Xingxing Zhang

In article number 2006957, Nomi L. A. N. Sorgenfrei and co‐workers demonstrate that a weak optical excitation creates electrons in the conduction band of p‐doped semiconducting molybdenum disulfide, which travel toward the surface layer. They accumulate in the top layer and concomitantly drive it from the semiconducting toward the metallic phase. The selectivity of synchrotron time‐resolved electron spectroscopy traces this effect. This surface modification influences the properties and functionality of MoS₂. Frontispiece Art: Martin Künsting.

Novel and phase‐pure layered iron diselenide (FeSe₂) nanocrystals, confirmed by X‐ray diffraction and atomic resolution scanning transmission electron microscopy, are epitaxially grown on mica by sublimed‐salt‐assisted chemical vapor deposition. They exhibit metallic behavior with high electrical conductivity and a phase transition at ≈11 K.

Abstract

Layered iron chalcogenides (FeX, X = S, Se, Te) provide excellent platforms to study intertwined phase transitions, superconductivity, and magnetism. However, layered iron dichalcogenides (FeX₂, X = S, Se, Te) are rarely reported and their intrinsic properties are still unknown. Here, phase‐pure layered iron diselenide (FeSe₂) nanocrystals are epitaxially grown on mica by the sublimed‐salt‐assisted chemical vapor deposition method at atmospheric pressure. The layered atomic structure of FeSe₂ is confirmed by X‐ray diffraction and atomic‐resolution scanning transmission electron microscopy. Electrical transport shows that the layered FeSe₂ is a metal with high conductivity and a phase transition at ≈11 K. The phase transition manifests itself as a kink in the temperature‐dependent resistivity, as well as anomalous magnetoresistance (MR) appearing around the phase‐transition temperature. The MR changes from negative to positive, accompanied by large hysteresis near the phase‐transition temperature upon cooling. The negative MR and hysteresis might originate from magnetic field suppression scattering of spin fluctuations and competition of magnetic interactions induced by the phase transition, respectively. Layered iron dichalcogenide will be potential candidate to explore novel quantum phenomena and other applications.

Transition metal dichalcogenide stacking‐engineered heterostructures, from controllable fabrication to typical characterization, are reviewed in detail and the stacking‐correlated physical behaviors are presented. Furthermore, recent advances in stacking design, such as stacking sequence, twist angles, and moiré superlattice heterojunctions, are also comprehensively summarized. Finally, the remaining challenges and possible strategies for using stacking engineering to tune the properties of 2D materials are outlined.

Abstract

The layer‐by‐layer assembly of 2D transition metal dichalcogenide monolayer blocks to form a 3D stack, with a precisely chosen sequence/angle, is the newest development for these materials. In this way, one can create “van der Waals heterostructures (HSs),” opening up a new realm of materials engineering and novel devices with designed functionalities. Herein, a detailed systematic review of transition metal dichalcogenide stacking‐engineered heterostructures, from controllable fabrication to typical characterization, and stacking‐correlated physical behaviors is presented. Furthermore, recent advances in stacking design, such as stacking sequence, twist angles, and moiré superlattice heterojunctions, are also comprehensively summarized. Finally, the remaining challenges and possible strategies for using stacking engineering to tune the properties of 2D materials are also outlined.

Large‐area patterned transition‐metal dichalcogenide (TMDC) films with centimeter size and good controllability of the thickness and TMDC heterostructures are achieved by a facile, low‐cost strategy involving printing using an industrial inkjet‐printer with precisely tuned inkjet‐printing parameters followed by a rapid heating process. High‐quality single‐domain monolayer TMDCs with millimeter size can be easily synthesized within 30 s by this method.

Abstract

Chemical vapor deposition (CVD) has been widely used to synthesize high‐quality 2D transition‐metal dichalcogenides (TMDCs) from different precursors. At present, quantitative control of the precursor with high precision and good repeatability is still challenging. Moreover, the process to synthesize TMDCs with designed patterns is complicated. Here, by using an industrial inkjet‐printer, an in situ aqueous precursor with robust usage control at the picogram (10⁻¹² g) level is achieved, and by precisely tuning the inkjet‐printing parameters, followed by a rapid heating process, large‐area patterned TMDC films with centimeter size and good thickness controllability, as well as heterostructures of the TMDCs, are achieved facilely, and high‐quality single‐domain monolayer TMDCs with millimeter‐size can be easily synthesized within 30 s (corresponding to a growth rate up to 36.4 µm s⁻¹). The resulting monolayer MoS₂ and MoSe₂ exhibits excellent electronic properties with carrier mobility up to 21 and 54 cm² V⁻¹ s⁻¹, respectively. The study paves a simple and robust way for the in situ ultrafast and patterned growth of high‐quality TMDCs and heterostructures with promising industrialization prospects. Moreover, this ultrafast and green method can be easily used for synthesis of other 2D materials with slight modification.

