2D Materials Research Research Articles

2D material-based surface plasmon resonance biosensors for applications in different domains: an insight.

The design of surface plasmon resonance (SPR) sensors has been greatly enhanced in recent years by the advancements in the production and integration of nanostructures, leading to more compact and efficient devices. There have been reports of novel SPR sensors having distinct nanostructures, either as signal amplification tags like gold nanoparticles (AuNPs) or as sensing substrate-like two-dimensional (2D) materials including graphene, transition metal dichalcogenides (TMDCs), MXene, black phosphorus (BP), metal-organic frameworks (MOFs), and antimonene. Such 2D-based SPR biosensors offer advantages over conventional sensors due to significant increases in their sensitivity with a good figure of merit and limit of detection (LOD). Due to their atomically thin structure, improved sensitivity, and sophisticated functionalization capabilities, 2D materials can open up new possibilities in the field of healthcare, particularly in point-of-care diagnostics, environmental and food monitoring, homeland security protection, clinical diagnosis and treatment, and flexible or transient bioelectronics. The present study articulates an in-depth analysis of the most recent developments in 2D material-based SPR sensor technology. Moreover, in-depth research of 2D materials, their integration with optoelectronic technology for a new sensing platform, and the predicted and experimental outcomes of various excitation approaches are highlighted, along with the principles of SPR biosensors. Furthermore, the review projects the potential prospects and future trends of these emerging materials-based SPR biosensors to advance in clinical diagnosis, healthcare biochemical, and biological applications.

Tuning the Poisson’s ratio of two-dimensional model network materials with application to 2D-silica bilayer structures

A fundamental understanding of the elastic properties of 2D network structures is highly beneficial for research and applications of 2D materials. In this context, Poisson’s ratio has been studied elaborately for different crystallographic structures of crystalline phases of bilayer silica structures. This paper focuses on the elastic structure–property relationship of crystalline and vitreous polymorphs of plane network materials. We generate defective crystalline network states and seven sets of disordered states, each with a different level of network heterogeneity, using a dual Monte Carlo bond switching algorithm. Firstly, uniaxial tensile deformation is applied to a model material in a two-dimensional framework, concentrating only on the in-plane network topologies. Secondly, uniaxial tensile deformation is applied to bilayer silica structures, considering the out-of-plane morphology of the material. Mechanical testing is performed using the athermal quasistatic deformation protocol, which allows one to study the influence of the network topology exclusively without any thermal disturbances. We show that manipulating the network heterogeneity allows one to directly tune the Poisson’s ratio of the material, creating possibilities for novel mechanical applications.

In Situ Growth of High-Quality Single-Crystal Twisted Bilayer Graphene on Liquid Copper.

Twisted bilayer graphene (TBG) generates significant attention in the fundamental research of 2D materials due to its distinct twist-angle-dependent properties. Exploring the efficient production of TBG with a wide range of twist angles stands as one of the major frontiers in moiré materials. Here, the local space-confined chemical vapor deposition growth technique for high-quality single-crystal TBG with twist angles ranging from 0° to 30° on liquid copper substrates is reported. The clean surface, pristine interface, high crystallinity, and thermal stability of TBG are verified by using comprehensive characterization techniques including optical microscopy, electron microscopy, and secondary-ion mass spectrometry. The proportion of TBG in bilayer graphene reaches as high as 89%. In addition, the stacking structure and growth mechanism of TBG are investigated, revealing that the second graphene layer develops beneath the first one. A series of comparative experiments illustrates that the liquid copper surface, with its excellent fluidity, promotes the growth of TBG. Electrical measurements show the twist-angle-dependent electronic properties of as-grown TBG, achieving a room-temperature carrier mobility of 26640 cm2 V-1 s-1 . This work provides an approach for the in situ preparation of 2D twisted materials and facilitates the application of TBG in the fields of electronics.

