Mechanically controlled reversible photoluminescence response in all-inorganic flexible transparent ferroelectric/mica heterostructures

The ability to reversibly control the luminescent properties of functional materials with diverse external stimuli, such as an electric field, strain, and temperature, is crucial for designing high-performance optical devices. Here, we demonstrate that a purely mechanical strain in a flexible mica substrate triggered by bending can be used to dramatically modify the photoluminescence response of a Pr-doped Ba0.85Ca0.15Ti0.9Zr0.1O3 epitaxial thin film in a stable and repeatable manner with a large gauge factor of up to 6853. The strong dependence of the photoluminescence performance on the mechanical bending arises from strain-induced variations in the lattice symmetry of the host film and the local crystal field around the Pr3+. In particular, because of the nature of mica, the film structure exhibits excellent antifatigue characteristics after 104 bending cycles as well as high optical transparency in the range of 450–780 nm. This study provides a viable route for exploring the correlation between structural symmetry and photoluminescence in ferroelectric thin-film systems and offers new possibilities for developing all-inorganic, reconfigurable, transparent and flexible light sources, photodetectors, and wearable sensors. A thin film device that can be triggered to glow simply by bending has been created by depositing luminescent ions on mineral surfaces. Rare earths such as lanthanide ions emit light in response to a stimulus without producing heat, making them popular for applications including biomedical imaging. Ming Zheng and coworkers from the Hong Kong Polytechnic University have now developed a technique for enhancing photo luminescent light using mechanical strain. The team used laser deposition techniques to grow lanthanide-based thin films on mica, a mineral that naturally forms flexible and transparent sheets. Experiments demonstrated that bending the device was sufficient to achieve a 10 times gain in luminescent intensity. The durability of mica enabled switching to proceed for over 10,000 cycles without device failure. An intriguing model system of flexible and transparent luminescent epitaxial thin film with highly tunable photoluminescence response is demonstrated by using the mica as substrate. The purely mechanical strain, instead of electric field, control of photoluminescence properties with superior stability, reversibility, and antifatigue behaviors shows promising applications in next-generation lightweight, transparent, flexible, and wearable passive optical devices.


Introduction
Dramatic advances in the photoluminescence (PL) properties of lanthanide ions embedded in ferroelectric oxides have occurred in recent decades. An example is the repeatable control of PL performance by exerting external physical stimuli (e.g., a magnetic field, an electric field, or temperature difference) [1][2][3][4][5][6][7][8] to elucidate the fundamental physics of the process and enable use in tunable optical waveguides, display screens, and biological imaging. With the rapid development of emerging technologies, ferroelectric luminescent materials and devices with unique properties, such as being light weight, transparent, flexible and wearable, are highly desirable for practical applications [9][10][11][12] . The conventionally used substrate materials for constructing flexible thin-film devices are polymers, ultrathin glasses, and metal foils 12 . However, the polymers (e.g., polyimide and polyethylene terephthalate) are not thermally stable, thus restricting the growth of high-quality thin films. The glasses are fragile, while the metal foils are nontransparent. The lack of a suitable flexible substrate material has become a bottleneck for this endeavor.
Recently, a breakthrough has been made whereby mechanically exfoliated mica with a thickness below 100 μm has been qualified as the substrate material for flexible and transparent oxide heteroepitaxy due to its atomically flat surface, high thermal stability, high transparency, and good mechanical pliability 13 19 , insulating VO 2 20 , and transparent conducting Ba 0.96 La 0.04 SnO 3 21 , have been successfully developed on mica substrates. The studies have substantiated the integration of oxide films with flexible mica substrates as an effective strategy for obtaining flexible functional devices. In addition, by bending the mica, the produced mechanical strain can markedly manipulate the physical properties (e.g., Curie temperature, coercive field, and ferromagnetic resonance) of the flexible thin films deposited on it 22,23 . Such a remarkable approach of using mica crystals as substrates makes it possible to prepare flexible and transparent luminescent thin films. If an appropriate rare-earth-doped ferroelectric thin film material is chosen, it is also highly likely to achieve mechanically tunable PL performance for wearable luminescent device applications.
