Large Piezoelectric Effect in a Lead-Free Molecular Ferroelectric Thin

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Large Piezoelectric Effect in a Lead-free Molecular Ferroelectric Thin Film Wei-Qiang Liao, Yuan-Yuan Tang, Peng-Fei Li, Yu-Meng You, and Ren-Gen Xiong J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10449 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 16, 2017

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Journal of the American Chemical Society

Large Piezoelectric Effect in a Lead-free Molecular Ferroelectric Thin Film Wei-Qiang Liao, Yuan-Yuan Tang, Peng-Fei Li, Yu-Meng You,* and Ren-Gen Xiong* State Key Laboratory of Bioelectronics, and Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China. Supporting Information ABSTRACT: Piezoelectric materials have been widely used in various applications, such as high-voltage sources, actuators, sensors, motors, frequency standard, vibration reducer, and so on. In the past decades, lead zirconate titanate (PZT) binary ferroelectric ceramics have dominated the commercial piezoelectric market due to their excellent properties near the morphotropic phase boundary (MPB), although they contain more than 60% toxic lead element. Here, we report a lead-free and one-composition molecular ferroelectric trimethylbromomethylammonium tribromomanganese (ΙΙ) (TMBM-MnBr3) with a large piezoelectric coefficient d33 of 112 pC/N along polar axis, comparable with those of typically one-composition piezoceramics such as BaTiO3 along polar axis [001] (~90 pC/N) and much greater than those of most known molecular ferroelectrics (almost below 40 pC/N). More significantly, the effective local piezoelectric coefficient of TMBM-MnBr3 films is comparable to that of its bulk crystals. In terms of ferroelectric performance, it is the low coercive voltages, combined with the multiaxial characteristic, that ensure the feasibility of piezo film applications. Based on these, along with the common superiorities of molecular ferroelectrics like lightweight, ﬂexibility, low acoustical impedance, easy and environment-friendly processing, it will open a new avenue for the exploration of next-generation piezoelectric devices in industrial and medical applications.

INTRODUCTION In 1880, the brothers Pierre Curie and Jacques Curie firstly discovered piezoelectric effect, which represents the ability of non-centrosymmetric crystalline materials to generate electrical charge under an applied mechanical force or conversely mechanical strain under an applied electrical field.1 From then on, such unique electromechanical interactions have made piezoelectric materials irreplaceable in the modern society, by endowing them with tremendous potential in applications like sensors, actuators, ultrasonic transducers, and so on.2 Onecomposition perovskite ferroelectric barium titanate (BTO) emerged as the first practically used piezoelectric ceramic with a moderate piezoelectric coefficient d33 of 90 and 190 pC/N along [001] and [111] crystal orientation.2b,3 Subsequently, owing to the presence of a morphotropic phase boundary (MPB), the extremely high piezoelectric properties superior than BTO can be obtained in the binary ferroelectric lead zirconate titanate (PZT), making it extensively utilized in almost all kinds of piezoelectric devices. Over the years, research interests had been centered on achieving high-performance PZT ceramics by modifying compositions or introducing different dopants. But, remarkably, the major drawback of these lead-based ceramics is that the toxic elements will bring hurt to human health and environment.2d For the sake of finding excellent alternatives for PZT ceramics and thus addressing the environmental issue, in recent years, there is growing concern regarding lead-free piezoelectric materials.2d,4 Currently, the promising candidates are mostly limited in the scope of inorganic ceramics, such as modified BTO, sodium niobate and bismuth titanate, or binary solid solution material compositions near MPBs.4a,5 Nevertheless, with the development of the times, the next-generation flexible

and wearable integrated devices lead the trend of piezoelectric/ferroelectric materials to thin, light-weight, soft, low-cost, and biocompatible. In this respect, both lead-based and leadfree inorganic ceramics that suffer from high processing temperature, structural rigidity, and high cost on large-area-filmprocessing are faced with huge challenges. In contrast, thanks to the advantages of low processing temperature, mechanical flexibility, light-weight, nontoxicity, biocompatibility, and easy film making, molecular piezoelectrics stand out and are expected to promote innovations in piezoelectric devices.6 During the past decade, molecular ferroelectrics began to revive, when their Tc and Ps can get close to or even exceed that of BTO.7 At the same time, however, the piezoelectric responses of those discovered cases are not satisfactory in terms of being catch up with the inorganic piezoelectric ceramics, hindering the application development of molecular ferroelectrics. More recently, the surprise was that we reported the discovery of an organic-inorganic perovskite ferroelectric with extraordinarily huge d33 of 185 pC/N approximating BTO, trimethylchloromethyl-ammonium trichloromanganese (ΙΙ) (TMCM-MnCl3), which points out the bright future of molecular piezoelectrics.2b Is molecular ferroelectric with large piezoelectric response can only be found by accident, because the d33 of most known molecular ferroelectrics is almost below 40 pC/N?2b If the cation still has a quasi-spherical geometry, which can change from the ordered state at low temperature to the totally dynamical disordered state at high temperature, whether the multiaxial nature and the resultant large piezoelectric response will get preserved? Here we present a lead-free and onecomposition molecular ferroelectric trimethylbromomethylammonium tribromomanganese (ΙΙ) (TMBM-MnBr3) that is capable of coupling outstanding ferroelectricity with excel-

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lent piezoelectric performance (d33 ≈ 112 pC/N), comparable to that of the [001]-poled one-composition tetragonal BTO single-domain crystal (90 pC/N).3a Undoubtedly, this work indicates that more and more high-performance molecular piezoelectrics can be rational designed and constructed, rather than be accidentally discovered. More importantly, the effective local piezoelectric coefficient of TMBM-MnBr3 films is comparable to that of its bulk crystals, and the coercive voltages of them that can be down to ~6 V are favorable to the low-energy fabrication processes. Based on all the superiorities of molecular ferroelectric thin films, TMBM-MnBr3 will be a great candidate for mechanical sensors using in flexible devices, biomedical devices, soft robotics, and other micromechanical applications.

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activated by the temperature. This transition breaks the 6/mmm symmetry and correspondingly results in a slight lattice deformation (β = 95.66(3)º at 293 K) and the displacements of the cations. Furthermore, the average shift of the positive charges carried by the N atoms along the c-axis is about 0.53 Å, which may induce a polarization of 3.05 µC/cm2.

RESULTS AND DISCUSSION TMBM-MnBr3 is of the typical ABX3-type linear chain compounds,8 consisting of face-sharing MnBr6 octahedra along c-axis separated by the organic cations (Figure 1). The anionic chains and the cations are arranged in the same manner as that in other ABX3-type (A = pyrrolidinium or 3pyrrolinium, B = Mn or Cd, X = Cl or Br) ferroelectric analogues,9 all of which adopt the BaNiO3-like hexagonal perovskite structure.10 Here, we focus on the structural characteristics related to the ferroelectric mechanism. At room temperature, TMBM-MnBr3 crystallizes in the polar monoclinic space group Cc, corresponding to the room-temperature phase (RTP). The difference between the crystal structures of TMBM-MnBr3 and those analogous ones is mainly in the cations (Figure 1a). The cation of TMBM-MnBr3, which is derived from the spherical tetramethylammonium cation by replacing the H atom with a Br atom, has a quasi-spherical geometry. Typically, three-dimensional (3D) spherical molecular structures like tetramethylammonium, adamantane, and dabco (1,4-diazabicyclo[2.2.2]octane) tend to exhibit dynamical disorder in the close packed crystals because of weak van der Waals interactions in the crystal lattice. In this case, the structural modification affecting the spherical symmetry raises the potential energy barrier of its tumbling motion, and meanwhile, it is the halogen···halogen interactions that contribute to the increasing intermolecular interactions. In TMBMMnBr3, Br atoms are involved in both the anionic chains (nucleophilic regions) and the cations (electrophilic region), and thus, C−Br···Br−Mn bonds occur.11 The short intermolecular Br···Br contact of 3.317(6) Å leads to the formation of type II12 C−Br···Br−Mn bonds (θ1 = 176.32(2)º, θ2 =105.10(3)º). These factors lead to the ordered state of cations retaining until a high temperature at around 415 K. The crystal structure of HTP (high-temperature phase) at 433 K belongs to the centrosymmetric hexagonal space group P63/mmc, with the same c-axis as that of the RTP (Figure 1b). The configuration of the [MnBr3] chains is similar to that in the RTP. The organic cations are located on the special sites with 6/mmm symmetry, while only the N atoms can be determined from the difference Fourier maps, indicating strong dynamic characteristic of the molecular tumbling. The so high site symmetry manifests that the cations perform free rotation in the crystals, as observed in plastic crystals.13 In order to satisfy the requirement of crystallographic symmetries, we have modeled the molecular geometry with 12-fold orientational disorder to constrain it in the same way as that in the ordered state. With this model, the ferroelectric mechanism can be attributed to the order-disorder transition of the cations