Reliable machine learning potentials based on artificial neural network for graphene

Graphene is one of the most researched two dimensional (2D) material in the past two decades due to its unique combination of mechanical, thermal and electrical properties. Special 2D structure of graphene enables it to exhibit a wide range of peculiar material properties like high Young’s modulus, high specific strength, electrical conductivity etc. which are critical for myriad of applications including lightweight structural materials, multi-functional coating and flexible electronics. As it is quite challenging and costly to experimentally investigate graphene and graphene based nanocomposites, computational simulations such as molecular dynamics (MD) simulations are widely adopted for understanding the microscopic origins of their unique properties. However, disparate results were reported from computational studies, especially MD simulations using various empirical inter-atomic potentials. In this work, an artificial neural network (ANN) based interatomic force field potential has been developed for graphene to represent the potential energy surface based on first principle calculations. The developed machine learning potential (MLP) facilitates high fidelity MD simulations to approach the accuracy of ab initio methods but with a fraction of computational cost, which allows larger simulation size and length, and thereby enables accelerated discovery and design of graphene-based novel materials. Lattice parameter, coefficient of thermal expansion (CTE), Young’s modulus and yield strength are estimated using machine learning accelerated MD simulations (MLMD), which are compared to experimental and first principle calculations from previous literatures. It is demonstrated that MLMD can capture the dominating mechanism governing the CTE of graphene, including effects from lattice parameter and out of plane rippling. The MLMD approach is highly scalable for 2D materials and can help in accelerating the research of novel 2D materials and 2D material hybrids with unique atomic structures.

Building intercalation structure for high ionic conductivity via aliovalent substitution

Two-dimensional (2D) materials, which possess robust nanochannels, high flux and allow scalable fabrication, provide new platforms for nanofluids. Highly efficient ionic conductivity can facilitate the application of nanofluidic devices for modern energy conversion and ionic sieving. Herein, we propose a novel strategy of building an intercalation crystal structure with negative surface charge and mobile interlamellar ions via aliovalent substitution to boost ionic conductivity. The Li2xM1–xPS3 (M = Cd, Ni, Fe) crystals obtained by the solid-state reaction exhibit distinct capability of water absorption and apparant variation of interlayer spacing (from 0.67 to 1.20 nm). The assembled membranes show the ultrahigh ionic conductivity of 1.20 S/cm for Li0.5Cd0.75PS3 and 1.01 S/cm for Li0.6Ni0.7PS3. This facile strategy may inspire the research in other 2D materials with higher ionic transport performance for nanofluids.

Special Section on 2D Materials for Electrochemical Energy Storage and Conversion

