Resolidified Chalcogen-Assisted Growth of Bilayer Semiconductors with Controlled Stacking Orders.

Bilayer semiconductors have attracted much attention due to their stacking-order-dependent properties. However, as both 3R- and 2H-stacking are energetically stable at high temperatures, most of the high-temperature grown bilayer materials have random 3R- or 2H-stacking orders, leading to non-uniformity in optical and electrical properties. Here, a chemical vapor deposition method is developed to grow bilayer semiconductors with controlled stacking order by modulating the resolidified chalcogen precursors supply kinetics. Taking tungsten disulfide (WS2 ) as an example, pure 3R-stacking (100%) and 2H-stacking dominated (87.6%) bilayer WS2 are grown by using this method and both show high structural and optical quality and good uniformity. Importantly, the bilayer 3R-stacking WS2 shows higher field effect mobility than 2H-stacking samples, due to the difference in stacking order-dependent surface potentials. This method is universal for growing other bilayer semiconductors with controlled stacking orders including molybdenum disulfide and tungsten diselenide, paving the way to exploit stacking-order-dependent properties of these family of emerging bilayer materials.

Resolidified Chalcogen Precursors for High-Quality 2D Semiconductor Growth.

Two-dimensional (2D) semiconductors including transition metal dichalcogenides (TMDCs) have gained attention in optoelectronics for their extraordinary properties. However, the large amount and locally distributed lattice defects affect the optical properties of 2D TMDCs, and the defects originate from unstable factors in the synthesis process. In this work, we develop a method of pre-melting and resolidification of chalcogen precursors (sulfur and selenium), namely resolidified chalcogen, as precursor for the chemical vapor deposition growth of TMDCs with ultrahigh quality and uniformity. Taking WS2 as an example, the monolayer WS2 shows uniform fluorescence intensity and a small full-width at half-maximum of photoluminescence peak at low temperatures with an average value of 13.6±1.9 meV. The defect densities at the interior and edge region are both low and comparable, i.e., (9±3)×1012  cm-2 and (10±4)×1012  cm-2 , indicating its high structural quality and uniformity. This method is universal in growing high quality monolayer MoS2 , WSe2 , MoSe2 , and will benefit their applications.

Layer-Dependent Raman Spectroscopy and Electronic Applications of Wide-Bandgap 2D Semiconductor ß-ZrNCl.

In recent years, 2D layered semiconductors have received much attention for their potential in next-generation electronics and optoelectronics. Wide-bandgap 2D semiconductors are especially important in the blue and ultraviolet wavelength region, although there are very few 2D materials in this region. Here, monolayer ß-type zirconium nitride chloride (ß-ZrNCl) is isolated for the first time, which is an air-stable layered material with a bandgap of ≈3.0 eV in bulk. Systematical investigation of layer-dependent Raman scattering of ZrNCl from monolayer, bilayer, to bulk reveals a blueshift of its out-of-plane A1g peak at ≈189 cm-1 . Importantly, this A1g peak is absent in the monolayer, suggesting that it is a fingerprint to quickly identify the monolayer and for the thickness determination of 2D ZrNCl. The back gate field-effect transistor based on few-layer ZrNCl shows a high on/off ratio of 108 . These results suggest the potential of 2D ß-ZrNCl for electronic applications.

Space-Confined One-Step Growth of 2D MoO2 /MoS2 Vertical Heterostructures for Superior Hydrogen Evolution in Alkaline Electrolytes.

2D material-based heterostructures are constructed by stacking or spicing individual 2D layers to create an interface between them, which have exotic properties. Here, a new strategy for the in situ growth of large numbers of 2D heterostructures on the centimeter-scale substrate is developed. In the method, large numbers of 2D MoS2 , MoO2 , or their heterostructures of MoO2 /MoS2 are controllably grown in the same setup by simply tuning the gap distance between metal precursor and growth substrate, which changes the concentration of metal precursors feed. A lateral force microscope is used first to identify the locations of each material in the heterostructures, which have MoO2 on the top of MoS2 . Noteworthy, the creation of a clean interface between atomic thin MoO2 (metallic) and MoS2 (semiconducting) results in a different electronic structure compared with pure MoO2 and MoS2 . Theoretical calculations show that the charge redistribution at such an interface results in an improved HER performance on the MoO2 /MoS2 heterostructures, showing an overpotential of 60 mV at 10 mA cm-2 and a Tafel slope of 47 mV dec-1 . This work reports a new strategy for the in situ growth of heterostructures on large-scale substrates and provides platforms to exploit their applications.

Locally Strained 2D Materials: Preparation, Properties, and Applications.

