Fig 1

Superior properties of 2D semiconductor Bi2O2Se. (a) Lattice and electronic band structure of Bi2O2Se. Bi2O2Se has a tetragonal structure (I4/mmm) with alternated positively charged [Bi2O2]n2n+ and negatively charged [Se]n2n− layers along the c-axis. It also exhibits zipper-like structure when exfoliated from bulk, leading to Se and Se-vacancy dimers at the surface. Bi2O2Se and its high-κ native-oxide Bi2SeO5 form atomically smooth interface. Electrically, Bi2O2Se is an indirect semiconductor with a band gap of about 0.8 eV. (b) Comparison of effective mass and band gap of 3D and 2D materials, illustrating that Bi2O2Se has a moderate band gap and small effective mass. The data are from Refs. 6,34−56. (c) Comparison of carrier mobility as a function of channel thickness of various semiconductors, showing Bi2O2Se has high mobility without severe degradation as the thickness shrinks. The data are from Refs. 6,33,35,39,50,57−76."

Fig 2

Systematic design and control of chemical vapor deposition (CVD) synthesis of 2D Bi2O2Se. The Bi-O-Se ternary phase diagram (left) indicates the potential precursors to synthesize Bi2O2Se. The key parameters, such as precursor type, growth temperature and substrate engineering, collectively affect CVD growing process (middle). They can further control the nucleation and quality of the products, resulting in different nucleation modes, which includes in-plane epitaxy of 2D nanoplate and film, vertical epitaxy of 2D fins and out-of-plane free-standing nanostructures (right)."

Fig 3

Controlled CVD synthesis of 2D Bi2O2Se on mica. (a) Schematic illustration of Van der Waals epitaxy between Bi2O2Se and mica 83. Adapted from John Wiley and Sons publisher. (b) Optical image of as-synthesized Bi2O2Se nanoplates on mica. Adapted with permission from Ref. 6, Copyright 2017 Springer Nature publisher. (c) AFM image of monolayer Bi2O2Se nanoplates, suggesting the thickness control of 2D Bi2O2Se can be achieved. Adapted with permission from Ref. 80, Copyright 2017 American Chemical Society publisher. (d) Statistics of growth rate and domain size of 2D Bi2O2Se with different growth temperatures. (e) Typical optical image of large-size single-crystal 2D Bi2O2Se up to 2 mm 22. Adapted from John Wiley and Sons publisher. (f) Schematic illustration of the two-stage reverse-flow-assisted CVD process. Stage I involves the reverse flow from valve A to valve B during the beginning temperature ramping stage. Stage Ⅱ involves the gas flow from C to D, carrying vaporized Bi2O2Se. (g) Typical image of a large-size Bi2O2Se single crystal with 750 μm in length and 8.3 nm thickness. (f–g) Adapted with permission from Ref. 81, Copyright 2021 American Chemical Society publisher."

Fig 4

Wafer-scale CVD synthesis of 2D Bi2O2Se on perovskite oxides. (a) Lattice structure of Bi2O2Se and perovskite oxides. (b) Schematic illustration of wafer-scale synthesis of 2D Bi2O2Se via Van der Waals epitaxy between Bi2O2Se and perovskite oxides with excellent lattice and symmetry matching, including (ⅰ) oriented nucleation, (ⅱ) boundary-free growth. (c) SEM images of Bi2O2Se crystals with different growth time of ~5, ~10 and ~20 min, respectively, showing the evolution of Bi2O2Se from single-oriented domains to uniform continuous single-crystal thin films. (d) Photograph of wafer-scale Bi2O2Se thin films grown on several perovskite oxide substrates (2-inch (La, Sr) (Al, Ta)O3, 1-inch LaAlO3, 1-inch SrTiO3, and 1.5 cm × 1.5 cm SrTiO3) with controlled thickness from few layers to 30 nm. (e) Atomic-resolution cross-sectional STEM-HAADF image (left) and corresponding lattice illustration (right) of Bi2O2Se/ SrTiO3 interface, showing perfect epitaxy and single crystallinity. (f) Fast Fourier transformation (FFT) image of e, showing diffraction patterns of Bi2O2Se and SrTiO3. (a–f) Adapted with permission from Ref. 23, Copyright 2019 American Chemical Society publisher."

Fig 5

Controlled CVD synthesis of vertical Bi2O2Se fins. (a) Schematic illustration of two approaches to synthesizing vertical Bi2O2Se fins: (ⅰ) seed-induced vertical growth, (ⅱ) edge-bonding epitaxial growth. Subsequently, site-specific vertical growth can be designed to achieve wafer-scale aligned 2D fin arrays. (b) SEM image of seed-induced vertical growth of Bi2O2Se fins on mica 83. Inset: schematic of 2D fin on mica. Adapted from John Wiley and Sons publisher. (c) Tilted SEM image of edge-bonding epitaxial growth of Bi2O2Se fins on LaAlO3 (100) surface. Inset: schematic of two perpendicular orientations of 2D fins on 4-fold symmetry LaAlO3 (100) surface. (d) Tilted SEM image of edge-bonding epitaxial growth of unidirectionally aligned Bi2O2Se fins on MgO (110) surface. Inset: schematic of single orientation of 2D fins on 2-fold symmetry MgO (110) surface. (e) Optical image of the vertical 2D fin arrays wafer. (f) Typical SEM images of aligned vertical 2D Bi2O2Se fin arrays on the 1-inch MgO (110) wafer (1.8 cm × 1.8 cm). (g) Orientation distribution of vertical 2D Bi2O2Se fin arrays on the wafer. Inset: schematic for the 2D Bi2O2Se fin orientation on epitaxy substrate. (c–g) Adapted with permission from Ref. 7, Copyright 2023 Springer Nature publisher."

