In the post-Moore era, traditional silicon-based semiconductors are gradually approaching their physical scaling limits, and the search for high-speed, low-power next-generation electronic devices has become a core focus of cutting-edge research. Two-dimensional semiconductors, with their atomic-scale thickness and superior electrostatic control capabilities, have opened new avenues for nanoelectronics and neuromorphic systems. Among them, bismuth oxyselenide (Bi₂O₂Se) combines high carrier mobility with excellent environmental stability. Its crystal structure consists of alternating stacks of positively charged Bi₂O₂ layers and negatively charged Se layers. This unique layered anisotropic structure gives rise to distinctly different electronic transport and resistive switching properties in the in-plane and out-of-plane directions, showing broad application prospects in neuromorphic computing and high-density memory devices.

However, this field still faces the serious challenge that the microscopic mechanisms are difficult to accurately reveal at the atomic scale. Ion migration is the key process driving resistive switching behavior, but how crystal anisotropy regulates ion migration pathways, energy barriers, and phase transition mechanisms remains unclear. Since ion migration typically occurs in nanometer or even sub-nanometer regions, and the process is highly transient and localized, accompanied by complex defect reconstruction and intermediate phase evolution, this places extremely high demands on the spatiotemporal resolution of characterization techniques. How to achieve atomic-precision real-time imaging and electrical measurements under in-situ bias voltage, thereby revealing the essence of anisotropic ion migration, is currently a key bottleneck constraining the rational design of direction-engineered two-dimensional resistive switching devices.

To address the above issues, a team composed of East China Normal University, Peking University, and others conducted systematic research using ZEPTOOLS' in-situ TEM measurement system. By combining in-situ electrical biasing with aberration-corrected transmission electron microscopy real-time imaging, supplemented by first-principles calculations, the team revealed for the first time at the atomic scale the microscopic mechanism of anisotropic ion migration-driven resistive switching and phase transitions in two-dimensional Bi₂O₂Se nanodevices, laying a solid foundation for the rational design of direction-engineered two-dimensional neuromorphic devices and high-density memories.

Title:Atomic-scale mechanism of anisotropic ion migration in 2D Bi₂O₂Se nanodevices
Journal:Nature Communications
URL:https://doi.org/10.1038/s41467-026-75431-9

In terms of device fabrication and construction of the in-situ observation architecture, the research team utilized focused ion beam micro-nano machining technology to precisely fabricate vertical (out-of-plane) and planar (in-plane) Bi₂O₂Se nanodevices with specific crystal orientations, establishing a well-defined anisotropic electrical testing model. To capture in real time the dynamic evolution of devices under electrical stimulation at the atomic scale, the team employed ZEPTOOLS' transmission electron microscope in-situ sample holder, which established stable electrical contact between probes and the nanodevices. This in-situ sample holder precisely applied DC bias voltage inside the transmission electron microscope and recorded current-voltage characteristics in real time, simultaneously achieving high-resolution transmission electron microscopy imaging and dynamic electrical signal monitoring. The stable voltage application and high-sensitivity electrical measurement capabilities of ZEPTOOLS' in-situ sample holder successfully established a direct causal link between abrupt changes or continuous modulation of macroscopic electrical properties and real-time atomic-scale structural phase transitions.

Under the vertical (out-of-plane) electric field, the research team revealed the microscopic mechanism of steep threshold resistive switching characteristics and the associated localized phase transition process. When a forward bias was applied to the vertical Bi₂O₂Se device, oxygen ions and selenium ions migrated upward under the electric field, forming an ordered defect phase D-Bi₂O₂Se near the interface. High-resolution imaging and atomic intensity analysis showed that selenium ions occupied oxygen vacancy sites, constructing reversible conductive channels. When a reverse bias was applied, selenium ions returned to their original positions, causing the defect phase to gradually recede and restore the original crystal structure. In thicker devices, anion migration-induced phase transitions also formed a Bi/BiOSe₂/Bi₂O₂Se triple-phase sandwich interface, exhibiting a zipper-like structural reconstruction. This highly localized ion migration pathway endows vertical devices with high stability and a steep switching ratio, making them highly suitable for high-density digital memory applications.

Under the in-plane electric field, the research team elucidated the microscopic mechanism of continuous conductance modulation characteristics and the associated topotactic phase transition sequence. When an in-plane bias was applied, because the in-plane migration energy barrier for oxygen ions was significantly lower than that for selenium ions, oxygen ions were preferentially extracted rapidly, forming oxygen vacancies, which subsequently triggered long-range selenium ion and oxygen ion diffusion along the electric field direction. As the electric field continued to act, the material evolved along the in-plane direction into a Bi/Bi₄₊₂ₙSe₃/Bi₂O₂Se multi-phase heterojunction, where the Bi₄₊₂ₙSe₃ phases (n=0, 1, 2) established a continuously varying selenium concentration gradient through periodic insertion of different numbers of bismuth layers. This gradual topotactic phase transition avoided abrupt current jumps and achieved continuous linear modulation of conductance, endowing planar devices with ideal characteristics for analog synaptic weight updates, suitable for neuromorphic computing applications.

Combining first-principles calculations with in-situ TEM observations, the research team further summarized from a physical perspective the general rules by which crystal anisotropy regulates ion migration energy barriers and resistive switching modes. Density functional theory calculations showed that due to interlayer electrostatic locking and steric hindrance effects, the out-of-plane migration energy barriers for oxygen ions and selenium ions were as high as 3.295 eV and 3.432 eV, respectively, while the in-plane migration energy barriers were reduced to 0.945 eV and 1.524 eV, respectively. The high out-of-plane energy barriers forced anions to migrate cooperatively only along localized vertical channels, forming a reversible D-Bi₂O₂Se phase, which determined the steep digital switching behavior of the devices. The low in-plane energy barriers and the significant difference between ion energy barriers facilitated preferential oxygen ion extraction and long-range topotactic phase transitions, determining the continuous analog synaptic behavior of the devices. This study reveals orientation-dependent ion migration and phase transition rules, providing a solid physical foundation for the rational design of multifunctional neuromorphic systems and high-density memory architectures based on two-dimensional anisotropic semiconductors.

ZEPTOOLS, as a Chinese high-end precision instrument company, is a first-class supplier of in-situ transmission electron microscopy characterization solutions. Its PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for significant research results from both domestic and international users. The following image shows the ZEPTOOLS in-situ TEM sample holder product used in this research: