Enabling efficient near-infrared emission in lead-free double perovskite via a codoping strategy

Abstract

Metal halides show great promise as a new generation of near-infrared (NIR) light-emitting materials. Compared with other light-emitting materials, double perovskites possess structures with different dimensionalities, which can support multiple emission centers, leading to varied photoluminescence. Among various doping centers, ytterbium(III) (Yb³⁺) has attracted attention because of its unique two-energy-level structure (²F_5/2 and ²F_7/2). However, the NIR emission of Yb³⁺ remains unsatisfactory because of poor resonance energy transfer between Yb³⁺ and sensitizers. Here, effective NIR-emitting lead-free perovskites are developed by co-doping antimony(III) (Sb³⁺) and lanthanide(III) ions into Cs₂NaInCl₆. Under excitation at 318 nm, Cs₂NaInCl₆:Sb³⁺/Yb³⁺ showed a broadband NIR emission peak at 1001 nm, whereas Cs₂NaInCl₆:Sb³⁺/Nd³⁺ exhibited three NIR emission peaks at 896, 1077, and 1358 nm. The exciton dynamics of the materials were investigated. Experiments and density functional theory calculations revealed that the NIR emission of Yb³⁺ originated from a charge-transfer state (CTS) and energy transfer, whereas that of Nd³⁺ arose from resonance energy transfer. Profiting from the high-energy self-trapped exciton (STE) emission and CTS with Yb³⁺, a high photoluminescence quantum yield of 48.95% was realized. The excellent NIR luminescence performance combined with high environmental stability demonstrates the potential of these metal halides for use in night-vision technologies.

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1. Introduction

All-inorganic metal halide perovskites are promising materials for light-emitting technologies because of their excellent photophysical properties such as high photoluminescence quantum yield (PLQY), tunable light emission wavelength, and cost-effective synthesis.^1–4 Presently, the development and preparation techniques for visible light-emitting perovskites are relatively mature, adequately meeting demands in areas such as illumination and displays.^5–7 However, the evolving needs of medical imaging, optical communications, optical storage, military reconnaissance, security monitoring, and sensing generate demand for lead-free near-infrared (NIR)-emitting metal halide perovskites.^8–11 This demand arises from the advantageous properties of NIR light sources, such as strong penetration, minimal biological damage, and high resolution.^12–14 NIR light-emitting diodes (LEDs) that can be used for full-spectrum (400–1100 nm) illumination, infrared security anti-counterfeiting, and infrared imaging have emerged as a burgeoning area of research.¹⁵ The most commonly used method for achieving NIR luminescence from perovskites involves doping with lanthanide or transition metal ions.^16–18 However, the traditional ABX₃ perovskite structure cannot meet the requirements of trivalent-ion dopants.¹⁹ Consequently, a strategy of heterovalent double-cation co-substitution (e.g., B⁺ + B^′3+ → Pb²⁺ + Pb²⁺) has been adopted to fabricate stable inorganic double-perovskite systems (A₂BB′X₆).^20–22 The challenges of low PLQY and restricted emission range have emerged as technological bottlenecks in the development of NIR-emitting perovskites.²³ Thus, the pursuit of an effective method to achieve high-efficiency, long-wavelength NIR emission holds paramount importance for expanding the applications of NIR technology.

