Mechanoluminescence Research Articles

Occlusal splint with mechanoluminescence properties based on BaSi2O2N2:Eu2+ cyan phosphor prepared by deposition-precipitation method

A series of Eu2+ activated BaSi2O2N2 oxynitride cyan phosphors with high crystallinity have been successfully synthesized by a facile deposition-precipitation (DP) method. Under excitation of 375 nm, the photoluminescence emission intensity at 495 nm was enhanced by 157% for BaSi2O2N2:Eu2+ prepared by DP method compared to the sample prepared by traditional solid-state reaction (SSR) method due to the uniform distribution of Eu2+ ions. Accordingly, BaSi2O2N2:0.02Eu2+ phosphor prepared by DP method exhibits a longer fluorescence lifetime (553.6 ns) than that (472.8 ns) prepared by SSR method. Interestingly, Eu2+ ions can incorporate in BaSi2O2N2 lattices through DP process even at a lower temperature (1000 °C) compared to the conventional temperature (1450 °C) during SSR process. Finally, the flexible occlusal splints based on BSON:Eu2+/PDMS composite elastomers has been fabricated by the excellent mechanoluminescent (ML) property of BaSi2O2N2:Eu2+ phosphors. The ML signal of BSON:Eu2+/PDMS occlusal splints under mechanical stimulation allows visualization of force magnitude during occlusal examination for the treatment of clinical oral disorders such as temporomandibular disorders.

Realizing Photoswitchable Mechanoluminescence in Organic Crystals Based on Photochromism.

Organic mechanoluminescent (ML) materials possessing photophysical properties that are sensitive to multiple external stimuli have shown great potential in many fields, including optic and sensing. Particularly, the photoswitchable ML property for these materials is fundamental to their applications but remains a formidable challenge. Herein, photoswitchable ML is successfully realized by endowing reversible photochromic properties to an ML molecule, namely 2-(1,2,2-triphenylvinyl) fluoropyridine (o-TPF). o-TPF shows both high-contrast photochromism with a distinct color change from white to purplish red, as well as bright blue ML (λML = 453nm). The ML property can be repeatedly switched between ON and OFF states under alternate UV and visible light irradiation. Impressively, the photoswitchable ML is of high stability and repeatability. The ML can be reversibly switched on and off by conducting alternate UV and visible light irradiation in cycles under ambient conditions. Experimental results and theoretical calculations reveal that the change of dipole moment of o-TPF during the photochromic process is responsible for the photoswitchable ML. These results outline a fundamental strategy to achieve for the control of organic ML and pave the way to the development of expanded smart luminescent materials and their applications.

Two Organic–Inorganic Manganese(II) Halide Hybrids Showing Compelling Photo- and Mechanoluminescence as well as Rewritable Anticounterfeiting Printing

Two organic-inorganic manganese(II) halide hybrids (OIMHs) with formulas of [(TEA)(TMA)]MnCl4 (1) and [(TPA)(TMA)3](MnCl4)2 (2) (TEA = tetraethylammonium, TMA = tetramethylammonium, and TPA = tetrapropylammonium) were synthesized by a mixed-ligand strategy. Both compounds crystallize in the acentric space group and are composed of isolated [MnCl4]2- tetrahedral units separated by two types of organic cations. They show high thermal stability and emit strong green light with different emission bandwidths, quantum yields, and high-temperature photostability. Remarkably, the quantum yield of 1 can reach up to 99%. Due to the high thermal stability and quantum yield of 1 and 2, green light-emitting diodes (LEDs) were fabricated. Furthermore, mechanoluminescence (ML) was observed in 1 and 2 when stress was applied. The ML spectrum of 1 is similar to the photoluminescence (PL) spectrum, suggesting ML and PL emissions come from the same transition of Mn(II) ions. Finally, rewritable anticounterfeiting printing and information storage were achieved by utilizing the outstanding photophysical properties and ionic features of the products. The printed images still remain clear after several cycles, and the information stored on the paper can be read out by a UV lamp and commercial mobile phones.

