Deriving 2D M2X3 (M = Mo, W, X = S, Se) by periodic assembly of chalcogen vacancy lines in their MX2 counterparts

Growth Anisotropy and Morphology Evolution of Line Defects in Monolayer MoS2 : Atomic-Level Observation, Large-Scale Statistics, and Mechanism Understanding

Understanding the growth behavior and morphology evolution of defects in 2D transition metal dichalcogenides is significant for the performance tuning of nanoelectronic devices. Here, the low-voltage aberration-corrected transmission electron microscopy with an in situ heating holder and a fast frame rate camera to investigate the sulfur vacancy lines in monolayer MoS2 is applied. Vacancy concentration-dependent growth anisotropy is discovered, displaying first lengthening and then broadening of line defects as the vacancy densifies. With the temperature increase from 20 °C to 800 °C, the defect morphology evolves from a dense triangular network to an ultralong linear structure due to the temperature-sensitive vacancy migration process. Atomistic dynamics of line defect reconstruction on the millisecond time scale are also captured. Density functional theory calculations, Monte Carlo simulation, and configurational force analysis are implemented to understand the growth and reconstruction mechanisms at relevant time and length scales. Throughout the work, high-resolution imaging is closely combined with quantitative analysis of images involving thousands of atoms so that the atomic-level structure and the large-area statistical rules are obtained simultaneously. The work provides new ideas for balancing the accuracy and universality of discoveries in the TEM study and will be helpful to the controlled sculpture of nanomaterials.

Halogen-Doped Chevrel Phase Janus Monolayers for Photocatalytic Water Splitting

Chevrel non-van der Waals crystals are promising candidates for the fabrication of novel 2D materials due to their versatile crystal structure formed by covalently bonded (Mo₆X₈) clusters (X-chalcogen atom). Here, we present a comprehensive theoretical study of the stability and properties of Mo-based Janus 2D structures with Chevrel structures consisting of chalcogen and halogen atoms via density functional theory calculations. Based on the analysis performed, we determined that the S₂Mo₃I₂ monolayer is the most promising structure for overall photocatalytic water-splitting application due to its appropriate band alignment and its ability to absorb visible light. The modulated Raman spectra for the representative structures can serve as a blueprint for future experimental verification of the proposed structures.

A Novel Membrane-like 2D A'-MoS2 as Anode for Lithium- and Sodium-Ion Batteries

Currently, new nanomaterials for high-capacity lithium-ion batteries (LIBs) and sodium- ion batteries (SIBs) are urgently needed. Materials combining porous structure (such as representatives of metal-organic frameworks) and the ability to operate both with lithium and sodium (such as transition-metal dichalcogenides) are of particular interest. Our work reports the computational modelling of a new A'-MoS₂ structure and its application in LIBs and SIBs. The A'-MoS₂ monolayer was dynamically stable and exhibited semiconducting properties with an indirect band gap of 0.74 eV. A large surface area, together with the presence of pores resulted in a high capacity of the A'-MoS₂ equal to ~391 mAg^-1 at maximum filling for both Li and Na atoms. High adsorption energies and small values of diffusion barriers indicate that the A'-MoS₂ is promising in the application of anode material in LIBs and SIBs.

High Anisotropic Optoelectronics in Monolayer Binary M8X12 (M = Mo, W; X = S, Se, Te)

Exploring high performance and excellent ambient stability in two-dimensional (2D) monolayer photoelectric materials is motivated by not only practical applications but also scientific interest. Here, a new 2D monolayer W8Se12 structure is synthesized via in situ electron-beam irradiation on 2D WSe2. Moreover, we systematically studied the photoelectric properties of the class of monolayer M8X12 (M = Mo, W; X = S, Se, and Te) materials by first principles. The results indicated that Mo8S12, Mo8Se12, W8S12, and W8Se12 monolayers possess desirable direct band gaps and remarkable anisotropic optical absorption in visible light, while Mo8Te12 and W8Te12 monolayers are metals. Impressively, the monolayer W8Se12 can result in a direct-indirect-metal transition under uniaxial strain. In addition, they show high anisotropic carrier mobilities (up to 104 cm2 V-1 s-1), significantly over those of transition-metal dichalcogenides. These new binary monolayer M8X12 structures can effectively broaden the 2D material family and may provide four potential candidates in photoelectric applications.

