Simultaneous NIR photoluminescence and mechanoluminescence from Cr³⁺ activated MgGa₂O₄ phosphors with multifunctions for optical sensing

Abstract

Light-emitting diodes (LEDs) combined with near-infrared (NIR) phosphors are rapidly becoming important light sources for food safety, anti-counterfeiting and biological imaging due to non-destructive and non-toxic advantages. Single-phase materials with multi-mode luminescence under different stimuli are crucial for the development and applications of smart NIR phosphors. Here, we have developed multifunctional Cr³⁺ activated MgGa₂O₄ (MGO:Cr³⁺) phosphors with simultaneous photoluminescence and mechanoluminescence for NIR sensing. Under 470 nm excitation, the phosphors can produce emission bands from 650 to 850 nm. Meanwhile, the flexible devices based on phosphors can exhibit strong emissions in response to mechanical deformation, such as shock and friction. In addition, optical manometers based on luminescence from phosphors can achieve pressure sensing from 7 to 64 KPa. More intriguingly, we combined the MGO:Cr³⁺ phosphor with a blue LED chip to fabricate an NIR phosphor-converted LED for anti-counterfeiting and biological tissue penetration. Our results suggest that the multifunctional NIR phosphors with simultaneous photoluminescence and mechanoluminescence have potential applications in various sensing devices.

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1. Introduction

Near-infrared (NIR) light occupies the electromagnetic spectrum region between visible and mid-infrared wavelengths.¹ Due to its notable penetration, imperceptibility, and non-destructive properties, NIR light is utilized in diverse fields such as food quality assessment, biological tissue monitoring, component analysis, and AI-based imaging. Based on photon energy, the International Commission on Illumination categorizes electromagnetic waves of 700 to 1400 nm wavelengths as near-infrared light.^2–4 Conventionally, this spectral region is divided into NIR I (700–950 nm) and NIR II (1000–1400 nm). Infrared products, with their extensive utility in both industrial processes and daily life, are anticipated to see consistent growth in the future market demand.⁵ For instance, in food biological analysis, near-infrared spectroscopy aids in the analysis and detection of key components. This capability stems from the unique ability of NIR light to interact with C–H, O–H, and N–H chemical groups, fundamental components of water, proteins, fat, and carbohydrates.⁶ Near-infrared luminescent materials can be excited by a variety of excitation processes, such as photoluminescence (PL), mechanical luminescence (ML), electroluminescence (EL), and thermoluminescence (TL).^7–9 Additionally, there are also some novel stimulation sources, such as modifying and improving some advanced magnetite nanomaterials to respond to external magnetic fields and produce bright near-infrared emissions.^10,11 Furthermore, sonoluminescence, a fascinating phenomenon, involves tiny bubbles in liquid emitting light when exposed to strong sound waves. This phenomenon has sparked significant interest among scientists due to its intriguing physical properties and potential applications across various fields. Consequently, the development of near-infrared luminescent materials adaptable to diverse stimuli is crucial for advancing luminescent material technology.¹²

Recent reports indicate that the active ions contributing to near-infrared (NIR) emission are mainly transition metal ions (Mn²⁺, Cr³⁺, and Ni²⁺),¹³ Ln³⁺ doped ions (Eu³⁺, Tb³⁺, Er³⁺, and Tm³⁺),¹⁴ and main group metal ions (Bi³⁺ and Bi²⁺).^15–17 Near-infrared phosphors can be categorized into three groups based on their activating ions: those activated by rare earth ions, main group ions, and transition metal ions.¹⁸ Cr³⁺ is extensively investigated as an activator of near-infrared luminescence, serving as a representative transition metal ion with partially filled d orbitals. The intensity of its emission spectrum relies on the crystal field environment of the host lattice.¹⁹ Additionally, Cr³⁺ ions possess notable light penetration properties. They can capture light signals passing through biological tissues, indicating potential for developing narrowband ML materials for the NIR I region.²⁰ Also, Cr³⁺ is efficiently excitable by blue light. Using blue LED chips with Cr³⁺ doped near-infrared phosphors can notably boost near-infrared emission efficiency.^21–23

In this study, we synthesized the MGO:Cr³⁺ phosphor emitting near-infrared light through a high-temperature solid-phase reaction, capitalizing on the luminescence characteristics of Cr³⁺ ions. Our investigation comprehensively analyzed the material's crystal structure, microstructure, and durability. Importantly, we investigated the phosphor's dual-mode response (PL and ML), specifically its pressure-sensing capabilities in ML mode when combined with polydimethylsiloxane (PDMS). Additionally, we combined MGO:Cr³⁺ with a commercial LED chip, forming a new pc-LED capable of penetrating biological tissues and expanding its potential applications, particularly in security.

