High performance NIR-I to NIR-II emission of a Cr³⁺-doped Cs₂NaLuCl₆ phosphor with an IQE and EQE of up to 92.9% and 60.75%

Abstract

Presently, there is a need to expand the near-infrared (NIR) emission range, especially in the bio-transparent window, to improve the internal quantum efficiency and external quantum efficiency, and to minimize the shortcomings of luminescence properties. In this work, an ultra-broadband NIR emission Cr³⁺-doped Cs₂NaLuCl₆ all-inorganic metal halide perovskite phosphor was synthesized by a grinding–sintering method. Under 300 nm excitation, Cs₂NaLuCl₆:Cr³⁺ phosphors exhibited ultra-broadband NIR emission of 800–1400 nm, which covers nearly the entire NIR-I (700–1000 nm) and NIR-II (1000–1400 nm) regions; the emission peaked at around 960 nm, with a full width at half maximum of ∼182 nm. Noticeably, the internal- and external-quantum efficiencies of the optimized Cs₂NaLuCl₆:Cr³⁺ phosphor were as high as ∼92.9% and ∼60.75%, respectively. We showed through analysis and simulation that the weak crystal field strength is conducive to increasing the odd term of the crystal field, and therefore it partially released the restriction imposed by the parity prohibition law and improved the energy absorption efficiency of Cr³⁺. As the size of the phosphor approaches the wavelength of NIR luminescence, the apparent Mie scattering effect starts to take effect, which enhances the light extraction of NIR luminescence in the radial direction.

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1. Introduction

Near-infrared (NIR) luminescence technology has many applications in the fields of optical anti-counterfeiting, night vision display, telecommunications, solar energy harvesting, etc. NIR light has the features of rapid responsiveness, non-destructiveness, biological transparency and high tissue penetration and is particularly useful in agriculture and in biomedical imaging and detection.^1–5 For example, the NIR first region (NIR-I, λ = 700–1000 nm), such as a λ = 730 nm NIR light source, is used for indoor plant lighting because it can react with phytochromes; the λ = 800–1000 nm range can be used for night vision monitoring and face recognition, while the NIR second region (NIR-II, λ = 1000–1500 nm) bandwidth is more suitable for high-resolution optical bioimaging and chemical analysis due to its low auto-fluorescence, high signal-to-noise ratio and deep tissue penetration.^6–9 Currently, commercial NIR light sources mainly include tungsten incandescent lamps, NIR LEDs, and NIR lasers. Among them, tungsten filament incandescent lamps provide ultra-broad continuous light from visible to NIR light, but they suffer from low energy efficiency, especially the NIR light fraction, high operating temperature, a short service life, and bulky sizes.¹⁰ Newer generation NIR LED and laser sources are compact and more energy efficient. They produce less heat and deliver NIR with higher luminous intensity, though the relative narrow emission wavelength with a full width at half-maximum (FWHM) of up to 40 nm restricts their use for broad spectroscopic analysis.^11,12 Therefore, a new NIR phosphor with efficient broadband NIR emission capacity is highly desirable. By combining NIR phosphors with commercial high-efficiency (ultraviolet) UV/blue LED chips, a new type of phosphor-converted near-infrared light-emitting diode (NIR pc-LED) light source can be built, which has the advantages of low cost, good stability, high power, energy saving and environmental protection.^13–17 Among them, the Cr³⁺ ion is a well-known NIR luminescence center, whose 3d³ electronic configuration and optical properties can be readily influenced by the crystal field environment and electron–phonon coupling; therefore, Cr³⁺-doped host materials often demonstrate a broad and tunable range for NIR emission.^18,19

