Oriented ion migration in dielectric Sb₄O₅Cl₂ single crystals for multifunctional two-dimensional electronics

Abstract

Exploring dielectrics with high gate capacitance and artificial tailoring of electronic states is highly desirable for the development of 2D electronics. Here, we report the synthesis of a single-crystalline ionic dielectric Sb₄O₅Cl₂ with aligned ionic channels, achieving a dielectric constant of 23.3 (at a thickness of ~ 25 nm) and non-volatile programmable regulation of the ionic migration. The fabricated 2D MoS₂ transistors display on/off ratios up to 10⁹_, leakage currents down to 10⁻¹⁴ A, and mobility of 33.4 cm²V⁻¹ s⁻¹. Oriented ionic channels facilitate efficient and controllable migration of Cl⁻ ions, thus realizing a non-structural-damage and reconfigurable transition between a quasi-metallic and semiconducting state in MoS₂, with a retention time exceeding 3000 s. During 100 cycles of state transitions, the quasi-metallic state exhibits a conductivity of 10⁻⁵ S, which is 10³ times that of the semiconducting state. Bio-inspired ionic channel further allows neuromorphic devices to integrate image storage, processing, and recognition capabilities, achieving a recognition accuracy improvement from 80.7% to 90.9% within just 5 training epochs through non-volatile preprocessing lasting over 300 s. Our study highlights the significant potential of this single-crystal ionic dielectric as a promising platform for fundamental research on electronic state modulation and advanced electronics.

Here, the authors report the synthesis of high-κ ionic dielectric Sb₄O₅Cl₂ thin films with aligned ionic channels, showing their application for the realization of high-performance 2D MoS₂ transistors with programmable non-volatile memory and synaptic behaviours enabled by the controllable ionic migration in Sb₄O₅Cl₂.

Introduction

Two-dimensional (2D) electronics have emerged as an ideal platform for studying exotic quantum physics, new functional transistors, and novel computing paradigms, owing to the quantum size effects, the tunable electronic structures, and the strong coupling interactions with external fields of 2D materials^1–8. To meet the demands of increasingly complex functionalities, gate dielectrics need not only high dielectric constants to address device-scaling challenges^9–12, but also artificial tailoring of electronic states^13,14. This dual functionality may allow monolithic integration of multiple operational modes within a single device architecture, such as combining high-performance field-effect transistors (FETs) for digital logic operations^15–17, reversible phase transition for analog memory programming^18,19, and neuromorphic computing^20,21 or other advanced functions. Yet now, conventional metal oxide dielectrics (such as Al₂O₃, SiO₂, and HfO₂) still face challenges including low dielectric constants, amorphous structures, and incompatible interfaces induced by dangling bonds, which hinders further miniaturization of devices, limited carrier modulation capabilities and lacking memory characteristics^22–26.

In recent years, ionic gate dielectrics have attracted significant attention due to their high-κ dielectric constants from ionic polarization^27–29 and the extremely high electric fields enabled by the electric double layer (EDL) of ions^30–33, which can significantly enhance the capability of modulating electronic states in 2D materials inducing discrete superconducting phases³⁴, strong Ising-type spin-orbit coupling³³, and quenching the bandgap of 2D semiconductors³². However, conventional ionic dielectrics (liquid or solid electrolytes, primarily amorphous or polycrystalline structures) still present formidable challenges including poor compatibility with 2D materials, disordered ion migration pathways leading to random electrical performance and irreversible lattice damage caused by ion-migration resulting in poor stability, thereby severely limiting further research^35–38.

In this work, we report a single-crystalline ionic dielectric Sb₄O₅Cl₂, possessing a high κ exceeding 23.3 ( ~ 25 nm) and oriented ion-migration paths to address the abovementioned long-term issues. Benefiting from the high κ value and van der Waals structure, 2D transistors exhibit high on/off ratio (10⁹) at a low operation voltage (V_ds = 0.5 V), low leakage current (10⁻¹⁴ A), and high room-temperature mobility of 33.4 cm²V⁻¹ s⁻¹, and realize NOT (inverter), AND, and NAND logic functions. The periodic ionic channels within the single-crystalline structure enhance the efficiency of ionic migration while achieving precise control and non-volatile characteristics. By electrically modulating the ionic states in Sb₄O₅Cl₂, a reversible transition between quasi-metallic and semiconducting states in MoS₂ channels has been achieved. This reconfigurability remains stable over 100 cycles and exhibits good retention under both room-temperature and elevated-temperature conditions (over 40 min). The quasi-metallic state shows a conductivity of ~10⁻⁵ S, surpassing the semiconducting state by three orders of magnitude, while their on/off ratios differ by six orders. Based on this, the device shows good memory properties with good retention (more than 3000 s) and endurance (over 1000 cycles). Furthermore, synapse-like functions integrating image storage, processing, and recognition have been realized. Owing to the non-volatile preprocessing capability lasting over 300 s, the recognition accuracy reaches 90.9% after just 5 training epochs. In summary, our research not only provides a material platform for high-performance 2D electronic devices but also demonstrates its potential in fields such as fundamental studies and multifunctional devices.

