Epitaxial growth of large-area and highly crystalline anisotropic ReSe<Subscript>2</Subscript> atomic layer

The anisotropic two-dimensional (2D) layered material rhenium disulfide (ReSe2) has attracted considerable attention because of its unusual properties and promising applications in electronic and optoelectronic devices.However,because of its low lattice symmetry and interlayer decoupling,anisotropic growth and out-of-plane growth occur easily,yielding thick flakes,dendritic structure,or flower-like structure.In this study,we demonstrated a bottom-up method for the controlled and scalable synthesis of ReSe2 by van der Waals epitaxy.To achieve controllable growth,a micro-reactor with a confined reaction space was constructed by stacking two mica substrates in the chemical vapor deposition system.Within the confined reaction space,the nucleation density and growth rate of ReSe2 were significantly reduced,favoring the large-area synthesis of ReSe2 with a uniform monolayer thickness.The morphological evolution of ReSe2 with growth temperature indicated that the anisotropic growth was suppressed at a low growth temperature (＜600 ℃).Field-effect transistors employing the grown ReSe2 exhibited p-type conduction with a current ON/OFF ratio up to 10s and a hole carrier mobility of 0.98 cm2/(V.s).Furthermore,the ReSe2 device exhibited an outstanding photoresponse to near-infrared light,with responsivity up to 8.4 and 5.1 A/W for 850-and 940-nm light,respectively.This work not only promotes the large-scale application of ReSe2 in high-performance electronic devices but also clarifies the growth mechanism of low-lattice symmetry 2D materials.     

拉曼光谱表征石墨烯结构的研究进展

石墨烯是一种只有一个原子层的二维原子晶体,它是构成零维富勒烯、一维碳纳米管和三维石墨等其他碳同素异形体的基本结构单元,具有很多独特的电子及力学性能,因而吸引了化学、材料及其他领域众多科学家的高度关注。拉曼光谱作为一种灵敏便捷的表征方法,在石墨烯的研究中起到重要的作用。该综述总结了近年来拉曼光谱在石墨烯表征中的应用,在对单层石墨烯的典型特征峰作详细介绍的基础上,通过对拉曼谱图中D峰、G峰和2D峰的强度、位置和半峰宽变化情况的分析,可以快速而准确地表征出石墨烯的层数,并可以对石墨烯的堆垛方式、边缘手性和掺杂程度进行判定。同时,也系统地分析了在石墨烯制备与测试过程中基底、掺杂、温度和激光功率等因素对拉曼谱图中D峰、G峰和2D峰的强度、位置和半峰宽的影响。     

Fatigue Behavior of Polymer Encapsulated Graphene to Mitigate Interfacial Fatigue Damage

In applications such as flexible electronic devices, graphene, and other 2D materials are frequently in contact with stretchable polymeric substrates. The interface between 2D materials and polymers is dominated by weak van der Waals forces and can eventually degrade due to the frequent dynamic mechanical loads that these devices experience. This can lead to significant local delamination and shear fracture of the 2D materials. Using the polydimethylsiloxane (PDMS) encapsulation method, it is shown that the damage in graphene is significantly mitigated when it is capped during dynamic loading. To track the spread of damage in both encapsulated and nonencapsulated graphene, in situ, cyclic loading is performed. The fundamental process driving this substantial reduction in damage propagation in the 2D lattice is explained by the conventional shear lag model. It is also observed that softer PDMS substrate and capping layer completely mitigate the fatigue damage in graphene for 100 cycles at 10% applied fatigue strain.     

三维石墨烯及其复合材料的制备及在超级电容器中的研究进展

三维(3D)石墨烯及其复合材料具有柔韧性好、比表面积大、功率密度高、力学性能稳定以及离子传输迅速等优良性能,成为材料科学领域备受关注的材料。概述了三维石墨烯材料的基本性质和性能,并对其多元复合材料的制备方法以及在超级电容器储能材料方面的应用研究进展进行了评述。三维(3D)石墨烯常用的制备方法有自组装法、模板导向法和3D打印法等,通过对制备方法进行改进,可以有效调控三维材料的多孔结构、孔径、柔韧性和电子传递速度等性能。三维(3D)石墨烯与过渡金属化合物及导电聚合物复合而成的多元复合物在超级电容器电极材料方面表现出广阔的应用前景。     

Two-dimensional dielectric nanosheets: novel nanoelectronics from nanocrystal building blocks

Two-dimensional (2D) nanosheets, which possess atomic or molecular thickness and infinite planar lengths, are regarded as the thinnest functional nanomaterials. The recent development of methods for manipulating graphene (carbon nanosheet) has provided new possibilities and applications for 2D systems; many amazing functionalities such as high electron mobility and quantum Hall effects have been discovered. However, graphene is a conductor, and electronic technology also requires insulators, which are essential for many devices such as memories, capacitors, and gate dielectrics. Along with graphene, inorganic nanosheets have thus increasingly attracted fundamental research interest because they have the potential to be used as dielectric alternatives in next-generation nanoelectronics. Here, we review the progress made in the properties of dielectric nanosheets, highlighting emerging functionalities in electronic applications. We also present a perspective on the advantages offered by this class of materials for future nanoelectronics.     

