Multiaxial Molecular Ferroelectric Thin Films Bring Light to Practical

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Perspective

Multiaxial Molecular Ferroelectric Thin Films Bring Light to Practical Applications Yuan-Yuan Tang, Peng-Fei Li, Wei-Qiang Liao, Ping-Ping Shi, Yu-Meng You, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04600 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Journal of the American Chemical Society

Multiaxial Molecular Ferroelectric Thin Films Bring Light to Practical Applications Yuan-Yuan Tang,‡ Peng-Fei Li,‡ Wei-Qiang Liao,† Ping-Ping Shi,‡ Yu-Meng You,‡ and Ren-Gen Xiong*,†,‡ †

Ordered Matter Science Research Center, Nanchang University, Nanchang 330031, P. R. China.

‡

Jiangsu Key Laboratory for Science and Applications of Molecular Ferroelectrics, Southeast University, Nanjing 211189, P. R. China. Supporting Information Placeholder

ABSTRACT: Though dominating most of the practical applications, inorganic ferroelectric thin films usually suffer from the high processing temperatures, the substrate limitation, and the complicated fabrication techniques that are high-cost, energy-intensive, and time-consuming. By contrast, molecular ferroelectrics offer more opportunities for the next-generation flexible and wearable devices due to their inherent flexibility, tunability, environmental-friendliness, and easy processibility. However, most of the discovered molecular ferroelectrics are uniaxial, one major obstacle for improving the thin-film performance and expanding the application potential. In this Perspective, we overview the recent advances on multiaxial molecular ferroelectric thin films, which is a solution to this issue. We describe the strategies for screening multiaxial molecular ferroelectrics and characterizations of the thin films, and highlight their advantages and future applications. Upon rational and precise design as well as optimizing ferroelectric performance, the family of multiaxial molecular ferroelectric thin films surely will get booming in the near future and inject vigor into the century-old ferroelectric field.

INTRODUCTION Over the past century, ferroelectrics, as an indispensable class of functional materials, have become key components in manifold areas such as ferroelectric random access memories (FeRAM), capacitors, sensors, infrared detectors, surface acoustic wave devices, and microactuators.1-2 Among their versatile physical properties including dielectricity, piezoelectricity, pyroelectricity, and nonlinear optical activity, the most intrinsic and valuable feature is of course the switchable spontaneous polarization (Ps) under an external electric field (E) greater than the coercive field (Ec). Based on this, FeRAM, the “real” application of ferroelectrics, is built by storing electronic information on the positive and negative remnant polarization (Pr) states.3 Nevertheless, it was realized in practice until the booming of integrated ferroelectrics during the 1980s, while the next decades have witnessed the shift in emphasis from single crystals and bulk ceramics to thin films.4-5 In addition to the smaller size, light weight, and diverse micro-level structures, the supreme advantage of ferroelectric thin films is

to overcome the initial barrier to the development of ferroelectric memories, i.e., lower operating voltage.6 According to the Kay-Dunn law: Ec = bd–2/3 and coercive voltage Vc = bd1/3 (d is film thickness and b is a d-independent constant), for typical ferroelectrics with Ec of about 50 kV/cm, only applying a Vc up to kV can lead to polarization switching in mm-level bulk devices; whereas for submicrometer films, the high Ec is able to be achieved by just a few volts, permitting integration into silicon technology.7 Apparently, the rise of ferroelectric thin films perfectly meets the growing demand for miniaturized multifunctional devices, and now they are being widely used in commercial products of memories, microdevices, and microwave electronic components. Although the history of ferroelectrics began with the molecular compound, Rochelle salt,8 it is the later inorganic ferroelectrics like BaTiO3 (BTO), Pb(Zr, Ti)O3 (PZT), and LiNbO3 (LNO) that have long occupied the mainstream with robust properties and great utilization potential. To date, a number of inorganic ferroelectric thin films have been intensively developed and broadly exploited, while the processing issue is always the main disadvantageous factor in successful device design.9 They not only contain environmentally harmful metals, but also suffer from the high processing temperatures that result in significant integration problems.10 Furthermore, it is particularly difficult to grow high-quality epitaxial or singlecrystalline films, without careful selection of appropriate substrates, because the lattice and thermal mismatch between them will cause strain and worsen the useful characteristics.11 Most importantly, the properties of the as-grown inorganic ferroelectric thin films rely heavily on the complicated, highcost, energy-intensive, and time-consuming fabrication techniques involving magnetron sputtering, pulsed laser deposition, molecular beam epitaxy, and metal-organic chemical vapor deposition.12 All of these unfavorable facts stimulate the desire for finding new ferroelectric systems as viable alternatives or supplements to the inorganic ones. Consequently, researchers renewed interest in molecular ferroelectrics with superiorities of light weight, low cost, environmental friendliness, mechanical flexibility, ease of low-temperature processing, structural tunability, low acoustic impedance, and biocompatibility.13-15 In the past decade, the emerging molecular ferroelectrics have definitely pointed out the bright future.16 Even the large

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Table 1. Division of the multiaxial and uniaxial ferroelectric phase transitions. Multiaxial

Uniaxial 1̅F1 (2) 2/mFm (2); 2/mF2 (2) 222F2 (2); mmmFmm2 (2) 4̅ F2 (2); 4/mF4 (2); 422F4 (2); 4̅ 2mFmm2 (2); 4/mmmF4mm (2) 3̅F3 (2); 32F3 (2); 3̅mF3m (2) 6̅ F3 (2); 6/mF6 (2); 622F6 (2); 6̅ m2F3m (2); 6/mmmF6mm (2)