MXenes have grown to prominence due to their impressive conductive and electrochemical behavior. Although the mechanical and tribological properties are critical in almost all MXenes applications, they are yet to be fully explored. An in‐depth perspective of the fundamental understanding of MXenes’ mechanical and tribological properties and their effects on current and future applications is provided.

Abstract

2D transition metal carbides, nitrides, and carbonitrides, known as MXenes, were discovered in 2011 and have grown to prominence in energy storage, catalysis, electromagnetic interference shielding, wireless communications, electronic, sensors, and environmental and biomedical applications. In addition to their high electrical conductivity and electrochemically active behavior, MXenes’ mechanical properties, flexibility, and strong adhesion properties play crucial roles in almost all of these growing applications. Although these properties prove to be critical in MXenes’ impressive performance, the mechanical and tribological understanding of MXenes, as well as their relation to the synthesis process, is yet to be fully explored. Here, a fundamental overview of MXenes’ mechanical and tribological properties is provided and the effects of MXenes’ compositions, synthesis, and processing steps on these properties are discussed. Additionally, a critical perspective of the compositional control of MXenes for innovative structural, low‐friction, and low‐wear performance in current and upcoming applications of MXenes is provided. It is established here that the fundamental understanding of MXenes’ mechanical and tribological behavior is essential for their quickly growing applications.

An overview of the synthesis of 2D transition metal chalcogenides (TMCs) by atomic layer deposition (ALD) is presented. While the ALD of thin films on 2D TMCs can modify the TMC properties, that of low-dimensional nanomaterials on 2D TMCs can enhance the device performance. The characteristics of ALD-based TMCs applied to nanoelectronics, sensors, and energy applications are discussed.

Abstract

Transition metal chalcogenides (TMCs) are a large family of 2D materials with different properties, and are promising candidates for a wide range of applications such as nanoelectronics, sensors, energy conversion, and energy storage. In the research of new materials, the development and investigation of industry-compatible synthesis techniques is of key importance. In this respect, it is important to study 2D TMC materials synthesized by the atomic layer deposition (ALD) technique, which is widely applied in industries. In addition to the synthesis of 2D TMCs, ALD is used to modulate the characteristic of 2D TMCs such as their carrier density and morphology. So far, the improvement of thin film uniformity without oxidation and the synthesis of low-dimensional nanomaterials on 2D TMCs have been the research focus. Herein, the synthesis and modulation of 2D TMCs by ALD is described, and the characteristics of ALD-based TMCs used in nanoelectronics, sensors, and energy applications are discussed.

Our examination of the interplay of ultrafast charge dynamics and electron–phonon interaction in the AA′ stacked bilayer MoS 2 provides a microscopic basis for understanding the features (two peaks) in the emission spectrum. We demonstrate that while the initial accumulation of excited charge occurs at and near the Q point of the two-dimensional Brillioun zone, emission takes place predominantly through two pathways: direct charge recombination at the K point and indirect phonon-assisted recombination of electrons at the ##IMG## [http://ej.iop.org/images/2053-1583/8/2/025018/tdmabd6b5ieqn1.gif] {${\text{K}}$} valley and holes at the ##IMG## [http://ej.iop.org/images/2053-1583/8/2/025018/tdmabd6b5ieqn2.gif] {${{\Gamma }}$} hill of the Brillouin zone. Analysis of the wave vector dependencies of the electron–phonon interaction traces the higher energy peak to phonon-assisted relaxation of the excited electrons from the Q to the ...

The two-dimensional layered material MoTe 2 has aroused extensive research interests in its rich optoelectronic properties in various phases. One property of particular interest is the circular photogalvanic effect (CPGE): a conventional second order nonlinear optical effect that is related to the chirality of materials. It has been demonstrated in T d -MoTe 2 , a type-II topological Weyl semimetal candidate, while it has been unclear so far whether it exists in the semimetallic 1T’ phase, another interesting phase that hosts a quantum spin hall state. In this article, we report a clear experimental observation of in-plane CPGE in 1T’-MoTe 2 . The observation is confirmed under various experimental designs with excitation by normally incident mid-infrared laser, and we find it to be related to an in-plane internal DC electric field. We attribute the circular photogalvanic response to a third-order nonlinear optical effect involving this DC el...

