An intense elastico-mechanoluminescence material CaZnOS:Mn2+ for sensing and imaging multiple mechanical stresses

Realizing Near Infrared Mechanoluminescence Switch in LAGO:Cr Based on Oxygen Vacancy

Mechanoluminescence (ML) materials are featured with the characteristic of "force to light" in response to external stimuli, which have made great progress in artificial intelligence and optical sensing. However, how to effectively enable ML in the material is a daunting challenge. Here, a Lu3Al2Ga3O12:Cr3+ (LAGO: Cr3+) near infrared (NIR) ML material peaked at 706 nm is reported, which successfully realizes the key to unlock ML by the lattice-engineering strategy Ga3+ substitution for Al3+ to "grow" oxygen vacancy (Ov) defects. Combined with thermoluminescence measurements, the observed ML is due to the formation of defect levels and the ML intensity is proportional to it. It is confirmed by X-ray photoelectron spectroscopy and electron paramagnetic resonance that such a process is dominated by Ov, which plays a crucial role in turning on ML in this compound. In addition, potential ML emissions from 4T2 and 2E level transitions are discussed from both experimental and theoretical aspects. This study reveals the mechanism of the change in ML behavior after cation substitution, and it may have important implications for the practical application of Ov defect-regulated turn-on of ML.

Visible-to-Near-Infrared Mechanoluminescence in Bi-Activated Spinel Compounds for Multiple Information Anticounterfeiting

Mechanoluminescence (ML) is the nonthermal luminescence generated in the process of force-to-light conversion, which has broad prospects in stress sensing, wearable devices, biomechanics, and multiple information anticounterfeiting. Multivalence emitter ions utilize their own self-reduction process to realize multiband ML without introducing another dopant, such as Eu3+/Eu2+, Sm3+/Sm2+, and Mn4+/Mn2+. However, self-reduction-induced ML in bismuth-activated materials has rarely been reported so far. In this work, a novel visible-to-near-infrared (vis-NIR) ML induced by the self-reduction of Bi3+ to Bi2+ in the spinel-type compound (MgGa2O4) is reported. The photoluminescence (PL) spectra, PL excitation (PLE) spectra, and PL lifetime curves demonstrate that Bi3+/Bi2+ ions are the main luminescence centers. Notably, the possible self-reduction model is proposed, where a magnesium vacancy (VMg″) is considered as the driving force for the self-reduction of Bi3+ to Bi2+. Furthermore, an oxygen vacancy (VO••) is confirmed by electron paramagnetic resonance (EPR) spectroscopy. Combined with thermoluminescence (TL) glow curves and ML spectra, a plausible trap-controlled ML mechanism is illustrated, where electron-hole (VO••/VMg″) pairs play a significant role in capturing electrons and holes. It is worth noting that the proof-of-concept dual-mode electronic signature application is implemented based on the flexible ML film, which improves the capabilities of signature anticounterfeiting for high-level security applications. Besides, multistimulus-responsive luminescence behaviors of the ML film are realized under the excitation of a 254 nm UV lamp, thermal disturbance, 980 nm laser, and mechanical stimuli. In general, this study provides new insights into designing vis-NIR ML materials toward wider application possibilities.

Mechanical force induced luminescence ratiometric thermometry in CaZnOS:Dy3

The 4I15/2-6H15/2 and 4F9/2-6H15/2 transitions of Dy3+ are usually used for luminescent ratiometric thermometry in the form of photoluminescence. However, here we demonstrate the possibility of using this pair of lines for luminescent ratiometric thermometry in the model of mechanoluminescence (ML) in CaZnOS:Dy3+. Upon stimulation of an external mechanical force rather than light, CaZnOS:Dy3+ emits bright yellow luminescence. The intensity ratio of 4I15/2-6H15/2 to 4F9/2-6H15/2 transitions of Dy3+ is found to increase gradually with the rise of temperature, which makes Dy3+ a qualified temperature indicator. Our work enriches the family of optical thermometry.