Triclinic Monoclinic Orthorhombic

2F1 (2/2); mF1 (2); 2/mF1 (4) 222F1 (4/2); mm2F1 (4/2); mm2Fm (2/2); mmmF1 (8); mmmFm (4) Tetragonal 4F1 (4/2); 4̅ F1 (4/2); 4/mF1 (8); 4/mFm (4); 422F1 (8/2); 422F2(s) (4); 4mmF1 (8/2); 4mmFm (4/2); 4̅ 2mF1 (8/2); 4̅ 2mF2(s) (4); 4̅ 2mFm (4/2); 4/mmmF1 (16); 4/mmmFm (s) (8); 4/mmmFm(p) (8); 4/mmmFmm2(s) (4) Trigonal 3F1 (3/2); 3̅F1 (6); 32F1 (6/2); 32F2 (3/2); 3mF1 (6/2); 3mFm (3/2); 3̅mF1 (12); 3̅mF2 (6); 3̅mFm (6) Hexagonal 6F1 (6/2); 6̅ F1 (6/2); 6̅ Fm (3/2); 6/mF1 (12); 6/mFm (6); 622F1 (12/2); 622F2(s) (6); 6mmF1 (12/2); 6mmFm (6/2); 6̅ m2F1 (12/2); 6̅ m2Fm(s) (6/2); 6̅ m2Fm(p) (6/2); 6̅ m2F mm2 (3/2); 6/mmmF1 (24); 6/mmmFm(s) (12); 6/mmmFm(p) (12); 6/mmmFmm2(s) (6) Cubic 23F1 (12/2); 23F2 (6); 23F3 (4/2); m3̅F1 (24); m3̅Fm(12); m3̅Fmm2 (6); m3̅F3 (8); 432F1 (24/2); 432F2(s) (12); 432F4 (6); 432F3 (8); 4̅ 3mF1 (24/2); 4̅ 3mFm (12/2); 4̅ 3mFmm2 (6); 4̅ 3m F3m (4/2); m3̅mF1 (48); m3̅mFm(s) (24); m3̅mFm (p) (24); m3̅mFmm2 (12); m3̅mF4mm (6); m3̅mF3m (8) * “F” means ferroelectric phase transition. The number of polarization directions are included in the round brackets, and those ones with “/2” represent reorientable ferroelectrics, in which the spontaneous polarization can’t be switched by 180°. Ps and high Curie temperature (Tc), once regarded as the major obstacles to the development of molecular ferroelectrics, have been successfully attained.17-18 Benefiting from the easy deposition of thin-films by simple, low-cost, and low-temperature solution methods, molecular ferroelectric thin films become a hot topic as a matter course.19-20 Nonetheless, it is a wonder that they are still far below the practical application level now, despite the ongoing efforts. Taking croconic acid as an example, its single crystal has a large Ps of 20 μC·cm−2,21 comparable to that of BTO, however, the Pr of thin film is as small as 0.4 μC·cm−2.22 Actually, not just in molecular ferroelectrics, such limited performance of thin films has also been found in LNO, whose use in turn is restricted to bulk crystals.23 What they have in common is uniaxial nature, so that under an applied electric field the random grains in polycrystalline state can only be polarized between two opposite polarization directions to show a very weak macroscopic polarization. By contrast, for the majority of inorganic ferroelectrics like BTO, the polarization in each grain can be switched more easily between multiple directions to achieve an excellent ferroelectric performance. In general, the maximum remnant polarization (Pr)max that may be gained in polycrystalline state is dependent on the available ferroelectric axes. For those cases with one, three, four, and six axes, the corresponding (Pr)max are 0.25, 0.83, 0.87, 0.91Ps for the single-crystal sample, respectively.4 Given this fact, introducing multiaxial nature into molecular ferroelectrics will make considerable contribution to enhancing usability in polycrystalline thin-film form. It is no doubt that, only if the performance of molecular ferroelectric thin films could be much improved, it is possible to envision their future uses as promising alternatives to the conventional inorganic family.

Aimed at moving molecular ferroelectrics from curiositydriven discoveries to real-world applications, the principal problem to solve is how to rationally design and effectively identify outstanding multiaxial molecular ferroelectrics as well as thin films. Herein, this Perspective focuses on the recent developments and future directions of multiaxial molecular ferroelectric thin films, being helpful to address the grand challenges facing the inorganic ones. In the light of the developed key design strategies and the vital explorations on highquality thin films, the family of multiaxial molecular ferroelectrics will get greatly enriched, and thus inspire technological evolution in the next-generation flexible and wearable devices. WHAT ARE FERROELECTRICS?

