Mechanoluminescence and Photoluminescence Heterojunction for Superior Multimode Sensing Platform of Friction, Force, Pressure, and Temperature in Fibers and 3D‐Printed Polymers

Endowing a single material with various types of luminescence, that is, exhibiting a simultaneous optical response to different stimuli, is vital in various fields. A photoluminescence (PL)‐ and mechanoluminescence (ML)‐based multifunctional sensing platform is built by combining heterojunctioned ZnS/CaZnOS:Mn2+ mechano‐photonic materials using a 3D‐printing technique and fiber spinning. ML‐active particles are embedded in micrometer‐sized cellulose fibers for flexible optical devices capable of emitting light driven by mechanical force. Individually modified 3D‐printed hard units that exhibit intense ML in response to mechanical deformation, such as impact and friction, are also fabricated. Importantly, they also allow low‐pressure sensing up to ≈100 bar, a range previously inaccessible by any other optical sensing technique. Moreover, the developed optical manometer based on the PL of the materials demonstrates a superior high‐pressure sensitivity of ≈6.20 nm GPa−1. Using this sensing platform, four modes of temperature detection can be achieved: excitation‐band spectral shifts, emission‐band spectral shifts, bandwidth broadening, and lifetime shortening. This work supports the possibility of mass production of ML‐active mechanical and optoelectronic parts integrated with scientific and industrial tools and apparatus.


Introduction
[3] Depending on the source of excitation, for example, electromagnetic radiation, stress (mechanical force), electric current or field, temperature, or magnetic field, different types of luminescence can be observed, such as photoluminescence (PL), [4,5] mechanoluminescence (ML), [6,7] electroluminescence (EL), [8,9] thermoluminescence (TL), [10] and magnetic-induced luminescence (MIL), [11] respectively.[14] ML is a phenomenon in which photon emissions are generated in response to mechanical stimuli occurring in solids, such as impact, friction, compression, fracture, grinding, and stretching. [6,15,16]Studies on ML have drawn special attention for widespread applications, including artificial skins, data storage devices, flexible optoelectronics, mechanical sensing, and magneto-optical coupling. [17,18]The fascinating mechanical force-to-photon conversion principle enables a new type of remote stress, force, and pressure sensing without additional energy input.MLbased pressure/stress sensing technologies have advantages such as stress/force distribution visualization, remote detection, and self-powering.Thus, for the development of nextgeneration sensing systems, this approach is regarded as one of the main routes.[33][34][35] However, vital challenges remain, for example, improving sensitivity for temperature or pressure, achieving a visual sensing approach, and widening the temperature or pressure sensing range.Thus, improving the sensitivity, response speed, and structural stability of the sensing materials is very important.Mn 2+ ions are considered suitable activators because of their low cost, broad excitation band, intense emission, abundance or availability of Mn 2+ sources, non-toxicity, etc. [36,37] Mn 2+ emission can be easily influenced by changing the polyhedral environments, such as geometry, bond lengths, and volumes, which consequently produce different magnitudes of crystal field splitting. [38]Mn 2+ has five d-electrons in its unfilled 3d shell with 3d 5 outer electron configuration and acts as an activator, endowing the matrices with good luminescence properties.Over the past few decades, Mn 2+ -based materials have been extensively investigated.For instance, Song et al. reported a dual-wavelengthemitting material, Li 2 ZnSiO 4 :Mn 2+ , as a wearable fiber temperature sensor with good accuracy (±0.2 °C) and repeatability. [39]hi et al. reported a multimode luminescence temperature sensor, Zn 2 GeO 4 :Mn 2+ phosphor, with a very high relative sensitivity of 12.2% K −1 . [40]In spite of recent progress, to the best of our knowledge, the reports about Mn 2+ -doped materials based on bifunctional materials for optical temperature and pressure sensing still lack.
Advances in 3D-printing technology have created new tools for modern construction, industrial design, automotive engineering, food processing and preservation, developing mock-ups, prototypes, etc. [41,42] 3D-printing technique has many advantages, such as ease of duplicating products, low cost, privacy considerations, and time efficiency. [41]Regarding the combination of ML materials with precise fabrication techniques for micron-sized resolution, Song et al. reported a 3D-printing technology combined with SiO 2 -modified ZnS:Cu/Mn@Al 2 O 3 @PDMS composites, which can achieve skin-attachable stress sensors and facial expression capture. [16]However, most printed models are made of materials lacking flexibility and elasticity, which are still sufficient for replicating models with fidelity.Cellulose is an easily accessible and environmentally friendly biopolymer that is widely used in textiles, cosmetics, paper, biodegradable wound dressings, etc. Cellulose-based fibers with special modifiers or fillers not only exhibit photoluminescence and catalytic or magnetic properties, but can also be incorporated into fabrics with good breathability, stretchability, flexibility, and inerrability, which are crucial for mimicking a variety of irregular movements of the human body.In contrast to EL-based devices in fabrics, [43] ML-based devices in fabrics can emit photons without using an electrical power supply, [44,45] that is, light emission is driven by mechanical force rather than a battery.
