Rare earth (RE) ions activated inorganic phosphors with multi-color emissions have received considerable attention because of their emerging applications in solid-state lighting, backlight displays, lasers, biomedical imaging, and so on. This tutorial review concerns the design principles for Eu2+ and Eu3+ activated red-emitting inorganic solids and highlights the influencing factors on the luminescence performance. Based on the recent advances in structural design of inorganic RE phosphors, we proposed several design principles for achieving red emission in Eu2+/Eu3+ ions doped solid-state materials. On the one hand, for the realization of red emission from Eu2+ ion, the used strategies include the following: (1) designed synthesis of new Eu2+-doped nitride or sulfide phosphors, (2) anionic substitution achieves large centroid shift, (3) Eu2+ occupies polyhedrons with small coordination numbers to obtain large ɛcfs, (4) doping concentration controls the distribution of Eu2+ ions, (5) mixed ligands induce large ΔS, and (6) doping Eu2+ in nitrides with UCr4C4-type structure to achieve narrow-band red emission. On the other hand, for the red emission originating from a Eu3+ ion, the design principles are listed as follows: (i) designed synthesis of Eu3+-doped phosphors with small CT energy, (ii) realization of low excitation energy by doping sensitizer ions, (iii) Eu3+ luminescence enhancement by charge compensation, and (iv) occupation of unsymmetrical sites to maintain high color purity of Eu3+. Finally, we discuss and look at the future opportunities for Eu2+/Eu3+ activated red phosphors.
Doping metal ions with luminescent behaviors into inorganic solids has been regarded as an effective as well as significant method for the design and discovery of new luminescent materials, and, therefore, doped inorganic phosphors contain two parts: host lattice and luminescent center.1,2 Normally, small amounts of rare earth (RE) ions or transition metal ions can act as the doped luminescent centers, and the host lattice provides certain crystallographic sites for the doped activators. Among all types of phosphors, RE ion doped phosphors have attracted much interest due to their abundant energy levels and tremendous allowed energy level transitions. RE elements are comprised of 17 chemical elements including the 15 lanthanides (Ln, from lanthanum to lutetium), scandium, and yttrium.3 According to the transition characteristics, the luminescent mechanism of Ln doped phosphors can be classified into two types: f–d transition with broad band emissions and f–f transition with sharp line emission.4 In fact, direct excitation of Ln3+ ion is a relatively inefficient process due to the parity forbidden character of the f–f transitions.5 Nevertheless, some host materials (such as and based materials) or other co-doping ions with a higher absorption coefficient than Ln3+ emitters can effectively transfer the absorbed energy to Ln3+ ions and thus enhance their luminescence.6 In contrast to the parity forbidden f–f transitions, the 4fn–4fn−15d1 electrical–dipole allowed transitions possess high emissive probability and short decay times. Moreover, unlike the fixed sharp emission line within f–f type transition, the f–d transition is sensitive to the local crystal field and thus features much broader absorption and emission spectra.7
Enlightened by the factors described above, RE-doped phosphors have been applied in different optoelectronic devices.8–10 The commonly used RE ions in phosphors are Eu2+/Eu3+, Ce3+, Tb3+, Gd3+, Yb3+, Dy3+, Sm3+, Tm3+, Er3+, Pr3+, and Nd3+. Particularly, Eu possesses both divalent and trivalent states, which can realize f–d transition (Eu2+) or f–f (Eu3+) transition in different hosts, respectively.4 As for the 4f–5d transition of Eu2+ ions, the electronics of 5d levels are exposed to the surface of electronic shell; thus, the excitation and emission spectra are strongly affected by the surrounding environment of Eu2+ ions, such as crystal symmetry, atom coordination, and crystal field strength.11 As for the f–f transition of Eu3+ ions, the 4f orbital is located in the interior of the ion and is shielded from the surrounding environment by the filled 5s2 and 5p6 orbitals.12 Therefore, the host lattice has a small (but important) influence on the optical transitions within the 4fn configuration.1 Thus, the emission spectra of Eu3+/Eu2+ doped phosphor can be designed by modifying the chemical composition of the host or controlling local structures around activators (Fig. 1).13 Consequently, Eu2+ and Eu3+ doped phosphors have become a research hotspot and have been widely used in white LED lighting (WLED), backlight displays, plant lighting, biomedical detection regions, and so on.14–17
Thanks to the rapid development of LED chips with high efficiency and low cost, phosphor converted white LEDs (PC-WLED) based on blue InGaN chips have become the state-of-art lighting devices and prominent backlight source for liquid crystal displays (LCDs). On the one hand, commercial WLEDs are manufactured by combining a blue LED chip with yellow emitting Ce-doped yttrium aluminum garnet (YAG:Ce) phosphor.18,19 However, the lack of a red component leads to a cool white light emission with a high correlated color temperature (CCT > 4000 K) and low color rendering index (CRI < 75) [Fig. 2(a)].20 High-quality warm-white light demands a color rendering index of more than 80 and a low correlated color temperature of 2700–4000 K.21 To achieve this goal, two modified ways could be adopted: one is to add red phosphors into blue chip pumped YAG phosphor systems; the other is to use a blue LED pumped with mixtures of green and red phosphors, as shown in Figs. 2(b) and 2(c).20 Obviously, either way, it is crucial to develop highly efficient red phosphors. On the other hand, the most commercial PC-WLED backlight for LCDs is fabricated by a combination of blue InGaN chip with green-emitting phosphor (β-SiAlON:Eu2+) and narrow-band red-emitting phosphor (K2SiF6:Mn4+) [Fig. 2(d)].15,22,23 However, the unstable chemical property and high cost of K2SiF6:Mn4+ restrict its popular applications, and massive evaporation of HF gas during the sample preparation process pollutes the environment. Hence, a novel red phosphor with narrow-band emission is also urgently required.
