# Lead Monoxide Nanostructures for Nanophotonics: A Review
## 3 Properties and Applications in Nanophotonics
### Photodetection Performance  
As a narrow bandgap metal oxide, the structure of PbO is similar to that of black phosphorus, which has great potential in high-performance optoelectronic devices, such photodetectors and field-effect transistors [7; 19; 20]. For example, 2D PbO NSs showed excellent self-powered photoresponse behavior (Figure 8a) [19]. The 2D PbO NSs exhibited an obvious on/off switching behavior at 0 V (Figure 8a) and 0.4 V (Figure 8a), and these signals can be significantly boosted by increasing the light power, which can be attributed to the piezo-phototronic effect [90]. However, a naked indium-tin oxide (ITO) glass without 2D PbO NSs displays a negligible signal, even if the light power density has been largely augmented (Figure 8a,b), indicating that the on/off switching photoresponse behavior indeed originates from the 2D PbO NSs themselves [19]. The similar on/off switching photoresponse behavior of the as-fabricated 2D PbO-based photodetector can be easily triggered under lasers with a wavelength value of \(<\) 650 nm, which is consistent with the UV-Vis-NIR result (Figure 6a). The stronger photoresponse signal can be observed under a laser with lower-wavelength light (e.g., 365, 400, and 475 nm), which is attributed to the higher photon energy. In addition, the 0D PbO QDs similarly fabricated by an LPE method also have the same trend in the photoresponse behaviors as the 2D PbO NSs (Figure 8c,d) [20]. The photocurrent signal keeps almost stable at a wavelength of \(\leq\)400 nm and obviously declines as the wavelength rises up to \(>\)400 nm in 0.05 M Na\({}_{2}\)SO  
Figure 6: Optical properties of PbO nanostructures. (**a**) UV–Vis–NIR absorption spectrum of the 2D PbO nanosheets fabricated by an LPE method in an IPA solvent. The inset shows photos of the 2D PbO/IPA dispersion (left) and the Tyndall effect (right). Reproduced with permission from Reference [19]. Copyright 2018, American Chemical Society. (**b**) Optical absorption of PbO nanocrystals prepared by a one-pot colloidal synthesis method. Reproduced with permission from Reference [88]. Copyright 2010, The Royal Society of Chemistry. (Figure 8d). Such discrepancy in the KOH and Na\({}_{2}\)SO\({}_{4}\) electrolytes can be mainly ascribed to the different functionalities between the PbO nanostructures on the substrate surface and the different types of electrolytes [20]. More importantly, the cycling stability of the PbO QD-based photodetector was traced under a simulated light in 0.01 M KOH at 118 mW cm\({}^{-2}\) (Figure 8e,f). It can be observed that the PbO QD-based photodetector displays a strong on/off switching behavior, and a mere 37.2% reduction of the photocurrent density is obtained after one month of continuous measurement in 0.01 M KOH, suggesting its superior stability compared to the previously reported nanostructures, such as black phosphorus NSs [90], SnS NSs [91], and Bi QDs [34], which demonstrates that this black-phosphorus-analog PbO QD-based photodetector can shed new light on the construction of high-performance PEC-type devices. Figure 7: First-principles density functional theory calculations and experimental results of PbO. (**a**) Orbital-projected G0W0-corrected band structure of monolayer, double-layer, triple-layer, and bulk PbO. The VBM is dominated by the oxygen p orbitals, and the lead p orbitals take more part in the CBM region. (**b**) Relationship between the \(E_{\text{g}}\) and the number of layers. The estimated layer number of 16 is indicated by the blue circle. Reproduced with permission from Reference [19]. Copyright 2018, American Chemical Society. (**c**) Bandgap of Mn-doped PbO thin films with a variation in the Mn concentration, calculated based on the fundamental absorption. Reproduced with permission from Reference [89]. Copyright 2018, Elsevier B. V.  
Figure 8: Photoresponse behavior of the as-prepared PbO nanostructures.