Optimizing the Mechanoluminescent Properties of CaZnOS:Tb via Microwave-Assisted Synthesis: A Comparative Study with Conventional Thermal Methods

Recent developments in lighting and display technologies have led to an increased focus on materials and phosphors with high efficiency, chemical stability, and eco-friendliness. Mechanoluminescence (ML) is a promising technology for new lighting devices, specifically in pressure sensors and displays. CaZnOS has been identified as an efficient ML material, with potential applications as a stress sensor. This study focuses on optimizing the mechanoluminescent properties of CaZnOS:Tb through microwave-assisted synthesis. We successfully synthesized CaZnOS doped with Tb3+ using this method and compared it with samples obtained through conventional solid-state methods. We analyzed the material's characteristics using various techniques to investigate their structural, morphological, and optical properties. We then studied the material's mechanoluminescent properties through single impacts with varying energies. Our results show that materials synthesized through microwave methods exhibit similar optical and, primarily, mechanoluminescent properties, making them suitable for use in photonics applications. The comparison of the microwave and conventional solid-state synthesis methods highlights the potential of microwave-assisted methods to optimize the properties of mechanoluminescent materials for practical applications.

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.

Self-Charging Persistent Mechanoluminescence with Mechanics Storage and Visualization Activities

Persistent mechanoluminescence (ML) with long lifetime is highly required to break the limits of the transient emitting behavior under mechanics stimuli. However, the existing materials with persistent ML are completely trap-controlled, and a pre-irradiation is required, which severely hinders the practical applications. In this work, a novel type of ML, self-charging persistent ML, is created by compositing the Sr₃ Al₂ O₅ Cl₂ :Dy³⁺ (SAOCD) powders into flexible polydimethylsiloxane (PDMS) matrix. With no need for any pre-irradiation, the as-fabricated SAOCD/PDMS elastomer could exhibit intense and persistent ML under mechanics stimuli directly, which greatly facilitates its applications in mechanics lighting, displaying, imaging, and visualization. By investigating the matrix effects as well as the thermoluminescence, cathodoluminescence, and triboelectricity properties, the interfacial triboelectrification-induced electron bombardment processes are demonstrated to be responsible for the self-charged energy in  SAOCD under mechanics stimuli. Based on the unique self-charging processes, the SAOCD/PDMS further exhibits mechanics storage and visualized reading activities, which brings novel ideas and approaches to deal with the mechanics-related problems in the fields of mechanical engineering, bioengineering, and artificial intelligence.

Improving the Sensitivity of the Mechanoluminescence Composite through Functionalization for Structural Health Monitoring

Over the past few years, considerable effort has been directed toward the development and improvement of mechanoluminescence (ML)-based stress sensing as an efficient nondestructive inspection technique. One of the challenges in ML stress sensing is the limited luminescent intensity and sensitivity of the ML-epoxy composite film to the local stress field. Herein, we present a novel approach for increasing the sensitivity of ML composites made of an epoxy resin matrix and SrAl₂O₄:Eu²⁺, Dy³⁺ particles functionalized with (3-aminopropyl)triethoxysilane. We performed a tensile test on an epoxy/ML composite specimen to investigate the effect of surface modification of ML particles on the luminescent sensitivity. A series of characterization analyses were performed on the modified surface to investigate the interfacial bonding. In addition, we applied the modified ML/epoxy paint to one side of the tensile specimen with an artificial invisible notch on the other side to visualize the stress field via light intensity (LI) distribution and then compared the results through a finite-element analysis (FEA). Surface modification of ML particles increased the sensitivity and introduced new chemical bonds, corresponding to a larger stress transfer through interfacial bonding rather than mere mechanical locking. In addition, the applied ML film on the notched specimen could visualize the specific pattern of LI reflecting the presence of a crack, which was confirmed by the FEA simulation. This implies that the proposed method of enhancing the ML film is promising for nondestructively predicting the presence, shape, and residual life of a crack in a specimen.

Mechanoluminescence from an Ion-Irradiated Single Crystal of Lithium Niobium Oxide

Mechanoluminescence (ML) is a well-known phenomenon that has a wide range of applications in security monitoring, biomechanical sensing, and displays. Although several mechanisms relating to ML have been proposed, significant ambiguity persists due to the coexistence of crystal boundaries, luminescence centers, and defects within the samples, making them hard to disentangle. Here we preclude such ambiguity by using a Kr⁺-irradiated single crystal of lithium niobium oxide (LiNbO₃) as the ML materials so that oxygen vacancies are retained to modulate the ML properties. We explore the ion concentration- and species-dependent ML properties along with the band calculations to explicitly reveal that it is the trapped electrons at the oxygen vacancies that are transferred to the conduction band under the piezopotentials of LiNbO₃, which combine with holes in the valence band and emit photons. This in-depth understanding not only clarifies the long-standing obscurity of the ML mechanism but also paves a rational and scalable way for the design of advanced ML materials with superior performances.

