Progress in the materials science of silicene

In its freestanding, yet hypothetical form, the Si counterpart of graphene called silicene is predicted to possess massless Dirac fermions and to exhibit an experimentally accessible quantum spin Hall effect. Such interesting electronic properties are not realized in two-dimensional (2D) Si honeycomb lattices prepared recently on metallic substrates where the crystal and hybrid electronic structures of these ‘epitaxial silicene’ phases are strongly influenced by the substrate, and thus different from those predicted for isolated 2D structures. While the realization of such low-dimensional Si π materials has hardly been imagined previously, it is evident that the materials science behind silicene remains challenging. In this contribution, we will review our recent results that lead to an enhanced understanding of epitaxial silicene formed on diboride thin films, and discuss the remaining challenges that must be addressed in order to turn Si 2D nanostructures into technologically interesting nanoelectronic materials.     

Micromechanism study on electronic and magnetic properties of silicene regulated by oxygen

Silicene, a two-dimensional (2D) silicon counterpart of graphene with attractive electronic properties, has attracted increasing attention. Understanding of its interaction with oxygen is of fundamental importance for nano-electronics in silicon-based technology. Here, we have systematically studied the structural, electronic and magnetic properties of silicene with oxygen atoms adsorption by using an unbiased structure search method coupled with First-principles calculations. The results show that the most favorable oxygen adsorption site on silicene surface is bridge site and oxygen atoms tend to chemisorb on silicene. A detailed analysis of the electronic band structure and density of state (DOS) suggests that there is a band gap opening near Fermi level after oxygen adsorption, which lead to pristine silicene changing from a gapless semiconductor to a direct or indirect bandgap semiconductor. The important finding is that two and six oxygen atoms adsorbed silicene are more advantageous due to the relatively large direct band gaps at the K point. The calculated magnetic moments and spin density isosurfaces reveal that the total magnetic moments are mostly localized on silicene sheet. This finding provides new insights for further materials design based on two-dimensional silicon systems.     

Chemically Integrated Inorganic‐Graphene Two‐Dimensional Hybrid Materials for Flexible Energy Storage Devices

State‐of‐the‐art energy storage devices are capable of delivering reasonably high energy density (lithium ion batteries) or high power density (supercapacitors). There is an increasing need for these power sources with not only superior electrochemical performance, but also exceptional flexibility. Graphene has come on to the scene and advancements are being made in integration of various electrochemically active compounds onto graphene or its derivatives so as to utilize their flexibility. Many innovative synthesis techniques have led to novel graphene‐based hybrid two‐dimensional nanostructures. Here, the chemically integrated inorganic‐graphene hybrid two‐dimensional materials and their applications for energy storage devices are examined. First, the synthesis and characterization of different kinds of inorganic‐graphene hybrid nanostructures are summarized, and then the most relevant applications of inorganic‐graphene hybrid materials in flexible energy storage devices are reviewed. The general design rules of using graphene‐based hybrid 2D materials for energy storage devices and their current limitations and future potential to advance energy storage technologies are also discussed.     

Fabrication of Millimeter‐Scale,Single‐Crystal One‐Third‐Hydrogenated Graphene with Anisotropic Electronic Properties

Periodically hydrogenated graphene is predicted to form new kinds of crystalline 2D materials such as graphane, graphone, and 2D C_xH_y, which exhibit unique electronic properties. Controlled synthesis of periodically hydrogenated graphene is needed for fundamental research and possible electronic applications. Only small patches of such materials have been grown so far, while the experimental fabrication of large‐scale, periodically hydrogenated graphene has remained challenging. In the present work, large‐scale, periodically hydrogenated graphene is fabricated on Ru(0001). The as‐fabricated hydrogenated graphene is highly ordered, with a √3 × √3/R30° period relative to the pristine graphene. As the ratio of hydrogen and carbon is 1:3, the periodically hydrogenated graphene is named “one‐third‐hydrogenated graphene” (OTHG). The area of OTHG is up to 16 mm². Density functional theory calculations demonstrate that the OTHG has two deformed Dirac cones along one high‐symmetry direction and a finite energy gap along the other directions at the Fermi energy, indicating strong anisotropic electrical properties. An efficient method is thus provided to produce large‐scale crystalline functionalized graphene with specially desired properties.     

