Tunable broadband near-infrared luminescence from Cr³⁺-doped gallium oxide-based phosphors for advanced sensing and LED applications

Abstract

Near-infrared (NIR) phosphor-converted light emitting diodes (pc-LEDs) highly demand broadband NIR phosphors for applications in food safety and smart detection. However, the lack of tunable NIR luminescent materials limits their development. Herein, we constructed a series of NIR Cr³⁺ doped Ga₂O₃-based materials with broadband, tunable, efficient emissions by utilizing [Zn²⁺–Ge⁴⁺] and [Ga³⁺–Ga³⁺] co-unit substitution. The obtained results show that the emission peak of NIR phosphors can be modulated from 713 to 765 nm. Meanwhile, the excitation peak remains in the blue light region. NIR phosphor-converted devices were prepared with 450 nm LED chips, which were studied using NIR imaging, penetration imaging, anti-counterfeiting, and pupil recognition technologies. Interestingly, NIR mechanoluminescence (ML) was realized based on the developed phosphors, which were successfully applied for non-destructive measurements of solids and in vivo bioimaging. Our study proposes a series of tunable and broadband luminescent materials and devices for NIR applications.

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1. Introduction

Broadband NIR light sources have characteristics of high penetration depth and invisibility.^1,2 In particular, they span the NIR region of 700 to 1100 nm, which covers the characteristic absorption signal of the normal vibration modes of C–H, O–H, and N–H.^3,4 Therefore, NIR light devices have been widely used in biomedicine, pesticide residue detection, artificial intelligence, unmanned aerial vehicle imaging, and security monitoring.^5,6 At present, the state-of-the-art device that produces NIR luminescence is the pc-LED. This device has advantages such as adjustable emission peak, low cost, long lifespan, energy conservation and environmental protection.⁷ At the same time, tunable broadband NIR emissions can be achieved with equipment design.⁸ However, the output performance of pc-LEDs depends on phosphor coating.⁹ Therefore, the development of broadband and long wavelength NIR phosphors is crucial for advancing this technology.^10,11

Recently, NIR phosphors doped with rare-earth ions and transition metals have gained much attention owing to their promising applications.¹² Rare-earth ions (Pr³⁺, Er³⁺, Yb³⁺ and Tm³⁺) have limited applications because of their typical narrow band and weak absorption.¹³ Trivalent chromium doped phosphors are among the best NIR emitters for LED applications today.^14,15 This is because the Cr³⁺ substituted phosphor exhibits excellent blue light absorption (λ_ex = 450 nm) and the ability to tune the position of the emission peak.^16,17 These functions require a weak octahedral crystal field to be realized.¹⁸ However, a weak crystal field emission peak also shifts the excitation peak, resulting in a mismatch between the phosphor and the blue LED chip. Meanwhile, according to the multiphoton radiation transition theory, the reduction of the energy gap between the ground state and the excited state would lead to the enhancement of non-radiative transition.¹⁹ The emission efficiency would inevitably decrease as the crystal field weakens.²⁰ Therefore, preparing phosphors that can maintain effective blue excitation while achieving the redshift of the emission spectrum remains a major challenge.²¹

In recent years, in order to prepare NIR phosphors with excellent luminous phosphors, researchers have developed a series of strategies to optimize the luminescence of phosphors.^22–24 The main strategies are ion substitution, crystal field engineering, and energy transfer, among which ion substitution strategy has been widely studied. For example, Liu et al.²⁵ prepared Ga_2−x(Al_0.68In_0.32)_xO₃:Cr³⁺ series phosphors by replacing Ga³⁺ with (Al_0.68In_0.32)³⁺. The increased cation radius deviation in substitution leads to an increased electron–phonon coupling. The resulting emission spectrum covers the 650 to 1000 nm range, with a 30% increase in the full width at half maxima (FWHM) while having 80% internal quantum efficiency. Wang et al.²⁶ introduced Al³⁺ into Cs₂KInF₆:0.1Cr³⁺ to enhance the crystallinity and structural stiffness, thus increasing thermal stability by 72.54%. Zeng et al.²⁷ modified simple oxide Ga₂O₃ by [Mg²⁺–Ge⁴⁺] and [Ga³⁺–Ga³⁺] units to prepare a series of Ga_1.97−2xMg_xGe_xCr_0.03O₃ phosphors, achieving tunable emission peaks from 726 to 830 nm while being efficiently excited by blue light. These results suggest that ion substitution is a very effective means of modulating NIR-emission phosphors.

