The discovery of innovative mechanoluminescence materials of SrAl2O4 and ZnS, which emit repeatable light [repeatable mechanoluminescence (ML), hereafter simply ML] even by soft touch, has trigged intense research interest in material/device/system development for applications across various fields. This perspective presents an overview of the crystal structures, mechanisms, and ML behaviors of most promising systems, namely, SrAl2O4-, ZnS-, LiNbO3-, and Sr3Sn2O7-based ferroelectric materials. These multipiezo materials, which simultaneously exhibit intrinsic piezoluminescence (true elastic deformation induced ML and no friction effect) and piezoelectricity, show distinct and valuable characteristics by integrating mechanical force, electric field, and light for stress sensing and other applications. Recent studies indicated the critical role of crystal structure, doping, and piezoelectric properties in achieving robust and reliable ML performance. These findings suggest that ML materials hold substantial promise for applications in stress/force sensing, structural health monitoring, mechanically activated lighting, and advanced imaging techniques. Further investigation and advancement of multipiezo materials could yield breakthroughs, further augmenting their usefulness across various industries and scientific domains. Exploring ferroelectric ML materials offer new prospects for developing advanced materials with unique electro-mechano-optical properties.

The phenomenon of light emission caused by mechanical stimuli, known as mechanoluminescence (ML), has been a subject of fascination for more than 400 years since Francis Bacon’s initial observation of triboluminescence in the 17th century. Yet, the potential of ML remained relatively untapped, with the light emission being weak and irreversible. A turning point came, as shown in Fig. 1, when Xu et al. discovered a phenomenon known as elastic deformation-induced repeatable ML (elasticoluminescence, also called piezoluminescence) in 1999 with ZnS:Mn and SrAl2O4:Eu.1–3 This repeatable light emission occurs in response to non-destructive small mechanical stimuli. It can be demonstrated by finger-touching a balloon and soccer ball coated with the ML material to emit visible light in daylight, as shown in Fig. 1, which got the highest audience rating on television and news magazines.4–6 They have also introduced the principle and applications of hybrid inorganic–organic composites, especially for sensors and sensitive artificial skins.7 For example, when ML inorganic particles were dispersed in a polymer, the intensity of the emitted light was proportional to the applied strain energy.8–12 These ML materials can emit light in response to the internal stress within structures or to external stimuli, such as ultrasound,13,14 showing immense practical value for applications in various multi-scales from large architectures (∼km) to neurons (∼nm). The capability of stress distribution visualization,15,16 failure prediction,17 and crack diagnosis18,19 makes it possible to enhance the safety of social infrastructures, including micro-components and large structures of bridges and tanks and, in situ monitoring of the components of industrial machines and artificial bones.7,20–32 In addition, recent studies have exploited a potential use of health care by delivering light inside deep living organisms.33 

FIG. 1.

History and new trends in ML and multipiezo materials,1,2,6,14,18,33–38,43 according to the citation report from Web of Science (keywords: piezoluminescence, elasticoluminescence, and mechanoluminescence) with typical materials and concept illustrated. The two inset illustrations indicate (i) a phototherapy for deep living body using ultrasonic-induced ML14 and (ii) safety diagnosis of a highway joint section using a scalable ML sensor.18 

FIG. 1.

History and new trends in ML and multipiezo materials,1,2,6,14,18,33–38,43 according to the citation report from Web of Science (keywords: piezoluminescence, elasticoluminescence, and mechanoluminescence) with typical materials and concept illustrated. The two inset illustrations indicate (i) a phototherapy for deep living body using ultrasonic-induced ML14 and (ii) safety diagnosis of a highway joint section using a scalable ML sensor.18 

