2D Materials Research Research Articles

A Mini Review of the Preparation and Photocatalytic Properties of Two-Dimensional Materials.

The successful preparation and application of graphene shows that it is feasible for the materials with a thickness of a single atom or few atomic layers to exist stably in nature. These materials can exhibit unusual physical and chemical properties due to their special dimension effects. At present, researchers have made great achievements in the preparation, characterization, modification, and theoretical research of 2D materials. Because the structure of 2D materials is often similar, it has a certain degree of qualitative versatility. Besides, 2D materials often carry good catalytic performance on account of their more active sites and adjustable harmonic electronic structure. In this review, taking 2D materials as examples [graphene, boron nitride (h-BN), transition metal sulfide and so on], we review the crystal structure and preparation methods of these materials in recent years, focus on their photocatalyst properties (carbon dioxide reduction and hydrogen production), and discuss their applications and development prospects in the future.

High-throughput screening of two-dimensional materials and prediction of photocatalytic performances

<p indent=0mm>In recent years, a variety of excellent two-dimensional (2D) materials have been successfully prepared by mechanical isolation, liquid-phase exfoliation and molecular beam epitaxial growth, which greatly promotes the exploration of the special properties of 2D materials. Traditional “trail and error methods” are facing challenges such as low efficiency, high cost and lack of guidance, so it is extremely difficult to explore novel 2D materials. In contrast, first-principles calculations can not only compute and simulate a large number of new 2D materials to predict their structure and physical properties, but also design their possible synthesis paths and methods by referring to the predicted properties. With the continuous improvement of supercomputers and the establishment of a large number of material databases, potential 2D structures can be screened out by using different algorithms, and targeted design of 2D materials can be achieved by combining first-principles calculations. The first-principles calculations can simulate the structure of 2D materials, predict their stability, and rationally design the synthesis route according to the simulation results. At present, many 2D material structures have been predicted by theoretical calculations and many of them have been synthesized by experiments. For example, the graphene analogue silicene has attracted much attention. The results of first-principles calculations show that itexhibitsa buckled honeycomb structure. Based on the calculation of phonon spectrum and molecular dynamics simulation, the structure is stable below<sc>1000 K.</sc> The calculation results show that Si can be well adsorbed on the Ag(110) surface and silicene was successfully obtained on the Ag(111) surface several years later. Compared with the traditional “trial and error method”, although the first-principles calculations can accelerate the discovery of 2D materials, it still does not get rid of the constraints of “intuition” and causes a waste of resources. First-principles calculations heavily rely on scientific intutition, which would lead to the waste of calcualtion resources. The emergence of high-throughput computing can reduce the waste of computing resources. It provides an effective method for the discovery and screening of novel 2D materials. Through various algorithms, novel 2D materials are excavated from theconstructed databases, such as inorganic crystal structure database, crystal open database, Materials Project database, etc. High-throughput screening combined with first-principles calculations can realize the targeted design of materials. For example, through high-throughput calculations, potential 2D materials are screened from materials databases. After that, the stability, band gap, carrier mobility, light absorption and the position of band edge for the screened 2D materials can be predicted by first-principles calculations. Therefore, the design of 2D photocatalytic materials with guidance and the evaluation of photocatalytic performance greatly promote the development of 2D photocatalytic materials. Based on high-throughput screening and first-principles calculations, predicting the structural and physical properties of 2D materials exhibits advantages that traditional experiments can not achieve. These advantages have greatly promoted the discovery and research of 2D materials. We introduced the methods of theoretical calculation and summarized the research results of structure design and performance prediction of 2D materials. Finally, based on these research advances, the future development of this field is prospected.

Synthesis of a two‐dimensional porphyrin framework connected by 1,3‐diyne linkages via in situ deprotection of trimethylsilicon at a water/oil interface

AbstractDeveloping two‐dimensional (2D) porphyrin frameworks is a hot topic in the research of 2D materials. Glaser reaction between terminal alkyne monomers at an interface is promising for creating 2D polymers, such as graphdiyne. However, the coupling reaction between terminal alkyne–porphyrin monomers is impossible due to insolubility in low‐polarity solvents. Herein we used a trimethylsilicon (TMS)‐protected precursor as monomer directly and the above problem was solved by in situ deprotection of TMS at a water/toluene interface. The obtained 2D porphyrin framework connected by 1,3‐diyne linkages exhibits nanometer thickness and can be free‐standing under ambient conditions. This method not only reduces the workload of monomer synthesis, but also overcomes the limitation of Glaser reaction for producing 2D polymer films at an interface when the terminal alkyne monomer is insoluble in nonpolar solvents. © 2020 Society of Industrial Chemistry