Herein, we present a PL-and ML-based multifunctional platform by combining heterojunctioned ZnS/CaZnOS mechanophotonic materials with a 3D-printing technique and fiber spinning.By introducing ML particles into the cellulose matrix, we could prepare PL/ML-active cellulose fibers that can emit light driven by photoexcitation or mechanical force.Finally, we fabricated the modified 3D-printed units that exhibit intense ML in response to mechanical deformations, such as impact and friction.Importantly, the developed materials also allowed low-pressure sensing up to ≈100 bar, a range previously inaccessible by any other optical sensing technique.Moreover, different strategies for temperature detection can be achieved for a single material based on the excitation/emission band shift, bandwidth, and lifetime.Notably, the pressure-driven PL spectral shift of the materials studied demonstrates a superior high-pressure sensitivity of ≈6.20 nm GPa −1 , as well.Owing to the possibility of scaling up the fabrication process of the 3D-printed polymer items and fiberbased materials containing ML compounds, our study supports the possibility of the mass production of ML-active flexible mechanical and optoelectronic parts integrated with scientific and industrial tools and apparatuses.

Strategy and Design
To obtain optimal ML properties, we chose and synthesized heterojunction materials with a combination of the hexagonal phases Zn 0.99 S:0.01Mn 2+ and CaZn 0.99 OS:0.01Mn 2+ (synthetic product ZnS/CaZnOS = 3:2), both of which belong to the space group P63mc.The details of the material synthesis are provided in Supporting Information.Previous research reported that the ZnS/CaZnOS:Mn 2+ heterojunction exhibits significantly more intense ML, compared to both commercial ZnS (considered state-of-the-art ML material) and CaZnOS material, with an ML enhancement of ≈2.2 and 3.5 times, respectively. [18]The design strategy for high-performance ML-and PL-based multifunctional sensing platforms is shown in Figure 1.Two types of sensing platforms were developed: I) A stress-induced ML sensing mode; and II) a highly sensitive simultaneous temperature-and pressureinduced PL sensing mode.It is shown that interfacial bonding creates band offsets that mitigate the inter-electronic level excitation barrier toward the conduction band (CB), leading to an improved efficiency of electron-hole recombination from the electronic interface levels of the activators (e.g., Mn 2+ ).Thus, combining the synthesized heterojunction with 3D-printed polymers and cellulose fibers can result in ML-manufactured objects working as stress sensors of various shapes or flexible ML fibers that can be knitted into fabrics or cloths showing intense ML signals.
On the other hand, as shown in Figure 1 (right), pressure can influence the energy level structure, and thus the emission band is usually redshifted spectrally due to the following two effects: I) The enhanced nephelauxetic effect.It is due to a reduction in the free-ion parameters, such as coulombic and spin orbits, and an enhancement in the covalent bonding nature that is caused by a shorter bond length.This structural change results in a decrease in the energy gap between ground and excited multiplets, as well as the energy gap between configurations.II) The stronger crystal-field effect.With an increase in crystal-field strength due to pressure or temperature, the ground and excited configurations experience a more distinct splitting, resulting in enhanced interactions between 3d electrons of the Mn 2+ ion and the valence electrons of oxygen ligands.Moreover, the spectroscopic characterization of d-block ions or Ln 2+/3+ -doped materials can be influenced by temperature variations, [32,[46][47] such as the spectral shift (redshift or blueshift) of excitation or emission bands, variation in bandwidth, luminescence lifetime (rise and decay times), band intensity, emission intensity ratio, etc.

Structural and PL Properties at Ambient Condition
In Figure S1 (Supporting Information), it is shown that the powder X-ray diffraction (XRD) patterns of the synthesized ZnS/CaZnOS:Mn 2+ heterojunction fit well with the reference patterns of hexagonal-phase ZnS (card no.JCPDS 00-001-0677) and hexagonal CaZnOS (card no.JCPDS 04-011-1217).As shown in the Raman spectra (Figure S2, Supporting Information), in the lower-energy range from 200 to 650 cm −1 , it depicts Raman-active modes at ≈269, 285, 365, and 539 cm −1 , which are characteristic of CaZnOS structure. [48,49]Moreover, the Raman-active modes at ≈322 and 394 cm −1 are characteristics of ZnS structure. [50]he two additional bands ≈495 and 1728 cm −1 are plausibly associated with the formation of the complex ZnS/CaZnOS heterojunction.This is because neither in pure ZnS nor in pure CaZnOS, these bands can be found.53] The excitation (monitored at  em = 590 nm) and emission spectra (under  ex = 330 nm excitation) of the ZnS/CaZnOS:Mn 2+ heterojunction are shown in Figure S3 (Supporting Information).The excitation and emission bands correspond to the 4 T 1 (4G)→ 6 A 1 (6S) transitions of Mn 2+ in the ZnS and CaZnOS phases, respectively.Based on the luminescence decay curve measured at room temperature (RT), excitation at  ex = 340 nm, and monitoring of the emission at  em = 590 nm (Figure S4, Supporting Information), the average emission lifetime () was determined to be ≈1 ms, where  = ∫ t ⋅ Idt∕ ∫ Idt).SEM images of the particles are shown at different scales (see Figure S5a,b, Supporting Information).These materials contained irregular, aggregated microparticles.The microparticle size distribution histogram corresponding to the SEM images is presented in Figure S5c (Supporting Information).To determine the size distributions, over 100 nanoparticles were randomly selected from the SEM images, and their diameters were measured.The results showed that the synthesized particles had an average diameter of ≈5.02 ± 4.07 μm.Such a relatively broad particle size distribution is due to the hightemperature solid-state method applied during the synthesis.However, the synthesized materials exhibit very strong ML activity, which compensates for the imhomogeneity of the particles.

