Magnetic and electronic properties of bulk and two-dimensional FeBi_(2)Te_(4):A first-principles study

Layered magnetic materials,such as MnBi_(2)Te_(4),have drawn much attention owing to their potential for realizing twodimensional(2D)magnetism and possible topological states.Recently,FeBi_(2)Te_(4),which is isostructural to MnBi_(2)Te_(4),has been synthesized in experiments,but its detailed magnetic ordering and band topology have not been clearly understood yet.Here,based on first-principles calculations,we investigate the magnetic and electronic properties of FeBi_(2)Te_(4)in bulk and 2D forms.We show that different from MnBi_(2)Te_(4),the magnetic ground states of bulk,single-layer,and bilayer FeBi_(2)Te_(4)all favor a 120°noncollinear antiferromagnetic ordering,and they are topologically trivial narrow-gap semiconductors.For the bilayer case,we find that a quantum anomalous Hall effect with a unit Chern number is realized in the ferromagnetic state,which may be achieved in experiment by an external magnetic field or by magnetic proximity coupling.Our work clarifies the physical properties of the new material system of FeBi_(2)Te_(4)and reveals it as a potential platform for studying magnetic frustration down to 2D limit as well as quantum anomalous Hall effect.

Review of Raman spectroscopy of two-dimensional magnetic van der Waals materials

Ultrathin van der Waals(vdW)magnets provide a possibility to access magnetic ordering in the two-dimensional(2D)limit,which are expected to be applied in the spintronic devices.Raman spectroscopy is a powerful characterization method to investigate the spin-related properties in 2D vdW magnets,including magnon and spin–lattice interaction,which are hardly accessible by other optical methods.In this paper,the recent progress of various magnetic properties in 2D vdW magnets studied by Raman spectroscopy is reviewed,including the magnetic transition,spin-wave,spin–lattice interaction,symmetry tuning induced by spin ordering,and nonreciprocal magneto-phonon Raman scattering.

Waveguide-integrated optical modulators with two-dimensional materials

Waveguide-integrated optical modulators are indispensable for on-chip optical interconnects and optical computing.To cope with the ever-increasing amount of data being generated and consumed,ultrafast waveguide-integrated optical modulators with low energy consumption are highly demanded.In recent years,two-dimensional(2D)materials have attracted a lot of attention and have provided tremendous opportunities for the development of high-performance waveguide-integrated optical modulators because of their extraordinary optoelectronic properties and versatile compatibility.This paper reviews the state-of-the-art waveguide-integrated optical modulators with 2D materials,providing researchers with the developing trends in the field and allowing them to identify existing challenges and promising potential solutions.First,the concept and fundamental mechanisms of optical modulation with 2D materials are summarized.Second,a review of waveguide-integrated optical modulators employing electro-optic,all-optic,and thermo-optic effects is provided.Finally,the challenges and perspectives of waveguide-integrated modulators with 2D materials are discussed.

Field-effect transistors based on two-dimensional materials for logic applications

Field-effect transistors （FETs） for logic applications, graphene and MoS2, are discussed. These materials have based on two representative two-dimensional （2D） materials, drastically different properties and require different consider- ations. The unique band structure of graphene necessitates engineering of the Dirac point, including the opening of the bandgap, the doping and the interface, before the graphene can be used in logic applications. On the other hand, MoS2 is a semiconductor, and its electron transport depends heavily on the surface properties, the number of layers, and the carrier density. Finally, we discuss the prospects for the future developments in 2D material transistors.

Thermal transport in semiconductor nanostructures, graphene,and related two-dimensional materials

We review experimental and theoretical results on thermal transport in semiconductor nanostructures（multilayer thin films, core/shell and segmented nanowires）, single-and few-layer graphene, hexagonal boron nitride, molybdenum disulfide, and black phosphorus. Different possibilities of phonon engineering for optimization of electrical and heat conductions are discussed. The role of the phonon energy spectra modification on the thermal conductivity in semiconductor nanostructures is revealed. The dependence of thermal conductivity in graphene and related two-dimensional（2 D） materials on temperature, flake size, defect concentration, edge roughness, and strain is analyzed.

Magnetic van der Waals materials:Synthesis,structure,magnetism,and their potential applications

As the family of magnetic materials is rapidly growing,two-dimensional(2D)van der Waals(vdW)magnets have attracted increasing attention as a platform to explore fundamental physical problems of magnetism and their potential applications.This paper reviews the recent progress on emergent vd W magnetic compounds and their potential applications in devices.First,we summarize the current vd W magnetic materials and their synthetic methods.Then,we focus on their structure and the modulation of magnetic properties by analyzing the representative vd W magnetic materials with different magnetic structures.In addition,we pay attention to the heterostructures of vd W magnetic materials,which are expected to produce revolutionary applications of magnetism-related devices.To motivate the researchers in this area,we finally provide the challenges and outlook on 2D vd W magnetism.

