Ionic-liquid-induced large-area polarization switching. CIPS is an excellent model system to study the diverse coupling effects between ferroelectric dipoles and external stimuli due to its high Curie temperature (Tc ~ 315 K). Below Tc, ferroelectric phase is generated by the antiparallel displacement of Cu and In sublattices along with a symmetry reduction from non-polar C2/c to polar Cc space group47. As shown in Fig. 1a, the off-centered ordering of Cu sublattice corresponds to upward polarization. The room-temperature ferroelectricity of CIPS samples used in this work has been examined in our previous studies25. For this study, about 84 nm thick CIPS nanoflakes were obtained by mechanically exfoliation onto an ultra-flat Pt/SiO2 substrate. The initial polarization states feature both upward (yellow domains) and downward polarization (purple domains) that were characterized by piezoresponse force microscopy (PFM). After exposure to N,N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium bis-(trifluoromethanesulfonyl)imide ([DEME][TFSI]), a kind of room-temperature ionic liquid composed of large organic cations [DEME] and anions [TFSI] (Fig. 1a), the initial upward polarization states were fully switched to downward polarization (Fig. 1b, c). In contrast, the original downward polarization states remained unchanged. It has been reported that large crystallites can be formed at surface of CIPS when electrically switching ferroelectric domains32, 33. In this work, we noted that no external electric field was applied and no surface deformations were observed (see Supplementary Fig. 1). Experimental details can be found in Supplementary Note 1 and Supplementary Fig. 2. It is worth noting that this interesting phenomenon is highly reproducible for CIPS flakes with varying thickness (see Supplementary Figs. 1, 3 and 4).
To further study the dynamics of polarization inversion induced by ionic liquid, local domain structures changing over time has been carefully investigated. Figure 1d shows the time evolution of PFM phase (top panel) and amplitude signals (bottom panel) of CIPS nanoflake with a thickness of 94 nm. By increasing duration of exposure to ionic liquid [DEME][TFSI], the downward ferroelectric domains expanded dramatically, which is clearly verified by the corresponding evolution of amplitude images. Up to now, the ionic liquid-induced large-area ferroelectric inversion in vdW CIPS single crystals has been evidenced by repeated experiments.
Interfacial effect at CIPS-ionic liquid interfaces. For layered transition-metal chalcogenophosphates, another attractive feature is the large vdW gap offering sufficient space for ion intercalation48. It has been reported that the intercalation of organic molecules plays a significant role in controlling structurally, electronically and magnetically ordered states49. To examine the possibility of intercalation of [DEME] or [TFSI] ions, the thickness and morphology of CIPS flakes before and after ionic liquid treatment have been carefully checked. The flat surface and unchanged thickness evidently excluded the ion intercalation mechanism (see Supplementary Figs. 1, 3 and 4) because ion intercalation can lead to surface wrinkle and bulk swelling of layered materials. To prove this point, we carried out a contrast experiment. Figure 2a shows the photograph of ionic liquid treatment process. Before exposure to ionic liquid, a 4 nm thick MoS2 nanoflake was placed on the surface of CIPS to prevent ionic liquid from contacting with CIPS (Fig. 2b). Note that we can easily detect the response signals of ferroelectric domains beneath such ultra-thin MoS2 film (see Supplementary Fig. 5). It can be observed clearly that the region covered by MoS2 owns identical upward polarization state with surrounding areas (Fig. 2c). After ionic liquid treatment, large-area polarization switching was induced by ionic liquid, except the coverage domains giving a sharply defined identical domain shape with that of MoS2 (Fig. 2d). These results reveal an interfacial effect evidently rather than a bulk effect with respect to ion intercalation.
Figure 2e shows a strip-shaped CIPS nanosheet partially covered by ionic liquid. Only the coverage area experienced a polarization reversal process yet again proving the role of solid-liquid interface (Fig. 2f, g). More surprisingly, after removing the ionic liquid, the switched domain pattern displays almost no change after 1 year (Fig. 2h), suggesting robust stability. The super long retention time is comparable to the longest retention time of classical perovskite-type oxide ferroelectrics electrically poled by a scanning probe50, 51.