Lithiation/Delithiation Synthesis of Few Layer Silicene Nanosheets for Rechargeable Li–O2 Batteries

Silicene has recently received increasing interest due to its unique properties. However, the synthesis of silicene remains challenging, which limits its wide applications. In this work, a top‐down lithiation and delithiation process is developed to prepare few layer silicene‐like nanosheets from ball‐milled silicon nanopowders. It is found that delithiation solvent plays a critical role in the structure evolution of the final products. The use of isopropyl alcohol renders 2D silicene‐like products 30–100 nm in length and ≈2.4 nm in thickness. The electrochemical characterization analysis suggests that the product shows high performance for rechargeable Li–O₂ batteries with 73% energy efficiency and high stability. The top‐down synthesis strategy proposed in this work not only provides a new solution to the challenging preparation issue of few layer silicene but also demonstrates the feasibility of producing 2D materials from nonlayered starting structures.     

A review on three‐dimensional graphene: Synthesis,electronic and biotechnology applications‐The Unknown Riddles

In the last decade, carbon‐based nanostructures such as buckyball (C₆₀), carbon nanotube (CNT), graphene and three‐dimensional (3D) graphene have been identified as promising materials for electronic, electrochemical energy storage (batteries and supercapacitors), optical and sensing applications. Since the discovery of graphene in 2004, scientists have devised mass production techniques and explored graphene as a promising material for a wide range of applications. Most of the electronic and solar cell applications require materials with good electronic conductivity, mobility and finite bandgap. Graphene is a zero bandgap material which prevents it from the mainstream applications. On the other hand, 3D graphene has good electronic conductivity, mobility, bandgap and electrochemical properties. This review article will focus on the synthesis of the 3D graphene, its structure‐property relationships, biotechnology and electronic applications and the hidden properties that are yet to be explored fully.     

From Flatland to Spaceland: Higher Dimensional Patterning with Two‐Dimensional Materials

The creation of three‐dimensional (3D) structures from two‐dimensional (2D) nanomaterial building blocks enables novel chemical, mechanical or physical functionalities that cannot be realized with planar thin films or in bulk materials. Here, we review the use of emerging 2D materials to create complex out‐of‐plane surface topographies and 3D material architectures. We focus on recent approaches that yield periodic textures or patterns, and present four techniques as case studies: (i) wrinkling and crumpling of planar sheets, (ii) encapsulation by crumpled nanosheet shells, (iii) origami folding and kirigami cutting to create programmed curvature, and (iv) 3D printing of 2D material suspensions. Work to date in this field has primarily used graphene and graphene oxide as the 2D building blocks, and we consider how these unconventional approaches may be extended to alternative 2D materials and their heterostructures. Taken together, these emerging patterning and texturing techniques represent an intriguing alternative to conventional materials synthesis and processing methods, and are expected to contribute to the development of new composites, stretchable electronics, energy storage devices, chemical barriers, and biomaterials.     

Recent development in 2D materials beyond graphene

Discovery of graphene and its astonishing properties have given birth to a new class of materials known as “2D materials”. Motivated by the success of graphene, alternative layered and non-layered 2D materials have become the focus of intense research due to their unique physical and chemical properties. Origin of these properties ascribed to the dimensionality effect and modulation in their band structure. This review highlights the recent progress of the state-of-the-art research on synthesis, characterization and isolation of single and few layer nanosheets and their assembly. Electronic, magnetic, optical and mechanical properties of 2D materials have also been reviewed for their emerging applications in the area of catalysis, electronic, optoelectronic and spintronic devices; sensors, high performance electrodes and nanocomposites. Finally this review concludes with a future prospective to guide this fast evolving class of 2D materials in next generation materials science.     