Results and discussion

Characterization of Sb₄O₅Cl₂ and theory calculations

The Sb₄O₅Cl₂ single crystal, a type of ionic layered material crystallizing in the monoclinic system with space group P2₁/c (No. 14, a = 6.16 Å, b = 5.14 Å, c = 13.62 Å), features an alternating stack of negatively charged [Cl]_2n^2n- anion layers and positively charged [Sb₄O₅]_n²ⁿ⁺ layers along the a-axis, exhibiting an interlayer spacing of approximately 6.16 Å (the left part of Fig. 1a). Notably, the Sb and O atoms in the [Sb₄O₅]_n²ⁿ⁺ layers are not densely packed. Instead, they form [SbO] eight-membered rings, which align precisely with the interlayer Cl⁻ ions, thereby creating a pathway for Cl⁻ ions migration along the a-axis between layers (the right part of Fig. 1a). Compared with amorphous and polycrystalline ionic materials, the oriented ion-migration within such a single-crystal system under external stimulus facilitates higher ionic concentration accumulation at interfaces, thereby inducing enhanced modulation and coupling effects. The synthesis method of Sb₄O₅Cl₂ single crystals is provided in Methods and Supplementary Fig. 1. The as-synthesized centimeter-scale Sb₄O₅Cl₂ single crystals exhibit three intense and sharp diffraction peaks assigned to the {100} crystallographic planes (Fig. 1b). Although the interlayers in Sb₄O₅Cl₂ are bonded via weak electrostatic interactions-making it more difficult to exfoliate than traditional van der Waals 2D materials-nanosheets were still obtained using a thermally assisted tape exfoliation and transfer method (see Supplementary Fig. 2 and Methods). The obtained Sb₄O₅Cl₂ nanosheets were further confirmed by Raman spectroscopy, and the vibrational modes of their characteristic peaks are assigned via angle-resolved polarized Raman measurements (Supplementary Figs. 3 and 4). These characterizations indicate the highly crystalline layered structure of the as-synthesized Sb₄O₅Cl₂ crystals.

Fig. 1. Structure, characterizations, and theory calculations of Sb₄O₅Cl₂ single crystal.

a Crystal structure of Sb₄O₅Cl₂. The yellow and blue dotted boxes on the left in the figure represent the structures correspond respectively to [Sb₄O₅]_n²ⁿ⁺ and [Cl]_2n^2n-. The blue dotted ring on the right side of the figure represents the [SbO] octahedral ring. b X-ray diffraction (XRD) patterns of Sb₄O₅Cl₂ single crystal. c Cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Sb₄O₅Cl₂ nanosheet. The white dashed lines denote the (002) and (100) crystal planes and the inset displays the side view of the atomic structure model. d HAADF-STEM image of (100) plane. The white dashed lines denote the (002) and (010) crystal planes. Scale bars in (c and d), 1 nm. e HAADF-STEM image of (010) plane and the corresponding high-resolution (Energy dispersive x-ray spectroscopy) EDS mapping. Scale bars, 1 nm. f Contrast intensity profiles along the orange and pink dash lines in (c). g Calculated energy band structure of Sb₄O₅Cl₂ with an indirect band gap of ~3.7 eV. h Absorption spectrum of Sb₄O₅Cl₂ nanosheets transferred onto quartz substrate. The inset shows the fitting of its optical band gap by Tauc’s law, where α, h, and ν are the absorption coefficient, Planck constant, and frequency, respectively.

To further elucidate the structure of Sb₄O₅Cl₂, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterization was conducted on Sb₄O₅Cl₂ nanosheets. As shown in Fig. 1c, d and Supplementary Fig. 5, we performed HAADF-STEM imaging along different crystallographic orientations of Sb₄O₅Cl₂, namely, [010], [100], and [001], all of which match well with the structure model and lattice spacing of Sb₄O₅Cl₂. Given the contrast dependence of HAADF-STEM on the atomic number (Z) and atomic occupancy^39–41, atomic columns containing different quantities of Sb (Z = 51) are clearly identified. Notably, the unique interlayer Cl⁻ ions sandwiched between [Sb₄O₅]_n²ⁿ⁺ layers can be distinctly observed (Fig. 1c and Supplementary Fig. 6) with an interlayer spacing of 6.16 Å. The measured lattice spacings of 6.81 Å and 5.14 Å correspond to the (002) and (010) lattice planes, respectively. While O atoms remain indiscernible due to their weak contrast arising from a low atomic number, Cl⁻ ions confined within the [SbO] rings are clearly resolved with discernible intensity (Supplementary Fig. 7). Energy-dispersive X-ray spectroscopy (EDS) mapping shows a uniform elemental distribution with an atomic ratio of Sb/O/Cl close to 4: 5: 2 (Supplementary Fig. 8). Furthermore, high-resolution EDS analysis along the [010] zone axis reveals an alternating stacked distribution of Sb and Cl elements (Fig. 1e), in agreement with the corresponding TEM image. Figure 1f displays the contrast intensity profile extracted along the orange dashed line crossing the [Sb₄O₅]_n²ⁿ⁺ layers and the pink dashed line situated within the interlayer regions from Fig. 1c. The profile distinctly reveals the periodic arrangement of Sb and Cl, along with the occupancy of Cl within the interlayer positions. The chemical states of Sb₄O₅Cl₂ were further characterized by X-ray photoelectron spectroscopy (XPS), where the Cl 2p spectral feature centered at a binding energy of 197.7 eV provides conclusive evidence for the existence of interlayer Cl⁻ ions (Supplementary Fig. 9).

