Open Access
Add to BibSonomyAbstract
We report on a novel elastico-mechanoluminescence (EML) phosphor of CaZr(PO4)2:Eu2+ for simultaneous luminescent sensing and imaging to mechanical load by the light-emitting of Eu2+ ions. The EML properties of CaZr(PO4)2:Eu2+ show an intense luminance (above 15 mcd m−2), a low load threshold (below 5 N), a broad measurement range for the dynamic load (up to 2000 N), and an accurate linear relationship of EML intensity against the applied load. The excellent EML characteristics are considered to originate from the piezoelectric crystal structure and the multiple trap levels with appropriate depths. An EML mechanism based on the electrons as the main charge carriers is proposed.
©2013 Optical Society of America
1. Introduction
Mechanoluminescence (ML) materials, a type of phosphors, can convert the local mechanical energy into light emission with the application of various stresses, such as deformation, friction, impact, and vibration [1]. As a category of ML materials, elastico-mechanoluminescence (EML) materials present an accurate linearity of ML intensity against load in the elastic region, in addition to the mechano-optical conversion [2]. The potential of EML materials has been recognized as a stress probe to monitor the two-dimensional stress distribution in artificial skin, engineering structure, and living body in view of the advantages of wireless, non-destructive, reproducible, real-time and reliable stress sensing [3–6]. For the practical applications in the detection and monitoring of load with different intensities, from the weak load such as touch to the strong load in parts of machinery or aircraft, the EML materials are required to have the characteristics of intense EML, low load threshold and wide measurement range for the dynamic load at the same time. To overcome these challenges, researchers have devoted their careers to searching and investigating new EML materials. To date more than ten kinds of inorganic materials emitting light of various colors and exhibiting EML performance have been developed [6]. However, only a few EML materials, including ZnS:Mn2+ (yellow) [3], (Ba,Ca)TiO3:Pr3+ (red) [7], SrAl2O4:Eu2+ (green) [8], Sr3Sn2O7:Sm3+ (reddish-orange) [9], and BaSi2O2N2:Eu2+ (bluish-green) [10], have been found to possess intense EML. Moreover, the EML load threshold for certain materials is too high to sense the weak mechanical force, such as that for (Ba,Ca)TiO3:Pr3+ (350 N) [11]. In addition, the linear measurement ranges of dynamic compressive load are different for various EML materials. For some materials the EML intensity reaches a saturated value when even for a relatively weaker excited load, which is not suitable for the application in the field of stronger compressive load detection, e.g. that for Sr3Sn2O7:Sm3+ [9].
Previous researches have indicated that the developed EML materials belong to the defect-controlled type piezoelectric materials and the EML mechanism has been explained using the piezoelectrically induced carrier de-trapping model [2,3,7–11]. Consequently, the EML performance above mentioned should be closely related to the concentration and depth of trap levels. On the one hand, the EML intensity is mainly influenced by the concentration of trap levels. If the concentration of trap levels whose depths are suitable for EML is relatively higher, more trapped carriers will be released and then recombine with the luminescence centers under the applied stress, and then an intense EML is obtained, e.g. in the cases of Srn + 1SnnO3n + 1:Sm3+ (n = 1, 2, ∞) and (Ca,Sr,Ba)Si2O2N2:Eu2+ [9,10]. On the other hand, the depth of trap levels is responsible for the load threshold and the measurement range of dynamic load. For example, SrAl2O4:Eu2+ exhibits an intense EML even under the application of a weak mechanical force, such as that generated by scratching the material with a finger. The shallow trap levels (0.2 ± 0.1 eV) is one of the most important reasons for its sensitivity to the weak load [8]. Nevertheless, it should be also pointed out that if there are only shallow (accessible) or less deep trap levels, the carriers in the traps can be released and then be emptied under the weak load, which results in the absence or weakness of EML under the strong load. On the contrary, if there are only deep or less shallow trap levels, the energy necessary for the carriers to release from the traps is so high that the trapped carriers cannot be excited by the weak load, resulting in a high load threshold for EML. Therefore, to search and design novel EML materials with suitable multiple trap levels is an effective and promising method to obtain the excellent EML performance.