Close modal

Figure 1 shows the citation report with history and new trends in ML and multipiezo materials. Inspired by the discovery of ZnS:Mn2+ and SrAl2O4:Eu2+,1–3 a new field of “Electro-Mechano-Optical Conversions” has been developed.36 Piezoelectricity, a material capability that generates an electric potential via applying stress in an elastic deformation range (no triboelectricity), and piezoluminescence, the repeatable light emission also induced by elastic deformation (no triboluminescence), serve as the crucial bridges that connect the mechanical, electrical, and optical conversions. Worldwide efforts have been made to develop materials that exhibit efficient piezoluminescence and piezoelectricity, 39–41 such as (Ba,Ca)TiO3, Pb(Zr,Ti)O3, CaZnOS, and SrZnOS. However, strong piezoluminescence within a single-phase piezoelectric matrix was not realized until 2017, when an elegant “multipiezo” system emerged, as shown in Fig. 1. The initial multipiezo material LiNbO3:Pr3+ achieved a “non-threshold” elasticoluminescence, greatly expanding practical application uses.35 Since then, several multipiezo host materials have been discovered,34,37,42 such as Sr3Sn2O7, wherein the Sr3Sn2O7 ferroelectric matrix was doped with the rare earth ion Nd3+. This material displays highly sensitive and sustainable, high-performance NIR (near-infrared, 800–1500 nm) piezoluminescence to penetrate biological tissues, expanding the ML to biotechnological applications. Furthermore, the ML intensity of the multipiezo materials has been enhanced 30 times by upgrading LiNbO3 through strategically controlling the crystal structure.43 This was achieved by partially replacing Li with Na in (Li,Na)NbO3. The substitution significantly improved the material’s piezoelectric properties and piezoluminescence near its morphotropic phase boundary (MBP) between R3c and orthorhombic P21ma in (Li,Na)NbO3.43,44 This material makes it possible to record and read the stress history quantitatively without additional devices by observing the afterglow.45 These advances have initiated a new trend toward developing new multipiezo materials over the past few years. Since there has been a rapid increase in societal demands for IoT (Internet of Things) sensors, structural diagnostics, and health care, asking for new materials, sensors, and light sources, the research related to ML and multipiezo materials is rapidly gaining international attention, as reflected in the accelerating growth in the citation report shown in Fig. 1.

To meet the demands of applications in relation to the ML wavelength, the manipulation of the ML color has been achieved by controlling the luminescent center ion and the host crystal structure (Fig. 2). The emission color in SrAl2O4:Eu2+ is predominantly green owing to the 4f7-4f65d1 electron transition in Eu2+. This results in a broad emission band of around 520 nm, which is a characteristic provoked by its susceptibility to the encompassing crystal field. Notably, by employing various host materials and luminescent centers, such as those exemplified in SrAl2O4:Ce3+, Ho3+ (UV),46 SrMg2(PO4)2:Eu2+ (purple),47 (Sr,Ca)Al2Si2O8:Eu2+ (blue),12,48 CaYAl3O7:Eu2+ (blue),49 (Sr,Ca)MgSi2O7:Eu2+ (light blue),50 Ca2MgSi2O7:Eu2+ (green),51 SrAl2O4:Eu2+ (green),2 Sr2SiO4:Eu2+ (yellow),52 ZnS:Mn2+ (yellow),1 Sr3Sn2O7:Sm3+ (orange),42 (Li,Na)NbO3:Pr3+ (red),43 (Ba,Ti)O3:Pr3+ (red),36 and SrGa12O19:Cr3+, Pr3+ (red),53 the emission colors can be generated spanning from 300 nm (UV) to 800 nm (Red). Moreover, a near-infrared elasticoluminescent material has been successfully synthesized, which can emit near-infrared light by combining multiple luminescent center ions, as shown in Fig. 2. Sr3Sn2O7:Nd3+(900–1400 nm),34 SrAl2O4:Eu2+, Er3+, Cr3+, and Nd3+(800–1550 nm),10,29,54 can exhibit emission wavelengths ranging from 800 to 1550 nm within the in vivo optical window, allowing them to be utilized as a light source for acquiring bio-images of living organisms.

FIG. 2.

Emission wavelength of ML covering the broad range from UV to NIR by controlling the host matrix and luminescent center. SrAl2O4:Ce3+, Ho3+ (UV);46 SrMg2(PO4)2:Eu2+ (purple);47 (Sr, Ca)Al2Si2O8:Eu2+ (blue);12,48 CaYAl3O7:Eu2+ (blue);49 (Sr, Ca)MgSi2O7:Eu2+ (light blue);50 Ca2MgSi2O7:Eu2+ (green);51 SrAl2O4:Eu2+ (green);2 Sr2SiO4:Eu2+ (yellow);52 ZnS:Mn2+ (yellow);1 Sr3Sn2O7:Sm3+ (orange);42 (Li, Na)NbO3:Pr3+ (red);43 (Ba, Ti)O3:Pr3+ (red);36 and SrGa12O19:Cr3+,Pr3+ (red),53 Sr3Sn2O7:Nd3+ (NIR),34 and SrAl2O4:Eu2+, Er3+ (NIR).10,29,54

FIG. 2.

