Multiple Color Emission of Mechanoluminescence and Photoluminescence from SrZnSO: Bi3+ for Multi-mode Anti-counterfeiting

: Mechanoluminescence materials that emit light under mechanical stimulation have attracted widespread attention in sensing, anti-counterfeiting and imaging applications. In this study, a series of novel Sr 1- x Bi x ZnSO (0.001 ≤ x ≤ 0.1) samples were synthesized by the method of high temperature solid-state reaction. It is worth noting that the distortion degree of SrO 3 S 3 octahedron was increased with increasing Bi 3+ concentration, and the color manipulated Sr 1- x Bi x ZnSO which can emit different photoluminescence (blue to dark blue and finally red) and mechanoluminescence (orange to red) colors are obtained. Moreover, the deep traps can stably store and provide electronic

sensors [11]. Peng et al. developed a highly stretchable ML fiber by coating the mixture of ZnS particles and PDMS onto an existing PDMS fiber, and the resulting composite fiber emitted an intensity-tunable light upon stretching and releasing, which can be used for wearable lighting devices [12]. Wang et al. developed a flexible pressure sensor based on the ML of ZnS: Mn particles can record the signing habits of people, which can be used for electric signature [13]. To date, many different kinds of ML materials have been developed, such as SrMgAl10O17:Eu [14], ZnAl2O4:Mn [15], Ca2Al2SiO7:Eu [16], SrCaMgSi2O7:Eu [17], Ca2Al2SiO7:Ce [18] and so on. However, there are still some problems in the existing ML materials, such as weak ML intensity, limited ML color and structural damage during stress application [19]. Hence, to develop new ML materials with strong ML intensity and tunable emission colors is pressing.
Recently, Liu et al. successfully synthesized a novel polar SrZnSO compound crystallized in a polar, non-centrosymmetric hexagonal space group of P63mc with lattice parameters a = 3.90442 Å, c = 11.6192 Å and Z = 2 by the method of high temperature solid state reaction [19]. SrZnSO is isostructural to CaZnSO. The substitution of Ca 2+ by Sr 2+ in MZnSO (M = Ca or Sr) causes a band gap reduction resulting in an influence on the photophysical properties and enhances the optic activity toward the visible region [20].
After the work of Liu et al., Reshak has detailed photophysical properties of SrZnSO and performed a comprehensive calculation to investigate the influence of substitution of Ca 2+ (180 pm) by larger Sr 2+ (200 pm) in MZnSO on the structural properties [21].
Bi can exhibit a variety of valence states, such as Bi 0 , Bi + , Bi 2+ , Bi 3+ , and so on [22][23][24]. Among these valence states, Bi 3+ is the most stable valence state and have many interesting properties. It is known that Bi 3+ exhibits a broad and tunable emission band with the electronic configuration of [Xe] 4f 14 5d 10 6s 2 6p 3 [25]. Moreover, due to the naked 6s and 6p electrons, Bi 3+ is sensitive to the surrounding coordination environment [26,27]. 1 S0 is the ground state and 3 P0, 3 P1, 3 P2 and 1 P1 are the excited states of Bi 3+ . The electron transitions from 1 S0 to 3 P0 and 3 P2 are spin forbidden, but the electron transitions from 1 S0 to 3 P1 and 1 P1 are allowed. Due to the strong sensitivity to the coordination environment, the emission of Bi 3+ can be tuned from ultraviolet to the yellow region due to the different bond lengths, coordination number, lattice symmetry, and so on [28,29]. Thus, further exploration on the luminescence performance of Bi 3+ in different hosts is interesting.
In this work, Bi-activated SrZnSO phosphors were synthesized, and their phase purity, site occupation and coordination environment of Bi 3+ in SrZnSO were investigated by powder X-ray diffraction (XRD) and Rietveld refinement. The morphology and the elemental analysis of Sr1-xBixZnSO were examined to study the particle size and elemental composition. The luminescence properties, optical band gap and trap depth of Sr1-xBixZnSO were investigated in detail to establish the ML mechanism for ML of Sr1-xBixZnSO.