In the last two decades, efforts have been made by researchers to explore red phosphors that can realize high-quality white light illumination or broaden color-gamut of LCDs. Among them, Eu2+/Eu3+-doped red phosphors have been widely investigated. The previous warm WLED was fabricated by packaging CaS:Eu2+ phosphor as the red component.24–26 These Eu2+ doped sulfide phosphors have attractive luminescent properties, but also show serious drawbacks, such as low thermal stability and poor chemical instability. Alternatively, Eu3+ doped red phosphors including Y2O3:Eu3+ and Y2O2S:Eu3+ have been extensively used in lighting and television screens due to its narrow-band red emission around 610 nm.27–29 Nevertheless, the weak absorption in the blue light region leads to a low external quantum efficiency and further limits their application. Recently, Eu2+ doped nitride phosphors, such as Sr2Si5N8:Eu2+, Ca2Si5N8:Eu2+, and CaAlSiN3:Eu2+, have been commercialized in warm WLEDs.30–32 Theses nitrides can be effectively excited by blue LEDs and show intense red emission due to the large polarizability of N3− ligand and strong covalent of Eu–N bonds. Unfortunately, such warm LED devices have lower luminous efficiencies because of the broad emission band that extends to the deep red region beyond the human eye's long-wavelength sensitivity limit (∼700 nm). Moreover, their broad excitation spectra always overlay with the emission spectra of YAG, which induces cascade excitation and decreases the efficiency of WLEDs.33 Though some nitride phosphors such as Sr(LiAl3N4):Eu2+ possess high luminescence efficiency and narrow-band emission with a full-width at half-maximum (FWHM) of 50 nm, the cascade excitation and poor chemical stability hinder its commercialization.34 It is, therefore, necessary to discover new Eu2+/Eu3+-doped red phosphors to meet the needs of high-quality lighting and display applications.
Indeed, many researchers have recently focused their attention on the design principles to tune or improve the luminescent properties of phosphors.35 For example, Liu's group adopted some useful ways, such as “Cation-Size-Mismatch effect” and “Neighboring-Cation Substitution effect,” to modify luminescent properties of the existed phosphors.36–38 Kalaji et al. found that the distortion of the local crystal structure leads to a large redshift in Ca2SiO4:Ce.39 In addition, Ye et al. reviewed the adjustment principles of excitation and emission spectra based on the crystal field theory.40 Lin's group summarized the current methods of achieving spectral control from the perspective of local crystal structure changes.20 Besides, our group proposed the mineral-inspired phosphor discovery strategy, which expounds the design principles of developing new phosphors based on the existed structural models.7 Furthermore, our group recently reviewed the structural strategies on manipulation of Eu2+ emission in silicates.8 However, few papers have summarized the design principles of Eu2+/Eu3+-activated red phosphors. To fill in this blank, herein we will summarize the design principles for Eu2+/Eu3+-doped red-emitting phosphors based on the dominant factors that contribute to the luminescent properties of Eu2+/Eu3+ ions. Moreover, the present challenges in red phosphors are highlighted, together with the prediction of future development trend.