2. Experimental section

Materials and synthesis: MgGa₂O₄:x%Cr³⁺ (0.5 ≤ x ≤ 2.0) powder was synthesized via the traditional high-temperature solid-state reaction method, where x represents the atomic percentage. MgO (99.99% purity, Aladdin), Ga₂O₃ (99.99% purity, Aladdin), and Cr₂O₃ (99.99% purity, Aladdin) were chosen as the initial raw materials to guarantee sample quality. Subsequently, the samples were poured into an agate mortar, fully and completely ground for 30 minutes, and finally transferred to a corundum crucible and placed in a tubular sintering furnace for 8 h. After the sample was naturally cooled to room temperature, it was removed from the furnace and poured into the mortar to continue to fully grind, and finally the required sample MgGa₂O₄:Cr³⁺ was obtained.

Preparation of MGO:x%Cr³⁺/PDMS composite elastomers: Elastic ML films were produced by blending the prepared phosphor and PDMS (Dow Corning) in a weight ratio of 3 : 2. Initially, 8 g of the PDMS substrate, 0.8 g of a curing agent, and 12 g of MGO:2%Cr³⁺ were accurately weighed into specialized Petri dishes with a 55 mm diameter, thoroughly mixed, and continuously stirred for 15 minutes. Subsequently, the solution was dried and cured in an oven at 70 °C for 1 hour to yield an elastomer exhibiting ML properties.

3. Results and discussion

3.1 Crystal structure and microstructure

Both crystal structure and microstructure are essential for material characterization. The X-ray diffraction (XRD) patterns of prepared MGO:x%Cr³⁺ (x = 0, 0.5, 1, 1.5, 2) are shown in Fig. 1a. The results shown align well with the MGO's standard card (PDF#10-0113). At the same time, no secondary phase and out of phase responses of the material were observed, indicating that Cr³⁺ doping on this surface did not affect the composition of the main phase of the material, and its purity and crystallinity were also high. A clear angular shift is observed when compared with the standard spectrum, indicating the successful doping of Cr³⁺ ions into MGO. MGO is a typical spinel structure, and its crystal structure is shown in Fig. 1b. Mg²⁺, Ga³⁺ and O²⁻ ions are represented by blue, yellow and red spheres respectively. The Ga³⁺ ion has 6 coordination and the Mg²⁺ ion has 4 coordination. The [GaO₆] octahedra are interconnected by shared edges, in addition to which they are connected to the [MgO₄] tetrahedra by sharing vertexes to form a rigid lattice structure.²⁴ Considering that the doping process follows the principle of similar ionic radii, Cr³⁺ (r = 0.615 Å) is more inclined to occupy the Ga³⁺ (r = 0.662 Å) site.^25–29 We conjecture the presence of [CrO₆] octahedra within the MGO:Cr³⁺ phosphor. The crystal structure diagram illustrates the sequential coordination of Ga³⁺ ions with Ga–O, Ga–Ga, and Ga–Mg shells.

Fig. 1 (a) The XRD patterns of MGO:Cr³⁺ samples and the standard pattern of MgGa₂O₄. (b) Crystal structure of the MGO:Cr³⁺ sample. (c) The energy scatter spectrum of 2%Cr³⁺ doped phosphors. (d) Elemental mapping images of O, Ga, Mg and Cr in MGO:2%Cr³⁺samples. (e) The XPS spectrum of the 2%Cr³⁺ doped phosphor.

Examination of the SEM images of the MGO:2%Cr³⁺ sample (Fig. S1, ESI†) reveals that the phosphor we synthesized possesses a regular cylindrical shape with a central hollow core. Overall, MGO exhibits a combination of agglomeration and dispersion. Fig. 1c presents the energy dispersion spectrum of MGO, clearly indicating the presence of Cr element in the prepared phosphor. Analysis of the elemental diagram in Fig. 1d reveals that Cr³⁺ ions are present in small quantities in MGO, with Mg, Ga, Cr, and O elements uniformly dispersed in a columnar form. X-ray photoelectron spectroscopy (XPS) analysis reveals the site occupancy and localized structure of Cr³⁺. The XPS spectra of Cr³⁺ in MGO reveal a peak at 578.6 eV (Fig. S2, ESI†), aligning closely with the binding energy characteristics of trivalent chromium ions within the [CrO₆] octahedra.^30,31 We also analyzed and measured the XPS patterns of other elements, which enhances our understanding of the energy levels of other elements.

