Modulation of the near-infrared-I and -II luminescence of thulium-incorporated lead-free double perovskites

Abstract

Currently, lanthanide-doped metal halide perovskites with near-infrared (NIR) luminescence are receiving considerable attention. However, they suffer from low NIR efficiency, particularly in the NIR-II emission range, and are primarily excited by high-energy ultraviolet light. In this work, high-performance NIR-I and -II emissions were realized by co-doping a Cs₂NaInCl₆ lattice with Tm³⁺ ions as luminescent emitters and Sb³⁺ and Te⁴⁺ ions as sensitizer ions, where Sb³⁺ ions can enable short-wavelength high-energy excitation and Te⁴⁺ ions can enable long-wavelength blue-light excitation. The NIR photoluminescence quantum yield (PLQY) reaches an unprecedented 58.0% in Cs₂NaInCl₆:Sb³⁺/Tm³⁺ (NIR-II emission: 1224 nm–48.9%), which is attributed to the presence of the ¹G₄ energy level of the Tm³⁺ ion acting as a bridge, efficiently transferring energy from high-energy states of Sb³⁺ to NIR-emissive states. Moreover, Tm³⁺ ions can be sensitized by Te⁴⁺ ions that have broadband absorption in the blue region, which enable a high NIR PLQY of 10.3% in Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ under 420 nm excitation. The excellent NIR luminescence performance combined with excellent environmental stability endows Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ with considerable potential for application in night-vision devices.

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Introduction

Near-infrared-I (NIR-I: 700–1000 nm) and near-infrared-II (NIR-II: 1000–1400 nm) luminescence is crucial in various fields related to human activities due to their low thermal effect, nondestructive nature, and suitable penetration.^1–4 The prevailing commercial NIR light sources including halogen and tungsten lamps are plagued by multiple shortcomings such as low efficiency, bulky design, and brief operational lifespans, providing the impetus for developing new high-performance NIR emitters.^5,6 Traditionally, trivalent chromium (Cr³⁺ ion)-doped phosphors have been considered desirable NIR emitters; however, most of the reported Cr³⁺ emissions only partially cover the NIR region, and the rest of them are located within the far-red region.^7–12 Nevertheless, lanthanide (Ln³⁺) doping enables them to emit various wavelengths of the NIR regions, taking advantage of their abundant metastable excitation energy levels.^13–15 In particular, for emerging optical communication technologies, NIR emissions at around 1.2 μm for the ³H₅ → ³H₆ electron transition of Tm³⁺ ions are more attractive. Several of these Ln³⁺-doped NIR emitters have been successfully commercialized. However, Ln³⁺-doped NIR emitters generally suffer from weak light absorption confined by forbidden f–f transitions.^16,17 Therefore, sensitizing Ln³⁺ luminescence by constructing appropriate energy transfer channels is expected to be an effective strategy to achieve efficient NIR-I and NIR-II emissions.

Recently, lead-free metal halide double perovskites have been explored as hosts for doping with Ln³⁺ to generate NIR photoluminescence (PL), which has gained tremendous interest for extensive research in the field of photoelectric materials,^18–24 since halide double perovskites can provide octahedral coordination (CN = 6), which is beneficial for Ln³⁺ incorporation. For example, Nag et al. doped Yb³⁺/Er³⁺ into Cs₂AgInCl₆ double perovskites, and the NIR emissions at 994 and 1550 nm from doped ions were realized.²⁵ Unfortunately, the NIR PL quantum yield (PLQY) of these Ln³⁺-doped Cs₂AgInCl₆ is not satisfactory owing to the inefficient energy transfer. Although many studies have reported the sensitization and enhancement of Ln³⁺-related NIR emission by the energy transfer from self-trapped excitons (STEs) to Ln³⁺ ions, the sensitization efficiency and luminescence of Ln³⁺ ions are low and weak because STEs take away part of the absorbed energy and cannot efficiently transfer their energy to Ln³⁺ ions. Due to the lack of resonance condition (limited overlap between STE emission and Ln³⁺ absorption profiles), tuning the emission spectra of the sensitized centers for better resonance with suitable Ln³⁺ ions is well worth exploring. Currently, the NIR-II emission of Ln³⁺ ions in reported double perovskites still suffers from the problems of low efficiency and high-energy ultraviolet light excitation, which considerably hinders their further application.^21,22,26,27 In this regard, it remains an important material design challenge to modulate the excitation and emission energies while investigating and optimizing the energy transfer efficiency in conjunction with the different energy level structures of Ln³⁺ ions to improve the power conversion efficiency of such NIR-II emitters.