In addition to the durability, structure stability is a very important aspect in practical applications especially for 2D layered ferroelectric materials. As shown in Fig. 2i, variable-temperature Raman spectra have been performed on the CIPS samples treated by ionic liquid for 72 hours. With temperature increasing, the sharp peak at 313 cm− 1 gradually disappears indicating a typical ferroelectric-paraelectric transition and vice versa25. The clear phase contrast within the white box (Fig. 2j) written by a PFM tip verifies the well-preserved switchable dipoles in CIPS. The distinct 180° phase reversal and well-defined butterfly loop in Fig. 2k also prove this point. Moreover, the Raman spectra make no difference before and after ionic liquid treatment (Fig. 2l). These results not only demonstrate that the polar crystal structure and switchable ferroelectricity of CIPS have not been damaged by ionic liquid, but also help us ruling out the case of ion intercalation reaction in terms of unchanged structures. Therefore, we attribute domain inversion mechanism to a solid-liquid interface effect. Since ionic liquids are entirely composed by large ions, the physical adsorption of ions at CIPS surface may account for the above polarization switch phenomenon.
Mechanism discussion and reversible inversion. In the material systems of perovskite-type oxide ferroelectrics, it has been reported that H+ adsorption and chemical bonding across ferroelectric–water interface can induce polarization switch from upward to downward state40. In that case, the formation of chemical bonds (metal-O-H at ferroelectric surface) rather than physical adsorption of molecules gives rise to a significant displacement of B-site cations. While, it is hard to form chemical bonding at the surface of vdW layered ferroelectrics at room temperature due to the absence of dangling bonds. To test whether H+ play a role in the polarization reversal process, CIPS nanoflakes were immersed into acidic aqueous solution with pH = 3. As a result, after 4 hours, both surface morphology and polarization states did not change ruling out the possible effect of H+ and H2O experimentally (see Supplementary Fig. 6). The good surface morphology, distinguishable domain structures and constant thickness clearly suggest the high quality polar structures of CIPS flakes before and after acidic solution treatment.
In order to make it clear which kind of ions interact with ferroelectric dipoles at the solid-liquid interface, consecutive experiments have been performed on the same CIPS sample as shown in Fig. 3a-d. We note that a small amount of water is inevitable in the as-received ionic liquid (typically in the order of ppm). To obtain further insights into the possible effect of H2O, deionized water was used to cover the CIPS surface directly. After 5.5 hours exposure to water, no obvious domain inversion phenomenon has been observed indicting that water molecules cannot efficiently affect the polarization states yet again (Fig. 3a, b). In contrast, ionic liquid [DEME][TFSI] can switch the polarization states remarkably only in 0.5 h (see Supplementary Fig. 7) and realize a full reversal after 2 h (Fig. 3c). It should be noted that, to eliminate the impact of residual moisture content, the experiments were conducted in vacuum and the ionic liquid used in this work has been heated at 423 K for 2 h in a vacuum environment. These results confirm that only the organic cations/anions inside ionic liquid play an integral role at ferroelectric interfaces.
Now the question turns to be which kind of ions inside ionic liquid plays a role. Given that anions [TFSI] adsorb on the positively charged surface (upward polarization) preferentially, the anions [TFSI] can screen the intrinsic surface charge of CIPS and offer an opposite electric field to the depolarization field (Fig. 3e). For clarity, all kinds of cations/anions are represented by positively/negatively charged pellets, respectively. The energy of this depolarization field can be reduced by the negative charges at CIPS-ionic liquid interfaces. Actually, this case has been reported for polarized ferroelectrics in ionic liquids/aqueous solutions45, 52. Charged ferroelectric surfaces can control the formation of electric double layer (EDL) via long-range electrostatic interaction. A typical compact layer (Helmholtz layer) is formed by counter ions at first, to which diffusion layer is developed adjacently. However, if this scenario holds true, the initial upward polarization state should be more stable and polarization inversion process will not happen, which is in contradiction with our experimental observations. Therefore, it is natural to take cations [DEME] into consideration. Assuming that cations [DEME] adsorb on the positively charged surface with upward polarization state (Fig. 3f), the adsorbed like-charges provide an additional electric field with the same direction of depolarization field driving a significant displacement of Cu atom in layers of CIPS. Consequently, the polarization direction could be changed, which is well consistent with our experiment phenomena although the attraction behavior between positively charged CIPS surfaces and [DEME] cations is apparently counterintuitive. In a broader perspective, the attraction phenomena between like-charged objects have been extensively studied in many systems such as macroions ranging from colloids, polymers to DNA solutions53. In the case of ionic liquids, both molecular dynamic simulations54 and experimental evidences55 have revealed a cation-rich interface between ionic liquid and positively charged surface because of the specific adsorption of cations in ionic liquid. These reports give us a hint that the EDL structures would be determined not only by long-range electrostatic forces, but also by some short-range interactions. Moreover, it is well known that ionic liquids with surface-active cations often serve as cationic surfactants due to their good surface adsorbability56, 57. It is reasonable to infer that the used cations [DEME] and its analogues may exhibit strong adsorption effect to CIPS surface. To emphasize the general nature of adsorption effect of cations, we performed similar experiments utilizing ionic liquids [DEME][BF4] with new anions [BF4] and [EMIM][BF4] containing new cations [EMIM]. The domain switching phenomena resembling the experimental observations in [DEME][TFSI] are displayed in Supplementary Figs. 8 and 9. These results demonstrate the universality of ionic liquid-induced polarization reversion in CIPS.
