Sustainable Mechanoluminescence by Designing Novel Pinning-Trap

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Sustainable Mechanoluminescence by Designing Novel Pinning-Trap in Crystals Dong Tu, Ryouta Hamabe, and Chaonan Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06714 • Publication Date (Web): 20 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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The Journal of Physical Chemistry

Sustainable Mechanoluminescence by Designing Novel Pinning-trap in Crystals Dong Tu, a,c Ryouta Hamabe, a,b Chao-Nan Xu a,b,* a

National Institute of Advanced Industrial Science and Technology (AIST), Saga, 841-0052,

Japan b

Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, 816-

8580, Japan c

School of Physics and Technology, Wuhan University, Wuhan 430072, China

Corresponding Author *

E-mail: [email protected]

ABSTRACT Mechanoluminescence (ML) from purely inorganic materials is basically triggered by a pre-UV-irradiation process, which largely limits its practical applications. Here, by designing a pining-trap structure, a novel sustainable ML is developed from Ruddlesden–Popper perovskite Sr3Sn2O7:Sm3+ even without the pre-UV-irradiation process. This ML of Sr3Sn2O7:Sm3+ could keep stable and remain a certain intensity even extending the wait time to 50 h. ML mechanism is also investigated by analysis of trap structure through the thermoluminescence method. A thermoluminescence peak at 420 K showed no attenuation after 50 h waiting time, which could be ascribed to a high-density pining trap, result in the increasement of ML intensity and ML intensity maintenance ratio.

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1. Introduction Mechanoluminescence (ML) is a luminescence phenomenon where photons are emitted by mechanical stimuli. The most important form of ML, elasticoluminescence (ESL), offers the advantages of wireless detection, nondestructive analysis, and repeatability, which is of interest for applications such as stress sensing and damage diagnosis, especially for dynamic visualization of stress distributions in engineering testing.1-8 Up to now, a certain amount of ML materials with a variety of luminescent colors have been developed in many inorganic compounds, for instance ZnS:Cu,9-11 SrMg2(PO4)2:Eu12 (blue), SrAl2O4:Eu,2-4 BaSi2O2N2:Eu13 (green), ZnS:Mn,14,15 CaZnOS:Mn16 (red) and so on. However, most of ML materials need to be pre-irradiated by light source and it is still difficult to achieve sustainable and intense ML especially after a long waiting time, which limits the quantitative evaluation and applications of ML. Thus, the goal of achieving sustainable and intense ML for use in applications for instance as a new class of light source remains unresolved. Recently, Ruddlesden – Popper perovskite phase Srn+1SnnO3n+1 (n=1, 2, ∞ ) have been attracting a great deal of interest due to their functional optical properties, such as photoluminescence, photochromism, long-persistent luminescence, and mechanoluminescence1720

. In previous research21, we have revealed that the ML properties in Srn+1SnnO3n+1:Sm3+ (n=1,

2, ∞) were altered by the crystal structure, which indicated that this material with Ruddlesden– Popper perovskite phase showed the potential to become next-generation ML material with high ML intensity and stability. In this study, for the first time we realized high-performance and sustainable ML of Sr3Sn2O7:Sm3+ (SSOS) from 3 h to even longer than 50 h waiting time. The previous researches


