Unidirectional Electric Field Enables Reversible Ferroelectric Domain Engineering

Unidirectional Electric Field Enables Reversible Ferroelectric Domain Engineering | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Unidirectional Electric Field Enables Reversible Ferroelectric Domain Engineering Weiyou Yang, Xingan Jiang, Muzhi Li, Yuanyuan Cui, Xiao Wu, Zunyi Deng, and 8 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5661145/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The deterministic control of ferroelectric polarization via an external field is critical for advancing the technologies of modern information storage. Conventionally, reversible and cyclic polarization switching in ferroelectric materials requires alternating electric fields with opposite (±) orientations. The present work demonstrates that this scenario can be circumvented in the van der Waal (vdW) ferroelectrics CuInP₂S₆. Conceptionally, by a fixed unidirectional electric field, effective ferroelectric domain engineering with reversible and cyclic switching is accomplished, relying on highly controllable Cu ion migration across vdW gaps. It further unveils the remarkable “shape memory” effect of manipulated domains, and the unprecedented ability to electrically “write” domain configurations. Our findings not only deepen the understanding of ion-coupled ferroelectric behavior, but also provide insights into origin of negative capacitance and the quantized charge transport, paving the way for emerging storage technologies and low-power neuromorphic applications. Physical sciences/Materials science/Condensed-matter physics/Ferroelectrics and multiferroics Physical sciences/Materials science/Condensed-matter physics/Electronic properties and materials Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The ferroelectric materials possess a switchable spontaneous polarization, which attracts a diverse portfolio of applications, such as memory devices 1-3 , sensors 4-6 , and low-power electronic devices 7-9 . Van der Waals (vdW) ferroelectrics have attracted significant interest due to their unique attributes, such as atomically thin layers with dangling-bond-free surfaces and weak interlayer interactions 10,11 . These features enable ferroelectricity to be realized at the atomic scale 12-15 , and facilitate vdW integration with diverse substrates as well as the construction of vdW heterojunctions without lattice limitation and interfacial damage 16 , which paves the way for wafer-scale fabrication of high-performance 2D/3D device architectures 17,18 . The ferroelectric polarization is a fundamental order parameter that responds to external electric fields 10,11 . The deterministic control of polarization switching with an external electric field, is essential for emerging functionalities 19 . Traditional ferroelectric materials feature displacive ions confined in the lattice, giving rise to double-well energy landscape. In such case, the polarization aligns along the identical direction to external electric field, implying that the reversible polarization switching must require two opposite (±) electric fields 20 . However, this scenario might be circumvented by taking highly mobile ions into account 21-23 , exemplified by the vdW ferroionic CuInP 2 S 6 . The ion-coupled ferroelectric behavior offers rich energy landscapes including quadruple-well and sextuple-well configurations, enabling the versatile polarization control and negative capacitance 21,24,25 . Recently, Vanderbilt et al. 26 theoretically proposed the scenario of quantized charge transport—polarization evolving by a quantum as the mobile Cu ions shift to neighboring layer, delivering great potential for low-power neuromorphic applications 27,28 . Yet, the experimental realization of cyclic polarization control in such systems, as a critical piece of evidence, has remained elusive due to structural degradation and domain-switching challenges at the nanoscale using scanning tip 29,30 . This work bridges this gap by experimental validation of ferroelectric domain engineering with cyclic and alternative switching, facilely by unidirectional electric fields in the vdW ferroionic CuInP 2 S 6 . It further unveils the highly controllable Cu ion migration, enabling a remarkable “shape memory” effect during cyclic domain switching and the unprecedented ability to electrically “write” domain pattern. Our findings not only deepen the understanding of the ion-coupled ferroelectric behavior, but also provide insights into the origin of negative capacitance and quantized nature of charge transport in this system. Results Cyclic domain switching under unidirectional electric field Fig. 1a illustrates the layered structure of CuInP 2 S 6 (CIPS) with vdW gaps. Two distinct Cu ions sites are identified near the upper and lower sulfur atoms, giving rise to two states with upward ( P↑ , ⊙) and downward ( P↓ , ⊕) polarizations, respectively 29 . Under electric field, the Cu ions can undergo long-range migration, holding significant promise for memristor-based neuromorphic applications 31-34 . However, Cu ions migration also easily causes substantial topographical damage in the scanned area and leads to the failed manipulation of ferroelectric domain during nanometer tip-bias scanning, posing significant challenge for in-situ domain manipulation. Herein, we employ a method that applies short-duration pulse bias at a single point location using a conductive tip (Fig. 1b), enabling the effective domain manipulation near the tip, as shown in Fig. 1c. More importantly, the results clearly demonstrate that a fixed unidirectional electric field allows bidirectional domain switching. Accordingly, a series of pulse biases (6 V for 0.5 s) are applied at 16 grid points arranged in a 4×4 matrix, with a bias direction from the bottom to top ( E↑ , ⊙), where the numbered #1-16 refers to the pulse numbers at grid points location. From the phase panel, it is evident that, upon applying fewer bias pulses, the purple-black ferroelectric domains ( P↓ ) are switched to yellow analogs ( P↑ ), with the polarization aligning with electric field direction ( E↑P↑ ). Remarkably, as the pulses number increases, particularly after 13 pulses, the yellow domains ( P↑ ) can be switched to purple-black analogs ( P↓ ), with the polarization opposite to electric field direction ( E↑P↓ ). Interestingly, the formation speed of topographical protrusions does not increase linearly with the bias voltage, rather increases sharply as the polarization aligns oppositely to electric field (see Supplementary Fig. 1). This suggests that such anomalous polarization switching behavior is closely related to the enhanced Cu ions migration. Further, we fix the scanning tip location and apply 3 sets of bias (6 V for 5 s) at the central point location to manipulate the domain switching, as illustrated in Fig. 1d. After the 1 st bias, the purple ferroelectric domains ( P↓ ) near the tip are switched to yellow analogs ( P↑ ). By continuously applying 2 nd and 3 rd bias pulses, the yellow domains ( P↑ ) can be reversibly switched back to purple-black analogs ( P↓ ). These results provide compelling evidence that the fixed unidirectional electric field enables the cyclic domain switching of P↓ → P↑ → P↓ . Ion migration-mediated cyclic switching mechanism