Large-Scale Fabrication of Lateral MoS₂ – MoS₂/WS₂ –WS₂ Heterostructures by Two-Step Chemical Vapor Deposition

In the research on two-dimensional (2D) crystals new directions and possibilities have evolved by combining different monolayer materials to lateral and vertical heterostructures. These new functionalities have opened up the fields that lead to field-effect transistors, <i>pn</i>- and Schottky solar cells [1] and light-emitting diodes employing transition metal dichalcogenides (TMDs) heterostructures. In the presented work two-dimensional TMDs are grown on Si – SiO₂ substrates using a chemical vapor deposition (CVD) technique with two separated steps as a simple large-scale fabrication methodology. Despite of its simplicity, the two-step CVD technique presented here allows the fabrication of well-defined interfaces over large areas.<br/>In the first step MoS₂ monolayer triangles are grown and in the second step the heterostructures are formed by growing WS₂ at the perimeter of the MoS₂ triangles. Optical microscopy, Raman scattering and mapping show that the two-step CVD process can be used to grow in-plane heterostructures with three-dimensional growth observed at the MoS₂ – WS₂ interface. The change in growth mode from two-dimensional to layered growth is determined by the change of the chemical potential D<i>m</i> of the growing layer. A major contribution to D<i>m</i> is the presence of strain at the interface between the two different materials, leading to either Frank-van der Merwe growth or Volmer-Weber growth [2]. The presented work provides a deeper understanding of the growth mechanisms taking place at the interface and gives a good starting point to tune the growth parameters for a large-scale fabrication applicable in the field of energy conversion and photocatalysis [3].<br/><br/>[1] T. Akama, W. Okita, R. Nagai, C. Li, T. Kaneko, T. Kato, Schottky solar cell using few-layered transition metal dichalcogenides toward large-scale fabrication of semitransparent and flexible power generator, Sci. Rep. 7 (2017) 11967. https://doi.org/10.1038/s41598-017-12287-6.<br/>[2] I.V. Markov, Crystal Growth for Beginners: fundamentals of nucleation, crystal growth and epitaxy, 3rd ed., World Scientific, Hackensack, New Jersey, 2016.<br/>[3] T. Su, Q. Shao, Z. Qin, Z. Guo, Z. Wu, Role of Interfaces in Two-Dimensional Photocatalyst for Water Splitting, ACS Catal. 8 (2018) 2253–2276. https://doi.org/10.1021/acscatal.7b03437.