Janus monolayers of transition metal dichalcogenides

Our recent demonstration in vapor phase growth of TMD monolayers and their lateral heterojunctions has stimulated the research in growth and applications (1,2). In addition to the commonly discussed symmetric 2D materials, we have also developed a method that can precisely manipulate arrangement of chalcogenide atoms (S and Se) along the vertical direction of TMD. This new strategy allows us to fabricate a MoSSe Janus structure, where the transition metals are sandwiched by selenium at upmost and sulfur at bottom. We confirm the Janus structure of MoSSe directly by means of scanning transmission electron microscopy and energy-dependent X-ray photoelectron spectroscopy, and prove the existence of vertical dipoles by second harmonic generation and piezoresponse force microscopy measurements. (3). References: (1) Adv. Mater. 24, 2320 (2012). (2) Science 349, 524 (2015). (3) Nature Nanotech. (2017) doi:10.1038/nnano.2017.100