Communication pubs.acs.org/cm
Correlated Proton Transfer and Ferroelectricity along Alternating Zwitterionic and Nonzwitterionic Anthranilic Acid Molecules Sachio Horiuchi,*,†,‡ Yuki Noda,†,‡ Tatsuo Hasegawa,†,‡ Fumitaka Kagawa,§,‡ and Shoji Ishibashi∥,‡ †
Flexible Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8562, Japan ‡ CREST, Japan Science and Technology Agency (JST), Tokyo 102-0076, Japan § RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan ∥ Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan S Supporting Information *
E
ring is located on the inversion symmetry, these three polymorphs cannot be ferroelectrics. The latest X-ray diffraction studies24,25 revealed that the form I crystal undergoes the phase transition to form III upon heating, not to form II as previously noted. The orthorhombic form I (Figure 1b) is the only polymorph that involves the ammonium/carboxylate tautomers, that is, zwitterions. Brown et al. performed both X-ray18 and neutral diffraction studies19 and found the polar crystal structure of the space group P21cn (#33, nonstandard setting of Pna21). This crystal symmetry poses the uniaxial polarity along the adirection. Figure 1c displays the crystal structure drawn after the reported neutron diffraction data set.19 Both zwitterionic and nonzwitterionic species are alternately connected by the hydrogen bonds. This hydrogen-bonded molecular sequence forms a zigzag chain parallel to the crystallographic [110] or [1̅10] direction (Figure 1d). The −CO2− group of the same zwitterion A is involved in the strong O−H···O− hydrogen bond (O···O distance = 2.497(7) Å) with the CO2H group of a neighboring nonzwitterionic molecule (denoted as B). The −N+H3 group of zwitterion (denoted as A in Figure 1) forms ionic N+−H···N hydrogen bond (N···N distance = 2.872(5) Å) to −NH2 group of the molecule B′ in the opposite neighbor. It should be noted that the crystal polar axis is parallel to the entire path of proton transfer. These features are promising for the possible ferroelectricity. The commercially available anthranilic acid was purified by vacuum sublimation. Single crystals of form I were exclusively obtained from cold acetic acid solution. The colorless and elongated hexagonal tablets of typically a few millimeter length developed the (100) plane (Figure 1b). The change of electric polarization P on the bulk crystal was measured at room temperature with a configuration sandwiched by a pair of electrodes. The applied electric field E was parallel to the polar a-axis. The pristine crystals exhibited a linear P−E curve until a hysteretic curve characteristic of ferroelectricity appears after several bipolar cycling. In some above-room temperature organic ferroelectrics, the pinned
lectric switching of spontaneous polarizations in the polar crystalline solids is called ferroelectricity and this principle has increasingly been applied to the information storages, sensors, electromechanical devices, and so on.1−4 In typical ferroelectric materials such as BaTiO3 and PbTiO3, the multiple charges on high-valence elements are densely packed, and their relative displacement can generate high electric polarization, which satisfies the most essential and common requirements for many electronic applications. The potential advantages of organic ferroelectrics are lightweight, soft, printable, and biocompatible characteristics required for the future flexible and/or wearable electronic devices.5−11 In the past few years, the electric field-induced proton tautomerism has been highlighted for the design of ferroelectrics with excellent polarization properties, as demonstrated by croconic acid, phenylmalondialdehyde, and benzimidazoles.12−14 Although electrically neutral molecules are inferior to the inorganic ferroelectrics in the spatial density and magnitude of static charges, the ferroelectric polarizations are improved by the dynamic charges traveling with the intermolecular proton transfer and intramolecular π-bond reorganizations. Here the proton-transfer principle is examined on zwitterions (inner salts) in order to emphasize the dipole moment with increasing ionic character of single-component organic ferroelectrics. Amino acids and sulfanilic acids are the typical examples of amphoteric molecules that contain both acidic and basic groups and can exist as zwitterions. The o-aminobenzoic acid (Figure 1a) examined herein is known as anthranilic acid and has been previously referred as vitamin L1.15 Previous crystallographic studies exhibited four polymorphs.16−22 Furthermore, the rarer analogue was reported on its meta isomer revealing as many as five polymorphs.23 Except for the low-temperature orthorhombic form (denoted form I),18 all the anthranilic acid molecules crystallize in the amine/carboxylic acid tautomer, which will be called nonzwitterionic form. The crystal structures are centrosymmetric for the high-temperature orthorhombic form II (space group, Pbca)20 and the two kinds of monoclinic forms III21 and IV22 (the same space group, P21/c). As for the latter three polymorphs, the carboxylic acid groups construct the cyclic dimer of an eight-membered ring with the intermolecular O−H···O hydrogen bonds. Because the center of each dimer © 2015 American Chemical Society
Received: August 2, 2015 Revised: September 3, 2015 Published: September 4, 2015 6193
DOI: 10.1021/acs.chemmater.5b02957 Chem. Mater. 2015, 27, 6193−6197
Communication
Chemistry of Materials
Figure 2. Ferroelectric properties of anthranilic acid (form I). (a) Electric polarization (P) vs electric field (E) hysteresis loops of [100]direction polarization measured at room temperature with triangular ac electric field of 50 Hz in the endurance measurements. (b) Change of the remanent polarization Pr and polarity-averaged coercive field |Ec| as a function of total cycles N obtained by the P−E hysteresis measurements repeated with constant interval on a logarithmic time scale. See the inset to the panel b for the schematic waveform.