Recently, all-inorganic metal halide perovskite matrices have attracted much attention for the research and development of photo-optical materials due to their high light absorption coefficient, excellent carrier mobility, high tolerability towards structural defects and high quantum efficiency. The twin perovskite structure has a general chemical formula such as A₂B^IB^IIIX₆ (A = Cs⁺; B^I = Cu⁺, Ag⁺, Na⁺; B^III = Ln³⁺, Bi³⁺, In³⁺; X = Cl⁻, Br⁻, I⁻), A, B and X occupy fixed positions. There are 6 halogen atoms surrounding each B atom in a regular octahedron [BX₆] form. Cr³⁺ preferentially enters the center of the octahedron, and its broadband NIR luminescence properties can be adjusted according to the strength of the crystal field.^20–22 So far, the reported NIR phosphors with Cr³⁺ in the octahedron as the luminescence center have suffered from having a low external quantum efficiency (EQE) value, despite having a substantially high internal quantum efficiency (IQE) value, as indicated in Fig. 1a, which lists both IQE and EQE test values of a panel of Cr³⁺-doped NIR luminescent materials. EQE, but not IQE, ultimately determines the performance of a lighting device.^23–90 Few have reported a Cr³⁺-doped NIR luminescent material that covers the NIR-I and NIR-II regions with high IQE and EQE values.

Fig. 1 (a) Survey of reported Cr³⁺-doped NIR phosphors. Taking the IQE/EQE value as the Y-axis, the value of IQE on the left side and the value of EQE on the right side are summarized. (b) Measured XRD pattern and (c) the magnified XRD patterns in the angle range of 23.2–24° of CNLC:xCr (x = 0, 0.005, 0.008, 0.01, 0.012, and 0.017), respectively. (d) The Rietveld refinement of the Cs₂NaLuCl₆ host. (e) Variation of lattice parameters a and V of CNLC:xCr (x = 0, 0.005, 0.008, 0.01, 0.012, and 0.017), respectively. (f) Schematic diagram of the crystal structure of Cs₂NaYCl₆.

Here we report the synthesis of Cr³⁺-doped Cs₂NaLuCl₆ NIR phosphors with an all-inorganic metal halide double perovskite structure by a grinding and sintering method. Excited by UV light (@ 300 nm), this phosphor effectively emitted 800–1400 nm ultra-broadband NIR covering the NIR-I to NIR-II regions. The emission peak was located at 960 nm and had a half-peak width of ∼182 nm. The IQE and EQE were 92.9% and 60.75%, respectively. We showed that the unique broadband emission originated from the ⁴T₂(F) → ⁴A₂(F) allowed transition of Cr³⁺ in the octahedra of the double perovskite structure. The structure–optical property relationship between the NIR luminescence centers and the host materials led to a high IQE and EQE, and the principles behind the ultra-broadband emission covering the NIR-I to NIR-II regions were explored. We also demonstrated the potential applications of NIR pc-LED light sources for the purpose of night vision imaging acquisition.

2. Experimental

2.1 Materials

CsCl (99.99%), NaCl (99.99%), LuCl₃·6H₂O (99.99%), and CrCl₃·6H₂O (99.99%) were purchased from Aladdin Chemical Reagent Co., Ltd. All chemicals were used as received without any further purification.

2.2 Synthesis of the Cs₂NaLuCl₆:xCr³⁺ phosphor

CsCl (2 mmol), NaCl (1 mmol), LuCl₃·6H₂O (1 − x mmol), and CrCl₃·6H₂O (x mmol) (x = 0, 0.005, 0.008, 0.01, 0.012, and 0.017) were thoroughly mixed by grinding in an agate mortar with a small amount of deionized water as the wetting agent for 30 minutes until a uniform powder was obtained. It was transferred to an alumina crucible, covered with a lid, sintered in air at 623 K for 2 hours, then slowly cooled to room temperature, and finally ground to powder before use. These samples were referred to as CNLC:xCr (x = 0, 0.005, 0.008, 0.01, 0.013, 0.015, and 0.017).

