Pressure-Induced Emission (PIE) of One-Dimensional Organic Tin

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Pressure-Induced Emission (PIE) of OneDimensional Organic Tin Bromide Perovskites Yue Shi, Zhiwei Ma, Dianlong Zhao, Yaping Chen, Ye Cao, Kai Wang, Guanjun Xiao, and Bo Zou J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02568 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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Pressure-Induced Emission (PIE) of One-Dimensional Organic Tin Bromide Perovskites Yue Shi, Zhiwei Ma, Dianlong Zhao, Yaping Chen, Ye Cao, Kai Wang, Guanjun Xiao*, and Bo Zou* State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, P. R. China. Supporting Information Placeholder ABSTRACT: Low-dimensional halide perovskites easily suffer from the structural distortion related to significant quantum confinement effects. Organic tin bromide perovskite C4N2H14SnBr4 is an unique one-dimensional (1D) structure in which the edge sharing octahedral tin bromide chains [SnBr42-]∞ are embraced by the organic cations C4N2H142+ to form the bulk assembly of core-shell quantum wires. Some unusual phenomena under high pressure are accordingly expected. Here, an intriguing pressure-induced emission (PIE) in C4N2H14SnBr4 was successfully achieved by means of diamond anvil cell. The observed PIE is greatly associated with the large distortion of [SnBr6]4− octahedral motifs resulting from a structural phase transition, which can be corroborated by in situ high-pressure photoluminescence, absorption, and angle-dispersive X-ray diffraction spectra. The distorted [SnBr6]4− octahedra would accordingly facilitate the radiative recombination of self-trapped excitons (STEs) by lifting the activation energy of detrapping of self-trapped states. First-principles calculations indicate that the enhanced transition dipole moment and the increased binding energy of STEs are highly responsible for the remarkable PIE. This work will improve the potential applications in the fields of pressure sensors, trademark security and information storage.

The ever-increasing demand for energy urges to discover highly efficient materials capable of saving energy in solid-state lighting applications.1-2 Hybrid organic−inorganic perovskites have attracted much attention owing to their exceptional properties, such as high ultraviolet−visible (UV−Vis) light absorption, tunable band gap, and long electron−hole diffusion length.3-4 Because the properties of three-dimensional (3D) perovskite are always affected by moisture, oxidation, thermal sensitivity and photodegradation,5 recent studies tend to favor low-dimensional perovskites due to the relative strong quantum confinement and the introduction of self-trapped excited states.6 However, only very few studies on one-dimensional (1D) organic tin bromide perovskite C4N2H14SnBr4 crystals have been reported thus far. Lead-free organic tin bromide perovskite C4N2H14SnBr4 exhibits a typical 1D structure, serving as the assembly of core−shell quantum wires that consist of [SnBr4]2− cations. The inorganic frameworks are composed of double edge sharing octahedral [SnBr6]4− double chains coated with [C4N2H14]2+ units.7 Pressure, as an independent thermodynamic parameter, can significantly tailor the electronic structure, bonding patterns, and hence chemical behavior, of compounds.8-10 Success in discovery of some new materials and novel phenomena was achieved under

high pressure.11-18 Especially, the development of high-pressure optical techniques facilitated the study of band structures and phase stabilities of semiconductors, including Si, MoS2, InP, CsPbBr3 and CsPbI3 nanocrystals etc.19-24 However, these materials are plagued by stress-induced PL decrease, which usually limits their potential applications in pressure sensors, trademark security and information storage. Therefore, overcoming the general pressure quenching of emission remains a great challenge to date. Here, we achieve an unexpected emission in intrinsically nonemissive 1D perovskite C4N2H14SnBr4 by high-pressure processing. This exotic pressure-induced emission (PIE) was believed to be greatly related to the large distortion of [SnBr6]4− octahedral motifs as a result of structural phase transition from monoclinic to triclinic. In situ high-pressure experiments, including PL, absorption, and angle-dispersive X-ray diffraction spectra (ADXRD), were conducted to investigate the comprehensive pressure response of C4N2H14SnBr4. Firstprinciples calculations corroborate that the enhanced transition dipole moment and the increased binding energy of self-trapped excitons (STEs) are highly responsible for the observed PIE. The works indicate that pressure processing gives us an effective means to tailor low-dimensional perovskite with enhanced functional properties. As shown in Figure 1a and Figure S1b, the pressure-dependent PL spectra of C4N2H14SnBr4 were recorded up to 20.02 GPa. It is observed that C4N2H14SnBr4 initially exhibited no PL response to the external pressure below 2.06 GPa (Figure S1a). Above this pressure point, a broadband emission with a full width at half maximum (FWHM) of ~277 nm appears (Figure S1c). As the pressure increased, the broadband emission underwent a gradually distinct emission. Intriguingly, the PL intensity experienced a persistent increase with increasing pressure, until it reached the maximum at about 8.01 GPa (Figure 1a). In addition, the optical micrographs of C4N2H14SnBr4 vs pressures in the DAC chamber clearly demonstrated the changes of PL brightness (Figure 1b). The PL color changed from dark yellow to luminous yellow. Although the emission is significantly decreased with increasing pressure, the emission in present 1D perovskite C4N2H14SnBr4 was able to persist up to 20.02 GPa, which is rarely observed in other 3D perovskite systems (Figure S1c). The emission weakening should be caused by the deviatoric stress resulting from nonhydrostatic conditions.25 In addition, the PL intensity decreases gradually, which should be attributed to the appearance of the progressive amorphization.26-28 Upon the complete release of pressure, no emission was observed, indicating the pressurerelated reversibility of 1D perovskite C4N2H14SnBr4 (Figure S2).