3.2 Photoluminescence properties

We employed the conventional solid-phase sintering method to prepare the MGO:Cr³⁺ phosphor. For a more in-depth exploration of its optical properties, Fig. 2a displays the excitation and emission spectra of the MGO:Cr³⁺ phosphor. When pumped with a 460 nm laser, it is found that the PL spectra of MGO:Cr³⁺ are composed of wide-band near-infrared light in the range of 600 to 900 nm, with a peak emission peak centered at 710 nm, which is in line with the ²E → ⁴A₂ transition of Cr³⁺. At the same time, the PLE spectrum of MGO:Cr³⁺ detected at 710 nm consists of two strong absorption bands centered at 440 (⁴A₂ → ⁴T₁) and 590 nm (⁴A₂ → ⁴T₂), which matches the commercial blue LED chip and can be better used as a device in the later stage.^32–34Fig. 2b shows the PL spectrum of MGO:x%Cr³⁺ (x = 0–2). It can be seen from the figure that the emission intensity increases when the concentration of Cr³⁺ increases. At the same time, in order to better know the concentration of Cr³⁺ ions with the best emission intensity, we increase x from 2 to 4. The emission intensity of the phosphor gradually increases, and then, due to the concentration quenching effect, reaches a maximum value at x = 2 and begins to gradually decrease (Fig. S3, ESI†). Thus, MGO:2%Cr³⁺ achieves the strongest overall emission intensity. In addition, we also observed that when we excited at 460 nm, the full-width at half-maximum (FWHM) also changed with the increase of Cr³⁺ concentration. As shown in Fig. 2c, when x = 0.2, FWHM reaches a maximum value of about 158 nm, which is the result of the combined contribution of ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ in Cr³⁺. Fig. 2d shows the diffuse reflection spectra of MGO:Cr³⁺ samples. It is evident from Fig. 2d that the peaks of MGO:Cr³⁺ at 440 and 590 nm correspond to two different absorptions, belonging to the ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₂ transitions of Cr³⁺, which are highly consistent with the emission peaks in the PLE spectrum. At the same time, with the increase of Cr³⁺ doping concentration in the sample, the reflection intensity of MGO:Cr³⁺ also gradually increased. Based on the diffuse reflection spectral data shown in Fig. S4 (ESI†), we calculated the band gap of the MGO:2%Cr³⁺ sample to be 2.43 eV.

Fig. 2 (a) The photoexcitation and photoluminescence spectra of 0.5%Cr³⁺ doped phosphors. (b) Emission spectra of MGO:Cr³⁺ samples excited at 460 nm. (c) Full-width at half-maximum (FWHM) change of MGO:Cr³⁺ samples under excitation at 460 nm. (d) Diffuse reflection spectra of MGO:Cr³⁺samples. (e) The diagram of the T–S level of MGO:Cr³⁺ phosphors. (f) Schematic diagram of the PL decay curve of MGO:Cr³⁺ phosphors (after excitation at 460 nm, monitored at 710 nm).

In order to better understand the luminescence mechanism of MGO:Cr³⁺, we explained it theoretically by using the Tanabe–Sugano theory and using Dq/B to appraise the crystal field intensity. The crystal field parameters are represented as Dq, and the Racah parameters are represented as B.

According to the calculation, we finally determine that Dq/B = 3.2, as shown in Fig. 2e. According to the data, when Dq/B > 2.3, the main body is a strong crystal field and ²E → ⁴A₂ of Cr³⁺ ions dominates.^35–37 At this point, the NIR light exhibits narrowband emission. When Dq/B < 2.3, indicating a weak crystal field, a ⁴T₂ → ⁴A₂ transition occurs, leading to broadband emission. Simultaneously, the emission spectra of Cr³⁺ doped NIR phosphors predominantly exhibit narrow bands (²E → ⁴A₂ transition) in strong crystal fields and wide bands (⁴T₂ → ⁴A₂ transition) in weak crystal fields.³⁸ In medium-intensity crystal fields, the coexistence of these two bands can be observed in the spectrograph, as demonstrated by our preparation of MGO:Cr³⁺. In addition, we monitored the correlation between the Cr³⁺ content and the photoluminescence lifetime of the prepared MGO:x%Cr³⁺ samples (Fig. 2f). We observed that when x = 2, the fluorescence lifetime of MGO reached its maximum value. Subsequently, with the continuous increase in concentration, the lifetime gradually decreased due to the concentration quenching effect.^39–41 The specific reason is that the non-radiative transition between Cr³⁺ ions increases with the continuous increase of Cr³⁺ concentration, leading to the shortening of its lifetime. In summary, MGO:Cr³⁺ has the best performance when the concentration of Cr³⁺ is 2%.