By the switching spectroscopy PFM (SS-PFM) technique 35 , the ion-mediated cyclic switching dynamics under external electric fields is further investigated. As presented in Fig. 2a, the polarization switching loops are consecutively measured at the identical location by varying the bias magnitude. In a small window of ±5 V dc , phase shows one standard switching loop with ~180° phase difference and the amplitude also presents one single “butterfly” loop, where the polarization aligns with the bias direction, and can be reversibly switched between P ↑ and P ↓ by applying opposite bias direction. As the bias window extends, especially up to ±13 V dc , both phase and amplitude exhibit three switching loops, with an additional switching loop emerging at higher positive ( E ↓ ) and negative ( E ↑ ) bias magnitudes, respectively. As positive bias ( E ↓ ) increases, P ↑ is firstly switched to P ↓ , then reversibly switched back to P ↑ with a switching cycle of P↑ → P↓ → P↑ . Vice versa, a switching cycle of P↓ → P↑ → P↓ also occurs under the negative bias ( E ↑ ). In this scenario, increasing the bias magnitude aligns the polarization opposite to the E -field direction ( E ↑ P ↓ or E ↓ P ↑ ), mainly attributed to the Cu ions migration across the vdW gap 21 , which will be discussed in following section. The repeated cycles further unveil the highly controllable Cu ion migration behaviors. Additionally, Fig. 2b presents the statistical results of polarization switching loop measured across 6×6 grid points, and the detailed polarization switching curves are included in Supplementary Figs. 2-6. Here, we use the terms “Success” and “Failure” to describe whether Cu ions migrate across the vdW gap or not. Notably, the results indicate that a higher bias significantly increases the probability of mobile Cu ions across the vdW gap, thus aligning the polarization opposite to the E -field. The energy barriers during Cu ion migration across vdW gap are calculated in Fig. 2c. Two minimum potential wells correspond to two Cu positions within the adjacent layers, situating near the upper S atoms in layer 1 and the lower sulfur atoms in layer 2, respectively. The barriers for the intralayer and interlayer Cu ions migration are ~0.33 and 0.85 eV, respectively. The difference in barriers can be overcome by increasing the electric fields, thereby enabling control over the Cu ion migration pathways. As illustrated in Fig. 2d, in the case of low E -field or short time duration, the Cu ions can only hop up and down in the intralayer without the energy across the vdW gap. In this scenario, the polarization aligns with the E -field direction, corresponding to the switching process from ① to ② state. Once the E -field increases and/or the time duration extends, Cu ions can migrate across the vdW gaps into adjacent layer with polarization opposite to E -field direction, as shown by the process from ② to ③ state. Ferroelectric domain engineering with cyclic and alternative switching To manipulate the domain switching over large scale with minimal surface damage, another approach has been utilized using a nanometer tip, as demonstrated in Fig. 3a. Correspondingly, a short-duration bias (-100 V, 5 s) is applied at a central point location from bottom to top ( E↑ ) through a conductive tip, maintaining an electrical open-circuit condition. As shown in Fig. 3b, the switching scale of ferroelectric domains is remarkable up to 5 µm × 5 µm, which is ~ 4 orders of magnitude greater than the tip’s contact area (tip radius: ~25 nm). Yet, the topographical data show negligible surface damage, with protrusion heights of just ~2 nm (see Extended Data Fig. 1a). In the top-left region, the yellow domains ( P↑ ) are switched to large areas of purple-black analogs ( P↓ ), with polarization opposite to the field direction ( E↑P↓ ). Vice versus, in the bottom-right side, the purple-black domains ( P↓ ) can be switched to large areas of yellow analogs ( P↑ ), with polarization along the electric field direction ( E↑P↑ ). In the entire scanned region, the domain percentages of yellow ( P↑ ) and purple-black domains ( P↓ ) remains nearly balanced, with a ratio close to 1:1—51%/49% before the bias and 48%/52% after the bias, respectively. These results confirm the domains engineering with reversible switching at a large scale under a unidirectional electric field. Moreover, these switched domains maintain their stability after the bias withdrawal (see Extended Data Fig. 1b). Fig. 3b investigates the domain switching behavior after applying the unidirectional bias ranging from -10 to -140 V ( E ↑ ) for 5 s in the central point location. Additional details on the surface topography, phase and amplitude before and after bias application are included in Supplementary Figs. 7-8. The two regions, highlighted with respective yellow and blue boxes, are selected to monitor the domain evolution. Correspondingly, the domain percentages of purple-black ( P↓ ) and yellow domains ( P↑ ), in response to various bias magnitudes in two selected regions are extracted, as presented in Fig. 3c. It discloses that, in yellow box region, the increased bias switches the ferroelectric domains in alternative sequence of P↑ → P↓ → P↑ → P↓ , which includes two cyclic switching of P↑ → P↓ → P↑ and P↓ → P↑ → P↓ . Similarly, the blue box region also reveals an alternative switching of P↓ → P↑ → P ↓ → P ↑ with two cyclic switching. These results verify the domain engineering with cyclic and alternative switching facilely enabled by unidirectional electric fields, providing solid evidence of highly controllable Cu ions migration layer-by-layer across the vdW gaps, as illustrated in Fig. 3d. Interestingly, the ferroelectric domains are almost equally switched (1:1 ratio) in two boxes but in opposite directions. As a result, the percentages of purple-black ( P↓ )and yellow domains ( P↑ ) in the entire scanning region remains almost constant with the increase of bias magnitude (see Extended Data Fig. 2). Moreover, through ultrafast scan imaging (~26 s per image) and repeated bias application, it captures this rapid relaxation process occurring in a small portion of switched domains, due to the imprint field 36,37 (see Extended Data Fig. 3). During a cyclic polarization switching of P↑ → P↓ → P↑ , it further unveils that ferroelectric domains exhibit a remarkable “shape memory” effect, disappearing and reappearing with recoverable shape effect 38 (see Extended Data Fig. 4, Supplementary Fig. 9). As far as we are aware, such domain memory effect is rarely reported in conventional ferroelectrics, which not only indicates the highly controllable Cu ions migration, but also inspires the dynamic control for reservoir computing applications 39 . Electrically “writing” domain patterns Finally, the unidirectional electric field is utilized to “write” a grid pattern of ferroelectric domains with alternating P ↑ and P↓ configurations. Fig. 4a shows the bias map for electrically “writing” domains, under alternating -25 V ( E↑ ) and -50 V ( E↑ ) electric fields, as indicated by the red line profiles in Fig. 4b. The pristine domains prior to applied bias are shown in Fig. 4c, revealing the randomly distributed domain patterns. As demonstrated in Fig. 4d, the unidirectional bias can realize the precise control over the