Spontaneous heteroassembly of 2D semiconducting van der Waals materials in random solution phase

van der Waals heterostructures (vdWHs), consisting of more than one type of atomically thin 2D crystal layers are emerging platforms for interesting electrical, optical, and catalytic applications. High yield production of vdWHs with atomic scale precision is crucial prerequisite for practical utilization. Here we present a generalized approach of random solution phase, high yield heteroassembly of semiconducting vdWHs by exploiting inherent surface charge states of 2D materials as well as chemical affinity of specific ligand end-functionalities. Facile removal of noncovalent functionalized ligands via simple pH reversal enables clean interfaces within vdWHs, yielding outstanding optoelectrical and electrochemical properties driven by fluent interfacial charge transfer among the layered 2D structures. The generality of this procedure is demonstrated by the formation of a series of different vdWHs such as WSe₂-MoS₂, graphene–MoS₂ - and phospherene–WSe₂ heterostructures. Atomically thin WSe₂–MoS₂ phototransistor displayed an exceptionally fast response time with high sensitivity. Graphene–MoS₂ overcomes the inherent charge transfer issue of MoS₂ for electrochemical catalyst. Phospherene–WSe₂ successfully addresses poor ambient stability of phospherene together with enhanced surface activity towards chemical sensing.     

Wrap‐Around Core–Shell Heterostructures of Layered Crystals

Engineered heterostructures create new functionality by integrating dissimilar materials. Combining different 2D crystals naturally produces two distinct classes of heterostructures, vertical van der Waals (vdW) stacks or 2D sheets bonded laterally by covalent line interfaces. When joining thicker layered crystals, the arising structural and topological conflicts can result in more complex geometries. Phase separation during one‐pot synthesis of layered tin chalcogenides spontaneously creates core–shell structures in which large orthorhombic SnS crystals are enclosed in a wrap‐around shell of trigonal SnS₂, forcing the coexistence of parallel vdW layering along with unconventional, orthogonally layered core–shell interfaces. Measurements of the optoelectronic properties establish anisotropic carrier separation near type II core–shell interfaces and extended long‐wavelength light harvesting via spatially indirect interfacial absorption, making multifunctional layered core–shell structures attractive for energy‐conversion applications.     

Direct Synthesis of a Self‐Assembled WSe2/MoS2 Heterostructure Array and its Optoelectrical Properties

Functional van der Waals heterojunctions of transition metal dichalcogenides are emerging as a potential candidate for the basis of next‐generation logic devices and optoelectronics. However, the complexity of synthesis processes so far has delayed the successful integration of the heterostructure device array within a large scale, which is necessary for practical applications. Here, a direct synthesis method is introduced to fabricate an array of self‐assembled WSe₂/MoS₂ heterostructures through facile solution‐based directional precipitation. By manipulating the internal convection flow (i.e., Marangoni flow) of the solution, the WSe₂ wires are selectively stacked over the MoS₂ wires at a specific angle, which enables the formation of parallel‐ and cross‐aligned heterostructures. The realized WSe₂/MoS₂‐based p–n heterojunction shows not only high rectification (ideality factor: 1.18) but also promising optoelectrical properties with a high responsivity of 5.39 A W^?1 and response speed of 16 µs. As a feasible application, a WSe₂/MoS₂‐based photodiode array (10 × 10) is demonstrated, which proves that the photosensing system can detect the position and intensity of an external light source. The solution‐based growth of hierarchical structures with various alignments could offer a method for the further development of large‐area electronic and optoelectronic applications.     