II. ELECTRONIC TRANSITIONS OF Eu2+ AND Eu3+ IONS
The luminescence of the Eu2+ ion comes from the parity-allowed electronic transitions from the 4f65d1 excited state to the 4f7 ground state. In general, Eu2+-based phosphors feature broad absorption and emission spectra owing to the high sensitivity of 5d orbital to the surrounding environment. For a free Eu2+ ion, there is a large energy gap (∼4.2 eV, 34 000 cm−1) between the lowest 5d excited state and the 4f ground state.7 Therefore, no visible emission could be observed for the free Eu2+ ion. When the Eu2+ ion is doped in a given crystalline material, the 5d energy level will be strongly affected by the crystal lattice environment, such as covalence, polarizability, crystal field strength, bond length, atom coordination numbers (CNs), and crystal symmetry.7 As a result, the 5d energy level is lowered and presented as redshift in emission spectrum including the centroid shift ɛc, crystal field splitting (ɛcfs), and Stokes shift (ΔS) [Fig. 3(a)].3 The emission energy of Eu2+-activated phosphors can be written as Eem = Efree − ɛc − ɛcfs − ΔS.41 Therefore, different emission spectra of Eu2+ ions varying from near ultraviolet (NUV) to near infrared (NIR) could be achieved by controlling the environment surrounding Eu2+ ions.
In contrast to the Eu2+ ion, the crystal field has a small effect on the emission of the Eu3+ ion because the partially filled 4f shell is shielded the from 5s25p6 orbital.42 Thus, Eu3+-based phosphors are famous for their commonly line-shaped red emission originated from the forbidden 5D0–7FJ transitions (J = 0, 1, 2, 3, 4), as shown in Fig. 3(b).43,44 Actually, other high energy emissions were also observed in some Eu3+-doped materials, such as the transitions from 5D1 (green), 5D2 (green, blue), and 5D3 (blue) level to 7FJ levels.1 However, these transitions are often neglected because the higher 5D1,2,3 emissions are quenched by cross relaxation (induced by short Eu3+–Eu3+ distance) when the doping concentration is high. In addition to the characteristic sharp excitation line originated from the transition of 7FJ → 5D1,2,3, Eu3+ ion doping compounds, especially Eu3+-doped nitrides or oxides, also possess a broad charge transfer band (CTB) formed by the electronic transition from the 2p orbital of O2− or N3− to the 4f orbital of the Eu3+ ion.45,46 Unlike the weak absorption of f–f transitions, the broad CTB makes commercialization become possible. For example, as one of the most promising red phosphors, Y2O3:Eu3+ has been widely applied to field emission display applications.28
III. DESIGN PRINCIPLES OF Eu2+-DOPED RED PHOSPHOR
As mentioned above, the emission energy of the Eu2+ ion is mainly determined by three elements: centroid shift ɛc, crystal field splitting ɛcfs and Stokes shift ΔS. Therefore, realizing larger ɛc, ɛcfs, or ΔS values becomes the key factor to design new Eu2+-doped red phosphor. Some proposed design principles are summarized as follows.
A. Synthesis of new Eu2+-doped nitride or sulfide phosphors
The relatively short distance and strong covalency of Eu–N and Eu–S chemical bonds in nitride or sulfide solids result in strong crystal field strength and large nephelauxetic effect, then yielding large redshifts in both emission and excitation spectra. Thus, research studies on red-emitting phosphors previously focused on the Eu2+-doped nitride or sulfide phosphors, and many commercial red phosphors such as CaS:Eu2+, SrS:Eu2+, Sr2Si5N8:Eu2+, and CaAlSiN3:Eu2+ have been successfully synthesized.24,26,31,32 As the representatives, SrS:Eu2+ and Sr2Si5N8:Eu2+ can be effectively excited by blue light and exhibit extensive red emission peaking at 615 and 618 nm, respectively, as shown in Figs. 4(a) and 4(b). However, nitride or sulfide phosphors also have some serious drawbacks. Normally, sulfide phosphors are chemically unstable in the atmosphere due to their deliquescent behaviors. Therefore, it should continue to explore the emerging sulfide phosphors with new crystal structures and promising luminescent properties. Moreover, phosphor coating technology is urgently needed to improve the chemical stability of sulfide phosphors. With regard to nitride phosphors, they are usually synthesized under tough conditions such as high temperature and high pressure, which causes the energy loss. In addition, the particle size, morphology, and crystallinity of nitride phosphors are out of control. Thus, the discovery of novel Eu2+-doped nitride and sulfide red phosphors is still an endless mission for scientists.
