A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric

Subscriber access provided by UNIV OF DURHAM

Article

A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric Xiu-Ni Hua, Wei-Qiang Liao, Yuan-Yuan Tang, Peng-Fei Li, Ping-Ping Shi, Dewei Zhao, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08286 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric Xiu-Ni Hua,† Wei-Qiang Liao,*,‡ Yuan-Yuan Tang,‡ Peng-Fei Li,‡ Ping-Ping Shi,† Dewei Zhao,‡ and Ren-Gen Xiong*,†,‡ †Jiangsu

Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, P. R. China. ‡Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, P. R. China. Supporting Information ABSTRACT: Organic-inorganic hybrid perovskite, [CH3NH3]PbI3, holds a great potential for next-generation solar devices. However, whether the ferroelectricity exists in [CH3NH3]PbI3 and results in the ultrahigh performance are not at present clear. Beyond that, no hybrid lead iodide perovskite ferroelectric has yet been found. Here, using precise molecular modifications, we successfully designed a room-temperature hybrid perovskite ferroelectric, [(CH3)3NCH2I]PbI3. Due to the high-symmetry and nearly spherical shape of [(CH3)4N]+ cation, [(CH3)4N]PbI3 crystalizes in a centrosymmetric space group P63/m at room temperature and undergoes a structural phase transition at 184 K. Accompanied by the introduction of halogen atoms on the cation from F to I, the phase transition temperature gradually increases to 312 K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of [(CH3)3NCH2I]-I and the largest volume of [(CH3)3NCH2I]+ among these compounds might be possible reasons for the stabilization of ordered [(CH3)3NCH2I]+ cation array and further reservation of its ferroelectricity at relatively high temperature. This work provides an efficient molecular design strategy toward the targeted harvest of room-temperature organic-inorganic perovskite ferroelectrics, and should inspire further exploration of the interplay between structure and ferroelectricity. The discovery of lead iodide perovskite ferroelectric also offers a foothold to the possibility for the existence of ferroelectricity in [CH3NH3]PbI3.

INTRODUCTION The remarkably versatile compositions and structures of perovskites (A and B are different metal cations, X is an anion) render a wide range of fascinating physical properties, including ferroelectricity, piezoelectricity, dielectricity, high-temperature superconductivity and colossal magnetoresistance.1 Organic-inorganic hybrid perovskites (OHPs), in which the A-site is occupied by organic component, have been known as an important branch of perovskite materials over more than 100 years. The past decade has witnessed the arise of “perovskite fever”, when such great enthusiasm to the study of OHPs extended the applications ranging to varied electronic, photovoltaic and optoelectronic devices.2 [CH3NH3]PbI3, a prototype of OHPs, is attracting worldwide interest due to the extraordinary light-harvesting property and high efficiency above 20%.3 The polar nature and orderdisorder transition of [CH3NH3]+ cations raise the possibility of ferroelectricity that is supposed to account for the superior performance in solar cells. Nevertheless, so far, experimental efforts to get the direct proof of the ferroelectric property yield only contradictory results.4 Fortunately, thanks to the unusual degree of structureproperty flexibility, beyond the [CH3NH3]PbI3 are countless unexplored candidates for new ferroelectric OHPs. What is urgently needed is to find the key to this treasure, that is, developing useful constructing avenues. According to the Neumann’s Principle, crystal structure controls properties, including ferroelectricity that occurs

only in one of the ten polar point groups: 1 (C1), 2 (C2), m (C1h), mm2 (C2v), 4 (C4), 4mm (C4v), 3 (C3), 3m (C3v), 6 (C6) and 6mm (C6v). A molecular design approach is consequently proposed for addressing the issue of introducing ferroelectricity into OHPs, since subtle alternation of a molecular structure will have direct influence on the crystal structures and properties. In contrast to their rigid inorganic counterparts, OHPs have high tunability through the halides and organic and metal cations, so their structural, electrical, and optical properties can be easily tuned and modulated by facile chemical modification, building up a promising platform for the design and optimization of diverse functional materials.5 By selecting polar and dynamic organic cations, the possibility of orientational disorder and polarization, or even bulk ferroelectric ordering, can be smoothly induced. Moreover, especially noteworthy is that both organic component and inorganic framework have templating influence on each other, since the latter will affect the ordering, orientation, and conformation of the former, and likewise, the former may distort the latter. It enables a certain degree of control over the overall structural characteristics and properties of the material. Under precise molecular modifications or tailoring, even those non-polar candidates can be endowed with ferroelectricity. [(CH3)4N]PbI3 (tetramethylammonium tris(μ2-iodo)-lead, (TM)PbI3), is an example of OHP derived from [CH3NH3]PbI3, where the three-dimensional (3D) symmetry is disrupted to 1D by replacing the cation

