Ohmic Contact in 2D Semiconductors via the Formation of a Benzyl Viologen Interlayer

The fabrication of a polymeric Ohmic contact interlayer between a metal and a 2D material using solution‐processed benzyl viologen (BV) is reported here. Predoping of the polymer alters the contact surface to obtain electron‐doped materials with ultrahigh work functions that significantly enhance the current density across the contact and reduce the contact resistance and Schottky barrier height. The fabrication of solution‐processed polymeric contacts for the preparation of high mobility MoS₂, WSe₂, MoTe₂, and BP (black phosphorous) FETs with significantly lowered contact resistance is demonstrated. Ohmic contacts are achieved and produce 3‐, 700‐, 3000‐, and 17‐fold increases in electron mobilities, respectively, when the bottom gate voltage is 10 V compared to those respective materials alone. Ambipolar and p‐type 2D material based FETs could, therefore, be transformed into n‐type FETs. Most importantly, the devices exhibit excellent stability in both ambient and vacuum.     

The Art of Integrated Functionalization: Super Stable Black Phosphorus Achieved through Metal‐Organic Framework Coating

Black phosphorus (BP) is a promising 2D nanomaterial with a great potential in various areas, while its intrinsic instability greatly suppresses practical applications, particularly under harsh conditions (e.g., high temperature). Herein, BP functionalization with Al ion is achieved in an integrated manner through MIL‐53 metal‐organic framework (MOF) coating, which greatly improves both ambient and thermal stability of BP. For the obtained MIL‐53 coated BP (BP@MIL‐53), abundant Al ion within MIL‐53 interacts with the lone pair electrons of BP, and subsequently decreases the BP surface electron density, reducing the reactivity of BP toward O₂ and H₂O. The MOF growth crosslinks the Al ion on the BP surface, and achieves integrated functionalization to withstand the detachment of individual Al ion from the BP surface. The noncovalent bond of BP? Al and highly porous structure of MIL‐53 preserve the physical/chemical properties of BP to the maximum, and render BP@MIL‐53 with super‐stability. This functionalization strategy extends the applications of BP based devices under high temperature conditions. As a proof of concept, BP@MIL‐53 is further utilized as a NO₂ gas sensor under relatively high operating temperatures. The BP@MIL‐53 sensor exhibits fast response, outstanding selectivity, and high recovery dynamic process in contrast to bare BP sensor.     

Creating an Air‐Stable Sulfur‐Doped Black Phosphorus‐TiO2 Composite as High‐Performance Anode Material for Sodium‐Ion Storage

With the increasing demand for low cost, long lifetime, high energy density storage systems, an extensive amount of effort has recently been focused on the development of sodium‐ion batteries (SIBs), and a variety of cathode materials have been discovered. However, looking for the most suitable anode material for practical application is a major challenge for SIBs. Herein, a high capacity sulfur‐doped black phosphorus‐TiO₂ (TiO₂‐BP‐S) anode material for SIBs is first synthesized by a feasible and large‐scale high‐energy ball‐milling approach, and its stability in air exposure is investigated through X‐ray photoelectron spectroscopy. The morphology of TiO₂‐BP‐S is characterized using transmission electron microscopy, indicating that the TiO₂ nanoparticles produce P? Ti bonds with BP. The TiO₂‐BP‐S composite with P? S and P? Ti bonds exhibits excellent stability in air and the superior electrochemical performance. For example, the discharge specific capacity is up to 490 mA h g^?1 after 100 cycles at 50 mA g^?1, and it remains at 290 mA h g^?1 after 600 cycles at 500 mA g^?1. Meanwhile, the scientific insight that the formation of stable P? S and P? Ti bonds can provide a guide for the practical large‐scale application of SIBs in other titanium base and black phosphorus materials is looked forward.     

Solution‐Processable Ultrathin Black Phosphorus as an Effective Electron Transport Layer in Organic Photovoltaics

2D van der Waals crystals, possessing excellent electronic and physical properties, have been intriguing building blocks for organic optoelectronic devices. Most of the 2D materials are served as hole transport layers in organic devices. Here,it is reported that solution exfoliated few layers black phosphorus (BP) can be served as an effective electron transport layer (ETL) in organic photovoltaics (OPVs) for the first time. The power conversion efficiencies (PCEs) of the BP‐incorporated OPVs can be improved to 8.18% in average with the relative enhancement of 11%. The incorporation of BP flakes with the optimum thickness of ≈10 nm can form cascaded band structure in OPVs, which can facilitate electron transport and enhance the PCEs of the devices. This study opens an avenue in using solution exfoliated BP as a highly efficient ETL for organic optoelectronics.     