MULTIAXIAL

MOLECULAR

Ferroelectricity is a bulk property that is hard to predict and design via a bottom-up approach. However, there have been some empirical guidelines for identifying ferroelectrics: (i) the ferroelectric phase belongs to one of the 10 polar point groups: 1 (C1), 2 (C2), m (C1h), mm2 (C2v), 4 (C4), 4mm (C4v), 3 (C3), 3m (C3v), 6 (C6), and 6mm (C6v); and (ii) there is a structural phase transition occurring at Tc.24 The condition (i) is essential for the occurrence of ferroelectricity while (ii) is generally true in most ferroelectrics. Accompanying the transition from the high-temperature, high-symmetry paraelectric phase to the low-temperature, low-symmetry ferroelectric phase, symmetry breaking happens with losing some symmetry elements, and hence Ps emerges. In view of the Curie principle, the ferroelectric point group should be a subgroup of both paraelectric point group and symmetry group of Ps.16 In other words, a paraelectric point group normally owns several isomorphic subgroups, whose polar direction corresponds to the direction


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Figure 1. Schematic illustration for the variation of equivalent polarization orientations n of BTO undergoing the m3̅mF4mm (n = 6), m3̅mFmm2 (n = 12), m3̅mF3m (n = 8) ferroelectric transitions. of Ps, and the maximal one comprising the most symmetry elements along the polarization direction is the ferroelectric point group. Accordingly, given a known paraelectric point group, the number of polarization directions in the ferroelectric phase can be defined as n = Np/Nf, where Np and Nf represent the respective orders of paraelectric and ferroelectric point groups, known as the sum of symmetry elements. In this manner, when n = 2, the ferroelectrics are uniaxial with only two opposite polarization directions, and certainly multiaxial ferroelectrics are characterized by more than two polarization directions. On account of the group-to-subgroup relationship determined by the Curie principle, Aizu has derived 88 species of potential ferroelectric phase transitions.25 This enables one to judge directly the occurrence of multiaxial ferroelectricity by evaluating the n, since the Np and Nf are already got for all the 32 crystallographic point groups.26 As depicted in Table 1, the greater the symmetry change between paraelectric phase and ferroelectric phase is, the more equivalent polarization directions a ferroelectric will have. In the case of LNO, since both the paraelectric space group R3̅c and the ferroelectric R3c belong to the trigonal crystal system, their lattice symmetries are so close that uniaxial nature arises.27 Likewise, diisopropylammonium bromide, as a representative molecular ferroelectric with a high Tc and a large Ps that can get on par with BTO, is also uniaxial with an Aizu notation of 2/mF2.17 Notably, it is found that those ferroelectric transitions from cubic crystal system to the one with much lower symmetry are generally multiaxial, and the n can even reach 48 (Table 1). For instance, because of the successive phase transitions from cubic Pm3̅m (Np = 48) to tetragonal P4mm (Nf = 8) at 393 K, to orthorhombic Amm2 (Nf = 4) at 278 K, and then to R3m (Nf = 6) at 183 K, BTO is a typical multiaxial ferroelectric having 6, 12, and 8 equivalent polarization directions in the three ferroelectric phases, respectively (Figure 1). STRATEGIES FOR SCREENING MOLECULAR FERROELECTRICS

MULTIAXIAL

Molecular ferroelectrics, which possess diverse structural and chemical variability exceeding the inorganic systems, can offer substantial opportunities for introducing or tuning physical properties by modulations on the molecular structures. Thanks to the prominent structural tunability, a much broader design flexibility can be expected for multiaxial molecular ferroelectrics. Experimentally, there have most re-

cently been obtained a dozen multiaxial molecular ferroelectrics (Table 2), from which several accessible screening strategies are proposed and summarized preliminarily as follows. As described above, a structural phase transition associated with significantly reduced crystal symmetry is necessary for multiaxial molecular ferroelectrics. Taking advantage of this characteristic enables multiaxial ferroelectricity, which researchers previously ignored, to be identified by reviewing reported ferroelectrics. Typically, early in 1999, some simple organic salts of mono-protonated 1,4-diazabicyclo [2.2.2]octane (Hdabco) were confirmed as ferroelectrics.28-30 In the vicinity of the respective Tc of 377 and 374 K, [Hdabco]ClO4 and [Hdabco]BF4 experience symmetry breaking from the tetragonal paraelectric space group P4/mmm to the orthorhombic ferroelectric Pm21n. From Table 1, we recognized that the two with the same Aizu notation of 4/mmmFmm2 should be biaxial and own 4 equivalent polarization directions.31-32 Their ferroelectric properties come from the ordering and relative displacement of the components in crystal lattice, which is common for molecular ferroelectrics. In particular, dabco is a spherical molecule that often displays orientational disorder or conformational transformation, and is thus capable of inducing a high symmetry. With a drop in temperature, the broken high lattice symmetry, stemming from the freezing of molecular rotations, is especially suitable for accessing multiaxial feature. By focusing on such type of spherical moieties and considering the crystal symmetry requirement, more examples are about to be obtainable during searching the database. In fact, the above spherical molecule-based ferroelectrics could have had the possibility to crystallize in the space group with a higher symmetry, rather than in the tetragonal one. It is supposed that [Hdabco]ClO4 and [Hdabco]BF4 may melt or decompose before further reaching the possible high-symmetry paraelectric phase. Instead, owing to the tremendously good thermal stability, their following analogue, [Hdabco]ReO4, does undergo two distinct phase transitions from the paraelectric Pm3̅m to the ferroelectric P4mm at 499.6 K and to the monoclinic ferroelectric Cm at 377.1 K.33 The Tc of 499.6 K is the highest one among the current molecular ferroelectrics, and above it, the detected cubic phase endows [Hdabco]ReO4 a multiaxial nature surpassing that of [Hdabco]ClO4 and [Hdabco]BF4.34 Specifically, the ferroelectric phase transitions with Aizu notations of m 3̅ mFm and m3̅mF4mm give rise to 24 and 6 equivalent polarization directions, respectively (Figure 2). The spherical molecules are nearly freely rotating in the high-temperature paraelectric phase to get in an isotropic plastic state, and when the