polarization directions ( P↑ and P↓ ), and the well-defined grid patterns of ferroelectric domains are achieved under the given bias. Fig. 4e further shows the line profiles of given bias and the obtained domain configurations along the red line for multiple “writing” processes (see the details in Supplementary Fig. 10). It discloses that, in the -25 V ( E↑ ) poled region, the purple-black domains ( P↓ ) are switched to yellow analogs ( P↑ ), identical to the bias direction. Meanwhile, in the -50 V ( E↑ ) poled region, the yellow domains ( P↑ ) can be switched to purple-black analogs ( P↓ ), opposite to the bias direction. To the best of our knowledge, this is the first report of electrically “writing” ferroelectric domain patterns by a unidirectional electric field. The well-defined domain configurations in the 8×8 box suggest the highly controllable electric field-driven Cu ions migration to different occupations, underscoring its potential for domain engineering and ferro-ionic device applications. Discussion Conventionally, ferroelectric materials must require opposite (±) electric fields for reversible and cyclic domain switching. However, this scenario can be circumvented in vdW ferroelectrics CuInP₂S₆, solely facilely by a unidirectional electric field. This work unveils highly controllable Cu ion migration layer by layer across vdW gap and experimentally confirms the effective domain engineering with cyclic and alternative switching. For instance, the direction of polarization ( P ) can be alternatively switched by a unidirectional electric field following the sequence of P↑ → P↓ → P↑ → P↓ , with two cyclic switching P↑ → P↓ → P↑ and P↓ → P↑ → P↓ . It further confirms the remarkable “shape memory effect” during a cyclic switching, as well as the unprecedented ability to electrically “write” grid-patterned domain configurations enabled by the unidirectional field. These exotic polarization switching behaviors stem from in the vdW structure and bias-controlled ion migration across vdW gap. In principle, such phenomena should happen in all vdW ferroelectric systems sustaining such ionic motion. Our findings substantially deepen the understanding on the origin of negative capacitance. Correspondingly, to access negative capacitance, the polarization can be intentionally switched opposite to the electric field, according to the relationship of C = d Q /d V ∝ d P /d E ＜ 0 , where V is the voltage, Q is the charge, P is the polarization, and E is the electric field, respectively 22 . Furthermore, the realized cyclic and alternative polarization switching paves the way for emerging storage technologies and low-power neuromorphic applications. Methods Crystal growth and sample preparation CIPS single crystals were synthesized using the chemical vapor transport (CVT) technique 40 . In a typical process, the stoichiometric elemental precursors were placed in a vacuum-sealed quartz tube, maintained at a vacuum of ~ 10 −3 Pa. In the given two-zone tube furnace, the hot and cool zones were kept at ~ 650 and ~600 °C, respectively. The reaction was proceeded for ~212 hours. Thin CIPS flakes were obtained by mechanically exfoliating the CIPS crystal with the adhesive tape, subsequently transferred onto silicon substrates for subsequent PFM and SS-PFM characterizations. Piezoresponse force microscopy measurement Piezoresponse force microscopy (PFM) was measured on thin flakes using a commercial atomic force microscope (Cypher S, Oxford Instruments, USA), equipped with a high-voltage module. The vector PFM mode was used to image the ferroelectric domains by Pt/Ir-coated Si cantilever tip driven with an AC voltage ( V ac = 1 V) with a frequency of ~350 kHz. The hysteresis loops were measured using the Switching Spectroscopy PFM (SS-PFM) mode. Pulsed triangular DC voltage ( V dc ) was used with a pulse width of 10 ms. A high-frequency AC voltage ( V ac = 1 V) was superimposed on the DC voltage. Multiple cycles of hysteresis loops were measured at a point location with the time duration of 1 s for each loop, consisting of 100 pulses. The off-field data were used for the analysis. To obtain the statistical results of bias-meditated polarization switching, SS-PFM measurements were conducted across a 6×6 grid point location. In the litho-PFM, the time is fixed at 128 s per image in the litho-PFM for electrically “writing” grid-patterned domains. First-principles calculations Density functional theory (DFT) was conducted using the Vienna ab-initio simulation package (VASP) 41–43 . The Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) 44 was chosen to describe the exchange-correlation interaction of electrons, and the DFT-D3 (Becke-Johnson) vdW formulation was used to describe vdW interactions between the layers. The wave functions were expanded in a plane-wave basis set, with an energy cutoff of 680 eV for structural relaxation and 520 eV for electronic structure computations. The force convergence criterion for each ion was set to less than 10 -3 eV/Å, and a precision of 10 -6 eV was adopted for a total energy minimization. The migration energy barriers for Cu ions were computed through the nudged elastic band method (NEB) 45 for the supercell. To accommodate the migrating ion, one Cu ion was removed, and the force convergence criterion was tightened to below 0.04 eV/Å. The valence electrons were defined: Cu: 3p 6 3d 10 4s 1 , In 4d 10 5s 2 5p 1 , P 3s 2 3p 3 and S 3s 2 3p 4 . The Crystal structures were visualized using VESTA software 46 . Declarations Acknowledgements We acknowledge Dr. Ruizhi Yu for helpful discussion. This work is supported by the Ningbo Yongjiang Talent Introduction Programme (2023A-390-G) and Ningbo Top Talent Project (Grant No. 2020-DST-003), the National Natural Science Foundation of China (Grant Nos. 52372063, 12202056, 92163101, 12374080, 12474101, 12202056, 12304120). Author contributions X. W. and W. Y. conceived the idea and directed the project. Y. C. and J.D provided the single crystal sample. X. J. performed the PFM measurements. M. Li gave the assistance with data collection. Z. D. conducted the DFT calculations. X. J., Z. L., X. W. and W. Y. carried out the data analyses and cowrote the manuscript. All authors discussed the data and contributed to the manuscript. Competing interests The authors declare no competing interest. 