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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Tuning Pressure-Induced Phase Transitions, Amorphization and Excitonic Emissions of 2D Hybrid Perovskites via Varying Organic Amine Cations Yan Qin, Zhengxing Lv, Shuguang Chen, Wei Li, Xiang Wu, Lei Ye, Neng Li, and Pei-Xiang Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06169 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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The Journal of Physical Chemistry

Tuning Pressure-Induced Phase Transitions, Amorphization and Excitonic Emissions of 2D Hybrid Perovskites via Varying Organic Amine Cations Yan Qin,†,‡ Zhengxing Lv,§ Shuguang Chen,§ Wei Li*,†,‡ Xiang Wu, *,§ Lei Ye,*,⊥Neng Li*,#, Peixiang Lu†

†School

of Physics, Huazhong University of Science and Technology, Wuhan 430074,

China.

‡School

of Materials Science and Engineering & TKL of Metal and Molecule-Based

Material Chemistry, Nankai University, Tianjin 300350, China.

§State

Key Laboratory of Geological Processes and Mineral Resources, China

University of Geosciences, Wuhan 430074, China.

ACS Paragon Plus Environment

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⊥

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School of Optical and Electronic Information, Huazhong University of Science and

Technology, Wuhan 430074, China.

#State

Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, Wuhan 430070, China.

ABSTRACT

The existence of organic components in 2D hybrid organic-inorganic perovskites gives them significant structural flexibility in response to pressure. Here we study the highpressure behavior and associated physical properties of a family of 2D hybrid perovskites, [CnH2n+1NH3]2PbI4 (n = 4, 8, 12). Our high-pressure synchrotron X-ray diffraction experiments and ab initio molecular dynamics simulations show that the critical pressures for driving structural transitions and amorphization are proportional to the length of alkylamine cations, which arise from their scaling counterbalance to compression energy via conformational rearrangements. In addition, we reveal that their photoluminescence alters from free excitons to bound excitons, then to self-trapped


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excitons under successive pressure manipulation. Furthermore, we discovered that high-pressure treatment could transform the samples from bulk micrometer powders to thin nanosheets. Our findings suggest that these 2D hybrid perovskites structurally evolve and amorphize in a remarkably different way compared with conventional 2D materials because of their high degrees of structural freedom.


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INTRODUCTION

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) have attracted significant interest recently since their abundant structural and chemical diversity can offer tremendous possibilities for achieving desired functionalities applicable in the wide field of optoelectronics.12

Notably, these 2D Ruddlesden−Popper HOIPs show significantly improved photo- and

environmental stability which can overcome drawbacks of their three-dimensional (3D) counterparts.3-5

These

2D

semiconductors

adopt

a

general

formula

of

(RNH3)2(CH3NH3)n-1MnX3n+1, in which RNH3+ is an organic amine cation, M2+ is a divalent metal cation, and X‒ is a halide, respectively. In a monolayer, the inorganic perovskite layer is capped by two RNH3+ layers; and the adjacent monolayers are further adhered via the van der Waals interactions to form a 3D structure.

The optoelectronic properties of these 2D HOIPs are primarily dependent on their semiconducting inorganic layers, however, the electronically inert organic amine cations have also been realized to play a very important role.6-9 For example, varying the length or size of organic amine cations can induce the distortion of the inorganic skeleton or alteration of the lattice dynamics, which result in tunable bandgaps and optical


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emissions.10-11 Moreover, the existence of abundant organic components in 2D HOIPs leads to more prominent van der Waals interactions over conventional 2D materials.1-2,9 Accordingly, these layered hybrid perovskites respond to external stimuli in a markedly different manner, and their high flexibility offers additional degrees of freedom for engineering physical properties using thermodynamic parameters.12-14 To explore such possibilities, pressure, as an effective perturbation, can be utilized to manipulate the structures and electronic nature of these flexible 2D HOIPs.15-17 A handful of emerging studies have demonstrated that moderate pressure can induce phase transitions in these layered HOIPs, and further increase of the hydrostatic compression up to few tens of GPa eventually results in amorphization.18-22 These structural transitions involve substantial alterations of M-X-M bond angles and M-X bond lengths, which can be used to engineer the bandgap.23-25 More importantly, such successive process is fully reversible upon the compression-decompression cycle that are facilitated by the substantial compliance enabled by the organic components through van der Waals interactions.26-28 Nevertheless, the detailed structural evolution of the organic amine cations from the ambient to high-pressure phase, then to the amorphous state, has yet


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been fully understood, and their specific modulation in the inorganic quantum wells and associated optoelectronic properties during the structural evolution also need to be quantified.