Black phosphorus junctions and their electrical and optoelectronic applications

Black phosphorus(BP),an emerging two-dimensional material,is considered a promising candidate for next-genera-tion electronic and optoelectronic devices due to in-plane anisotropy,high mobility,and direct bandgap.However,BP devices face challenges due to their limited stability,photo-response speed,and detection range.To enhance BP with powerful electric-al and optical performance,the BP heterostructures can be created.In this review,the state-of-the-art heterostructures and their electrical and optoelectronic applications based on black phosphorus are discussed.Five parts introduce the perform-ance of BP-based devices,including black phosphorus sandwich structure by hBN with better stability and higher mobility,black phosphorus homojunction by dual-gate structure for optical applications,black phosphorus heterojunction with other 2D materials for faster photo-detection,black phosphorus heterojunction integration with 3D bulk material,and BP via As-doping tunable bandgap enabling photo-detection up to 8.2 μm.Finally,we discuss the challenges and prospects for BP electric-al and optical devices and applications.     

Field‐Induced n‐Doping of Black Phosphorus for CMOS Compatible 2D Logic Electronics with High Electron Mobility

Black phosphorus (BP) has been considered as a promising two‐dimensional (2D) semiconductor beyond graphene owning to its tunable direct bandgap and high carrier mobility. However, the hole‐transport‐dominated characteristic limits the application of BP in versatile electronics. Here, we report a stable and complementary metal oxide semiconductor (COMS) compatible electron doping method for BP, which is realized with the strong field‐induced effect from the K⁺ center of the silicon nitride (Si_xN_y). An obvious change from pristine p‐type BP to n type is observed after the deposit of the Si_xN_y on the BP surface. This electron doping can be kept stable for over 1 month and capable of improving the electron mobility of BP towards as high as ~176 cm² V^–1 s^–1. Moreover, high‐performance in‐plane BP p‐n diode and further logic inverter were realized by utilizing the n‐doping approach. The BP p‐n diode exhibits a high rectifying ratio of ~10⁴. And, a successful transfer of the output voltage from “High” to “Low” with very few voltage loss at various working frequencies were also demonstrated with the constructed BP inverter. Our findings paves the way for the success of COMS compatible technique for BP‐based nanoelectronics.     

Dual Emission of Water‐Stable 2D Organic–Inorganic Halide Perovskites with Mn(II) Dopant

Moisture‐delicate and water‐unstable organic–inorganic halide perovskites (OI‐HPs) create huge challenges for the synthesis of highly efficient water‐stable light‐emitting materials for optoelectronic devices. Herein, a simple acid solution–assisted method to synthesize quantum confined 2D lead perovskites through Mn doping is reported. The efficient energy transfer between host and dopant ions in orange light‐emitting Mn²⁺‐doped OI‐HPs leads to the most efficient integrated luminescence with a photoluminescence quantum yield over 45%. The Mn²⁺ substitution of Pb²⁺ and passivation with low dielectric constant molecules such as phenethylamine, benzylamine, and butylamine enhance water resistivity, leading to water stability. The dual emission process of this water‐stable 2D Mn‐doped perovskite will help in developing highly efficient 2D water‐stable perovskites for practical applications.     

Catalase Nanocapsules Protected by Polymer Shells for Scavenging Free Radicals of Tobacco Smoke

Free radicals in tobacco smoke trigger numerous lung diseases, which are worldwide health considerations. The ideal free‐radical, tobacco‐smoke scavenger must be highly reactive and temperature resistant. Catalases (CATs) show attractive potential to scavenge free radicals in tobacco smoke, because of their higher reaction rate compared to that of non‐catalyzed reactions. Their fragile nature, however, diminishes their catalytic activity in hot tobacco smoke. Therefore, it is essential to enhance the structural integrity and catalytic stability of these enzymes under severe environmental conditions. In order to improve the thermal stability of CATs, we have developed a facile approach to produce CAT nanocapsules (nCATs) by encapsulating a single enzyme in a polyacrylamide (PAAM) shell. The rigid polymer shells on the CATs surface prevents their free deformation. The secondary structure of the enzyme is retained and their dissociation is almost nil even under high operational temperatures. As a result, the structural stability and thermal resistance of the enzyme are significantly enhanced. The nCATs are covalently bound on cellulose acetate fibers to enable the enzyme sticking to the cigarette filters.The electron paramagnetic resonance (EPR) and Saltzman procedure reveal that the nCATs are able to efficiently scavenge 90% of the free radicals in tobacco smoke. The use of such nCATs with improved enzyme thermal stability opens up new opportunities for future application in cigarette filters.     