3.3 Thermal stability

Thermal stability is one of the important parameters to characterize phosphors. In this work, in order to evaluate the thermal stability, we studied the temperature-dependent PL spectrum of MGO:Cr³⁺ (Fig. 3a). As temperature rises, the luminous intensity gradually decreases, but the emission wavelength of the MGO:Cr³⁺ NIR phosphor, prepared by us, remains relatively constant. This suggests that the crystal field intensity of Cr³⁺ in the phosphor does not significantly change with temperature. At the same time, we studied the spectral intensity of MGO:Cr³⁺ at 710 nm under 460 nm excitation, and cumulatively drew the data into a histogram (Fig. 3b). With the gradual increase of temperature, the spectral emission intensity of MGO:Cr³⁺ gradually decreases. The emission intensity at 303 K is 87.42% of the initial level, at 473 K is 52.31% of the initial level, and at 523 K is 38.34% of the initial level. This indicates that the prepared MGO:Cr³⁺ phosphor also has a certain stability at ultra-high temperatures. In addition to stability, reversibility and repeatability checks are also extremely important for phosphor testing. We repeated 8 cycles of heating and cooling of MGO:2%Cr³⁺ at 303, 473 and 523 K, and it can be seen from Fig. 3c that after several cycles of experiments, the spectral intensity is basically at the same level, and the samples prepared on this surface are also reproducible.

Fig. 3 (a) The spectrum of the MGO phosphor doped with 2%Cr³⁺ across the temperature range 300–523 K, excited at 460 nm. (b) Intensity histogram of MGO:2%Cr³⁺ at a 710 nm wavelength in different temperature ranges. (c) The temperature cycle stability experiment of 2%Cr³⁺ doped MGO. (d) The schematic configuration coordinate diagram of MGO.

To better understand the factors affecting the thermal stability of phosphors, we extensively reviewed the data. Our investigation shows that lower concentrations of active ions tend to increase structural rigidity and lead to wider main band gaps, which helps develop phosphors with stronger thermal stability.^42–44 In addition to this, another key factor affecting thermal stability is the thermal quenching performance of the material. The thermal quenching performance of MGO:Cr³⁺ can be illustrated by the configuration diagram. As can be seen from Fig. 3d, at high temperatures, the electrons of the ⁴T₂ state will reach the intersection of the ⁴T₂ excited state and the ⁴A₂ ground state (process IV, activation energy is provided in the form of ΔE₂) and then return to the ground state by non-radiative transition (process V), which is the thermal quenching process of the ⁴T₂ state.^45–47 The heat quenching process of the ²E state is completely different. After excitation, it is easier to lift the electrons of the ²E state to the intersection of the ²E and ⁴T₂ excited states (process III, where the activation energy is provided in the form ΔE₁), after which the electrons will return to the ground state via radiative transitions (process II) and non-radiative transitions (processes IV and V). For MGO:Cr³⁺, due to large Dq/B (Fig. 2e), we can clearly observe the narrowband emission of ²E → ⁴A₂. As temperature increases, the ²E state will not have enough electrons to compensate for the electrons lost through the non-radiative transition, resulting in a decrease in thermal stability. However, we found that when ⁴T₂ → ⁴A₂ is dominant, thermal stability is higher than when ²E → ⁴A₂ is dominant. Therefore, we can allow the coexistence of sharp ²E → ⁴A₂ and wideband ⁴T₂ → ⁴A₂ to significantly improve the thermal stability of Cr³⁺ activated NIR light emission, which also provides a new idea for our subsequent work.

3.4 ML properties

In order to explore whether MGO:Cr³⁺ has multi-mode response, we further examined its ML characteristics. Using the special stress sensing characteristics of the PDMS film, we mixed PDMS and MGO:Cr³⁺ to produce a new near-infrared fluorescent film. We applied different forces to the film and measured its spectrum through a fiber optic spectrometer. Fig. 4a shows that as the force we applied increased, the ML strength of MGO:Cr³⁺ also increased. Meanwhile, we measured its FWHM and observed that FWHM increased with the increasing force (Fig. 4b). In addition to the applied stress, the concentration of doping ions is also a key factor that can affect the strength of ML. Fig. 4c shows that with increasing Cr³⁺ ion concentration, the ML strength of MGO:x%Cr³⁺ (0 ≤ x ≤ 2.0) first increases and then decreases, reaching a maximum at x = 2 (at which point the force fixed to the material is 10 N).

Fig. 4 (a) The ML spectrum of MGO:2%Cr³⁺ under different forces (F = 1–10 N). (b) FWHM change of MGO:2%Cr³⁺ under different forces. (c) The ML intensity of MGO:x%Cr³⁺ (x = 0–2) with 10 N intercept. (d) The schematic diagram of the ML process.