Molten Salt Shielded Synthesis of Monodisperse Layered CaZnOS-Based Semiconductors for Piezophotonic and X-Ray Detection Applications

CaZnOS-based semiconductors are the only series of material system discovered that can simultaneously realize a large number of dopant elements to directly fulfill the highly efficient full-spectrum functionality from ultraviolet to near-infrared under the same force/pressure. Nevertheless, owing to the high agglomeration of the high temperature solid phase manufacturing process, which is unable to control the crystal morphology, the application progress is limited. Here, the authors report first that CaZnOS-based fine monodisperse semiconductor crystals with various doping ions are successfully synthesized by a molten salt shielded method in an air environment. This method does not require inert gas ventilation, and therefore can greatly reduce the synthesis cost and more importantly improve the fine control of the crystal morphology, along with the crystals' dispersibility and stability. These doped semiconductors can not only realize different colors of mechanical-to-optical energy conversion, but also can achieve multicolor luminescence under low-dose X-ray irradiation, moreover their intensities are comparable to the commercial NaI:Tl. They can pave the way to the new fields of advanced optoelectronic applications, such as piezophotonic systems, mechanical energy conversion and harvesting devices, intelligent sensors, and artificial skin as well as X-ray applications.

Mechanoluminescence Rebrightening the Prospects of Stress Sensing: A Review

The emergence of new applications, such as in artificial intelligence, the internet of things, and biotechnology, has driven the evolution of stress sensing technology. For these emerging applications, stretchability, remoteness, stress distribution, a multimodal nature, and biocompatibility are important performance characteristics of stress sensors. Mechanoluminescence (ML)-based stress sensing has attracted widespread attention because of its characteristics of remoteness and having a distributed response to mechanical stimuli as well as its great potential for stretchability, biocompatibility, and self-powering. In the past few decades, great progress has been made in the discovery of ML materials, analysis of mechanisms, design of devices, and exploration of applications. One can find that with this progress, the focus of ML research has shifted from the phenomenon in the earliest stage to materials and recently toward devices. At the present stage, while showing great prospects for advanced stress sensing applications, ML-based sensing still faces major challenges in material optimization, device design, and system integration.

Construction of a novel mechanoluminescent phosphor Li2MgGeO4:xMn2+ by defect control

Lattice defect plays a significant role in the optical properties of elastic mechanoluminescent materials, which could be modulated by cationic non-equivalent replacement. Here, a series of novel mechanoluminescent phosphors Li2-xMgGeO4:xMn2+ (0 ≤ x ≤ 0.025) were synthesized via a high-temperature solid-state reaction method in an ambient atmosphere. The defect type and its relationship with optical perfomance were clarified via X-ray photoelectron spectroscopy, electron spin resonance, and thermoluminescent spectroscopy. Along with the introduction of Mn ions, the trap levels of oxygen vacancies become shallow, which are beneficial to produce long afterglow and mechanoluminescence. This study offers a feasible approach for developing new functional materials via defect control in self-reduction systems.

Near-infrared mechanoluminescence crystals: a review

Due to the in situ, real-time, and non-destructive properties, mechanoluminescence (ML) crystals have been considered as intelligent stress sensors, which demonstrate potential applications such as in inner crack visualization, light source, and ultrasonic powder recording. Thereinto, it is highly expected that near-infrared (NIR) MLs can realize the visualization of inner biological stress because mechanically induced signals from them can penetrate biological tissues. However, such an energy conversion technique fails to work in biomechanical monitoring due to the limited advances of NIR ML materials. Based on those, some research groups have begun to focus on this field and initially realized this idea in vitro while related advances are still at the early stage. To advance this field, it is highly desirable to review recent advances in NIR ML crystals. In this review, to our knowledge, all the NIR ML crystals have been included in two main groups: oxysulfides and oxides. Besides, the present and emerging trends in investigation of such crystals were discussed. In all, the aim is to advance NIR ML crystals to more practical applications, especially for that of biomechanical visualization in vivo.

