Lead-free lanthanide-based Cs₃LnCl₆ metal halides

Abstract

All-inorganic lead-free luminescent metal halides have attracted considerable interest for their special optical properties in various optoelectronic applications. Among them, lanthanide-based Cs₃LnCl₆ metal halides, with low phonon energies and suitable crystallographic features, have been in the spotlight due to their appealing optical properties. However, it remains challenging for us to know how to simply and efficiently synthesize lanthanide-based Cs₃LnCl₆ metal halides, and how to optimize their low absorption and emission efficiencies owing to the parity-forbidden f–f transitions. In this minireview, we summarized several synthesis approaches towards both Cs₃LnCl₆ polycrystals and nanocrystals. The association of the crystal/electronic structure, optical properties and applications is discussed in three sections: self-trapped emission, down-conversion luminescence, and upconversion luminescence. We provide not only the synthesis strategies and recent progress in the luminescence properties of Cs₃LnCl₆, but also their promising future as alternatives to their toxic and poorly stable lead-based counterparts.

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1. Introduction

In recent years, a large number of studies have shown that replacing lead with non-toxic elements can solve the issue of lead toxicity in metal halide perovskites (MHPs) with excellent photoelectric properties.^1–3 Based on the similarities in ion radius, charge, chemical reactivity, and structural tolerance of elements, lead is replaced to form lead-free metal halide perovskite (LFMHP) materials.^4,5 The LFMHPs are categorized into equivalent substitution and heterovalent substitution of Pb²⁺. Equivalent substitution involves replacing Pb²⁺ with a bivalent cation (Sn²⁺, Eu²⁺, and Mn²⁺) to form AMX₃ type perovskites.^6–9 Heterovalent substitution mainly includes the following types. First, replacing two Pb²⁺ lattice positions with a univalent cation (Ag⁺, Na⁺, and K⁺) and a trivalent cation (In³⁺, Bi³⁺, and Ln³⁺), resulting in a double perovskite structure with the structural formula A₂B(I)B(III)X₆.^10–12 Second, substituting two Pb²⁺ lattice positions with a tetravalent cation (Sn⁴⁺, Zr⁴⁺, and Hf⁴⁺) and a lattice vacancy, leading to a vacancy ordered double perovskite structure with the structural formula A₂B(IV)X₆.^13–15 Third, replacing three Pb²⁺ lattice positions with two trivalent cations (Bi³⁺ and Sb³⁺) and one vacancy, forming a triple perovskite structure with the structural formula A₃B(III)₂X₉.^16,17 Fourth, substituting four Pb²⁺ lattice positions with two trivalent cations (Bi³⁺ and Sb³⁺) and one divalent cation (Mn²⁺, Zn²⁺, and Ca²⁺) results in a quadruple perovskite structure, also known as a layered double perovskite structure with the structural formula A₄B(II)B(III)₂X₁₂.^18–21 Among them, lanthanide doping and lanthanide-based halide perovskite luminescent materials have attracted extensive research interest due to their stable characteristic emission.^22–25 However, there are a number of difficulties that hinder their further development. First, the optical properties of lanthanide-doped nanocrystals (NCs) and polycrystals (PCs) are very sensitive to the ratios of lanthanide dopants, sensitizers and substrates, and figuring out the optimal ratios is time-consuming and laborious.^26,27 Second, the emission of lanthanide ions is usually accompanied by bright broadband self-trapped exciton (STE) emission or sensitizer emission,^28–30 which may overlap with the luminescence of the lanthanide ions, thus masking the advantage of the narrowband emission of the lanthanide ions.^31,32 STEs refer to excited states in insulators and semiconductors where an electron and a hole become localized in the same region due to their mutual Coulomb attraction. This localization leads to unique optical properties, such as enhanced luminescence efficiency.¹⁰ Furthermore, the introduction of lanthanide ions as dopants leads to concentration bursts and increased defects in perovskite NCs. In the field of atomic physics, f–f transitions are the movements of electrons between the f subshells of heavy elements known as rare earth elements. The Laporte rule states that transitions involving orbitals with the same angular momentum quantum number (l) are not allowed, whereas transitions between orbitals of different parity are permitted. Yet, for specific rare earth elements, including lanthanides and actinides, the f–f transitions are prohibited by symmetry because the f orbitals are centrosymmetric. Finally, the luminescence of doped lanthanide ions depends heavily on the energy transfer (ET) from the host to the lanthanide ions, and the energy level mismatch between the host and the lanthanide ions leads to a low ET efficiency, which affects the overall luminescence performance. Therefore, it is crucial to simplify the chemical composition and reduce the interference of other emissions. It is worth mentioning that all-inorganic zero-dimensional (0D) lanthanide-based Cs₃LnCl₆ (Ln = Sc, Y, La, etc.) halide NCs and SCs, with excellent luminescence performance, have been reported recently.^33–35 They have low phonon energies and suitable crystallographic lattices for potential applications in light-emitting diodes (LEDs),^36–38 X-ray scintillators,^39–41 photodetectors⁴² and chemical sensors.^43–45 Their crystal and electronic structure can be easily adjusted without changing the chemical elements. 0D lanthanide halide NCs and PCs are some of the best research objects to discuss and analyze the structure–optical property relationship. However, the low-temperature wet synthesis of Cs₃LnCl₆ crystals remains challenging due to the tendency of rare-earth halides to crystallize into hydrated phases in solution.^27,46 In addition, the luminescence efficiency and stability of lanthanide halides are still relatively poor and need to be improved.^47–49 Therefore, it is significant and challenging to find an efficient and stable lanthanide halide luminescent material, to elucidate its luminescence mechanism and excited state dynamics, and to apply it in the field of optoelectronics.^50,51

In this review, we aim to summarize the recent progress of Cs₃LnCl₆ (Ln: Sc, Y, La, Ce, Eu, Gd, Tb, Ho, Er, Tm, Yb, Lu) luminescent materials and discuss their synthesis, characterization, and structural and optical properties. More specifically, we summarize the effects of Sb³⁺ alloying, small organic molecule modification, and lanthanide ion doping on the luminescence properties of Cs₃LnCl₆. These strategies regulate luminescence of Cs₃LnCl₆, achieving highly efficient lanthanide emission and multi-mode luminescence based on upconversion and down-conversion luminescence. Finally, the current challenges and future opportunities for the research of Cs₃LnCl₆ are proposed, and an outlook on the prospects for its further applications is provided.

2. Synthesis methods of Cs₃LnCl₆

The control of the synthesis of either PCs or NCs plays an important role in the design optimization of the luminescent properties of the material, which certainly applies to Cs₃LnCl₆.^52–54 In this section, we summarize several synthesis methods for Cs₃LnCl₆ polycrystals and nanocrystals.

2.1. Cs₃LnCl₆ polycrystals

The solid-state reaction route is usually reported to synthesize Cs₃LnCl₆ PCs. As shown in Fig. 1a, a starting material consisting of stoichiometric amounts of CsCl and LnCl₃·6H₂O was ground in an agate mortar under an inert atmosphere. The mixture was then placed in an alumina crucible and compressed into granular form using a hydraulic press. Finally, the samples were fired at 450 °C for 24 h. Cs₃GdCl₆ and Cs₃LuCl₆ were synthesized by solid-state reaction, respectively.^50,55 The solid-state reaction method is characterized by its simplicity in operation and the controllability of the synthesis process; however, it is associated with high energy consumption and the requirement of high temperatures. Recently, Im et al. reported an innovative mechanochemical approach for the synthesis of Cs₃LnCl₆. This method involves the continuous grinding of precursors, which reduces their particle size and exposes reactive sites on the material surface, thereby facilitating the reaction of the precursors to yield the desired products. This mechanochemical synthesis is characterized by its straightforward reaction conditions and a relatively low generation of by-products, which simplifies the purification process and enhances the overall efficiency of the synthesis. Compared to solution-phase methods, this approach allows for a larger reaction scale, which is advantageous for scaling up the production process. The mechanochemical method not only reduces the use of solvents but also minimizes environmental impact, aligning with the principles of green chemistry.^56,57 Cs₃LnCl₆ single crystals can be prepared through a facile solvothermal method, which involves the direct reaction of CsCl and LnCl₃·6H₂O or Ln₂O₃ in a concentrated HCl solution. The requisite quantities of CsCl and LnCl₃·6H₂O or Ln₂O₃, in accordance with the specified stoichiometric ratio and with the addition of an appropriate amount of concentrated HCl, were loaded into a polytetrafluoroethylene (PTFE) container. The aforementioned PTFE container was then sealed within a stainless-steel Parr autoclave, which was heated to a temperature of 180 °C for a period of 10 hours. Upon cooling to room temperature, crystals were observed to precipitate. This synthesis method is simple and at low temperature, but it produces unknown impurity phases, so the synthesis process of the promising Cs₃LnCl₆ needs to be optimized.

Fig. 1 Synthesis methods for Cs₃LnCl₆ polycrystals and nanocrystals. (a) Synthesis of Cs₃LnCl₆ single crystals or microcrystals. (b) Hot-injection synthesis using metal chlorides as well as oleic acid, oleylamine, and 1-octadecene. (c) Cs₃TbCl₆ with different synthesis temperatures (180–240 °C). Reproduced with permission from ref. 58. Copyright 2023, Wiley. Reproduced with permission from ref. 59. Copyright 2024, Wiley. Reproduced with permission from ref. 60. Copyright 2022, Wiley. (d) TEM images of Cs₃ScCl₆:Sb³⁺ with the different reaction temperatures (120, 140, and 220 °C). Reproduced with permission from ref. 61. Copyright 2023, Wiley.

2.2. Cs₃LnCl₆ nanocrystals

The decrease in crystal size leads to a large number of attractive properties of the material, which are also exhibited by Cs₃LnCl₆.⁶² They were prepared by thermal injection and solvothermal methods. Through different synthesis processes and conditions, different Cs₃LnCl₆ crystal structures can be prepared and the product can be rapidly obtained at low temperature. Liu et al. synthesized Cs₃TbCl₆ quantum dots using a thermal injection method, in which Tb atoms occupy the B site in the cube and are bonded to Cl atoms.⁶⁰ Cs₃TbCl₆ exhibit a cubic crystal structure with the Fm3̄m space group. As shown in Fig. 1b, Cs₃TbCl₆ synthesized using a modified thermal injection method adopts a monoclinic crystal structure with the C2/c space group.²⁵ In both structures, each Tb atom is coordinated with six neighboring Cl atoms to form an isolated [TbCl₆]³⁻ octahedron.⁶⁰ The Cs atoms are filled in the interlayer formed by the [TbCl₆]³⁻ octahedral structure. The prepared cesium precursors are usually injected into oleic acid and TbCl₃ solutions at high synthesis temperatures (180–240 °C) and under a nitrogen atmosphere (Fig. 1c).

The injection temperature plays a crucial role in obtaining pure phase materials and determining their luminescent properties. For example, the phase stability of lead halide chalcogenides can be enhanced by simply changing the reaction temperature. The reaction temperature for the synthesis of Cs₃ScCl₆:Sb³⁺ NCs was thus determined. When the reaction temperature increased from 120 to 220 °C, the NCs changed from spherical to cylindrical rods as shown in Fig. 1d.⁶¹ Through this thermal injection route, the synthesis parameters, including temperature, precursor, ligand, and vacuum or not, can be easily adjusted to produce different photoluminescence (PL) properties.

During the heating process, the amount of oleylamine (OAm) and oleic acid (OA) in the reaction had a significant effect on the crystal structure and size distribution of the products. Taking Cs₃YbCl₆ as an example, monoclinic Cs₃YbCl₆ NCs with a size larger than several hundred nanometers were formed when the amount of OA was more than 2 mL, suggesting that OA induces the growth or agglomeration of Cs₃YbCl₆ NCs. In contrast, when the relative content of OAm is larger than that of OA, the byproducts form colloidal CsCl NCs with a size of ∼6 nm. The relative content of CsCl NCs in the final product increases with the relative volume amount of OAm, indicating that OAm promotes the formation of colloidal CsCl NCs. The size and surface properties of these surfactant-covered CsCl NCs were similar to those of Cs₃LnCl₆ NCs, which made it difficult to separate these NCs during purification. Therefore, optimization of the reaction is crucial for the selective synthesis of phase-pure Cs₃LnCl₆ NCs. The reaction temperature is also a key factor in controlling its size and phase purity. For example, large polydisperse Cs₃YbCl₆ NCs with nanoflake impurities were formed when the reaction temperature was higher than 300 °C, whereas CsCl NCs were formed as by-products with Cs₃YbCl₆ NCs when it was lower than 230 °C.⁶³

3. Crystal/electronic structure of Cs₃LnCl₆

Cs₃LnCl₆ has suitable crystal positions and is an efficient host material with good luminescence properties. The lead-free zero-dimensional (0D) Cs₃LnCl₆ mostly belongs to the monoclinic C2/c space group. Each Ln atom is coordinated to six neighboring Cl atoms to form an isolated [LnCl₆]³⁻ octahedron (Fig. 2a). Three Cs atoms occupy the interstices of the structure, and each Cl atom shares a corner with the other octahedra. In addition, Cs₃LnCl₆ has two distinct [LnCl₆]³⁻ octahedra with different bond lengths, as shown in Fig. 2b. Taking Cs₃YbCl₆ as an example, its crystal structure, bond lengths, and bond angles were calculated by Lee et al.⁶³ Yb1 and Yb2 are each coordinated to six adjacent Cl atoms to form distorted octahedra with Yb–Cl bond lengths ranging from 2.548–2.881 Å (Fig. 2c). The distance between the Yb centers ranges from 7.374 to 7.879 Å, which is more than twice as long as the Yb–Cl bond. In addition, the closest Cl–Cl distance between neighboring octahedra is 3.556 Å, which is much longer than the covalent bond distance of the Cl₂ molecule. Thus, the [YbCl₆]³⁻ octahedra are spatially isolated from each other within the Cs₃YbCl₆ NCs, forming a 0D metal halide structure. To understand the electronic structure of Cs₃LnCl₆, their partial charge densities, orbital projected state densities, and electronic energy band structures have been studied. Han et al. reported the electronic structures of Cs₃TbCl₆ and Rb₃TbCl₆, giving the charge of the Cl 3p orbital and the charges of the Tb 5d and 4f orbitals.²⁵ The former is mainly involved in the valence band states and are distributed at the edges of the [TbCl₆]³⁻ octahedron (Fig. 2d–f). For the latter, the 5d orbitals mainly contribute to the conduction band state and are distributed around the [TbCl₆]³⁻ octahedron, and the 4f orbitals are distributed around the Tb atoms in [TbCl₆]³⁻ between the band gaps. As shown in Fig. 2g and h, the effect of Cs and Rb cations on the frontier orbitals is negligible.^61,64,65 The direct bandgap of both Cs₃TbCl₆ and Rb₃TbCl₆ at the Γ-point is 5.1 eV. In addition, the Cs₃LnCl₆ also exhibit a cubic crystal structure with an octahedral [LnCl₆]³⁻. For example, Liu et al. reported different Cs₃TbCl₆ with a cubic crystal structure with the Fm3̄m space group (Fig. 2i). The Tb atoms occupy the B sites interacting with the Cs atoms.⁶⁰ For this cubic Cs₃TbCl₆, the valence band mainly consists of the d and f orbitals of Tb and the p orbitals of Cl by first-principles density functional theory (DFT) calculations, and the band gap of Cs₃TbCl₆ was calculated to be 3.21 eV (Fig. 2j and k).