Tunable SnSe2/WSe2 Heterostructure Tunneling Field Effect Transistor

The burgeoning 2D semiconductors can maintain excellent device electrostatics with an ultranarrow channel length and can realize tunneling by electrostatic gating to avoid deprivation of band‐edge sharpness resulting from chemical doping, which make them perfect candidates for tunneling field effect transistors. Here this study presents SnSe₂/WSe₂ van der Waals heterostructures with SnSe₂ as the p‐layer and WSe₂ as the n‐layer. The energy band alignment changes from a staggered gap band offset (type‐II) to a broken gap (type‐III) when changing the negative back‐gate voltage to positive, resulting in the device operating as a rectifier diode (rectification ratio ~10⁴) or an n‐type tunneling field effect transistor, respectively. A steep average subthreshold swing of 80 mV dec^?1 for exceeding two decades of drain current with a minimum of 37 mV dec^?1 at room temperature is observed, and an evident trend toward negative differential resistance is also accomplished for the tunneling field effect transistor due to the high gate efficiency of 0.36 for single gate devices. The I _ON/I _OFF ratio of the transfer characteristics is >10⁶, accompanying a high ON current >10^?5 A. This work presents original phenomena of multilayer 2D van der Waals heterostructures which can be applied to low‐power consumption devices.     

VI3—a New Layered Ferromagnetic Semiconductor

2D materials are promising candidates for next‐generation electronic devices. In this regime, insulating 2D ferromagnets, which remain rare, are of special importance due to their potential for enabling new device architectures. Here the discovery of ferromagnetism is reported in a layered van der Waals semiconductor, VI₃, which is based on honeycomb vanadium layers separated by an iodine–iodine van der Waals gap. It has a BiI₃‐type structure ( $R\overline{3}$, No.148) at room temperature, and the experimental evidence suggests that it may undergo a subtle structural phase transition at 78 K. VI₃ becomes ferromagnetic at 49 K, below which magneto‐optical Kerr effect imaging clearly shows ferromagnetic domains, which can be manipulated by the applied external magnetic field. The optical bandgap determined by reflectance measurements is 0.6 eV, and the material is highly resistive.     

Mobility Engineering in Vertical Field Effect Transistors Based on Van der Waals Heterostructures

Vertical integration of 2D layered materials to form van der Waals heterostructures (vdWHs) offers new functional electronic and optoelectronic devices. However, the mobility in vertical carrier transport in vdWHs of vertical field‐effect transistor (VFET) is not yet investigated in spite of the importance of mobility for the successful application of VFETs in integrated circuits. Here, the mobility in VFET of vdWHs under different drain biases, gate biases, and metal work functions is first investigated and engineered. The traps in WSe₂ are the main source of scattering, which influences the vertical mobility and three distinct transport mechanisms: Ohmic transport, trap‐limited transport, and space‐charge‐limited transport. The vertical mobility in VFET can be improved by suppressing the trap states by raising the Fermi level of WSe₂. This is achieved by increasing the injected carrier density by applying a high drain voltage, or decreasing the Schottky barrier at the graphene/WSe₂ and metal/WSe₂ junctions by applying a gate bias and reducing the metal work function, respectively. Consequently, the mobility in Mn vdWH at +50 V gate voltage is about 76 times higher than the initial mobility of Au vdWH. This work enables further improvements in the VFET for successful application in integrated circuits.     

Symmetry‐Reduction Enhanced Polarization‐Sensitive Photodetection in Core–Shell SbI3/Sb2O3 van der Waals Heterostructure

Structural symmetry is a simple way to quantify the anisotropic properties of materials toward unique device applications including anisotropic transportation and polarization‐sensitive photodetection. The enhancement of anisotropy can be achieved by artificial symmetry‐reduction design. A core–shell SbI₃/Sb₂O₃ nanowire, a heterostructure bonded by van der Waals forces, is introduced as an example of enhancing the performance of polarization‐sensitive photodetectors via symmetry reduction. The structural, vibrational, and optical anisotropies of such core–shell nanostructures are systematically investigated. It is found that the anisotropic absorbance of a core–shell nanowire is obviously higher than that of two single compounds from both theoretical and experimental investigations. Anisotropic photocurrents of the polarization‐sensitive photodetectors based on these core–shell SbI₃/Sb₂O₃ van der Waals nanowires are measured ranging from ultraviolet (UV) to visible light (360–532 nm). Compared with other van der Waals 1D materials, low anisotropy ratio (I_max/I_min) is measured based on SbI₃ but a device based on this core–shell nanowire possesses a relatively high anisotropy ratio of ≈3.14 under 450 nm polarized light. This work shows that the low‐symmetrical core–shell van der Waals heterostructure has large potential to be applied in wide range polarization‐sensitive photodetectors.     