ML Properties and Applications
Under mechanical stress, the ZnS/CaZnOS:Mn 2+ sample exhibits an immediate, short-lasting, and intense ML signal.The samples were exposed to UV irradiation prior to ML characterization.Figure 2a shows the friction-induced ML spectra of the sample under a loading force of 12 N measured 1, 72, and 232 h after irradiation.From 1 to 72 h, the integrated intensity of the ML signal decreased to 0.5 times, and then from 72 to 232 h, to 0.08 times the initial intensity.Figure 2b depicts the ML spectra as a function of applied force.The dependence of the total ML intensity on applied force is shown in Figure 2c.The total ML intensity of the sample exhibits a linear trend with increasing ap-plied force.Please note that, within the elastic limit of the materials, the ML intensity of most ML materials is linearly related to the applied stress.This property is also one of the prerequisites for the ML material that can be used for strain sensing.Beyond the elastic range, the luminous intensity will be saturated. [54]he saturation will also depend on whether the stress can completely release the carriers that can be used for luminescence.If the stress/impact is large enough, the luminescent center is fully excited through energy transfer, for example, if the groundstate d-electrons of manganese ions are completely excited by deformation, then there will be saturation.As the mechanism of ML, including the relationship between light intensity and applied stress, is usually related to the joint influence of factors from different aspects, such as defects in the host materials, crystal structures, carrier type, quantity, etc. ZnS and CaZnOS are semiconductor-type hosts, and the ML of semiconductors, such as manganese-doped ZnS.Chandra did some theory research; [55] while the experimental phenomena were independently reported by Xu and Jeong et al. [21,56] The total ML intensity was determined from the spectra as the integral of intensity over wavelengths and time.In general, the ML of the ZnS/CaZnOS:Mn 2+ sample was highly sensitive to mechanical stress and emitted ML even when a mild force was applied (below 1 N). Figure 2d shows the results of ML measurements at 0.25, 24, 48, 72, 112, and 232 h after the UV irradiation of the sample.In each of the ML signals shown in Figure 2d, the six strong peaks correspond to six rod movements across the sample.An example set of ML spectra obtained 72 h after irradiation is shown in Figure 2e.As shown in Figure 2d,e, each of the six movements of the rod across the sample plate generated an ML signal of the same shape and similar intensity.In Figure 2f, the average values (with their respective standard deviations) of the most robust ML signals generated by the six rod movements as a function of the time elapsed from the end of the UV irradiation stage are calculated based on the results shown in Figure 2d.The per-day decrease in ML intensity is relatively small, and the sample retains ML ability even after a long time after irradiation.Notably, the ML intensity depends on the time elapsed after irradiation and the applied force; however, the spectral range and shape of the ML spectra remain unchanged and are similar to those of the PL spectrum.Another essential advantage of the ZnS/CaZnOS:Mn 2+ material is that after repeated mechanical loading, ML can be emitted without sample recharging with UV irradiation.Figure 2g depicts the results of ML signal sustainability and repeatability under a series of mechanical stimuli.
When the ZnS/CaZnOS:Mn 2+ powder is introduced into a polymer host, such as a 3D-printed polymer or cellulose fibers, its ML can still be observed.However, as expected, its intensity is lower because of the limited light transmission and scattering effects in these materials, as well as because the ML-active components are diluted in the polymer and fiber matrices.Mechanical properties such as the stiffness and viscoelasticity of the polymer matrix are also crucial for the effectiveness of transferring the external mechanical load exerted on the polymer sample to the ML particles distributed inside.Figure 2h shows a comparison of the ML spectra of the ZnS/CaZnOS:Mn 2+ powder layer, 3D-printed polymer, and cellulose fibers.For both ZnS/CaZnOS:Mn 2+ in the polymer and ZnS/CaZnOS:Mn 2+ in the fibers, the samples were placed on a PMMA plate, and the ML phenomenon was induced by the movement of a glass rod pressed against a plate with a force of 30 N. Despite the different sensitivities (signal intensities) of the tested samples to mechanical loading, the shapes of the ML spectra were very similar (inset of Figure 2h).
