Lattice vibration and Raman scattering of two-dimensional van der Waals heterostructure

Research on two-dimensional(2D) materials and related van der Waals heterostructures(vdWHs) is intense and remains one of the leading topics in condensed matter physics.Lattice vibrations or phonons of a vdWH provide rich information,such as lattice structure,phonon dispersion,electronic band structure and electron–phonon coupling.Here,we provide a mini review on the lattice vibrations in vdWHs probed by Raman spectroscopy.First,we introduced different kinds of vdWHs,including their structures,properties and potential applications.Second,we discussed interlayer and intralayer phonon in twist multilayer graphene and MoS2.The frequencies of interlayer and intralayer modes can be reproduced by linear chain model(LCM)and phonon folding induced by periodical moiré potentials,respectively.Then,we extended LCM to vdWHs formed by distinct 2D materials,such as MoS2/graphene and hBN/WS2 heterostructures.We further demonstrated how to calculate Raman intensity of interlayer modes in vdWHs by interlayer polarizability model.

A facile and efficient dry transfer technique for two-dimensional Van der Waals heterostructure

Two-dimensional （2D） Van der Waals heterostructures have aroused extensive concerns in recent years. Their fabrica- tion calls for facile and efficient transfer techniques for achieving well-defined structures. In this work, we report a simple and effective dry transfer method to fabricate 2D heterostructures with a clean interface. Using Propylene Carbonate （PC） films as stamps, we are able to pick up various 2D materials flakes from the substrates and unload them to the receiving substrates at an elevated temperature. Various multilayer heterostructures with ultra-clean interfaces were fabricated by this technique. Furthermore, the 2D materials can be pre-pattemed before transfer so as to fabricate desired device structures, demonstrating a facile way to promote the development of 2D heterostructures.

Magnetic two-dimensional van derWaals materials for spintronic devices

Magnetic two-dimensional(2D)van derWaals(vdWs)materials and their heterostructures attract increasing attention in the spintronics community due to their various degrees of freedom such as spin,charge,and energy valley,which may stimulate potential applications in the field of low-power and high-speed spintronic devices in the future.This review begins with introducing the long-range magnetic order in 2D vdWs materials and the recent progress of tunning their properties by electrostatic doping and stress.Next,the proximity-effect,current-induced magnetization switching,and the related spintronic devices(such as magnetic tunnel junctions and spin valves)based on magnetic 2D vdWs materials are presented.Finally,the development trend of magnetic 2D vdWs materials is discussed.This review provides comprehensive understandings for the development of novel spintronic applications based on magnetic 2D vdWs materials.

Raman scattering study of two-dimensional magnetic van der Waals compound VI3

The layered magnetic van der Waals materials have generated tremendous interest due to their potential applications and importance in fundamental research.Previous x-ray diffraction(XRD)studies on the magnetic van der Waals compound VI3,revealed a structural transition above the magnetic transition but output controversial analysis on symmetry.In this paper we carried out polarized Raman scattering measurements on VI3 from 10 K to 300 K,with focus on the two Ag phonon modes at^71.1 cm^-1 and 128.4 cm-1.Our careful symmetry analysis based on the angle-dependent spectra demonstrates that the crystal symmetry can be well described by C2h rather than D3d both above and below structural phase transition.We further performed temperature-dependent Raman experiments to study the magnetism in VI3.Fano asymmetry and anomalous linewidth drop of two Ag phonon modes at low temperatures,point to a significant spin-phonon coupling.This is also supported by the softening of 71.1-cm^-1 mode above the magnetic transition.The study provides the fundamental information on lattice dynamics and clarifies the symmetry in VI3.And spin-phonon coupling existing in a wide temperature range revealed here may be meaningful in applications.

