1. Introduction

The sense of touch of human beings can respond to skin surface mechanical contact and stimuli, including touch, impact, vibration, friction and compression, and can detect the corresponding stress distribution. As the level of sophistication and performance of smart skin technology and humanoid robots increases, great effort has been made to obtain stress sensors that can perceive contact, motion, shape and texture [1–4].

Currently stress sensors can be divided into two categories according to the stress feedback signal, i.e. electrical signal and optical signal [4]. One conventional method that measures and analyzes the electrical characteristics of magneto-resistive, piezoelectric or strain sensitive conducting materials and structures is based on electrodes and physical contact [5–7]. Thus stress sensing is restricted to single point measurements, which is time consuming while producing data with limited spatial resolution, in practice limited to areas of maximally 5 * 5 cm and to planar surfaces [1]. Furthermore, it is difficult to measure the stress distribution of a dynamic moving part due to the requirement for physical contact. Optical imaging techniques have been proposed in recent years in order to solve these critical problems. One type of stress sensor using optical data images the contact stress distribution by calculation from the change in shape of the deformable sensor surface obtained using a camera [8]. However, the analysis is complex and time consuming. Another vision-based approach takes advantage of light emission from photoluminescent [9], electroluminescent [10], piezochromic luminescent [11], piezo-phototronic [12], mechanoluminescent (ML) [13–15] and elastico-mechanoluminescent (EML) [16] materials and devices. High resolution luminescent images can be efficiently obtained and easily manipulated.

EML materials play a significant role in converting the local contact force into a measurable luminescence signal when mechanical energy is applied [17]. This type of material has attracted considerable interest due to their potential application as stress sensors and in health monitoring since the EML properties of SrAl₂O₄:Eu²⁺ (green) and ZnS:Mn²⁺ (yellow) were first reported in 1999 [18,19]. To date more than ten kinds of inorganic materials exhibiting excellent EML performance have been successfully developed. These inorganic materials are common silicate, titanate, aluminate and phosphate systems, including Ca₂Al₂SiO₇:Ce³⁺ (blue) [20], BaTiO₃-CaTiO₃:Pr³⁺ (red) [21], SrAl₂O₄:Ce³⁺(Ho³⁺) (ultraviolet) [22], CaMgSi₂O₇:Eu²⁺(Dy³⁺) (green) [23], CaAl₂Si₂O₈:Eu²⁺ (blue) [24], CaYAl₃O₇:Eu²⁺ (blue) [25] and SrMg₂(PO₄)₂:Eu²⁺ (purple) [26]. Recently, blue-green emitting oxynitride phosphor BaSi₂O₂N₂:Eu²⁺ became a new member of the EML family [27,28]. For the practical application in the field of touch sensors, the EML materials are required to have the characteristics of the intense EML and the ability to sense multi-stress at the same time. To the best of our knowledge, however, only SrAl₂O₄:Eu²⁺, one of the most promising EML material, has been reported to demonstrate a prompt and intense luminescent response to several kinds of stress stimulus, such as compression [18], stretching [29], bending [14], impact [30], friction [30,31], torsion [16] and ultrasonic vibration [32]. Moreover, as the above mentioned, most of EML materials including SrAl₂O₄:Eu²⁺ are doped rare-earth elements as the luminescent centers. It is well known that the rapid growth in applications of electronic products that utilize rare-earth elements will lead to their scarcity in the near future with corresponding increases in their prices. Therefore, there is an urgent need to develop novel EML materials with strong luminance and high sensitivity to multi-stress, but without doping rare-earth elements.

