Phonon resonance modulation in weak van der Waals heterostructures:Controlling thermal transport in graphene-silicon nanoparticle systems

The drive for efficient thermal management has intensified with the miniaturization of electronic devices.This study explores the modulation of phonon transport within graphene by introducing silicon nanoparticles influenced by van der Waals forces.Our approach involves the application of non-equilibrium molecular dynamics to assess thermal conductivity while varying the interaction strength,leading to a noteworthy reduction in thermal conductivity.Furthermore,we observe a distinct attenuation in length-dependent behavior within the graphene-nanoparticles system.Our exploration combines wave packet simulations with phonon transmission calculations,aligning with a comprehensive analysis of the phonon transport regime to unveil the underlying physical mechanisms at play.Lastly,we conduct transient molecular dynamics simulations to investigate interfacial thermal conductance between the nanoparticles and the graphene,revealing an enhanced thermal boundary conductance.This research not only contributes to our understanding of phonon transport but also opens a new degree of freedom for utilizing van der Waals nanoparticle-induced resonance,offering promising avenues for the modulation of thermal properties in advanced materials and enhancing their performance in various technological applications.

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.

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.

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.

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.

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.

Supplementary Materials: Ultrafast charge transfer in dual graphene-WS_2 van der Waals quadrilayer heterostructures

1. The transient absorption spectra of the WS2 monolayer sample.In the measurement of the transient absorption spectra of the WS2 monolayer sample, A 400-nm (3.1eV) pump pulse with a peak fluence of about 10μJ/cm2excites the electrons from the valence band into the conduction band,the

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.

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 charge transfer in dual graphene-WS_2 van der Waals quadrilayer heterostructures

Using dual graphene–WS2 quadrilayer heterostructures as an example, we find that the ultrafast transfer of electrons from WS2 to graphene takes place within 114 fs, and the Coulomb field of the charge can effectively affect the interlayer electron transfer. This effect illustrates that the charge transfer in such van der Waals heterostructures may be controlled by an externally applied electric field for promising applications in photoelectric 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.

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.

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.

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.

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.

Electrically-driven ultrafast out-of-equilibrium light emission from hot electrons in suspended graphene/hBN heterostructures

Nanoscale light sources with high speed of electrical modulation and low energy consumption are key components for nanophotonics and optoelectronics.The record-high carrier mobility and ultrafast carrier dynamics of graphene make it promising as an atomically thin light emitter which can be further integrated into arbitrary platforms by van der Waals forces.However,due to the zero bandgap,graphene is difficult to emit light through the interband recombination of carriers like conventional semiconductors.Here,we demonstrate ultrafast thermal light emitters based on suspended graphene/hexagonal boron nitride(Gr/hBN)heterostructures.Electrons in biased graphene are significantly heated up to 2800 K at modest electric fields,emitting bright photons from the near-infrared to the visible spectral range.By eliminating the heat dissipation channel of the substrate,the radiation efficiency of the suspended Gr/hBN device is about two orders of magnitude greater than that of graphene devices supported on SiO2or hBN.Wefurther demonstrate that hot electrons and low-energy acoustic phonons in graphene are weakly coupled to each other and are not in full thermal equilibrium.Direct cooling ofhigh-temperature hot electrons to low-temperature acoustic phonons is enabled by the significant near-field heat transfer at the highly localized Gr/hBN interface,resulting in ultrafast thermal emission with up to 1 GHz bandwidth under electrical excitation.It is found thatsuspending the Gr/hBN heterostructures on the SiO2trenches significantly modifies the light emission due to the formation of the optical cavity and showed a~440%enhancement inintensity at the peak wavelength of 940 nm compared to the black-body thermal radiation.The demonstration of electrically driven ultrafast light emission from suspended Gr/hBNheterostructures sheds the light on applications of graphene heterostructures in photonicintegrated circuits,such as broadband light sources and ultrafast thermo-optic phase modulators.

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.

Electronic property and topological phase transition in a graphene/CoBr_(2) heterostructure

Recently,significant experimental advancements in achieving topological phases have been reported in van der Waals(vdW)heterostructures involving graphene.Here,using first-principles calculations,we investigate graphene/CoBr_(2)(Gr/CoBr_(2))heterostructures and find that an enhancement of in-plane magnetic anisotropy(IMA)energy in monolayer CoBr_(2) can be accomplished by reducing the interlayer distance of the vdW heterostructures.In addition,we clarify that the enhancement of IMA energy primarily results from two factors:one is the weakness of the Co-d_(xy) and Co-d_(x^(2)-y^(2)) orbital hybridization and the other is the augmentation of the Co-d_(yz) and Co-d_(z)2 orbital hybridization.Meanwhile,calculation results suggest that the Kosterlitz–Thouless phase transition temperature(TKT)of a 2D XY magnet Gr/CoBr_(2)(23.8 K)is higher than that of a 2D XY monolayer CoBr_(2)(1.35 K).By decreasing the interlayer distances,the proximity effect is more pronounced and band splitting appears.Moreover,by taking into account spin–orbit coupling,a band gap of approximately 14.3 meV and the quantum anomalous Hall effect(QAHE)are attained by decreasing the interlayer distance by 1.0 A.Inspired by the above conclusions,we design a topological field transistor device model.Our results support that the vdW interlayer distance can be used to modulate the IMA energy and QAHE of materials,providing a pathway for the development of new low-power spintronic devices.

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.