Highly Efficient Nonvolatile Magnetization Switching and Multi-Level States by Current in Single Van der Waals Topological Ferromagnet Fe3GeTe2

Robust multi-level spin memory with the ability to write information electrically is a long-sought capability in spintronics, with great promise for applications. Here, nonvolatile and highly energy-efficient magnetization switching is achieved in a single-material device formed of van-der-Waals (vdW) topological ferromagnet Fe₃GeTe₂, whose magnetic information can be readily controlled by a tiny current. Furthermore, the switching current density and power dissipation are about 400 and 4000 times smaller than those of the existing spin-orbit-torque magnetic random access memory based on conventional magnet/heavy-metal systems. Most importantly, multi-level states, switched by electrical current are also demonstrated, which can dramatically enhance the information capacity density and reduce computing costs. Thus, the observations combine both high energy efficiency and large information capacity density in one device, showcasing the potential applications of the emerging field of vdW magnets in the field of spin memory and spintronics.     

Emergence of Multispinterface and Antiferromagnetic Molecular Exchange Bias via Molecular Stacking on a Ferromagnetic Film

Heterointerfaces may exhibit unexpected physical properties distinct from intrinsic properties of component materials. In particular, metal–organic interfaces can drive unique interfacial spin moments, which are often called molecular spinterface. Here, van der Waals stacking of molecular layers may lead to variations in the intra/interlayer exchange coupling resulting in multiple ground states, which is highly desired for multifunctional magnetic devices. In this report, the emergence of molecular multispinterface of paramagnetic cobalt‐octaethyl‐porphyrin (CoOEP) layers in a Fe/CoOEP heterostructure is demonstrated through the interfacial layer and a successive antiferromagnetic molecular spin chain. The disentangled interfacial ferromagnetic spins lead to multiple magnetic ground states and behave as additional spin‐dependent scattering centers, as evidenced through the magnetotransport study. In addition, the antiferromagnetic molecule spin chain derives tunable exchange bias, which signifies the dominance of the antiferromagnetic interfacial interaction. Theoretical calculations demonstrate spin configurations of the molecular chain and the antiferromagnetic interfacial coupling through oxygen intermediaries. The development of the molecular multispinterface and controllable exchange bias therein will provide a promising route for the active control of multivalued data processing at the nanoscale.     

InSe Schottky Diodes Based on Van Der Waals Contacts

2D semiconductors are excellent candidates for next‐generation electronics and optoelectronics thanks to their electrical properties and strong light‐matter interaction. To fabricate devices with optimal electrical properties, it is crucial to have both high‐quality semiconducting crystals and ideal contacts at metal‐semiconductor interfaces. Thanks to the mechanical exfoliation of van der Waals crystals, atomically thin high‐quality single‐crystals can easily be obtained in a laboratory. However, conventional metal deposition techniques can introduce chemical disorder and metal‐induced mid‐gap states that induce Fermi level pinning and can degrade the metal‐semiconductor interfaces, resulting in poorly performing devices. In this article, the electrical contact characteristics of Au–InSe and graphite–InSe van der Waals contacts, obtained by stacking mechanically exfoliated InSe flakes onto pre‐patterned Au or graphite electrodes without the need for lithography or metal deposition is explored. The high quality of the metal‐semiconductor interfaces obtained by van der Waals contact allows to fabricate high‐quality Schottky diodes based on the Au–InSe Schottky barrier. The experimental observation indicates that the contact barrier at the graphite–InSe interface is negligible due to the similar electron affinity of InSe and graphite, while the Au–InSe interfaces are dominated by a large Schottky barrier.     