We calculated the band structures of Sb₄O₅Cl₂ using density functional theory (Fig. 1g). Sb₄O₅Cl₂ exhibits a large indirect band gap of ~3.7 eV, whose conduction band minimum (CBM) and valence band maximum (VBM) are located at Ζ and Γ point of the first Brillouin zone, respectively. Furthermore, the band structure of Sb₄O₅Cl₂ was experimentally determined by ultraviolet-visible absorption spectrum and ultraviolet photoelectron spectrum (UPS) with a bandgap of ~3.9 eV (Fig. 1h and Supplementary Fig. 10), in good agreement with the calculated band structures. Such a wide bandgap renders Sb₄O₅Cl₂ an insulator, as ascertained by the conductive test of a two-terminal device and its resistivity of over 10¹⁰ Ω·cm at 300 K can be extracted from the I_ds-V_ds curve (Supplementary Fig. 11).

Dielectric properties of Sb₄O₅Cl₂

The theoretical dielectric constant of Sb₄O₅Cl₂ was also calculated in Fig. 2a. Sb₄O₅Cl₂ exhibits high dielectric constants across all three crystallographic directions, predominantly originating from ionic polarization (with electronic polarization contributing merely ~5 to the total permittivity). This observation highlights the strong ionic character of Sb₄O₅Cl₂, which arises from its unique crystal structure. Notably, the highest dielectric constant of 32.9 is observed along the a-axis (perpendicular to the [Sb₄O₅]_n²ⁿ⁺ layers), which is consistent with the direction of the Sb₄O₅Cl₂ nanosheets obtained by mechanical exfoliation, suggesting a high dielectric controllability of electronic states. The parallel-plate capacitors with vertical metal-insulator-metal (MIM) structure were further fabricated¹⁶, as shown in the inset of Fig. 2b. The effective dielectric constant (ε_eff) of Sb₄O₅Cl₂ nanosheets can be calculated by the equation C = Aε_effε₀/t, where C is the measured capacitance, A is the overlapping area of top and bottom electrodes, ε₀ is the vacuum dielectric constant and t is the physical thickness of Sb₄O₅Cl₂. The ε_eff of the 190 nm Sb₄O₅Cl₂ nanosheet, derived from capacitance-frequency (C-f ) measurement (Fig. 2b), reaches around 29.5 at 100 kHz and this experimental value aligns remarkably well with theoretical calculations. Thickness-dependent ɛ_eff is shown in Fig. 2c. Despite thickness-dependent reduction, Sb₄O₅Cl₂ achieves a dielectric constant of up to 43.2 and maintains a value above 17.5 even at ~15 nm, attributable to its strong ionic polarization mechanism. Additionally, the leakage current and corresponding breakdown field strength of Sb₄O₅Cl₂ were demonstrated in Supplementary Fig. 12, revealing a breakdown field of 3 MV cm⁻¹ while maintaining leakage current below the dynamic random-access memory (DRAM) limit⁹. The relationship between the band gap and corresponding dielectric constant of currently reported dielectrics was plotted in Fig. 2d^{9–11,17,28,29,39,42–47}, demonstrating that Sb₄O₅Cl₂ single crystal possesses a relatively higher dielectric constant (as evidenced by ε_eff = 3.3 for h-BN, ε_eff = 16.5 for β-Bi₂SeO₅, ε_eff = 11.5 for Sb₂O₃, ε_eff = 8.4 for CaF₂ and ε_eff = 15.3 for GdOCl).

Fig. 2. Band gap and dielectric constant of Sb₄O₅Cl₂ nanosheets, as well as the MoS₂ field-effect transistors (FETs) based on Sb₄O₅Cl₂ dielectrics.

a Calculated dielectric constant of Sb₄O₅Cl₂ along different cell directions. b Typical frequency-dependent capacitance (C-f) characteristics of the common metal-insulator-metal device, and the configuration and optical micrograph (OM) image of device are inset in (b) (scale bars, 10 μm). c The thickness-dependent effective permittivity (ε_eff) of Sb₄O₅Cl₂ nanosheets. Solid dots denote the measured values, and the red curve serves as a visual guide. d Energy band gap versus dielectric constant of representative dielectric materials^{9–11,17,28,29,39,42–47}. e Output curves at different gate votage (V_g). Solid lines represent the curves in linear coordinates, and dashed lines represent those in logarithmic coordinates. f Transfer characteristics of the back-gated MoS₂/Sb₄O₅Cl₂ FETs. I_g denotes the leakage current. The inset is an OM image of the device. Scale bars, 10 μm. g Performance comparison among 2D FETs with state-of-the-art high-κ dielectrics in terms of on/off ratios at different V_ds^{9–11,17,28,29,39,43,44}.

