Triboluminescence: Materials, Properties, and Applications

Triboluminescence is one of the types of luminescence that could be activated by mechanical stress. Considering the rising research efforts and achievements in recent years, this chapter provides an overview on the study of triboluminescence. The first part gives a background description regarding the history, research status, and advantages of triboluminescence. Then, we summarize the material systems for triboluminescence in both organics and inorganics. In the third part, we review the properties of triboluminescence, particularly on the unique characteristics and their improvements. Finally, we give a comprehensive summary on the developments of triboluminescent devices for applications in various fields in terms of mechanical engineering, energy, biological monitoring, and sensors as well as lighting, imaging, and displaying.


Introduction
Triboluminescence (TL) refers to the phenomenon that materials could emit light when they are mechanically stimulated, such as rubbing, grinding, impact, stretching, and compression [1][2][3]. TL was first recorded by Francis Bacon in 1605 when breaking the sugar crystals [4]. After that, TL has been found in many solids, such as rocks, quartz, alkaline halide, molecular crystals, and some organic materials [5]. It is estimated that nearly 50% of inorganic compounds and 30% of organic molecular solids have been confirmed to have TL [6]. Because TL could be directly activated by the widely existed mechanical activities in daily life without requiring artificial optical/electrical sources, TL shows great advantages in energy saving and environmental protection [7].
In general, TL could be classified into three types, i.e., fracture TL, plastic TL, and elastic TL [8], as illustrated in Figure 1. Among them, the elastic TL has gained the most attention because of its structure nondestructive characteristic which is crucial for practical applications. The present researches of TL are mainly focused on the development of novel TL materials and the performance improvement in terms of brightness, color manipulation, and cyclic stability [9][10][11]. Based on the efforts in the above aspects, a variety of decent applications of TL materials have been achieved in recent years, covering the fields of mechanical engineering, energy, biological monitoring, and sensors as well as lighting, imaging, and displaying.
In this chapter, we provide an overview of TL, regarding the materials, properties, and applications. Since TL covers a large range from organics to inorganics with emitting types from fracture TL to plastic TL and elastic TL, most of the content of the chapter was focused on the elastic TL of inorganic solids in which the most significant progresses have been made during recent years. We hope that this chapter could provide a deep understanding of TL and stimulated new ideas for further researches.
Organometallic compounds, including rare earth and transition metal complexes, have also featured TL. The typical examples are some β-diketone complexes of Ln III ion (Ln = Eu, Sm, Pr, Yb, Tb, Gd, or Nb). Among them, the europium complexes (EuD 4 TEA and its doped forms) generate extremely bright and daylightvisible red-orange TL, which is much stronger than that of the others [19]. But these  complexes show a very sharp emission band corresponding to the f-f transition of Eu 3+ ions. The transition metal-based complexes are mainly Mn II , copper I , and Pt II complexes, such as Mn(Ph 3 PO) 2 X 2 (X = Cl, Br), (MePh 3 P) 2 MnCl 4 , Cu(NCS) (py) 2 (PPh 3 ), and Pt(ipyim)(bipz), which give a broad emission band [20].

Spectral characteristics
In many organic and inorganic systems, the TL spectra are consistent with the PL spectra, suggesting they possess the same emitting processes. The differences between TL and PL lie in the excitation/activation processes that TL originates from the release of the trapped carries or the piezoelectric effect under mechanical stimuli. In some systems, like BaZnOS:Mn 2+ [25], the compression-induced TL and rubbing-induced TL exhibit 24 nm and 48 nm blueshift, respectively, compared to that of PL (Figure 2). Such phenomenon could be ascribed to the conduction band and valence band tailoring by piezoelectric fields.
In piezoelectric materials, there is also obvious difference on the concentration quenching between PL and TL. For example, the quenching concentrations of Pr 3+ in CaNb 2 O 6 , Ca 2 Nb 2 O 7 , and Ca 3 Nb 2 O 8 for TL are 0.25 mol%, 0.1 mol%, and 0.075 mol%, respectively, while the values for PL are 0.5 mol%, 0.3 mol%, and 0.1 mol%, respectively [31]. The decreased quenching concentration of TL was attributed to the participation of piezoelectric field in delivering the energy from traps to quenching centers.