In this work, high-performance NIR-I and NIR-II light was realized by co-doping Tm³⁺ and sensitizer ions (Sb³⁺ and Te⁴⁺) into a Cs₂NaInCl₆ lattice where Sb³⁺ ions can enable short-wavelength high-energy excitation and Te⁴⁺ ions can enable long-wavelength low-energy excitation. Through Sb³⁺ sensitization, efficient NIR-I emission at 808 nm and NIR-II emission at 1224 nm with a high NIR PLQY of up to 58.0% was realized in Cs₂NaInCl₆:Sb³⁺/Tm³⁺ under 320 nm excitation. The energy transfer efficiency in the Sb³⁺/Tm³⁺ co-doped sample is up to 56.1%, which may be attributed to the relatively large overlap between Sb³⁺ emission and Tm³⁺ absorption profiles. The presence of the ¹G₄ energy level of the Tm³⁺ ion acts as a bridge, facilitating the transfer of energy from Sb³⁺ to Tm³⁺. Based on the sensitization of Te⁴⁺ ions, similar NIR emissions with a high NIR PLQY of up to 10.3% have been realized in Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ under blue light 420 nm excitation. The experiments and DFT calculations reveal the origin of the luminescence and demonstrate the possibility of energy transfer. Profiting from the efficient energy transfer channels, highly efficient NIR-I and NIR-II emissions of Tm³⁺ ions can be realized. The excellent NIR-I and NIR-II luminescence performance combined with excellent environmental stability endows Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ with great potential for application in night-vision devices. This work provides inspiration for the design of excitation state modulation and efficient NIR-emitting Ln³⁺-doped halide perovskites, demonstrating their considerable potential as NIR LEDs for night vision technologies and other related fields.

Results and discussion

The Cs₂NaInCl₆ double perovskite is environmentally friendly and serves as an excellent host material owing to the wide bandgap that does not interact with most of the ultraviolet-visible radiation, resulting in photostability. We obtained Tm³⁺-doped halide double crystals by a hydrothermal method. The corresponding structures of Cs₂NaInCl₆ and Cs₂NaInCl₆:Tm³⁺–Sb³⁺ (or Te⁴⁺) double perovskites are shown in Fig. 1a, where Tm³⁺, Sb³⁺, and Te⁴⁺ randomly occupy the position of In³⁺ when doped into the lattice. The powder X-ray diffraction (XRD) patterns of the samples show that the XRD patterns of Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ samples can correspond well to the standard card (ICSD#132718), as shown in Fig. 1b. The diffraction peaks of Sb³⁺/Tm³⁺ or Te⁴⁺/Tm³⁺-doped samples shift toward a lower angle as the Tm³⁺ concentration increases, as shown in Fig. S1.† This shift occurs because the ionic radius of Tm³⁺ (88 pm) is larger than that of In³⁺ (80 pm), which indicates that Tm³⁺ was successfully doped into the lattice. The actual doping concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS), and the results are listed in Tables S1 and S2.†

Fig. 1 (a) Structural schematic diagram of Cs₂NaInCl₆, Cs₂NaInCl₆:Sb³⁺/Tm³⁺, and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ double perovskites. (b) XRD patterns of pure, 5%Sb³⁺/30%Tm³⁺, and 1%Te⁴⁺/30%Tm³⁺ doped Cs₂NaInCl₆.

