Magnetic‐Force‐Induced‐Luminescent Effect in Flexible ZnS:Cu/PDMS/NdFeB Composite

The force‐induced light‐emitting phenomenon in polymer composites plays an important role in the soft electronic field due to its display function. Here, a magnetic‐force‐induced‐luminescence (MFIL) effect is reported in ZnS:Cu particle‐doped polydimethylsiloxane incorporated with a NdFeB magnetic tip mass for real‐time incident magnetic field strength light‐emitting display. Investigations show that the luminescence intensity increases nearly linear in response to the applied AC magnetic field, HAC; meanwhile, the minimum HAC for inducing MFIL is as low as 0.1 mT (1 Oe) at the resonance. The MFIL effect is 1000 times better and more energy‐efficient than the best result published previously. The findings, thus, indicate that the MFIL effect could serve as an effective method for light‐emitting display triggered by HAC; MFIL essentially originates from the donor–acceptor recombination between shallow donor level and the t2 level of Cu2 in ZnS:Cu semiconductor particles. The present results could, thus, provide a viable pathway toward multifunctional flexible electronic designs and applications, especially toward those for the real‐time visualization of remote magnetic field sensing.


Introduction
Normally, magnetic sensing is a conversion process of magnetic field input and electric signal output based on Hall, electromagnetic induction, magnetoresistive, magnetoelectric effects, etc. [1] Conventional magnetic meters are also mechanically rigid and bulk; and therefore, it is difficult to be integrated into a flexible microsystem. [2] Soft electronic devices with self-powered function have emerged as new technologies, which go beyond the boundaries and limitations of conventional rigid devices; so they are more suitable for wearable and implantable applications. [3] www.advmatinterfaces.de flexible PDMS polymer matrix embedded with phosphor semiconductor microparticles has the features of superior flexible/ stretchable characteristics without any loss of luminescence functionality. [9] In particular, Cu-doped ZnS (ZnS:Cu) phosphors semiconductor also possess an excellent piezoelectric nature, which means that under a small external stress, it can generate an internal electric field. On the other hand, the internal electric field induced piezoelectric potential is capable of triggering the luminescent center of metal-ion dopants inside, leading to the light emissions in the flexible PDMS composites embedded with metal-ion-doped ZnS microparticles. [10] The ZnS:Cu particles also display repetitive luminescence under mechanical stress over one hundred-thousand cycles without requiring additional treatment. [11] Electroluminescence coupled with piezoelectric potential in this material allows us to tune the PL emission through energy band engineering. In other words, the intensity of light emission is a direct result of the internal piezoelectric potential that creates a tilted energy band.
According to the type of excitation source, it is well-known that luminescence can be classified as photoluminescence (PL), electroluminescence, ML, triboelectrification-induced electroluminescence (TIEL), and magnetic-induced luminescence (MIL). For instance, white and green light emissions were achieved under different external stimuli, including electric field, uniaxial strains of stretch and mechanical writing, and piezoelectric biaxial strain. [8c] TIEL of the ZnS:Cu due to the triboelectric leakage field was introduced via a gently horizontal sliding between a ZnS:Cu particle-doped PDMS film and a polytetrafluoroethylene, which could be used in advanced optoelectronic devices. [10c,e] Wong et al. reported the MIL phenomenon based on a double-layered composite: one layer consisted of metal-iondoped ZnS microparticles in PDMS (served as a luminescence layer); the other one consisted of magnetic Fe-Co-Ni alloy particles embedded PDMS matrix (serving as a magnetic actuation layer). In response to a switching alternative magnetic field (H AC ) in the range of 0-3.5 kOe, the MIL intensity (≈509 nm) is enlarged; then it tends to its saturated value. However, the minimum field for triggering the MIL is as high as ≈2000 Oe; this poor luminescence response to incident magnetic field apparently hinders its potential for practical real time magnetic field detection. [12] It would be highly beneficial for future magnetic systems if a high-sensitivity or high energy-efficient magneticinduced-luminescence device is conceived.
In this work, we introduce a novel magnetic-force-inducedluminescence (MFIL) coupling effect that occurs among magnetic, stress, electric and luminescence subsystems, as shown in Figure 1. This proposed MFIL coupling effect is based on a flexible doped ZnS:Cu microparticle/PDMS composite film (belt) incorporated with a NdFeB magnetic tip mass. In order to enhance the luminescence response, the composite belt was clamped at its two ends. In response to an external magnetic field applied perpendicularly to the magnetization direction of the magnetic tip mass, a bending vibration mode of ZnS:Cu-PDMS composite belt can be excited (due to the magnetic force effect of the tip mass), which causes a stress applied to ZnS:Cu particles; correspondingly, the magnetic force induced piezopotential results in a tilted energy band in ZnS:Cu particle semiconductor. Finally, electrons are detrapped from the conduction band and recombined to emit light on the impurity state of Cu 2 . The designed system exhibits a great MFIL effect even under 0.1 mT (≈1 Oe) AC magnetic field excitation. In contrast to conventional magnetic sensors, the proposed flexible MFIL-based device opens the possibility to perform a realtime, remote, self-powered visualization magnetic sensing without making an electric contact.

