Fundamental principles and materials considerations

The piezophotonic effect is a two-way coupling between piezoelectricity and photoexcitation properties as shown in Figure 1a.^{Reference Wong, Chen, Tsang, Zhang and Hao14} The piezophotonic effect uses the piezopotential to tune the energy band, carrier detrapping, and recombination and hence induces photon processes such as light emission. Apart from the two-way coupling effect mentioned, coupling between other external stimuli and the piezophotonic effect can lead to additional physical effects. As an example, magnetic-induced luminescence (MIL), shown in Figure 1a, is observed via strain-mediated coupling, which will be described in the subsequent section.

Figure 1.

(a) Schematic showing the coupling for piezophotonics and magnetic-induced luminescence.^{Reference Wong, Chen, Tsang, Zhang and Hao14} (b) Metal ions in piezophotonic materials.^{Reference Bai, Tsang and Hao6} (c) Energy diagram showing the piezophotonic effect-initiated luminescence in ZnS:Mn.^{Reference Wang, Zhang, Yu, Dong, Peng, Zhang, Zhang, Liu, Pan and Wang21} (i) Detrapping of electrons is facilitated by strain-induced piezopotential. (ii) Nonradioactive recombination occurs between these detrapped electrons and holes. (iii) Transfer of energy to Mn²⁺ ions. (iv) Excited Mn²⁺ ions fall back to the ground state with (v) the emission of visible light. (d) Ultrasonic-, impact-, tribo-, and compression-luminescence of CaZnOS:Mn²⁺ polymer films.^{Reference Zhang, Xu, Kamimura, Terasawa, Yamada and Wang12} (e) Dual-mode emission of piezophotonic luminescent and ultrasonic waves from ZnS:Mn film grown on Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMN-PT) under an AC electric field.^{Reference Zhang, Gao, Chan, Dai, Wang and Hao18} Note: ITO, indium tin oxide; E _piezo, piezoelectric potential inside the phosphor crystal lattice induced from external pressure; F, force applied to the ZnS:Mn phosphor; PMT, photomultiplier tube; CCD, charge-coupled device; ⁴T₁ and ⁶A₁, positron energy levels of Mn²⁺ ions when it doped in a II–IV semiconductor; CB, conduction band; VB, valence band; V, voltage.

Figure 1b shows commonly used luminescent ions in a periodic table.^{Reference Bai, Tsang and Hao6} Two classes of metal ions are mainly used as activators for piezophotonics, lanthanide (Ln) ions and transition-metal (TM) ions. The luminescence of Ln and TM ion-doped phosphors can cover a broad optical spectral range, including ultraviolet, visible, and infrared regions.^{Reference Bai, Tsang and Hao6} Ln ions, including Pr³⁺, Sm³⁺, co-doped Ce³⁺ and Ho³⁺, and Eu²⁺ have been used in ESL phosphors.^{Reference Botterman, Eeckhout, Baere, Poelman and Smet9,Reference Tu, Xu, Yoshida, Fujihala, Hirotsu and Zheng11,Reference Kamimura, Yamada and Xu15–Reference Liu and Xu17} Because of the larger spatial extension of the spectroscopically active d electrons, the energy-level structures of TM ions are sensitive to their coordination environment associated with external perturbations, such as mechanical strain. To date, TM ions such as Mn²⁺, Cu⁺, and co-doped Mn²⁺ and Cu⁺ have been used by different groups.^{Reference Zhang, Xu, Kamimura, Terasawa, Yamada and Wang12,Reference Wong, Chen, Tsang, Zhang and Hao14,Reference Zhang, Gao, Chan, Dai, Wang and Hao18–Reference Wang, Zhang, Yu, Dong, Peng, Zhang, Zhang, Liu, Pan and Wang21}

Using the typical example of ZnS:Mn, the fundamental principle of light emission under mechanical stress is shown in Figure 1c. An imposed stress gives rise to a piezopotential in wurtzite-structured ZnS, which tilts the bands of ZnS. The detrapped electrons at the defect states can then escape to the conduction band, followed by the nonradiative recombination of electrons and holes. The released energy excites the Mn²⁺ dopant ions, and photon emission occurs when the excited Mn²⁺ ions return to the ground state.^{Reference Wang, Zhang, Yu, Dong, Peng, Zhang, Zhang, Liu, Pan and Wang21,Reference Zhang, Jie, Chen, Liu and Hao22}