Two-Dimensional van der Waals Superconductor Heterostructures:Josephson Junctions and Beyond

The past few years have witnessed prominent progress in two-dimensional(2D)van der Waals heterostructures.Vertically assembled in an artificial manner,these atomically thin layers possess distinctive electronic,magnetic,and other properties,which have provided a versatile platform for both fundamental exploration and practical applications in condensed matter physics and materials science.Within various potential combinations,a particular set of van der Waals superconductor(SC)heterostructures,which is realized by stacking fabrication based on two-dimensional SCs,is currently attracting intense attention.For example,the Josephson junction,a specific structure in which a nonsuperconducting barrier is inserted between two proximity-coupled SCs,shows phenomena and outstanding properties with atomic-scale thickness.In this Perspective,we first review this emerging research area of van der Waals SC heterostructures,especially progress on the 2D van der Waals Josephson junctions,from the aspects of preparation,performance,and application,and also propose our vision for the future direction and potential innovation opportunities.

Two-Dimensional Materials for Highly Efficient and Stable Perovskite Solar Cells

Perovskite solar cells(PSCs)offer low costs and high power conversion efficiency.However,the lack of long-term stability,primarily stemming from the interfacial defects and the sus-ceptible metal electrodes,hinders their practical application.In the past few years,two-dimensional(2D)materials(e.g.,graphene and its derivatives,transitional metal dichalcogenides,MXenes,and black phosphorus)have been identified as a promising solution to solving these problems because of their dangling bond-free surfaces,layer-dependent electronic band structures,tunable functional groups,and inherent compactness.Here,recent progress of 2D material toward efficient and stable PSCs is summarized,including its role as both interface materials and electrodes.We discuss their beneficial effects on perovskite growth,energy level alignment,defect passivation,as well as blocking external stimulus.In particular,the unique properties of 2D materials to form van der Waals heterojunction at the bottom interface are emphasized.Finally,perspectives on the further development of PSCs using 2D materials are provided,such as designing high-quality van der Waals heterojunction,enhancing the uniformity and coverage of 2D nanosheets,and developing new 2D materials-based electrodes.

Centered Waves for the Two-dimensional Pseudo-Steady van der Waals Gas Satisfied Maxwell's Law Around a Sharp Corner

In this paper,the authors study the centered waves for the two-dimensional(2D for short)pseudo-steady supersonic flow with van der Waals gas satisfied Maxwell's law around a sharp corner.In view of the initial value of the specific volume and the properties of van der Waals gas,the centered waves at the sharp corner are constructed by classification.It is shown that the supersonic incoming flow turns the sharp corner by a centered simple wave or a centered simple wave with right-contact discontinuity or a composite wave(jump-fan,fan-jump or fan-jump-fan),or a combination of waves and constant state.Moreover,the critical angle of the sharp corner corresponding to the appearance of the vacuum phenomenon is obtained.

Flexible electronics and optoelectronics of 2D van der Waals materials

Flexible electronics and optoelectronics exhibit inevitable trends in next-generation intelligent industries,including healthcare and wellness,electronic skins,the automotive industry,and foldable or rollable displays.Traditional bulk-material-based flexible devices considerably rely on lattice-matched crystal structures and are usually plagued by unavoidable chemical disorders at the interface.Two-dimensional van der Waals materials(2D VdWMs)have exceptional multifunctional properties,including large specific area,dangling-bond-free interface,plane-to-plane van der Waals interactions,and excellent mechanical,electrical,and optical properties.Thus,2D VdWMs have considerable application potential in functional intelligent flexible devices.To utilize the unique properties of 2D VdWMs and their van der Waals heterostructures,new designs and configurations of electronics and optoelectronics have emerged.However,these new designs and configurations do not consider lattice mismatch and process incompatibility issues.In this review,we summarized the recently reported 2D VdWM-based flexible electronic and optoelectronic devices with various functions thoroughly.Moreover,we identified the challenges and opportunities for further applications of 2D VdWM-based flexible electronics and optoelectronics.

Strain drived band aligment transition of the ferromagnetic VS_(2)/C_(3)N van der Waals heterostructure

Exploring two-dimensional(2D)magnetic heterostructures is essential for future spintronic and optoelectronic devices.Herein,using first-principle calculations,stable ferromagnetic ordering and colorful electronic properties are established by constructing the VS_(2)/C_(3)N van der Waals(vdW)heterostructure.Unlike the semiconductive properties with indirect band gaps in both the VS2 and C3N monolayers,our results indicate that a direct band gap with type-Ⅱband alignment and p-doping characters are realized in the spin-up channel of the VS_(2)/C_(3)N heterostructure,and a typical type-Ⅲband alignment with a broken-gap in the spin-down channel.Furthermore,the band alignments in the two spin channels can be effectively tuned by applying tensile strain.An interchangement between the type-Ⅱand type-Ⅲband alignments occurs in the two spin channels,as the tensile strain increases to 4%.The attractive magnetic properties and the unique band alignments could be useful for prospective applications in the next-generation tunneling devices and spintronic devices.