B. Anionic substitution achieves large centroid shift
For inorganic phosphors, the nearest neighbor anions around Eu2+ ions have a great effect on the 5d levels. This effect was described by centroid shift ɛc, which is related to the nephelauxetic effect. The nephelauxetic effect is directly linked to the covalency between Eu2+ ions and anion ligands in the host lattice.47 For an isomorphous structure, the shorter ionic radius and the higher negative charge will lead to the stronger covalency, thus causing a larger centroid shift. The nephelauxetic effect for different anion tends to decrease in the sequence of Se2− > S2− > N3− > Cl− > O2− > F−. Dorenbos surveyed the luminescence of Eu2+ in a variety of inorganic compounds and demonstrated that the ɛc values tend to follow the nephelauxetic effect sequence as fluorides < chlorides < bromides < iodides < oxides < nitrides < sulfides (Fig. 5).48 Normally, with the strengthening of nephelauxetic effect or covalent strength, both the bonding and antibonding orbitals will move to lower energies, leading to redshift emission.
Considering the relatively large centroid shift and highly chemical stability of Eu2+-doped nitride phosphors, N3− anion was often used as a promising candidate for the anion substitutions to improve the luminescence properties of oxide phosphors. Take Sr2SiO4:Eu2+ for example, the emission peaks shifted from 563 to 583 nm through partial co-substitution of [Sr2+–O2−] with [Lu3+–N3−] unit [Fig. 6(a)].49 The reason is that N3− has less electronegative and stronger polarization than O2−, which increases the strength of the covalent bond. By singly substituting O2− with N3−, Zhao et al. successfully synthesized red-emitting phosphor Sr2Si(N,O)4:Eu2+.50 Compared with the emission peak of Sr2SiO4:Eu2+ at 560 nm, the maximum emission peak at 617 nm in Sr2Si(N,O)4:Eu2+ phosphor is attributed to the formation of strong Eu2+–N3− covalent bond [Fig. 6(b)]. Thus, introducing N3− into Eu2+-doped oxide phosphors can be served as a strategy to adjust centroid shift and further realizes red emission.
C. Eu2+ occupies polyhedrons with small coordination numbers to obtain large ɛcfs
For the free Eu2+ ion, the five 5d-orbitals have the same energy. When the Eu2+ ion is doped into a crystalline lattice, the 5d-orbitals of Eu2+ will be affected by the crystal field and the degenerate 5d levels will be split into groups with different energies.51 The degree of crystal field splitting can be described by ɛcfs, which is generally related to the type, size, and coordination numbers (CNs) of polyhedrons occupied by Eu2+. The crystal field splitting appears to behave as .52 Herein, is a constant depends on the type of polyhedron and Rav is related to the individual bond length and ionic radius of cations occupied by Eu2+ ion. The shorter bond length, smaller CNs, and higher distortion index of polyhedron will lead to stronger crystal field splitting and longer emission wavelength. Thus, red emission can be realized by adjusting crystal field splitting ɛcfs.
Typically, the CNs of transition metal elements and main-group elements are about 2–6, while the CNs of rare earth ions in inorganic hosts are greater than or equal to 6. If we can manipulate sites occupation of Eu2+ ion in a host with small CNs, red or even near infrared (NIR) emission will be realized. Based on this assumption, our group has established a new design principle on the discovery of red phosphors, i.e., site-selective occupation of Eu2+ at polyhedrons with small CNs to achieve blue light-excited red/NIR emission. For example, K3YSi2O7 unit cell contains five different polyhedrons K1O6, K2O8, K3O9, Y1O6, and Y2O6, and Eu2+ will selectively occupy both the highly distorted K2O8 polyhedrons and Y2O6 polyhedrons with small CNs.53 Consequently, a large crystal field splitting in 5d energy levels is realized and the orange–red-emitting for K3YSi2O7:Eu phosphor could be observed [Fig. 7(a)]. In addition, Rb3YSi2O7 crystal belongs to the P63/mcm space group and contains three different polyhedrons. The selective occupation of Eu2+ in YO6 and Rb2O6 polyhedrons with small CN results in large crystal field splitting, and further leads to the broad red emission under blue light excitation, as shown in Fig. 7(b).54 Moreover, we also successfully designed and synthesized the abnormal Eu2+-doped NIR phosphor K3LuSi2O7:Eu2+ by substituting the large Rb+ and Y3+ ions in Rb3YSi2O7:Eu2+ with small K+ and Lu3+ ions. This substitution further increases the crystal field splitting effect, and the first Eu2+-doped NIR phosphor that gives a broad band NIR emission (600–900 nm) under 460 nm blue light excitation was obtained [Fig. 7(c)].55 The small CNs’ principles have been used to design other red/NIR-emitting phosphors. For example, Li et al. successfully achieved NIR emission in Bi3+-activated XAl12O19 (X = Ba, Sr, Ca) phosphor by constructing the selective site occupation of Bi3+ in AlO4 polyhedrons.56 Therefore, occupying polyhedrons with small CNs can be employed as an effective design principle for the discovery of red/NIR phosphor. Recently discovered Sr3LnAl2O7.5 (Ln = Y or Lu) and Mg2Al4Si5O18:Eu2+ phosphors by our group could further support this, with red emission originating from Eu2+ at the six-coordinated YO6 and LuO6 polyhedrons and vacant channels, respectively.57,58