Single-crystalline ionic dielectric Sb₄O₅Cl₂ for 2D transistors

To examine the potential of Sb₄O₅Cl₂ as a dielectric in devices, MoS₂ FETs with Sb₄O₅Cl₂ as the bottom-gate dielectric were fabricated on the SiO₂/Si substrate, utilizing exfoliated few-layer graphene (FLG) as gate electrode and In/Au as the source and drain electrodes. Figure 2e shows liner I_ds-V_ds curves under different gate voltages (V_g), indicating well electrical contacts. The transfer characteristics are presented in Fig. 2f, presenting a high on/off ratio (I_on/I_off) exceeding 10⁹ at a low operation voltage (V_ds = 0.5 V). It is worth noting that even when the operation voltage of V_ds is as low as 0.05 V, the transistor still maintains a high I_on/I_off of 10⁸. Moreover, the device exhibits a low leakage current (~10⁻¹⁴ A) throughout the entire testing process. Furthermore, field-effect mobility of the device reaches 33.4 cm²V⁻¹ s⁻¹ by linear fitting the transfer curve (V_ds = 1 V). The high I_on/I_off and charge carrier mobility are fundamentally attributed to the superior dielectric constant of Sb₄O₅Cl₂, which originates from robust ionic polarization effects enabled by its ordered ionic migration channels. And the performance of 20 FETs devices was tested and statistically analyzed, as shown in Supplementary Fig. 13. The results demonstrate that the devices exhibit relatively excellent performance in terms of I_on/I_off, threshold voltage, and mobility, with relatively small variations observed between different devices. Additionally, we fabricated and characterized a top-gated MoS₂ device with a Sb₄O₅Cl₂ single-crystal dielectric layer thinner than 100 nm (Supplementary Fig. 14), which also exhibits a high I_on /I_off exceeding 10⁸. Compared with the 2D transistors with state-of-the-art high-κ dielectrics, our devices exhibit stronger gate controllability (as shown in Fig. 2g), exhibiting higher I_on /I_off under low operation voltage and much lower leakage current^{9–11,17,28,29,39,43,44}. To evaluate the stability of the Sb₄O₅Cl₂ ionic single-crystal dielectric layer, we conducted positive bias temperature instability (PBTI) and time-dependent dielectric breakdown (TDDB) tests on the MoS₂/Sb₄O₅Cl₂/FLG device, and the results are presented in Supplementary Fig. 15.

We also fabricated logic circuits based on MoS₂/Sb₄O₅Cl₂ and MoTe₂/Sb₄O₅Cl₂ FETs. Supplementary Fig. 16 illustrates the most essential NOT gate (also known as an inverter), which exhibits sharp switching characteristics at different V_DD values. More complicated logic units such as the AND gate and the NAND gate were also fabricated (Supplementary Fig. 17). The AND gate and NAND gate operate at V_DD of 2.0 V to perform their logical functions. Our results confirm the application potential of the Sb₄O₅Cl₂ ionic single-crystal dielectric layer in logic circuits.

Reconfigurable transition of MoS₂

The ion-electron coupling between Sb₄O₅Cl₂ and MoS₂ can effectively modulate the carrier concentration in MoS₂, thereby further inducing changes in the electrical properties of MoS₂. As illustrated in Fig. 3a, the top Sb₄O₅Cl₂ layer functions as a modulation layer, not merely a dielectric. It regulates the direction of Cl⁻ ion migration within itself by adjusting the top ionic gate voltage (V_ionic), thus controlling the carrier concentration in MoS₂. The SiO₂/Si back-gate, in contrast, is used to characterize the modulated electrical behavior of the MoS₂ channel. The I_ds-V_ds and back-gate transfer characteristics under varying V_ionic (+10 V and −10 V for 20 s) are characterized, with the results presented in Fig. 3b, c, respectively. From the I_ds-V_ds curves, it can be observed that, compared to pristine MoS₂, a positive V_ionic suppresses I_ds while a negative V_ionic enhances I_ds, which indicates that applying V_ionic can effectively modulate the conductivity of MoS₂. From the back-gate transfer characteristics, a positive V_ionic enhances the n-type semiconductor behavior of MoS₂, increasing the I_on/I_off from 10⁶ to 10⁷. In contrast, after applying a negative V_ionic, the device maintains a high conductance state (I_ds ~ 10⁻⁵ A) throughout the back-gate sweep process, and the I_on /I_off is less than 10, indicating the transition from semiconducting to quasi-metallic characteristics. Importantly, unlike structure-destructive and irreversible atomic doping^48,49, the transition of MoS₂ between quasi-metallic and semiconducting state enabled by the modulated Cl⁻ ions of Sb₄O₅Cl₂ exhibits non-destructive nature and high reversibility. By applying gradually increasing negative and positive V_ionic to the Sb₄O₅Cl₂ modulation layer, a stepwise transition of MoS₂ between semiconducting and quasi-metallic state was achieved, as shown in Fig. 3d, e, respectively. We also tested the retention characteristics of both states at room temperature and elevated temperatures (Supplementary Fig. 18). The state could be maintained for over 40 min even at 100 °C. Furthermore, (I_ds-T) curves after different pulsed V_ionic demonstrate that the conductance retention can exceed 3000 s (Supplementary Fig. 19). These results indicate that the reconfigurable modulation exhibits a certain degree of non-volatility, and based on this, we successfully fabricated a memory device with good retention (>3000 s) and endurance (>1000 cycles), as shown in Supplementary Fig. 20.

Fig. 3. Reconfigurable transition of MoS₂.

a Configuration of the dual-gate device. b, c I_ds-V_ds and back-gate transfer characteristic curves of MoS₂ before and after applying different ionic gate voltages (V_ionic). d, e Back-gate transfer characteristics under stepwise increasing negative and positive V_ionic. f Back-gate transfer characteristics of 100 state transition cycles. The holding time for both positive and negative V_ionic is 20 s and the insets are OM images of the device before and after cycling. Scale bars, 10 μm. g, h The current on/off ratios (I_on/I_off) and electrical conductivity of two states during 100 cycles extracted from (f).

