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.     

Symmetric Ultrafast Writing and Erasing Speeds in Quasi‐Nonvolatile Memory via van der Waals Heterostructures

Due to the large gap in timescale between volatile memory and nonvolatile memory technologies, quasi‐nonvolatile memory based on 2D materials has become a viable technology for filling the gap. By exploiting the elaborate energy band structure of 2D materials, a quasi‐nonvolatile memory with symmetric ultrafast write‐1 and erase‐0 speeds and long refresh time is reported. Featuring the 2D semifloating gate architecture, an extrinsic p–n junction is used to charge or discharge the floating gate. Owing to the direct injection or recombination of charges from the floating gate electrode, the erasing speed is greatly enhanced to nanosecond timescale. Combined with the ultrafast write‐1 speed, symmetric ultrafast operations on the nanosecond timescale are achieved, which are ≈10⁶ times faster than other memories based on 2D materials. In addition, the refresh time after a write‐1 operation is 219 times longer than that of dynamic random access memory. This performance suggests that quasi‐nonvolatile memory has great potential to decrease power consumption originating from frequent refresh operations, and usher in the next generation of high‐speed and low‐power memory technology.     

Dynamics of Antimonene–Graphene Van Der Waals Growth

Van der Waals (vdW) heterostructures have recently been introduced as versatile building blocks for a variety of novel nanoscale and quantum technologies. Harnessing the unique properties of these heterostructures requires a deep understanding of the involved interfacial interactions and a meticulous control of the growth of 2D materials on weakly interacting surfaces. Although several epitaxial vdW heterostructures have been achieved experimentally, the mechanisms governing their synthesis are still nebulous. With this perspective, herein, the growth dynamics of antimonene on graphene are investigated in real time. In situ low‐energy electron microscopy reveals that nucleation predominantly occurs on 3D nuclei followed by a self‐limiting lateral growth with morphology sensitive to the deposition rate. Large 2D layers are observed at high deposition rates, whereas lower growth rates trigger an increased multilayer nucleation at the edges as they become aligned with the Z2 orientation leading to atoll‐like islands with thicker, well‐defined bands. This complexity of the vdW growth is elucidated based on the interplay between the growth rate, surface diffusion, and edges orientation. This understanding lays the groundwork for a better control of the growth of vdW heterostructures, which is critical to their large‐scale integration.     

Number‐Resolved Single‐Photon Detection with Ultralow Noise van der Waals Hybrid

Van der Waals hybrids of graphene and transition metal dichalcogenides exhibit an extremely large response to optical excitation, yet counting of photons with single‐photon resolution is not achieved. Here, a dual‐gated bilayer graphene (BLG) and molybdenum disulphide (MoS₂) hybrid are demonstrated, where opening a band gap in the BLG allows extremely low channel (receiver) noise and large optical gain (≈10¹⁰) simultaneously. The resulting device is capable of unambiguous determination of the Poissonian emission statistics of an optical source with single‐photon resolution at an operating temperature of 80 K, dark count rate 0.07 Hz, and linear dynamic range of ≈40 dB. Single‐shot number‐resolved single‐photon detection with van der Waals heterostructures may impact multiple technologies, including the linear optical quantum computation.     

Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near-infrared (NIR) to visible light upconversion in a metal–insulator–semiconductor (MIS) vdW heterostructure tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS₂, which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS₂, followed by formation and radiative recombination of excitons in WS₂. The photocarrier transfer rate can be described by Fowler–Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro-optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.     

P‐GaSe/N‐MoS2 Vertical Heterostructures Synthesized by van der Waals Epitaxy for Photoresponse Modulation

The important role of p–n junction in modulation of the optoelectronic properties of semiconductors is widely cognized. In this work, for the first time the synthesis of p‐GaSe/n‐MoS₂ heterostructures via van der Waals expitaxial growth is reported, although a considerable lattice mismatching of ≈18% exists. According to the simulation, a significant type II p–n junction barrier located at the interface is expected to be formed, which can modulate optoelectronic properties of MoS₂ effectively. It is intriguing to reveal that the presence of GaSe can result in obvious Raman and photoluminescence (PL) shift of MoS₂ compared to that of pristine one, more interestingly, for PL peak shift, the effect of GaSe‐induced tensile strain on MoS₂ has overcome the p‐doping effect of GaSe, evidencing the strong interlayer coupling between GaSe and MoS₂. As a result, the photoresponse rate of heterostructures is improved by almost three orders of magnitude compared with that of pristine MoS₂.     

Uncovering the Conduction Behavior of van der Waals Ambipolar Semiconductors

A long‐standing puzzle about van der Waals semiconductors (vdWS) is regarding the origin(s) of the conduction behavior they exhibit. Of particular interest are those with ambipolar conduction, which may provide an alternative choice for practical applications when considering the difficulties of doping the ultrathin bodies of vdWS. Here, the conduction behavior of ambipolar vdWS is analytically and theoretically studied. Using numerical simulation, it is shown that ambipolar vdWS can be fully captured by a Schottky‐barrier FET model. Based on this, it is found that the widely observed conduction polarity transition while changing the body thickness mainly comes from the tuning of band alignment at the metal/vdWS interfaces. This transition can be suppressed/inversed by introducing an inert hBN layer between the vdWS and the substrate. Through first‐principles calculations, it is demonstrated that metal/vdWS/substrate interactions play a crucial role in tuning the Schottky‐barrier heights, which finally determines the conduction behavior that ambipolar vdWS exhibit.     