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Table 2. Summary of recently discovered multiaxial molecular ferroelectrics. Formula[a]

Tc (K)

Symmetry change

Aizu notation (n)

Pr / Vc of film[b]

Ref

[Hdabco]ClO4

377

Pm21n↔P4/mmm

4/mmmFmm2 (4)

4.0 / 25 (298 K)

31

[Hdabco]BF4

374

Pm21n↔P4/mmm

4/mmmFmm2 (4)

4.9 / 10 (298 K)

32

[Hdabco]ReO4

377.1, 499.6

Cm↔P4mm↔Pm3̅m

m3̅mFm (24),

9.0 / 10 (298 K)

34

[gua]ClO4

454

R3m↔Pm3̅m

m3̅mF3m (8)

8.1 / 30 (298 K)

35

[Et4N]ClO4

378

Fm3̅m↔Cc

m3̅mFm (24)

7.0 (298 K)

37

[qui]IO4

322

Pmn21↔Pm3̅m

m3̅mFmm2 (12)

6.7 (298 K)

38

[qui]ReO4

345, 367

Pmn21↔R3m↔Pm3̅m

m3̅mFmm2 (12),

5.2 / 50 (298 K)

39

m3̅mF4mm (6)

m3̅mF3m (8) [hqu]Cl

340

F432↔P41

432F4 (6)

1.7 / 15 (298 K)

40

[apd]RbBr3

440

Ia*↔Pm3̅m

m3̅mFm (24)

2.3 / 38 (303 K)

43

[MeHdabco]RbI3

430

R3↔P432

432F3 (8)

6.8 / 39 (298 K)

44

[tmno]2[KFe(CN)6]

402

Cc↔ Fm3̅m

m3̅mFm (24)

1.25 / 35 (298 K)

45

[tmcm]MnCl3

406

Cc↔ P63/mmc

6/mmmFm (12)

4.0 (363 K)

47

[tmbm]MnBr3

415

Cc↔ P63/mmc

6/mmmFm (12)

3.5 (348 K)

48

[a]

Hdabco = mono-protonated 1,4-diazabicyclo[2.2.2]octane; gua = guanidinium; qui = quinuclidinium; hqu = (R)(‒)-3-hydroxlyquinuclidinium; apd = 3-ammoniopyrrolidinium; MeHdabco = protonated N-methyl-1,4-diazoniabicyclo[2.2.2]octane; tmno = protonated trimethylamine N-oxide; tmcm = trimethylchloromethylammonium; tmbm = trimethylbromomethylammonium. [b] μC·cm−2 / V. * Nonstandard space group. The corresponding chemical structures for these compounds are shown in Table S1 of the Supporting Information. temperature decreases, their rotations are slowing down to arouse an identical orientation and the resultant multiaxial ferroelectricity. At this point, plastic phase with the highest cubic symmetry, in which the molecules show rotator-like motions and lose orientational order, becomes a simple design approach to acquire more ferroelectric axes. Remarkably, when looking back to the discovered molecular ferroelectrics through this idea, the scope is not just confined in spherical molecules, but should be extended to other components that are easy to be rotational or severely orientationally disordered in the similar way. The multiaxial nature of [gua]ClO4 (gua = guanidinium), where the planar [gua]+ cation usually has a C3 reorientation, is thereby distinguished.35 Years ago, it has been found to crystallize in the polar space group R3m at room temperature and to undergo a ferroelectric phase transition at a high Tc of 454 K.36 On the basis of variable-temperature powder X-ray diffraction (PXRD) measurements, the paraelectric phase is refined in the space group of Pm3̅m and deduced as plastic. The Aizu notation is m3̅mF3m, so there exist 8 equivalent polarization directions, resembling what happens to BTO below 183 K. As a consequence of the replacement of [gua]+ by [Et4N]+ cation that also facilitates intense molecular dynamics, a new multiaxial molecular ferroelectric is designed and constructed smoothly. Similar to [gua]ClO4, [Et4N]ClO4 adopts a cubic space group in the plastic phase above Tc of 378 K, that is, Fm3̅m, but upon cooling it is broken into the monoclinic Cc.37 With the same m 3̅ mFm Aizu notation as [Hdabco]ReO4, as many as 24 equivalent polarization directions are available in [Et4N]ClO4.

Figure 2. Crystal structures in ferroelectric Cm (a) and P4mm phase (c) of [Hdabco]ReO4. (b, d) The 24 and 6 equivalent polarization directions arise in the two ferroelectric phases. Except for dabco, another famous spherical molecule, quinuclidinium (qui), which bears a very analogous structure to the former, has been developed as desirable unit for evoking multiaxial ferroelectricity, too.38-40 For [qui]IO4, the ferroelectric phase transition occurs at around 322 K, and it is no suprise that in the paraelectric phase it belongs to the cubic Pm3̅m, just as [Hdabco]ReO4 does.38 To satisfy such high