Notably, this reconfigurable transition exhibits good cycling stability. As shown in Fig. 3f, the device underwent 100 cycles of positive-negative V_ionic switching while maintaining stable state transfer characteristics. With increasing cycling times, both the quasi-metallic and semiconducting state exhibit minimal fluctuations in I_on /I_off (Fig. 3g). After 100 cycles, the quasi-metallic state retains an I_on /I_off below 10, while the semiconducting state maintains a high I_on /I_off of 10⁵. As demonstrated in Fig. 3h, the quasi-metallic state consistently maintains high conductance (>10⁻⁵ S), which is 10³ times higher than that of the semiconducting state, while a seven-order-of-magnitude conductance contrast between the two states persists even after 100 cycling tests (V_bg = −10 V). Furthermore, the reconfiguration endurance was extended to 2000 cycles with stable performance. The performance of Sb₄O₅Cl₂-based top-gate FETs was compared before and after cycling, and negligible degradation was observed (Supplementary Fig. 21). These results demonstrate that Sb₄O₅Cl₂ exhibits a remarkably strong non-volatile modulation capability on the electronic states of MoS₂, and owing to its unique crystal structure, the physical structural damage caused by this modulation is negligible⁵⁰.

Mechanism of oriented ion-migration in Sb₄O₅Cl₂

To elucidate the mechanism of tailoring the electronic states, theoretical calculations and systematic experiments were conducted. Piezoresponse force microscopy (PFM) measurements on Sb₄O₅Cl₂ nanosheets ruled out ferroelectric behavior, supported by negligible phase hysteresis and butterfly amplitude loops⁴⁰, consistent with its centrosymmetric structure (Supplementary Fig. 22). Therefore, we proposed that the migration of Cl⁻ ions at the Sb₄O₅Cl₂/MoS₂ interface under the top ionic gate leads to the tailoring of electronic states in MoS₂, enabling reconfigurable transitions between quasi-metallic and semiconducting state, as illustrated in Fig. 4a. Under negative V_ionic, Cl⁻ ions migrate and accumulate at the Sb₄O₅Cl₂/MoS₂ interface, which captures holes in the MoS₂ channel, thereby generating more electrons and driving the transition of MoS₂ from the initial n-type semiconductor to a quasi-metal. While under positive V_ionic, Cl⁻ ions at the interface are depleted, leading to the accumulation of Cl⁻ vacancies (V_Cl). These V_Cl capture electrons in the MoS₂ channel, reducing the electron concentration and restoring semiconducting behavior. Subsequently, we conducted band structure calculations for MoS₂ at two distinct Sb₄O₅Cl₂ interfaces-the Cl⁻ ions interface and the V_Cl interface-as well as for isolated MoS₂, as shown in Fig. 4b, c and Supplementary Fig. 23, respectively. The Fermi level (E_f) of isolated MoS₂ is located at the VBM. However, MoS₂ shows an upward E_f shift into the conduction band with Cl⁻ ion interface, indicating n-type degenerate doping⁴⁸. In contrast, the E_f of MoS₂ returns to the VBM position with Cl⁻ vacancy interfaces closely matching that of the isolated MoS₂, indicating that MoS₂ exhibits semiconducting properties at this state. The theoretical calculation results are in excellent agreement with the experimental data and the proposed mechanism.

Fig. 4. Mechanism of oriented ions migration in Sb₄O₅Cl₂.

a Schematic illustrations of the mechanism for modulating MoS₂ channel carriers by Cl⁻ ions migration in Sb₄O₅Cl₂ under different V_ionic. Red arrows denote the direction of the electric field (E). b Calculated band structure of MoS₂ at the Sb₄O₅Cl₂/MoS₂ interface with Cl⁻ ions accumulation. The inset shows the schematic of the structure. c Calculated band structure of MoS₂ at the Sb₄O₅Cl₂/MoS₂ interface with Cl⁻ vacancy (V_Cl). The inset shows the schematic of the structure. d Calculated migration pathways of Cl⁻ ions and the barrier heights. The insets are the migration processes of the ions from different perspectives. e Comparison of the barrier heights for ions passing through the planar layers in other 2D material systems. f OM image of the device and (Kelvin probe force microscopy) KPFM mapping after applying different V_ionic for 1 min. Black dashed lines denote the boundary of the bottom gate region, and the potential distribution curves were extracted along the red and yellow arrows, respectively. g Curves of the electric potential distribution extracted along the direction of the yellow arrow in (f), and the dashed lines and arrows in the figure are used to extract and calculate the potential difference of MoS₂ between the gate region and the non-gate region.