Electrochemical energy storage and conversion are currently one of the most critical challenges due to the increasing energy demand. Therefore, discovering novel materials to develop low-cost and more efficient energy storage technologies is urgently necessary. Among various novel materials, two-dimensional (2D) materials have attracted intensive research activities in multiple fields due to their fascinating physical and chemical properties. The 2D materials having a higher surface-to-volume ratio are beneficial to developing low-cost and large-scale energy storage systems for practical applications. There have been many promising concepts of 2D material-based real-life energy applications recently in batteries, supercapacitors, fuel cells, solar cells, thermoelectric, triboelectric generators, etc. Despite recent progress, significant efforts are still needed to investigate the fundamentals of 2D materials for electrochemical energy storage and conversion. Over the past few years, there has been substantial progress in modeling, theories, and experimental characterizations of 2D materials for energy storage and conversion. This timely Special Section issue addressed some recent advances in this critical area. We have selected eight papers covering a gamut of electrochemical-centric research in 2D materials for energy storage and conversion.In this issue, Qu et al. reported the electrochemical evaluation of helical carbon nanofibers (HCNFs) prepared by the ethanol flame method as anode materials of lithium-ion batteries. Their results show that HCNFs possess high reversible capacity, good rate performance, and excellent cycling stability. Farma et al. investigated a simple and cost-effective method to generate porous carbon activation from Palmae plant waste biomass, namely, areca leaf midrib (ALM). They showed that the electrochemical properties of activated carbon supercapacitor cells derived from ALM biomass have the highest specific capacitance value and scan rate in a two-electrode system. Arumugam et al. discussed a ternary composite made up of graphene oxide, polyaniline, and zinc oxide as an electrode material for supercapacitors with its structural and electrochemical properties. The ternary composite exhibited the highest specific capacitance. Metzger et al. focused on using graphene-coated proton exchange membrane (PEM) to reduce fuel crossover. They found that the adhesion of graphene on PEMs is insufficient for prolonging fuel cell operation, resulting in graphene delamination at high temperatures and higher fuel crossover values compared to lower temperature testing. Zaidi et al. reported the superior performance of graphene nanosheet (GNS) materials over Vulcan XC incorporated as a cathode catalyst in Li-O2 battery. The GNS catalysts demonstrated promising performance at higher current densities and with various organic electrolytes. Pandey et al. synthesized a mechanically stable, proton-conducting, and very cost-effective nanocomposite membrane using a simple and scalable phase-inversion approach. The synthesized proton-conducting nanocomposite membrane was demonstrated as a potential advanced functional solid electrolyte for possible application in proton exchange membrane fuel cells. Fauzi et al. reported the thermophysical properties of N,N-diethylethanolammonium chloride/ethylene glycol-DES (deep eutectic solvent) for the replacement of ionic liquid. They studied the physical properties of DES, which are thermal conductivity, viscosity, and surface tension. Finally, the review article by Zeng et al. provided an overview of the application of conductive diamond in electrocatalytic reduction and outlined the improvement of electrochemical properties by employing metal particles to modify the surface.We believe that this Special Section issue will be a valuable contribution to the energy storage and conversion literature and open new frontiers for researchers. The interdisciplinary field of 2D material-based energy storage is rapidly evolving. Our Special Section issue will motivate many researchers to implement novel techniques, such as artificial intelligence, digital twins, and automated advanced manufacturing, in this field and initiate new collaborations between experimentalists, theorists, and modelers. We would like to thank all contributors to this Special Section issue for submitting their latest high-impact work. Also, special thanks would go out to the invited reviewers for helping us further enhance the quality of the articles.

Sustainable and mild exfoliation of bulk crystalline carbon nitride into ultrathin nanosheets via ion-exchange in pure-water

Semiconducting two-dimensional (2D) materials, or nanosheets, have been gaining wide attention in photocatalysis field due to their abundant active sites, short charge diffusion length, and more powerful photo-reduction/-oxidation ability than that of the bulk counterparts. The heptazine-based crystalline carbon nitride, named poly(heptazine imide) (PHI), is a polymer semiconductor being extensively investigated in the past decade. PHI nanosheets have demonstrated the superiority in photocatalytic reactions over bulk PHI, however, the current strategies to derive PHI nanosheets are generally costly, energy-consuming, as well as destructive. Herein, PHI nanosheets are sustainably and mildly synthesized by simply agitation of the bulk PHI in deionized water, which acts as both ion-exchange and stabilization agents. As-prepared PHI nanosheets exhibit significantly improved photocatalytic hydrogen evolution activity than that of the bulk PHI under visible light irradiation. The sustainable and large-scale synthesis of PHI nanosheets was proposed to stimulate the exfoliation research of other 2D materials.

