1. Introduction

Piezoluminescence is a phenomenon of mechano-optical conversion, in which materials respond to the pressure-related mechanical actions by photon emission [1–3]. The breakthrough in strong and nondestructive piezoluminescence is the first report of the material system of ZnS:Mn²⁺ and SrAl₂O₄:Eu²⁺ in 1999 by the Xu group [4,5], which brings the materials of combining piezoelectricity, luminescent centers and carrier traps into the studies on mechanoluminescence [6,7]. Since then, the piezoluminescent process is recognized to involve the piezoelectric-induced detrapping and recombination of carriers captured by trap states [8]. The high integrity of material structure, recyclable charge-discharge of carrier traps and in situ emission endows piezoluminescence with the ability to image and read-out dynamic variation in pressure distribution via reproducible luminescence. During the past 20-years, the broad prospects of piezoluminescence in stress sensors, structural health diagnosis, and mechano-driven displays and excitation sources have attracted much attention to exploit novel piezoluminescent materials exhibiting high performance [9–24]. In view of aforementioned three fundamental elements forming piezoluminescence, two strategies have been employed to discover new piezoluminescent materials. One is to introduce proper luminescent ions into objective piezoelectrics with a view of creating luminescent centers and carrier traps in materials. The representative materials include SrAl₂O₄:Eu²⁺ [4], ZnS:Mn²⁺ [5] and LiNbO₃:Pr³⁺ [25] developed by the Xu group, and mCaO·Nb₂O₅:Pr³⁺ (m = 1, 2 and 3) exploited by our group recently [26]. Another is to screen persistent phosphors exhibiting piezoelectricity, in which the luminescent centers and trap levels in persistent phosphors could be directly available for piezoluminescence [27], providing convenience for material development. Several piezoluminescent materials have been identified accordingly, including CaAl₂Si₂O₈:Eu²⁺ [28], Ca₂Al₂SiO₇:Ce³⁺ [29], BaSi₂O₂N₂:Eu²⁺ [30] and Sr₃Sn₂O₇:Sm³⁺ [31], as well as CaZr(PO₄)₂:Eu²⁺ developed by our group [32].

In this work, we employed the perovskite-type compound Ca₃Ti₂O₇ as the model material. It belongs to the Ruddlesden-Popper titanate with composition [n(CaTiO₃)·CaO, n = 2], were primarily recognized as hybrid improper ferroelectrics due to the distinctive crystal structure [33–35]. With the ever-growing requirements for the integration and intelligence of electronic devices, efforts have been made to realize multifunction in an individual material. By means of Eu³⁺ and Bi³⁺ dopants, photoluminescence was realized in Ca₃Ti₂O₇, broadening a promising application in phosphor-converted LEDs [36]. More attractively, long-persistent luminescence was reported afterwards in Pr³⁺-activated Ca₃Ti₂O₇ [37]. Considering that all ferroelectrics are also piezoelectric materials, we have reasons to infer an efficient piezoluminescent performance in Ca₃Ti₂O₇:Pr³⁺. As a proof-of-concept experiment, herein, we report a novel piezoluminescent phosphor – Ca₃Ti₂O₇:Pr³⁺ that sense compression and friction by intense red emission. The systematical characterizations reveal the underlying piezoluminescent mechanism involving non-centrosymmetric structure, luminescent centers and trap levels. Our research is expected to guide an expanded scope of developing novel piezoluminescent materials, thereby promoting further application of piezoluminescence.

2. Experimental

Single-phase phosphor Ca₃Ti₂O₇:Pr³⁺ was synthesized using the solid-state reaction among CaCO₃, TiO₂ and Pr₆O₁₁ (Aladdin, ≥99.9%) via multiple steps of pre-calcination + two-step sintering. The optimum amount of Pr doping for Ca site was 0.3 mol% on account of piezoluminescent performance. Firstly, a stoichiometric mixture of raw materials was thoroughly ground and pre-calcined at 900 °C for 4 h in air. Secondly, the pre-calcined samples were ground, pressed into pellets, and subsequently sintered at 1400 °C for 6 h in air. Thirdly, the sintered pellets were ground, pressed into pellets, re-sintered at 1550 °C for 6 h in air, and finally pulverized for further use.