Fig. 2 Structural and morphological properties of Cs₃LnCl₆ metal halides. (a) Crystal structure of 0D Cs₃TbCl₆. Reproduced with permission from ref. 25. Copyright 2023, American Chemical Society. (b) and (c) Distinct crystallographic sites of Yb with sixfold coordination. (d)–(f) Partial charge density of Cl 3p, Tb 5d, and 4f orbitals in Cs₃TbCl₆, respectively. (g) and (h) Density of states and calculated electronic band structures of Cs₃TbCl₆ and Rb₃bCl₆, respectively. Reproduced with permission from ref. 25. Copyright 2023, American Chemical Society. (i) The crystalline structure of Cs₃TbCl₆, Cs₃TbCl₆ and Rb₃TbCl₆. (j) and (k) The first-principles calculations (PDOS) of Cs₃TbCl₆ and the electronic band structure obtained by theoretical calculation. Reproduced with permission from ref. 60. Copyright 2022, Wiley. (l) The optimized unit cells of Cs₃Ce_1−xGd_xCl₆ for x are 0, 0.5 (with red dashed box), and 1, where different Wyckoff positions for Ce/Gd sites are notated. (m) Internal energy of mixing as a function of the alloying concentration of Gd³⁺ ions in Cs₃Ce_1−xGd_xCl₆. Reproduced with permission from ref. 66. Copyright 2024, Wiley.

By partially replacing Ln(1)³⁺ with Ln(2)³⁺ ions, a significant improvement in their optical properties and stability might be achieved. Fig. 2l illustrates the optimized unit cells of Cs₃Ce_1−xGd_xCl₆, which exhibit two potential Ce/Gd sites, 4d and 4e, in accordance with the Wyckoff position.⁶⁶ In this case, x is equal to 0 or 1, and all Ce³⁺ (or Gd³⁺) ions occupy both the 4d and 4e sites. In the case of x being neither 0 nor 1, the 4d and 4e sites are found to be partially occupied by the Ce³⁺ and Gd³⁺ ions. For example, three potential configurations of Ce³⁺ and Gd³⁺ ions are observed in Cs₃Ce_0.5Gd_0.5Cl₆: I (in which all 4e sites are occupied by Ce³⁺ and all 4d sites are occupied by Gd³⁺). The second configuration (half of the 4d and 4e sites are occupied by Ce³⁺ while the remaining sites are occupied by Gd³⁺) and the third configuration (all 4d sites are occupied by Ce³⁺ and all 4e sites are occupied by Gd³⁺) are also possible. The aforementioned site-preference can be evaluated by calculating the internal energy of mixing (U_mix) using the following equation:

U_mix = E₁ − (1 − x)E₂ − xE₃

E₁, E₂, and E₃ represent the total energies of Cs₃Ce_1−xGd_xCl₆, Cs₃CeCl₆, and Cs₃GdCl₆, respectively. Fig. 2m illustrates the calculated U_mix as a function of x. A negative U_mix of −45 meV per cation was obtained for Cs₃Ce_0.5Gd_0.5Cl₆ when all 4e sites were occupied by Gd³⁺ and all 4d sites were occupied by Ce³⁺. This indicates that the configuration is thermodynamically stable and that it will not decompose into pure Cs₃CeCl₆/Cs₃GdCl₆ phases. In the alternative configuration (i.e., Cs₃Ce_0.5Gd_0.5Cl₆ when all Ce³⁺ ions were at 4e sites and all Gd³⁺ ions were at 4d sites), a positive U_mix of 217 meV per cation was obtained, indicating that the configuration is thermodynamically unstable and will decompose. The simulation results corroborate the hypothesis that Ce³⁺ invariably occupies 4d sites, whereas Gd³⁺ consistently occupies 4e sites. Consequently, when Cs₃CeCl₆ is alloyed with Gd³⁺ ions, the latter will occupy the 4d sites, which are thermodynamically unstable for Ce³⁺. This phenomenon can enhance the overall stability of the system.^67,68 The addition of Gd³⁺ to the Cs₃CeCl₆ NCs led to not only a concomitant improvement in phase stability but also a notable enhancement in PLQY.^69,70 The PLQY of the Cs₃CeCl₆ NCs was elevated from 57% to 96% by increasing the concentration of Gd³⁺.

4. Optical properties, optimized strategies and applications

Here are several effective strategies to endow functional materials with new and improved optical properties. By doping or alloying Sb³⁺ with ns² electrons, Cs₃LnCl₆ can become a new type of highly emissive lanthanide halide for various luminescence applications. In addition, Cs₃LnCl₆ NCs can achieve impressive PLQYs through the introduction of multifunctional small organic molecules as intermediate states. All the above strategies mainly focus on the down-conversion luminescence. At the same time, Cs₃LnCl₆ can realize the upconversion luminescence regulation of the material based on the stable characteristic emission of different lanthanide ions. It is meaningful to explore the luminescence regulation mechanism and multi-functional applications.