Inkjet Electrodes for Developing Wearable Sensors for the Detection of Peptides and Neurotransmitters in Sweat Using Flexible Materials

In recent years, there has been an increased interest in the research and development of 2D materials for use and application in different fields, such as new technologies, medicine, health care, flexible biosensors, and wearable devices. Our goal is based on developing a flexible wearable biosensor for the skin to detect in real-time neurotransmitters and neuropeptides in human sweat. The development of flexible biosensors for detecting molecules in sweat has been of great interest. However, the problem is that most research has focused on a limited number of molecules such as lactate, chloride and potassium ions, glucose, and pH, and there is a great need to develop flexible biosensors that can monitor real-time molecules that are not yet studied in sweat. One of our molecules of great interest is neuropeptide Y (NPY). NPY has an essential role in the energy balance and is related to different diseases and conditions such as diabetes, obesity, depression, anxiety, and sleep problems and has recently been found to be associated with cardiovascular problems.Our biosensor consists of flexible, low-cost materials that do not require much manufacturing time. The proposed flexible sensor system consists of paper-based microfluidics that serve as our system's passive flow method and silver inkjet electrodes printed on the PET surface. Microfluidic systems can be characterized as active or passive, depending on the force applied to the sample or flow. We can perform minimal flow or particles in a minimum amount of liquid or a sample flow with paper-based microfluidics systems. Among the passive microfluidic systems, paper-based microfluidics, also known as micro-pads, are among the most promising methods because they are easy to manufacture, inexpensive, and have low sample volume. One of the most popular techniques to develop electrochemical wearable biosensors is the fabrication of microelectrode arrays through inkjet printing. To fabricate our electrode arrays, we first created the design in AutoCAD and then printed it using silver conductive ink and a commercial inkjet printer. Since sweat is a complex fluid, it is necessary to modify the working electrode's surface to give the electrode's selectivity towards the molecule to be analyzed or detected. To achieve the detection of NPY, we first add carboxyl groups to the surface of the working electrode, which will serve as an anchor for the specific aptamer for NPY. We use different buffer solutions like artificial cerebrospinal fluid, phosphate buffer solutions, and artificial sweat to perform electrochemical characterization on our silver inkjet electrodes through cyclic voltammetry and electrochemical impedance spectroscopy (EIS). We decreased the oxidation of the surface of our electrodes by changing the buffer and modifying the surface for greater selectivity towards NPY. Using 2D materials, we managed to manufacture and electrochemically characterize our flexible biosensor system prototypes to detect neuropeptides and neurotransmitters in sweat, applying the use of paper-based microfluidics and silver inkjet electrodes printed on PET and paper surfaces.

Emergent Phenomena in Magnetic Two-Dimensional Materials and van der Waals Heterostructures

Two-dimensional (2D) materials and their van der Waals (vdW) heterostructures provide powerful platforms for exploring fascinating physical phenomena and implementing intriguing applications. The recent discovery of 2D magnetic materials opens up many exciting topics in the research of 2D materials. 2D magnetic materials not only exhibit diverse properties on their own, which can be effectively controlled via external stimuli, but also can serve as a part of vdW heterostructures, creating a variety of unprecedented properties and functionalities. The field of 2D magnetism has grown rapidly in the past few years. In this review, we summarize the recent progress on the emergent phenomena in this active field. We review the well-established understanding of typical 2D magnetic materials and the vdW heterostructures and introduce the manipulations of their magnetic properties. In addition, we discuss the interplay between magnetism and topology in some recently discovered 2D materials. Moreover, we also address the valley relevant physics and the topological spin textures in 2D magnetic materials. Finally, we highlight some future interests in this promising field.

Exfoliated 2D Antimonene‐Based Structures for Light‐Harvesting Photoactive Layer of Highly Stable Solar Cells

2D materials have shown great promise in various applications including solar cells, but their use as light‐harvesting active layers in photovoltaic (PV) devices is limited. Herein, surface‐oxidized antimonene sheets are prepared using a liquid‐phase exfoliation method and employed as an active light absorber material after functionalization. It is shown that 2D antimonene possesses unique surface chemistry that allows it to form photoactive Sb2S3 light absorbers for solar cells under ambient conditions. Under the standard PV testing conditions (AM1.5G), devices fabricated with 2D antimonene‐based light‐harvesting materials deliver a power conversion efficiency (PCE) of up to 4.28%. This novel type of solar cell exhibits outstanding operational stabilities, preserving >90% of the initial PCE after aging for 60 min at a temperature of 85 °C, retaining >95% of the initial efficiency after exposure to continuous light illumination under 1 sun for 180 min, and maintaining its functionality after being underwater for 25 min. This work opens new avenues for research in 2D materials and photovoltaics.

