Atomic-scale mechanism of anisotropic ion migration in 2D Bi₂O₂Se nanodevices

Abstract

Progress in 2D memristive technologies is increasingly constrained by a limited understanding of how crystallographic anisotropy governs ion migration and resistive switching. Bi₂O₂Se offers a compelling model system in which in-plane and out-of-plane devices display strikingly different electrical behaviors, yet the atomic-scale origins of this disparity remain unknown. Here, we engineer orientation-defined Bi₂O₂Se nanodevices using focused ion beam fabrication coupled with in situ aberration-corrected transmission electron microscopy, enabling simultaneous electrical probing and real-time imaging of structural evolution under bias. Supported by density functional theory (DFT) calculations, we demonstrate that anisotropic migration barriers for O^2- and Se^2- ions give rise to two fundamentally distinct switching pathways. Vertical fields, constrained by strong interlayer electrostatic locking, lead to localized vertical migration and the formation of a reversible, ordered conductive D-Bi₂O₂Se phase, producing abrupt, threshold-type switching. By contrast, lateral fields enable long-range ion diffusion, generating extended Bi/Bi_4+2nSe₃/Bi₂O₂Se heterostructures through a topotactic sequence with continuously evolving Se concentration, yielding smooth and linear conductance modulation. These results establish the microscopic principles that underpin direction-dependent transport and phase transformation in Bi₂O₂Se memristors. By revealing how crystallographic orientation dictates functionality, our work provides a mechanistic foundation for the rational design of directionally engineered 2D neuromorphic and memory systems with enhanced versatility and integration potential.

The development of two-dimensional memristors is limited by unclear links between crystal direction, ion motion, and switching. Qu et al. build orientation controlled Bi₂O₂Se devices and combine real time imaging and theory to reveal distinct direction dependent ion migration and switching pathways.

Introduction

As information technology advances toward the post-Moore era, the demand for faster, smaller, and more energy-efficient electronic devices continues to intensify, while conventional silicon-based semiconductors are approaching their physical scaling limits^1–5. Two-dimensional (2D) semiconductors, with atomically thin structures and excellent electrostatic control, provide a promising route for next-generation nanoelectronics and neuromorphic systems^6–10. Among emerging 2D semiconductors, Bi₂O₂Se has attracted extensive attention because it combines high carrier mobility, environmental stability, and process compatibility within a unique layered crystal framework^11–18. Benefiting from its characteristics, Bi₂O₂Se has been widely explored in field-effect transistors, photodetectors, and particularly memristive devices^19–22. Its alternating positively charged [Bi₂O₂]²⁺ layers and negatively charged [Se]²⁻ layers form a highly anisotropic structure, resulting in distinct in-plane and out-of-plane electronic transport behaviors. Various Bi₂O₂Se-based memristors, including planar, vertical, self-rectifying, twisted-bilayer, and reservoir-array architectures, have demonstrated excellent resistive switching and neuromorphic functionalities, establishing Bi₂O₂Se as a versatile platform for multifunctional 2D electronics and high-density memory integration^23–29.

Ion migration is the fundamental process governing resistive switching behaviors in memristors, and in anisotropic layered materials such as Bi₂O₂Se, the migration process strongly depends on transport direction^30,31. Due to the intrinsic difference between relatively weak interlayer interactions and strong intralayer chemical bonding, ion migration pathways, migration barriers, defect evolution, and associated structural phase transitions along the in-plane and out-of-plane directions are fundamentally distinct^32,33. Consequently, planar and vertical Bi₂O₂Se memristors exhibit markedly different switching mechanisms and device characteristics^34,35. Out-of-plane transport is typically associated with short-range vertical migration of anions, metal ions, or defects through interlayer gaps, favoring filamentary switching with fast response and high integration density³⁵. In contrast, in-plane transport is dominated by lateral diffusion of ions and defects within the crystal lattice, leading to spatially distributed and continuously tunable conductance modulation suitable for neuromorphic computing and artificial synapses³⁴. Although substantial progress has been achieved in device engineering and functional optimization, the microscopic mechanisms underlying anisotropic ion-migration-induced resistive switching in Bi₂O₂Se remain poorly understood. A comprehensive understanding of these direction-dependent ion migration processes is therefore essential for revealing the physical origin of resistive switching and for guiding the rational design of high-performance 2D memristive systems.

