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  • [ May 05, 2020]

    A breakthrough about the CVD synthesis of centimeter-scale single-crystal MoS2 films

  • Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) have emerged as attractive platforms in next-generation nanoelectronics and optoelectronics for reducing device sizes down to ten nanometer scale. To achieve this, the controlled synthesis of wafer-scale single-crystal TMDs with high crystallinity has been a continuous pursuit. Although polycrystalline films have been achieved by various routes, grain boundaries and defects usually exist and cause charge scatterings and trap sites, leading to dramatically reduced electrical properties. To propel their applications in highly integrated electronic devices, the wafer-scale synthesis of single-crystal TMD films that are free of grain boundaries or defects is highly desired, however, still challenging.

    Yanfeng Zhang’s team in the College of Engineering is one of the earliest research groups in the related fields, and has made great progress in the controllable syntheses of TMD monolayers and their heterostructures. For example, centimeter-scale uniform monolayer MoS2 films were firstly obtained on the lattice-matching mica substrates via the van der Waals epitaxial growth mode (Nano Lett. 2013, 13, 3870). Through a designed face-to-face metal precursor supply route, 6-inch uniform monolayer MoS2 films were successfully synthesized on the soda-lime glass substrates, where the uniformly distributed sodium was proven to be effective catalyst for the growth of MoS2 layers (Nat. Commun. 2018, 9, 979). In order to be compatible with the onsite characterization techniques (such as scanning tunneling microscopy/spectroscopy (STM/STS)) and the direct electrochemical test, the conductive gold foils were utilized as substrates for the direct synthesis of domain-size tunable monolayer MoS2 (ACS Nano 2014, 8, 10196; ACS Nano 2015, 9, 4017) and various TMDs related heterostructures, such as MoS2/Gr、MoS2/WS2 and WS2/MoS2 (Adv. Mater. 2015, 27, 7086; Adv. Mater. 2016, 28, 10664). Furthermore, the novel metallic 2H-TaS2 flakes and centimeter-sized ultrathin films were also realized on such gold foils, which were demonstrated as highly efficient catalysts for the hydrogen evolution reaction (Nat. Commun. 2017, 8, 958).

    Based on these previous work, recently, Yanfeng Zhang’s group has made important progress in the growth of centimeter-scale single-crystal MoS2 film. They have realized, for the first time, the epitaxial growth of centimeter-scale uniform, single-crystal MoS2 monolayer on a lattice matching single-crystal Au (111) substrate, via a facile chemical vapor deposition (CVD) route. The centimeter-scale Au (111) films were obtained by means of melting and resolidification commercial Au foils on the tungsten templates. On the Au (111) substrates, single-oriented growth of monolayer MoS2 domains, as well as the centimeter-scale (1-inch) uniform monolayer MoS2 film was successfully obtained through a van der Waals epitaxial growth mode. Subsequently, the unidirectional alignment and seamless stitching of the MoS2 domains were comprehensively demonstrated using atomic- to centimeter-scale characterization techniques, such as low energy electron diffraction (LEED), transmission electron microscopy (TEM) and scanning tunneling microscope (STM), etc. The high crystalline quality of the single-crystal film was further confirmed by electronical performance test, which was carried out by the cooperated research group of Professor Qing Chen from Peking University. Besides, by utilizing onsite STM characterizations combined with first-principles calculations (cooperated with Professor Xiaolong Zou from Tsinghua University), it was revealed that, the nucleation of MoS2 monolayer was dominantly guided by the steps on Au (111), which leads to highly-oriented growth of MoS2 along the <110> step edges. This work, thereby, introduces a novel method/system for the preparation of large-area single-crystal MoS2 monolayers by using single-crystal metallic substrate and epitaxial growth mode, which makes a significant step towards the practical applications of semiconducting 2D materials and the large-scale integration of 2D electronics.

    Controllable synthesis of centimeter-scale single-crystal MoS2 monolayers on Au (111) substrate

    This paper was published online on April 8, 2020 on ACS Nano (DOI: 10.1021/acsnano. 0 c01478). The corresponding authors are Professor Yanfeng Zhang (from College of Engineering, Peking University), Professor Xiaolong Zou (from Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University), and Professor Qing Chen (from Department of Electronics, Peking University). The research was financially supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Beijing Natural Science Foundation, Shenzhen Basic Research Projects, and Guangdong Innovative and Entrepre-neurial Research Team Program.