Surface Modification of 2D Photocatalysts for Solar Energy Conversion.

2D materials show many particular properties, such as high surface-to-volume ratio, high anisotropic degree, and adjustable chemical functionality. These unique properties in 2D materials have sparked immense interest due to their applications in photocatalytic systems, resulting in significantly enhanced light capture, charge-transfer kinetics, and surface reaction. Herein, the research progress in 2D photocatalysts based on varied compositions and functions, followed by specific surface modification strategies, is introduced. Fundamental principles focusing on light harvesting, charge separation, and molecular adsorption/activation in the 2D-material-based photocatalytic system are systemically explored. The examples described here detail the use of 2D materials in various photocatalytic energy-conversion systems, including water splitting, carbon dioxide reduction, nitrogen fixation, hydrogen peroxide production, and organic synthesis. Finally, by elaborating the challenges and possible solutions for developing these 2Dmaterials, the review is expected to provide some inspiration for the future research of 2D materials used on efficient photocatalytic energy conversions.

Recent progress on two-dimensional materials confining single atoms for CO2 photoreduction

Photoreduction of CO2 into value-added products offers a promising approach to overcome both climate change and energy crisis. However, low conversion efficiency, poor product selectivity, and unclear mechanism limit the further advancement of CO2 photoreduction. The development of two-dimensional (2D) materials and construction of single atom sites are two frontier research fields in catalysis. Combining the advantages of 2D materials and single atom sites is expected to make a breakthrough in CO2 photoreduction. In this review, we summarized the design and application, proposed challenges and opportunities, and laid a foundation for further research and application of 2D materials confining single atoms (SACs@2D) for CO2 photoreduction.

Applications and Challenges of 2D Materials in Lithium Metal Batteries

Owing to the high theoretical specific capacity and low electrochemical potential, lithium (Li) metal is considered as the most promising anode material for next-generation batteries. However, the commercial application of lithium metal batteries (LMBs) is restricted by Li dendritic growth and infinite volume change. Generally, introducing lithiophilic sites and constructing artificial solid-electrolyte interphase (SEI) layer are regarded as effective ways to induce uniform deposition of Li and inhibit growth of Li dendrites. 2D materials, due to their unique planar structure, high specific surface area, high mechanical strength and rich surface chemistry, are expected to achieve high performance LMBs with high stability and safety. Herein, the current progress of 2D materials for LMBs is summarized, focusing on constructing lithiophilic sites and artificial solid-electrolyte interphase (SEI) layer. Perspectives of future directions for LMBs are discussed. With the continuous research of 2D materials in LMBs, it is predictable that 2D materials will have great application prospects and make a difference in high-energy-density LMBs.

2D Indium Phosphorus Sulfide (In2 P3 S9 ): An Emerging van der Waals High-k Dielectrics.

2D van der Waals (vdW) semiconductors hold great potentials for more-than-Moore field-effect transistors (FETs), and the efficient utilization of their theoretical performance requires compatible high-k dielectrics to guarantee the high gate coupling efficiency. The deposition of traditional high-k dielectric oxide films on 2D materials usually generates interface concerns, thereby causing the carrier scattering and degeneration of device performance. Here, utilizing a space-confined epitaxy growth approach, the authors successfully obtained air-stable ultrathin indium phosphorus sulfide (In2 P3 S9 ) nanosheets, the thickness of which can be scaled down to monolayer limit (≈0.69nm) due to its layered structure. 2D In2 P3 S9 exhibits excellent insulating properties, with a high dielectric constant (≈24) and large breakdown voltage (≈8.1MVcm-1 ) at room temperature. Serving as gate insulator, ultrathin In2 P3 S9 nanosheet can be integrated into MoS2 FETs with high-quality dielectric/semiconductor interface, thus providing a competitive electrical performance of device with subthreshold swings (SS) down to 88mVdec-1 and a high ON/OFF ratio of 105 . This study proves an important strategy to prepare 2D vdW high-k dielectrics, and greatly facilitates the ongoing research of 2D materials for functional electronics.