Herein, the simple oxide Ga₂O₃ was modified by an ion substitution strategy. By using the similar sized [Zn²⁺–Ge⁴⁺] (r_6-coor. (Zn²⁺) + r_4-coor. (Ge⁴⁺) = 1.13 Å) to replace part of [Ga³⁺–Ga³⁺] (r_6-coor. (Ga³⁺) + r_6-coor. (Ga³⁺) = 1.09 Å), a series of Ga_2−2xZn_xGe_xO₃:Cr³⁺ (x = 0–0.2) phosphors were fabricated. Although the proportion of co-substitution is only 20%, frequency band spreading and tunable emission peak are achieved. Notably, the excitation peak remains in the blue light region. A comprehensive study of the local structure and crystal field environment of Cr³⁺ provides insight into the mechanism underlying the change in optical properties. The crystal structure, photoluminescence and fluorescence lifetime properties were also systematically investigated. Moreover, this material also has the property of ML, and a series of systematic studies on ML were carried out. Compared with the previous ML property of Cr³⁺-doped Ga₂O₃, the intensity of the ML is enhanced by 353.16% through the strategy of co-substitution, and the mechanism of the enhancement of the luminescence intensity is explained.

2. Results and discussion

Crystal structure and morphology

Fig. 1a depicts X-ray diffraction (XRD) patterns of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors. The spectra match well with that of the Ga₂O₃ standard card (PDF #76-0573), indicating that phase-pure samples have been successfully synthesized. Meanwhile, no spurious peaks were generated, indicating that part of the [Ga³⁺–Ga³⁺] unit was successfully substituted by the [Zn²⁺–Ge⁴⁺] unit. Fig. S1† presents the SEM image of the Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor, and it can be observed that the phosphor is mainly at the micrometer level and has an irregular block shape. The crystal structures of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors are shown in Fig. 1b. There are two different cationic sites in Ga₂O₃. One is [GaO₆], where the Ga³⁺ ion is coordinated with six oxygen atoms to form an octahedron, and the other is [GaO₄], where the Ga³⁺ ion is coordinated with four oxygen atoms to form a tetrahedron. The doped cations prefer to replace ions with similar ionic radii. Therefore, in the lattice of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃, Zn²⁺ (r = 0.6 Å, CN = 6) and Cr³⁺ (r = 0.615 Å, CN = 6) ions prefer to replace Ga³⁺ in [GaO₆] (r = 0.62 Å, CN = 6) and Ge⁴⁺ (r = 0.53 Å, CN = 4) prefers to replace Ga³⁺ in [GaO₄] (r = 0.39 Å, CN = 4). The energy dispersion spectrum of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ (x = 0–0.2) phosphors is shown in Fig. 1c. It can be seen from the figure that the prepared samples did contain elements Ga, Zn, Ge, Cr and O.²⁸ The table in the figure lists the weight and atomic percentages of each element, where the specific weight and atomic ratio of each element is close to the nominal ones, indicating that the synthesis of phosphors was successful. Fig. 1d shows the elemental mapping images of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ with uniform distribution of Ga, Zn, Ge, O, Cr without obvious segregation, which further directly demonstrated that the [Zn²⁺–Ge⁴⁺] unit and Cr³⁺ are successfully doped into the Ga₂O₃ lattice. Raman spectroscopy is also an excellent way to characterize the morphology and structure of phosphors. The results are illustrated in Fig. 1e. The strong bands of the Raman spectrum in the regions 150–230 cm⁻¹ and 730–820 cm⁻¹ were assigned to the [GaO₆] octahedron and [GaO₄] tetrahedron-associated stretching vibrations. From the figure, the peaks in the 150–230 cm⁻¹ band are red-shifted, while the peaks in the 730–820 cm⁻¹ band are blue-shifted. The red-shifted peak at 150–230 cm⁻¹ is associated with the increased Ga–O bond length. The enlargement of the Ga–O bond is caused by the substitution of Cr³⁺ and Zn²⁺ ions for a part of [GaO₆]. Similarly, the incorporation of Ge⁴⁺ in a portion of [GaO₄] lead to the increase in wavenumbers for the peak at 730–820 cm⁻¹.²⁹

Fig. 1 (a) XRD patterns of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors. (b) The crystal structure of the Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor. (c) The energy dispersion spectra of the Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor. (d) Elemental mapping images of Ga, O, Ge, Zn and Cr in the Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor. (e) Raman diagrams of the Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors.