Ratiometric mechanoluminescence in LiSrPO4:Eu2+/Eu3+ for stress sensing: Dopant concentration dependent sensitivity

Ratiometric mechanoluminescence (ML) has been recently proposed for stress-sensing, which is regarded to be superior to the traditional ML-intensity based sensing route in terms of accuracy and cycle stability. The dual-activator strategy to realize the ratiometric ML takes advantage of instantaneous change of local crystal field environment induced by stress, where the structural characteristics of host are supposed to play major role. Herein, LiSrPO4:Eu2+/Eu3+ phosphors have been explored to realize the ratiometric ML, with the focus on the influence of Eu dopant concentration on the sensitivity of the stress-dependent ratiometric ML. The dependence between the dopant concentration and sensitivity can be interpreted in terms of host phase structure evolution with dopant concentration.

Dynamic anti-counterfeiting and information encryption of Sr3Y2Ge3O12: Tb3+, Er3+ phosphor via carriers filling and release processes

Complex and high-security-level anti-counterfeiting strategies with multiple luminescent modes are extremely critical for meeting the requirement of constantly developing information storage and information security. In this work, Tb3+ ions doped Sr3Y2Ge3O12 (SYGO) and Tb3+/Er3+ co-doped SYGO phosphors are successfully fabricated and are unitized for anti-counterfeiting and information encoding under distinct stimuli sources. The green photoluminescence (PL), long persistent luminescence (LPL), mechano-luminescence (ML), and photo-stimulated luminescence (PSL) behaviors are respectively observed under the stimuli of ultraviolet (UV), thermal disturbance, stress, and 980 nm diode laser. Based on the time-dependence of the filling and releasing rate of the carriers from the shallow traps, the dynamic information encryption strategy is proposed by simply changing the UV pre-irradiation time or shut-off time. Moreover, a tunable color from green to red is realized by prolonging the 980 nm laser irradiation time, which is attributed to the elaborate cooperation of the PSL and upconversion (UC) behaviors. The anti-counterfeiting method based on SYGO: Tb3+ and SYGO: Tb3+, Er3+ phosphors herein possess an extremely high-security level with attractive performance for designing advanced anti-counterfeiting technology.

Mechanoluminescence Affected by Trap Types and Excitation State Positions in Mg3Ca3(PO4)4:Eu2+/Mn2+/Ce3+ for Multimode Anticounterfeiting.

Mechanoluminescence (ML) materials with tunable emissions can serve in many practical applications; however, their underlying mechanism still needs further clarification. Herein, we developed Eu2+-/Mn2+-/Ce3+-activated Mg3Ca3(PO4)4 (MCP) phosphors and studied their luminescence properties by device fabrication. The intense blue ML is obtained by fabricating MCP:Eu2+ into the polydimethylsiloxane elastomer matrix. The red ML of relatively weak intensity is received in Mn2+ activator, but the ML for the Ce3+ dopant is nearly quenched in the same host. The possible reason is proposed from the analysis of the relative positions between the excitation state and conduction band, together with the trap types. The appropriate location of the excited energy levels in the band gap allows for a larger probability of efficient ML when shallow traps near the excitation states are created synchronously as an effective energy transfer (ET) channel. The concentration-dependent ML for the MCP:Eu2+,Mn2+-based devices indicates that the emitting light color can be tailored, where several ET processes among oxygen vacancies, Eu2+, Ce3+, and Mn2+, occur. The luminescence manipulation with dopants and excitation sources demonstrates the potential applications in visualized multimode anticounterfeiting. These findings open up many possibilities for constructing new ML materials by introducing appropriate traps into the band structures.