Surface-enhanced Raman spectroscopy chips based on two-dimensional materials beyond graphene

Surface-enhanced Raman spectroscopy (SERS) based on two-dimensional (2D) materials has attracted great attention over the past decade.Compared with metallic materials,which enhance Raman signals via the surface plasmon effect,2D materials integrated on silicon substrates are ideal for use in the fabrication of plasmon-free SERS chips,with the advantages of outstanding fluorescence quenching capability,excellent biomolecular compatibility,tunable Fermi levels,and potentially low-cost material preparation.Moreover,recent studies have shown that the limits of detection of 2D-material-based SERS may be comparable with those of metallic substrates,which has aroused significant research interest.In this review,we comprehensively summarize the advances in SERS chips based on 2D materials.As several excellent reviews of graphene-enhanced Raman spectroscopy have been published in the past decade,here,we focus only on 2D materials beyond graphene,i.e.,transition metal dichalcogenides,black phosphorus,hexagonal boron nitride,2D titanium carbide or nitride,and their heterostructures.We hope that this paper can serve as a useful reference for researchers specializing in 2D materials,spectroscopy,and diverse applications related to chemical and biological sensing.     

Opportunities and Challenges in Precise Synthesis of Transition Metal Single-Atom Supported by 2D Materials as Catalysts toward Oxygen Reduction Reaction

Transition metal single-atom catalysts (SACs) are currently a hot area of research in the field of electrocatalytic oxygen reduction reaction (ORR). In this review, the recent advances in transition metal single-atom supported by 2D materials as catalysts for ORR with high performance are reported. Due to their large surface area, uniformly exposed lattice plane, and adjustable electronic state, 2D materials are ideal supporting materials for exploring ORR active sites and surface reactions. The rational design principles and synthetic strategies of transition metal SACs supported by 2D materials are systematically introduced while the identification of active sites, their possible catalytic mechanisms as well as the perspectives on the future of transition metal SACs supported by 2D materials for ORR applications are discussed. Finally, according to the current development trend of ORR catalysts, the future opportunities and challenges of transition metal SACs supported by 2D materials are summarized.     

Controlled Assembly of MXene Nanosheets as an Electrode and Active Layer for High-Performance Electronic Skin

MXenes are an emerging class of 2D transition metal carbides and nitrides. They have been widely used in flexible electronics owing to their excellent conductivity, mechanical flexibility, and water dispersibility. In this study, the electrode and active layer applications of MXene materials in electronic skins are realized. By utilizing vacuum filtration technology, few-layer MXene electrodes are integrated onto the top and bottom surfaces of the 3D polyacrylonitrile (PAN) network to form a stable electronic skin. The fabricated flexible device with Ti₃C₂T_x MXene electrodes outperforms those with other electrodes and exhibits excellent device performance, with a high sensitivity of 104.0 kPa⁻¹, fast response/recovery time of 30/20 ms, and a low detection limit of 1.5 Pa. Furthermore, the electrode and the constructed MXene/PAN-based flexible pressure sensor exhibit robust mechanical stability and can survive 240 bending cycles. Such a robust, flexible device can be enlarged or folded like a jigsaw puzzle or origami and transformed from 2D to 3D structures; moreover, it can detect tiny movements of human muscles, such as movements corresponding to sound production and intense movements during bending of fingers.     

Black Phosphorus and Polymeric Carbon Nitride Heterostructure for Photoinduced Molecular Oxygen Activation

Metal‐free heterostructure photocatalysts composed of black phosphorus (BP) and polymeric carbon nitride (CN) are successfully synthesized via a one‐step liquid exfoliation method assisted by sonication. The combination of BP with CN strengthens the visible‐light harvesting ability, facilitates the charge separation in the photocatalytic process, and renders promoted activity of photoinduced molecular oxygen activation, such as superoxide radicals (·O₂^?) evolution and H₂O₂ production. This work highlights that coupling semiconductors with well‐matched band levels provide a flexible route to enhance the performance of photocatalysts for producing reactive oxygen species, and gives ideas for the design of highly active and metal‐free materials toward sustainable solar‐to‐chemical energy conversion and environmental remediation.     