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Figure 1. (a) Pressure-dependence of PL spectra of C4N2H14SnBr4. (b) Photographs of C4N2H14SnBr4 under different pressure points. (c) Pressure-dependent chromaticity coordinates of the emissions. Likewise, the chromaticity coordinates of emission with the increase of pressure from 2.06 to 8.01 GPa was recorded (Figure 1c). Based on the chromaticity coordinates of emission, the emission at 2.06 GPa belongs to the yellow component (0.36, 0.30) of white-light in the temperature range of 2750–4000 K. As pressure increased, the emission gradually moved towards the monochromatic yellow light (0.48, 0.48), which was well consistent with the gradual narrowing of the peak's FWHM (Figure S1d). The pressure effect on optical properties of C4N2H14SnBr4 is of great significance to improve the application of optical devices. Likewise, in order to exclude the influence of ultraviolet irradiation on the occurrence of emission in 1D perovskite C4N2H14SnBr4, since it has been reported that photoinduced structural transformation could produce fluorescence.7 Thereby, the samples were suffered from continuous ultraviolet irradiation at 0.21 GPa. As depicted in Figure S3, we can see that the fluorescence failed to experience any increase as the exposure time increased. Accordingly, the observed emission of 1D perovskite C4N2H14SnBr4 was reasonable to be dominated by the applied pressure. Moreover, the modulated band gap was also traced by absorption spectroscopy up to 19.02 GPa. As shown in Figure S4a, the products in the ambient condition show an absorption edge located at about 400 nm consistent with previous report.7 As the increase in pressure, the absorption edge exhibited a gradual red shift, accompanied by a sustainable band gap narrowing of ~0.25 eV until 2.07 GPa. Upon further compression, the profile of the absorption edge underwent a tiny blue shift. This pressure point was in good accordance with the sudden appearance of broadband emission. After the pressure increased to 3.52 GPa, the absorption edge happened to continuously red shift again up to 19.02 GPa. Upon releasing pressure, the absorption spectrum of C4N2H14SnBr4 restored back to the initial state. The band gap of C4N2H14SnBr4 was assessed by extrapolating the linear portion of the (αhν)2 versus the hν curve, where α is the absorption coefficient, and hν is the photon energy.29 The ambient band gap was estimate about 2.96 eV (Figure S4b). The pressuredependentband gap evolution of 1D perovskite C4N2H14SnBr4 can be illustrated in Figure S4c. To definitely confirm the correlation between the optical properties and structure evolution of C4N2H14SnBr4 upon compression, we performed in situ high-pressure ADXRD pattern up to ca. 20.14 GPa (Figure S5a). It is found that the splitting of the diffraction peak at ~3.5° of C4N2H14SnBr4 occurred at 1.99 GPa, which indicated the beginning of pressure-induced phase

transition. Note that this pressure point matched well with the onset of both PIE and the tiny blue shift of absorption spectra under high pressure. Meanwhile the clearly defined two peaks in ADXRD pattern at 3.50 GPa manifested the complete phase transition. Rietveld refinements of the ADXRD patterns at 0.14 and 3.50 GPa confirmed that C4N2H14SnBr4 experienced a structural phase transition from the monoclinic (I2/M) phase to the triclinic (P−1) phase. When the compression exceeded about 14.80 GPa, some original reflections disappeared in the diffraction pattern, and the anisotropic peaks became wider and weaker, indicating the beginning of amorphous structure with a considerable degree of structural distortion. Furthermore, the asymmetry decreases in lattice constants with increasing pressure indicated the structural distortion of 1D C4N2H14SnBr4 upon compression (Table S1).

Figure 2. Pressure-induced octahedral chain deformation. (a-b) Crystal structure of C4N2H14SnBr4 before and after phase transition. (c) Transformation of octahedral chain views of C4N2H14SnBr4 under high pressure. Stage I and stage II are octahedral chains before and after the phase transition, respectively. The axes show the orientation of the crystal structure. (d-e) Schematic illustrations of Sn−Br bond length and Br−Sn−Br

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bond angle within SnBr6 octahedral framework before and after phase transition.