Towards controlled synthesis of 2D crystals by chemical vapor deposition (CVD)

The emergence of two-dimensional (2D) materials has captured the imagination of researchers since graphene was first exfoliated from graphite in 2004. Their exotic properties give rise to many exciting potential applications in advanced electronic, optoelectronic, energy and biomedical technologies. Scalable growth of high quality 2D materials is crucial for their adoption in technological applications the same way the arrival of high quality silicon single crystals was to the semiconductor industry. A huge amount of effort has been devoted to grow large-area, highly crystalline 2D crystals such as graphene and transition metal dichalcogenides (TMDs) through various methods. While CVD growth of wafer-scale monolayer graphene and TMDs has been demonstrated, considerable challenges still remain. In this perspective, we advocate for the focus on the crystal growth morphology as an underpinning for understanding, diagnosing and controlling the CVD process and environment for 2D material growth. Like snowflakes in nature, 2D crystals exhibit a rich variety of morphologies under different growth conditions. The mapping of crystal shapes in the growth parameter space “encodes” a wealth of information, the deciphering of which will lead to better understanding of the fundamental growth mechanism and materials properties. To this end, we envision a collective effort by the 2D materials community to establish the correlation between crystal shapes and the intrinsic thermodynamic and kinetic parameters for CVD reactions through integrated crystal growth experiment, database development and machine learning assisted predictive modeling, which will pave a robust path towards controlled synthesis of 2D materials and heterostructures.     

Defect Healing in Graphene via Rapid Thermal Annealing with Polymeric “Nanobandage”

Overcoming throughput challenges in current graphene defect healing processes, such as conventional thermal annealing, is crucial for realizing post-silicon device fabrication. Herein, a new time- and energy-efficient method for defect healing in graphene is reported, utilizing polymer-assisted rapid thermal annealing (RTA). In this method, a nitrogen-rich, polymeric “nanobandage” is coated directly onto graphene and processed via RTA at 800 °C for 15 s. During this process, the polymer matrix is cleanly degraded, while nitrogen released from the nanobandage can diffuse into graphene, forming nitrogen-doped healed graphene. To study the influence of pre-existing defects on graphene healing, lattice defects are purposefully introduced via electron beam irradiation and investigated by Raman microscopy. X-ray photoelectron spectroscopy reveals successful healing of graphene, observing a maximum doping level of 3 atomic nitrogen % in nanobandage-treated samples from a baseline of 0–1 atomic % in non-nanobandage treated samples. Electrical transport measurements further indicate that the nanobandage treatment recovers the conductivity of scanning electron microscope-treated defective graphene at ≈85%. The reported polymer-assisted RTA defect healing method shows promise for healing other 2D materials with other dopants by simply changing the chemistry of the polymeric nanobandage.     

Programmed Synthesis of Freestanding Graphene Nanomembrane Arrays

Freestanding graphene membranes are unique materials. The combination of atomically thin dimensions, remarkable mechanical robustness, and chemical stability make porous and non‐porous graphene membranes attractive for water purification and various sensing applications. Nanopores in graphene and other 2D materials have been identified as promising devices for next‐generation DNA sequencing based on readout of either transverse DNA base‐gated current or through‐pore ion current. While several ground breaking studies of graphene‐based nanopores for DNA analysis have been reported, all methods to date require a physical transfer of the graphene from its source of production onto an aperture support. The transfer process is slow and often leads to tears in the graphene that render many devices useless for nanopore measurements. In this work, we report a novel scalable approach for site‐directed fabrication of pinhole‐free graphene nanomembranes. Our approach yields high quality few‐layer graphene nanomembranes produced in less than a day using a few steps that do not involve transfer. We highlight the functionality of these graphene devices by measuring DNA translocation through electron‐beam fabricated nanopores in such membranes.     

Etching of two-dimensional materials

Etching is one of the key steps in materials processing in the semiconducting industry and recent progress in this field has shown that etching can be used to create various structures of two-dimensional (2D) materials, and can be viewed as a complementary technique to growth. Besides, etching has many other applications in 2D materials synthesis, processing and characterization, such as helping to understand the growth mechanism in-depth, identifying point and line defects in a 2D material, improving the quality of 2D materials by combining the growth process, and for fabricating 2D heterojunctions. Here, we review the current progress on the etching of graphene and other 2D materials; the content of this review includes: (i) etching of single-crystalline 2D materials; (ii) etching of multilayer 2D materials; (iii) etching of polycrystalline 2D materials; (iv) experimental factors, such as partial pressure of etchants, etchant species, and substrate, that affect 2D materials etching; (v) applications of etching in graphene and other 2D materials synthesis, characterization and processing and (vi) the challenges and opportunities in 2D materials etching.     