However, directly probing anisotropic ion migration in Bi₂O₂Se at the atomic scale remains extraordinarily challenging. Ion migration generally occurs within nanometer or even sub-nanometer regions and proceeds in a highly transient and localized manner, imposing stringent requirements on both the spatial and temporal resolution of characterization techniques^36–39. Moreover, the migration process is often accompanied by intermediate phases, defect reconstruction, interfacial evolution, and lattice distortions, thereby requiring simultaneous atomic-resolution imaging and dynamic-tracking. Therefore, advanced in situ transmission electron microscopy (TEM) techniques capable of combining electrical biasing with atomic-resolution imaging provide a uniquely powerful platform for directly visualizing ion migration dynamics and structural evolution during resistive switching. Achieving such atomic-scale observation is of great scientific significance, as it can not only uncover the physical nature of anisotropic resistive switching in 2D materials, but also establish a critical materials-physics foundation for developing next-generation low-power, high-reliability memristors, self-rectifying memory arrays, and multifunctional neuromorphic devices³⁹.

In this work, we fabricated out-of-plane and in-plane Bi₂O₂Se nanodevices using a combination of focused ion beam (FIB) processing and in situ aberration-corrected scanning/transmission electron microscopy (S/TEM). This approach enabled the real-time, atomic-scale observation of resistive switching dynamics. Our results reveal that these switching behaviors are primarily governed by the distinct energy barriers for anion and cation migration. Specifically, the insulating nature of Bi₂O₂Se along the out-of-plane direction, coupled with the confinement of O^2- and Se^2- ions by interlayer electrostatic interactions, results in a cross-layer migration barrier significantly higher than that of in-plane migration. Under an out-of-plane electric field, the migration of O^2- and Se^2- ions induces the formation of an ordered defective phase (D-Bi₂O₂Se), creating a reversible conducting channel. Conversely, in-plane fields drive the rapid extraction of O^2- ions, facilitating the formation of Bi_4+2nSe₃ (n = 0, 1, 2) structures, and the resulting lateral Se concentration gradient then triggers a lateral topotactic phase transition. These findings underscore the necessity of understanding orientation-dependent transport at the atomic level to enable the rational design of directionally engineered Bi₂O₂Se memristors for multifunctional neuromorphic and high-density memory applications.

Results

Design of Bi₂O₂Se-based nanodevices with controllable orientation under in situ TEM

Bi₂O₂Se films were epitaxially deposited on (001)-oriented Nb-doped SrTiO₃ (NSTO) substrate using pulsed laser deposition (PLD) (Supplementary Fig. 1). Atomic-resolution analysis confirms the film’s quality and intrinsic structure. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image shown in Fig. 1a reveals the intrinsic layered structure of the Bi₂O₂Se thin film. This experimental image corresponds closely to the simulated HAADF image generated using Dr. Probe software⁴⁰ (shown on the right in Fig. 1a). The accompanying atomic structure model (Fig. 1b) illustrates that Bi₂O₂Se consists of alternating positively charged (Bi₂O₂)_n²ⁿ⁺ and negatively charged (Se)_n^2n- layers. This architecture yields a quasi-two-dimensional (2D) configuration, leading directly to inherent anisotropy in its crystal structure and macroscopic properties along both the in-plane and out-of-plane directions.

Fig. 1. Atomic structure, in situ TEM setup, and electrical characterization of Bi₂O₂Se nanodevices under out-of-plane and in-plane electric fields.

a, b Atomic-resolution HAADF-STEM image accompanied by a simulated HAADF-STEM image (a), and atomic structural model (b) of Bi₂O₂Se viewed along [100] zone axis. c Schematic diagram of the experimental setup for in situ TEM observation, illustrating the application of an electric field to the sample via a biasing probe. d, e HAADF-STEM image (d) and corresponding schematic illustration (e) of the Bi₂O₂Se nanodevice under an out-of-plane electric field. f Typical current-voltage (I-V) curve of the vertically configured Bi₂O₂Se nanodevice (out-of-plane geometry). g, h HAADF-STEM image (g) and corresponding schematic illustration (h) of the Bi₂O₂Se nanodevice under an in-plane electric field. i, Typical I-V curve of the laterally configured Bi₂O₂Se nanodevice (in-plane geometry). Scale bars, 1 nm (a); 100 nm (d); 100 nm (g).

