ABSTRACT
Phosphorene is a recently developed two-dimensional (2D) material that has attracted tremendous attention owing to its unique anisotropic manner, 1-7 layer-dependent direct bandgaps, 8-10 and quasi-one-dimensional (1D) excitonic nature, 11-13 which are all in drastic contrast with the properties of other 2D materials, such as graphene 14 and transition metal dichalcogenide (TMD) semiconductors. 15-17 Monolayer phosphorene has been of particular interest in exploring technological applications and investigating fundamental phenomena, such as 2D quantum confinement and many-body interactions. 11, 18 However, this unique 2D material is unstable in ambient conditions and degrade quickly. 9, 19-24 Particularly, monolayer phosphorene is expected to be much less stable than few-layer phosphorene, 19 and hence its identification and characterization are extremely challenging. So far, 48 to identify the layer number of thin-layer phosphorene, atomic force microscopy (AFM), 2, 5, 6, 9, 13, 19, 22, 24-27 phase-shifting interferometry (PSI), 7, 9, 10, 12, 21 Raman spectroscopy, 5, 19-21, 27-30 photo-luminescence (PL) spectroscopy 9-11, 21, 31, 32 and optical contrast method 6, 27, 29, 31, 33 have been used by researchers. And by combining results from above-mentioned techniques, the layer number of thin-layer phosphorene can be precisely determined. The other intriguing property of phosphorene is its layer-dependent direct bandgap in the infrared range 9-11, 21, 31, 32 With this unique property, phosphorene can bridge the zero-bandgap graphene 14, 34 and comparatively large-bandgap TMD semiconductors, 35-37 leading it to potential applications in infrared range, such as infrared photodetectors and absorber. 28, 30, 38-40 Besides, unlike TMD semiconductors, which show an indirect-to-indirect bandgap transition only when the layer number is thinned down to monolayer, 35-37 phosphorene is a direct-bandgap semiconductor from monolayer to its bulk form, 5, 9, 31 resulting in high internal quantum efficiency and making it a promising candidate for future optoelectronic applications. The anisotropic nature of phosphorene, another unique property of phosphorene originated from its puckering structure, has also been characterized by using Raman spectroscopy. 2, 6, 13, 25, 33, 40-42 Further-more, X-ray diffraction (XRD) 43 X-ray photoelectron spectroscopy (XPS), 23, 28, 43-45 scanning tunneling microscopy (STM), 46 and trans-mission electron microscopy (TEM) 20, 23, 25, 26, 42, 45, 47 are also applied to characterize the structure and component of fresh phosphorene and to detect its degradation in ambient atmosphere. In this chapter, we will focus on the layer number characterization of few-layer phosphorene with PSI method, and temperature/angle-dependent Raman spectroscopy and layer-dependent PL measurements of few-layer phosphorene.