Based on the above results, we can conclude the potential ML mechanism of MGO:Cr³⁺ (Fig. 4d). Upon external mechanical stimulation of MGO:Cr³⁺, it produces an internal piezoelectric field, capturing charges through the piezoelectric effect. The piezoelectric field-induced band bending enables the release of electrons from the trap to the conduction band. Additionally, they can directly achieve the dopant level of Cr³⁺ by leveraging the tunneling effect. Subsequently, non-radiative energy is released through electron–hole recombination, exciting Cr³⁺ ions and facilitating electron relaxation to achieve ML. In summary, the ML phenomenon in MGO:Cr³⁺ is contingent on the induction of electrons into the trap state, disrupting the electron distribution.^48–52 This process leads to the creation of a localized piezoelectric field within the inter-electronic energy level, facilitating electron transfer and energy conversion.

3.5 Stress sensing

Additionally, it should be noted that certain fluorescent powders exhibit sensitivity to pressure, such as Sr₈Si₄O₁₂Cl₈:Eu²⁺, which emits yellow light above 7 GPa and undergoes a significant red shift with increasing pressure.⁵³ Utilizing upconversion, sub-micron-sized YVO₄:Yb³⁺ Er³⁺ demonstrates high sensitivity and accuracy in ultra-low-pressure ranges.⁵⁴ A single lanthanide-doped upconversion material, YPO₄:Yb³⁺ Er³⁺, can also be employed for optical sensing of low and high-pressure values.⁵⁵ At the same time, the MGO:Cr³⁺ fluorescent powder we prepared is also sensitive to dynamic loading. By leveraging this characteristic, we can transform this fluorescent powder into a pressure sensor. As shown in Fig. 5a, the MGO:Cr³⁺ phosphor and PDMS solvent are fully mixed in a mass ratio of 3 : 2 and then cut into a normal rectangle (50 × 18 × 2 mm) after the process of drying and cooling. After fixing one end, the other part was connected to a tensiometer, and different degrees of tensile force were applied to it. At the same time, we irradiated pre-made PDMS + MGO:Cr³⁺ films with a 460 nm laser and measured their spectral data by placing a fiber optic spectrometer in the vertical direction. To counteract the 460 nm laser, we placed a 550 nm filter in front of the fiber probe to eliminate the inherent effects of the 460 nm laser. With a fixed 460 nm laser power, the tension force applied to the film was systematically varied, and the resulting data were collected using a fiber spectrometer. The obtained results are presented in Fig. 5b and c. From the above figure, it can be seen that the spectral intensity of the film gradually increases as the pressure applied to the side of the film increases. The emission bands at 710 and 770 nm showed a linear increment trend. The spectral intensity data of 710 and 770 nm were taken out separately for linear fitting. It can be seen from Fig. 5d that the spectral intensity and force basically have a linear relationship, and the final fitting value is R² = 0.998. The better fitting results can prove that our prepared phosphors have good application prospects in stress sensing in the future. By harnessing the sensitivity of MGO:Cr³⁺ thin film to dynamic loading and the imperceptibility of near-infrared light, we captured the real-time single-point dynamic pressure trajectory of personal handwriting on the thin film, as depicted in Fig. S5 (ESI†). The pressure visualization of handwritten “CJLU” is indistinguishable to the naked eye but becomes discernible under near-infrared cameras.

Fig. 5 (a) The schematic representation of the stress sensing device. (b) Spectral variations of MGO:2%Cr³⁺/PDMS composites within the pressure range of 7–64 KPa. (c) The spectral intensities under various forces at 710 and 770 nm. (d) The force-dependent properties located at the selected 710 and 770 nm emission bands were tested respectively. (e) Applying a weight of 20 g on the prepared MGO:Cr³⁺ composite to examine its spectral intensity. (f) Altering the weight (20–150 g) to observe changes in its spectral intensity.

At the same time, we studied the sensitivity of the phosphor to static load. We placed 20 g weights on the MGO:Cr³⁺/PDMS film to provide pressure. After the 460 nm laser excitation, the data were measured with the fiber spectrometer, as shown in Fig. 5e. It can be seen that the obtained spectra were basically similar to those in Fig. 5b. Repetitive experiments were conducted by varying the applied weight (20–150 g), and the results are presented in Fig. 5f. As can be seen from the figure, as the weight of the weight applied to the film increases, the light intensity presented by MGO:Cr³⁺ also increases, which is highly similar to the above experimental conclusions obtained by us and better proves that the MGO:Cr³⁺ phosphor has the function of stress sensing. This provides new ideas for our follow-up work.