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have revealed the effect of designing and controlling of crystal structure on ML enhancement, here we report the totally new phenomenon by designing a pining trap. Sm3+ ion substitutes into the Sr2+ site and it acts as a luminescent center for the ML. By designing a pining-trap structure of SSOS through different atmosphere sintering (nitrogen, argon and air), we achieve a sustainable ML without directly pre-UV-irradiation process, which shows great potential to be used for stress sensing, especially the quantitative detection of stress with high stability 2. Experimental SSOS was obtained by a solid-phase synthesis. H3BO3 (99.99%) was added as a flux to commercial SrCO3 (99.9%), SnO2 (99.9%), and Sm2O3 (99.9%), and the resulting powder sample was weighed, crushed, and mixed. The mixture was annealed for 1 h at 1073 K in air, and then was sintered for 5 h at 1773 K in air, nitrogen and argon atmospheres. The sintered samples were crushed in a mortar to produce mechanoluminescent powder samples. The crystal phases of the sintered samples were identified with a powder X-ray diffractometer (RINT2000, Rigaku) at 40 kW, 40 mA, and with Cu Kα radiation (1.5418 Å). After checking for impurities, the photoluminescence (PL) properties were measured at room temperature with a spectrophotometer equipped with a Xe lamp (FP-6600, JASCO). ML properties were evaluated by using our lab-built ML evaluation device consisting of a photomultiplier tube (R645, Hamamatsu Photonics) and photon counter (C5410, Hamamatsu Photonics). Cylindrical samples (diameter 25 mm, thickness 9 mm) were used for ML testing, and these were fabricated by molding and fixing the surface of the powder samples with epoxy resin. The ML intensity of the powder sample surface was measured at room temperature while applying compressive stress with a triangular waveform from 0 to a maximum of 1000 N at a constant speed (3 mm/min) by


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using a material tester (RTC-1310A, Orientec Corporation). To understand the defects in the internal crystal structure, thermoluminescence measurements were performed using a heating/cooling stage and a fluorescence spectrophotometer (FP-6600, JASCO). 3. Results and discussion

Figure 1. (a) XRD patterns of SSOS sintered under argon, nitrogen, and air; (b) Crystal structure of Sr3Sn2O7:Sm3+. Figure 1 shows the XRD patterns of SSOS samples synthesized by changing the sintering atmosphere (air, nitrogen, and argon). These SSOS samples should originate from a noncentrosymmetric structure of the Sr3Sn2O7 with A21am space group. All the diffraction peaks match well with the noncentrosymmetric structure Sr3Sn2O7, and no impurity phases are found, indicating that single-phase SSOS can be successfully synthesized. As shown in Fig. 1(b), the noncentrosymmetric structure of SSOS contains two layers of corner-sharing SnO6 octahedra with Sr ions located at the body center, which are stacked along the c-direction with two Sr sheets inserted between the adjacent perovskite blocks. The combination of SnO6 octahedra inphase rotation around the c-axis and out-of-phase tilting around the orthorhombic b-axis leads to


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Sr displacements along the a-direction and a net bulk electric polarization along the a-direction. This polar structure makes it easy to produce polarized electrons under the stress.

Figure 2. (a) Photoluminescence excitation and emission spectra with addition of 1 mol% H3BO3; (b) ML response curves of samples with addition of 1 mol% H3BO3; (c) ML intensity of SSOS sintered with 0 and 1 mol% H3BO3 under nitrogen, argon, and air when load at 800 N. To fully exploring the ML mechanism, the luminescence behaviors of SSOS samples were investigated. Figure 2(a) shows the PL excitation and emission spectra of SSOS powders synthesized under the air, nitrogen, and argon sintering atmosphere. All the three samples


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exhibited a single broad peak at 263 nm in the excitation spectra, whereas five peaks were observed in the emission spectra at 574, 583, 611, 625, and 665 nm, no obvious peak shift could be seen. The broadband excitation peak should be ascribed to the host absorption, and the emission peaks were attributed to electron transitions from the 4G5/2 excited state of the Sm ion emitting centers to the ground state 6Hj (J = 5/2, 7/2, 9/2)21,22. Considering the PL intensity, which of N2-SSOS and Ar-SSOS were weaker than air-SSOS. This is perhaps due to that oxygen defects may form via oxygen escaping during sintering in the oxygen-free atmospheres of nitrogen and argon, and which will be further discussed afterwards. Figure 2(b) shows the response curves of the ML intensity versus compressive load for the SSOS samples. The ML intensity increased as the applied load increased, and the ML intensity of N2-SSOS was 3 times larger than that of air-SSOS under an applied load of 800 N. Thus, higher ML intensities could be obtained by sintering SSOS in a nitrogen atmosphere. Figure 2(c) shows the dependence of the ML intensities on the 0 and 1 mol% H3BO3 content for SSOS samples sintered in air, nitrogen, and argon. The N2-SSOS samples exhibited a large increase in ML intensity and showed the highest intensity for 1 mol % H3BO3 addition, which was obviously stronger than Ar-SSOS and air-SSOS sample, indicating that nitrogen atmosphere sintering played an important role in obtaining high ML intensity instead of in air or argon, and the addition of a certain amount of H3BO3 could help to enlarge the influence of sintering atmosphere on ML intensity. However, when the concentration of H3BO3 is 0%, the effect of atmosphere on the defect is not obvious because of the low crystallinity of SSOS, resulting in little change of ML intensity.23,24


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Intensity (a.u.)

