Fig. 1: Polar structure of the Janus MoSSe monolayer.

a Structure of the Janus MoSSe monolayer and the generation of the electron-hole pairs (circles with + and – signs) under optical illumination (left). The intrinsic polarization (P) existing in the MoSSe monolayer results in the spontaneous separation of electrons and holes (right). Typical Raman (b) and photoluminescence (PL) (c) spectra of MoS₂, MoSSe and MoSe₂ monolayers. d Scanning transmission electron microscopy (STEM) image of the cross-section of the MoSSe monolayer, showing the asymmetric structure with Se (brown) on top and S (green) at the bottom. e Incident angle-dependent second-harmonic generation (SHG) intensity ratio (\({I}_{{{{\rm{p}}}}}/{I}_{{{{\rm{s}}}}}\)) between the p and s polarizations in a MoSSe monolayer. The inset shows a schematic of the out-of-plane SHG measurement of the MoSSe sample. The incident angle \(\theta\) can be tuned accordingly by moving the beam position at the objective back aperture using a translation stage. The incident light and SHG signal are denoted with \(\omega\) and \(2\omega\), respectively.