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Supplementary Files ExtendedData.docx Extended Data Figures SupplementaryInformation.docx Supplementary Information File Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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University","correspondingAuthor":false,"prefix":"","firstName":"Jianming","middleName":"","lastName":"Deng","suffix":""},{"id":397378641,"identity":"ae4fe538-46ab-4000-a283-83307f46cccb","order_by":8,"name":"Xiaolei Wang","email":"","orcid":"https://orcid.org/0000-0002-6964-2453","institution":"Beijing University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiaolei","middleName":"","lastName":"Wang","suffix":""},{"id":397378642,"identity":"10385eec-f91e-4092-b32d-5b75ec9c9cbd","order_by":9,"name":"Dongdong Zhang","email":"","orcid":"https://orcid.org/0000-0002-0091-1803","institution":"Hunan University","correspondingAuthor":false,"prefix":"","firstName":"Dongdong","middleName":"","lastName":"Zhang","suffix":""},{"id":397378643,"identity":"3ee55e5c-ab82-4366-a0a2-18a905e73d88","order_by":10,"name":"Xiangdong Yang","email":"","orcid":"https://orcid.org/0000-0002-0797-8293","institution":"Ningbo University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiangdong","middleName":"","lastName":"Yang","suffix":""},{"id":397378644,"identity":"1ff64d65-1f05-4ea6-ae19-0f4ae0339dd1","order_by":11,"name":"Zhuoyin Peng","email":"","orcid":"","institution":"Changsha University of Science \u0026 Technology","correspondingAuthor":false,"prefix":"","firstName":"Zhuoyin","middleName":"","lastName":"Peng","suffix":""},{"id":397378645,"identity":"d22d6ad0-e138-4f5b-8356-f228c5c2e16d","order_by":12,"name":"Zhao Liang","email":"","orcid":"","institution":"Ningbo University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Zhao","middleName":"","lastName":"Liang","suffix":""},{"id":397378646,"identity":"5b8d706f-71f5-493a-8a09-9cfd09f34631","order_by":13,"name":"Xueyun Wang","email":"","orcid":"https://orcid.org/0000-0001-5264-9539","institution":"Beijing Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Xueyun","middleName":"","lastName":"Wang","suffix":""}],"badges":[],"createdAt":"2024-12-17 11:10:27","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5661145/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5661145/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":81495958,"identity":"5c0b3a7b-b83d-4a9d-b509-33c4540d6065","added_by":"auto","created_at":"2025-04-28 02:15:01","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":758591,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eReversible and cyclic domain switching under unidirectional electric fields.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Crystalline structure with two Cu sites in the sulfur octahedron, showing the upward and downward polarization. \u003cstrong\u003eb\u003c/strong\u003e A diagram of applying a short-duration pulse bias at a central point location under a conductive tip within a closed electric circuit. \u003cstrong\u003ec\u003c/strong\u003e Ferroelectric domain switching with increased numbers of bias pulses (6 V, 0.5 s) at 16 different locations. \u003cstrong\u003ed\u003c/strong\u003e Ferroelectric domain switching by 3 sets of bias pulses (6 V, 5 s) at a fixed location. The white dashed circle indicates the region influenced by the tip’s electric field.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5661145/v1/8f7ac178b27878dfd26fea96.png"},{"id":81495959,"identity":"eff7ec20-3601-4f55-a839-fcd20691152d","added_by":"auto","created_at":"2025-04-28 02:15:01","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":330631,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eIon migration-mediated domain switching mechanism.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e Consecutive SS-PFM hysteresis loops measured at a fixed location by tuning the tip bias. \u003cstrong\u003eb\u003c/strong\u003eThe statistical results of SS-PFM measured across 6×6 grid points location. \u003cstrong\u003ec\u003c/strong\u003eEnergy profile for Cu ion migration within the layer or across vdW gap, with local minima corresponding to \u003cem\u003e\u003cstrong\u003eP↑\u003c/strong\u003e\u003c/em\u003e and \u003cem\u003e\u003cstrong\u003eP↓\u003c/strong\u003e\u003c/em\u003e, and migration barriers of 0.33 and 0.85 eV for intralayer and interlayer migration, respectively. \u003cstrong\u003ed\u003c/strong\u003e Cu ion migration pathways and polarization switching under unidirectional electric field.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5661145/v1/4c8ad8ebd84f9b448c24eeed.png"},{"id":81496155,"identity":"9d97ff16-4b36-4414-b520-39609033b2dc","added_by":"auto","created_at":"2025-04-28 02:23:01","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":943597,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCyclic and alternative switching of ferroelectric domains. a-b \u003c/strong\u003eSchematic setup and bidirectional domain switching using a conductive tip.\u003cstrong\u003e c\u003c/strong\u003e The domain switching under a unidirectional electric field with progressively increased magnitude.\u003cstrong\u003e d \u003c/strong\u003ePercentages of purple-black (\u003cem\u003e\u003cstrong\u003eP↓\u003c/strong\u003e\u003c/em\u003e) and yellow domains (\u003cem\u003e\u003cstrong\u003eP↑\u003c/strong\u003e\u003c/em\u003e) in the yellow- and blue- box regions. \u003cstrong\u003ee\u003c/strong\u003e Schematic Cu ions migration layer-by-layer across the vdW gaps for cyclic and alternative polarization switching.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5661145/v1/bac63d2e3b3d2afdcb2ed8d3.png"},{"id":81495961,"identity":"9b15ee45-9e75-4f66-9e52-4679a55cbc89","added_by":"auto","created_at":"2025-04-28 02:15:01","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":578766,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectrical “writing” operation of ferroelectric domainconfigurations.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e The unidirectional bias map for electrically “writing” grid-patterned ferroelectric domains. \u003cstrong\u003eb\u003c/strong\u003e The red line profiles of applied bias in panel \u003cstrong\u003ea\u003c/strong\u003e. \u003cstrong\u003ec\u003c/strong\u003ePristine domains with randomly distributed domain patterns. \u003cstrong\u003ed\u003c/strong\u003e Grid-patterned ferroelectric domain arrangement after applying unidirectional bias. \u003cstrong\u003ee\u003c/strong\u003eThe line profiles of applied bias and the domain arrangement along the red line for multiple “writing” processes.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5661145/v1/87aff416875bb366b88fa220.png"},{"id":81496505,"identity":"d1f0c354-b8b8-44b9-a15c-605871a47cea","added_by":"auto","created_at":"2025-04-28 02:31:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3367917,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5661145/v1/eab1e966-3625-4e3f-a4c7-79bc236854b1.pdf"},{"id":81495963,"identity":"21ea17bb-d5a3-4154-a2bd-4458bbf879d2","added_by":"auto","created_at":"2025-04-28 02:15:01","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":9765550,"visible":true,"origin":"","legend":"Extended Data Figures","description":"","filename":"ExtendedData.docx","url":"https://assets-eu.researchsquare.com/files/rs-5661145/v1/233b72a0eb0bd2fbfd9e8cef.docx"},{"id":81495964,"identity":"da2be02c-0e35-4191-b077-148be652b108","added_by":"auto","created_at":"2025-04-28 02:15:02","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":16516110,"visible":true,"origin":"","legend":"Supplementary Information File","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5661145/v1/21c1dffe2ee375cb13362a8c.