Rational design for broad near-infrared emission from a two-sited Rb₂LiAlF₆:Cr³⁺ phosphor with high efficiency and thermal stability for spectroscopic applications

Abstract

The exploration of high-performance near-infrared phosphors has attracted widespread attention. In this work, a brand new Rb₂LiAlF₆:Cr³⁺ (denoted as RLAF:Cr) phosphor has been constructed by the substitution of Al³⁺ ions with Cr³⁺ ions. Evidence shows that two sets of near-infrared emission bands, which originated from two types of Cr³⁺ sites, were observed upon blue light excitation. These emission bands merged into a wide emission band locating in the region of 650 nm–1050 nm, with a full width at half maximum (FWHM) of 125 nm. In addition, a high quantum efficiency of 77.7% and an excellent thermal stability at 417 K, with a retention rate of 90.5% of that at room temperature (RT), were witnessed. Profiting from the luminescence properties of the NIR phosphor, clear images of biological tissues and human palm veins were obtained using a light-emitting diode (LED) as a lighting source, which was constructed using an RLAF:Cr phosphor and a blue InGaN chip. These images showed the large potential of the RLAF:Cr phosphor for night vision and bioimaging in LED devices.

---

1. Introduction

Near-infrared (NIR) light has been widely used as an energy source in several application fields, such as biological imaging, night vision, food quality analysis, and plant lighting. The reasons for these applications are the favourable benign features of the NIR light, such as low energy, minimum damage, and deep penetration.^1–3 Among the latest technologies, the NIR phosphor-converted light-emitting diode (pc-LED) is an emerging lighting source that is based on the combination of an NIR phosphor and a blue LED chip, and it is regarded as the best technical solution for a quick and portable application of the NIR light. This is due to its attractive properties, such as small size, high efficiency, and long lifetime.^4–6 Although it is a crucial component of the NIR pc-LED device, the high-performance NIR phosphor, with its high efficiency and excellent thermal stability, is still a challenge to explore and has consequently attracted worldwide research interest.^7–9

A number of NIR phosphors have been realized by the incorporation of different metal ions, including lanthanide ions and transition-metal ions, typically Eu²⁺ and Cr³⁺, into suitable host structures.^10–12 In particular, the trivalent Cr³⁺ ion has been considered a preferable NIR activator since its 3d³ electronic configuration enables tuneable emissions, from narrowband R-line emission (²E → ⁴A₂) to wideband emission (⁴T₂ → ⁴A₂).^13,14 Recently, fluorides have gained increasing attention, which is due to the fact that their low phonon energies favour weak electron–phonon coupling (EPC) effects.¹⁵ A Cr³⁺-doped fluoride-induced high efficiency and excellent thermal stability are then expected. For example, K₂NaAlF₆:Cr³⁺ shows a quantum efficiency (QE) of 68.08% and an excellent anti-thermal quenching of 96.5% at 150 °C but has a small NIR spectral range (FWHM = 96 nm).¹⁶ As is known, emission bandwidth is an important parameter for the evaluation of NIR phosphors since a wider NIR spectrum is beneficial for the detection of more substances.¹⁷ Experimentally, different crystal field strengths give various emission bandwidths. This means that only one type of occupied Cr³⁺ site will result in a narrow NIR emission band. In other words, a wide NIR emission band can be achieved when the Cr³⁺ ions occupy several types of lattice sites. According to this observation, substitution of different lattice sites with Cr³⁺ ions is an effective way to widen the spectral range of the phosphor.^18,19 For fluorides, LiMgGaF₆:Cr³⁺ has a two-site Cr³⁺ ion occupation with an ultra-broad NIR emission (FWHM = 189.9 nm); however, the unsatisfactory QE value (42.3%) and thermal stability (50% at 400 K) limit its further applications.²⁰ That is to say, the design of a high-performance fluoride phosphor with wideband NIR emission, an enhanced QE value, and low thermal quenching is still difficult to achieve.^21,22 Therefore, to address this issue, a newly developed Cr³⁺-doped fluoride NIR phosphor with a wide emission band, high efficiency, and excellent luminescence thermal stability is of great interest to explore.

In this work, a highly efficient and thermally stable RLAF:Cr phosphor has been designed by the substitution of two different types of Al³⁺ ions with Cr³⁺ ions. A broad NIR emission band composed of two NIR emission bands was then observed upon blue light excitation. An NIR pc-LED was also fabricated using the RLAF:Cr phosphor and a blue LED chip and used as an NIR lighting source to show the potential use of the RLAF:Cr phosphor.

2. Experimental

2.1 Materials and preparation

The starting materials, including aluminium hydroxide (Al(OH)₃, AR), rubidium fluoride (RbF, AR), lithium carbonate (Li₂CO₃, AR), chromium fluoride (CrF₃, AR), ammonium fluorohydride (NH₄HF₂, AR), hydrofluoric acid (HF, 40 wt%), and absolute ethanol, were purchased from Shanghai Aladdin Biochemical Technology Co. Ltd, China. All chemicals were used as supplied without further purification.

(NH₄)₃CrF₆ was prepared in advance according to the method described in the literature.²³ The synthesis of the RLAF:Cr phosphor was performed in a plastic tube at ambient temperature using a co-precipitation method. For the typical RLAF:0.02Cr sample, 5.0 mmol of Al(OH)₃, 15.0 mmol of RbF, 2.8 mmol of Li₂CO₃, and 0.1 mmol of (NH₄)₃CrF₆ were weighed and added to 4.5 mL of HF solution. This mixture was magnetically stirred for 8.0 h, which was followed by centrifugation. Subsequently, the precipitate was washed five times with absolute ethanol and finally dried at 80 °C for 10 h for further use. A schematic diagram of the preparation process is shown in Fig. S1 in the ESI.†

The LED device was fabricated by coating the surface of a blue InGaN chip (3 W, 445 nm–450 nm; Guhoon Optoelectronics Co., Ltd, China) with the as-prepared RLAF:Cr phosphor, using epoxy resin (OE-6550A/B, Dow Corning, USA) as a binder. The device was dried at 150 °C for 60 min, which was followed by photoelectric tests.