Phase composition for the fabricated powders was characterized with X-ray diffraction (XRD, D8 Advance, Bruker AXS GmbH). Diffuse reflectance spectra were measured using a UV/vis/NIR spectrophotometer (V570, Jasco). Spectra of photoluminescence (PL) and afterglow (AG), and AG decay curves were recorded on a fluorescence spectrometer (F-4600, Hitachi). Thermoluminescent (ThL) curves of as-synthesized powders were measured using a ThL meter (FJ427A1, Beijing Nuclear Instrument Factory). To characterize piezoluminescence and triboluminescence of Ca₃Ti₂O₇:Pr³⁺ phosphors triggered by different mechanical stimuli, cylinder-shaped (25 mm in diameter and 15 mm in thickness) composites were prepared by mixing the as-synthesized powders with an optical epoxy resin. Compression was applied along the diameter direction of cylinder by a lab-built universal testing machine. Friction was exerted on the upper surface of cylinder by a lab-made friction machine. Signals of piezoluminescence and triboluminescence were captured using an in-house assembled photon-counting system. Spectra of piezoluminescence and triboluminescence were recorded by a fiber optic spectrometer (QE65000, Ocean Optics). Before the measurements of piezoluminescence, triboluminescence and ThL, the samples were exposed to UV light (308 nm) for 1 min and sat in the dark for 2 min. Photographs of PL, AG, piezoluminescence and triboluminescence were recorded using a Canon 7D camera. All measurements except ThL were performed at room temperature.

3. Results and discussion

As mentioned in the experimental section, we synthesized Ca₃Ti₂O₇:Pr³⁺ via two-step sintering of 1400 °C + 1550 °C. The XRD pattern of product synthesized under such condition reveals a single phase of Ca₃Ti₂O₇ [Fig. 1(a)]. No detectable shift of XRD peaks is observed before and after activation with Pr³⁺ dopants due to similar ionic radius of Pr³⁺ (0.99 Å) and Ca²⁺ (1.00 Å). Moreover, we also attempted the synthesis methods described by the literature [36,37], including one-step conditions of 1350 °C + HBO₃, 1400 °C and 1550 °C, however the XRD patterns indicate the generation of wrong phases and impurity phases [Fig. 1(a)]. Figure 1(b) illustrates the crystal structure of Ca₃Ti₂O₇, crystallizing in orthorhombic space group Ccm2₁ (36) with a = 5.4172 Å, b = 19.4169 Å, c = 5.4334 Å and Z = 4 [33]. It is composed of double layers of CaTiO₃ perovskite units separated by CaO layers, in which the perovskite layers are built up of corner sharing TiO₆ octahedra, and Ca atoms are arranged in the body center of the perovskite layers. The tilted and rotated TiO₆ octahedra lead to Ca displacements in adjacent perovskite slabs, further resulting in a net polarization [35]. Such structure provides a prerequisite for the piezoelectric response of Ca₃Ti₂O₇ on the pressure-related stimuli.

 

Fig. 1 (a) Powder XRD patterns of products synthesized using different conditions. (b) Crystal structure of Ca₃Ti₂O₇.

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Figure 2(a) illustrates the diffuse reflection spectrum of as-synthesized Ca₃Ti₂O₇:Pr³⁺. It shows an intense drop near the 300 nm UV region, attributing to the valence-to-conduction absorption of host, namely, O(2p)-Ti(3d) charge transfer transition [38]. In the range of 400-700 nm (visible light region), the refection presents a ramp lift as increasing the wavelength. Owing to the stronger absorption in blue-green visible region compared to that in red region, the sample of Ca₃Ti₂O₇:Pr³⁺ exhibits a faint yellow body color under daylight lighting [Fig. 2(a), top-left). Furthermore, on the basis of Kubelka-Munk function and the Tauc relation [39,40], we estimated the value of band gap to be 3.66 eV [Fig. 2(a), bottom-right], which is close to the reported value of 3.62 eV [37], and a little less than that of 3.94 eV [38]. The PL excitation peaks of Ca₃Ti₂O₇:Pr³⁺ are identical with the diffuse reflectance spectrum [Fig. 2(b)]. The broadband excitation located at ~280 nm is ascribed to the lowest component of 5d state of Pr³⁺ [41]. The intense peak of 327 nm is corresponding to O-Ti charge transfer (CT) [38]. The shoulder band ranging from 350 to 400 nm is attributed to a low-lying Pr-O-Ti intervalence charge transfer state (IVCT), which could induce a complete depopulation of higher excited levels of Pr³⁺, such as ³P_J (J = 2, 1, 0), leading to the absence of blue and green emission [42]. The weak peaks in range of 450-500 nm originate from ³H₄ to ³P_2,1,0 transition of Pr³⁺. Upon 327 nm UV excitation, a bright red emission located at 615 nm is observed [Fig. 2(b), inset], with a CIE ordinate of (0.66, 0.33) which is close to the ideal red light (0.67, 0.33).