Collective Dipole‐Dominated Doping of Monolayer MoS2: Orientation and Magnitude Control via the Supramolecular Approach

Molecular doping is a powerful, tuneable, and versatile method to modify the electronic properties of 2D transition metal dichalcogenides (TMDCs). While electron transfer is an isotropic process, dipole‐induced doping is a collective phenomenon in which the orientation of the molecular dipoles interfaced to the 2D material is key to modulate and boost this electronic effect, despite it is not yet demonstrated. A novel method toward the molecular functionalization of monolayer MoS₂ relying on the molecular self‐assembly of metal phthalocyanine and the orientation‐controlled coordination chemistry of axial ligands is reported here. It is demonstrated that the subtle variation of position and type of functional groups exposed on the pyridinic ligand, yields a molecular dipole with programed magnitude and orientation which is capable to strongly influence the opto‐electronic properties of monolayer MoS₂. In particular, experimental results revealed that both p‐ and n‐type doping can be achieved by modulating the charge carrier density up to 4.8 10¹² cm^?2. Density functional theory calculations showed that the doping mechanism is primarily resulting from the effect of dipole‐induced doping rather than charge transfer. The strategy to dope TMDCs is a highly modulable and robust, and it enables to enrich the functionality of 2D materials‐based devices for high‐performance applications in optoelectronics.     

二硫化钼-石墨烯垂直异质结的制备与表征

以采用化学气相沉积法(CVD)生长的单层石墨烯为导电电极、四硫代钼酸铵水溶液为电解质,通过电化学沉积法合成了二硫化钼/石墨烯(MoS2/graphene)垂直异质结。将合成的MoS2/graphene垂直异质结通过CVD在氢气(H2)和氩气(Ar)环境下进行退火处理。利用拉曼光谱、X射线衍射仪(XRD)、扫描电子显微镜(SEM)、原子力显微镜(AFM)系统地分析了样品的物质成分、表面形貌和厚度等。这种简单、环保、低成本的制备大面积MoS2/graphene垂直异质结的方法具有普遍适用性,为其他垂直异质结的制备开辟了新途径。     

Synthesis of 2D Layered BiI3 Nanoplates,BiI3/WSe2van der Waals Heterostructures and Their Electronic,Optoelectronic Properties

Two–dimensional layered materials (2DLMs) have attracted considerable recent interest as a new material platform for fundamental materials science and potential new technologies. Here we report the growth of layered metal halide materials and their optoelectronic properties. BiI₃ nanoplates can be readily grown on SiO₂/Si substrates with a hexagonal geometry, with a thickness in the range of 10–120 nm and a lateral dimension of 3–10 µm. Transmission electron microscopy and electron diffraction studies demonstrate that the individual nanoplates are high quality single crystals. Micro‐Raman studies show characteristic A _g band at ≈115 cm^?1 with slight red‐shift with decreasing thickness, and micro‐photoluminescence studies show uniform emission around 690 nm with blue‐shift with decreasing thickness. Electrical transport studies of individual nanoplates show n‐type semiconductor characteristics with clear photoresponse. Further, the BiI₃ can be readily grown on other 2DLMs (e.g., WSe₂) to form van der Waals heterostructures. Electrical transport measurements of BiI₃/WSe₂ vertical heterojunctions demonstrate p–n diode characteristics with gate‐tunable rectification behavior and distinct photovoltaic effect. The synthesis of the BiI₃ nanoplates can expand the library of 2DLMs and enable a wider range of van der Waals heterostructures.     

Predicting 2D silicon allotropes on SnS<Subscript>2</Subscript>

A first principles study on the stability and structural and electronic properties of two-dimensional silicon allotropes on a semiconducting layered metal-chalcogenide compound,namely SnS2,is performed.The interactions between the two-dimensional silicon layer,commonly known as silicene,and the layered SnS2 template are investigated by analyzing different configurations of silicene.The calculated thermodynamic phase diagram suggests that the most stable configuration of silicene on SnS2 belongs to a family of structures with Si atoms placed on three different planes;so-called dumbbell silicene.This particular dumbbell silicene structure preserves its atomic configuration on SnS2 even at a temperature of 500 K or as a "flake" layer (i.e.,a silicene cluster terminated by H atoms),thanks to the weak interactions between the silicene and the SnS2 layers.Remarkably,an electric field can be used to tune the band gap of the silicene layer on SnS2,eventually changing its electronic behavior from semiconducting to (semi)metallic.The stability of silicene on SnS2 is very promising for the integration of silicene onto semiconducting or insulating substrates.The tunable electronic behavior of the silicene/SnS2 van der Walls heterostructure is very important not only for its use in future nanoelectronic devices,but also as a successful approach to engineering the bang-gap of layered SnS2,paving the way for the use of this layered compound in energy harvesting applications.     

