Fig. 1. CVD synthesis method and mechanism of Bi₂O₂Se^{[31-33, 67-68]}. (a) Schematic of CVD growth^[31-32]. Copyright © 2021, AIP Publishing. (b) Optical image of Bi₂O₂Se nanosheets grown at 550 °C for 30 min. (c) Average domain size of synthesized 2D Bi₂O₂Se crystals as a function of growth temperature. Copyright © 2017, American Chemical Society^[32]. (d) Schematic of the c-MOCVD growth. (e) Crystal structure of Bi₂O₂Se in the [100] direction (the z-axis direction of Bi₂O₂Se) observed by atomic-resolution ABF-STEM (left) and HAADFSTEM (right). Copyright © 2021 American Chemical Society^[67]. (f) Schematic of the RF CVD growth^[33]. (g) The relationship between the average size of the samples and the RF holding time. The dotted line is the fitted curve. Copyright © 2021, American Chemical Society

Fig. 2. Fabrication of Bi₂O₂Se by wet-chemical process^[34-35]. (a) Growth mechanism of ultrathin Bi₂O₂Se nanosheets in LiNO₃ solution. (b) Morphology of Bi₂O₂Se nanosheets obtained with different LiNO₃ masses (ⅰ–0g, ⅱ–3g, ⅲ–6g, ⅳ–10g). Copyright © 2020, Royal Society of Chemistry. (c) Schematic of the two-step synthesis of Bi₂O₂Se nanosheets. (d) The morphology of Bi₂O₂Se nanosheets obtained by adjusting the solvent types in solution 1 (sample 1—water, sample 2—HNO₃ solution, sample 3—mannitol solution, sample 4—ethylene glycol and water mixture). Copyright © 2021, Elsevier

Fig. 3. (a) MBE fabrication and mechanism of Bi₂O₂Se. Bi source and Se source co-evaporated in the dilute oxygen atmosphere. Copyright © 2021, American Chemical Society. (b) AFM image of single-layer Bi₂O₂Se nanosheet. (c) AFM image for thickness measurement results of single-layer Bi₂O₂Se nanosheet. Copyright © 2019, John Wiley and Sons. (d) Schematic of PLD method preparation. Copyright © 2021, AIP Publishing. (e) A diagram of the substrate temperature T_s dependent phase and out-of-plane orientation for PLD-grown Bi₂O₂Se films on STO. (f) Dependence of Bi₂O₂Se film thickness on deposition duration. (g-k) Schematic of Bi₂O₂Se films grown on STO substrates by PLD method. Copyright © 2021, IOP Publishing^{[37, 38, 73, 74]}

Fig. 4. (a) Body-center tetragonal crystal structure of Bi₂O₂Se, consisting of alternating Bi₂O₂ and Se layers. (b) Cross-section examination of the Bi₂O₂Se/STO film deposited at T_s = 425 °C by low-magnification TEM and HAADF-STEM characterizations. Copyright © 2020, IOP Publishing. (c) HRTEM image and SAED image of Bi₂O₂Se. Copyright © 2021, John Wiley and Sons. (d) (i) Calculated band gaps as a function of the number of layers for ultrathin Bi₂O₂Se films. Direct band gap (Γ-Γ) (circles) and indirect band gap (X-Γ) (triangles) and experimental optical band gap (Expt.) (Crosses). (ii) Energy of band edge with respect to the vacuum level for the highest valence band at VB-Γ (Diamonds) and at VB-X (Stars), and lowest conduction band at CB-Γ (Squares). Orbital-projected band structures for (e) monolayer, (f) bilayer, (g) tri-layer, (h) bulk Bi₂O₂Se. Band weights of Bi 6p, Se 4p and O 2p orbitals are sized by symbols. Valence band maximum (VBM) is set as 0 eV, as marked by the horizontal dashed lines. Copyright © 2021, Elsevier.^{[25, 38-39, 77]}(a) Bi₂O₂Se的四方晶体结构，由交替的 层和 层组成。(b) 在T_s = 425 °C下沉积的Bi₂O₂Se/STO纳米片的横截面低倍和高倍分辨透射电镜图。(c) Bi₂O₂Se的HRTEM图像和SAED图。(d) (i) 计算得到的带隙与超薄Bi₂O₂Se纳米片层数的函数关系。直接带隙(圆形)、间接带隙(三角形)和实验光学带隙(十字)。(ii) VB-Γ(菱形)和VB-X(五角星)处最高价带以及CB-Γ(正方形)处最低导带的能级。(e) 单层、(f) 双层、(g) 三层、(h) 块体Bi₂O₂Se的能带结构。VBM设置为0 eV，由水平虚线标记。^{[25, 38-39, 77]}

