Natural Science Foundation of China (51832005).

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Graphical abstract:

Discovery of new phosphors with new crystal structure and promising properties by single crystal analysis, modified combinatorial chemistry, high throughput calculations, etc.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: