In the past few years, metal halide perovskites as luminescent materials have been widely studied because of their intriguing optoelectronic properties, including spectral tenability and high photoluminescence quantum yields (PLQYs) [1-4]. As the key to the development of next-generation displays, luminescent materials used as light-emitting diode (LED) are required not only to meet the color purity standards [5-7], but also to be low-cost and environmental friendliness [8-10]. Although great progresses have been achieved, the stability issue of perovskite is still a bottleneck [11-14]. Furthermore, lead-free metal halide perovskites with intense emission in deep-red spectral region (> 660 nm) are very rare and remain great developing challenge [15, 16], although they are more desirable for display, photoelectric detection, biological tissue imaging, and solid-state lighting [17-20]. Therefore, it is of great significance to develop environmental-friendly, stable metal halides with highly efficient deep-red luminescence.

Recently, the zero-dimensional (0D) metal halide Cs₂ZnCl₄ with wide bandgap of 4.2 eV and negligible luminescence [21] are once again becoming excellent candidates as active materials, because the luminescence properties of the materials used Cs₂ZnCl₄ as host can be greatly improved by doping strategy. For example, Cheng et al. [22] reported the Cu⁺-doped blue emitting Cs₂ZnX₄ with PLQY of 65.3%. Su et al. [23] developed a near-infrared emission single crystal by incorporating Sb³⁺ into the Cs₂ZnCl₄ matrix with high PLQY of 69.9%. Although the PLQYs of these materials have been greatly enhanced compared with that of the pristine Cs₂ZnCl₄, their properties are still unsatisfactory, especially their structural stability towards humidity and thermal. Generally, there are many factors that determine the color tunability and PL efficiency of emitting materials, and the structure of luminescent center is a key one of them [24-28]. Because Cs₂ZnCl₄ has an orthorhombic crystal structure with disconnected [ZnCl₄]²⁻ tetrahedrons, when the doped metal ion M^x+ (x = 1, 2) is introduced into Cs₂ZnCl₄, a part of [ZnCl₄]²⁻ tetrahedrons should be replaced by [MCl₄]^y− (y = 2, 3) tetrahedrons, which is the origin of strong emission for doped Cs₂ZnCl₄ [22, 29]. Previous reports [30, 31] have demonstrated that the four coordinated Sn²⁺ ions are prone to induce red emission. When Sn²⁺ is used as a dopant, more luminescent centers associated with Sn²⁺ ions are introduced into the lattice of Cs₂ZnCl₄, which induces the generation of self-trapped excitons (STEs), ultimately leading to the formation of STE states with a deeper self-trapping depth [32-34]. Therefore, it is anticipated that the band gap of Cs₂ZnCl₄ would be reduced further and the deep-red emission can thus be realized by introducing Sn²⁺ into the lattice. The Sn²⁺-doped Cs₂ZnCl₄ microcrystal has recently been developed by Wu and co-workers [35]. While its PL efficiency is unknown and the behind PL mechanism is poorly understood. For Cs₂ZnCl₄: Sn, the synthesis of its single crystals with lower defect densities is challenging. Thus, efficient deep-red emission is highly expected in Cs₂ZnCl₄: Sn single crystals, and the corresponding photophysical properties remain to be unveiled further.

Herein, we report a Sn²⁺-doped 0D metal halide Cs₂ZnCl₄ (Cs₂ZnCl₄: Sn) with a broadband deep-red emission peaked at about 700 nm. By introducing Sn²⁺ into Cs₂ZnCl₄, an unprecedented improvement of PLQY (~99.4%) and outstanding stability (the PLQY of Cs₂ZnCl₄: Sn only decreases 4% when it is exposed to the air with relative humidity of 80% for 370 days) were realized. To the best of our knowledge, this is the best performance reported to date for any lead-free metal halides with deep-red emission. Detailed spectral characterizations and density functional theory (DFT) calculations revealed that the bright deep-red emission in Cs₂ZnCl₄: Sn originates from the STEs induced by the doped Sn²⁺ ion. For comparison, the Sn²⁺-doped Cs₂ZnBr₄ (Cs₂ZnBr₄: Sn) emitting deep-red light (710 nm) was also synthesized using the same method, and its PLQY (~33.9%) and stability are also very prominent.

Cs₂ZnCl₄ and Sn²⁺-doped Cs₂ZnCl₄ (i.e., Cs₂ZnCl₄: Sn) single crystals were obtained through solvothermal reaction. The detailed synthesis and characterization are presented in Supporting information. It is reported that the crystal of Cs₂ZnCl₄ is orthorhombic space group Pnam [21]. The [ZnCl₄]²⁻ tetrahedrons are isolated from each other by Cs⁺ cations and thus form a 0D structure (Fig. 1a). Powder X-ray diffraction (PXRD) patterns of Cs₂ZnCl₄ and Cs₂ZnCl₄: Sn were measured and the main diffraction peaks were found to be unique to Cs₂ZnCl₄ (Fig. 1b). Compared with Cs₂ZnCl₄, the diffraction peak of Cs₂ZnCl₄: Sn at 20.5° (2θ) moves to a smaller angle, indicating that Sn²⁺ is incorporated into the Cs₂ZnCl₄ successfully. Moreover, Cs₂ZnBr₄ and Cs₂ZnBr₄: Sn were also synthesized and verified by PXRD (Fig. S1 in Supporting information). The oxidation state of Sn²⁺ was confirmed to be positively divalent by X-ray photoelectron spectroscopy (XPS) (Fig. S2 in Supporting information). As shown in Fig. S2d, the main bands located at 487 and 496 eV are derived from the Sn²⁺ 3d⁵ state. Inductively coupled plasma optical emission spectroscopy (ICP-OES) reveals the actual content of Sn²⁺ in various Cs₂ZnCl₄: Sn and Cs₂ZnBr₄: Sn samples. The results shown in Table S1 (Supporting information) unveil the actual Sn²⁺ content of the maximum feeding ratio samples for Cs₂ZnCl₄: Sn and Cs₂ZnBr₄: Sn is only 1.42% and 1.12%, respectively, implying that the feeding Sn²⁺ precursor was only partially embedded into the Cs₂ZnX₄ lattice. Moreover, the slight shift of the XRD peaks towards a small angle further demonstrates the incorporation of trace amounts of Sn²⁺ in Cs₂ZnX₄. Nevertheless, the photophysical properties of Cs₂ZnX₄ have been modulated greatly after Sn²⁺ doping, which will be discussed later. In this article, unless otherwise specified, the samples with the highest actual doping rate (i.e., Cs₂ZnCl₄: 1.42%Sn and Cs₂ZnBr₄: 1.12%Sn) were chosen for all tests, and abbreviated as Cs₂ZnCl₄: Sn and Cs₂ZnBr₄: Sn for simplicity. To shed new light on the structural changes of doped samples, Raman spectra of Cs₂ZnCl₄ and Cs₂ZnCl₄: Sn were measured. As shown in Figs. 1c and d and Table S2 (Supporting information), the Raman peaks of Cs₂ZnCl₄ and Cs₂ZnCl₄: Sn are very similar, indicating that the introduction of Sn²⁺ does not destroy the lattice of Cs₂ZnCl₄. Here, the relative intensity and the positions of Zn-Cl stretching mode (ν₁-ν₄) and Cl-Zn-Cl bending mode (ν₅-ν₉) are changed slightly for Cs₂ZnCl₄: Sn due to the distortion of [ZnCl₄]²⁻ tetrahedron after Sn²⁺ doping. The above analysis confirmed that Sn²⁺ was successfully doped into the lattice of Cs₂ZnCl₄ by substituting a small part of Zn²⁺.

The optical properties of these materials were characterized by UV–vis absorption spectroscopy and PL spectroscopy. The solid-state UV–vis spectrum of Cs₂ZnCl₄ in Fig. 2a exhibits absorption below 400 nm, with an intense absorption peak at ~270 nm, indicating the negligible absorption in visible-light region. While a shoulder peak between 300 and 400 nm emerges after Sn²⁺ doping, which can be attributed to the absorption of [SnCl₄]²⁻ tetrahedron, suggesting the successful embedding of Sn²⁺ in Cs₂ZnCl₄ [36]. As shown in Fig. 2b, an additional narrow-band peak (~350 nm) appears in the PL excitation (PLE) spectrum, which is the main feature of the red emission observed for the Sn²⁺-doped materials [37, 38]. Figs. 2b and c and Table S3 (Supporting information) provide the room-temperature PL properties of Cs₂ZnCl₄ with different Sn²⁺ content, revealing that the PLQY gradually increases with the increase of Sn²⁺ doping concentration. This is because that more luminescent centers associated with Sn²⁺ ions are introduced into the lattice of Cs₂ZnCl₄ and the optimum luminescent performance is obtained when the actual doping amount reaches n_Sn: n_Zn = 1.42%. After that, impurity such as CsSnCl₃ was detected. For Cs₂ZnCl₄: 1.42%Sn, a deep-red emission centered at ~700 nm is observed under 280 nm excitation with PLQY of 99.4% and the corresponding Commission Internationale de L'Eclairage (CIE) coordinates is (0.58, 0.39). It is worth adding that, to the best of our knowledge, such PLQY is the highest value of deep-red light among the reported metal-halide single crystals (Table S4 in Supporting information). The PL decay curves of Cs₂ZnCl₄: Sn with different Sn²⁺ concentrations were shown in Fig. 2d, and the corresponding data were presented in Table S5 (Supporting information). The average PL lifetime in Fig. 2d indicates a slight increase in PL lifetime with the increase of doping concentration, and the lifetime of Cs₂ZnCl₄: 1.42%Sn is up to 18.3 µs. Such long lifetime, large Stokes shift and broad PL line suggest the deep-red emission might be from the radiative recombination of triplet STEs in [SnCl₄]²⁻ tetrahedron, by inhibiting the non-radiative recombination in Cs₂ZnCl₄.

To gain more insight into the effect of Sn²⁺ on the photophysical properties of Cs₂ZnCl₄, first principle calculations were carried out. As displayed in Fig. 3a, the calculated energy bandgap of Cs₂ZnCl₄ is 4.62 eV, while that of Sn²⁺-doped Cs₂ZnCl₄ is reduced to 3.42 eV (Fig. 3b), which is in agreement with the experimentally reduced trend of bandgap (4.15 eV for Cs₂ZnCl₄ and 3.58 eV for Cs₂ZnCl₄: Sn). It is worth noting that the calculated band structure of Sn-doped Cs₂ZnCl₄ is more flatter, compared to that of the undoped Cs₂ZnCl₄, suggesting the excited carriers are more localized after the incorporation of Sn²⁺. Upon examining the projected density of states (DOS) (Fig. S3 in Supporting information), the resultant valence band maximum (VBM) of Cs₂ZnCl₄ is mainly composed of Cs-p, Zn-d and Cl-p orbitals, and the conduction band minimum (CBM) consists of Cs-d, Zn-s and Cl-p orbitals. Conversely, for Cs₂ZnCl₄: Sn, its VBM are mainly contributed by Sn-s and Cl-p orbitals, and CBM are mainly composed of Sn-p and Cl-p orbitals. This verifies that the embedded Sn²⁺ ions change the position of VBM and CBM of Cs₂ZnCl₄ and reduce the band gap, resulting in the deep-red emission in Sn²⁺-doped Cs₂ZnCl₄. In addition, the calculated charge density (Figs. 3c and d) demonstrates that the carriers are delocalized in both VBM and CBM of Cs₂ZnCl₄, whereas those are localized in [SnCl₄]²⁻ tetrahedrons in Sn²⁺-doped Cs₂ZnCl₄, agreeing perfectly with the DOS results. The highly localized charge density induces a statically expressed distortion of the [SnCl₄]²⁻ tetrahedrons due to the presence of stereoactive 5s² lone pair in Sn²⁺ [33, 39], which is conducive to the formation of STEs, and thereby leading to the broadband deep-red emission [40].

The radiative recombination in 0D metal halides can originate from many aspects, such as permanent defects, free excitons (FE) and STEs. To probe into the origin of the deep-red emission, PLE and PL spectra of Cs₂ZnCl₄: Sn monitored at different wavelengths were carried out, and the consistency of the peaks shown in Fig. S4 (Supporting information) indicates the deep-red emission origins from an identical excited states. Further, upon the excitation power dependent PL measurements, the PL intensity of deep-red emission for Cs₂ZnCl₄: Sn is linear with the excitation power (Fig. 4a), eliminating the possibility that the deep-red emission is resulted from permanent defects. In contrast to FE emission that typically features with narrow PL band, small Stokes shifts and short lifetime is at nanosecond level, the PL spectrum of Sn²⁺-doped Cs₂ZnCl₄ exhibits an ultra-broad full width at half maximum (FWHM) over 158 nm, a large Stokes shifts of 420 nm and a long PL lifetime up to 18.3 µs at room temperature, ruling out the possibility of FE emission. Combined with the calculated results, it can be reasonably speculated that the strong deep-red light of Cs₂ZnCl₄: Sn is originated from the triplet STE emission induced by Sn²⁺ dopant. To prove such conjecture, temperature-dependent PL spectra of Cs₂ZnCl₄: Sn was measured and the corresponding results were depicted in Fig. 4b and Fig. S5 (Supporting information). With the decrease of temperature, the maximum peak position gradually redshifts from 700 nm to 740 nm, as a result of the intense electron-phonon interaction [41]. Meanwhile, the PL intensity enhances with the decrease of temperature, suggesting the suppressed of nonradiative recombination. When the temperature is lower than 150 K, an obvious emission centered at 470 nm appears. Furthermore, as shown in Fig. S6 (Supporting information), the exciton binding energy (E_b) can be calculated with 190 meV, which is much higher than the thermal energy at room temperature (26 meV), providing a high probability of radiative combination in Cs₂ZnCl₄: Sn [42]. Meanwhile, the relationship between FWHM and temperature can be fitted, with a calculated Huang-Rhys factor (S) value of 62.6. Such a large S indicates the presence of a strong electron-phonon coupling effect in Cs₂ZnCl₄: Sn, which enables the formation of STEs and the broadening of PL band. To further probe into such emission, PL decay curves of Cs₂ZnCl₄: Sn were measured at 80 K (Fig. 4c and Table S6 in Supporting information). For comparison, the PL and PL decay lifetimes of Cs₂ZnCl₄ were also measured at the 80 K (Fig. S7 in Supporting information and Table S6). As shown in Table S6, the lifetime of the emission peak at 450 nm in Cs₂ZnCl₄ is 20 ns, much shorter than that of the emission peak at 470 nm in Cs₂ZnCl₄: Sn (10.33 µs), indicating that the emission at 470 nm also comes from the triplet STEs. For Sn²⁺ ions with 5s² electronic configuration, its excited states can be composed of one singlet state (¹P₁) and three triplet states (³P₀, ³P₁ and ³P₂), where the triplet states ³P₀, ³P₁ and ³P₂ can be further split due to spin-orbit interactions [43]. Undoubtedly, for Cs₂ZnCl₄: Sn, the emission peak at 740 nm in 80 K is attributed to the ³P₁→¹S₀ transition of Sn²⁺ ion [33], while the emission peak at 470 nm in 80 K may be assigned to the transition of ³P₂→¹S₀, as a result of the break of its forbidden nature at low temperature. In this situation, the crystal symmetry might be modulated or broken to convert the forbidden transition of ³P₂→¹S₀ to an allowed transition, thus further enabling the radiative emission, which has also been seen in bismuth-based materials [44].

Following the above results, the possible photophysical process of Cs₂ZnCl₄: Sn is illustrated in Fig. 4d. Briefly, upon the photoexcitation, electrons in the ground state are promoted to the excited states of [SnCl₄]²⁻. And simultaneously, the highly localized electrons in excited states combining with the stereoactive 5s² lone pair in Sn²⁺ induce structural distortion of [SnCl₄]²⁻ polyhedron that further leads to the formation of STEs [45]. Then, the excitons will undergo an intersystem crossing (ISC) process to triplet STE state, where some of them are quickly self-trapped to relatively high energy states, enabling the blue light (~470 nm) emission, while the rest will transfer to low energy STE state, finally resulting in the highly efficient deep-red emission with a large Stokes shift and long lifetime.

To expand the scope of materials, Cs₂ZnBr₄: Sn was also synthesized and studied to uncover the effect of halogen on the PL properties, details of which can be found in Supporting information. Furthermore, as an excellent luminescent material, except for the outstanding luminescent efficiency, stability is also important for practical applications. As illustrated in Fig. S10a (Supporting information), the thermal stability of Cs₂ZnCl₄: Sn and Cs₂ZnBr₄: Sn was evaluated by thermogravimetry analysis (TGA), and the results reveal that both of the two materials are thermally stable up to 610 ℃, indicating their excellent thermal stability that is extremely important for optoelectronic applications. Moreover, their humidity stability was also investigated. As shown in Fig. S10b (Supporting information), the XRD profiles of the two materials after exposing to the atmosphere (relative humidity > 80%) for one year exhibit almost no significant change compared with that of their freshly prepared samples, manifesting that both Cs₂ZnCl₄: Sn and Cs₂ZnBr₄: Sn are resistant to the humidity. Surprisingly, after exposing to atmosphere for one year, the PLQYs of Cs₂ZnCl₄: Sn decreased by only 4% (~30% for Cs₂ZnBr₄: Sn) compared with its initial values (Fig. S10c in Supporting information), further confirming its excellent PL stability towards humidity. Generally, Sn²⁺ is unstable and can be easily oxidized to Sn⁴⁺ in air. In these two materials, the actual doping concentration of Sn²⁺ is extremely low (1.42% for Cs₂ZnCl₄: Sn and 1.12% for Cs₂ZnBr₄: Sn), thus Sn²⁺ is hardly oxidized in atmosphere with high humidity. Moreover, it is noteworthy that the Sn-X (X = Cl, Br) bond lengths here (2.28 Å for Cs₂ZnCl₄: Sn and 2.43 Å for Cs₂ZnBr₄: Sn) are significantly shorter than that in the previous reports (both Sn-Cl and Sn-Br are > 2.5 Å) (Table S7 in Supporting information), which allows for stronger interactions between the Sn²⁺ ion and the halogen, resulting in the excellent stability of the studied materials.

In summary, we have successfully prepared the Sn²⁺-doped 0D free-lead metal halides Cs₂ZnX₄: Sn (X = Cl, Br) with bright deep-red emission. The Cs₂ZnCl₄: Sn exhibits deep-red emission (700 nm) with ultra-high PLQY (99.4%) owing to the introduction of Sn²⁺ ions which can reduce the band gap of the native and induce triplet STE emission. Such a superior PL performance is attributed to the 5s² lone pair electron expression of the Sn²⁺ ion. Most importantly, Cs₂ZnCl₄: Sn possess outstanding PL and structural stability, maintaining the PLQY under ambient air with relative humidity over 80% for more than 370 days. The present work not only sheds insight into the luminescent mechanism of Cs₂ZnX₄: Sn (X = Cl, Br), but also provides an efficient, stable and environment-friendly deep-red emitting material for promising optoelectronic devices and biological imaging.

The authors report no declarations of interest.

We acknowledge the financial supports from National Natural Science Foundation of China (Nos. 91741105, 22109130), Chongqing Municipal Natural Science Foundation (Nos. cstc2018jcyjAX0625, cstc2021jcyj-msxmX1180), and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (No. CXTDX201601011).

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.070.