# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
## 3 Production of 2D Bismuthene
### Bottom-Up Approach  
In addition to the top-down methods, a series of bottom-up methods have also been utilized in the fabrication of 2D Bi layers. CVD is one the most widely used bottom-up methods used to synthesis 2D materials. In 2020, [PERSON] et al. successfully grew 2D Bi films on both rigid (SiO\({}_{2}\)/Si) and flexible (polyimide) substrates using CVD. As shown in Figure 2d, Bi powder in a silica boat is placed in the heating zone of the furnace. Using the flow of N\({}_{2}\) gas in the tube, the Bi film will formed on the substrates, which are located downstream of the system [56]. The size and thickness of the CVD-grown 2D bismuth nanosheets can be controlled by tuning the key preparation parameters, such as reaction atmosphere, gas flow rate, processing temperature, and reaction time [57]. Over the last 5 years, [PERSON] et al. have synthesized high quality Bi nanoflakes on Cu foil substrate using CVD. By introducing an h-BN top layer, the structural transformation of 2D Bi is effectively restricted. After removing the h-BN layer by mechanical exfoliation, 2D Bi nanoflakes show extraordinary thermal stability evidenced by the phenomenon of its ability to resist oxidization after annealing at 500 \({}^{\circ}\)C for 10 min. It is also significant that the obtained 2D Bi nanoflakes could be applied in electrochemical CO\({}_{2}\) reduction reactions, retaining high crystallinity beyond 15 h.  
PLD is one of the most effective physical methods to synthesize large-scale 2D materials and heterostructures [55]. As shown in Figure 2e, during the PLD procedure, focused laser pulses strike the bulk target, and the generated energetic plasma plume is collected on a pre-heated substrate. In contrast to CVD, the processing temperature of 2D materials using PLD is usually relatively low. In addition, PLD exhibits the advantages of a high growth rate, better controllability of the film, and stoichiometric growth. In 2019, our group grew high crystallinity centimeter-scale Bi layers using PLD [17]. The 2D Bi(111) and Bi(110) thin films can be both produced by tuning the processing temperature at 100 \({}^{\circ}\)C and room temperature, respectively The thickness of Bi(111) and Bi(110) can be precisely controlled by the number of laser pulses. Meanwhile, the large scale and uniform surface give PLD-grown bismuthene great potential for developing practical device applications [55]. MBE is another commonly used bottom-up method to synthesis high quality 2D films and nanostructures [58; 59]. The preparation process is realized in an ultrahigh vacuum chamber, which could prevent contamination by impurities. MBE is suitable for producing large-scale films with a consistent thickness, and can be processed with several types of in situ characterization equipment, such as reflection high-energy electron diffraction (RHEED) systems, to monitor the fabrication process, which can rarely be achieved in other bottom-up methods. In order to synthesis bismuthene, bulk Bi is heated to sublimation in an MBE system, forming a 2D Bi thin film on the substrates [47; 60]. By tuning the substrate temperature, different phases of bismuthene are synthesized [61]. For example, [PERSON] et al. have grown multiple-layer 2D Bi films on Si(111) substrate using MBE [62]. The ultrathin Bi films with a (012)-oriented phase were generated above the wetting layer. This bismuth structure, discovered for the first time, is similar to the teratoid phase of BP. Besides PLD and MBE, e-beam evaporation is also feasible for the synthesis of 2D-layered Bi [48; 63]. Table 1 shows the comparison of the growth conditions and the material quality of 2D materials obtained using different fabrication techniques. In summary, the bottom-up synthesis of bismuthene is a significant complement to the top-down methods. The successful realization of wafer-scale few- and mono-layer bismuth films builds a solid foundation for the future development of high-performance photonic applications based on bismuthene.