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.     

In situ microscopy techniques for characterizing the mechanical properties and deformation behavior of two-dimensional (2D) materials

Due to the atomic thickness and planar characteristics, two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) are considered to be excellent electronic materials, which endow them with great potential for future device applications. The robust and reliable application of their functional devices requires an in-depth understanding of their mechanical properties and deformation behavior, which is also of fundamental importance in nanomechanics. Considering their exceedingly small sizes and thicknesses, this is a very challenge task. In situ microscopy techniques show great superiority in this respect. This review focuses on the progress in in situ microscopy techniques (including atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)) in characterizing the mechanical properties and deformation behavior of 2D materials. The technical characteristics, advantages, disadvantages, and main research fields of various in situ AFM, SEM, and TEM techniques are analyzed in detail, and the corresponding mechanical scenarios from point to plane are realized, including local indentation, planar stretching, friction sliding between atomic layers and atomic movement mechanisms. By virtue of their complementary advantages, in situ integrated microscopy techniques enable the simultaneous study of various mechanical properties, nanomechanical behavior, and inherent atomic mechanisms of 2D materials. Based on the present research, we look forward to further optimized in situ integrated microscopy techniques with high spatiotemporal atomic resolution that can reveal the dynamic structure-performance correlations and corresponding atomic mechanisms between the physical properties, such as mechanical, electrical, optical, thermal, and magnetic properties of 2D materials and their crystal structures, electronic structures, atomic layers, defect densities and other influencing factors under multifield coupling conditions. This will provide beneficial predictions and guidance for the design, construction and application of 2D material-based mechanoelectronic, piezoelectric, photoelectric, thermoelectric, etc. nanoelectronic devices.     

Effects of Physically Adsorbed Films on Conductivity of?Two-Dimensional Metal Surfaces and Graphene

The electronic properties of two-dimensional (2D) materials, like graphene, are of considerable fundamental interest. This paper describes how physically adsorbed films probe these properties. The cases of an ideal 2D free electron gas and graphene are examined, with explicit results found for an adsorbed He film. In the free electron case, the scattering is proportional to the 2D structure factor of the film. Results are presented for He on graphene and the free electron gas in both the low coverage and fully commensurate limits.     

电弧法制备石墨烯材料的表征与评价

针对一种电弧法制备的石墨烯材料,通过X射线光电子能谱、拉曼光谱、红外光谱和扫描电镜分析等方法进行了表征与评价。结果表明:石墨烯中C元素含量为89.77%,O元素和N元素含量分别为3.03%和7.21%;通过拉曼光谱中G峰和2D峰的位置确定石墨烯样品以多层石墨烯为主;红外光谱显示石墨烯样品吸附了水分子;显微结构观察确定了石墨烯片层结构的层数及尺寸。根据实验方法总结了一套石墨烯的测试表征方案,为石墨烯在复合材料中的推广应用提供了评价依据。     

Direct imaging of a two-dimensional silica glass on graphene

Large-area graphene substrates provide a promising lab bench for synthesizing, manipulating, and characterizing low-dimensional materials, opening the door to high-resolution analyses of novel structures, such as two-dimensional (2D) glasses, that cannot be exfoliated and may not occur naturally. Here, we report the accidental discovery of a 2D silica glass supported on graphene. The 2D nature of this material enables the first atomic resolution transmission electron microscopy of a glass, producing images that strikingly resemble Zachariasen's original 1932 cartoon models of 2D continuous random network glasses. Atomic-resolution electron spectroscopy identifies the glass as SiO(2) formed from a bilayer of (SiO(4))(2-) tetrahedra and without detectable covalent bonding to the graphene. From these images, we directly obtain ring statistics and pair distribution functions that span short-, medium-, and long-range order. Ab initio calculations indicate that van der Waals interactions with graphene energetically stabilizes the 2D structure with respect to bulk SiO(2). These results demonstrate a new class of 2D glasses that can be applied in layered graphene devices and studied at the atomic scale.     