Origin of itinerant ferromagnetism in two-dimensional Fe_(3)GeTe_(2)

Magnetic order in two-dimensional systems was not supposed to exist at finite temperature.In recent years,the successful preparation of two-dimensional ferromagnetic materials such as CrI_(3),Cr_(2) Ge_(2) Te_(6),and Fe_(3)GeTe_(2) opens up a new chapter in the remarkable field of two-dimensional materials.Here,we report on a theoretical analysis of the stability of ferromagnetism in Fe_(3)GeTe_(2).We uncover the mechanism of holding long-range magnetic order and propose a model to estimate the Curie temperature of Fe_(3)GeTe_(2).Our results reveal the essential role of magnetic anisotropy in maintaining the magnetic order of two-dimensional systems.The theoretical method used here can be generalized to future research of other magnetic two-dimensional systems.

Two-dimensional hexagonal Zn3Si2 monolayer:Dirac cone material and Dirac half-metallic manipulation

The fascinating Dirac cone in honeycomb graphene,which underlies many unique electronic properties,has inspired the vast endeavors on pursuing new two-dimensional(2D)Dirac materials.Based on the density functional theory method,a 2D material Zn3Si2 of honeycomb transition-metal silicide with intrinsic Dirac cones has been predicted.The Zn3Si2 monolayer is dynamically and thermodynamically stable under ambient conditions.Importantly,the Zn3Si2 monolayer is a room-temperature 2D Dirac material with a spin-orbit coupling energy gap of 1.2 meV,which has an intrinsic Dirac cone arising from the special hexagonal lattice structure.Hole doping leads to the spin polarization of the electron,which results in a Dirac half-metal feature with single-spin Dirac fermion.This novel stable 2D transition-metal-silicon-framework material holds promises for electronic device applications in spintronics.

Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials

With a large number of researches being conducted on two?dimen?sional(2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that pro?vides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high?quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of di erent types of 2D heterostruc?tures, followed by the discussions on present challenges and perspectives of further investigations.

Room temperature synthesis of two-dimensional multi layer magnets based on α-Co^(Ⅱ) layered hydroxides

Research on two-dimensional(2D) materials is one of the most active fields in materials science and nanotechnology. Among the members of the 2D family, layered hydroxides(LHs) represent an exceptional case of study due to their unparalleled chemical versatility which allows the modulation of their physicochemical properties at will. Nowadays, LHs based on earth-abundant metals are key materials in the areas of energy storage and conversion, hybrid materials or magnetism. α-Co hydroxides(Simonkolleite-like structures) are promising phases with tuneable electronic and magnetic properties by ligand modification. However, even in the simple case of α-Co^(Ⅱ) hydroxychlorides, the preparation of well-defined large 2D crystals is not straightforward, hindering the development of fundamental studies. Herein, we present the synthesis of 2D hexagonal crystals with outstanding sizethickness relationship(diameter > 5 μm and thickness of 20 ± 7 nm) by a simple homogeneous synthesis taking place at room temperature. In structural terms, no differences are observed between our layered materials and those obtained hydrothermally. However, dynamic susceptibility measurements alert about different arrangements of the magnetic sublattices, which have been rationalized with structural DFT calculations. This work provides an extremely easy bottom-up method to obtain high-quality 2D crystals based on α-CoIIhydroxides,paving the way for the development of fundamental studies and applications.

Two-dimensional Cr_(2)Cl_(3)S_(3) Janus magnetic semiconductor with large magnetic exchange interaction and high-T_(C)

The two-dimensional(2D)Janus monolayers are promising in spintronic device application due to their enhanced magnetic couplings and Curie temperatures.Van der Waals CrCl_(3) monolayer has been experimentally proved to have an in-plane magnetic easy axis and a low Curie temperature of 17 K,which will limit its application in spintronic devices.In this work,we propose a new Janus monolayer Cr_(2)Cl_(3)S_(3) based on the first principles calculations.The phonon dispersion and elastic constants confirm that Janus monolayer Cr_(2)Cl_(3)S_(3) is dynamically and mechanically stable.Our Monte Carlo simulation results based on magnetic exchange constants reveal that Janus monolayer Cr_(2)Cl_(3)S_(3) is an intrinsic ferromagnetic semiconductor with TC of 180 K,which is much higher than that of CrCl_(3) due to the enhanced ferromagnetic coupling caused by S substitution.Moreover,the magnetic easy axis of Janus Cr_(2)Cl_(3)S_(3) can be tuned to the perpendicular direction with a large magnetic anisotropy energy(MAE)of 142eV/Cr.Furthermore,the effect of biaxial strain on the magnetic property of Janus monolayer Cr_(2)Cl_(3)S_(3) is evaluated.It is found that the Curie temperature is more robust under tensile strain.This work indicates that the Janus monolayer Cr_(2)Cl_(3)S_(3) presents increased Curie temperature and out-of-plane magnetic easy axis,suggesting greater application potential in 2D spintronic devices.

