Correlation between persistent luminescence and mechano-luminescence of Sm³⁺-doped YPO₄

Abstract

Persistent luminescence (PersL) and mechanoluminescence (ML) are two interesting optical phenomena, which are initiated by trapped charge carriers in some materials upon applying heat or force stimulations. Therefore, it is meaningful to jointly investigate PersL and ML in a single material to better understand the correlation between them. Herein, we focused on Sm³⁺-doped YPO₄ because it can emit both PersL and ML. On the one hand, we demonstrated that after being charged by X-ray, YPO₄:Sm³⁺ emitted orange PersL for over one hundred minutes, which can be attributed to the ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺. During the PersL stage, Sm³⁺ ions consistently served as luminescent centers. Using thermoluminescence measurements, we confirmed a continuous distribution of traps in YPO₄:Sm³⁺, with trap depths ranging from 0.59 to 0.81 eV. On the other hand, during the PersL process, we observed a significant enhancement in YPO₄:Sm³⁺ upon receiving an external force stimulation. It suggested that in addition to heat, external force stimulation enabled the release of the same trapped charge carriers in YPO₄:Sm³⁺, resulting in ML. Additionally, we confirmed that the ML of YPO₄:Sm³⁺ belonged to the trap-controlled type, with both piezoelectric and triboelectric effects playing pivotal roles in the force-activated luminescence process. Our work offers insights into the coupling between PersL and ML, laying the foundation for the rational design of future multi-stimuli responsive luminescent materials.

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1. Introduction

Persistent luminescence (PersL) materials are capable of emitting photons for extended periods after being charged by various excitation sources, such as lasers, light-emitting diodes, xenon lamps, mercury lamps, high-energy rays, and even sunlight.^1–5 To date, the wavelength of PersL has been significantly extended from the ultraviolet-C to near-infrared-II region.^6–12 Therefore, PersL materials have broad application prospects in many fields, ranging from initial emergency lighting and decoration to emerging high-contrast bioimaging and three-dimensional flexible X-ray imaging.^13–16 For instance, Pei et al. achieved imaging of a murine tumour using PersL located in the near-infrared-II region from NaY(Gd)F₄:Er³⁺/Ho³⁺/Tm³⁺/Nd³⁺ nanoparticles.¹⁷ Compared with conventional biological imaging that relies on real-time laser excitation, PersL-assisted imaging presented a higher signal-to-noise ratio, laying the foundation for future applications.

Mechanoluminescence (ML) has recently attracted considerable attention because of its unique ability to convert force stimuli into light emission.^18–20 Specifically, some inorganic and even organic materials can emit light when subjected to external stress stimuli, such as bending, pressing, rubbing, and friction.^21–23 Mechanoluminescent materials have found their applications in various fields, including safety signage, pressure sensors, and novel lighting technologies.^24–26 In general, mechanoluminescent materials can be divided into two main categories: self-recovering and non-self-recovering materials. ZnS:Cu²⁺/Mn²⁺ and CaZnOS doped with different lanthanides and transition elements are the typical examples of self-recovering mechanoluminescent materials, which can emit light when mechanically stressed.^27,28 In comparison, there are more types of non-self-recovering mechanoluminescent materials, such as the representative SrAl₂O₄:Eu²⁺.²⁹ When irradiated with a light source for recharging, SrAl₂O₄:Eu²⁺ can provide continuous ML emission. The fundamental reason lies in the fact that non-self-recovering mechanoluminescent materials are all defect-contained systems that trap charge carriers upon exposure to light irradiation.³⁰ The trapped charge carriers are released when subjected to external force stimuli, resulting in ML. Considering this, we can conclude that non-self-recovering ML materials are closely related to PersL materials in terms of the origin of ML and PersL.^31,32 Therefore, it is meaningful to jointly investigate the PersL and ML properties of the two-in-one material to better understand the relationship between PersL and ML, as well as the potential applications in more fields, such as optoelectronics. Despite growing interest in the topic, related research is very limited, highlighting the need for further investigation.

