Xenes as an Emerging 2D Monoelemental Family: Fundamental Electrochemistry and Energy Applications

As an emerging subclass of 2D materials, Xenes (e.g., borophene, silicene, germanene, stanene, phosphorene, arsenene, antimonene, and bismuthene) consist of one single element and have opened the door for various important applications. Benefiting from their impressive characteristics, including ultrathin folded structure, ultrahigh surface–volume ratio, excellent mechanical strength and flexibility, Xenes are considered as promising electrode materials in the field of electrochemical energy with large capacity, high rate, and high safety. This review provides a comprehensive summary of selected properties, synthetic challenges, and the latest theoretical and experimental advances in the energy‐related applications of Xenes, including Li/Na ion batteries, Li–S batteries, electrocatalysis, and supercapacitors. Finally, the challenges and outlook of this emerging field are discussed.     

Sensing Applications of Atomically Thin Group IV Carbon Siblings Xenes: Progress,Challenges, and Prospects

The 2D graphene (G) nanosheets (NSs) discovery is amound the foremost revolutionary incidents in materials science history. This discovery has stimulated huge attention in the study of other novel 2D materials (2DMs). This trend might be called modern day “alchemy,” where the basic aim is to convert most of periodic table elements into G like 2D structures. Monoelemental, atomically thin 2DMs, called “Xenes” (“X” = group (III–VI)A elements, “ene” suffix that indicates one atom thick 2D layer of atoms) which are a newly invented family among nanomaterials. The number of predicted and experimentally synthesized 2D Xene materials of group IVA, i.e., G's siblings, has gained attention in nanosize devices. Such materials involve buckle structures that have recently been experimentally fabricated. The 2D Xene materials analog to G offer exciting potential for novel sensing applications. The group IVA Xenes, in cooperation with their ligand-functionalized derivatives, arrange in a honeycomb lattice analogous to G but through a changeable degree of buckling. Their electronic structure ranges from trivial insulators passing via semiconductors with tunable gaps, to semimetallic, depending on substrate, chemical functionalization, and strain. In this review, different potential synthesis methods for group IVA 2D Xenes are briefly presented. A brief overview of their properties obtained theoretically and experimentally is presented, and finally their potential sensing applications are discussed.     

铋烯材料生长控制及光电子器件应用研究进展

石墨烯和其他二维材料凭借其自身独特的物理和化学性能,引起了科学和工程领域的广泛关注。探索新型二维材料体系并扩展其应用范围是研究人员的热点研究内容。其中,第五主族单元素二维烯(二维磷烯、二维砷烯、二维锑烯、二维铋烯)具有较窄的且可调节的能带宽度、高的载流子迁移率、良好的透光性和优异的光电子学性能,成为二维材料及其在光电子应用领域的新的研究对象。鉴于此方面,从基本物理结构、材料的制备方法和在光电子方面的应用深入分析二维铋烯的相关理论以及实验研究的工作进展。在材料的可控制备方面,重点围绕二维铋烯的电化学剥离法展开相应论述。最后讨论了二维铋烯在光电子学应用领域的现状,包括在超快光电子学器件的应用,并且对二维铋烯未来的发展进行了展望。     

Progress,Challenges, and Opportunities for 2D Material Based Photodetectors

2D material based photodetectors have attracted many research projects due to their unique structures and excellent electronic and optoelectronic properties. These 2D materials, including semimetallic graphene, semiconducting black phosphorus, transition metal dichalcogenides, insulating hexagonal boron nitride, and their various heterostructures, show a wide distribution in bandgap values. To date, hundreds of photodetectors based on 2D materials have been reported. Here, a review of photodetectors based on 2D materials covering the detection spectrum from ultraviolet to infrared is presented. First, a brief insight into the detection mechanisms of 2D material photodetectors as well as introducing the figure‐of‐merits which are key factors for a reasonable comparison between different photodetectors is provided. Then, the recent progress on 2D material based photodetectors is reviewed. Particularly, the excellent performances such as broadband spectrum detection, ultrahigh photoresponsivity and sensitivity, fast response speed and high bandwidth, polarization‐sensitive detection are pointed out on the basis of the state‐of‐the‐art 2D photodetectors. Initial applications based on 2D material photodetectors are mentioned. Finally, an outlook is delivered, the challenges and future directions are discussed, and general advice for designing and realizing novel high‐performance photodetectors is given to provide a guideline for the future development of this fast‐developing field.     