Facile fabrication of functionalized graphene sheets (FGS)/ZnO nanocomposites with photocatalytic property

Functionalized graphene sheets (FGS)/ZnO nanocomposites were fabricated via thermal treatment method, using graphene oxide as a precursor of graphene, Zn(NH(3))(4)CO(3) as a precursor of zinc oxide, and poly(vinyl pyrrolidone) as an intermediate to combine zinc with carbon materials. Thermogravimetric analysis, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) were used to characterize crystal structure and morphology of FGS/ZnO nanocomposites. It was shown that the well-dispersed ZnO nanoparticles were deposited on FGS homogeneously. The composites exhibited photocatalytic activity to decompose rhodamine 6G efficiently under low-power ultraviolet (UV) light. This facile and low-cost method makes the composite a perfect candidate in applications of catalysis and other areas.     

Dimensionality effects of carbon-based thermal additives for microporous adsorbents

We present a systematic study of carbon nanomaterials with different geometries and thermal properties, including few-layer graphene (FLG), graphene oxide (GO), and functionalized carbon nanotubes (fCNT) as additives to enhance the thermal conductivity of microporous adsorbent materials. The dimensionality and intrinsic thermal conductivity of the additives were found to be critical for both maximizing the thermal conductivity enhancement, and minimizing the reduction in the adsorption capacity of the active materials. We demonstrated that two-dimensional (2D) FLG was the most effective thermal additive for zeolite (ZT) adsorbents due to its high thermal conductivity and preferential 2D geometry. Meanwhile, negligible enhancement was observed from one-dimensional (1D) additives such as fCNTs, which is consistent with the predictions from a modified effective medium analysis (EMA). Our work provides insights for the development of additives to enhance the thermal performance of porous materials in applications such as adsorption heat pumps, gas storage, and separation processes.     

Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices

By stacking various two-dimensional (2D) atomic crystals on top of each other, it is possible to create multilayer heterostructures and devices with designed electronic properties. However, various adsorbates become trapped between layers during their assembly, and this not only affects the resulting quality but also prevents the formation of a true artificial layered crystal upheld by van der Waals interaction, creating instead a laminate glued together by contamination. Transmission electron microscopy (TEM) has shown that graphene and boron nitride monolayers, the two best characterized 2D crystals, are densely covered with hydrocarbons (even after thermal annealing in high vacuum) and exhibit only small clean patches suitable for atomic resolution imaging. This observation seems detrimental for any realistic prospect of creating van der Waals materials and heterostructures with atomically sharp interfaces. Here we employ cross sectional TEM to take a side view of several graphene-boron nitride heterostructures. We find that the trapped hydrocarbons segregate into isolated pockets, leaving the interfaces atomically clean. Moreover, we observe a clear correlation between interface roughness and the electronic quality of encapsulated graphene. This work proves the concept of heterostructures assembled with atomic layer precision and provides their first TEM images.     

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.     

3D Mesoporous van der Waals Heterostructures for Trifunctional Energy Electrocatalysis

The emergence of van der Waals (vdW) heterostructures of 2D materials has opened new avenues for fundamental scientific research and technological applications. However, the current concepts and strategies of material engineering lack feasibilities to comprehensively regulate the as‐obtained extrinsic physicochemical characters together with intrinsic properties and activities for optimal performances. A 3D mesoporous vdW heterostructure of graphene and nitrogen‐doped MoS₂ via a two‐step sequential chemical vapor deposition method is constructed. Such strategy is demonstrated to offer an all‐round engineering of 2D materials including the morphology, edge, defect, interface, and electronic structure, thereby leading to robustly modified properties and greatly enhanced electrochemical activities. The hydrogen evolution is substantially accelerated on MoS₂, while the oxygen reduction and evolution are significantly improved on graphene. This work provides a powerful overall engineering strategy of 2D materials for electrocatalysis, which is also enlightening for other nanomaterials and energy‐related applications.     

石墨烯基复合材料的三维组装与应用研究进展

三维石墨烯结构体不仅继承了二维石墨烯片完美的碳晶体结构,还展现出超低的密度、极高的孔隙率和较大的比表面积等特点,具有导电、导热、吸附等优异性能,是近年来石墨烯功能材料中的一颗新星。目前,石墨烯与聚合物、无机纳米材料组装成三维结构复合材料的研究已经取得了实质性进展,研究者通过丰富的化学和物理路径实现了石墨烯与功能组分的三维有序组装,并赋予该材料奇特的结构特点和性能优势。这些特性使材料在能量储存、环境保护、传感器等研究领域表现出不错的应用前景。根据当前研究热点,综述了石墨烯基复合材料的三维组装与应用的研究进展,包括三维石墨烯/聚合物复合材料与三维石墨烯/无机纳米复合材料两种体系。重点总结了两种体系的三维组装方法,并分析了复合材料中石墨烯与功能组分的结构特点,简要概括了当前三维石墨烯基复合材料在环境保护、超级电容器等不同领域的应用进展,并对三维石墨烯基复合材料的三维结构设计与多样化应用进行了展望。