In recent years, with the development of the light emitting diode (LED) industry, alkaline earth sulfide and oxysulfide phosphors, such as CaS:Eu²⁺ (red) [33], SrS:Eu²⁺ (orange) [34], SrLaGa₃S₆O:Eu²⁺ (yellowish-green) [35], CaZnOS:Mn²⁺ (red) [36], CaZnOS:Eu²⁺ (red) [37] and Sr₈Al₁₂O₂₄S₂:Eu²⁺ (yellowish-orange) [38], have received increasing attention because of their strong absorption in the blue region. However, there has been no investigation of their mechanoluminescence and EML properties except for ZnS:Mn²⁺ [19], ZnS:MnTe [39,40] and ZnS:Cu [41]. Recently the multi-stress sensitive EML characteristics of CaZnOS:Mn²⁺ phosphor were for the first time discovered in our laboratory. The polar compound CaZnOS was first prepared by Petrova et al. in 2003 [42] and its structure and physical properties were investigated in detail by Clarke et al. in 2007 [43]. Because of its chemical and thermal stability CaZnOS is considered to be good host material for Mn²⁺ luminescence. Subsequently the photoluminescence properties of CaZnOS:Mn²⁺ phosphor were reported by Hintzen et al. in 2009 [36]. In this work we have successfully developed rare-earth-ion free CaZnOS:Mn²⁺ with excellent EML characteristics. The EML indicator CaZnOS:Mn²⁺ particles were encapsulated into an optically transparent epoxy resin matrix for test purposes. This material can sense various types of mechanical stress, including ultrasonic vibration, impact, friction and compression by the intense EML emission. Moreover, an accurate linear relation between the emission intensity and different stress parameters (such as intensity, energy, and deformation rate) was determined. The EML mechanism is further discussed with respect to the crystal structure and trap levels. All of the results indicate that the investigated CaZnOS:Mn²⁺ material has great potential as a stress intensity and distribution sensor.

2. Experimental

Polycrystalline phosphors with the composition CaZn_1-xMn_xOS (0≤x≤0.10) were synthesized using a solid state reaction. The raw materials CaCO₃, ZnS and MnCO₃ (99.99%) were thoroughly ground, pressed into pellets, and subsequently sintered at 1100 °C for 3 h in an argon atmosphere. The CaZnOS:Mn²⁺ product was ground and screened through a 20 μm sieve. The EML sensing film with a thickness of 200 μm were prepared by mixing the CaZnOS:Mn²⁺ screened powder and an optical epoxy resin at a weight ratio of 1:1. For EML characterization under friction and pressure the film was prepared on a plastic disk 25 mm in diameter and 15 mm thick.

The sample crystallization behavior was examined by X-ray diffraction (XRD) (Rint2000, Rigaku Corp.) and field emission scanning electron microscopy (FESEM) using an energy dispersive X-ray spectrometer (JSM-7001F, JEOL Ltd.). Photoluminescence (Photo-L) was recorded with a fluorescence spectrometer (FP-6600, Jasco Co.). Thermoluminescence (ThL) was measured by combining the above mentioned fluorescence spectrometer with an in-house made temperature control unit.

The ultrasonic vibration excited luminescence (Ultrasonic-L) intensity was measured using a photomultiplier tube (R649, Hamamatsu Photonics K.K.) and a photoncounter (C9692, Hamamatsu Photonics K.K.). Ultrasonic-L images were recorded using an intensified charge-coupled device (ICCD) camera (FocusScope SV200-i, Photron Corp.). Ultrasonic waves were generated by a 20 MHz single element piezoelectric transducer (NDK Corp.) and an ultrasonic pulser-receiver (Panametrics-NDT 5072PR, Olympus Corp.). The impact stress-excited luminescence (Impact-L) was excited by a free-falling ZrO₂ ball (18 mg). The impact energy could be adjusted from 0 to 54 mJ. The friction-excited luminescence (Tribo-L) was excited using an in-house made friction machine. The compression stress-excited luminescence (Compress-L) was excited with a universal testing machine (RTC-1310A, Orientec Corp.). The Impact-L, Tribo-L and Compress-L intensities were measured using the same photon-counting system that consists of a photomultiplier tube (R649, Hamamatsu Photonics K.K.) and a photoncounter (C3866, Hamamatsu Photonics K.K.). The Ultrasonic-L, Impact-L, Tribo-L and Compress-L measurements were all repeated five times to reduce error. All of the relative standard deviations were less than 10%. The mean values were used for linear fitting. The images and spectra of Impact-L, Tribo-L and Compress-L were recorded by a Nikon D3 camera with a Nikkor 50 mm f1.2 lense and a photon multi-channel analyzer system (PMA-100, Hamamatsu Photonics K.K.). Before measurements of the EML intensity and spectra the specimens were irradiated with 254 nm UV light for 1 min. All measurements except ThL were performed at room temperature.