The PL excitation (PLE) and PL spectra of Cs₂NaInCl₆ doped with different types of ions are studied to explore the luminescence properties of Tm³⁺ ion-doping systems. The PLE spectra of both Sb³⁺ and Sb³⁺/Tm³⁺-doped Cs₂NaInCl₆ samples have a strong excitation band at around 320 nm (Fig. 2a), corresponding to the ¹S₀ → ³P₁ transition of Sb³⁺.^28,29 The Sb³⁺ single-doped Cs₂NaInCl₆ samples exhibit a broadband blue emission at 450 nm under 320 nm excitation, while Sb³⁺/Tm³⁺-doped Cs₂NaInCl₆ samples exhibit considerable NIR-I emission peaks at 808 nm, where the 808 nm NIR-I emission is attributed to the f–f transition process of Tm³⁺ ions.²⁷ The doping of Tm³⁺ ions can induce the NIR luminescence of Cs₂NaInCl₆ perovskite materials. The PL/PLE pseudo-color maps demonstrate that the excited state origin of these emission bands was the same (Fig. 2c), suggesting that the NIR luminescence of Tm³⁺ ions primarily depends on the sensitization of Sb³⁺ ions.

Fig. 2 (a) PLE and PL spectra of 5%Sb³⁺ and 5%Sb³⁺/60%Tm³⁺-doped Cs₂NaInCl₆. (b) PLE and PL spectra of 1%Te⁴⁺ and 1%Te⁴⁺/50%Tm³⁺-doped Cs₂NaInCl₆. The PLE/PL mapping of (c) 5%Sb³⁺/60%Tm³⁺ and (d) 1%Te⁴⁺/50%Tm³⁺-doped Cs₂NaInCl₆.

Although NIR luminescence can be achieved in the Sb³⁺/Tm³⁺ systems, the excitation energy is high; therefore, we introduced other sensitizers to modulate the excitation wavelength. The PLE spectra of Cs₂NaInCl₆ samples doped with Te⁴⁺ and Te⁴⁺/Tm³⁺ exhibit similar excitation bands, with the optimal excitation wavelength appearing at 437 nm (Fig. 2b). The PLE peak closely agrees with the absorption spectrum and has a strong absorption cross-section with the addition of sensitizers (Fig. S2†). The Te⁴⁺ singly doped Cs₂NaInCl₆ sample exhibits a broadband emission at 620 nm under 437 nm excitation with a full width at half maximum of 152 nm, which was attributed to the Te⁴⁺-state via³P_0,1 → ¹S₀ transition or related STE emission.^30,31 The Te⁴⁺/Tm³⁺-doped Cs₂NaInCl₆ sample demonstrates an additional NIR-I emission peak at 808 nm, which is attributed to the electronic transition process of Tm³⁺ ions. Accordingly, the NIR emission exhibited a millisecond-scale lifetime of 2.91 ms for Sb³⁺/Tm³⁺ and 3.23 ms for Te⁴⁺/Tm³⁺ (Fig. S3†), which is attributed to Laporte forbidden f–f transitions. The introduction of Tm³⁺ ions could reduce the broadband emission intensity (500–800 nm) while generating NIR emission. The PL/PLE pseudo-color maps confirm that the excited state origin of these emission bands was the same (Fig. 2d), indicating that the energy transfer process through Te⁴⁺ is the main pathway to achieve NIR emission of Tm³⁺ ions.

In addition to introducing Te⁴⁺ ion sensitizers to achieve long-wavelength excitation, we introduced other ions in addition to the original Sb³⁺/Tm³⁺ doping to modulate the excited state of Sb³⁺. After introducing Bi³⁺ ions, the PLE of Cs₂NaInCl₆:Sb³⁺/Bi³⁺/Tm³⁺ splits further, with the splitting width caused by the Jahn–Teller deformation increasing with Bi³⁺ ion concentration (Fig. S4†). With the increase in Bi³⁺ ion doping concentration, the excitation peak can be effectively modulated from 330 to 360 nm, although the luminescence intensity is slightly reduced, as shown in Fig. S5.† Furthermore, the introduction of Bi³⁺ into the Te⁴⁺/Tm³⁺ system did not change the spectra, and the excitation peaks were consistent with those of Te⁴⁺ and the Te⁴⁺/Tm³⁺ doping systems (Fig. S6†), indicating that the introduction of Bi³⁺ did not modulate the excited state of Te⁴⁺.