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. The space groups at room temperature and phase transition temperatures for compounds (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3, (TMBM)PbI3 and (TMIM)PbI3, respectively. with an excessively large one. The high-symmetric and nonpolar [(CH3)4N]+ cation triggers a centrosymmetric space group P63/m, disallowing the possibility of hightemperature ferroelectricity, while its disorder-to-order change is responsible for the structural phase transition at 184 K. To change this, we proposed the idea of incorporating more electronegative species like halogen atoms on the cation to induce molecular dipole and asymmetry. Three new 1D OHPs, [(CH3)3NCH2F]PbI3 (trimethylfluoromethylammonium tris(μ2-iodo)-lead, (TMFM)PbI3), [(CH3)3NCH2Cl]PbI3 (trimethylchloromethylammonium tris(μ2-iodo)-lead, (TMCM)PbI3) and [(CH3)3NCH2Br]PbI3 (trimethylbromomethylammonium tris(μ2-iodo)-lead, (TMBM)PbI3) were therefore obtained. The structural modifications raise the potential energy barrier of the tumbling motion of [(CH3)4N]+ cation, so the phase transition temperatures of (TMFM)PbI3, (TMCM)PbI3 and (TMBM)PbI3 are increased to 189, 269 and 291 K, respectively. However, because the F, Cl and Br atoms are not large or heavy enough to make such tumbling motion freezing, the [(CH3)3NCH2Cl]+ and [(CH3)3NCH2Br]+ cations still show severe disorder similar to the original [(CH3)4N]+ cation. As a result of the cancelled molecular dipoles, (TMFM)PbI3, (TMCM)PbI3 and (TMBM)PbI3, disappointingly, adopt the centrosymmetric space group P63/m and P63/mmc at room temperature, respectively. By way of analogy, tying a feather to the balloon is not enough to hinder its rise or move, only tying a rock could work. Hence, upon proceeding down group VIIA, we noted the I atom that is much heavier than F, Cl and Br, and successfully designed a ferroelectric OHP, [(CH3)3NCH2I]PbI3 (trimethyliodomethylammonium tris(μ2-iodo)-lead, (TMIM)PbI3). As expected, the TMIM cation is well ordered at room temperature, the alignment of molecular dipoles does give rise to a polar space group C2, and the ferroelectric phase transition occurs at 312 K with an Aizu notation of 2/mF2. By exploring many possibilities for systematically determining structure-property relationships, this work opened up a fine design path toward the targeted harvest of ferroelectric OHPs. And, the lowtemperature solution processability will make them sub-

stantially exceptional and cost-effective for large-scale ferroelectric applications.

RESULTS AND DISCUSSION (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3, (TMBM)PbI3 and (TMIM)PbI3 were prepared as light yellow crystals by slow evaporation of a clear HI solution containing stoichiometric amounts of PbI2 and corresponding organic iodide salts. Their phase purities were confirmed by powder X-ray diffraction (PXRD) (Figure S1) and infrared (IR) analysis (Figure S2). Infrared characteristic peak of the C−F bond from TMFM cation, the C−Cl bond from TMCM cation, the C−Br bond from TMBM cation, and the C−I bond from TMIM cation was clearly shown at around 1090 cm-1, 806 cm-1, 702 cm-1, and 645 cm-1, respectively. We then directly checked the phase transition behaviors by differential scanning calorimetry (DSC) measurements (Figure 1a). (TM)PbI3 was found to exhibit a low-temperature phase transition at around 184 K with an obvious endothermic peak at this temperature upon heating. (TMFM)PbI3 shows a little higher phase transition temperature at around 189 K after one H atom of TM cation substituted by a halogen atom F. When replacing one H atom of TM cation with a halogen atom Cl, the phase transition temperature of (TMCM)PbI3 increases significantly to 269 K, but it is still below room temperature. When further substituting the H atom of TM cation by one halogen atom Br, the phase transition of (TMBM)PbI3 occurs near room temperature at 291 K. The phase transition temperature was then further successfully increased above room temperature in (TMIM)PbI3 by replacing an H atom of TM cation with another halogen atom I. As shown in the DSC curves of (TMIM)PbI3, one small endothermic peak at around 312 K was observed in the heating run, indicating a secondorder phase transition. The phase transitions of these compounds were also verified by the temperaturedependent real part (εʹ) of complex dielectric permittivity (Figure 1b). Apparent dielectric anomalies were presented near the phase transition temperatures in all the compounds.