D. Doping concentration controls the distribution of Eu2+ ions
As mentioned in Sec. III C, phosphor is composed of “host + activator.” Normally, there are more than one type of polyhedrons in host lattices. Eu2+ ions can distribute in different polyhedrons by controlling the doping concentration, and thus not only the luminescence efficiency can be optimized but also the emission peak can be tuned. For example, Sato et al. reported a new dicalcium silicate phosphor Ca2−xEuxSiO4, which realized tunable emission from 580 to 650 nm by increasing doping concentration.59 When the doping level is low (x < 0.2), Ca2−xEuxSiO4:Eu2+ shows extensively yellow emission peaking at 580 nm, by virtue of the large bond length caused by the occupation of Eu2+ ions in Ca1nO7 sites. When x > 0.2, the emission color gradually transforms to red because more Eu2+ ions tend to occupy Ca2nO6 sites with shorter bond length [Fig. 8(a)]. The substitution of Eu2+ ions in the small Ca2n sites causes strong crystal field splitting and further leads to the red emission. The similar phenomenon has also observed in (SrBa)2SiO4:xEu2+ phosphor. XRD refinement results revealed that the Eu2+ ion is distributed in two different crystallographic sites and Eu2+ located at the nine-coordination sites at low doping concentration. As shown in Fig. 8(b), with increasing concentration of Eu2+, the ratio of the red emission with lower energy increases, whereas the high energy ones decrease, resulting in redshifts of photoluminescence (PL) and photoluminescence excitation (PLE) spectra.60 However, very few phosphors possess this property, and high doping concentrations will lead to luminescence quenching. Thus, this design principle has certain limitations and deep research studies are needed in the future.
E. Mixed ligands induce large ΔS
The Stokes shift is defined as the energy difference between excitation and emission spectra, which is related to the parabola offset that is induced by lattice relaxation at the excited states.61 According to the previous work of Gettinger et al.,62 the Stokes shift has an inversely proportional correlation with the rigidity of the host lattice, whereas Daicho et al. believe that ΔS is related to the symmetry of Eu-doping sites, and the higher distorted condition will result in the larger Stokes shift due to a significant reorganization of the atoms around the luminescent center.63 For example, K2Ca(PO4)F:Eu2+ has a large ΔS value about 1.38 eV, which is large enough to convert NUV light to red luminescence without the absorption at the region of blue and green light [Fig. 9(a)].63 For the sites of Eu2+ in K2Ca(PO4)F:Eu2+, the 5d1 orbital can hardly extend toward the F− ion side because that F− is more electronegative than the O2− ion. The O–Eu–O angle increased due to the rotation of the PO4 tetrahedra to alleviate the electrostatic repulsion with the 5d1 electron [Fig. 9(b)]. As a result, 5d1 electrons localized in the asymmetrical space and created a large ΔS. This method should be used to adjust the small ΔS in Eu2+-activated nitride phosphors.
F. Emerging narrow-band red emitters in Eu2+ doped UCr4C4-type nitrides
Narrow-band emitters play a key role in WLED backlights that are made by the combination of blue emitting LED chip with narrow green- and red-emitting phosphors. Nowadays, β-SiAlON:Eu2+ phosphor has been commercialized in LCDs owing to its excellent luminescence properties of narrow green emission band (FWHM 55 nm) and high quantum efficiency.9 However, the narrow-band red-emitting phosphors activated by the Eu2+ ion are still in vacancy. Up to now, only a few Eu2+ doped nitride/oxynitride phosphors with the UCr4C4-type structure exhibit narrow-band red emission. The first narrow-band red-emitting phosphor Sr[LiAl3N4]:Eu2+ possesses a triclinic crystal structure with UCr4C4-type, which contains a highly rigid network formed by order corner- and edge-sharing AlN4 and LiN4 tetrahedra [Fig. 10(a)].34 Sr2+ sites are filled in ring channels with high symmetric cubic polyhedrons. The occupation of Eu2+ in Sr2+ sites with highly symmetric cubic polyhedrons and rigid network results in abnormal narrow-band red emission (FWHM 50 nm). Soon afterward, Schnick and co-workers discovered other UCr4C4-type narrow red-emitting phosphors, such as Ca[LiAl3N4]:Eu2+ (FWHM = 60 nm) and Sr[Mg3SiN4]:Eu2+ (FWHM = 43 nm).64,65 Recently, Hoerder et al. reported a new red-emitting phosphor Sr[Li2Al2O2N2]:Eu2+, which shows a narrow-band emission peaking at 614 nm with a FWHM of 48 nm [Fig. 10(b)].66 It is worth noting that the crystal structure of Sr[Li2Al2O2N2] also belongs to the ordered variant of UCr4C4-type. Therefore, wide attention should be paid to UCr4C4-type nitrides in future research on Eu2+ activated narrow-band red-emitting phosphors.