Two dimensional nanomaterials-enabled smart light regulation technologies: Recent advances and developments

For a long time, human beings have been yearning for free regulation of light waves. But it is limited by poor performances of the natural materials. The emergence of two-dimensional(2D) synthetic nanomaterials greatly promotes the developments of light control technologies, which has dramatically improved the ability of human beings to control light. People can realize flexible regulation of the propagation speed or propagation direction of light wave in material, optical response and frequency, polarization state of light wave based on a series of novel photoelectric properties of 2D synthetic nanomaterials. So, 2D nanomaterials enabled light control technologies have become a research hotspot in recent years. In this review, we summarize the various technologies realized by 2D materials and introduce the related applications. We further analyze, compare these technologies. This review is aimed to give a generalized knowledge of 2D nanomaterials-enabled smart light regulation technologies and their current devices applications, and thus inspiring the exploration and development of other kinds of new light regulation devices and various novel applications. The challenges and the prospects of the future, especially the application of artificial intelligence (AI) in research and development of 2D materials, are also given.

Ultrathin Scattering Spin Filter and Magnetic Tunnel Junction Implemented by Ferromagnetic 2D van der Waals Material

AbstractEmerging research in 2D materials has promoted the development of nanoelectronics. Ferromagnetic van der Waals (vdW) layered materials can be utilized to implement ultrathin spintronic devices with new functionalities. The theoretical investigation of 2D vdW scattering spin filters and magnetic tunnel junctions consisting of atomically thin Fe3GeTe2 (FGT) are reported. By the nonequilibrium Green's function technique, the spin polarization of ballistic transport through single‐/double‐layer FGT sandwiched between two Cu electrodes is predicted to be 53/85%. In ultrathin FGT‐hBN‐FGT heterostructures, remarkable magnetoresistance is observed, in which maximum (minimum) resistance occurs when the magnetization of two FGT layers is parallel (antiparallel) to each other. For heterostructures consisting of single‐/double‐layer FGT, the magnetoresistance reaches 183/252% at zero‐bias limit. The parallel state of a FGT magnetic tunnel junction exhibits spin polarization larger than 75%. These results suggest the application of magnetic vdW layered materials in ultrathin spintronics.

Nonlinear optical ASnX (A = Na, H; X = N, P) nanosheets with divalent tin lone electron pair effect by first-principles design.

We propose a series of novel ASnX (A = Na, H; X = N, P) layered structures based on first-principles modeling and simulation. These nanosheet materials are all derived from the rational design for the parent structures of NaSnX (X = N, P) with unique divalent tin lone electron pairs. First-principles calculations show that the polar divalent tin effect can lead to strong second-order nonlinear optical (NLO) and large birefringence response in bulk materials, and can also greatly enhance the second harmonic generation (SHG) effect in the corresponding two-dimensional (2D) nanostructures by hydrogen passivation or strain engineering. The correlation between structural evolution and bandgap is illustrated through detailed analysis of the electronic structure and charge transfer model. The SHG effect and its origin can be further evaluated based on the SHG-weighted density scheme, which will facilitate the NLO research of 2D materials in the nitride and phosphide systems.

Alkali Metal Arenides as a Universal Synthetic Tool for Layered 2D Germanene Modification.

The rediscovery of graphene in 2004 has started an enormous chase in the research of 2D materials. A new family of layered 2D materials consisting of the 14th group elements beyond carbon has already been reported. Here, a new methodology in germanene chemistry is presented using germanane (Ge6 H6 ) as a stable and easily accessible starting material for effective synthesis of novel germanene derivatives. The modification procedure utilizing strong bases-alkali metal arenides-for deprotonation of germanane and its subsequent functionalization with p-nitrobenzyl bromide is described. Functionalization of germanene is confirmed by FT-IR, Raman, and XPS spectroscopy as well as by X-ray diffraction analysis.

Alkali Metal Arenides as a Universal Synthetic Tool for Layered 2D Germanene Modification

AbstractThe rediscovery of graphene in 2004 has started an enormous chase in the research of 2D materials. A new family of layered 2D materials consisting of the 14th group elements beyond carbon has already been reported. Here, a new methodology in germanene chemistry is presented using germanane (Ge6H6) as a stable and easily accessible starting material for effective synthesis of novel germanene derivatives. The modification procedure utilizing strong bases—alkali metal arenides—for deprotonation of germanane and its subsequent functionalization with p‐nitrobenzyl bromide is described. Functionalization of germanene is confirmed by FT‐IR, Raman, and XPS spectroscopy as well as by X‐ray diffraction analysis.

Type-II Ising Superconductivity in Two-Dimensional Materials with Spin-Orbit Coupling.