2.3 Characterization

The phases of the samples were examined by X-ray diffraction (XRD) measurements on a Bruker AXS ADVANCE diffractometer using Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 40 mA. The diffraction data were collected in the range of 10°–80° with a step size of 0.01 (2θ) at a step time of 0.2 s. Rietveld refinement was employed using Fullprof software. X-ray photoelectron spectra (XPS) were recorded on an ESCALAB 250 photoelectron spectrometer (Thermo Fisher Scientific, USA) with Al Kα (1486.6 eV) as the X-ray source. The morphological characteristics were evaluated using a scanning electron microscope (SEM, SU8000, Hitachi) equipped with an energy dispersive X-ray spectroscopy (EDS) system. Photoluminescence (PL) and excitation photoluminescence (PLE) spectra were recorded on a spectrofluorometer (Horiba 8000, Horiba) equipped with a 75 W Xe lamp as excitation, and an InGaAs DDS detector was connected. Diffuse reflection (DR) spectra were recorded by a UV-Vis-NIR spectrophotometer (U-4100, Hitachi) with an integration sphere. The electron spin resonance (EPR) spectra were recorded at 293 K on a Bruker EMX PLUS EPR spectrometer operating in the X-band (about 9.205 GHz) with a field modulation frequency of 100 kHz. The magnetic field was scanned from 260 to 360 mT and the microwave power used was 10 mW. A powdered phosphor of 20 mg was taken in a quartz tube for EPR measurements. The NIR internal quantum yield (IQE), absorption efficiency value, and NIR luminescence decay time were measured using an integrated sphere on the Edinburgh FLS1000 spectrophotometer. The temperature-dependent PL spectra were collected by a measurement system containing a heating stage, which gives time-integrated intensities. Electroluminescence (EL) spectra and the performances of the fabricated NIR pc-LED devices were measured using an integrating sphere (Labsphere), and data were collected using a multichannel photodetector (MCPD-9800, Otsuka Photal Electronics). Visible images and NIR images were obtained using a visible camera (Canon EOS6D Mark II, respond wavelength: 300–700 nm) and an NIR camera (VIR APR1, respond wavelength: 400–1100 nm), respectively.

2.4 Computational model

The CASTEP package of Material Studio was used to simulate the electron density of a Cs₂NaLuCl₆ lattice based on parameters of the crystal structure obtained from Rietveld refinement analysis. A generalized gradient approximation with the Perdew–Burke–Ernzerhof functional was adopted. We defined the convergence criteria for the energy task as ultra-fine quality, including a custom energy cutoff of 660.00 eV, SCF tolerance of 5 × 10⁻⁷ eV per atom, and K-point of 4 × 4 × 4. These computational parameters were derived by optimization based on trial and error. The forward-direction scattering by the single micro-particle was simulated using the finite element method (COMSOL). The outer frame of the simulation model consists of a sphere (diameter = 30 μm) surrounded by a perfectly matched layer (PML) (thickness = 3 μm). The sphere model was divided into two parts: all part composed of air (n = 1.00), and single micro-particle (diameter = 1.5 μm, n = 1.75) in the xy-plane was located on the central point of the sphere. The incident light (λ = 960 nm) from the port was allowed to propagate from the bottom to the upper side at θ_in = 0°. The far-field scattering intensity was integrated along the boundary edge between the PML and air in the xz-plane.

2.5 Fabrication of a NIR pc-LED device

A NIR pc-LED device was fabricated using the optimized NIR phosphor and a commercial UV LED chip (@ ∼300 nm). Typically, the as-prepared CNLC:0.008Cr phosphor was thoroughly mixed with epoxy resin (1 : 1) and coated on the chip; then it was heated at 353 K for 10 h to obtain a NIR pc-LED device.