Currently, lanthanide ions (Ln³⁺; e.g., Er³⁺, Ho³⁺, Tm³⁺, Sm³⁺, Dy³⁺, Yb³⁺, Nd³⁺) and transition metal ions (e.g., Mn⁴⁺, Cr³⁺, Cr⁴⁺) are popular activators for NIR luminescence.^24–26 However, achieving high PLQYs in halide matrices with these activator ions through single ion-doping strategies is rarely possible. Energy transfer (ET) is an effective approach to improve NIR PLQY and extend the emission wavelength range of metal halides.^27,28 For example, NIR luminescence was achieved by co-doping Te⁴⁺ and Nd³⁺ into a Cs₂ZrCl₆ host; ET from yellow-emitting Te⁴⁺ to NIR-emitting Nd³⁺ was clearly observed.²⁹ The simple energy-level structure (²F_5/2 and ²F_7/2) of Yb³⁺, which helps to suppress non-radiative energy losses, has led to it becoming a popular activator ion in the field of NIR luminescence.^30–32 However, the NIR PLQYs and ET efficiency of Yb³⁺-doped materials remain low because of the reliance of the traditional resonant ET mechanism on the degree of overlap between the emission band of the photoreceptor and absorption band of the activator. For example, while Bi³⁺/Yb³⁺-co-doped Cs₂Na_0.6Ag_0.4InCl₆ double perovskites showed enhanced NIR emission because of ET from self-trapped excitons (STEs) to Yb³⁺, the ET efficiency was not ideal.^33,34 Therefore, suitable transfer strategies need to be identified to facilitate ET and obtain high-efficiency NIR emission.

Herein, high-efficiency NIR emission is achieved by co-doping Sb³⁺ and Ln³⁺ in Cs₂NaInCl₆. The Jahn–Teller deformation of the [SbCl₆]³⁻ octahedron in the STE process leads to efficient formation of ET channels, enabling efficient NIR luminescence from Ln³⁺ in Cs₂NaInCl₆:Sb³⁺. With the help of a charge-transfer state (CTS) of Yb³⁺, efficient NIR emission at 1001 nm with a high PLQY of 48.95% is obtained from Cs₂NaInCl₆:Sb³⁺/Yb³⁺ under 318 nm excitation. In addition, three NIR emission peaks at 896, 1077, and 1358 nm with a high PLQY of 11.29% are exhibited by Cs₂NaInCl₆:Sb³⁺/Nd³⁺ under 318 nm excitation. Experimental and density functional theory (DFT) calculation results reveal that the NIR emission of Yb³⁺ could be derived from a CTS and ET, whereas that of Nd³⁺ could originate from resonance ET.

2. Results and discussion

Cs₂NaInCl₆ crystallizes into a cubic structure (space group: Fd3̄m) consisting of alternating [InCl₆]³⁻ and [NaCl₆]⁵⁻ octahedra with large Cs⁺ occupying the center of the cuboctahedral cavities (Fig. 1a).³⁵ High-quality Cs₂NaInCl₆:Sb³⁺/Ln³⁺ (Ln³⁺ = Nd³⁺ or Yb³⁺) single crystals were prepared via a facile hydrothermal synthesis method (Fig. S1†). Sb³⁺ and Ln³⁺ tend to occupy the In³⁺ sites because of their similar ionic radii and valence.³⁶ Powder X-ray diffraction (PXRD) measurements showed that all diffraction peaks matched well with the standard pattern of Cs₂NaInCl₆ (ICSD#132718). With increasing Nd³⁺ or Yb³⁺ content, the diffraction peaks shifted to lower angles because the ionic radii of Nd³⁺ (98 pm) and Yb³⁺ (87 pm) are larger than that of In³⁺ (80 pm), as shown in Fig. S2 and S3.†³⁷ Scanning electron microscopy (SEM) images showed that the samples were polyhedral with mean sizes of about 5 μm. Energy-dispersive X-ray spectroscopy (EDS) elemental mapping revealed that the dopants were homogeneously distributed in the samples (Fig. 1c and S4–S6†). These results suggest the successful incorporation of Sb³⁺ and Ln³⁺ in the double perovskite crystal lattice. The dopant concentrations were estimated by inductively coupled plasma mass spectrometry (ICP-MS); the results are listed in Tables S1 and S2.† The ICP-MS results are similar to the EDS element content results, and the accuracy of the measurement is verified again (Table S3†). The actual content of Ln³⁺ in the samples is shown in Fig. 1d. There was a considerable difference between the actual doping concentration of the Cs₂NaInCl₆:Sb³⁺/xLn³⁺ samples and the feed ratio of Yb³⁺ and Nd³⁺. Similar trends have been reported for doped halides such as Cs₂KInCl₆:Sb³⁺/Ho³⁺, Cs₂AgInCl₆:Cr³⁺/Er³⁺, and Cs₂ZrCl₆:Sb³⁺/Nd³⁺.^38–40 The doping ratio stated in this study refers to the feed ratio of Ln³⁺ rather than the actual doping concentration.

Fig. 1 (a) Schematic illustrations of Cs₂NaInCl₆ crystal structures with Sb³⁺ and Ln³⁺ (Ln = Yb, Nd) doping. (b) The PXRD patterns of host, Sb³⁺-doped, Sb³⁺/Yb³⁺ co-doped, and Sb³⁺/Nd³⁺ co-doped Cs₂NaInCl₆. (c) SEM image and EDX elemental mappings (Cs, Na, In, Cl, Sb, Yb, and merge) of Cs₂NaInCl₆:Sb³⁺/Yb³⁺. (d) ICP-MS results of Ln³⁺ amount in Cs₂NaInCl₆:Sb³⁺/Yb³⁺ and Cs₂NaInCl₆:Sb³⁺/Nd³⁺.

To explore the photophysical process of Cs₂NaInCl₆:Sb³⁺/Ln³⁺ in detail, photoluminescence spectra in the visible and NIR regions were recorded. The PLQY of the Cs₂NaInCl₆ matrix at RT was less than 1%. The Sb³⁺-doped sample showed a high-energy blue emission peak at 446 nm with a full width at half maximum (FWHM) of 128 nm (Fig. S7†), which was attributed to the STE emission in the [SbCl₆]³⁻ octahedra caused by strong Jahn–Teller lattice distortion.⁴¹ Under 318 nm excitation, in addition to the blue emission, an intense NIR emission at 1001 nm was observed in Sb³⁺/Yb³⁺-co-doped Cs₂NaInCl₆, which was assigned to the ²F_5/2 → ²F_7/2 transitions of Yb³⁺ (Fig. 2a and b).³¹ As shown in Fig. 2c, the STE emission intensity decreased and the NIR emission intensity greatly increased as the Yb³⁺ doping content increased to 60%. Higher Yb³⁺ doping concentration than 60% led to a decrease in NIR efficiency because of concentration quenching. The Sb³⁺/Nd³⁺-co-doped Cs₂NaInCl₆ samples displayed a blue emission peak at 446 nm and three sharp NIR emission bands at 896, 1077, and 1358 nm, which were assigned to the ⁴F_3/2 → ⁴I_9/2, ⁴F_3/2 → ⁴I_11/2, and ⁴F_3/2 → ⁴I_13/2 transitions of Nd³⁺, respectively (Fig. 2d and e).⁴² The NIR emission signals exhibited millisecond-scale lifetimes of 3.29 ms for that originating from Yb³⁺ and 4.08 ms for that derived from Nd³⁺ (Fig. S8 and S9†), which were characteristic of Laporte forbidden f–f transitions.^32,40 The NIR emission intensity greatly increased and STE emission intensity decreased with increasing Nd³⁺ content, as depicted in Fig. 2e. Both Sb³⁺/Yb³⁺ and Sb³⁺/Nd³⁺ co-doped samples showed excellent NIR luminescence properties. The highest NIR emission intensity of Sb³⁺/Yb³⁺-co-doped samples with a PLQY of 48.95% was obtained when the Yb³⁺ feed ratio was 60% (Fig. S10†). It has shown good performance in the recently reported NIR PLQY of metal halides (Table S4†). The highest NIR PLQY of Sb³⁺/Nd³⁺-co-doped Cs₂NaInCl₆ of 11.29% was obtained when the Nd³⁺ feed ratio was 70% (Fig. S11†). The sensitization induced by Sb³⁺ caused the NIR emission intensity of Sb³⁺/Yb³⁺ and Sb³⁺/Nd³⁺ co-doped samples to increase considerably compared to that of Yb³⁺ or Nd³⁺ singly doped samples (Fig. S12 and S13†).

