Xingxing Zhang

Controllable synthesis of 2D layered 3‐rhombohedral phase WS₂ and WSe₂ atomic layers with full‐covered top layers is realized by physical vapor deposition. Compared to the 2‐hexagonal phase, 3‐rhombohedral phase layers show a unique photoluminescence and Raman spectra, and more importantly quadratically increasing second harmonic generation intensity with respect to layer numbers, which is promising for nonlinear optics.

Abstract

2D layered 3‐rhombohedral (3R) phase transition metal dichalcogenides (TMDs) have received significantly increased research interest in nonlinear optical applications due to their unique crystal structures and broken inversion symmetry. However, controlled growth of 2D 3R phase TMDs still remains a great challenge. In this work, a direct growth of large‐area WS₂ and WSe₂ atomic layers with controllable crystal phases via a developed temperature selective physical vapor deposition route is reported. Large‐area triangular 3R phase layers are synthesized at a lower deposition temperature. Steady state and time‐resolved photoluminescence spectroscopy and Raman spectroscopy are used to study the unique properties of 3R phase layers due to different layer stacking and interlayer coupling. More importantly, with broken inversion symmetry, 3R phase layers show a quadratically increased second harmonic generation (SHG) intensity with respect to layer numbers. Furthermore, by polarization‐resolved SHG, a uniform polarization preference is observed in bilayer and trilayer 3R phase WS₂, which could be a benefit for practical applications. The results not only contribute to the controlled growth of 2D TMDs layers with different phases but also pave the way to promising nonlinear optical devices.

Large‐domain‐size ultrathin MTe₂ (M = V, Nb, Ta) nanoplates with the thickness down to the monolayer regime are prepared using 2D WSe₂ or WS₂ as the growth substrate. The atomically flat dangling‐bond‐free surface of WSe₂ (WS₂) ensures a minimized diffusion barrier for the successful realization of atomically thin 2D metallic MTe₂ (M = V, Nb, Ta) nanosheets.

Abstract

2D metals have attracted considerable recent attention for their special physical properties, such as charge density waves, magnetism, and superconductivity. However, despite some recent efforts, the synthesis of ultrathin 2D metals nanosheets down to monolayer thickness remains a significant challenge. Herein, by using atomically flat 2D WSe₂ or WS₂ as the growth substrate, the synthesis of atomically thin 2D metallic MTe₂ (M = V, Nb, Ta) single crystals with the thickness down to the monolayer regime and the creation of atomically thin MTe₂/WSe₂ (WS₂) vertical heterojunctions is reported. Comparison with the growth on the SiO₂/Si substrate under the same conditions reveals that the utilization of the dangling‐bond‐free WSe₂ or WS₂ as the van der Waals epitaxy substrates is crucial for the successful realization of atomically thin MTe₂ (M = V, Nb, Ta) nanosheets. It is further shown that the epitaxial grown 2D metals can function as van der Waals contacts for 2D semiconductors with little interface damage and improved electronic performance. This study defines a robust van der Waals epitaxy pathway to ultrathin 2D metals, which is essential for fundamental studies and potential technological applications of this new class of materials at the 2D limit.

Van der Waals bipolar junction transistors based on vertically stacked 2D materials (V2D‐BJT) are proposed, and experimental studies are conducted on the V2D‐BJT using an MoS₂/WSe₂/MoS₂ heterostructure in an n‐p‐n configuration. The V2D‐BJT shows excellent gas sensing performance with a low power dissipation (≈2 nW), a fast response (9 s), and a fast recovery (35 s) time.

Abstract

The majority of microelectronic devices rely on a p‐n junction. The process of making such a junction is complicated, and it is difficult to make layers that form a junction with an atomic thickness. In this study, bipolar junctions are made by using 2D atomic crystalline layers and even a single layer in which 2D layers adhere together to form a heterostructure via van der Waals forces. A vertical 2D bipolar junction transistor (V2D‐BJT) is studied for the first time. It uses an MoS₂/WSe₂/MoS₂ heterostructure and has an n‐p‐n configuration that exhibits a maximum common‐base current gain of ≈0.97 and a stable common‐emitter current gain (β) of 12 with a nanowatt power consumption. In the first attempt at gas sensing, it shows outstanding performance, exhibiting a very fast response and recovery time (9 and 35 s, respectively) with a power dissipation of only 2 nW. This study demonstrates the potential application of the V2D‐BJT in nanowatt power amplifiers as well as fast‐response and low‐power gas sensors.