4D Printing at the Microscale

3D printing of adaptive and dynamic structures, also known as 4D printing, is one of the key challenges in contemporary materials science. The additional dimension refers to the ability of 3D printed structures to change their properties—for example, shape—over time in a controlled fashion as the result of external stimulation. Within the last years, significant efforts have been undertaken in the development of new responsive materials for printing at the macroscale. However, 4D printing at the microscale is still in its early stages. Thus, this progress report will focus on emerging materials for 4D printing at the microscale as well as their challenges and potential applications. Hydrogels and liquid crystalline and composite materials have been identified as the main classes of materials representing the state of the art of the growing field. For each type of material, the challenges and critical barriers in the material design and their performance in 4D microprinting are discussed. Importantly, further necessary strategies are proposed to overcome the limitations of the current approaches and move toward their application in fields such as biomedicine, microrobotics, or optics.     

2D Metallic Transition-Metal Dichalcogenides: Structures,Synthesis, Properties,and Applications

2D materials and the associated heterostructures define an ideal material platform for investigating physical and chemical properties, and exhibiting new functional applications in (opto)electronic devices, electrocatalysis, and energy storage. 2D transition metal dichalcogenides (2D TMDs), as a member of the 2D materials family including 2D semiconducting TMDs (s-TMDs) and 2D metallic/semimetallic TMDs (m-TMDs) have attracted considerable attention in the scientific community. Over the past decade, the 2D s-TMDs have been extensively researched and reviewed elsewhere. Because of their distinctive physical properties including intrinsic magnetism, charge-density-wave order and superconductivity, and potential applications, such as high-performance electronic devices, catalysis, and as metal electrode contacts, 2D m-TMDs have grabbed widespread attention in recent years. However, reviews demonstrating the m-TMDs systematically and comprehensively have been rarely reported. Here, the recent advances in 2D m-TMDs in the aspects of their unique structures, synthetic approaches, distinctive physical properties, and functional applications are highlighted. Finally, the current challenges and perspectives are discussed.     

Salt‐Assisted Synthesis of 2D Materials

2D materials have demonstrated good chemical, optical, electrical, and magnetic characteristics, and offer great potential in numerous applications. Corresponding synthesis technologies of 2D materials that are high‐quality, high‐yield, low‐cost, and time‐saving are highly desired. Salt‐assisted methods are emerging technologies that can meet these requirements for the fabrication of 2D materials. Herein, the recent process for the salt‐assisted synthesis of 2D materials and their typical applications are summarized. First, the properties of salt crystals and molten salts are briefly introduced, and then some examples of 2D materials synthesis with the assistance of salt as well as their representative applications are presented. The underlying mechanisms of salts with different states on the formation of 2D morphology are discussed to aid in the rational design of synthetic route of 2D materials. At last, the challenges and future perspectives for salt‐assisted methods are briefly described. This review provides guidance for the controllable synthesis of 2D materials based on the salt‐assisted approaches.     

Fabrication,Applications, and Prospects of Aramid Nanofiber

Aramid nanofibers (ANFs) are of great interest in various applications due to its 1D nanoscale, high aspect ratio, high specific surface area, excellent strength, and modulus as well as impressive chemical and thermal stabilities. It is considered as one of the most promising nano‐sized building blocks with excellent properties and has therefore drawn increasing attention since 2011. However, no review has summarized the research progress and the prospective challenges of ANF. Herein, the methods of ANF fabrication and their relative merits are comprehensively discussed together with the challenges and progress in the deprotonation method for preparing ANF. The fabrication methods and development of ANF‐based advanced materials with different macroscopic morphologies, including the 1D ANF aerogel fiber, 2D ANF film/nanopaper/coating, and 3D ANF gel and particle are also described. Furthermore, the applications of ANF in nanocomposite reinforcement, battery separators, electrical insulation nanopaper, flexible electronics, and adsorption and filtration media are presented. Additionally, the possible challenges and outlooks toward the future development of ANF are highlighted. This review indicates that the ANF and ANF‐based materials mentioned herein will boost the development of next‐generation advanced functional materials.     

