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Due to the unique optical responses induced by vertical atomic asymmetry inside a monolayer, two-dimensional Janus structures have been conceived as promising building blocks for nanoscale optical devices. In this paper, second harmonic generation (SHG) in Janus transition metal chalcogenide oxide monolayers is systematically investigated by the first-principles calculations. Second-order nonlinear susceptibilities are theoretically determined for Janus MXO (M = Mo/W, X = S/Se/Te) monolayers. The calculated values are comparable in magnitude with Janus MoSSe monolayer. X-M-O symmetry breaking leads to non-zero components in vertical direction, compared with the non-Janus structure. Focusing on the SHG induced by incident light at 1064 nm, polarization-dependent responses of six Janus MXO monolayers are demonstrated. The symmetry of p-polarization changes from six-fold to three-fold with acute incidence angle. Moreover, the effects of biaxial strain on band structures and SHG are further investigated, taking MoSO as an exemplary case. We expect these results to bring in recipes for designing nonlinear optical devices based on Janus transition metal chalcogenide oxide monolayers. [ABSTRACT FROM AUTHOR]

The unique physical properties exhibited by one-dimensional nanoscrolls assembled from nanosheets have propelled them into the spotlight of two-dimensional materials research. However, the self-scrolling mechanism of transition metal dichalcogenides has not been unveiled with an appropriate theoretical approach. In this paper, we systematically investigate the spontaneous formation of MoS 2 nanoscrolls from flat monolayers by molecular dynamics simulations based on a reactive force field. The sulfur vacancies on one side break the atomic symmetry and the reconstruction acts as the driving force for the curling of the flat nanoribbon. If sulfur vacancies are arranged in a line, clear bending angles of the nanoribbon can be obtained and the angle relies on the direction of the line vacancy. With random sulfur vacancies on the top, spontaneous curling and a time-dependent scrolling process of the nanoribbon can be observed. The interplay between dangling bonds and van der Waals (vdW) interactions plays a pivotal role in the formation process of MoS 2 nanoscrolls. With an increasing density of sulfur vacancies, the curvature of the nanoscrolls increases. Meanwhile, the scrolling rate accelerates and the time required for the formation of vdW structures decreases. These results provide theoretical insights into the fabrication of nanoscrolls and pave avenues for tailoring nanoscrolls with different morphologies.