Environmentally Stable and Self-Recovery Flexible Composite Mechanical Sensor Based on Mechanoluminescence

Mechanoluminescent materials featuring mechanical-to-optical conversion have been vigorously studied as a carrier of optical sensing technology in the field of mechanical sensing due to their self-recovery, visualization, and non-contact sensing characteristics. However, the impact of the human living environment such as temperature, humidity, corrosive substances, etc. on optical sensing materials cannot be ignored, which greatly affects the accuracy of optical sensing. To overcome these barriers to the accuracy of these factors, a flexible mechanoluminescent composite device with high brightness in a bright and dark environment based on Ce3+-doped garnet structure phosphors Tb3Al5O12 was proposed. Under the action of repeated mechanical stimulation, the mechanoluminescence (ML) intensity presents good self-recovery performance, and the ML intensity shows a zero-thermal-quenching performance in the range of 0–100 °C. In addition, the evaluation of moisture resistance was performed under different durations in the water, proving that the intensity of ML can hardly be affected by the humid environment. Accordingly, a transmission system based on a robot finger was proposed to simulate the application of the human tactile system. The results showed great potential for Tb3Al5O12: Ce3+-based ML composite devices in advanced optical multimode sensors to promote their diversified application.

Research of Stress Detection Based on Mechanoluminescence

Stress and stress concentration are one of the main factors of invalidating load-bearing structural members. Stress detection becomes an important part of industrial production. Mechanoluminescent (ML), which is produced by mechanical stimulation acting on materials, has been suggested to use in stress detection. This work focuses on the development and mechanism of ML, concludes with the applications of ML on the stress measurement, and discusses the specific challenges to the future directions of ML.

<i>In Situ</i> Quantitative Measurement of Stress Distribution in Tensile Specimen Using a Detachable Mechanoluminescence Film

Mechanoluminescence (ML) materials, e.g., SrAl2O4:Eu2+, emit visible light when mechanical stress is applied. It was reported that the intensity of the light is in proportion to the applied von Mises stress. Therefore, acting von Mises stress can be measured from the intensity using a calibration curve. To make the ML measurements easier and more flexible, a detachable ML film has been newly developed. One surface of the base sheet was coated with detachable adhesive, while the other was coated with ∼5 µm thick ML layer. Two ML films were taken from one ML sheet so that the two pieces have identical ML properties. One piece was pasted on an A6061 aluminum alloy test piece (JIS13B, t = 1.0 mm) to obtain a calibration curve by the tensile test. The other piece was pasted on another A6061 test piece with a circular opening at the center to reveal the nonuniform stress distribution. The von Miese stress converted from the ML intensity showed a good agreement with the prediction by finite element analysis (FEA). It is concluded that the detachable ML film is sufficiently accurate and practical means to measure stress distribution in the target material.

Recoverable Dual-Modal Responsive Sensing Materials Based on Mechanoluminescence and Thermally Stimulated Luminescence toward Noncontact Tactile Sensors.

Tactile sensing with stress and temperature sensing as core elements have shown promising prospects in intelligent robots and the human-machine interface. Mechanoluminescence (ML)-based stress sensing can realize the direct sensing of mechanical stimulation, whereas indirect temperature sensing based on luminescent sensing materials usually requires the stimulation of extra light or force. Herein, a trap-controlled material Sr2MgAl22O36:Mn2+ with bifunctional mechano/thermal sensing applications was developed and investigated in detail. Visualized bright green-emitting ML and thermally stimulated luminescence (TSL) directly and rapidly responded to mechano/thermal dual stimulation in the Sr2MgAl22O36:Mn2+/PDMS composite film. It is worth mentioning that this study proposed a new idea of direct temperature sensing by the initial intensity of TSL due to thermal-photo energy conversion, unlike previous temperature sensor technology. Based on this, we designed a flexible optical skin with a simple structure and verified its application prospect as a tactile sensing material with dual mechano/thermal response, establishing a unique imaging mode and providing a convenient, reliable, and sensitive way to remotely visualize the distribution of stress and temperature. This study paves a new way for the development of optical skins with simple structures and sensitive visibility in the application of intelligent robot tactile sensing.