The advantages of 3D-printing technology are the high adaptability of materials, relatively low cost, simplicity, minimal waste of raw materials, and ability to fabricate pieces with more complex geometries than those fabricated by traditional manufacturing methods.The schematic drawings and digital photographs of the 3D printer used to fabricate the polymeric 3D shapes are presented in Figure S6(Supporting Information).Details of the preparation and fabrication of the ML-active materials embed-ded in polymeric items, that is, 3D-printed polymers containing a ZnS/CaZnOS:Mn 2+ heterojunction, can be found in Supporting Information.Figure 3a illustrates the digital visualization of the 3D-printed folded structures and the distribution of ML-active components in their internal structures.Notably, the folded shape of the fabricated composites amplified the ML effect, for example, when the surface is rubbed with metal items.The uniform ML signal through the 3D-printed polymer materials indicates an even distribution of the inorganic heterojunction in the polymer, which initially consisted of urethane acrylate, an acrylic monomer, and a photoinitiator.Figure 3b presents a simplified scheme of the 3D-printer device and images of the fabricated pure (transparent) polymer items and corresponding composites containing the ML-active components (10 wt%), which are opaque and white due to the light scattering effects.Figure 3c shows photographs of the pure polymer resins under UV light irradiation (excitation at  ex = 365 nm), revealing the bright blue emission coming from the organic material.Figure 3d shows photographs of the bright orange ML of the 3D-printed polymers triggered by rubbing their surfaces with a metal spatula, taken in darkness and daylight (see the inset).A 3D visualization and digital photograph of the real device used for force-controlled triggering of ML are presented in Figure 3e.Depending on the initial height to which the lever ending in a metal rod is raised, the vertical stress system is elevated, and its mass, different forces can be generated, while the resulting pressure depends on the contact area with the material under study.An extremely bright orange ML light triggered during the impact of the metal rod on the polymer surface can be observed in the photograph, as shown in Figure 3f.Please note that videos of the ML phenomenon of the 3D-printed polymer under the stimulus of friction and impact can be found in Video S1 and Video S2 (Supporting Information).
The emission spectra of both the unmodified and modified polymers were recorded under 340 nm excitation light, as shown in Figure 3g.The unmodified polymer only exhibits a broad emission band located at ≈420 nm (blue region), whereas the modified polymer shows not only the same band located in the blue region corresponding to the organic components, but also a less intense band located at ≈590 nm, corresponding to the inorganic components.The PL images of the samples show different colors (see the insets of Figure 3g), that is, blue and orange luminescence for the unmodified and modified polymers, respectively.The bright blue emission originates from the organic part, whereas the orange one (observed through a long pass filter) comes from luminescence of the inorganic ZnS/CaZnOS:Mn 2+ component upon UV excitation.During ML measurements, the generated ML light was collected using a photomultiplier detector placed close to the test sample.The resulting ML signal intensities of pure ZnS/CaZnOS:Mn 2+ powder and the corresponding polymer composite as a function of the applied force and resulting pressure are presented in Figure 3h.In contrast to the linear dependence of the ML intensity on force for the powder sample, the polymer composite exhibited a parabolic dependence under the same measurement conditions, which is plausibly due to the high flexibility and elasticity of the 3D-printed materials.Importantly, using the 3D-printed ML-active polymer materials, pressure variations in a relatively low-pressure regime can be optically monitored, from a few up to ≈100 bar, a range previously -printing device used for printing the ML polymer and the 3D-printed polymer was doped with 10 wt% (white) and 0 wt% of the ML particles (transparent, in a different shape).c) Photos of the pure 3D-printed polymers under UV light irradiation (365 nm).d) Digital photograph of the friction-driven ML response of the 3D-printed polymer observed in darkness and in daylight (inset).e) Schematic configuration of the vertical stress system for applying impact on the ML polymer.f) The digital photograph of the corresponding impact-driven ML response.The inset shows the magnified photograph.g) The emission spectra of the unmodified and modified polymers under an excitation light of 340 nm.h) The integral ML intensity (IInte) in the range of ≈450-800 nm upon different loading pressures (MPa) (or loading forces) in 3D-printed polymer (red) and ML powder (blue).The error bars represented the standard deviations of the data sets.
inaccessible by any other optical sensing technique.Chandra proposed an ML technique to analyze elastic and plastic regions and fractures in different materials. [28]He found that the total intensity I T due to the impact of a sphere on a mechanoluminescent elastic material is proportional to the square of the pressure P exerted upon impact, which is assumed to be over than the threshold pressure for emission.
Considering that the pressure is the force per unit area of the surface under impact, and this area remains constant, the intensity is proportional to the square of the force.In the experiments, we observed a power of ≈1.8, close to the theoretical prediction (Figure 3h).A possible explanation for this small deviation is as follows: For each run of the experiment, the initial energy (E) is E ≈ mgh, where m is the mass, g is the gravity acceleration, and h is the initial height.If we assume that all the energy upon impact is transferred in the form of work, then Fd = mgh, where F is the force at impact and d is the deformation length of the elastic material.If A is the impact surface, the pressure upon impact is P = mgh/Ad.As d can be an increasing function of h, the intensity as a function of the force will show a power smaller than 2.However, in the case of the ZnS/CaZnOS:Mn 2+ powder (see Figures 2c and 3h), the dependence of the ML intensity with the applied pressure is linear.As pressure increases, a saturation of the ML intensity is expected.In this sense, the ML intensity versus pressure will initially be a parabolic function and finally saturate at some value.For the 3D-printed ML polymer, the measured pressure range is in the initial parabolic regime, while for the ML powder, it is far from that regime, and consequently the dependence of intensity with pressure should be smaller than parabolic.Further studies are required for exploring the limited regime, which may be our future works.