2D materials are promising for strain engineering due to their atomic thickness and exceptional mechanical properties. In particular, non-uniform and localized strain can be induced in 2D materials by generating out-of-plane deformations, resulting in novel phenomena and properties, as witnessed in recent years. Therefore, the locally strained 2D materials are of great value for both fundamental studies and practical applications. This review discusses techniques for introducing local strains to 2D materials, and their feasibility, advantages, and challenges. Then, the unique effects and properties that arise from local strain are explored. The representative applications based on locally strained 2D materials are illustrated, including memristor, single photon emitter, and photodetector. Finally, concluding remarks on the challenges and opportunities in the emerging field of locally strained 2D materials are provided.

Growth of 2D Cr2 O3 -CrN Mosaic Heterostructures with Tunable Room-Temperature Ferromagnetism.

2D magnets have generated much attention due to their potential for spintronic devices. Heterostructures of 2D magnets are interesting platforms for exploring physical phenomena and applications. However, the controlled growth of 2D room-temperature ferromagnetic heterostructures is challenging. Here, one-pot chemical vapor deposition growth of stable 2D Cr2 O3 -CrN mosaic heterostructures (MHs) is reported with a controlled ratio of components that possess robust room-temperature ferromagnetism. The 2D MHs consist of Cr2 O3 flakes with embedded CrN subdomains and the CrN:Cr2 O3 ratio can be tuned from 0% to 100% during growth. By changing the CrN:Cr2 O3 ratio, the ferromagnetism of the MHs (e.g., saturation magnetization, coercive field), which originates from the interfacial coupling between Cr2 O3 and CrN, can be controlled. Importantly, the obtained Cr2 O3 -CrN MHs are stable in air at elevated temperatures and have robust ferromagnetism with Curie temperature >400 K. This work presents a facile method for fabricating 2D MHs with tunable magnetism which will benefit high-temperature spintronics.

Controlled Growth of Wafer-Scale Transition Metal Dichalcogenides with a Vertical Composition Gradient for Artificial Synapses with High Linearity.

Artificial synapses are promising for dealing with large amounts of data computing. Great progress has been made recently in terms of improving the on/off current ratio, the number of states, and the energy efficiency of synapse devices. However, the nonlinear weight update behavior of a synapse caused by the uncertain direction of the conductive filament leads to complex weight modulation, which degrades the delivery accuracy of information. Here we propose a strategy to improve the weight update behavior of synapses using chemical-vapor-deposition-grown transition metal dichalcogenides (TMDCs) with a vertical composition gradient, where the sulfur concentration decreases gradually along the thickness direction of TMDCs and thus forms a certain direction of the conduction filament for synapse devices. It is worth noting that the devices show an excellent linear conductance of potentiation and depression with a high linearity of 0.994 (surpassing most state-of-the-art synapses), have a large number of states, and are able to fabricate synapse arrays with wafer-scale. Furthermore, the devices based on the TMDCs with the vertical composition gradient exhibit an asymmetric feature of potentiation and depression behaviors with high linearity and follow the simulated linear Leaky ReLU function, resulting in a high recognition accuracy of 94.73%, which overcomes the unreliability issue in the Sigmoid function due to the vanishing gradient phenomenon. This study not only provides a universal method to grow TMDCs with a vertical composition gradient but also contributes to exploring highly linear synapses toward neuromorphic computing.

Dual-metal precursors for the universal growth of non-layered 2D transition metal chalcogenides with ordered cation vacancies.

Two-dimensional (2D) transition metal chalcogenides (TMCs) are promising for nanoelectronics and energy applications. Among them, the emerging non-layered TMCs are unique due to their unsaturated dangling bonds on the surface and strong intralayer and interlayer bonding. However, the synthesis of non-layered 2D TMCs is challenging and this has made it difficult to study their structures and properties at thin thickness limit. Here, we develop a universal dual-metal precursors method to grow non-layered TMCs in which a mixture of a metal and its chloride serves as the metal source. Taking hexagonal Fe1-xS as an example, the thickness of the Fe1-xS flakes is down to 3 nm with a lateral size of over 100 µm. Importantly, we find ordered cation Fe vacancies in Fe1-xS, which is distinct from layered TMCs like MoS2 where anion vacancies are commonly observed. Low-temperature transport measurements and theoretical calculations show that 2D Fe1-xS is a stable semiconductor with a narrow bandgap of ∼60 meV. In addition to Fe1-xS, the method is universal in growing various non-layered 2D TMCs containing ordered cation vacancies, including Fe1-xSe, Co1-xS, Cr1-xS, and V1-xS. This work paves the way to grow and exploit properties of non-layered materials at 2D thickness limit.