In this work, we systematically study the pressure-induced phase transitions in three analogous layered HOIPs with increasing length of alkylamine cations, namely [C4H9NH3]2PbI4 (C4-Pb), [C8H17NH3]2PbI4 (C8-Pb) and [C12H25NH3]2PbI4 (C12-Pb). Our in-situ high-pressure synchrotron powder X-ray diffraction (HP-PXRD) and ab initio molecular dynamics (AIMD) simulations reveal that the organic components dominate the structural and optical stabilities under high pressure conditions in these 2D HOIPs. In addition, we disclose that the pressure manipulation can enable broadband tuning of photoluminescence (PL) and morphological transformation of these layered perovskite materials.

METHODS


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General Information and Synthesis. All chemicals and solvents were of reagent grade and used as received. [C4H9NH3]2PbI4, [C8H17NH3]2PbI4 and [C12H25NH3]2PbI4 were synthesized according to the literature method.29-31

High-Pressure Generation. The hydrostatic pressure was exerted by the systematic diamond anvil cells (DACs) with a culet diameter of 400 μm. The samples in well ground powder form were placed in a hole of about 190 μm diameter in a pre-indented stainless steel gasket with a thickness about 40 μm. The silicone oil was served to act as the pressure-transmitting medium and ruby spheres were placed for pressure calibration by measuring the fluorescence shift as the function of pressure.32

In situ High-Pressure X-Ray Diffraction. The synchrotron high-pressure powder X-Ray Diffraction (HP-XRD) experiments were performed at the 4W2 beam line of Beijing Synchrotron Radiation Facility (BSRF). The X-ray beam with a wavelength of 0.6199 Å was focused into a 36×12 μm2 spot using Kirkpatrick–Baez mirrors. The diffraction patterns were collected by a Pilatus 2M detector and integrated via FIT2D suit of package. Le Bail analysis was carried out using the TOPAS program. The HP phases of


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C4-Pb, C8-Pb and C12-Pb were identified using their low temperature (LT) structures, and they were refined respectively in the Pbca, P21/a, and P21/a space groups as listed in Table S1.

ab initio Molecular Dynamics (AIMD). The ab initio molecular dynamics (AIMD) involved in Cambridge Sequential Total Energy Package (CASTEP)33 is employed for the global structure relaxation for each pressure. AIMD calculations in this work within isothermal– isobaric ensemble (NPT ensemble) with each ionic MD time step of 2 femtosecond (fs) for duration up to 5 picoseconds (ps). The more details of the calculated here as following parameters: (1) The projector augmented wave with Perdew–Burke–Ernzerhof (PAW-PBE) potentials34 with the generalized gradient approximation (GGA);35 (2) electronic convergence criterion is set at 10-4 eV; (3) a high energy cutoff of 450 eV; (4) Gama-point sampling is used owing to enough size with more than two hundred atoms. (5) the DFT-D36 empirical correction of Grimme was carried out for describing the dispersion correction in the models.


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High Pressure Photoluminescence (HP-PL). The spectra were measured by a Raman spectrometer (Horiba, LabRAM HR Evolution) under a 325 nm excitation laser with reflection method. A long working distance 15× microscope objective focused light on the sample mounted in the DAC. The PL spectra were dispersed by a 1800 groove per millimeter diffraction grating and accumulated 5 times with exposing 10s.21 The pressure exerting and calibration were alike to the aforementioned methods.