With this as the motivation, we fabricated transparent and flexible Pr-doped Ba 0.85 Ca 0.15 Ti 0.9 Zr 0.1 O 3 [abbreviated as BCTZ:Pr] epitaxial films on mica substrates and investigated the effect of mechanical strain on their PL properties. The BCTZ host is one of the most widely studied lead-free piezoelectrics in the past decade 24 . Pr 3+ ions are selected as luminescent activators because their PL emission is very sensitive to the crystallographic symmetry of the BCTZ host through the alteration of the 4f5d configuration level 25 . Due to the high sensitivity, the emission of Pr 3+ ions has been utilized as a probe for investigating the structural transition of ferroelectric hosts 5,7,25 . Our results reveal that the PL response, including the intensity and splitting of the emission peak, can be adjusted efficiently by mechanical bending with excellent stability and durability. This work provides a promising direction for the design of future all-inorganic transparent wearable optical devices.

Materials and methods
The (Ba 0.85 Ca 0.15 ) 0.998 Pr 0.02 Ti 0.9 Zr 0.1 O 3 thin film was grown on a (001)-cut fluorophlogopite mica substrate (KMg 3 (AlSi 3 O 10 )F 2 , Changchun Taiyuan Co., LTD., China) using the pulsed laser deposition technique. The BCTZ:Pr ceramic target was ablated at 700°C under 10 Pa pure oxygen with a KrF excimer laser (λ = 248 nm). After deposition, the film was postannealed in situ at 700°C in an oxygen pressure of 10 3 Pa for 30 min to reduce oxygen vacancies and engender good crystalline quality. Then, the sample was cooled to room temperature at a rate of 5°C/min.
The crystal structure and epitaxial nature of the asgrown BCTZ:Pr/mica sample were examined by X-ray diffraction (XRD, SmartLab, Rigaku Co., Japan) equipped with Cu K α1 (λ = 1.5406 Å) radiation. The thickness (120 nm) of the film was determined via X-ray reflectivity. The surface morphology was characterized using scanning probe microscopy (SPM, Asylum MFP 3D Infinity). The transmittance spectra were recorded using a UV-vis spectrometer (Shimadzu Co.). The PL excitation and emission spectra were measured using an Edinburgh FLSP920 spectrophotometer with a 450-nm xenon arc lamp (Xe900) as the excitation source. Prior to the measurements, the two-dimensional layered mica substrate was mechanically exfoliated, like graphene, to a thickness of 10 μm using a sharp blade to acquire good mechanical flexibility. By taping the BCTZ:Pr/mica sample onto the polyester convex molds with different radii of curvature (ROCs), a tunable tensile mechanical strain was induced in the BCTZ:Pr thin film. Note that the sample was firmly attached to the mold to avoid slipping and fracturing during the measurement. All measurements were performed at ambient temperature. Figure 1a shows a typical out-of-plane XRD θ-2θ scan spectrum of the BCTZ:Pr/mica structure. The appearance of only BCTZ:Pr (lll) (l = 1, 2) reflections indicates the preferred orientation growth of the BCTZ:Pr film with a single-phase structure. The out-of-plane interplanar spacing d 111 (2.323 Å) of the BCTZ:Pr film is larger than that of the BCTZ:Pr bulk value (2.321 Å), suggesting that the film has an out-of-plane tensile strain (0.086%) accompanied by an in-plane compressive strain. XRD ϕ scans were performed on the BCTZ:Pr (101) and mica (202) reflection peaks to further reveal the in-plane crystallographic orientation of the film with the substrate. As shown in Fig. 1b, compared with the trifold symmetry of mica, sixfold symmetric reflection peaks are observed at 60°intervals from the BCTZ:Pr due to the occurrence of two sets of structural domains in the films. The epitaxial relationship of the film with respect to the substrate is determined to be (111) BCTZ:Pr ||(001) Mica and [01−1] BCTZ: Pr ||[010] Mica . The BCTZ:Pr film also exhibits a flat surface, sharp interface, and uniform thickness, as evidenced by the surface morphology image (inset) and the clear interference pattern of the X-ray reflectivity curve (Fig.  1c). A full width at half maximum of~0.51°is observed, indicating excellent crystallinity in the BCTZ:Pr film. Moreover, the transmittance of the BCTZ:Pr/mica structure is over 70% in the visible light region ranging from 450 to 780 nm (Fig. 1d), which is higher than that of other ferroelectric/mica heterostructures 14,16 . As shown in the insets of Fig. 1d, the sample possesses excellent transparency and mechanical flexibility. All the above characterizations demonstrate a high-quality surface and good epitaxial heterointerface of the flexible and transparent BCTZ:Pr/mica structure, which provide strong elastic coupling at the interface of the structure either in a flat or bent state.