Centrosymmetric materials with spin-degenerate bands are generally considered to be trivial for spintronics and related physics. In two-dimensional (2D) materials with multiple degenerate orbitals, we find that the spin-orbit coupling can induce spin-orbital locking, generate out-of-plane Zeeman-like fields displaying opposite signs for opposing orbitals, and create novel electronic states insensitive to the in-plane magnetic field, which thus enables a new type of Ising superconductivity applicable to centrosymmetric materials. Many candidate materials are identified by high-throughput first-principles calculations. Our work enriches the physics and materials of Ising superconductivity, opening new opportunities for future research of 2D materials.

Rapid identification of two-dimensional materials via machine learning assisted optic microscopy

A combination of Fresnel law and machine learning method is proposed to identify the layer counts of 2D materials. Three indexes, which are optical contrast, red-green-blue, total color difference, are presented to illustrate and simulate the visibility of 2D materials on Si/SiO2 substrate, and the machine learning algorithms, which are k-mean clustering and k-nearest neighbors, are employed to obtain thickness database of 2D material and test the optical images of 2D materials via red-green-blue index. The results show that this method can provide fast, accurate and large-area property of 2D material. With the combination of artificial intelligence and nanoscience, this machine learning assisted method eases the workload and promotes fundamental research of 2D materials.

Controllable Fabrication of Graphene and Related Two-Dimensional Materials on Liquid Metals via Chemical Vapor Deposition.

Due to the confinement of the charge, spin, and heat transport in the plane, graphene and related two-dimensional (2D) materials have been demonstrated to own many unique and excellent properties and witnessed many breakthroughs in physics. They show great application potential in many fields, especially for electronics and optoelectronics. However, a bottleneck to widespread applications is precise and reliable fabrication, in which the control of the layer number and domain assembly is the most basic and important since they directly determine the qualities and properties of 2D materials. The chemical vapor deposition (CVD) strategy was regarded as the frontrunner to achieve this target, and the design of the catalytic substrate is of great significance since it has the most direct influence on the catalysis and mass transfer, which can be the most essential elemental steps. In recent years, as compared to traditional solid metal catalysts, the emergence of liquid metal catalysts has brought a brand-new perspective and contributes to a huge change and optimization in the fabrication of 2D materials. On one hand, strictly self-limited growth behavior is discovered and is robust to the variation of the growth parameters. The atoms in the liquid metal tend to move intensely and arrange in an amorphous and isotropic way. The liquid surface is smooth and isotropic, and the vacancies in the fluidic liquid phase enable the embedding of heteroatoms. The phase transition from liquid to solid will facilitate the unique control of the mass-transfer path, which can trigger new growth mechanisms. On the other hand, the excellent rheological properties of liquid metals allow us to explore self-assembly of the 2D materials grown on the surface, which can activate new applications based on the derived collective properties, such as the integrated devices. Indeed, liquid metals show many unique behaviors in the catalytic growth and assembly of 2D materials. Thus, this Account aims to highlight the controllable fabrication of graphene and related 2D materials on liquid metals. By utilizing the phase transition of liquid metals, the segregation of precursors in the bulk can be controlled, leading to self-limited growth. By utilizing the fluidity of the liquid metals, 2D material crystals can achieve self-assembly on their surface, including oriented stitching, ordered assembly, and heterostacking, which enables the creation of new multilevel or hybrid structures, leading to property and function extension and even the emergence of new physics. Finally, the unique liquid characteristic of liquid metals can also offer us new ideas about the transfer process. By utilizing the shear transformation of liquid metals, the direct sliding transfer of 2D materials onto arbitrary substrates can be realized. The research concerning the self-limited growth, self-assembly, and sliding transfer of 2D materials on liquid metals is just raising the curtain on the behavioral study of 2D materials on liquid metals. We believe these primary technology developments revealed by liquid metals will establish a solid foundation for both fundamental research and practical application of 2D materials.

Optimising the visibility of graphene and graphene oxide on gold with multilayer heterostructures

Metals have been increasingly used as substrates in devices based on two-dimensional (2D) materials. However, the high reflectivity of bulk metals results in low optical contrast (<3%) and therefore poor visibility of transparent mono- and few-layer 2D materials on these surfaces. Here we demonstrate that by engineering the complex reflectivity of a purpose-designed multilayer heterostructure composed of thin Au films (2–8 nm) on SiO2/Si substrate, the optical contrast of graphene and graphene oxide (GO) can be significantly enhanced in comparison to bulk Au, up to about 3 and 5 times, respectively. In particular, we achieved ∼17% optical contrast for monolayer GO, which is even 2 times higher than that on bare SiO2/Si substrate. The experimental results are in good agreement with theoretical simulations. This concept is demonstrated for Au, but the methodology is applicable to other metals and can be adopted to design a variety of high-contrast metallic substrates. This will facilitate research and applications of 2D materials in areas such as plasmonics, photonics, catalysis and sensors.