Characterization
Powder X-ray diffraction (XRD) designs of arranged tests for stage immaculateness examination were collected on a Rigaku D/max 2500 X-ray diffractometer utilizing Cu Kα (λ = 1.54178 Å) radiation worked at 40 kV and 250 mA in 2θ extend from 10 to 80 ° with a step estimate of 0.02 ° at room temperature. The information for Rietveld examination were collected in a step-scanning mode with a step measure of 0.02 ° and 2 s checking time per step over a 2θ extend from 5 to 130 °, and the Rietveld refinement was performed by utilizing FullProf computer program package [30]. Morphologies, molecule sizes, and energy-dispersive spectroscopy (EDS) natural mapping of sintered tests were inspected employing a filtering electron magnifying instrument (SEM; Zeiss Gemini SEM 300). UV-Vis diffuse reflectance spectra of tests were measured at room temperature on a U-3900H UV/Vis spectrophotometer (HITACHI) and reflection spectra were calibrated with BaSO4.
Photoluminescence (PL) and photoluminescence excitation (PLE) spectra were measured with FLS1000 prepared with a xenon light at room temperature. Radiance rot bends were gotten utilizing an FLS1000 fluorescence spectrometer (Edinburgh Rebellious Ltd., U.K.) and an μF2 streak light was utilized as excitation asset. To assess ML properties of tests, circular composite disks with a distance across of 25 mm and a thickness of 15 mm were arranged by blending Sr1-xBixZnSO powders and optical epoxy tar. These disks were uncovered to bright light (320 nm) for 5 min some time recently location. ML properties were measured with a lab-made framework comprising of a widespread testing machine (AGS-X 10KN STD, Shimadzu), a photomultiplier tube (C13796, Hamamatsu Photonics), and a spectrometer (SR-500i-B1-R, Andor) controlled by a computer. ML pictures of Sr1-xBixZnSO beneath compressive stack were taken with a computerized camera.
Thermoluminescence (ThL) spectrum was identified with TOSL-3DS. The test was to begin with uncovered to X-ray for 15 min. It was at that point warmed from room temperature to 500 ℃ with a warming rate of 1 ℃·s -1 . The powder XRD patterns of several samples Sr1-xBixZnSO (0.001 ≤ x ≤ 0.10) and

Results and discussion
SrZnSO theoretical pattern (ICSD No: 431819) are shown in Fig. 1(a). The comparison between Sr1-xBixZnSO patterns and calculated diffraction patterns appears that SrZnSO is the main phase of these samples with a trace of SrS as an impurity phase. The radius of Bi 3+ (r = 1.03 Å, CN = 6) ion is little smaller than Sr 2+ (r = 1.18 Å, CN = 6) ion, but the radius of Bi 3+ ion is much larger than Zn 2+ (r = 0.74 Å, CN = 6) ion. According to the ionic radius similarity mechanism, when Bi 3+ ions are introduced into the matrix lattice, Bi 3+ ions may occupy the sites of Sr 2+ ions. As a consequence, substitution of larger Sr 2+ by smaller Bi 3+ will lead to lattice shrinkage, as demonstrated in Fig. 1(b). These results indicate Bi 3+ were successfully incorporated into the samples while maintaining the crystal structures intact. As the representative of Sr1-xBixZnSO, we performed the Rietveld refinement for Sr1-xBixZnSO (x = 0, 0.005, 0.04 and 0.08), adopting crystal structure of SrZnSO (ICSD No: 431819) as the initial structure model. As shown in Fig. 1(c) and Fig.   S1(a) -(f), the experimental patterns are in good agreement with the theoretical patterns.