Emission wavelength of ML covering the broad range from UV to NIR by controlling the host matrix and luminescent center. SrAl2O4:Ce3+, Ho3+ (UV);46 SrMg2(PO4)2:Eu2+ (purple);47 (Sr, Ca)Al2Si2O8:Eu2+ (blue);12,48 CaYAl3O7:Eu2+ (blue);49 (Sr, Ca)MgSi2O7:Eu2+ (light blue);50 Ca2MgSi2O7:Eu2+ (green);51 SrAl2O4:Eu2+ (green);2 Sr2SiO4:Eu2+ (yellow);52 ZnS:Mn2+ (yellow);1 Sr3Sn2O7:Sm3+ (orange);42 (Li, Na)NbO3:Pr3+ (red);43 (Ba, Ti)O3:Pr3+ (red);36 and SrGa12O19:Cr3+,Pr3+ (red),53 Sr3Sn2O7:Nd3+ (NIR),34 and SrAl2O4:Eu2+, Er3+ (NIR).10,29,54

Close modal

As highlighted by the multipiezo materials, high performance can be anticipated in ferroelectric systems. The crystal structure of a ferroelectric material plays a crucial role in determining its properties. They typically lack a center of symmetry, known as a non-centrosymmetric crystal structure, allowing for a permanent electric dipole moment. Considering that all ferroelectric materials are also piezoelectric, it is natural to expect elasticoluminescence and multipiezo performance in ferroelectric systems.

The present perspective primarily focuses on multipiezo in ferroelectric materials, emphasizing the crystal structure, mechanism, and their performance. We believe that a systematic consideration of the structural evolution and the capabilities of ferroelectrics in multipiezo is a timely request. Section II of this perspective discusses the basic concepts of ferroelectric multipiezo compounds, and Sec. III concentrates on the recently discovered ferroelectric multipiezo compounds. Their technical features were evaluated for potential use of the sensing technology and new applications. Section IV subsequently concludes this perspective and provides a forward-looking assessment. The term ML denotes elasticoluminescence (elastic deformation-induced luminescence) throughout the subsequent text, unless explicitly indicated otherwise.

General dynamic of the ML mechanism of piezoelectricity-induced emission is shown in Fig. 3. According to the crystal structure and properties, three are three dynamic ML models, i.e., [(i), (ii), and (iii)].

  • Trap-controlled model; charge carriers are generated and trapped at a shallow trap (Trap1) and deep trap (Trap2) when the material is exposed to an excitation light (process ①). The trapped electrons at Trap1 can be thermally released to the conduction band and observed as the afterglow by recombination with the luminescent center. In contrast, charge carriers at Trap2 are stable and deep in the bandgap, making it difficult to thermally excite the electrons to the conduction band. Applying stress to the material creates a local piezoelectric field, causing the trapped electrons at Trap2 to be released (process ②) and recombined with the luminescent centers to give subsequent light emission (process ③).39 

  • Energy transfer model; during dynamic loading, a local piezoelectric field is generated, resulting in the piezoelectric polarization charges and the separation of the electron–hole pair (process ①). Subsequently, the partial electrons and holes can break free from the restraint, that is, the electrons are promoted to the conduction band and the holes enter the valence band. Then, electrons and holes recombine by transferring energy to the luminescent center, causing the emission (processes ② and ③).86 

  • Tunnel model; the pinning trap is stable, concentrated and time-independent, which makes it difficult for the carriers to escape. Upon the application of a mechanical load (process ①), polarization charges are induced in the material caused by the enhanced piezoelectric field during deformation. Immediately thereafter, the trapped carriers are released by the tunneling effect (process ②) and recombined with the luminescent centers with the emission (process ③).34,37,56

FIG. 3.

Schematic diagram of the ML mechanism of piezoelectricity-induced emission. (i) Trap model, (ii) energy transfer model, and (iii) tunnel model.34,37,39,56,86

FIG. 3.

Schematic diagram of the ML mechanism of piezoelectricity-induced emission. (i) Trap model, (ii) energy transfer model, and (iii) tunnel model.34,37,39,56,86

Close modal

In addition to the above-mentioned mechanism, a dislocation movement model has also been proposed, which is found to be effective for pseudoelastic materials, such as SrAl2O4:Eu.55 

The monoclinic crystal structure of strontium aluminate (SrAl2O4:Eu2+), also known as α-SrAl2O4:Eu2+ or SAOE, is characterized by the space group P21 and has been the subject of extensive research due to its exceptional ML properties.18,39,55,57–66,94 This crystal structure of staffed-tridymite is comprised of a three-dimensional network of linked corner-sharing [AlO4] tetrahedra and Sr that contributes to the stability of the structure. Sr2+ ions are relatively small compared to the large cavities in the undistorted framework [Fig. 4(a)]. As a result, they occupy two non-equivalent sites and form irregular polyhedra with the neighboring O.59,65,66 It is important to note that the lattice of α-SrAl2O4 is highly anisotropic, with thermal expansion coefficients along the a and b axes about ten times larger than that along the c axis.65 In addition, it has been demonstrated that the P21 type SAO exhibits intrinsically ferroelectric and super-elastic properties.55,64 Although there are still very few reports on the quantitative piezoelectric coefficient, future experiments and theoretical calculations for single crystals will provide more insights into this field. It has been revealed that only the ferroelectric SAOE phase demonstrates strong ML with green emission at 520 nm,57,58,64 distinguishing it from other strontium aluminates, such as SrAl12O19, Sr4Al14O25, SrAl4O7, β-SrAl2O4, and Sr3Al2O6. This correlation between the ML and crystal structure is particularly significant. At the time of the discovery of ML in SAOE, these materials were not recognized as ferroelectric systems. In 2004, SAOE was confirmed to be ferroelectric by experimental observations of P-E hysteresis loops.64 Because piezoelectricity derives from ferroelectricity, it is expected that SAOE possesses piezoelectric properties. These findings have paved the way for the targeted design of strong ML in ferroelectric materials.