2.2 Characterization and calculations

The X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance powder diffractometer with Cu Kα radiation (λ = 0.15406 nm) and a graphite monochromator that operated at 40 mA and 40 kV from 10° to 70°, with a step size of 0.02°. The scanning rate was 10° min⁻¹. The same diffractometer was used for the Rietveld refinements, at a scanning rate of 1° min⁻¹ in the range of 5°–120°. Scanning electron microscopy (SEM) was performed with a FEI Nova NanoSEM-450 microscope that was equipped with an energy-dispersive X-ray spectrometer (EDS). It was used for the analyses of the morphologies and elemental compositions of the as-prepared samples. The diffuse reflectance spectra (DRS) were recorded using a UV-Vis-NIR spectrophotometer (UV-3600i Plus, Shimadzu, Japan), using BaSO₄ as a reference. The photoluminescence excitation (PLE), photoluminescence (PL) spectra, and QEs were recorded using a steady-state fluorescence spectrophotometer (FLS1000, Edinburgh Instruments, UK) with an additional integrating sphere. The aging test was performed in a constant temperature and humidity chamber (BPS-50CL, Blue pard) maintaining a high temperature (85 °C) and high humidity (85%) atmosphere. In addition, the thermal quenching behaviour was analyzed using a fiber spectrophotometer (AVANTES Avaspec Mini 2048CL-SHB3), with a 460 nm laser diode as the excitation source and a constant temperature heating instrument (JF-956S). The electron paramagnetic resonance (EPR) spectra of the samples were recorded using a Bruker EMXplus-6/1 spectrometer. Furthermore, a fast-scan spectrophotometer (OHSP-350M, Hangzhou Hopoo Light & Color Technology Co., Ltd, China) was used to record the electroluminescence spectra of the as-assembled LED devices. An infrared thermal imager (FLIR E8) was used to capture the thermographs of the operational pc-LED. Also, an industrial NIR camera (XA10E, Canon, Japan) was used for the demonstration images.

Rietveld XRD refinement was performed with the Fullprof software and the crystallographic data of the RLAF host (ICSD no. 22115) was used as the initial structural model. The crystalline structure was constructed by using the Visualization for Electronic and Structural Analysis (Ver. 3.4.4) software. The electronic structure was obtained using a CASTEP module of the Materials Studio package on the basis of the density functional theory (DFT). Specifically, the Perdew–Burke–Ernzerhof (PBE) functional and the generalized gradient approximation (GGA) were used in these calculations, in addition to the electronic configurations of Rb (4p⁶5s¹), Li (2s¹), Al (3s²3p¹), and F (2s²2p⁵) for the electronic structural determinations. Furthermore, the distribution of Cr³⁺ ions in the RLAF host was evaluated by using a 2 × 2 × 2 supercell and a cutoff energy of 520 eV in the theoretical calculations.

3. Results and discussion

Fig. 1a shows the XRD patterns of the RLAF:xCr products and the standard diffraction card of the RLAF host. The enhanced diffraction peaks of each sample matched well with the R3̄m (166) space group of trigonal RLAF (JCPDS no. 27-0534) in the standard powder diffraction file. The impurity-free patterns and the sharp diffraction peaks showed single phases and high crystallinities of the products. They also indicated that the crystalline structures were stable after the incorporation of Cr³⁺ ions into RLAF, even for the highest doping contents.²⁴ With the same coordination number of 6, the ionic charges and radii of Rb⁺ (1.52 Å), Li⁺ (0.76 Å), Al³⁺ (0.535 Å), and Cr³⁺ (0.615 Å) indicated that the lattice sites of Al³⁺ ions in RLAF were preferable sites for Cr³⁺ doping. As can be seen in Fig. 1a, the right panel presents some parts of the magnified XRD patterns of the RLAF:xCr products. The (040) peak that was located at ∼31° slowly moved towards a smaller diffraction angle with an increasing value of x (from 0.00 to 0.05). This was a result of the gradual expansions of the cell volumes and the increase in lattice spacings.²⁵

Fig. 1 (a) XRD patterns of RLAF:xCr (x = 0, 0.005, 0.01, 0.02, 0.03, 0.04, and 0.05). (b) Rietveld XRD patterns, (c) crystalline structure, and (d and e) formation energies (E_fs) of RLAF:0.02Cr for two different substitution modes.

The Rietveld refinement results of the RLAF:0.02Cr sample are presented in Fig. 1b and Table S1 in the ESI.† The R factors and goodness-of-fit values indicated a high correspondence between the calculated results and observed data (Fig. 1b), which proved the high reliability of the refinement.²⁶ The refined parameters (a = b = 5.8092(3) Å, c = 28.0803(9) Å, and V = 851.5471 ų) were higher than those of the RLAF host, which is due to the larger ionic radius of Cr³⁺ compared to that of Al³⁺ (Table S2†). The crystalline structure of RLAF:0.02Cr is presented in Fig. 1c, where each Al³⁺ or Li⁺ ion was coordinated with 6 F⁻ ions and located at the centre of an octahedron. On the other hand, each Rb⁺ ion was coordinated by 12 F⁻ ions to form a [RbF₁₂] polyhedron. Thus, there were three different types of octahedra (i.e., [LiF₆], [AlF₆(I)], and [AlF₆(II)]) and one type of [RbF₁₂] polyhedron in the RLAF structural network. The Al(I)³⁺ ions predominantly occupied the corner sites of the unit cells to form the [AlF₆(I)] octahedra that were connected with [RbF₁₂] polyhedra through face-sharing F⁻ anions. In addition, each of the [AlF₆(II)] octahedra was connected with two neighbouring [LiF₆] octahedra through face-sharing. The average bond lengths of Al–F in [AlF₆(I)] and [AlF₆(II)] were 1.8024 Å and 1.8113 Å, respectively.