Recently, we have successfully synthesized a novel zirconium phosphate luminescent material Eu2+-activated CaZr(PO4)2 with multiple trap levels and the desired EML performance. In addition to the accurate linear relationship of EML intensity against the applied load, it shows an intense EML emission (above 15 mcd m−2), a low load threshold (below 5 N) for EML, and a broad measurement range for dynamic load (up to 2000 N). The host material CaZr(PO4)2 was first prepared by Bettinali et al. (1962) [12], while the luminescent property of rare earth ions doped CaZr(PO4)2 was seldom reported except for the x-ray and vacuum ultraviolet-ultraviolet (VUV-UV) excited luminescent properties of CaZr(PO4)2:Re3+ (Re3+ = Eu3+, Tb3+, Tm3+) [13]. In the present study, we report the EML characteristics of the CaZr(PO4)2:Eu2+ phosphors for the first time. Moreover, the multiple trap levels in CaZr(PO4)2:Eu2+ are confirmed through the afterglow decay curves and the thermoluminescence (ThL) glow curves. The trap depths are derived and the nature of trap levels is discussed. Finally, a possible EML mechanism is proposed according to the crystal structure and the multiple trap levels.
2. Experimental
The CaZr(PO4)2:Eu2+ pulverized phosphors were synthesized using a two-stage solid-state reaction. In the first step, stoichiometric amounts of starting reagents CaCO3, ZrO2 and Eu2O3 (≥99.9%) were weighed and thoroughly mixed. Appropriate amounts of Ca were substituted by 1.0 atom% Eu. The obtained mixtures were then sintered at 1300°C for 3 h in air to synthesize the phase of CaZrO3:Eu3+. In the second step, the ground product CaZrO3:Eu3+ was mixed with the appropriate amount of NH4H2PO4 (>99%) and pressed into pellets, and subsequently sintered at 1200 °C for 10 h in a reducing atmosphere (argon containing 5% H2) to synthesize CaZr(PO4)2:Eu2+. Since the europium ions are incorporated in divalent lattice sites, and the reaction atmosphere is reducing, europium is predominantly incorporated into the host lattice as Eu2+. The preparation scheme is shown in Fig. 1.
The phase purity and crystal structure of the obtained materials were investigated by an X-ray diffractometer (XRD; D/max-2400, Rigaku Co.) using Cu Kα radiation. The photoluminescence (PL) spectra and afterglow decay curves were measured using a spectrofluorometer (F-4600, Hitachi Co.). Thermoluminescence (ThL) was measured using a fluorescence spectrometer (FP-6600, Jasco Co.) connected with an in-house made temperature control unit.
To evaluate the EML properties of CaZr(PO4)2:Eu2+, powders were mixed in a transparent epoxy resin (SpeciFix, Struers GmbH) at a weight ratio of 1:9 to form circular disk with a diameter of 25 mm and a thickness of 15 mm. The layer of CaZr(PO4)2:Eu2+ powders about 1 mm thickness was encapsulated in one underside of the circular disk. A varying compressive load was applied on the composite samples along the diameter direction of circular disk by means of a universal testing machine (WES-50, Jinan New Century Testing Machine Manufacture Co.). The EML intensity was measured with a computer-driven photon-counting system that consists of a photomultiplier tube (R2949, Zolix Instruments Co.) and a photon counter (DCS103, Zolix Instruments Co.). The EML images were recorded by a Canon 7D camera with a 50 mm f/1.4 lens. The EML spectra were recorded with a photon multi-channel analyzer system (PMA-100, Hamamatsu Photonics Co.). 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. Structural characterization