Results and Discussion
Figure 2b schematically shows the setup of the proposed MFIL system. A standard Helmholtz coil driven by an AC current supply was used to generate the excitation magnetic field. The MFIL system includes a flexible, doped ZnS:Cu particle embedded PDMS composite film and a NdFeB magnetic tip mass attached at the center of the composite film. In order

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to effectively realize the conversion of the incident magnetic energy to mechanical stress, both ends of the composite film were fixed. The attached magnetic tip mass experiences a large deflection due to the magnetic torque effect under a small AC magnetic field excitation, which results in a bending vibration mode of the ZnS:Cu/PDMS composite film; further, it causes the large deformation of the film. Finally, the magnetic-forceinduced-luminescence phenomenon was observed due to the deformation/strain induced stress. The generated light was recorded by an optical fiber spectrometer, as illustrated in Figure 2b. The whole setup is available in Figure S1 in the Supporting Information. The optical fiber could collect the emission light and directs it to a spectrometer for spectrum analysis.
Clearly, the MFIL effect is a product effect of the magnetic/ mechanical effect in the magnetic phase, mechanical/electric effect and then followed by the electric/luminescent effect in a two-phase composite of ZnS:Cu/PDMS film attached with magnetic tip mass

MFIL effect Magnetic Mechanical
Mechanical Electric (1) Figure 2ci shows the photograph of the proposed MFIL system. A magnetic torque generated by tip magnet (NdFeB) could induce a bending vibration mode of the film, which would result in emitting luminescent light, as shown in Figure 2cii.
To investigate the MFIL effect, we have synthesized the twophase composite consisting of Cu-doped ZnS microparticles dispersed in a PDMS matrix by a spin coating technique and an attached magnetic tip mass. Experimental results showed that the photoluminescent intensity was enhanced with increasing the volume content of ZnS:Cu and it reaches to its maximum at the weight ratio 7:3 of ZnS:Cu/PDMS, which is available in Figure S2 in the Supporting Information. Figure 3a,b illustrates the scanning electron microscopy (SEM) images of the microparticle and its resultant composite. It was found that the ZnS:Cu phosphor microparticles exhibited with irregular and spherical shapes with sizes ranging from 10 to 70 µm and an average size of 28 µm, as presented in Figure 3a,c. The image of the top surface of the composite revealed that the microparticles were well dispersed and wrapped in the PDMS matrix (as shown in Figure 3b). The X-ray diffraction (XRD) pattern clearly showed that all the diffraction peaks of the composite were similar to the wurtzite-type ZnS pattern (Joint Committee on Powder Diffraction Standards (JCPDS) #36-1450). The three peaks are corresponded to the (002), (110), and (112) lattice planes, respectively, and the characteristic peaks of copper impurity (such as CuS and CuO) were also observed. To evaluate the luminescence characteristics, we constructed a measurement setup, as shown in Figure 2c.
The peak of luminescent spectrum was observed to locate at 512 nm, which is corresponded to the Commission Internationale de Eclairage (CIE) coordinates of (0.2025, 0.4963) (as shown in Figure 3e,f). The narrow emission spectrum suggested that the Cu dopant levels could provide an effective radiative recombination path for the excitations.
To further elucidate the mechanism of the magnetic-forceinduced-luminescence phenomenon, the finite element method software Comsol was employed to simulate the stress-strain distribution and deformation of the PDMS composite film when it was clamped at both ends, as can be seen in Figure 4a. By applying an external AC magnetic field, H AC , perpendicular to the magnetization direction of the NdFeB tip mass, a magnetic force torque (moment) could be generated, resulting in