Fig. 2 (a) Visible and (b) NIR emission spectra of Cs₂NaInCl₆:Sb³⁺/Yb³⁺. (c) Integrated PL intensities of visible and NIR emission of Cs₂NaInCl₆:Sb³⁺/Yb³⁺ under 318 nm excitation. (d) Visible and (e) NIR emission spectra of Cs₂NaInCl₆:Sb³⁺/Nd³⁺. (f) Integrated PL intensities of visible and NIR emission of Cs₂NaInCl₆:Sb³⁺/Nd³⁺ under 318 nm excitation.

To reveal the luminescence mechanism of the samples, absorption and photoluminescence excitation (PLE) spectra of the co-doped samples were collected. As shown in Fig. 3a, Yb³⁺ singly doped Cs₂NaInCl₆ exhibited an additional absorption peak at about 270 nm compared to the absorption spectra of undoped and Nd³⁺ singly doped Cs₂NaInCl₆, matching closely with the PLE spectra (Fig. 3b). The excitation band at 266 nm corresponded to the Yb³⁺ absorption of the Cl⁻ → Yb³⁺ CTS.^29,43,44 Sb³⁺/Yb³⁺ and Sb³⁺/Nd³⁺ co-doped Cs₂NaInCl₆ samples exhibited an intense absorption peak at 320 nm, which was attributed to Sb³⁺. As the feed ratio of Ln³⁺ rose, the actual doping concentration of Sb³⁺ increased (Tables S1 and S2†), and thus the corresponding absorption peak of Sb³⁺ increased in intensity at high Ln³⁺ doping concentration, as shown in Fig. S14 and S15.† The PLE spectrum monitored at 446 nm displayed characteristic excitation bands at 318 and 333 nm, which were assigned to the ¹S₀–³P₁ transition of Sb³⁺ (PLE spectra are shown in Fig. S16 and S17†). The PLE spectrum of Sb³⁺/Nd³⁺ co-doped Cs₂NaInCl₆ monitored at 1077 nm contained two weak narrow excitation peaks at 534 and 588 nm, consistent with that of Nd³⁺ singly doped Cs₂NaInCl₆, which were attributed to the direct excitation of Nd³⁺.⁴² The PLE spectra of visible (446 nm) and NIR (1077 nm) emissions were similar (Fig. 3c), indicating that these emissions have the same excited-state origin. Notably, the emission of Ln³⁺ originating from the f–f transition is usually sensitive to the excitation wavelength, while the emission spectra were almost constant with the variation of the excitation wavelength (Fig. 3e), suggesting that the NIR luminescence of Nd³⁺ mainly depends on the sensitization of Sb³⁺. In addition, the excitation intensities at 534 and 588 nm were much lower than that at 318 nm, indicating that Nd³⁺ excitation through ET was more efficient than direct excitation. Because of the wide excitation range and high absorption strength of STE emission in the Sb³⁺/Nd³⁺ co-doped Cs₂NaInCl₆ system, the weak absorption of the f–f transition can be overcome by the efficient ET, thus realizing efficient NIR luminescence. Notably, however, the NIR efficiency is still relatively low, which may be related to the small overlap between the emission region of Sb³⁺ and absorption region of Nd³⁺ limiting the ET efficiency.

Fig. 3 (a) Optical absorbance spectra of undoped, Sb³⁺-doped, Yb³⁺-doped, Nd³⁺-doped, Sb³⁺/Yb³⁺ co-doped, and Sb³⁺/Nd³⁺ co-doped Cs₂NaInCl₆. Normalized PLE spectra of (b) Yb³⁺-doped and Sb³⁺/Yb³⁺ co-doped Cs₂NaInCl₆, (c) Nd³⁺-doped and Sb³⁺/Nd³⁺ co-doped Cs₂NaInCl₆. Contour plot of the excitation-dependent NIR emission of (d) Cs₂NaInCl₆:Sb³⁺/Yb³⁺, and (e) Cs₂NaInCl₆:Sb³⁺/Nd³⁺.