4.1. STE emission in Sb³⁺-doped Cs₃LnCl₆ (Ln: Sc, Y, La, Gd, Lu)

Sb³⁺ dopant occupies the position of octahedral Ln³⁺ sites in Cs₃LnCl₆ to form a series of new efficient 0D luminescent materials. Due to the spin orbitals of Sb³⁺ allowing ³P₁ → ¹S₀ leaps, these materials not only exhibit broadband photoluminescence, but also have excellent resistance to thermal burst and photostability.^71–73 Liang et al. developed Sb³⁺-doped 0D Cs₃GdCl₆ PCs.⁷⁴ The lattice contraction of Cs₃GdCl₆ PCs with the increase in Sb³⁺ concentration indicates that the smaller Sb³⁺ successfully substitutes the larger Gd³⁺ (Fig. 3a). Samanta et al. investigated the luminescence properties of Sb³⁺-doped Cs₃ScCl₆ NCs.⁶¹ Upon illumination with 345 nm ultraviolet (UV) light, the Cs₃ScCl₆:Sb³⁺ NCs exhibit asymmetric yellow emission, which is characteristic of STE-assisted emission (Fig. 3b). The broadband emission is attributed to the ³P₁ → ¹S₀ and ¹P₁ → ¹S₀ transitions of Sb³⁺ ions, followed by STE levels. Typically, STEs are observed in 0D MH structures due to lattice deformation, and similar STE-assisted emission is observed in Cs₃ScCl₆:Sb³⁺ through Sb³⁺ doping. Sb³⁺-activated MHs typically emit in the blue, green, or red regions, with emission unlikely to extend across the entire visible spectrum.⁷⁵ Interestingly, the Cs₃ScCl₆:Sb³⁺ NCs exhibit bright white emission across the entire visible spectrum when excited with 365 nm UV light (Fig. 3c). The emissions from Cs₃ScCl₆:Sb³⁺ NCs have different origins from their corresponding photoluminescence excitation (PLE) spectra (Fig. 3d). Upon illumination with 365 nm UV light, electrons in the ground state of ¹S₀ are simultaneously excited to the higher- and lower-energy excited states of the carbon dots, ¹P₁ and ³P₁, respectively (Fig. 3e). Excitons present in the energy states of carbon dots radiatively decay with luminescent centers. Subsequently, the higher and lower energy states of ¹P₁ and ³P₁ transfer the energy to the STE1 and STE2 levels, respectively, and radiatively decay to ¹S₀, thereby emitting white light. From the absolute photoluminescence quantum yield (PLQY) of Cs₃ScCl₆:Sb³⁺ NCs at different doping levels, the optimal Sb³⁺ ion concentrations under excitation wavelengths of 345 and 365 nm were ∼1.5 and ∼3 mol%, respectively (Fig. 3f). The difference between the two optimal concentrations may be attributed to excessive carbon dot emission. Furthermore, upon illumination with 365 nm UV light, Cs₃ScCl₆:Sb³⁺ NCs exhibit higher PLQY values than those observed with 345 nm UV light. This is because of a reduction in the Stokes shift (i.e., energy loss) from 1.34 to 0.72 eV. The PL lifetime of Cs₃ScCl₆:Sb³⁺ NCs, as measured as a function of Sb³⁺ concentration (Fig. 3g), is consistent with the concentration-dependent PLQY results (Fig. 3f, yellow emission). Moreover, a typical broadband STE-assisted emission upon doping of Sb³⁺ ions into the host lattice of Cs₃LnCl₆ (Ln: La, Gd, Y, Lu) can be observed with the emission wavelength shift from 570 to 525 nm (Fig. 3h) and similar PLE spectral shape.⁷⁶ In general, the emission color observed in Sb³⁺-doped zero-dimensional metal halides is dependent on the degree of asymmetry inherent to the Sb³⁺-doped lattice site. To illustrate, Rb₃InCl₆:Sb³⁺ exhibits green emission, while Cs₂InCl₅·H₂O:Sb³⁺ displays red emission. The diverse emission colors can be attributed to variations in the degree of asymmetry within each Sb³⁺-doped lattice site.^77–79 Furthermore, the distinct crystal structures of these materials contribute to different excitation spectra. However, in similar crystal structure systems, the emission color is primarily governed by the degree of asymmetry present in the Sb³⁺-doped lattice site. Upon changing the Ln ion type from Lu to La, the PLQY progressively increased (Fig. 3i). A mechanism for tuning the emission energies in Cs₃LnCl₆:Sb³⁺ NCs is proposed in the structure systems shown in Fig. 3j, the emission color is primarily governed by the degree of asymmetry present in the Sb³⁺-doped lattice site. Upon changing the Ln ion type from Lu to La, the PLQY progressively increased (Fig. 3i). A mechanism for tuning the emission energies in Cs₃LnCl₆:Sb³⁺ NCs is proposed in Fig. 3j. Irradiation of the Cs₃LnCl₆:Sb³⁺ NCs with 320 nm light results in the excitation of electrons in the ground state of ¹S₀ into the ¹P₁ and ³P₁ states. Subsequently, electrons from the higher energy excited state, ¹P₁, are transferred to the STE1 state and radiatively recombine with ¹S₀, thereby causing the 450 nm emission band. In contrast, a lower energy excited state transfers the energy to STE2 and radiatively recombines with ¹S₀, resulting in emissions in the green and yellow regions, respectively.⁸⁰ This is dependent on the degree of asymmetry present in the LnCl₆ octahedra. The doping of the host material with Sb³⁺ ions typically results in the excitation of two distinct types of STE states. The first type corresponds to a higher energy STE state, resulting from the electronic transition from ¹P₁ to ¹P₀, which is a spin-allowed transition. The second type of transition involves the formation of lower-lying STE states, which arise from the electronic transition from ³P_n (where n can take values of 0, 1, or 2) to ¹P₀. This transition is classified as a parity-spin-allowed transition. Additionally, the spin-allowed transition from ¹P₀ to ¹P₁ is comparatively weak in comparison to the parity-allowed transition from ³P₁ to ¹P₀ in zero-dimensional metal halides. This disparity in transition strengths contributes to the observation of a broad emission band within the blue-green-yellow spectral regions.^81–83

Fig. 3 (a) Schematic of the crystal structure of orthorhombic Cs₃GdCl₆ and the crystallographic site for the Sb³⁺ dopant. PL and PLE spectra of (b) yellow-emitting and (c) white-emitting Cs₃ScCl₆:Sb³⁺ NCs, and the insets show the bright yellow and white light under 345 nm UV light excitation and 365 nm UV light excitation, respectively. (d) PLE spectra of Cs₃ScCl₆:Sb³⁺ NCs by monitoring the 420 and 555 nm emission peaks. (e) Schematic for the proposed mechanism of the origin of white-light-emission. (f) PLQY values and (g) PL lifetime of Cs₃ScCl₆:Sb³⁺ NCs as a function of Sb³⁺ concentration. Reproduced with permission from ref. 61. Copyright 2023, Wiley. (h) PL and PLE spectra and (i) PLQY values of Cs₃LnCl₆:Sb³⁺ (Ln: La, Gd, Y, Lu) NCs under 320 nm UV light excitation. (j) Schematic for the proposed mechanism for the origin of tuning the emission wavelengths in Cs₃LnCl₆:Sb³⁺ NCs. Reproduced with permission from ref. 80. Copyright 2024, Elsevier.

4.2. Down-conversion luminescence in Cs₃LnCl₆ (Ln: Ce, Tb, Eu)

Cs₃LnCl₆ (Ln: Ce, Tb, Eu) usually exhibits blue, green and red emission, and their luminescence properties can be improved through Sb³⁺ doping and the introduction of multifunctional small organic molecules.⁵⁹ Recently, Zhou et al. synthesized Sb³⁺-doped Cs₃TbCl₆ NCs and investigated their PL properties and compared them with undoped Cs₃TbCl₆ NCs.⁵⁸ For the Sb³⁺-doped NCs, a new absorption peak appeared, which was attributed to the 5s² → 5s¹5p¹ (S → P) transition of Sb³⁺ in the [SbCl₆]³⁺ octahedral unit (Fig. 4a). The optimal PLQY of Cs₃TbCl₆:2.5% Sb NCs is about three times higher than that of the undoped NCs (Fig. 4b), which is a new record for the PLQY of Tb-emissive halide perovskite and variant NCs. To further explore this luminescence process, they provide two photophysical pathways for radiative recombination of Tb³⁺-PL emission in this system. The charge carrier dynamics model is shown in Fig. 4c. Carriers generated by band-edge photons are directly trapped by [TbCl₆]³⁻ under UV light excitation, leading to PL emission, and carriers generated by photons can also be directly trapped by [SbCl₆]³⁻ octahedra via the 5s² → 5s¹5p¹ (S → P) transition of Sb³⁺. Subsequently, the thermally enhanced energy transfer pathway assisted by the more matched [SbCl₆]³⁻ induced STE states play a dominant role, which is more favorable for the narrower and more efficient intrinsic f-energy level PL emission of Tb³⁺.²⁵ Moreover, Im's group demonstrated the optical properties of Cs₃TbCl₆ and Rb₃TbCl₆ polycrystals (PCs). They show bright green emission (λ_em = 548 nm, ⁵D₄ → ⁷F₅ transition) of Tb³⁺ ions under 280 nm UV light excitation (Fig. 4d and e). The broad absorption band ranges from 250 to 300 nm, corresponding to the charge transfer band (CTB) of Cl⁻ → Tb³⁺, with the maximum peak at 280 nm.^84,85 The narrow bands ranging from 300 to 400 nm are assigned to the spin-forbidden 4f–4f transitions of the Tb³⁺ ions.⁸⁶ The scintillation process of Cs₃TbCl₆ and Rb₃TbCl₆ can be divided into three phases, namely, the conversion, the transport and the luminescence (Fig. 4f).