Emerging MXene-Based Memristors for In-Memory,Neuromorphic Computing,and Logic Operation

Confronted by the difficulties of the von Neumann bottleneck and memory wall, traditional computing systems are gradually inadequate for satisfying the demands of future data-intensive computing applications. Recently, memristors have emerged as promising candidates for advanced in-memory and neuromorphic computing, which pave one way for breaking through the dilemma of current computing architecture. Till now, varieties of functional materials have been developed for constructing high-performance memristors. Herein, the review focuses on the emerging 2D MXene materials-based memristors. First, the mainstream synthetic strategies and characterization methods of MXenes are introduced. Second, the different types of MXene-based memristive materials and their widely adopted switching mechanisms are overviewed. Third, the recent progress of MXene-based memristors for data storage, artificial synapses, neuromorphic computing, and logic circuits is comprehensively summarized. Finally, the challenges, development trends, and perspectives are discussed, aiming to provide guidelines for the preparation of novel MXene-based memristors and more engaging information technology applications.     

2D Covalent Organic Frameworks for Biomedical Applications

Covalent organic frameworks (COFs) are an emerging class of organic crystalline polymers with well‐defined molecular geometry and tunable porosity. COFs are formed via reversible condensation of lightweight molecular building blocks, which dictate its geometry in two or three dimensions. Among COFs, 2D COFs have garnered special attention due to their unique structure composed of two‐dimensionally extended organic sheets stacked in layers generating periodic columnar π‐arrays, functional pore space, and their ease of synthesis. These unique features in combination with their low density, high crystallinity, large surface area, and biodegradability have made them an excellent candidate for a plethora of applications ranging from energy to biomedical sciences. In this article, the evolution of 2D COFs is briefly discussed in terms of different types of chemical linkages, synthetic strategies of bulk and nanoscale 2D COFs, and their tunability from a biomedical perspective. Next, the biomedical applications of 2D COFs specifically for drug delivery, phototherapy, biosensing, bioimaging, biocatalysis, and antibacterial activity are summarized. In addition, current challenges and emerging approaches in designing 2D COFs for advanced biomedical applications are discussed.     

Review of Broadband Metamaterial Absorbers: From Principles,Design Strategies,and Tunable Properties to Functional Applications

Metamaterial absorbers have been widely studied and continuously concerned owing to their excellent resonance features of ultra-thin thickness, light-weight, and high absorbance. Their applications, however, are typically restricted by the intrinsic dispersion of materials and strong resonant features of patterned arrays (mainly referring to narrow absorption bandwidth). It is, therefore essential to reassert the principles of building broadband metamaterial absorbers (BMAs). Herein, the research progress of BMAs from principles, design strategies, tunable properties to functional applications are comprehensively and deeply summarized. Physical principles behind broadband absorption are briefly discussed, typical design strategies in realizing broadband absorption are further emphasized, such as top-down lithography, bottom-up self-assembly, and emerging 3D printing technology. Diversified active components choices, including optical response, temperature response, electrical response, magnetic response, mechanical response, and multi-parameter responses, are reviewed in achieving dynamically tuned broadband absorption. Following this, the achievements of various interdisciplinary applications for BMAs in energy-harvesting, photodetectors, radar-IR dual stealth, bolometers, noise absorbing, imaging, and fabric wearable are summarized. Finally, the challenges and perspectives for future development of BMAs are discussed.     

High‐Mobility Carriers in Germanene Derivatives

Buckled elemental analogs of graphene—2D‐Xenes silicene, germanene, and stanene—and their derivatives are predicted to host high‐mobility carriers. Experiments, however, have not as yet confirmed the predictions. Here, high‐mobility (exceeding 10⁴ cm² V^?1 s^?1) carriers are discovered in intercalated multilayer germanene. Epitaxial films of antiferromagnetic and diamagnetic MGe₂ are synthesized via topochemical reactions, followed by extensive studies of the atomic and magnetic structures. Quantum oscillations in MGe₂ resistance manifest quasi‐2D Fermi‐surface pockets with effective masses of carriers as low as 0.015 m_e, comparable to graphene. The detected signature of the chiral anomaly in magnetoresistance and nonzero Berry phases may indicate the topological nature of the MGe₂ electronic structure and charge transport. The discovery bridges the gap between theory and experiment, thus establishing 2D‐Xenes as promising building blocks in materials engineering. Concurrently, the combination of magnetism and high mobility in Eu‐intercalated germanene is attractive for spintronic applications.     