Bilayer graphene (BLG) with 30°-twist (30°-tBLG) has been proven to possess a quasicrystal structure potentially providing novel applications. Despite the growth of BLG, especially the AB-stacking bilayer, has gained great attention, the growth of 30°-tBLG has been rarely achieved. Herein, for the first time, the decaborane-assisted synchronous growth of millimeter-sized single-crystalline 30°-tBLG was achieved on Cu foil by controlling the nucleation density and growth kinetics of graphene during chemical vapor deposition using diluted methane gas as the carbon source. The synchronous growth kinetics and decaborane-assisted co-catalysis mechanism are revealed by monitoring the growth process from the initial stage of graphene seeds to the millimeter-size scale. A 30°-tBLG based field effect transistor was fabricated and was found to possess a field-effect carrier mobility as high as 3671.3 cm 2 V −1 s −1 at room temperature. Thus, this work provide...

We analyze the lineshape of the quasiparticle photoluminescence of monolayer (ML) and bilayer (BL) molybdenum ditelluride in temperature- and excitation intensity-dependent experiments. We confirm the existence of a negatively charged trion in the BL based on its emission characteristics and find hints for a coexistence of intra- and interlayer trions with a few meV splitting in energy. From the lineshape analysis of exciton and trion emission we extract values for exciton and trion deformation potentials as well as acoustical and optical phonon-limited mobilities in MoTe 2 . We estimate an acoustical phonon limited mobility of 6000 and 4300 cm 2  Vs −1 for the exciton at low temperature for ML and BL, respectively, which corresponds to an electron mobility of 10 5 cm 2  Vs −1 . At higher temperatures, the optical phonons limit the mobility to 1100 and 250 cm 2  Vs −1 for ML and BL.

The progress in fabrication of twisted bilayer graphene by the mainstream methods and the topological physical properties applied in different applications is comprehensively presented. The discovery of killer applications with suitable preparation method for twisted bilayer graphene is systematically discussed by listing the advantages and disadvantages of preparation methods and various applications.

Abstract

Twisted bilayer graphene (tBLG) exhibits a host of innovative physical phenomena owing to the formation of moiré superlattice. Especially, the discovery of superconducting behavior has generated new interest in graphene. The growing studies of tBLG mainly focus on its physical properties, while the fabrication of high‐quality tBLG is a prerequisite for achieving the desired properties due to the great dependence on the twist angle and the interfacial contact. Here, the cutting‐edge preparation strategies and challenges of tBLG fabrication are reviewed. The advantages and disadvantages of chemical vapor deposition, epitaxial growth on silicon carbide, stacking monolayer graphene, and folding monolayer graphene methods for the fabrication of tBLG are analyzed in detail, providing a reference for further development of preparation methods. Moreover, the characterization methods of twist angle for the tBLG are presented. Then, the unique physicochemical properties and corresponding applications of tBLG, containing correlated insulating and superconducting states, ferromagnetic state, soliton, enhanced optical absorption, tunable bandgap, and lithium intercalation and diffusion, are described. Finally, the opportunities and challenges for fabricating high‐quality and large‐area tBLG are discussed, unique physical properties are displayed, and new applications inferred from its angle‐dependent features are explored, thereby impelling the commercialization of tBLG from laboratory to market.

The single‐step growth of alloy/alloy (MoS_{2(1‐ x )}Se_{2 x} /SnS_{2(1‐ y )}Se_{2 y} ) vertical heterostructures is demonstrated and the heterostructure exhibits a nearly intrinsic van der Waals (vdW) interface in terms of a near‐atomically sharp and defect‐free boundary along the interface as well as a well‐aligned epitaxial relationship. The almost‐ideal interface enables the identification of the intrinsic behavior of the heterostructures such as the band alignment, charge transfer, and carrier transport.

Abstract

The interfacial tunable band alignment of heterostructures is coveted in device design and optimization of device performance. As an intentional approach, alloying allows band engineering and continuous band‐edge tunability for low‐dimensional semiconductors. Thus, combining the tunability of alloying with the band structure of a heterostructure is highly desirable for the improvement of device characteristics. In this work, the single‐step growth of alloy‐to‐alloy (MoS_{2(1‐ x )}Se_{2 x} /SnS_{2(1‐ y )}Se_{2 y} ) 2D vertical heterostructures is demonstrated. Electron diffraction reveals the well‐aligned heteroepitaxial relationship for the heterostructure, and a near‐atomically sharp and defect‐free boundary along the interface is observed. The nearly intrinsic van der Waals (vdW) interface enables measurement of the intrinsic behaviors of the heterostructures. The optimized type‐II band alignment for the MoS_{2(1‐ x )}Se_{2 x} /SnS_{2(1‐ y )}Se_{2 y} heterostructure, along with the large band offset and effective charge transfer, is confirmed through quenched PL spectroscopy combined with density functional theory calculations. Devices based on completely stacked heterostructures show one or two orders enhanced electron mobility and rectification ratio than those of the constituent materials. The realization of device‐quality alloy‐to‐alloy heterostructures provides a new material platform for precisely tuning band alignment and optimizing device applications.