Water-Resistant Mechanoluminescent Electrospun Fabrics with Protected Sensitivity in Wet Condition via Plasma-Enhanced Chemical Vapor Deposition Process

Mechanoluminescence (ML), which emits light upon external mechanical stress, was applied to fibrous composites. Herein, ML particles were incorporated into poly(vinylidene fluoride) (PVDF) and polyacrylonitrile (PAN) electrospun webs to prepare ML/PVDF and ML/PAN composite fabrics. The produced fabrics were treated with O2 and C4F8 plasma to modify the wetting properties, then the effects of composite wettability on the light-emitting response in dry and wet conditions were investigated. The light intensity was greatly decreased when the composite fabrics absorbed water. When the composites were hydrophobized by the C4F8 plasma-enhanced chemical vapor deposition process, the original light intensity was protected in wet conditions, while maintaining the water vapor transmission rate. As the clothing material would be exposed to moisture in varied situations, the reduced ML sensitivity in wet conditions may limit the application of ML composite fabrics. The findings suggest a facile strategy to fabricate moisture-resistant, breathable mechanoluminescence composite fabrics.

Ferroelectric Sr3 Sn2 O7 :Nd3+ : A New Multipiezo Material with Ultrasensitive and Sustainable Near-Infrared Piezoluminescence

Ultrasensitive and sustainable near-infrared (NIR)-emitting piezoluminescence is observed from noncentrosymmetric and ferroelectric-phase Sr₃ Sn₂ O₇ doped with rare earth Nd³⁺ ions. Sr₃ Sn₂ O₇ :Nd³⁺ (SSN) with polar A2₁ am structure is demonstrated to emit piezoluminescence of wavelength of 800-1500 nm at microstrain levels, which is enhanced by the ferroelectrically polarized charges in the multipiezo material. These discoveries provide new research opportunities to study luminescence properties of multipiezo and piezo-photonic materials, and to explore their potential as novel ultrasensitive probes for deep-imaging of stress distributions in diverse materials and structures including artificial bone and other implanted structures (in vivo, in situ, etc).

A ZnS/CaZnOS Heterojunction for Efficient Mechanical-to-Optical Energy Conversion by Conduction Band Offset

Actively collecting the mechanical energy by efficient conversion to other forms of energy such as light opens a new possibility of energy-saving, which is of pivotal significance for supplying potential solutions for the present energy crisis. Such energy conversion has shown promising applications in modern sensors, actuators, and energy harvesting. However, the implementation of such technologies is being hindered because most luminescent materials show weak and non-recoverable emissions under mechanical excitation. Herein, a new class of heterojunctioned ZnS/CaZnOS piezophotonic systems is presented, which displays highly reproducible mechanoluminescence (ML) with an unprecedented intensity of over two times higher than that of the widely used commercial ZnS (the state-of-the-art ML material). Density functional theory calculations reveal that the high-performance ML originates from efficient charge transfer and recombination through offset of the valence and conduction bands in the heterojunction interface region. By controlling the ZnS-to-CaZnOS ratio in conjunction with manganese (Mn²⁺ ) and lanthanide (Ln³⁺ ) doping, tunable ML across the full spectrum is activated by a small mechanical stimulus of 1 N (10 kPa). The findings demonstrate a novel strategy for constructing efficient ML materials by leveraging interface effects and ultimately promoting practical applications for ML.

CaZnOS:Nd3+ Emits Tissue-Penetrating near-Infrared Light upon Force Loading

Mechanoluminescent (ML) materials are mechano-optical converters that can emit light under an external mechanical stimulus. All the existing ML materials can only emit light from near ultraviolet to red, which is outside the near-infrared (NIR) windows desired for biomechanical imaging. No studies have been done on doping rare earth (RE) ions with photoluminescence (PL) in the NIR region into a compound to form a ML material that emits NIR light in response to an external force. Here, we show that doping RE ions with a NIR PL into an inorganic compound does not usually result in the formation of a NIR ML material, which can only be achieved in the combination of Nd³⁺ ions and a CaZnOS compound among the combinations we studied. The newly discovered NIR ML material (CaZnOS:Nd³⁺) is biocompatible and can efficiently convert mechanical stress into NIR light over the first and second tissue-penetrating bioimaging window. Its NIR ML emission appeared at a very low force threshold (even when the material was shaken slightly), increased sensitively and linearly with the increase in the force (up to >5 kN), and could penetrate the tissue as deep as >22 mm to enable biomechanical detection. Such a force-responsive behavior is highly reproducible. Hence, CaZnOS:Nd³⁺ is a new potential ultrasensitive biomechanical probe and expands the ML application horizons into in vivo bioimaging.

