Out-of-plane piezoresponse of monolayer MoS2 on plastic substrates enabled by highly uniform and layer-controllable CVD

Two-dimensional (2D) layered materials have unique electromechanical properties in contrast to their bulk counterparts. In particular, the out-of-plane piezoresponse of 2D layered materials is still veiled according to their properties and mechanisms, whereas the in-plane piezoelectricity has been well confirmed. Herein, a large-area MoS₂ monolayer was deposited on a SiO₂/Si substrate by chemical vapor deposition, and subsequently, it was transferred to a flexible plastic substrate. The number of MoS₂ layers (1L, 2L, and 3L) was controlled by the synthesis time (10, 20, and 30 min). The layer number of MoS₂ was confirmed using Raman and photoluminescence spectra. To focus on the piezoelectric property of the 2D material, we observed the surface morphology of the MoS₂ monolayer consisting of both large-corrugated and small-corrugated regions using atomic force microscopic characterization. Piezoresponse force microscopic measurements revealed that the out-of-plane surface charge distribution of the MoS₂ monolayer was attributed to the corrugation of the MoS₂ layer. Further, the local piezoresponse of the MoS₂ showed that the out-of-plane piezoelectricity can be invoked by flexoelectric effects. This study shows the possibilities of controlling the synthesis and piezoelectricity of large-area 2D materials for piezoelectric applications.