 

Fig. 2 (a) Diffuse reflection spectrum of as-synthesized Ca₃Ti₂O₇:Pr³⁺. The top-left inset shows the optical image in a lit environment. The bottom-right inset presents the derived band gap. (b) PL excitation (λ_em = 615 nm) and emission (λ_ex = 327 nm) spectra of Ca₃Ti₂O₇:Pr³⁺. The inset presents the photograph of Ca₃Ti₂O₇:Pr³⁺ irradiated by 254 nm UV light.

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We further assessed the persistent luminescence of as-synthesized Ca₃Ti₂O₇:Pr³⁺ [Fig. 3]. The AG excitation spectrum was plotted by marking AG intensity at 615 nm after the stoppage of monochromatic excitation light ranging from 200 to 500 nm in a step of 5 nm [Fig. 3(a)]. By the comparison of PL and AG excitation spectra [Fig. 3(a), inset], the excitations corresponding to valence-to-conduction and IVCT show a significant contribution on AG, indicating an efficient charging process of trap states under such two excitation conditions, namely a closely matched energy among these three states. Under the optimized excitation of 327 nm, the AG decay curve of Ca₃Ti₂O₇:Pr³⁺ is composed of tri-exponential components with time constants of 0.18 s, 2.02 s and 26.31 s [Fig. 3(b)]. The red persistent luminescence can last over 10 min in the limit of light perception for naked eye (0.32 mcd/m²), twice the time reported by Ref. 37, which is possibly attributed to the better growth of grains under higher sintering temperature in this work.

 

Fig. 3 (a) AG excitation spectrum of Ca₃Ti₂O₇:Pr³⁺ obtained by plotting AG intensity (I_30s) after 1 min irradiation with wavelengths between 200 and 500 nm in 5-nm step. Inset shows spectra of PL excitation (PLE) and AG excitation (AGE) for comparison. (b) AG decay curve of Ca₃Ti₂O₇:Pr³⁺. The inset presents the optical images taken at different times (10-600 s) after light irradiation of 308 nm for 1 min by a hand-held UV lamp. The red AG is visible to naked eyes for more than 600 s.

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As expected, ferroelectric Ca₃Ti₂O₇:Pr³⁺ long-persistent phosphor shows the characteristic of piezoluminescence [Fig. 4(a)]. We firstly studied the profile of piezoluminescence during a compression test – within the pressure range of 0-200 N, the intensity of piezoluminescence shows a swift and nearly linear increase with the amplification of compression; with further enlarging the compression to 1000 N, the luminescent intensity exhibits a relatively slow linear increase. The staged response curve in Fig. 4(a) can be influenced by many factors, such as the distribution of the particle size, the hardness of the composite, the uniformity of particles in the composite, and the mechanical-coupling between the particles and the epoxy resin, which need further investigation. In the process of loading, we can directly observe a fusiform red-emitting light distribution [Fig. 4(a), inset]. In addition to piezoluminescence, Ca₃Ti₂O₇:Pr³⁺ composite shows a triboluminescent response in a sustained period of rod scratching experiment – the luminescent signals increase steeply after turning on the friction, following by the periodic and attenuated oscillations, and vanish immediately on cutting off the frictional stimulus [Fig. 4(b)]. As applying a mechanical friction, we can observe a bright red light-spot at the contact position of rod and sample, and also a circular friction track of persistent luminescence [Fig. 4(b), inset]. We found that both piezoluminescence and triboluminescence from Ca₃Ti₂O₇:Pr³⁺ display a reproducible performance after the UV irradiation of 308 nm, suggesting the recharging of traps by UV excitation, in spite of the possibility of concurrent fracture in the particle surfaces during the friction process. The complexity of friction action involved in the triboluminescence from Ca₃Ti₂O₇:Pr³⁺ still needs further investigation. These mechano-optical conversions from Ca₃Ti₂O₇:Pr³⁺ demonstrate the promising prospects in multiscale artificial skins, visualization of stress distribution, and mechano-sensitive devices.

