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.     

Piezoelectricity in Multilayer Black Phosphorus for Piezotronics and Nanogenerators

Recently, piezoelectric characteristics have been a research focus for 2D materials because of their broad potential applications. Black phosphorus (BP) is a monoelemental 2D material predicted to be piezoelectric because of its highly directional properties and non-centrosymmetric lattice structure. However, piezoelectricity is hardly reported in monoelemental materials owing to their lack of ionic polarization, but piezoelectric generation is consistent with the non-centrosymmetric structure of BP. Theoretical calculations of phosphorene have explained the origin of piezoelectric polarization among P atoms. However, the disappearance of piezoelectricity in multilayer 2D material generally arises from the opposite orientations of adjacent atomic layers, whereas this effect is limited in BP lattices due to their spring-shaped space structure. Here, the existence of in-plane piezoelectricity is experimentally reported for multilayer BP along the armchair direction. Current–voltage measurements demonstrate a piezotronic effect in this orientation, and cyclic compression and release of BP flakes show an intrinsic current output as large as 4 pA under a compressive strain of −0.72%. The discovery of piezoelectricity in multilayer BP can lead to further understanding of this mechanism in monoelemental materials.     

Large-Area Black Phosphorus/PtSe2 Schottky Junction for High Operating Temperature Broadband Photodetectors

High operating temperature (HOT) broadband photodetectors are urgently necessary for extreme condition applications in infrared-guided missiles, infrared night vision, fire safety imaging, and space exploration sensing. However, conventional photodetectors show dramatic carrier mobility decreases and carrier losses with low photoresponsivity at HOT due to the increased carrier scattering in channels at high temperatures. Herein, the HOT broadband photodetectors from room temperature to 470 K are developed for the first time by large-area black phosphorus (BP)/PtSe₂ films device arrays via a depletion-enhanced photocarrier dynamics strategy. Attributed to the 2D Schottky junction at BP/PtSe₂ interface and resulting in full depleted working channels, the BP/PtSe₂ photodetector arrays exhibit high tolerance to carrier mobility decrease during the increasing operating temperature in a wide wavelength range from 532 to 2200 nm. Thus, the photodetector shows a state-of-the-art operating temperature at 470 K with the photo-responsivity (R) and specific detectivity (D*) of 25 A W⁻¹ and 6.4 × 10¹¹ Jones under 1850 nm illumination, respectively. Moreover, BP/PtSe₂ photodetector arrays show high-uniformity photo-response in a large area. This work provides new strategies for high-performance broadband photodetector arrays with HOT by Schottky junction of large-area BP/PtSe₂ films.     

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.     

In‐Plane Uniaxial Strain in Black Phosphorus Enables the Identification of Crystalline Orientation

Identification of the crystalline axis of anisotropic black phosphorus (BP) is important for investigating its physical properties, as well as for optical and electronic applications. Herein, it is showed that by applying in‐plane uniaxial strain and measuring the changes of the Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on the Raman shifts are angle‐dependent, and they can be expressed as a combination of the Raman responses under zigzag and armchair strain. Differing from previous polarized optical spectroscopic methods where the Raman intensity is analyzed, the proposed method uses the Raman frequency shift, which is less affected by laser polarization, excitation wavelength, the sample thickness, and the substrate. The effective strain applied on BP from the stretched substrate is estimated, and the results show that only 20 to 40% of the strain can be effectively transferred to BP flakes from a polyethylene terephthalate substrate. Our method provides not only an effective and robust approach to identify the crystalline orientation of layered BP, but it is also a model to extract additional information in strain‐related studies. It can also be extended to other 2D anisotropic materials.     

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.     

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.     

单层黑磷和蓝磷扩展分子结的电子输运特性

本工作应用非平衡格林函数理论和密度泛函理论研究了二维材料单层黑磷扩展分子结和蓝磷扩展分子结的电子输运特性,以及在应力作用下能隙和伏安特性的变化特点。结果发现,两种扩展分子结在发生应变(拉伸和压缩)过程中,随着应变(拉伸和压缩)的增加,HOMO能量和LUMO能量逐步靠近,导致能隙逐渐降低,分别减小了0.67eV和1.33eV。能隙降低,同时导通轨道间隔和导通轨道迁移率也降低,导致伏安特性曲线中量子化台阶逐渐消失,出现了类似金属的I-V曲线特点。此外,单层黑磷扩展分子结在0.75~2.00V范围内产生稳定电流,有望应用于电路中的稳流装置。     

Quasi‐Monolayer Black Phosphorus with High Mobility and Air Stability

Black phosphorus (BP) exhibits thickness‐dependent band gap and high electronic mobility. The chemical intercalation of BP with alkali metal has attracted attention recently due to the generation of universal superconductivity regardless of the type of alkali metals. However, both ultrathin BP, as well as alkali metal‐intercalated BP, are highly unstable and corrode rapidly under ambient conditions. This study demonstrates that alkali metal hydride intercalation decouples monolayer to few layers BP from the bulk BP, allowing an optical gap of ≈1.7 eV and an electronic gap of 1.98 eV to be measured by photoluminescence and electron energy loss spectroscopy at the intercalated regions. Raman and transport measurements confirm that chemically intercalated BP exhibits enhanced stability, while maintaining a high hole mobility of up to ≈800 cm² V^?1 s^?1 and on/off ratio exceeding 10³. The use of alkali metal hydrides as intercalants should be applicable to a wide range of layered 2D materials and pave the way for generating highly stable, quasi‐monolayer 2D materials.     

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.     

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.     

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.     

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.     

Black Phosphorus/Platinum Heterostructure: A Highly Efficient Photocatalyst for Solar‐Driven Chemical Reactions

A 2D black phosphorus/platinum heterostructure (Pt/BP) is developed as a highly efficient photocatalyst for solar‐driven chemical reactions. The heterostructure, synthesized by depositing BP nanosheets with ultrasmall (≈1.1 nm) Pt nanoparticles, shows strong Pt–P interactions and excellent stability. The Pt/BP heterostructure exhibits obvious P‐type semiconducting characteristics and efficient absorption of solar energy extending into the infrared region. Furthermore, during light illumination, accelerated charge separation is observed from Pt/BP as manifested by the ultrafast electron migration (0.11 ps) detected by a femtosecond pump‐probe microscopic optical system as well as efficient electron accumulation on Pt revealed by in situ X‐ray photoelectron spectroscopy. These unique properties result in remarkable performance of Pt/BP in typical hydrogenation and oxidation reactions under simulated solar light illumination, and its efficiency is much higher than that of other common Pt catalysts and even much superior to that of conventional thermal catalysis. The 2D Pt/BP heterostructure has enormous potential in photochemical reactions involving solar light and the results of this study provide insights into the design of next‐generation high‐efficiency photocatalysts.