Cycling stability
The TL of organic molecules or complexes mostly originates from the fracture of crystals, and thus there is no cycling stability for such materials. For the TL along with the nondestructive structure, mainly referring to the piezoelectric effect and de-trapping-induced TL, the cycling stability is particularly important. The TL aroused by piezoelectric effects usually exhibits stable luminescence when activated by cyclic mechanical tests [37]. For example, the Pr 3+ -doped LiNbO 3 could keep its TL intensity for more than 100 cycles [32]. ZnS:Cu/PDMS composites could maintain the TL intensity up to 30,000 cycles of stretching, and the intensity still reached 65% of the initial one without a color change even after 100,000 cycles of tests [38]. However, for the TL aroused by the de-trapping of carriers in structure, intensity degradation would be serious during cycling tests, i.e., such materials showed poor cycling stability [39,40]. To overcome the above issue, great efforts have been made based on the TL mechanism in terms of the de-trapping processes. Researchers proposed a strategy to improve the cycling stability of the de-trappinginduced TL by applying an extra UV irradiation source to ensure the balance between the trapping and de-trapping of carries [11]. The power density played a key role to stabilize the TL intensity, and the effective power density was determined to be 1000 mW/cm 2 as shown in Figure 3 [41].

Intrinsic structure-dependent TL
The TL characteristics could be directly modulated by varying the concentration of luminescent centers. Generally, there is a concentration quenching phenomenon in terms of TL intensity. In CaZnOS:Mn 2+ , the increase of the doping concentration of Mn 2+ could not only vary the TL intensity with a trend that increases first and then decreases but also arouse a redshift on the TL spectra with the emitting color manipulated from orange to red [10]. In addition, the chemical composition of the hosts, namely, the variation of the defect phases or traps, could also cause significant variations on the TL intensity and color. In (Ba,Ca)TiO 3 :Pr 3+ , the co-dopant of trivalent rare earth ions, such as La 3+ , Y 3+ , Nd 3+ , Gd 3+ , Yb 3+ , and Lu 3+ , could greatly improve the TL intensity, in which Gd 3+ could enhance the intensity more than 61% [30]. This is because that the co-dopant of the above ions could increase the concentration of the carries in traps and thus lead to more luminescence emitted under mechanical stimuli. In Sr 2 MgSi 2 O 7 :Eu 2+ , when part of Sr 2+ was substituted by Ca 2+ or Ba 2+ , the TL intensity and emitting color could be adjusted simultaneously [26]. SrBaMgSi 2 O 7 :Eu 2+ showed the lowest TL intensity compared to that of Sr 2 MgSi 2 O 7 :Eu 2+ and SrCaMgSi 2 O 7 :Eu 2+ . The replacement of Sr 2+ by Ca 2+ or Ba 2+ in Sr 2 MgSi 2 O 7 :Eu 2+ could further manipulate the emission band in a wide range from 440 nm to 499 nm. Researches also showed that the TL performance is dependent on the crystal size. In sucrose crystals, the TL intensity significantly increased with increasing crystal size (Figure 4), which could be explained by piezoelectric mechanism [42].

External factor-dependent TL
The TL powders could be directly stimulated by ultrasonication or impact. The ultrasonic TL is dependent on the ultrasonic power with a linear relationship [43]. The impact-induced TL is strongly affected by the impact velocity or impact energy [2,44]. When TL powders were composited in various matrices, other mechanical  TL integrated intensity on dependent of particle sizes of sucrose crystals. Reproduced by permission of the American Chemical Society [42].
actions, such as rubbing, stretching, and compression, would be employed for TL. The intensity of the rubbing-induced TL shows relationships to both the applied normal load and the friction velocity [43]. For the stretching-induced TL, the elastic modulus plays a key role on the critical strain [45]. In addition, TL intensity varies along with the change of strain levels and stretching speeds [7]. For the compression-induced TL, it depends on the applied load as well as the deformation rate [31,37].

Applications
TL materials could be composited in a variety of hosts, such as polymer matrices and metal bulk materials. The as-fabricated TL composites could emit light under the stimulus of mechanical behaviors for various applications. Because SrAl 2 O 4 :Eu 2+ and ZnS:Mn 2+ /Cu + are the well-recognized intense TL materials, the present applications almost focus on them.

