Volume 33 Issue 5
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Xiao-xia Li, Shen-long Jiang, Qun Zhang. Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 561-568. doi: 10.1063/1674-0068/cjcp2005071
Citation: Xiao-xia Li, Shen-long Jiang, Qun Zhang. Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 561-568. doi: 10.1063/1674-0068/cjcp2005071

Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites

doi: 10.1063/1674-0068/cjcp2005071
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  • Corresponding author: Qun Zhang, E-mail: qunzh@ustc.edu.cn
  • Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
  • Received Date: 2020-05-18
  • Accepted Date: 2020-06-02
  • Publish Date: 2020-10-27
  • Two thin-film 2D organic-inorganic hybrid perovskites, i.e., 2-phenylethylammonium lead iodide (PEPI) and 4-phenyl-1-butylammonium lead iodide (PBPI) were synthesized and investigated by steady-state absorption, temperature-dependent photoluminescence, and temperature-dependent ultrafast transient absorption spectroscopy. PBPI has a longer organic chain (via introducing extra ethyl groups) than PEPI, thus its inorganic skeleton can be distorted, bringing on structural disorder. The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics. In addition, the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy, suggesting its correlation with inorganic skeleton rather than organic chain. Moreover, the photoexcited coherent phonons were observed in both PEPI and PBPI, pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton. This work provides valuable insights into the optical properties, excitonic behaviors and dynamics, as well as coherent phonon effects in 2D hybrid perovskites.
  • Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
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Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites

doi: 10.1063/1674-0068/cjcp2005071

Abstract: Two thin-film 2D organic-inorganic hybrid perovskites, i.e., 2-phenylethylammonium lead iodide (PEPI) and 4-phenyl-1-butylammonium lead iodide (PBPI) were synthesized and investigated by steady-state absorption, temperature-dependent photoluminescence, and temperature-dependent ultrafast transient absorption spectroscopy. PBPI has a longer organic chain (via introducing extra ethyl groups) than PEPI, thus its inorganic skeleton can be distorted, bringing on structural disorder. The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics. In addition, the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy, suggesting its correlation with inorganic skeleton rather than organic chain. Moreover, the photoexcited coherent phonons were observed in both PEPI and PBPI, pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton. This work provides valuable insights into the optical properties, excitonic behaviors and dynamics, as well as coherent phonon effects in 2D hybrid perovskites.

Part of the special issue for "the Chinese Chemical Society's 16th National Chemical Dynamics Symposium".
Xiao-xia Li, Shen-long Jiang, Qun Zhang. Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites†[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 561-568. doi: 10.1063/1674-0068/cjcp2005071
Citation: Xiao-xia Li, Shen-long Jiang, Qun Zhang. Impact of Structural Disorder on Excitonic Behaviors and Dynamics in 2D Organic-Inorganic Hybrid Perovskites[J]. Chinese Journal of Chemical Physics , 2020, 33(5): 561-568. doi: 10.1063/1674-0068/cjcp2005071
  • Owing to their outstanding optoelectronic properties, organic-inorganic hybrid perovskites have recently stimulated great interest in the fields of photovoltaics, light-emitting diodes, and lasers [1-4]. Layered two-dimensional (2D) hybrid perovskites can be regarded as a type of quantum well, in which the metal-halide octahedra act as potential well and the organic-cation layers as potential barrier [5, 6]. Given the strong quantum and dielectric confinement effects, excitons with large binding energy are confined in the inorganic layers [7]. Such features of forming stable excitons in 2D hybrid perovskites, in conjunction with spectral tunability via manipulation of their organic cations, have been proven to be beneficial for applications such as light-emitting diodes [8, 9]. In addition, modification of the organic layers can also bring on useful properties such as hydrophobicity and stability [10, 11].