Scanning Electron Microscopy (SEM). The morphology and size of pristine, after ground for high pressure experiments and after high pressure treatment samples were investigated by a field-emission scanning electron microscope (Sirion 200 SEM, FEI; Nova NanoSEM 450, FEI; Zeiss G300). The acceleration voltage was adjusted from 5 KeV to 20 KeV in order to get clear pictures. The as-prepared samples were sprayed with platinum for 5 min in precision etching coating system (gatan model 682).

Transmission Electron Microscopy (TEM). The powder after released was dispersed in 0.1ml methanol and manually shocked several times, then the resulting suspension was dipped onto a holey Cu TEM grid prior to TEM testing. A Titan 60-300 Cs Corrected


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transmission electron microscope or Talos f200x (FEI) field-emission transmission electron microscope operating at 200 kV was used to capture the bright-field images on the high dynamic range image plates. The nanosheets were very sensitive to electron beam and lost their crystallinity within 5s.

RESULTS AND DISCUSSION

All three perovskites crystallize in the orthorhombic system with the Pbca space group, and the cell parameters are a = 8.8764(1), b = 8.6925(1), c = 27.6014(5) Å for C4-Pb,29

a = 8.9817(4), b = 8.6886(3), c = 37.4821(18)Å for C8-Pb30 and a = 8.8645(2), b = 8.5149(1), c = 49.0253(9) Å for C12-Pb,31 respectively. Clearly, the a- and b-axis have the similar dimensions in all three compounds due to their identical arrangement of the corner-shared [PbI4]2- layer, while the c-axis is proportional to the length of the organic amine cations as seen in Figure 1(a).


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Figure 1. (a) Crystal structures of C4-Pb (upper), C8-Pb (middle) and C12-Pb (bottom) at ambient conditions. Color scheme: C, black; N, blue; I, red; Pb, turquiose. H, gray. (b) Relative changes of interlayer distance with increasing pressure extracted from HPXRD results. (c-e) HP-PXRD patterns of (c) C4-Pb, (d) C8-Pb and (e) C12-Pb, respectively. Different phases are represented as different colors.


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The pressure dependent HP-PXRD patterns are shown in Figure 1(c-e). Obviously, all these 2D HOIPs undergo phase transitions at low pressure and reach amorphization ultimately upon further compression (Figures S1-3 ， Table S1). At ~0.5 GPa, C4-Pb transforms to the HP phase, Pbca II, through an isostructural transition. However, C8Pb turns into a mixed phase of LP (Pbca) and HP (P21/a) structures, and C12-Pb remains in the original LP phase (Pbca) at 0.51 GPa. These results suggest that the phase transition pressure is proportional to the length of organic amine cation in this family of layered perovskites. By further increasing pressure, the diffraction peaks of C4-Pb, C8-Pb and C12-Pb exhibit significant weakening and broadening respectively at ~4.77, ~7.29 and ~10.17 GPa, suggesting the onset of structural disorder.37 As applied pressure on C4-Pb, C8-Pb and C12-Pb further rise to ~8.13, ~16.71 and ~20.59 GPa, only three broad peaks associated with crystallographic planes composed of Pb atoms are remained, implying the occurrence of complete disorder, hence amorphization, of all three structures. The amorphization pressure also scales with the length of protonated organic amine. As seen in Figure 1b, the interlayer distance between adjacent monolayers

exhibits

a

continuous

contraction

under

compression,

and

the


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compressibility of lead iodides along the layer stacking direction is proportional to the length of organic amine cations. In addition, the above pressure-induced structural transition and amorphization exhibit a highly reversible nature upon decompression, which could be attributed to the strong resilience of the van der Waals interacted organic components. In addition, the broadening of XRD peaks originate from the pressure-induced grain size reduction as evidenced by SEM and TEM (Figure 5), and the hump of diffraction patterns could arise from incomplete recovery due to the limitation of released time.37


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Figure 2. (a-c) Crystal structures obtained from AIMD simulations of (a) C4-Pb, (b) C8Pb and (c) C12-Pb at 10 GPa, respectively. (d) Representative scheme for quantifying the corrugation of [PbI4]2- layer in C4-Pb through angles (θ1, θ2, θ3 and θ4) between the crystallographical plane composed of displaced Pb atoms and the initial ab-plane under 20 GPa. The maximum angle (θmax) is selected to describe the degree of corrugation. The turquiose planes are parallel to (001) plane. (e) Pressure–dependent θmax obtained from AIMD calculations between 0 to 40 Gpa.