A Review of Mechanoluminescence in Inorganic Solids: Compounds, Mechanisms, Models and Applications

Mechanoluminescence (ML) is the non-thermal emission of light as a response to mechanical stimuli on a solid material. While this phenomenon has been observed for a long time when breaking certain materials, it is now being extensively explored, especially since the discovery of non-destructive ML upon elastic deformation. A great number of materials have already been identified as mechanoluminescent, but novel ones with colour tunability and improved sensitivity are still urgently needed. The physical origin of the phenomenon, which mainly involves the release of trapped carriers at defects with the help of stress, still remains unclear. This in turn hinders a deeper research, either theoretically or application oriented. In this review paper, we have tabulated the known ML compounds according to their structure prototypes based on the connectivity of anion polyhedra, highlighting structural features, such as framework distortion, layered structure, elastic anisotropy and microstructures, which are very relevant to the ML process. We then review the various proposed mechanisms and corresponding mathematical models. We comment on their contribution to a clearer understanding of the ML phenomenon and on the derived guidelines for improving properties of ML phosphors. Proven and potential applications of ML in various fields, such as stress field sensing, light sources, and sensing electric (magnetic) fields, are summarized. Finally, we point out the challenges and future directions in this active and emerging field of luminescence research.

"Energy Relay Center" for doped mechanoluminescence materials: a case study on Cu-doped and Mn-doped CaZnOS

We unraveled the mechanisms of transition metal-doped mechanoluminescent materials through a case study of CaZnOS. We found that the native point defect levels in Cu or Mn-doped CaZnOS system acted as energy relay centers for luminescence energy transfer. In combination with native point defect levels, discussed in a previous study [Phys. Chem. Chem. Phys., 2016, 18, 25946], we found that phosphor luminescence belongs to two different mechanisms. For Cu-doping, it occurs by the path via the conduction band minimum to the Cu-t_2g level of the 3d orbital localized in the band gap. The hole-drifting effect was found to support the reported red-shifting of the emission. Both reversible and irreversible mechanical quenching were attributed to the spatially separated electrons recombining with the hole localized on the Cu-t_2g level within the gap at levels below or above respectively. For Mn-doping, this occurs by a collaborative luminescence assisted by native point defects, and the excited states of Mn²⁺ overlap with the conduction band edge. The coexistence of Mn_Zn and Mn_Ca was confirmed, but was relatively low in Mn_Ca. The concentration quenching effect, as well as the red-shift of absorption, shows a strong correlation with native point defect levels and the relative position of the 4T₁(⁴G) state for both Mn_Zn and Mn_Ca. Further simplified approximations were used for modeling such concentration quenching effects.

LiNbO3 :Pr3+ : A Multipiezo Material with Simultaneous Piezoelectricity and Sensitive Piezoluminescence

Red-emitting piezoluminescence (elasticoluminescence) is achieved by doping rare earth Pr³⁺ into the well-known piezoelectric matrix, LiNbO₃ . By precisely tuning the Li/Nb ratio in nonstoichiometric Li _x NbO₃ :Pr³⁺ , a material that exhibits an unusually high piezoluminescence intensity, which far exceeds that of any well-known piezoelectric material, is produced. Li _x NbO₃ :Pr³⁺ shows excellent strain sensitivity at the lowest strain level, with no threshold for stress sensing. These multipiezo properties of sensitive piezoluminescence in a piezoelectric matrix are ideal for microstress sensing, damage diagnosis, electro-mechano-optical energy conversion, and multifunctional control in optoelectronics.

Charge transfer induced energy storage in CaZnOS:Mn – insight from experimental and computational spectroscopy

A spectroscopic study shows that energy storage prior to mechanoluminescence and thermoluminescence in CaZnOS:Mn can be effectuated by a ligand-to-Mn charge transfer.

Enhanced persistent properties of Mn2+ activated CaZnOS

The red emitting CaZnOS:Mn²⁺ long persistent phosphor was successfully prepared via a conventional high-temperature solid-state reaction method.