Structural health monitoring
The TL composites could be directly stimulated by the inner stress, showing application perspectives in structural health monitoring of devices, machines, and buildings [46][47][48]. To date, TL materials have been well employed to visualize and monitor the stress distribution as well as the fatigue crack initiation and propagation of matrices [1,49,50]. The sensitization of stress distribution in solids was first conducted by C-N. Xu et al. [1] They composited the green-emitting SrAl 2 O 4 :Eu 2+ TL powders in epoxy resins and confirmed that the TL behaviors of the SrAl 2 O 4 :Eu 2+ /epoxy composites under a compressive load of 1000 N could reflect the stress distribution based on the experimental and simulative results. The SrAl 2 O 4 :Eu 2+ /epoxy composites were further employed to realize the measurements of instantaneous R-curves and bridging stress in a fast-propagating crack system ( Figure 5) [51].
Based on the above pioneering achievements, researchers successfully developed the structural health monitoring applications of TL materials in steel box girders [52], hydrogen storage cylinders [53], and gas pipelines [54]. Compared with the conventional monitoring methods by electrical and magnetic signals, the approach by TL signals shows advantages of contactless, wireless, convenient, and visualization [49,52].

Impact/load sensor
When TL materials undergo loading or impact, the emitted luminescence shows one-to-one correspondence between the emission intensity and impact/loading energy, which could be utilized to develop impact/load sensors to record the related mechanical information [2]. However, for the sensors fabricated from SrAl 2 O 4 :Eu 2+ , the prominent problem is that the TL intensity will be decreased along with the increase of impact times or loading time, i.e., SrAl 2 O 4 :Eu 2+ shows poor cycling stability that goes against for its applications as impact/load sensors [41,44]. Researchers further found that when an ultraviolet (UV) irradiation source with a certain power density was applied, SrAl 2 O 4 :Eu 2+ could keep the TL intensity stably based on the balance of trapping and de-trapping of the carriers in structure [41,44]. The proposed SrAl 2 O 4 :Eu 2+ -based sensor under UV irradiation could stably sensitize the applied load both in dynamic and static states (Figure 6) [41]. Differing from SrAl 2 O 4 :Eu 2+ , ZnS:Mn/Cu showed almost no TL intensity degradation along with the increase of cycle numbers because of the piezoelectric effect, which could be directly used for impact/load sensor applications without needing an extra UV irradiation [37,55].

Lighting, imaging, and displaying
The exploited devices for lighting, imaging, and displaying are mainly fabricated from ZnS:Mn/Cu and elastomer matrices. The as-fabricated ZnS:Cu/PDMS flexible composites showed bright and durable TL under stretching with a brightness of ca. 120 cd/m 2 and durability over 10,000 cycles [38]. The composites could be further fabricated into fabrics with patterns that could be applied for imaging and displaying as presented in Figure 7 [56].
In addition to the stimulus of stretching and rubbing, the TL composites could also be activated by various mechanical sources, such as wind [5], magnetic field [57], and ultrasonic wave [58], which fulfill the requirements of green and sustainable developments. For practical applications in lighting and displaying, the TL flexible devices with a white light or multicolored emissions are required, and a variety of strategies have been proposed. For example, Jeong et al. employed ZnS:Cu, Mn and ZnS:Cu as the orange and green TL materials, respectively, and fabricated ZnS-based flexible composites, in which TL color manipulation including a warm white light was demonstrated by adjusting the component ratios of Figure 6 Load responsiveness of SAOED in static and dynamic states with UV irradiation turned on and turned off. Reproduced by permission of the OSA Publishing [41].
ZnS:Cu, Mn and ZnS:Cu [36]. They further presented a strategy for the TL color manipulation of doped ZnS by physically combining fluorescent dyes in PDMS elastomers based on the energy transfer between the TL of doped ZnS and the PL of dyes [59]. Hao and his co-workers also realized the remote tuning of TL color of ZnS:Al, Cu/PDMS composites by modulating the frequency of magnetic field [60]. In addition, flexible devices with dual-mode emissions, i.e., EL and TL, have also been developed for imaging and displaying [59,61].