Results and discussions
To explore the mechanical strain effect on the PL response, predesigned polyester convex molds with various ROCs were employed to introduce different bending strains in the film by attaching the flexible BCTZ:Pr/mica structure onto them (inset of Fig. 2a). The photoluminescence excitation spectrum of the flat BCTZ:Pr/ mica structure monitored at 578 nm was first measured, giving the results shown in Fig. 2a. Three excitation peaks corresponding to the 3 P 0 ! 3 H 5 , 3 P 1 ! 3 H 6 and 3 P 0 ! 3 F 2 transitions of Pr 3+ are observed at 440, 468, and 495 nm, respectively. Based on the results, the PL emission spectra of the structure in different bent states (with different ROCs) as well as the flat state were recorded under an excitation of 468 nm. As shown in Fig. 2b, similar to earlier reports on other Pr-doped materials 24,26 , the BCTZ:Pr film exhibits strong PL emissions of Pr 3+ ions. The emission peaks centered at 549, 578, 596, 616, and 647 nm originate from the 3 P 0 ! 3 H 5 , 3 P 1 ! 3 H 6 , 1 D 2 ! 3 H 4 , 3 P 0 ! 3 H 6 , and 3 P 0 ! 3 F 2 transitions, respectively. The multiplets associated with these transitions stem from crystal field-induced Stark splitting, which has been observed for Pr 3+ ions in Ba 0.77 Ca 0.23 TiO 3 and CaTiO 3 hosts 25,27 . Obviously, the PL response is strongly dependent on the bending radius (i.e., radius of curvature (ROC)) for all of the emission peaks from green to red. It is noted that the strongest emission peak corresponding to the 3 P 1 ! 3 H 6 transition split into two peaks upon bending. At a ROC > 5 mm, the split peak at a low wavelength is stronger than the other one at a high wavelength, whereas it becomes weaker at a ROC < 5 mm. The bending-induced relative change in the PL intensity (ΔI/I) of the strongest emission peak as a function of the ROC is shown in Fig. 3a. Here, ΔI/I is defined as ΔI/I = [I(ROC) − I(ROC = ∞)]/I(ROC = ∞). The ΔI/I first decreases gradually with decreasing ROC and then increases sharply after reaching a minimum value at a ROC = 5 mm. As illustrated in the inset of Fig. 3b, a It is thus inferred that a close relationship between the PL performance and bending-induced mechanical strain exists.