Fig. 4 (a) Normalized steady-state absorption spectra of undoped and Sb-doped Cs₃TbCl₆ NCs. (b) PL spectra of undoped and Sb-doped Cs₃TbCl₆ NCs. (c) The charge-carrier dynamics model and photophysical processes of Cs₃TbCl₆:Sb NCs. (1) Transfer process assisted by Cl⁻–Tb³⁺ charge transition in [TbCl₆]³⁻ and (2) thermally boosting energy transfer assisted by the [SbCl₆]³⁻-induced STE state. Reproduced with permission from ref. 58. Copyright 2023, Wiley. (d) PL and PLE spectra of Cs₃TbCl₆ and Rb₃TbCl₆. (e) Schematic illustration of the luminescence mechanism of Cs₃TbCl₆ and Rb₃TbCl₆. (f) Schematic diagram of the scintillator conversion mechanism in Cs₃TbCl₆ and Rb₃TbCl₆. The process is divided into conversion, transport, and luminescence. Reproduced with permission from ref. 25. Copyright 2023, American Chemical Society. (g) PL and PLE spectra of Cs₃CeCl₆ NCs along with the schematic luminescence mechanism. The inset shows the image of Cs₃CeCl₆ NC colloids in hexane under white light (left) and 365 nm UV irradiation (right). (h) The PLQYs as a function of the Gd³⁺ concentration of Cs₃CeGdCl₆ NCs. Reproduced with permission from ref. 66. Copyright 2024, Wiley. (i) PL spectra of Cs₃CeCl₆ NCs, Cs₃TbCl₆ NCs, and Cs₃Tb_xCe_1−xCl₆ (x = 0.1, 0.3, 0.5, 0.7, 0.9) alloy NCs. (j) Ce³⁺ → Tb³⁺ energy transfer mechanism in Cs₃Tb_xCe_1−xCl₆ alloy NCs. (k) PL decay curves for Cs₃Tb_xCe_1−xCl₆ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 0.99) alloy NCs. Reproduced with permission from ref. 51. Copyright 2024, Wiley.

PL and PLE spectra of Cs₃CeCl₆, along with its luminescence mechanism, are illustrated in Fig. 4g. Upon UV excitation, Cs₃CeCl₆ NCs exhibited intense luminescence in the UV to visible region, characterized by asymmetric luminescence bands at 376 and 408 nm. In the case of the Ce³⁺ ion, the 4f¹ energy level undergoes a splitting into ²F_5/2 and ²F_7/2 as a consequence of spin–orbit coupling. Additionally, the 5d¹ (²D) energy level experiences a splitting into T_2g and E_g, which is attributed to the influence of the crystal field effect.^87,88 The principal luminescence bands at 379 nm and 408 nm are derived from the transition from the lowest 5d¹ (T_2g Γ₈) to the 4f¹ (²F_5/2 and ²F_7/2) multiple levels.⁸⁹ The PLE spectra of the Cs₃CeCl₆ NCs were determined by monitoring the 376 nm emission wavelength. Excitation bands were observed at 264 and 336 nm, due to the 4f–5d electronic transition.⁹⁰ A salient feature of Ce³⁺-based luminescent materials is the parity-allowed f–d transition exhibited by Ce³⁺ ions, which typically manifests as a broad emission band, a short PL lifetime (commonly on the nanosecond scale), and a tunable emission spectrum. These PL properties render Ce³⁺-based materials promising candidates for optoelectronic devices. The [CeCl₆]³⁻ octahedra, comprising Cl⁻ ions as ligands, exert a dominant influence on the crystal field splitting strength. With a sixfold coordination number, the octahedra exhibit strong splitting in the 5d of Ce³⁺ ions.^91,92 A gradual increase in the PLQY of the Cs₃CeCl₆ NCs was observed with an increase in the Gd³⁺ concentration, as illustrated in Fig. 4h. This represents a significant enhancement in comparison to the initial PLQY of 57% in the absence of Gd³⁺. This improvement in PLQY is ascribed to the substitution of Ce³⁺ ions with Gd³⁺ ions in [CeCl₆]³⁻ octahedra, which indicates the suppression of concentration quenching by providing higher distances between Ce³⁺ ions. Furthermore, this process reduces radiation reabsorption.

However, following the complete substitution of Ce³⁺ with Gd³⁺ ions, the PLQY of the Cs₃GdCl₆ NCs decreased to 6.2%, which is because of the surfactant ligands. It can be postulated that the only possible reason why Cs₃GdCl₆ NCs emit in the UV region would be oleic acid and oleylamine on the surface of NCs. Fig. 4i illustrates the PL spectra of the Cs₃Tb_xCe_1−xCl₆ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1) NCs. Upon excitation in the CTB band (290 nm), the Cs₃TbCl₆NCs emit a pronounced green luminescence at 546 nm. Furthermore, additional peaks were observed at 489, 588, and 621 nm. The emission peaks observed at 489, 546, 588, and 621 nm are assigned to ⁵D₄ → ⁷F₆, ⁵D₄ → ⁷F₅, ⁵D₄ → ⁷F₄, and ⁵D₄ → ⁷F₃ transitions, respectively.⁹³ PLE spectra of the Cs₃TbCl₆ NCs were obtained by monitoring the 546 nm emission wavelength. A broad PLE band was observed in the 250–300 nm range, which is attributed to the charge transfer from Cl⁻ to Tb³⁺. The PLE bands corresponding to the 4f–4f transition in the UV-A region were observed to be extremely weak. It is noteworthy that the Cs₃Tb_xCe_1−xCl₆ (x = 0.1, 0.3, 0.5, 0.7, and 0.9) NCs exhibited intense blue-green luminescence upon excitation at 340 nm. In addition to the emission peaks at 489, 546, 588, and 621 nm from Tb³⁺ ions, blue-violet emission peaks (360–410 nm) were observed. The blue-violet peaks of Ce³⁺ ions are also characterized by asymmetric luminescent bands at 375 and 308 nm, which originate from the transition from the lowest 5d¹ (T_2g Γ₈) to the 4f¹ (²F_5/2 and ²F_7/2) multiple.⁹⁴ The 4f¹ energy level of Ce³⁺ ions undergoes a splitting into ²F_5/2 and ²F_7/2, while the 5d¹ (²D) energy level undergoes a splitting into T_2g and E_g as a result of the crystal field effect. It is noteworthy that an increase in the concentration of Ce³⁺ resulted in an enhancement in the intensity of the blue-violet peak and a concomitant reduction in the intensity of the Tb³⁺ ion luminescence. In the case of Cs₃TbCl₆ NCs, a broad excitation band was observed in the UV-B region (260–310 nm). Upon the incorporation of Ce³⁺ ions into the Cs₃TbCl₆ (Cs₃Tb_xCe_1−xCl₆) NCs, an additional peak emerged in the UV-A region (340–370 nm), while the UV-B PLE peak of Cs₃TbCl₆ was no longer discernible for the Cs₃CeCl₆ NCs. A schematic representation of the energy transfer process from Ce³⁺ to Tb³⁺ is presented in Fig. 4j. Upon excitation at 340 nm, the Ce³⁺ ions absorbed photons and were excited to the excited state (5d¹). A proportion of the excitons underwent radiative decay from the 5d¹ state to the ²F_5/2 and ²F_7/2 states, resulting in the emission of blue-violet luminescence. The excited state of the Ce³⁺ ions exhibited an overlap with a 5d¹ energy level of Tb³⁺ ions. Consequently, efficient energy transfer occurs from the 5d¹ state to the ⁵G₁ (Tb³⁺) energy level. Subsequently, a non-radiative energy transfer occurs from the ⁵D₁ (Tb³⁺) to ⁵D₄ (Tb³⁺) energy level. The hot excitons were radiatively transferred from the ⁵D₄ level to the ⁷F_J (J = 0, 1, 2, 3, 4, 5, and 6) levels. Fig. 4k illustrates the time-correlated PL (TRPL) Cs₃Tb_xCe_1−xCl₆ (x = 0, 0.1, 0.3, 0.5, 0.7, and 0.9) alloy NCs. The average lifetime τ_ave of Cs₃CeCl₆ NCs was calculated. As a result, the average lifetime decreased from 36.80 ns (Cs₃CeCl₆ NCs) to 7.27 ns (Cs₃Tb_0.1Ce_0.9Cl₆ NCs), which can be attributed to the Ce³⁺ → Tb³⁺ energy transfer. Recently, a new type of Ce³⁺-based metal halide Cs₃CeCl₆·3H₂O was presented. Single-crystal XRD measurements reveal that the material exhibits a distinctive one-dimensional crystalline structure, characterized by H₂O–Ce–Cl tetrakaidecahedral units linked together in a linear chain. The synthesized compound exhibits the characteristic luminescence of Ce³⁺ ions, featuring a near-unity PLQY.⁹⁵