Page 2 of 8

Page 3 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Figure 1. (a) DSC curves in a heating−cooling cycle and (b) temperature dependence of the real part of complex dielectric permittivity for polycrystalline samples of compounds (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3, (TMBM)PbI3 and (TMIM)PbI3, respectively.

Structurally, (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3, (TMBM)PbI3 and (TMIM)PbI3 are isostructural, having a hexagonal ABX3 perovskite structure, in which the organic cations occupy the cavities between adjacent [PbI3]-n chains of face-sharing PbI6 octahedra (Figures 2a and S3). Above phase transition temperature, (TM)PbI3 and (TMFM)PbI3 crystalize in the same hexagonal space group P63/m and point group C6h at 298 K (Table S1). The TM and TMFM cations, lying on a special position with 6/m symmetry, show highly orientational disorder (Figure S3). Resembling (TM)PbI3 and (TMFM)PbI3, (TMCM)PbI3 and (TMBM)PbI3 also crystalize in the hexagonal crystal system at 298 K, with the different space group P63/mmc and point group D6h (Table S1). The TMCM and TMBM cations situated on the 6/mmm symmetry positions are severely disordered (Figure S3). Both the C6h and D6h point groups are centrosymmetric, meaning that it is impossible for (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3 and (TMBM)PbI3 to have ferroelectricity above room temperature. We tried to determine their structures below phase transition temperature to check the space groups of low-temperature phase. But we failed because of the very poor diffraction data, induced by the common multiple twining in hybrid perovskites.6

Figure 2. (a) Packing diagram of (TMIM)PbI3 at 298 K along b axis, containing three nonequivalent TMIM cations. The pink dashed lines stand for the I· · · I edges interactions. Hydrogen atoms were omitted for clarity. Comparison of packing views of (TMIM)PbI3 along [101] direction at (b) 298 K and (c) 318 K. The TMIM cations occupy a general symmetry position at 298 K while a special position of the m symmetry plane (blue dashed lines) at 318 K.

Fortunately, compared with the high-symmetry centrosymmetric hexagonal space groups in (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3 and (TMBM)PbI3, (TMIM)PbI3 adopts a low-symmetry noncentrosymmetric monoclinic space group C2 at 298 K (Table S1), which belongs to one of the 10 polar point groups, C2. The crystal structure is comprised of three independent TMIM cations and infinite [PbI3]-n chains of four nonequivalent face-sharing PbI6 octahedra running along [101] direction (Figure 2a). The hexagonal ABX3 perovskite structure in (TMIM)PbI3 is distinct from the cubic ABX3 perovskite structure in [CH3NH3]PbI3, which contains [CH3NH3]+ cation and three-dimensional [PbI3]- framework of corner-sharing PbI6 octahedra. Different from the 6/m symmetry position of TM cation in (TM)PbI3 and TMFM cations in (TMFM)PbI3, as well as the 6/mmm symmetry positions of TMCM cation in (TMCM)PbI3 and TMBM cations in (TMBM)PbI3, each of three TMIM cations is located on a general position. Consequently, the TMIM cation is ordered with one orientation in the ferroelectric phase. As shown in Figure 2b, all the C−I bonds of TMIM cations point bottom right or left. Such an alignment should result in a spontaneous polarization along b axis from the viewpoint of polarization. Based on a point charge model, the spontaneous polarization in the direction of b axis is estimated to be about 0.67 μC/cm2 (Table S2). The PbI6 octahedra show weak distortion with the average trans I–Pb–I angles of 171.83° , 175.25° , 172.73° , and 177.03°for Pb1, Pb2, Pb3, and Pb4 center, respectively. The TMIM cation is situated in the free space between adjacent [PbI3]-n chains. Interestingly, the organic cation links the inor-