To investigate the performance differences and underlying physical mechanisms resulting from this structural anisotropy during device operation, we fabricated two orientations of Bi₂O₂Se-based nanodevices using FIB. We then characterized their resistive switching properties in both the out-of-plane and in-plane directions. This in situ TEM nanodevice design enables the simultaneous dynamic characterization of electrical performance and microstructural evolution, allowing for a real-time correlation between macroscopic device performance and microscopic physical mechanisms (Fig. 1c). As shown in Fig. 1d and g, utilizing FIB’s precise fabrication capabilities, we successively created island-shaped vertical (out-of-plane) and lateral (in-plane) Bi₂O₂Se-based nanodevices, with the corresponding schematic models presented in Fig. 1e and h. In vertical configurations, NSTO is used as the bottom electrode, and a tungsten (W) probe acts as the top electrode, directly contacting the Bi₂O₂Se thin film. Conversely, in the lateral configuration, W is deposited on the left and the W probe functions as the left and right-side electrodes, respectively. By altering the electrode structure and the contact position/orientation between the W probe and the Bi₂O₂Se thin film, electric biasing was applied along both the out-of-plane and in-plane directions, realizing Bi₂O₂Se-based memristors with distinct geometric orientations.

Subsequently, an in situ TEM-STM holder was used to apply electrical bias to the nanodevices in both directions to test the differences in resistive switching performance of the Bi₂O₂Se thin film in the in-plane and out-of-plane directions. Figure 1f and i show the DC current-voltage (I-V) characteristics of the Bi₂O₂Se nanodevices in the out-of-plane and in-plane directions, respectively. The results show that both exhibit typical reversible resistive switching behavior, but their switching modes, conduction characteristics, and hysteresis shapes differ significantly. Overall, the device corresponding to Fig. 1f exhibits an abrupt switching process, characterized by a clear threshold voltage and a rapid current surge, suitable for high-stability digital storage applications; while Fig. 1i shows a more gradual, progressively changing conductance modulation, with a continuous slope change in the I-V curve, exhibiting distributed, linearly adjustable synaptic weight update characteristics, suitable for simulation scenarios such as neuromorphic computing. These fundamental performance disparities stem from differences in ion migration energy barriers, channel dimensions, and the localization of migration paths. To further elucidate the atomic-scale origins of these differences, we monitored the real-time structural evolution of the film under applied electric fields using spherical aberration-corrected TEM.

Resistive switching and reversible phase transition mechanisms in vertical Bi₂O₂Se nanodevices

To clarify the ion-migration-induced phase transition during resistive switching in Bi₂O₂Se nanodevices, simultaneous I-V measurements and real-time TEM imaging were performed (Fig. 2). Figure 2a–c and 2d–g present the reversible structural evolution of a 15-nm-thick vertical Bi₂O₂Se nanodevice during the switching process, with the corresponding I-V characteristics shown in Fig. 2h. Upon application of a + 3.9 V bias to the top surface of the Bi₂O₂Se thin film, anions (O^2-, Se^2-) migrate upward toward the surface. This process leads to the formation of a defective ordered structure (D-Bi₂O₂Se) near the Bi₂O₂Se/Nb: SrTiO₃ interface, as detailed in Fig. 2l-n. Under the applied electric field, oxygen vacancies first migrate and accumulate near the interface, thereby forming preferential ion migration pathways. Subsequently, Se ions migrate upward along these pathways and occupy the oxygen-vacancy sites, resulting in the formation of D-Bi₂O₂Se. Figure 2i-k and 2l-n show the HAADF-STEM images, corresponding crystal structure models, and atomic intensity profiles extracted from the white boxed regions in Fig. 2i and 2l for pristine Bi₂O₂Se and D-Bi₂O₂Se, respectively. The enhanced contrast at the O sites (highlighted by yellow stars in Fig. 2n) provides direct evidence for the occupation of oxygen sites by Se atoms.

Fig. 2. Structural evolution and atomic-scale characterization of Bi₂O₂Se nanodevices during resistive switching in a 15 nm out-of-plane device.

a–c In situ TEM image series recorded from the same sample region, showing reversible phase transitions in an out-of-plane Bi₂O₂Se nanodevice during resistive switching. d–g In situ TEM images showing partially reversible transitions in the D-Bi₂O₂Se structure under high cutoff voltage conditions. h Representative I-V characteristics of the vertically configured (out-of-plane) nanodevice. i–k High-resolution HAADF-STEM image (i), corresponding atomic model (j), and line-scan intensity profile (k) of pristine Bi₂O₂Se. l–n HAADF-STEM image (l), atomic model (m), and line-scan intensity profile (n) of D-Bi₂O₂Se, illustrating periodic modulation structures formed during the cycling process. Scale bars, 10 nm (a–g); 1 nm (i, l).