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crystallographic symmetry, severely orientational disorder is manifested by the spherical cations and anions, meaning a collaborative flipping (Figure 3). In the low-temperature ferroelectric phase, the disappearance of disorder is accompanied by the symmetry breaking to the orthorhombic space group Pmn21. Obviously, the same Aizu notation of m 3̅ mFmm2 as that of BTO is indicative of 12 equivalent polarization directions. Similarly, the transition from Pm3̅m above 367 K to Pmn21 below 345 K is also presented in [qui]ReO4.39 The difference is the presence of an intermediate ferroelectric phase between them, which adopts the R3m space group and allows 8 equivalent polarization directions. Based on the two examples, research goes further into the homochiral [hqu]Cl (hqu = (R)-(‒)-3-hydroxlyquinuclidinium), showing the valuable role of molecular modifications.40 Among the 10 ferroelectric point groups, 5 ones are enantiomorphic, i.e., 1 (C1), 2 (C2), 4 (C4), 3 (C3), and 6 (C6). The compounds comprising homochiral molecules are more likely to crystallize in these enantiomorphic polar point groups, thereby helping to introduce ferroelectricity more effectively. Herein, upon a H atom at the 3-site of [qui]+ is substituted by a hydroxyl group, the spherical geometry gets modified slightly, and a homochiral [hqu]+ molecule is achieved. By assembling it with a carefully chosen Cl– anion, [hqu]Cl adopts the expected enantiomorphic polar space group P41 in the ferroelectric phase. Then, above 340 K, the severely disordered [hqu]+ cations experience highly dynamic reorientation and are modelled with spherical structure. Thus, a cubic-enantiomorphic space group F432 is generated in the plastic paraelectric phase. The Aizu notation of 432F4 signifies the multiaxial nature of [hqu]Cl, in which the polarization can be switched between 6 equivalent directions. It is believed that the unique availability of holding homochiral molecules in molecular ferroelectrics, which is impossible in the inorganic cousins, will provide an easy path to develop multiaxial ferroelectricity.

flourished and emerged as competitive ferroelectric candidates.41 Given the distinctive structural variability and tunability,42 they shed light on the possible bottom-up approach to design multiaxial organic-inorganic perovskite ferroelectrics. With an eye to this, excitedly three excellent cases were harvested, namely [apd]RbBr3 (apd = 3-ammoniopyrrolidinium),43 [MeHdabco]RbI3 (MeHdabco = protonated N-methyl-1,4-diazoniabicyclo[2.2.2] octane),44 (Figure 4) and [tmno]2[KFe(CN)6] (tmno = protonated trimethylamine N-oxide).45 This type of structure is characterized by dynamic organic cations confined in the 3D anionic cages that afford enough spacious cavity for their rotations or motions. In the paraelectric phase, the confined cation undergoes a highly dynamic disorder in the cubic anionic cage, while in the ferroelectric phase the cation becomes totally ordered with specific orientations. A cubic-to-distorted phase transition, triggered by disordering-to-ordering of the cationic guests and tilting of the anionic octahedra, evolves a multiaxial ferroelectricity, like the inorganic perovskite ferroelectrics. Along with the phase transitions occurring at 440, 402, and 430 K, respectively, both [apd]RbBr3 and [tmno]2[KFe(CN)6] bear the Aizu notation of m3̅mFm and have 24 equivalent polarization directions, and those of [MeHdabco]RbI3 are 432F3 and 8. Naturally, these achievements in multiaxial 3D organic-inorganic perovskite ferroelectrics are all under precise structural design, particularly for [MeHdabco]RbI3. At first, what caught our interest is [H2dabco]RbX3 (X = Cl, Br, I),44,46 unfortunately they adopt the nonpolar space groups and experience non-ferroelectric phase transitions. To improve that, a N-methyl is added on the nonpolar [H2dabco]2+ to reduce the high molecular symmetry of D3h down to C3v and to bring in a molecular dipole moment. Finally, the resulting [MeHdabco]2+ molecule is packed in the larger RbI3 framework. Just as expected, such a subtle molecular design strategy did induce crystallization in polar space group and permit ferroelectricity.

Figure 3. Comparison of crystal structures of [qui]IO4 in paraelectric (a) and ferroelectric phase (b).

Figure 4. Crystal structures of [MeHdabco]RbI3 in ferroelectric phase and paraelectric phase. The blue arrows in the paraelectric phase indicate 8 equivalent polarization directions.

Thus far, the above presented examples basically center on simple organic salts, while the origin of multiaxial ferroelectricity is principally attributed to the relative large displacement between cations and anions. However, beyond them there remain countless intriguing structures and complicated ferroelectric mechanisms to explore. In this respect, ABX3 perovskites (A, B are two different cations, and X is anion), where the A cations are enclosed in the three-dimensional (3D) framework of corner-sharing BX6 octahedrons, deserve special attention. It is well-known that the kinds of extraordinary merits of the star inorganic ferroelectrics like BTO and PZT are inseparable from such unique 3D perovskite structure. In recent years, note that organic-inorganic perovskites have also

Besides the intrinsic ferroelectricity, piezoelectricity also definitely stands out as the most dominant and extensively studied property of ferroelectrics.4 A number of inorganic ferroelectrics like BTO and PZT display large piezoresponse and have long been put into practice.2 Indeed, whether before the flourishment of ferroelectric memories or even until now, various piezoelectric devices such as sensors, transducers and actuators keep occupying an important position in the ferroelectric markets. In this regard, the combination of exceptional ferroelectric and piezoelectric performances should be a persistent goal for molecular ferroelectrics. Therefore, the piezoelectric properties of multiaxial molecular ferroelectrics become a matter of concern. Intriguingly, the above-mentioned