To explore the effect of Tm³⁺ ion doping concentration on the luminescence performance of samples, the emission spectra of Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ are recorded at different Tm³⁺ ion doping concentrations (Fig. 3a and b). Cs₂NaInCl₆:Sb³⁺ only demonstrates a strong broadband blue emission peak, which could be attributed to the inter-configurational transitions of Sb³⁺ dopants.^26,32 After introducing Tm³⁺ ions, the PL spectra exhibit a distinct NIR-I emission peak attributed to the ³H₄ → ³H₆ electron transition process of Tm³⁺ ions under ultraviolet (UV) light excitation. The detailed trends of blue and NIR-I emission intensities of the Cs₂NaInCl₆:Sb³⁺/Tm³⁺ sample with the increase in Tm³⁺ doping concentration are shown in Fig. 3c. Under excitation of 320 nm, the NIR emission of Tm³⁺ ions gradually increased with the increase in Tm³⁺ concentration. However, the intensity of the blue emission of Sb³⁺ decreases, indicating the presence of energy transfer process from Sb³⁺ to Tm³⁺ in Cs₂NaInCl₆ perovskites. Similarly, the Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ sample exhibits a NIR-I emission peak at 808 nm (Tm³⁺: ³H₄ → ³H₆) on the basis of the broadband emission peak of Te⁴⁺ with the introduction of Tm³⁺ ions. As shown in Fig. 3d, the Te⁴⁺ emission intensity decreased, while the NIR emission intensity greatly increases for up to 50% Tm³⁺ doping. Higher doping concentrations lead to a decrease in NIR efficiency, which is caused by concentration quenching.

Fig. 3 PL spectra of (a) Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and (b) Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ with different Tm³⁺ ion concentrations, (c) 450 nm and 808 nm emission intensity of Cs₂NaInCl₆:Sb³⁺/Tm³⁺ samples, and (d) 620 nm and 808 nm emission intensity of Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ samples.

In addition to the NIR-I emission at 808 nm, we found that the Tm³⁺ ion-doped samples exhibited intense NIR-II emissions at 1224 nm, which was attributed to the ³H₅ → ³H₆ electron transition process of Tm³⁺ ions. As shown in Fig. 4a, the NIR-II emission intensity considerably increases for up to 60% Tm³⁺ doping. The PLE spectra of NIR-II emission show the same profiles, which indicate that the NIR-II emission could originate from Sb³⁺ ions (Fig. 4b). The NIR emission spectra were almost constant with the variation in the excitation wavelength (Fig. 4c), indicating that the NIR-II luminescence of Tm³⁺ ions primarily depends on the sensitization of Sb³⁺ ions. Similarly, the Te⁴⁺/Tm³⁺-codoped Cs₂NaInCl₆ exhibited the same NIR-II emission, with the NIR-II emission intensity increasing for up to 50% Tm³⁺ doping, as shown in Fig. 4d. The PLE spectra of NIR-II emission show the same profiles, indicating that the NIR-II emission could originate from Te⁴⁺ ions (Fig. 4e). The PL and PLE pseudo-color maps confirm that the NIR-II emission is realized by an energy transfer process from Te⁴⁺ to Tm³⁺ ions (Fig. 4f). The highest NIR-II emission of Sb³⁺/Tm³⁺-codoped samples with a PLQY up to 48.9% was obtained when the Tm³⁺ ion feed concentration was 60%, as shown in Fig. S7.† Similarly, Te⁴⁺/Tm³⁺-codoped Cs₂NaInCl₆ also demonstrated a high NIR-II PLQY of up to 8.3% when the Tm³⁺ ion feed concentration was 50% (Fig. S8†). Based on the sensitization of Sb³⁺ or Te⁴⁺ ions, the NIR-II emission intensity of Sb³⁺/Tm³⁺ and Te⁴⁺/Tm³⁺ co-doped samples exhibited a huge improvement compared to the Tm³⁺ singly doped sample (Fig. 4b and d). The PLQY of NIR (NIR-I and NIR-II) emissions of Sb³⁺/Tm³⁺-codoped samples was up to 58.0%, while the NIR of Te⁴⁺/Tm³⁺-codoped samples showed a PLQY as high as 10.3%, showing excellent NIR luminescence performance. Note that the PLQY of NIR emission of this Sb³⁺/Tm³⁺-codoped Cs₂NaInCl₆ reaches the unprecedented 58.0%, which is the record in reported all-inorganic lead-free halide perovskites with a narrow NIR-II emission band (Table 1).

