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.     

2D Black Phosphorus for Energy Storage and Thermoelectric Applications

Recent progress in the currently available methods of producing black phosphorus bulk and phosphorene are presented. The effective passivation approaches toward improving the air stability of phosphorene are also discussed. Furthermore, the research efforts on the phosphorene and phosphorene‐based materials for potential applications in lithium ion batteries, sodium ion batteries, and thermoelectric devices are summarized and highlighted. Finally, the outlook including challenges and opportunities in these research fields are discussed.     

Black Phosphorus: Optical Characterization,Properties and Applications

The rise of black phosphorus (BP) as a new family member of two‐dimensional (2D) materials brings new concepts and applications to the field, because of the infrared band gap and anisotropic properties of such materials. Among many excellent properties of BP, the optical property attracts special attention in recent years. Optical methods have been widely and successfully used in characterizing BP, not only to obtain the structural information (such as thickness and crystalline orientation), but also to probe the fundamental properties of BP in terms of the behavior of electrons, phonons, excitons etc. In this Review, a comprehensive understanding about the optical characterization of BP such as Raman, absorption, and photoluminescence is presented. Also, the unique optical properties and applications explored in recent years are reviewed.     

Black Phosphorus Nanosheets: Synthesis,Characterization and Applications

Black phosphorus (BP) is an emerging two‐dimensional (2D) material with a natural bandgap, which has unique anisotropy and extraordinary physical properties. Due to its puckered structure, BP exhibits strong in‐plane anisotropy unlike other layered materials. The bandgap tunability of BP enables a wide range of ultrafast electronics and high frequency optoelectronic applications ranging from telecommunications to thermal imaging covering the nearly entire electromagnetic spectrum, whereas no other 2D material has this functionality. Here, recent advances in the synthesis, fabrication, anisotropic physical properties, and BP‐based devices including field effect transistors (FETs) and photodetectors, are discussed. Recent passivation approaches to address the degradation of BP, which is one of the main challenges to bring this material into real world applications, are also introduced. Finally, a comment is made on the recent developments in other emerging applications, future outlook and challenges ahead in BP research.     

Recent Advances on Black Phosphorus for Energy Storage,Catalysis, and Sensor Applications

As a new type of 2D semiconductor, black phosphorus (BP) possesses high charge‐carrier mobility and theoretical capacity, thickness‐dependent bandgap, and anisotropic structure, which has attracted tremendous attention since early 2014. To explore its full application in all aspects, studies based on BP nanostructures are swiftly expanding from the electronic field to energy storage and even biochemistry. The mechanism and application of BP in Li‐/Na‐ion battery anodes, oxygen evolution reaction/hydrogen evolution reaction catalysis, photocatalytic hydrogen production, and selective sensors are summarized. Based on the solid research on this topic, feasible improvements and constructive suggestions regarding these four fields are put forward.     

Direct Solvent‐Derived Polymer‐Coated Nitrogen‐Doped Carbon Nanodots with High Water Solubility for Targeted Fluorescence Imaging of Glioma

Cancer imaging requires biocompatible and bright contrast‐agents with selective and high accumulation in the tumor region but low uptake in normal tissues. Herein, 1‐methyl‐2‐pyrrolidinone (NMP)‐derived polymer‐coated nitrogen‐doped carbon nanodots (pN‐CNDs) with a particle size in the range of 5–15 nm are prepared by a facile direct solvothermal reaction. The as‐prepared pN‐CNDs exhibit stable and adjustable fluorescence and excellent water solubility. Results of a cell viability test (CCK‐8) and histology analysis both demonstrate that the pN‐CNDs have no obvious cytotoxicity. Most importantly, the pN‐CNDs can expediently enter glioma cells in vitro and also mediate glioma fluorescence imaging in vivo with good contrast via elevated passive targeting.     

Single Pixel Black Phosphorus Photodetector for Near‐Infrared Imaging

Infrared imaging systems have wide range of military or civil applications and 2D nanomaterials have recently emerged as potential sensing materials that may outperform conventional ones such as HgCdTe, InGaAs, and InSb. As an example, 2D black phosphorus (BP) thin film has a thickness‐dependent direct bandgap with low shot noise and noncryogenic operation for visible to mid‐infrared photodetection. In this paper, the use of a single‐pixel photodetector made with few‐layer BP thin film for near‐infrared imaging applications is demonstrated. The imaging is achieved by combining the photodetector with a digital micromirror device to encode and subsequently reconstruct the image based on compressive sensing algorithm. Stationary images of a near‐infrared laser spot (λ = 830 nm) with up to 64 × 64 pixels are captured using this single‐pixel BP camera with 2000 times of measurements, which is only half of the total number of pixels. The imaging platform demonstrated in this work circumvents the grand challenges of scalable BP material growth for photodetector array fabrication and shows the efficacy of utilizing the outstanding performance of BP photodetector for future high‐speed infrared camera applications.     