As reported by Zhao et al. and Zhang et al. 28,29 , the bending strain ε in the BCTZ:Pr film can be described as ε = [(t + R)θ -Rθ]/Rθ = t/R, where t is the half of the total thickness of the film and the mica substrate and R is the ROC (inset of Fig. 3b). The bending strain at each ROC was then calculated, and its relationship with ΔI/I is presented in Fig. 3b. Similar to the dependence on the ROC, the ΔI/I decreases first with increasing ε and then increases abruptly after reaching a minimum value of −52% at ε = 0.1%. It is worth noting that the value of ΔI/I reaches a very large value of 445% at ε = 0.25%. It is known that the PL response is closely related to the structural symmetry of the host and the crystal field around rare-earth ions 30,31 . Based on the Judd-Ofelt (J-O) theory, a decrease in the structural symmetry of the BCTZ host triggers an increase in uneven components of the crystal field around the Pr 3+ dopant ions. The uneven crystal field can mix opposite-parity states into the 4f configuration level and enhance the electronic coupling between 4f energy levels and the higher 4f5d configuration, thereby leading to an increase in the 4f-4f electric dipolar transition probabilities of Pr 3+ ions 32 . In other words, the lower the structural symmetry, the stronger the Fig. 3 The relative change in PL intensity (ΔI/I) as a function of a the bending radius and b the bending strain for the structure. The inset in b shows the illustration of the structure with a tensile in-plane strain under outward bending Fig. 2 The PL a excitation spectrum and b emission spectra of the BCTZ:Pr/mica structure at various ROCs. The inset in a shows the schematic of the bending configuration of the structure on a convex mold PL emission. This result has been verified for many lanthanide-doped ferroelectric ceramic bulk and thin film materials, where the PL response is mediated by the variation in structural symmetry of the host through the electric-field-induced biaxial strain, polarization, or phase transition 3,5-8 . As discussed above (Fig. 1a), an initial compressive in-plane strain (i.e., epitaxial strain) is produced in the BCTZ:Pr film because of a lattice mismatch. Using the Poisson relation ε zz = −2ν/(1 − ν)ε xx 33-35 and Poisson ratio ν = 0.3 36 , the epitaxial strain is estimated to be −0.1%. This compressive epitaxial strain can be canceled out by bending the substrate outward (inset of Fig.  3b). As shown in Fig. 3a, b, the effective in-plane strain ε eff (i.e., the sum of the epitaxial strain and bending strain) of the film should become almost zero at a ROC = 5 mm, thus leading to the smallest distortion, the highest structural symmetry, and then the weakest PL emission. For a ROC > 5 mm (in-plane compressive) or ROC < 5 mm (inplane tensile), the PL intensity is increased due to a reduction in the structural symmetry of the host. The mechanically tunable PL response is attributed to strainmediated symmetries of the host and crystal fields around From the perspective of practical applications, a long durability and high stability are essential for a long operational lifetime. In this work, the durability was assessed by investigating the PL response of the flexible BCTZ:Pr/mica structure subjected to cyclic bending (up to 10 4 cycles), while the stability was evaluated by monitoring the PL intensity of the strongest emission peak, both in the flat (ROC = ∞) and bent (ROC = 2 mm) states, as a function of time. A photograph illustrating the cyclic bending is shown in the inset of Fig. 4b. As shown in Fig. 4a, the PL emission spectra of the film, both in the flat and bent states, remain almost unaltered after 10 4 cycles of bending. Similarly, the PL intensities of the strongest emission peak in the flat and bent states decrease only slightly during the cyclic bending and retain more than 95% of the initial values after 10 4 cycles of bending (Fig. 4b). These clearly demonstrate the switching endurance and excellent durability of the BCTZ:Pr/ mica structure. It has been reported that heteroepitaxy involving two-dimensional materials (e.g., mica) can be categorized as van der Waals epitaxy 12,13,[18][19][20] . The excellent durability of the PL response of the BCTZ:Pr films upon bending the mica can be attributed to the weak van der Waals interaction between the film and twodimensional layered substrate instead of the strong chemical bonding, which can reduce the substrate clamping effect 12,13 . The PL retention properties of the structure in both states are depicted in the inset of Fig. 4b. No obvious decay of the PL intensities is observed over a long period of 10 4 s. Compared with the electric/magnetic field-driven luminescent devices, the mechanical strain-induced dynamic and continuous PL switching in the flexible BCTZ:Pr/mica structure indicates the possibility of practical use for high-performance wearable optical devices due to its excellent stability, reliability, and antifatigue behavior.
