# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
## 5 Conclusions and Outlook  
Recent 2D bismuthene research progress has generated significant interest in exploring its features and potential applications. The atomically thin nature and layered structure of 2D bismuthene endow it with beneficial properties distinctive from its bulk. Bismuthene's layer-dependent optical band gaps, high carrier mobility, and excellent stability in air make it suitable for applications in both electronics and optoelectronics. In this review, we summarize the recent advances in photonic applications based on 2D bismuthene. The crystalline and electronic structure, as well as the nonlinear properties, of bismuthene are firs reviewed, demonstrating a non-trivial 2D topological insulator and a suitable saturable absorption ability. Next, the techniques commonly used to realize bismuthene are outlines from both a top-down and bottom-up approach. A variety of photonic devices based on bismuthene are then discussed, including photodetectors, ultrafast photonic applications, and photocatalysts, each exhibiting attractive performance and great potential for modern photonic technologies. It is worth noting that bismuthene-based lasers exhibit a shorter pulse width than those of other 2D materials. Meanwhile, the responsivity and response time are comparable to those of graphene or TMDs-based heterostructures. Therefore, bismuthene is desirable for designing high performance photonic devices. Currently, the design and device demonstration of bismuthene are still in the initial stages. Although the research has already achieved considerable progress, some challenges and opportunities for approaching the creation of practical devices remain:  
Figure 6: Schematic diagram and key parameters of the Bi-based photodetector. (**a**) Schematic figure of the working mechanism of Bi/Si under illumination [22]. (**b**) Schematic figure of the flexible PEC-type photodetector based on 2D Bi. (**c**) Band diagram of Bi nanosheets. (**d**) Photocurrent density of the PEC-type Bi-based device as a function of bending cycles [91]. (**e**) Band diagram of Bi/WS2/Si heterostructure. (**f**) Detectivity and responsivity of Bi/WS2/Si-based photodetector as a function of wavelength, respectively [93]. The large-area synthesis method is one of the significant prerequisites for integrating 2D materials into practical applications. As we introduced in the article, the primary methods for the scalable growth of 2D bismuthene are mainly carried out by bottom-up techniques, including CVD, PLD, MBE, etc. However, currently, the low crystallinity and large number of defects of the obtained bismuthene limit its application in photonic and optoelectronic devices. Meanwhile, the poor controllability of the fabrication process, its slow growth rate, and its current high cost does not meet the criteria for commercialization. Therefore, developing a direct-synthesis method for realizing high quality 2D bismuthene films would be a high priority. 2. During the photocatalytic experiments, 2D bismuthene is easily oxidized to amorphous bismuth oxide due to its atomically thin nature and semimetal features, which will degrade the efficiency of photocatalysts. Thus, it is a crucial to discover a method to solve the oxidation issue of 2D Bi. A straightforward approach is to package the bismuthene into porous materials, which could reduce its exposure to oxidizing conditions. Another possible method is to integrate bismuthene photocatalysts into a photoelectrochemical system, which may also prevent the oxidation of 2D Bi. 3. Thanks to the weak vdW interaction between the adjacent layers and the dangling-bond-free surface, 2D materials can form heterostructures by combining with another 2D candidate, or even materials of different dimensionality, which greatly expand the properties, functionalities, and applications of the 2D family. Considering the tunable band alignment and the combination of merits from different materials, 2D bismuthene heterostructures provide new strategies for designing high performance photonic applications.