FIG. 4.

(a) Crystal structure of SAOE. (b) Light emission from a single particle during the application of a small force (nano–micro N) by AFM. ML signal was observed in synchronization with the force.63 (c) In situ TEM observation of SAOE domain boundary changes with loading. The white arrowheads indicate an area where twin interfaces disappeared during loading.55 

FIG. 4.

(a) Crystal structure of SAOE. (b) Light emission from a single particle during the application of a small force (nano–micro N) by AFM. ML signal was observed in synchronization with the force.63 (c) In situ TEM observation of SAOE domain boundary changes with loading. The white arrowheads indicate an area where twin interfaces disappeared during loading.55 

Close modal

A quantitative evaluation of the emission efficiency is necessary to comprehend the fundamental characteristics of luminescent particles for practical application. The emission in the elastic deformation region was quantitatively observed from a single particle, which provides the opportunity for the use of a probe. Figure 4(b) shows the successful observation of repeated light emission from an SAOE micro- and nano-single particle upon cyclic force load by the atomic force microscopy (AFM) cantilever for the first time.61–63 To better elucidate the relationship between ML and its structure, further investigation was conducted to analyze the microstructure, phase transformation behavior, and structural changes in the SAOE crystal under load [Fig. 4(c)].55 Crystal structural changes under load were observed using an in situ TEM nanoindentation, revealing a pseudoelastic deformation in SAOE. Twin boundaries were observed before applying the load and then moved and disappeared under loading. Subsequently, upon releasing the load, the twin boundaries re-emerged. This observation indicates that the applied load influences the movement of twin boundaries, with their displacement corresponding to the load magnitude. These observations can be viewed as the manifestation of twinning pseudoelastic deformation. This creates a local electric field around the twin interfaces, which releases the trapped carriers.

Figure 5(a) shows an illustrative overview of ML technology for detecting and visualizing various mechanical stresses.67 Due to its non-destructive and non-invasive attributes, a ML film can conform to intricate surfaces, such as an artificial femoral posterior, as shown in Fig. 5(b).4,30,68 According to the scheme, the 2D stress distribution can be quantitatively assessed by analyzing the ML pattern of the artificial femoral posterior. It facilitates a comparative examination of stress distributions after the attachment of differently designed artificial joints, thereby elucidating the impact of each design on the dynamic environment of the bone, showing its potential for deployment as touch sensors or smart skin for complex architectural structures and robots. Meanwhile, it proves useful for experimental finite element method (FEM) validation. In addition, a non-destructive evaluation technique was introduced using an ML sensor to detect inner cracks within high-pressure hydrogen vessels [Fig. 5(c)].31 The ML sensor, affixed to the outer surface of the storage cylinder, successfully visualized an artificial notch on the inner surface. Notably, finite element method (FEM) calculations revealed a discernible ML pattern that depicted an equivalent strain distribution pertinent to stress concentration at the crack tip. Furthermore, the analysis of ML images facilitated the estimation of the cracks’ depth on the inner surface, thereby enabling the assessment of the device’s life. Hence, the developed ML sensor demonstrates considerable potential as a non-destructive evaluation technique for identifying inner cracks within high-pressure storage cylinders employed at hydrogen filling stations.

FIG. 5.

(a) Schematic illustration for detection and visualization of various mechanical stress.67 (b) The quantitative visualization of strain/stress distributions for designing artificial femoral posterior from an ML image and comparison with simulation.68 (i) The ML response to the load was analyzed at the ROI (①–⑧) in a pseudo color image. (c) A non-destructive evaluation technique for identifying inner cracks within high-pressure hydrogen storage cylinders and life-time evaluation using an ML sensor.31 

FIG. 5.