Fig. 4 (a) NIR-II emission spectra of Cs₂NaInCl₆:Sb³⁺/Tm³⁺. (b) PLE spectra of Cs₂NaInCl₆:5%Sb³⁺/60%Tm³⁺; PL spectra of Cs₂NaInCl₆:60%Tm³⁺ and Cs₂NaInCl₆:5%Sb³⁺/60%Tm³⁺. (c) Contour plot of the excitation-dependent NIR-II emission of Cs₂NaInCl₆:5%Sb³⁺/60%Tm³⁺. (d) NIR-II emission spectra of Cs₂NaInCl₆:1%Te⁴⁺/50%Tm³⁺. (e) PLE spectra of Cs₂NaInCl₆:1%Te⁴⁺/50%Tm³⁺; PL spectra of Cs₂NaInCl₆:50%Tm³⁺ and Cs₂NaInCl₆:1%Te⁴⁺/50%Tm³⁺. (f) Contour plot of the excitation-dependent NIR-II emission of Cs₂NaInCl₆:1%Te⁴⁺/50%Tm³⁺.

Table 1 Summary of the PL characteristics of NIR-emission lead-free metal halides

Compound	PLQY [%]	Excitation wavelength [nm]	Emission wavelength [nm]	Ref.
Cs2AgBiCl6:Yb^3+	—	365	1000	33
Cs2AgInCl6:Yb^3+	3.6	300	996	34
Cs2AgBiBr6:Yb^3+	28.0	430	1000	35
Cs2Ag0.2Na0.8BiCl6:Yb^3+	19.0	360	995	36
Cs2Ag0.2Na0.8BiCl6:Er^3+	4.3	360	1540
Cs2ZrCl6:Te^4+/Yb^3+	0.2	392	1002	37
Cs2ZrCl6:Te^4+/Er^3+	6.1	392	1539
Cs4MnBi2Cl12:Yb^3+	1.4	360	1000	38
Cs2NaBiCl6:Mn^2+/Er^3+	14.2	360	1540	39
Cs2AgInCl6:Cr^3+	22.0	760	1010	40
Cs2AgInCl6:Cr^3+/Yb^3+	45.0	365	1000	41
Cs2NaSbCl6:Er^3+	0.3	335	1543	42
Cs2NaScCl6:Er^3+	28.3	365	1540	43
Cs2NaInCl6:Sb^3+/Tm^3+	58.0	320	808, 1224	This work
Cs2NaInCl6:Te^4+/Tm^3+	10.3	420	808, 1224