Mechanoluminescent Device: In Situ Renewable Carbazole Derivatives Sandwiched by Self-Healing Disulfide-Containing Polyurethane for Mechanical Signals Detection

Mechanoluminescent (ML) materials can emit visible light by utilizing mechanical energy, which shows unique advantages in visual mechanical sensing, displays, and biomechanical monitoring due to the correlation between force stimulation and luminescence intensity. Most organic ML materials exhibit luminescence intensity attenuation, disappearing completely with force stimulation and failing to recover. Here, organic luminogens (Cz-alkyl6) can be synthesized by introducing a soft alkyl chain into the carbazole, which exhibits ML emission with self-assembly units. Furthermore, organic luminogens can be generated repeatedly by simply recrystallizing the fracture crystal in situ after a short thermal treatment (70 °C) within 14 s. More importantly, the quantitative correlation between force pressure and ML intensity has been established by a sandwich-type ML device based on a novel carbazole derivative (Cz-alkyl6). The ML device presents a capacity for detecting mechanical signals up to 13 N according to its ML intensity (≤275 a.u.), exhibiting potential application value in engineering damage detection, anticounterfeiting, and advanced visual mechanical sensing.

Modeling and validation of mechanoluminescent strain sensing mechanism at quasi-static loading rates

Mechanoluminescence (ML) sensors offer full-field strain/stress measurements and have the advantages of easy implementation, non-invasiveness, and low cost. However, knowledge of the mechanism of carrier detrapping owing to mechanical forces remains elusive as it may include one or more complicated processes. Several attempts to develop calibration models for converting the light emitted by ML sensors into mechanical stress have been reported, but few studies have tested the ML response under static or quasi-static loading rates. In this study, we built a real-time measurement system to evaluate the constitutive strain-luminescence relationship of ML sensors. The ML response was investigated at loading rates as low as 5 µm/s (i.e., strain rates as low as 16.2 μst/s). The results showed that the light of ML sensors was still excited under quasi-static loading rates. A double-exponential function was applied to describe nonlinear changes in light intensity over time. We used this function to modify the previous model and predict the ML responses under variable loading rate conditions. The results predicted by the model proposed in this paper are in good agreement with the experimental results.

Mechanoluminescent Visualization of Crack Propagation for Joint Evaluation.

In this study, methods for the mechanoluminescent (ML) visualization of crack propagation and mechanical behavior to evaluate adhesive joints are demonstrated and explained. The first step involved sample preparation; an air spray was used to apply ML paint to the surface of the adhesive joint specimens. The performance of the ML sensor was described to examine the measurement conditions. The results of ML sensing during a double cantilever beam (DCB) test and a lap-shear (LS) test are demonstrated as these are the most frequently and widely used methods for evaluating adhesives. Originally, it was difficult to directly quantify the crack tip and strain/stress distribution and concentration because the crack tip was too small, and the effects of the strain could not be observed. The mechanoluminescence, crack propagation, and mechanical behavior during mechanical testing can be visualized via the ML pattern during the adhesive evaluation. This allows for the recognition of the precise position of the crack tips and other mechanical behaviors related to structural failure.

Promoting intense mechanoluminescence by strengthening C-H⋯π interactions in thioxanthene derivatives

Pure organic mechanoluminescent (ML) materials are a kind of new environmentally friendly material that can be mechanically stimulated by external force, which has the potential to apply in stress imaging, health monitoring and structural defect detection. However, the mechanism of ML process in organic materials is still unclear and the molecular designing strategies for these materials remain insufficient. Herein, through simply strengthening intermolecular C-H⋯π interactions in the ML-inactive thioxanthene derivatives TxPh, the thiophene modified compound TxPhTh showed bright ML. The stronger C-H⋯π interactions suppressed the nonradiative transition channels during the ML process, and also boosted the photoluminescence quantum yield (PLQY) of crystalline TxPhTh. In addition, the molecular arrangements were switched from centrosymmetric (TxPh) to non-centrosymmetric (TxPhTh). Photophysical studies, detailed single-crystal and TD-DFT calculations were performed. These results provided a new path for designing ML materials and promoted the investigation of ML mechanism.