Versatile, Low-Cost, and Portable 2D Material Transfer Setup with a Facile and Highly Efficient DIY Inert-Atmosphere Glove Compartment Option.

Research in van der Waals heterostructures has been rapidly progressing in the past decade, thanks to the art of sequential and deterministic placement of one two-dimensional (2D) material over another. The successful creation of heterostructures however has relied largely on expensive transfer systems that are not easily accessible to researchers. Although a few reports on low-cost systems have recently surfaced, the full functionality, portability features, and overall effectiveness of such systems are still being explored. In this work, we present an “all-in-one” low-cost transfer setup that is compact, lightweight, and portable and which can be quickly installed with a facile and do it yourself (DIY)-style anaerobic glovebox option that performs at par with commercial anaerobic systems. The “installable” glovebox option means the user has the convenience of quickly converting the working environment into an inert one when air-sensitive 2D materials are used. The lowest RH values obtained in our glovebox is <3%, and the O2 levels rapidly drop from 21% to less than 0.1% in just a few minutes of purging the chamber with inert gas. The transfer system is also equipped with a light-weight PID-controlled substrate heating option that can be easily assembled within just a few hours. We test the versatility of our low-cost system by the successful creation of hexagonal boron nitride (hBN)-encapsulated graphene and hBN-encapsulated molybdenum disulphide (MoS2) heterostructures using the hot pickup technique and graphene-hBN, MoS2-hBN, twisted MoS2, and twisted MoS2 on hBN stacks using the wetting technique, and a MoS2-hBN-graphene vertical tunneling heterostructure was formed using a combination approach. The effectiveness of the DIY glovebox is proven with the demonstration of extended stability of freshly exfoliated black phosphorous (BP) flakes, their encapsulation between thin hBN layers, and the formation of electrically contacted BP devices with a protective hBN top layer. At an overall price point of approximately 1000 $, the versatile setup presented here is expected to further contribute to the growth of research in 2D materials, in particular, for researchers initially faced with overcoming a huge entry-level threshold to work in the field of 2D materials and van der Waals heterostructures.

Magneto-optics of layered two-dimensional semiconductors and heterostructures: Progress and prospects

Beginning with the “conventional” two-dimensional (2D) quantum wells based on III–V and II–VI semiconductors in the 1970s, to the recent atomically thin sheets of van der Waals materials such as 2D semiconducting transition metal dichalcogenides (TMDCs) and 2D magnets, the research in 2D materials is continuously evolving and providing new challenges. Magneto-optical spectroscopy has played a significant role in this area of research, both from fundamental physics and technological perspectives. A major challenge in 2D semiconductors such as TMDCs is to understand their spin-valley-resolved physics and their implications in quantum computation and information research. Since the discovery of valley Zeeman effects, deep insights into the spin-valley physics of TMDCs and their heterostructures have emerged through magneto-optical spectroscopy. In this Perspective, we highlight the role of magneto-optics in many milestones such as the discovery of interlayer excitons, phase control between coherently excited valleys, determination of exciton-reduced masses, Bohr radii and binding energies, physics of the optically bright and dark excitons, trions, and other many-body species such as biexcitons and their phonon replicas in TMDC monolayers. The discussion accompanies open questions, challenges, and future prospects in the field including comments on the magneto-optics of van der Waals heterostructures involving TMDCs and 2D magnets.