The X-ray diffraction (XRD) pattern of CaZr(PO4)2:Eu2+ (Eu: 1.0 atom%) together with the Powder Diffraction File (PDF) card No. 35-0159 are shown in Fig. 2(a). From a comparison between them, the position and intensity of the peaks are the same. No obvious impurity lines are observed, and the pattern could be indexed to a CaZr(PO4)2 single phase. Substituting a small number of Eu ions in the crystal lattice does not have a detectable influence on the XRD spectra and thus we assume that for low doping concentration the crystal structure is unaltered. The crystal structure of CaZr(PO4)2 belongs to the orthorhombic system with symmetry class 222 and space group P212121, Z = 4 (a = 14.488 Å, b = 6.721 Å, c = 6.235 Å), as shown in Fig. 2(b). It consists of CaO7, ZrO7, P1O4, and P2O4 polyhedra, which share edges to form infinite chains with the composition of [CaO3ZrO3P2O8]12− along the [010]. Individual chains are linked together, forming a two-dimensional sheet parallel to (100). These sheets are stacked in the [100] direction to form a three-dimensional structure. The Ca atom and surrounding seven oxygen atoms form a distorted capped octahedron, while the ZrO7 coordination polyhedron is a distorted pentagonal bipyramid [14]. When adding a small content of Eu2+ activators into CaZr(PO4)2, the Eu2+ ions are considered to enter into the interior host lattice by replacing Ca2+ ions in the polar octahedron due to their similar ionic radii. Ionic radii for Ca2+, Zr4+, and Eu2+ in the sevenfold coordination are r(Ca2+) = 1.06 Å, r(Zr4+) = 0.78 Å, and r(Eu2+) = 1.20 Å, respectively [15]. Of the 32 crystal classes, 20 exhibit direct piezoelectricity [10,16]. These classes are: 1; 2; m; 222; mm2; 4; ; 422; 4mm; 2m; 3; 32; 3m; 6; ; 622; 6mm; 2m; 23; 3m. It is obviously that the space group 222 of CaZr(PO4)2 belongs to one of the 20 piezoelectric classes. Therefore, The piezoelectricity of the host material CaZr(PO4)2 provides a prerequisite for the EML in CaZr(PO4)2:Eu2+ according to the piezoelectrically induced de-trapping model for EML [10,11,17], i.e. the application of pressure will produce a local piezoelectric field on the defects and activators to either reduce the trap depth or cause band bending to release the trapped carriers.
Fig. 2 (a) XRD pattern of CaZr(PO4)2:Eu2+ and standard XRD pattern of CaZr(PO4)2. (b) crystal structure of calcium zirconium diorthophosphate.
3.2. Photoluminescence (PL) and elastico-mechanoluminescence (EML)
The PL spectra of CaZr(PO4)2:Eu2+ are shown in Fig. 3. There are two principle excitation bands: a strong peak located at 236 nm, which is related to the O to Zr charge transfer transition [18], while a broad shoulder band peaking at 300 nm, which is attributed to the 4f7→4f65d1 (4f-5d) transition of Eu2+ ions. Both excitations at 236 nm and 300 nm exhibit a cyan band emission centered around 474 nm, which is similar with that in BaAl2S4:Eu2+ (476 nm) [19]. The emission band is assigned to the 4f65d1→4f7 (5d-4f) transition on Eu2+ ions. In addition, a small shoulder peak located at 610 nm is observed, which is considered to originate from the remaining trivalent europium (Eu3+) [20].
Fig. 3 Photoluminescence (PL) excitation (λem = 474 nm) and emission (λex = 236 nm and 300 nm) spectra of CaZr(PO4)2:Eu2+.
Figure 4 shows the EML images and spectrum obtained from the CaZr(PO4)2:Eu2+ powders/epoxy resin composite pellet subjected to a compressive load. When the compressive load is applied [Fig. 4(a)], a cyan EML is clearly observable with naked eyes [Fig. 4(b)]. As shown, the highest value of EML is found at the contact points of the pellet and the testing machine where the applied compressive load is the strongest. It suggests that the EML brightness is consistent with the stress distribution [4,8,21]. The strongest light intensity induced is above 15 mcd m−2, which is roughly 5000 times higher than the light perception of dark-adapted human eyes (3.2 × 10−3 mcd m−2) [22]. By comparing the EML spectrum from the CaZr(PO4)2:Eu2+ sample in Fig. 4(c) with its PL spectrum, it is obvious that the EML spectrum consists of only a broad emission band peaking at 472 nm, none of 613 nm light emission (Eu3+ ions). The results indicate that the EML is emitted from the emitting centers of Eu2+ ions, which is produced by the transition of Eu2+ ions between the 8S7/2(4f7) ground state and the excited 4f65d1 as the process of PL.
Fig. 4 (a) and (b) Elastico-mechanoluminescence (EML) images at bright and dark environment, respectively. (c) EML spectra of CaZr(PO4)2:Eu2+.