Dirac Semimetal Heterostructures: 3D Cd3As2 on 2D Graphene

Dirac semimetal is an emerging class of quantum matters, ranging from 2D category, such as, graphene and surface states of topological insulator to 3D category, for instance, Cd₃As₂ and Na₃Bi. As 3D Dirac semimetals typically possess Fermi‐arc surface states, the 2D–3D Dirac van der Waals heterostructures should be promising for future electronics. Here, graphene–Cd₃As₂ heterostructures are fabricated through direct layer‐by‐layer stacking. The electronic coupling results in a notable interlayer charge transfer, which enables us to modulate the Fermi level of graphene through Cd₃As₂. A planar graphene p–n–p junction is achieved by selective modification, which demonstrates quantized conductance plateaus. Moreover, compared with the bare graphene device, the graphene–Cd₃As₂ hybrid device presents large nonlocal signals near the Dirac point due to the charge transfer from the spin‐polarized surface states in the adjacent Cd₃As₂. The results enrich the family of van der Waals heterostructure and should inspire more studies on the application of Dirac/Weyl semimetals in spintronics.     

Twinned Growth of Metal‐Free,Triazine‐Based Photocatalyst Films as Mixed‐Dimensional (2D/3D) van der Waals Heterostructures

Design and synthesis of ordered, metal‐free layered materials is intrinsically difficult due to the limitations of vapor deposition processes that are used in their making. Mixed‐dimensional (2D/3D) metal‐free van der Waals (vdW) heterostructures based on triazine (C₃N₃) linkers grow as large area, transparent yellow‐orange membranes on copper surfaces from solution. The membranes have an indirect band gap (E _g,opt = 1.91 eV, E _g,elec = 1.84 eV) and are moderately porous (124 m² g^?1). The material consists of a crystalline 2D phase that is fully sp² hybridized and provides structural stability, and an amorphous, porous phase with mixed sp²–sp hybridization. Interestingly, this 2D/3D vdW heterostructure grows in a twinned mechanism from a one‐pot reaction mixture: unprecedented for metal‐free frameworks and a direct consequence of on‐catalyst synthesis. Thanks to the efficient type I heterojunction, electron transfer processes are fundamentally improved and hence, the material is capable of metal‐free, light‐induced hydrogen evolution from water without the need for a noble metal cocatalyst (34 µmol h^?1 g^?1 without Pt). The results highlight that twinned growth mechanisms are observed in the realm of “wet” chemistry, and that they can be used to fabricate otherwise challenging 2D/3D vdW heterostructures with composite properties.     

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

To explore new constituents in two‐dimensional (2D) materials and to combine their best in van der Waals heterostructures is in great demand as being a unique platform to discover new physical phenomena and to design novel functionalities in interface‐based devices. Herein, PbI₂ crystals as thin as a few layers are synthesized, particularly through a facile low‐temperature solution approach with crystals of large size, regular shape, different thicknesses, and high yields. As a prototypical demonstration of band engineering of PbI₂‐based interfacial semiconductors, PbI₂ crystals are assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS₂ is enhanced in MoS₂/PbI₂ stacks, while a dramatic photoluminescence quenching of WS₂ and WSe₂ is revealed in WS₂/PbI₂ and WSe₂/PbI₂ stacks. This is attributed to the effective heterojunction formation between PbI₂ and these monolayers; type I band alignment in MoS₂/PbI₂ stacks, where fast‐transferred charge carriers accumulate in MoS₂ with high emission efficiency, results in photoluminescence enhancement, and type II in WS₂/PbI₂ and WSe₂/PbI₂ stacks, with separated electrons and holes suitable for light harvesting, results in photoluminescence quenching. The results demonstrate that MoS₂, WS₂, and WSe₂ monolayers with similar electronic structures show completely distinct light–matter interactions when interfacing with PbI₂, providing unprecedented capabilities to engineer the device performance of 2D heterostructures.     

