Intercalation is an effective method to modify physical properties and induce novel electronic states of transition metal dichalcogenide (TMD) materials. However, it is difficult to reveal the microscopic electronic state evolution in the intercalated TMDs. Here we successfully synthesize the copper-intercalated 1T-TaS₂ and characterize the structural and electronic modification combining resistivity measurements, atomic-resolution scanning transmission electron microscopy (ADF-STEM), and scanning tunneling microscopy (STM). The intercalated Cu atom is determined to be directly below the Ta atom and suppresses the commensurate charge density wave (CCDW) phase. Two specific electronic modulations are discovered in the near-commensurate (NC) CDW phase: the electron doping state near the defective star of Davids (SDs) in metallic domains and the spatial evolution of the Mott gap in insulating domains. Both modulations reveal that intercalated Cu atoms act as a medium to enhance the interaction between intralayer SDs, in addition to the general charge transfer effect. It also solidifies the Mott foundation of the insulating gap in pristine samples. The intriguing electronic evolution in Cu-intercalated 1T-TaS₂ will motivate further exploration of novel electronic states in the intercalated TMD materials.

Mirror twin boundary (MTB) brings unique one-dimensional (1D) physics and properties into two-dimensional (2D) transition metal dichalcogenides (TMDCs), but they were rarely observed in non-Mo-based TMDCs. Herein, by post-growth Nb doping, high density 4|4E-W and 4|4P-Se mirror twin boundaries (MTBs) were introduced into molecular beam epitaxy (MBE) grown WSe₂ monolayers. Of them, 4|4E-W MTB with a novel structure was discovered experimentally for the first time, while 4|4P-Se MTBs present a random permutations of W and Nb, forming a 1D alloy system. Comparison between the doped and non-doped WSe₂ confirmed that Nb dopants are essential for MTB formation. Furthermore, quantitative statistics reveal the areal density of MTBs is directly proportional to the concentration of Nb dopants. To unravel the injection pathway of Nb dopants, first-principles calculations about a set of formation energies for excess Nb atoms with different configurations were conducted, based on which a model explaining the origin of MTBs introduced by excess metal was built. We conclude that the formation of MTBs is mainly driven by the collective evolution of excess Nb atoms introduced into the lattice of host WSe₂ crystal and subsequent displacement of metal atoms (W or Nb). This study provides a novel way to tailor the MTBs in 2D TMDC materials via proper metal doping and presents new opportunities for exploring the intriguing properties.

The crystallographic shapes of nanocrystals play critical roles in determining their physical and chemical properties. Liquid phase synthesis serves as one of the most important approaches for preparing shape-controlled nanocrystals, therefore, understanding the formation mechanisms of the thermodynamic equilibrium structures of nanocrystals in liquid solution is important. Using in situ liquid cell transmission electron microscopy (TEM), we observe for the first time the shape transformation of individual palladium nanocrystals from energy unfavored spherical shapes into equilibrium truncated octahedrons in aqueous solution. Via quantitative analysis of the shape evolution dynamics of an individual Pd nanocrystal, we find that about 10% of nanocrystal atoms were relocated during the shape transformation. The mass transport is attributed to the synergetic effect of electron beam irradiation and water environment.

Edge structures are highly relevant to the electronic, magnetic, and catalytic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) and their one-dimensional (1D) counterparts, i.e., nanoribbons, and should be precisely tailored for the desired application. In this work, we report the formation of novel Mo₆S₆ nanowire (NW)-terminated edges in monolayer molybdenum disulfide (MoS₂) via an e–beam irradiation process combined with high temperature heating. The atomic structures of the NW-terminated edges and the dynamic formation process were observed experimentally using scanning transmission electron microscopy. Further analysis showed that the NW-terminated edge could be formed on both the Mo-zigzag (ZZ) edge and S-ZZ edge and could exhibit a stability superior to that of the pristine ZZ and armchair (AC) edges. In addition, analogous edge structures could also be formed in MoS₂ nanoribbons and other TMD materials such as Mo_xW_1-xSe₂. We believe that these novel edge structures may impart novel properties to the 2D and 1D TMD materials and provide new opportunities for their applications in catalytic, spintronic, and electronic devices.

In the current extensive studies of layered two-dimensional (2D) materials, compared to hexagonal structures such as graphene, hBN, and MoS₂, low- symmetry 2D materials have shown great potential for applications in anisotropic devices. Rhenium diselenide (ReSe₂) possesses the bulk space group P1 and belongs to the triclinic crystal system with a deformed cadmium-iodide-type structure. Here, we propose an electron diffraction-based method to distinguish the monolayer ReSe₂ membrane from multilayer ReSe₂ and its two different vertical orientations. Our method is also applicable to other low-symmetry crystal systems, including both triclinic and monoclinic lattices, as long as their third unit-cell basis vectors are not perpendicular to the basal plane. Our experimental results are well explained by kinematical electron diffraction theory and the corresponding simulations. Generalization of our method to other 2D materials, such as graphene, is also discussed.

Phosphorus atomic chains, the narrowest nanostructures of black phosphorus (BP), are highly relevant to the in-depth development of BP-based one-dimensional (1D) nano-electronics components. In this study, we report a top-down route for the preparation of phosphorus atomic chains via electron beam sculpturing inside a transmission electron microscope (TEM). The growth and dynamics (i.e., rupture and edge migration) of 1D phosphorus chains are experimentally captured for the first time. Furthermore, the dynamic behavior and associated energetics of the as-formed phosphorus chains are further investigated by density functional theory (DFT) calculations. It is hoped that these 1D BP structures will serve as a novel platform and inspire further exploration of the versatile properties of BP.

There have been continuous efforts to seek novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and a subsequent Ar⁺ plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast spectra combined with atomic force microscopy (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of the A_2g mode stiffens, and the intensity ratio of A_2g to A_1g modes shows a monotonic discrete increase with the decrease of phosphorene thickness down to a monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor. This work on monolayer phosphorene fabrication and thickness determination will facilitate future research on phosphorene.