By considering the same valence states and similar ionic radii of the dopant and acceptor ions and the quite similar coordination environments of the Al(I)³⁺ and Al(II)³⁺ ions, the possibility for two different types of occupation sites for Cr³⁺ in the RLAF host has been investigated here. To achieve more information about the occupation of Cr³⁺ ions in the RLAF host, a 2 × 2 × 2 supercell with 48 formula units of RLAF was selected for DFT calculations. This supercell contained in total 480 atoms. There were two possible types of substitutions: M1 (Fig. 1d) and M2 (Fig. 1e). For the substitution of the [AlF₆(I)] (substitution type M1) or [AlF₆(II)] (substitution type M2) octahedra in RLAF with [CrF₆] octahedra, the formation energies were calculated according to the equation E_f = E(doped) − E(pure) + μ_Al − μ_Cr, where E(doped) and E(pure) are the total energies of the Cr³⁺-doped system and the perfect system, respectively. In addition, μ_Al and μ_Cr are the chemical potentials for the Al and Cr atoms, respectively. The formation energies of the models for the two substitution types M1 and M2 were determined to be 3.59 eV and 3.41 eV, respectively. The comparable formation energies indicated that both [AlF₆(I)] and [AlF₆(II)] octahedra tended to be replaced by the [CrF₆] octahedra.

The SEM images, elemental mappings, and EDS spectra showed that the RLAF:Cr sample adopted a regular hexagonal prism shape with the presence of Rb, Al, F, and Cr elements (Fig. S2†). However, the absence of the Li signal could be explained by its light element nature. The EPR spectrum was used to verify the presence and the valence state of chromium in the as-prepared RLAF:0.02Cr phosphor (Fig. S3†). The calculated g-factor of 1.997 ensured the presence of trivalent chromium ions with an electron spin S = 3/2 in this case, and the intense EPR signal originated from the transition of Cr³⁺ from the m_s = −1/2 to 1/2 state located in the octahedral crystal field.^16,27 The band structure that can be seen in Fig. S4a† showed that the host material had a direct bandgap, with the conduction band minimum (CBM) and the valence band maximum (VBM) located at the same point.²⁸ The resulting bandgap (E_q = 6.292 eV) was wide enough to accommodate the excited and ground states that originated from the crystal field splitting of the Cr³⁺ 3d state, implying that the generation of NIR luminescence can be expected from the Cr³⁺-doped RLAF phosphor.^28,29 Besides, the CBM was composed of Al 3p states, and the VBM was composed of F 2p states (Fig. S4b†), which implied that the luminescence properties induced by the Cr³⁺ ions were highly dependent on the [AlF₆] group in the RLAF host.³⁰

Fig. 2a shows the RT PLE and PL spectra of the RLAF:0.02Cr phosphor, in which three excitation bands were observed at ∼291 nm, 450 nm, and 637 nm in the UV, blue, and red regions, respectively. These bands correspond to the spin-allowed transitions of Cr³⁺ from the ⁴A₂ state to the ⁴T₁ (⁴P), ⁴T₁ (⁴F), and ⁴T₂ (⁴F) states, respectively.³¹ The three d–d transitions of Cr³⁺ from the ground state to the excited states were consistent with the three noticeable DRS absorption bands (Fig. S5†).³² The broad blue excitation band that ranged from 350 nm to 540 nm, with a FWHM of 82 nm, was much wider than that of the blue InGaN chip, implying that the RLAF:Cr phosphor could be used together with the commercial blue LED chip.³³ Upon 450 nm blue light excitation, a wide NIR emission band that peaked at 781 nm in the range of 650 nm–1050 nm, with a FWHM of 125 nm, was observed. It originated from the spin-allowed ⁴T₂ (⁴F) → ⁴A₂ transition. The concentration-dependent PL spectra, which were recorded at RT, showed that the RLAF:0.02Cr phosphor had the strongest PL intensity (Fig. 2b). A further increase in the x value resulted in a concentration quenching effect of the degeneration on the PL intensity (inset of Fig. 2b).³⁴

Fig. 2 (a) PLE and PL spectra of the RLAF:0.02Cr sample. (b) PL spectra of the RLAF:xCr samples recorded at RT. (c) PLE and (d) PL spectra, in addition to fitted Gaussian profiles, of the RLAF:0.02Cr sample, as obtained at 77 K. Decay curves and average lifetimes of the RLAF:0.02Cr sample, as monitored at (e) 750 nm and (f) 800 nm from the temperature of 77 K to 477 K.