3. Results and discussion

3.1 Structure, composition and morphology characterization

The XRD patterns of the CaZn_1-xMn_xOS (0≤x≤0.10) samples were investigated. Figure 1(a) shows the XRD patterns of the CaZnOS and CaZn_0.997Mn_0.003OS samples and the standard PDF card for CaZnOS for comparison. The XRD results reveal that the synthesized CaZn_1-xMn_xOS (0≤x≤0.10) was composed of a main CaZnOS phase combined with a small amount of impurities, i.e. CaO and CaS. Energy dispersive X-ray spectrometry (EDS) data for the CaZn_0.997Mn_0.003OS sample depicted in Fig. 1(b) confirm that it is composed of Ca, Zn, O and S. Mn was not detected due to its low content (0.3 at.%). FESEM observation showed that the size of the CaZn_0.997Mn_0.003OS particles was approximately 1-4 μm and these grains tightly bound to each other, as shown in the inset of Fig. 1(b).

 

Fig. 1 (a) XRD patterns of the CaZnOS and CaZn_0.997Mn_0.003OS samples. (b) EDS spectrum of the CaZn_0.997Mn_0.003OS pellet sample. (Inset) An FESEM picture of the CaZn_0.997Mn_0.003OS pellet sample.

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3.2 Elastico-mechanoluminescence (EML) for sensing and imaging multiple mechanical stresses

A multiple stress sensitive film was successfully prepared by the homogeneous dispersion of EML indicator CaZnOS:Mn²⁺ particles into an optical epoxy resin matrix, as shown in Fig. 2. The CaZnOS:Mn²⁺ particles in this film are like a neuron which responds to external stimuli, while the epoxy resin matrix serves as a packaging material to retain the network of neurons, and transmits external mechanical contacts to the CaZnOS:Mn²⁺ particles. When the stress-sensitive film was subjected to various mechanical stresses (such as ultrasonic vibration, impact, friction and compression) the local contact stress was converted into a measurable EML signal by the CaZnOS:Mn²⁺ particles, thus the EML image and the luminescence signal could be recorded by both the digital camera and photomultiplier tube (PMT) detector. The recorded EML (including ultrasonic-L, Impact-L, Tribo-L and Compress-L) intensities are modulated on the application of various mechanical stresses to the film, which results in a stress distribution image. This EML film, just like skin, could be distributed over a model surface with an arbitrary shape. Its nondestructive and non-invasive characteristics indicate promising potential as touch sensors or smart skin in a complex structure.

 

Fig. 2 Schematic representation of the EML material for sensing and imaging various mechanical stresses.

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Figures 3-6 show the Ultrasonic-L, Impact-L, Tribo-L and Compress-L behaviors, respectively, of CaZnOS:Mn²⁺ (0.3 at.% Mn²⁺) under four typical kinds of stress excitation.

 

Fig. 3 (a) Ultrasonic-L response signal induced by an ultrasonic output power of 2.94 mW. (Inset) Ultrasonic-L image recorded during ultrasonic vibration. (b) Relationship between Ultrasonic-L intensity and ultrasonic output power.

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Ultrasonic-L was excited by an applied ultrasonic fountain used for medical applications at a frequency of 20 MHz with an output power of no more than 2.94 mW. As shown in Fig. 3(a), an apparent luminescence signal was immediately measured when medical ultrasonic vibration was applied on the film. The Ultrasonic-L intensity increased sharply with increasing time of applied ultrasonic vibration. When the ultrasonic power was turned off the EML luminescence was rapidly attenuated. The Ultrasonic-L image could be directly recorded by an ICCD gray scale camera, although the power of the ultrasonic fountain was weak. A circular luminescent region was observed in the center where the ultrasonic power was applied [inset in Fig. 3(a)]. The diameter of the circular luminescent region was about 4 mm, which is consistent with that of the piezoelectric ceramic patch. The quantitative relationship between Ultrasonic-L intensity and ultrasonic output power is shown in Fig. 3(b). The Ultrasonic-L intensity increased linearly, proportional to the output power of the applied ultrasonic fountain, which implies that the CaZnOS:Mn²⁺ film can sense ultrasonic vibration and at the same time evaluate the corresponding ultrasonic work power. This proposed technique has potential in low intensity ultrasonic measurements on biological tissues in medical applications. It should be noted that a more intense Ultrasonic-L is obtained when ultrasonic vibrations of greater power (40 kHz, 4 W cm⁻²) are applied to the film, levels usually used in glass and ceramic vessel cleaning. These results indicate that this EML material may sense ultrasonic vibration with a higher power and wider frequency range.

