# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
## 3 Production of 2D Bismuthene
### Top-Down Approach  
With the assistance of the proper chemical solvents and sonication processes, the interlayer _vdWs_ force of bulk Bi can be broken to obtain 2D Bi nanosheets [24]. A typical convenient liquid exfoliation process is shown in Figure 2a. Bulk bismuth was ground into power using isopropyl alcohol. Subsequently, the bismuth isopropyl solution was subjected to both bath and probe sonication for 10 h, respectively. Next, 2D bismuthene was obtained after the centrifugation process. It should be noted that the type of chemical solvent used is of great significance for the efficient production of bismuthene nanosheets. The exfoliation efficiency can be optimized when the solvent's surface tension component ratios are comparable to those of bismuthene [49].  
As an efficient and low-cost method, liquid exfoliation can fabricate high-quality Bi with high efficiency and yield [50]. The strong interaction between bulk Bi and a suitable solution (such as isopropyl alcohol) leads to the dispersion of the bismuth nanosheets in the solvent [51]. In 2019, [PERSON] et al. exfoliated Bi nanosheets from Bi powder in an ethanol solution using sonication-assisted liquid exfoliation [50]. Few-layer Bi was prepared after 1 h of sonication at 950 W and centrifugation at 500 rpm. According to the atomic force microscope (AFM) image (Figure 2b), monolayer bismuth nanosheets were successfully fabricated. However, owing to defects in the exfoliation process, the exfoliated layers are not of high quality. In addition, the lateral size of the as-produced nanosheets ranges from  
Figure 1: (**a**,**b**) Schematics of \(\alpha\)-bismuthene and \(\beta\)-bismuthene from the top and side views [30]. (**c**) AFM image of \(\beta\)-bismuthene [34]. (**d**) Calculated band diagram of \(\alpha\)- and \(\beta\)- bismuthene [44]. (**e**,**f**) The diffraction ring patterns and number of rings in the bismuthene solution under different powder intensites at illuminations of 532 nm and 633 nm, respectively [24].  
the nm scale to the multiple \(\upmu\)m scale. The optimization of the exfoliation process in order to better control the size of the obtained samples should be addressed in the future.  
In addition to the liquid exfoliation process, 2D Bi can also be fabricated by the electrochemical exfoliation method due to its high chemical durability and the low cost of bulk metallic Bi [52; 53]. Thanks to the advantages of short production time, moderate fabrication conditions, and high yield, the electrochemical exfoliation method provides a favorable platform for the mass production of 2D nanosheets. When bismuthene is prepared using a standard electrochemical exfoliation method based on a DC voltage apparatus, bulk Bi works as the cathode, and platinum foil works as the anode. During the fabrication process, both are soaked in the proper organic solution. The exerted bias facilitates the insertion of cations between the molecular layers, which can effectively increase the interlayer spacing. In 2019, 2D Bi, with a lattice fringe of 0.23 nm, was successfully exfoliated using a rapid electrochemical cathodic exfoliation method, resulting in Bi nanosheets with a large reactive surface area, beneficial for electrocatalytic performance (Figure 2c) [53]. Thanks to the significant field influence and the capacity to precisely tune the voltage, electrochemical exfoliation is an effective method to produce 2D Bi with good crystallinity and high yield [52; 54]. However, owing to the introduction of organic solvents, the residues of the solution restrict the quality of the exfoliated samples.  
Figure 2: (**a**) Schematic diagram of the bismuthene liquid exfoliation process. [24]. (**b**) AFM image of the bismuthene layer [50]. (**c**) High resolution TEM image of the electrochemical exfoliated bismuth nanosheets [53]. (**d**) Schematic figure of the CVD process for preparing bismuthene [22]. (**e**) Schematic picture of the PLD system [55].