The two different types of Al³⁺ sites indicated the possibility of two types of Cr³⁺ activators in the RLAF crystalline structure. To experimentally verify the Cr³⁺ occupancy in the RLAF matrix, low temperature PLE and PL spectra of the RLAF:0.02Cr sample were recorded at 77 K. Each of the spin-allowed transitions consisted of two wide bands according to the fitted Gaussian peak splitting results (Fig. 2c and d).^20,35 Specifically, the ⁴A₂ → ⁴T₁ transition was composed of two blue excitation bands that peaked at 433 nm and 464 nm. Also, the ⁴A₂ → ⁴T₂ transition was composed of two excitation bands that peaked at 626 nm and 674 nm in the red region. The ⁴T₂ → ⁴A₂ transition could, therefore, be divided into two NIR emission bands with peak locations at 749 nm and 796 nm. That is to say, the occupations of Cr³⁺ ions in two different types of sites in the RLAF facilitated a wider emission band as compared with a single site substitution. Generally, the longer emission wavelength is always associated with a weaker crystal field.³⁶ According to the averaged bond lengths of Al–F in the [AlF₆(I)] and [AlF₆(II)] octahedra, when a Cr³⁺ ion replaced the Al³⁺ ion in the [AlF₆(II)] octahedron, it resulted in a longer emission at 796 nm. Therefore, the two types of six-coordinated Al³⁺ lattice sites in the RLAF crystalline structure, which were available occupation sites for the Cr³⁺ activators, could be further verified by analysing the decay trends of the Cr³⁺ ions at the different luminescent sites. The PL decay curves of the RLAF:0.02Cr phosphor, which were obtained by different λ_em values of 750 nm, 800 nm, and 770 nm at various temperatures, are displayed in Fig. 2e, f, and S8,† respectively. The double-site occupancies of the Cr³⁺ ions were then verified by fitting the PL decay curves, monitored with a different detection wavelength for both luminescent centres, to a double-exponential function (eqn (S1) and (S2) in the ESI†).³⁷ The listed average lifetimes presented enhanced luminescence decay trends as the temperature increased from 77 K to 477 K. This was attributed to the enhanced thermal vibrations and the increased nonradiative transition probabilities among the Cr³⁺–Cr³⁺ pairs at higher temperatures. This was also the reason why the dual NIR emissions, which originated from the two types of Cr³⁺ lattice sites, slowly decreased and gradually merged into a wider single NIR emission band at an increased temperature (Fig. S6†).³⁸ In addition, based on the fitted Gaussian profiles with two groups of peak energies for the ⁴T₁ and ⁴T₂ states, the local crystal field strengths of the two types of Cr³⁺ lattice sites were estimated by using the ⁴T₁ and ⁴T₂ peak energies at the PLE maxima.³⁹ As can be seen in the Tanabe–Sugano diagram (Fig. S7†), the Dq/B ratios that were calculated from eqn (S3)–(S5)† were 2.15 and 2.10, respectively. This implied that the RLAF host offered weak crystal fields for the incorporation of Cr³⁺ activators into two different types of occupation sites.²⁸ Two sets of wide emission bands in the NIR region were, thereby, generated.

Luminescence always declines at higher temperatures, which is due to nonradiative transitions. To determine the thermal quenching properties of the RLAF:0.02Cr phosphor, temperature-dependent PL spectra were recorded from 297 K to 477 K under blue light excitation (Fig. 3a). Obviously, only one emission band could be observed at 297 K. As the temperature increased, the PL intensity gradually declined. This was attributed to the increasing number of nonradiative transitions at higher temperatures.⁴⁰ The PL intensity preserved 93.5% (of the PL intensity at 297 K) at 377 K and 90.5% at 417 K (Fig. 3b). These intensities were much better than most of the intensities reported for Cr³⁺-doped fluoride NIR phosphors (Table S3†), thereby proving the outstanding PL thermal stability of the as-prepared RLAF:Cr phosphor.

Fig. 3 For the RLAF:0.02Cr phosphor: (a) temperature-dependent PL spectra recorded from 297 K to 477 K. (b) Integral PL intensity versus temperature. (c) Configurational coordinate diagram explaining the thermal-quenching behaviour. (d) Normalized PL spectra. (e and f) Relationships between peak location, FWHM, and temperature. Relationships between (g) ln[(I₀/I_T) − 1] and 1/kT and (h) FWHM² and 2kT.

The configurational coordinate diagram presented in Fig. 3c could be used to explain the low thermal-quenching behavior of the RLAF:Cr phosphor. Upon blue light excitation, the Cr³⁺ electrons in the ground state ⁴A₂ absorbed energy and became excited to the ⁴T₂ energy level via pathway I. At RT, most of the electrons in the ⁴T₂ excited state returned to their ground state via pathway II, which was a radiative transition. Nevertheless, as the temperature increased, the electrons in the ⁴T₂ state could be excited to the intersection point of the ⁴T₂ and ⁴A₂ energy levels and, thereafter, return to the ground state via the nonradiative pathway III. The energy difference between the lowest position of the ⁴T₂ excited state and the intersection point of the ⁴T₂ and ⁴A₂ states was defined as the activation energy ΔE (Fig. 3c), which was an important parameter for the description of the thermal-quenching behaviour of luminescence materials.⁴¹ In this case, the ΔE value could be calculated using the modified Arrhenius equation (eqn (S6)†). By plotting the relationship between ln[(I₀/I_T) − 1] and 1/kT, a straight fitting line with a correlation coefficient of 0.988 and a slope of −0.228 was obtained. This indicated that the fitting result was trustworthy and that ΔE was 0.228 eV (Fig. 3g). This value was much larger than the thermal disturbance energy (0.026 eV) at RT, indicating that the energy barrier for the nonradiative relaxation was hard to overcome and therefore, the RLAF:Cr phosphor had excellent thermal stability.⁴²