Smart Semiconductor-Heterojunctions Mechanoluminescence for printable and wearable sports light sources

Mechanoluminescence (ML) materials, which directly convert mechanical energy into light emission, have emerged as an emerging class of light-emitting materials with various applications.Although significant progress has been made in the development of new ML materials and devices, their practical application is limited by their low intensity and homogenously structured host structures. Thus, multiple-structured host ML materials with high ML intensity and reproducibility are needed, particularly for wearable and printable ML light sources with high brightness.In this study, various Mn-doped ternary heterojunction ML materials, CaZnOS-ZnS-SrZnOS, are synthesised by high-temperature solid-phase reactions, and their luminescence properties are systematically tested. The result indicates that the ternary heterojunction materials have a higher ML intensity over 60 times stronger than the state-of-art SrZnOS: Mn2+, which is currently the brightest sample we have obtained. ML linearly depends on the magnitude of the applied force. Based on the synthesised ternary ML heterojunctions, repetitive self-driving luminescence of the zipper is achieved for the first time through powder modification and inlay; the clothes composed of our prepared zippers are pulled and bent when worn on the body to emit bright visible light. Additionally, a Velcro that can achieve light emission is developed; it provides a new idea for the development of wearable flexible self-luminous devices in the future. Simultaneously, movement-driven stress luminous printable objects that can be printed on the surface of various commodities or their packaging are demonstrated. The preparation of self-driven, battery-free, and electrode-free stress luminescence wearable low light sources has an important application trend, which can avoid the interference of the circuit and improve the stability of the devices, thus promising a wide range of applications in smart wear, energy-saving displays, safety and rescue, robotic skin, and information security.

Controlled mechano-luminescence properties of SrGa2O4:Tb3+ co-doping with Dy3+ and Eu3+ ions.

Trap-controlled mechano-luminescence (ML) featuring photon emission under mechanical stimuli provides promising applications such as dynamic imaging of force, integrated optical sensing, information storage, and anti-counterfeiting encryption. However, the corresponding emission with a single color still limits the application of ML materials. Here, a trap-controlled ML phosphor of SrGa2O4:Tb3+ (SGO:Tb3+) with a green-emission is investigated with an adjustable ML color. The relationship between ML and thermoluminescence (TL) is verified by co-doping with Dy3+ and Eu3+ ions for the manipulation of the constructed traps. Accordingly, the as-explored ML phosphor with multicolor output is employed to create encrypted anticounterfeiting patterns, which produces bright and spatially resolvable optical codes under the single-point dynamic pressure of a ballpoint pen. Hence, it provides a new approach to achieve ML with multicolor and gives us an insight into understanding the mechanism of the ML procedure.

Cation-defect-induced self-reduction towards efficient mechanoluminescence in Mn2+-activated perovskites.

Mechanoluminescent (ML) materials have shown promising prospects for various applications, e.g. in stress sensing, information anti-counterfeiting and bio stress imaging fields. However, the development of trap-controlled ML materials is still limited, because the trap formation mechanism is not always clear. Here, inspired by a defect-induced Mn4+ → Mn2+ self-reduction process in suitable host crystal structures, a cation vacancy model is creatively proposed to determine the potential trap-controlled ML mechanism. Combined with the theoretical prediction and experimental results, both the self-reduction process and ML mechanism are clarified in detail, where the contribution of and defects dominates the ML luminescent process. Electrons/holes are mainly captured by the anionic/cationic defects, followed by the combination of electrons and holes to transfer energy to the Mn2+ 3d states under mechanical stimuli. Based on the multi-mode luminescent features excited by X-ray, 980 nm laser and 254 nm UV lamp, together with the excellent persistent luminescence and ML, a potential application in advanced anti-counterfeiting is demonstrated. These results will deepen the understanding of the defect-controlled ML mechanism, and inspire more defect-engineering strategies to develop more high-performance ML phosphors for practical application.