In the conversion stage, the scintillator converts the incident radiant energy into energetic electrons and holes due to the photoelectric effect. The generated electrons and holes induce secondary electrons and holes. In the transport phase, the generated electrons and holes are transferred to the luminescent center. In the luminescence stage, the transferred electrons and holes from the excited and ground states recombine at the luminescence center to produce light. The high radioluminescence (RL) performance of Cs₃TbCl₆ and Rb₃TbCl₆ can be explained due to their low exciton binding energy to produce more free carriers by thermalization, which helps Cs₃TbCl₆ and Rb₃TbCl₆ to produce strong radiation at their Tb³⁺ luminescence centers. This allows them to be applied to homemade X-ray imaging systems. Thick films of Cs₃TbCl₆ and Rb₃TbCl₆ were fabricated as shown in Fig. 5a. Cs₃TbCl₆ and Rb₃TbCl₆ PCs were mixed with a polydimethylsiloxane (PDMS) binder, and then large area (25 cm²) films were fabricated using a spin-coating method. The bottom images in Fig. 5a are photographs of a scintillator film in daylight and under a 254 nm UV light in the dark. Due to the high ductility of PDMS, the scintillator films could be well stretched and bent while maintaining homogeneous green emission under a 254 nm UV light (Fig. 5b and c). Fig. 5d shows the schematic device structure of the indirect X-ray detector constructed using Cs₃TbCl₆ and Rb₃TbCl₆ scintillator films. The indirect X-ray detector consists of a carbon fiber reinforced polymer (CFRP) film/scintillator film/arrayed CMOS photodiode, which acquires X-ray images via a data interface with a digital video output and a frame grabber. Another important indicator of the performance of an X-ray detector is its spatial resolution, which can be determined using the modulation transfer function (MTF). The spatial resolution of the X-ray detector is affected by the spatial resolution of the photodiode array and the thickness and performance of the scintillator film. The MTF was measured using the oblique edge method and the spatial resolution of the commercial GOS film was measured in the same system as the Cs₃TbCl₆ and Rb₃TbCl₆ scintillator films for comparison (Fig. 5e). The spatial resolutions of the Cs₃TbCl₆ and Rb₃TbCl₆ films at MTF values of 0.2 were 3.3 and 3.9 lp mm⁻¹ respectively. These values are slightly higher than the spatial resolution of the commercial GOS film reported in a previous study (2.7 lp mm⁻¹). Static X-ray imaging was then performed using Cs₃TbCl₆ and Rb₃TbCl₆ scintillator films. As shown in Fig. 5f, normal and bent clips cannot be identified with the naked eye when under a key ring. However, they could be clearly distinguished in the radiographs obtained, indicating the application potential in homemade X-ray imaging systems.

Fig. 5 X-Ray imaging of Cs₃TbCl₆ and Rb₃TbCl₆ scintillator films. (a) Fabrication method of Cs₃TbCl₆ and Rb₃TbCl₆ scintillator films and photographs of a scintillator film under daylight and a 254 nm UV lamp. (b) and (c) Photographs of stretched and bent scintillator films, respectively. (d) Device schematic of the X-ray imaging system. (e) Spatial resolution measurement of Cs₃TbCl₆, Rb₃TbCl₆, and GOS scintillator films. (f) Photographs of normal (left) and bent (right) clips and a key ring. Reproduced with permission from ref. 25. Copyright 2023, American Chemical Society.

Recently, Sun et al. achieved highly efficient red emission by introducing multifunctional coumarin small molecules into Cs₃EuCl₆ NCs as intermediate states.⁵⁹ The coumarin-modified Cs₃EuCl₆ exhibits pure red emission with 92.4% PLQY and excellent energy transfer efficiency. The energy transfer pathway in the Cs₃EuCl₆ system is shown in Fig. 6a. Upon excitation at 365 nm UV light, the electron leaps from the ground state to the excited state of the Cs₃EuCl₆ NCs, and the incident photon is absorbed by the ethyl 7-hydroxycoumarin-3-carboxylate (EHC), which facilitates the electron leaps from the ground state to S₁.^96–98 Subsequently, strong electron–phonon coupling induces lattice deformation, leading to free exciton capture by the self-trapped excited state.^99–101 At the same time, energy transfer occurs between (i) the self-trapped exciton and Eu³⁺, (ii) the self-trapped exciton and S₁ of the EHC, and (iii) the EHC and S₁ of Eu³⁺. This complex network of energy transfer pathways achieves efficient multichannel energy transfer to Eu³⁺, leading to emission at 593 and 617 nm (⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂). Remarkably, the STE emission was largely suppressed due to the efficient energy transfer from the STE to Eu³⁺ and S₁ of the EHC. The PL intensity was maximized when 4% of EHC molecules were added, and the PLQY was increased from 51% to 92% (Fig. 6b). The coumarin molecules improved the PLQY of Cs₃LnCl₆ NCs by passivating the halogen vacancy defects and increasing the crystallinity of the materials. By UV photoelectron spectroscopy (UPS) testing, they found that the coumarin molecule post-treated films had a low figure of merit (3.6 eV) and a high energy shift between the figure of merit and valence band (2.3 eV) (Fig. 6c). To extend the investigation, EHC molecules were introduced during the synthesis of Cs₃LnCl₆ (Ln: La, Pr, Nd, Sm, Eu, Tb, Er, Yb) NCs, resulting in lead-free perovskite materials with tunable emission wavelengths from the visible to the near-infrared range, together with a high PLQY (Fig. 6d). When excited by 365 nm light, Cs₃LnCl₆ NCs exhibit exciton emissions at ∼438 nm, consistent with previous studies in a similar context.¹⁰² In addition, the PL spectra show transitions related to the intrinsic electron transitions of lanthanide ions. These include the ¹D₂–³F₄ and ¹G₄–³H₅ transitions for Pr³⁺, ⁴F_3/2–⁴I_11/2 and ⁴F_3/2–⁴I_3/2 transitions for Nd³⁺, ⁴G_5/2–⁶H_J(5,7,9)/2 transition for Sm³⁺, ⁵D₀–⁷F_J(1,2,3,4) transition for Eu³⁺, ⁵D₄–⁷F_J(6,5,4,3) transition for Tb³⁺, ⁴I_13/2–⁴I_15/2 transition for Er³⁺ and the ²F_5/2–²F_7/2 transition for Yb³⁺.^103–105 Moreover, the introduction of EHC molecules significantly increased the overall PLQY of Cs₃LnCl₆ NCs compared to Cs₃LnCl₆ NCs without EHC (Fig. 6e).