Figure 5 displays the dependence of EML intensity of CaZr(PO4)2:Eu2+ on the compressive load at a rate of 3 mm min−1. The increase in compressive load induces a corresponding linear increase in EML intensity, presenting a typical EML characteristic, which is extremely useful for the sense of the stress intensity and distribution. More importantly, two superiorities of EML in CaZr(PO4)2:Eu2+ should be pointed out. Firstly, the load threshold for the EML in CaZr(PO4)2:Eu2+ is only 4.86 N, much lower than that of (Ba,Ca)TiO3:Pr3+ (350 N) [11], and there is almost no non-linear ML region for CaZr(PO4)2:Eu2+ sample once the compressive load above the threshold is applied [inset of Fig. 5]. Secondly, EML from CaZr(PO4)2:Eu2+ sample could be excited by a broad dynamic range of compressive force. The EML reaches an unsaturated peak value even for the excited load up to 2000 N, showing the trend of sustained growth. It is much higher than the previously reported peak load of other EML materials, e.g. Sr3Sn2O7:Sm3+ (250 N) and (Ba,Ca)TiO3:Pr3+ (1000 N) [9,11]. All the results indicate that the excellent EML properties from CaZr(PO4)2:Eu2+ can provide high sensitivity, accuracy and tolerance for smart-skin, mechano-optical sensor and self-diagnosis applications.
Fig. 5 Linear dependence of elastico-mechanoluminescence (EML) intensity of CaZr(PO4)2:Eu2+ on the compressive load.
3.3. Elastico-mechanoluminescence (EML) mechanism
Currently, the EML phenomenon is usually explained by the piezoelectrically induced trapped carrier de-trapping model, in which carriers (electron or hole) trapped at the trap levels are released under the piezoelectric field induced by mechanical stimulus and then recombines with the luminescence centers, resulting in photon emission [6,8–11]. Thus, the trap levels play an important role in EML process. On this point of view, we will discuss the reason that CaZr(PO4)2:Eu2+ has such excellent EML performance. It is well known that the phenomena of long afterglow and ThL glow are both governed by the liberation of carriers trapped in the trap levels due to thermal excitation. More importantly, for several established EML materials, such as BaSi2O2N2:Eu2+ [10], (Ba,Ca)TiO3:Pr3+ [11,23], and Ca2MgSi2O7:Eu2+(Dy3+) [24], it is considered that the same traps are responsible for the afterglow luminescence and EML. Accordingly, to clarify the trap levels in CaZr(PO4)2:Eu2+, the afterglow decay curve and ThL glow curves were measured, respectively.
Figure 6 presents the afterglow image and decay curve of CaZr(PO4)2:Eu2+ at room temperature. As expected, the cyan long persistent afterglow can be obviously seen by naked eyes after switching off the excitation of UV light (254 nm). The afterglow curve monitoring the emission of Eu2+ ions can be well fitted into a tri-exponential decay function, indicating at least three decay processes: a fast decay (14.8 s), a middle decay (91.2 s), and a slow decay (503.6 s). The three decay components imply that there are at least three types of trap levels in the CaZr(PO4)2:Eu2+ material [25].
Figure 7 shows the ThL glow curves of CaZr(PO4)2:Eu2+ irradiated by 254 nm UV light for 1 min, which are measured from 0 to 400 °C at various heating rates of 10, 30, 60, and 90 °C min−1, respectively. There exist several ThL peaks for each ThL curve. In the ThL glow curve, each peak represents one type of trap centers present in the system. The existence of different ThL peaks indicates that there are several kinds of traps in this material. Thus, the sub-peak fitting of ThL curves is carried out. Each ThL curve can be well separated to four peaks using Lorentz fit. It suggests that there are four types of traps in the CaZr(PO4)2:Eu2+ phosphor. The inset of Fig. 7 shows the the sub-peak fitting results of ThL curve measured at the heating rate of 90 °C/min. Four ThL peaks, i.e. Peak 1 located at 53 °C, Peak 2 located at 136 °C, Peak 3 located at 214 °C, and Peak 4 located at 255 °C, are obtained. This result is consistent with the analysis of afterglow curve, since the ThL intensity of Peak 3 is relatively weak.
Fig. 7 Thermoluminescence (ThL) glow curves of the CaZr(PO4)2:Eu2+ phosphor at different heating rates. (Inset) The fitting curves for ThL spectrum at the heating rate of 90 °C/min.