Based on the above discussions, we attribute the ionic liquid-induced polarization reversion to the adsorption of cations at the CIPS-ionic liquid interface. However, the lack of in-situ microscopic techniques makes it difficult to trace ferroelectric–liquid interfacial structures. To understand this mechanism, we conducted a simple experiment circumventing the technical difficulties. If the adsorption of large quaternary ammonium cations [DEME] can induce the switching of upward polarization state, the downward polarized domains can be further switched by the surface adsorption of large anions in the same way (Fig. 3h). In other words, we can artificially construct desirable solid–liquid interfacial states just by selecting the appropriate adsorbed ions to switch the ferroelectric polarization states reversibly. Sodium dodecylbenzenesulphonate (Na[DDBS]), a famous anionic surfactant (the main ingredient of detergent), was chosen to provide suitable large anions [DDBS]. The [DDBS] anion has a long alkyl chain exhibiting strong adsorption capacity for solids58. Its molecular structures can be found in Fig. 3i. As expected, downward polarized domains were reversed back after immersed into aqueous solutions of Na[DDBS] (Fig. 3d). The direction of back-switched polarization was verified by applying an external electric filed (see Supplementary Fig. 7r). We also carried out repeated experiments on other CIPS flakes and observed analogous phenomena (see Supplementary Fig. 10). Similarly, these results cannot be explained by the scenario of cation (Na+) adsorption as shown in Fig. 3g. Raman spectra suggest that the polar structures of CIPS are well preserved after Na[DDBS] treatment (see Supplementary Fig. 11).
Simulation of the adsorption behaviors at CIPS-liquid interfaces. In order to obtain molecular insights into the adsorption behaviors of [DEME] cations and [DDBS] anions on CIPS, first-principles calculations have been performed using the DMol3 code59, 60 (Fig. 4). The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional and the DFT semi-core pseudo-potentials (DSPP) with double numerical atomic basis set plus polarization (DNP) were employed61. To compensate the poor description of the weak vdW interactions by the popular PBE functional, an empirical dispersion-corrected density functional theory (DFT-D) approach proposed by Grimme was used62. All the calculations are based on the 24.4 Å × 21.2 Å × 30 Å supercells to accommodate the large-size [DEME] and [DDBS] ions. The adsorption energy can be expressed as:
E ad = Esurface + Eion – Esurface−ion (Eq. 1)
where Esurface, Eion and Esurface−ion is the total energy of bare CIPS surface, an isolated ion, and the optimized CIPS with adsorbed species, respectively. As shown in Fig. 4b, e, the [DEME] cations and [DDBS] anions are preferred to be adsorbed by CIPS surface with initial upward and downward polarizations, respectively. During the ion adsorption, the adsorption energy differences between positive and negative surfaces make polarization switching energetically favorable leading to a significant displacement of Cu atom with respect to P2S6-defined framework. The driving force is the reduction in the total energy of [DEME]-CIPS and [DDBS]-CIPS systems. One might question that, before polarization switching, there is always coulomb repulsion between adsorbed [DEME] cations (DDBS anions) and positive (negative) surface charges of CIPS with initial upward (downward) polarization state to prevent adsorption. Actually, besides vdW forces between ions and surfaces, the sulfur-mediated C–H···S bonds (dotted lines in Fig. 4) may provide attractive forces to balance the coulomb repulsion effect. It should note that the strength of C–H···S bonds lies between strong chemical bonds (e.g. covalent/ionic bonds) and vdW forces63. Our first-principles calculations indicate that the bond lengths of C–H···S bonds are vary from 2.607 to 3.696 Å. On one hand, the long alkyl chains of [DEME] and [DDBS] ions forms many hydrogen bonds with surface S atoms giving rise to the large adsorption energy. On the other hand, the net charges of adsorbed [DEME] and [DDBS] introduce an electric field analogous to applied voltage and reverse the ferroelectric dipoles. These results pave a new way to control the polarization states of layered ferroelectrics dynamically, and the further understanding of the complex interactions between adsorbed ions and ferroelectric surfaces promises to artificially construct desirable ferroelectric-liquid interfacial structures with specific functions or effects.