Rational design of vitamin C/defective carbon van der Waals heterostructure for enhanced activity,durability and storage stability toward oxygen reduction reaction

Metal-free defective carbon materials with abundant active sites have been widely studied as low-cost and efficient oxygen reduction reaction(ORR)electrocatalysts in metal-air batteries.However,the active sites in defective carbon are easily subjected to serious oxidation or hydroxylation during ORR or storage,leading to rapid degradation of activity.Herein,we design a van der Waals heterostructure comprised of vitamin C(VC)and defective carbon(DC)to not only boost the activity but also enhance the durability and storage stability of the DC-VC electrocatalyst.The formation of VC van der Waals between DC and VC is demonstrated to be an effective strategy to protect the defect active sites from oxidation and hydroxylation degradation,thus significantly enhancing the electrochemical durability and storage anti-aging performance.Moreover,the DC-VC van der Waals can reduce the reaction energy barrier to facilitate the ORR.These findings are also confirmed by operando Fourier transform infrared spectroscopy and density functional theory calculations.It is necessary to mention that the preparation of this DC-VC electrocatalyst can be scaled up,and the ORR performance of the largely produced electrocatalyst is demonstrated to be very consistent.Furthermore,the DC-VC-based aluminum-air batteries display very competitive power density with good performance maintenance.

Two Step Chemical Vapor Deposition of In2Se3/MoSe2 van der Waals Heterostructures

Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide in- terest not only for the fundamental research, but also for the application of next generation electronic and optoelectronic devices. Herein, we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In2Se3/MoSe2 heterostructures. Transmission electron microscopy characterization reveals clearly that the In2Se3 has well-aligned lattice orientation with the substrate of monolayer MoSe2. Due to the interaction between the In2Se3 and MoSe2 layers, the heterostructure shows the quench- ing and red-shift of photoluminescence. Moreover, the current rectification behavior and photovoltaic effect can be observed from the heterostructure, which is attributed to the unique band structure alignment of the heterostructure, and is further confirmed by Kevin probe force microscopy measurement. The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.

Fabrication and applications of van der Waals heterostructures

Van der Waals heterostructures(vdWHs) are showing considerable potential in both fundamental exploration and practical applications. Built upon the synthetic successes of(two-dimensional) 2D materials, several synthetic strategies of vdWHs have been developed,allowing the convenient fabrication of diverse vdWHs with decent controllability, quality, and scalability. This review first summarizes the current state of the art in synthetic strategies of vdWHs, including physical combination, deposition, solvothermal synthesis, and synchronous evolution. Then three major applications and their representative vdWH devices have been reviewed, including electronics(tunneling field effect transistors and 2D contact),optoelectronics(photodetector), and energy conversion(electrocatalysts and metal ion batteries), to unveil the potentials of vdWHs in practical applications and provide the general design principles of functional vdWHs for different applications. Besides, moiré superlattices based on vdWHs are discussed to showcase the importance of vdWHs as a platform for novel condensed matter physics. Finally, the crucial challenges towards ideal vdWHs with high performance are discussed, and the outlook for future development is presented. By the systematical integration of synthetic strategies and applications, we hope this review can further light up the rational designs of vdWHs for emerging applications.

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.

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.

InSe-Te van der Waals heterostructures for current rectification and photodetection

As the basis of modern electronics and optoelectronics,high-performance,multi-functional p-n junctions have manifested and occupied an important position.However,the performance of the silicon-based p-n junctions declines gradually as the thickness approaches to few nanometers.The heterojunction constructed by two-dimensional(2D)materials can significantly improve the device performance compared with traditional technologies.Here,we report the In Se-Te type-II van der Waals heterostructures with rectification ratio up to 1.56×10^(7) at drain-source voltage of±2 V.The p-n junction exhibits a photovoltaic and photoelectric effect under different laser wavelengths and densities and has high photoresponsivity and detectivity under low irradiated light power.Moreover,the heterojunction has stable photo/dark current states and good photoelectric switching characteristics.Such high-performance heterostructured device based on 2D materials provides a new way for futural electronic and optoelectronic devices.