IV. DESIGN PRINCIPLES OF Eu3+-DOPED RED PHOSPHOR
Compared with Eu2+ doped red phosphors, Eu3+ activated red phosphors have better color purity due to their sharp line emission originating from f–f transition of Eu3+ ions.67 However, there are some defects for Eu3+ activated phosphors. Take the commercial Y2O3:Eu3+ red phosphor for example, it can be only excited by UV light and shows red emission peaking at 610 nm.68 Nevertheless, the PLE spectra of Y2O3:Eu3+ cannot be matched well with the blue or NUV chips. Therefore, the biggest challenge for Eu3+ doped red phosphors is adjusting the position of CTB. In addition, the 5D0 → 7F1 (590 nm) and 5D0 → 7F2 (610 nm) transitions are insensitive and sensitive to chemical environment, respectively, and the 5D0 → 7F2 transition will be dominant when Eu3+ occupies an non-symmetric site. Thus, high color purity can be achieved by controlling the sites occupation of Eu3+ ions in an unsymmetric site. Furthermore, the luminescent efficiencies of Eu3+ activated phosphors are still low and need to further optimize. Considering that the above problems existed in Eu3+ doped phosphors, four design principles to improve luminescence properties of Eu3+ are summarized.
A. Synthesis of Eu3+-doped phosphors with small CT energy
Charge transfer (CT) describes the process in which Eu3+ ions obtain electrons from the coordination anions like O2− or N3− ions, and the energy of CT can be interpreted by Jorgensen's empirical formula69
where and are the optical electronegativity of the anion and metal ion, respectively. Based on this formula, Dorenbos presented a compendium of CT energy of Eu3+ in different types of host materials, as shown in Fig. 11(a). Obviously, the CT energy changes with the type of anions and tends to decrease in a sequence: fluorides > oxides > nitrides > sulfides. Moreover, for the materials with same type of anions, the CT energy decreases gradually with the increase of average distance to the surrounding anions [Fig. 11(b)]. Therefore, developing Eu3+ doped nitride or sulfide phosphors seems to have higher probability of obtaining low CT energy, and the occupation of Eu3+ in a larger crystalline site tends to realize lower CT energy. Take α-M3B2N4 (M = Ca, Sr):Eu3+ for example, both of them show low CT energy (300–400 nm) compared with oxide phosphor (250–310 nm), which is due to the relatively high optical electronegativity of N3− than O2− [Fig. 12(a)].70 In addition, the excitation wavelength of α-Ca3B2N4:Eu3+ is shorter than that of α-Sr3B2N4:Eu3+. This is because the electron cloud of Sr2+–N3− group is smaller than that of Ca2+–N3− group, and thus Sr2+–N3− group needs lower energy compared with the Ca2+–N3− group [Fig. 12(b)]. The broad CTB peaking at the NUV region was also observed in other nitrides phosphors such as LiCaAlN2:Eu3+.71 These results indicate that the CTB of Eu3+-activated nitride or sulfide phosphors has more probability to match with the commercial NUV LEDs.