Figure 3. Illustration of PIE mechanism associated with exciton self-trapping in C4N2H14SnBr4. Calculated absorption oscillator strengths using the excited-state structure associated with self-trapped excitons (STEs) at 0.17 GPa (a) and 8.01 GPa (b), respectively. Configuration coordinate models for the C4N2H14SnBr4 at ambient pressure (c) and high pressure (d). Therein, the route from A to B is depicted the transition upon excitation. The STE recombination is described via green arrows in Figure 3c. The path (red arrows) refers to exciton selftrapping (blue arrow) and detrapping (green arrow). Edetrap, activation energy for detrapping; Eex, bound exciton state; ΔE, energy among the ST states; ST, self-trapped state; G, ground state; S1/2, Huang-Rhys parameter. Figure 2a and b demonstrated the monoclinic and triclinic structures of C4N2H14SnBr4 before and after phase transition. A typical 1D core-shell quantum wire structure of C4N2H14SnBr4 can be observed where the [SnBr42-]∞ octahedral chains are surrounded by the organic cations. Meanwhile, the octahedral chain exhibited obvious distortion change with zigzag configuration as the pressure increased, accompanied by a distorted single octahedron (Figure 2c). Moreover, the Br-Sn-Br bond angle exhibits an obvious decrease with increasing pressure (Figure 2d, e). To make it clear what is going on inside the octahedron, we have described the changes of Sn-Br bond lengths and Br-Sn-Br bond angles under high pressure, as shown in Table S2-S4. Stronger rigidity along the long chain of 1D C4N2H14SnBr4 will accommodate pressure to a wider pressure range where the emission enhancement can be sustained. Moreover, the organic group in 1D C4N2H14SnBr4 may offer large steric hindrance and serve as a buffer layer. Accordingly, a much wider pressure range for the PIE effects in 1D C4N2H14SnBr4 can be expected than that of zero-dimensional (0D) perovskites of Cs4PbBr6 or Cs3Bi2I9.30-31 To elucidate the underlying mechanism of PIE in C4N2H14SnBr4, we adopted a first-principles calculations associated with STEs (Figure 3a, b). It is obderved that the oscillator strengths at 0.17 GPa is 0.014 a.u., whereas, it undergo a significant increase to 0.58 a.u. at 8.01 GPa after phase transition, indicating an enhanced transition dipole moments under high pressure. Consequently, the pressure-strengthened transition dipole moments are greatly related to the observed PIE in 1D perovskite C4N2H14SnBr4. In addition, pressure can play a role in increasing binding energy of STEs, which also contributed to the enhancement of fluorescence, as recently established in 0D perovskites upon compression.32 Therefore, the essential increase of both transition dipole moments and binding energy of STEs should be responsible for the appearance of PIE in 1D perovskite

C4N2H14SnBr4. Figure 3c and d depict the schematic illustration of the PIE in C4N2H14SnBr4. Due to the specifically intrinsic 1D structure of C4N2H14SnBr4, the excited carriers at ambient conditions are readily localized to the conduction band to form bound excitons because of the strong quantum confinement. The formed bound excitons can subsequently relax to self-trapped (ST) states via the route of red arrow. However, the STEs are easily detrapped from the multiple ST states to the bound exciton state by thermal activation owing to lower activation energy for self-trapping (green arrow).31 Therefore, we cannot observe the emission of 1D perovskite C4N2H14SnBr4 at ambient conditions. Upon compression, the electron-phonon coupling strength associated with Huang-Rhys parameter S1/2 increases accompanied by distortions of both structure and excited state of C4N2H14SnBr4. This improves the activation energy of detrapping, thus effectively preventing STEs to convert into bound excitons. The change subsequently increases the concentration of STEs, and the according possibility of radiation recombination. Eventually, broadband emission associated with the transition of STEs to valence bands via radiation recombination occurs under the external pressure. At much higher pressures, the decreased energy difference E among the ST states leads to the improved color purity. In conclusion, the pressure-modulated structure and optical properties of C4N2H14SnBr4 were methodically studied with a symmetric DAC apparatus. It is worth noting that an unprecedented PIE of 1D perovskite C4N2H14SnBr4 was successfully achieved, accompanied by a great distortion of [SnBr6]4− octahedral motifs. Comprehensive experiments regarding in situ high-pressure PL, absorption and ADXRD manifested that the sample experienced a structural transformation from monoclinic structure to triclinic phase at pressure. Firstprinciples calculations indicated that the enhanced transition dipole moment and the increased binding energy of the STEs after

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structural phase transition were highly responsible for the fascinating PIE. Our work proved the possibility of fluorescence engineering in low-dimensional lead-free perovskites with enhanced functionality through high-pressure structural modulation.

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ASSOCIATED CONTENT Supporting Information Details of the experimental; Figures S1−S5 with detailed discussions; Table S1 to S4 with detailed discussions. The Supporting Information is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION

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Corresponding Author [email protected] [email protected]

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ACKNOWLEDGMENT This work is supported by the National Science Foundation of China (Nos. 21725304, 11774125, and 21673100), the Chang Jiang Scholars Program of China (No. T2016051), Changbai Mountain Scholars Program (No. 2013007), and National Defense Science and Technology Key Laboratory Fund (No. 6142A0306010917), Jilin Provincial Science & Technology Development Program (No. 20190103044JH), Scientific Research Planning Project of the Education Department of Jilin Province (No. JJKH20180118KJ). This work was performed at 4W2 HPStation, Beijing Synchrotron Radiation Facility (BSRF) which is supported by Chinese Academy of Science (grant nos. KJCX2SW-N20 and KJCX2-SW-N03).

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