Here, we take Sm³⁺-doped YPO₄ as an example to jointly study the related PersL and ML properties, aiming to reveal the intrinsic connection between these two optical phenomena. For one thing, we demonstrate that YPO₄:Sm³⁺ is a kind of PersL material capable of continuously emitting red luminescence after being charged with an X-ray. The red PersL is assigned as the ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺ ions, which always act as luminescent centers during the entire PersL stage. Using thermoluminescence (TL) measurements, we investigated the trap properties of YPO₄:Sm³⁺. In another study, we observed the ML of YPO₄:Sm³⁺ when external mechanical stimulation was applied to the samples. On this basis, we further disclosed the release of the trapped charge carriers upon excitation by external mechanical stimulation. Based on all the experimental evidence mentioned above, we confirmed that the PersL and ML of YPO₄:Sm³⁺, although generated under different external stimuli, come from the same trapped charge carriers, laying the foundation for further design of new multi-stimulated materials that exhibit enhanced properties.

2. Results and discussions

The powder X-ray diffraction (XRD) patterns of YPO₄:xSm³⁺ (x = 0, 0.5%, 1%, 2%, 3% and 4%) powders accorded well with the referenced ones (PDF#11-0254), preliminarily indicating that all prepared samples had a tetragonal crystal structure (Fig. 1a). Such deduction was further verified by the Rietveld refinement results (Fig. 1b). In the tetragonal crystal structure, each P atom is coordinated with four O atoms to form a [PO₄] tetrahedron. Each Y atom is adjacently connected with eight O atoms, resulting in [YO₈] polyhedral states. The [PO₄] tetrahedron and [YO₈] polyhedra are combined by sharing the same O atom or O–O bond (Fig. 1c). The ion radii of Y³⁺ and Sm³⁺ were 0.093 and 0.096 nm, respectively. Considering the similar ion radius and the same valence state between Y³⁺ and Sm³⁺, Sm³⁺ ions were expected to enter the YPO₄ host and occupy the Y³⁺ sites. The lattice parameters, a, b and c, of YPO₄:xSm³⁺ (x = 0, 0.5%, 1%, 2%, 3% and 4%) powders were found to increase gradually with rising the content of Sm³⁺ (Fig. 1d), suggesting a slight expansion of the YPO₄ structure due to the larger ion radius of Sm³⁺ than that of Y³⁺.

Fig. 1 Basic characterization of YPO₄:Sm³⁺. (a) X-ray diffraction patterns of YPO₄:xSm³⁺ (x = 0, 0.5%, 1%, 2%, 3% and 4%). (b) Rietveld refinement of YPO₄:1%Sm³⁺. (c) Crystal structure of YPO₄:Sm³⁺. (d) Evolution of a, b and c cell parameters of YPO₄ as a function of Sm³⁺ content. (e) and (f) X-ray photoelectron spectra of YPO₄:1%Sm³⁺. (g) Scanning electron microscopy image and elemental mappings of YPO₄:1%Sm³⁺.

X-ray photoelectron spectroscopy (XPS) successfully detected the peak signals belonging to the Sm 3d, O 1s, Y 3d and P 2p orbitals with success (Fig. 1e). We also measured the fine XPS spectrum over the 1070–1125 eV region and obtained a double-peak profile (Fig. 1f). The peaks with binding energy at 1083.2 and 1110.4 eV were assigned to the Sm 3d_5/2 and Sm 3d_3/2 orbitals, respectively.³³ The images of YPO₄:1%Sm³⁺ obtained by scanning electron microscopy (SEM) suggest that four kinds of elements, Y, P, O and Sm, are distributed uniformly over a single particle (Fig. 1g). The above results demonstrate the successful synthesis of the target materials.