 

Fig. 4 Piezoluminescence and triboluminescence from Ca₃Ti₂O₇:Pr³⁺. (a) Dependence of piezoluminescent intensity on compression load and photograph of piezoluminescence at peak load. (b) Triboluminescent response on the mechanical friction with a metal rod of 1 mm diameter and the corresponding optical image.

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In a further set of experiments, we characterized the spectra of AG and piezoluminescence [Fig. 5] and ThL curves [Fig. 6]. The AG and piezoluminescent spectra, as well as triboluminescent spectrum (not shown here) are identical with the PL one, indicating the same origin of luminescence from the ¹D₂-³H₄ transition of Pr³⁺. The ThL curves were measured at different heating rates of 1, 2, and 3 °C/s [Fig. 6(a)]. Each ThL curve is well fitted by one Gaussian peak, suggesting one kind of Gaussian-distributed traps in Ca₃Ti₂O₇:Pr³⁺. According to the Hoogenstraaten method [43], the most probable trap depth is estimated to be 0.65 eV [Fig. 6(b)], which is close to the value estimated using Eeckhout et al.’s method [44]. Considering the nonequivalent substitution of Pr³⁺ for Ca²⁺, the luminescent center related defects [Pr_Ca]^o should act as the trap centers of electrons to involve in the processes of persistent luminescence and piezoluminescence, and even triboluminescence [41].

 

Fig. 5 Spectra of afterglow and piezoluminescence (Piezo-L) from Ca₃Ti₂O₇:Pr³⁺.

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Fig. 6 (a) ThL curves of Ca₃Ti₂O₇:Pr³⁺ at different heating rate (1, 2 and 3 °C/s). The inset illustrates the Gauss fit of single peak. (b) Hoogenstraaten plots for ThL peak.

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On the basis of above investigation, the underlying luminescent processes in Ca₃Ti₂O₇:Pr³⁺ are illustrated in Fig. 7. Upon the excitation of UV irradiation, the host absorbs energy and subsequently transfers to Pr³⁺, thus inducing an electronic transition of Pr³⁺ to Pr-O-Ti IVCT. There are possibly three effective pathways for these excited electrons. The first one is to relax stepwisely via high-energy states to ¹D₂ level of Pr³⁺, followed by a red PL from the transition of ¹D₂ to ³H₄. The second and third ones are to be captured by the [Pr_Ca]^o defects through indirect transfer of CB and direct tunneling effect, respectively [26]. Under the thermal stimulus, the trapped electrons can be slowly released to return IVCT through CB or tunneling, and then relaxed to ¹D₂ level as the PL process, leading to a red persistent luminescence. Under the mechanical action, a local electric field generated by the piezoelectric effect would act on the traps to release and relax the captured electrons as the process of persistent luminescence, resulting in piezoluminescence. It is worth of noting that the discharging speed during mechanical stimulus is more rapid than that under ambient temperature due to the extra import of mechanical energy. Therefore, piezoluminescence, compared to persistent luminescence, presents a stronger intensity, but an identical emission wavelength.

 

Fig. 7 Schematic illustration of the luminescent processes in Ca₃Ti₂O₇:Pr³⁺.

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4. Conclusion

In summary, according to the ferroelectricity and persistent luminescence of Ca₃Ti₂O₇:Pr³⁺, we inferred the characteristic of piezoluminescence in the same material, and verified the deduction experimentally. The single-phase Ca₃Ti₂O₇:Pr³⁺ was synthesized using solid-state reaction via a two-step sintering of 1400 °C + 1550 °C. Under the multiscale mechanical stimuli including compression and friction, Ca₃Ti₂O₇:Pr³⁺ emits a bright red light from ¹D₂-³H₄ transition of Pr³⁺. The investigation on persistent luminescence indicates a significant excitation contribution of valence-to-conduction and IVCT on the trap-charging process, showing an observable red afterglow for more than 600 s. The measurement and analysis of thermoluminescence reveal the existence of traps with the most probable depth of 0.65 eV, which is responsible for the behaviors of persistent luminescence and piezoluminescence. Finally, a possible mechanism is proposed to elucidate the photo-, thermo-, and piezoluminescence in Ca₃Ti₂O₇:Pr³⁺.