Except the CTB formed by Eu–N and Eu–O groups, other CTBs formed by Mo–O, V–O, and W–O in Eu3+-doped molybdate, vanadate, and tungstate phosphors have also attracted increasing interest owing to their broad and intense absorption in the UV/NUV region.72–74 Importantly, the captured energy by Mo–O, V–O, and W–O CTB can be efficiently transferred to the Eu3+ ion.75 As shown in Fig. 12(c), MgWO4:Eu3+ and MgMoO4:Eu3+ phosphor possess broad W4+–O2− and Mo4+–O2− CTB blended with the Eu3+–O2− CTB.76 However, their CTBs are still located in the UV region and are insufficient to be effectively pumped by NUV or blue LED chips. Recently, researchers found that the position of CTB in MoOx-based phosphors is related to the coordination number x, and the higher value of x contributes to the longer excitation wavelength.77 By comparing the position of CTB in Figs. 12(c) and 12(d), it can be seen that the CTBs of A2MoO6:Eu3+ (A = Y, Gd) phosphors shift obviously to the longer wavelength than that of MgWO4:Eu3+ and MgMoO4:Eu3+ phosphors.76 This is attributed to the higher coordination number of transition metal ion resulting in a longer and weaker Mo6+–O2− bond, and thus the energy of the Mo6+–O2− bond extends the CTB edge from 330 nm (for MoO4 groups) to 420 nm (for MoO6 groups). Therefore, synthesizing Eu3+-doped molybdate phosphors with a high coordination number (MoO6 groups) is an effective way to reduce CTB energy.
B. Realization of low excitation energy by doping sensitizer ions
Energy transfer (ET) among lanthanide ions has been used to develop efficient phosphors for lighting, lasers, optical markers, and solar cells. It is necessary to explore sensitizers for Eu3+ to realize appropriate CTB that overlaps with the commercial InGaN chips. A group of possible transitions leading to effective luminescence under near-UV excitation are the allowed 4fn → 4fn−15d1 transitions in Eu2+ or Ce3+. However, Eu2+/Ce3+ is unlikely to directly sensitize Eu3+ luminescence in virtue of the metal–metal charge transfer (MMCT) quenching.78 One way to simultaneously avoid the MMCT quenching and sensitize Eu3+ emission is adding intermediate ions that can act as a bridge for energy transfer between Eu2+/Ce3+ and Eu3+. Blasse first used Gd3+ as an intermediate in (Y,Gd)F3:Ce3+, Eu3+ phosphor and realized Ce3+ → (Gd3+)n → Eu3+ energy transfer.79 Replacing Gd3+ by Tb3+, YBO3:Ce, Tb, Eu phosphor exhibits strong absorption bands in the UV and NUV regions produced by the Ce3+ → (Tb3+)n → Eu3+ ET scheme.78 Some other novel red phosphors, such as Na2Y2B2O7:Ce3+, Tb3+, Eu3+, Y10Al2Si3O18N4:Ce3+, Tb3+, Eu3+, and Y2SiO5:Ce3+, Tb3+, Eu3+ have been synthesized by using this strategy.80–82 Take Y2SiO5:Ce3+, Tb3+, Eu3+ for example, Y2SiO5:Ce3+ possesses several broad absorption bands at 270, 300, and 355 nm [Fig. 13(a)]. When Tb3+ was employed as the dopant, the corresponding PLE spectra monitored at 545 nm are similar with that of Y2SiO5:Ce3+, which evidences the existence of energy transfer between Ce3+ and Tb3+. Moreover, Eu3+ is effectively sensitized by Ce3+ → (Tb3+)n → Eu3+ energy transfer [Fig. 13(b)].82 As a result, not only the luminescence of Eu3+ was enhanced but also the excitation wavelength was tuned to 360 nm attributed to the allowed 4f–5d absorption of Ce3+ ions.
C. Eu3+ luminescence enhancement by charge compensation
Though Eu3+ shows luminescence in most host materials, the luminescence intensity is still low and needs to be further improved. Particularly for the phosphors in which Eu3+ occupies the divalent crystalline sites, the doping concentration is restricted for the sake of charge unbalance. Alkali metal ions Li+, Na+, and K+ with distinct ionic radii and low oxidation states were often used as charge compensation ions to modify the local site symmetry and improve the Eu3+ luminescence efficiency.83–86 For example, the substitution of Eu3+ at Mg2+ sites in MgAl2O4:Eu3+ nanophosphor gives rise to a net positive charge in the system due to different valence states of Eu3+ and Mg2+. Hence, some Eu3+ ions are incapable of entering the lattice sites and tend to aggregate together, which results in low efficiency. On this condition, charge compensation becomes important. Hence, Li+ ions were introduced to achieve an electrically neutral environment. Obviously, the excitation intensity of MgAl2O4:Eu3+ were enhanced with the charge balance mechanism of , as shown in Fig. 14(a).87 Moreover, by co-doping Na+ into CaWO4:Eu3+ phosphor, the quantum efficiency of CaWO4:Eu3+, Na+ reaches 92% [Fig. 14(b)].88 The charge balance of Eu3+ substitution in Ca2+ sites can be realized via paths: 2Ca2+ → Eu3+ + Na+. Similar luminescence enhancements were also observed in other Eu3+ activated phosphors, such as Li+ doped Y2O3:Eu3+ and (Y,Gd)2O3:Eu3+ phosphors.89 However, there is no charge balance problem in these systems and thus Li+ may only act as a flux in these phosphors.