(a) Schematic illustration for detection and visualization of various mechanical stress.67 (b) The quantitative visualization of strain/stress distributions for designing artificial femoral posterior from an ML image and comparison with simulation.68 (i) The ML response to the load was analyzed at the ROI (①–⑧) in a pseudo color image. (c) A non-destructive evaluation technique for identifying inner cracks within high-pressure hydrogen storage cylinders and life-time evaluation using an ML sensor.31 

Close modal

Wurtzite-type ZnS has a hexagonal crystal structure with space group P63mc [Fig. 6(a), inset]. The tetrahedral coordination of Zn and S atoms defines the local structure, resulting in a uniform Zn–S bond length and nearly ideal tetrahedral bond angles. In addition, wurtzite-type ZnS exhibits piezoelectric properties due to its non-centrosymmetric structure,1,69 allowing it to generate an electric field in response to mechanical stress, which is useful in various technological applications. Indeed, P63mc type ZnS belongs to the polar point group 6 mm, and its piezoelectric coefficient d33 is 3.2 pC/N in a single crystal.64 It is crucial to note that the piezoelectric properties can be dramatically increased by doping rare earth or transition metal ions into many piezoelectric matrices, such as piezoelectric transducer [Pb(Zr,Ti)O3]. This intriguing aspect of the field keeps researchers engaged and motivated. When pressure is applied to the [ZnS4] tetrahedron, it displaces the center of gravity of the total charge, resulting in a dipole moment. Furthermore, recent research has revealed the ferroelectric behavior in ZnS:Mn quantum dots.70 

FIG. 6.

(a) SEM image of ZnS:Mn nanoparticles (inset) wurtzite-type crystal structure. (b) Response to compression stress of 500 N, which was applied by using a material test machine.1 Corresponding to the increase and release of stress, two luminescent pulses, I and I′, respectively, were generated during each cycle of applying stress, all of which were reproducible, as shown in II and II′. (c) Left: a schematic illustration of an optogenetics technique through neural modulation via ultrasound-triggered light emission from ML particles circulating in the blood circulation. Right: a schematic illustration of in vivo sono-optogenetic stimulation setup, showing the intact scalp and skull of the mouse, reproduced with permission from Wu et al. Proc. Natl. Acad. Sci. U. S. A. 116(52), 26332–26342 (2019), Copyright 2019 Author(s), licensed under a Creative Commons Attribution License 4.0 (CC BY).

FIG. 6.

(a) SEM image of ZnS:Mn nanoparticles (inset) wurtzite-type crystal structure. (b) Response to compression stress of 500 N, which was applied by using a material test machine.1 Corresponding to the increase and release of stress, two luminescent pulses, I and I′, respectively, were generated during each cycle of applying stress, all of which were reproducible, as shown in II and II′. (c) Left: a schematic illustration of an optogenetics technique through neural modulation via ultrasound-triggered light emission from ML particles circulating in the blood circulation. Right: a schematic illustration of in vivo sono-optogenetic stimulation setup, showing the intact scalp and skull of the mouse, reproduced with permission from Wu et al. Proc. Natl. Acad. Sci. U. S. A. 116(52), 26332–26342 (2019), Copyright 2019 Author(s), licensed under a Creative Commons Attribution License 4.0 (CC BY).

Close modal

ZnS is one of the first ML materials discovered at nearly the same time as SrAl2O41 and is still a hot subject of research.70–72,74–79 Transition metal ions, including Mn2+, Cu+ and co-doped Mn2+ and Cu+, have been documented as luminescent centers.71–74 The artificial skin capable of sensing mechanical stress through visible light emission is composed of a highly luminescent piezoelectric material, specifically the nanoparticles of ZnS doped with 1.5 at.% Mn [Fig. 6(a)]. The Mn acts as the luminescent center, and the film emits strong visible light when mechanical stress is applied [Fig. 6(b)]. The luminescence intensity correlates directly with the stress applied, making it a reliable indicator of mechanical forces. The light emission was also reversible and reproducible even in 100k ML cycles, ensuring a consistent performance.76 

Beyond stress-optical sensing, optogenetics, a technique employing visible light to regulate cells genetically engineered with light-gated ion channels, constitutes a potent method for precisely dissecting neural circuitry with specific neuron-subtype targeting [Fig. 6(c), left]. However, the limited tissue penetration of visible light necessitates invasive craniotomy and intracranial implantation of tethered optical fibers for in vivo optogenetic modulation. Recent research introduces ML nanoparticles designed to function as localized light sources in the brain when activated by brain-penetrant focused ultrasound (FUS) through the intact scalp and skull [Fig. 6(c), right].78 These nanoparticles can be administered into the systemic circulation via intravenous injection, recharged by 400 nm photoexcitation light in superficial blood vessels during circulation, and activated by FUS to repeatedly emit 470 nm light within the intact brain for optogenetic stimulation. In contrast to conventional “outside-in” optogenetics approaches utilizing fiber implantation, this method offers an “inside-out” strategy to deliver nanoscale light emitters through the natural circulatory system and to activate and deactivate them at any desired time and location within the brain through a minimally invasive ultrasound interface, eliminating the need for extravasation.