3. Results and discussion

Fig. 1b shows the X-ray diffraction patterns of CNLC:xCr (x = 0, 0.005, 0.008, 0.01, 0.012, and 0.017) with different doping concentrations of Cr³⁺ ions. The diffraction peaks of the measured samples corresponded well with the standard diffraction pattern, indicating that the synthesized CNLC:xCr had relative phase purity and Cr³⁺ doping did not have a significant impact on the phase purity of the matrix.^91,92 In addition, Fig. 1c shows a partially enlarged XRD diffraction peak of CNLC:xCr in 23.2–24° corresponding to the (220) crystal plane position. It shows that with the increase of Cr³⁺ doping amount, the diffraction peak gradually shifts to a larger angle, which suggests that ions with smaller ionic radii replaced the larger ones, which caused the lattice to shrink. Considering the ionic radii of Cs⁺ (CN = 12, r = 1.88 Å), Na⁺ (CN = 6, r = 1.02 Å), Lu³⁺ (CN = 6, r = 0.86 Å) and Cr³⁺ (CN = 6, r = 0.615 Å) in the CNLC crystal (Å), together with the similarity in valence and Bragg's law, it is most likely that it was Cr³⁺ that replaced Lu³⁺ and occupied the center position in octahedra. To gain a better understanding on the crystal structure of the Cs₂NaLuCl₆ phosphor, we carried out Rietveld refinement on the host phosphor with Fullprof software. Fig. 1dshows the refined spectrum of the matrix sample. The refined data showed the Cs₂NaLuCl₆ host as a pure phase, and the R_p and R_wp values were 5.87% and 5.22%, respectively, and the χ² value was 2.64%. The powder diffraction data fitted well with the data listed in the standard card. To further probe into the crystal structure of a CNLC:xCr phosphor and clarify the nature of ion replacement by Cr³⁺, we did Rietveld refinement on the CNLC:xCr phosphor. The fitting results are shown in Fig. S1,† and the crystallographic parameters obtained from the refinement are listed in Table S1.† These results suggest that the CNLC:xCr phosphors belong to the cubic phase in the space group Fm3̄m (225). As shown in Fig. 1e, the unit cell constant (a) and unit cell volume (V) of the Cs₂NaLuCl₆ lattice decreased with the increase of Cr³⁺ doping amount, which is consistent with the notion that Cr³⁺ with smaller ionic radius (CN = 6, r = 0.615 Å) instead of Lu³⁺ (CN = 6, r = 0.86 Å) with larger ionic radius, occupies the center of octahedra in the matrix lattice, causing the apparent shrinkage. Fig. 1f shows the spatial configuration of the crystal structure of Cs₂NaLuCl₆, that is each of the Na⁺ and Lu³⁺ ions bonds with six Cl⁻ ions to form regular octahedra [NaCl₆] and [LuCl₆], and these octahedra are connected at the same vertices, along three directions spatially to form a cubic structure; Cs⁺ ions are located in the gap positions of the octahedra to fill in, forming a three-dimensional double perovskite structure.

Fig. 2a shows schematically the preparation process of Cs₂NaLuCl₆:xCr³⁺ phosphors. All chlorides were mixed according to the stoichiometric ratio with the help of the appropriate amount of deionized water by grinding at room temperature and sintering in a muffle furnace at 623 K. Fig. 2b shows the XPS spectra of the Cs₂NaLuCl₆ matrix and the CNLC:0.01Cr phosphor. The peaks of Cs, Na, Lu, Cl, and Cr can be identified. Two peaks of XPS fine spectrum at 577.1 and 586.6 eV in Fig. 2c were from Cr³⁺ in 2p_3/2 and 2p_1/2 states, which indicates that the valence state of Cr in CNLC:0.01Cr is +3.^93,94 ESR is an appropriate method to reflect the valence state and the occupation status of paramagnetic transition metal ions. Fig. 2d shows the ESR spectrum of the CNLC:0.01Cr phosphor at room temperature, and an obvious resonance signal is present (g = 1.97), which is mainly caused by the Cr³⁺–Cr³⁺ coupled exchange as ion pairs; and the results reflect the fact that Cr ions finally entered the Cs₂NaLuCl₆ matrix lattice in the +3 state.⁹⁵ The morphology, particle size and size distribution are known to have a significant impact on the luminous efficiency of phosphor powder and can influence the outcome of the encapsulation process during LED casting. The SEM picture showed that CNLC:0.01Cr powder consisted of relatively smooth spherical particles of 1.5–2 μm in size, which are agglomerated by smaller nano-sized particles of 50–70 nm. The submicron size is susceptible to Mie scattering of NIR luminescence, which could improve the external quantum efficiency of NIR emission (Fig. 2e).^96–99 The SEM-EDS mapping showed that Cs, Na, Lu, Cr and Cl elements were evenly distributed on the surface of the particles. The ESD spectrum of CNLC:0.008Cr showed that the atomic ratio is Cs : Na : Lu : Cl : Cr = 2 : 1 : 0.932 : 0.0075 : 6, which is consistent with the chemical formula within the error range (Fig. S2†).