Black Phosphorus,a Rising Star 2D Nanomaterial in the Post‐Graphene Era: Synthesis,Properties, Modifications,and Photocatalysis Applications

Semiconductor photocatalysis, a sustainable and renewable technology, is deemed to be a new path to resolve environmental pollution and energy shortage. The development of effective photocatalysts, especially the metal‐free photocatalysts, is a critical determinant of this technique. The recently emerged 2D material of black phosphorus with distinctive properties of tunable direct bandgap, ultrahigh charge mobility, fortified optical absorption, large specific surface area, and anisotropic structure has captured enormous attention since the first exfoliation of bulk black phosphorus into mono‐ or few layered phosphorene in 2014. In this article, the state‐of‐the‐art preparation methods are first summarized for bulk black phosphorus, phosphorene, and black phosphorus quantum dot and then the fundamental structure and electronic and optical properties are analyzed to evaluate its feasibility as a metal‐free photocatalyst. Various modifications on black phosphorus are also summarized to enhance its photocatalytic performance. Furthermore, the multifarious applications such as solar to energy conversion, organic removal, disinfection, nitrogen fixation, and photodynamic therapy are discussed and some of the future challenges and opportunities for black phosphorus research are proposed. This review reveals that the rising star of black phosphorus will be a multifunctional material in the postgraphene era.     

Wavelength-Tunable Mid-Infrared Lasing from Black Phosphorus Nanosheets

Van der Waals layered semiconductor materials own unique physical properties and have attracted intense interest in developing high-performance electronic and photonic devices. Among them, black phosphorus (BP) is distinct for its layer number-tuned direct band gap which spans from near- to mid-infrared (MIR) waveband. In addition, the puckered honey comb crystal lattice endows the material with highly linear-polarized emission and marked anisotropy in carrier transportation. These unique material properties render BP as an intriguing and promising building block for constructing mid-infrared-ranged coherent light sources. Here, a room temperature surface-emitting MIR laser based on single crystalline BP nanosheets coupled with a distributed Bragg reflector cavity is reported. MIR stimulated emission at 3611 nm is achieved with a near-unity linear polarization, which exhibits robust thermal stability up to 360 K. Most importantly, the lasing wavelength can be tuned from 3425 to 4068 nm by varying the cavity length via thickness control of BP layer. The demonstrated highly polarized lasing output and wavelength-tunable capacity of the proposed device scheme in MIR spectral range opens up promising opportunities for a broad array of applications in polarization-resolved IR imaging, range-finding, and free space quantum communications.     

Growth of Black Phosphorus Nanobelts and Microbelts

Black phosphorus nanobelts are fabricated with a one‐step solid–liquid–solid reaction method under ambient pressure, where red phosphorus is used as the precursor instead of white phosphorus. The thickness of the as‐fabricated nanobelts ranges from micrometers to tens of nanometers as studied by scanning electron microscopy. Energy dispersive X‐ray spectroscopy and X‐ray diffraction indicate that the nanobelts have the composition and the structure of black phosphorus, transmission electron microscopy reveals a typical layered structure stacked along the b‐axis, and scanning transmission electron microscopy with energy dispersive X‐ray spectroscopy analysis demonstrates the doping of bismuth into the black phosphorus structure. The nanobelt can be directly measured in scanning tunneling microscopy in ambient conditions.     

Synthesis of Crystalline Black Phosphorus Thin Film on Sapphire

Black phosphorus (BP) has recently attracted significant attention due to its exceptional physical properties. Currently, high‐quality few‐layer and thin‐film BP are produced primarily by mechanical exfoliation, limiting their potential in future applications. Here, the synthesis of highly crystalline thin‐film BP on 5 mm sapphire substrates by conversion from red to black phosphorus at 700 °C and 1.5 GPa is demonstrated. The synthesized ≈50 nm thick BP thin films are polycrystalline with a crystal domain size ranging from 40 to 70 µm long, as indicated by Raman mapping and infrared extinction spectroscopy. At room temperature, field‐effect mobility of the synthesized BP thin film is found to be around 160 cm² V^?1 s^?1 along armchair direction and reaches up to about 200 cm² V^?1 s^?1 at around 90 K. Moreover, red phosphorus (RP) covered by exfoliated hexagonal boron nitride (hBN) before conversion shows atomically sharp hBN/BP interface and perfectly layered BP after the conversion. This demonstration represents a critical step toward the future realization of large scale, high‐quality BP devices and circuits.     

Black Phosphorus Nanosheets Passivation Using a Tripeptide

In the past several years, 2D black phosphorus (BP) has captured the research community's interest because of its unique electronic, photonic, and mechanical properties. However, the intrinsic instability of BP limits its preservation and practical application. Despite kinds of BP passivation strategies being well‐documented, the use of metal ligand coordination or polymer modification may have potential long‐term detrimental effects on human bodies. Here, a tailored tripeptide Fmoc‐Lys‐Lys‐Phe (Fmoc‐KKF) is synthesized for surface modification of BP nanosheets. Compared with bare BP with rapid degradation, the BP@FKK complex exhibits excellent stability, thereby significantly increasing the life span. Significantly, the BP@FKK shows favorable cell compatibility and enhanced cellular uptake compared to the bare BP.