RESUMEN
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.
RESUMEN
The van der Waals (vdW) interface provides two important degrees of freedom-twist and slip-to tune interlayer structures and inspire unique physics. However, constructing diversified high-quality slip stackings (i.e., lattice orientations between layers are parallel with only interlayer sliding) is more challenging than twisted stackings due to angstrom-scale structural discrepancies between different slip stackings, sparsity of thermodynamically stable candidates and insufficient mechanism understanding. Here, using transition metal dichalcogenide (TMD) homobilayers as a model system, this work theoretically elucidates that vdW materials with low lattice symmetry and weak interlayer coupling allow the creation of multifarious thermodynamically advantageous slip stackings, and experimentally achieves 13 and 9 slip stackings in 1Tâ³-ReS2 and 1Tâ³-ReSe2 bilayers via direct growth, which are systematically revealed by atomic-resolution scanning transmission electron microscopy (STEM), angle-resolved polarization Raman spectroscopy, and second harmonic generation (SHG) measurements. This work also develops modulation strategies to switch the stacking via grain boundaries (GBs) and to expand the slip stacking library from thermodynamic to kinetically favored structures via in situ thermal treatment. Finally, density functional theory (DFT) calculations suggest a prominent dependence of the pressure-induced electronic band structure transition on stacking configurations. These studies unveil a unique vdW epitaxy and offer a viable means for manipulating interlayer atomic registries.
RESUMEN
Developing a convenient method to determine the complete structure of single-walled carbon nanotubes (SWNTs) is important to achieve the fully controlled growth of this nanomaterial. However, approaches that can identify handedness at the atomic level with simple equipment, operation, and data analysis are still lacking. Here, the SWNTs/graphene (Gr) vertical heterostructures are artificially constructed with aligned interfaces to realize the lattice interpretation of SWNT upper and lower walls separately by only one transmission electron microscopy image, thus transforming the 3D handedness information to projected 2D space. Gr displays prominent out-of-plane deformation at the interface, promoting the energetic advantage for the aligned interface construction. The interfacial alignment between the SWNT and Gr shows no obvious dependence on either the helical angle or diameter of SWNTs. The half-wrapping of SWNTs by deformed Gr also triggers diversified alterations in electronic structures based on theoretical calculations. 27 specimens with SWNTs prepared by two disparate methods are examined, implying equal handedness distribution in the randomly aligned SWNTs grown on quartz and potential handedness enrichment in horizontal SWNT arrays grown on a-sapphire. This work provides a simple strategy for chiral discrimination and lays a characterization foundation for handedness-selective growth of nanomaterials.
RESUMEN
Developing promoters that can boost the growth quality, efficiency, and robustness of two-dimensional (2D) transition metal dichalcogenides is significant for their industrial applications. Herein a new group (group IIA) of promoters in the periodic table has been disclosed, whose chlorides (especially CaCl2 and SrCl2) exhibit a versatile promoting effect on the CVD growth of various TMD monolayers, including hexagonal MoS2, MoSe2, Re doped MoS2, and triclinic ReS2. The promoting effect of group IIA promoters relies on the appropriate dose and is strongly substrate-dependent. The performances of five typical group IA-IIA metal chlorides are ranked by quantitative investigations, displaying periodic variations closely related to the electronegativities of the metal elements. A brand-new acid-base match model is proposed, attributing the promoting mechanism to an increase of the substrate basicity due to the usage of promoters, thus leading to the sufficient adsorption of the acidic precursor. Aberration-corrected annular dark field scanning transmission electron microscopy (ADF-STEM) was applied, unveiling anomalous grain boundaries (GBs) with a low density of coincident sites in the as-grown ReS2 and detailed atomic configurations of Re doped MoS2. This work expands the promoter library and gives an insight into GB engineering for the CVD growth of 2D TMDs.