Furthermore, ML particles were used to develop an ML fabric, which was achieved by modifying cellulose fibers with a ZnS/CaZnOS:Mn 2+ heterojunction and their subsequent processing into a knitted fabric.Figure 4a shows a simplified schematic of the ML fabric.Basic information about the mechanical, structural, and physical properties of the ML fabric is shown in Tables S2-S4 (Supporting Information).The ML particle content in the cellulose fibers was ≈2.2 wt%.The SEM images and EDX mapping (Figure 4b-f) also undoubtedly confirmed the presence of ML particles in the fibers.Moreover, the PL properties of the modified fabric were investigated and compared with those of unmodified cellulose fibers.Upon exposure to 360 nm UV light, the fabric made of ML fibers exhibited bright orange luminescence (see Figure 4g), which overcame the faint blue luminescence (derived from cellulose) observed in the case of unmodified cellulose fibers (see the inset in Figure 4g).In the emission spectra (see Figure 4h), both the ML fabric and unmodified cellulose fibers were excited at 335 nm, and a broadband in the blue region of the spectrum (≈420 nm) was observed.In contrast, a second, much more intense broadband in the orange region of the spectrum (≈590 nm) was observed only for the ML fabric.Figure 4i presents digital photographs of the bright orange ML of the prepared fabric made of ML cellulose fibers, triggered by rubbing their surface with a glass rod in reciprocal motion.
The corresponding video of the ML phenomenon is shown in Video S3 (Supporting Information).The superior PL and ML performances indicate that the developed materials have the potential for modern applications in PL/ML multimode cellulose-fiberbased devices.

Luminescent Thermometry
The emission of Mn 2+ ions originates from the 4 T 1 → 6 A 1 transition, where the lowest 4 T 1 emission level is highly sensitive to the surrounding environment through crystal field interactions.Therefore, it can be affected by internal and external factors, such as temperature or pressure, leading to a change in spectroscopic features.To explore the feasibility of ZnS/CaZnOS:Mn 2+ as a PL temperature-sensing material, we examined the PL excitation, emission spectra, and lifetimes of the 4 T 1 excited state as functions of temperature.Its temperature detection performance was analyzed based on the excitation spectra measured in the temperature range of 25-500 K.As shown in Figure 5a, in the contour map of the excitation spectra recorded at the emission wavelength of 590 nm, it is clear that an increase in temperature resulted in a decline in the intensity of excitation bands, as the probability of nonradiative transitions increases.Moreover, in the normalized excitation spectra (see the inset in Figure 5b), all the excita- tion bands show a spectral shift to longer wavelengths.The most intense excitation band, which was initially centered at 339 nm at 25 K, exhibited the most significant shift.Therefore, we correlated the band-centroid values of the most intense excitation band with temperature.The excitation band centroid ( ex-centroid ) versus temperature was well fitted to third-order polynomial functions with R 2 = 0.999.For practical temperaturesensing applications, sensitivity must be investigated to quantitatively evaluate the temperature-sensing ability.For luminescent thermometry based on the spectral shift or bandwidth, absolute sensitivity (S a ) is commonly used, which is usually expressed in nm or cm −1 and represents the spectral shift or bandwidth change per 1 K of absolute temperature.However, when the luminescence thermometer is based on the band intensity ratio or lifetime (decay curve), the relative sensitivity (S r ) is commonly to indicate the sensitivity of a measured thermometric parameter, specifically the percentage change in response for every 1 K increases in absolute temperature. [57]The S a and S r values with respect to temperature are presented by the following functions: [57] where MP denotes the measured thermometric parameter.Table S1 (Supporting Information) lists the fitting parameters that were obtained.Figure 5c illustrates the temperature sensitivities determined for  ex-centroid with respect to temperature are shown in Figure 5c.With increasing temperature, the absolute sensitivity (S a ) for  ex-centroid , that is, |d( ex−centroid )/dT|, shows an increasing trend with a maximum shift rate of 0.168 nm K −1 at 500 K (≈15.93 cm −1 K −1 ).The temperature-dependent PL spectra of the examined materials are shown in Figure 5d.The PL emission intensity showed a monotonically decreasing trend as the temperature increased from 20 to 600 K.As demonstrated in the inset of Figure 5d, the normalized emission spectra show that the emission band attributed to the 4 T 1 → 6 A 1 d-d transition of the Mn 2+ ion presents a significant band broadening with increasing temperature.Phonon-assisted processes largely govern the absorption and emission band profiles, that is, band shift and width, when the temperature intervenes in the optical properties of transition metal ions. [58]Within the Coordinate Configurational Model, the temperature-induced broadening of the absorption and emission bands is caused by the electronphonon interaction and their difference between the initial and final states of the transitions.The theoretical expression developed, that is, where S is the Huang-Rhys parameter, related to the electronphonon coupling.This equation predicts that the width of the band increases with temperature. [58]ecause of the unilateral broadening of the emission band as a function of temperature, the centroid of the emission