Highly Symmetrical Six-Transition Metal Ring Units Promising High Air-Stability of Layered Oxide Cathodes for Sodium-Ion Batteries

Layered oxides are the most prevalent cathodes for sodium-ion batteries (SIBs), but their poor air stability significantly limits their practical application owing to the rapid performance degradation of aged materials and the cost increase for material storage and transportation. Here, an effective strategy of constructing stable transition metal (TM) layers with a highly symmetrical six-TM ring is suggested to enhance structure stability, thus hindering ambient air corrosion. The density functional theory calculations reveal that the higher symmetry ensures a higher thermodynamic energy for H₂O insertion into Na layer. The combined analyses of selected area electron diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and chemical titration indicate that the six-TM ring structure can effectively suppress the series of aging processes including water insertion, the spontaneous loss of lattice sodium, TM valence increment and residual alkali formation. Benefiting from the overall suppression of aging process, the strategy results in an excellent improvement in capacity retention after air exposure from 13.57% to 95.59%, and exhibits a good universality for both P2- and O3-cathodes, which are the two most common structures of Na-based layered oxides with different aging mechanism. These findings provide new insight to design high-performance cathodes for SIBs.     

Fluorination of Black Phosphorus—Will Black Phosphorus Burn Down in the Elemental Fluorine?

Black phosphorus (BP) represents a promising tunable bandgap alternative to graphene and other 2D materials in the field of semiconductors. However, its reactivity toward covalent modification of its surface (as a key to its bandgap adjustment) is scarcely reported. Here a method of covalent modification of BP involving reaction with fluorine is reported. Other allotropes of phosphorus are known to react violently with fluorine resulting in its complete burning down and formation of gaseous phosphorus pentafluoride. The results of our fluorination experiments conducted in analogy to the procedures used for fluorination of graphene indicate a successful binding of fluorine to BP. This route of modification of BP opens new possible ways toward covalent modification of the surface of this promising material.     

3D Chemical Cross‐Linking Structure of Black Phosphorus@CNTs Hybrid as a Promising Anode Material for Lithium Ion Batteries

The existence of rechargeable lithium ion batteries with high operating voltage, high energy density, and excellent cycling performance are drawing increasing attention due to their viability to be used as portable power and in electrical applications. However, there is a considerable problem that the conductivity of the active material becomes poor due to the volume expansion under the condition of repeated circulation, which reduces the performance of the device, thus hindering its practical application. As an emerging 2D material, black phosphorus (BP) has drawn significant attention in the field of Li‐ion battery energy storage due to its large theoretical capacity of 2596 mA h g^?1 and ability to absorb large amount of Li atoms. Here, a unique 3D conductive structure with the BP and carbon nanotubes (CNTs), displaying good stability and high conductivity for the fabrication of BP@CNTs hybrid‐based Li‐ion batteries is described. With strong trapping, good affinity, structure stable, and high adsorption for polyphosphorus, the developed BP@CNTs hybrid electrodes display high capacity, good electrical conductivity, and a stable cycle life. Additionally, the lithium ion batteries can illuminate the light emitting diode, proving that the materials have great potential for development of energy storage devices.     

Electron‐Beam Manipulation of Silicon Impurities in Single‐Walled Carbon Nanotubes

The recent discovery that impurity atoms in crystals can be manipulated with focused electron irradiation has opened novel perspectives for top‐down atomic engineering. These achievements have been enabled by advances not only in electron optics and microscope stability but also in the preparation of suitable materials with impurity elements incorporated via ion and electron‐beam irradiation or chemical means. Here it is shown that silicon heteroatoms introduced via plasma irradiation into the lattice of single‐walled carbon nanotubes (SWCNTs) can be manipulated using a focused 55–60 keV electron probe aimed at neighboring carbon sites. Moving the silicon atom mainly along the longitudinal axis of large 2.7 nm diameter tubes, more than 90 controlled lattice jumps are recorded and the relevant displacement cross sections are estimated. Molecular dynamics simulations show that even in 2 nm diameter SWCNTs, the threshold energies for out‐of‐plane dynamics are different than in graphene, and depend on the orientation of the silicon‐carbon bond with respect to the electron beam as well as the local bonding of the displaced carbon atom and its neighbors. Atomic‐level engineering of SWCNTs where the electron wave functions are more strictly confined than in 2D materials may enable the fabrication of tunable electronic resonators and other devices.     

Electron Field Emission of Geometrically Modulated Monolayer Semiconductors

Electron field emission, electrons emitted from solid surfaces under high electric field, offers significant scientific interests in materials sciences and potential optoelectronics applications. 2D atomic layers, such as MoS₂, exhibit fascinating properties for diverse applications in next‐generation nanodevices and rich physical phenomena for fundamental research. However, the study on field emission of semiconducting monolayers is lacking owing to its low efficiency and stability of electron emission. Here, electron field emission of the geometrically modulated monolayer semiconductors suspended with 1D nanoarrays is demonstrated. Chemical vapor deposition synthesis of two prototype monolayers of transition metal dichalcogenides (TMD), MoS₂ and MoSe₂, is presented and their diverse band structures offer an ideal platform to explore the fundamental process of the electron emission in the TMD. Geometrical modulation and charge transfer of the semiconducting monolayers can be significantly tuned with the structural suspension with the 1D ZnO nanoarrays. Possible mechanisms on the enhanced electron emission of the 2D monolayers are discussed. With geometrical control of the monolayers, a highly efficient and stable electron emission of TMD monolayers is achieved in low turn‐on electric fields, enabling applications on electrons sources and opening a new avenue toward geometrically tuned atomic layers.     