Ultrasound-activated luminescence with color tunability enabled by mechanoluminescent colloids and perovskite quantum dots.

Ultrasound represents a wireless and non-contact route for energy delivery and device control, owing to its ability to propagate and focus in various mediums including biological tissue. Specifically, ultrasound-activated mechanoluminescence from a colloidal suspension of mechanoluminescent (ML) nanocrystals offers a wireless means to remotely control a light source, such as wirelessly addressing a multicolor display. However, the limited color purity and tunability, as well as the large sizes of conventional ML materials prevent their use in an ultrasound-mediated flexible color display. Here, we apply a biomineral-inspired suppressed dissolution approach to synthesize ML colloids with bright blue emission under ultrasound and small sizes down to 20 nm. In addition, we leverage the bandgap engineering strategy of all-inorganic perovskite quantum dots (PQDs) to achieve wavelength tunability of the mechanoluminescence of ML colloid/PQD composites. Remarkably, the ultrasound-activated emission of the ML colloid/PQD composites exhibits a highly saturated color gamut covering the entire visible spectrum. Based on these advantages, we assembled a pixel array composed of different ML colloid/PQD composites in a silicone elastomer and demonstrated the proof-of-concept of a flexible and wireless multicolor display with each pixel individually addressed by scanning focused ultrasound.

Biosafety evaluation of BaSi2O2N2:Eu2+/PDMS composite elastomers.

In recent years, mechanoluminescent (ML) materials have shown great potential in stress sensing, mechanical energy collection and conversion, so they have attracted wide attention in the field of stomatology. In the early stage of this study, BaSi2O2N2:Eu2+ ML phosphors were synthesized by two-step high temperature solid state method, and then mixed with Polydimethylsiloxane (PDMS) in different proportions to obtain BaSi2O2N2:Eu2+/PDMS ML composites with different mass fractions (10%,20%,30%,40%,50%). Then its biosafety was evaluated by Cell Counting Kit-8 (CCK-8), Calcein-AM/PI fluorescence staining, hemolysis, oral mucosal irritation, acute and subacute systemic toxicity tests. The experimental results show that the biosafety of BaSi2O2N2:Eu2+/PDMS ML composite elastomers with different mass fraction is in line with the existing standards, and other related properties can be further studied.

Mechanoluminescence, thermoluminescence, optically stimulated luminescence and photoluminescence in SrAl2O4:Eu micro- and nanophosphors: effect of particle size and annealing in different atmospheres.

SrAl2O4:Eu in microcrystalline form was prepared by a combustion method. The formation of the material in a single phase was confirmed by XRD analysis. The material was crushed and sieved to get particles with different particle size ranges. It was further ball milled for 1-7 days to get particles in the nanosize ranges. The broadening of the XRD peaks of the phosphor material in nanocrystalline form was used to determine average particle sizes. The shapes and sizes of these particles could also be seen in FESEM images. The materials thus obtained were annealed in reducing (10% H2 in Ar) and oxidizing (in air) atmospheres at different temperatures for 1.0 h. The increase in the mechanoluminescence (ML) intensity on annealing in a reducing atmosphere at different temperatures and decrease on annealing in an oxidizing atmosphere could be attributed to redox reactions. This was further confirmed by PL measurements. Mechanoluminescence (ML), thermoluminescence (TL), and optically stimulated luminescence (OSL) of the materials were studied. In all three cases (i.e., ML, TL, and OSL), the intensities are found to decrease with the particle size. A large shift of approximately 20 °C in the main peak of TL glow curves of micro- and nanocrystalline materials shows a widening of the band gap due to the particle size effect. A decrease in piezoelectric constant (d33) and field (F V m-1) with particle size was also observed. The present systematic study of particle size effect (over a wide range of particle sizes) on ML has great importance from a technological and application point of view for developing stress sensors.