Few‐Layered Black Phosphorus: From Fabrication and Customization to Biomedical Applications

As a new kind of 2D material, black phosphorus has gained increased attention in the past three years. Although few‐layered black phosphorus nanosheets (BPs) degrade quickly under ambient conditions to phosphate anions, which greatly hampers their optical and electronic applications, this property also makes BPs highly biocompatible and biodegradable, and is regarded as an advantage for various biomedical applications. This Concept summarizes the state‐of‐art progresses of BPs, from fabrication and surface modification to biomedical applications. It is expected that BPs with such fascinating properties will encourage more scientists to engage in expanding its biomedical applications by tackling the scientific challenges involved in their development.     

Lanthanide‐Coordinated Black Phosphorus

Black phosphorus (BP) possesses unique physical properties and, owing to its intrinsic instability, the proper surface and chemical coordination is the key point in many applications. Herein, a facile and efficient surface lanthanide‐coordination strategy based on lanthanide (Ln) sulfonate complexes is designed to passivate and functionalize different BP‐based nanostructures including quantum dots, nanosheets, and microflakes. By means of Ln–P coordination, the lone‐pair electrons of phosphorus are occupied, thus preventing oxidation of BP, and the LnL₃@BP exhibits excellent stability in both air and water. Furthermore, accompanied by the original photothermal performance of BP nanostructures, the Gd‐coordinated BP has high R₁ relativities in magnetic resonance (MR) imaging, and other Ln (Tb, Eu, and Nd) coordinated BP structures exhibit fluorescence spanning the visible to near‐infrared regions. Not only is LnL₃ surface passivation an efficient method to enhance the stability of BP, but also the MR or fluorescence derived from lanthanide ions extends the application of BP to optoelectronics and biomedical engineering.     

Few‐Layer Black Phosphorus Carbide Field‐Effect Transistor via Carbon Doping

Black phosphorus carbide (b‐PC) is a new family of layered semiconducting material that has recently been predicted to have the lightest electrons and holes among all known 2D semiconductors, yielding a p‐type mobility (≈10⁵ cm² V^?1 s^?1) at room temperature that is approximately five times larger than the maximum value in black phosphorus. Here, a high‐performance composite few‐layer b‐PC field‐effect transistor fabricated via a novel carbon doping technique which achieved a high hole mobility of 1995 cm² V^?1 s^?1 at room temperature is reported. The absorption spectrum of this material covers an electromagnetic spectrum in the infrared regime not served by black phosphorus and is useful for range finding applications as the earth atmosphere has good transparency in this spectral range. Additionally, a low contact resistance of 289 Ω µm is achieved using a nickel phosphide alloy contact with an edge contacted interface via sputtering and thermal treatment.     

二维黑磷物理性质及化学稳定性的研究进展

1914年科研工作者首次合成了黑磷的块体形式。在黑磷沉寂了100年之后,2014年人们成功地将其薄化到少层状态,得到了新型二维纳米材料——二维黑磷。它是由磷原子堆叠而成的单一元素的二维层状半导体材料,具有合适的可控直接带隙、高的载流子迁移率、高的漏电流调制率、较高的开关电流比、良好的导电导热能力和明显的平面各向异性等性质,在低维无机半导体领域备受关注。二维黑磷是一种具有众多优异性质的内禀p型材料,但在空气中的不稳定性使得目前还不能分离得到大面积磷烯(即单原子层黑磷)薄膜,这限制了二维黑磷的应用。本文综述了自2014年以来国内外关于二维黑磷的研究进展,系统介绍了二维黑磷的结构、制备与性质,重点归纳了包括电学性质、光学性质、力学性质、热学性质和磁学性质在内的物理性质及化学稳定性。最后总结并展望了二维黑磷作为电子材料的广阔前景。     

Metal‐Ion‐Modified Black Phosphorus with Enhanced Stability and Transistor Performance

Black phosphorus (BP), a burgeoning elemental 2D semiconductor, has aroused increasing scientific and technological interest, especially as a channel material in field‐effect transistors (FETs). However, the intrinsic instability of BP causes practical concern and the transistor performance must also be improved. Here, the use of metal‐ion modification to enhance both the stability and transistor performance of BP sheets is described. Ag⁺ spontaneously adsorbed on the BP surface via cation–π interactions passivates the lone‐pair electrons of P thereby rendering BP more stable in air. Consequently, the Ag⁺‐modified BP FET shows greatly enhanced hole mobility from 796 to 1666 cm² V^?1 s^?1 and ON/OFF ratio from 5.9 × 10⁴ to 2.6 × 10⁶. The mechanisms pertaining to the enhanced stability and transistor performance are discussed and the strategy can be extended to other metal ions such as Fe³⁺, Mg²⁺, and Hg²⁺. Such stable and high‐performance BP transistors are crucial to electronic and optoelectronic devices. The stability and semiconducting properties of BP sheets can be enhanced tremendously by this novel strategy.     

Recent Progress on Stability and Passivation of Black Phosphorus

From a fundamental science perspective, black phosphorus (BP) is a canonical example of a material that possesses fascinating surface and electronic properties. It has extraordinary in‐plane anisotropic electrical, optical, and vibrational states, as well as a tunable band gap. However, instability of the surface due to chemical degradation in ambient conditions remains a major impediment to its prospective applications. Early studies were limited by the degradation of black phosphorous surfaces in air. Recently, several robust strategies have been developed to mitigate these issues, and these novel developments can potentially allow researchers to exploit the extraordinary properties of this material and devices made out of it. Here, the fundamental chemistry of BP degradation and the tremendous progress made to address this issue are extensively reviewed. Device performances of encapsulated BP are also compared with nonencapsulated BP. In addition, BP possesses sensitive anisotropic photophysical surface properties such as excitons, surface plasmons/phonons, and topologically protected and Dirac semi‐metallic surface states. Ambient degradation as well as any passivation method used to protect the surface could affect the intrinsic surface properties of BP. These properties and the extent of their modifications by both the degradation and passivation are reviewed.     

Tuning the Optical and Electrical Properties of Few‐Layer Black Phosphorus via Physisorption of Small Solvent Molecules

Black phosphorus (BP) is recently becoming more and more popular among semiconducting 2D materials for (opto)electronic applications. The controlled physisorption of molecules on the BP surface is a viable approach to modulate its optical and electronic properties. Solvents consisting of small molecules are often used for washing 2D materials or as liquid media for their chemical functionalization with larger molecules, disregarding their ability to change the opto‐electronic properties of BP. Herein, it is shown that the opto‐electronic properties of mechanically exfoliated few‐layer BP are altered when physically interacting with common solvents. Significantly, charge transport analysis in field‐effect transistors reveals that physisorbed solvent molecules induce a modulation of the charge carrier density which can be as high as 10¹² cm^?2 in BP, i.e., comparable to common dopants such as F₄‐TCNQ and MoO₃. By combining experimental evidences with density functional theory calculations, it is confirmed that BP doping by solvent molecules not only depends on charge transfer, but is also influenced by molecular dipole. The results clearly demonstrate how an exquisite tuning of the opto‐electronic properties of few‐layer BP can be achieved through physisorption of small solvent molecules. Such findings are of interest both for fundamental studies and more technological applications in opto‐electronics.