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.     

Tuning the Carrier Confinement in GeS/Phosphorene van der Waals Heterostructures

Van der Waals (vdW) heterostructures, which have the advantage of integrating excellent properties of the stacked 2D materials by vdW interactions, have gained increasing attention recently. In this work, within the framework of density functional theory calculations, the electronic properties of vdW heterostructure composed of phosphorene (BP) in black phosphorus phase and GeS monolayer are systematically explored. The results show that the carriers are not separated for both lattice‐match and lattice‐mismatch heterostructures. For the lattice‐match heterostructure, it is found that changing monolayer of GeS to bilayer can increase the energy difference of valence band offsets between GeS and BP, thus realizing electron–hole separation. For the lattice‐mismatch heterostructure, altering the layer distance can transform the heterostructure into a typical type‐I alignment, but applying the electric field or doping with 2, 3, 5, 6‐tetrafluoro‐7, 7, 8, 8‐tetracyanoquinodimethane (F4TCNQ) can make it display a perfect desirable type‐II alignment, where holes migration and electrons transfer are revealed to account respectively for the phenomenon of carrier separation. It is believed that the work would greatly enlarge the potential application of the BP‐based heterostructures in photoelectronics and further stimulate the investigation enthusiasms on other fashionable heterostructures and even unassuming heterostructures in which the charming electronic properties can be modulated to emerge by various general methods.     

Two‐Terminal Multibit Optical Memory via van der Waals Heterostructure

2D van der Waals (vdWs) heterostructures exhibit intriguing optoelectronic properties in photodetectors, solar cells, and light‐emitting diodes. In addition, these materials have the potential to be further extended to optical memories with promising broadband applications for image sensing, logic gates, and synaptic devices for neuromorphic computing. In particular, high programming voltage, high off‐power consumption, and circuital complexity in integration are primary concerns in the development of three‐terminal optical memory devices. This study describes a multilevel nonvolatile optical memory device with a two‐terminal floating‐gate field‐effect transistor with a MoS₂/hexagonal boron nitride/graphene heterostructure. The device exhibits an extremely low off‐current of ≈10^?14 A and high optical switching on/off current ratio of over ≈10⁶, allowing 18 distinct current levels corresponding to more than four‐bit information storage. Furthermore, it demonstrates an extended endurance of over ≈10⁴ program–erase cycles and a long retention time exceeding 3.6 × 10⁴ s with a low programming voltage of ?10 V. This device paves the way for miniaturization and high‐density integration of future optical memories with vdWs heterostructures.     

A New Opportunity for 2D van der Waals Heterostructures: Making Steep-Slope Transistors

The use of a foreign metallic cold source (CS) has recently been proposed as a promising approach toward the steep-slope field-effect-transistor (FET). In addition to the selection of source material with desired density of states–energy relation (D(E)), engineering the source:channel interface for gate-tunable channel-barrier is crucial to CS-FETs. However, conventional metal:semiconductor (MS) interfaces generally suffer from strong Fermi-level pinning due to the inevitable chemical disorder and defect-induced gap states, precluding the gate tunability of the barriers. By comprehensive materials and device modeling at the atomic scale, it is reported that 2D van der Waals (vdW) MS interfaces, with their atomic sharpness and cleanness, can be considered as general ingredients for CS-FETs. As test cases, InSe-based n-type FETs are studied. It is found that graphene can be spontaneously p-type doped along with slightly opened bandgap around the Dirac-point by interfacing with InSe, resulting in superexponentially decaying hot carrier density with increasing n-type channel-barrier. Moreover, the D(E) relations suggest that 2D transition-metal dichalcogenides and 2D transition-metal carbides are a rich library of CS materials. Graphene, Cd₃C₂, T-VTe₂, H-VTe₂, and H-TaTe₂ CSs lead to subthreshold swing below 60 mV dec⁻¹. This work broadens the application potentials of 2D vdW MS heterostructures and serves as a springboard for more studies on low-power electronics based on 2D materials.     

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.     

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.     

Quasi van der Waals epitaxy of ZnSe on the layered chalcogenides InSe and GaSe

ZnSe has been grown on van der Waals surfaces of the layered chalcogenides InSe and GaSe. Its growth morphology, epitaxial relation and electronic band lineup were studied using XPS, UPS, LEED and HRSEM. The II–VI materials showed a strong tendency to form clusters on the low energy van der Waals surfaces. LEED measurements showed a ZnSe(111)/Ga(In)Se(0001) epitaxial relation with strong facetting of the II–VI clusters deposited on GaSe. There was no evidence for a reaction layer between substrate and film, as deduced from UPS and XPS measurements. The band lineups for the ZnSe/InSe and ZnSe/GaSe heterointerface have been determined.