On the Origin of Sinter-Resistance and Catalyst Accessibility in Raspberry-Colloid-Templated Catalyst Design

Nanoparticle (NP) sintering is a major cause of the deactivation of supported catalysts. Raspberry-colloid-templated (RCT) catalysts are an emerging class of materials that show an unprecedented level of sinter-resistance and exhibit high catalytic activity. Here a comprehensive study of the origin of NP stability and accessibility in RCT catalysts using theoretical modeling, 3D electron microscopy, and epitaxial overgrowth is reported. The approach is showcased for silica-based RCT catalysts containing dilute Pd-in-Au NPs previously used in hydrogenation and oxidation catalysis. Modeling of the contact line of the silica precursor infiltrating into the assembled raspberry colloids suggests that a large part of the particles must be embedded into silica, which is confirmed by quantitative visualization of >200 individual NPs by dual-axis electron tomography. The RCT catalysts have a unique structure in which all NPs reside at the pore wall but have >50% of their surface embedded in the matrix, giving rise to the strongly enhanced thermal and mechanical stability. Importantly, epitaxial overgrowth of Ag on the supported NPs reveals that not only the NP surface exposed to the pore but the embedded interface as well remained chemically accessible. This mechanistic understanding provides valuable guidance in the design of stable catalytic materials.     

Toward Bioelectronic Nanomaterials: Photoconductivity in Protein–Porphyrin Hybrids Wrapped around SWCNT

The development of sophisticated ordered functional materials is one of the important challenges in current science. One of the keys to enhance the properties of these materials is the control of the organization and morphology at different scales. This work presents a novel bioinspired methodology to achieve highly ordered donor/acceptor bio‐nanohybrids using a designed repeat protein as scaffold, endowed with photoactive and electron donating porphyrin (P) units, to efficiently wrap around electron accepting single wall carbon nanotubes (SWCNT). A systematic experimental and theoretical study to evaluate the effect of the length of the protein reveals that longer proteins wrap around the SWCNT in a more efficient manner due to the stronger supramolecular interaction existing between the inner concave surface of the protein (namely Trp and His residues) and the convex surface of the (7,6)‐SWCNT. Interestingly, spectroscopy and X‐ray diffraction data further confirm that the so‐called protein‐P–SWCNT donor–acceptor bio‐nanohybrids retain the original protein structure. Finally, the new bio‐nanohybrids show a remarkable enhancement on the photoconductivity values by flash‐photolysis microwave conductivity (FP‐TRMC technique) demonstrating that the major charge carriers of electrons are injected into the SWCNTs and move along the 1D‐structures.     

Highly Efficient Electrocatalysts for Oxygen Reduction Reaction Based on 1D Ternary Doped Porous Carbons Derived from Carbon Nanotube Directed Conjugated Microporous Polymers

One‐dimensional (1D) porous materials have shown great potential for gas storage and separation, sensing, energy storage, and conversion. However, the controlled approach for preparation of 1D porous materials, especially porous organic materials, still remains a great challenge due to the poor dispersibility and solution processability of the porous materials. Here, carbon nanotube (CNT) templated 1D conjugated microporous polymers (CMPs) are prepared using a layer‐by‐layer method. As‐prepared CMPs possess high specific surface areas of up to 623 m² g^?1 and exhibit strong electronic interactions between p‐type CMPs and n‐type CNTs. The CMPs are used as precursors to produce heteroatom‐doped 1D porous carbons through direct pyrolysis. As‐produced ternary heteroatom‐doped (B/N/S) 1D porous carbons possess high specific surface areas of up to 750 m² g^?1, hierarchical porous structures, and excellent electrochemical‐catalytic performance for oxygen reduction reaction. Both of the diffusion‐limited current density (4.4 mA cm^?2) and electron transfer number (n = 3.8) for three‐layered 1D porous carbons are superior to those for random 1D porous carbon. These results demonstrate that layered and core–shell type 1D CMPs and related heteroatom‐doped 1D porous carbons can be rationally designed and controlled prepared for high performance energy‐related applications.