3D Microprinting of Super-Repellent Microstructures: Recent Developments,Challenges, and Opportunities

Liquid super-repellent surfaces, characterized by a low liquid–solid contact fraction, allow various liquids to bead up and freely roll off. Apart from liquid repellency, these surfaces feature several unique properties, including inter alia, self-cleaning, low-friction, anti-icing, and anti-biofouling, making them valuable for a vast array of applications involving liquids. Essential to achieve such super-repellency is the bio-inspired reentrant or doubly reentrant micro-topography. However, despite their unique interfacial properties, the fabrication of these delicate 3D topographies by conventional microfabrication methods is extremely challenging. Recently, emerging 3D microprinting technologies, particularly two-photon lithography, have brought new scope to this field. With unparalleled design freedom and flexibility, 3D microprinting greatly facilitates the design, testing, and studying of complex 3D microstructures. Here, applications of 3D microprinting in the design and fabrication of super-repellent microstructures are summarized, with a focus on their remarkable properties, and new functionalities offered by these intricate 3D topographies. Current challenges and new opportunities of emerging 3D microprinting techniques to further advance liquid super-repellent materials are also discussed.     

Engineering 2D Multifunctional Ultrathin Bismuthene for Multiple Photonic Nanomedicine

2D monoelemental nanomaterials (Xenes) have shown tremendous potential for versatile biomedical applications. Bismuth, as a heavy element in pnictogens, has acquired massive research interest due to its unique optical performance, high biocompatibility, stability, and relatively low cost. However, the utilization of 2D bismuthene in nanomedicine has not been achieved because of the difficulty in engineering bismuthene with crucial structural/compositional characteristics for satisfying strict biomedical requirements. Herein, to address this Gordian knot, a facile strategy to intercalate and delaminate Bi bulk for generating mass few-layered 2D bismuthene with high yield by employing a water molecule mediated freezing–thawing process and sodium borohydride-triggered reduction treatment is proposed. The resulting 2D bismuthene displays good optical performance in the near-infrared (NIR) biowindow and can be excited via red light for reactive oxygen species generation, enabling applications in multiple photonic cancer nanomedicine settings, including photothermal hyperthermia and photodynamic therapy. Utilizing the intrinsic desirable optical absorbance and strong X-ray attenuation of bismuthene, dual photonic therapy can be conducted under the supervision of photoacoustic/computed tomography guided multimodal imaging. This research not only offers a potential mass-production ready, cost-effective, and eco-efficient methodology for engineering 2D Xenes, but also exploits an innovative 2D bismuthene based photonic cancer nanomedicine.     

2D Molybdenum Compounds for Electrocatalytic Energy Conversion

The development of advanced nanomaterials is urgent for electrocatalytic energy conversion applications. Recently, 2D nanomaterial-derived heterogeneous electrocatalysts have shown great promise for both fundamental research and practical applications owing to their extremely high surface-to-volume ratio and tunable geometric and electronic properties. Because of their unique electronic structure and physicochemical properties, molybdenum (Mo)-based 2D nanomaterials are emerging as one of the most attractive candidates among the nonprecious materials for electrocatalysts. This review provides a comprehensive overview of the recent advances in the synthesis and modulation of 2D Mo compounds for applications in electrocatalytic energy conversion. The categories based on different compositions and corresponding synthetic approaches of 2D Mo compounds are first introduced. Subsequently, various atomic/plane/synergistic engineering strategies, along with catalytic optimization in the electrochemical process that involves the cycles of water, carbon, and nitrogen, are discussed in detail. Finally, the current challenges and future opportunities for the development of 2D Mo-based electrocatalysts are proposed with the goal of shedding light on these promising 2D nanomaterials for electrocatalytic energy conversion.     

Wearable Biofuel Cells: Advances from Fabrication to Application

The booming field of wearable devices has nourished progress in developing multifunctional wearable energy sources that can withstand deformations while maintaining their electrochemical functions. Unlike energy storage systems such as rechargeable batteries and supercapacitors, wearable biofuel cells (w-BFCs) generate green electricity from energy-dense carbon-neutral fuels via highly efficient bioelectrochemical reactions, delivering excellent biocompatibility, remarkable environmental sustainability, and exceptional capability of miniaturization. These desirable merits give w-BFCs great potential in the field of wearable applications. Moreover, emerging studies of w-BFCs in self-powered biosensing, controlled drug delivery, and wound dressings have greatly expanded their possible fields of application. Recent progress and strategies to accomplish flexible and stretchable w-BFCs are summarized here. Novel materials and configurations with tailored features that can be employed to fabricate w-BFCs are elaborated and discussed. Current applications and near-future applications of w-BFCs in health-monitoring and medical treatment fields are outlined. Furthermore, challenges and perspectives regarding this emerging field of materials science and engineering are also emphasized.     