Figure 1. Packing views of the structures in (a) ferroelectric and (b) paraelectric phases of TMBM-MnBr3, showing the similarities of the crystal structures and the differences of orientation states of the organic TMBM cations.

The phase transition behavior of TMBM-MnBr3 was detected by differential scanning calorimetry (DSC) measurements. As shown in Figure 2a, the DSC curves show a pair of endothermic and exothermic peaks in heating and cooling processes, respectively, revealing a reversible phase transition at Tc = 415 K, which is a little higher than that of TMCM-MnCl3 (406 K).2b The large thermal hysteresis of 23 K and sharp peaks are indicative of a typically first-order phase transition. Meanwhile, the real part (ε′) of the dielectric permittivity of TMBM-MnBr3 at several frequencies display prominent dielectric anomalies at about 415 K upon heating, in good agreement with the DSC results and further confirming the existence of a phase transition (Figure 2b). The step-like dielectric anomalies indicate that it is an improper ferroelectric, which is different from normal proper ferroelectrics with typical large λ-shaped dielectric anomalies at around Tc. In addition, the ε′ values in the vicinity of Tc show obvious increase with the frequency decreasing, whereas the temperatures where the dielectric anomalies appear are frequency-independent, suggesting no dielectric relaxation. Particularly, second harmonic generation (SHG) technique is very important and useful in detecting the breaking of space-inversion symmetry and the emergence of ferroelectricity, because the SHG signal exists only in noncentrosymmetric materials, unless a magnetic dipole or an electric quadrupole contributes to it. Figure 2c clearly shows that the SHG intensity of TMBM-MnBr3 is basically zero above Tc and then sharply increases to a saturation value below Tc, consistent well with the centrosymmetric-to-noncentrosymmetric transi-


