  Surfaces and interfaces of ferroelectric oxides exhibit enhanced
functionality, and therefore serve as a platform for novel nano and quantum
technologies. Experimental and theoretical challenges associated with examining
the subtle electro-chemo-mechanical balance at metal-oxide surfaces have
hindered the understanding and control of their structure and behavior. Here,
we combine advanced electron-microscopy and first-principles thermodynamics
methods to reveal the atomic-scale chemical and crystallographic structure of
the surface of the seminal ferroelectric BaTiO3. We show that the surface is
composed of a native < 2-nm thick TiOx rock-salt layer in epitaxial registry
with the BaTiO3. Using electron-beam irradiation, we successfully patterned
artificially TiOx sites with sub-nanometer resolution, by inducing Ba escape.
Therefore, our work offers electro-chemo-mechanical insights into ferroelectric
surface behavior in addition to a method for scalable high-resolution
beam-induced chemical lithography for selectively driving surface phase
transitions, and thereby functionalizing metal-oxide surfaces.