Furthermore, the ion-migration paths have been calculated, as shown in Fig. 4d. The total energy of the system at each step as the Cl⁻ ions moved through the [Sb₄O₅]_n²ⁿ⁺ layer was obtained by relaxing the atomic positions until the maximum force on each atom was less than 10⁻² eV/Å. To enable efficient Cl⁻ ions migration, the Sb-O bonds surrounding the [SbO] octagonal ring undergo sequential dissociation, driving structural reconfiguration from an octagonal ring through decagonal to dodecagonal configurations, with the entire process exhibiting an energy barrier of 2.10 eV. Compared to ion-migration barriers in other 2D material systems (Fig. 4e)^51–53, the Cl⁻ ions migration barrier in Sb₄O₅Cl₂ single crystals is relatively lower (e.g., K⁺/MoS₂: 2.46 eV and Ag⁺/graphene: 4.27 eV) indicating the inherent migration. Meanwhile, the periodic lattice potential of the single-crystalline Sb₄O₅Cl₂ constrains Cl⁻ migration, imparting a certain degree of non-volatility.

To further confirm this modulation mechanism, we fabricated a MoS₂/Sb₄O₅Cl₂/FLG device and employed Kelvin probe force microscopy (KPFM) to characterize the surface potential evolution (Supplementary Fig. 24). As shown in Fig. 4f and Supplementary Fig. 25, the change in the work function is expressed by the contact potential difference (CPD) between the sample surface and the probe tip, as defined by the formula V_CPD = (φ_tip − φ_sample)/q, where q is the elementary charge, φ_sample and φ_tip are the work functions of the sample surface and probe tip^54,55. Compared with the pristine KPFM mapping, at V_ionic = −20 V, the potential of Sb₄O₅Cl₂ in the edge regions of MoS₂ exhibits a significant decrease, which correlates with an increase in the φ_sample and a downward shift of the E_f, attributed to the reduction in electron concentration resulting from the accumulation of Cl⁻ ions. Conversely, at V_ionic = +20 V, the potential of Sb₄O₅Cl₂ at the MoS₂ edges exhibits a significant increase, accompanied by an upward shift of the E_f, as Cl⁻ ions migrate away from the interface, resulting in hole depletion and electron accumulation. Notably, the potential variation in Sb₄O₅Cl₂ was exclusively observed within the overlap regions between the bottom FLG and the top MoS₂ layer. This phenomenon is attributed to the migration of Cl⁻ ions occurring only in areas subjected to a vertical electric field, whereas ions migration remains negligible in non-gated regions. (as demarcated by the black dashed lines in Fig. 4f). The potential-distance profiles extracted along the yellow and red arrows (Fig. 4f) are shown in Fig. 4g and Supplementary Fig. 26. Initially, the regulated (left part of the black dashed line) and non-regulated (right part of the black dashed line) MoS₂ regions exhibited a minor potential difference (−18.2 mV). At V_ionic = −20 V, the regulated MoS₂ region showed an E_f upshift and potential increase, resulting in a 110.6 mV difference compared to the non-regulated region. Conversely, at V_ionic = +20 V, the potential difference reversed to −84.3 mV, indicating an E_f downshift in the regulated MoS₂ region that exceeded its initial state (Fig. 4g). The Sb₄O₅Cl₂ at MoS₂ edges displayed significant potential inversion (>2 V, Supplementary Fig. 26) under alternating V_ionic, driven by Cl⁻ ions depletion and accumulation. KPFM results reveal the band structure evolution of the Sb₄O₅Cl₂/MoS₂ heterojunction under varying V_ionic (Supplementary Fig. 27), with the E_f shift in MoS₂ in excellent agreement with electrical and theoretical data. This highlights that the reconfigurable transition between quasi-metallic and semiconducting state in MoS₂ is achieved through efficient Cl⁻ ions migration enabled by the unique crystal structure of Sb₄O₅Cl₂.

Neuromorphic image preprocessing and classification

Leveraging the distinct periodic crystal potential field and the tunable Cl⁻ ions migration behavior of Sb₄O₅Cl₂, we emulate ion-mediated conductance modulation analogous to synaptic plasticity. The detailed biological analogy and transport mechanism are provided in Supplementary Information (Supplementary Fig. 28)^56,57. Under positive or negative pulse voltages (V_p), Cl⁻ ions drift directionally within the dielectric, accumulating near or depleting from the MoS₂ interface through [SbO] ring channels, as illustrated in Fig. 5a(i), b(i). This dynamic redistribution modulates the local Cl⁻ concentration, thereby tuning the device conductance (Fig. 5a (ii) and Fig. 5b (ii)). The process mimics the depolarization and hyperpolarization behaviors observed in synaptic systems, offering a physically interpretable framework for neuromorphic signal modulation.

Fig. 5. Demonstration of Sb₄O₅Cl₂ based synaptic functions.

a (i) MoS₂/Sb₄O₅Cl₂/FLG device undergoing Cl⁻ ions migration and depolarization with negative pulse voltage (V_p). (ii) Current variation under negative V_p. b (i) Device undergoing Cl⁻ ions migration and hyperpolarization with positive V_p. (ii) Current variation under positive pulse V_p. c Conductance retention under varying V_p amplitudes. d Linear conductance changes with applied V_p pulses (100 negative pulses at −7 V and 100 positive pulses at +5.5 V). e Device pulse programming stability over 45 cycles at fixed readout voltages (−0.5 V, 0 V, and 0.5 V), demonstrating robustness for artificial synapse applications. f Fashion Modified National Institute of Standard and Technology (Fashion-MNIST) image (with σ = 0.8 random background noise) preprocessing and integration with the (artificial neural network) ANN model system. g Recognition accuracy versus epoch curves before and after preprocessing of Fashion-MNIST images (with σ = 0.8 random background noise).