Synthesis of Ultrathin Metallic MTe2 (M = V,Nb, Ta) Single‐Crystalline Nanoplates

Two‐dimensional materials with intrinsic magnetism have recently drawn intense interest for both the fundamental studies and potential technological applications. However, the studies to date have been largely limited to mechanically exfoliated materials. Herein, an atmospheric pressure chemical vapor deposition route to ultrathin group VB metal telluride MTe₂ (M = V, Nb, Ta) nanoplates with thickness as thin as 3 nm is reported. It is shown that the resulting nanoplates can be systematically evolved from mostly thicker hexagonal domains to thinner triangular domains with an increasing flow rate of the carrier gas. X‐ray diffraction and transmission electron microscopy studies reveal MTe₂ (M = V, Nb, Ta) nanoplates are high‐quality single crystals. High‐resolution scanning transmission electron microscope imaging reveals the VTe₂ and NbTe₂ nanoplates adopt the hexagonal 1T phase and the TaTe₂ nanoplates show a monoclinic distorted 1T phase. Electronic transport studies show that MTe₂ single crystals exhibit metallic behavior. Magnetic measurements show that VTe₂ and NbTe₂ exhibit ferromagnetism and TaTe₂ shows paramagnetic behavior. The preparation of ultrathin few‐layered MTe₂ nanoplates will open up exciting opportunities for the burgeoning field of spintronics, sensors, and magneto‐optoelectronics.     

Self‐Assembled Heterostructures: Selective Growth of Metallic Nanoparticles on V2–VI3 Nanoplates

Precise control of the selective growth of heterostructures with specific composition and functionalities is an emerging and extremely challenging topic. Here, the first investigation of the difference in binding energy between a series of metal–semiconductor heterostructures based on layered V₂–VI₃ nanostructures is investigated by means of density functional theory. All lateral configurations show lower formation energy compared with that of the vertical ones, implying the selective growth of metal nanoparticles. The simulation results are supported by the successful fabrication of self‐assembled Ag/Cu‐nanoparticle‐decorated p‐type Sb₂Te₃ and n‐type Bi₂Te₃ nanoplates at their lateral sites through a solution reaction. The detailed nucleation–growth kinetics are well studied with controllable reaction times and precursor concentrations. Accompanied by the preserved topological structure integrity and electron transfer on the semiconductor host, exceptional properties such as dramatically increased electrical conductivity are observed thanks to the pre‐energy‐filtering effect before carrier injection. A zigzag thermoelectric generator is built using Cu/Ag‐decorated Sb₂Te₃ and Bi₂Te₃ as p–n legs to utilize the temperature gradient in the vertical direction. Synthetic approaches using similar chalcogenide nanoplates as building blocks, as well as careful control of the dopant metallic nanoparticles or semiconductors, are believed to be broadly applicable to other heterostructures with novel applications.     

Tunneling Diode Based on WSe2/SnS2 Heterostructure Incorporating High Detectivity and Responsivity

van der Waals (vdW) heterostructures based on atomically thin 2D materials have led to a new era in next‐generation optoelectronics due to their tailored energy band alignments and ultrathin morphological features, especially in photodetectors. However, these photodetectors often show an inevitable compromise between photodetectivity and photoresponsivity with one high and the other low. Herein, a highly sensitive WSe₂/SnS₂ photodiode is constructed on BN thin film by exfoliating each material and manually stacking them. The WSe₂/SnS₂ vdW heterostructure shows ultralow dark currents resulting from the depletion region at the junction and high direct tunneling current when illuminated, which is confirmed by the energy band structures and electrical characteristics fitted with direct tunneling. Thus, the distinctive WSe₂/SnS₂ vdW heterostructure exhibits both ultrahigh photodetectivity of 1.29 × 10¹³ Jones (I_ph/I_dark ratio of ≈10⁶) and photoresponsivity of 244 A W^?1 at a reverse bias under the illumination of 550 nm light (3.77 mW cm^?2).     