Fig. 2 (a) Schematic illustration of the synthesis process used for the CNLC:xCr phosphor via the grinding–sintering method. (b) Full XPS curves and (c) fine spectra of the CNLC and CNLC:0.01Cr phosphors, respectively. (d) Room temperature EPR spectrum of the CNLC:0.01Cr phosphor. (e) SEM image and element mapping images of the CNLC:0.01Cr sample.

Fig. 3a shows the diffuse reflectance spectra of CNLC:0.008Cr (x = 0, 0.005, 0.008, 0.01, 0.012, and 0.017) phosphors. Three obvious absorption bands were present from the ultraviolet to near-infrared range, located in the 250–350 nm, 530–670 nm, and 700–900 nm ranges, respectively. These absorption peaks correspond to ⁴A₂(F) → ⁴T₁(P), ⁴A₂(F) → ⁴T₁(F), and ⁴A₂(F) → ⁴T₂(F) electronic transitions of Cr³⁺ ions, respectively. Based on the method of Tauc plot,¹⁰⁰ we used the data of diffuse reflectance of the Cs₂NaLuCl₆ matrix and derived that the band gap of the matrix is 5.37 eV (Fig. S3†), and no additional defect level absorption was observed in the matrix, indicating that Cs₂NaLuCl₆ is a suitable matrix material for NIR phosphor. Fig. 3b shows the excitation and emission spectra of CNLC:0.008Cr phosphor at room temperature. Two obvious excitation peaks located at 300 nm and 555 nm were present in the excitation spectrum, which correspond to the electronic transitions of ⁴A₂(F) → ⁴T₁(P) and ⁴A₂(F) → ⁴T₁(F) of Cr³⁺ ions, respectively. These data were consistent with the diffuse reflectance spectrum. Under ∼300 nm UV light excitation, the emission spectrum exhibited an ultra-broadband emission peak (700–1400 nm) that covers the NIR-I to NIR-II region, with a FWHM of 182 nm, which corresponds to the Cr³⁺:⁴T₂(F) → ⁴A₂(F) spin allowed transition. Fig. 3c shows the emission spectrum of the CNLC:xCr (x = 0, 0.005, 0.008, 0.01, 0.012, and 0.017) phosphors under 300 nm excitation. As the doping contents of Cr³⁺ ions gradually increased, the emission intensity of NIR first increased, peaked at 0.8 mol%, and then decreased due to a concentration-dependent quenching effect. The optical properties of Cr³⁺ are very sensitive to the strength of the crystal field, and the parameters of the peak position and peak width of an emission peak are also closely related to the strength of the crystal field. According to the theory of crystal field, the Tanabe–Sugano energy level diagram is valuable to analyze the energy level distribution of electron configuration. The energy level distribution of Cr³⁺ in the 3d³ configuration is shown in Fig. 3d, which can be described by the Tanabe–Sugano schematic diagram as:^101–106

10D_q = E(⁴T₂) (1)