Families of magnetic semiconductors——an overview

The interplay of magnetic and semiconducting properties has been in the focus for more than a half of the century. In this introductory article we briefly review the key properties and functionalities of various magnetic semiconductor families, including europium chalcogenides, chromium spinels, dilute magnetic semiconductors, dilute ferromagnetic semiconductors and insulators, mentioning also sources of non-uniformities in the magnetization distribution, accounting for an apparent high Curie temperature ferromagnetism in many systems. Our survey is carried out from today's perspective of ferromagnetic and antiferromagnetic spintronics as well as of the emerging fields of magnetic topological materials and atomically thin 2D layers.

Highly stable two-dimensional graphene oxide:Electronic properties of its periodic structure and optical properties of its nanostructures

According to first principle simulations, we theoretically predict a type of stable single-layer graphene oxide（C_2O）.Using density functional theory（DFT）, C_2O is found to be a direct gap semiconductor. In addition, we obtain the absorption spectra of the periodic structure of C_2O, which show optical anisotropy. To study the optical properties of C_2O nanostructures, time-dependent density functional theory（TDDFT） is used. The C_2O nanostructure has a strong absorption near 7 eV when the incident light polarizes along the armchair-edge. Besides, we find that the optical properties can be controlled by the edge configuration and the size of the C_2O nanostructure. With the elongation strain increasing, the range of light absorption becomes wider and there is a red shift of absorption spectrum.

Modulating p-type doping of two-dimensional material palladium diselenide

The van der Waals heterostructures have evolved as novel materials for complementing the Si-based semiconductor technologies.Group-10 noble metal dichalcogenides(e.g.,PtS_(2),PtSe_(2),PdS_(2),and PdSe_(2))have been listed into two-dimensional(2D)materials toolkit to assemble van der Waals heterostructures.Among them,PdSe_(2) demonstrates advantages of high stability in air,high mobility,and wide tunable bandgap.However,the regulation of p-type doping of PdSe_(2) remains unsolved problem prior to fabricating p–n junction as a fundamental platform of semiconductor physics.Besides,a quantitative method for the controllable doping of PdSe_(2) is yet to be reported.In this study,the doping level of PdSe_(2) was correlated with the concentration of Lewis acids,for example,SnCl_(4),used for soaking.Considering the transfer characteristics,the threshold voltage(the gate voltage corresponding to the minimum drain current)increased after SnCl_(4) soaking treatment.PdSe_(2) transistors were soaked in SnCl_(4) solutions with five different concentrations.The threshold voltages from the as-obtained transfer curves were extracted for linear fitting to the threshold voltage versus doping concentration correlation equation.This study provides in-depth insights into the controllable p-type doping of PdSe_(2).It may also push forward the research of the regulation of conductivity behaviors of 2D materials.

In-plane optical anisotropy of two-dimensional VOCl single crystal with weak interlayer interaction

Transition-metal oxyhalides MOX(M=Fe,Cr,V;O=oxygen,X=F,Cl,Br,I),an emerging type of two-dimensional(2D)van der Waals materials,have been both theoretically and experimentally demonstrated to possess unique electronic and magnetic properties.However,the intrinsic in-plane anisotropic properties of 2D VOCl still lacks in-depth re-search,especially optical anisotropy.Herein,a systematic Raman spectroscopic study is performed on VOCl single-crystal with different incident laser polarization at various temperatures.The polarized-dependent Raman scattering spectra reveal that the Ag mode of VOCl show a 2-lobed shape in parallel polarization configuration while a 4-lobed shape in vertical configuration.In addition,the temperature-dependent and thickness-dependent Raman scattering spectra confirm a rela-tively weak van der Waals interaction between each layers among VOCl single crystal.These findings might provide better understanding on the in-plane anisotropic phenomenon in VOCl layers,thus will accelate further application of 2D single crystals for nanoscale angle-dependent optoelectronics.

