Observation of Piezoelectricity in Monolayer Molybdenum Disulfide
  Piezoelectricity offers precise and robust conversion between electricity and mechanical force. Here we report the first experimental evidence of piezoelectricity in a single layer of molybdenum disulfide (MoS2) crystal as a result of inversion symmetry breaking of the atomic structure, with measured piezoelectric coefficient e11 = 2.9e-10 C/m. Through the angular dependence of electro-mechanical coupling, we uniquely determined the two-dimensional (2D) crystal orientation. We observed that only MoS2 membranes with odd number of layers exhibited piezoelectricity, in sharp contrast to the conventional materials. The piezoelectricity discovered in single molecular membrane promises scaling down of nano-electro-mechanical systems (NEMS) to single atomic unit cell - the ultimate material limit. 
Hanyu Zhu, Yuan Wang, Jun Xiao, Ming Liu, Shaomin Xiong, Zi Jing Wong,   Ziliang Ye, Xiaobo Yin and Xiang Zhang
cond-mat.mtrl-sci