Under excitation at 407 nm, corresponding to the ⁴K_11/2/⁶P_3/2 ← ⁶H_5/2 absorption of Sm³⁺,YPO₄:1%Sm³⁺ emitted photoluminescence (PL) over the 550–750 nm range (Fig. 2a).^34,35 Four emission bands appeared at 565, 606, 648 and 706 nm, corresponding to the ⁴G_5/2 → ⁶H_5/2, ⁴G_5/2 → ⁶H_7/2, ⁴G_5/2 → ⁶H_9/2 and ⁴G_5/2 → ⁶H_11/2 transitions of Sm³⁺, respectively (Fig. 2b). The four emission lines are characteristic transitions of Sm³⁺. The main emission peak of YPO₄:1%Sm³⁺ occurred at 606 nm, and the CIE chromaticity coordinates were located in the red region (Fig. 2c). By monitoring the emission intensity at 606 nm, we obtained the photoluminescence excitation (PLE) spectrum consisted of several sharp lines (Fig. 2a). The main PLE line at 407 nm is attributed to the ⁴K_11/2 ← ⁶H_5/2 transition of Sm³⁺, which matches well with the diffuse reflectance absorption spectrum (Fig. S1, ESI†).

Fig. 2 Steady optical property of YPO₄:Sm³⁺. (a) PL (λ_ex = 407 nm) and PLE (λ_em = 606 nm) spectra of YPO₄:1%Sm³⁺. (b) Possible excitation and emission processes of Sm³⁺. (c) CIE chromaticity coordinates of PL (λ_ex = 407 nm) of YPO₄:1%Sm³⁺. (d) Emission intensity of YPO₄ (integration region: 584–637 nm) as a function of Sm³⁺ content (λ_ex = 407 nm). (e) Emission decay curve of YPO₄:1%Sm³⁺ (λ_em = 606 nm). (f) PL spectrum of YPO₄:1%Sm³⁺ under X-ray excitation.

We then optimized the doping concentration of Sm³⁺ in YPO₄. The spectra of all samples had the same profile, confirming the role of Sm³⁺ ions as luminescent centers (Fig. S2, ESI†). The optimal doping concentration of Sm³⁺ in YPO₄ was in the 1–3% range in molar ratio (Fig. 2d). This conclusion was further confirmed by measuring the PLE spectra and calculating the related PLE intensity (Fig. S3 and S4, ESI†). In addition, the emission quantum yield of YPO₄:1%Sm³⁺ was 6.12% (Fig. S5, ESI†). We also collected the luminescence decay curves of the ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺, which fitted well with a double-exponential fitting function (Fig. 2e and Fig. S6, ESI†). The lifetime of the ⁴G_5/2 state of Sm³⁺ is on the order of milliseconds. Moreover, such lifetime decreased markedly with increasing concentrations of Sm³⁺ (Fig. S6, ESI†), which was probably attributed to cross relaxation among Sm³⁺ ions due to the constantly shortening distance between Sm³⁺ ions, according to a previous report.³⁵ The above discussions are all related to the direct excitation of the 4f energy levels of Sm³⁺. In addition to this strategy, YPO₄:Sm³⁺ was able to emit orange luminescence upon band-to-band excitation via X-ray (Fig. 2f), laying the foundation for the subsequent discussion of the PersL of YPO₄:Sm³⁺.

We then focused on the PersL property of YPO₄:Sm³⁺. Due to the huge bandgap, it is impossible to charge YPO₄:Sm³⁺via commonly used light sources such as mercury lamps, flashlights and LEDs.^36,37 Therefore, an X-ray source was used to charge the as-prepared YPO₄:Sm³⁺. As mentioned above, YPO₄:1%Sm³⁺ emits a characteristic orange luminescence upon X-ray irradiation. After cessation of the X-ray, YPO₄:1%Sm³⁺ continued to emit orange luminescence for a long time, resulting in the so-called PersL phenomenon (Fig. 3a). The profile of the PersL spectra remained unchanged, indicating that Sm³⁺ ions always acted as luminescent centers during the entire PersL stage. We thus monitored the PersL intensity at 606 nm, which corresponded to the ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺. After half an hour, the PersL signal could also be distinguished from the noise (Fig. 3b). We also prepared a bird pattern made of YPO₄:1%Sm³⁺. After pre-exposure to X-rays, the bird pattern could be captured by a common camera even after an hour and a half, thanks to the orange PersL from YPO₄:1%Sm³⁺ (Fig. 3c).