Observation of magnetoresistance in CrI_(3)/graphene van der Waals heterostructures

Two-dimensional ferromagnetic van der Waals(2D vdW)heterostructures have opened new avenues for creating artificial materials with unprecedented electrical and optical functions beyond the reach of isolated 2D atomic layered materials,and for manipulating spin degree of freedom at the limit of few atomic layers,which empower next-generation spintronic and memory devices.However,to date,the electronic properties of 2D ferromagnetic heterostructures still remain elusive.Here,we report an unambiguous magnetoresistance behavior in CrI_(3)/graphene heterostructures,with a maximum magnetoresistance ratio of 2.8%.The magnetoresistance increases with increasing magnetic field,which leads to decreasing carrier densities through Lorentz force,and decreases with the increase of the bias voltage.This work highlights the feasibilities of applying two-dimensional ferromagnetic vdW heterostructures in spintronic and memory devices.

Reversible doping polarity and ultrahigh carrier density in two-dimensional van der Waals ferroelectric heterostructures

Van der Waals semiconductor heterostructures(VSHs)composed of two or more two-dimensional(2D)materials with different band gaps exhibit huge potential for exploiting high-performance multifunctional devices.The application of 2D VSHs in atomically thin devices highly depends on the control of their carrier type and density.Herein,on the basis of comprehensive first-principles calculations,we report a new strategy to manipulate the doping polarity and carrier density in a class of 2D VSHs consisting of atomically thin transition metal dichalcogenides(TMDs)andα-In_(2)X_(3)(X=S,Se)ferroelectrics via switchable polarization field.Our calculated results indicate that the band bending of In_(2)X_(3)layer driven by the FE polarization can be utilized for engineering the band alignment and doping polarity of TMD/In_(2)X_(3)VSHs,which enables us to control their carrier density and type of the VSHs by the orientation and magnitude of local FE polarization field.Inspired by these findings,we demonstrate that doping-free p–n junctions achieved in MoTe2/In2Se3 VSHs exhibit high carrier density(1013–1014 cm–2),and the inversion of the VHSs from n–p junctions to p–i–n junctions has been realized by the polarization switching from upward to downward states.This work provides a nonvolatile and nondestructive doping strategy for obtaining programmable p–n van der Waals(vdW)junctions and opens the possibilities for self-powered and multifunctional device applications.

Two-dimensional capillaries assembled by van der Waals heterostructures

Research on two-dimensional materials in the past decades has brought many insights of low-dimensional science on a wide range of related topics.As a novel two-dimensional structure,the atomic-scale capillaries which can conceptually be seen as the empty space left by removing few layers of two-dimensional materials from their bulk van der Waals crystals offer a unique platform of investigating physical and chemical processes of ions,molecules,and atoms under two-dimensional confinements.Investigation of many important problems,such as capillary condensation and water network structure that are difficult to be explored experimentally in other confinement structures,has now been accessible;two-dimensional migration of ions,water,and gases shows abnormal transport properties beyond conventional theory prediction;influence of quantum effect to molecule permeation is observable even at room temperature.All these discoveries greatly extend our fundamental understandings of nano-science,and stimulate the development of potential applications.We review the fabrication of these two-dimensional capillaries which are created by the assembly of van der Waals heterostructures,and discuss the ultimate steric effects in the smallest possible confinements.Exotic interactions between capillary interior and confined particles are also summarized.When coupled with external stimuli,these channels exhibit tunable mass transport behaviors,which not only gives feedback to the mechanism understanding but in turn guides the channel structure optimization.