D. Occupation of unsymmetrical sites to maintain high color purity of Eu3+
In general, the electron dipole transition of 5D0–7F2 is very sensitive to the environment, whereas the magnetic dipole 5D0–7F1 transition is irrelevant to the environment. Thus, Eu3+ is often used to probe the local structure changes in the crystal lattice.90 When Eu3+ occupies an unsymmetrical site, the dominant emission located at around 620 nm could be obtained with high color purity. On the contrary, symmetric site occupation of Eu3+ allows dominant emission peaked around 592 nm with low color purity. For example, the occupation of Eu3+ at symmetric sites in α-Sr3B2N4 crystals leads to the sharp emission line at 593 nm [Fig. 15(a)], while Eu3+ ions occupy the unsymmetric Ca site in α-Ca3B2N4 achieving red emission with high color purity at 613 nm [Fig. 15(b)].70 Furthermore, researchers also found that Ba2La2/3TeO6:Eu3+ exhibits orange–red color with chromaticity coordinates of (0.629, 0.369), ascribed to Eu3+ ions occupying the symmetric octahedral LaO6 sites [Fig. 15(c)].90 On the contrary, the unsymmetric YO8 sites occupied by Eu3+ ions in CaYAl3O7 host lattice leads to a high color purity red emission with chromaticity coordinates of (0.6303, 0.3607), as exhibited in Fig. 15(d).91 From the perspective of discovering new Eu3+-doped red phosphors, it is necessary to select host materials with unsymmetric crystalline sites.
V. CONCLUSION AND OUTLOOK
Researchers have made significant efforts to improve and develop Eu2+ and Eu3+ activated red phosphors to meet the various demands in high-quality LED lighting and high-definition displays. In this perspective, we mainly focus on the design principles of Eu2+/Eu3+ activated red-emitting phosphors. As for Eu2+ ion, the 5d–4f luminescence in the solids depends greatly on the surrounding environments. Based on this characteristic, we summarized six types of design principles for Eu2+ to realize red emission in a host lattice, including the synthesis of new Eu2+-doped nitride or sulfide phosphors, anionic substitution achieves large centroid shift, Eu2+ occupies polyhedrons with small CNs to obtain large ɛcfs, doping concentration controls the distribution of Eu2+ ions, mixed ligands induce large ΔS, and emerging narrow-band red emitters in Eu2+ doped UCr4C4-type nitrides. As for the Eu3+ ion, the main challenge is the weak absorption in NUV and blue light region due to the forbidden f–f transition. Thus, we proposed the design principles to adjust the charge transfer band and realize low energy excitation, including synthesis of Eu3+-doped nitride, sulfide, or molybdate phosphors to reduce CTB energy and doping sensitizer ions to realize low energy excitation. In addition, design principles of charge compensation and unsymmetrical sites occupation are proposed to improve the luminous efficiency and optimize the color purity of Eu3+ doped phosphors.
These design principles herein will prompt the development and optimization of Eu2+ and Eu3+ doped red phosphors for future studies. Looking forward, much effort will be required in further work, including but are not limited to the following: (1) although many design principles have been proposed to improve the luminescence properties of Eu2+ and Eu3+, there is still a lack of general rules applicable to all phosphor systems. Thus, more concerns should be focused on the deep understanding of structure-related luminescence mechanisms. (2) Currently, the main commercial Eu2+ doped phosphors are nitrides or sulfides. However, these phosphors suffer from some disadvantages, such as high cost and poor chemical stability. Therefore, much effort should be made to develop of Eu2+ activated oxide phosphors with a new design viewpoint. (3) The main problem of Eu3+ activated phosphor is the weak absorption from f–f transitions. Compared with the CT energy Eu3+–O2−, Eu3+–N3− and Eu3+–S2− have lower CT energy due to large covalency of N3− and S2−. Hence, the research on Eu3+ doped nitride or sulfide phosphors should be concerned and encouraged to obtain low CT energy.
The authors acknowledge the support received from the National Natural Science Foundations of China (Grant Nos. 51972118 and 51961145101), International Cooperation Project of National Key Research and Development Program of China (No. 2021YEF0105700), Guangzhou Science & Technology Project (No. 202007020005), the State Key Laboratory of Luminescent Materials and Devices (No. Skllmd-2021-09), and the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01X137).
The data that support the findings of this study are available from the corresponding author upon reasonable request.