Fig 6

Dimensionality control of Bi2O2Se via CVD synthesis. (a) Schematic illustration of dimension control of Bi2O2Se: (ⅰ) square nanoplates via isotropic growth; (ⅱ) 1D nanoribbons via anisotropic growth. Adapted from Acta Physico-Chimica Sinica publisher. (b) Dimensional phase diagram of Bi2O2Se, determined by Bi2O3/Bi2Se3 ratio and growth temperature collectively 77. Inset, typical optical images of square shape Bi2O2Se and Bi2O2Se nanoribbons. Adapted from John Wiley and Sons publisher. (c, d) Typical SEM images of 1D nanoribbons via epitaxial growth (c) and vapor-liquid-solid (VLS) growth (d), respectively. (c) Adapted with permission from Ref. 84, Copyright 2019 AIP publishing. (d) Adapted with permission from Ref. 85, Copyright 2019 American Physical Society publisher. (e) Length and width control of 1D Bi2O2Se nanoribbons. Adapted with permission from Ref. 86, Copyright 2020 Acta Physico-Chimica Sinica publisher. (f, g) Thickness control of 1D Bi2O2Se nanoribbons, showing both monolayer (f) and multilayer (g) nanoribbons can be synthesized 77. Adapted from John Wiley and Sons publisher."

Fig 7

Approaches for preparing ordered 2D Bi2O2Se arrays. (a, b) Illustration of selective-area chemical etching to prepare ordered 2D Bi2O2Se crystal arrays (a) and the optical microscopic (OM) image of corresponding result (b) 88. (b) Adapted from John Wiley and Sons publisher. (c, d) Illustration of selective-area growth for the preparation of ordered 2D Bi2O2Se crystal arrays via pretreatment of substrate (c) and the optical microscopic (OM) image of corresponding result (d) 80. (d) Adapted from American Chemical Society publisher. (e, f) Illustration of site-controlled growth for the preparation of ordered 2D Bi2O2Se fin arrays via pre-patterned nucleation sites (e) and the scanning electron microscopy (SEM) image of corresponding result (f) 7. (f) Adapted from Springer Nature publisher."

Fig 8

Defect control and self-modulation doping effect of 2D Bi2O2Se via CVD synthesis. (a) Illustration of five possible defects in Bi2O2Se shown by atomic structure. (b) Calculated band structure of 2D Bi2O2Se with VSe defects. Fermi level is at 0 and the red dots show the anticipated bands on the nearest Bi and Se atoms close to the VSe defects. (a, b) Adapted with permission from Ref. 89, Copyright 2018 American Physical Society publisher. (c) Defect transition levels for Bi2O2Se. The blue areas represent conduction and valence bands. The solid and hollow circles show donor and acceptor defects, respectively. Adapted with permission from Ref. 90, Copyright 2018 AIP Publishing. (d) The relation between formation energy for defects in Bi2O2Se and different Se-richness conditions 89. (e) Statistics for Hall mobility at 2 K and residual resistance ratios of Bi2O2Se obtained under Se-rich and Se-poor conditions 89. The plot shows that Bi2O2Se synthesized under Se-poor condition has much higher mobility and residual resistance ratio than under Se-rich condition. (d, e) Adapted from American Physical Society publisher. (f) Comparison of carrier concentration and the corresponding mobility of Bi2O2Se via different growth methods (precursors) 79,81."

Fig 9

Improved electrical properties of 2D Bi2O2Se via strain-free out-of-plane CVD synthesis. (a, b) Schematic illustration (a) and AFM image (b) of internal strain of in-plane 2D Bi2O2Se nanoplate caused by wrinkles during cooling process after CVD growth. (c, d) Schematic illustration (c) and SEM image (d) of strain-free 2D Bi2O2Se crystals obtained by free-standing CVD synthesis with a self-catalyzed VLS mechanism. (a–c) Adapted with permission from Ref. 51, Copyright 2022 American Chemical Society publisher. (d) Adapted with permission from Ref. 91, Copyright 2020 American Chemical Society publisher. (e) Statistics and contrast of Hall mobility at 2 K and residual resistance ratio of free-standing strain-free Bi2O2Se crystals and previously reported in-plane strained Bi2O2Se nanoplates 51,92. (f) Statistics and contrast of field-effect mobility of free-standing Bi2O2Se crystals 7,51,86,91 and previously reported in-plane strained Bi2O2Se nanoplates 6,8,11,22,23,77,81,93–97."