Luminous properties of phosphors

The diffuse reflection (DR) spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0 and 0.2) phosphors are shown in Fig. 2a. It can be seen from the DR diagram that the position of two absorption bands, ⁴A₂ → ⁴T₂ (⁴F) and ⁴A₂ → ⁴T₁ (⁴F), of Cr³⁺ is basically unchanged regardless of the change in x value. On the contrary, the absorbance of phosphors increases with the value of x. It arises from a distortion in the local coordination environment. The introduction of odd-parity breaks the parity-forbidden nature of Cr³⁺ 3d–3d electronic transitions.³⁰ As a result, the absorbance of phosphors increases. Fig. 2b shows the excitation spectrum of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors. Although the value of x increases from 0 to 0.2, there is only a small change in the position of the excitation peak in the blue region.³¹Fig. 2c shows the normalized photoluminescence (PL) spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors. It can be seen from the figure that the emission spectra of the phosphor are mainly in the NIR region and the position of emission peaks of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors are red-shifted. As shown in Fig. S2,† the unit ratio of [Zn²⁺–Ge⁴⁺] is adjusted from x = 0 to x = 0.2, and the emission peak of phosphors is red-shifted from 713 nm to 767 nm. Meanwhile, as shown in Fig. S3,† the FWHM of phosphors expands from 92 to 155 nm with an increasing co-unit substitution ratio, increasing the spectral coverage in the NIR region.³² It is well known that the surface temperature of blue chips can be as high as 423 K during normal operation. The thermal stability of phosphors will affect the overall optoelectronic performance of the packaged NIR pc-LED devices. Therefore, to investigate the thermal stability of samples, NIR phosphors are necessary. Fig. 2d shows the down-conversion emission spectrum of the phosphor at 450 nm in the temperature range of 303–423 K with a step of 20 K. The emission intensity decreases with increasing temperature. Fig. S4† represents the relative PL intensities of phosphors at different temperatures. Nevertheless, the NIR emission intensity at 423 K remains at 58.34% of the emission intensity at 303 K, indicating that the sample has excellent thermal stability and is suitable for practical application.²⁶Fig. 2e shows the photoluminescence quantum yield (PLQY) histograms of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ in the temperature range of 303–423 K. The quantum yield decreases from 68% to 47% with increasing temperature. The decrease in PLQY of the phosphor is due to thermal burst. With increasing temperature, the non-radiative transitions of excited electrons are significantly enhanced. In the Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) series samples, the relationship between Cr³⁺ doping concentration and decay time is shown in Fig. 2f. The fluorescence lifetime decreases with the increase in the [Zn²⁺–Ge⁴⁺] unit ratio, which stems from stronger non-radiative leaps between Cr³⁺ ions as a result of the increase in the proportion of co-unit substitution. It is known that the symmetry and strength of the crystal field lead to energy level splitting and center-of-mass shift, respectively, and thus affect the 3d–3d leap in Cr³⁺ emission. Weakening the crystal field strength is a way to modulate the Cr³⁺ emission and excitation position to a longer wavelength. The NIR emission of Cr³⁺ in an octahedral environment is determined by the relative energies of the two lowest energy excited states, ²E and ⁴T₂. The energies of these energy levels strongly depend on the crystal field strength and electron cloud rearrangement effects. The relative energy magnitudes of the ²E and ⁴T₂ energy levels of Cr³⁺ can be visually compared by the T–S (Tanabe–Sugano) diagram. The strength of the crystal field is usually evaluated by D_q/B. D_q/B can be calculated from eqn (1)–(3),³³ where D_q is the crystal field parameter and B is the Racah parameter.

Fig. 2 (a) The DR spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0 and 0.2) phosphors. (b) The photoluminescence excitation spectra and (c) the normalized PL emission spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors. (d) The down-conversion emission spectra of the Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor in the temperature range of 303–423 K. (e) The PLQY histogram of the Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor at the temperature range of 303 K–423 K. (f) Decay curve of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors.

The value of E(⁴A₂–⁴T₁) is 16 393 cm⁻¹ (610 nm), and the value of E(⁴A₂–⁴T₂) is 22 727 cm⁻¹ (440 nm). The calculated values of D_q, B and D_q/B are 1639 cm⁻¹, 625 cm⁻¹ and 2.62, respectively. As shown in Fig. 3a, the ²E and ⁴T₂ energy levels of Cr³⁺ have similar energies at the crossover point D_q/B = 2.3.³⁴ When D_q/B > 2.3 in materials with strong crystal fields, the ²E energy level is lower than the ⁴T₂ energy level, and the ²E → ⁴A₂ leap of Cr³⁺ ions dominate, which produces a narrow-band emission. At the same time, the excitation peaks of phosphors remained almost unchanged, indicating that the strength of the crystal field is also almost invariant. This suggests that the change in the strength of the crystal field is not the cause of the band spreading and the occurrence of a redshift. Fig. 3b depicts the all elemental XPS spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0 and 0.2) phosphors. Comparison of two xps spectrum shows that the [Ga³⁺-Ga³⁺]co-substitution of the [Zn²⁺-Ge⁴⁺] unit is successful. Fig. S5 and S6† are high-resolution Cr 2p and Ga 2p spectra. It is noteworthy that in samples, the Cr 2p peak shifts toward lower binding energies, and in Ga 2p spectra, peaks at 1117.77 eV and 1144.63 eV are shifted toward lower binding energies. The unit of [Zn²⁺–Ge⁴⁺] leads to an increase in defects, and the arrangement of atoms in the Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) lattice becomes relaxed. This implies that the co-substitution of [Zn²⁺–Ge⁴⁺] units reduces the Cr–O and Ga–O bonding energies. The reduced Cr–O and Ga–O bonding energies may favorably affect the luminescent properties of Cr³⁺-doped Ga₂O₃ phosphors, such as band spreading and redshift. Here, as stated earlier, we exclude the strength of the crystal field effect, causing the redshift of the emission position. Therefore, the local structural distortion should be the cause of the redshift of the emission spectrum (Fig. 3c). In this mechanism, the degenerate level splits, while the other energy levels exhibit greater splitting by the addition of [Zn²⁺–Ge⁴⁺] unit substitution (negligible center-of-mass shift).³⁵ This splitting allows the excited electron to relax to a lower energy level, which produces a red-shifted emission spectrum and frequency band spreading, but the excitation energy nearly keeps constant.³⁶