Pressure sensor
The TL flexible composites exhibit luminescent signals dependent on the applied pressure. Based on such performance, Wang et al. developed a ZnS:Mnbased pressure sensor for both single-point dynamic pressure recording and 2D planar pressure mapping with a high spatial resolution of 100 μm and a fast response time less than 10 ms [24]. The pressure sensor was further used as a flexible handwriting device that could collect the information of both signatures and signing habits as shown in Figure 8, exhibiting high-level security compared with the existing technologies. They further introduced the single-electrode triboelectric nanogenerator in the ZnS:Mn-based flexible composites and obtained a full dynamic-range pressure sensor for the visualization of pressure distribution both in low pressure regimes (< 100 kPa) and high-pressure regimes (> 1 MPa) with an excellent pressure sensitivity of 6 MPa −1 [62]. In addition, CaZnOS:Er 3+ thin-film was prepared, which possessed the pressure and temperature sensing based on its TL and upconversion luminescence [63]. [56].

Stress/strain sensor
When the TL materials are introduced in elastic matrices, stress/strain sensor could be obtained. At present, the widely employed TL materials for fabricating stress/strain sensors are ZnS:Mn, ZnS:Cu, and SrAl 2 O 4 :Eu, because of their prominent TL properties as well as the one-to-one correspondence between the TL intensity and stress/strain. Yun et al. [64] further found that the co-dopant of Dy 3+ in SrAl 2 O 4 :Eu could improve its performance as stress sensor based on the sensitivity. In addition to sense the stress or strain by analyzing the TL intensity, the risetime and decay time of TL during cyclic elastic deformation of SrAl 2 O 4 :Eu were also demonstrated to be suitable for evaluating the change of the strain energy [65]. Moreover, a calibration method for SrAl 2 O 4 :Eu, Dy-based thin-film sensor was proposed to enable quantitative full-field strain measurements in pixel-level resolution [66]. Qian et al. [45] prepared ZnS:Mn/Cu@Al 2 O 3 /PDMS flexible composites and adjusted the elastic modulus by introducing SiO 2 nanoparticles. They finally obtained a TL stress/strain sensor that could be driven by weak mechanics of skin movements.
In the very recent work [7], Sr 3 Al 2 O 6 :Eu with bright and tunable PL and TL was presented when it was composited in PDMS elastomers. By combining the wavelength selectivity of PL and dynamic stress responsiveness of TL, a multimode stretching/strain sensor was developed by a bilayered structure design of Sr 3 Al 2 O 6 :Eu/PDMS composites with coating a light-shielding layer of Au atop (as shown in Figure 9). The fabricated sensor could sense the stretching states and strain levels simultaneously, breaking the limit of static strain sensing in previous researches.

Mechanics-light-electricity conversion
The TL materials could convert mechanics into light, which could be further utilized to generate electricity for various applications. When the SrAl 2 O 4 :Eu/epoxy TL composites were combined in a commercial silicon solar cell, the mechanicslight-electricity conversion could be achieved [67]. In addition to the generation of electricity by utilizing the mechanics-induced luminescence, TL materials could be combined with a nanogenerator and convert the input mechanical stimuli to electric and light simultaneously [68]. The TL materials could also be composited with a photocatalyst to realize the catalysis activity in dark under the stimuli of mechanics [69]. The above conversion systems based on TL show great perspectives for applications in dark environments, such as deep sea and polar night region.

Biological applications
Because some of the TL materials show good biocompatibility, such as the rare earth-doped oxide ceramics, they are promising for the detection of mechanical behaviors in biological tissues/organs. The SrAl 2 O 4 :Eu TL powders was applied in the synthetic bone, and the related mechanical dynamic environment was monitored with a high-definition and high-speed visualization [70]. SAOED powders were also applied in artificial tooth for occlusal examination [71]. The composition of SAOED in the commercial denture base resin (DBR) could not only endow with bright TL but also improve its mechanical performance. As a result, an artificial tooth model with SAOED was made in which bright and sensitive TL could be directly observed to guide clinicians to purposefully adjust the occlusal surface until a balanced occlusion established.

Conclusions
In summary, we present a comprehensive overview on the study of TL. The material systems in both organics and inorganics, unique spectral characteristics, and TL performance, as well as the representative applications in various fields, are included. We hope that this chapter could help researchers in the field to gain a comprehensive and in-depth understanding of TL and stimulate continued interests and endeavors in this area to promote more innovative applications.