Fig. 3 PersL property of YPO₄:Sm³⁺. (a) PersL spectra of YPO₄:1%Sm³⁺ after being charged by X-ray. (b) Evolution of PersL intensity (606 nm) of YPO₄:1%Sm³⁺. (c) Photographs of bird patterns made of YPO₄:1%Sm³⁺ during the PersL stage. (d) TL curve of YPO₄:1%Sm³⁺ (λ_em = 606 nm). (e) TL curves of YPO₄:1%Sm³⁺ with different cleaning temperatures (λ_em = 606 nm). (f) Trap depths of YPO₄:1%Sm³⁺. For all the above measurements, the X-ray charging time was 5 min.

In general, PersL is associated with traps, the information of which can be disclosed via TL technology.^38–40 By monitoring the PersL intensity at 606 nm, the TL curve of YPO₄:1%Sm³⁺ was collected, showing a broad band mainly covering the 323–423 K range (Fig. 3d). This suggests that continuously distributed traps existed inside the sample. The partial emptying method was used to further evaluate the trap distribution.⁴¹ With the gradually increasing peak emptying temperature, the TL curves went toward high temperatures, revealing once again the continuously distributed nature of the traps (Fig. 3e). The trap depth of YPO₄:1%Sm³⁺ was located in the 0.59–0.81 eV range (Fig. 3f).

By varying the doping concentration of the luminescent center and charging time, we successfully achieved the tuning of PersL performance. All YPO₄:x%Sm³⁺ (x = 0.5, 1, 2, 3 and 4) samples presented observable PersL for over half an hour (Fig. 4a). YPO₄:1%Sm³⁺ exhibited the best PersL performance (Fig. 4b). When the content of Sm³⁺ was higher than 1%, the distance between the two Sm³⁺ ions became closer. The cross relaxation among Sm³⁺ ions and energy migration from Sm³⁺ ions to quenching centers were probably enhanced, which was detrimental to PersL. We measured the TL curves of the YPO₄:x%Sm³⁺ (x = 0.5, 1, 2, 3 and 4) samples by monitoring the intensity at 606 nm (Fig. 4c). All TL curves had a consistent line shape, but the intensities were different. The results revealed that the role of Sm³⁺ was to adjust the density of traps. The optimal concentration of Sm³⁺ was also 1%, consistent with the dependence of PersL on the content of Sm³⁺ (Fig. 4b).

Fig. 4 PersL regulation of YPO₄:Sm³⁺. (a) PersL decay curves, (b) PersL and TL intensity and (c) TL curves of YPO₄ doped with different contents of Sm³⁺. X-ray charging time: 5 min. (d) PersL decay curves, (e) PersL and TL intensity and (f) TL curves of YPO₄:1%Sm³⁺ with different X-ray charging times. For the above measurements, the monitoring wavelength was 606 nm.

The PersL decay curves of YPO₄:1%Sm³⁺ also showed a dependence on the charging time (Fig. 4d and e). Extending the exposure time to X-rays was beneficial for longer PersL times and stronger PersL intensity, which could be attributed to the increment of trapped charge carriers (Fig. 4e and f). Moreover, after 5 min of X-ray charging, the density of trapped charge carriers nearly reached saturation, indicating that further exposure may not lead to a significant increase in the trap population.

In addition to heat, force stimulation like friction could release the trapped charge carriers of YPO₄:Sm³⁺.¹⁸ We built a hand-made device to perform the related measurements (Fig. 5a). YPO₄:Sm³⁺ powder was sealed with two pieces of plastic and then fixed on a glass sheet. Thereafter, a metal tip was whipped across the sealed YPO₄:Sm³⁺ to both the forward and back sides and the signal was collected by the fiber spectrometer on the other side. Without pre-charging, self-recoverable ML was not obtained (Fig. 5b). When YPO₄:1%Sm³⁺ was exposed to photo-excitation at 254 and 365 nm for 5 min, it did not emit luminescence (Fig. 5c and d), suggesting the failure of non-band-to-band pre-charging.

Fig. 5 ML property of YPO₄:Sm³⁺. (a) Schematic experimental setup for the ML test. ML spectrum of YPO₄:1%Sm³⁺ (b) without pre-excitation and under pre-excitation at (c) 254 nm and (d) 365 nm for 5 min. (e) Image of YPO₄:1%Sm³⁺ tablet with a slight scratch stimulation during PersL. (f) PersL decay curve of YPO₄:1%Sm³⁺ with intermittent stimulation (monitoring: 606 nm) after 5-min X-ray charging. (g) Comparison of PersL and ML spectra of YPO₄:1%Sm³⁺.