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an in-plane elastic deformation of the composite film parallel to the imposed stress, as presented in Figure 4b,c. The secondorder intrinsic vibration mode is given in Figure 4c through finite element modeling (COMSOL Multiphysics). It is worth noting that the average young's modulus of ZnS:Cu/PDMS composite was around 7.5 MPa; therefore, it had a high enough elasticity to bear a large elastic deformation. The stress-strain curve of the ZnS:Cu/PDMS composite is available in Figure S3 in the Supporting Information. The film tended to experience a stress in Cu-doped ZnS-PDMS composite film due to bending vibration. Once this mechanical stress was coherently transferred to the Cu-doped ZnS semiconductor particle inside the flexible PDMS composite film, it induced a piezopotential inside the semiconductor due to the piezoelectric effect. Note that formation of electric dipoles in ZnS semiconductor particle could be influenced by Cu-dopant element, which would induce a local electric field, and the ionized Cu 2+ could act as the luminescent center. The magnetic-force-torque induced local piezopotential could tilt the energy band of the ZnS:Cu semiconductor. Figure 4di,ii illustrates the schematic energy level diagrams of the Cu-doped ZnS semiconductor. The sulfur vacancies in the ZnS compound could build two energy levels: 1) shallow donor levels by trapping electrons and 2) acceptor t 2 level of Cu 2+ by trapping holes. This effect would result in trapping of electrons from the valance band to the impurity shallow donor level. The trapped electrons at the impurityinduced shallow donor levels are most likely to be recombined with the acceptors (holes) in the t 2 level of Cu 2+ under applied AC magnetic field H AC . Therefore, the observed green emission originates from the carrier recombination between the shallow donor level (sulfur vacancy) and the t 2 level of Cu 2+ ions.
The photoluminescent intensities could be governed by two factors: i) piezopotential under applied H AC and ii) ionization center of impurity ions (like Cu dopant). By increasing the magnetic input force, the MFIL system could experience a greater stress-strain behavior, leading to the excited photoluminescent effect due to the enhanced piezopotential.
This research demonstrates that an MFIL system consisting of ZnS:Cu microparticles embedded PDMS polymer and an incorporated NdFeB magnetic tip mass could exhibit much better luminescence responses to a small, varying external magnetic field H AC . In this system, besides of flexibility, optical transparency, and chemical stability, the PDMS polymer also exhibits excellent elasticity character to deliver the stress-strain generated by the magnetic force torque of NdFeB to the Cudoped ZnS microparticles for producing green luminescence under applied H AC . A detailed torque effect on a magnet bar placed in a uniform magnetic field and the loading profile of both ends clamped film are available in Figures S4 and S5 in the Supporting Information.
During measurements, the Cu doped ZnS/PDMS composite film (belt) was clamped at its both ends and a NdFeB magnet with the optimized mass was attached at the center of the composite belt for generating a magnetic force moment under a weak H AC . Because the PDMS composite belt is quite soft with constant compliance coefficient, the resonant frequency of the MFIL system is mainly determined by the magnetic tip mass. The NdFeB magnetic material is widely used in magnetic field energy harvesting, because of its giant coercive and high saturation magnetization properties. Figure 5a shows the luminescence spectra obtained from the Cu-doped ZnS-PDMS composite belt attached with a tip magnetic mass of 3.4 g as a function of the excitation frequency of AC magnetic field with an amplitude of H AC = 0.7 mT. Clearly, the luminescence intensity of the MFIL system was proportional to the operating frequency. It can be clearly seen from