Upon application of a reverse bias of −5 V, the D-Bi₂O₂Se ordered structure gradually disappears. This recovery process occurs because the reverse electric field drives Se^2- ions from the oxygen-vacancy sites back to their original lattice positions, while oxygen vacancies migrate upward, thereby restoring the pristine Bi₂O₂Se structure. Furthermore, under a larger positive bias, D-Bi₂O₂Se rapidly propagates upward and eventually penetrates the entire sample thickness. Subsequent application of a negative bias induces a partial transformation of D-Bi₂O₂Se back to pristine Bi₂O₂Se, further confirming the reversibility of the ion-migration-induced phase transition during resistive switching.

Atomic-scale ion migration and structural evolution in thick vertical Bi₂O₂Se memristors

Moreover, the thickness of the vertical device was found to play a critical role in the phase transition dynamics under an external electric field. As shown in Supplementary Fig. 2, this is attributed to non-uniform ion migration, which becomes more pronounced in thicker devices. As shown in Fig. 3a, the W probe contacted the Bi₂O₂Se sample (~ 90 nm, Supplementary Fig. 1) at its top surface and a negative bias was applied. As shown in Supplementary Movie 1, a significant phase transition process that occurred under an applied bias voltage is recorded in real-time, with key frames extracted from the movie presented in Fig. 3a-f. The initial Bi₂O₂Se film on the NSTO substrate maintains an intact layered structure, with the surface of Bi₂O₂Se marked by orange dashed lines (Fig. 3a). When the applied voltage reaches the threshold (−2.5 V), a phase transition begins from the surface of the Bi₂O₂Se film (a region of lighter contrast appears on the surface, tentatively referred to as the ‘surface phase’, which is subsequently identified as Bi), and gradually expands downwards (Fig. 3b). When the thickness of the surface phase transition region reaches approximately 10 nm, new phase transitions are triggered on both sides of the surface phase transition region, generating two new phases with different structures. Subsequently, as the voltage is maintained, the surface phase transition region of the Bi₂O₂Se film continues to expand, accompanied by the continuous appearance and disappearance of the new phases on both sides. It seems that the two phases compete with each other, appearing and disappearing repeatedly (Fig. 3b–f), which should be caused by the balance of the horizontal electric field component and the diffusion barrier of anions (Se^2-, O^2-). Finally, the surface phase engulfs the new phases on both sides, and the new phases on both sides completely disappear, leaving only the lighter contrast phase on the surface (Fig. 3f). The fine structural characterization and confirmation of these new phases will be discussed in detail later. Here, we can see that when the Bi₂O₂Se nanodevice operates vertically, ion migration leads to a phase transition and eventually forms a two-phase interface inside the film (marked by the yellow dashed line in Fig. 3a–f). The formation of this interface hinders further phase transitions, thus directly affecting the resistive switching behavior. However, since the electric field applied by the W probe inevitably diffuses within a certain range, although the electric field is mainly applied along the vertical direction, there is still a certain in-plane electric field component. This results in the phase transition process under the action of the electric field, eventually forming a strip-shaped region.

Fig. 3. In situ TEM observation and atomic-scale mechanism of out-of-plane electric-field-driven phase transitions in a 90 nm Bi₂O₂Se nanodevice.

a–f In situ TEM image series acquired from the same sample region (Supplementary Movie 1), showing the evolution of the phase transition as the bias voltage was increased from 0 V to −2.5 V. g, Atomic-scale HAADF-STEM image of the region marked by the white dashed box in Supplementary Fig. 3a, revealing the tri-phase interface of Bi/BiOSe₂/Bi₂O₂Se. h, Schematic diagram illustrating the phase transition mechanism of the BiOSe₂ film along the out-of-plane direction under an external electric field. i-l, Atomic-resolution HAADF-STEM images of the BiOSe₂/Bi interface and the BiOSe₂/Bi₂O₂Se interface, corresponding to the in-plane direction. m, n Magnified HAADF-STEM images from the area in Supplementary Fig. 4c, compared with simulated images based on the BiOSe_1.2 structure (m) and the BiOSe₂ structure with 65% Bi_Se occupancy (n), corresponding to the interface configuration of the tri-phase interface (Bi/BiOSe₂/Bi₂O₂Se) along the out-of-plane direction. Scale bars, 10 nm (a–f); 2 nm (g, i, k); 1 nm (j, l, m, n).