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Unidirectional Electric Field Enables Reversible Ferroelectric Domain Engineering","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe ferroelectric materials possess a switchable spontaneous polarization, which attracts a diverse portfolio of applications, such as memory devices\u003csup\u003e1-3\u003c/sup\u003e, sensors\u003csup\u003e4-6\u003c/sup\u003e, and low-power electronic devices\u003csup\u003e7-9\u003c/sup\u003e. Van der Waals (vdW) ferroelectrics have attracted significant interest due to their unique attributes, such as atomically thin layers with dangling-bond-free surfaces and weak interlayer interactions\u003csup\u003e10,11\u003c/sup\u003e. These features enable ferroelectricity to be realized at the atomic scale\u003csup\u003e12-15\u003c/sup\u003e, and facilitate vdW integration with diverse substrates\u0026nbsp;as well as the construction of vdW heterojunctions without lattice limitation and interfacial damage\u003csup\u003e16\u003c/sup\u003e, which paves the way for\u0026nbsp;wafer-scale fabrication of high-performance\u0026nbsp;2D/3D device architectures\u003csup\u003e17,18\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe ferroelectric polarization is a fundamental order parameter that responds to external electric fields\u003csup\u003e10,11\u003c/sup\u003e. The deterministic control of polarization switching with an external electric field, is essential for emerging functionalities\u003csup\u003e19\u003c/sup\u003e. Traditional ferroelectric materials feature displacive ions confined in the lattice, giving rise to double-well energy landscape. In such case, the polarization aligns along the identical direction to external electric field, implying that the\u0026nbsp;reversible polarization switching must require two opposite (\u0026plusmn;) electric fields\u003csup\u003e20\u003c/sup\u003e. However, this scenario\u0026nbsp;might\u0026nbsp;be circumvented by taking highly mobile ions into account\u003csup\u003e21-23\u003c/sup\u003e, exemplified by the vdW ferroionic\u0026nbsp;CuInP\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e.\u0026nbsp;The ion-coupled ferroelectric behavior offers rich energy landscapes including quadruple-well and sextuple-well configurations, enabling the versatile polarization control and negative capacitance\u003csup\u003e21,24,25\u003c/sup\u003e. Recently, Vanderbilt \u003cem\u003eet al.\u003c/em\u003e\u003csup\u003e\u0026nbsp;26\u003c/sup\u003e theoretically proposed the scenario of quantized charge transport\u0026mdash;polarization evolving by\u0026nbsp;a quantum as the mobile Cu ions shift to neighboring layer, delivering great potential for low-power neuromorphic applications\u003csup\u003e27,28\u003c/sup\u003e. Yet, the experimental realization of cyclic polarization control in such systems, as a critical piece of evidence, has remained elusive due to structural degradation and domain-switching challenges at the nanoscale\u0026nbsp;using scanning tip\u003csup\u003e29,30\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThis work bridges this gap by experimental validation of\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eferroelectric domain engineering with cyclic and alternative switching, facilely by unidirectional electric fields in the vdW ferroionic CuInP\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e. It further unveils the highly controllable Cu ion migration, enabling a remarkable \u0026ldquo;shape memory\u0026rdquo; effect during cyclic domain switching and the unprecedented ability to electrically \u0026ldquo;write\u0026rdquo; domain pattern. Our findings not only deepen the understanding of the ion-coupled ferroelectric behavior, but also provide insights into the origin of negative capacitance and quantized nature of charge transport in this system.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eCyclic domain switching under unidirectional electric field\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFig. 1a illustrates the layered structure of CuInP\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e6\u003c/sub\u003e (CIPS) with vdW gaps. Two distinct Cu ions sites are identified near the upper and lower sulfur atoms, giving rise to two states with upward (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e, ⊙) and downward (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e, \u0026oplus;) polarizations, respectively\u003csup\u003e29\u003c/sup\u003e. Under electric field, the Cu ions can undergo long-range migration, holding significant promise for memristor-based neuromorphic applications\u003csup\u003e31-34\u003c/sup\u003e. However, Cu ions migration also easily causes substantial topographical damage in the scanned area and leads to the failed manipulation of ferroelectric domain during nanometer tip-bias scanning, posing significant challenge for in-situ domain manipulation. Herein, we employ a method that applies short-duration pulse bias at a single point location using a conductive tip (Fig. 1b), enabling the effective domain manipulation near the tip, as shown in Fig. 1c. More importantly, the results clearly demonstrate that a fixed unidirectional electric field allows bidirectional domain switching. Accordingly, a series of pulse biases (6 V for 0.5 s) are applied at 16 grid points arranged in a 4\u0026times;4 matrix, with a bias direction from the bottom to top (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;\u003c/em\u003e\u003c/strong\u003e, ⊙), where the numbered #1-16 refers to the pulse numbers at grid points location. From the phase panel, it is evident that, upon applying fewer bias pulses, the purple-black ferroelectric domains (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e) are switched to yellow analogs (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e), with the polarization aligning with electric field direction (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;P\u0026uarr;\u003c/em\u003e\u003c/strong\u003e). Remarkably, as the pulses number increases, particularly after 13 pulses, the yellow domains (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) can be switched to purple-black analogs (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e), with the polarization opposite to electric field direction (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;P\u0026darr;\u003c/em\u003e\u003c/strong\u003e). Interestingly, the formation speed of topographical protrusions does not increase linearly with the bias voltage, rather increases sharply as the polarization aligns oppositely to electric field (see Supplementary Fig. 1). This suggests that such anomalous polarization switching behavior is closely related to the enhanced Cu ions migration.\u003c/p\u003e\n\u003cp\u003eFurther, we fix the scanning tip location and apply 3 sets of bias (6 V for 5 s) at the central point location to manipulate the domain switching, as illustrated in Fig. 1d. After the 1\u003csup\u003est\u003c/sup\u003e bias, the purple ferroelectric domains (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e) near the tip are switched to yellow analogs (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e). By continuously applying 2\u003csup\u003end\u003c/sup\u003e and 3\u003csup\u003erd\u003c/sup\u003e bias pulses, the yellow domains (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) can be reversibly switched back to purple-black analogs (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e). These results provide compelling evidence that the fixed unidirectional electric field enables the cyclic domain switching of \u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eIon migration-mediated cyclic switching mechanism\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBy the switching spectroscopy PFM (SS-PFM) technique\u003csup\u003e35\u003c/sup\u003e, the ion-mediated cyclic switching dynamics under external electric