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

Abstract

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.

Fig. 1. CVD growth of layered 2D Bi₂TeO₅ single crystals with ferroelectricity.

a Optical image of Bi₂TeO₅ single crystals. b, c AFM topography and corresponding lateral PFM images of single Bi₂TeO₅ flake. Height profile in b shows a step of 1.2 nm, consistent to the thickness of monolayer Bi₂TeO₅. d Schematic of Bi₂TeO₅ crystal structure along a axis. e Top oblique view of layered Bi₂TeO₅ crystal structure. The blue pyramids correspond to the BiO₅ cages. The blue arrows represent the polarization direction of BiO₅ cages. The sandwiched Bi-O-Bi sublayers are not shown here for better presentation. f Schematic of Bi₂TeO₅ crystal structure along c axis. An enlarged image in green rectangle shows the calculated Bi displacements (D_Bi) and lattice rotation angle (θ). g Atomically resolved HAADF-STEM image of Bi₂TeO₅ along c axis. Lower left inset: Simulated STEM image of Bi₂TeO₅. Oxygen columns are invisible due to their weak scattering. h Extracted D_Bi and θ distribution in g. The error bars correspond to the standard deviation of D_Bi and θ. i Superposition of Bi³⁺ displacement vectors with g. For simplicity, only the large displacements of Bi³⁺ sites (A-rows) are depicted in the image.

Fig. 2. Intercalated buffer layer as 180° domain wall in Bi₂TeO₅ single crystal.

a Lateral PFM images showing the striped FE domain in Bi₂TeO₅. b Atomically resolved HAADF-STEM image of typical 180° domain wall. The purple and orange circles denote Bi³⁺ and Te⁴⁺ atoms, respectively. Each row is labelled as either A (Bi column) or B (mixed Bi/Te column) row accordingly. Note that additional B row is intercalated (labelled in red and highlighted by white dashed rectangle) and act as a buffer layer in the switch of polarization. c Extracted D_Bi and θ in b. The error bars correspond to the standard deviation of D_Bi and θ. d Superposition of Bi³⁺ displacement vectors with b. e Colored ion displacements mapping in d. f Schematic structure of 180° domain wall. The blue and orange pyramids correspond to the opposite displacements of BiO₅ cages. The buffered B-row is marked by the black dotted square.

Fig. 3. Buffered B-rows controlled domain width in Bi₂TeO₅.

a–d Atomically resolved HAADF-STEM images of Bi₂TeO₅ with different domain widths. e–h Calculated Bi³⁺ displacements color map of a–d. The up and down polarizations are marked with red and blue areas, respectively.

Fig. 4. Antiferroelectric transition induced by the maximum limit of intercalated buffer domain wall.

a Lateral PFM images of bismuth tellurite with antiferroelectricity. Upper left inset is the corresponding topography image. b Atomically resolved HAADF-STEM image of the AFE phase along c axis. Yellow arrows denote the Bi³⁺ displacement vectors for each site. The purple and orange circles denote Bi³⁺ and Te⁴⁺ atoms, respectively. Each row is labelled as A and B accordingly. The intercalated buffered B-row is labelled in red and denoted by white dotted lines. Lower right inset: Simulated STEM annular dark field image of bismuth tellurite with antiferroelectricity. c Extracted Bi³⁺ displacements color map of b. d Extracted D_Bi and θ in b. The error bars correspond to the standard deviation of D_Bi and θ. e DFT calculated structure of the AFE phase. The buffered B-rows are labelled with black dotted lines.

Fig. 5. Kinked buffer layer induced terraced domain wall in Bi₂TeO₅.

a, b Lateral PFM images at perpendicular cantilever angle showing fan-shaped domains in Bi₂TeO₅. c Zoom-in HAADF-STEM image of the black square in b. d, e Atomically resolved HAADF-STEM image and corresponding polarization distribution at the kink. f DFT calculated structure of the kink at 180° domain wall. The blue and orange areas represent the FE domains with opposite polarization directions. The positions of the 180° domain walls are marked with green areas. g Te and O concentration dependent formation enthalpy of the kinks at 180° domain wall.