As shown in Fig. 3g-h and Supplementary Fig. 3, we performed atomic-scale characterization of the phase transition region. Figure 3g reveals that the phase boundary exhibits a Bi/BiOSe₂/Bi₂O₂Se sandwich structure, which can be attributed to the migration of O^2- and Se^2- away from the probe tip under a negative electric field. Although the Bi region is not immediately visible in Fig. 3g due to a lack of epitaxial alignment with the BiOSe₂/Bi₂O₂Se layers, tilting the sample along the zone axis confirms that the elemental Bi exists as a perfect single crystal (Supplementary Fig. 3b). Under point voltage source stimulation, the Bi/BiOSe₂/Bi₂O₂Se three-phase interface exhibits a large-area flat structure, with a uniform phase transition depth along the vertical direction, further indicating significant differences between the in-plane and out-of-plane properties of Bi₂O₂Se. The evolution of the original Bi₂O₂Se lattice to the Bi/BiOSe₂/Bi₂O₂Se three-phase structure under the action of an out-of-plane electric field is illustrated in Fig. 3h. Furthermore, in situ biasing experiments on smaller Bi₂O₂Se island films (Supplementary Figs. 4-7 and Fig. 3i-n) demonstrate phase transition behaviors consistent with these observations.

Further analysis of the horizontal and vertical heterointerfaces elucidates the atomic-scale mechanism of the phase transition. On either side of the phase transition region, horizontal heterointerfaces between BiOSe₂/Bi and BiOSe₂/Bi₂O₂Se were observed (Fig. 3i and k). As illustrated in Fig. 3i, the thickness of a single BiOSe₂ layer (approximately 0.955 nm) is about equal to 2.5 Bi layers (0.401 nm). Consequently, each additional BiOSe₂ layer results in a relative vertical increment of roughly half a Bi layer. The transition is initiated by oxygen loss and bismuth reorganization. At the BiOSe₂/ Bi₂O₂Se interface (Fig. 3k), a dislocation-free, coincident-site lattice mechanism gives perfect epitaxy, where two BiOSe₂ layers match three Bi₂O₂Se layers. Specifically, Se diffusion in Bi₂O₂Se causes two neighboring Bi layers (marked by the yellow box) to collapse and form the Bi₂O₂ layer found in BiOSe₂, resulting in a ‘zipper-like’ structural reconfiguration. We further examined the phase transition mechanism along the out-of-plane direction (Fig. 3m-n). Compared to the Se atomic signal intensity in the BiOSe₂ structure in Supplementary Fig. 4b, the Se signal from BiOSe₂ at the three-phase interface within the central phase transition region in Fig. 3m is significantly weaker. Analysis of the intensity ratio between Se and Bi atoms (I_Se/I_Bi) using Dr. Probe software indicated apparent Se vacancies at the BiOSe₂ interface between elemental Bi and the Bi₂O₂Se thin film, forming a BiOSe_1.2-like structure (Fig. 3m and Supplementary Figs. 8 and 10a). This implies that the Se^2- ions diffusion barrier along the in-plane direction is much lower than that along the out-of-plane direction. Furthermore, in the more central region (Fig. 3n), we observed an intermediate phase indicative of the Bi-migration-driven transition from BiOSe₂ to Bi along the out-of-plane direction. The stronger contrast in the yellow circle marked area of Fig. 3n primarily stems from the partial substitution of Se atoms by Bi (Supplementary Figs. 9 and 10b). Additionally, the presence of interstitial Bi atoms immediately above the interface indicates that this process involves the gradual occupation of a Se layer by Bi ions, which forces the displaced Bi ions to migrate upward under the negative electric field. This mechanism is consistent with the phenomena observed in Fig. 3h.

Atomic-scale ion migration and microstructure evolution of lateral Bi₂O₂Se memristors

To further explore the in-plane ion diffusion characteristics of Bi₂O₂Se, the W probe was positioned at the notched edge of the film (Supplementary Fig. 11). This configuration captures the reversible phase transition of Bi₂O₂Se driven by the bidirectional migration of O^2- and Se^2- ions under an external electric field. To examine the resulting new phases and their structural evolution in greater detail, a higher electric field was applied to trigger further phase transitions, as documented in Fig. 4 and Supplementary Movie 2. As can be seen in Fig. 4e-f, under the influence of an in-plane electric field, the original Bi₂O₂Se film transforms into Bi in the region where the W probe contacts the film. With increasing voltage, Se^2- and O^2- ions migrate to the left along the electric field direction, undergoing further phase transition and generating a striped modulated structure phase (Fig. 4g-h). Atomic-scale HAADF images and simulated images confirm that this modulated structure is the Bi₄Se₃ phase, which can be understood as an alternating and regular arrangement of Bi₂Se₃ and Bi layers, as shown in Fig. 4k and l. Under a continuous electric field, the phase transition further extends, ultimately forming a Bi/Bi₄Se₃/Bi₂O₂Se three-phase interface. Atomic-scale HAADF images of the Bi₂O₂Se/Bi and Bi₂O₂Se/Bi₄Se₃ interfaces are shown in Fig. 4i and j. Figure 4e-h shows the inverse Fast Fourier Transform (iFFT) images corresponding to Fig. 4a-d, respectively, highlighting the phase transition and the interface evolution process.

Fig. 4. In-plane electric field-driven phase transition in Bi/Bi₂O₂Se heterostructure observed by in situ TEM.

a–d In situ TEM image series acquired from the same sample region (Supplementary Movie 2) and e–h corresponding inverse Fast Fourier Transform (iFFT) images, showing the evolution of the phase transition as the bias voltage was increased from 0 V to −5.7 V. i, Atomic-resolution HAADF-STEM image of the Bi/Bi₂O₂Se interface. j Atomic-resolution HAADF-STEM image of the Bi₄Se₃/Bi₂O₂Se interface. k–l Atomic-resolution HAADF-STEM image (k) and atomic model (l) of Bi₄Se₃. Scale bars, 20 nm (a-h); 5 nm (i); 2 nm (j); 1 nm (k).

We further investigated the microstructure of the multiphase interfaces following the removal of the electric field (Fig. 5). A long-range phase transition is evident along the in-plane direction, driven by a gradient in Se concentration. Specifically, the region near the W probe consists of elemental Bi, while the area further away exhibits a Bi_4+2nSe₃ (n = 0, 1, 2) structure. The formation of a Se-rich Bi₄Se₃ (n = 0) phase in the immediate vicinity of the Bi₂O₂Se should be caused by Se accumulation induced by the negative electric field. Adjacent to this phase, Bi layers intercalate into the Bi₄Se₃ lattice to form Bi_4+2nSe₃ (n = 1, 2), leading to the decrease in Se concentration. This structure likely results from Se extraction from the Bi₄Se₃ phase, which triggers periodic Bi layer precipitation. As shown in Figs. 5k-m and 5n-p, the number of intercalated Bi layers increases from 2 near the Se-rich phase to 4 further away. This establishes a gradual Se concentration gradient, realizing a topotactic phase transition sequence: from Bi₄Se₃ to Bi₆Se₃, Bi₈Se₃, and finally to elemental Bi. As illustrated in Fig. 5b-p, we further examined the out-of-plane interface between the phase transition region and Bi₂O₂Se at the atomic scale. The results indicate that the crystal structures in the phase transition region are all connected to the Se atoms in Bi₂O₂Se. Figure 5b-d and 5e-g show two typical interfacial configurations between Bi and Bi₂O₂Se: a Bi-deficient interface (Bi-Poor) and a Bi-rich interface (Bi-Rich). In both interface types, the Bi atoms exhibit either regular vacancy ordering or occupation behavior. Furthermore, the out-of-plane interface between the Bi_4+2nSe₃-connected Bi and Bi₂O₂Se also exhibits highly ordered epitaxial growth (Fig. 5h-p). Based on these observations, we find that, in sharp contrast to the very thin out-of-plane transition layer, the lateral migration of ions establishes a relatively long-range Se concentration gradient. This gradient, in turn, facilitates the lateral topotactic phase transition and thus clearly demonstrates the highly anisotropic migration property of Bi₂O₂Se.

Fig. 5. Structural characterization of interfacial configurations after removal of the in-plane electric field.

a iFFT image of Bi₂O₂Se acquired after removing the in-plane bias voltage. b–g Atomic-resolution HAADF-STEM image (b, e), enlarged images extracted from panels b and e (c, f), and the corresponding interfacial atomic structure model (d, g) of the interface between Bi [210] and Bi₂O₂Se. b–d and e–g show Bi-poor and Bi-rich regions, respectively. h–j Atomic-resolution HAADF-STEM image (h), enlarged image extracted from h (i), and the corresponding interfacial atomic structure model (j) of the Bi [100] and Bi₂O₂Se interface. k–m Atomic-resolution HAADF-STEM images of the interface between Bi_4+2nSe₃ and Bi₂O₂Se, and of Bi_4+2nSe₃, together with the corresponding atomic model. n–p Atomic-resolution HAADF-STEM image (n), enlarged image extracted from panel n (o), and interfacial atomic model (p) of the Bi_4+2nSe₃/Bi₂O₂Se interface in a region far from the W probe. Scale bars, 5 nm (a); 3 nm (b, h, k, n); 2 nm (e); 1 nm (c, f, i, l, o).