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To determine the interaction between Tm³⁺ and Sb³⁺ (or Te⁴⁺) dopants and the regulatory effect of Tm³⁺ on NIR emission, the PLE spectra of the Sb³⁺/Tm³⁺-doped sample monitored at 450, 808 and 1224 nm and the PLE spectra of the Te⁴⁺/Tm³⁺-doped sample monitored at 620, 808 and 1224 nm were recorded (Fig. S9†). The PLE spectra of the Cs₂NaInCl₆:Sb³⁺/Tm³⁺ sample at 450, 808 and 1224 nm were comparable (with the same excitation bands at 320 and 333 nm), providing evidence for the energy transfer from Sb³⁺ to Tm³⁺ ions.^14,44 Similar phenomena are observed in the PLE spectra of the Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ sample, indicating the presence of an energy transfer process. As shown in Fig. S9,† a weak narrow excitation peak at 470 nm, which is attributed to the direct excitation of Tm³⁺ ions (³H₆ → ¹G₄ transition), was observed. As shown in Fig. S10,† the PL spectra of the Cs₂NaInCl₆:Sb³⁺/Tm³⁺ or Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ sample display that the clear NIR emission peak still appears at different excitation wavelengths. The PLE spectra of visible and NIR emissions were comparable, which indicated that the excited state origin of these emission bands was the same. The emission of Tm³⁺ ions with f–f transitions is usually sensitive to the excitation wavelength, while the emission spectra are almost constant with the change in excitation wavelength, indicating that the NIR luminescence of Tm³⁺ ions mainly depends on the sensitization of Sb³⁺ or Te⁴⁺ ions. Moreover, the excitation intensity at 470 nm was lower than the excitation band of Sb³⁺ or Te⁴⁺ ions, indicating that the Tm excitation through the energy transfer was more efficient than direct excitation. Therefore, the energy transfer process through the ns² ion is the significant main pathway to achieve NIR emission of Tm³⁺ ions.

To determine the energy transfer process of Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ samples, the time-resolved PL of ns² emissions were measured. As shown in Fig. 5a–d, the PL lifetimes monitored at 450 or 620 nm decreased with the increase in Tm³⁺ content (Tables S3 and S4†), indicating that the improved energy transfer from the ns² (at the [SbCl₆]³⁻ or [TeCl₆]²⁻ octahedron) to the Tm³⁺ ions promoted the decay of the ns² emission. The energy transfer efficiency (η_T) in the can be expressed as follows:⁴⁵

where τ₀ is the lifetime of ns² emission in the absence of Tm³⁺ ions and τ_x is the lifetime with different Tm³⁺ concentrations. The η_T values increase with the concentration of Tm³⁺ ions, reaching up 56.1% at 5%Sb³⁺/70%Tm³⁺ doping and 43.7% at 1%Te⁴⁺/70%Tm³⁺ doping (Fig. 5c and d). The efficient energy transfer efficiency can be attributed to the presence of Tm³⁺ and Sb³⁺ or Te⁴⁺ ions occupying the In³⁺ sites in the Cs₂NaInCl₆ lattice. This results in a reduced distance between the Sb³⁺ or Te⁴⁺ sensitizer and the Tm³⁺ activator, thereby favoring the occurrence probability of the energy transfer process. Moreover, for Cs₂NaInCl₆:Sb³⁺/Tm³⁺, a relatively large overlap between ns² emission and Tm³⁺ absorption profile (Fig. S11†) and a slight relaxation will facilitate the transfer of energy from Sb³⁺ to Tm³⁺. The presence of the ¹G₄ energy level (∼470 nm) of Tm³⁺ ions is close to the emission of Sb³⁺ (∼450 nm), which can act as a bridge to achieve a highly efficient energy transfer efficiency.

Fig. 5 PL lifetimes of (a) Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and (b) Cs₂NaInCl₆:Te⁴⁺/Tm³⁺. PL lifetimes (450 or 620 nm) and energy transfer efficiency as a function of Tm³⁺ concentration in (c) Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and (d) Cs₂NaInCl₆:Te⁴⁺/Tm³⁺. (e) Energy-level diagram of Sb³⁺/Tm³⁺ and Te⁴⁺/Tm³⁺ co-doped Cs₂NaInCl₆ and the proposed luminescence mechanism.

