  Piezoelectric and ferroelectric properties in the two dimensional (2D) limit
are highly desired for nanoelectronic, electromechanical, and optoelectronic
applications. Here we report the first experimental evidence of out-of-plane
piezoelectricity and ferroelectricity in van der Waals layered
${\alpha}$-In2Se3 nano-flakes. The non-centrosymmetric R3m symmetry of the
${\alpha}$-In2Se3 samples is confirmed by scanning transmission electron
microscopy, second-harmonic generation, and Raman spectroscopy measurements.
Domains with opposite polarizations are visualized by piezo-response force
microscopy. Single-point poling experiments suggest that the polarization is
potentially switchable for ${\alpha}$-In2Se3 nano-flakes with thicknesses down
to ~ 10 nm. The piezotronic effect is demonstrated in two-terminal devices,
where the Schottky barrier can be modulated by the strain-induced
piezopotential. Our work on polar ${\alpha}$-In2Se3, one of the model 2D
piezoelectrics and ferroelectrics with simple crystal structures, shows its
great potential in electronic and photonic applications.