To clarify the relationship between EML and carrier traps, the depths of trap levels inside this material are evaluated. In case the recombination process involved follows the first-order reaction, the activation energy Et of ThL, which corresponds to the trap depth, can be estimated from the following Hoogenstraaten equation [26,27]:
Here, β denotes the constant heating ratio, Tm the maximum peak temperature of the ThL curve, and k the Boltzmann constant. Et equals the slope of a straight line in a (1/Tm) versus ln(β/Tm2) plot. The Hoogenstraaten plots for different fitting peaks allow us to derive Et = 0.687 eV (Peak 1), 0.829 eV (Peak 2), 1.171 eV (Peak 3) and 1.873 eV (Peak 4) for different kinds of traps, respectively. According to the report of Sakai et al. [28], 0.6-0.7 eV are the most suitable trap depths for a long afterglow. Significantly, the traps corresponding to Peak 1 of the ThL curves indicate the reason why there is a long persistent afterglow and a low load threshold for CaZr(PO4)2:Eu2+, but the traps related to EML are emptied faster than in the case of long persistent afterglow due to the application of mechanical stimulation. On the other hand, the existence of consecutive multiple trap levels guarantees that EML could be excited by the mechanical loads with different intensities, i.e. a broad measurement range of the dynamic load.According to the results above obtained, the oxygen vacancies (VO), Eu3+ in Ca2+ site ([EuCa]+), and calcium vacancies (VCa) are proposed to act as the main traps in CaZr(PO4)2:Eu2+. Oxygen vacancies are formed because of the reducing preparation condition, which has been reported in many storage phosphors [29,30]. The Eu3+ in Ca2+ site and calcium vacancies are formed by Eu2+ doping, since some Eu3+ ions are always present in the materials even if the reducing preparation condition has been used [31]. This is indicated by the Eu3+ luminescence (Fig. 3). Because the Eu3+ ions (CN = 7, r = 1.01 Å) replace the Ca2+ ions to form [EuCa]+ in the CaZr(PO4)2:Eu2+ material [15], the natural consequence is the formation of charge compensating lattice defects, e.g. Ca2+ vacancies. The oxygen vacancy and [EuCa]+ are both positively charged relative to the environment and can trap electrons, while calcium vacancy is negatively charged and can trap a hole as well as aggregate with VO or/and [EuCa]+. VO and [EuCa]+ would repulse each other without VCa. The aggregation of lattice defects has been confirmed to be a usual phenomenon in solid state [32,33]. On the other hand, there are eight oxygen sites in the CaZr(PO4)2 lattice [14], and thus several kinds of oxygen vacancies are formed. The many oxygen vacancies and the complex aggregation of lattice defects in CaZr(PO4)2 may offer an explanation to the consecutive distribution of multiple trap levels.
In order to investigate the EML process of CaZr(PO4)2:Eu2+, the energy-band structure and the energy levels of Eu2+ ion are also discussed. Vacuum ultraviolet studies by Kaneyoshi reveal the fundamental absorption edge of CaZr(PO4)2 at Efa = 6.9 eV (180 nm) [18]. Since the band gap energy is normally ca. 0.5 eV higher than the fundamental absorption edge due to the exciton creation energy [34], the band gap of CaZr(PO4)2 can be estimated as 7.4 eV. The 4f ground levels of the Eu2+ ion could be estimated according to the charge transfer energy (from ligand to the Eu3+ ion) of one Eu3+ ion [20], because there is only one calcium site in the CaZr(PO4)2 lattice [14]. The maximum of the O(2p)-Eu3+ charge-transfer absorption band is observed at 4.4 eV (280 nm) in CaZr(PO4)2 [13]. The 4f ground level of Eu2+ is thus located 3.0 eV below the conduction band. Since the luminescence band of Eu2+ is observed at 474 nm (2.6 eV), the lowest 5d level of Eu2+ is then located 0.4 eV below the bottom of the conduction band, but the higher 5d levels are in the conduction band. Otherwise, if all Eu2+ 5d bands are higher than the conduction, the Eu2+ luminescence should be quenched. This condition that part of 5d electron’s energy for rare earth ions is above the host conduction band also happens in SrAl2O4:Eu2+ [20], SrAl2O4:Ce3+ [35], and CaAl2O4:Eu2+ [20].