ganic anionic framework through weak halogen· · · halogen interaction instead of hydrogen-bonding interaction found in most of the organic-inorganic metal halides, such as N−H···I hydrogen bonds in [CH3NH3]PbI3. That is, the I atom of TMIM cation forms C−I···I−Pb bond with the I atom of [PbI3]-n chain, with average I· · · I distance of 3.624 Å. At 318 K, (TMIM)PbI3 also crystalizes in a monoclinic but centrosymmetric space group C2/m with the point group of C2h (Table S1), corresponding to the paraelectric phase. From the Aizu rule, (TMIM)PbI3 is classified as the 2/mF2 type ferroelectric.7 In the structure, the PbI6 octahedra are still distorted, with a little increase in the average trans I–Pb–I angle of 177.72° . However, the symmetry site of three nonequivalent TMIM cations changes to a special one of m symmetry plane perpendicular to b axis. Two of them are orientationally disordered over two equivalent positions related by m symmetry plane (Figure 2c). Their C−I bonds point to the bottom right and upper right of this plane, while the C−I bond of other TMIM cation lies on the symmetry plane. Thus, the dipole moment along b axis cancels each other out, leading to the disappearing of spontaneous polarization.

the utilization of the intermolecular interactions for the development of new molecular crystals with desired physical properties. A halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. Due to the unique chemical nature of halogen bonding, this intermolecular interaction serves as an indispensable tool for the development of organic-inorganic hybrid crystal systems. In the TMXMPbI3 (X = F, Cl, Br and I) crystal system, all the halogen bonds tend to form at approximately 180°angles. The strength of halogen bonds are found to be increased from 8.3*10-4 to 2.89*10-3 Hartree with increased atom number (Figure 3). The strongest halogen bond energy (2.89*10-3 Hartree) in TMIM-I may be responsible for the formation of ordered TMIM cation array in onedimensional PbI3 chains at room temperature phase. On the other hand, compared with the molecular volume (Table 1) of TMFM (134.70 Å3), TMCM (141.27 Å3) and TMBM (159.70 Å3), the much larger volume of TMIM (182.01 Å3) could also be another possible reason for the stabilization of ordered TMIM cation array and further reservation of its ferroelectricity in such organicinorganic hybrid crystalline systems.

Figure 3. Calculated halogen bond energy in the system of TMXM-I (X = F, Cl, Br and I). Table 1. Structural parameters for TM cation series. (The computed molecular volume is defined as the volume inside a contour of 0.001 electrons/Bohr3 density.) Molecule

Bond length (Å)

Dipole (D)

Volume (Å3)

Symmetry

TM

1.08864 (C - H)

0

134.18

Td

TMFM

1.35551 (C - F)

3.1617

134.70

Cs

TMCM

1.77049 (C - Cl)

4.6531

141.27

Cs

TMBM

1.93731 (C - Br)

6.7510

159.70

Cs

TMIM

2.16621 (C - I)

8.3246

182.01

Cs

Halogen bonding effect is being exploited for an important element in crystal engineering, which involves

Figure 4. Temperature-dependent SHG intensity of (TMIM)PbI3, revealing a noncentrosymmetric-tocentrosymmetric phase transition at around 312 K.

The symmetry change accompanying the phase transition in (TMIM)PbI3 was further investigated by the measurement of second harmonic generation (SHG) response as a function of temperature, which is a reliable method to detect the change of inversion symmetry. As shown in Figure 4, (TMIM)PbI3 is SHG active with detectable intensities below 312 K, which indicates a noncentrosymmetric phase, consistent with the ferroelectric C2 point group having symmetry elements of E and C2. The SHG intensity at 298 K is approximately 0.65 times that of KH2PO4 (KDP) (Figure S4). No SHG signal is observable above 312 K due to the appearing of inversion symmetry in the centrosymmetric C2/m (symmetry elements: E, C2, i, and σh) space group. Thus, the SHG results combined with the structure analyses clearly reveal that the phase transition around 312 K is from a noncentrosymmetric ferroelectric phase to a centrosymmetric


Page 4 of 8

Page 5 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society paraelectric one. In addition, the continuous variation of SHG intensity in the vicinity of 312 K suggests a secondorder phase transition feature, matching well with the DSC results. Similar to that observed in the DSC cures of (TMIM)PbI3, there is a small thermal hysteresis between the heating and cooling runs, which is related to the temperature changing rates of 10 K/min in the testing process. We also examined the SHG response of (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3 and (TMBM)PbI3 at room temperature. All of them are SHG inactive, confirming the centrosymmetric structure (Figure S4).