In addition to the simple hexagonal ZnS system, related compounds of M(Ca, Sr, Ba)ZnxSxO (x = 1, 2)-based materials have also attracted much attention.56,67,96 While we are aware of the near-infrared ML for these materials, it is now under discussion whether they intrinsically show piezoluminescence (true elastic-deformation-induced ML) or triboluminescence (triboelectricity-induced ML).34 Future development is needed to clarify relations between elastic deformation and the light emission without friction effect.

ABO3 perovskites comprise [BO6] octahedra that share corners and form a primitive cubic unit cell. Cations at site A are placed in 12-coordinated interstices and can accommodate two types of cations as long as their combined oxidation states equal 6. The adjustability of perovskite compounds, achieved by modifying the crystal structure and defect configurations through changes in chemical composition or synthesis methods, has contributed to the success of perovskites in various applications.80 LiNbO3 crystallizes within the trigonal crystal system and is classified under the space group R3c, signifying a rhombohedral lattice system with a centro-symmetric point group. The [NbO6] and [LiO6] octahedra exhibit slight distortion due to the crystal’s polar nature and the different ionic radii of Li and Nb, resulting in a distorted perovskite-type structure (Fig. 7, inset). This material demonstrates spontaneous polarization along the c axis (hexagonal axis) due to its non-centrosymmetric arrangement of a displacement of hexagonally closed-packed oxygens, resulting in a permanent dipole moment and ferroelectricity. As the R3c space group belongs to the polar point group 3m, it intrinsically has piezoelectricity from the viewpoint of the crystal structure, and its piezoelectric coefficient d33 is about 6.0 pC/N in a single crystal.95 LiNbO3 has attracted considerable interest from researchers owing to its impressive ferroelectric properties.81–83 Its substantial potential has led to extensive research on synthesizing ceramics, single crystal, nanoparticles, and films in recent decades.84,85

FIG. 7.

Non-threshold red-color piezoluminescence was achieved in multipiezo LiNbO3:Pr with incorporated pictures of the ML intensity changes with strain under the tensile test. The inset plot in the following shows the crystal structure of LiNbO3.35 

FIG. 7.

Non-threshold red-color piezoluminescence was achieved in multipiezo LiNbO3:Pr with incorporated pictures of the ML intensity changes with strain under the tensile test. The inset plot in the following shows the crystal structure of LiNbO3.35 

Close modal

Figure 7 shows a groundbreaking study on the first multipiezo material demonstrating piezoelectricity and efficient elastic ML in a single-phase LiNbO3 with a red-emitting piezoluminescence achieved by incorporating rare earth Pr3+ into the well-known piezoelectric matrix. This material shows exceptional strain sensitivity at the lowest strain level without a threshold for stress sensing. This discovery is expected to improve the dynamic scale and accuracy of ML detection significantly and will benefit bio-imaging development with single-cell resolution.35 The study emphasizes the influence of the Li/Nb ratio on the PL and ML properties. It was observed that slightly Li-rich nonstoichiometric LixNbO3 demonstrates an optimal ML performance with a red emission at 619 nm. The stress-sensing capabilities of LixNbO3 position it as a promising candidate for stress or force sensors even at micro-strain or pico-N scales. By leveraging the piezoelectric and optoelectronic properties of LiNbO3, the efficient electro-mechano-optical conversion and mutual control is highly expected.

Further studies were undertaken to enhance the performance of ML and ferroelectricity in the multipiezo material LiNbO3:Pr3+.35,43,45 It has been revealed that by controlling the Li/Na ratio, four crystal phases, LiNbO3-R3c, NaNbO3-R3c, NaNbO3P21ma, and NaNbO3-Pbma, can coexist in Li1−xNaxNbO3:Pr3+ (LNNO:Pr). The highest ML intensity and piezoelectric constant were achieved near the morphotropic phase boundary (MBP) of R3c and P21ma [Fig. 8(a)], and a prominent ferroelectric behavior was observed from P-E hysteresis loops [Fig. 8(b)]. In addition, a strong correlation between ML intensity and piezoelectric property d33 were observed [Fig. 8(c)]. The piezoluminescence reached 30 times higher than that of the LiNbO3 system with a piezoelectric coefficient d33 45 pC/N in the ceramics.43,44 This research provides valuable insights into designing and synthesizing efficient ML materials by using the MBP region. Recently, an innovative approach has been developed to quantitatively assess the stress history by analyzing the afterglow characteristics of the MBP region [Fig. 8(d)]. While it is commonly assumed that afterglow characteristics are independent of mechanical loading history, this new study revealed a strong correlation between LNNO:Pr and stress distribution following UV excitation. This correlation allowed accurate interpretation of the recorded stress from the material’s afterglow image. This unique memory retention function was attributed to the elimination of shallow trap sites, which influenced the afterglow characteristics when subjected to a mechanical load.

FIG. 8.