Interestingly, the PLE spectra of the Sb³⁺/Yb³⁺-co-doped sample for the emissions at 446 and 1001 nm showed different patterns. Sb³⁺/Yb³⁺-co-doped Cs₂NaInCl₆ showed a broadband excitation above 300 nm, which was consistent with the excitation of Sb³⁺-doped Cs₂NaInCl₆. This result implies that STE and NIR emissions could originate from the same excited state, and an ET process occurs from Sb³⁺ to Yb³⁺.⁴⁵ The PLE spectrum of Sb³⁺/Yb³⁺-co-doped Cs₂NaInCl₆ displayed an additional excitation peak at 273 nm, which may originate from the Cl⁻ → Yb³⁺ CTS. As shown in Fig. 3d, the photoluminescence and PLE pseudo-color maps further confirmed that the NIR emission can be realized by ET and CTS. The characteristic excitation peaks of Yb³⁺ are different to those of other Ln³⁺, as shown in Fig. S18.† Yb³⁺ has only two energy levels, ²F_5/2 and ²F_7/2, whereas other Ln³⁺ (e.g., Er³⁺, Ho³⁺, Nd³⁺) typically possess abundant 4f electronic states (Fig. S19†). Because Yb³⁺ has no energy levels that match the ³P₁ state of Sb³⁺, Förster resonance ET from Sb³⁺ to Yb³⁺ was difficult. The spectral characteristics of the Sb³⁺/Yb³⁺-co-doped Cs₂NaInCl₆ system exhibit certain similarities to those reported for Cs₂ZrCl₆:Te⁴⁺/Yb³⁺ halides.²⁹

To further determine the ET process in the co-doped samples, time-resolved photoluminescence measurements of STE emissions were recorded. As shown in Fig. 4a and b, the excitation lifetime monitored at 446 nm shortened as the Ln³⁺ content increased (Tables S5 and S6†), implying that the improved ET from the STEs to Ln³⁺ promoted decay of the STE emission.⁴⁶ The ET efficiency (η_T) from STE to Ln³⁺ can be calculated using the following equation:

where τ₀ and τ_x are the lifetimes of STE emission in the absence and presence of Ln³⁺, respectively. η_T increases with the concentration of Yb³⁺ or Nd³⁺, reaching 43.96% at 70% Yb³⁺ doping and 6.35% at 70% Nd³⁺ doping (Fig. 4c and d). It is noticed that, η_T of Sb³⁺/Yb³⁺ is considerably higher than that of Sb³⁺/Nd³⁺, which is likely due to the possible CTS of Yb³⁺ as reported previously. For the Sb³⁺/Yb³⁺-co-doped system, it is difficult to achieve resonance ET because of the mismatched excitation energy-level between Yb³⁺ and Sb³⁺. However, efficient NIR luminescence can be achieved via CTS of Yb³⁺, which is similar to the ET process between Ce³⁺ (∼400 nm) and Yb³⁺ (∼980 nm) in a traditional phosphor system.^47–49 Schematic diagrams of the possible light emission mechanism are presented in Fig. 4e and f. Upon ultraviolet (UV) excitation at 300–380 nm, electrons are excited from the ground state to the excited states of Sb³⁺ and then trapped in [SbCl₆]³⁻ octahedra through nonradiative relaxation, resulting in high-energy blue STE emission.⁵⁰ Simultaneously, benefiting from the efficient ET channel from Sb³⁺ to Ln³⁺, part of the energy can be transferred to Yb³⁺ or Nd³⁺ to emit NIR luminescence at 1001 nm (²F_5/2 → ²F_7/2 transition of Yb³⁺) or at 896, 1077, and 1358 nm (the transitions of Nd³⁺ from ⁴F_3/2 to ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 energy levels, respectively). Additionally, NIR emission at 1001 nm can also be achieved by Cl⁻ → Yb³⁺ CTS under excitation around 270 nm. On the basis of this CTS and ET, efficient NIR luminescence can be obtained in Sb³⁺/Yb³⁺-co-doped Cs₂NaInCl₆.