Electronic Transport in Two-Dimensional Materials.

Two-dimensional (2D) materials have captured the attention of the scientific community due to the wide range of unique properties at nanometer-scale thicknesses. While significant exploratory research in 2D materials has been achieved, the understanding of 2D electronic transport and carrier dynamics remains in a nascent stage. Furthermore, because prior review articles have provided general overviews of 2D materials or specifically focused on charge transport in graphene, here we instead highlight charge transport mechanisms in post-graphene 2D materials, with particular emphasis on transition metal dichalcogenides and black phosphorus. For these systems, we delineate the intricacies of electronic transport, including band structure control with thickness and external fields, valley polarization, scattering mechanisms, electrical contacts, and doping. In addition, electronic interactions between 2D materials are considered in the form of van der Waals heterojunctions and composite films. This review concludes with a perspective on the most promising future directions in this fast-evolving field.

Metal Thio- and Selenophosphates as Multifunctional van der Waals Layered Materials.

Since the discovery of Dirac physics in graphene, research in 2D materials has exploded with the aim of finding new materials and harnessing their unique and tunable electronic and optical properties. The follow-on work on 2D dielectrics and semiconductors has led to the emergence and development of hexagonal boron nitride, black phosphorus, and transition metal disulfides. However, the spectrum of good insulating materials is still very narrow. Likewise, 2D materials exhibiting correlated phenomena such as superconductivity, magnetism, and ferroelectricity have yet to be developed or discovered. These properties will significantly enrich the spectrum of functional 2D materials, particularly in the case of high phase-transition temperatures. They will also advance a fascinating fundamental frontier of size and proximity effects on correlated ground states. Here, a broad family of layered metal thio(seleno)phosphate materials that are moderate- to wide-bandgap semiconductors with incipient ionic conductivity and a host of ferroic properties are reviewed. It is argued that this material class has the potential to merge the sought-after properties of complex oxides with electronic functions of 2D and quasi-2D electronic materials, as well as to create new avenues for both applied and fundamental materials research in structural and magnetic correlations.

Controlled Synthesis of High-Mobility Atomically Thin Bismuth Oxyselenide Crystals.

Non-neutral layered crystals, another group of two-dimensional (2D) materials that lack a well-defined van der Waals (vdWs) gap, are those that form strong chemical bonds in-plane but display weak out-of-plane electrostatic interactions, exhibiting intriguing properties for the bulk counterpart. However, investigation of the properties of their atomically thin counterpart are very rare presumably due to the absence of efficient ways to achieve large-area high-quality 2D crystals. Here, high-mobility atomically thin Bi2O2Se, a typical non-neutral layered crystal without a standard vdWs gap, was synthesized via a facial chemical vapor deposition (CVD) method, showing excellent controllability for thickness, domain size, nucleation site, and crystal-phase evolution. Atomically thin, large single crystals of Bi2O2Se with lateral size up to ∼200 μm and thickness down to a bilayer were obtained. Moreover, optical and electrical properties of the CVD-grown 2D Bi2O2Se crystals were investigated, displaying a size-tunable band gap upon thinning and an ultrahigh Hall mobility of >20000 cm2 V-1 s-1 at 2 K. Our results on the high-mobility 2D Bi2O2Se semiconductor may activate the synthesis and related fundamental research of other non-neutral 2D materials.

Density functional theory study of bulk and single-layer magnetic semiconductorCrPS4

Searching for two-dimensional (2D) materials with multifunctionality is one of the main goals of current research in 2D materials. Magnetism and semiconducting are certainly two desirable functional properties for a single 2D material. In line with this goal, here we report a density functional theory (DFT) study of bulk and single-layer magnetic semiconductor ${\mathrm{CrPS}}_{4}$. We find that the ground-state magnetic structure of bulk ${\mathrm{CrPS}}_{4}$ exhibits the A-type antiferromagnetic ordering, which transforms to ferromagnetic (FM) ordering in single-layer ${\mathrm{CrPS}}_{4}$. The calculated formation energy and phonon spectrum confirm the stability of single-layer ${\mathrm{CrPS}}_{4}$. The band gaps of FM single-layer ${\mathrm{CrPS}}_{4}$ calculated with a hybrid density functional are within the visible-light range. We also study the effects of FM ordering on the optical absorption spectra and band alignments for water splitting, indicating that single-layer ${\mathrm{CrPS}}_{4}$ could be a potential photocatalyst. Our work opens up ample opportunities of energy-related applications of single-layer ${\mathrm{CrPS}}_{4}$.