Fig. 6 (a) Schematic diagram of the luminescence mechanism of Cs₃EuCl₆. (b) PLQY of Cs₃EuCl₆-NCs and Cs₃EuCl₆-NC-treated EHC. (c) Energy level diagram of the pristine film and EHC post-treated film obtained from parameters derived from UPS spectra (EVAC, vacuum level; EF, Fermi level). (d) Normalized PL spectra and (e) PLQY values of 4% EHC-treated Cs₃LnCl₆ (λ_ex = 365 nm). Reproduced with permission from ref. 59. Copyright 2023, Wiley. (f) Luminescence photographs of Cs₃EuCl₆ and Cs₃TbCl₆ NC films under excitation wavelengths from 260 to 360 nm with a 10 nm interval. (g) Direct writing using brush transfer printing under ambient light, 254 nm UV light, and 365 nm UV light. Reproduced with permission from ref. 35. Copyright 2023, The Royal Society of Chemistry.

In addition, Cs₃EuCl₆ and Cs₃TbCl₆ NC films also show the excitation-wavelength dependent emission colors, attributed to the presence of two different emission mechanisms. For instance, Cs₃EuCl₆ NCs exhibit blue emission under the excitation range of 310 to 360 nm and red emission under the excitation range from 260 to 330 nm, both of which depend on the excitation wavelength. This blue emission may be from the nanocrystals’ organic components or the d–f transition of Eu²⁺, and red emission is from the f–f transition of Eu³⁺. Consequently, the relative PL intensity ratio of blue and red PL can be systematically varied by changing the excitation wavelength, which enables the emission color to be fine-tuned over a wide spectrum range between blue and red, as illustrated in Fig. 6f. The dynamic color changes observed under conventional UV light suggest that Cs₃EuCl₆ and Cs₃TbCl₆ NCs may be suitable for use as writeable and printable security inks for high-level anti-counterfeiting applications. Fig. 6g illustrates the direct writing of a flower image on printing paper using Cs₃EuCl₆ and Cs₃TbCl₆ NCs and a writing brush. The marked image was invisible under ambient light, indicating that the images can be effectively hidden. Upon 365 nm UV light irradiation, a blue flower image emerges due to the luminescence of both the NCs. This image color changes to red and green under 254 nm UV light irradiation. Notably, this multicolor image is clearly visible under UV irradiation using a conventional hand UV lamp, and not using a high-power laser or time-gating equipment. The stamped images clearly show blue/red (Eu) or blue/green (Tb) dual-mode emission, depending on the excitation wavelength.^106–108

4.3. Upconversion luminescence in Cs₃LnCl₆ (Ln: Ho, Er, Tm, Yb)

Upconversion luminescent materials have attracted significant interest in the field of sensors due to their ability to penetrate deep into light-absorbing materials through the use of infrared excitation sources, while causing minimal damage to living tissues.

Cs₃LnCl₆ (Ln: Ho, Er, Tm, Yb) upconversion luminescent material has potential in the field of water detection and information encryption. Fig. 7a shows the upconversion luminescence (UCL) of different Er³⁺-doped Cs₃Gd_0.8−yYb_0.2Er_yCl₆ (y = 0.005, 0.01, 0.02 and 0.04) PCs. In particular, the 2% Er³⁺-doped sample (Cs₃Gd_0.78Yb_0.2Er_0.02Cl₆) showed the most intense luminescence compared to the other Er³⁺ doped samples. The UCL intensities of the 525, 550, and 660 nm peaks (green and red peak) were determined at varying Er³⁺ dopant concentrations. Consequently, green luminescence is generated from the ⁴H_11/2 and ⁴S_3/2 manifolds. For red luminescence, the ⁴F_9/2 (Er³⁺) manifold is populated via multiple phonon relaxations from the ⁴H_11/2/⁴S_3/2 manifold. In general, green luminescence and red luminescence occur via a two-photon process. Under 980 nm laser excitation, Cs₃Gd_(0.8−y)Yb_0.2Ho_yCl₆ NCs (y is 0.005, 0.01, 0.02 and 0.04, respectively) exhibit the typical UCL of Ho³⁺ ions. The characteristic emission peaks are 540 nm (⁵S₂, ⁵F₄ → ⁵I₈) and 650 nm (⁵F₅ → ⁵I₈), which are capable of emitting green and red light, respectively. Fig. 7b shows Ho³⁺ doped Cs₃Gd_(0.8−y)Yb_0.2Ho_yCl₆ NCs, the 0.5%Ho³⁺ sample (Cs₃Gd_0.795Yb_0.2Ho_0.005Cl₆) has the highest luminescence intensity. Excitation of Cs₃Gd_(0.8−x)Yb_0.2Tm_xCl₆ (x = 0.001, 0.003, 0.005, 0.01, and 0.02) PCs with a 980 nm laser resulted in the emission of intense near infrared (NIR) UCL at 815 nm (³H₄ → ³H₆) of Tm³⁺. In addition to the 815 nm peak, a weak peak at 770 nm was also observed. However, the Cs₃Gd_0.797Yb_0.2Tm_0.003Cl₆ PCs exhibited the most intense UCL compared with the other Tm³⁺-doped samples (Fig. 7c). The UCL mechanism in Cs₃Gd_0.78Yb_0.2Er_0.02Cl₆ NCs has been the subject of extensive research and is now well understood (Fig. 7d). In accordance with this mechanism, the Yb³⁺ ion, acting as a sensitizer, absorbs incoming NIR photons (with a wavelength of 980 nm) and efficiently transfers energy to the ⁴I_11/2 energy level of the Er³⁺ ion. Subsequently, the ⁴F_7/2 level of the Er³⁺ ion absorbs another photon from the ⁴I_11/2 level through an energy transfer up-conversion process. Consequently, green luminescence is generated by the ⁴H_11/2 and ⁴S_3/2 energy levels. With regard to red emission, excitation of the ⁴F_9/2 level of the Er³⁺ ion occurs as a result of relaxation of the ⁴H_11/2/⁴S_3/2 level of multiple phonons.^42,109 Analogous to the aforementioned example, the sensitizer Yb³⁺ absorbed NIR photons at 980 nm, leading to the excitation of the ⁵S₂ and ⁵F₄ energy levels of Ho³⁺ ions and the ³H₄ of Tm³⁺. The ⁵S₂ and ⁵F₄ energy levels of Ho³⁺ ions as well as the ³H₄ energy levels of Tm³⁺ ions could be populated either through energy transfer up-conversion or via energy transfer followed by excited state absorption.^110–113

Fig. 7 (a) Different Er³⁺ dopant concentrations in Cs₃Gd_1−x−yYb_xEr_yCl₆ PCs under 980 nm laser excitation; (b) different Ho³⁺ dopant concentrations in Cs₃Gd_(1−x−y)Yb_xHo_yCl₆ PCs upon 980 nm laser excitation; (c) different Tm³⁺ dopant concentrations in Cs₃Gd_(1−x−y)Yb_xTm_yCl₆ PCs upon 980 nm laser excitation. (d) Mechanism of energy transfer from Yb³⁺ to Er³⁺, Ho³⁺, and Tm³⁺ ions for UCL. (e) CIE coordinates of UCL from Er³⁺, Ho³⁺, and Tm³⁺. (f) The UCL mechanism of Cs₃GdCl₆:Yb³⁺,Er³⁺ PCs in the presence and absence of water molecules. Reproduced with permission from ref. 55. Copyright 2023, Wiley.