Fig. 4 PL decay curves (λ_ex = 318 nm, λ_em = 446 nm) of (a) Cs₂NaInCl₆:Sb³⁺/Yb³⁺, (b) Cs₂NaInCl₆:Sb³⁺/Nd³⁺. Lifetime of 446 nm and ET efficiency as a function of (c) Yb³⁺ concentration, and (d) Nd³⁺ concentration. Schematic representation of the photophysical process in (e) Cs₂NaInCl₆:Sb³⁺/Yb³⁺, (f) Cs₂NaInCl₆:Sb³⁺/Nd³⁺.

To further unveil the photophysics of the Sb³⁺–Ln³⁺ pair, DFT calculations were performed to reveal the electronic structure of Cs₂NaInCl₆:Sb³⁺/Ln³⁺. In the pure Cs₂NaInCl₆ system, the conduction-band minimum (CBM) is mainly composed of Cl p states and In s states and the valence-band maximum (VBM) mainly consists of Cl p orbitals, as shown in Fig. S20.† For the Sb³⁺/Yb³⁺- and Sb³⁺/Nd³⁺-co-doped systems, a new band consisting of the Cl p states and Sb lone-pair s states appears above the host valence band, while the SbCl₆ CBM appears in a gap deep in the host conduction band, as illustrated in Fig. 5. The highest occupied molecular orbitals are composed of Sb s and Cl p states and the lowest unoccupied molecular orbitals are composed of Sb p and Cl p states. Meanwhile, the 4f bands of Yb³⁺ or Nd³⁺ are located in the gap between the CBM and VBM of SbCl₆, which provides the possibility to realize ET.^51–53 These calculation results demonstrate that the introduced Sb³⁺ can modulate the host electronic structure and the introduced Ln³⁺ are located in the gap between the CBM and VBM of SbCl₆, confirming the possibility of ET.

Fig. 5 DFT simulations. DOS of (a) Cs₂NaInCl₆:Sb³⁺/Yb³⁺ sample and (b) Cs₂NaInCl₆:Sb³⁺/Nd³⁺ sample.

To illustrate a potential application of Cs₂NaInCl₆:Sb³⁺/Ln³⁺, NIR LEDs were fabricated. The prepared Cs₂NaInCl₆:Sb³⁺/Yb³⁺ was coated on a commercially available 365 nm UV-LED chip. The as-fabricated LEDs exhibited strong luminescence, as shown in Fig. 6a. The NIR emission intensity increased with the drive current, showing excellent NIR emission performance (Fig. 6b). Fig. 6c shows NIR applications of Cs₂NaInCl₆:Sb³⁺/Yb³⁺ using different cameras (visible and NIR) and light sources (natural and NIR). To demonstrate NIR anti-counterfeiting, a picture with the acronyms “SZU”, “DLB”, “NIR”, and “LED” in different colors exhibited distinct patterns when illuminated by natural and NIR light. The principle behind this behavior is that the black “SZU” letters contained carbon black, resulting in strong absorption of NIR light. For infrared detection applications, the area of a picture covered by the long-wavelength filter was not detected by the visible camera under natural light. No image was captured by the NIR camera when the NIR LEDs were switched off. In contrast, when the NIR LEDs were switched on, a clear and complete image was detected by the NIR camera. Similar detection applications can also be found in biology, where yellow-green leaf veins can be clearly captured using the NIR LEDs. To demonstrate night-vision applications, black-and-white photographs of fruit were captured upon illumination with the NIR LEDs. The image clarity was relatively high, with the images clearly displaying the contours of the photographed objects. These results reveal the great potential of the prepared Cs₂NaInCl₆:Sb³⁺/Yb³⁺ samples for use in NIR anti-counterfeiting, infrared detection, and night-vision technologies. The environmental stability of the Cs₂NaInCl₆:Sb³⁺/Yb³⁺ and Cs₂NaInCl₆:Sb³⁺/Nd³⁺ samples was evaluated. As shown in Fig. S21,† the PXRD peaks of the Cs₂NaInCl₆:Sb³⁺/Yb³⁺ and Cs₂NaInCl₆:Sb³⁺/Nd³⁺ samples remained unchanged after continuous illumination under 365 nm UV light. As shown in Fig. S22,† the PL performance of Cs₂NaInCl₆:Sb³⁺/Yb³⁺ and Cs₂NaInCl₆:Sb³⁺/Nd³⁺ samples were almost unchanged even upon heating at 420 K for 6.5 h. These experimental results indicate that Cs₂NaInCl₆:Sb³⁺/Ln³⁺ perovskites are promising for NIR LED applications in advanced night-vision detection and anti-counterfeiting.