fields is further investigated. As presented in Fig. 2a, the polarization switching loops are consecutively measured at the identical location by varying the bias magnitude. In a small window of \u0026plusmn;5 V\u003csub\u003edc\u003c/sub\u003e, phase shows one standard switching loop with ~180\u0026deg; phase difference and the amplitude also presents one single \u0026ldquo;butterfly\u0026rdquo; loop, where the polarization aligns with the bias direction, and can be reversibly switched between \u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e and \u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026darr;\u003c/em\u003e\u003c/strong\u003e by applying opposite bias direction. As the bias window extends, especially up to \u0026plusmn;13 V\u003csub\u003edc\u003c/sub\u003e, both phase and amplitude exhibit three switching loops, with an additional switching loop emerging at higher positive (\u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026darr;\u003c/em\u003e\u003c/strong\u003e) and negative (\u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) bias magnitudes, respectively. As positive bias (\u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026darr;\u003c/em\u003e\u003c/strong\u003e) increases, \u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e is firstly switched to \u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026darr;\u003c/em\u003e\u003c/strong\u003e, then reversibly switched back to \u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e with a switching cycle of \u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e. Vice versa, a switching cycle of \u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e also occurs under the negative bias (\u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e). In this scenario, increasing the bias magnitude aligns the polarization opposite to the \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e-field direction (\u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026darr;\u003c/em\u003e\u003c/strong\u003e or \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e), mainly attributed to the Cu ions migration across the vdW gap\u003csup\u003e21\u003c/sup\u003e, which will be discussed in following section. The repeated cycles further unveil the highly controllable Cu ion migration behaviors.\u003c/p\u003e\n\u003cp\u003eAdditionally, Fig. 2b presents the statistical results of polarization switching loop measured across 6\u0026times;6 grid points, and the detailed polarization switching curves are included in Supplementary Figs. 2-6. Here, we use the terms \u0026ldquo;Success\u0026rdquo; and \u0026ldquo;Failure\u0026rdquo; to describe whether Cu ions migrate across the vdW gap or not. Notably, the results indicate that a higher bias significantly increases the probability of mobile Cu ions across the vdW gap, thus aligning the polarization opposite to the \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e-field. The energy barriers during Cu ion migration across vdW gap are calculated in Fig. 2c. Two minimum potential wells correspond to two Cu positions within the adjacent layers, situating near the upper S atoms in layer 1 and the lower sulfur atoms in layer 2, respectively. The barriers for the intralayer and interlayer Cu ions migration are ~0.33 and 0.85 eV, respectively. The difference in barriers can be overcome by increasing the electric fields, thereby enabling control over the Cu ion migration pathways. As illustrated in Fig. 2d, in the case of low \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e-field or short time duration, the Cu ions can only hop up and down in the intralayer without the energy across the vdW gap. In this scenario, the polarization aligns with the \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e-field direction, corresponding to the switching process from ① to ② state. Once the \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e-field increases and/or the time duration extends, Cu ions can migrate across the vdW gaps into adjacent layer with polarization opposite to \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e-field direction, as shown by the process from ② to ③ state.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFerroelectric domain engineering with cyclic and alternative switching\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo manipulate the domain switching over large scale with minimal surface damage, another approach has been utilized using a nanometer tip, as demonstrated in Fig. 3a. Correspondingly, a short-duration bias (-100 V, 5 s) is applied at a central point location from bottom to top (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) through a conductive tip, maintaining an electrical open-circuit condition. As shown in Fig. 3b, the switching scale of ferroelectric domains is remarkable up to 5 \u0026micro;m \u0026times; 5 \u0026micro;m, which is ~ 4 orders of magnitude greater than the tip\u0026rsquo;s contact area (tip radius: ~25 nm). Yet, the topographical data show negligible surface damage, with protrusion heights of just ~2 nm (see Extended Data Fig. 1a). In the top-left region, the yellow domains (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) are switched to large areas of purple-black analogs (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e), with polarization opposite to the field direction (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;P\u0026darr;\u003c/em\u003e\u003c/strong\u003e). Vice versus, in the bottom-right side, the purple-black domains (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e) can be switched to large areas of yellow analogs (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e), with polarization along the electric field direction (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;P\u0026uarr;\u003c/em\u003e\u003c/strong\u003e). In the entire scanned region, the domain percentages of yellow (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) and purple-black domains (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e) remains nearly balanced, with a ratio close to 1:1\u0026mdash;51%/49% before the bias and 48%/52% after the bias, respectively. These results confirm the domains engineering with reversible switching at a large scale under a unidirectional electric field. Moreover, these switched domains maintain their stability after the bias withdrawal (see Extended Data Fig. 1b).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFig. 3b investigates the domain switching behavior after applying the unidirectional bias ranging from -10 to -140 V (\u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) for 5 s in the central point location. Additional details on the surface topography, phase and amplitude before and after bias application are included in Supplementary Figs. 7-8. The two regions, highlighted with respective yellow and blue boxes, are selected to monitor the domain evolution. Correspondingly, the domain percentages of purple-black (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e) and yellow domains (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e), in response to various bias magnitudes in two selected regions are extracted, as presented in Fig. 3c. It discloses that, in yellow box region, the increased bias switches the ferroelectric domains in alternative sequence of \u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e, which includes two cyclic switching of \u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003eand \u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cem\u003e.