A review on Mn⁴⁺ activators in solids for warm white light-emitting diodes

Abstract

Currently, the major commercial white light-emitting diodes consist of a blue-emitting chip and Y₃Al₅O₁₂:Ce³⁺ yellow phosphor. However, the shortage of a red emitting component in the constructed device makes it difficult to realize warm white light with a high color-rendering index and low correlated color temperature. In this mini-review article, we provide an overview of recent progresses in developing Mn⁴⁺ doped red phosphors for promising applications in warm white light-emitting diodes. Firstly, the spectroscopic properties of Mn⁴⁺ in solids, including electronic and vibronic energy-level structures, crystal-field parameters as well as thermal stability, were briefly discussed. And then the related physical and chemical synthesis strategies were introduced in detail. Afterwards, Mn⁴⁺ doped phosphors, such as oxides, fluorides as well as glass ceramic composites, and their impact on improving the photoelectric performance of white light-emitting diodes were summarized. Finally, several challenges and perspectives for exploring novel and high-performance Mn⁴⁺ doped red phosphors will be presented.

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Daqin Chen

Daqin Chen is a professor of Materials Science at Hangzhou Dianzi University (HDU). He obtained his Bachelor (2001) and Master (2004) degrees in Materials Science and Engineering from Central South University, P. R. China, and received his Ph.D degree (2008) in Condensed Matter Physics from Fujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy of Science. Since then, he has been working as an assistant, associate and full professor in FJIRSM (2008–2014). After that, he joined HDU as a full professor. His current research interests mainly concern luminescent materials (including nanoparticles, phosphors and glass ceramic composites) targeting at sensor, PV and LED applications. He has published over 100 peer-reviewed scientific papers in the field of luminescent materials, and has more than 3000 citations.

Yang Zhou

Yang Zhou is a Master student at Hangzhou Dianzi University under the supervision of Prof. Daiqn Chen. He obtained his Bachelor degree in Materials and Chemical Engineering from Chongqing University of Arts and Sciences, P. R. China in 2014. His research interests mainly focus on novel Mn^4+ doped red phosphors for warm WLEDs.

Jiasong Zhong

Jiasong Zhong is an assistant professor of Materials Science at Hangzhou Dianzi University (HDU). He received Ph.D. degree (2014) in Materials Science and Engineering from Tongji University, P. R. China. After that, he joined the group of Prof. Daqin Chen as an assistant professor. His research interests include nonlinear quantum dot glasses and Mn^4+ doped red phosphors for warm WLEDs.

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Introduction

Over the years, white light-emitting diodes (WLEDs) have been paid increasing attention due to the advantages over traditional incandescent or fluorescent lightings such as high luminous efficiency, long persistence, energy saving and therefore environmental benefits etc.^1–4 The commercial strategy for WLED is mainly based upon the combination of LED chips with red/green/blue (RGB) tricolor phosphors. Nowadays, the commercial and most popular approach is the conjunction of the InGaN blue LED (typically showing an emission wavelength between 450 and 480 nm) with Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) yellow phosphors. However, YAG:Ce³⁺ exhibits weak emission in the red spectral region, and this makes it difficult to realize warm WLEDs with a high color-rendering index (CRI, R_a > 80) and low correlated color temperature (CCT < 4500 K).^5,6 In order to generate the white light with the warm perception similar to incandescent light, one of the best routes is to mix a highly efficient red phosphor showing strong blue absorption with YAG:Ce³⁺ yellow phosphor.

Currently, the searching of red phosphors is mainly based on rare-earth-doped strategy, and the most successfully materials are the Eu²⁺- or Ce³⁺-doped oxynitride and nitride phosphors, typically commercial Eu²⁺:CaAlSiN₃ and Eu²⁺:Sr₂Si₅N₈.^7,8 Unfortunately, it is well known that most of rare earth ions are costly and some rare-earth chlorides/citrates are even toxic and harmful.⁹ Furthermore, the synthesis of (oxy)nitrides has to be performed at high temperature and high pressure, for example at 1900 °C and 1 MPa N₂ atmosphere, which limits their practical applications in WLEDs.^4,7 On the other hand, semiconductor quantum dots (QDs) have recently been regarded as the potential replacement for the rare-earth doped phosphors owing to their relatively narrow emission bands (25–60 nm) and broadband absorption.^10–16 However, several issues, including the improvement of chemical/thermal/photo-stability, the development of Cd-free QDs and the avoidance of instinct self-absorption effect, are still required to be well solved for their practical application in WLEDs.¹⁷ Therefore, considerable efforts have been devoted to the development of the non-rare-earth-based phosphors usable in warm WLEDs.