Anisotropic 2D SiAs for High-Performance UV-Visible Photodetectors.

Recently, extensive efforts have been directed at finding novel 2D-layered structures with anisotropic crystal structures. Herein, the in-plane anisotropic optical and (photo)electrical properties of 2D SiAs nanosheets synthesized using a solid-state reaction and subsequent mechanical exfoliation are reported. The angle-resolved polarized Raman spectrum shows high in-plane anisotropy of the phonon vibration modes, which are consistent with the theoretical prediction. Field-effect transistor devices fabricated using the SiAs nanosheets demonstrate significant anisotropy in the hole mobility with an anisotropic ratio as high as 5.5. Photodetectors fabricated with single SiAs nanosheet exhibit high sensitivity in the UV-visible region, and the anisotropic ratio of the photocurrent reaches 5.3 at 514.5 nm and 2.3 at 325 nm. This work lays the foundation for future research in anisotropic 2D materials.

REVIEW OF THE APPLICATION OF ATOMIC FORCE MICROSCOPY IN TESTING THE MECHANICAL PROPERTIES OF TWO-DIMENSIONAL MATERIALS

Graphene and other two-dimensional (2D) materials possess various excellent properties and hold great promises for next generation of electronic devices and other applications. The mechanical properties are of fundamental importance in the research and application of 2D materials. Despite the fact that 2D materials have been extensively investigated in the past two decades, efforts on the mechanical properties are strikingly lacking and vastly needed. Atomic force microscopy (AFM) is one of the most widely used tools for the mechanical characterizations of low-dimensional materials. Particularly, the AFM-based nano-indentation technique has been extensively employed to explore the mechanical properties of 2D materials. In this review, we first introduce the basic backgrounds of 2D materials and atomic force microscopy. The mechanism and theoretical background of AFM-based nano-indentation are then demonstrated. In the second part, we review the research work by employing nano-indentation on studying the in-plane mechanical properties of 2D materials. The measurement errors of AFM-based nano-indentation and their origins are also discussed. Nano-indentation is perfectly suitable for the in-plane/intralayer mechanical measurement but also greatly limited in probing the out-of-plane/interlayer elasticity, due to the extreme anisotropy of 2D materials. Therefore, in the third part, we introduce an unconventional AFM-based technique - Angstrom-indentation which allows for sub-nm deformation on 2D materials. With such a shallow indentation depth comparable to the interlayer spacing of 2D materials, Angstrom-indentation is capable of measuring and tuning the interlayer van der Waals interactions in 2D materials. The interlayer elasticities of graphene and graphene oxide measured by Angstrom-indentation are discussed as examples in the third part. In the final part, we give a quick overview of a new type of 2D material - van der Waals heterostructure and its novel mechanical properties. We also discuss the potential application of A-indentation in the investigation of the mechanical properties of van der Waals heterostructures.

Controllable preparation and photoelectric applications of two-dimensional in-plane and van der Waals heterostructures

&lt;sec&gt;Since the discovery of graphene, two-dimensional (2D) materials have received continuous attention and carried out in-depth exploration and development due to their excellent properties. With the exploration of the preparation of new 2D materials, one began to consider the synergistic effects produced by the in-plane junction and interlayer stacking to compensate for the defects of a single material and obtain some new properties. Matching the lattice structure to achieve specific functionalization, or using van der Waals force to achieve stacking, helps to introduce a new degree of freedom by combining different 2D materials, and open a new window for the research and practical application of 2D materials.&lt;/sec&gt;&lt;sec&gt;From the perspective of atomic manufacturing, in this article we introduce the controllable preparation and optoelectronic applications of 2D planar and van der Waals heterojunction materials. First, we briefly introduce the common 2D materials such as graphene, hexagonal boron nitride, transition metal dichalcogenides and black phosphorus used in the preparation of heterojunctions and related concepts of heterojunctions. Second, we review, in principle, the commonly used characterization methods including scanning probe-based techniques, spectrum-based, electron-based imaging techniques and others. Third, we summarize the preparation methods of planar and vertical heterojunctions. Basically, mechanical transfer method such as wet or dry method can be used to produce various vertical heterostructures of 2D materials, but usually lack the scalability. On the other hand, chemical vapor deposition method provides a scalable route to producing the planar heterostructure and vertical structure of 2D materials. Several strategies have been developed to produce various heterostructures. In addition, the recent development of twist-angle and quasi-crystalline bi-layer graphene is briefly reviewed. Fourth, the properties and applications of 2D van der Waals heterostructures such as field-effect transistor, light emitting diode, solar cell, flexible optoelectronic devices and plasmonic applications are introduced. Finally, the problems in the field are discussed, and the outlook is provided. &lt;/sec&gt;