Introducing Spin Polarization into Mixed-Dimensional Van der Waals Heterostructures for High-Efficiency Visible-Light Photocatalysis

The low separation efficiency of the photogenerated carrier and the poor activity of the surface redox reaction are the main barrier to further improvement of photocatalytic materials.To address these issues,introducing spin-polarized electrons in single-component photocatalytic materials emerged as a promising approach.However,the decreased redox ability of photocarriers in these materials becomes a new challenge.Herein,we mitigate this challenge with a carbon nitride sheet(CNs)/graphene nanoribbon(GNR)composite material that has a van der Waals heterostructures(vdWHs)and spin-polarized electron properties.Experimental results and theoretical calculations show that the heterostructure has a strong redox ability,high carrier-separation efficiency,and enhanced surface catalytic reaction.Consequently,the mixed-dimensional CNs/GNR vdWHs exhibit remarkable performance for H_(2)and O_(2)generation as well as CO_(2)production under visible-light irradiation without any cocatalyst.The spin-polarized vdWHs discovered in this study revealed a new type of photocatalytic materials and advanced the development of spintronics and photocatalysis.

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.

Ultrafast interlayer photocarrier transfer in graphene–MoSe2 van der Waals heterostructure

We report the fabrication and photocarrier dynamics in graphene–MoSe2 heterostructures. The samples were fabricated by mechanical exfoliation and manual stacking techniques. Ultrafast laser measurements were performed on the heterostructure and MoSe2 monolayer samples. By comparing the results, we conclude that photocarriers injected in MoSe2 of the heterostructure transfer to graphene on an ultrafast time scale. The carriers in graphene alter the optical absorption coefficient of MoSe2. These results illustrate the potential applications of this material in optoelectronic devices.

Near-zero Poisson's ratio and suppressed mechanical anisotropy in strained black phosphorene/SnSe van der Waals heterostructure:a first-principles study

Black phosphorene(BP)and its analogs have attracted intensive attention due to their unique puckered structures,anisotropic characteristics,and negative Poisson’s ratio.The van der Waals(vdW)heterostructures assembly by stacking different materials show novel physical properties,however,the parent materials do not possess.In this work,the first-principles calculations are performed to study the mechanical properties of the vdW heterostructure.Interestingly,a near-zero Poisson’s ratio ν_(zx)is found in BP/SnSe heterostructure.In addition,compared with the parent materials BP and SnSe with strong in-plane anisotropic mechanical properties,the BP/SnSe heterostructure shows strongly suppressed anisotropy.The results show that the vdW heterostructure has quite different mechanical properties compared with the parent materials,and provides new opportunities for the mechanical applications of the heterostructures.

Excitonic devices based on two-dimensional transition metal dichalcogenides van der Waals heterostructures

Excitonic devices are an emerging class of technology that utilizes excitons as carriers for encoding, transmitting, and storing information. Van der Waals heterostructures based on transition metal dichalcogenides often exhibit a type II band alignment, which facilitates the generation of interlayer excitons. As a bonded pair of electrons and holes in the separation layer, interlayer excitons offer the chance to investigate exciton transport due to their intrinsic out-of-plane dipole moment and extended exciton lifetime. Furthermore, interlayer excitons can potentially analyze other encoding strategies for information processing beyond the conventional utilization of spin and charge. The review provided valuable insights and recommendations for researchers studying interlayer excitonic devices within van der Waals heterostructures based on transition metal dichalcogenides. Firstly, we provide an overview of the essential attributes of transition metal dichalcogenide materials, focusing on their fundamental properties, excitonic effects, and the distinctive features exhibited by interlayer excitons in van der Waals heterostructures. Subsequently, this discourse emphasizes the recent advancements in interlayer excitonic devices founded on van der Waals heterostructures, with specific attention is given to the utilization of valley electronics for information processing, employing the valley index. In conclusion, this paper examines the potential and current challenges associated with excitonic devices.