After being pre-irradiated by X-ray for 5 min, the YPO₄:1%Sm³⁺ sheet showed orange PersL. The region whipped by the metal tip exhibited a much brighter orange emission compared to the rest regions (Fig. 5e). Moreover, the PersL signal suffered from a huge surge upon stimulation by an external force (Fig. 5f). A comparison between the ML and PersL spectra revealed that the additional signal burst came from the luminescent centers of Sm³⁺ (Fig. 5g). These experimental results suggest that external force stimulation accelerates the release of trapped charge carriers of YPO₄:Sm³⁺.

The concentration of Sm³⁺ was examined in terms of the ML performance of YPO₄:Sm³⁺. As shown in Fig. 6(a), the ML spectral profile remained consistent in response to the change in Sm³⁺. In contrast, the integrated ML intensity exhibited a clear dependence on the concentration of Sm³⁺ (Fig. 6b). YPO₄:1%Sm³⁺ exhibited the best ML performance, consistent with the previous results shown in Fig. 4(b). With increasing stimulation of the external force, the ML spectral profile of YPO₄:1%Sm³⁺ remained unchanged (Fig. 6c), similar to the PL and PersL spectra separately shown in Fig. 2(a) and 3(a). The ML intensity of YPO₄:1%Sm³⁺ increased linearly in response to the change in the external force, suggesting the potential of YPO₄:1%Sm³⁺ for stress sensing (Fig. 6d).

Fig. 6 ML regulation of YPO₄:Sm³⁺. (a) ML spectra and (b) ML intensity of YPO₄:xSm³⁺ (x = 0.5%, 1%, 2%, 3% and 4%) after release of room temperature PersL. (c) ML spectra and (d) ML intensity of YPO₄:1%Sm³⁺ under stimulation of different forces. (e) ML images of YPO₄:1%Sm³⁺ in pure powder and different polymers during the PersL stage. For the above measurements, the samples were pre-irradiated with X-rays for 5 min.

We conducted more experiments to explore the release mechanism of trapped charge carriers of YPO₄:1%Sm³⁺ upon stimulation by an external force. After being charged by X-ray for 5 min, the YPO₄:1%Sm³⁺ sample, even in pure powdered form, exhibited a phenomenon of instantaneous enhancement in luminescence, suggesting strong coupling between mechanical stress and trapped charge carriers (Fig. 6e). We embedded YPO₄:1%Sm³⁺ powders in polymers of polydimethylsiloxane (PDMS), polyurethane (PU) and epoxy resin (ER) to fabricate several different composites. When an external force was applied along the radial direction of YPO₄:1%Sm³⁺@ER, the contact area was brighter than the rest region. In this case, the friction effect can be neglected. These results indicate that the piezoelectric effect is responsible for the ML phenomenon. In other words, the introduction of Sm³⁺ probably caused the local piezoelectric property. In addition, orange ML emissions were also observed from YPO₄:1%Sm³⁺@PDMS, YPO₄:1%Sm³⁺@PU and YPO₄:1%Sm³⁺@ER, suggesting that triboelectrification also played a role in the generation of ML from YPO₄:1%Sm³⁺.

3. Conclusions

We prepared a series of YPO₄:Sm³⁺ capable of emitting PersL for over one hundred minutes using X-rays as the charging source. The main peak at 606 nm was attributed to the ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺ ions, which consistently acted as luminescent centers throughout the PersL stage. TL measurements further revealed that the trap depth for YPO₄:Sm³⁺ was in the 0.59–0.81 eV range. In addition, we observed ML of YPO₄:Sm³⁺ upon the addition of external mechanical stimulation to the samples. On this basis, we further revealed the pivotal role of the piezoelectric effect as the main mechanism for releasing trapped charge carriers upon excitation by external mechanical stimulation. These results indicate that the PersL and ML of YPO₄:Sm³⁺, although generated under different external stimuli, originate from the same trapped charge carriers. This work contributes to the future design of new multi-stimulated materials with enhanced properties.