Controlling Magnetic and Optical Properties of the van der Waals Crystal CrCl3−xBrx via Mixed Halide Chemistry

Magnetic van der Waals (vdW) materials are the centerpiece of atomically thin devices with spintronic and optoelectronic functions. Exploring new chemistry paths to tune their magnetic and optical properties enables significant progress in fabricating heterostructures and ultracompact devices by mechanical exfoliation. The key parameter to sustain ferromagnetism in 2D is magnetic anisotropy—a tendency of spins to align in a certain crystallographic direction known as easy‐axis. In layered materials, two limits of easy‐axis are in‐plane (XY) and out‐of‐plane (Ising). Light polarization and the helicity of topological states can couple to magnetic anisotropy with promising photoluminescence or spin‐orbitronic functions. Here, a unique experiment is designed to control the easy‐axis, the magnetic transition temperature, and the optical gap simultaneously in a series of CrCl_3?xBr_x crystals between CrCl₃ with XY and CrBr₃ with Ising anisotropy. The easy‐axis is controlled between the two limits by varying spin–orbit coupling with the Br content in CrCl_3?x Br_x. The optical gap, magnetic transition temperature, and interlayer spacing are all tuned linearly with x. This is the first report of controlling exchange anisotropy in a layered crystal and the first unveiling of mixed halide chemistry as a powerful technique to produce functional materials for spintronic devices.     

WSe2 Photovoltaic Device Based on Intramolecular p–n Junction

High quality p–n junctions based on 2D layered materials (2DLMs) are urgent to exploit, because of their unique properties such as flexibility, high absorption, and high tunability which may be utilized in next‐generation photovoltaic devices. Based on transfer technology, large amounts of vertical heterojunctions based on 2DLMs are investigated. However, the complicated fabrication process and the inevitable defects at the interfaces greatly limit their application prospects. Here, an in‐plane intramolecular WSe₂ p–n junction is realized, in which the n‐type region and p‐type region are chemically doped by polyethyleneimine and electrically doped by the back‐gate, respectively. An ideal factor of 1.66 is achieved, proving the high quality of the p–n junction realized by this method. As a photovoltaic detector, the device possesses a responsivity of 80 mA W^?1 (≈20% external quantum efficiency), a specific detectivity of over 10¹¹ Jones and fast response features (200 µs rising time and 16 µs falling time) at zero bias, simultaneously. Moreover, a large open‐circuit voltage of 0.38 V and an external power conversion efficiency of ≈1.4% realized by the device also promises its potential in microcell applications.     

Magnetic Proximity Effect in Graphene/CrBr3 van der Waals Heterostructures

2D van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. Efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through the magnetic proximity effect (MPE). Here, the investigation of MPE in 2D graphene/CrBr₃ van der Waals heterostructures is reported, which is probed by the Zeeman spin Hall effect through non-local measurements. Quantitative estimation of the Zeeman splitting field demonstrates a significant MPE field even in a low magnetic field. Furthermore, the observed anomalous longitudinal resistance changes at the Dirac point R_XX,D with increasing magnetic field near ν = 0 may be attributed to the MPE-induced new ground state phases. This MPE revealed in the graphene/CrBr₃ van der Waals heterostructures therefore provides a solid physics basis and key functionality for next-generation 2D spin logic and memory devices.     

Ultra-High Interfacial Thermal Conductance via Double hBN Encapsulation for Efficient Thermal Management of 2D Electronics

Heat dissipation is a major limitation of high-performance electronics. This is especially important in emerging nanoelectronic devices consisting of ultra-thin layers, heterostructures, and interfaces, where enhancement in thermal transport is highly desired. Here, ultra-high interfacial thermal conductance in encapsulated van der Waals (vdW) heterostructures with single-layer transition metal dichalcogenides MX₂ (MoS₂, WSe₂, WS₂) sandwiched between two hexagonal boron nitride (hBN) layers is reported. Through Raman spectroscopic measurements of suspended and substrate-supported hBN/MX₂/hBN heterostructures with varying laser power and temperature, the out-of-plane interfacial thermal conductance in the vertical stack is calibrated. The measured interfacial thermal conductance between MX₂ and hBN reaches 74 ± 25 MW m⁻² K⁻¹, which is at least ten times higher than the interfacial thermal conductance of MX₂ in non-encapsulation structures. Molecular dynamics (MD) calculations verify and explain the experimental results, suggesting a full encapsulation by hBN layers is accounting for the high interfacial conductance. This ultra-high interfacial thermal conductance is attributed to the double heat transfer pathways and the clean and tight vdW interface between two crystalline 2D materials. The findings in this study reveal new thermal transport mechanisms in hBN/MX₂/hBN structures and shed light on building novel hBN-encapsulated nanoelectronic devices with enhanced thermal management.     