(a) Crystal structure changes with Na substitution in Li1−xNaxNbO3:Pr3+. (b) P-E hysteresis loops in the MPB region of Li1−xNaxNbO3:Pr3+ (LNNO:Pr). (c) Correlation between piezoelectric constant d33 of LNNO:Pr and ML intensity.43 (d) Left: read-out image of the stress history and correlation between recorded stress and recorded intensity. Right: FEM analysis result of Mises stress.45 

FIG. 8.

(a) Crystal structure changes with Na substitution in Li1−xNaxNbO3:Pr3+. (b) P-E hysteresis loops in the MPB region of Li1−xNaxNbO3:Pr3+ (LNNO:Pr). (c) Correlation between piezoelectric constant d33 of LNNO:Pr and ML intensity.43 (d) Left: read-out image of the stress history and correlation between recorded stress and recorded intensity. Right: FEM analysis result of Mises stress.45 

Close modal

Structures with the formula Srn+1SnnO3n+1 (n = 1, 2 ∞) are known as Ruddlesden–Popper structures.87 The initial investigation involved the introduction of doping Sm3+ into Srn+1SnnO3n+1 (n = 1, 2 ∞) with the CmCm space group, although they are not intrinsically piezoelectric. It was observed that the intensity of ML was closely correlated with the number of perovskite units within the layer [Fig. 9(a), left]. Under a compressive load, Sr3Sn2O7:Sm3+ (SSS) exhibited a visible orange light emission, making it a promising candidate for stress sensors [Fig. 9(b), right]. On the other hand, by controlling the syntheses process, Sr3Sn2O7 can be controlled to crystallize in the orthorhombic crystal system and belongs to the space group A21am,34,37 which is non-centrosymmetric and ferroelectric. As the A21am space group belongs to the polar point group mm2, it intrinsically has piezoelectricity from the viewpoint of the crystal structure. Although there are still very few reports on the piezoelectric coefficient, future experiments and theoretical calculations for single crystals will provide more insights into this area. As shown in Fig. 9(b), the multipiezo A21am-type Sr3Sn2O7:Nd3+ (SSN) has been successfully developed,34 demonstrating exceptional sensitivity and sustainability in converting mechanical stress into near-infrared (NIR) light [Fig. 9(b), up-right]. This pioneering study allows for stress sensing at a microstrain level, even under standard lighting conditions. SSN has exhibited significant potential for high-contrast imaging and biomechanical stress sensing in biological tissues. Its unique properties are attributed to its non-centrosymmetric structure, which enhances its ability to emit NIR light when subjected to mechanical stress, as confirmed with ferroelectric properties by analyzing the P-E hysteresis loop [Fig. 9(b), up-left]. The feature of SSN, emitting NIR light without external excitation, can be utilized to analyze stress distribution in implanted biomaterials. Furthermore, SSN can generate both NIR-I and NIR-II emissions, which can be recorded and quantitatively analyzed [Fig. 9(b), below]. On the other hand, this potential can also be used to advance the fields of on-site structural diagnosis under bright illuminated environments.

FIG. 9.

(a) Left: comparison of ML intensities for Sm3+ doped Srn+1SnnO3n+1 (n = 1, 2 ∞). The inset shows the crystal structure of Srn+1SnnO3n+1 (n = 1, 2 ∞). Right: PL and ML spectra for Sr3Sn2O7:Sm3+. The inset picture shows a typical image of ML obtained from the pellet of Sr3Sn2O:Sm3+ subjected to a compressive load,42 (b) Up-left: P-E hysteresis loops in Sr3Sn2O7:Nd3+. Up-right: NIR ML spectrum by the optical windows in biological tissues. The lower panel is an illustration of the novel ultrasensitive probe Sr3Sn2O7:Nd3+ for deep-imaging stress distributions in diverse materials and structures.34 

FIG. 9.

(a) Left: comparison of ML intensities for Sm3+ doped Srn+1SnnO3n+1 (n = 1, 2 ∞). The inset shows the crystal structure of Srn+1SnnO3n+1 (n = 1, 2 ∞). Right: PL and ML spectra for Sr3Sn2O7:Sm3+. The inset picture shows a typical image of ML obtained from the pellet of Sr3Sn2O:Sm3+ subjected to a compressive load,42 (b) Up-left: P-E hysteresis loops in Sr3Sn2O7:Nd3+. Up-right: NIR ML spectrum by the optical windows in biological tissues. The lower panel is an illustration of the novel ultrasensitive probe Sr3Sn2O7:Nd3+ for deep-imaging stress distributions in diverse materials and structures.34 

