Recently, SUSTech Associate Professor Chun Cheng’s group from the Department of Materials Science and Engineering reported their latest work of synthesizing monolayer MoS2 dendrite by processing the substrate with adhesive tapes. This work entitled with “Twin Defect Derived Growth of Atomically Thin MoS2 Dendrites” was published on ACS Nano (impact factor: 13.942). PhD student Jingwei Wang from Prof. Cheng’s group is the first author of this paper and Department of Materials Science and Engineering of SUSTech is the first corresponding affiliation.
Transitional-metal dichalcogenides (TMDCs) with the formulas of MX2 (M refers to Mo or W; and, X to S, Se, or Te) have attracted great attention among two-dimensional (2D) materials due to their distinct physical and chemical properties. Possessing a direct band-gap at single layer, spin and valley physics and active sites at edges make TMDCs ideal candidates for solar cells, gas sensors, nonlinear and electro-optical devices, and, for valleytronics, optoelectronics and hydrogen evolution reactions, etc.
As a representative of TMDCs, the growth of monolayer molybdenum disulfide (MoS2) has been greatly promoted by using the chemical vapor deposition (CVD) method. Many achievements have been made in growth mechanism, layer number, orientation, coverage, domain size and position control, while the progress in morphology control is far from researchers’ expectations. Limited by the structure symmetry, the most common shapes of CVD grown monolayer MoS2 are dominated by compact triangles. The mature growth mechanisms of traditional nanostructures may provide useful hints for the shape-engineering of monolayer MoS2. Besides the intrinsic symmetry of crystals, structural defects, such as hetero-phases, screw dislocations, twins, etc., have been reported in engineering the morphology of nanostructures. Among these, the twinned setup can be an effective method to connect building blocks for intentionally organizing complex nanostructures. Recently, the formation of monolayer MoS2 crystals in polygonal shapes has been reported by merging the triangular domains across faceted tilt and mirror twin boundaries, including tetragon, pentagon, hexagon and butterfly shapes. These polygonal monolayer MoS2 flakes present a shape dependent photoluminescence behavior due to their high dense defects accumulated in twin boundaries. Therefore, twinned structures in the atomic crystals of MoS2 not only create abundant morphologies, but also strongly affect the electronic, magnetic, optical and mechanical properties of MoS2. However, there are few robust approaches in controlling twin defects during the growth of monolayer MoS2 and the understanding of the twin defects derived growth mechanism is desired. Considering the above facts, it is essential to develop simple and facile methods for reliable shape-engineering of CVD-grown monolayer MoS2.
Prof. Chun Cheng’s group applied the adhesive tapes to treat the substrate for synthesizing monolayer MoS2 dendrite. Twin defects were intentionally introduced at the initial nucleation stage and/or the growth process for realizing the shape modulation of monolayer MoS2 dendrite. The obtained MoS2 crystals were featured with hexagonal backbones and tunable degrees of fractal shapes. The shape evolution was attributed to the synergistic result of twin defect nucleation derived by the adhesive seed and the variation of local S: Mo vapor ratio. In addition, strong and localized enhancement of photoluminescence emission was observed in the cyclic twin regions due to the accumulated sulfur vacancies. This work provides a robust strategy and simple protocol for synthesizing monolayer MoS2 with controllable shapes. It also contributes greatly to the understanding of twin defect growth mechanism and gives rise to electrocatalytic and optoelectronic applications. This work has been highly evaluated and reported by authoritative official accounts such as “Na Mi Ren” and “Jiangsu Nano Innovation Center” (see links at the end).
Figure 1 (a)Schematic of forming MoS2 dendrite by introducing adhesive seed from tape on the substrate; (b) Large area monolayer six pointed star-like MoS2; (c) Snowflake-like monolayer MoS2.
Figure 2 (a) Optical image of product, the S: Mo vapor ratio increases along the sulfur vapor flow; (b-f) Product images corresponding to the areas in Figure 2a; (g) The relationship between fractal dimension and S: Mo vapor ratio.
Prof. Chun Cheng’s group focuses on the study of smart materials, energy materials and 2D inorganic flexible electronic materials and devices. Since the establishment of research group in 2013, they have published over 40 SCI papers (including the top journals in nanoscience and energy materials such as ACS Nano, Adv. Opt. Mater., J Mater. Chem. A, ACS Appl. Mater. Inter., Solar RRL etc.) and one ESI highly cited paper, applied 6 patents and rewarded with Shenzhen youth science and technology award in 2016.
This work was supported by the Guangdong-Hong Kong joint innovation project, the National Natural Science Foundation of China, the Guangdong Natural Science Funds for Distinguished Young Scholars, starting grants from Southern University of Science and Technology and the Principal's Fund. Part of work on transmission electron microscopy characterization is supported by Professor Ning Wang’s Group from Hong Kong University of Science and Technology. Associate Prof. Chun Cheng also gratefully acknowledges the colleagues in SUSTech for their continued support and his students for their great efforts.
The paper links: http://pubs.acs.org/doi/abs/10.1021/acsnano.7b07693
Prof. Chun Cheng's Group website: http://chengc.mse.sustech.edu.cn/index.php/en/homeen
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