Among several transition metal ions, Mn⁴⁺ is a suitable activator for red phosphors as the dominant spin-forbidden ²E → ⁴A₂ transition of Mn⁴⁺ is usually located in the red spectral region owing to its high effective positive charge and thus the influence of strong local crystal-field.¹⁸ In general, Mn⁴⁺ luminescence exhibits both broad excitation bands in the ultraviolet-blue region and sharp emission lines, which makes Mn⁴⁺ doped red phosphors excitable by blue chips and lessens the re-absorption effect when mixing with other green or yellow phosphors.^4,19 These unique optical features meet the requirement of an ideal red phosphor for warm LEDs. Moreover, the cheap and easy-to-obtain manganese raw materials are beneficial to reduce the cost of red phosphors. As a consequence, many researchers have focused on developing the Mn⁴⁺ activated red phosphors, especially, Mn⁴⁺ doped oxides and fluorides, which can be prepared under milder conditions and are cost-effective for commercial applications. Through appropriately modifying the crystal-field environments, the spectral position of Mn^{4+ 2}E_g → ⁴A_2g transition can be easily tuned over a wide range from 620 nm to 723 nm.

In this feature article, we try to make a general survey on the recent progresses of Mn⁴⁺ doped red phosphors for warm WLEDs. This differs from previously published mini-reviews written by M. G. Brik et al., reporting the spectroscopic features of Mn⁴⁺ ion from the perspective of theoretical analysis.^18,20 Firstly, we discussed the optical parameters of Mn⁴⁺ activators in the octahedral crystal field of solids, followed by the introduction of both physical and chemical synthesis strategies for Mn⁴⁺ red phosphors. Then the optical performance of Mn⁴⁺ doped oxide and fluoride red phosphors and potential application in warm WLEDs were summarized. Finally, the related challenges and perspectives will be pointed out.

Mn⁴⁺ ions in solids

The understanding of electronic and vibronic energy-level structures of Mn⁴⁺ in solids is essential for the design of Mn⁴⁺-based red phosphors. There are 120 possibilities to distribute the 3d³ electrons of Mn⁴⁺ in the five d-orbitals and the electrostatic interactions among these electrons result in eight LS-coupling free-ion terms, i.e., ⁴F ground state and ⁴P, ²G, ²P, ²D(1), ²F, ²D(2), ²H excited states. When Mn⁴⁺ ions are doped into the crystal lattice, these LS-coupling terms will be split into several energy levels, which highly depend on the local site symmetry of Mn⁴⁺. For example, detailed energy levels of Mn⁴⁺ at site symmetries of O_h and D_3d were deduced and plotted in Fig. 1a.^4,18 After considering the spin–orbit interaction, it can be seen that all these energy levels can further split into several sublevels. Typically, both ⁴A₂ ground state and ²E emitting-state split into two sublevels, and emissions from lower and upper sublevels of ²E were named as R₁ and R₂ zero phonon lines (ZPLs), respectively.

Fig. 1 (a) Energy level splitting of Mn⁴⁺ ions at the crystallographic site of D_3h symmetry with the effect of spin–orbit interaction. (b) Tanabe–Sugano diagram for the d³ electron configuration in the octahedral crystal field; inset schematically shows Mn⁴⁺ ion locating in octahedral site of host. (c) RT PL (λ_ex = 460 nm) and PLE (λ_em = 630 nm) spectra of K₂SiF₆:Mn⁴⁺ phosphor, the calculated ZPL and vibration sidebands are also provided. (d) Low-temperature (3–30 K) high-resolution PL spectra of K₂TiF₆:Mn⁴⁺ phosphor, inset is the enlarged R₁ ZPL at 3 K (reproduced from ref. 4 and 22 with permission from Nature Publishing and the Electrochemical Society).

Generally, Mn⁴⁺ emitting center prefers to stay in the octahedral or modified octahedral site of host owing to the strong ligand-field stabilization energy of Mn⁴⁺ in the 6-fold coordination. In the ideal octahedral site, the dependence of Mn⁴⁺ energy levels on crystal-field (CF) strength can be well illustrated by Tanabe–Sugano (T–S) energy diagram (Fig. 1b).²¹ Notably, the energies of most multiplets are strongly related to the CF strength except ²T₁ and ²E states. Room temperature (RT) photoluminescence (PL) and PL excitation (PLE) spectra of Mn⁴⁺ are well known, as shown in Fig. 1c.^22,23 Two intense broad excitation/absorption bands, assigned to spin-allowed ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions of Mn⁴⁺, are observed. Upon excitation into ⁴T₁ and ⁴T₂ states, electrons in these intermediate states will non-radiatively relax to ²E one, from which sharp emission lines, assigned to spin-forbidden ²E → ⁴A₂ transition, are detected.