where B is the Racah electron repulsion parameter, D_q is the crystal field parameter, E(⁴T₂) and E(⁴T₁) were determined by the energy positions of the excitation peaks ⁴A₂ → ⁴T₂ and ⁴A₂ → ⁴T₁ energy levels, respectively. The calculated 10D_q/B value of CNLC:xCr phosphors was 20.2, which indicates that the Cs₂NaLuCl₆ matrix provides a weak crystal field environment for Cr³⁺. To further explain the mechanisms of phosphor luminescence and energy transfer, the NIR luminescence decay time at 960 nm, under 300 nm excitation of CNLC:0.008Cr and CNLC:0.017Cr phosphors, was separately monitored; as shown in Fig. 3e, the average fluorescence lifetimes were 40.85 and 40.47 μs, which are close to the reported NIR luminescence lifetimes of ⁴T₂(F) → ⁴A₂(F) spin allowed transitions from Cr³⁺ located in the octahedra. In addition, the lifetimes of CNLC:xCr phosphors fit very well to the pattern of the mono-exponential decay curve, which shows that the energy transfer between Cr³⁺ ions is negligible, which could contribute to a higher luminescence internal quantum efficiency.^107,108 Thermal stability is an important factor affecting the performance of luminescent materials, especially for phosphor materials used in NIR pc-LED devices expected to be exposed to high temperature under high output power. Generally, the reason for the temperature-dependent decrease in luminous intensity is that when the temperature rises, the lattice vibration intensifies, so is the lattice relaxation of the luminescence center, which increases the probability of no radiation transition, and thus a decrease in luminous efficiency. Fig. 3f shows the temperature–wavelength emission spectrum of the prepared CNLC:0.008Cr phosphor from room temperature (@ 293 K) to 373 K under the excitation of 300 nm ultraviolet light. As the temperature was raised, the peak shape and peak position of the emission spectra remained basically unchanged, and the luminous intensity decreased due to thermal quenching caused by an increased probability of a non-radiative transition at high temperature. To further study the thermal quenching process, the thermal activation energy (E_a) of the CNLC:0.008Cr phosphor was evaluated using the Arrhenius formula. As shown in Fig. S4.† The value of E_a was calculated to be 0.175 eV by plotting the function of In[(I₀/I_T) − 1] and 1/K_BT.^109,110 The luminescence quantum efficiency of the phosphor is crucial to the performance of NIR pc-LED light source devices. As shown in Fig. 3g, the NIR IQE value of the CNLC:0.008Cr phosphor that showed the highest NIR emission intensity is 92.9%, and the absorption rate of the phosphor is 0.6, so its NIR EQE value is as high as 60%, which is much higher than the known Cr³⁺- doped NIR phosphor materials. Next, we analyzed the possible mechanisms that led to the high IQE and EQE values of the CNLC:0.008Cr phosphor. The charge density of octahedra and the unit cell symmetry were simulated with the Material Studio software. The distribution of charge density of a CNLC:0.008Cr unit cell around the center of the [Y/CrCl₆] octahedron is shown in Fig. 3h. It shows a high charge density, but the distribution of charge density in the center of the octahedron is highly symmetric. Also, we evaluated the degree of distortion of the octahedra before and after Cr³⁺ doping based on the refined XRD results and found that the distortion of the CrCl₆ octahedra was negligible. The results support the conclusion derived from the visual simulation that the octahedra are highly symmetric. For this reason, the Cr³⁺ ions are weakly influenced by the crystal field strength. This results in a broadband NIR emission with a large Stokes shift, which is consistent with the results of the Tanabe–Sugano theoretical calculation. It is important to point out that a weak strength of crystal field is beneficial for increasing the odd term of the crystal field, thereby partially releasing the restriction by the parity prohibition law, improving the efficiency of energy absorption by Cr³⁺ ions in Cs₂NaLuCl₆ crystals, and contributing to the high IQE and absorptivity, thus a high EQE value of CNLC:0.008Cr.^111,112 On the other hand, we used COMSOL Multiphysics software to simulate the far-field scattering effect on NIR fluorescence of CNLC:xCr phosphor (Fig. 3i). The apparent Mie scattering effect occurs when particles of 1 μm in diameter meet with NIR emission (the wavelength is about 0.98 μm); the NIR light is strongly scattered radially, which greatly reduces the loss of NIR light and boosts both the IQE and EQE.^113,114

Fig. 3 (a) Diffuse reflectance spectra of the as-prepared CNLC:xCr phosphor. (b) Photoluminescence excitation and emission spectra of the CNLC:Cr phosphor. (c) Measured NIR spectra of the CNLC:xCr phosphor excited at 300 nm, x = 0, 0.005, 0.008, 0.01, 0.012, and 0.017, respectively. (d) Tanabe–Sugano energy level diagram for Cr³⁺ ions in the octahedral crystal field of Cs₂NaLuCl₆ compounds. (e) NIR luminescence decay curves of the CNLC:0.008Cr and CNLC:0.017Cr phosphors (λ_ex = 300 nm and λ_em = 960 nm), respectively. (f) Relative integral intensity of temperature-dependent emission spectra of the CNLC:0.008Cr phosphor. (g) Measured NIR PLQY of the reference and the MTO:0.06Ni sample, respectively. (h) Simulated electron density of Cr³⁺-doped CNLC system. (i) COMSOL Multiphysics software simulation of the far-field radiation distribution of a single micro-particle at λ_em = 960 nm light.