Fig. 3d–f show the normalized temperature-dependent PL spectra of the RLAF:0.02Cr sample in the temperature region of 297 K to 477 K, in addition to the relationships between the PL peak positions, FWHMs, and temperatures. Apparently, as the working temperature increased, the PL peak position became instantly redshifted towards a longer wavelength in the region of 781 nm to 805 nm (Fig. 3e). This was due to the unit cell expansion in the hot environment.⁴³ In addition, the emission band became gradually wider, with an increase in FWHM from 125 nm to 158 nm (Fig. 3f), which originated from the stronger EPC effect at higher temperatures.⁴⁴ This spectral broadening phenomenon could be explained by the Huang–Rhys factor with different electron-vibrational interactions, which could be estimated using eqn (S7) and (S8).† ⁴⁵ Based on the temperature-dependent PL spectra, a fitting line with a high correlation coefficient of 0.998 was obtained by plotting FWHM²versus 2kT (Fig. 3h), indicating that the fitting results were highly reliable. According to the slope of the line, the hω and S values were determined to be 14.9 meV and 2.64, respectively, which indicated that Cr³⁺ activators experienced a weak EPC effect in RLAF host.⁴⁶ This was also assumed to be the reason for the remarkable PL thermal stability of RLAF:Cr. Moreover, the QE of RLAF:0.02Cr upon excitation with a wavelength of 450 nm was found to be 77.7%. The integral PL intensity of RLAF:0.02Cr was maintained at 88.1% of the initial value after being aged at 85 °C and 85% relative humidity for 24 h and still maintained at 83.7% for 168 h (Fig. S9†), indicating the high reliability of the RLAF:0.02Cr phosphor in a high temperature and high humidity environment when it is used as a colour converter in NIR pc-LED devices. It is at present difficult to achieve both high efficiency and superb thermal stability of a Cr³⁺-doped fluoride (Table S3†). Therefore, in comparison with other one- or two-sited Cr³⁺-doped fluorides, a high-performance near-infrared two-sited Cr³⁺-doped RLAF phosphor with outstanding luminescence, thermal stability and high efficiency has rarely been reported.

Due to the different types of occupation sites for the Cr³⁺ activators, the low thermal-quenching, high-efficiency RLAF:Cr NIR phosphor was successfully used in the construction of NIR pc-LEDs with excellent optical performances. The NIR pc-LEDs were assembled by coating blue InGaN chips with RLAF:Cr powders. Fig. 4a shows the LED photos and the current-dependent PL spectra of a packaged LED device. The weak blue emission band in the region of 425 nm–475 nm originated from the LED chip, and the dominant broad NIR emission band in the region of 650 nm–1050 nm originated from RLAF:Cr. With a continuous increase in the drive current from 20 mA to 320 mA, both of the blue and NIR PL intensities gradually increased with stable spectral shapes. Thus, these results showed that RLAF:Cr was promising for applications in high-power NIR LED devices. As monitored using an NIR imaging camera, the temperature and thermographs of the working LED device for four different drive currents are shown in Fig. 4b. As induced by the accumulated heat at higher drive currents, the working temperature of the LED device increased from 17.0 °C to 90.9 °C. Thus, it was concluded that the RLAF:Cr phosphor could be stably used in an LED, which was due to its high PL thermal stability. Moreover, as the drive current increased from 20 mA to 320 mA, the NIR output power of the LED device continuously increased from 2.52 mW to 37.25 mW, and the photoelectric conversion efficiency changed from 4.84% to 3.69% (Fig. S10†), in which the latter one is mainly attributed to the decreasing external QE of the blue LED chip at higher temperatures.⁴⁴ Subsequently, to validate the role of the RLAF:Cr phosphor in the LED device and to verify the non-invasive and penetration properties of the NIR light, the potential use of the NIR pc-LED for various applications was investigated. Fig. 4c–e present a series of photos of some objects (i.e., flowers, an apple and a doll), which were taken with an NIR camera with its natural and near-infrared modes and by using the assembled NIR LED device as a lighting source. Very clear photos of the objects are presented in Fig. 4c1 and d1. These photos were taken with the camera in its natural mode and under natural light irradiation. The colourful photos were then visible to the human eye. On the other hand, nothing was captured when the natural light was missing (Fig. 4c2 and d2), while the camera took clear photos of the objects when the NIR pc-LED was working (Fig. 4c3 and d3). Additionally, the NIR light (700 nm–1100 nm) showed strong penetration characteristics for biological tissues. The blood vessel distribution in a palm can be clearly observed in Fig. 4e, with the packaged LED as a lighting source. These results have strongly illustrated that the highly efficient and thermally stable RLAF:Cr phosphor has great potential for night vision and biological imaging applications in NIR pc-LEDs.

Fig. 4 (a) PL spectra of the NIR pc-LED for various drive currents. The insets show the photos of the as-fabricated pc-LED, lighted pc-LED, and lighted pc-LED with an optical filter. (b) Thermographs of the LED device, which are obtained at 20 mA, 100 mA, 200 mA, and 320 mA. Photos of an apple, a flower, and a doll taken (c1 and d1) under natural light, (c2 and d2) with no lighting source and (c3 and d3) with the as-prepared NIR pc-LED device. (e) Photo of the NIR light-penetrating palm.

4. Conclusions

In this work, a highly efficient and thermally stable RLAF:Cr phosphor with two occupation sites has been presented by incorporating Cr³⁺ as luminescent centres into the RLAF host. The structural analysis and calculated formation energies disclose that both Al³⁺ sites are presumed to be substituted by Cr³⁺ activators. The spectral features confirm that two groups of Cr³⁺:⁴T₂ (⁴F) → ⁴A₂ transitions with two sets of NIR emission bands are witnessed, which merge into a broad NIR emission band (FWHM = 125 nm) spanning the range of 650–1050 nm. Upon blue light excitation, RLAF:Cr demonstrates the capability to produce an excellent thermal stability at 417 K, with a ratio of 90.5% of that at RT, which originates from the subdued EPC effect around Cr³⁺ activators and the high activation energy of the RLAF:Cr phosphor. Due to the high thermal stability and high QE (77.7%) of the NIR phosphor, clearly visible photographs of actual objects and human hand veins were obtained using the fabricated NIR pc-LED as a lighting source. These results showed the potential applications of the RLAF:Cr phosphor in NIR pc-LED devices.