Currently, luminescent metal ion-doped hosts have been fabricated in different forms, including crystals, powders, nanowires, nanoparticles, and thin films. Figure 1d displays the ultrasonic-, impact-, tribo- and compression-luminescence of CaZnOS:Mn²⁺ polymer films under different types of stress excitation.^{Reference Zhang, Xu, Kamimura, Terasawa, Yamada and Wang12} A method of controllable piezoelectric-induced strain from Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMN-PT) has been developed, which has been used for piezophotonics. With this novel technique, dual-mode emissions can be generated and modulated precisely (Figure 1e).^{Reference Zhang, Gao, Chan, Dai, Wang and Hao18} Through dual-mode emissions, light and ultrasound waves can be simultaneously generated from a single system. One can tune the luminescence and ultrasound signal from this system via a converse piezoelectric effect in PMN-PT upon the application of either an AC or a DC electric field. Recently, Fang et al. monolithically integrated a triboelectric nanogenerator with a mechanoluminescent film.^{Reference Fang, Wang, Li, Peng, Yan and Pan23} These approaches utilize readily available ambient mechanical energy to generate either electrical or light output. The pursuit of multifunctionality has become a new important trend in recent years and such devices promote the development of smart systems to efficiently utilize mechanical energy in many ways and demonstrates great potential for a variety of applications.

Recent prototypical demonstrations and applications

Recently, there has been a trend to apply piezophotonic materials to demonstrate interesting multifunctional applications. This section briefly describes select examples of the latest research in this field. Generally, it is still difficult to predict and quantify the risk level of fracture by stress/strain concentration for fabricated structures with microstrains below 1000 μst (1 μST = 10^–6 strain) using conventional ESL materials. Therefore, it is important to develop novel materials with thresholdless high sensitivity and quantify emission intensity for microstrains.

Recently, a high-performance piezo-multifunctional material named as “multi-piezo” has been found that exhibits both piezoelectricity and efficient luminescence.^{Reference Tu, Xu, Yoshida, Fujihala, Hirotsu and Zheng11} The study was performed by doping lanthanide Pr³⁺ ions into a LiNbO₃ host by precisely tuning the Li/Nb ratio in nonstoichiometric LiNbO₃:Pr³⁺, a material that exhibits a high luminescence intensity. Figure 2a shows the light emission of a LiNbO₃:Pr³⁺ sheet during tensile strain tests. In particular, LiNbO₃:Pr³⁺ shows excellent strain sensitivity at the lowest strain levels, with no threshold for stress sensing, as shown in the inset of Figure 2a. These multiple piezoproperties are useful for multifunctional device applications. As previously discussed, it is interesting that a PMN-PT actuator used to produce piezopotential demonstrated the piezophotonic effect. In addition to the earlier study on proving the piezophotonic effect by this strategy,^{Reference Zhang, Gao, Chan, Dai, Wang and Hao18} composite-based hybrid devices have been further developed that are capable of sensing different types of external stimuli, including electric fields, uniaxial strains for stretching and mechanical writing, and piezoelectric biaxial strain (Figure 2b).^{Reference Chen, Wong, Bai, Jie and Hao24}

Figure 2.

(a) Mechanoluminescence (ML) response of the LiNbO₃:Pr³⁺ sheet. The inset shows the mechanoluminescence response of the sheet in the strain range from 0 to 300 μst, where the unit μst (microstrain) = 10^–6 strain.^{Reference Tu, Xu, Yoshida, Fujihala, Hirotsu and Zheng11} The ML curves showed excellent reproducibility during the five strain cycles. Meanwhile, LiNbO₃:Pr³⁺ exhibits a much higher piezoluminescence intensity, which far exceeds other piezoluminescence materials such as CaZnOS:Mn, ZnS:Mn, and ZnS:Cu.^{Reference Zhang, Xu, Kamimura, Terasawa, Yamada and Wang12,Reference Chen, Wong, Bai, Jie and Hao24} (b) Composite phosphors and devices for sensing different types of external stimuli. (i) Structure of the device. The inset shows a photograph of the white light emission of the device biased at 100 V and 1 kHz. (ii) (Left) Sample mounted on the stretch-release system and (right) the corresponding white light emission from the sample. (iii) (Left) Photograph of sample under mechanical writing and (right) green ML emission under mechanical writing. (iv) Schematic of the piezophotonic luminescence device.^{Reference Chen, Wong, Bai, Jie and Hao24} (c) Schematic of the pressure mapping process by the piezophotonic effect.^{Reference Wang, Ling, Zhang, Que, Peng and Pan26} Note: PET, poly(ethylene terephthalate); PDMS, poly(dimethylsiloxane); PMN-PT, Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃; V, voltage.