Carbon‐Nanotube‐Confined Vertical Heterostructures with Asymmetric Contacts

Van der Waals (vdW) heterostructures have received intense attention for their efficient stacking methodology with 2D nanomaterials in vertical dimension. However, it is still a challenge to scale down the lateral size of vdW heterostructures to the nanometer and make proper contacts to achieve optimized performances. Here, a carbon‐nanotube‐confined vertical heterostructure (CCVH) is employed to address this challenge, in which 2D semiconductors are asymmetrically sandwiched by an individual metallic single‐walled carbon nanotube (SWCNT) and a metal electrode. By using WSe₂ and MoS₂, the CCVH can be made into p‐type and n‐type field effect transistors with high on/off ratios even when the channel length is 3.3 nm. A complementary inverter was further built with them, indicating their potential in logic circuits with a high integration level. Furthermore, the Fermi level of SWCNTs can be efficiently modulated by the gate voltage, making it competent for both electron and hole injection in the CCVHs. This unique property is shown by the transition of WSe₂ CCVH from unipolar to bipolar, and the transition of WSe₂/MoS₂ from p–n junction to n–n junction under proper source–drain biases and gate voltages. Therefore, the CCVH, as a member of 1D/2D mixed heterostructures, shows great potentials in future nanoelectronics and nano‐optoelectronics.     

A High‐On/Off‐Ratio Floating‐Gate Memristor Array on a Flexible Substrate via CVD‐Grown Large‐Area 2D Layer Stacking

Memristors such as phase‐change memory and resistive memory have been proposed to emulate the synaptic activities in neuromorphic systems. However, the low reliability of these types of memories is their biggest challenge for commercialization. Here, a highly reliable memristor array using floating‐gate memory operated by two terminals (source and drain) using van der Waals layered materials is demonstrated. Centimeter‐scale samples (1.5 cm × 1.5 cm) of MoS₂ as a channel and graphene as a trap layer grown by chemical vapor deposition (CVD) are used for array fabrication with Al₂O₃ as the tunneling barrier. With regard to the memory characteristics, 93% of the devices exhibit an on/off ratio of over 10³ with an average ratio of 10⁴. The high on/off ratio and reliable endurance in the devices allow stable 6‐level memory applications. The devices also exhibit excellent memory durability over 8000 cycles with a negligible shift in the threshold voltage and on‐current, which is a significant improvement over other types of memristors. In addition, the devices can be strained up to 1% by fabricating on a flexible substrate. This demonstration opens a practical route for next‐generation electronics with CVD‐grown van der Waals layered materials.     

3D Mesoporous van der Waals Heterostructures for Trifunctional Energy Electrocatalysis

The emergence of van der Waals (vdW) heterostructures of 2D materials has opened new avenues for fundamental scientific research and technological applications. However, the current concepts and strategies of material engineering lack feasibilities to comprehensively regulate the as‐obtained extrinsic physicochemical characters together with intrinsic properties and activities for optimal performances. A 3D mesoporous vdW heterostructure of graphene and nitrogen‐doped MoS₂ via a two‐step sequential chemical vapor deposition method is constructed. Such strategy is demonstrated to offer an all‐round engineering of 2D materials including the morphology, edge, defect, interface, and electronic structure, thereby leading to robustly modified properties and greatly enhanced electrochemical activities. The hydrogen evolution is substantially accelerated on MoS₂, while the oxygen reduction and evolution are significantly improved on graphene. This work provides a powerful overall engineering strategy of 2D materials for electrocatalysis, which is also enlightening for other nanomaterials and energy‐related applications.