Fig. 6 (a) Emission spectrum of as-fabricated LED based on Cs₂NaInCl₆:Sb³⁺/Yb³⁺. Inserts show the photographs of as-fabricated LED. (b) NIR emission spectrum for different operational voltages of as-fabricated LED. (c) Photographs captured by visible camera or NIR camera under daylight, in dark, and upon illumination with the LED combined with Cs₂NaInCl₆:Sb³⁺/Yb³⁺.

3. Conclusions

In summary, Cs₂NaInCl₆:Sb³⁺/Yb³⁺ and Cs₂NaInCl₆:Sb³⁺/Nd³⁺ single crystals that displayed efficient NIR emission were synthesized. Under excitation at 318 nm, Cs₂NaInCl₆:Sb³⁺/Yb³⁺ showed a broadband NIR emission peak at 1001 nm with a high PLQY of 48.95%, whereas Cs₂NaInCl₆:Sb³⁺/Nd³⁺ exhibited three NIR emission peaks at 896, 1077, and 1358 nm with a PLQY of 11.29%. The results of experiments and DFT calculations revealed that the NIR emission of Yb³⁺ was derived from a CTS and ET, whereas the NIR emission of Nd³⁺ originated from resonance ET. Profiting from the high-energy STE emission and CTS of Yb³⁺, the high NIR PLQY was realized by efficient ET from STEs to Yb³⁺. The excellent NIR luminescence performance and high environmental stability of Cs₂NaInCl₆:Sb³⁺/Yb³⁺ endow it with good application prospects in NIR anti-counterfeiting and night-vision devices.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the National Natural Science Foundation of China (Grant No. 61874074), Science and Technology Project of Shenzhen (Grant No. JCYJ20220531100815034), and Guangdong Basic and Applied Basic Research Foundation (General Program, Grant No. 2022A1515012055).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, PLQYs testing and calculation, photographs of Cs₂NaInCl₆:Sb³⁺/Ln³⁺ (Ln³⁺ = Yb³⁺, Nd³⁺) single crystals, PXRD patterns, SEM image and EDX elemental mappings, PLE spectra, NIR PL decay, PLQYs, Ultraviolet–visible absorption spectra of Cs₂NaInCl₆:Sb³⁺/Ln³⁺ (Ln³⁺ = Yb³⁺, Nd³⁺) single crystals, PL and PLE mapping of Cs₂NaInCl₆:Sb³⁺/Ln³⁺ (Ln³⁺ = Yb³⁺, Nd³⁺) single crystals, PLE spectra of Cs₂NaInCl₆:Sb³⁺/Ln³⁺ (Ln³⁺ = Er³⁺, Ho³⁺, Nd³⁺, Yb³⁺), energy-level diagrams, PDOS, ICP elemental analysis of Cs₂NaInCl₆:Sb³⁺/Ln³⁺ (Ln³⁺ = Yb³⁺, Nd³⁺) single crystals, NIR PLQYs of metal halides (PDF). See DOI: https://doi.org/10.1039/d4qi01518e

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