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300

Trap-1

N2 Trap-2 Ar Trap-3

air 350

400

450

500

Temperature (K)

Figure 3. Thermoluminescence of SSOS sintered under nitrogen, argon, and air after a waiting time of 5 min with heating speed of 30 K/min. Table 1. Thermoluminescence peak positions and activation energies of SSOS sintered under nitrogen, argon, and air. Sample

Peak (K)

Activation Energy (eV)

Peak 1

Peak 2

Peak 3

Peak 1

Peak 2

Peak 3

air-SSOS

358

--

431

0.882

--

0.960

N2-SSOS

362

419

438

0.791

3.517

1.089

Ar-SSOS

357

421

436

1.261

9.040

1.433

In order to analyze the reason of high ML intensity of SSOS sintered in nitrogen atmosphere, the analysis of trap states in SSOS is very necessary. Figure 3 shows the thermoluminescence glow curves (common trap analysis method) for each SSOS sample. Table 1 shows the peak positions and activation energies in each glow curve obtained by the Hoogenstraaten method25. None of the glow peaks were single and symmetric, which indicated that several different defect levels existed in the SSOS crystals. The glow curve for air-SSOS contained peaks at 361 and 441


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K, whereas a peak was also observed at 420 K in the N2-SSOS and Ar-SSOS curves. This suggests that new defects were created in the crystal in SSOS by sintering in a nitrogen or an argon atmosphere. The generation of new defects was accompanied by an increase in the trapped carrier density and an increase in the ML intensity; thus, the defect levels and carrier density strongly affected the ML properties, and the increase in ML intensity arose from the increase in carrier density trapped by these defects. The glow curve of the air-SSOS sample was separated into two peaks that followed a Gaussian distribution, with activation energies calculated to be E1 = 0.882 and E3 = 0.96 eV. The glow curves of N2-SSOS and Ar-SSOS were separated into three peaks following a Gaussian distribution with activation energies calculated to be E1 = 0.791, E2 = 3.517, and E3 = 1.089 eV for N2-SSOS, and E1 = 1.261, E2 = 9.04, and E3 = 1.433 eV for ArSSOS (Table 1). The thermal activation energy at room temperature was around 0.03 eV; thus, the carriers captured by the lattice defects in each of the SSOS samples were metastable at room temperature. However, the activation energy of the sharp peak near 420 K in the N2-SSOS and Ar-SSOS samples was abnormally large, which should be due to the high-density pining trap. Therefore, analysis by trap depth alone was insufficient to analyze the lattice defect properties that produced this peak.


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Figure 4. (a) Thermoluminescence after various wait times; (b) ML after various wait times of SSOS sintered in nitrogen. ML intensity against waiting time when load at 1000 N is shown in the inset. We further investigated the effect of the wait time after illumination with 254 nm UV radiation on the thermoluminescence measurements. The samples were first exposed to UV light (254 nm) for 1 min, after a pause with a different time (5, 30 min and 3, 15, 24, 50 h), the thermoluminescence started to be measured at 625 nm. Figure 4(a) shows the thermoluminescence measurements of N2-SSOS samples measured at a temperature increase rate of 30 K/min for wait times from 5 min to 50 h after illumination with 254 nm UV radiation. The peak at 361 K shifted towards the high-temperature side and the intensity decreased as the wait time increased compared with the thermoluminescence intensity measured after a wait time of 5 min. A similar decrease was observed for the peak at 441 K, although it was smaller than that for the peak at 361 K. However, for the sharp peak at 420 K, the peak position and thermoluminescence intensity was unchanged by the length of the wait time. Therefore, defects existed that captured carriers so stably that they were not emitted during the wait time. Figure 4(b) shows the results of ML intensity measurements of N2-SSOS after a long wait time corresponding to the Fig. 4(a). Normal ML intensity measurements involve illuminating a pellet