Primary refinement parameters and point by point crystallographic information are organized in Table 1. The converged refinement results demonstrate that the statistically substituted Bi 3+ ions on Sr 2+ sites satisfy the refinements. The shrinkage of cell volume which is agreement with the smaller ionic radius of Bi 3+ with respect to Sr 2+ and further indicates that Bi 3+ ions occupy the sites of Sr 2+ ions. As shown in the inset graph of Fig.   1(b), the crystal structure of Sr0.995Bi0.005ZnSO is composed of ZnOS3 tetrahedrons and SrO3S3 octahedrons. These tetrahedrons and octahedrons are connected to each other to form a three-dimensional layered structure [31]. Rietveld refinement results show that Sr1-xBixZnSO is the hexagonal structure with the space group P63mc, and the unit cell parameters are: a = 3.91001(2) Å, c = 11.6177(4) Å and V = 153.815(8) Å 3 . The refinement results in Table 1, the bond length of Sr-S bonds is inconsistent with Sr-O bonds, indicating that SrO3S3 octahedron is distorted. The distortion degree of SrO3S3 octahedron can be estimated using the following formula [32]: where D, n, Li, and Lav are distortion degree, number of bonds in SrO3S3 octahedron, bond length of Sr|Bi-S or Sr|Bi-O, and average bond length in SrO3S3 octahedron, respectively.
The distortion degree of SrO3S3 octahedron was increased with increasing Bi 3+ concentration, as shown in Fig. 1  Compounds (2) 3.9070 (9) 3.9053 (5) 3.9023(1) 3.9028 (7) c /Å 11.6141(6) 11.6177(4) 11.6207 (7) 11.6096(5) 11.6187(4) 11.6117 (4) R is the diffuse reflectance of the sample, K is the absorption coefficient and S is the scattering coefficient. The absorption spectrum of Sr0.999Bi0.001ZnSO calculated with the formula of Kubelka-Munk is shown in the inset of Fig. 2(b). The optical bandgap of Sr0.999Bi0.001ZnSO was calculated to be 3.61 eV on the premise of extrapolation from the 11 equation of Kubelka-Munk condition to K / S = 0 [34]. It is seen that the values of optical band gap of Sr1-xBixZnSO have increased from 3.61 to 3.78 eV when the concentration of Bi 3+ is lower than x = 0.04, and then decreased to 3.70 eV when the concentration of Bi 3+ is higher than x = 0.04, which is shown in Fig. 2(b). As the electronegativity of Bi 3+ ion is stronger than that of Sr 2+ , the covalence of the ligands gradually decreases when Bi 3+ concentration increased, which leads to the nephelauxetic effect getting weaker. Therefore, the optical band gap of Sr1-xBixZnSO increased when the concentration of Bi 3+ is lower than x = 0.04. However, when the concentration of Bi 3+ ions is higher, the band gap of Sr1-xBixZnSO decreases due to the appearance of impurity bands and band tails [29].  Fig. 3(b). The blue emission band derived from 3 P1, 0 → 1 S0 transitions of Bi 3+ , and the inset shows the luminescence intensity first increased and then decreased with increasing Bi 3+ doping concentration. The optimal Bi 3+ concentration is x = 0.04. The luminescence intensity of Sr1-xBixZnSO decreased when Bi 3+ concentration is higher than x = 0.04, which is attributed to the concentration quenching effect. As the concentration of Bi 3+ ions increase, the distance between the Bi 3+ ions become smaller, and the non-radiative transition increases, resulting in a decrease in luminescence intensity. The critical distance between the Bi 3+ ions can be estimated with the following equation [35]: Rc, N, Χc, and V are critical distance, the number of sites accessible for the dopant within the unit cell, quenching concentration of activator ion, and unit cell volume, respectively.
Consequently, the Rc is estimated to be 15.4 Å, which is quite similar to the results reported in other Bi-activated materials [36,37]. When concentration quenching effect occurs in Sr1-xBixZnSO phosphors, the electrical multipole interaction between the Bi 3+ ions play a major role [26,35]. As shown in Fig. 3 increases. Therefore, the luminescence color of Sr1-xBixZnSO changes from dark blue to red as the concentration of Bi 3+ increases as shown in Fig. 3(e).
