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.