The booming development of 2D homojunctions has received tremendous attention over the past years. The recent research progress on the construction strategies and device applications in electronics and optoelectronics is comprehensively summarized.

Abstract

In the post‐Moore era, 2D materials with rich physical properties have attracted widespread attention from the scientific and industrial communities. Among 2D materials, the 2D homojunctions are of great promise in designing novel electronic and optoelectronic devices due to their unique geometries and properties such as homogeneous components, perfect lattice matching, and efficient charge transfer at the interface. In this article, a pioneering review focusing on the structural design and device application of 2D homojunctions such as p–n homojunctions, heterophase homojunctions, and layer‐engineered homojunctions is provided. The preparation strategies to construct 2D homojunctions including vapor‐phase deposition, lithium intercalation, laser irradiation, chemical doping, electrostatic doping, and photodoping are summarized in detail. Specifically, a careful review on the applications of the 2D homojunctions in electronics (e.g., field‐effect transistors, rectifiers, and inverters) and optoelectronics (e.g., light‐emitting diodes, photovoltaics, and photodetectors) is provided. Eventually, the current challenges and future perspectives are commented for promoting the rapid development of 2D homojunctions.

The recent experimental and computational research progress on modulation of MnO₂ single‐species and MnO₂‐based composites for environmental applications are summarized. MnO₂ single‐species can be modified through morphology control, structure construction, facet engineering, and element doping while MnO₂‐based composites can be tuned by construction of homojunctions, MnO₂/semiconductor/conductor binary heterojunctions, and MnO₂‐based ternary heterojunctions.

Abstract

Manganese dioxide (MnO₂) is a promising photo–thermo–electric‐responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂‐based composites via the construction of homojunctions and MnO₂/semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂‐based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high‐efficiency MnO₂‐based materials for comprehensive environmental applications is provided.

A single graphene sheet is stacked on top of monolayer MoS₂. The 2D heterostructure is buckled out‐of‐plane, inducing bending in both layers. In article number 2007269, Arend M. van der Zande, Pinshane Y. Huang, and co‐workers study the impact of interfacial engineering on the tunability of the bending stiffness in 2D multilayers. A bending model is developed to predict and design the deformability of any arbitrary 2D heterostructure.

The atomic sawtooth Au surface allows the anisotropic adsorption energy of transition metal dichalcogenide (TMdC) clusters to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming a single‐crystal TMdC film, regardless of the Miller indices. Growth using the atomic sawtooth gold surface as a universal growth template is further demonstrated for several TMdC monolayer films, including WS₂, WSe₂, MoS₂, the MoSe₂/WSe₂ heterostructure, and W_{1− x} Mo _x S₂ alloys.

Abstract

Growth of 2D van der Waals layered single‐crystal (SC) films is highly desired not only to manifest the intrinsic physical and chemical properties of materials, but also to enable the development of unprecedented devices for industrial applications. While wafer‐scale SC hexagonal boron nitride film has been successfully grown, an ideal growth platform for diatomic transition metal dichalcogenide (TMdC) films has not been established to date. Here, the SC growth of TMdC monolayers on a centimeter scale via the atomic sawtooth gold surface as a universal growth template is reported. The atomic tooth‐gullet surface is constructed by the one‐step solidification of liquid gold, evidenced by transmission electron microscopy. The anisotropic adsorption energy of the TMdC cluster, confirmed by density‐functional calculations, prevails at the periodic atomic‐step edge to yield unidirectional epitaxial growth of triangular TMdC grains, eventually forming the SC film, regardless of the Miller indices. Growth using the atomic sawtooth gold surface as a universal growth template is demonstrated for several TMdC monolayer films, including WS₂, WSe₂, MoS₂, the MoSe₂/WSe₂ heterostructure, and W_{1− x} Mo _x S₂ alloys. This strategy provides a general avenue for the SC growth of diatomic van der Waals heterostructures on a wafer scale, to further facilitate the applications of TMdCs in post‐silicon technology.