Direct Growth of High Mobility and Low‐Noise Lateral MoS2–Graphene Heterostructure Electronics

Reliable fabrication of lateral interfaces between conducting and semiconducting 2D materials is considered a major technological advancement for the next generation of highly packed all‐2D electronic circuitry. This study employs seed‐free consecutive chemical vapor deposition processes to synthesize high‐quality lateral MoS₂–graphene heterostructures and comprehensively investigated their electronic properties through a combination of various experimental techniques and theoretical modeling. These results show that the MoS₂–graphene devices exhibit an order of magnitude higher mobility and lower noise metrics compared to conventional MoS₂–metal devices as a result of energy band rearrangement and smaller Schottky barrier height at the contacts. These findings suggest that MoS₂–graphene in‐plane heterostructures are promising materials for the scale‐up of all‐2D circuitry with superlative electrical performance.     

The Role of Graphene and Other 2D Materials in Solar Photovoltaics

2D materials have attracted considerable attention due to their exciting optical and electronic properties, and demonstrate immense potential for next‐generation solar cells and other optoelectronic devices. With the scaling trends in photovoltaics moving toward thinner active materials, the atomically thin bodies and high flexibility of 2D materials make them the obvious choice for integration with future‐generation photovoltaic technology. Not only can graphene, with its high transparency and conductivity, be used as the electrodes in solar cells, but also its ambipolar electrical transport enables it to serve as both the anode and the cathode. 2D materials beyond graphene, such as transition‐metal dichalcogenides, are direct‐bandgap semiconductors at the monolayer level, and they can be used as the active layer in ultrathin flexible solar cells. However, since no 2D material has been featured in the roadmap of standard photovoltaic technologies, a proper synergy is still lacking between the recently growing 2D community and the conventional solar community. A comprehensive review on the current state‐of‐the‐art of 2D‐materials‐based solar photovoltaics is presented here so that the recent advances of 2D materials for solar cells can be employed for formulating the future roadmap of various photovoltaic technologies.     

Band Engineering for Novel Two‐Dimensional Atomic Layers

The discovery of graphene has sparked much interest in science and lead to the development of an ample variety of novel two‐dimensional (2D) materials. With increasing research interest in the field of 2D materials in recent years, the researchers have shifted their focus from the synthesis to the modification of 2D materials, emphasizing their electronic structures. In this review, the possibilities of altering the band structures are discussed via three different approches: (1) alloying 2D materials, so called ternary 2D materials, such as hexagonal carbonized boron nitrides (h‐BCN) and transition metal dichalcogenides (TMDs) ternary materials; (2) stacking 2D materials vertically, which results in 2D heterostructures named van der Waals (vdW) solids (using hexagonal boron nitrides (h‐BN)/graphene and TMDs stacking as examples), and growing lateral TMDs heterostructrues; (3) controlling the thickness of 2D materials, that is, the number of layers. The electronic properties of some 2D materials are very sensitive to the thickness, such as in TMDs and black phosphorus (BP). The variations of band structures and the resulting physical properties are systematically discussed.     

Graphene Oxide,Highly Reduced Graphene Oxide,and Graphene: Versatile Building Blocks for Carbon‐Based Materials

