  The control of the in-plane domain evolution in ferroelectric thin films is
not only critical to understanding ferroelectric phenomena but also to enabling
functional device fabrication. However, in-plane polarized ferroelectric thin
films typically exhibit complicated multi-domain states, not desirable for
optoelectronic device performance. Here we report a strategy combining
interfacial symmetry engineering and anisotropic strain to design
single-domain, in-plane polarized ferroelectric BaTiO$_3$ thin films.
Theoretical calculations predict the key role of the BaTiO$_3$ / PrScO$_3$
(110)$_O$ substrate interfacial environment, where anisotropic strain,
monoclinic distortions, and interfacial electrostatic potential stabilize a
single-variant spontaneous polarization. A combination of scanning transmission
electron microscopy, piezoresponse force microscopy, ferroelectric hysteresis
loop measurements, and second harmonic generation measurements directly reveals
the stabilization of the in-plane quasi-single-domain polarization state. This
work offers design principles for engineering in-plane domains of ferroelectric
oxide thin films, which is a prerequisite for high performance optoelectronic
devices.