Fig. 3 (a) Tanabe–Sugano energy level diagram of Cr³⁺ in the Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor crystal field. (b) The all elemental XPS spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0 and 0.2) phosphors. (c) Models of crystal field splitting due to local structural distortion of Cr³⁺ and center-of-mass displacement induced by crystal field weakening.

ML properties and principles of phosphors

In addition to the excellent PL properties of these phosphors, we have also intriguingly discovered their ML properties. Fig. 4a is the ML spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) under a force of 4 N. The ML intensity, FWHM and luminescence peak position of phosphors change with the increasing proportions of co-unit substitution. Fig. 4b shows the relative ML intensity of phosphors with different substitution ratios. It is worth noting that the ML intensity after co-substitution is stronger than the unsubstituted Ga₂O₃. Compared with the ML property of Cr³⁺-doped Ga₂O₃, the intensity of the ML is enhanced by 353.16% through the strategy of co-substitution. This is because the crystal produces more traps when [Ga³⁺–Ga³⁺] are substituted by a pair of [Zn²⁺–Ge⁴⁺].³⁷ However, as the proportion of co-unit substitution rises to a certain value, the ML intensity starts to decrease instead. This is because the arrangement of the atoms in the Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) lattice becomes relaxed as the x-value rises. Doping also requires more mechanical energy for the leap, leading to weaker ML intensity.³⁸ The inset in Fig. 4b is a live image of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ taken with a NIR camera under a force of 4 N. From the inset, the NIR ML produced by the phosphor can be clearly seen. Fig. 4c shows the peak position and FWHM of the ML emission spectrum as a function of x in Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2). Like the PL of phosphors, the ML of phosphors also undergoes redshift and band spreading with the increase in the proportion of co-substitution. The red-shifted ML and band spreading are also caused by local structural distortion. Fig. 4d is the ML spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) under an applied force of 2–20 N. As we can see from the graph, the ML peak position does not change as the force increases, and the intensity increases with the applied force. Specifically, as shown in Fig. 4e, the ML intensity of the phosphor increases linearly with increasing force. Fig. 4f is the schematic diagram of the ML spectral mechanism. When [Ga³⁺–Ga³⁺] is replaced by a pair of [Zn²⁺–Ge⁴⁺], defects are created in the crystal. There are many possible candidates for these defects (V_Ga, V_Ge, V_O, etc.). Then, due to the non-centrosymmetric nature of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) crystals, traps can be charged through the piezoelectric field. Under pressure, a localized piezoelectric potential is generated in asymmetric crystals, leading to traps above the valence band (VB) and below the conduction band (CB) to capture the separated electrons and holes, respectively. In addition, due to the energy band tilting induced by the stress-induced piezoelectric potential, the recombination of holes and electrons activates the Cr³⁺ ions via non-radiative energy transfer (NET), resulting in ML behavior. Relaxation of Cr ion excitation electrons leads to ML. In short, the ML of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) is attributed to the perturbation of induced trap states by local piezoelectric field, electron redistribution and non-radiative energy transfer and subsequent radiative relaxation of the excited state. We performed thermoluminescence (TL) tests of Ga_1.89Zn_0.04Cr_0.03O₃ and Ga_1.97Cr_0.03O₃ phosphors, and the test results are shown in Fig. S7.† The TL intensity of the Ga_1.89Zn_0.04Cr_0.03O₃ phosphor is higher, which also implies that there are more traps in the Ga_1.89Zn_0.04Cr_0.03O₃ phosphor when compared with the unsubstituted Ga_1.97Cr_0.03O₃ phosphor. This indicates that when a pair of [Zn²⁺–Ge⁴⁺] replaces [Ga³⁺–Ga³⁺], the crystal produces more traps. Meanwhile, the TL curve after co-substitution shifts toward higher temperature, which proves that the trap depth of the phosphor increases after co-substitution.

Fig. 4 (a) The ML spectra of Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) phosphors under a force of 4 N. (b) Intensity, (c) peak position and FWHM of ML spectra as a function of x in Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2). (d) The ML spectra of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphors under an applied force of 2–20 N. (e) The ML spectra of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ phosphor under an applied force of 2–20 N. (f) The schematic diagram of the ML mechanism.

NIR pc-LED fabrication and application

In view of the excellent luminescence properties of the synthesized NIR phosphors Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2), phosphors were homogeneously mixed with PDMS in a 1 : 1 mass ratio. The mixture was coated on the blue LED chip (450 nm) and left to dry naturally at room temperature for 12 hours to prepare NIR pc-LED devices. The reason for choosing this ratio is shown in Fig. S8,† and it can be seen that the mass ratio of Ga_1.57Zn_0.2Ge_0.2Cr_0.03O₃ : PDMS = 1 : 1 has the highest spectral intensity, so we chose the mass ratio of 1 : 1 in preparing the pc-LED chips for the experiments. Fig. 5a shows the emission spectra of the NIR pc-LED device at 20–300 mA drive current. The emission intensity of pc-LED increases almost linearly with the increase in current, with no oversaturation phenomenon. Several practical application experiments prove that the NIR pc-LED prepared from Ga_1.97−2xZn_xGe_xCr_0.03O₃ phosphors have promising applications. Fig. 5b shows the schematic diagram of the anti-counterfeiting experimental setup. The key information was printed out with carbon ink with strong absorption of NIR and a layer of non-carbon ink, which does not have strong absorption of NIR, was overlaid on the surface of the key information. Then, the paper surface was irradiated with the prepared NIR pc-LED, and the obscured key information could be easily obtained using the NIR camera. We also tested the output power and photoelectric conversion efficiency of the NIR pc-LED, as shown in Fig. 5c. As the driving current gradually increased from 20 mA to 300 mA, the output power of the NIR pc-LED device also increased from 65.02 mW to 892.4 mW with a linear increase. However, the photoelectric conversion efficiency decreased from 11.4% to 5.88%, which is mainly because as the current increases, the luminous efficiency of the blue LED chip decreases. Broadband NIR light sources with different absorption of chemical components, high penetration depth and invisibility have a wide range of applications in fields such as anti-counterfeiting and bio-imaging. Fig. 5d shows the NIR pc-LED application demonstration diagram. In the left part, under regular fluorescent light, only the black rectangular pattern can be seen. However, under NIR illumination, the key information of the panda pattern is clearly seen using the NIR camera. In the middle of the right part, nine dots were partially printed with carbon and partially painted with non-carbon ink. Under visible light, one cannot distinguish the holes printed with carbon from those with non-carbon ink. On the contrary, under NIR illumination, the difference can be easily identified. A demonstration of pupil tracking was also performed. Under visible light, it was almost impossible to distinguish the pupil from the iris. In contrast, with the use of NIR pc-LED irradiation, the pupil was clearly distinguished by reflecting NIR light from the pupil with the help of a NIR camera. When a black bottle is filled with a certain liquid, one cannot find the liquid height under visible light. However, it could be visualized with the NIR camera under the illumination of NIR pc-LED. Fig. 5e shows a photograph of the NIR light emitted with the NIR pc-LED device through the finger with the help of the NIR camera. Since the chromophores in the blood absorb NIR light, the blood vessels in the fingers can be clearly observed. These application examples convincingly demonstrate that phosphors have promising applications in anti-counterfeiting, pupil tracking, rapid identification and non-invasive bioimaging. Fig. 5f displays the temperature change of the NIR pc-LED device under different driving currents recorded using a thermal imaging camera. As the driving current increases from 20 to 300 mA, the device operating temperature increases from 24.1 to 78.9 °C. Although the temperature change is large, the photoelectric conversion efficiency shows that only a small portion of the input electrical energy is converted to heat, which further suggests the promising aspects of the NIR pc-LED device.³⁹

Fig. 5 (a) The emission spectra of the NIR pc-LED under 20–300 mA current. (b) The schematic diagram of the anti-counterfeiting experimental setup. (c) The output power and light point conversion efficiency diagrams of NIR pc-LED. (d) Application diagrams of the NIR pc-LED in anti-counterfeiting, pupil tracking and non-destructive testing. (e) The NIR light transmission photograph of fingers. (f) Thermal images of the NIR pc-LED at different operating currents from 20 to 300 mA.