Zhang et al. recently presented addressable and color-tunable piezophotonic light-emitting stripes by replacing the PMN-PT crystal with piezoelectric films. Each actuator can be manipulated independently, offering an opportunity to control the light emission in a single pixel.^{Reference Chen, Zhang, Karnaushenko, Chen, Hao, Ding and Schmidt25} Interestingly, the Pan group demonstrated a pressure sensor matrix composed of poly(dimethylsiloxane) mixed with ZnS:Mn ML phosphor.^{Reference Wang, Zhang, Yu, Dong, Peng, Zhang, Zhang, Liu, Pan and Wang21,Reference Wang, Ling, Zhang, Que, Peng and Pan26} Figure 2c shows a schematic of the pressure mapping process using the piezophotonic effect.^{Reference Wang, Ling, Zhang, Que, Peng and Pan26} The dynamic pressure mapping device promises prospective applications in real-time pressure sensing and three-dimensional handwriting.

Apart from stress-optical sensing, materials have been explored for other emerging applications.^{Reference Peng, Chen and Wang27} Figure 3a illustrates a ML light source for a fluorescent probe molecule in bioimaging and phototherapy, even in the dark environment inside biotissues.^{Reference Terasaki, Zhang, Yamada and Xu28} A light source induced by ultrasonic waves was also demonstrated for photo-activation of TiO₂ photocatalyst.^{Reference Terasaki, Yamada and Xu29} Jeong et al. reported a wind-driven device that is capable of emitting warm/neutral/cool white light from Cu-doped ZnS-based composites.^{Reference Jeong, Song, Joo, Kim, Hwang, Jeong and Kim20} Such novel displays/lighting systems could pave the way to new environmentally friendly lighting, reducing energy waste and promoting sustainability (Figure 3b).

Figure 3.

(a) Mechanoluminescence (ML) light source for a fluorescent probe molecule in a potential biological application.^{Reference Terasaki, Zhang, Yamada and Xu28} Inset images are the photographs before and during application of a load on the pellet, which consists of blue ML particles of CaYAl₃O₇ with grain size of ca. 1 μm. Upper image was taken with lighting before application of the load, while the lower image was recorded during application of the load without light. The fluorescence material of FITC was excited by the ML light source. (b) Conceptual image of a wind-driven ML device that produces significant brightness and emits light over a range of tunable white color. The inset demonstrated patterned, colorful ML driven by wind.^{Reference Jeong, Song, Joo, Kim, Hwang, Jeong and Kim20} (c) Full-color display by temporal and remote tuning of piezophotonic-effect-induced color gamut from a piezo-phosphor (ZnS:Al,Cu) coupled with other phosphors. (i) The red curves indicate the instantaneous magnetic flux (B) though the magnetic-induced luminescence (MIL) sample in (left) low and (right) high frequency regimes. Donor (V_s,Al_Zn) and acceptor (Cu_Zn) states are created when specific metal ions are doped into the host piezophosphor lattice. Under a strain-induced piezo-potential, electron detrapping occurs, and electrons recombine with holes in the Cu_Zn acceptor levels, resulting in MIL emission. (ii) Photographs of white emission with tunable color temperature (CCT) under different modulation frequencies of the magnetic field from a MIL composite mixed with cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor. (iii) The red and green MIL emission of a multicolor MIL flexible composite combined with (Ca_x Sr_1−x)S:Eu displaying the logo of The Hong Kong Polytechnic University. (iv) A display of red-green blue MIL emission.^{Reference Wong, Chen, Bai, Huang and Hao31} Note: CCT, correlated color temperature; FITC, fluorescein isothiocyanate isomer-I.

Compared to PL and EL, the realization of light emission stimulated by magnetic fields faces a great challenge and the phenomenon of intrinsic MIL is extremely rare. The Hao group first proposed a strategy to observe MIL by coupling the magnetic effect to the piezo-photonic effect,^{Reference Wong, Chen, Tsang, Zhang and Hao14,Reference Graydon30} as shown in Figure 1a. Most recently, remote and temporal tuning of luminescence was also demonstrated. Tuning is achieved by modulating the frequency of magnetic-field excitation at room temperature, where the emission spectrum from the phosphors in magnetic sensitive flexible composites can be reversibly controlled in real time and in situ. Based on this intriguing property, proof-of-concept devices, including tunable white-light and red-green-blue full-color displays, were fabricated, and they demonstrated promising opportunities for numerous applications, as shown in Figure 3c.^{Reference Wong, Chen, Bai, Huang and Hao31}