Multiple 2D Phase Transformations in Monolayer Transition Metal Chalcogenides

Phase transformation lies at the heart of materials science because it allows for the control of structural phases of solids with desired properties. It has long been a challenge to manipulate phase transformations in crystals at the nanoscale with designed interfaces and compositions. Here in situ electron microscopy is employed to fabricate novel 2D phases with different stoichiometries in monolayer MoS₂ and MoSe₂ . The multiphase transformations: MoS₂  → Mo₄ S₆ and MoSe₂  → Mo₆ Se₆ which are highly localized with atomically sharp boundaries are observed. Their atomic mechanisms are determined as chalcogen 2H ↔ 1T sliding, cation shift, and commensurate lattice reconstructions, resulting in decreasing direct bandgaps and even a semiconductor-metal transition. These results will be a paradigm for the manipulation of multiphase heterostructures with controlled compositions and sharp interfaces, which will guide the future phase engineered electronics and optoelectronics of metal chalcogenides.

Novel Janus group III chalcogenide monolayers Al2XY2(X/Y = S, Se, Te): first-principles insight onto the structural, electronic, and transport properties

Motivated by the recent successful synthesis of 2D quintuple-layer atomic materials, for the first time, we design and investigate the electronic and transport properties of Janus Al₂XY₂(X/Y = S, Se, Te; X ≠ Y) monolayers by using the density functional theory. Our calculations demonstrate that most of the models of Al₂XY₂(except for Al₂STe₂monolayer) are dynamically and mechanically stable. By using the hybrid functional, all models of Al₂XY₂are semiconductors with an indirect bandgap. Meanwhile, Al₂TeS₂monolayer is found to be metal at the Perdew-Burke-Ernzerhof level. Due to the vertical asymmetry structure, an intrinsic built-in electric field exists in the Al₂XY₂and leads to a difference in the vacuum levels between the two sides of the monolayers. Carrier mobilities of Al₂XY₂monolayers are high directional anisotropic due to the anisotropy of their deformation potential constant. Al₂XY₂monolayers exhibit high electron mobility, particularly, the electron mobility of Al₂SeS₂exceeds 1 × 10⁴cm²V^-1 s^-1, suggesting that they are suitable for applications in nanometer-sized electronic devices.

Electron beam triggered single-atom dynamics in two-dimensional materials

Controlling atomic structure and dynamics with single-atom precision is the ultimate goal in nanoscience and nanotechnology. Despite great successes being achieved by scanning tunneling microscopy (STM) over the past a few decades, fundamental limitations, such as ultralow temperature, and low throughput, significantly hinder the fabrication of a large array of atomically defined structures by STM. The advent of aberration correction in scanning transmission electron microscopy (STEM) revolutionized the field of nanomaterials characterization pushing the detection limit down to single-atom sensitivity. The sub-angstrom focused electron beam (e-beam) of STEM is capable of interacting with an individual atom, thereby it is the ideal platform to direct and control matter at the level of a single atom or a small cluster. In this article, we discuss the transfer of energy and momentum from the incident e-beam to atoms and their subsequent potential dynamics under different e-beam conditions in 2D materials, particularly transition metal dichalcogenides (TMDs). Next, we systematically discuss the e-beam triggered structural evolutions of atomic defects, line defects, grain boundaries, and stacking faults in a few representative 2D materials. Their formation mechanisms, kinetic paths, and practical applications are comprehensively discussed. We show that desired structural evolution or atom-by-atom assembly can be precisely manipulated by e-beam irradiation which could introduce intriguing functionalities to 2D materials. In particular, we highlight the recent progress on controlling single Si atom migration in real-time on monolayer graphene along an extended path with high throughput in automated STEM. These results unprecedentedly demonstrate that single-atom dynamics can be realized by an atomically focused e-beam. With the burgeoning of artificial intelligence and big data, we can expect that fully automated microscopes with real-time data analysis and feedback could readily design and fabricate large scale nanostructures with unique functionalities in the near future.