3. Conclusions

In summary, Ga_1.97−2xZn_xGe_xCr_0.03O₃ (x = 0–0.2) NIR phosphors were successfully synthesized using a high-temperature solid-phase method. The redshift and band spreading were achieved by adjusting the value of x. In addition, we found that the phosphors have the ability of ML and found that ML also has the effect of red-shift and band spreading. Furthermore, the NIR pc-LED device was prepared by packaging phosphors and PDMS 1 : 1 mixture on a blue LED chip. The device has excellent output power and conversion efficiency. Under the illumination of NIR pc-LED light, applications in anti-counterfeiting, pupil recognition, rapid identification and non-destructive imaging were successfully achieved. This work not only suggests a series of tunable and broadband luminescent materials but also provides a promising phosphor for NIR pc-LED applications.

4. Experimental section

Materials preparation

Polycrystalline powder samples were synthesized using a high-temperature solid phase reaction method. The Cr³⁺ concentration is fixed at 0.03. First, raw materials such as Ga₂O₃ (Aladdin, 99.99%), ZnO (Aladdin, 99.99%), GeO₂ (Aladdin, 99.99%) and Cr₂O₃ (Aladdin, 99.99%) are weighed according to stoichiometric ratios. Then, the powder is mixed and ground in an agate mortar and pestle for 30 minutes. This mixture is transferred to the Al₂O₃ crucible and put into the Muffle furnace. The furnace first raised the temperature to 1350 °C at a heating rate of 7 °C min⁻¹ and then kept it under air for 6 hours. After naturally cooling to room temperature in the furnace, the product is removed and ground into a fine powder with an agate mortar and pestle for further measurement.

Relevant affirmations

All individuals and their parents provided informed consent to participate in this study, and approval was provided by China Jiliang University.

Material characterization

The phase purity of the sample is checked by powder X-ray diffraction collected by X-ray diffractometer (XRD-6100, Japan). Samples were photographed using a field emission scanning electron microscope (SU-8010, Japan). Visualization and crystal structure analysis were performed with VESTA. Diffuse reflection (DR) spectra were collected with an ultraviolet-visible spectrophotometer (UV-3600, Japan). The PLQY of the phosphors was measured using the Absolute PL Quantum Yield Spectrometer (C11347-11, England). Photoluminescent excitation and emission spectra and attenuation curves at different temperatures have been obtained on a Fluorolog fluorescence spectrometer (FL3-211, France). Three-dimensional thermoluminescence (3D-TL) color mappings were measured on a thermoluminescence spectrometer (TOSL-3DS, China), and the two-dimensional thermoluminescence (2D-TL) curves were obtained via the respective iso-wavelength 3D-TL contours. Prototype NIR pc-LED devices are made by coating a mixture of phosphors and polydimethylsiloxane (PDMS) in a 1 : 1 weight ratio on a 450 nm blue LED chip. The input and output parameters and electroluminescence spectra of the prepared NIR pc-LED were measured by an LED photoelectric measurement system (HAAS-2000, China).

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LR24F050002) and the National Natural Science Foundation of China (62175225).