In addition, it is found that the two excitation bands recorded at RT consists of a number of components with a spacing of a few hundred kaysers. Such phonon replica-like structure can be well understood as a vibronic progression of the fundamental frequency combined with an unsymmetrical vibration of the octahedron of host superimposed on the electronic transition. The intensity of the nth order vibration sideband, I^ex_n, is related to that of the ZPL, I^ex₀, by the expression^23,24

where n̄ is the average phonon (local vibration) number. The vertical bars in Fig. 1c are the estimated results using eqn (1) with n̄ = 4 and 7 for ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions, respectively. This analysis can determine the ZPL (n = 0) energy. The ²E_g →⁴A₂ energy separation is close to the ²G → ⁴F energy interval of the free ion and the energies of the free ion electrostatic terms are determined by the Racah parameters B and C. Low temperature (3–30 K) high-resolution PL spectra (Fig. 1d) clearly evidence the dominant phonon-coupled vibronic transitions, i.e., phonon sidebands of Mn⁴⁺ in solid.⁴ As theoretical predicted in Fig. 1a, R₁ and R₂ ZPLs of the ²E → ⁴A₂ transition are observed at 30 K, and the luminescence of R₂ line (hot band) gradually weakens with decrease of temperature and finally disappears at 3 K owing to the thermal deexcitation from upper sublevel of Mn⁴⁺:²E.

Table 1 lists the values of the related parameters (n̄, ZPL, E_²E→⁴A2 and D_q) coming from different fluoride phosphors. The outer 3d → 3d transition of Mn⁴⁺ is sensitive to local crystal-field environments in the host and can be tuned by various substitutions. Therefore, the emission of Mn⁴⁺ could cover the entire red light region in solid. The local crystal-field strength D_q can be determined by the mean peak energy of the ⁴A₂ → ⁴T₂ transition according to the following equation^4,9,21

Table 1 The spectroscopic parameters of Mn⁴⁺ in different fluoride phosphors. n̄ represents phonon number, ZPL is the zero-phonon line, E_²E→⁴A2 is the energy of the Mn^{4+ 2}E level, and D_q is the crystal field strength

Host	Crystal structure	n̄		ZPL (eV)		E^2E→^4A2 (eV)	Dq (cm^−1)	Ref.
c-K2MnF6	Cubic	3	10	2.57	2.90	1.99	2070	25
K2SiF6	Cubic	4	7	2.52	3.09	2.00	2030	26
KNaSiF6	Orthorhombic	3	9	2.52	2.93	2.00	2030	27
Na2SiF6	Trigonal	4	9	2.44	2.90	2.01	1970	28
K2GeF6	Trigonal	4	11	2.44	2.80	1.99	1970	29
BaSiF6	Trigonal	4	9	2.43	2.86	1.99	1960	30
K2SnF6	Orthorhombic	4	9	2.42	2.87	2.0	1950	31
BaTiF6	Trigonal	4	9	2.42	2.84	1.99	1950	32
Na2SnF6	Tetragonal	4	9	2.39	2.38	2.00	1930	33
Cs2SnF6	Trigonal	4	10	2.83	2.76	1.99	1920	33
h-KMnF6	Hexagonal	6	13	2.37	2.67	2.00	1910	23

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On the basis of the peak energy difference between the ⁴A₂ → ⁴T₂ and ⁴A₂ → ⁴T₁ transitions, the Racah parameter B can be evaluated from the expression

where the parameter x is defined as

According to the peak energy of ²E_g → ⁴A_2g derived from the emission spectrum, the Racah parameter C can be calculated by the expression

E(²E_g → ⁴A_2g)/B = 3.05C/B + 7.9 − 1.8B/D_q (5)

The thermal stability is important in ensuring a high efficiency of phosphor-converted devices as phosphors usually suffer from the impact of aggregated heat emitted from LED chip. Higher thermal stability generally improves the lifetime of WLED. Fig. 2a depicts the typical temperature-dependent PL spectra. The decrease in emission intensity with elevation of temperature can be explained by thermal quenching at the configurational coordinate diagram (Fig. 2b). The thermal quenching activation energy is usually expressed by the following equation^34,35

where I₀ and I_T are the initial emission intensity and the luminescent intensity at temperature T, respectively, c is a constant, E_a is the quenching activation energy and k is Boltzmann constant. The experimental data can be well-fitted by eqn (6), as demonstrated in Fig. 2b. The value of E_a reflects the thermal stability of phosphors, and the higher the quenching activator energy, the better the thermal stability.

Fig. 2 (a) Typical temperature-dependent emission spectra of Y₃Al₅O₁₂:Mn⁴⁺ red phosphor. (b) Integrated red PL intensity of Mn⁴⁺ as a function of temperature. Solid line represents the fitting result. Inset is the configurational coordinate diagram of Mn⁴⁺ in YAG host, showing the possible thermal quenching process (reproduced from ref. 21 with permission from Royal Society of Chemistry).