Figure 5c presents the conductance retention of the device under single V_p modulation at varying amplitudes. Owing to the migration potential barrier of Cl⁻ ions, the device demonstrates robust retention exceeding 300 s. Nonlinear fitting of the conductance states under different V_p from −5 to −9 V reveals a distinct nonlinear correlation between V_p and conductance states, facilitating linear mapping between image pixels and V_p for effective background noise suppression (Supplementary Figs. 29 and 30). Furthermore, single V_p modulation with a nonlinear conductance response was applied for denoising preprocessing on an image of the word ‘HUST’, embedded with random background noise (σ = 0.8). The processed image exhibited significantly enhanced contrast between the letters and background, with the effect sustained for over 300 s (Supplementary Fig. 31). To further explore conductance modulation, a pulse sequence of 100 negative V_p (−7 V) pulses and 100 positive V_p (5.5 V) pulses were applied, each with a duration and interval of 0.2 s (Fig. 5d). This sequence effectively emulated postsynaptic current (PSC) plasticity, exhibiting signal enhancement (depolarization) and suppression (hyperpolarization) characteristics. The observed PSC variations mimic key neurobiological behaviors, specifically pulse-number-dependent long-term potentiation (LTP) and long-term depression (LTD). Figure 5e illustrates the current variation of the device over 45 cycles of 9000-pulse programming, demonstrating excellent robustness at fixed read voltages (−0.5 V, 0 V, and 0.5 V). Moreover, the device performance shows almost no degradation before and after 9000 pulse operations (Supplementary Fig. 32). These results highlight its potential for artificial synapse applications.

By leveraging the nonlinear conductance characteristics of V_p programming control (Supplementary Fig. 29) and the linear LTP/LTD behavior of V_p programming control (Fig. 5d), we developed an integrated preprocessing system coupled with a three-layer artificial neural network (ANN) to recognize the Fashion Modified National Institute of Standard and Technology (Fashion-MNIST) images (Fig. 5f). A detailed description of the system model is presented in Supplementary Fig. 33. To evaluate the device’s potential for background noise suppression, we introduced random background noise (σ = 0.8) to the Fashion-MNIST images, with the preprocessed images (Supplementary Fig. 34a and 34b). The results demonstrate that preprocessing significantly facilitated convergence. After just 5 training epochs, the classification accuracy of preprocessed images reached 90.9%, notably surpassing the 80.7% achieved by unprocessed images (Fig. 5g). The corresponding confusion matrix for classification accuracy is depicted in Supplementary Fig. 35. By 20 epochs, the model attained an impressive classification accuracy of approximately 99.1%.

In summary, we present a single-crystalline ionic dielectric, Sb₄O₅Cl₂, featuring oriented ion-migration channels and high ionic polarization, enabling a high dielectric constant of 23.3 ( ~ 25 nm) and precise carrier modulation. Through van der Waals integration with MoS₂, the fabricated 2D FETs achieve good performance, including a high I_on/I_off (10⁹), low leakage current (10⁻¹⁴ A), and room-temperature mobility of 33.4 cm²V⁻¹ s⁻¹ and realize various logic functions. The material further facilitates controllable and non-volatile transition between quasi-metallic and semiconducting state in MoS₂, demonstrating 100-cycle reversibility, driven by electric-field-modulated Cl⁻ ions accumulation and depletion at the Sb₄O₅Cl₂/MoS₂ interface, which shifts the E_f of MoS₂. Based on this mechanism, memory functionality has been achieved with good retention (more than 3000 s), and endurance (over 1000 cycles). Leveraging these properties, we realized neuromorphic devices with synapse-like functions for integrated image storage, processing, and recognition, achieving 90.9% accuracy after 5 training epochs and sustaining non-volatile preprocessing for over 300 s. Our work provides an ionic dielectric material with precise carrier regulation capabilities for developing functional devices and manufacturing advanced electronics.

Methods

Sample synthesis

Single crystals of bulk Sb₄O₅Cl₂ were synthesized by the chemical vapor transport method. The mixture of Sb₂O₃ powder (Aladdin, 99.9%) and SbCl₃ lump (Aladdin, 99.9%) in a molar ratio of 5:2 were placed into a sealed quartz tube under vacuum 10⁻² Pa. The tube was slowly heated to 823 K on the source zone (T₁), while the sink zone (T₂) was kept at 723 K. After 7 days, the white plate-like crystals were obtained on the sink zone. For more reaction route details see Supplementary Fig. 1. The nanosheets were isolated from bulk crystals by mechanical exfoliation and transferred to a 300 nm SiO₂/Si substrate.

Characterizations

As-grown Sb₄O₅Cl₂ crystals were examined by XRD (D2 phase, Bruker) and XPS (AXIS-ULTRA DLD-600W). The morphology, thickness, and microstructure of the exfoliated Sb₄O₅Cl₂ nanosheets were characterized by optical microscope (BX51, Olympus), atomic force microscopy (AFM, Dimension Icon, Bruker), HAADF-STEM and EDS (Tecnai G2 F30, FEI). Raman spectra were performed by a confocal Raman system (Alpha 300 R, WITec) equipped with a 532 nm laser source. PFM and KPFM measurements were carried out on AFM platform, and different V_ionic were applied using a digital source meter (Keithley 2400).

Device fabrication and measurements

To achieve efficient exfoliation and targeted transfer, a two-step method was adopted (as shown in Supplementary Fig. 2). Firstly, scotch tape was used to mechanically exfoliate the Sb₄O₅Cl₂ and transfer them onto the thermally releasable adhesive. Subsequently, the adhesive was heated to release the nanosheets onto a 300 nm SiO₂/Si substrate. In this way, more and thinner Sb₄O₅Cl₂ nanosheets could be obtained. Then, to enable targeted transfer, polypropylene carbonate (PPC) solution was first spin-coated and dried at 100 ^oC for 5 min. After that, polydimethylsiloxane (PDMS) was used to simultaneously transfer the PPC/Sb₄O₅Cl₂ onto the target 2D material (mechanically exfoliated FLG or MoS₂).

The metal-insulator-metal capacitors were fabricated as follows: exfoliated FLG was used as bottom electrodes on 300 nm SiO₂/Si. The Sb₄O₅Cl₂ nanosheets were transferred onto the bottom electrode using the method described above. The top electrodes were deposited with In/Au (10/40 nm) after electron-beam lithography (EBL). For the back-gate MoS₂/Sb₄O₅Cl₂/FLG FETs, MoS₂, Sb₄O₅Cl₂, and FLG nanosheets were sequentially transferred and stacked on 300 nm SiO₂/Si. Subsequently, after EBL, source-drain electrodes and back-gate electrodes were evaporated onto MoS₂ and FLG. For the dual-gate Sb₄O₅Cl₂/MoS₂/SiO₂ FETs, the fabrication method is the same as back-gate FETs. The electrical measurements were carried out with a semiconductor characterization system (4200SCS, Keithley) and a cryogenic probe station (CRX-6.5, Lake Shore).

Computational methods

We perform the first-principles calculations within the frame of the generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerhof (PBE)⁵⁸ in the Vienna ab initio simulation package (VASP)⁵⁹. The 5p, 5 s were taken as valence states for Sb, 3 s and 3p for Cl, and 2 s, 2p for O atoms. The cut off energy was chosen to 600 eV. The Γ centered k-grids were adopted 6 × 6 × 2. The convergence criterion of energy in relaxation was set to be 10⁻⁵ eV. We fully relaxed the atomic positions until the maximum force on each atom was less than 10⁻² eV/Å. The Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional⁶⁰ was employed for the validation of band structures. The Brillouin zone path of the energy band is Γ-X-S-Y-Γ. In order to compute energy barriers, the CINEB method with 8 intermediate images was used to find the transition pathways⁶¹. The climbing image method allows for more accurate finding of saddle points with fewer images than the regular NEB method. To simulate the vertical Cl⁻ migration path, a 2 × 1 × 1 supercell was constructed along a direction.

Implementation of pattern preprocessing and recognition

To integrate image preprocessing and classification, we employ the Fashion MNIST dataset with σ = 0.8 random background noise and preprocess the images using a voltage-dependent nonlinear conductance function (Supplementary Fig. 30). In forward propagation, the preprocessed images are flattened and transformed via a nonlinear transformation using the ReLU activation function: $h^{\prime }$= max (0, x), yielding the hidden layer output $h^{\prime }$. The hidden layer output is subsequently processed by a Softmax activation function to obtain the final probability distribution: Softmax($y_i^{\prime }$) = $\frac{e^{y_i^{\prime }}}{{\sum }_j{e}^{y_j^{\prime }}}$. The classification error is measured using the cross-entropy loss function: L = $-{\sum }_{i=1}^C{y}_i\log \left(p_i\right)$, where C = 10 denotes the total number of classes, y_i is the true label in one-hot encoding, and p_i is the predicted probability. In backpropagation, the Adam optimizer updates model parameters, following W = W − η$\frac{\partial L}{\partial W}$, where η = 0.001 is the learning rate, and $\frac{\partial L}{\partial W}$ denotes the gradient of the loss function with respect to the weights. Notably, all network weights are mapped to conductance values based on the LTP/LTD characteristics (Supplementary Fig. 33, blue dashed box), ensuring adherence to the physical properties of the device.

Author contributions

X.Z. and T.Z. conceived and designed the experiments. Z.L. synthesized the bulk Sb₄O₅Cl₂ single crystal. Z.L., G.G., X.X., and H.W. performed the AFM, Raman, and XPS characterizations of the samples. Z.L., H.W., and T.L. finished the UPS and absorption spectroscopy measurement. Z.L., H.L., and J.W. performed the HAADF-STEM characterizations and analyzed the results. Z.L., G.G., and H.W. finished device fabrication and electronic measurements. T.L. and. X.X. performed the theoretical calculations and analyzed the results. Z.L., G.G., and T.L. performed the KPFM measurement and analyzed the mechanism. Z.L. and G.G. wrote the paper with inputs from J.W., K.L., L.G., T.Z., and X.Z. All authors participated in discussions and approved the manuscript.

Peer review

Peer review information

Nature Communications thanks Congwei Tan, Michael Waltl, and the other, anonymous, reviewer for their contribution to the peer review of this work. A peer review file is available.

Data availability

Relevant data supporting the key findings of this study are available within the article and the Supplementary Information file. All raw data generated during the current study are available from the corresponding authors upon request.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-69869-0.