Twistronics and moiréexcitonic physics in van der Waals heterostructures

Heterostructures composed of two-dimensional van der Waals(vdW)materials allow highly controllable stacking,where interlayer twist angles introduce a continuous degree of freedom to alter the electronic band structures and excitonic physics.Motivated by the discovery of Mott insulating states and superconductivity in magic-angle bilayer graphene,the emerging research fields of“twistronics”and moiréphysics have aroused great academic interests in the engineering of optoelectronic properties and the exploration of new quantum phenomena,in which moirésuperlattice provides a pathway for the realization of artificial excitonic crystals.Here we systematically summarize the current achievements in twistronics and moiréexcitonic physics,with emphasis on the roles of lattice rotational mismatches and atomic registries.Firstly,we review the effects of the interlayer twist on electronic and photonic physics,particularly on exciton properties such as dipole moment and spin-valley polarization,through interlayer interactions and electronic band structures.We also discuss the exciton dynamics in vdW heterostructures with different twist angles,like formation,transport and relaxation processes,whose mechanisms are complicated and still need further investigations.Subsequently,we review the theoretical analysis and experimental observations of moirésuperlattice and moirémodulated excitons.Various exotic moiréeffects are also shown,including periodic potential,moiréminiband,and varying wave function symmetry,which result in exciton localization,emergent exciton peaks and spatially alternating optical selection rule.We further introduce the expanded properties of moirésystems with external modulation factors such as electric field,doping and strain,showing that moirélattice is a promising platform with high tunability for optoelectronic applications and in-depth study on frontier physics.Lastly,we focus on the rapidly developing field of correlated electron physics based on the moirésystem,which is potentially related to the emerging quantum phenomena.

Flower-like CuS/γ-Fe_(2)O_(3) van der Waals heterostructures with highefficient electromagnetic wave absorption

The escalating electromagnetic(EM)pollution issues and the demand to elevate military stealth technology make it imperative to develop cost-effective and high-performance electromagnetic wave(EMW)absorbing materials.In this paper,the flower-like CuS/γ-Fe_(2)O_(3) van der Waals(vdW)heterostructures have been synthesized via a facile two-step solvothermal approach.The flower-like CuS skeleton increases the attenuation path of EMW while reducing the material density.Different contents ofγ-Fe_(2)O_(3) nanoparticles anchor between the flower-like CuS nanosheets to constitute a heterogeneous structure,which enables dielectric and magnetic loss synergistically to optimize impedance matching and remarkably improve the EMW absorption performance.The minimum reflection loss(RLmin)is-49.36 dB with a thickness of only 1.6 mm and the effective absorption bandwidth(EAB)reaches 4.64 GHz(13.36–18 GHz).By adjusting the thickness of the absorber,the EAB can cover 96%of the GHz band.Notably,the superior absorption of-61.53 dB at middle frequency band can be obtained by adjusting the amount of Fe_(2)O_(3) addition.In this study,the adjustment of EM parameters and the optimization of impedance matching have been achieved by constructing a novel vdW heterogeneous structure,which provides fresh ideas and references for the design of high-performance EMW absorbing materials.

Reconfigurable logic and in-sensor encryption operations in an asymmetrically tunable van der Waals heterostructure

Reconfigurable devices can be used to achieve multiple logic operation and intelligent optical sensing with low power consumption,which is promising candidates for new generation electronic and optoelectronic integrated circuits.However,the versatility is still limited and need to be extended by the device architectures design.Here,we report an asymmetrically gate two-dimensional(2D)van der Waals heterostructure with hybrid dielectric layer SiO_(2)/hexagonal boron nitride(h-BN),which enable rich function including reconfigurable logic operation and in-sensor information encryption enabled by both volatile and non-volatile optoelectrical modulation.When the partial gate is grounded,the non-volatile light assisted electrostatic doping endowed partially reconfigurable doping between n-type and p-type,which allow the switching of logic XOR and not implication(NIMP).When the global gate is grounded,additionally taking the optical signal as another input signal,logic AND and OR is realized by combined regulation of the light and localized gate voltage.Depending on the high on/off current ratio approaching 105 and reliable&switchable logic gate,in-sensor information encryption and decryption is demonstrated by manipulating the logic output.Hence,these results provide strong extension for current reconfigurable electronic and optoelectronic devices.