Isolated graphene, a nanometer‐thick two‐dimensional analog of fullerenes and carbon nanotubes, has recently sparked great excitement in the scientific community given its excellent mechanical and electronic properties. Particularly attractive is the availability of bulk quantities of graphene as both colloidal dispersions and powders, which enables the facile fabrication of many carbon‐based materials. The fact that such large amounts of graphene are most easily produced via the reduction of graphene oxide—oxygenated graphene sheets covered with epoxy, hydroxyl, and carboxyl groups—offers tremendous opportunities for access to functionalized graphene‐based materials. Both graphene oxide and graphene can be processed into a wide variety of novel materials with distinctly different morphological features, where the carbonaceous nanosheets can serve as either the sole component, as in papers and thin films, or as fillers in polymer and/or inorganic nanocomposites. This Review summarizes techniques for preparing such advanced materials via stable graphene oxide, highly reduced graphene oxide, and graphene dispersions in aqueous and organic media. The excellent mechanical and electronic properties of the resulting materials are highlighted with a forward outlook on their applications.     

Chemical modification of group IV graphene analogs

Mono-elemental two-dimensional (2D) crystals (graphene, silicene, germanene, stanene, and so on), termed 2D-Xenes, have been brought to the forefront of scientific research. The stability and electronic properties of 2D-Xenes are main challenges in developing practical devices. Therefore, in this review, we focus on 2D free-standing group-IV graphene analogs (graphene quantum dots, silicane, and germanane) and the functionalization of these sheets with organic moieties, which could be handled under ambient conditions. We highlight the present results and future opportunities, functions and applications, and novel device concepts.     

Solution Adsorption Formation of a π‐Conjugated Polymer/Graphene Composite for High‐Performance Field‐Effect Transistors

Semiconducting polymers with π‐conjugated electronic structures have potential application in the large‐scale printable fabrication of high‐performance electronic and optoelectronic devices. However, owing to their poor environmental stability and high‐cost synthesis, polymer semiconductors possess limited device implementation. Here, an approach for constructing a π‐conjugated polymer/graphene composite material to circumvent these limitations is provided, and then this material is patterned into 1D arrays. Driven by the π–π interaction, several‐layer polymers can be adsorbed onto the graphene planes. The low consumption of the high‐cost semiconductor polymers and the mass production of graphene contribute to the low‐cost fabrication of the π‐conjugated polymer/graphene composite materials. Based on the π‐conjugated system, a reduced π–π stacking distance between graphene and the polymer can be achieved, yielding enhanced charge‐transport properties. Owing to the incorporation of graphene, the composite material shows improved thermal stability. More generally, it is believed that the construction of the π‐conjugated composite shows clear possibility of integrating organic molecules and 2D materials into microstructure arrays for property‐by‐design fabrication of functional devices with large area, low cost, and high efficiency.     

Postsynthesis of h‐BN/Graphene Heterostructures Inside a STEM

Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in‐plane graphene growth from the step‐edge of hexagonal BN (h‐BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source. The growth interface between h‐BN and graphene is atomically identified as largely N–C bonds. This postgrowth method can form graphene nanoribbons connecting two h‐BN domains with different twisting angles, as well as isolated carbon islands with arbitrary shapes embedded in the h‐BN layer. The electronic properties of the vertically stacked h‐BN/graphene heterostructures are investigated by electron energy‐loss spectroscopy (EELS). Low‐loss EELS analysis of the dielectric response suggests a robust coupling effect between the graphene and h‐BN layers.     

Ultrathin 2D Photocatalysts: Electronic‐Structure Tailoring,Hybridization, and Applications

As a sustainable technology, semiconductor photocatalysis has attracted considerable interest in the past several decades owing to the potential to relieve or resolve energy and environmental‐pollution issues. By virtue of their unique structural and electronic properties, emerging ultrathin 2D materials with appropriate band structure show enormous potential to achieve efficient photocatalytic performance. Here, the state‐of‐the‐art progress on ultrathin 2D photocatalysts is reviewed and a critical appraisal of the classification, controllable synthesis, and formation mechanism of ultrathin 2D photocatalysts is presented. Then, different strategies to tailor the electronic structure of ultrathin 2D photocatalysts are summarized, including component tuning, thickness tuning, doping, and defect engineering. Hybridization with the introduction of a foreign component and maintaining the ultrathin 2D structure is presented to further boost the photocatalytic performance, such as quantum dots/2D materials, single atoms/2D materials, molecular/2D materials, and 2D–2D stacking materials. More importantly, the advancement of versatile photocatalytic applications of ultrathin 2D photocatalysts in the fields of water oxidation, hydrogen evolution, CO₂ reduction, nitrogen fixation, organic syntheses, and removal pollutants is discussed. Finally, the future opportunities and challenges regarding ultrathin 2D photocatalysts to bring about new opportunities for future research in the field of photocatalysis are also presented.     