Discussion

To gain a deeper understanding of ion diffusion and structural transformations, we performed density functional theory (DFT) calculations to elucidate the anisotropic ion migration barriers and the phase transition mechanism. Figure 6 summarizes the ion migration mechanisms and phase-transition processes in Bi₂O₂Se under both in-plane and out-of-plane electric fields by combining the results of first-principles calculations with the in situ TEM observations. As illustrated in the schematic, the distinct electrical behaviors of the in-plane and out-of-plane devices originate primarily from the strongly anisotropic migration barriers of O^2- and Se^2- ions within the layered Bi₂O₂Se structure. For the out-of-plane device, O^2- and Se^2- ions must migrate across the vertically stacked [Bi₂O₂]²⁺ and Se^2- layers, thereby overcoming significant steric constraints and interlayer coulombic interactions. Consequently, the out-of-plane migration barriers are substantially higher than those in the in-plane direction. Specifically, the migration barrier of O^2- is calculated to be 3.295 eV along the out-of-plane direction, compared with only 0.945 eV for in-plane migration (Fig. 6a and f), whereas Se^2- exhibits migration barriers of 3.432 eV and 1.524 eV for the out-of-plane and in-plane directions, respectively (Fig. 6b and g). Owing to the comparable out-of-plane migration barriers of O^2- and Se^2-, oxygen-ion migration is frequently accompanied by selenium-ion migration within the same vertical diffusion channels, leading to the formation of the defect-modulated D-Bi₂O₂Se structure. Furthermore, before extensive precipitation of elemental Bi occurs, these vertical ion migration channels can still be reversibly reconstructed under an opposite electric field, thereby enabling reversible phase transitions and resistive switching behavior, as illustrated in Fig. 6c-e.

Fig. 6. First-principles calculations of ion migration paths and phase transition mechanisms under out-of-plane and in-plane electric fields.

a, b Migration paths and energy barriers of O^2- and Se^2- ions along the out-of-plane direction in Bi₂O₂Se obtained from first-principles calculations. c–e HAADF-STEM images, atomic models, and corresponding simulated images showing the phase transition path from [100] Bi₂O₂Se (c) to [100] D-Bi₂O₂Se (d) and finally to [210] Bi (e) in a vertical device. f, g Migration paths and energy barriers of O^2- and Se^2- ions along the in-plane direction in Bi₂O₂Se obtained from first-principles calculations. h-j, HAADF-STEM images, atomic models, and corresponding simulated images showing the phase transition path from [100] Bi₂O₂Se (h) to [100] Bi_4+2nSe₃ (i) and finally to [100] Bi (j) in a lateral device. Scale bars: 1 nm (c–e) and 1 nm (h–j).

In contrast, for the in-plane Bi₂O₂Se device, O^2- and Se^2- ions mainly migrate laterally along their respective in-plane diffusion channels, resulting in significantly reduced migration barriers. More importantly, the in-plane migration barrier of O^2- is substantially lower than that of Se^2-, enabling the preferential and rapid migration of oxygen ions under the applied electric field. The resulting oxygen depletion promotes the formation of oxygen vacancies and subsequently induces the evolution of Bi-rich selenide phases (Bi_xSe_y) (Fig. 6h and i). With continued electrical biasing, Se^2- ions also begin to migrate, eventually leading to the gradual formation of elemental Bi (Fig. 6j). At the early stage of ion migration, when only limited redistribution of O^2- and Se^2- has occurred, the migrated ions can partially return upon reversing the electric field, allowing the structure to recover to the original Bi₂O₂Se phase. However, under prolonged or high-intensity electrical biasing, extensive elemental Bi precipitation occurs, accompanied by severe depletion of O^2- and Se^2- in the local region. Under these conditions, the phase transition becomes irreversible, and the material can no longer recover its original Bi₂O₂Se structure.

In summary, we developed Bi₂O₂Se nanodevices with well-defined in-plane and out-of-plane orientations using FIB fabrication combined with in situ aberration-corrected TEM. This experimental platform enables simultaneous electrical probing and real-time visualization of structural evolution under applied bias. Our findings reveal that resistive switching in the two directions is fundamentally governed by direction-dependent migration barriers of O^2- and Se^2- ions. Out-of-plane operation, constrained by strong interlayer electrostatic interactions, leads to localized vertical ion migration and the formation of a reversible, ordered conductive D-Bi₂O₂Se phase, enabling resistive switching. In contrast, the in-plane direction supports long-range ion diffusion, producing extended Bi/Bi_4+2nSe₃/Bi₂O₂Se heterostructures via topotactic phase transitions with continuously tunable Se concentration. These distinct pathways give rise to abrupt, threshold-type switching in the vertical configuration and gradual, linear synaptic modulation in the lateral configuration. Our findings uncover the atomic-scale mechanisms responsible for orientation-dependent resistive switching in Bi₂O₂Se and establish how anisotropic ion migration governs phase evolution, interface formation, and device functionality. This work provides a mechanistic foundation for directionally engineered 2D memristors, enabling the rational design of multifunctional neuromorphic systems and high-density memory architectures based on anisotropic 2D semiconductors.

