  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.