Author contributions

S. Qing: investigation, methodology, data curation, and writing – original draft. J. Wan: writing – review and editing. T. Yang: writing – review and editing. Q. Zhou: conceptualization, supervision, and writing – review and editing. Y. Y. Zhou: writing – review and editing. Z. L. Wang: supervision and writing – review and editing. D. W. Wen: writing – review and editing. M. M. Wu: project administration, resources, and writing – review and editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (grant nos. 22365034, 22065039, 52272174 and U22A20135) and the Yunnan Fundamental Research Projects (202101AT070072).

References

1. P. M. Pattison, J. Y. Tsao, G. C. Brainard and B. Bugbee, LEDs for photons, physiology and food, Nature, 2018, 563, 493–500.
2. D. J. Liu, G. G. Li, P. P. Dang, Q. Q. Zhang, Y. Wei, H. Z. Lian, M. M. Shang, C. C. Lin and J. Lin, Simultaneous broadening and enhancement of Cr³⁺ photoluminescence in LiIn₂SbO₆ by chemical unit cosubstitution: night-vision and near-infrared spectroscopy detection applications, Angew. Chem., Int. Ed., 2021, 60, 14644–14649.
3. V. Rajendran, M. H. Fang, W. T. Huang, N. Majewska, T. Lesniewski, S. Mahlik, G. Leniec, S. M. Kaczmarek, W. Pang, V. K. Peterson, K. M. Lu, H. Chang and R. S. Liu, Chromium ion pair luminescence: a strategy in broadband near-infrared light-emitting diode design, J. Am. Chem. Soc., 2021, 143, 19058–19066.
4. M. Vasilopoulou, A. Fakharuddin, F. P. G. de Arquer, D. G. Georgiadou, H. Kim, A. R. bin, M. Yusoff, F. Gao, M. K. Nazeeruddin, H. J. Bolink and E. H. Sargent, Advances in solution-processed near-infrared light-emitting diodes, Nat. Photonics, 2021, 15, 656–669.
5. J. W. Qiao, G. J. Zhou, Y. Y. Zhou, Q. Y. Zhang and Z. G. Xia, Divalent europium-doped near-infrared-emitting phosphor for light-emitting diodes, Nat. Commun., 2019, 10, 5267.
6. Z. W. Jia, C. X. Yuan, Y. F. Liu, X. J. Wang, P. Sun, L. Wang, H. C. Jiang and J. Jiang, Strategies to approach high performance in Cr³⁺-doped phosphors for high-power NIR-LED light sources, Light: Sci. Appl., 2020, 9, 86.
7. G. H. Wei, P. L. Li, R. Li, Y. Wang, S. X. He, J. H. Li, Y. W. Shi, H. Suo, Y. B. Yang and Z. J. Wang, How to achieve excellent luminescence properties of Cr ion-doped near-infrared phosphors, Adv. Opt. Mater., 2023, 11, 2301794.
8. S. W. Wang, R. Pang, T. Tan, H. Y. Wu, Q. Wang, C. Y. Li, S. Zhang, T. X. Tan, H. P. You and H. J. Zhang, Achieving high quantum efficiency broadband NIR Mg₄Ta₂O₉:Cr³⁺ Phosphor through lithium-ion compensation, Adv. Mater., 2023, 35, 2300124.
9. L. You, R. Tian, T. Zhou and R. J. Xie, Broadband near-infrared phosphor BaMgAl₁₀O₁₇:Cr³⁺ realized by crystallographic site engineering, Chem. Eng. J., 2021, 417, 129224.
10. J. W. Qiao, S. Zhang, X. Q. Zhou, W. B. Chen, R. Gautier and Z. G. Xia, Near-infrared light-emitting diodes utilizing a europium-activated calcium oxide phosphor with external quantum efficiency of up to 54.7%, Adv. Mater., 2022, 34, 2201887.
11. X. Zhou, J. M. Xiang, J. M. Zheng, X. Q. Zhao, H. Suo and C. F. Guo, Ab initio two-sites occupancy and broadband near-infrared emission of Cr³⁺ in Li₂MgZrO₄, Mater. Chem. Front., 2021, 5, 4334–4342.
12. H. H. Lin, G. X. Bai, T. Yu, M. K. Tsang, Q. Y. Zhang and J. H. Hao, Site occupancy and near-infrared luminescence in Ca₃Ga₂Ge₃O₁₂:Cr³⁺ persistent phosphor, Adv. Opt. Mater., 2017, 5, 1700227.
13. F. Y. Zhao, Z. Song and Q. L. Liu, Advances in chromium-activated phosphors for near-infrared light sources, Laser Photonics Rev., 2022, 16, 2200380.
14. Y. Y. Liang, Q. T. Cao, Y. Q. Zhou, W. L. Zhou, J. L. Zhang, L. P. Yu, S. X. Lian and Z. X. Qiu, Towards control facilities for mimicking plant growth optimum action spectrum: efficient near-ultraviolet to far-red light-conversion in Cr³⁺-doped rare-earth aluminate phosphors, Chem. Eng. J., 2023, 454, 140235.
15. Y. J. Jia, Y. X. Pan, Y. Q. Li, L. J. Zhang, H. Z. Lian and J. Lin, Improved moisture-resistant and luminescence properties of a red phosphor based on dodec-fluoride K₃RbGe₂F₁₂:Mn⁴⁺ through surface modification, Inorg. Chem., 2021, 60, 231–238.