Methods

Thin film preparation

The synthesis of the Bi₂O₂Se film on a 0.5 wt% Nb: SrTiO₃ /SrTiO₃ substrate was achieved by pulsed laser deposition. A commercial Bi₂O₂Se target was ablated using a KrF excimer laser (λ = 248 nm, repetition rate = 10 Hz). Key deposition parameters included a base pressure of 1×10⁻⁶ Torr, a substrate temperature of 450 °C, and an oxygen working pressure of 100 mTorr. A post-deposition cooling stage was implemented at a rate of 0.4 °C/s.

Nanodevices preparation

The in situ TEM specimens were prepared using a focused ion beam scanning electron microscope (Helios G4 UX, Thermo Fisher). The FIB preparation procedure consisted of the following steps: During rough milling, performed at an acceleration voltage of 30 kV, the ion beam current was initially set at 2.5 nA and progressively decreased to thin the cross-section to approximately 200 nm. This was followed by a fine milling stage at a reduced acceleration voltage of 5 kV, which further thinned the specimen to about 50 nm. Finally, an ultralow-voltage milling step at 2 kV was applied to form a comb-like structure and remove the surface damage layer.

In situ TEM experiments

Atomic-scale characterization of the Bi₂O₂Se thin film was conducted on a 300 kV JEM-ARM300 double Cs-corrected TEM (JEOL) equipped with a PicoFemto in situ electrical biasing system (ZepTools). In the experimental setup, the grid of Cu remains grounded, while a W probe is used to directly contact the sample surface to apply an electric field. For Bi₂O₂Se devices with out-of-plane structures, the tip of the W probe is used for contact; whereas for in-plane structured Bi₂O₂Se devices, the side of the W probe is employed for contact. HAADF-STEM images were recorded at collection angles of 90-370 mrad, and image simulations were carried out using Dr. Probe software⁴⁰. EDS mappings were acquired using a JEM-ARM300F STEM equipped with a cold field emission gun and a JEOL dual 158 mm² SDD detector.

DFT calculations

First-principles calculations were conducted using the projector augmented-wave (PAW) method as implemented in the Vienna Ab initio Simulation Package (VASP). Exchange-correlation interactions were described within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional. A 2×1×1 supercell was constructed to minimize spurious periodic interactions between defect images. The plane-wave cutoff energy was set to 500 eV, and a Г-centered 3×6×2 k-point mesh was used for Brillouin zone sampling. The minimum energy paths and transition states for oxygen (O) and selenium (Se) ion migration were determined using the climbing-image nudged elastic band (CI-NEB) method. All structures were fully relaxed until the forces on each atom were less than 0.02 eV/Å, ensuring accurate high-fidelity mapping of the anisotropic migration barriers.

Author contributions

K.Q., Z.Z.Y., C.G.D., and P.G. conceived and designed the research. K.Q., J.W.Z., J.C.C., J.Z., H.N.W., and Z.Z.Y. conducted the in situ TEM experiments and performed data analysis. C.S.Y. and C.G.D. completed the theoretical calculations. W.T.C. and Y.H.C. grew the thin film samples. T.J.X., P.L., and Y.C. conducted the electrical tests on nanodevices. K.Q., J.W.Z., and Z.Z.Y. wrote the manuscript. H.B.Y. provided conceptualization and supervision. All the authors participated in the discussion and interpretation of results.

Peer review

Peer review information

Nature Communications thanks Bilu Liu, Rongjun Zhang and the other anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Funding

This work was supported by the National Key R&D Program of China [grant no. 2022YFA1402902 (C.G.D.)], the National Natural Science Foundation of China [grant no. 52572129 (Z.Z.Y.), and W2412084 (Y.C.)], and the Shanghai Committee of Science and Technology [grant no. 24JD1401200 (Z.Z.Y.)]. P.G. acknowledges the support from the New Cornerstone Science Foundation through the XPLORER PRIZE.

Data availability

All data needed to evaluate the conclusions of this study are present in the Article, its Supplementary Information and Source Data file. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-75431-9.