\u0026nbsp;\u003c/em\u003eSimilarly, the blue box region also reveals an alternative switching of \u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e with two cyclic switching. These results verify the domain engineering with cyclic and alternative switching facilely enabled by unidirectional electric fields, providing solid evidence of highly controllable Cu ions migration layer-by-layer across the vdW gaps, as illustrated in Fig. 3d. Interestingly, the ferroelectric domains are almost equally switched (1:1 ratio) in two boxes but in opposite directions. As a result, the percentages of purple-black (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e)and yellow domains (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) in the entire scanning region remains almost constant with the increase of bias magnitude (see Extended Data Fig. 2).\u003c/p\u003e\n\u003cp\u003eMoreover, through ultrafast scan imaging (~26 s per image) and repeated bias application, it captures this rapid relaxation process occurring in a small portion of switched domains, due to the imprint field\u003csup\u003e36,37\u003c/sup\u003e (see Extended Data Fig. 3). During a cyclic polarization switching of \u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e, it further unveils\u003cem\u003e\u0026nbsp;\u003c/em\u003ethat ferroelectric domains exhibit a remarkable \u0026ldquo;shape memory\u0026rdquo; effect, disappearing and reappearing with recoverable shape effect\u003csup\u003e38\u003c/sup\u003e (see Extended Data Fig. 4, Supplementary Fig. 9). As far as we are aware, such domain memory effect is rarely reported in conventional ferroelectrics, which not only indicates the highly controllable Cu ions migration, but also inspires the dynamic control for reservoir computing applications\u003csup\u003e39\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eElectrically \u0026ldquo;writing\u0026rdquo; domain patterns\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFinally, the unidirectional electric field is utilized to \u0026ldquo;write\u0026rdquo; a grid pattern of ferroelectric domains with alternating \u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003e\u0026uarr;\u003c/em\u003e\u003c/strong\u003e and \u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e configurations. Fig. 4a shows the bias map for electrically \u0026ldquo;writing\u0026rdquo; domains, under alternating -25 V (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) and -50 V (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) electric fields, as indicated by the red line profiles in Fig. 4b. The pristine domains prior to applied bias are shown in Fig. 4c, revealing the randomly distributed domain patterns. As demonstrated in Fig. 4d,\u0026nbsp;the unidirectional bias can realize the precise control over the polarization directions (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e and \u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e), and the well-defined grid patterns of ferroelectric domains are achieved under the given bias. Fig. 4e further shows the line profiles of given bias and the obtained domain configurations along the red line for multiple \u0026ldquo;writing\u0026rdquo; processes (see the details in Supplementary Fig. 10). It discloses that, in the -25 V (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) poled region, the purple-black domains (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e) are switched to yellow analogs (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e), identical to the bias direction. Meanwhile, in the -50 V (\u003cstrong\u003e\u003cem\u003eE\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) poled region, the yellow domains (\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e) can be switched to purple-black analogs (\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e), opposite to the bias direction. To the best of our knowledge, this is the first report of electrically \u0026ldquo;writing\u0026rdquo; ferroelectric domain patterns by a unidirectional electric field. The well-defined domain configurations in the 8\u0026times;8 box suggest the highly controllable electric field-driven Cu ions migration to different occupations, underscoring its potential for domain engineering and ferro-ionic device applications.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eConventionally, ferroelectric materials must require opposite (\u0026plusmn;) electric fields for reversible and cyclic domain switching. However, this scenario can be circumvented in vdW ferroelectrics CuInP₂S₆, solely facilely by a unidirectional electric field. This work unveils highly controllable Cu ion migration layer by layer across vdW gap and experimentally confirms the effective domain engineering with cyclic and alternative switching.\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eFor instance, the direction of polarization (\u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e) can be alternatively switched by a unidirectional electric field following the sequence of \u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u0026rarr;\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e,\u003cstrong\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/strong\u003ewith two cyclic switching \u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003eand\u003cem\u003e\u0026nbsp;\u003cstrong\u003eP\u0026darr;\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026uarr;\u003c/em\u003e\u003c/strong\u003e\u003cstrong\u003e\u0026rarr;\u003c/strong\u003e\u003cstrong\u003e\u003cem\u003eP\u0026darr;\u003c/em\u003e\u003c/strong\u003e\u003cem\u003e.\u0026nbsp;\u003c/em\u003eIt further confirms the remarkable \u0026ldquo;shape memory effect\u0026rdquo; during a cyclic switching, as well as the unprecedented ability to electrically \u0026ldquo;write\u0026rdquo; grid-patterned domain configurations enabled by the unidirectional field. These exotic polarization switching behaviors stem from in the vdW structure and bias-controlled ion migration across vdW gap. In principle, such phenomena should happen in all vdW ferroelectric systems sustaining such ionic motion. Our findings substantially deepen the understanding on the origin of negative capacitance. Correspondingly, to\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eaccess negative capacitance, the polarization can be intentionally switched opposite to the electric field, according to the relationship of \u003cstrong\u003e\u003cem\u003eC\u003c/em\u003e\u003c/strong\u003e=\u003cem\u003ed\u003cstrong\u003eQ\u003c/strong\u003e/d\u003cstrong\u003eV\u003c/strong\u003e\u003c/em\u003e\u0026prop;\u003cem\u003ed\u003cstrong\u003eP\u003c/strong\u003e/d\u003cstrong\u003eE\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e＜\u003c/em\u003e\u003cem\u003e0\u003c/em\u003e, where \u003cstrong\u003e\u003cem\u003eV\u003c/em\u003e\u003c/strong\u003e is the voltage, \u003cstrong\u003e\u003cem\u003eQ\u003c/em\u003e\u003c/strong\u003e is the charge, \u003cstrong\u003e\u003cem\u003eP\u003c/em\u003e\u003c/strong\u003e is the polarization, and \u003cstrong\u003e\u003cem\u003eE\u003c/em\u003e\u003c/strong\u003e is the electric field, respectively\u003csup\u003e22\u003c/sup\u003e. Furthermore, the realized cyclic and alternative polarization switching paves the way for emerging storage technologies and low-power neuromorphic applications.