Figure 5. Vertical and lateral PFM phase (a, c) and amplitude images (b, d) for the (TMIM)PbI3 thin film. (e, f) Vertical PFM phase and amplitude signals as functions of the tip voltage for a selected point, showing local PFM hysteresis loops.

The most intrinsic property of ferroelectrics is that they possess a spontaneous, stable and electrically switchable polarization. Piezoresponse force microscopy (PFM) is a powerful tool for addressing the statics and dynamics of ferroelectric polarization, which can nondestructively visualize a microscale domain structure with very high spatial resolution.8 Each PFM image contains the amplitude and phase parameters that provide information on the relative strength of the piezoelectric coefficient and the polarization direction in each individual domain, respectively. Figure S5 presents the vertical PFM phase and amplitude images and the topographic images for the (TMIM)PbI3 thin film. The top half demonstrates the classical lamellar domain structure and the bottom half shows the stripe one, like that in triglycine sulfate (TGS).9 For a more unambiguous visualization of the domain structures, the vertical and lateral PFM images have been simultaneously acquired and analyzed in the left bottom area (Figure 6). Note that it is almost the same for the domain patterns in the two components. The 180°phase contrast in both phase im-

ages demonstrates the polarizations in the two adjacent domains should be anti-parallel. Meanwhile, the amplitude contrasts for the adjacent domains separated by domain walls are very close, which provides a solid proof for the existence of 180°domain structure, supporting its crystal structure determination (2/mF2). When the vertical and lateral PFM mappings were overlaid on threedimensional (3D) topography, it is clear that the piezoresponse has no obvious correlation with the local topology of the sample surface, which excludes topographic crosstalk and further demonstrates the presence of ferroelectric polarization in the (TMIM)PbI3 thin film (Figure S6).

Figure 6. Topographic images (top) and vertical PFM amplitude (middle) and phase (bottom) images of the (TMIM)PbI3 film surface. (a) Initial state. (b) After the first switching with positive bias at +90 V. (c) After the succeeding back-switching with negative bias at -90 V. The yellow and blue regions in phase images indicate the polarizations oriented upward and downward, respectively.

To explore the polarization switching behavior, local PFM-based hysteresis loop measurement was performed on the (TMIM)PbI3 thin film. As shown in Figure 5e,f, the obvious 180°reversal of phase signal and the characteristic butterfly loop of amplitude signal can be observed on the film surface, which is a typical demonstration for the switching of ferroelectric polarization. Subsequently, we carried out local poling test to visualize the domain switching process (Figure 6). In the initial state, the vertical PFM images of as-grown film show a distinct domain pattern, just like a flying elephant (Figure 6a). When a tip bias of +90 V was used to scan the whole region, the polarization directions of domains were completely switched downward, as the color contrast in the phase image changed from yellow to blue and the domain walls disappeared in the amplitude image (Figure 6b). After a poling with opposite bias of -90 V on the tip, the polarization direction of central region can also be switched upward, demonstrating a reproducible polarization switching process (Figure 6c). All the above evi-



dence unambiguously establish the presence of stable and switchable polarization in the (TMIM)PbI3 thin film.

Page 6 of 8

foothold to the possibilities for enriching the family of room-temperature organic-inorganic perovskite ferroelectrics.

EXPERIMENTAL SECTION

Figure 7. UV−vis absorption spectrum of (TMIM)PbI3. The inset shows the Tauc plot. The estimated band gap is 2.82 eV regarded as a direct band gap semiconductor.

The optical property of (TMIM)PbI3 was investigated by the solid state UV-vis diffuse reflectance spectrum at room temperature. As shown in Figure 7, the UV−vis absorbance spectrum displays an intense absorption at the band edge onset 470 nm (~2.64 eV), corresponding well with its yellow appearance of bulk crystal. Considering the power law for the variation of absorption coefficient versus photo energy (Figure 7, inset), the feature of direct band gap semiconductor can be concluded. From the Tauc plot, the measured optical band gap for the direct transition is 2.82 eV, slightly larger than initial absorption photon energy of the band edge. Another significant character of direct band gap is that a strong PL (photoluminescence) emission can be found with photon energy centered at the value of energy gap owing to the high quantum efficiency of the radiative recombination in direct band gap materials. In the case of (TMIM)PbI3, an obvious PL emission was observed with photon energy centered at 2.33 eV (Figure S7), whose emission maximum does not shift with different excitation wavelengths, indicates that the band gap type of (TMIM)PbI3 is more likely to be direct.