The tunable mechanical strain effect due to bending provides great potential for the design and development of transparent wearable mechanic-luminescent sensing systems. As illustrated in Fig. 5, we can detect and adjust the ROC of the device by recording the evolution of the PL spectra (e.g., the intensity and wavelength of the emission peak), corresponding to process ➀, which can be adopted in flexible optical deformation sensors. On the other hand, a specific PL signal can be obtained through accurately controlling the ROC (i.e., process ➁), which provides an intriguing approach for flexible display lighting, optical filters, and medical imaging. Although we have proven the validity of the mechanically controlled PL response in BCTZ:Pr films by using flexible mica as a substrate, we anticipate that similar modulation is achievable in other perovskite oxide thin films, such as BaTiO 3 3 , (Bi, Na)TiO 3 4,6 , and (Li, Na, K)NbO 3 32 , doped with lanthanide ions and epitaxially grown on the mica substrates. In addition, the mechanical strain in tension/ compression induced by outward/inward bending can also be diverted into NaGdF 4 and metal halide nanocrystals and nanowires by implementing them on mica substrates 2,37-39 , and these structures are also promising candidates for mechanically tunable PL properties. Table 1 summarizes the relative change of PL emission intensity of several lanthanide ions when the corresponding ferroelectric hosts are under different external Fig. 5 Illustrations of the transparent wearable mechanic-luminescent sensing system for the flexible BCTZ:Pr/mica structure. In process ➀, the PL signal change can be used to detect the curvature variation. In process ➁, changing the curvature can obtain a specific PL signal stimuli. It is clear from the table that the BCTZ:Pr/mica system has the best performance metrics among all the lanthanide-doped ferroelectric elements. It is interesting to compare the BCTZ:Pr/mica structure with the BCTZ: Pr/Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT) structure in our previous study 7 . Both systems allow a gradual and continuous evolution in the strain of the films, which enables us to precisely fine-tune the PL response. In the case of the BCTZ:Pr/PMN-PT structure, the PL modulation (6.6%) is mediated by the external electric-field-generated relatively small piezostrain (0.055%) of the PMN-PT, where we only shed light on lattice-coupled PL performance for the lateral compressively strained thin films (considering the sum of the epitaxial strain and piezostrain) due to much smaller lattice parameters of the La 0.7 Sr 0.3 MnO 3 buffer layer than those of the overlying BCTZ:Pr film. In the BCTZ:Pr/mica structure, the strain switching for PL is produced by purely mechanical bending without being driven by an electric signal. The inplane strain state of the thin film can be changed from compressive to tensile (considering the sum of the epitaxial strain and bending strain) due to the good pliability of the mica, which gives rise to a rich physical process of energy transition and a large PL tuning effect. We note that the gauge factor (6853) of the BCTZ:Pr film on mica is much larger than that (120) on PMN-PT. This finding is ascribed to a reduction in the substrate clamping effect induced by the weak van der Waals interaction between the film and two-dimensional layered mica substrate 12,13 .

Conclusions
In summary, we report a purely mechanical straininduced PL and repeatable control of the PL response using a simple BCTZ:Pr/mica epitaxial structure. Owing to the nature of mica, the structure possesses high optical transmittance and excellent mechanical flexibility. The PL switching shows no noticeable decay after being bent to a ROC of 2 mm for 10 4 cycles, demonstrating the robust functionality of the BCTZ:Pr films against mechanical bending. Our finding is informative for achieving mechanically controlled PL properties in lattice-sensitive ferroelectric oxides and designing next-generation allinorganic, reconfigurable, light weight, transparent, and flexible passive luminescent devices. The high-quality heteroepitaxy on mica is expected to open a new and simple approach for expanding the material database, such as colossal magnetoresistive manganites and topological insulators, for future soft technology.  7 Wang et al. 8 Hao et al. 3 Sun et al. 5 Zhang et al. 25 Khatua et al. 4 Host