3.6 Near infrared applications

In view of the excellent NIR luminescence performance of the MGO:Cr³⁺ phosphor we prepared, we combined it with a blue LED chip (460 nm) to fabricate an NIR LED device. The preparation process is as follows: (1) MGO:Cr³⁺ and PDMS were merged evenly according to a 3 : 2 ratio of mass, and bubbles were removed at rest. (2) Then, they were smeared on the LED chip. The blue LED chip was energized to facilitate subsequent experiments. Fig. 6a shows the emission spectra of the fabricated NIR pc-LED chip, whose spectral intensity is gradually increased by changing the current applied to the chip. The bright deep red emission light can be observed by using a near-infrared camera. In order to explore the application of this device in anti-counterfeiting, we designed an experiment as shown in Fig. 6b. Firstly, we used a black ball-point pen to cover our designed pattern completely, and then irradiated it with a near-infrared pc-LED chip afterward, and finally with the help of a near-infrared camera, we observed the original pattern clearly. Secondly, we surrounded the reagent bottle with black gauze and again irradiated it with the NIR pc-LED chip, so that the reagent capacity in the original reagent bottle could be observed under the NIR camera. Finally, we irradiated eggs with NIR light from the NIR pc-LED. The separation of egg yolk and egg white could not be observed with the naked eye, but could be clearly observed using the NIR camera, which was consistent with our expected idea.

Fig. 6 (a) Emission spectrum of the NIR pc-LED at 20–310 mA. (b) The application of the NIR pc-LED in an anti-counterfeiting scenario. (c) Relationship between output power and luminous efficiency of the NIR pc-LED with respect to driving current. (d) The transilluminated photograph of the palm of the hand under near-infrared light. (e) Near-infrared light transmission photos of flowers and fruits.

Fig. 6c provides the output power and photoelectric conversion efficiency of our prepared blue LED chip in the driving current range of 20–310 mA. As the driving current increased, the output power of the NIR pc-LED also rose gradually from 92.45 to 952.3 mW. However, the photoelectric conversion efficiency of the NIR pc-LED decreased proportionally. This is presumed to be primarily due to the reduction in the luminous efficiency of the blue LED chip caused by the surge in current. As shown in Fig. 6d and e, the NIR light emitted by our prepared MGO:Cr³⁺ was also used to illuminate palms, fruits, and flowers. When observed through the NIR camera, the blood vessels in palms are clearly visible. The experimental results indicate that the MGO:Cr³⁺ near-infrared pc-LED prepared by us has significant potential applications in penetrating animal and plant tissues for area coverage and anti-counterfeiting purposes.

4. Conclusions

In summary, we have successfully synthesized MGO:Cr³⁺ NIR phosphors using the high temperature solid phase method. The optimal doping concentration of Cr³⁺ was experimentally determined to be 2%. The phosphors can be efficiently excited by a 460 nm laser to produce an emission band centered at 710 nm. In addition to traditional photoluminescence, MGO:Cr³⁺ exhibits ML properties. Varying the applied force yields different spectral patterns, enabling a dual-mode response. It also has good temperature stability and repeatability. In addition, the device fabricated by mixing the phosphor and PDMS film shows excellent stress sensing performance. Most importantly, we encapsulated MGO:Cr³⁺/PDMS on a blue LED chip. Under the illumination of the NIR pc-LED chip and by using NIR camera, different hidden objects are clearly observable, demonstrating its penetration capability. This study enhances the potential applications of NIR phosphors and introduces novel theoretical concepts for designing multi-mode responsive NIR luminescent materials.

Author contributions

Yaowu Wang: conceptualization, methodology, investigation, data curation, writing original draft; Gongxun Bai: resources, writing – reviewing and editing, supervision, project administration, funding acquisition, formal analysis; Guocheng Pan: methodology, investigation; Yinyan Li: resources, data curation; Jianfeng Wang: funding acquisition, investigation; Zhenping Wu: project administration, formal analysis; and Shiqing Xu: resources, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Zhejiang Provincial Natural Science Foundation of China (LR24F050002), the Provincial Professional Degree Graduate Student Research Program (012013) and Tunable Intelligent Luminescent Material Design and its Optical Detection Application (230034).