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tion from the paraelectric phase to the ferroelectric one. The symmetry breaking in TMBM-MnBr3 was further investigated by Curie symmetry principle analysis, which means the ferroelectric space group Cc should be a subgroup of the paraelectric P63/mmc. For P63/mmc, there are a set of maximal nonisomorphic subgroups (P6 2c, P6 m2, P63mc, P6322, P63/m, P3 m1, P3 1c and Cmcm), while for Cc there are a set of minimal non-isomorphic supergroups (C2/c, Cmc21, Ccc2, Ama2, Aea2, Fdd2, Iba2, Ima2, P3c1, P31c and R3c). Since C2/c, Cmc21 and Ama2 are all maximal non-isomorphic subgroups of Cmcm, Cc is consequently a subgroup of P63/mmc, obeying the Curie symmetry principle. In the case of TMBM-MnBr3, symmetry breaking occurs with an Aizu notation of 6/mmmFm, with a decrease in macroscopic symmetry elements from 24 (E, 2C6, 2C3, C2, 3C'2, 3C"2, i, 2S3, 2S6, σh, 3σv, and 3σd) in paraelectric point group 6/mmm (D6h) to 2 (E and σh) in ferroelectric point group m (C1h), and thus there are 12 crystallographically equivalent polarization directions, corresponding to 6 ferroelectric axes. This is well consistent with that in TMCM-MnCl3. Of the 21 noncentrosymmetric crystal classes, 20 show direct piezoelectricity (the exception is the cubic class 432), 10 of which are polar. Hence, piezoelectric behavior of TMBMMnBr3 was investigated by determining the direct piezoelectric coefficient (d33) using the simple and straightforward Berlincourt method on the as-grown single crystal. The maximum d33 at the tapping frequency of 110 Hz we obtained is 112 pC/N along the vicinity of the [102] direction of the crystal at room temperature (Figure S2). Along the opposite direction, the maximum value can also reach -112 pC/N. Upon changing the tapping frequency between 30 Hz and 300 Hz, the d33 exhibits a slight increment in the higher frequency range (Figure 2d). The d33 for crystal TMBM-MnBr3 is smaller than that for its analogue TMCM-MnCl3 (185 pC/N) but much greater than those for other organic ferroelectric crystals including Rochelle salt (7 pC/N), triglycine sulfate (22 pC/N), potassium titanyl phosphate (6 pC/N), PVDF (30 pC/N), croconic acid (5 pC/N), and diisopropylammonium bromide (11 pC/N).2b

Figure 2. (a) Temperature dependence of DSC obtained on a heating-cooling cycle. (b) Temperature-dependent data of the real part (ε′) of dielectric permittivity (ε = ε′ – iε′′, where ε′′ is imaginary part of ε). (c) Temperature-dependent SHG intensity. (d) Piezoelectric coefficient (d33) of TMBM-MnBr3 as a function of frequency.

As is well known, the d33 values of most organic ferroelectrics are below 40 pC/N, so it is very fascinating that TMBMMnBr3 and TMCM-MnCl3 can have such large d33, more than 100 pC/N. Considering their similar structures, the large d33 might result from the multiaxial characteristic induced by the significant symmetry change between paraelectric and ferroelectric phases. Taking the classical multiaxial ferroelectric BaTiO3 as an example, its ferroelectric point group 4mm is not a maximal non-isomorphic subgroup of the paraelectric m3 m, so the phase transition between them might be consisting of a ferroelastic transition m 3 mF4/mmm and a ferroelectric one 4/mmmF4mm. On the contrary, with respect to the uniaxial ferroelectric LiNbO3 with an Aizu notation of 3 mF3m, the ferroelectric 3m is a maximal non-isomorphic subgroup of the

Table 1. Left cosets of point group m divided from point group 6/mmm. j

left cosets

1 2 3 4 5 6 7 8 9 10

a

b

||

||

1; c(x,x,z) 1; 2(x,̅ ,0)

[112] [112]

[1122] [1122]

2(0,0,z); m(x,̅ ,z)

[112] [112]

[1122] [1122]

[102] [102]

[2112] [2112]

[012] [012]

[1212] [1212]

2(x,x,0); m(x,y,0) +

c(0,y,z); 3 (0,0,z) 2(2x,x,0); 3+(0,0,z) −

c(x,0,z); 3 (0,0,z) −

2(x,2x,0); 3 (0,0,z) +

m(x,2x,z); 6 (0,0,z) 2(x,0,0); 6+(0,0,z) −

11

m(2x,x,z); 6 (0,0,z)

12

−

2(0,y,0); 6 (0,0,z)

[012] [012] [102] [102]

[1212] [1212] [2112] [2112]

Spontaneous deformation

∡PiPj

U(I)

0°

U

(I)

180°

U

(II)

53°

U

(II)

127°

U

(III)

45°

U

(III)

135°

U

(IV)

45°

U

(IV)

135°

U

(V)

26°

U

(V)

154°

U

(VI)

26°

U

(VI)

154°

a, b

These polarization directions in the left and right rows are expressed using three and four Miller indices, respectively.