A Mechanoluminescent ZnS:Cu/Rhodamine/SiO2/PDMS and Piezoresistive CNT/PDMS Hybrid Sensor: Red-Light Emission and a Standardized Strain Quantification

We developed a hybrid strain sensor by combining mechanoluminescent ZnS:Cu/rhodamine/SiO₂/PDMS composites and piezoresistive CNT/PDMS for qualitative and quantitative analysis of onsite strain. The former guarantees a qualitative onsite measure of strain with red-light emission via mechanoluminescence (ML) and the latter takes part in accurate quantification of strain through the change in electrical resistance. The PDMS matrix enabled a strain sensing in a wider range of strain, spanning up to several hundred percent in comparison to the conventional rigid matrix composites and ceramic-based ML sensors. Red-light emission would be much more effective for the visualization of strain (or stress) when ML is used as a warning sign in actual applications such as social infrastructure safety diagnosis, emergency guide lighting, and more importantly, in biomedical applications such as in the diagnosis of motility and peristalsis disorders in the gastrointestinal tract. Despite the realization of an efficient red-light-emitting ML in a ZnS:Cu/rhodamine/SiO₂/PDMS composite, the quantification and standardization of strain throughout the ML has been far from complete. In this regard, the piezoresistive CNT/PDMS compensated for this demerit of mechanoluminescent ZnS:Cu/rhodamine/SiO₂/PDMS composites.

Hysteresis compensation of photoluminescence in ZnS:Cu for noncontact shaft torque sensing

This paper presents a preliminary investigation of loading rate-dependent hysteresis of photoluminescence (PL) by phosphorescence quenching of copper-doped zinc sulfide (ZnS:Cu) microparticles in response to dynamic torsional loading. Precision sinusoidal torque waveforms in the frequency range of 0.5-3 Hz are used to identify the loading rate-dependent (i.e., frequency-dependent) nonlinear hysteresis phenomenon. The potential of the application of PL is demonstrated by successfully measuring the sinusoidal torque applied to a rotational shaft by evaluating the loading rate-dependent PL intensity signature using a photomultiplier tube. In addition, the potential of noncontact shaft torque sensing is demonstrated successfully by the simple compensation derived from ad hoc heuristic characterization.

Energy harvesting and conversion mechanisms for intrinsic upconverted mechano-persistent luminescence in CaZnOS

Vacancy defects acting as native activators, e.g. V2+ZnO and V2+CaZnOS, function as energy conversion centers to transfer energy into photons.

Mechanoluminescence of ZnS:Mn phosphors and its correlation to impact energy and contact geometry

The influence of the impulsive mechanical energy and the contact geometrical parameters on stress-induced light emission of ZnS-based phosphors dispersed in polymeric matrix is investigated using modified impact test equipment. At a low to moderate energy region, luminescent pulses consist of two distinguishable peaks that can be assigned to a loading and unloading sequence, respectively. Transient characteristics are found to be independent of the external excitation. An increment of peak emission intensity with higher impact energy and smaller tip diameter indicates a significant relationship of mechanoluminescence to the geometry of contact. Intense emission deviating from the linearity and an absence of the secondary peak are suggested to be the consequence of fractoluminescence.

Metal crack propagation monitoring by photoluminescence enhancement of quantum dots

A visualization method for monitoring minor metal crack propagation is presented in this paper. Through CdS@ZnS core-shell quantum dots (QDs) enhanced emission of photoluminescence (PL), this crack detection method provides a visualization signal in real time and through a noncontact fashion. The crack of the CdS@ZnS core-shell QDs-epoxy resin kept a synchronous propagation with the metal crack. Detection of the tip growth in the film layers demonstrated that the actual crack propagation on the metal surface could be deduced from the tips in the film layers. The fluorescence peak tended to increase along the crack from the initial opening to the tip. Crack width as small as 10 μm can be detected with a precision of 0.1 μm and the minimum crack tip width of the QDs-epoxy resin was measured as 0.72 μm.

Recent advances in energy transfer in bulk and nanoscale luminescent materials: from spectroscopy to applications

We discuss optical energy transfer involving ions, QDs, molecules etc., together with the relevant applications in different areas.

Controlling elastico-mechanoluminescence in diphase (Ba,Ca)TiO3:Pr3+ by co-doping different rare earth ions

Elastico-mechanoluminescence (EML) of diphase (Ba,Ca)TiO₃:Pr³⁺,RE (RE includes all rare-earth-ions except Sc and Pm) is systematically investigated.