Van der Waals Heterostructure Devices with Dynamically Controlled Conduction Polarity and Multifunctionality

Controlling the conduction behavior of 2D materials is an important prerequisite to achieve their electronic and optoelectronic applications. However, most of the reported approaches are aware of the shortcomings of inflexibility and complexity, which limits the possibility of multifunctional integration. Here, taking advantage of van der Waals heterostructure engineering, a simple method to achieve a dynamically controlled binary channel in a semivertical MoTe₂/MoS₂ field effect transistor is proposed. It is enabled by the high switchability between tunneling and thermal transports through simply changing the sign of voltage bias. In addition, the proposed system allows for multifunctional integration of transistor with on/off ratio >10⁷ and diode with rectification ratio >10⁶. Moreover, the devices show screen capability to negative photoresponse effect that is widely observed in ambipolar materials, hence improving the photodetection reliability and sensitivity. This study broadens the functionalities of van der Waals heterostructures and opens up more possibilities to realize multifunctional devices.     

Ultrathin Non‐van der Waals Magnetic Rhombohedral Cr2S3: Space‐Confined Chemical Vapor Deposition Synthesis and Raman Scattering Investigation

Two dimensional (2D) magnetic materials display enormous application potential in spintronic fields. However, most of currently reported magnetic materials are van der Waals layered structure that is easy to be isolated via exfoliation method. By contrast, the studies on non‐van der Waals ultrathin magnetic materials are rare, largely due to the difficulty in fabrication. Rhombohedral Cr₂S₃, an intensively studied antiferromagnetic transition metal chalcogenide with Neel temperature of ≈120 K, has a typical non‐van der Waals structure. Restricted by the strong covalent bonding in all the three dimensions of non‐van der Waals structure, the synthesis of ultrathin Cr₂S₃ single crystals is still a challenge that is not achieved yet. Besides, the study on the Raman modes of rhombohedral Cr₂S₃ is also absent. Herein, by employing space‐confined chemical vapor deposition strategy, ultrathin rhombohedral Cr₂S₃ single crystals with a thickness down to ≈2.5 nm for the first time are successfully grown. Moreover, a systematically investigation is also conducted on the Raman vibrations of ultrathin rhombohedral Cr₂S₃. With the aid of angle‐resolved polarized Raman technique, the Raman modes of rhombohedral Cr₂S₃ for the first time based on crystal symmetry and Raman selection rules are rationally assigned.     

AsP/InSe Van der Waals Tunneling Heterojunctions with Ultrahigh Reverse Rectification Ratio and High Photosensitivity

Van der Waals heterojunctions made of 2D materials offer competitive opportunities in designing and achieving multifunctional and high‐performance electronic and optoelectronic devices. However, due to the significant reverse tunneling current in such thin p–n junctions, a low rectification ratio along with a large reverse current is often inevitable for the heterojunctions. Here, a vertically stacked van der Waals heterojunction (vdWH) tunneling device is reported consisting of black arsenic phosphorus (AsP) and indium selenide (InSe), which shows a record high reverse rectification ratio exceeding 10⁷ along with an unusual ultralow forward current below picoampere and a high current on/off ratio over 10⁸ simultaneously at room temperature under the proper band alignment design of both the Schottky junction and the heterojunction. Therefore, the vdWH tunneling device can function as an ultrasensitive photodetector with an ultrahigh light on/off ratio of 1 × 10⁷, a comparable responsivity of around 1 A W^?1, and a high detectivity over 1 × 10¹² Jones in the visible wavelength range. Furthermore, the device exhibits a clear photovoltaic effect and shows a spectral detection capability up to 1550 nm. The work sheds light on developing future electronic and optoelectronic multifunctional devices based on the van der Waals integration of 2D materials with designed band alignment.