    In terms of the organic cations in 2D hybrid perovskites, previous studies have revealed that their manipulation can not only alter bond length/angle in inorganic skeleton [12], but also affect deformation of perovskite layers (with varied penetration depths of organic cations) and hence structural stiffness [13]. Such a deformation usually leads to poor crystallinity due to introduction of structural disorder [14-16]. It has been found that structural disorder in 2D or 3D hybrid perovskites can result in modifications of electronic and excitonic structures as well as photoluminescence (PL) behaviors [17, 18]. As is well known, PL performances of 2D or 3D hybrid perovskites are of close relevance to the involved exciton dynamics, drawing increased attention in recent years [19-22]. Nevertheless, we notice that systematic examination of the impact of structural disorder on excitonic behaviors and dynamics remains meager, which is the focus of our work presented here.

    The 2D organic-inorganic hybrid perovskites we investigated here are 2-phenylethylammonium lead iodide (PEPI) [i.e., (C$ _6 $H$ _5 $-CH$ _2 $-CH$ _2 $-NH$ _3 $)$ _2 $PbI$ _4 $] and 4-phenyl-1-butylammonium lead iodide (PBPI) [i.e., (C$ _6 $H$ _5 $-CH$ _2 $-CH$ _2 $-CH$ _2 $-CH$ _2 $-NH$ _3 $)$ _2 $PbI$ _4 $], both of which were prepared in the form of thin films. Introduction of extra -CH$ _2 $ groups in PBPI, making its organic chain longer than that of PEPI, can result in effective crystalline distortion in its inorganic skeleton (i.e., PbI$ _6 $ octahedra) [23], as schematically illustrated in FIG. S1 in supplementary materials. By means of steady-state absorption, temperature-dependent PL, and temperature-dependent ultrafast transient absorption spectroscopy, we performed a set of systematic, comparative measurements on the two thin-film samples of PEPI and PBPI. The obtained results provided valuable insights into the impact of structural disorder on exciton dynamics, fine-structuring of excitonic resonances, and low-frequency coherent phonons in 2D organic-inorganic hybrid perovskites.

  • Lead iodide (PbI$ _2 $, 99%), phenylethylammonium iodide (PEAI, 99%), and 4-phenyl-1-butylammonium iodide (PBAI, 99%) were purchased from Xi'an Polymer Light Technology Corp. Dimethylformamide (DMF, 99.8%) was purchased from Sigma-Aldrich. All of the chemicals were used without further purification.

  • In a typical synthesis of the thin-film PEPI sample, 0.08 mmol of PEAI and 0.04 mmol of PbI$ _2 $ were dissolved in a mixture of DMF (1 mL) and $ n $-octylamine (2 $\mathtt{ μ} $L) to form a precursor solution. Then, 20 $\mathtt{ μ} $L of this precursor solution was quickly dropped on a quartz plate and spun at 2000 r/min for 40 s and at 4000 r/min for 20 s, resulting in formation of a smooth PEPI film. The thin-film PBPI sample was prepared by following the same procedures as in PEPI, differing only in the organic chemical used (i.e., PBAI instead of PEAI).

  • Steady-state absorption spectra were recorded on a TU-1901 spectrophotometer (Persee). PL emission spectra were recorded on an FLS920 fluorescence spectrometer (Edinburgh) operating in a temperature range of 297-77 K. PL quantum yields (PLQYs) were measured using a C13534 UV/NIR absolute PL quantum yield spectrometer (Hamamatsu). Femtosecond time-resolved transient absorption (fs-TA) measurements were performed on a Helios pump-probe system (Ultrafast Systems LLC) operating in a temperature range of 297-77 K. Pump pulses (400 nm, $ \sim $40 nJ/pulse at the thin-film sample on a quartz substrate) were provided by frequency-doubling of the 800-nm output from a Ti:sapphire regenerative amplifier (Coherent). White-light continuum probe pulses (450-600 nm) were generated by focusing the 800-nm beam (split from the regenerative amplifier, $ \sim $400 nJ/pulse) onto a sapphire plate. The instrument response function was measured to be $ \sim $100 fs by a routine cross-correlation procedure. The temporal and spectral profiles (chirp-corrected) of pump-induced absorbance changes were registered using an optical fiber-coupled multichannel spectrometer and further processed by Surface Xplorer software. During the temperature-dependent PL and fs-TA measurements, a nitrogen cryostat (Optistat DN2, Oxford Instruments) equipped with a controller (MercuryiTC, Oxford Instruments) was used to control the temperature.

