Progress of fabrication and surface modification of 2D black phosphorus

Abstract

Two-dimensional (2D) materials have attracted broad interest because of their low-dimensional effect. Black phosphorus (BP), a new 2D layered material as graphene, has attracted worldwide attention since its appearance in 2014. As the most thermodynamic stable allotrope of phosphorus, BP is a natural semiconductor with adjustable bandgap and predominant electrical properties, and is considered to be able to replace silicon as the core material of semiconductor industry. The band gap in bulk BP is 0.3 eV and can be expanded to 2.0 eV depending on the layer numbers, and its band gap is highly sensitive to the strain either in-plane or out-of-plane. The BP based field effect transistor (FET) exhibits high mobility and appreciably high on/off ratios, and the mobility is thickness dependent. The optical property of BP is also superior to other semiconductors. The range of BP band gap corresponds to an absorption spectrum from visible light to infrared. With a direct bandgap, which means the bottom of conduction band and the top of valence band are at the same position, BP can couple with photons directly and construct a new generation of photoelectric device, and its in-plane anisotropy makes it suitable for the detecting of polarized light. Moreover, BP has good biocompatibility because about 1% of human weight is made up by phosphorus. All these attractive properties make BP a promising material in electronic, optical and biomedical applications. However, as always, opportunities and challenges coexist in the study and utilization of new materials, and such is the case for BP. There are still fundamental obstacles hampering the application of BP, such as mass production and stability. The thickness of BP can be scaled down to the atomic layer scale known by mechanical exfoliation or liquid exfoliation, but it is still difficult to realize the massive production of high-quality BP with accurate control of size and height. Furthermore, although BP is the most stable allotrope of elemental phosphorus and bulk BP can be stable under ambient conditions for months, the monolayer or few-layer BP is found to be unstable in an atmospheric environment, being subject to severe degradation by moisture and oxygen in air. It has been demonstrated that BP is very reactive to oxygen and water under ambient conditions, resulting in compositional and physical changes and consequently considerable degradation in the electronic and optical properties. Long-term exposure of BP to humid air or water can even completely etch the materials away. This poses a severe limitation to the adoption of BP in flexible electronics and photoelectronics, and its instability in water further limits potential electrochemical and biomedical applications. In this perspective, we review recent progress in the research of BP, touching upon topics on properties, fabrication and surface modification. We highlight that the surface modification to gain BP with good stability again air and water is important to its application. Finally, the development trend and application perspective of BP are mentioned.