According to the above-mentioned investigations, the possible PL mechanism for Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ is presented in Fig. 5e. For Cs₂NaInCl₆:Sb³⁺/Tm³⁺, the electrons can be excited from the ground state to the excited states of Sb³⁺ ions under 320 nm UV excitation, and then undergo a dynamic Jahn–Teller distortion, leading to broadband blue emission. After Bi³⁺ codoping, there is an increase in the splitting width of the PLE band, which can reduce the excitation energy to 330–390 nm range. The energy is then transferred to Tm³⁺ ions through the energy transfer channel from Sb³⁺ to Tm³⁺ ions, which results in the intrinsic emission of Tm³⁺ ions, achieving a blue emission at 470 nm (Tm³⁺: ¹G₄ → ³H₆), an NIR-I emission at 808 nm (Tm³⁺: ³H₄ → ³H₆), and an NIR-II emission at 1224 nm (Tm³⁺: ³H₅ → ³H₆). Similarly, the electrons in the ground state of the Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ sample are excited to the excited state under low-energy blue light excitation, emitting a broadband emission at 500–800 nm through the inter-configurational transitions. Meanwhile, some of the electrons are transferred to the ³H₄ and ³H₅ levels of Tm³⁺ ions, achieving an NIR-I emission at 808 nm and an NIR-II emission at 1224 nm. The doping of Tm³⁺ ions provides a pathway for efficient NIR-I and NIR-II emissions, while the selection of Sb³⁺ or Te⁴⁺ offers diverse sensitization routes.

To further unveil the photophysics process, density functional theory (DFT) calculations were carried out to determine the electronic structure of the Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ systems, which are shown in Fig. 6. For the undoped Cs₂NaInCl₆ system, the conduction band minimum (CBM) is mainly composed of Cl p states and In s states, while the valence-band maximum (VBM) mainly consists of Cl p orbitals, as shown in Fig. S12.† In the Sb-doped system, a new band composed of the Sb lone-pair s states and Cl p states appears above the host valence band, while the SbCl₆ CBM emerges in a gap deep in the host conduction band, as shown in Fig. 6a. Similarly, the Te s states and Cl p states appear above the host valence band, while the TeCl₆ CBM emerges in front of the host conduction band (Fig. 6b). The SbCl₆ CBM–VBM gap (∼3.7 eV) is higher than the TeCl₆ CBM–VBM gap (∼3.3 eV), which matches the experimental results (a higher excitation energy for Sb than for Te). The calculations confirmed that the observed visible emission band originated from [SbCl₆] or [TeCl₆] octahedrons, respectively. Furthermore, as shown in Fig. 6c and d, the 4f bands of Tm are located in the CBM–VBM of SbCl₆ or TeCl₆, providing the possibility for the realization of energy transfer. These calculation results demonstrate that the introduced sensitizer Sb³⁺ or Te⁴⁺ ions can modulate the host electronic structure, while the introduced Tm³⁺ ions are located in the CBM–VBM of SbCl₆ or TeCl₆, which confirms the possibility of energy transfer.

Fig. 6 Density of states (DOS) of (a) Cs₂NaInCl₆:Sb³⁺, (b) Cs₂NaInCl₆:Te⁴⁺, (c) Cs₂NaInCl₆:Sb³⁺/Tm³⁺, and (d) Cs₂NaInCl₆:Te⁴⁺/Tm³⁺.