band exhibits a significant spectral shift.We correlated the determined bandwidth (full width at half maximum, FWHM) and band centroid ( em-centroid ) of the emission band with the temperature in the measured temperature range of 20-600 K, as shown in Figure 5e.The determined FWHM shows a monotonic increasing trend with temperature, from 51.9 nm at 20 K to 78.23 nm at 600 K. On the other hand, the  em-centroid shows an increasing tendency with temperature, that is, 587.23 nm at 20 K to 597.45 nm at 140 K. Afterward,  em-centroid decreases continuously with a further increase in the temperature to 592.01 nm at 600 K. Transition metal ions are strongly coupled to the immediate ligands through the crystal-field interaction, in which the vibrational modes of these bonds play an important role.As in the case of Cr 3+ ion in ruby, the most used pressure sensor below 500 K, the temperature-induced energy shift of the emission lines is caused by the electron-phonon interaction.This energy shift changes with the phonon states occupied in the initial and final states of the transition and their different depends on the different change of the mean square displacement of atoms, related to the Huang-Rhys parameter, with temperature. [59]This parameter is also related to the intensity difference between the two components of the 4 T 1 → 6 A 1 d-d transition of the Mn 2+ ion in ZnS/CaZnOS:Mn 2+ material, leading to the change in the centroid value.The temperature-dependent FWHM and emission band centroid ( em-centroid ) were determined, as shown in Figure 5e.Notably, temperature sensing based on  em-centroid was not effective below 140 K, while that for FWHM could be used in a broader temperature range, that is, down to 40 K.Both the temperature dependence of FWHM and  em-centroid can be well fitted to the 3rd-order polynomial functions, with R 2 ≥ 0.999.Table S1 (Supporting Information) presents the fitting parameters that correspond to the data.Figure 5f shows the S a values as a function of temperature based on the FWHM and  em-centroid .The maximal S a values for FWHM and  em-centroid are 0.057 nm K −1 (≈1.71 cm −1 K −1 ) at 373.6 K and 0.015 nm K −1 (≈0.43 cm −1 K −1 ) at 496.1 K, respectively.The luminescence decay curves as a function of temperature for the 4 T 1 → 6 A 1 d-d transition of the Mn 2+ ion in ZnS/CaZnOS:Mn 2+ at  em = 430 nm and  ex = 280 nm, as depicted in Figure 5g.Due to technical limitations in the availability of a high-power, focusable light source, the sample was excited at a wavelength of 280 nm, which was in close proximity to the optimal excitation wavelength.A strong temperature dependence is observed in the recorded emission decay profiles in the temperature range of 15-500 K (Figure 5g).The decay profiles were non-exponential; thus, the average decay times for the Mn 2+ emission were calculated using the following equation: where I(t) denotes the PL signal intensity.The calculated average decay times are shown in Figure 5h.The average decay time decreased with increasing temperature, similar to the intensitytemperature curve.Above this temperature, in the tested temperature range up to 500 K, the decay time practically did not change.However, the decay time change as a function of temperature is similar to the intensity of the emission excited at 430 nm, which is caused by the temperature quenching of the excited state of Mn 2+ .We employed the single-barrier model to fit the emission lifetime determined for temperatures ranging from 10 to 325 K. [60,61] Table S1 (Supporting Information) lists the corresponding fitting parameters.A decrease in the lifetime was observed from 1.51 to 0.94 ms as the temperature was increased from 10 to 325 K. To analyze the temperaturesensing performance based on PL lifetime, the corresponding temperature-dependent S r value was calculated using Equation 2, as shown in Figure 5i.When the temperature ranges from 80 to 325 K, S r shows an increasing trend, reaching a maximal value of 0.40% K −1 at 325 K.However, when the temperature was below 80 K, the relationship between lifetime and temperature did not reveal a clear trend, showing a very small S r (close to 0).Notably, the developed sensing material can be applied to multiparameter thermometry to overcome the disadvantages of single-mode temperature sensors.Good temperature sensitivity can be achieved by selecting different temperature-sensing approaches (excitation/emission band shift, emission band broadening, and lifetime).The temperature-sensing performances obtained using different thermometric parameters are summarized in Figure 5. Table S4 (Supporting Information) presents a comparative analysis of the sensing performance of the developed temperature sensor with that of other reported thermometers.Clearly, our material can not only realize 4-approach temperature sensing (i.e., excitation band shift, emission band shift, FWHM, and PL lifetime) but is also one of the most sensitive thermometers in each category.In particular, the temperature sensitivity of the developed sensor based on the excitation band shift shows a superior S a of 15.93 cm −1 K −1 , which is approximately six times higher than that of the previously reported leader, Na 4 Mg(WO 4 ) 3 . [62]Noteworthy, the developed multiparameter temperature sensing materials can overcome the drawback of a single temperature sensing approach, ensuring satisfactory S r in the whole temperature range studied.Such a good temperature-sensing performance indicates that the developed 4-parameter temperature sensing material with good thermal sensitivity allows temperature detection over the entire measured temperature range.

