Unraveling Piezoelectricity of Two-Dimensional Ferroelectric Metal 1T′′-MoS2

The recent discovery of ferroelectric behavior in two-dimensional (2D) van der Waals materials has spurred interest in their piezoelectric properties, which are determined by the relative strength of the inter- and intralayer interactions. However, progress in this field is hindered by the high electrical conductivity and weak piezoelectricity of 2D ferroelectrics: establishing not only a magnitude but also a sign of the piezoelectric coefficient in these materials via measurements of the electrically induced strain as a function of polarization proved to be extremely challenging. Here, we report investigation of the longitudinal piezoelectric coefficient in the recently discovered 2D ferroelectric 1T′′-MoS2 by means of local probe microscopy techniques. The electromechanical response and the surface potential of the flexoelectrically poled 1T′′-MoS2 have been tested by piezoresponse and Kelvin probe force microscopies, respectively. The comparative interferometric displacement spectroscopy studies provide solid evidence of the mechanically induced polarization direction in 1T′′-MoS2 and allow quantification of its piezoelectric response. It is found that 1T′′-MoS2 exhibits negative piezoelectricity with a d33 value of the order of −3 pm/V. First-principles density-functional theory calculations support the experimental findings for d33 in both sign and magnitude.