References

1. 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.
2. B. Su, M. Li, E. Song and Z. Xia, Sb³⁺-Doping in Cesium Zinc Halides Single Crystals Enabling High-Efficiency Near-Infrared Emission, Adv. Funct. Mater., 2021, 31, 2105316.
3. 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.
4. Y. Zhuang, X. Li, F. Lin, C. Chen, Z. Wu, H. Luo, L. Jin and R.-J. Xie, Visualizing Dynamic Mechanical Actions with High Sensitivity and High Resolution by Near-Distance Mechanoluminescence Imaging, Adv. Mater., 2022, 34, 2202864.
5. D. Liu, G. Li, P. Dang, Q. Zhang, Y. Wei, H. Lian, 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.
6. Y. J. Yu, C. Zou, W. S. Shen, X. Zheng, Q. S. Tian, Y. J. Yu, C. H. Chen, B. Zhao, Z. K. Wang and D. Di, Efficient Near–Infrared Electroluminescence from Lanthanide–Doped Perovskite Quantum Cutters, Angew. Chem., Int. Ed., 2023, 62, e202302005.
7. W. T. Huang, C. L. Cheng, Z. Bao, C. W. Yang, K. M. Lu, C. Y. Kang, C. M. Lin and R. S. Liu, Broadband Cr³⁺, Sn⁴⁺-Doped Oxide Nanophosphors for Infrared Mini Light-Emitting Diodes, Angew. Chem., 2019, 131, 2091–2094.
8. Y. Xiao, W. Xiao, D. Wu, L. Guan, M. Luo and L. D. Sun, An extra–broadband VIS–NIR emitting phosphor toward multifunctional LED applications, Adv. Funct. Mater., 2022, 32, 2109618.
9. Q. Shao, H. Ding, L. Yao, J. Xu, C. Liang and J. Jiang, Photoluminescence properties of a ScBO₃: Cr³⁺ phosphor and its applications for broadband near-infrared LEDs, RSC Adv., 2018, 8, 12035–12042.
10. J. Qiao, G. Zhou, Y. Zhou, Q. Zhang and Z. Xia, Divalent europium-doped near-infrared-emitting phosphor for light-emitting diodes, Nat. Commun., 2019, 10, 5267.
11. E. Song, H. Ming, Y. Zhou, F. He and Q. Zhang, Cr³⁺ Doped Sc³⁺ Based Fluoride Enabling Highly Efficient Near Infrared Luminescence: A Case Study of K₂NaScF₆: Cr³⁺, Laser Photonics Rev., 2020, 15, 2000410.
12. C. Yuan, R. Li, Y. Liu, L. Zhang, J. Zhang, G. Leniec, P. Sun, Z. Liu, Z. Luo and R. Dong, Efficient and broadband LiGaP₂O₇: Cr³⁺ phosphors for smart near–infrared light–emitting diodes, Laser Photonics Rev., 2021, 15, 2100227.
13. Y. Z. Zhu, X. S. Wang, J. W. Qiao, M. S. Molokeev, H. C. Swart, L. X. Ning and Z. G. Xia, Regulating Eu²⁺ Multisite Occupation through Structural Disorder toward Broadband Near-Infrared Emission, Chem. Mater., 2023, 35, 1432–1439.
14. M. H. Fang, P. Y. Huang, Z. Bao, N. Majewska, T. Lesniewski, S. Mahlik, M. Grinberg, G. Leniec, S. M. Kaczmarek, C. W. Yang, K. M. Lu, H. S. Sheu and R. S. Liu, Penetrating Biological Tissue Using Light-Emitting Diodes with a Highly Efficient Near-Infrared ScBO₃:Cr³⁺ Phosphor, Chem. Mater., 2020, 32, 2166–2171.
15. Q. Q. Zhang, D. J. Liu, P. P. Dang, H. Z. Lian, G. G. Li and J. Lin, Efficient and broadband LiGaP₂O₇: Cr³⁺ phosphors for smart near–infrared light–emitting diodes, Laser Photonics Rev., 2022, 16, 2100459.
16. H. B. Xu, G. X. Bai, K. He, S. X. Tao, Z. L. Lu, Y. Zhang and S. Q. Xu, Multifunctional optical sensing applications of luminescent ions doped perovskite structured LaGaO₃ phosphors in near-infrared spectroscopy, Mater. Today Phys., 2022, 28, 100872.
17. J. M. Xiang, X. Zhou, X. Q. Zhao, Z. Y. Wu, C. H. Chen, X. J. Zhou and C. F. Guo, Ab Initio Site-Selective Occupancy and Luminescence Enhancement in Broadband NIR Emitting Phosphor Mg₇Ga₂GeO₁₂: Cr³⁺, Laser Photonics Rev., 2023, 10, 2200965.
18. R. D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr., 1976, 32, 751–767.
19. L. P. Jiang, X. Jiang, J. H. Xie, H. Y. Sun, L. L. Zhang, X. L. Liu, Z. H. Bai, G. C. Lv and Y. J. Su, Ultra-broadband near-infrared Gd₃MgScGa₂SiO₁₂: Cr, Yb phosphors: Photoluminescence properties and LED applications, J. Alloys Compd., 2022, 920, 165912.
20. T. Liu, H. Cai, N. Mao, Z. Song and Q. Liu, Efficient near–infrared pyroxene phosphor LiInGe₂O₆: Cr³⁺ for NIR spectroscopy application, J. Am. Ceram. Soc., 2021, 104, 4577–4584.
21. F. Y. Zhao, H. Cai, Z. Song and Q. L. Liu, Structural Confinement for Cr³⁺ Activators toward Efficient Near-Infrared Phosphors with Suppressed Concentration Quenching, Chem. Mater., 2021, 33, 3621–3630.