  • FIG. 1(a) shows the steady-state absorption spectra (450-600 nm) recorded on the PEPI and PBPI films at room temperature. The absorption profiles peaking at $ \sim $515 nm (2.408 eV) for PEPI and $ \sim $503 nm (2.465 eV) for PBPI can be assigned to the excitonic X band [24], and the 57-meV blue-shift arises as a result of the enlarged bandgap due to enhanced 2D confinement effect in PBPI [23]. FIG. 1(b) shows the room-temperature PL emission spectra (450-600 nm) recorded on PEPI and PBPI under 400-nm excitation. The Stokes shifts for the emission bands of PEPI (peaking at $ \sim $523 nm, 2.371 eV) and PBPI (peaking at $ \sim $511 nm, 2.427 eV) are $ \sim $8 nm. The blue-shift of PBPI with respect to PEPI is $ \sim $56 meV. Notably, the excitonic X band for PBPI appears broader and more asymmetric than that for PEPI. Moreover, it was estimated that the PLQY value of PBPI ($ \sim $0.5%) is only a half of that of PEPI ($ \sim $1%), as shown in FIG. S2 in supplementary materials. These observations suggest that PBPI may possess more defect states than PEPI, which is most likely caused by its higher degree of structural disorder associated with the longer organic chain-induced crystalline distortion of the inorganic skeleton.

    Figure 1.  (a) Steady-state absorption and (b) PL emission spectra recorded on the thin-film samples of PEPI and PBPI at room temperature.

    To further make a quantitative evaluation on the spectral profiles of PL emissions, we performed temperature-dependent PL measurements at temperatures ranging from 297 K down to 77 K (with a 10-K interval). The results collected on PEPI and PBPI are displayed in FIG. 2 (a) and (b), respectively. Interestingly, the biexcitonic XX band (peaking at $ \sim $534 nm, 2.322 eV) [24] emerges for PEPI when temperatures are lowered to around 100 K, while it is always silent for PBPI in this temperature range. The absence of such a biexcitonic XX band found in our PBPI sample could be linked to the local energy fluctuations induced by disorder in thin-film perovskites [24, 25], reflecting the existence of a greater structural disorder in PBPI than in PEPI. Given that particular care is taken to the X band, we executed its retrieval from the bimodal profiles of PEPI (recorded at several low temperatures) via Lorentzian decomposition [26], as exemplified in FIG. 2 (c). FIG. 2(d) plots the extracted full width at half maximum (FWHM, in meV) as a function of temperature for both PEPI and PBPI. The two sets of data (i.e., $ \Gamma $($ T $) vs. $ T $) were fitted according to the following equation [27]

    Figure 2.  Temperature-dependent PL emission spectra recorded on the thin-film samples of (a) PEPI and (b) PBPI in the temperature range from 297 K down to 77 K (at an interval of 10 K). (c) Representative retrieval of the excitonic X band from the bimodal profile (through Lorentzian decomposition) for PEPI at 77 K. (d) The plot of FWHM as a function of temperature for both PEPI and PBPI. The solid curves are the fits.