Luminescent Manometry and Pressure Stability
In order to investigate the impact of pressure on the PL features of the ZnS/CaZnOS:Mn 2+ material and explore the feasibility of its application as an optical pressure sensor, an optical measurement setup was built; the corresponding schematic configuration is shown in Figure 6a.We used a 532 nm laser equipped with an optical bandpass filter and lens as a highly efficient excitation light source.High-pressure luminescence measurements were conducted in a diamond anvil cell (DAC) at RT for up to ≈19.20 GPa.The normalized emission spectra as functions of pressure and the corresponding determined band centroids are shown in Figure 6b,c, respectively.The broad emission band of the 4 T 1 → 6 A 1 transition gradually shifts to the higher wavelength with increasing pressure.The observed spectral shift can be attributed to an overall contraction of the ground and excited configurations, and thus, the photon energy from the 4 T 1 → 6 A 1 spin forbidden d-d transition, as well as the reduction in the energy gap between them, that is, the inter-configurational emissions between the multiplets of the ground and first excited configurations.
The calibration curves for the determined band centroids in the compression-decompression cycles are presented in Figure 6b.Under compression from 0.36 to 19.2 GPa, the emission band shift from 618.57 to 736.76 nm undergoes a total spectral redshift of ≈118.2 nm (2593.3cm −1 ).The corresponding pressure dependence of the band centroid was well fitted (R 2 = 0.998) with a linear response and a calculated shift rate (d/dP) of 6.20 nm GPa −1 .The corresponding fitting parameters are listed in Table S2 (Supporting Information).[65][66] Compared to recent leaders in terms of sensitivity, for example, BaLi 2 Al 2 SiN 6 :Eu 2+ material (d/dP = 1.58 nm GPa −1 ), [67] YVO 4 :Er 3+ /Yb 3+ (d/dP = 1.766 nm GPa −1 ), [68] and Ca 2 Gd 8 Si 6 O 26 :Ce 3+ (d/dP = 3.00 nm GPa −1 ), [69] the ZnS/CaZnOS:Mn 2+ material still shows its superiority.This result indicates that the developed pressure sensor can be applied as a pressure sensor with high pressure sensitivity.
In addition, the Commission Internationale de I'Eclairage (CIE) chromaticity diagram for the developed pressure sensor and the corresponding luminescence image of the samples (taken under different pressure conditions) under irradiation with a 532 nm laser are shown in Figure 6d,e, respectively.The luminescence color presented in the CIE diagram (based on the PL emission spectra under pressure) agreed well with the luminescence images.These results indicate that the developed sensor can be used as a visual pressure sensor.The corresponding CIE chromaticity coordinates are illustrated in Table S6 (Supporting Information).
As shown above, the luminescence properties of the developed materials changed significantly when high pressure was applied.Because pressure stability is crucial for the developed pressure sensor, the question of the phase transition of the material occurring under high-pressure conditions arises.The pressuredependent Raman spectra of the ZnS/CaZnOS:Mn 2+ samples are shown in Figure S7a (Supporting Information).As the pressure increased from 0.36 to 19.2 GPa, a linear shift toward higher energy values was observed in all Raman modes.This indicates that the energy of the phonon modes also increased, which is related to the bond shortening that occurs in a compressed structure.However, the behavior of the Raman modes changed and showed abnormal shifts when the pressure exceeded 19.2 GPa.This result agrees well with the phase transition of CaZnOS materials reported in the literature, [70] that is, a reversible phase transition from a hexagonal crystal to a triclinic system under pressure occurs at ≈19 GPa. Figure S7b (Supporting Information) shows that the Raman mode centroids exhibit a consistent shift rate from 0.36 to 19.2 GPa.In other words, linear shifts to higher frequencies were observed for all the Raman modes initially located at 1730.4,1827.90, 2064.82, and 2107.81cm −1 , exhibiting shift rates of 6.92, 8.44, 9.65, and 9.05 cm −1 GPa −1 , respectively.Notably, owing to the increasing strain and crystal defects in the compressed material, an overall intensity deterioration in the Raman signal was observed with pressure.These results demonstrate good pressure stability for pressures below 19.2 GPa and that the ZnS/CaZnOS:Mn 2+ phosphors are suitable for visual high-sensitivity pressure sensing.

Conclusion
We have demonstrated the development of an advanced PLand ML-based multifunctional sensing platform based on heterojunctioned ZnS/CaZnOS:Mn 2+ mechano-photonic materials.In the case of PL properties of the materials studied, the pressure-driven giant spectral shift of the emission band demonstrates a superior high-pressure sensitivity, being as high as ≈6.20 nm GPa −1 .Whereas, for temperature detection, four different strategies were employed, that is, excitation/emission band shift, bandwidth change, and lifetime shortening.On the other hand, by incorporating ML particles into the cellulose matrix, we prepared ML cellulose-based fibers capable of emitting light, which can be generated by mechanical stimuli in ML materials.We also fabricated optically active 3D-printed units that exhibit intense ML in response to mechanical deformations, such as impact and friction.Importantly, the ML affect also allows lowpressure sensing up to ≈100 bar, which is a range previously inaccessible by any other optical sensing technique.These results indicate that the ZnS/CaZnOS heterojunction can be used as a multifunctional and multimodal sensing platform for friction, force, pressure, and temperature in the form of powder, fibers, and 3D-printed polymers.Because of the possibility of scaling up the fabrication process of 3D-printed polymer items and fiberbased materials containing ML compounds, our study also substantiates the possibility of the mass production of ML-active mechanical and optoelectronic parts integrated with scientific and industrial tools and apparatus.