Decay curves of Sr1-xBixZnSO (0.001 ≤ x ≤ 0.1) phosphors excited at 345 nm and monitored at 467 nm are shown in Fig. 3(f). All the decay curves can be well fitted by the second-order exponential equation [38]:   which implies the complexity of carrier transport in the current ML system [39].
Furthermore, the peak bands at about 80 °C and the trap depth about 0.72 eV are benefited to afterglow [40]. ThL curves of Sr0.999Bi0.001ZnSO as a function of temperature are depicted in Fig. 5(d). The trap depth can be estimated by the multi-peak fitting equation where I(T) is the ThL intensity, s is the frequency factor, n0 is the concentration of trapped  [5]. In, the schematic diagram of the formation of proposed defects in Sr1-xBixZnSO is shown in Fig. 5(e), the luminescence centers were introduced by doping Bi 3+ ions that replace Sr 2+ ions in the host lattice [42]. Due to a non-equivalent substitution, two kinds of defects can be simultaneously created as described by equation: The Bi 3+ -related defects Sr × carrying one positive charge act as the trapping centers of electrons to participate in the trap-related luminescent processes. Furthermore, there are vacancies between Sr 2+ [43]. To compensate the +3 charge, •• (where V is denoted as vacancy) with opposite charge should be formed in order to keep the local neutrality [44].
Through non-equivalent substitution and charge-compensated to achieve charge balance, it is expected to expand the design range of ML materials and to study the mechanism. The schematic diagram of mechanism for ML of Sr1-xBixZnSO is shown in Fig. 5  As aforementioned, the Sr1-xBixZnSO phosphors with different incorporation concentrations of Bi 3+ exhibit different colors in CIE chromaticity diagram under the excitation of 297 nm wavelength, and the same phenomenon is also recorded when the applied compression load is 5000 N. Inspired by this interesting phenomenon, as shown in Fig. 6(a), the synthesized Sr1-xBixZnSO (x = 0.001, 0.01, 0.08) phosphors will be well dispersed in the handmade "ink" which is the mixture of glycerin and ethylene glycol and this "ink" is used to draw a "SHU" pattern on the filter paper. Since the ThL peak bands at 80 °C, the dried pattern will be transferred to the dark environment and heated to 80 ℃ for recording its performance. As presented in Fig. 6(b), all the patterns drawn with the "inks" with different Bi 3+ concentrations emit strong blue-green light under the excitation of 365 nm wavelength. Interestingly, the different colors pattern "SHU" which can emit purple, yellow and red light after turnning off the UV lamp can be obtained with the x = 0.001, 0.08 and 0.1, respectively. Moreover, it can be observed that when heated to 80 ℃ these patterns will emit orange, orange-red, and red light, respectively, which can be attributed to ThL (Fig. 5). This phenomenon is attributed to under heating conditions, the electrons and holes will be released from the traps and finally trapped luminescent center. Moreover, as shown in Fig. 6(c), a film made by dispersing phosphor in PDMS serves as a signature system. The writing photo and the stress distribution map of "L" pattern in handwritten "ML" are collected and shown in the inset. The signature systems with different Bi 3+ concentrations exhibit different color emissions while handwriting process, which is corresponding to the emission of these samples under loading. Furthermore, with increasing handwriting pressure the writing trace appears to be more precise and brighter.
Therefore, thanks to the multi-color emission, the devices made by SrZnSO: Bi 3+ phosphor and PDMS can be used as multi-mode anti-counterfeiting, which has a great potential in the field of anti-counterfeiting technology.

Conclusion
As a summary, novel colorful ML phosphors Sr1-xBixZnSO (0.001 ≤ x ≤ 0. potential in the field of mechanical-optical conversion, stress sensing and stress imaging.

Conflicts of interest
There are no conflicts to declare.