References

1. J. Qiao, G. Zhou, Y. Zhou, Q. Zhang and Z. Xia, Nat. Commun., 2019, 10, 5267.
2. B. Su, M. Li, E. Song and Z. Xia, Adv. Funct. Mater., 2021, 31, 2105316.
3. S. Yu, S. Fang, L. Zhao, Y. Bai, R. Wang and Z. Wang, Chem. Eng. J., 2023, 474, 145542.
4. S. Liu, Y. Zheng, D. Peng, J. Zhao, Z. Song and Q. Liu, Adv. Funct. Mater., 2023, 33, 2209275.
5. X. Zou, H. Zhang, W. Li, M. Zheng, M. S. Molokeev, Z. Xia, Y. Zheng, Q. Li, Y. Liu, X. Zhang and B. Lei, Adv. Opt. Mater., 2022, 10, 2200882.
6. B. Kumar Rajwar, J. Manam and S. Kumar Sharma, Spectrochim. Acta, Part A, 2023, 293, 122512.
7. Y. Zhang, S. Miao, Y. Liang, C. Liang, D. Chen, X. Shan, K. Sun and X. J. Wang, Light: Sci. Appl., 2022, 11, 136.
8. R. S. Liu, Chem. Mater., 2023, 35, 6179–6183.
9. R. Shi, S. Miao, Y. Zhang, X. Lv, D. Chen and Y. Liang, J. Mater. Chem. C, 2023, 11, 2748–2755.
10. M. Runowski and S. Lis, J. Lumin., 2016, 170, 484–490.
11. M. Skwierczyńska, P. Woźny, M. Runowski, M. Perzanowski, P. Kulpiński and S. Lis, J. Alloys Compd., 2020, 829, 154456.
12. J. Xiang, X. Zhou, X. Zhao, Z. Wu, C. Chen, X. Zhou and C. Guo, Laser Photonics Rev., 2023, 17, 2200965.
13. S. M. Jeong, S. Song, K. I. Joo, J. Kim, S. H. Hwang, J. Jeong and H. Kim, Energy Environ. Sci., 2014, 7, 3338–3346.
14. M. Runowski, A. Shyichuk, A. Tymiński, T. Grzyb, V. Lavín and S. Lis, ACS Appl. Mater. Interfaces, 2018, 10, 17269–17279.
15. G. H. Li, J. B. Huang, Q. H. Yang, J. Wang, G. M. Cai and X. J. Wang, J. Mater. Chem. C, 2023, 11, 16578–16586.
16. X. Zhang, X. Hou, J. Gao, Z. Wang, X. Zhao, C. Xu and D. Gao, J. Mater. Chem. C, 2023, 11, 16631–16637.
17. E. Song, H. Ming, Y. Zhou, F. He, J. Wu, Z. Xia and Q. Zhang, Laser Photonics Rev., 2021, 15, 2000410.
18. J. Xue, Z. Yu, H. M. Noh, B. R. Lee, B. C. Choi, S. H. Park, J. H. Jeong, P. Du and M. Song, Chem. Eng. J., 2021, 415, 128977.
19. C. Jin, R. Li, Y. Liu, L. Zhang, J. Zhang, P. Sun, Z. Luo and J. Jiang, Adv. Opt. Mater., 2023, 11, 2300772.
20. S. Liu, R. Yang, H. Cai, Y. Zhuang, Z. Song, L. Ning and Q. Liu, Laser Photonics Rev., 2023, 17, 2200999.
21. K. Ye, Z. Yan, X. Yang and S. Xiao, Opt. Mater., 2021, 121, 111480.
22. J. Deng, Z. Guan, D. Fu, Y. Zheng, Z. Chen, H. Li and X. Liu, Adv. Opt. Mater., 2023, 11, 2300207.
23. J. H. Han, Y. Yoon, Y. M. Park, H. J. Kim, N. S. M. Viswanath, H. B. Cho, S. H. Jang, Y. R. Kim, J. M. Seo, J. Y. Moon, K. Kim and W. B. Im, ECS J. Solid State Sci. Technol., 2023, 12, 076006.
24. S. Hajra, S. Panda, S. Song, B. K. Panigrahi, P. Pakawanit, S. M. Jeong and H. J. Kim, Nano Energy, 2023, 114, 108668.
25. G. Tiwari, N. Brahme, R. Sharma, D. P. Bisen, S. K. Sao and A. Khare, J. Lumin., 2017, 183, 89–96.
26. A. Abdukayum, J. T. Chen, Q. Zhao and X. P. Yan, J. Am. Chem. Soc., 2013, 135, 14125–14133.
27. E. Song, X. Jiang, Y. Zhou, Z. Lin, S. Ye, Z. Xia and Q. Zhang, Adv. Opt. Mater., 2019, 7, 1901105.
28. J. Zuo and X. Lin, Laser Photonics Rev., 2022, 16, 2270025.
29. G. Wei, P. Li, R. Li, Y. Wang, S. He, J. Li, Y. Shi, H. Suo, Y. Yang and Z. Wang, Adv. Opt. Mater., 2023, 11, 2301794.