    where $ \Gamma_0 $ term is the inhomogeneous broadening (at 0 K) and the $ \Gamma_{ \rm{LO}} $ term accounts for the homogeneous broadening due to longitudinal optical (LO) phonon scattering (via Fröhlich interaction), in which $ \gamma_{ \rm{LO}} $, $ E_{ \rm{LO}} $, and $ k_ \rm{B} $ stand for coupling strength, dominant (or average) phonon energy, and Boltzmann constant, respectively. Note that another homogeneous broadening due to acoustic phonon scattering is negligible [26] and hence was not involved here. The fitting results (FIG. 2(d)) are as follows (all in the unit of meV): $ \Gamma_0 $ = 22$ \pm $1, $ \gamma_{ \rm{LO}} $ = 59$ \pm $13, and $ E_{ \rm{LO}} $ = 42$ \pm $6 for PEPI; while $ \Gamma_0 $ = 36$ \pm $3, $ \gamma_{ \rm{LO}} $ = 103$ \pm $26, and $ E_{ \rm{LO}} $ = 41$ \pm $7 for PBPI. Apparently, PBPI exhibits a larger inhomogeneous broadening ($ \Gamma_0 $) for the excitonic X band than PEPI (i.e., increased by $ \sim $64%), pointing to a larger density of defect states induced by a higher degree of structural disorder in PBPI than in PEPI [28].

    To gain more mechanistic insights into how such structural disorder would affect exciton dynamics invol ved in these hybrid perovskites, we resorted to fs-TA spectroscopy, a powerful tool for probing ultrafast dynamics occurring in nanomaterials [29-31]. In the measurements, we adopted a pump-probe configuration with the white-light continuum probe (450-600 nm) being variably time-delayed relative to the preceding 400-nm pump that is suited to induce interband transitions for both PEPI and PBPI. FIG. 3 (a) and (b) show the room-temperature fs-TA spectra taken at several representative probe delays for PEPI and PBPI, respectively. Not surprisingly, we observed negatively-valued photobleaching of the excitonic X bands peaking at $ \sim $515 and $ \sim $503 nm for PEPI and PBPI, respectively, alongside with two shoulder bands featuring positively-valued, photoinduced absorption. Obviously, the broadening of the photobleaching band for PBPI with respect to PEPI is consistent with what was observed in the steady-state absorption spectra (refer to FIG. 1(a)).

    Figure 3.  Representative room-temperature fs-TA spectra (probe delays at 0.5, 5, 20, and 500 ps) recorded on the thin-film samples of (a) PEPI and (b) PBPI. Decay-associated spectra (via a global analysis) for (c) PEPI and (d) PBPI, with all the derived characteristic time constants being annotated therein.

    To avoid unwanted influence of spectral congestion on relaxation kinetics, we performed a global analysis to retrieve the decay-associated spectra, as shown in FIG. 3 (c) and (d). The resulting characteristic time constants are as follows: $ \tau_1 $ = 518$ \pm $82 fs, $ \tau_2 $ = 4.5$ \pm $0.5 ps, $ \tau_3 $ = 91$ \pm $10 ps, and $ \tau_4 $$ > $1 ns for PEPI; while $ \tau_1 $ = 461$ \pm $81 fs, $ \tau_2 $ = 3.5$ \pm $0.6 ps, $ \tau_3 $ = 46$ \pm $7 ps, and $ \tau_4 $$ > $1 ns for PBPI. Obviously, the $ \tau_1 $, $ \tau_2 $, and $ \tau_3 $ components are all shortened for PBPI with respect to PEPI. Albeit it seems not indisputable to assign the multiple components to certain processes unambiguously, they can be attributed to consecutive processes of exciton relaxation including thermalization, annihilation, trapping/detrapping, and recombination [32-34]. Importantly, most of such relaxation pathways can be sensitive to the situation of (band gap and/or intraband) trap states [35, 36]. Thus the observed acceleration of exciton relaxations for PBPI (with respect to PEPI) likely originates from the increased density of trap states, since normally such an increase can enhance the coupling strengths for the relevant state-to-state transitions. In this particular case, the poorer crystallinity and hence the higher degree of structural disorder in PBPI can inevitably introduce into the system more defect states, serving as trap states with increased densities so as to promote exciton relaxation dynamics as observed here.