Interestingly, upon exposure to atmospheric moisture, these PCs absorbed water molecules to influence their UCL properties. Taking Cs₃Gd_0.78Yb_0.2Er_0.02Cl₆ as a case, with exposure time, the PL intensity of the red peak (λ_em = 660 nm) increased, and the green peak (λ_em = 550 nm) intensity decreased gradually. Thus, the UCL color changed from green to red with increasing exposure time (Fig. 7e). In addition, in order to understand the amount of moisture absorption of Cs₃GdCl₆:Yb³⁺,Er³⁺, the change in the intensity of green and red luminescence peaks with the change in the mass of Cs₃GdCl₆:Yb³⁺,Er³⁺ was measured. Also note that the vibrational energy of water (–OH group) overlaps with the ²F_5/2 energy level of the Yb³⁺ ions; consequently, the absorption of water molecules near the 980 nm wavelength causes the excitation photons to be attenuated. Thus, the overall UCL is quenched by the absorption of more water molecules. The role of water molecules in the upconversion process is shown in Fig. 7f.

Fig. 8a shows the variation of red-green UCL ratio with increasing water content. The UCL color of the solution starts to change from green to red, and when the water content of the tetrahydrofuran (THF) solution is 0.08%, the UCL color changes completely from green to red. Subsequently, a film in the form of a seven-segment display was fabricated, as shown in Fig. 8b, applying Cs₃GdCl₆:Yb³⁺,Er³⁺ PCs in a form that allows for information encryption and wet sensing applications. The prepared film is white in daylight, as shown in Fig. 8c, however, at 980 nm, the UCL reversibly turns red upon hydration and green upon dehydration. As shown in Fig. 8d, the pattern could not be distinguished under daylight conditions. The seven-segment display film was fabricated into a separable shape and separated in the desired pattern. By partial dehydration and hydration, a dial pattern image of 0–9 was obtained at 980 nm, confirming the information encryption effect of the film. In addition, the mechanical stability of the film was measured by folding and unfolding the film. This confirmed the mechanical stability of the film. In addition, Cs₃GdCl₆:Yb³⁺,Er³⁺ PC was cipher encrypted using a 10 mW cm⁻¹ laser. The Cs₃GdCl₆:Yb³⁺,Er³⁺ PC was processed into a plate and the ciphertext was written with the laser. As shown in Fig. 8e, when writing with the laser, the water in the area where the laser passes through will locally evaporate, after which the hidden information will no longer be visible to the naked eye but will be examined with 980 nm light. In addition, when the material is again hydrated by the moisture in the air, the message naturally disappears to remain secure. In order to confirm the possibility of encrypting the ciphertext, the initials “HY” of Hanyang University were laser-engraved here. A heart-shaped ciphertext was also drawn on the board as shown in Fig. 8f. After 15 minutes, the ciphertext disappeared. This confirms the feasibility of encrypting and re-encrypting the ciphertext using a Cs₃GdCl₆:Yb³⁺,Er³⁺ pc machine and a laser.

Fig. 8 Applications of Cs₃GdCl₆:Yb³⁺,Er³⁺ for sensing moisture and encrypting information. (a) Hydrochromic UCL in Cs₃GdCl₆:Yb³⁺,Er³⁺ nanoparticles in THF varies with water concentration. (b) Diagrams of the creation of a 7-segment display-like moisture-sensing film. (c) Color changes in the moisture-sensing film due to hydrochromic UCL when exposed to 980 nm light. (d) Pattern changes in the moisture-sensing film under both daylight and 980 nm irradiation. (e) A schematic illustrating the encryption of ciphertext using laser writing. (f) Visual representation of the encrypted information using 980 nm laser excitation to decrypt the ciphertext. Reproduced with permission from ref. 55. Copyright 2023, Wiley.

5. Conclusion and outlook

In this review, we briefly introduced several synthetic methods and optical properties of Cs₃LnCl₆ PCs and NCs. In order to better understand the intrinsic mechanism, luminescence modulation and optimization of Cs₃LnCl₆ luminescence, the crystal and electronic structures of Cs₃LnCl₆ are summarized. In particular, alloying and modification strategies are proposed to improve the PLQY and tunable emission. Challenges and perspectives are briefly summarized as follows.

(1) The synthesis of Cs₃LnCl₆ is complex, mainly due to the underdeveloped synthesis conditions and incomplete understanding of optimal reaction parameters. The lack of established experimental conditions and a thorough investigation of these parameters is a key challenge. Addressing these is crucial for research progress. The scarcity of literature on Gibbs free energy changes during the reaction underscores the need for a deeper exploration of the synthesis's thermodynamics and kinetics. Combining chemical reaction thermodynamics with kinetics can establish a robust theoretical framework to guide systematic condition exploration. This approach may identify optimal parameters for efficient Cs₃LnCl₆ synthesis, overcoming challenges and enabling more effective research methods.

(2) For Cs₃LnCl₆, the luminescence efficiency is too low for practical application, especially for nanocrystals. Therefore, various luminescence modulation or luminescence optimization strategies are used, such as doping with ns²-cationic metals/lanthanides or modifying the crystal surface with small organic molecules. Therefore, future research should focus on further exploring these methods to enhance luminescence performance and delving into their mechanisms.

(3) In addition to the existing lead-free metal halide application scenarios, the luminescent properties of Cs₃LnCl₆ can be more suitable for application. Cs₃LnCl₆ has a high degree of compatibility with rare earth elements, which extends beyond traditional visible light research. It can be used to study near-infrared luminescence and imaging applications, as well as for multi-mode anti-counterfeiting through up-conversion luminescence. In addition, Cs₃LnCl₆ undergoes a phase change when in contact with water, a property that suggests potential applications in anti-counterfeiting and environmental moisture monitoring. Referring to the current application methods and combining with practical needs, it is better to make people's life better while enriching the application scenes.

In conclusion, the lead-free lanthanide metal halide Cs₃LnCl₆ has garnered increasing scholarly interest due to its superior chemical stability, commendable lattice adaptability, and distinctive characteristics within the realms of optics and photovoltaics. Despite the existence of extant methodologies for the modification and enhancement of PLQYs, the domain necessitates the engagement of a broader research community to further refine the intrinsic properties of these materials or to diversify their applications. Additionally, computational approaches like ICSD data or high-throughput screening DFT for phase competition can be utilized to support the analysis of synthetic routes and reaction conditions, in the pursuit of determining the most efficient synthesis pathway for Cs₃LnCl₆. Employing such methods to systematically explore reaction conditions and identify suitable synthetic routes can streamline the experimental process and prevent resource wastage, making this approach highly worthwhile to pursue. Furthermore, Cs₃LnCl₆ exhibits a favourable interaction with water. Materials such as Cs₂YbCl₅·HO and Cs₃CeCl₆·3H₂O are progressively garnering the attention of researchers.^95,114 These materials frequently demonstrate high PLQY and robust stability, undergo phase transition in the presence of water, and can be more effectively utilised in anti-counterfeiting applications.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (2023YFB3506600), the National Natural Science Foundation of China (NSFC No. 12374386, 12374388, 12304461, 52172166, 52072349 and 51932009), and the Project for Science and Technology Development Plan of Jilin Province (20240101316JC).

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