Static response of functionally graded multilayered two-dimensional quasicrystal plates with mixed boundary conditions

The unusual properties of quasicrystals(QCs)have attracted tremendous attention from researchers.In this paper,a semi-analytical solution is presented for the static response of a functionally graded(FG)multilayered two-dimensional(2 D)decagonal QC rectangular plate with mixed boundary conditions.Based on the elastic theory of FG 2 D QCs,the state-space method is used to derive the state equations composed of partial differential along the thickness direction.Besides,the Fourier series expansion and the differential quadrature technique are utilized to simulate the simply supported boundary conditions and the mixed boundary conditions,respectively.Then,the propagator matrix which connects the field variables at the upper interface to those at the lower interface of any homogeneous layer can be derived based on the state equations.Combined with the interface continuity condition,the static response can be obtained by imposing the sinusoidal load on the top surfaces of laminates.Finally,the numerical examples are presented to verify the effectiveness of this method,and the results are very useful for the design and understanding of the characterization of FG QC materials in their applications to multilayered systems.

Moirésuperlattice engineering of two-dimensional materials for electrocatalytic hydrogen evolution reaction

Vertically stacking two-dimensional(2D)materials with small azimuthal deviation or lattice mismatch generate distinctive global structural periodicity and symmetry,revealed as the moirésuperlattices(MSLs).Manipulating the interlayer twist angle enables the modification of the electronic structure of 2D materials to explore the advanced applications.Although extraordinary progress has been achieved in the unique structure and emergent properties of MSLs,the investigation of the catalytic applications of MSLs materials is still in its infancy.It is therefore very urgent to summarize the advanced development of MSLs in the field of catalysis.In this review,we firstly summarize the advanced fabrication and high-resolution characterization techniques of the MSLs materials,as well as their novel properties related to catalysis represented by electrocatalytic hydrogen evolution reaction(HER).Then,all the MSLs materials such as MoS_(2),WS_(2),and Ru serving as electrocatalysts for HER are further reviewed in detail.Finally,we outline the current challenges as well as the experimental and theoretical strategies to advance the development of function-oriented MSLs materials for catalysis.This review aims to provide profound insight into the wide applications of this novel material platform in catalytic field.

Ferromagnetic and ferroelectric two-dimensional materials for memory application

The discoveries of ferromagnetic and ferroelectric two-dimensional(2D)materials have dramatically inspired intense interests due to their potential in the field of spintronic and nonvolatile memories.This review focuses on the latest 2D ferromagnetic and ferroelectric materials that have been most recently studied,including insulating ferromagnetic,metallic ferromagnetic,antiferromagnetic and ferroelectric 2D materials.The fundamental properties that lead to the long-range magnetic orders of 2D materials are discussed.The low Curie temperature(Tc)and instability in 2D systems limits their use in practical applications,and several strategies to address this constraint are proposed,such as gating and composition stoichiometry.A van der Waals(vdW)heterostructure comprising 2D ferromagnetic and ferroelectric materials will open a door to exploring exotic physical phenomena and achieve multifunctional or nonvolatile devices.

Advance in two-dimensional twisted moirématerials:Fabrication,properties,and applications

Two-dimensional(2D)twisted moirématerials,a new class of van der Waals(vdW)layered heterostructures with different twist angles between neighboring layers,have attracted tremendous attention due to their rich emerging properties.In this review,we systematically summarize the recent progress of 2D twisted moirématerials.Firstly,we introduce several representative fabrication methods and the fascinating topographies of the twisted moirématerials.Specifically,we discuss various remarkable physical properties related to twisted angles,including flat bands,unconventional superconductivity,ferromagnetism,and ferroelectricity.We also analyze the potential applications in various twisted moirésystems.Finally,the challenges and future perspectives of the twisted moirématerials are discussed.This work would spur edge-cutting ideas and related achievements in the scientific and technological frontiers of 2D twisted moirématerials.

Epitaxially grown monolayer VSe2： an air-stable magnetic two-dimensional material with low work function at edges

Recent experimental breakthroughs open up new opportunities for magnetism in few-atomic-layer twodimensional(2 D) materials, which makes fabrication of new magnetic 2 D materials a fascinating issue.Here, we report the growth of monolayer VSe_2 by molecular beam epitaxy(MBE) method. Electronic properties measurements by scanning tunneling spectroscopy(STS) method revealed that the asgrown monolayer VSe_2 has magnetic characteristic peaks in its electronic density of states and a lower work-function at its edges. Moreover, air exposure experiments show air-stability of the monolayer VSe_2. This high-quality monolayer VSe_2, a very air-inert 2 D material with magnetism and low edge work function, is promising for applications in developing next-generation low power-consumption, high efficiency spintronic devices and new electrocatalysts.