To demonstrate the potential application of the NIR emission of Cs₂NaInCl₆:Sb/Tm and Cs₂NaInCl₆:Te/Tm materials, the simple NIR detection system is designed, as shown in Fig. 7a. The prepared Cs₂NaInCl₆:Sb³⁺/Tm³⁺ samples were coated on a commercially available 310 nm UV-LED chip, while the Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ samples were coated on a commercially available 410 nm LED chip. The as-fabricated LED devices exhibited intense NIR luminescence, as shown in Fig. 7b. The intense NIR emissions demonstrate great potential in night vision applications, in which the photographs printed paper upon illumination with the NIR-LEDs as shown in Fig. 7c. Under natural light, photographs of the “Jiaying University” pattern were obtained using a visible camera. No image was captured by the cameras when the LEDs were off. In contrast, black-and-white images were recorded using an NIR camera when the NIR LEDs were ignited. The fabricated NIR LED can also be used for night vision surveillance and penetration applications by virtue of the unique spectral characteristics of NIR-II light, such as being invisible to the naked eye and special penetration ability. Under natural light, part of the emission band covered by the 750 nm filter was not detected by the visible camera. When the NIR LED is turned on, the obscured hand is clear in the NIR camera (Fig. 7d). Furthermore, the strong NIR emission enables a clear fruit image by the NIR camera under the irradiation of an NIR LED lamp, which reveals the great potential of the prepared samples in night vision technologies and other related fields. The Tm³⁺-doped NIR materials also exhibit excellent environmental stability, which does not degrade significantly after 2 months, and the luminescence intensity is basically consistent with the original value under ambient conditions (Fig. S13a and b†). The XRD measurements showed that the Tm³⁺-doped sample structure can remain stable for more than 90 days (Fig. S13c and d†). Moreover, the PL performances of Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ samples were almost unchanged even upon heating at 400 K for 6 h, demonstrating excellent thermal stability, as shown in Fig. S14.† These results demonstrated that Tm³⁺-doped perovskites are promising for NIR LED in advanced night vision detection.

Fig. 7 (a) Schematic diagram of the NIR photography device. The LED is illuminated using a portable lamp colorimeter, and a 900 nm filter is installed at the LED light source to filter visible light. (b) Emission spectra of the as-fabricated NIR emitting LED composed of Cs₂NaInCl₆:5%Sb³⁺/60%Tm³⁺ and Cs₂NaInCl₆:1%Te⁴⁺/50%Tm³⁺ phosphors. (c) Photographs under daylight, in the darkness, and upon illumination with the NIR LED. (d) Vis and NIR images illuminated by fluorescent light and the fabricated NIR LED light.

Conclusions

In summary, Cs₂NaInCl₆:Sb³⁺/Tm³⁺ and Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ single crystals have been successfully synthesized, which enable efficient NIR-I and NIR-II emissions. Cs₂NaInCl₆:Sb³⁺/Tm³⁺ shows efficient blue emissions at 450 nm, and an NIR-I emission peak at 808 nm and an NIR-II emission peak at 1224 nm under 320 nm excitation, while Cs₂NaInCl₆:Te⁴⁺/Tm³⁺ exhibits broadband orange emissions at 620 nm, and similar NIR-I and NIR-II emission peaks at 808 and 1224 nm under 420 nm excitation. A high NIR (NIR-I and NIR-II) PLQY of up to 58.0% was obtained in Cs₂NaInCl₆:Sb³⁺/Tm³⁺, while a high NIR PLQY of up to 10.3% was obtained in Cs₂NaInCl₆:Te⁴⁺/Tm³⁺. The experiments and theoretical calculations reveal the origin of the luminescence as well as demonstrate the possibility of energy transfer. The efficient energy transfer channel design from Sb³⁺ enables us to realize efficient NIR emissions, and the energy transfer channel from Te⁴⁺ enables us to realize long-wavelength low-energy excited NIR emission. The excellent optical performance combined with excellent environmental stability makes Tm³⁺-doped halide perovskites have a good application in NIR LED and night-vision devices. These findings provide a design strategy to tune the excited state based on different sensitizers, which can achieve efficient infrared emission under different excitation energies for versatile photoelectric applications.

Data availability

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

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the University Key Laboratory of Guangdong (2024KSYS021), Universities Special Project in Key Fields of Guangdong (no. 2022ZDZX3027), Natural Science Foundation of Guangdong Province (no. 2022A1515010813), University Engineering Technology Center of Guangdong (no. 2022GCZX007), Inorganic Optical Functional Materials and Application Innovation Team of Guangdong (no. 2023KCXTD033).

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Footnote

† Electronic supplementary information (ESI) available: Detailed description of the experimental synthesis, and characterizations; XRD pattern, ultraviolet–visible absorption spectra, PL decay curves, PLE spectra, PL spectra, PLQY, DOS, photoluminescence stability, ICP elemental analysis (PDF). See DOI: https://doi.org/10.1039/d4qi01772b

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