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sample with 254 nm UV radiation and a wait time of 5 min before applying compressive stress to the powder sample surface and performing measurements at room temperature with an optical measurement system consisting of a photomultiplier tube and photon counter. In this work, we investigated the ML intensity maintenance ratio by increasing the wait time to 30 min to 50 h before applying the compressive stress. For the N2-SSOS sample, the ML intensity when 1000 N was applied after a wait time of 15 h decreased to 56.7% of the ML intensity after a wait time of 5 min, suggesting that this ML could be partly attributed to the afterglow. Interestingly, the ML intensity kept stable from then on, and remained a certain intensity from 3 h even extending the wait time to 50 h (as shown in inset Fig. 4(b)), demonstrating the achievement of a sustainable ML and a different ML mechanism. This suggests a relationship with the sharp peak at 420 K where the temperature and thermoluminescence intensity of the peak remained constant for long wait time after measuring the thermoluminescence following excitation. The metastable capture of electrons in the pining trap corresponding to this peak contributed to increasing the ML intensity and the ML intensity maintenance ratio. Meanwhile, the sustainability measurements of ML have been also carried out in Ar-SSOS and Air-SSOS, and the Ar-SSOS samples showed sustainable ML similar as the N2-SSOS, but can not be obtained from Air-SSOS, which again proved the relationship between the sharp thermoluminescence peak and the sustainable ML. Based on the experiment results and discussion, we propose a mechanism for ML in SSOS. There should be three main trap levels (Trap-1, Trap-2 and Trap-3) which is ascribed to the thermoluminescence results. As shown in Fig. 5, after the UV excitation, the electrons are photoionized and become captured by trap levels. The trapped electrons from Trap-1 and Trap-3 can be thermally released to the conduction band and then transferred to the transition from 4G5/2 to 6Hj levels of Sm3+. According to the thermoluminescence results, the Trap-1 and Trap-3


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become deeper over the waiting time, so it is difficult to achieve sustainable ML through these two traps. However, the Trap-2 is stable and concentrated, which makes the electrons hard to escape from Trap-2 during the wait time in the afterglow process. When a mechanical load is applied on SSOS, a spontaneous polarization was arisen due to symmetry breaking by trilinear coupling of two types of octahedron rotation of SSOS, and the electrons from Trap-2 can be directly excited to the energy levels of Sm3+ trough the tunneling effect, which results in the maintained ML intensity even after the long wait time until 50 h.

Figure 5. Schematic diagram of the mechanisms for ML in SSOS.

4. Conclusions In summary, we developed the sustainable ML from layered perovskite Sr3Sn2O7:Sm3+ and investigated the trap states and ML mechanism in depth. The thermoluminescence results showed that the sintering atmosphere affected the spectra. To compare with the glow curve of the sample sintered in air, the samples sintered in nitrogen and in argon exhibited an additional peak at 420 K. For the Sr3Sn2O7:Sm3+ sample sintered in nitrogen, the temperature and thermoluminescence intensity of the peak remained almost unchanged and the sample exhibited


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strong ML intensity from 3 h to even longer than 50 h waiting time. These results indicated that the peak at 420 K was related to a high-density pining trap, which increased the ML intensity and ML intensity maintenance ratio, result in a sustainable and intense ML. This finding will reduce the adverse effects of pre-UV-irradiation process on ML detection and highly improve the quantitative stress sensing with high stability.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was partially supported by JSPS KAKENHI Grant Numbers 25249100, JP16F16076, JP17H06374, JP18H01453, JSPS Postdoctoral Fellowship for Overseas Researchers (Grant No. P16076) and Cross-ministerial Strategic Innovation Promotion Program (SIP) [Developing hybrid mechanoluminescence materials for visualization of structural health] (Funding agency: JST). The authors deeply thank all colleagues at National Institute of Advanced Industrial Science and Technology (AIST) and Kyushu University for their technical support and valuable discussion

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Sustainable Mechanoluminescence by Designing Novel Pinning-Trap

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