Mechanism of mechanical quenching and mechanoluminescence in phosphorescent CaZnOS:Cu

Abstract

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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