CONCLUSION In conclusion, we report a room-temperature organicinorganic hybrid perovskite ferroelectric, [(CH3)3NCH2I]PbI3. The spherical [(CH3)4N]+ cation tends to exhibit dynamical disorder at room temperature, allowing [(CH3)4N]PbI3 crystalize in a centrosymmetric space group P63/m and undergo a structural phase transition at relatively low temperature (184 K). Through precise molecular modification by replacing a H atom with a halogen atom from F to I, the phase transition temperature gradually increases to 312 K and the space group transforms into a polar C2 at room temperature. The strongest halogen bond energy of [(CH3)3NCH2I]-I and the largest volume of [(CH3)3NCH2I]+ among these compounds might be possible reasons for the stabilization of ordered [(CH3)3NCH2I]+ cation array and further reservation of its ferroelectricity at relatively high temperature. This fine molecular design strategy offers a

Materials. Tetramethylammonium iodide (TM· I) was commercially available. Trimethylfluoromethylammonium iodide (TMFM· I) was synthesized from trimethylfluoromethylammonium chloride (TMFM· Cl), which was first prepared by reacting equimolar amounts of trimethylamine (30 wt % in water) with chlorofluoromethane in ethanol at room temperature. Then, excess Ag2CO3 was added to a TMFM· Cl aqueous solution with stirring, resulting in the formation of AgCl precipitates. After filtering, the clear solution was acidized by excess HI solution. TMFM· I was obtained as colorless solid after removing the solvent under reduced pressure. For trimethylchloromethylammonium iodide (TMCM· I) and trimethylbromomethylammonium iodide (TMBM· I), the synthetic procedure was similar to that of TMFM· I, with the chlorofluoromethane replaced by dichloromethane and dibromomethane, respectively. Trimethyliodomethylammonium iodide (TMIM· I) was directly synthesized by the reaction of equimolar amounts of trimethylamine (30 wt % in water) and diiodomethane in ethanol. Light yellow crystals of (TM)PbI3, (TMFM)PbI3, (TMCM)PbI3, (TMBM)PbI3, and (TMIM)PbI3 were obtained by slowly evaporating clear HI solutions containing equal molar amounts of PbI2 and TM· I, TMFM· I, TMCM· I, TMBM· I, and TMIM· I, respectively. (TMIM)PbI3 shows excellent thermal stability (Figure S8) and air stability (Figure S9). PXRD (Figure S1) and elemental analyses confirm the phase purity of (TMIM)PbI3 (Table S3). Measurements. PXRD, IR, DSC, dielectric, SHG and PFM measurements were described elsewhere.10 For PFM measurements, the samples of thin film crystals of (TMIM)PbI3 were prepared by spreading a drop (20 µL) of the precursor solution containing 10 mg (TMIM)PbI3 in 1 mL N,N-dimethylmethanamide on a clean ITOcoated glass substrate. Isolated thin film crystals were grown by slowly evaporating the solvent. UV-vis diffuse-reflectance spectra measurements were performed at room temperature using a Shimadzu UV2450 spectrophotometer mounted with ISR-2200 integrating sphere operating from 200 to 900 nm. BaSO4 was used as a 100% reflectance reference. Powdered crystal of (TMIM)PbI3 was prepared for measurement. The generated reflectance-versus-wavelength data were used to estimate the band gap of the material by converting reflectance data to absorbance according to the Kubelka−Munk equation: F(R∞) = (1− R∞)2/2R∞. Therefore, the optical band gap can be determined by the variant of the Tauc equation: (hν· F(R∞))1/n = A(hν – Eg), Where: h: Planck's constant, ν: frequency of vibration, F(R∞): Kubelka−Munk equation, Eg: band gap, A: proportional constant. The value of the exponent n denotes the nature of the sample transition. For direct allowed transition, n = 1/2; for indirect allowed transition, n = 2. Hence, the optical band gap Eg can be obtained from a Tauc plot by plotting (hν·F(R∞))1/n against the energy in eV and extrapolation of the linear region to the X-axis intercept.


Page 7 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society Calculation condition. All the geometric optimization, harmonic vibration analysis and rigid potential energy scan are performed at b3lyp/6-311G** (Lanl2DZ for iodine) level using Gaussian 09 package. The initial structures before geometric optimization are obtained from single crystal X-ray diffraction data.

ASSOCIATED CONTENT Supporting Information. Figures S1–S9, Tables S1–S3, and discussion. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]; *[email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21427801, 21703033) and Natural Science Foundation of Jiangsu Province (BK20170658).

