Atomically thin transition metal chalcogenides (TMDs), members of two-dimensional (2D) material family, have received great attention due to their novel optoelectronic and electronic properties. CVD technique have been well developed to synthesize monolayer TMDs, such as MoS₂, WS₂, WSe₂, and MoSe₂ where transition metals are sandwiched by the same chalcogenide layers¹. Band gas of those intrinsic materials can be further manipulated by precisely compositional control of TMD alloy structure². For example, band gap of MoS_xSe_2-x sat in between MoS₂ and MoSe₂. Although great effort had paid in fabricating such complex 2D structures had been fabricated, compositional control of TMD along the vertical direction remains challenging. Such asymmetrical composition distribution could result in additional polarity along the vertical direction and extend its application to spintronics³. Here, we develop a method that can precisely manipulate arrangement of chalcogenide atoms (S and Se) along the vertical direction of TMD by applying plasma thinning along with selenization (or sulfurization). This new strategy is not only to control the composition of upmost chalcogenide layer but also able to fabricate MoSSe Janus structure. In this Janus structure, the transition metals are sandwiched by selenium at upmost and sulfur at bottom. Raman spectra is applied to track and investigate the mechanism. Furthermore, this approach can be generalized to other 2D materials. Janus WSSe is also successfully achieved by applying similar plasma treatment and corresponding chalcogenization.

References:

1. Nat. Nanotechnol. 2012, 7, 699.

2. Adv. Mater. 2014, 26, 2648.

3. EPL, 2013, 102, 57001.