[gua]ClO4 is the first example of molecular ferroelectrics displaying piezoelectric activity in the powder form after poling, with a piezoelectric coefficient (d33) of 10 pC·N−1 being comparable to those of its single crystals (15 pC·N−1) and LNO (8 pC·N−1).35 This behavior is analogous to inorganic ferroelectrics and might be bounded up with the multiaxial nature. More impressively, the largest d33 (185 pC·N−1, Figure 5) among known molecule-based peizoelectrics (e.g., ~20 pC·N−1 for PVDF), close to that of BTO (190 pC·N−1 along [111] direction), has been found in the single crystal of a one-dimensional organic-inorganic perovskite [tmcm]MnCl3 (tmcm = trimethylchloromethylammonium) very recently.47 Above 406 K, its paraelectric phase belongs to a centrosymmetric hexagonal space group P63/mmc, in which the molecular tumbling of [tmcm]+, derived from the spherical [Me4N]+ cation, is modelled with 12-fold orientational diorder. Conversely, the cations get freezed in the ferroelectric phase to give a polar space group Cc and 12 equivalent polarization directions (Aizu notation 6/mmmFm). In addition, its analogue [tmbm]MnBr3 (tmbm = trimethylbromomethylammonium) also has a large d33 of 112 pC/N along polar axis, which undergoes a phase transition from a hexagonal (P63/mmc) to a monoclinic (Cc) phase at 415 K.48 In these two multiaxial ferroelectrics, the noteworthy piezoresponsivity is supposed to come from a striking change of the polarization due to the rotation of the polarization direction. These findings denote that the well-designed multiaxial molecular ferroelectrics have much to offer in not only ferroelectric field, but also the uses of molecular piezoelectrics in microrobotics, bioimplanted sensors, flexible and wearable devices, and so on.49-50

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edge-pinned-crystallization. Therefore, there is no need to grow single-crystalline film on specific substrates.

MOLECULAR FERROELECTRIC THIN FILMS Molecular ferroelectric thin films can be prepared via a simple and low-cost full-solution procedure, in contrast to the complicated procedures of inorganic ferroelectric films.33 The common procedure is illustrated by uniaxial ferroelectric (Im)ClO4 (Im = imidazolium) via a spin-coating approach.34 The as-grown single-crystalline film on substrate is about 2 μm in thickness and shows different crystal planes, including the preferred (102̅) plane which is approximately perpendicular to the polar axis. According to the polarization vector analysis, the measurement on the (102̅) plane can show comparable performance of its bulk single crystal sample. Apart from this lucky preferred orientation of (Im)ClO4 thin film, it is generally hard to control the specific polarization orientations of single-crystalline thin films in uniaxial molecular ferroelectrics. This difficulty can be largely overcome by multiaxial molecular ferroelectric thin films. For example, the plastic crystal (qui)(IO4) undergoes a ferroelectric phase tranistion between the paraelectric phase Pm3̅m and ferroelectric phase Pmn21. It has twelve symmetrically equivalent polarization orientations, i.e., six-fold polar axes. Such a multiaxial ferroelectricity does not require any orientation-controlled growth of the film on specific substrate. Therefore, the film growth is greatly simplified. The aqueous precursor solution containing 200 mg (qui)(IO4) per mililiter was spreaded on various substrates, including ITO-coated glass or flexible polyethylene terephthalate (PET) (conductive ITO as the bottom electrode), SiO2/Si, single-crystalline silicon and so on. The as-prepared thin-films with large area, high uniformity, and high-coverage were obtained with controlled substrate temperature and

Figure 5. Crystal structures of [tmcm]MnCl3 in ferroelectric (a) and paraelectric phase (b). (c) Illustration of the 12 equivalent polarization directions. (d) Temperature and frequency dependent piezoelectric coefficient (d33). CHARACTERIZATIONS AND PERFOMANCES OF MULTIAXIAL MOLECULAR FERROELECTRIC THIN FILMS Just like all the other functional materials, the final goal of multiaxial ferroelectrics is to be applied in practice in the near future. When moving forward to device fabrication, the first level of control is achieved by modulating the molecular structure, and the second one is achieved by optimizing thin film properties. For that matter, characterizations and performances of multiaxial molecular ferroelectric thin films are of top priority. With respect to the above screened multiaxial molecular ferroelectrics, one of the key advantages resides in the fact that their Tc are all above room temperature (as listed in Table 1), varying between 322 and 500 K. Some of them have even higher Tc than BTO (393 K), such as [Hdabco]ReO4, [gua]ClO4, [apd]RbBr3, and [MeHdabco]RbI3.34,43-44 The high Tc means that these ferroelectrics can be applicable in a wider range of temperatures and is quite an important characteristic for forthcoming device design. From the application point of view, polarization switching performance maintaining in the thin films is surely worth exploring in the first place. Unique features that are highly desirable for FeRAM devices include the large Pr for high-density