REFERENCES (1) Tejuca, L. G.; Fierro, J. Properties and applications of perovskite-type oxides; CRC Press: New York, 2000. (2) Hodes, G. Science 2013, 342, 317. (3) (a) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. J. Am. Chem. Soc. 2009, 131, 6050; (b) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Nature 2013, 499, 316; (c) Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.;

Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; Gratzel, M.; Han, H. Science 2014, 345, 295. (4) (a) Rakita, Y.; Bar-Elli, O.; Meirzadeh, E.; Kaslasi, H.; Peleg, Y.; Hodes, G.; Lubomirsky, I.; Oron, D.; Ehre, D.; Cahen, D. Proc. Natl. Acad. Sci. 2017, 114, E5504; (b) Röhm, H.; Leonhard, T.; Hoffmann, M. J.; Colsmann, A. Energy Environ. Sci. 2017, 10, 950; (c) Strelcov, E.; Dong, Q.; Li, T.; Chae, J.; Shao, Y.; Deng, Y.; Gruverman, A.; Huang, J.; Centrone, A. Sci. Adv. 2017, 3, e1602165. (5) (a) You, Y.-M.; Liao, W.-Q.; Zhao, D.; Ye, H.-Y.; Zhang, Y.; Zhou, Q.; Niu, X.; Wang, J.; Li, P.-F.; Fu, D.-W.; Wang, Z.; Gao, S.; Yang, K.; Liu, J.-M.; Li, J.; Yan, Y.; Xiong, R.-G. Science 2017, 357, 306; (b) Tang, Y.-Y.; Li, P.-F.; Liao, W.-Q.; Shi, P.-P.; You, Y.-M.; Xiong, R.-G. J. Am. Chem. Soc. 2018, 140, 8051; (c) Shi, P.-P.; Tang, Y.-Y.; Li, P.-F.; Liao, W.-Q.; Wang, Z.-X.; Ye, Q.; Xiong, R.-G. Chem. Soc. Rev. 2016, 45, 3811; (d) Dou, L.; Wong, A. B.; Yu, Y.; Lai, M.; Kornienko, N.; Eaton, S. W.; Fu, A.; Bischak, C. G.; Ma, J.; Ding, T.; Ginsberg, N. S.; Wang, L.-W.; Alivisatos, A. P.; Yang, P. Science 2015, 349, 1518; (e) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Nat. Mater. 2014, 13, 897. (6) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Inorg. Chem. 2013, 52, 9019. (7) Aizu, K. J. Phys. Soc. Jpn. 1969, 27, 387. (8) (a) Garcia, V.; Fusil, S.; Bouzehouane, K.; Enouz-Vedrenne, S.; Mathur, N. D.; Barthelemy, A.; Bibes, M. Nature 2009, 460, 81; (b) Tang, Y.-Y.; Li, P.-F.; Shi, P.-P.; Zhang, W.-Y.; Wang, Z.-X.; You, Y.M.; Ye, H.-Y.; Nakamura, T.; Xiong, R.-G. Phys. Rev. Lett. 2017, 119, 207602; (c) Kalinin, S. V.; Rodriguez, B. J.; Jesse, S.; Karapetian, E.; Mirman, B.; Eliseev, E. A.; Morozovska, A. N. Annu. Rev. Mater. Res. 2007, 37, 189; (d) Ye, H.-Y.; Tang, Y.-Y.; Li, P.-F.; Liao, W.-Q.; Gao, J.-X.; Hua, X.-N.; Cai, H.; Shi, P.-P.; You, Y.-M.; Xiong, R.-G. Science 2018, 361, 151; (e) Tang, Y.-Y.; Li, P.-F.; Zhang, W.-Y.; Ye, H.-Y.; You, Y.-M.; Xiong, R.-G. J. Am. Chem. Soc. 2017, 139, 13903. (9) Nakatani, N. Ferroelectrics 2011, 413, 238. (10) Zhang, W.-Y.; Tang, Y.-Y.; Li, P.-F.; Shi, P.-P.; Liao, W.-Q.; Fu, D.-W.; Ye, H.-Y.; Zhang, Y.; Xiong, R.-G. J. Am. Chem. Soc. 2017, 139, 10897.




Page 8 of 8

8

A Room-Temperature Hybrid Lead Iodide Perovskite Ferroelectric

Recommend Documents