30. J. Ning, Y. Zheng, Y. Ren, L. Li, X. Shi, D. Peng and Y. Yang, Sci. Bull., 2022, 67, 707–715.
31. T. Zheng, M. Runowski, I. R. Martín, K. Soler-Carracedo, L. Peng, M. Skwierczyńska, M. Sójka, J. Barzowska, S. Mahlik, H. Hemmerich, F. Rivera-López, P. Kulpiński, V. Lavín, D. Alonso and D. Peng, Adv. Mater., 2023, 35, 2304140.
32. T. Y. Hwang, Y. Choi, Y. Song, N. S. A. Eom, S. Kim, H. B. Cho, N. V. Myung and Y. H. Choa, J. Mater. Chem. C, 2018, 6, 972–979.
33. T. Gao, Y. Liu, R. Liu and W. Zhuang, Materials, 2023, 16, 3145.
34. C. Chen, Z. Lin, H. Huang, X. Pan, T. Zhou, H. Luo, L. Jin, D. Peng, J. Xu, Y. Zhuang and R. Xie, Adv. Funct. Mater., 2023, 33, 2304917.
35. L. Li, C. Cai, X. Lv, X. Shi, D. Peng, J. Qiu and Y. Yang, Adv. Funct. Mater., 2023, 33, 2301372.
36. B. Malysa, A. Meijerink and T. Jüstel, J. Lumin., 2018, 202, 523–531.
37. B. Chen, C. Li, D. Deng, F. Ruan, M. Wu, L. Wang, Y. Zhu and S. Xu, J. Alloys Compd., 2019, 792, 702–712.
38. M. Szymczak, M. Runowski, V. Lavín and L. Marciniak, Laser Photonics Rev., 2023, 17, 2200801.
39. Y. Zhuang, D. Chen, W. Chen, W. Zhang, X. Su, R. Deng, Z. An, H. Chenand and R. J. Xie, Light: Sci. Appl., 2021, 10, 132.
40. L. Guo, P. Xia, T. Wang, A. N. Yakovlev, T. Hu, F. Zhao, Q. Wang and X. Yu, Adv. Funct. Mater., 2023, 33, 2306875.
41. Z. Liu, X. Yu, Q. Peng, X. Zhu, J. Xiao, J. Xu, S. Jiang, J. Qiu and X. Xu, Adv. Funct. Mater., 2023, 33, 2214497.
42. Q. Zhang, D. Liu, P. Dang, H. Lian, G. Li and J. Lin, Laser Photonics Rev., 2022, 16, 2100459.
43. Y. Yan, S. Fang, Y. Li, Y. Xu, Y. Song, Z. Ma, Y. Shi, L. Zhao and Z. Wang, J. Mater. Chem. C, 2023, 11, 11509–11517.
44. Q. Du, J. Ueda and S. Tanabe, J. Mater. Chem. C, 2023, 11, 16225–16233.
45. Z. Zhu, Z. Luo, Y. Xie, Y. Sun, L. Xu and Q. Wu, Adv. Funct. Mater., 2023, 24, 2313701.
46. D. Huang, H. Zhu, Z. Deng, H. Yang, J. Hu, S. Liang, D. Chen, E. Ma and W. Guo, J. Mater. Chem. C, 2021, 9, 164–172.
47. E. Song, J. Wang, S. Ye, X. Yang, M. Peng, Q. Zhang and L. Wondraczek, Adv. Opt. Mater., 2017, 5, 1700070.
48. J. Lai, W. Shen, J. Qiu, D. Zhou, Z. Long, Y. Yang, K. Zhang, I. Khan and Q. Wang, J. Am. Ceram. Soc., 2020, 103, 5067–5075.
49. M. Jiao, N. Guo, W. Lü, Y. Jia, W. Lv, Q. Zhao, B. Shao and H. You, Inorg. Chem., 2013, 52, 10340–10346.
50. D. Liu, G. Li, P. Dang, Q. Zhang, Y. Wei, L. Qiu, M. S. Molokeev, H. Lian, M. Shang and J. Lin, Light: Sci. Appl., 2022, 11, 112.
51. A. Lay, C. Siefe, S. Fischer, R. D. Mehlenbacher, F. Ke, W. L. Mao, A. P. Alivisatos, M. B. Goodman and J. A. Dionne, Nano Lett., 2018, 18, 4454–4459.
52. Q. Zhang, X. Wei, J. Zhou, B. Milićević, L. Lin, J. Huo, J. Li, H. Ni and Z. Xia, Adv. Opt. Mater., 2023, 11, 2300310.
53. T. Zheng, M. Runowski, J. Xue, L. Luo, U. R. Rodríguez-Mendoza, V. Lavín, I. R. Martín, P. Rodríguez-Hernández, A. Muñoz and P. Du, Adv. Funct. Mater., 2023, 33, 2214663.
54. M. Runowski, P. Woźny, S. Lis, V. Lavín and I. R. Martín, Adv. Mater. Technol., 2020, 5, 1901091.
55. M. Runowski, P. Woźny and I. R. Martín, J. Mater. Chem. C, 2021, 9, 4643–4651.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc00059e

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