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memories, the low-voltage operation (all Si devices work at ≤ 5V), and the fast polarization switching for high-speed operation. The Pr is extracted from the polarization−electric field (P−E) hysteresis loops measured by standard Sawyer-Tower circuit usually. The ferroelectric thin film is made into a capacitor with a typical sandwich-like architecture (GaIn/thin film/ITO) where the bottom electrode is the conductive ITO coated on the substrate while the top electrode is liquid GaIn eutectic with the diameter of ~0.5 mm. High-quality polycrystalline thin films of these multiaxial samples are found to exhibit comparable Pr values with the single-crystal samples, distinct from the uniaxial ones which perform considerably more poorly in thin film than in bulk. For example, the cold-pressed powder and polycrystalline thin film samples of [gua]ClO4 have the large Pr of 5.1 and 8.1 μC·cm−2, respectively, comparable with the measured Ps (8.4 μC·cm−2) of the single crystal.3536 On the other hand, among the reported multiaxial molecular ferroelectrics, [Hdabco]ReO434 has the highest Pr of 9 μC·cm−2 at 298 K while [qui]IO4,38 [Et4N]ClO4,37 and [Medabco]RbI344 have the similiar Pr values between 6.7 and 8.1 μC·cm−2 at 298 K (Table 1). These values are comparable with 8 μC·cm−2 for the ferroelectric polymer poly(vinylidenefluoride) (PVDF) and could be an important step toward the development of cheap and flexible nonvoltatile memory.13 For a number of ferroelectric candidates, the Ec is always so high that verifying the ferroelectricity in the single-crystal form becomes somewhat difficult. From the Kay-Dunn law, the magnitude of the Ec may be as much as an order of magnitude higher in thin films than in bulk materials, but the corresponding low Vc will make the detection of ferroelectricity easier and provide hope for low-voltage operation. Most of the reported multiaxial molecular ferroelectrics display large Ec values, because the reorientational motions of the polar cations in the crystal lattice is energy-demanding under electric field, similar to the PVDF (Ec = 500 kV·cm−1).13 However, it is the high-quality films with suitable thickness that facilitate easy realization of polarization switching at low voltage. For example, polarization switching in [Hdabco]BF432 and [Hdabco]ReO434 thin films with respective thicknesses of ~150 and ~600 nm can be achieved at a low voltage down to 4.2 and 10 V. In the cases of other multiaxial ferroelectric thin films, the fine P−E hysteresis loops are all detectable at relatively lower Vc than 50 V. Of particular note is [qui]ReO4 that previously demonstrated ferroelectricity in just a small temperature window of 22 K (from 345 to 367 K), owing to the presence of a large Ec in the bulk crystal sample.51 At that time, for the sake of extending its working temperature range and expanding the application potential, one should determine that whether the room-temperature phase below 345 K is ferroelectric. Not surprisingly, it turned out that a 1.5 μm thick film of [qui]ReO4 displays P−E hysteresis loops with good rectangularity at room temperature, with a considerably large Pr of 5.2 μC·cm−2 and a low Vc of 50 V.39 In the past, it was thought that molecular ferroelectrics had few advantages in fast polarization switching, while most of them can be operated at frequencies no more than 500 Hz. This situation has changed in multiaxial molecular ferroelectrics, and the thin films of [Hdabco]ClO4, [Hdabco]BF4, and [Hdabco]ReO4 show well-defined rectangular ferroelectric

loops at relatively high frequencies of 10, 20 and 100 kHz, respectively, at room temperature (Figure 6). Such high polarization switching rates are unprecedented in molecular ferroelectric thin films but will open a new avenue for FeRAM applications.

Figure 6. Room-temperature ferroelectric hysteresis loops of the thin film of [Hdabco]ReO4 at different AC frequencies (Hz). Ferroelectric domain structures of the thin films are studied by piezoresponse force microscopy (PFM) imaging at the nanometer scale which affords the information on the orientation of the domain polarization and piezoelectric coefficient.52-55 A typical domain structure is the stripe pattern, where the adjacent domains should be 180° when they have the equal amplitude signals and 180° phase contrasts in both out-of-plane and in-plane PFM images, and non-180° when the amplitude signals are significantly different. Figure 7a shows the non-180° stripe domains in the [Hdabco]BF4 thin film, like that found in BiFeO3 (BFO).56 In some cases, two types of non-180° stripe domains with different directions would intersect each other to form the herringbone pattern. For example, [tmcm]MnCl3, [tmbm]MnBr3 and [Hdabco]ReO4 all can exhibit the beautiful herringbone domains, as depicted in Figure 7b and 7c, which also exist in BTO.57 In addition to these, some irregular domain patterns would also appear due to the coexistence of multiple domains with various directions. Using PFM, we can further understand the polarization switching behavior, the most important feature for ferroelectric materials. Firstly, local PFM spectroscopic experiments would be measured, where the characteristic bipolar piezoelectric hysteresis and butterfly loops are typical for the successful switching of ferroelectric domains (Figure 7d). Subsequently, by applying DC bias tip voltage, the domain switching process of thin film can be intuitively observed. Figure 7e displays the classical box-in-box bipolar domain patterns in the [tmno]2[KFe(CN)6] thin film after applying reversed DC bias in the centre. This result demonstrates that the polarization of ferroelectric domain can be switched forth and back. Moreover, we also can visualize the non-180° rotation of polarization direction by employing PFM. As shown in Figure 7f, when a tip bias under -25 V was