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To fully understand the pressure-induced structural evolution of these layered perovskites, AIMD simulations were performed on their ambient phases using the dispersion corrected potentials by considering the existence of van der Waals interactions.33-36 The validity of selected potentials was confirmed by reproducing the unit cell versus pressure trend of the HP-PXRD experiments on C4-Pb between 0.5 to 4 GPa (Figure S4), as well as the comparable contraction of interlayer distance versus pressure between HP-XRD measurements and AIMD simulation (Figure S5) in all three perovskites . More detailed simulations were carried out from ambient to 40 GPa at an interval of 10 GPa and the optimized structures were displayed in Figures S6-7. The simulated structures of these three HOIPs at 10 GPa, are shown in Figure 2(a-c), which disclose the significant differences in their compressed states. For C4-Pb, both the inorganic skeletons and organic chains are largely displaced, while regular inorganic skeletons and disordered organic chains exist in C8-Pb. For C12-Pb, the structure almost resembles to the ambient one apart from only slightly dislocated organic amine cations. The complete structural evolutions are shown in Figures S6-7, which demonstrate that the organic chains become disordered firstly because they adsorb


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most of the hydrostatic stress through conformational reorganization. Upon further compression, the PbI6 octahedra are distorted and finally lead to displaced Pb atoms away from their original positions within the ab-plane. The maximum angle (θmax, Figure 2(d-e), Tables S2-4), between the plane composed of displaced Pb atoms and the initial ab-plane, is selected to quantify the corrugation of the [PbI4]2- layer under pressure. Clearly, the θmax will be increased constantly with increase of the pressure for a given 2D perovskite, which indicates it gradually reduces its structural order. More importantly, the θmax has a strong inverse relation to the length of organic amine cation at the same pressure, which is strongly supported by the HP-PXRD results that 2D HOIPs with longer alkylamine cation chains are much harder to be hydrostatically amorphized. The reason stems from the strongly anisotropic structural arrangement of these 2D HOIPs constructed by compliant organic amine cations but rigid inorganic [PbI4]2- sheets. Under compression, the organic layer counterbalance significantly more hydrostatic stress than the inorganic sheets. Considering the identical inorganic component in all three compounds, the extended length of alkylamine cations would result in enhanced resistance to amorphization. Specifically, longer alkylamine cations could absorb more


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compression energy by substantially reorganizing their conformations under same compression, which lead to larger pressure-induced contractions of interlayer spacing as seen in Figure S5. As a result, the [PbI4]2- inorganic layers show less deformation with longer charge-balancing alkylamine cations in these 2D perovskites. To achieve a fully amorphized state, both the [PbI4]2- inorganic layers and the alkylamine cations in these 2D perovskites need to be in complete disorder. In this regard, the increased length of alkylamine cation leads to less ease of structural disorder, hence enhanced resistance to amorphization.


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Figure 3. Pressure dependent PL spectra of (a) C4-Pb, (b) C8-Pb and (c) C12-Pb. The shadows in (b) represent the reflected background from diamond. Considering the significantly different structural transitions under pressure in these 2D halide perovskites, their semiconducting properties and corresponding optical emissions are expected to be dissimilar. In-situ high-pressure PL experiments were performed on these three compounds and the normalized steady-state spectra in dependence of pressure are shown in Figure 3. At ambient pressure, the PL peaks of C4-Pb, C8-Pb and C12-Pb are respectively located at about 519.5, 520.5 and 499 nm, all originating from the emissions of free excitons (FE, Figure 4a).38-39