We fabricated a NIR pc-LED device with a commercial 300 nm UV LED chip and CNLC:0.008Cr phosphors. To ensure a high performance, a heat-dissipating aluminum sheet was welded on the back of the UV chip. The CNLC:0.008Cr phosphor was mixed with silica gel at a weight ratio of 1 : 1 and applied on the surface of the semiconductor chip and cured to complete the packaging.^115–126Fig. 4a shows the electroluminescence (EL) spectrum of the packaged NIR pc-LED driven under 60 mA direct current (voltage @ 3 V). It showed a narrow band of the UV LED at 300 nm and a broadband NIR emission of the phosphor at 800–1400 nm. Given the unique nature of shortwave NIR, we proposed a range of applications for the NIR pc-LED. Under the illumination of a white LED, the luminescence of the NIR pc-LED device underneath a long-wavelength filter could not be displayed with an ordinary digital camera (Fig. 4b, left); however, the NIR luminescence underneath the same long-wavelength pass filter was clearly captured with a NIR camera, because visible light was not blocked by this filter, highlighting a potential NIR anti-counterfeiting application. The images in Fig. 4c and d are the pictures under natural light taken with an ordinary camera and with a NIR camera, respectively. Fig. 4e shows the picture taken with a NIR camera using NIR produced by NI-LED as the light source. The objects are metal screws (left side of Fig. 4c), color patterns (left side of Fig. 4d) and a logo (upper side of Fig. 4e), respectively. The corresponding imaging captured by the NIR camera clearly revealed the outlines of these objects, indicating that the NIR light source emitted from a NIR pc-LED (1 W) prepared from a 300 nm UV chip and the CNLC:0.008Cr phosphor performed well for night vision imaging under a very weak luminescent condition.

Fig. 4 (a) Light-emitting spectrum of a NIR pc-LED device combined the as-prepared CNLC:Cr phosphor with a UV chip @ 300 nm. (b) NIR anti-counterfeiting application with a long-wavelength pass filter and (c–e) night vision imaging were recorded in daylight with a normal camera and in the dark with a NIR pc-LED camera as lighting only with a NIR camera, respectively.

4. Conclusion

In summary, we synthesized a series of Cr³⁺-doped all-inorganic metal halide perovskite Cs₂NaLuCl₆ phosphors by a grinding and sintering method. The phosphors could be effectively excited by UV (@ 300 nm) light and demonstrated an ultra-broadband NIR emission that peaked at 960 nm and covered the NIR-I to NIR-II region of 800–1400 nm and had a FWHM of ∼182 nm. It showed a very high IQE and EQE value of luminous NIR at 92.9% and 60.75%, respectively. Based on the understanding of the crystal structure, the NIR optical properties and the results of numerical simulation, we conclude that the weaker strength of the crystal field surrounding Cr³⁺ is conducive to an increased odd term of the crystal field and partially release the restriction by the parity prohibition law, which improves the absorption efficiency of Cr³⁺ ions. As the size of the agglomerate of the particles is close to the wavelength of NIR emission, it triggers an obvious Mie scattering effect, which is beneficial for the light extraction of radial emission from NIR luminescence. A NIR pc-LED was prepared with the Cs₂NaLuCl₆:Cr³⁺ phosphor and a commercial UV LED chip which emitted an efficient broadband NIR, and it was demonstrated as a NIR light source for potential applications such as anti-counterfeiting and night vision imaging. The synthesis and characterization studies on Cr³⁺-doped Cs₂NaLuCl₆ broadband NIR phosphors with very high IQE and EQE values could provide a basis for the development and applications of high-lumen broadband NIR light sources.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article [and/or its ESI†].

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China-Yunnan Joint Fund (U2241236), the National Natural Science Foundation of China (12204206), the Yunnan Fundamental Research Projects (202301AT070149, 202201BE070001-029), the Major Scientific and Technological Projects of Yunnan Province (202202AG050016), the International Joint Innovation Platform of Yunnan Province (202203AP140004), the Fund for Testing and Analyzing of Kunming University of Science and Technology (2022T20220051), and the Young Elite Scientists Sponsorship Program by CAST (2024QNRC001). YG acknowledges the support from the XingDian Youth Talent Plan of Yunnan Province.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01418a

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