22. D. L. Gao, Q. Q. Kuang, F. Gao, H. Xin, S. N. Yun and Y. H. Wang, Achieving opto-responsive multimode luminescence in Zn_1+xGa_2-2xGe_xO₄: Mn persistent phosphors for advanced anti-counterfeiting and information encryption, Mater. Today Phys., 2022, 27, 100765.
23. S. C. Peng, P. Xia, T. Wang, L. Lu, P. Zhang, M. Zhou, F. Zhao, S. Q. Hu, J. T. Kim, J. B. Qiu, Q. Y. Wang, X. Yu and X. H. Xu, Mechano-luminescence Behavior of Lanthanide-Doped Fluoride Nanocrystals for Three-Dimensional Stress Imaging, ACS Nano, 2023, 17, 9543–9551.
24. S. Liu, Z. Wang, H. Cai, Z. Song and Q. Liu, Highly efficient near-infrared phosphor LaMgGa₁₁O₁₉: Cr³⁺, Inorg. Chem. Front., 2020, 7, 1467–1473.
25. C. Y. Chang, N. Majewska, W. K. Pang, V. K. Peterson, D. H. Cherng, K. M. Lu, S. Mahlik and R. S. Liu, Broadening Phosphor-Converted Light-Emitting Diode Emission: Controlling Disorder, Chem. Mater., 2022, 34, 10190–10199.
26. J. Wang, X. X. Han, Y. Y. Zhou, Z. X. Wu, D. X. Liu, S. Cao and B. S. Zou, Ion Substitution Strategy toward High-Efficiency Near-Infrared Photoluminescence of Cs₂Kin_1-yAlyF₆:Cr³⁺ Solid Solutions, J. Phys. Chem. Lett., 2023, 14, 1371–1378.
27. J. Y. Zhong, L. W. Zeng, W. R. Zhao and J. Brgoch, Producing Tunable Broadband Near-Infrared Emission through Co- Substitution in (Ga_1-XMgX)(Ga_1-XGeX)O₃: Cr³⁺, ACS Appl. Mater. Interfaces, 2022, 14, 51157–51164.
28. Z. Liu, X. Yu, Q. Peng, X. Zhu, J. Xiao, J. Xu, S. Jiang, J. Qiu and X. Xu, NIR Mechanoluminescence from Cr³⁺ Activated Y₃Al₅O₁₂ with Intense Zero Photon line, Adv. Funct. Mater., 2023, 33, 2214497.
29. P. X. Xiong, B. L. Huang, D. F. Peng, B. Viana, M. Y. Peng and Z. J. Ma, Self–Recoverable Mechanically Induced Instant Luminescence from Cr³⁺–Doped LiGa₅O₈, Adv. Funct. Mater., 2021, 31, 2010685.
30. B. Bai, P. Dang, D. Huang, H. Lian and J. Lin, Broadband Near-Infrared Emitting Ca₂LuScGa₂Ge₂O₁₂:Cr³⁺ Phosphors: Luminescence Properties and Application in Light-Emitting Diodes, Inorg. Chem., 2020, 59, 13481–13488.
31. X. Xu, Q. Shao, L. Yao, Y. Dong and J. Jiang, Highly efficient and thermally stable Cr³⁺-activated silicate phosphors for broadband near-infrared LED applications - ScienceDirect, Chem. Eng. J., 2020, 383, 123108.
32. J. Zhong, Y. Zhuo, H. Zhang, W. Zhao and J. Brgoch, Efficient and Tunable Luminescence in Ga_2–xIn_xO₃:Cr³⁺ for Near-Infrared Imaging, ACS Appl. Mater. Interfaces, 2021, 13, 31835–31838.
33. D. Chen, Y. Chen, H. Lu and Z. Ji, A bifunctional Cr/Yb/Tm:Ca₃Ga₂Ge₃O₁₂ phosphor with near-infrared long-lasting phosphorescence and upconversion luminescence, Inorg. Chem., 2015, 45, 8638–8645.
34. 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.
35. H. Wang, X. H. Chen, J. L. Li, M. Li, K. Liu, D. L. Yang, S. Peng, T. T. Zhao, B. H. Zhao, Y. C. Li, Y. G. Wang, C. L. Lin and W. E. Yang, Pressure- and Rate-Dependent Mechanoluminescence with Maximized Efficiency and Tunable Wavelength in ZnS: Mn²⁺, Eu³⁺, ACS Appl. Mater. Interfaces, 2023, 15, 28204–28214.
36. W. Meng, P. B. Cai, X. Y. Fu and H. W. Zhang, Sensitive mechanoluminescence from Eu²⁺, Tm³⁺ co-doped Sr₃Al₂O₅Cl₂ phosphors, J. Lumin., 2022, 248, 118983.
37. S. Q. Liu, Y. T. Zheng, D. F. Peng, J. Zhao, Z. Song and Q. L. Liu, Near-Infrared Mechanoluminescence of Cr³⁺ Doped Gallate Spinel and Magnetoplumbite Smart Materials, Adv. Funct. Mater., 2022, 9, 2209275.
38. L. J. Li, L. Wondraczek, M. Y. Peng, Z. W. Ma and B. Zou, Force-induced 1540 nm luminescence: Role of piezotronic effect in energy transfer process for mechanoluminescence, Nano Energy, 2020, 69, 104413.
39. L. R. Hirsch, R. J. Stafford, J. A. Bankson, S. R. Sershen, B. Rivera, R. E. Price, J. D. Hazle, N. J. Halas and J. L. West, Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance, Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 13549–13554.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi00846d

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