On the basis of these results and previous works [10,11,20,36–39], an EML mechanism is proposed (Fig. 8). When CaZr(PO4)2:Eu2+ is irradiated with UV light (254 nm), the electrons are firstly excited from the 4f7 ground level of Eu2+ to 4f65d1 level. Because the 5d levels of Eu2+ are partly in the conduction band of CaZr(PO4)2, some electrons can escape to the conduction band with the aid of thermal energy (i.e. lattice vibrations) or photoexcitation energy. Eu2+ stays now as Eu2+-h+. Then the electron is trapped from the conduction band to an electron trap (VO or [EuCa]+) and migrate from one trap to another with the aid of acquiring (or releasing) thermal energy. When a stress is applied, the CaZr(PO4)2:Eu2+ lattice is deformed, inducing a strain energy and generating a local electric field in the piezoelectric region, and then these trapped electrons are de-trapped. Electrons in the shallow trap levels could be excited to the conduction band by the weak strain energy, while under the strong strain energy electrons in the deep trap levels could be excited to shallower contiguous trap levels, then to the conduction band, or directly to the conduction band by tunneling. The de-trapped electrons subsequently come back to the 5d levels of Eu2+-h+. Finally, the relaxation of the electrons back to the ground level of Eu2+ produces a cyan light emission (474 nm).
In the early studies, the holes originating from the reduction of Eu2+ to Eu+ had also been assumed to be the charge carriers in the persistent luminescence of MAl2O4:Eu2+ (M = Ca and Sr) [8,40], and ML of SrAl2O4:Eu2+ [41]. However, the presence of Eu+ has not been observed by the X-ray absorption measurements of SrAl2O4:Eu2+,Dy3+ [41]. Furthermore, electron paramagnetic resonance (EPR) measurements have proved the existence of electrons in anion vacancies in CaAl2O4:Eu2+ [33]. Consequently, this work suggests that the electrons are the main charge carriers in the EML process, although there is a possibility that the holes trapped in the hole traps could escape to the valence band, thereby returning to the luminescent centers with light emission.
4. Conclusions
In summary, the excellent EML properties are investigated in the novel phosphor CaZr(PO4)2:Eu2+. It shows an intense luminance (above 15 mcd m−2), a low load threshold (below 5 N), a broad measurement range (up to 2000 N) for the dynamic load, and the accurate linear relationship of EML intensity against the applied load, which could be used to simultaneously sense and image the stress intensity and distribution. The luminescence spectra indicate that the EML is emitted by the transition of Eu2+ ions between the 8S7/2(4f7) ground state and the excited 4f65d1. The results of afterglow and ThL confirm the existence of multiple trap levels in CaZr(PO4)2:Eu2+. The excellent EML characteristics are considered to originate from the piezoelectric crystal structure and the multiple trap levels with appropriate depths. Finally, an EML mechanism based on the electrons as the main charge carriers is proposed according to the trap types, energy-band structure, and energy levels of Eu2+ ion in CaZr(PO4)2:Eu2+. These findings will provide an effective and promising method to develop novel stress sensitive EML materials and extend their practical applications.
Acknowledgments
This work was supported by the CREST program of JST, the Natural Science Foundation of China (Grant No. 11074138), the State Scholarship Fund of China (Grant No. 2010626112), the Shandong Provincial Natural Science Foundation for Distinguished Young Scholars (Grant No. JQ201103), the Taishan Scholars Program of Shandong Province, and Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.