Few‐Layer GeAs Field‐Effect Transistors and Infrared Photodetectors

The family of 2D semiconductors (2DSCs) has grown rapidly since the first isolation of graphene. The emergence of each 2DSC material brings considerable excitement for its unique electrical, optical, and mechanical properties, which are often highly distinct from their 3D counterparts. To date, studies of 2DSC are majorly focused on group IV (e.g., graphene, silicene), group V (e.g., phosphorene), or group VIB compounds (transition metal dichalcogenides, TMD), and have inspired considerable effort in searching for novel 2DSCs. Here, the first electrical characterization of group IV–V compounds is presented by investigating few‐layer GeAs field‐effect transistors. With back‐gate device geometry, p‐type behaviors are observed at room temperature. Importantly, the hole carrier mobility is found to approach 100 cm² V^?1 s^?1 with ON–OFF ratio over 10⁵, comparable well with state‐of‐the‐art TMD devices. With the unique crystal structure the few‐layer GeAs show highly anisotropic optical and electronic properties (anisotropic mobility ratio of 4.8). Furthermore, GeAs based transistor shows prominent and rapid photoresponse to 1.6 µm radiation with a photoresponsivity of 6 A W^?1 and a rise and fall time of ≈3 ms. This study of group IV–V 2DSC materials greatly expands the 2D family, and can enable new opportunities in functional electronics and optoelectronics based on 2DSCs.     

Advances in Subcritical Hydro‐/Solvothermal Processing of Graphene Materials

Many promising graphene‐based materials are kept away from mainstream applications due to problems of scalability and environmental concerns in their processing. Hydro‐/solvothermal techniques overwhelmingly satisfy both the aforementioned criteria, and have matured as alternatives to wet‐chemical methods with advances made over the past few decades. The insolubility of graphene in many solvents poses considerable difficulties in their processing. In this context hydro‐/solvothermal techniques present an ideal opportunity for processing of graphenic materials with their versatility in manipulating the physical and thermodynamic properties of the solvent. The flexibility in hydro‐/solvothermal techniques for manipulation of solvent composition, temperature and pressure provides numerous handles to manipulate graphene‐based materials during synthesis. This review provides a comprehensive look at the subcritical hydro‐/solvothermal synthesis of graphene‐based functional materials and their applications. Several key synthetic strategies governing the morphology and properties of the products such as temperature, pressure, and solvent effects are elaborated. Advances in the synthesis, doping, and functionalization of graphene in hydro‐/solvothermal media are highlighted together with our perspectives in the field.     

Polymeric Graphene Bulk Materials with a 3D Cross‐Linked Monolithic Graphene Network

Although many great potential applications are proposed for graphene, till now none are yet realized as a stellar application. The most challenging issue for such practical applications is to figure out how to prepare graphene bulk materials while maintaining the unique two‐dimensional (2D) structure and the many excellent properties of graphene sheets. Herein, such polymeric graphene bulk materials containing three‐dimensional (3D) cross‐linked networks with graphene sheets as the building unit are reviewed. The theoretical research on various proposed structures of graphene bulk materials is summarized first. Then, the synthesis or fabrication of these graphene materials is described, which comprises mainly two approaches: chemical vapor deposition and cross‐linking using graphene oxide directly. Finally, some exotic and exciting potential applications of these graphene bulk materials are presented.