Room-temperature tunable tunneling magnetoresistance in Fe_(3)GaTe_(2)/WSe_(2)/Fe_(3)GaTe_(2) van der Waals heterostructures

The exceptional properties of two-dimensional(2D)magnet materials present a novel approach to fabricate functional magnetic tunnel junctions(MTJ)by constructing full van der Waals(vdW)heterostructures with atomically sharp and clean interfaces.The exploration of vdW MTJ devices with high working temperature and adjustable functionalities holds great potential for advancing the application of 2D materials in magnetic sensing and data storage.Here,we report the observation of highly tunable room-temperature tunneling magnetoresistance through electronic means in a full vdW Fe_(3)GaTe_(2)/WSe_(2)/Fe_(3)GaTe_(2) MTJ.The spin valve effect of the MTJ can be detected even with the current below 1 nA,both at low and room temperatures,yielding a tunneling magnetoresistance(TMR)of 340%at 2 K and 50%at 300 K,respectively.Importantly,the magnitude and sign of TMR can be modulated by a DC bias current,even at room temperature,a capability that was previously unrealized in full vdW MTJs.This tunable TMR arises from the contribution of energy-dependent localized spin states in the metallic ferromagnet Fe_(3)GaTe_(2) during tunnel transport when a finite electrical bias is applied.Our work offers a new perspective for designing and exploring room-temperature tunable spintronic devices based on vdW magnet heterostructures.

Performance optimization of thermionic refrigerators based on van der Waals heterostructures

In this paper, an irreversible thermionic refrigerator model based on van der Waals heterostructure with various irreversibilities is established by utilizing combination of non-equilibrium thermodynamics and finite time thermodynamics. The basic performance characteristics of the refrigerator are obtained. The effects of key factors, such as bias voltages, Schottky barrier heights and heat leakages, on the performance are studied. Results show that cooling rates and coefficients of performances(COPs) can attain the double maximum with proper modulation of barrier heights and bias voltages. Increasing cross-plane thermal resistance as well as decreasing electrode-reservoir thermal resistance and reservoir-reservoir thermal resistance can enhance the performance of the device. The optimal performance region is the interval between the maximum cooling rate point and the maximum COP point. By modulating the bias voltage, the working state of the device can fall into the optimal performance region. The optimal performance of the refrigerator when using single layer graphene and a few layers graphene as electrode material is also compared.

Graphitic carbon nitride/antimonene van der Waals heterostructure with enhanced photocatalytic CO_(2) reduction activity

Photocatalytic reduction of CO_(2) into valuable fuels is one of the potential strategies to solve the carbon cycle and energy crisis.Graphitic carbon nitride(g-C_(3)N_(4)),as a typical two-dimensional(2D)semiconductor with a bandgap of∼2.7 eV,has attracted wide attention in photocatalytic CO_(2) reduction.However,the performance of g-C_(3)N_(4) is greatly limited by the rapid recombination of photogenerated charge carriers and weak CO_(2) activation capacity.Construction of van der Waals heterostructure with the maximum interface contact area can improve the transfer/seperation efficiency of interface charge carriers.Ultrathin metal antimony(Sb)nanosheet(antimonene)with high carrier mobility and 2D layered structure,is a good candidate material to construct 2D/2D Sb/g-C_(3)N_(4) van der Waals heterostructure.In this work,the density functional theory(DFT)calculations indicated that antimonene has higher carrier mobility than g-C_(3)N_(4) nanosheets.Obvious charge transfer and in-plane structure distortion will occur at the interface of Sb/g-C_(3)N_(4),which endow stronger CO_(2) activation ability on di-coordinated N active site.The ultrathin g-C_(3)N_(4) and antimonene nanosheets were prepared by ultrasonic exfoliation method,and Sb/g-C_(3)N_(4) van der Waals heterostructures were constructed by self-assembly process.The photoluminescence(PL)and time-resolved photoluminescence(TRPL)indicated that the Sb/g-C_(3)N_(4) van der Waals heterostructures have a better photogenerated charge separation efficiency than pure g-C_(3)N_(4) nanosheets.In-situ FTIR spectroscopy demonstrated a stronger ability of CO_(2) activation to^ (∗)COOH on Sb/g-C_(3)N_(4) van der Waals heterostructure.As a result,the Sb/g-C_(3)N_(4) van der Waals heterostructures showed a higher CO yield with 2.03 umol g^(−1) h^(−1),which is 3.2 times that of pure g-C_(3)N_(4).This work provides a reference for activating CO_(2) and promoting CO_(2) reduction by van der Waals heterostructure.