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Figure 5a that luminescence signal increased as a function of frequency, reaching to its maximum at the frequency of 10 Hz, which was the second-order resonance frequency of the MFIL system.
In order to evaluate luminescence response to H AC , we measured luminescence sensitivity response to a small AC magnetic field variation (0-0.7 mT) at its resonance frequency of 10 Hz, as shown in Figure 5b. MFIL peak appeared at the wavelength of 512 nm. With further increase of AC magnetic frequency, it then gradually diminished to zero, which could be attributed to the hysteresis effect of the bulk magnetic mass. However, MFIL increased as the enhancement of magnetic field strength, H AC , reaching to its saturated value at H AC = 0.7 mT, as shown in Figure 5bi,ii. It was also observed that the MFIL system was capable of responding to the weak AC magnetic field variation of ∆H AC as low as 0.1 mT at the resonance frequency of 10 Hz, which could be regarded as the best results ever reported. When the composite belt was clamped at its two ends, only second-order bending mode of the MFIL system can be exited (which was around 10 Hz) under H AC ; when the applied frequency matches the natural frequency of second-order bending mode, the impedance and damping of the MFIL system would be minimum, and the deflection and deformation of the MFIL system will become the maximum. It then will decrease when the frequency exceeds the natural resonance frequency. The frequency region outside the natural resonance is called as off-resonance, where it tends to vibrate with a lower amplitude.
The amount of generated deformation directly depends on the intensity of incident magnetic field working at the resonance and the magnetic torque force of the tip mass transferred into the polymer matrix. As illustrated in Figure 5c, the relationship between magnetic field strength and luminescence in the composite can be approximately assigned as a linear behavior.

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By transferring the greater magnetic torque effect into the elastomeric polymer matrix, it could cause a larger deformation and strain, inducing a larger stress applied to the metal-iondoped ZnS microparticles, subsequently generating luminescent light due to the electron-hole recombination between the shallow donor level and the t 2 acceptor level of Cu 2 . This can also be called "magnetic-piezo-photonic effect," which makes this novel MFIL system very promising for real-time magnetic sensing visualization, as well as flexible remote magneto-optic sensing devices without making an electric contact.
To summarize, in this study, a magnetic-force-inducedluminescence (MFIL), i.e., a magnetic-piezo-photonic coupling effect, was investigated based on Cu-ion-doped ZnS microparticles embedded in a transparent PDMS elastomer incorporated with a tip NdFeB magnet mass in this work. The prepared PDMS composite exhibited high flexibility and excellent MFIL performance under a magnetic field excitation. The strainstress triggered by the tip magnetic torque effect could create an internal piezoelectric potential in the Cu-ion-doped ZnS semiconductor microparticles, leading to a tilted energy band inside, which further induces the electron-hole charge carriers detrapping and then recombination between the shallow donor level and the t 2 acceptor level of Cu 2 ; this, in turn, could lead to the induced luminescence as a response to the applied alternative magnetic field.
Investigations further showed that the ZnS:Cu embedded PDMS composite had a high AC magnetic-field sensitivity of 0.1 mT at the resonance of 10 Hz at room temperature, which was at least one thousand times higher than the best result published before based on Magnetic-Induced Luminescence sensor. The proposed MFIL system, therefore, opens up a new approach for real-time magnetic sensing visualization and flexible remote magneto-optic sensing devices without making electric contact.

Experimental Section
ZnS:Cu microparticles were supplied by Shanghai Phosphor Technology Co., Ltd. Polydimethylsiloxane (Sylgard 184 Silicone Elastomer) with a crosslinker prepared from Dow Corning. The fabrication process of the ZnS:Cu-PDMS composite film/band is schematically illustrated in Figure 2a. Dow Corning 184 and curing agent were poured into a beaker, comprising a base prepolymer and a crosslinking agent (with a weight ratio of 10:1). ZnS:Cu microparticles with the average size of 23 µm were then uniformly mixed in PDMS with a weight ratio of 7:3. After ultrasonication of the solution consisting of ZnS:Cu-PDMS, it was spin coated on a precleaned glass substrate at 500 revolutions per minute (RPM) for 60 s. The mixture process of the PDMS base and curing agent could often lead to air bubbles within the prepolymer due to its chemical reaction. To remove the bubbles, the film was placed into the vacuum chamber for about 10 min. The last step was to cure the composite film at 70 °C for 2 h.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.