Two-dimensional (2D) transition metal dichalcogenides (TMDs) have gained much attention in virtue of their various atomic configurations and band structures. Apart from those thermodynamically stable phases, plenty of metastable phases exhibit interesting properties. To obtain 2D TMDs with specific phases, it is important to develop phase engineering strategies including phase transition and phaseselective synthesis. Phase transition is a conventional method to transform one phase to another, while phase-selective synthesis means the direct fabrication of the target phases for 2D TMDs. In this review, we introduce the structures and stability of 2D TMDs with different phases. Then, we summarize the detailed processes and mechanism of the traditional phase transition strategies. Moreover, in view of the increasing demand of high-phase purity TMDs, we present the advanced phase-selective synthesis strategies. Finally, we underline the challenges and outlooks of phase engineering of 2D TMDs in two aspects-high phase purity and excellent controllability. This review may promote the development of controllable phase engineering for 2D TMDs and even other 2D materials toward both fundamental studies and practical applications.

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To construct complex superstructures for electronics, [4] the disassembly process could be even more important, which can be utilized to create a variety of complicated systems. That is, the multinary structures and even superlattices could be achieved via the disassembly process. [5] Facing future electrical applications, improving the functional integration of electronics and integrated circuits is one of the researching focus, for which the disassembly must be the crucial way. In the case, a disassembly strategy for 2D materials is in urgent need of development.It is worth mentioning that, in order to achieve the disassembly of 2D materials, an appropriate energy should be applied to weakening the interactions between 2D monomers but not to result in the destruction of the building blocks. Oppositely, some traditional methods provide excessive external mechanical or thermal energy and superfluous high-energy particles, including micromechanical cleavage [6] and the post-treatment with ion beams, [7] laser, [8] plasma, [9] and some compounds, [10] which would inevitably lead to the crumbling of the 2D building blocks. Thus far, there are no effective methods to achieve the disassembly of 2D materials.Herein, we, for the first time, achieve the disassembly of 2D materials. As a demonstration, the disassembly of 2D vertical heterostructures (2DVHs) was realized with the inspiration of the genetic expression: as shown in Figure 1, interlayer van der Waals forces of 2DVHs are weakened after the combination with disassembly promoters (DPs), and then the 2DVHs are successfully disassembled. In situ Raman spectra and atomic force microscopy (AFM) were conducted to confirm the successful disassembly of 2DVHs. Cross-sectional transmission electron microscopy (TEM) was also utilized to characterize the disassembly of 2DVHs. Besides, density functional theory (DFT) calculations accompanied with controlled experiments were performed to elucidate the mechanism of the disassembly, which owes to the activation of 2DVHs and the weakening of interlayer van der Waals forces with the assistance of Te DPs. The disassembly of 2DVHs, to the best of our knowledge, is firstly reported, which is essential to novel electronics and optoelectronics with patterned channels. Such a novel structure tuning method could have an excellent prospect for multifunctional devices.The disassembly of 2DVHs could be described as the equation AB → A + B, in which A and B stand for the components of the 2DVHs AB. It means that the components of 2DVHs could As one of the most widely discussed fields, the assembly of nanomaterials has always been extensively studied. However, its inverse process, namely disassembly, is still limited in the ambit of biomolecules. Specifically, in the emerging 2D research field, disassembly still remains unexplored. Inspired by the disassembly of DNA molecules via breaking intermolecular hydrogen bonds, the disassembly of 2D vertical heterostructures (2DVHs) is first achieved through the weakening of the interlayer van ...

Germanium, the prime applied semiconductor, is widely used in solid‐state electronics and photoelectronics. Unfortunately, since the 3D diamond‐like structure with strong covalent bonds impedes the 2D anisotropic growth, only the examples of ultrathin Ge along the (111) plane have been investigated, much less to the controllable synthesis along another crystal surface. Meanwhile, Ge(111) flakes are limited in semiconductor applications because of their gapless property. Here, ultrathin Ge(110) single crystal is synthesized with semiconductive property via gallium‐associated self‐limiting growth. The obtained ultrathin Ge(110) single crystal exhibits anisotropic honeycomb structure, uniformly incremental lattice, wide tunable direct‐bandgap, blue‐shifted photoluminescence emission, and unique phonon modes, which are consistent with the previous theoretical predictions. It also confirms excellent second harmonic generation and high hole mobility of 724 cm2 V−1 s−1. The realization of ultrathin Ge(110) single crystal will provide an excellent candidate for application in electronics and optoelectronics.

Summary of main observation and conclusion Lattice symmetry is vital to the properties of two‐dimensional (2D) materials, yet their fixed symmetry cannot meet the increasing requirements in highly efficient and programmable electrical transport. If the structural diversity of 2D materials, as demonstrated by 1T’‐WTe2, is improved without any phase transition or structural reconstruction, excellent metallic 1T’‐WTe2 would be possibly used for integrated devices. Here, we realized meta symmetry of 1T’‐WTe2 by using an edge‐induced mechanism, which is recognized as the combination of the intrinsic C2v symmetry and sixfold axes. On account of the dynamically controlled growth, the meta symmetric 1T’‐WTe2 with ~94.9% purity is obtained for the first time. Meta symmetry will also keep the intrinsic electrical properties of 1T’‐WTe2 over the node. Such meta symmetry could not only enrich the structural diversity of 1T’‐WTe2, but also be extended to other low‐symmetry 2D materials, which would be promising for customized circuits and devices.