References and links
1. J. Walton, “Triboluminescence,” Adv. Phys. 26(6), 887–948 (1977). [CrossRef]
2. B. P. Chandra, “Mechanoluminescence,” in Luminescence of Solids, edited by D. R. Vij (Plenum Press, 1988), 361.
3. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Artificial skin to sense mechanical stress by visible light emission,” Appl. Phys. Lett. 74(9), 1236–1238 (1999). [CrossRef]
4. C. N. Li, C. N. Xu, Y. Imai, and N. Bu, “Xu, Y. Imai, and N. Bu, “Real-time visualisation of the Portevin-Le Chatelier effect with mechanoluminescent-sensing film,” Strain 47(6), 483–488 (2011). [CrossRef]
5. N. Terasaki, H. Yamada, and C. N. Xu, “Ultrasonic wave induced mechanoluminescence and its application for photocatalysis as ubiquitous light source,” Catal. Today 201, 203–208 (2013). [CrossRef]
6. V. K. Chandra and B. P. Chandra, “Dynamics of the mechanoluminescence induced by elastic deformation of persistent luminescent crystals,” J. Lumin. 132(3), 858–869 (2012). [CrossRef]
7. X. Wang, C. N. Xu, H. Yamada, K. Nishikubo, and X. G. Zheng, “Electro-mechano-optical conversions in Pr3+-doped BaTiO3-CaTiO3 ceramics,” Adv. Mater. 17(10), 1254–1258 (2005). [CrossRef]
8. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Direct view of stress distribution in solid by mechanoluminescenc,” Appl. Phys. Lett. 74(17), 2414–2416 (1999). [CrossRef]
9. S. Kamimura, H. Yamada, and C. N. Xu, “Strong reddish-orange light emission from stress-activated Srn+1SnnO3n+1:Sm3+ (n=1, 2, ∞) with perovskite-related structures,” Appl. Phys. Lett. 101(9), 091113 (2012). [CrossRef]
10. J. Botterman, K. V. Eeckhout, I. D. Baere, D. Poelman, and P. F. Smet, “Mechanoluminescence in BaSi2O2N2:Eu,” Acta Mater. 60(15), 5494–5500 (2012). [CrossRef]
11. J. C. Zhang, X. Wang, X. Yao, C. N. Xu, and H. Yamada, “Strong Elastico-mechanoluminescence in diphase (Ba,Ca)TiO3:Pr3+ with self-assembled sandwich architectures,” J. Electrochem. Soc. 157(12), G269–G273 (2010). [CrossRef]
12. C. Bettinali, A. Grandin, and M. Valigi, ““Calcium zirconium phosphate-preparation and crystallographic characteristics,” Lincei,” Classe Sci. Fis. Mat. Nat. 33, 472–476 (1962).
13. Z. J. Zhang, J. L. Yuan, X. J. Wang, D. B. Xiong, H. H. Chen, J. T. Zhao, Y. B. Fu, Z. M. Qi, G. B. Zhang, and C. S. Shi, “Luminescence properties of CaZr(PO4)2:RE (RE = Eu3+, Tb3+, Tm3+) under x-ray and VUV–UV excitation,” J. Phys. D Appl. Phys. 40(7), 1910–1914 (2007). [CrossRef]
14. K. Fukuda and K. Fukutani, “Crystal structure of calcium zirconium diorthophosphate, CaZr(PO4)2,” Powder Diffr. 18(4), 296–300 (2003). [CrossRef]
15. R. D. Shannon, “Revised effective ionicradii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. A 32(5), 751–767 (1976). [CrossRef]
16. J. F. Nye, Physical Properties of Crystals: Their Representation by Tensors and Matrices (Oxford Press, 1985)
17. Y. Liu and C. N. Xu, “Electroluminescent ceramics excited by low electrical field,” Appl. Phys. Lett. 84(24), 5016–5018 (2004). [CrossRef]
18. M. Kaneyoshi, “Luminescence of some zirconium-containing compounds under vacuum ultraviolet excitation,” J. Lumin. 121(1), 102–108 (2006). [CrossRef]
19. V. Petrykin and M. Kakihana, “Direct synthesis of BaAl2S4:Eu2+ blue emission phosphor by one-step sulfurization of highly homogeneous oxide precursor prepared via a solution-based method,” Chem. Mater. 20(16), 5128–5130 (2008). [CrossRef]
20. T. Aitasalo, J. Hölsä, H. Jungner, M. Lastusaari, and J. Niittykoski, “Thermoluminescence study of persistent luminescence materials: Eu2+- and R3+-doped calcium aluminates, CaAl2O4:Eu2+,R3+.,” J. Phys. Chem. B 110(10), 4589–4598 (2006). [CrossRef] [PubMed]
