# Recent Progress in Synthesis and Photonic Applications of Two-Dimensional Bismuthene
###### Abstract  
The emergence of phosphorene has generated significant interest in 2D group VA nanomaterials. Among this group, bismuthene exhibits layer-dependent direct bandgaps, high carrier mobility, and topological insulator properties because of its unique structure and ultrathin nature, distinguishing it as a promising candidate for photonic applications. Particularly, its outstanding stability in air makes bismuthene more advantageous than phosphorene for practical applications. Here, we provide a comprehensive review of recent advances regarding 2D bismuth by focusing on the aspects of methods of synthesis and photonic applications. First, the structure and fundamental properties of bismuthene are described, referring to its crystallinity and band structures, as well as to its nonlinear optical properties. Subsequently, the common synthesis methods for 2D bismuth are summarized, including both top-down and bottom-up approaches. Then, potential photonic applications based on 2D bismuth, involving nonlinear photonic devices, photocatalyst, and photodetectors, are illustrated. The performance, mechanisms, and features of the devices are discussed. Finally, the review is summarized and some challenges and future outlooks in this field are addressed.  
photonics; 2D materials; bismuthene; photodetectors; photocatalyst; 2D group-VA materials +
Footnote †: journal: _Applied Sciences_  
possess unique surface states and band structures, demonstrating layer-dependent topological properties [17; 19]. In 2017, [PERSON] et al. successfully synthesized a graphene-like bismuthene film on top of a SiC substrate, showing a topological energy gap of 0.8 eV, which renewed the research interest in atomically thin Bi [20]. Since bismuthene exhibits a small band gap, a large nonlinear refraction index, and ultrahigh carrier mobility, the material is favorable for photonic applications such as broadband photodetectors [19; 21; 22], mode-locked lasers [18; 23; 24], and all-optical switching [25; 26]. For example, [PERSON] et al. demonstrated an ultrasensitive terahertz photodetector based on 2D bismuth. The strong photoresponse observed was attributed to the asymmetric scattering of topological surface states, which was stimulated by the localized surface plasmon-induced terahertz field. Furthermore, 2D bismuthene shows a large specific surface area, a hexagonal lattice structure, and high stability in air, making it suitable for photocatalytic applications [27; 28; 29]. In addition, the saturable absorption ability of bismuthene makes it applicable in Q-switched lasers. Recent study has reported a universal photo-redox catalyst based on few-layer bismuthene nanosheets, demonstrating arrested catalytic activity in an organic transformation under various reaction conditions [28]. Although the number of original research articles on bismuthene and its photonic applications has increased rapidly in recent years, to the best of our knowledge, there is currently no specific review article focusing on this area.  
Herein, we present a comprehensive review of the recent progress in photonic applications based on 2D bismuthene. First, we will briefly outline the crystalline structure, electronic attributes, and nonlinear optical properties of bismuthene. Then, the common synthesis methods used to realize high quality bismuthene, from both top-down and bottom-up approaches, will be introduced. Next, we will review the typical photonic devices based on bismuthene, in the sequence of photodetectors, non-linear photonics devices, and photocatalytic applications. The performance and mechanisms of the devices will also be discussed. Finally, the review will be summarized, and some perspectives and future prospects of this field will be provided.