# Lead Monoxide Nanostructures for Nanophotonics: A Review
## 1 Introduction  
In recent years, a new family of materials which have similar folded structures to black phosphorus, termed black phosphorus analogs, not only show a tunable bandgap, superior on/off ratio, and carrier mobility, but also exhibit impressive environmental stability [1; 2; 3; 4; 5; 6; 7; 8; 9] and hold great promise in practical applications. Until now, a number of reviews on black phosphorus analogs, such as bismuthene [4; 10; 11], antimonene [12], tellurene [13; 14], etc., have been systematically summarized due to their fascinating properties. For example, our group recently summarized the controlled synthesis of monoelemental bismuth (Bi) nanomaterials with different shapes and sizes, and highlighted the recent advances of their intriguing applications [4]. In 2018, [PERSON] et al. [12] reviewed the different approaches and characterization techniques for antimonene and summarized the electronic band structure of antimonene for optoelectronics and thermoelectric applications in both monolayer and few-layer forms. In 2017, [PERSON] et al. [14] reviewed the controlled synthesis of tellurium (Te) nanostructures and summarized the latest progress in many applications, such as batteries, photodetectors, sensors, etc.  
Apart from that, black-phosphorus-analog lead monoxide (PbO) is also of research interest in recent years owing to its excellent photodetection and nonlinear optical properties [15; 16; 17; 18; 19; 20]. PbO is a layer compound which exists in two crystal forms: red tetragonal (\(\alpha\)) and yellow orthorhombic (\(\beta\)) forms [21], both of which have been experimentally synthesized [22; 23; 24; 22]. Due to the same folded structure as that of black phosphorus for the yellow orthorhombic crystal form, more attention was paid to \(\beta\)-PbO to pursue high-performance black-phosphorus-analog-based devices. The unique properties of PbO nanostructures for practical applications are predominantly dependent on their structural stability, as well as their electronic and spintronic characteristics. Moreover, the low cost, easy fabrication, and high environmental stability of PbO nanostructures are also of significance to provide themwith a great potential in a variety of applications, such as CO\({}_{2}\) photoreduction [25], gas detection [26], batteries [27], supercapacitors [28], and photovoltaics [29].  
Inspired by the recently published reviews on black phosphorus analogs, such as bismuthene [4; 11], antimonene [12], and tellurene [13; 14], we summarize and highlight the recent progress in the elaborate design and controlled synthesis of PbO nanostructures, their fascinating properties, and emerging applications, as shown in Figure 1. We firstly summarize PbO nanostructures with different dimensionalities, including zero-dimensional (0D) nanoparticles (NPs), one-dimensional (1D) nanostructures (nanowires (NWs), nanorods (NRs), etc.), two-dimensional (2D) nanosheets (NSs), and three-dimensional (3D) Bi-based nanostructures. Afterwards, the fundamental properties and optoelectronic/photonic performance of PbO nanostructures are reviewed. Last, but not least, the challenges and opportunities in future high-performance nanophotonic devices based on PbO-related nanostructures are discussed.