Figure 4(a) illustrates the strong Impact-L response of the CaZnOS:Mn²⁺ film, in which only one luminescence pulse is obtained corresponding to a single impact. When a ball freely drops onto the CaZnOS:Mn²⁺ film the Impact-L intensity initially increases with time, until it attains a peak value. The fitting calculation indicates that the decay time of Impact-L is less than 2.3 ms after the collision. The inset in Fig. 4(a) shows the corresponding Impact-L image, in which an intense red gleam is observed. The relationship between Impact-L intensity and impact energy (0-54 mJ) is illuminated in Fig. 4(b). It is clear that the Impact-L intensity increases linearly with impact energy. Correspondingly, the mechanical impact can be remotely monitored using this EML material through detecting the Impact-L intensity.

 

Fig. 4 (a) Impact-L intensity response induced by an impact energy of 28.2 mJ. (Inset) The Impact-L image. (b) Relationship between Impact-L intensity and impact energy.

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In addition to successfully sensing ultrasonic vibration and impact energy, the CaZnOS:Mn²⁺ film can also respond to friction (shear stress). A strong red Tribo-L is emitted in the contact position between the friction rod and film, and no EML trace line was observed by the naked eye [Figs. 5(a) and 5(b)], i.e. there is no Tribo-L hysteresis phenomenon for CaZnOS:Mn²⁺, which is different from SrAl₂O₄:Eu²⁺ [30,31] and BaSi₂O₂N₂:Eu²⁺ [28]. This performance is more suitable for the measurement of dynamic stress distribution. Figure 5(c) shows the Tribo-L intensity response. When friction is applied the Tribo-L intensity increases steeply and attains a peak value, then a periodic oscillation is observed, after which the luminescence intensity gradually decreases as the friction is maintained. The Tribo-L response immediately disappears when the friction force is turned off. The oscillation of Tribo-L has been shown to originate from inadequacies in the machine alignment and the non-uniformity of the sample setting [30]. Figure 5(d) shows, as expected, that the Tribo-L intensity increases linearly with an increase in the applied friction force, indicating that mechanical friction can also be monitored by the CaZnOS:Mn²⁺ film.

 

Fig. 5 (a) and (b) Tribo-L images, and (c) the Tribo-L intensity response induced by mechanical friction with a transparent resin rod of 5 mm diameter. d) Dependence of the Tribo-L intensity on load.

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Fig. 6 (a) and (b) Compress-L images with a compressive stress on the CaZnOS:Mn²⁺ composite disk. (c) Three-dimensional representation of the Compress-L intensity distribution in (b). (d) Compress-L behavior during the reapplication of load. (e) Influence of the deformation rate on the Compress-L intensity.

