# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
## 4 Photonic Applications Based on 2D Bismuth
### Photodetectors  
Thanks to its narrow bandgaps, high carrier mobility, and strong absorption ability, bismuthene shows great potential for use in high performance photodetection applications [21; 89; 90]. In 2020, [PERSON] et al. designed photodetectors based on 2D bismuth thin films on both rigid (SiO\({}_{2}\)/Si) and flexible (PI) substrates using vapor deposition [22]. The 2D Bi photodetectors on SiO\({}_{2}\)/Si not only showed a broadband photoresponse from 405 to 1064 nm, but also exhibited a self-powered, stable photodetection performance. According to Figure 6a, the 2D Bi layer on top of the device plays the role of a light absorption layer and a surface conductive channel due to its topological nature and its high carrier mobility. Some carriers prefer to transfer along the surface Bi channel because the thickness of the Bi layer is less than one mean free path, which is beneficial for photocurrent generation and quick response time (14 ms). Additionally, the Bi film on the PI substrates also showed similar photoresponse and demonstrated outstanding durability and reproducibility by bending the devices with different curvature radiuses. More recently, [PERSON] et al. fabricated a photoelectrochemical (PEC) photodetector based on 2D bismuth nanosheets by utilizing a solid electrolyte prepared using polyvinyl alcohol (PVA) (Figure 6b) [91]. By using asolid electrolyte instead of a traditional liquid electrolyte, the volume of the photodetectors will be reduced and the electrolyte leakage problem can be addressed during the folding process. As shown in Figure 6c, the heterojunction of 2D Bi nanosheets and the solid electrolyte will generate a built-in electric field, which could increase the separation efficiency of electron-hole pairs and enable the self-powered photodetection capability. In addition, the 2D Bi PEC photodetector was also fabricated on indium-tin oxide (ITO) substrate for designing a flexible and transparent photodetector. As shown in Figure 6d, the photocurrent remains at a steady value after bending 120 times, further verifying the excellent stability of 2D Bi-based flexible photodetection devices. Besides, the responsivity of photodetectors can be calculated using the following equation:  
\[R_{\lambda}=I_{ph}/P_{\lambda}S\]  
where \(R_{\lambda}\) stands for the photoresponsivity, \(I_{ph}\) represents the photocurrent of the photodetectors, \(P_{\lambda}\) is the incident power of the illumination, and \(S\) represents the effective illumination area. Thus, the obtained responsivity of Bi photodetectors is up to 9.7 uA/W, which is superior to that of a BP (5.4 uA/W)-based PEC-type photodetector under similar circumstance [92]. In the wake of improving the performance of the Bi detector, the strategy of combining 2D Bi with other materials to form heterostructure has garnered significant interest. [PERSON] et al. demonstrated a Bi/WS2/Si mixed dimensional heterostructure-based photodetector with outstanding photosensitivity up to 1.4 \(\times\) 10\({}^{8}\) cm\({}^{2}\)/W [93]. As illustrated in Figure 6e, the photogenerated carriers of Si are unable to tunnel into the Bi layer owing to the wide blocking barrier induced by the WS2 layer. This phenomenon could effectively weaken the recombination efficiency of the generated carriers. In addition, the WS  
Figure 5: Schematic diagram and mechanisms of Bi–based photocatalyst. (**a**) Catalytic process of bismuthene photoredox C-Hylation of (hetero)arenes [28]. (**b**) Schematic figure of the bismuthene photocatalytic CO2 reduction reaction process. (**c**) Schematic picture of the charge transformations of the type–II C3N4/BiOCI heterostructure and the Z-scheme C3N4/bismuthene/BiOCI heterostructure [29]. (**d**) Schematic figure of the catalytic process of water splitting [88]. dangling-bonds-free surface passivates the surface of Si, which leads to the small surface state. Therefore, Bi/WS2/Si possesses fewer dangling-bonds compared to Bi/Si, which reduces the recombination loss.