Atomic-Scale Visualization of Polar Domain Boundaries in Ferroelectric In2Se3 at the Monolayer Limit

Femtosecond Laser Manipulation of Multistage Phase Switching in Two-Dimensional In2Se3 Visualized via an In Situ Transmission Electron Microscope

Phase transitions critically determine material properties for applications, making them central to material science. The two-dimensional (2D) van der Waals material In2Se3 has been extensively studied as a model system for multiphase switching due to its intricate phase transition behaviors and outstanding ferroelectric properties for device applications. However, the lack of an efficient method for precise phase control and the poorly defined conditions for multiphase transitions have severely hindered its practical use. Here, we report that the femtosecond (fs) laser can serve as a potent tool for fast and precisely manipulating multiphase transitions in In2Se3 thin flakes. Using a transmission electron microscope capable of in situ fs laser irradiation, we realize controllable fast phase switching between four phases of 2D In2Se3 by controlling the laser fluence, including the transition from the ferroelectric α phase to the antiferroelectric β' or paraelectric β phase, reversible switching between antiferroelectric β' and paraelectric β phases at room temperature, as well as reversible transformation between the ferroelectric α' phase and antiferroelectric β' or paraelectric β phase at liquid nitrogen temperature. Notably, these multiphase transitions are accompanied by rapid formation and annihilation of domain structures and superlattices, resulting in fast changes in electric conductivity. Our first-principles calculations verify the multiphase transition pathways and reveal that the conductivity change stems from electronic band structure variation among the different phases. This work systematically investigates the phase transition behaviors in In2Se3 through spatially and temporally resolved characterization methods, providing foundational insights into memory device optimization.

Atomic-scale manipulation of polar domain boundaries in monolayer ferroelectric In2Se3

Domain boundaries have been intensively investigated in bulk ferroelectric materials and two-dimensional materials. Many methods such as electrical, mechanical and optical approaches have been utilized to probe and manipulate domain boundaries. So far most research focuses on the initial and final states of domain boundaries before and after manipulation, while the microscopic understanding of the evolution of domain boundaries remains elusive. In this paper, we report controllable manipulation of the domain boundaries in two-dimensional ferroelectric In2Se3 with atomic precision using scanning tunneling microscopy. We show that the movements of the domain boundaries can be driven by the electric field from a scanning tunneling microscope tip and proceed by the collective shifting of atoms at the domain boundaries. Our density functional theory calculations reveal the energy path and evolution of the domain boundary movement. The results provide deep insight into domain boundaries in two-dimensional ferroelectric materials and will inspire inventive applications of these materials.

Quantum Confinement and Electronic Structure at the Surface of van der Waals Ferroelectric α-In2Se3

Two-dimensional (2D) ferroelectric (FE) materials are promising compounds for next-generation nonvolatile memories due to their low energy consumption and high endurance. Among them, α-In2Se3 has drawn particular attention due to its in- and out-of-plane ferroelectricity, whose robustness has been demonstrated down to the monolayer limit. This is a relatively uncommon behavior since most bulk FE materials lose their ferroelectric character at the 2D limit due to the depolarization field. Using angle resolved photoemission spectroscopy (ARPES), we unveil another unusual 2D phenomenon appearing in 2H α-In2Se3 single crystals, the occurrence of a highly metallic two-dimensional electron gas (2DEG) at the surface of vacuum-cleaved crystals. This 2DEG exhibits two confined states, which correspond to an electron density of approximately 1013 electrons/cm2, also confirmed by thermoelectric measurements. Combination of ARPES and density functional theory (DFT) calculations reveals a direct band gap of energy equal to 1.3 ± 0.1 eV, with the bottom of the conduction band localized at the center of the Brillouin zone, just below the Fermi level. Such strong n-type doping further supports the quantum confinement of electrons and the formation of the 2DEG.

Continuously tunable ferroelectric domain width down to the single-atomic limit in bismuth tellurite

Emerging functionalities in two-dimensional materials, such as ferromagnetism, superconductivity and ferroelectricity, open new avenues for promising nanoelectronic applications. Here, we report the discovery of intrinsic in-plane room-temperature ferroelectricity in two-dimensional Bi2TeO5 grown by chemical vapor deposition, where spontaneous polarization originates from Bi column displacements. We found an intercalated buffer layer consist of mixed Bi/Te column as 180° domain wall which enables facile polarized domain engineering, including continuously tunable domain width by pinning different concentration of buffer layers, and even ferroelectric-antiferroelectric phase transition when the polarization unit is pinned down to single atomic column. More interestingly, the intercalated Bi/Te buffer layer can interconvert to polarized Bi columns which end up with series terraced domain walls and unusual fan-shaped ferroelectric domain. The buffer layer induced size and shape tunable ferroelectric domain in two-dimensional Bi2TeO5 offer insights into the manipulation of functionalities in van der Waals materials for future nanoelectronics.