from 8 to 10 until its discontinuous and irreversible drop at T = 357 K (Figure S1, Supporting Information). This temperature represents the high-temperature limit of the ferroelectric phase, because it coincides with the onset temperature of monotropic transition from polar form I to nonpolar III,24,25 as identified by the differential scanning calorimetry (DSC) (Figure S2, Supporting Information). The ferroelectricity was also confirmed by the real-space imaging of inverted domains using the piezoresponse force microscope (PFM). The PFM technique employs the scanning force microscopy which probes the piezoelectricity of ferroelectrics in mesoscopic and nanoscopic length scales by measuring the local mechanical deformation (strain) with an externally applied ac voltage underneath the scanning conductive tip.28−30 In this study, the phase and amplitude of vertical (out-of-plane) displacements were measured on the crystal (100) surface in order to distinguish the upward and downward polarizations, which would be different ideally by 180° in the PFM phase. Considering the coercive field above and the bias limit (±220 V) of the apparatus used, the crystal thickness should be less than ∼150 μm for ferroelectric switching. The thickness of the as-grown thin crystal was further slimmed to 40−80 μm by fractionally dissolving the surface with ethanol. Figures 3 depicts the out-of-plane PFM phase and amplitude images and the topographic images of a 50 μm thick single crystal. The piezoresponse of as-grown initial state is almost homogeneous in both phase and amplitude images (Figure 3a), indicating the uniform polarity as the single-domain state. First, the center of the selected area was poled for 100 s at −200 V to the tip. Then, the phase image revealed a clear contrast inside an ellipse of ∼20 μm diameter (Figure 3b), which coincides the region of faint contrast in the amplitude image. This observation manifests that a upward polarization was printed as a domain within the downward polarization region. The phase and amplitude images suggested somewhat incomplete propagation of polarization reversal around the lower right of the elliptic domain, probably due to a pinning of the side-way propagation.31 After the succeeding poling for the shorter period (5 s) with opposite bias at +200 V to the tip, the appearance of a concentric circle (Figure 3c) can be related to the back-switching in a 10 μm scale inside the upward polarization domain. It should be noted that the surface morphology shown by the topographic images unchanged
Figure 1. Ferroelectric polymorph (form I) of anthranilic acid. (a) Molecular structures of the zwitterionic (A) and nonzwitterionic (B) forms. (b) Photograph of the single crystals. (c) Crystal structure viewed along the crystallographic b-direction. (d) Hydrogen-bonded sequence viewed on the crystallographic [120]-direction. The dotted lines in (c) and (d) indicate the proton-transfer paths over the intermolecular O−H···O− and N+−H···N hydrogen bonds. The crystallographic drawings exploit the reported atomic coordinates.19
motion of ferroelectric domain boundaries (domain walls) often reduces the switchable polarization in bulk.26,27 The pinning arises from some crystal imperfections such as charged defects and impurities and can be often released with thermal annealing or repetitive polarization switching.12,26 In the anthranilic acid, the ferroelectric switching could be likewise improved by repetitive switching in the endurance measurements mode (Figure 2). Bipolar rectangular pulses were continuously applied with a cycling frequency of f = 100 Hz and an amplitude of bipolar field Ef = 35 kV cm−1, which was sufficiently higher than coercive field Ec (4−18 kV cm−1). The P−E hysteresis loops were collected by applying the triangular waveform voltage (f = 50 Hz, Em = 44 kV cm−1) with constant interval in a logarithmic time scale. See the waveform in the inset to Figure 2b. Both the remanent polarization Pr and polarity-averaged coercive field |Ec| were found to increase with total switching cycles N. The switching beyond 106 cycles fatigued the Pr from the maximum 4.5 μC cm−2 with the steeper increase in |Ec|. Under the same electric field configuration, the temperature variation of dielectric permittivity was measured to find out the ferroelectric-to-paraelectric phase transition above room temperature. The relative permittivity gradually increased 6194
DOI: 10.1021/acs.chemmater.5b02957 Chem. Mater. 2015, 27, 6193−6197
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Figure 3. Phase (top) and amplitude images (middle) of the piezoresponse force microscope and topographic images (bottom) of the crystal (100) surface. (a) Initial state. (b) After the first switching with negative bias at −200 V for 100 s. (c) After the succeeding back-switching with positive bias at +200 V for 5 s. The outer domain boundary was shrunk in area from that generated in the first switching (dashed outline in the phase image). Figure 4. Microscopic picture of polarization switching in anthranilic acid. (a) A schematic illustration of switching process involving the collective transfer of protons (filled circles) over the O−H···O− and N+−H···N hydrogen bonds. (b) Evolution of the a-direction polarization as the function of λ obtained from the first-principles calculations. (inset) The P−E hysteresis loop of optimized remanent polarization in good agreement with the theoretical value.
regardless the domain boundaries during the switching processes. Reversible switching is manifested by the restoration of the initial PFM phase (i.e., the downward polarization state) in the back-switched domain.32 During the back-switching, the outer elliptic domain boundary was found to shrink slightly as marked by the light-blue dashed outline, suggesting the metastable nature of the locally written domains. The spontaneous back-switching was also demonstrated by the time-lapse of phase image after poling at −200 V for 100 s (Figure S3, Supporting Information); most area of the inverted domain recovered to original phase image after 2 h. The metastability is likely attributed to the insufficient domain size relative to the crystal thickness (∼100 μm), which would prevent the penetration of inverted domain into a cylinder enough to gain the stability against the elastic energy of domain boundaries.33 The measurements successfully demonstrated the polarization switching under both bulk and local electric field on the anthranilic acid I crystal. The most plausible microscopic origin involves the proton-transfer process over the O−H···O− and N+−H···N hydrogen bonds as schematized by the bent arrows in Figure 4a. This picture is corroborated by the pseudosymmetric nature of hypothetical intermediate state in which the locations of acidic protons are centered in the respective hydrogen bond. One can see an inversion center on the middle of each O···O bond of the A−B and A′−B′ pairs, and a 2-fold axis crossing normal to each N···N bond of the A−B′ and A′−B pairs. Addition of this set of symmetry operations to the original group P21cn yields a supergroup Pccn (#56), which denotes the hypothetical paraelectric structure. The spontaneous polarization has been theoretically evaluated by the Berry phase formalism34,35 using our computational code QMAS,36 which is based on the projector augmented-wave (PAW) method37 and the planewave basis set
with the generalized gradient approximation (GGA)38 for the exchange-correlation energy, similarly to our previous work.39 The planewave energy cutoff was set to 20.0 Ha. The number of k points was set to 6 × 6 × 6 in the full Brillouin zone. The neutron diffraction data set19 was employed for the atomic positions in the ferroelectric state. The hypothetical paraelectric structure of the space group Pccn was constructed by imposing an inversion symmetry and accompanied by centered protons as noted above. We introduced the parameter λ to describe the intermediate crystal structure constructed through linear interpolation of the atomic positions between the reference paraelectric (λ = 0) and ferroelectric (λ = 1) states. The spontaneous polarization was calculated by increasing λ from zero to one, and the value of 5.97 μC cm−2 was obtained (Figure 4b). For comparison, the inset specifies the best optimized polarization and the corresponding P−E hysteresis exhibiting Pr = 5.6 μC cm−2 and Ec = 15 kV cm−1 at f = 50 Hz (obtained after 2 × 103 precycling with triangular waves at f = 50 Hz and Em = 67 kV cm−1 on the different specimen from Figure 2). The good agreement between the theoretical and experimentally optimized polarizations validates the collective proton-transfer for polarization switching as considered. The Pr is as large as those of the hydrogen-bonded ionic supramolecules and also close to that of PVDF system (∼8 μC cm−2). The observed Ec is as small as the typical values found in the ferroelectrics of the intermolecular proton-transfer type. 6195
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Chemistry of Materials Recently, Kholkin et al.40 revealed a different mechanism of ferroelectricity on the crystalline glycine of piezoelectric γform41 comprising only the H3N+CH2CO2− zwitterions. According to the local electromechanical PFM measurements, they found the ferroelectricity on the nanoscale level along with the switchable domain structure images. The polarization is reversed by the bulk flip motion of zwitterions themselves instead of the intermolecular proton-transfer process. The flipping is accompanied by break and repair of the hydrogen bonds and this would explain why it requires very high switching field (∼1 V nm−1 = 10 MV cm−1). Rather, the anthranilic acid has a partial resemblance to the triglycine sulfate (TGS),42,43 in which the complex switching mechanism involves the proton transfer between the H3N+CH2CO2− zwitterion and H3N+CH2CO2H cation. The difference is that out-of-plane bending motion of NH3+ group is present in TGS but absent in anthranilic acid. In summary, the time-honored anthranilic acid was revisited as a new ferroelectric as confirmed by the polarization switching under both bulk and local electric field. The correlated protontransfer over the sequence of alternating nonzwitterions and zwitterions can reasonably explain the optimized bulk polarization of 5.6 μC cm−2, as supported by the first-principles calculations. The modern PFM technique artificially imprinted polarization-inverted domains accompanied by some metastable or pinning characteristics.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b02957. Details for electric, thermal, and PFM experiments (PDF).
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AUTHOR INFORMATION
Corresponding Author
*S. Horiuchi. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Mr. H. Yoshida for thermal analysis. REFERENCES
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