Upon compression, the FE emission peak (peak 1) of C4-Pb red shifts to 521.5 nm at ~0.85 GPa, along with the emergence of a small peak (peak 2) at 499.0 nm associated with the phase transition.40 With the increase of pressure to 1.07 GPa, peak 1 disappears while peak 2 increases significantly by intensity. At 3.10 GPa, peak 2 red shifts to 515.9 nm, and a new peak (peak 3) at 559.1 nm appears. The intensity of peak 2 gradually decreases and completely disappears at ~7.80 GPa. Interestingly, peak 3


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strengthens with increasing pressure but is quenched at ~10.6 GPa (Figure S8). For C8-Pb, the predominant peak (peak 1) from FEs continuously red shifts from 520.5 to 629.6 nm until quenched at ~10.7 GPa. Notably, a new peak (peak 2) near 572.5 nm appears at ~1.48 GPa. With further increase of pressure, peak 1 and 2 respectively weakens and strengthens in a red-shifting manner, and become dramatically broadened at ~3.25 and ~6.0 GPa, respectively. At ~12.9 GPa, peak 2 is quenched (Figure S9).21 Moreover, C12-Pb shows a more complex PL evolution under pressure. The ambient FE emission at 499 nm (peak 1) exhibits an asymmetric feature and its lower energy tail could come from the radiative recombination.41 Surprisingly, the increase of pressure to 0.41 GPa leads to the occurrence of two new peaks located at 521.0 (peak 2) and 529.8 nm (peak 3) in addition to the red-shifted peak 1 (506.5nm), which could originate from the coexistence of mixed Pbca and P21/a phases. With increasing pressure, both peak 1 and 2 show gradual red-shift with reduced intensity, and finally disappear at ~0.82 and 4.20 GPa, respectively. However, peak 3 strengthens up to 2.12 GPa, and weakens afterwards. It then becomes remarkably broad at 8.20 GPa, and is ultimately quenched at 14.3 GPa. In addition, a new peak emerges near 568.9 nm (peak 4) at 1.79 GPa,


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which red-shifts quickly with elevated pressure and become finally quenched at 14.3 GPa (Figure S10). The photographs of these HOIPs upon compression and decompression cycle are shown in Figures S11-13, which indicate their reversible piezochromism and application potential as strain sensors.

Figure 4. Mechanistic scheme of pressure-induced excitonic emissions. The solid black arrow denotes photo excitation. The dotted black and blue arrows represent the formation of FE and BE, respectively. The solid green and blue arrows respectively depict the FE and BE emission; the yellow, red and brown arrows denote the STE emission. CB: conduction band; VB: valence band. There are some common features in the optical emissions of these layered HOIPs under high-pressure conditions. First of all, the general pressure-induced red-shifts of all PL peaks are mainly attributed to the contraction of Pb−I bond lengths and flattening of


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the Pb-I-Pb bond angles in the crystal structure, which enhances the orbital overlaps and band dispersions to reduce bandgaps.20,23-24 Secondly, all three compounds show new exciton emissions (bound exciton, BE, Figure 4b) with lowered energies under moderate pressure region. Pressure-induced band gap narrowing could activate the suppressed trap states near VBM and CBM.42 Along with the reduced distance and enhanced Coulomb-interactions between the electron-hole pairs, different emitting pathways could be enabled.43-44 Thirdly, broad emissions are observed for all three perovskites at high-pressure conditions, which could arise from the formation of extrinsic self-trapped excitons (STEs) due to the synergistic effects of both lattice deformation and materials defects.45-46 As discussed previously, the AIMD simulation results reveal the significant corrugation of the semiconducting [PbI4]2- layer and disorder of the alkylamine cations under high-pressure conditions, which could further modulate the bandgap structure and emitting dynamics, hence giving rise to different emission trajectories.47-51 It is worth of stressing that the quenching pressures for C4-Pb, C8-Pb and C12-Pb are respectively at 10.6, 12.9 and 14.3 GPa, which indicates the proportional effect to the length of the alkylamine cation for preserving STEs.


