Abstract
We explore ferroelectric properties of cleaved 2-D flakes of copper indium
thiophosphate, CuInP2S6 (CITP), and probe size effects along with limits of ferroelectric
phase stability, by ambient and ultra high vacuum scanning probe microscopy. CITP
belongs to the only material family known to display ferroelectric polarization in a van der
Waals, layered crystal at room temperature and above. Our measurements directly reveal
stable, ferroelectric polarization as evidenced by domain structures, switchable polarization,
and hysteresis loops. We found that at room temperature the domain structure of flakes
thicker than 100 nm is similar to the cleaved bulk surfaces, whereas below 50 nm
polarization disappears. We ascribe this behavior to a well-known instability of polarization
due to depolarization field. Furthermore, polarization switching at high bias is also
associated with ionic mobility, as evidenced both by macroscopic measurements and by
formation of surface damage under the tip at a bias of 4 Vlikely due to copper reduction.
Mobile Cu ions may therefore also contribute to internal screening mechanisms. The
existence of stable polarization in a van-der-Waals crystal naturally points toward new strategies for ultimate scaling of polar
materials, quasi-2D, and single-layer materials with advanced and nonlinear dielectric properties that are presently not found in
any members of the growing “graphene family”.