16. J. Li, H. F. Zhe, H. Ming, Y. Y. Zhou, Y. Q. Ye, Q. Zhou and Z. L. Wang, A near-infrared phosphor doped with Cr³⁺ towards zero-thermal-quenching for high-power LEDs, Mater. Today Chem., 2022, 24, 100839.
17. P. P. Dang, Y. Wei, D. J. Liu, G. G. Li and J. Lin, Recent advances in chromium-doped near-infrared luminescent materials: fundamentals, optimization strategies, and applications, Adv. Opt. Mater., 2023, 11, 2201739.
18. S. H. Miao, Y. J. Liang, D. X. Chen, R. Q. Shi, X. H. Shan, Y. Zhang, F. Xie and X. J. Wang, Site-selective occupancy control of Cr ions toward ultrabroad-band infrared luminescence with a spectral width up to 419 nm, ACS Appl. Mater. Interfaces, 2022, 14, 53101–53110.
19. Q. Q. Zhang, D. J. Liu, P. P. Dang, H. Z. Lian, G. G. Li and J. Lin, Two selective sites control of Cr³⁺-doped ABO₄ phosphors for tuning ultra-broadband near-infrared photoluminescence and multi-applications, Laser Photonics Rev., 2022, 16, 2100459.
20. W. D. Nie, J. X. Zuo, S. Yang, Y. Li, W. F. Zou, G. Chen, S. H. Wu, D. Wu, S. B. Liu, J. Q. Peng, L. Han and X. Y. Ye, Ultra-broad NIR emission from two-site Cr³⁺ occupation in fluoride phosphor via composition-driven structural transformation, Mater. Today Chem., 2022, 26, 101164.
21. J. Q. Fan, W. Y. Zhou, J. L. Zhang, P. C. Chen, Q. Pang, L. Y. Zhou, C. Y. Zhou and X. G. Zhang, A novel efficient broadband near-infrared phosphor LiGaGe₂O₆:Cr³⁺ with EQE enhancement and spectral tuning by Sc³⁺-Ga³⁺ substitution for NIR pc-LED application, Inorg. Chem. Front., 2023, 10, 511–521.
22. X. K. Zou, X. J. Wang, H. R. Zhang, Y. Y. Kang, X. Yang, X. J. Zhang, M. S. Molokeev and B. F. Lei, A highly efficient and suitable spectral profile Cr³⁺-doped garnet near-infrared emitting phosphor for regulating photomorphogenesis of plants, Chem. Eng. J., 2022, 428, 132003.
23. F. Q. He, E. H. Song, Y. Y. Zhou, H. Ming, Z. T. Chen, J. C. Wu, P. S. Shao, X. F. Yang, Z. G. Xia and Q. Y. Zhang, A general ammonium salt assisted synthesis strategy for Cr³⁺-Doped hexafluorides with highly efficient near infrared emissions, Adv. Funct. Mater., 2021, 31, 2103743.
24. G. H. Wei, P. L. Li, R. Li, J. H. Li, Y. W. Shi, Y. Wang, S. X. He, Y. B. Yang, H. Suo and Z. J. Wang, Chromium(III)-doped phosphors of high-efficiency two-site occupation broadband infrared emission for vessel visualization applications, Inorg. Chem., 2022, 61, 5665–5671.
25. T. Yang, L. X. Chu, Y. Qin, Q. Zhou, J. Wan, H. J. Tang, Y. Q. Ye and Z. L. Wang, An efficient and thermally stable source with Cr³⁺ near-infrared luminescence for non-destructive testing applications, Mater. Today Chem., 2024, 35, 101918.
26. T. Tan, S. W. Wang, J. Y. Su, W. H. Yuan, H. Y. Wu, R. Pang, J. T. Wang, C. Y. Li and H. J. Zhang, Design of a novel near-infrared luminescence material Li₂Mg₃TiO₆:Cr³⁺ with an ultrawide tuning range applied to near-infrared light-emitting diodes, ACS Sustainable Chem. Eng., 2022, 10, 3839–3850.
27. Q. M. Lin, Q. Wang, M. Liao, M. X. Xiong, X. Feng, X. Zhang, H. F. Dong, D. Y. Zhu, F. G. Wu and Z. F. Mu, Trivalent chromium ions doped fluorides with both broad emission bandwidth and excellent luminescence thermal stability, ACS Appl. Mater. Interfaces, 2021, 13, 18274–18282.
28. E. H. Song, H. Ming, Y. Y. Zhou, F. Q. He, J. C. Wu, Z. G. Xia and Q. Y. Zhang, Cr³⁺-doped Sc-based fluoride enabling highly efficient near infrared luminescence: a case study of K₂NaScF₆:Cr³⁺, Laser Photonics Rev., 2021, 15, 2000410.
29. H. S. Zhang, J. Y. Zhong, F. Du, L. Chen, X. L. Zhang, Z. F. Mu and W. R. Zhao, Efficient and thermally stable broad-band near-infrared emission in a KAlP₂O₇:Cr³⁺ phosphor for nondestructive examination, ACS Appl. Mater. Interfaces, 2022, 14, 11663–11671.
30. D. C. Huang, S. S. Liang, D. J. Chen, J. Hu, K. Y. Xu and H. M. Zhu, An efficient garnet-structured Na₃Al₂Li₃F₁₂:Cr³⁺ phosphor with excellent photoluminescence thermal stability for near-infrared LEDs, Chem. Eng. J., 2021, 426, 131332.
