Red-emitting piezoluminescence (elasticoluminescence) is achieved by doping rare earth Pr into the well-known piezoelectric matrix, LiNbO . By precisely tuning the Li/Nb ratio in nonstoichiometric Li NbO :Pr , a material that exhibits an unusually high piezoluminescence intensity, which far exceeds that of any well-known piezoelectric material, is produced. Li NbO :Pr 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.

This article reports a new phosphorescent material, CaZnOS:Cu, that exhibits two types of mechano-optical conversion: mechanical quenching and mechanoluminescence. An intense mechanical quenching of phosphorescence corresponding to mechanical stimuli can be achieved in CaZnOS:Cu within a short decay time period. Over time, it gradually changes to mechanoluminescence when a mechanical load is applied. We propose that the mechanical quenching and mechanoluminescence arise from the different roles of shallow and deep traps in CaZnOS:Cu. CaZnOS:Cu has promising applications in monitoring mechanical stress in industrial plants, structures, and living bodies. Keywords: mechanical quenching; mechanoluminescence; phosphorescence; traps INTRODUCTIONAdjusting and controlling the optical properties of materials by altering environmental factors are important in the development of various applications in sensing, memory, detection, and display devices [1][2][3][4][5][6] . Mechanical stress is the most common external stimulus, and thus materials that exhibit mechano-optical conversion are promising for practical applications in science and engineering [7][8][9][10][11][12] .Mechanoluminescent (ML) materials emit light when mechanical energy is applied and are effective for mechano-optical conversion 1,9,13,14 . Therefore, ML materials have been used as optical sensors for monitoring changes in mechanical stress. 25 , have been explored. Recently, our group has discovered another type of mechano-optical conversion, which is referred to as mechanical quenching (MQ) 26 . In contrast to ML, MQ is the quenching of phosphorescence intensity by using mechanical stimuli. Previously, we reported that CaZnOS:Cu exhibited MQ under applied mechanical stress. However, to understand the MQ process and mechanism further and use it for practical applications, the crystal structure, phosphorescence properties, and MQ properties of CaZnOS:Cu must be determined.Here, we carry out further study on MQ in a phosphorescent material, CaZnOS:Cu. We examine the change in crystal structure that is caused by changing the Cu concentration, the phosphorescence properties, and the MQ mechanism. We find that CaZnOS:Cu exhibits a variety of mechano-optical conversions depending on the experimental conditions. MATERIALS AND METHODSCaZnOS:Cu was synthesized by a solid-state reaction method. Highpurity CaCO 3 (50 mol% excess) and appropriate amounts of ZnS and Cu 2 O were weighed and ground in an agate mortar with ethanol and then sintered under an air flow at 1100 6 C for 5 h. The excess calcium compounds were removed from the sample by washing with an aqueous solution of acetic acid. After filtration and drying, CaZnOS:Cu was ground again and then pulverized for measurement.The crystalline phase of CaZnOS:Cu was characterized by using X-ray powder diffraction (XRD; RINT-2000, Rigaku Co., Tokyo, Japan) with CuKa radiation (1.5418 Å ; cathode voltage, 40 kV; current, 40 mA) at room temperature. As reported previously, CaZnOS has an unusual structure with the...

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structures attracted world-wide attention in order to ensure effective maintenance and avoid recurrence of such severe disasters. [1][2][3] In December 2017, a serious incident happened at a Shinkansen bullet train of Japan due to anomalous load caused fatigue cracks and rupture in an undercarriage, [4] and this sudden incident further enhanced the urgent necessity to predict the fracture (damage) of infrastructures. Since conventional magnetic particle testing (MT), penetrant testing (PT), and ultrasonic testing (UT) methods are difficult for precise largescale infrastructure diagnosis, many techniques were proposed on this topic in recent years, such as sensing techniques of electrical resistance or capacitive MEMS strain sensors and piezoelectric sensors were proposed for precise strain or anomaly detection, but these methods still had limitations for strain imaging on real world structures due to the sensor scale and/or spatial resolution. [5][6][7][8] On the other hand, new types of sensors based on photoelectric methods such as assembled nanowires/nanotubes or microstructured rubber layers, [9][10][11][12][13][14][15] stretchable and flexible strain sensor with conductive nanostructure for sensitive detection of human motion, [16] piezotronic/piezo-phototronic-effect enhanced light emitting smart sensors, [17,18] and flexible or bionic mechanosensors were proposed as effective sensing techniques for dynamic imaging of pressure or stress and diagnosing movement disorders in high-resolution. [19][20][21][22][23] However, fabrication methods of the abovementioned nanowire array or flexible smart sensors are complicated, and they are inconvenient for scalable precise stress/strain imaging and especially difficult for large-scale application and onsite real world infrastructures.Herein, we report another mechanism of largely scalable and flexible strain sensor using elasticoluminescent smart paint for challenge to solve worldwide structural diagnosis problems ranging from micro to macroscales. The elasticoluminescence presented in this study also called elastico-mechanoluminescence is a unique form of mechanoluminescence (ML) that can generate repeatable and reproducible light emission during elastic deformation. [24][25][26][27][28][29] The new scalable elasticoluminescent strain sensor has great significance on high sensitivity and precise dynamic stress/strain imaging, and onsite fracture inspection and diagnoses on large-scale infrastructures. The innovative scalable strain sensor would prospectively open a Precise dynamic stress/strain imaging is critical for a broad range of research and engineering analyses ranging from micrometer to meter-scales, but there is no precise multiscale strain sensor, especially for onsite real-time structural health monitoring. Here, a scalable elasticoluminescent strain sensor is presented for solving worldwide structural diagnosis problems ranging from micrometer to meter-scales. Significant progress has recently been made on both highly sensitive elasticoluminescent sensor...