Wafer-scale vertical van der Waals heterostructures

Wafer-scale van der Waals heterostructures(vdWHs),benefitting from the rich diversity in materials available and stacking geometry,precise controllability in devices structure and performance,and unprecedented potential in practical application,have attracted considerable attention in the field of twodimensional(2D)materials.This article reviews the state-of-the-art research activities that focus on wafer-scale vdWHs and their(opto)electronic applications.We begin with the preparation strategies of vdWHs with wafer size and illustrate them from four key aspects,that is,mechanical-assembly stack,successive deposition,synchronous evolution,and seeded growth.We discuss the fundamental principle,underlying mechanism,advantages,and disadvantages for each strategy.We will then review the applications of large-area vdWHs based devices in electronic,optoelectronic and flexible devices field,unveiling their promising potential for practical application.Ultimately,we will demonstrate the challenges they face and provide some viable solutions on waferscale heterostructure synthesis and device fabrication.

Band-tailored van der Waals heterostructure for multilevel memory and artificial synapse

Two-dimensional(2D)van der Waals heterostructure(vdWH)-based floating gate devices show great potential for next-generation nonvolatile and multilevel data storage memory.However,high program voltage induced substantial energy consumption,which is one of the primary concerns,hinders their applications in lowenergy-consumption artificial synapses for neuromorphic computing.In this study,we demonstrate a three-terminal floating gate device based on the vdWH of tin disulfide(SnS2),hexagonal boron nitride(h-BN),and few-layer graphene.The large electron affinity of SnS2 facilitates a significant reduction in the program voltage of the device by lowering the hole-injection barrier across h-BN.Our floating gate device,as a nonvolatile multilevel electronic memory,exhibits large on/off current ratio(105),good retention(over 104 s),and robust endurance(over 1000 cycles).Moreover,it can function as an artificial synapse to emulate basic synaptic functions.Further,low energy consumption down to7 picojoule(pJ)can be achieved owing to the small program voltage.High linearity(<1)and conductance ratio(80)in long-term potentiation and depression(LTP/LTD)further contribute to the high pattern recognition accuracy(90%)in artificial neural network simulation.The proposed device with attentive band engineering can promote the future development of energy-efficient memory and neuromorphic devices.

Q-switching of waveguide lasers based on graphene/WS_2 van der Waals heterostructure

We report on the operation of passively Q-switched waveguide lasers at 1 μm wavelength based on a graphene∕WS_2 heterostructure as a saturable absorber(SA). The gain medium is a crystalline Nd:YVO_4 cladding waveguide produced by femtosecond laser writing. The nanosecond waveguide laser operation at 1064 nm has been realized with the maximum average output power of 275 m W and slope efficiency of 37%. In comparison with the systems based on single WS_2 or graphene SA, the lasing Q-switched by a graphene∕WS_2 heterostructure SA possesses advantages of a higher pulse energy and enhanced slope efficiency, indicating the promisingapplications of van der Waals heterostructures for ultrafast photonic devices.