Synthetic strategy

Currently, the solid-state reaction method and wet chemical route were widely adopted to synthesize the Mn⁴⁺ red emitting phosphors.^32,34,36,37 Mn⁴⁺ activated oxide red phosphors were usually prepared by solid-state reaction and the sintered temperature maintained above 1000 °C. The starting materials of solid-state reaction were powders. The powders were weighed according to the desired ratio and mixed with an appropriate amount of ethanol in an alumina mortar with a pestle. After drying, the powder mixture was pressed into a block, which was placed on an alumina plate and heated at specific temperature in air using an electric furnace. Mn⁴⁺ activated fluoride red phosphors were usually prepared through a convenient chemical route. In general, KMnO₄ was firstly added to HF solution to synthesis K₂MnF₆ precursor. Then, K₂MnF₆ and adding SiO₂ reacted in HF solution to gain K₂SiF₆:Mn⁴⁺ phosphors. The reactants, such as KMnO₄ and SiO₂, could be replaced for synthesizing diverse fluoride phosphors.

The synthesis process of solid-state reaction is simple and convenient and could produce in enormous quantities, but suffers from the compositional uncertainty of phosphors and the existence of impurity phases due to partial evaporation of some components during high temperature reaction. In addition, the particle size is usually large and inhomogeneous. Wet chemical method provides an alternative approach to synthesis Mn⁴⁺ red phosphors. It has several advantages over solid-state reaction: lower processing temperature, easier composition control, and better chemical homogeneity of the product. However, the synthetic approach may be not practical for the low-cost mass production and has problems in controlling the valence states of manganese in the host. Moreover, surface inorganic/organic defects of phosphors could result in low luminescence efficiency.

Fig. 3a exhibits the most common chemical method to prepare K₂SiF₆:Mn⁴⁺. Pure SiO₂ powders were dissolved in HF at room temperature for 2 hours to form the H₂SiF₆ solution. A stoichiometric amount of KMnO₄ was dissolved in the H₂SiF₆ solution. After the color of the solution changed from colorless to a deep purple, H₂O₂ was added drop-by-drop, and subsequently the yellow precipitates of K₂SiF₆:Mn⁴⁺ were achieved. After completion of the reaction, the powders were filtered and dried at 80 °C in the oven.^38–40 The overall chemical reaction of K₂SiF₆:Mn⁴⁺ can be described as^22,41,42

SiO₂ + 4KMnO₄ + 12HF → K₂SiF₆ + K₂MnF₆ + 3MnO₂ + 6H₂O + 3O₂

Fig. 3 (a) Schematic diagram of the routine wet chemical synthesis process of K₂SiF₆:Mn⁴⁺ phosphors. (b) Schematic illustration of the cation exchange procedure for synthesizing Mn⁴⁺-activated fluoride compounds (reproduced from ref. 4 and 38 with permission from Royal Society of Chemistry and Nature Publishing).

Notably, the main difficulty for synthesizing Mn⁴⁺ activated fluoride compounds lies in that manganese has diverse valence states and its valence states often change during synthesis, resulting in undesired by-products.

Recently, a novel cation exchange procedure (Fig. 3b) was provided to prepare highly efficient K₂TiF₆:Mn⁴⁺ red phosphor and other Mn⁴⁺ doped fluorides (K₂SiF₆:Mn⁴⁺, NaGdF₄:Mn⁴⁺, NaYF₄:Mn⁴⁺) by X. Y. Chen, et al.⁴ The preparation was carried out by simply mixing K₂TiF₆ host with a small volume of HF solution dissolved with K₂MnF₆ powders. After stirring, a muddy mixture was formed, and then heated for a period time, which afforded the final product of K₂TiF₆:Mn⁴⁺. The strategy based on cation exchange shows two distinct advantages: (1) the consumption of HF is far lower than that of cocrystallization method, being beneficial for the waste processing of hazard HF; (2) the synthetic duration is obviously shortened and the reaction process is well controllable, being crucial for mass production.

In addition, J. Wang et al. reported a green synthetic route to prepare narrow red emitting K₂SiF₆:Mn⁴⁺ phosphors without the usage of toxic and volatile HF solution.⁴³ They show that K₂SiF₆:Mn⁴⁺ can be produced in common low-toxic H₃PO₄/KHF₂ liquid instead of high-toxic HF one. Firstly, it needs to synthesize soluble manganese(IV) (MnL₂, L = HPO₄²⁻). Then, SiO₂, KHF₂ and as-prepared MnL₂ solution were thoroughly mixed, sealed up in a stainless steel autoclave and reacted at 180 °C for 6 hours to obtain K₂SiF₆:Mn⁴⁺ red phosphor.

Finally, sol–gel method, showing several advantages, such as good mixing of starting materials, relatively low reaction temperature and more homogeneous products,^44–46 is also an efficient technique for preparing Mn⁴⁺ doped red phosphor. For example, magnesium titanate Mg₂TiO₄:Mn⁴⁺ was successfully synthesized via a sol–gel route by using butyl titanate and magnesium nitrate ethanol solution as reagents.⁴⁴ Typically, a solution of butyl titanate precursor in acetic acid was added into an ethanol solution of magnesium nitrate and manganese nitrate. After stirring at a certain temperature in a water-bath, the formed yellow solution was dried and then calcined to obtain final products.