31. Y. W. Zhu, Y. F. Yang, Y. Q. Zhu, Y. F. Chen, Z. Y. Liu, F. M. Lei, H. Yu, Q. N. Mao, J. S. Zhong and J. Wang, Enhanced photoluminescence of a HF free novel NIR phosphor of K₂ZnF₄:Cr³⁺ by lithium ion doping, J. Mater. Chem. C, 2023, 11, 10694–10702.
32. F. Y. Zhao, Z. Song, J. Zhao and Q. L. Liu, Double perovskite Cs₂AgInCl₆:Cr³⁺: broadband and near-infrared luminescent materials, Inorg. Chem. Front., 2019, 6, 3621–3628.
33. Q. Zhou, L. Dolgov, A. M. Srivastava, L. Zhou, Z. L. Wang, J. X. Shi, M. D. Dramicanin, M. G. Brik and M. M. Wu, Mn²⁺ and Mn⁴⁺ red phosphors: synthesis, luminescence and applications in WLEDs. A review, J. Mater. Chem. C, 2018, 6, 2652–2671.
34. H. J. Yu, J. Chen, R. Y. Mi, J. Y. Yang and Y. G. Liu, Broadband near-infrared emission of K₃ScF₆:Cr³⁺ phosphors for night vision imaging system sources, Chem. Eng. J., 2021, 417, 129271.
35. S. S. Ding, H. J. Guo, P. Feng, Q. F. Ye and Y. H. Wang, A new near-infrared long persistent luminescence material with its outstanding persistent luminescence performance and promising multifunctional application prospects, Adv. Opt. Mater., 2020, 8, 2000097.
36. S. W. Yuan, F. G. Wu, J. Wang, L. L. Lou, S. Zhao, D. Y. Zhu and Z. F. Mu, Ultra-broadband near infrared phosphor with wide spectral range and long peak wavelength achieved by double-site occupation, J. Rare Earths, 2023, 41, 1670–1677.
37. H. Li, J. K. Jiao, X. F. Xiang, J. Z. Wu, W. B. Hu, J. Y. Xie, S. P. Huang, H. Z. Zhang and J. Zhu, Directly Identifying Multiple Cr³⁺ Emitting Centers for Broad Near-Infrared Emission in an Efficient and Near-Zero Thermal Quenching Garnet-Type Phosphor, Adv. Opt. Mater., 2024, 12, 2302391.
38. H. T. Zeng, T. L. Zhou, L. Wang and R. J. Xie, Two-site occupation for exploring ultra-broadband near-infrared phosphor—double-perovskite La₂MgZrO₆:Cr³⁺, Chem. Mater., 2019, 31, 5245–5253.
39. Q. X. Pang, Y. C. Wang, L. Q. Yan, G. Zhu, S. Xu, J. S. Zhang, X. Z. Zhang, Y. Z. Cao and B. J. Chen, Cr³⁺-Cr³⁺ ion pair induced fast energy migration in Cr³⁺ doped Na-β-Al₂O₃ ultra-wide near-infrared phosphors for NIR spectroscopy application, Laser Photonics Rev., 2024, 18, 2301039.
40. D. C. Huang, X. G. He, J. R. Zhang, J. Hu, S. S. Liang, D. J. Chen, K. Y. Xu and H. M. Zhu, Efficient and thermally stable broadband near-infrared emission from near zero thermal expansion AlP₃O₉:Cr³⁺ phosphors, Inorg. Chem. Front., 2022, 9, 1692–1700.
41. Q. Zhou, J. Wan, Y. Y. Zhou, S. Zhang, D. X. Shi, X. L. Xie, H. Q. Pu, Y. Q. Ye and Z. L. Wang, Ultraintense zero-phonon line from a Mn⁴⁺ red-emitting phosphor for high-quality backlight display applications, Inorg. Chem., 2021, 60, 19197–19205.
42. Z. X. Wu, X. X. Han, J. Wang, Y. Y. Zhou, K. Xing, S. Cao, J. L. Zhao, B. S. Zou and R. S. Zeng, Highly efficient and thermally stable broadband near-infrared emitting fluoride Cs₂KGaF₆:Cr³⁺ for multiple LED applications, J. Mater. Chem. C, 2022, 10, 10292–10301.
43. S. Y. Guo, L. G. Ma, M. Abudureyimu, R. F. Wei, F. M. Lu, F. F. Hu and H. Guo, Improving and broadening luminescence in Gd_2−xAl_xGaSbO₇:Cr³⁺ phosphors for NIR LED applications, Inorg. Chem. Front., 2023, 10, 2197–2205.
44. Z. X. Wu, X. X. Han, Y. Y. Zhou, K. Xing, S. Cao, L. Chen, R. S. Zeng, J. L. Zhao and B. S. Zou, Efficient broadband near-infrared luminescence of Cr³⁺ doped fluoride K₂NaInF₆ and its NIR-LED application toward veins imaging, Chem. Eng. J., 2022, 427, 131740.
45. D. Wu, L. L. Liu, H. Liang, H. B. Duan, W. D. Nie, J. R. Wang, J. Q. Peng and X. Y. Ye, LiBAlF₆:Cr³⁺ (B = Ca, Sr) fluoride phosphors with ultra-broad near-infrared emission for NIR pc-LEDs, Ceram. Int., 2022, 48, 387–396.
46. J. H. Xie, J. H. Tian and W. D. Zhuang, Near-Infrared LuCa₂ScZrGa₂GeO₁₂:Cr³⁺ garnet phosphor with ultra-broadband emission for NIR LED applications, Inorg. Chem., 2023, 62, 10772–10779.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi00429a

---