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paraelectric 3 m. That may be why BaTiO3 has a large d33 of 190 pC/N but that of LiNbO3 is only 8 pC/N.14 Just like BaTiO3, the phase transition processes for TMBM-MnBr3 and TMCM-MnCl3 might include a ferroelastic transition 6/mmmFmmm, another ferroelastic one mmmF2/m and a ferroelectric one 2/mFm, or the other processes. Also, by observing the polarization vector and strain tensor, we can find that BaTiO3 and TMBM-MnBr3 are both fully ferroelectric/partially ferroelastic compounds, when LiNbO3 is ferroelectric/non ferroelastic. Therefore, for these multiaxial ferroelectrics, part of polarization states are also strain states. While the external stress switches the strain state, the corresponding polarization state would also be switched to induce a large change of the polarization and the striking piezoresponsivity. Knowing that point group m is a subgroup of point group 6/mmm, one can divide all the symmetry elements of point group 6/mmm into so-called left cosets. One of these left cosets consists of the elements of subgroup m itself (Table 1, j = 1). Every other left coset can be constructed by selecting a symmetry element gi of point group 6/mmm (gi ∉ m) and by multiplying it from the right by all the elements of subgroup m. Consequently, the resulting coset contains as many different elements as there are in subgroup m itself. The ratio of the a-axis and c-axis polarization components is approximately 1:2, we assume that the polarization direction occurs in [1122]. By dividing all the symmetry elements of point group 6/mmm into left cosets, twelve equivalent polarization directions can be obtained. Therefore, 180, 53, 127, 45, 135, 26 and 154° domain angles can be predicted according to their geometrical relationship. Piezoresponse force microscopy (PFM) is an effective tool for probing local ferroelectric polarization switching at the nanometer scale, noting that it enables the simultaneous mappings of phase and amplitude of piezoresponse signals to identify the orientation of domain polarization and the relative strength of piezoelectric coefficient, respectively.15 Figure S3 presents the vector PFM phase and amplitude images of the film surface for TMBM-MnBr3 in the annealed state over an area of 12 × 12 µm2, where the sample was heated up to the paraelectric phase and then cooled down to the ferroelectric phase again. It is clear that the domain patterns are different in the vertical (out-of-plane) and lateral (in-plane) components. From the lateral component, it is the irregular and lamellar shaped domains that were clearly observed. As for the vertical component, the emerging diverse herringbone shaped domains are very similar to but smaller than those in the TMCMMnCl3 crystals, in which all of the 12 polarization directions have been identified.2b In particular, high spatial resolution vector PFM was further performed in an area of 1 × 1 µm2 with herringbone shaped domain structures, as indicated by the black boxes. As shown in Figure 3, we can see that four kinds of domains are arranged periodically to form the herringbone pattern, where various amplitude intensities and phase contrasts in both components manifest the distinct polarization directions in these domains. These polarization directions will form different angles from each other, such as 180, 53, 127, 45, 135, 26 and 154°.

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Figure 3. Vertical and lateral PFM phase (a, c) and amplitude images (b, d) for the thin film of TMBM-MnBr3 over an area of 1 × 1 µm2. (e) Comparison of PFM resonance peaks of films of TMBM-MnBr3 and PVDF (2 V). (f) Comparison of local piezoresponse measured with PFM. The data are plotted against macroscopic d33 so as to demonstrate the linear dependence. (Inset) The local piezoresponse of TMBM-MnBr3 as a function of DC bias voltage.