Mn⁴⁺-Activated oxide/fluoride red phosphors

Compared to the ionic nature of metal–F-bonds, the oxide hosts with strong covalence, low thermal vibration, and weak polarizability are favorable to achieve red emission.⁶⁶ Fig. 4a shows typical PLE and PL spectra of YAG:Mn⁴⁺ red phosphor. PL spectrum exhibits two emission bands centered at 646 and 673 nm owing to the Mn⁴⁺ spin-forbidden ²E → ⁴A₂ transition. PLE spectrum monitored at 673 nm emission consists of two strong excitation peaks centered at 352 and 480 nm, respectively, assigning to ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions of Mn⁴⁺.²¹ Apart from different emission/excitation peak positions, similar PL and PLE spectra can be observed for other Mn⁴⁺ doped oxide phosphors.^{19,34,36,44,47–66} Table 2 collects a summary of the spectroscopic data, including excitation/emission wavelength and quantum efficiency (QE), for the Mn⁴⁺ ions in a number of crystalline solids. In fluorides the energy of the ²E_g level exhibits a restricted variation from 626 nm to 635 nm with an average value of 15 873 cm⁻¹ (630 nm). In oxides, however, the energy of the ²E_g level varies widely from 652 nm to 713 nm with an average value of 14 858 cm⁻¹ (673 nm). In more covalent hosts, the strong nephelauxetic effect makes the levels of oxide split more seriously than fluorides. Therefore, the emission wavelengths of oxides have a wider variation range and larger value. The data tabulated in Table 2 are grouped in such a way that all fluoride matrices are listed after the oxides. It becomes immediately clear that the ²E_g level in fluorides is at considerably higher energy than in oxides. As we all know, the Mn⁴⁺ activators prefer to occupy octahedral site. In oxide host, Mn⁴⁺ substitutes the cations, such as Al³⁺, Si⁴⁺, Zr⁴⁺, Ti⁴⁺, Ge⁴⁺ and so on, in the centre of octahedron. Charge compensations are required when Mn⁴⁺ ions substitutes the cations with different valences, such as Al³⁺. Therefore the charge compensating dopants were required to improve the luminescence of phosphors. Mg²⁺ as a kind of effective charge compensator was systematically investigated. Fig. 4b exhibits the proposed mechanism of Mg²⁺ as charge compensator to enhance the phosphor luminescence. Mn⁴⁺–Mg²⁺ pairs will be formed to replace Al³⁺–Al³⁺ pairs without the requirement of charge compensation and additional oxygen ions. Mg²⁺ ions can not only act as charge compensating centers but also suppress the adverse energy migration among neighboring Mn⁴⁺ ions.^21,74,75 Ge⁴⁺, Ca²⁺, Li⁺, Na⁺, K⁺, Cl⁻ could also serve as charge compensators to enhance the luminescence of red phosphors.^2,51,58,76 Fig. 4c depicts the influence of Li⁺, Na⁺, K⁺ and Mg²⁺ codoping on the fluorescence intensities of SrAl₁₂O₁₉:Mn⁴⁺. The different charge compensators have different effects for improving Mn⁴⁺ red emission.

Fig. 4 (a) PLE and PL spectral of Y₃Al₅O₁₂:Mn⁴⁺ (YAG:Mn⁴⁺) red phosphor. Inset shows the corresponding luminescent photograph of red phosphor under UV illumination. (b) Schematic illustration of the mechanism that Mn⁴⁺–Mn⁴⁺ pairs in connection with interstitial O²⁻ are transformed into isolated Mn⁴⁺ ions with charge compensation provided by Mg²⁺ ions. (c) Emission spectra and relative intensity of SrAl₁₂O₁₇:Mn⁴⁺, M (M = Li⁺, Na⁺, K⁺, Mg²⁺) red phosphors (reproduced from ref. 61 with permission from Wiley-VCH).

Table 2 Spectroscopic parameters of Mn⁴⁺ ions in various crystals (λ_ex/λ_em is the excitation/emission wavelength, QE is the quantum efficiency)^a