Fig. 5. Optical properties of Bi₂O₂Se^{[31, 40, 85]}. (a) OM images of CVD-grown atomically thin Bi₂O₂Se crystals with different number of layers (from 1L to 6L) on mica with the reflectance mode (top) and transmittance mode (bottom). (b) Micro-optical measurements for Bi₂O₂Se nanosheets with different various layers in (a). Copyright © 2017, American Chemical Society. (c) UV-Vis-NIR diffuse reflectance spectra of Bi_2-xCe_xO₂Se (0 ≤ x ≤ 0.15). Copyright © 2021, Elsevier. (d) Bi₂O₂Se of different thickness on mica (from 1 to 7). (e) Measurement of the optical contrast of Bi₂O₂Se nanosheets with different thicknesses in (d). (f) The relationship between the contrast value and thickness at 550 nm in (e)

Fig. 6. Raman spectrum of Bi₂O₂Se flake^{[32, 41, 43]}. (a) The A_1g vibration mode of Bi₂O₂Se. Copyright © 2019, AIP Publishing. (b) Raman spectra of Bi₂O₂Se flakes grown on quartz and mica substrates showing the characteristic A_1g Raman mode. Copyright © 2021, AIP Publishing. (c)~(d) Individual Raman spectra of the center and edges of the polygons: (c) 532 nm excitation mode. (d) 785 nm excitation mode. Copyright © 2022, American Chemical Society^[43]

Fig. 7. Polarized Raman spectra of Bi₂O₂Se (532 nm laser)^[41-42]. (a) Evolutions of Raman shift with uniaxial strain for bulk Bi₂O₂Se. (b, c) Raman frequencies changes with rotated uniaxial strain for degenerate (b) and (c) modes in bulk Bi₂O₂Se. (d) Optical microscope images of polarization-dependent Raman spectra collected at the center and the edge of the square sample and at the inner horizontal line defect of the right-angled triangle sample. Copyright © 2018, American Chemical Society. (e-f) Polar figures of the Raman intensity of (e) the square edge and (f) triangle defect corresponding to E_u modes located at ~ 55 cm⁻¹. (g-i) Polar of the Raman intensity: (g) the square center, (h) the square edge and (i) triangle defect at the A_1g mode (~160 cm⁻¹). Copyright © 2022, American Chemical Society Bi₂O₂Se的偏振拉曼光谱(532 nm激光)^[41-42]。(a) 块状Bi₂O₂Se拉曼峰位偏移与单轴应变的关系；(b) ，和(c) 两种简并模式的拉曼峰位偏移的变化速率与旋转单轴应变的关系；(d) 样品的光学显微镜图像；(e) 和(f) 分别是正方形边缘和直角三角形的内部水平线缺陷处位于~55 cm⁻¹的E_u模式拉曼强度的极坐标。(g-i) 分别是正方形中心、正方形边缘和直角三角形的内部水平线缺陷处位于~160 cm⁻¹的A_1g模式拉曼强度的极坐标^[41-42]

Fig. 8. Thermal effect and thermal conductivity of Bi₂O₂Se^{[32, 43]}. (a) Low-temperature Raman spectra and (b) corresponding Raman shift as a function of temperature in Bi₂O₂Se flakes. (c) Variation of FWHM with measurement temperature for the characteristic A_1g mode in the semiconducting Bi₂O₂Se nano-flakes. (d) Variation of Raman intensity of the A_1g mode as a function of measurement temperature. Copyright © 2021, AIP Publishing. (e) Schematic of a focused laser beam used as a steady-state heat source to heat a sample suspended on a Cu grid in a photothermal Raman measurement. (f) 3D numerical modeling of the heat conduction at the steady state. Temperature and Raman intensity distribution in the space domain are shown. (g) Power dependence of the A_1g peak frequencies of Bi₂O₂Se flakes with different thicknesses. (h) The average temperature rise of the ~ 8 nm Bi₂O₂Se flake with increasing thermal conductivity at the steady state. Copyright © 2019, AIP Publishing

Fig. 9. Ultrafast spectra of Bi₂O₂Se^{[44, 45, 115]}. (a) Schematics of non-degenerate ultrafast pump-probe experiment. (b) The measured ΔT/T₀ (black symbols) and the corresponding fits (red curves). The inset shows the semi-logarithmic plots with red dotted lines to present two different decay channels. Copyright © 2021, Elsevier. (c) Differential reflection signal of the 13 nm Bi₂O₂Se nanoplate as a function of both the probe delay and the probe position. (d) The squared width of the spatial profiles obtained by Gaussian fits as a function of the probe delay. The linear fit, shown as the red line, gives a diffusion coefficient of about 4.8 cm²s⁻¹. (e) Differential reflection signal of the monolayer Bi₂O₂Se sample as a function of both the probe delay and the probe position. (f) The squared width of the spatial profiles obtained by Gaussian fits as a function of the probe delay. The linear fit shown as the red line gives a diffusion coefficient of about 20 cm²/s. Copyright © 2020, John Wiley and Sons