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Typical Compress-L images are shown in Figs. 6(a) and 6(b). A bright red luminescence is clearly observable, and the Compress-L intensity exhibits an uneven distribution over the surface of the test sample. In order to quantitatively illustrate position dependence on the Compress-L intensity, Fig. 6(c) displays a three-dimensional representation of the Compress-L intensity distribution in pseudo-color, which exhibits a high spatial resolution. As shown, the highest value is found at the contact points where the applied compressive load is strongest. The strongest light intensity induced is about 30-40 mcd m⁻², which is roughly 104 times the human eye sensitivity to darkness [44]. The EML intensity for CaZnOS:Mn²⁺ was almost three times that of (Ba,Ca)TiO₃:Pr³⁺ (10-15 mcd m⁻²) which is one of the most intense red emitting EML materials [45]. It indicates that the EML luminance of CaZnOS:Mn²⁺ is intense enough for application in a dim environment. Figure 6(d) shows the Compress-L behavior during reapplication of a compressive stress (10 consecutive cycles) at a constant deformation rate of 3 mm min⁻¹. In the first cycle the Compress-L intensity increased linearly with increasing compressive load, reaching an unsaturated peak value at the peak load (1000 N), followed by sharp attenuation on releasing the compressive load. In the later cycles the Compress-L peak intensity decayed significantly with stressed time. However, in each cycle the Compress-L intensity always changed linearly with the applied compressive load and showed a Compress-L peak at the load peak. Since the relation between Compress-L intensity and compressive load is linear, the EML brightness distribution [Figs. 6(b) and 6(c)] reflects the stress distribution, which has been confirmed by previous simulation results [16,18,29]. This indicates that this stress distribution visualization technology could be used to decipher the stress distribution of a dynamic moving part without the restriction of physical contact. Moreover, another linear relation between the Compress-L intensity and deformation rate has been found, as shown in Fig. 5(e), which indicates that this EML material could also be used to sense deformation rate.

3.3 Elastico-mechanoluminescence (EML) mechanism of CaZnOS:Mn²⁺

To elucidate the emission features of luminescence induced by different stresses Impact-L, Tribo-L, Compress-L and Photo-L spectra of CaZnOS:Mn²⁺ were measured and are compared in Fig. 7. Only one broad emission band peaking at about 610 nm, originating from the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Mn²⁺ ions, can be observed in each of these EML spectra, which is the same process as is responsible for Photo-L [36]. Furthermore, no red emission was detected in the sample CaZnOS without Mn²⁺ doping when different stresses were applied. The result also confirms that the Mn²⁺ ions is the luminescence centers of EML in CaZnOS:Mn²⁺.

 

Fig. 7 Impact-L, Tribo-L, Compress-L and Photo-L spectra of CaZnOS:Mn²⁺.

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The above results suggest that the CaZnOS:Mn²⁺ material can simultaneously respond to various mechanical stresses, including ultrasonic vibration, impact, friction and compressive stress, and also indicate the existence of these applied stresses by the intense red light emission from Mn²⁺ ions. The response to the external stimuli, i.e. EML intensity, is linear with respect to the stress magnitude, energy and deformation rate. This EML material could be coated onto a complex structure to visually reflect and precisely diagnose the dynamic stress and stress distribution as a non-destructive and non-invasive touch sensor, smart skin, or self-diagnosis system.

Now we will discuss the possible principle of the excellent EML properties of the CaZnOS:Mn²⁺ material. From Figs. 5(c) and 6(c) it is apparent that the Tribo-L and Compress- L intensities decayed during cycles of friction and compression, respectively. This was also the case in the Ultrasonic-L and Impact-L experiments, i.e. the Ultrasonic-L and Impact-L intensities decreased with cycles of ultrasonic vibration and impact, respectively. Interestingly, the EML intensity of CaZnOS:Mn²⁺ recovered completely after irradiation with 254 nm UV light for 1 min, i.e. EML of CaZnOS:Mn²⁺ is reproducible. Therefore, CaZnOS:Mn²⁺ belongs to the so-called defect-controlled group of materials [16], in which deep energy traps can be filled by UV light irradiation.

In order to provide evidence for deep energy traps the ThL curves of CaZnOS:Mn²⁺ irradiated with 254 nm UV light for 1 min were measured at four different heating rates, 10, 30, 60 and 90 °C min^–1, the results of which are shown in Fig. 8. There are three peaks in the investigated temperature region (−193 °C to 200 °C), proving the existence of deep energy traps. Among them two peaks (peaks 1 and 2) occurred above room temperature. Based on the Hoogenstraaten method [46], the depths of these traps were about 0.18 (corresponding to the peak below 0 °C), 0.58 (peak 1) and 0.67 (peak 2) eV, respectively. The trap corresponding to the peaks below 0 °C will be emptied at room temperature because of the shallow depth, while the traps corresponding to peaks 1 and 2 with suitable trap depths could not be thermally activated at room temperature, and will be responsible for the reproducible EML phenomenon. Moreover, the trap levels corresponding to peaks 1 and 2 in the CaZnOS:Mn²⁺ sample are much higher than that of the SrAl₂O₄:Eu²⁺ sample (0.2 ± 0.1 eV) prepared by a solid-state reaction [18]. This is the reason why CaZnOS:Mn²⁺ gives rise to a much shorter EML hysteresis. Due to the existence of CaO and CaS impurity phases in the CaZnOS:Mn²⁺ material investigated lattice defects associated with Ca, O and S vacancies will form during the high temperature synthesis process. These lattice defects can act as traps of carriers (holes or electrons) in the band gap of CaZnOS:Mn²⁺. Thus the decrease in EML intensity with reapplication of a stress could be attributed to detrapping of these trapped carriers.