Red phosphors	λex (nm)		λem (nm)	QE (%)	Ref.
a * represents internal quantum efficiency, Δ represents external quantum efficiency.
Ca14Zn6Al10O35:Mn^4+	314	465	713	—	19
MgAl2Si2O8:Mn^4+	258	358	710	—	47
Ca14Al10Zn6O35:Mn^4+	318	462	708	50.7	34
La2LiTaO6:Mn^4+	330	495	707	21.4	48
Gd2ZnTiO6:Mn^4+	378	504	705	39.7*	49
Y3Al5O12:Mn^4+	352	480	673	—	21
LiAlO2:Mn^4+	330	430	670	48*, 33^Δ	50
SrMgAl10O17:Mn^4+	320	465	660	—	51
BaMg6Ti6O19:Mn^4+	338	475	660	12*	52
BaMgAl10O17:Mn^4+	335	465	660	—	36
MgO·MgF2·GeO2:Mn^4+	330	420	659	—	53 and 54
Sr2MgAl22O36:Mn^4+	312	468	658	80	55
Mg2TiO4:Mn^4+	350	475	657	38.3*	44, 56 and 57
CaAl12O19:Mn^4+	330	465	657	63*	52 and 58
CaAl4O7:Mn^4+	335	467	656	—	59
CaMg2Al16O27:Mn^4+	390	468	655	35.6*, 16^Δ	60
SrAl12O19:Mn^4+	325	475	655	—	61
Sr4Al14O25:Mn^4+	325	450	652	—	62–66
K2TiF6:Mn^4+	362	468	635	98	4 and 67
BaGeF6:Mn^4+	360	460	634	—	68
Cs2SnF6:Mn^4+	370	470	633	—	33
K2SiF6:Mn^4+	360	460	632	74	26, 37 and 69
K2GeF6:Mn^4+	360	460	632	—	29
BaSiF6:Mn^4+	365	466	632	—	30
BaTiF6:Mn^4+	360	470	632	—	32
ZnTiF6:Mn^4+	360	465	631	26.4*, 6.2^Δ	70
K2SnF6:Mn^4+	360	470	630	40^Δ	31
KNaSiF6:Mn^4+	360	460	630	—	27
ZnSiF6:Mn^4+	356	460	629	—	71
Na2SnF6:Mn^4+	370	470	627	—	33
Na2SiF6:Mn^4+	360	460	627	—	28, 72 and 73
NaGdF4:Mn^4+	363	469	627	—	4
NaYF4:Mn^4+	360	470	626	—	4

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The A₂XF₆:Mn⁴⁺ and BXF₆:Mn⁴⁺ fluorides, where A is the alkali metal ions, B = Ba and Zn and X = Si, Ge, Zr, Sn, and Ti, are the most common reported red phosphors. RT PL, PLE and temperature-dependent PL spectra were usually used to discuss optical properties of fluoride phosphors. Fig. 5 shows RT PL spectra obtained from (a) K₂GeF₆:Mn⁴⁺, (b) K₂SiF₆:Mn⁴⁺, (c) Na₂GeF₆:Mn⁴⁺ and (d) Na₂SiF₆:Mn⁴⁺.²⁹ All these spectra were measured by exciting at λ_ex = 325 nm (He–Cd laser). No obvious difference in the PL intensities was observed among these samples. The ZPLs in Fig. 5a–d correspond to (a) 1.990, (b) 1.993, (c) 2.005, and (d) 2.017 eV, respectively. The internal vibronic frequencies ν_i (i = 1–6) of the MnF₆²⁻ octahedron in dialkali and alkali earth hexafluorometallates can be principally determined by its molecular dynamics only. Fig. 5e shows the PL spectra for K₂GeF₆:Mn⁴⁺ phosphor measured at temperatures between 20 K and 300 K with a He–Cd laser as an excitation light source.²² As previously mentioned, the sharp red emissions can be assigned to the ²E_g → ⁴A_2g transition of the 3d³ electrons in the MnF₆²⁻ octahedron. The Mn 3d³ transitions are electronic dipole forbidden but gain intensity by the activation of vibronic modes. At T = 20 K, many sharp peaks dominate on the long-wavelength side of the zero-phonon line (ZPL). At T = 300 K, these sharp peaks become very broad and appear not only on the long-wavelength side but also on the short-wavelength one. The long- and short-wavelength emission peaks are known as the Stokes and anti-Stokes lines, respectively.

Fig. 5 RT PL spectra of (a) K₂GeF₆:Mn⁴⁺, (b) K₂SiF₆:Mn⁴⁺, (c) Na₂GeF₆:Mn⁴⁺ and (d) Na₂SiF₆:Mn⁴⁺ phosphors with a He–Cd laser as the excitation light source. (e) Temperature-dependent PL spectra for K₂SiF₆:Mn⁴⁺ phosphor between 20 and 300 K (reproduced from ref. 22 and 29 with permission from the Electrochemical Society and American Institute of Physics).

Generally, Mn⁴⁺ doped oxide phosphors exhibit deep red (λ ≥ 650 nm) and broadband emissions, and their absorption in the blue wavelength region is relatively weak. Therefore, the application of oxide red phosphors is restricted in term of blue-light-excited WLED. In the research of oxide red phosphors, the quantum efficiency (QE) is rarely mentioned. Due to high temperature sintering, oxide hosts usually have a large amount of defects leading to low QE of Mn⁴⁺ luminescence. These defects could also affect the thermal stability of phosphors. On the contrary, Mn⁴⁺ doped red fluoride phosphors have strong absorption for blue excitation light. In addition, the shorter emission wavelength and narrower emission band make them more suitable as the red component applying in the warm WLED. So far, the highest PL QE up to 98% was realized for the K₂TiF₆:Mn⁴⁺ red phosphor.⁴ Notably, further research must be carried out to realize mass production of Mn⁴⁺ doped red fluoride phosphors and improve their thermal stability as well as humidity resistance.