Structures and functions of two-dimensional materials: From theoretical prediction to experimental realization

<p indent="0mm">Two-dimensional (2D) materials have shown promising applications in optoelectronics, energy storage and conversion, electronic informatics, aerospace, and biomedicine due to their unique physical, chemical, and mechanical properties. The past decade has witnessed the increasing power and the more widely applications of theoretical approaches, especially those based on the density functional theory (DFT), in the research field of 2D materials, and the close interplay between theory and experiment has become a new paradigm. Herein we briefly reviewed the computational design of the structures and functions of 2D materials, and their experimental realizations, paying special attention to how theoretical predictions guided experimental explorations. The structural design strategy for 2D materials mainly includes bottom-up (constructing 2D materials from 0D or 1D materials) and top-down (exfoliating bulk to monolayers) approaches. In a bottom-up approach, 2D frameworks were constructed by linking macrocyclic conjugated molecules, such as metalloporphyrin and metallophthalocyanine, whose typical MN_{<italic>x</italic>} metal centers can serve as active sites of catalysts. In addition, nanoribbons, which can be viewed as quasi-2D materials or derivatives of 2D materials, were achieved by unzipping the 1D nanotubes. For example, the interaction between the B and its adjacent N atoms of a BN nanotube can be weakened by attaching an external lone electron pair to the B site, thus aqueous ammonia solution was used as an effective reagent to unzip boron nitride nanotubes into BN nanoribbons upon sonication treatment. In the experiment, the top-down approach is a straightforward manner to obtain 2D materials by exfoliating its bulk counterparts. Among others, guided by theoretical predictions, arsenene and antimonene have been exfoliated from the experimentally available bulk material, and various applications have been explored. In addition, the combination of DFT computations with the global minimum search, exemplified by the particle swarm optimization (PSO), in the 2D space, is also an effective method for searching new 2D materials. By this method, the novel pentagonal PdS₂ and PdSe₂ monolayers were theoretically predicted, shortly followed by the experimental fabrication of 2D penta-PdSe₂. We introduced several important methods to manipulate the band structure, magnetism, catalytic activity, etc. of 2D materials, including surface hydrogenation and halogenation, doping with heteroatoms, modulating single-atom catalytic centers, forming van der Waals heterojunctions. These functionalizations enable 2D materials for various applications, such as nanoelectronic devices, catalysts, gas separation, and electrode materials for rechargeable batteries. Among others, we discussed the hydrogenation of h-BN and how their electronic structures and magnetism performance were modulated. For example, fully hydrogenated armchair BN nanoribbons are nonmagnetic semiconductors, while the zigzag counterparts are magnetic and metallic. Moreover, single-doping, co-doping, and doping at different sites were theoretically explored to improve the catalytic performance of the 2D materials, and the corresponding experimental verifications were highlighted. Almost all the theoretically designed materials under review have been experimentally realized, demonstrating the power of the theoretical studies in the research field of 2D materials. With the further development of computer software and hardware and increasing usage of machine learning and big-data techniques, theoretical studies will play an even more important role and accelerate the research and development of 2D materials in a faster pace.