Figure 1 .
Figure 1.General mechanism of the sensing platform.Schematic of the proposed mechanism for the dual-mode pressure sensing in the ZnS/CaZnOS:Mn 2+ heterojunction: a stress-induced ML mode sensing mode (left) and a highly sensitive simultaneous temperature-and pressureinduced PL sensing mode (right).

Figure 2 .
Figure 2. ML properties.a) ML spectra measured 1, 72, and 232 h after irradiation, and b) generated with different force values.c) Dependence of total ML intensity on the applied force.ML signals were taken from two movements of a glass rod across the sample plate, with a time gap of 4 s.The error bars represent the maximum deviations from the set force values.d) Integrated ML signals collected from the same part of the sample, measured at different times after UV irradiation.ML was generated by six movements of the rod, pressed toward the sample plate with 12 N.The drawing speed was 6 mm s −1 .e) Set of ML spectra measured 72 h after UV irradiation.f) Dependence on time after UV irradiation of the mean of the integrated ML intensities, taken from six movements of the rod across the sample.The error bars represent the standard deviations of the data sets.g) Integrated ML signal, generated by 40 movements of the glass rod across the sample, pressed toward the sample with a force of 8 N. h) ML spectra of ZnS/CaZnOS:Mn 2+ powder layer, ZnS/CaZnOS:Mn 2+ introduced into 3D-printed polymer and cellulose fabric containing ZnS/CaZnOS:Mn 2+ .The inset shows the corresponding normalized ML spectra.The error bars represented the standard deviations of the data sets.

Figure 3 .
Figure 3. ML and PL properties of the 3D-printed polymer.a) Schematic configuration of the ML polymer in combination with ML hybrids and phosphor embedded within acrylic matrix.b) 3D-printing device used for printing the ML polymer and the 3D-printed polymer was doped with 10 wt% (white) and 0 wt% of the ML particles (transparent, in a different shape).c) Photos of the pure 3D-printed polymers under UV light irradiation (365 nm).d) Digital photograph of the friction-driven ML response of the 3D-printed polymer observed in darkness and in daylight (inset).e) Schematic configuration of the vertical stress system for applying impact on the ML polymer.f) The digital photograph of the corresponding impact-driven ML response.The inset shows the magnified photograph.g) The emission spectra of the unmodified and modified polymers under an excitation light of 340 nm.h) The integral ML intensity (IInte) in the range of ≈450-800 nm upon different loading pressures (MPa) (or loading forces) in 3D-printed polymer (red) and ML powder (blue).The error bars represented the standard deviations of the data sets.

Figure 4 .
Figure 4. ML and PL properties of the cellulose fabrics.a) A schematic configuration of the ML fabric composed of cellulose fibers modified with the ZnS/CaZnOS:Mn 2+ ML particles.b,c) The EDX elemental mapping of the ML cellulose fibers, showing the elemental distribution mapping of oxygen (b) and sulfur (c), which is a representative element for ML particles.d-f) The SEM image of the ML cellulose fibers at three different scales.g) Luminescence emission spectra of ML fabric and unmodified fibers,  ex = 335 nm.h) Luminescence photograph of modified fabric and unmodified cellulose fibers (the inset of (h)) under UV-light irradiation,  ex = 365 nm.i) Digital photographs of the knitted fabric made of ZnS/CaZnOS:Mn 2+ doped cellulose fabric upon rubbing with a glass rod.

Figure 5 .
Figure 5. Temperature sensing properties.a) Temperature dependence of the PL excitation spectra.b) The temperature-dependent band centroid of the 6 A 1 → 4 T 1 excitation band.c) The determined absolute temperature sensitivity (S a ).d)The PL emission spectra as a function of temperature.e,f) The temperature-dependent band centroid of the Mn 2+ : 4 T 1 → 6 A 1 transition emission band (e) and S a (f).g) PL kinetic profiles for the ZnS/CaZnOS:Mn 2+ material (Mn 2+ : 4 T 1 → 6 A 1 transition) at  em = 590 nm and  ex = 340 nm, measured in the temperature range of 15-500 K. h) Luminescence lifetimes of the Mn 2+ : 4 T 1 → 6 A 1 transition, as a function of temperature.i) The relative sensitivity, S r based on the Mn 2+ : 4 T 1 → 6 A 1 transition emission lifetime.The error bars represented the standard deviations of the data sets.

Figure 6 .
Figure 6.Pressure sensing properties.a) Simplified scheme of the high-pressure measurement setup.b) Pressure-dependent peak centroid of the Mn 2+ emission band.c) Normalized photoluminescence emission spectra, measured at various pressure values, upon a 532 nm laser excitation.d) CIE diagram for ZnS/CaZnOS:Mn 2+ materials measured at different pressures.e) Corresponding PL photographs under UV irradiation in a DAC at selected pressures of 0.36, 3.14, 11.06, and 19.2 GPa.The error bars represented the standard deviations of the data sets.