Application in white-LED

Currently, the major commercial WLED is the phosphor converted LED made of the InGaN blue-emitting chip and the YAG:Ce³⁺ yellow phosphor dispersed in the organic epoxy resin or silicon. Due to lack of red component, the CCT and CRI are unsatisfactory. Hence, the general solution needs to add excess red phosphor to organic epoxy resin or silicon. Fig. 6 shows the PLE and PL of CaAlSiN₃:Eu²⁺ red phosphor, YAG:Ce³⁺ yellow phosphor, Mn⁴⁺ active oxide and fluoride red phosphors. Obviously, PLE spectra of nitride red phosphors and PL of YAG:Ce³⁺ yellow phosphor have a large degree of overlap, which leads to serious reabsorption and reduces luminous efficiency of WLED. In comparison, this reabsorption effect is greatly lessened between YAG:Ce³⁺ yellow phosphor and Mn⁴⁺ doped oxide/fluoride red phosphors, which ensures their possible application in warm WLED as red component. Fig. 7a shows the WLEDs based on the combination of a UV LED chip with the mixture of blue (BaMgAl₁₀O₁₇:Eu²⁺), green (Sr₂SiO₄:Eu²⁺) and red GdZnTiO₆:Mn⁴⁺ phosphors.⁴⁹ By modifying the RGB ratio, the white light was found evolving from cool to warm with a tunable CCT from 6977 K to 4742 K, a CRI up to 82.9 and an improved R9 value to 43, verifying GdZnTiO₆:Mn⁴⁺ a promising red component for UV-based WLED. Fig. 7b exhibits the WLEDs consisting of a blue chip (455 nm), YAG:Ce³⁺ yellow (or aluminate green) and K₂TiF₆:Mn⁴⁺ red phosphors. Photographs of three typical lighted WLEDs are shown in Fig. 7c–e. Benefited from the incremental red light component in the electroluminescence spectrum of LED (owing to the increase of K₂TiF₆:Mn⁴⁺ content), a significantly warmer tone of the emitting white light was found. The color coordinates of these WLEDs with CCTs of 5954, 3556 and 2783 K, (0.322, 0.342), (0.400, 0.382) and (0.457, 0.416), are marked in CIE 1931 color spaces, respectively, and all three color points are close to the black body locus (inset of Fig. 7e). Table 3 presents the important photoelectric parameters for WLEDs by using several typical Mn⁴⁺ doped red phosphors, which clearly demonstrates the ability of these red phosphors to improve CCT and CRI of WLEDs.

Fig. 6 PLE and PL of Eu²⁺ doped CaAlSiN₃:Eu²⁺ red phosphors, YAG:Ce³⁺ yellow phosphor, Mn⁴⁺ activated oxide and fluoride red phosphors.

Fig. 7 (a) Photographs of lighted WLEDs fabricated by coupling the 365 nm UV chip with the mixture of blue (BaMgAl₁₀O₁₇:Eu²⁺), green (Sr₂SiO₄:Eu²⁺) and red GdZnTiO₆:Mn⁴⁺ phosphors and the corresponding chromaticity coordinations. (b) Photographs of WLEDs consisting of blue chip (455 nm), YAG:Ce³⁺ yellow (or aluminate green) and K₂TiF₆:Mn⁴⁺ red phosphors. (c–e) Lighted WLEDs exhibit CCTs of (c) 5954, (d) 3556 and (e) 2783 K as the content of K₂TiF₆:Mn⁴⁺ red phosphor increases. Inset of (e) shows the corresponding chromaticity coordinations of three typical WLEDs (reproduced from ref. 4 and 49 with permission from Royal Society of Chemistry and Nature Publishing).

Table 3 Important photoelectric parameters for WLEDs with different red phosphors

Red phosphor blend	LE (lm W^−1)	CCT (K)	CRI	Chromaticity coordinate	Ref.
—	45.21	6283	76	—	1 and 68
BaMgAl10O17:Mn^4+	55.1	3622	96	(0.43, 0.45)	36
Ca14Al10Zn6O35:Mn^4+	—	4130	—	(0.39, 0.44)	19
La2LiTaO6:Mn^4+	150	5500	72	(0.29, 0.32)	48
Mg2TiO4:Mn^4+	—	—	—	(0.41, 0.56)	56
CaMg2Al16O27:Mn^4+	58.3	3896	85.5	(0.40, 0.42)	60
Sr4Al14O25:Mn^4+	35	5406	93.23	(0.33, 0.36)	62
Na2SiF6:Mn^4+	77.6	6875	86	(0.31, 0.30)	28
K2SiF6:Mn^4+	116	3900	89.9	(0.38, 0.33)	39
K2TiF6:Mn^4+	116	3556	81	(0.46, 0.41)	4
BaGeF6:Mn^4+	52.12	4210	84	—	68
ZnTiF6:Mn^4+	92.21	3987	83.1	(0.39, 0.40)	70
K2GeF6:Mn^4+	—	3974	89	(0.40, 0.45)	77