The charge transfer phenomenon is identified to be a major factor determining exciton and trion characteristics of atomically thin MoS 2 layers in various stacking configurations. We report photoluminescence (PL) from CVD-grown layered MoS 2 in the presence of a skewed or a deformed triangular-shaped monolayer/bilayer (1L/2L) lateral interface. Integrated PL mapping images over the 1L and 2L MoS 2 regions revealed that the neutral exciton emission was significantly enhanced and exhibited an oscillatory behavior in its intensity in the 1L region near the 1L/2L boundary, whereas the negative trion emission remained unchanged. The interplays among the number of MoS 2 layers, a substrate, and a geometric boundary structure of the 1L/2L lateral interface turned out to be important in charge transfer due to a modulation in work functions. Density functional theory predicted that the work functions of 1L and 2L MoS 2 were strongly influ...

Sucrose‐grafted BN nanosheets (sucrose‐g‐BNNSs) are produced by a simple yet efficient sucrose‐assisted mechanochemical exfoliation process, and easily dispersed in polar liquids. Compared to pure poly(vinyl alcohol) (PVA), the sucrose‐g‐BNNS/PVA composites show remarkably improved tensile strength, thermal dissipation and flame‐retardancy. This method also works for the simultaneous exfoliation and functionalization of many other two‐dimensional (2D) materials.

Abstract

Due to their extraordinary properties, boron nitride nanosheets (BNNSs) have great promise for many applications. However, the difficulty of their efficient preparation and their poor dispersibility in liquids are the current factors that limit this. A simple yet efficient sugar‐assisted mechanochemical exfoliation (SAMCE) method is developed here to simultaneously achieve their exfoliation and functionalization. This method has a high actual exfoliation yield of 87.3%, and the resultant BNNSs are covalently grafted with sugar (sucrose) molecules, and are well dispersed in both water and organic liquids. A new mechanical force–induced exfoliation and chemical grafting mechanism is proposed based on experimental and density functional theory investigations. Thanks to the good dispersibility of the nanosheets, flexible and transparent BNNS/poly(vinyl alcohol) (PVA) composite films with multifunctionality is fabricated. Compared to pure PVA films, the composite films have a remarkably improved tensile strength and thermal dissipation capability. Noteworthy, they are flame retardant and can effectively block light from the deep blue to the UV region. This SAMCE production method has proven to be highly efficient, green, low cost, and scalable, and is extended to the exfoliation and functionalization of other two‐dimensional (2D) materials including MoS₂, WS₂, and graphite.

The strain engineering of 2D materials is particularly exciting, because an individual sheet can survive remarkably large mechanical strain and its atomic thinness allows mechanical deformations like a piece of paper. These exceptional circumstances create opportunities for the study of new fundamental physics and applications of 2D materials emerging at the large strain level.

Abstract

Triggered by the growing needs of developing semiconductor devices at ever‐decreasing scales, strain engineering of 2D materials has recently seen a surge of interest. The goal of this principle is to exploit mechanical strain to tune the electronic and photonic performance of 2D materials and to ultimately achieve high‐performance 2D‐material‐based devices. Although strain engineering has been well studied for traditional semiconductor materials and is now routinely used in their manufacturing, recent experiments on strain engineering of 2D materials have shown new opportunities for fundamental physics and exciting applications, along with new challenges, due to the atomic nature of 2D materials. Here, recent advances in the application of mechanical strain into 2D materials are reviewed. These developments are categorized by the deformation modes of the 2D material–substrate system: in‐plane mode and out‐of‐plane mode. Recent state‐of‐the‐art characterization of the interface mechanics for these 2D material–substrate systems is also summarized. These advances highlight how the strain or strain‐coupled applications of 2D materials rely on the interfacial properties, essentially shear and adhesion, and finally offer direct guidelines for deterministic design of mechanical strains into 2D materials for ultrathin semiconductor applications.