    To further look into what is hidden in the asymmetric spectral profile of X-band photobleaching, we registered temperature-dependent fs-TA spectra at the temperatures ranging from 275 K down to 77 K, as displayed in FIG. 4 (a) and (b) (probed at a typical early time of 0.5 ps) for PEPI and PBPI, respectively. With decreasing temperature, the predominant excitonic band undergoes an obvious red-shift. The red-shifts are $ \sim $27 meV (i.e., $ \sim $517 nm at 275 K shifts to $ \sim $523 nm at 77 K) for PEPI and $ \sim $36 meV (i.e., $ \sim $504 nm at 275 K shifts to $ \sim $512 nm at 77 K) for PBPI. Such a red-shift is caused by the bandgap narrowing as a result of the lattice contraction effect [37, 38]. As compared to PEPI, PBPI exhibits a larger red-shift (i.e., increased by $ \sim $33%), indicating a stronger lattice contraction effect that also hints at a higher degree of structural disorder therein. More importantly, the gradual lowering of temperature brings on the emergence of extra excitonic resonances toward higher energy.

    Figure 4.  Temperature-dependent fs-TA spectra (probe delay at 0.5 ps) recorded on the thin-film samples of (a) PEPI and (b) PBPI in the temperature range from 275 K down to 77 K. Representative 77-K spectral profiles (probe delays at 160, 300, and 890 fs) for (c) PEPI and (d) PBPI, with all the excitonic resonances being annotated therein.

    FIG. 4 (c) and (d) show the spectral evolution of the 77-K profiles for PEPI and PBPI, respectively, taken at several representative early-time probe delays. In this way one can easily uncover the progression of diverse excitonic resonances buried underneath the spectral profiles, especially for those being blended into the positively-valued, photoinduced absorption features. As indicated by the arrows, the regularly-spaced resonances for PEPI locate at $ \sim $508, 515, 522, and 530 nm, yielding a nearly constant interpeak energy spacing of $ \sim $34 meV, while those for PBPI locate at $ \sim $498, 506, 512, and 520 nm, corresponding to $ \sim $35 meV. Similar observations on PEPI were also reported in recent years, but with divergent opinions on its physical origin [39-43]. By ruling out some unlikely mechanisms, Kandada and coworkers attributed such a kind of excitonic fine-structuring in PEPI to polaronic effects, with each excitonic resonance having a distinct degree of polaronic character [41-43]. To the best of our knowledge, our current observation on PBPI represents a first one in this regard for this particular 2D hybrid perovskite system. Notably, PEPI and PBPI share exactly the same inorganic skeleton (i.e., PbI$ _6 $ octahedra). Therefore, it can be inferred from the above observation (i.e., they exhibit a nearly identical energy spacing) that such extraordinary excitonic resonances (and hence the related polaronic effects) most probably correlate with their inorganic (rather than organic) part in the two hybrid perovskites.

    Last but not least, the pump-probe configuration of our fs-TA spectroscopic experiments also allowed us to reveal the low-frequency Raman-active vibrations in both PEPI and PBPI. The preceding ultrashort pump pulses can be converted into impulsive force acting on the lattice via Raman interactions, generating coherent phonons (i.e., collective lattice displacements) whose oscillation dynamics can be tracked by the subsequently arriving probe pulse. Such a temporal response of coherent phonons can be readily extracted by subtraction from its superimposed background, i.e., the exponentially decaying trace of the fs-TA signal [44]. FIG. 5 (a) and (b) exhibit such a set of data collected from the thin-film PEPI sample (pump at 400 nm, probe at 528 nm, 77 K). The retrieved coherent phonon modes (via Fourier transformation) with low vibrational frequencies of 22, 34, and 45 cm$ ^{-1} $ (with the aid of Gaussian fitting) are shown in the inset of FIG. 5(b). Very recently, Thouin et al. [42] reported their experimental and computational results on a variety of low-frequency Raman-active vibrational modes for the PEPI system, where they assigned the modes with frequencies below 30 cm$ ^{-1} $ to the octahedral twist along an axis on the inorganic sheet, those within 30-40 cm$ ^{-1} $ to the octahedral twist, Pb displacement, or Pb-I-Pb bending, and those within 40-50 cm$ ^{-1} $ to the Pb-I-Pb bending, Pb-I stretching, and scissoring of Pb-I-Pb angle, all of which are associated with the inorganic skeleton (i.e., PbI$ _6 $ octahedra). Displayed in FIG. 5 (c) and (d) are another set of data collected from the thin-film PBPI sample (pump at 400 nm, probe at 519 nm, 77 K). It can be seen that the above three modes for PEPI (i.e., 22, 34, and 45 cm$ ^{-1} $) all experience a slight frequency-shift for PBPI (i.e., to 20, 30, and 50 cm$ ^{-1} $, respectively). The lower-frequency two ($ < $40 cm$ ^{-1} $) are red-shifted by 2 and 4 cm$ ^{-1} $, respectively, while the higher-frequency one ($ > $40 cm$ ^{-1} $) is blue-shifted by 5 cm$ ^{-1} $. Such a frequency-shift irregularity could be a reflection of the subtle impact of structural disorder (caused by organic-chain elongation) on the low-frequency vibrations of the inorganic skeleton in these hybrid perovskites, which evokes further experimental and theoretical investigations for gaining deeper insights into the physics behind.