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eCrystal growth and sample preparation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eCIPS single crystals were synthesized using the chemical vapor transport (CVT) technique\u003csup\u003e40\u003c/sup\u003e. In a typical process, the stoichiometric elemental precursors were placed in a vacuum-sealed quartz tube, maintained at a vacuum of ~ 10\u003csup\u003e\u0026minus;3\u003c/sup\u003e Pa. In the given two-zone tube furnace, the hot and cool zones were\u0026nbsp;kept at ~ 650 and ~600 \u0026deg;C, respectively. The reaction was proceeded for ~212 hours.\u0026nbsp;Thin CIPS flakes were obtained by mechanically exfoliating the CIPS crystal with the adhesive tape, subsequently transferred onto silicon substrates for subsequent PFM and SS-PFM characterizations.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePiezoresponse force microscopy measurement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePiezoresponse force microscopy (PFM) was measured on thin flakes using a commercial atomic force microscope (Cypher S, Oxford Instruments, USA), equipped with a high-voltage module. The vector PFM mode was used to image the ferroelectric domains by Pt/Ir-coated Si cantilever tip driven with an AC voltage (\u003cem\u003eV\u003csub\u003eac\u003c/sub\u003e\u003c/em\u003e = 1 V) with a frequency of ~350 kHz. The hysteresis loops were measured using the Switching Spectroscopy PFM (SS-PFM) mode. Pulsed triangular DC voltage (\u003cem\u003eV\u003csub\u003edc\u003c/sub\u003e\u003c/em\u003e) was used with a pulse width of 10\u0026thinsp;ms. A high-frequency AC voltage (\u003cem\u003eV\u003csub\u003eac\u003c/sub\u003e\u003c/em\u003e = 1 V) was superimposed on the DC voltage. Multiple cycles of hysteresis loops were measured at a point location with the time duration of 1 s for each loop, consisting of 100 pulses. The off-field data were used for the analysis. To obtain the statistical results of bias-meditated polarization switching, SS-PFM measurements were conducted across a 6\u0026times;6 grid point location. In the litho-PFM, the time is fixed at 128 s per image in the litho-PFM for electrically \u0026ldquo;writing\u0026rdquo; grid-patterned domains.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFirst-principles calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDensity functional theory (DFT) was conducted using the Vienna ab-initio simulation package (VASP)\u003csup\u003e41\u0026ndash;43\u003c/sup\u003e. The Perdew\u0026ndash;Burke\u0026ndash;Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA)\u003csup\u003e44\u003c/sup\u003e was chosen to describe the exchange-correlation interaction of electrons, and the DFT-D3 (Becke-Johnson) vdW formulation was used to describe vdW interactions between the layers. The wave functions were expanded in a plane-wave basis set, with an energy cutoff of 680 eV for structural relaxation and 520 eV for electronic structure computations. The force convergence criterion for each ion was set to less than 10\u003csup\u003e-3\u003c/sup\u003e eV/\u0026Aring;, and a precision of 10\u003csup\u003e-6\u003c/sup\u003e eV was adopted for a total energy minimization. The migration energy barriers for Cu ions were computed through the nudged elastic band method (NEB)\u003csup\u003e45\u003c/sup\u003e for the supercell. To accommodate the migrating ion, one Cu ion was removed, and the force convergence criterion was tightened to below 0.04 eV/\u0026Aring;. The valence electrons were defined: Cu: 3p\u003csup\u003e6\u003c/sup\u003e3d\u003csup\u003e10\u003c/sup\u003e4s\u003csup\u003e1\u003c/sup\u003e, In 4d\u003csup\u003e10\u003c/sup\u003e5s\u003csup\u003e2\u003c/sup\u003e5p\u003csup\u003e1\u003c/sup\u003e, P 3s\u003csup\u003e2\u003c/sup\u003e3p\u003csup\u003e3\u003c/sup\u003e and S 3s\u003csup\u003e2\u003c/sup\u003e3p\u003csup\u003e4\u003c/sup\u003e. The Crystal structures were visualized using VESTA software\u003csup\u003e46\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe acknowledge Dr. Ruizhi Yu for helpful discussion. This work is supported by the Ningbo Yongjiang Talent Introduction Programme (2023A-390-G) and Ningbo Top Talent Project (Grant No. 2020-DST-003), the National Natural Science Foundation of China (Grant Nos. 52372063, 12202056, 92163101, 12374080, 12474101, 12202056, 12304120).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eX. W. and W. Y. conceived the idea and directed the project. Y. C. and J.D provided the single crystal sample. X. J. performed the PFM measurements. M. Li gave the assistance with data collection. Z. D. conducted the DFT calculations. X. J., Z. L., X. W. and W. 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Crystallogr.\u003c/em\u003e\u003cstrong\u003e41\u003c/strong\u003e, 653\u0026ndash;658 (2008).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5661145/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5661145/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"The deterministic control of ferroelectric polarization via an external field is critical for advancing the technologies of modern information storage. Conventionally, reversible and cyclic polarization switching in ferroelectric materials requires alternating electric fields with opposite (±) orientations. The present work demonstrates that this scenario can be circumvented in the van der Waal (vdW) ferroelectrics CuInP₂S₆. Conceptionally, by a fixed unidirectional electric field, effective ferroelectric domain engineering with reversible and cyclic switching is accomplished, relying on highly controllable Cu ion migration across vdW gaps. It further unveils the remarkable “shape memory” effect of manipulated domains, and the unprecedented ability to electrically “write” domain configurations. Our findings not only deepen the understanding of ion-coupled ferroelectric behavior, but also provide insights into origin of negative capacitance and the quantized charge transport, paving the way for emerging storage technologies and low-power neuromorphic applications.","manuscriptTitle":"Unidirectional Electric Field Enables Reversible Ferroelectric Domain Engineering","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-28 02:14:56","doi":"10.21203/rs.3.rs-5661145/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"15e98cf4-cd77-46ea-b24a-d5ed576e446e","owner":[],"postedDate":"April 28th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":42336919,"name":"Physical sciences/Materials science/Condensed-matter physics/Ferroelectrics and multiferroics"},{"id":42336920,"name":"Physical sciences/Materials science/Condensed-matter physics/Electronic properties and materials"}],"tags":[],"updatedAt":"2025-04-28T02:14:56+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-28 02:14:56","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5661145","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5661145","identity":"rs-5661145","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}