21. C. N. Xu, in “Encyclopedia of Smart Materials,” edited by M. Schwartz (Wiley, New York, 2002) 1, 190.
22. K. V. D. Eeckhout, P. F. Smet, and D. Poelman, “Persistent luminescence in Eu2+-doped compounds: a review,” Materials 3(4), 2536–2566 (2010). [CrossRef]
23. J. C. Zhang, X. Wang, and X. Yao, “Enhancement of luminescence and afterglow in CaTiO3:Pr3+ by Zr substitution for Ti,” J. Alloy. Comp. 498(2), 152–156 (2010). [CrossRef]
24. H. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Green mechanoluminescence of Ca2MgSi2O7:Eu and Ca2MgSi2O7:Eu,Dy,” J. Electrochem. Soc. 155(2), J55–J57 (2008). [CrossRef]
25. T. Z. Zhan, C. N. Xu, H. Yamada, Y. Terasawa, L. Zhang, H. Iwased, and M. Kawaid, “Enhancement of afterglow in SrAl2O4:Eu2+ long-lasting phosphor with swift heavy ion irradiation,” RSC Advances 2(1), 328–332 (2011). [CrossRef]
26. J. R. Hird, A. Chakravarty, and A. J. Walton, “Triboluminescence from diamond,” J. Phys. D 40(5), 1464–1472 (2007). [CrossRef]
27. W. Hoogenstraaten, “Electron traps in zinc-sulfide phosphors,” Philips Res. Rep. 13(6), 515–693 (1958).
28. R. Sakai, T. Katsumata, S. Komuro, and T. Morikawa, “Effect of composition on the phosphorescence from BaAl2O4:Eu2+, Dy3+ crystals,” J. Lumin. 85(1–3), 149–154 (1999). [CrossRef]
29. A. Meijerink, W. J. Schipper, and G. Blasse, “Photostimulated luminescence and thermally stimulated luminescence of Y2SiO5-Ce,Sm,” J. Phys. D Appl. Phys. 24(6), 997–1002 (1991). [CrossRef]
30. N. Kodama, T. Takahashi, M. Yamaga, Y. Tanii, J. Qiu, and K. Hirao, “Long-lasting phosphorescence in Ce3+-doped Ca2Al2SiO7 and CaYAl3O7 crystals,” Appl. Phys. Lett. 75(12), 1715–1717 (1999). [CrossRef]
31. H. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Blue light emission from stress-activated CaYAl3O7:Eu,” J. Electrochem. Soc. 155(5), J128–J131 (2008). [CrossRef]
32. B. Henderson, “Defects in Crystalline Solids,” (London: Arnold, 1972), 2.
33. J. Hölsä, T. Aitasalo, M. Lastusaari, J. Niittykoski, and G. Spano, “Role of defect states in persistent luminescence materials,” J. Alloy. Comp. 374(1–2), 56–59 (2004). [CrossRef]
34. P. Dorenbos, “The Eu3+ charge transfer energy and the relation with the band gap of compounds,” J. Lumin. 111(1–2), 89–104 (2005). [CrossRef]
35. T. Kattsumata, R. Sakai, S. Komuro, and T. Morikawa, “Thermally stimulated and photostimulated luminescence from long duration phosphorescent SrAl2O4:Eu,Dy crystals,” J. Electrochem. Soc. 150(5), H111–H114 (2003). [CrossRef]
36. H. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Ultraviolet mechanoluminescence from SrAl2O4:Ce and SrAl2O4:Ce,Ho,” Appl. Phys. Lett. 91(8), 081905 (2007). [CrossRef]
37. K. Korthout, K. Van den Eeckhout, J. Botterman, S. Nikitenko, D. Poelman, and P. F. Smet, “Luminescence and x-ray absorption measurements of persistent SrAl2O4:Eu,Dy powders: evidence for valence state changes,” Phys. Rev. B 84(8), 085140 (2011). [CrossRef]
38. H. Yamamoto and T. Matsuzawa, “Mechanism of long phosphorescence of SrAl2O4:Eu2+,Dy3+ and CaAl2O4:Eu2+,Nd3+,” J. Lumin. 72–74, 287–289 (1997). [CrossRef]
39. V. K. Chandra, B. P. Chandra, and P. Jha, “Models for intrinsic and extrinsic elastico and plastico-mechanoluminescence of solids,” J. Lumin. 138, 267–280 (2013). [CrossRef]
40. K. Kato, I. Tsutai, T. Kamimura, F. Kaneko, K. Shinbo, M. Ohta, and T. Kawakami, “Thermoluminescence properties of SrAl2O4:Eu sputtered films with long phosphorescence,” J. Lumin. 82(3), 213–220 (1999). [CrossRef]
41. J. Qiu, M. Kawasaki, K. Tanaka, Y. Shimizugawa, and K. Hirao, “Phenomenon and mechanism of long-lasting phosphorescence in Eu2+-doped aluminosilicate glasses,” J. Phys. Chem. Solids 59(9), 1521–1525 (1998). [CrossRef]