Next, PFM was also used to measure the magnitude of the effective longitudinal piezoelectric coefficient (d33) of TMBMMnBr3 thin film.5c,16 To evaluate its d33, the commercially poled PVDF film (28 µm thickness) was used as a contrast, whose macroscopic d33 obtained by the Berlincourt method is 22 pC/N. Firstly, we drove each film across resonance using PFM (Figure 3e), while the higher resonance peak of TMBMMnBr3 thin film than that of PVDF film suggests the excellent piezoelectricity. The PFM amplitude-voltage butterfly loops of TMBM-MnBr3 and PVDF films were then acquired on the single domains. Significantly, the amplitude of the tip vibration that was derived by calibrating the relationship between cantilever deflection and vertical cantilever motion (i.e. the inverse optical lever sensitivity), can be divided by the applied AC bias to gain the piezoelectric coefficient. As reported in the last paper,2b such local piezoresponses achieved by PFM have a good linear relationship with macroscopic d33 measured by the Berlincourt method in LiNbO3, triglycine sulfate (TGS), BTO, and TMCM-MnCl3. In this case, TMBM-MnBr3 and PVDF films can also be well matched with the relationship between microscopic and macroscopic d33 (Figure 3f). All the above evidence points to the fact that the large piezoelectric coefficient in TMBM-MnBr3 film is comparable to those of TMBM-MnBr3 and BTO bulk crystals, making it a great candidate for piezo films in applications like microphones, loudspeakers, sonar systems, ultrasonic devices, shape recognition, optical shutter, robotics sensors and actuators.


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Figure 5. Calculated polarization of TMBM-MnBr3 as a function of dimensionless parameter λ.

Figure 4. The panels in each row are arranged as the sequence: the topographic image (left), the vertical PFM amplitude image (middle) and the phase image (right) of the surface. (a) Initial state. (b) After the first polarization switching with positive tipbias of +10 V. (c) After the second polarization switching in a smaller area with tip-bias of -10 V. ⊙ and ⊗ correspond to their antiparallel directions of ferroelectric polarizations oriented upward and downward, respectively.

Before considering practical applications, the “poling” process, which induces the film to act like a single crystal by aligning the internal dipoles of the crystallites (domains), should be addressed in the TMBM-MnBr3 thin film. We thus performed PFM spectroscopy and local polarization manipulation experiments to study the polarization switching behaviour in the TMBM-MnBr3 thin films with different thicknesses. In the 220 nm-thick film (Figure S4), the characteristic hysteresis and butterfly loops shown in Figure S4c indicate ferroelectric polarization switching with local coercive voltages below 10 V, that is, about -9.2/-10 and +8.4/+9.6 V. And then the box-in-box pattern with reversed DC biases in the centre was written on the film surface to directly visualize the domain switching process (Figure 4). A tip bias of +10 V was applied to pole the 6 µm × 6 µm square region (Figure 4b), followed by another poling with a tip bias of -10 V in the central area of 2 µm × 2 µm (Figure 4c). The resultant box-in-box pattern with large vertical phase contrast (~180º) discloses that the polarization of TMBM-MnBr3 film is switchable under an operating voltage as low as 10 V. Moreover, when the thickness is decreased to about 70 nm, the film is still uniform and continuous with numerous tiny dendritic crystals that cross each other (Figure S5). Notably, the corresponding local coercive voltages can be reduced to about -5.6/-6.8 and +4.8/+6.8 V, as indicated by the minima of the amplitude loop. In comparison, the coercive voltages in the TMBM-MnBr3 film are relatively smaller than those in the TMCM-MnCl3 film (~20 V).2b Such a low operating voltage makes it easy to be poling and is very desirable for piezoelectric devices and low-power information field.