Unipolar p-type monolayer WSe_(2) field-effect transistors with high current density and low contact resistance enabled by van der Waals contacts

High-performance field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors have demonstrated great promise in post-Moore integrated circuits. However, unipolar p-type 2D semiconductor transistors yet remain challenging and suffer from low saturation current density (less than 10 µA·µm^(−1)) and high contact resistance (larger than 100 kΩ·µm), mainly limited by the Schottky barrier induced by the mismatch of the work-functions and the Fermi level pinning at the metal contact interfaces. Here, we overcome these two obstacles through van der Waals (vdW) integration of high work-function metal palladium (Pd) as the contacts onto monolayer WSe2 grown by chemical vapor deposition (CVD) method. We demonstrate unipolar p-type monolayer WSe2 FETs with superior device performance: room temperature on-state current density exceeding 100 µA·µm^(−1), contact resistance of 12 kΩ·µm, on/off ratio over 107, and field-effect hole mobility of ~ 103 cm2·V^(−1)·s^(−1). Electrical transport measurements reveal that the Fermi level pinning effect is completely effectively eliminated in monolayer WSe2 with vdW Pd contacts, leading to a Schottky barrier-free Ohmic contact at the metal-semiconductor junctions. Combining the advantages of large-scale vdW contact strategy and CVD growth, our results pave the way for wafer-scale fabrication of complementary-metal-oxide-semiconductor (CMOS) logic circuits based on atomically thin 2D semiconductors.

Van der Waals ferroelectric transistors:the all-round artificial synapses for high-precision neuromorphic computing

State number,operation power,dynamic range and conductance weight update linearity are key synaptic device performance metrics for high-accuracy and low-power-consumption neuromorphic com-puting in hardware.However,high linearity and low power consump-tion couldn’t be simultaneously achieved by most of the reported synaptic devices,which limits the performance of the hardware.This work demonstrates van der Waals(vdW)stacked ferroelectric field-effect transistors(FeFET)with single-crystalline ferroelectric nanoflakes.Ferroelectrics are of fine vdW interface and partial polar-ization switching of multi-domains under electric field pulses,which makes the FeFETs exhibit multi-state memory characteristics and ex-cellent synaptic plasticity.They also exhibit a desired linear conduc-tance weight update with 128 conductance states,a sufficiently high dynamic range of G_(max)/G_(min)>120,and a low power consumption of 10 fJ/spike using identical pulses.Based on such an all-round device,a two-layer artificial neural network was built to conduct Modified Na-tional Institute of Standards and Technology(MNIST)digital num-bers and electrocardiogram(ECG)pattern-recognition simulations,with the high accuracies reaching 97.6%and 92.4%,respectively.The remarkable performance demonstrates that vdW-FeFET is of obvious advantages in high-precision neuromorphic computing applications.

Van der Waals magnets: Wonder building blocks for two-dimensional spintronics?

The unprecedented realization of two-dimensional(2D)van der Waals magnets excitingly extends the synergy between spintronics and 2D materials,started with graphene over the last decade.This article reviews the recent milestones in the development of 2D magnets and its derived heterostructures.In particular,a number of critical challenges centered around the scalability,ambient stability and Curie temperature of these atomically thin magnets are discussed.This mini-review also provides an outlook on what the future might hold for this integrated field of 2D spintronics,and assesses its potential in postsilicon electronics.

Realization of vertical and lateral van der Waals heterojunctions using two-dimensional layered organic semiconductors

Van der Waals （vdW） heterojunctions based on two-dimensional （2D） atomic crystals have been extensively studied in recent years. Herein, we show that both vertical and lateral vdW heterojunctions can be realized with layered molecular crystals using a two-step physical vapor transport （PVT） process. Both types of heterojunctions show clean and sharp interfaces without phase mixing under atomic force microscopy （AFM）. They also exhibit a strong interfacial built-in electric field similar to that of their inorganic counterparts. These heterojunctions have greater potential for device applications than individual materials. The lateral heterojunction （LHJ） devices show rectifying characteristics due to the asymmetric energy barrier for holes at the interface, while the vertical heterojunction （VHJ） devices behave like metal-insulator-semiconductor tunnel junctions, with pronounced negative differential conductance （NDC）. Our work extends the concept of vdW heterojunctions to molecular materials, which can be generalized to other layered organic semiconductors （OSCs） to obtain new device functionalities.