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Figure 5. SEM images before and after compression and bright field TEM images after compression of C4-Pb (a) (b) (c), C8-Pb (d) (e) (f), C12-Pb (g) (h) (i), respectively. The scale bars in (a) (d) (g) are 5μm, in (b) (e) (h) are 1μm, in (c) (f) (i) are 200 nm, respectively. We have also examined the alterations in grain sizes and morphologies of these three perovskite samples before and after compression. The collected SEM images for all three perovskite samples show that their lateral sizes before and after high pressure


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treatment are in marked difference, which transform from a few micrometres to a few hundred nanometres (Figure 5). Strikingly, the morphologies of these perovskite samples change from thick platelets to stepwise nanosheets upon the compression and decompression cycle. Bright field TEM images were conducted, which demonstrated that the thin nanosheets obtained from high-pressure treatment have lateral sizes of a few hundred nanometres, in accord with the SEM results. In addition, the HP-PXRD profiles upon decompression demonstrate significantly broadened reflection peaks which also supports the size reduction and delamination induced by pressure treatment.37 The fracture of bulk 3D hybrid perovskites crystals under hydrostatic compression has been previously observed,37,52 however, the underlying mechanism and especially the importance of van der Waals interactions in the process has not been specifically discussed. As seen in Figure S14, the inorganic layers start to break to compensate the hydrostatic stress that the compliant organic groups cannot resist anymore. Subsequently, bulk perovskite sheets are ruptured into small thin nanosheets which are severely corrugated and distribute randomly with highly disordered organic amine cations, hence leading to amorphization. Upon decompression, the disordered


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organic groups within each corrugated nanosheet and the irregular distances between adjacent [PbI4]2- layers are fully restored to their original states, which results in the recovery

of

crystallinity

(Figures

1(b-d)).

Further

PL

measurements

of

the

decompressed samples are consistent with the HP-PXRD results in terms of the reversibility (Figures S15-17). The trivial blue shifts of PL peaks (0.5 nm for C4-Pb, 1.0 nm for C8-Pb and 2.0 nm for C12-Pb; Figures S18-20 also support the above findings with respect to the reduction of thickness.53-54 Notably, the resilience of organic amine cations, facilitated by the van der Waals interactions, primarily determines the excellent reversibility and morphology transformation of these 2D perovskite samples.

CONCLUSIONS

In summary, we have examined the pressure-induced structural evolution and amorphization of a family of 2D HOIPs, [CnH2n+1NH3]2PbI4 (n = 4, 8, 12), which have alkylamine cations with different lengths. The combined synchrotron HP-PXRD experiments and AIMD simulations disclose that longer alkylamine cation leads to less ease of phase transition and amorphization, because of the substantially increased


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counterbalancing conformational rearrangements of the alkylamine cations. Moreover, we have demonstrated that the optical emissions of these layered perovskites can be successively modulated via pressure from free excitons to bound excitons, then to selftrapped excitons. Furthermore, we have discovered that the sample morphologies transform from bulk micrometre powders to thin nanosheets before and after compression. Our work demonstrates that the high degrees of structural freedom are enabling attributes to access unusual metastable phases and novel optoelectronic properties in 2D hybrid perovskites.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website Supplemental figures and tables (PDF) Le Bail refinements; AIMD calculated structures; pressure-dependent PL locations; PL spectra upon decompression; photographs of piezochromism; contrastive PL before and after high pressure treatment.


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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (W. Li) * E-mail: [email protected] (X. Wu) * E-mail: [email protected] (L. Ye) * E-mail: [email protected] (N. Li)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 21571072 and 11604249), the Fok Ying-Tong Education Foundation


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for Young Teachers in the Higher Education Institutions of China (No. 161008), and the Fundamental Research Funds for the Central Universities (Nankai University, No. 63196006). The work concerning in situ HP-PXRD measurements were performed at beamline 4W2, Beijing Synchrotron Radiation Facility (BSRF). We also thank the Analytical and Testing Center in Huazhong University of Science and Technology (HUST) for technical support. Special thanks to Dr. Zhao Lu at Analytical and Testing Center of HUST for the guidance of TEM characterizations.

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