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Figure 7. (a) Out-of-plane PFM amplitude image in the thin film of [Hdabco]BF4. (b) In-plane amplitude image in the thin film of [Hdabco]ReO4. (c) Out-of-plane phase image in the thin film of [tmcm]MnCl3. (d) Out-of-plane phase and amplitude signals as functions of the tip voltage for a selected point in the thin film of [qui]IO4, showing local PFM hysteresis loops. (e) Out-of-plane phase image recorded after writing an area of 15 × 15 µm2 with +70 V and then the central 8 × 8 µm2 square with -70 V using a biased conductive tip in the thin film of [tmno]2[KFe(CN)6]. (f) Out-of-plane amplitude image recorded after the switching produced by scanning with a tip bias of -25 V in the thin film of [MeHdabco]RbI3. used to pole the center region in the [MeHdabco]RbI3 film, a bidomain-pattern emerged with different amplitude intensities, indicating such polarization rotation has an angle of less than 180° and appears to be a ferroelastic switching. This means that a larger spontaneous polarization can be obatained in the corresponding polycrystalline samples after poling, promoting their practical applications in the thin-film form. OPPOTUNITIES AND CHALLENGES The current development of multiaxial molecular ferroelectric thin films is filling the gap between pure fundamental research and practical applications of molecular ferroelectrics. From the viewpoint of practical applications, some candidates show promising properties for the use in devices, e.g., [Hdabco]ReO4 and [tmcm]MnCl3. In the light of the useful design principles and the structural diversity, the family of multiaxial molecular ferroelectrics can get greatly enriched, providing deep insight to the structure-property relationship. Moreover, multiple properties would be introduced, along with the ferroelectricity, due to the diversity of molecules such as chirality. An interesting example is the homochiral molecular ferroelectrics with potential applications in electro-optics, molecular recognition and chiral induction. It is expected that the multiaxial molecular ferroelectric thin films with superior features will help to address the grand challenges facing the inorganic ones and inspire technological evolution in the next-generation flexible, wearable electronics. These developments will inject new vitality to the century-old ferroelectric field and attract the widespread interest of researchers from the multiple subjects.

In order to develop multiaxial molecular ferroelectrics, researchers are encouraged to pay more attention to the concept of “ferroelectric chemistry”, which refers not only to the chemical design of molecular ferroelectrics from the molecular level, but also to the potential applications of ferroelectricity in the chemical field, such as surface catalysis58. From a chemist's point of view, the design strategy and/or optimization of ferroelectricity lie in the delicate modification of flexible organic dipoles. Taking the nonpolar tetramethylammonium [Me4N]+ cation as a prototype, through slight structural substitution by one chlorine atom, the resultant polar trimethylchloromethylammonium [Me3NCH2Cl]+ could greatly influence the ferroelectric/piezoelectric properties in one-dimensional metal halides. However, the systematic study is further needed to clarify the underlying mechanisms. For example, through halogen substitution by fluorine [Me3NCH2F]+, chlorine [Me3NCH2Cl]+, bromine [Me3NCH2Br]+ and iodine [Me3NCH2I]+, or other organic substituent group like ethyl group [Me3NEt]+, together with the structural variability tuned by the metal and halide ions from the inorganic parts, we can constitute a huge class of potential multiaxial ferroelectrics. Based on these, superior ferroelectricity, piezoelectricity or nonlinear optical properties can be envisioned. Different from the aforementioned one-dimensional ABX3 organic-inorganic hybrid hexagonal perovskites, the development and rational design of three-dimensional ABX3 cubic perovskites seem more attractive due to their structural similarity to BTO and inherent multiaxial nature. For recently developed ferroelectric perovskites based on alkali halide framework, the embedded polar divalent organic cations can be extended to N-methyl-1,4-diazoniabicyclo[2.2.2]octane, triethylenediamine-N-oxide, even some divalent chiral organic cati-


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ons such as 3-ammonioquinuclidine and 3-ammoniopyrrolidine. In pursuit of more light-weight and structural flexible molecular ferroelectrics in perovskite system, the providential effective molecular radius affords NH4+ cation to be ideal to construct metal-free molecular perovskite ferroelectrics. The effective ion radius of NH4+ cation (1.46 Å ) falls in between the gap of K+ (1.38 Å ) and Rb+ (1.52 Å ), providing infinite combinations with halide anions and bivalent organic cation to form 3D metal-free ABX3 type perovskite ferroelectrics. Besides, benefited by the possible natural sublimation character of electroneutral single-molecule organic compounds, single-molecule organic ferroelectric will facilitate solventfree film fabrication process like thermal evaporation. The ability of thermal evaporation will bring the possibility of large-scale integration based on multiaxial molecular ferroelectrics. However, there are still some challenges to overcome. The number of multiaxial molecular ferroelectrics is limited. The overall performances of the thin films need further optimization. In particular, some advantages of the molecular ferroelectrics over the inorganic ceramics are practicably disadvantageous, such as aqueous solubility and thermal instability. It is becoming clear that the study of multiaxial molecular ferroelectric thin films needs a shift from the curiosity-driven research to problem-driven research. This trend calls for efficient collaborations among researchers in the fields of chemistry, physics, materials, microelectronics and so on. It is reasonable to believe that these efforts will pave the way for practicable applications of the molecular polycrystalline ferroelectrics in the future, as the inorganic ceramic ferroelectrics.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Tables S1 showing the chemical structures of recently discovered multiaxial molecular ferroelectrics.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by 973 project (2014CB932103) and the National Natural Science Foundation of China (21290172 and 91622113).

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Multiaxial Molecular Ferroelectric Thin Films Bring Light to Practical

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