The properties of ferroelectric switchable polarization are also investigated by measuring polarization–electric field (P– E) hysteresis loop in the TMBM-MnBr3 crystal. As shown in Figure S6, the well-defined rectangular hysteresis loop testifies to the ferroelectric property of TMBM-MnBr3. The observed saturate polarization Ps is around 3.5 µC/cm2 at 348 K, close to that of TMCM-MnCl3.2b The crystalline polarization was evaluated by the Berry phase method developed by KingSmith and Vanderbilt.17 First-principles calculations are performed within the framework of density functional theory (DFT) implemented in the Vienna ab initio Simulation Package (VASP).18 The exchange-correlation interactions were treated within the generalized gradient approximation of the Perdew-Burke-Ernzerhof type.19 The ground state of the experimental room temperature (293 K) ferroelectric structure is found to be antiferromagnetic along the MnBr3 chain with a magnetic moment of 4.4 µB for each Mn atoms, which is 0.13 eV per unit cell lower than the ferromagnetic state. The calculated polarization vector lies in the ac plane. The vector module is 4.57 µC/cm2 and its projection along c direction is 3.62 µC/cm2. The continuous evolution of spontaneous polarization (both module and projection in a, c direction) from the reference phase (λ=0) to the ferroelectric phase (λ=1) is plotted as a function of dimensionless parameter λ in Figure 5. Both the displacement and the rotation of the TMBM cations are included in λ.

CONCLUSION In summary, we discovered a lead-free multiaxial molecular ferroelectric, trimethylbromomethylammonium tribromomanganese (ΙΙ), which undergoes a distinct phase transition at 415 K. It crystallizes in the monoclinic space group Cc at room temperature and the hexagonal space group P63/mmc at high temperature (433 K), thus causing the presence of 6 polar axes in the ferroelectric phase. By analyzing the crystal symmetries in the two phases of TMBM-MnBr3, the 180, 53, 127, 45, 135, 26 and 154° domains in the RTP can be deduced. It exhibits a large piezoelectric coefficient d33 of 112 pC/N that might be ascribed to the multiaxial characteristic. Importantly, it can also maintain a high local piezoelectric coefficient in the form of film. Meanwhile, the low voltages for poling and the multiaxial nature allow the piezo film of TMBM-MnBr3 to be easily applied on various substrates including flexible polymer,



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transparent glasses, amorphous metal plates, etc. With those benefits and excellent piezoelectric properties, TMBM-MnBr3 shows great potential in applications like flexible devices, soft robotics, biomedical devices and so on.

EXPERIMENTAL SECTION Materials. (Bromomethyl)trimethylammonium bromide was synthesized by the reaction of equimolar amounts of trimethylamine (30 wt % in water) and dibromomethane in acetonitrile at room temperature for 24 h. The solvent was removed under reduced pressure to obtain colorless solid. Me3NCH2BrMnBr3 (TMBM-MnBr3) was prepared by dissolving equimolar amounts of (bromomethyl)trimethylammonium bromide and manganese(II) bromide in concentrated HBr solution (40%) under stirring. Pink block single crystals were grown by slow evaporation of the clear solution at 323 K in an oven. Thin-film preparation. Thin films of TMBM-MnBr3 were prepared by the conventional drop-casting method. A drop (20 uL) of a methanol solution containing 50 mg/ml TMBMMnBr3 was carefully spread onto a clean ITO (indium tin oxide) -coated glass substrate. Uniform and compact thin films were in situ grown on the substrate. The purity of the thin film was verified by the powder X-ray diffraction (Figure S1). Measurements. DSC, SHG, dielectric, and macroscopic piezoelectric measurements were described elsewhere.2b Nanoscale polarization imaging and local switching spectroscopy were carried out using a resonant-enhanced piezoresponse force microscopy (MFP-3D, Asylum Research). Conductive Pt/Ir-coated silicon probes (EFM, Nanoworld) were used for domain imaging and polarization switching studies.

ASSOCIATED CONTENT Supporting Information. Figures S1–S6 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 973 project (2014CB932103) and the National Natural Science Foundation of China (21290172, 91422301, and 21427801).

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