Two-dimensional van der Waals thin film transistors as active matrix for spatially resolved pressure sensing

The development of pressure sensor arrays capable of distinguishing the shape and texture details of objects is of considerable interest in the emerging fields of smart robots,prostheses,human-machine interfaces,and artificial intelligence(AI).Here we report an integrated pressure sensor array,by combining solution-processed two-dimensional(2D)MoS2 van der Waals(vdW)thin film transistor(TFT)active matrix and conductive micropyramidal pressure-sensitive rubber(PSR)electrodes made of polydimethylsiloxane/carbon nanotube composites,to achieve spatially revolved pressure mapping with excellent contrast and low power consumption.We demonstrate a 10×10 active matrix by using the 2D MoS2 vdW-TFTs with high on-off ratio>10^(6),minimal hysteresis,and excellent device-to-device uniformity.The combination of the vdW-TFT active matrix with the highly uniform micropyramidal PSR electrodes creates an integrated pressure sensing array for spatially resolved pressure mapping.This study demonstrates that the solution-processed 2D vdW-TFTs offer a solution for active-matrix control of pressure sensor arrays,and could be extended for other active-matrix arrays of electronic or optoelectronic devices.

Phonon anharmonicity and thermal conductivity of two-dimensional van der Waals materials: A review

Two-dimensional(2D) van der Waals(vdW) materials have extraordinary thermal properties due to the effect of quantum confinement, making them promising for thermoelectric energy conversion and thermal management in microelectronic devices.In this review, the mechanism of phonon anharmonicity originating from three-and four-phonon interactions is derived. The phonon anharmonicity of 2D vdW materials, involving the Grüneisen parameter, phonon lifetime, and thermal conductivity, is summarized and derived in detail. The size-dependent thermal conductivity of representative 2D vdW materials is discussed experimentally and theoretically. This review will present fundamental and advanced knowledge on how to evaluate the phonon anharmonicity in 2D vdW materials, which will aid the design of new structures and materials for applications related to energy transfer and conversion.

Controlled growth of two-dimensional InAs single crystals via van der Waals epitaxy

Two-dimensional(2D)indium arsenide(InAs)is promising for future electronic and optoelectronic applications such as highperformance nanoscale transistors,flexible and wearable devices,and high-sensitivity broadband photodetectors,and is advantageous for its heterogeneous integration with Si-based electronics.However,the synthesis of 2D InAs single crystals is challenging because of the nonlayered structure.Here we report the van der Waals epitaxy of 2D InAs single crystals,with their thickness down to 4.8 nm,and their lateral sizes up to~37μm.The as-grown InAs flakes have high crystalline quality and are homogenous.The thickness can be tuned by growth time and temperature.Moreover,we explore the thickness-dependent optical properties of InAs flakes.Transports measurement reveals that 2D InAs possesses high conductivity and high carrier mobility.Our work introduces InAs to 2D materials family and paves the way for applying 2D InAs in high-performance electronics and optoelectronics.

High-performance self-powered ultraviolet to near-infrared photodetector based on WS_(2)/InSe van der Waals heterostructure

van der Waals heterostructures(vdWHs)based on two-dimensional(2D)materials without the crystal lattice matching constraint have great potential for high-performance optoelectronic devices.Herein,a WS_(2)/InSe vdWH photodiode is proposed and fabricated by precisely stacking InSe and WS_(2)flakes through an all-dry transfer method.The WS_(2)/InSe vdWH forms an n–n heterojunction with strong built-in electric field due to their intrinsic n-type semiconductor characteristics and energy-band alignments with a large Fermi level offset between WS_(2)and InSe.As a result,the device displays excellent photovoltaic behavior with a large open voltage of 0.47 V and a short-circuit current of 11.7 nA under 520 nm light illumination.Significantly,a fast rising/decay time of 63/76μs,a large light on/off ratio of 105,a responsivity of 61 mA/W,a high detectivity of 2.5×10^(11) Jones,and a broadband photoresponse ranging from ultraviolet to near-infrared(325–980 nm)are achieved at zero bias.This study provides a strategy for developing high-performance self-powered broadband photodetectors based on 2D materials.