 

Fig. 8 Thermoluminescence (ThL) curves of CaZnOS:Mn²⁺ at the heating rates of 10, 30, 60 and 90 °C min⁻¹.

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Previous results have shown that the structure of the phosphor plays an important role in determining the EML properties [21,25,28]. It was reported that phosphors with low symmetry usually possess excellent EML performance, because these phosphors can produce stronger strain electric fields when a stress is applied. In the crystal structure of CaZnOS, each Zn is tetrahedrally coordinated by an O atom and three S atoms [ZnOS₃]. The crystal structure of CaZnOS is non-centrosymmetric and has few analogs. It is composed of isotypic puckered hexagonal ZnS and CaO layers arranged so that [ZnOS₃] tetrahedra are all aligned in parallel, resulting in a polar structure and piezoelectric behavior [42,43]. In particular, the piezoelectric coefficient (d₃₃) value (38 pm V⁻¹) is larger than those observed in a-SiO₂ (2.3 pm V⁻¹), wurtzite ZnO (10.6 pm V⁻¹), and CdS (10.3 pm V⁻¹) [43]. On adding a small content of transition metal activator (Mn²⁺) into CaZnOS the Mn²⁺ ions are considered to enter into the interior host lattice by replacing Zn²⁺ ions in the polar tetrahedron due to the similar ionic radii of Mn²⁺ (CN4, r = 0.80 Å) and Zn²⁺ (CN4, r = 0.74 Å) [47]. The local polar crystal field around Mn²⁺ ions will have a remarkable effect on the luminescence properties of Mn²⁺ ions. This unusual entirely non-centrosymmetric structure guarantees sensitive piezoelectric behavior of CaZnOS:Mn²⁺ when sensing stresses over a wide range of energies and orientations, not only for axes strain, but also for shear strain. Thus the EML characteristics of CaZnOS:Mn²⁺ could be explained using a model of piezoelectrically induced trapped carrier excitation. When a stress is applied the CaZnOS:Mn²⁺ lattice is deformed, inducing local atomic displacement and generating a local electric field in the piezoelectric region. The large electric field gradient around traps results in a decrease in the trap depth. The trapped carriers are excited, and then recombination of an electron and hole gives rise to energy which will be transferred to the luminescence center Mn²⁺ ions. Subsequently Mn²⁺ ions could be excited, giving rise to red emission based on the transition between ⁴T₁(⁴G) and ⁶A₁(⁶S).

4. Conclusions

In summary, excellent multi-stress sensitive properties of intense Ultrasonic-L, Impact-L, Tribo-L and Compress-L were found for the CaZnOS:Mn²⁺ material. This stress sensitive EML material can immediately sense and image multiple mechanical stresses due to the ⁴T₁(⁴G) → ⁶A₁(⁶S) transition of Mn²⁺ ions producing strong red emission. Moreover, linear relationships between emission intensity and different stress parameters (such as intensity, energy and deformation rate) were confirmed, which suggests that stress intensity and distribution can be determined from the EML intensity of the CaZnOS:Mn²⁺ material investigated. The crystal structure and ThL results indicate that the EML properties of CaZnOS:Mn²⁺ originate from its entirely non-centrosymmetric structure and the existence of carrier traps of appropriate depth. These findings will pave the way to developing novel multi-stress-sensitive EML materials and extend their practical applications.