Close modal

The research of materials with ferroelectric properties, particularly those used in ML applications, has shown significant progress and potential applications across various fields. This perspective summarized the crystal structures, mechanisms, and ML behaviors of the discovered ferroelectric ML materials, represented by SrAl2O4, ZnS, LiNbO3, and Sr3Sn2O7. These multipiezo materials exhibit unique and valuable characteristics by combining mechanical force, electric field, and light for stress sensing and other applications [Fig. 10(a)]. Recent research revealed the critical role of the crystal structure, doping, and piezoelectric properties in achieving robust and reliable ML performance. Further exploration and development of the multipiezo materials could lead to breakthroughs, expanding their utility across various industries and scientific domains, developing advanced materials with unique electro-mechano-optical properties.79 The preliminary results obtained from SrAl2O4-, ZnS-, LiNbO3-, and Sr3Sn2O7-based systems provide a start point for future research to materials development, driving innovative applications and multi-scale technological advancements in stress sensing as well as health care applications [Fig. 10(b)].

FIG. 10.

(a) Summary and the possible application of the multipiezo material;37 the photo of the light-emitting fiber on the hand was reproduced with permission from Yang et al. Science 384(6691), 74–81 (2024), Copyright 2024 Author(s), some rights reserved; exclusive licensee American Association for the Advancement of Science, distributed under the terms of the Science Journals Default License. (b) The promising application of the multipiezo material used in various multi-scale targets from nm to km.10,11

FIG. 10.

(a) Summary and the possible application of the multipiezo material;37 the photo of the light-emitting fiber on the hand was reproduced with permission from Yang et al. Science 384(6691), 74–81 (2024), Copyright 2024 Author(s), some rights reserved; exclusive licensee American Association for the Advancement of Science, distributed under the terms of the Science Journals Default License. (b) The promising application of the multipiezo material used in various multi-scale targets from nm to km.10,11

Close modal

Ferroelectric materials have been attracting the interest of researchers over the past decade due to their multiple functions of electro-optics, piezoelectric, mechano-optics, and dielectric effects. Conversely, ferroelectric polarization can also modulate the luminescent properties of lanthanide-doped ferroelectrics. Many efforts have been made to study the correlation between luminescence behavior and polarization (poling).88–93 For example, the polarization and phase transition of (Ba0.77Ca0.23)TiO3 was investigated in relation with the ML intensity.88 After poling, the red emission of 1D23H4 transition was enhanced by around 30% because of the variation of the crystal field during poling. The relationship between PL spectra of Er3+-doped KNbO3 was also investigated with the poling electric field.89 The polarization of KNbO3 effectively enhanced the fine Stark splitting components of 2H11/2, 4S3/24I15/2 transitions. The modulation of upconversion PL via an electric field in epitaxial Yb3+/Er3+-doped BaTiO3 thin films was realized by 2.7 times.90 Other researchers reported that ferroelectric polarization enhanced PL from Pr3+-doped ferroelectrics in (Bi0.5Na0.5)TiO391 and Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics.92 Of particular interest is the Ba1−xCaxTiO3 (x = 0.7) ceramics, which enhanced up to 100% after poling.93 These findings have revealed that ferroelectric polarization affects the optical and structural properties in the lanthanide-doped ferroelectric materials. Therefore, the ferroelectric polarization undoubtedly affects the multipiezo characteristics. In addition, the ferroelectric properties are depending on the orientation of the crystal lattice, thereby affecting the ML response about the direction and nature of the applied mechanical stimulus. Therefore, the electric field modulation based on ferroelectrics can establish a new avenue for tunable multipiezo materials.

One of the main challenges in developing multipiezo materials, particularly ferroelectrics, is accurately predicting and controlling material properties under different conditions. The poling effect, for example, can significantly influence the multipiezo properties by aligning atomic displacement. This displacement is believed to be crucial in adjusting emission wavelengths, enhancing emission intensity, and introducing anisotropy. Achieving this could allow for more precise tuning of material properties, leading to the development of ferroelectrics with enhanced performance and tailored characteristics. This, in turn, could result in more efficient and effective applications in various technologies, such as sensors, actuators, and energy harvesting devices, ultimately pushing the boundaries of what is possible in materials science. To address these challenges, integrating advanced density functional theory (DFT) calculations with high-throughput experimental data could enable more accurate predictions of material behavior. Furthermore, advanced time-resolved in situ spectroscopy might lead to a deeper understanding of new ferroelectric materials with optimized properties. Addressing these challenges will open new avenues for research and potentially lead to groundbreaking advancements in materials science.

This research was partially supported by a Grant-in Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS KAKENHI, Grant Nos. 19H00835 and 22H00269).

The authors have no conflicts to disclose.

Tomoki Uchiyama: Writing – original draft (lead). Xu-Guang Zheng: Writing – review & editing (lead). Chao-Nan Xu: Funding acquisition (lead); Project administration (lead); Supervision (lead); Writing – review & editing (lead).

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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