Synthesis,properties, and applications of large-scale two-dimensional materials by polymer-assisted deposition

Two-dimensional(2D) materials have attracted considerable attention because of their novel and tunable electronic,optical, ferromagnetic, and chemical properties. Compared to mechanical exfoliation and chemical vapor deposition, polymer-assisted deposition(PAD) is more suitable for mass production of 2D materials owing to its good reproducibility and reliability. In this review, we summarize the recent development of PAD on syntheses of 2D materials. First, we introduce principles and processing steps of PAD. Second, 2D materials, including graphene, MoS2, and MoS2/glassy-graphene heterostructures, are presented to illustrate the power of PAD and provide readers with the opportunity to assess the method. Last, we discuss the future prospects and challenges in this research field. This review provides a novel technique for preparing 2D layered materials and may inspire new applications of 2D layered materials.     

Pushing the Limit of Ordered Mesoporous Materials via 2D Self-Assembly for Energy Conversion and Storage

Material and energy efficiencies are two key parameters that benchmark the electrochemical energy conversion and storage devices (EECSDs). Maximizing both requires researchers to grasp the limits of the physiochemical properties of core electrode materials. Ordered mesoporous materials (OMMs) have been regarded as promising electrode materials; however, their intrinsic deficiencies (e.g., plugs, inaccessible pores, and surfaces) impose limits for wide applications. 2D ordered mesoporous materials (2DOMMs) featured with an extended lateral dimension and a nanometer thickness not only inherit the structure advantages of mesoporous materials, but also have a unique 2D ultrathin feature that can fully address the imperfections of conventional OMMs. Herein, recent achievements on the preparation of 2DOMMs by combining single micelle assembly strategy with 2D bottom-up patterning techniques including the molecular/space confined, interfacial orientated, and surface limited assembly are focused. Special focus is devoted to the newly developed synthetic strategies and their fundamental mechanisms for accurate control of some key structural parameters. Recent advances of 2DOMMs in EECSDs are also highlighted, which suggest that 2DOMMs are excellent material platforms for developing new battery chemistry as well as targeting performance optimization and cost reduction. Finally, the challenges and prospects are proposed based on current development.     

Dimensionality Concept in Solid‐State Reactions: A Way to Control Synthesis of Functional Materials at the Nanoscale

The concept of dimensionality is fundamental in physics, chemistry, materials science, etc. Low‐dimensional and layered materials are distinguished by their unique physical properties and applications. Concurrently, low‐dimensional reactants, products, and reaction spaces extend the toolbox of materials science considerably. Here, the concept of dimensionality is adapted to solid‐state reactions by counting the basic axes along which the unit cell undergoes significant expansion/shrinking. For illustration, 1D synthesis of layered ternary compounds MA₂X₂ via derivatives of 2D‐Xenes, silicene, and germanene, is demonstrated, and the reaction mechanism and the role of templates are determined. The approach is then extended to 1D synthesis of non‐layered compounds. The 1D nature of the reactions, established with structural studies, is explored by nanoscale confinement. The mutual orientation of the reaction and confinement—parallel (thus preventing the lattice expansion) or orthogonal—controls the reaction pathways and outcome. The work provides a proof‐of‐concept for anisotropic reactivity caused by directional confinement.     

Doping and Decorating 2D Materials for Biosensing: Benefits and Drawbacks

The rapid advancements in the field of materials science, especially nanoscience, have played a critical role in the advancement of sensors, in particular the development of novel transducer platforms. Sensors are devices that respond to specific phenomena with recordable and analytically useful output signals and have found their way into a myriad of applications in daily life. Some of these applications include the measurement of glucose and cholesterol levels and detection of emerging infectious diseases for biomedical purposes, environmental monitoring, and food analysis. 2D materials proffer numerous advantageous physical, chemical, electronic, and optical attributes such as large specific surface areas, excellent electrical and thermal conductivity, an abundance of catalytic sites, ease of functionalization, and tuneable electronic structures, allowing them to hold promising potential for the development of sensors with high sensitivity. Although layered materials demonstrate many beneficial attributes for the development of sensors, the properties and electronic structure of layered materials can be fine-tuned via doping or decorating to enhance the sensing performance. This review highlights the current progress of electrical and optical sensors based upon metal-decorated and metal-doped 2D materials and examines the effects of decorating and doping 2D materials for sensor developments.