    Figure 5.  (a) Typical fs-TA kinetic trace recorded on the thin-film sample of PEPI (probe at 528 nm, 77 K). (b) Oscillation dynamics of coherent phonons extracted from (a) via background subtraction. (c) Typical fs-TA kinetic trace recorded on the thin-film sample of PBPI (probe at 519 nm, 77 K). (d) Oscillation dynamics of coherent phonons extracted from (c) via background subtraction. The insets of (c) and (d) exhibit the corresponding low-frequency phonon modes derived via Fourier transformation.

  • In conclusion, we first constructed two thin-film samples of 2D organic-inorganic hybrid perovskites, i.e., 2-phenylethylammonium lead iodide (PEPI) and 4-phenyl-1-butylammonium lead iodide (PBPI). As compared to the PEPI system, the PBPI system features a longer organic chain (via the introduction of more ethyl groups), thereby leading to effective crystalline distortion in its inorganic skeleton. We then carried out a set of systematic measurements on the two systems and analyzed the comparative results acquired from the steady-state absorption, temperature-dependent PL, and temperature-dependent fs-TA spectroscopic characterizations. The major findings are as follows. Firstly, based on the analysis of spectral profiles (including steady-state absorption and temperature-dependent PL spectra), we revealed the increased density of defect states originating from the structural disorder due to organic-chain elongation in PBPI relative to PEPI. This was further verified by the fs-TA results, which indicated that the higher degree of structural disorder in PBPI can bring on more defect states serving as trap states with increased densities to enable promoted exciton relaxation dynamics therein. Secondly, the temperature-dependent fs-TA observations unraveled the fine-structuring of excitonic resonances (likely related to polaronic effects), which can be correlated to the inorganic skeleton (rather than the organic chain) in this type of hybrid perovskite system. Thirdly, we also addressed the interesting behaviors of low-frequency coherent phonons observed in both PEPI and PBPI, disclosing a subtle impact of structural disorder on the low-frequency Raman-active vibrations of the inorganic skeleton. Overall, the present work would offer valuable inputs for the fundamental understanding of the optical properties, excitonic behaviors and dynamics, as well as coherent phonon effects in 2D organic-inorganic hybrid perovskites.

    Supplementary materials: The schematic crystal structures and the PLQY results for both PEPI and PBPI are given.

  • This work was supported by the National Key Research and Development Program on Nano Science and Technology of the Ministry of Science and Technology of China (No.2016YFA0200602 and No.2018YFA0208702), the National Natural Science Foundation of China (No.21573211 and No.21633007), and the Anhui Initiative in Quantum Information Technologies (No.AHY090200).

  • Figure S1.  The schematic crystal structure of (a) PEPI and (b) PBPI.

    Figure S2.  The schematic crystal structure of (a) PEPI and (b) PBPI.

Reference (44)

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