4. Experimental section

4.1 Sample synthesis

YPO₄:Sm³⁺ was prepared by a mature high-temperature solid-state method. NH₄H₂PO₄ (Aladdin, 99%), Y₂O₃ (Aladdin, 99.99%) and Sm₂O₃ (Aladdin, 99.99%) were used as raw materials. First, the raw materials of NH₄H₂PO₄, Y₂O₃ and Sm₂O₃ were weighted according to the calculated stoichiometry ratio. The raw materials were then fully mixed and ground for half an hour to obtain a uniform mixture. Subsequently, the uniform mixture was placed in an alumina crucible, followed by calcination at 500 °C for 1 h and at 1200 °C for 2 h to obtain the final products.

For the samples measured in Fig. 5 and 6a–d, the YPO₄:Sm³⁺ powders were sealed with two pieces of polyethylene terephthalate (PET) membranes. For the ML test, the sealed YPO₄:Sm³⁺@PET film was fixed on a sheet of glass. During the PersL stage, a metal tip was whipped across the sealed YPO₄:Sm³⁺@PET film back and forth to apply force.

The preparation steps of YPO₄:Sm³⁺@ER were as follows. 0.5 g of YPO₄:1%Sm³⁺ powder was fully mixed with 3 g of glue, and the mixture was poured into a round mould with a diameter of 3.5 cm and a thickness of 3 mm. Subsequently, the mould was kept at 70 °C for two hours. After cooling to room temperature, a YPO₄:Sm³⁺@ER hard tablet with a diameter of 3.5 cm and a thickness of 3 mm was obtained.

The preparation steps of YPO₄:Sm³⁺@PU were as follows. 0.5 g of YPO₄:1%Sm³⁺ powder was fully mixed with 5 g of glue, and the mixture was poured into a round mould with a diameter of 5 cm and a thickness of 6 mm. Subsequently, the mould was kept at 70 °C for two hours. After cooling to room temperature, a YPO₄:Sm³⁺@PU flexibility tablet with a diameter of 5 cm and a thickness of 6 mm was obtained.

The preparation steps of YPO₄:Sm³⁺@PDMS were as follows. 0.5 g of YPO₄:1%Sm³⁺ powder was fully mixed with 5 g of PDMS, and the mixture was poured into a square mould with a length of 4 cm, a width of 6 cm and a thickness of 2 mm. Subsequently, the mould was kept at 70 °C for two hours. After cooling to room temperature, the YPO₄:Sm³⁺@PDMS flexibility tablet was obtained with a length of 4 cm, a width of 6 cm and a thickness of 2 mm.

4.2 Characterization and measurement

The crystal structures of the samples were confirmed using an X-ray diffractometer from Bruker (D8-ADVANCE, 40 kV, 40 mA). Morphology and elemental analysis were performed using a scanning electron microscope from FEI (Nova Nano SEM450) equipped with an X-ray energy spectrometer. The valence states of Sm, Y, P and O were determined using an X-ray photoelectron spectroscope from Thermo Scientific (ESCALAB 250Xi). The basic PL and PLE spectra of the samples were collected using an optical spectrometer from Hitachi (F-7100). The emission lifetimes and quantum yields of the samples were measured using an integrated spectrometer from HORIBA (FLuorolog-3). The charging source for the sample was an X-ray emitter (JF-2000, 40 kV, 30 mA). The PersL decay and TL curves were obtained using an optical spectrometer from ZhuoLiHanGuang (Omni-λ-500i). The PL spectra under X-ray irradiation, PersL spectra and ML spectra were collected by a fiber optic spectrometer from Ocean Optics (Maya-2000 Pro).

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

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

Acknowledgements

L. Li would like to thank the Science and Technology Project of Hebei Education Department (BJK2024084) and the financial support from the National Natural Science Foundation of China (12474401). Y. Yang acknowledges the financial support from the National Natural Science Foundation of China (12374373) and the Natural Science Interdisciplinary Research Program of Hebei University (No. DXK202301).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc00240k

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