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Furthermore, the easy aging of the organic binder ascribing to the accumulated heat emitting from the InGaN chip will reduce the long-term reliability and actual lifetime of WLEDs. Inorganic glass ceramic (GC), a kind of composite with YAG:Ce³⁺ micro-crystals distributing among glass matrix, exhibits excellent thermal resistance and easy formability, and may simultaneously play the key roles of luminescent convertor and encapsulating material as the traditional phosphor powder and organic resin respectively used in WLEDs.^{2,21,60,78,79} Recently, YAG:Mn⁴⁺ embedded inorganic glass ceramic was successfully fabricated to replace phosphor in organic silicone as the color converter, and a stacking geometric configuration by sequentially coupling a YAG:Ce³⁺ glass ceramic and a YAG:Mn⁴⁺ glass ceramic with an InGaN blue chip was designed to explore its possible application in warm white light-emitting diodes, as shown in Fig. 8a.²¹ As YAG:Mn⁴⁺ content increases in GC sample, red luminescence enhances gradually, leading to the decrease of CCT from 5707 K to 4671 K (Fig. 8b). On the other hand, dual-phase (such as YAG:Ce³⁺ yellow and BaMgAl₁₀O₁₇ (BMA):Mn⁴⁺ red phosphors) embedded GCs were designed by Y. S. Wang et al. and then coupled with InGaN blue chips, aiming to achieve warm white light, as shown Fig. 8c and d.³⁶ Scanning electron microscope (SEM) observations and energy-dispersive X-ray spectrometer (EDS) analyses clearly distinguish two kinds of embedded crystallites from their different contrasts (Fig. 8c). The constructed GC-based remote-type WLED is superior to the conventional conformal one because of the lower heat accumulation experienced by the phosphors as well as the reduced glare to human eyes (Fig. 8d). By simply modifying the weight ratio of BMA:Mn⁴⁺ red phosphor, the appearance of the lighted WLED (Fig. 8e) evolves from cold white to natural white and, finally, to warm white. As a consequence, the CIE chromaticity coordinates shift from (0.339, 0.349) to (0.427, 0.448), the LE decreases from 99.2 to 55.1 lm W⁻¹, the CCT decreases from 6608 to 3622 K, and the CRI increases from 68.4 to 86.0, meeting the requirements for indoor lighting applications. Similar results were also reported in YAG:Ce³⁺ and CaMg₂Al₁₆O₂₇:Mn⁴⁺ dual-phase GC-based WLEDs.⁶⁰

Fig. 8 (a) Photograph of the WLED device constructed by coupling YAG:Ce³⁺ GC and YAG:Mn⁴⁺ GC stacking structure with blue chips and (b) electroluminescence spectra of GC-based WLEDs with increase of YAG:Mn⁴⁺ content. (c) SEM observations and EDS analyses of GC embedded with BMA:Mn⁴⁺ red and YAG:Ce³⁺ yellow dual-phase phosphors. (d) Schematic illustration of the constructed remote-type WLED with dual-phase embedded GC as color converter and (e) the corresponding electroluminescent spectra of WLEDs with increase of BMA:Mn⁴⁺ content. Insets of (e) show luminescent photographs of the WLEDs in operation under 350 mA current (reproduced from ref. 21 and 36 with permission from Royal Society of Chemistry and American Chemical Society).

Challenges and perspectives

So far, Mn⁴⁺ activated oxide and fluoride red phosphors have drawn more and more attention due to their special advantages, such as relatively simple preparation route and low-cost. Depending on the host, the spectral position of the ²E_g → ⁴A_2g transition can be modified over a wide range and the absorption/excitation bands locate in the UV-blue wavelength region. Hence, these Mn⁴⁺ doped materials, being an alternative to traditional rare earth doped red phosphors, show greatly potential applications in warm WLEDs. Despite the fact that a large amount of Mn⁴⁺ doped red phosphors have been successfully developed, there are a number of research issues that are particularly interesting, but require concerted effort for success. For Mn⁴⁺ doped oxide phosphors, blue shifting of the emission wavelength from deep-red to orange/red and enhanced absorption/excitation in blue wavelength region are highly desired. One of the possible solutions is to reduce the covalency of oxide matrix through impurity doping. For Mn⁴⁺ doped fluoride phosphors, more concerns should be focused on thermal stability as well as humidity resistance. It is expected that surface controlling by coating or designing core@shell structure is an effective strategy to improve their thermal stability and humidity resistance.⁸⁰ Finally, the quantum efficiency should be further improved by optimizing synthesis procedure and decreasing matrix defects.

Acknowledgements

This work was supported by the Natural Science Foundation of Zhejiang Province for Distinguished Young Scholars (LR15E020001), National Natural Science Foundation of China (21271170, 61372025, 51402077 and 51572065) and the 151 talent's projects in the second level of Zhejiang Province.

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