  Some highly ordered compounds of graphene oxide (GO), e.g., the so-called
clamped and unzipped GO, are shown to have piezoelectric responses via
first-principles density functional calculations. By applying an electric field
perpendicular to the GO basal plane, the largest value of in-plane strain and
strain piezoelectric coefficient, d31 are found to be 0.12% and 0.24 pm/V,
respectively, which are comparable with those of some advanced piezoelectric
materials. An in-depth molecular structural analysis reveals that deformation
of the oxygen doping regions in the clamped GO dominates its overall strain
output, whereas deformation of the regions without oxygen dopant in the
unzipped GO determines its overall piezoelectric strain. This understanding
explains the observed dependence of d31 on oxygen doping rate, i.e., higher
oxygen concentration giving rise to a larger d31 in the clamped GO whereas
leading to a reduced d31 in the unzipped GO. As the thinnest two-dimensional
piezoelectric materials, GO has a great potential for a wide range of MEMS/NEMS
actuators and sensors.
