  Domain boundaries in ferroelectric materials exhibit rich and diverse
physical properties distinct from their parent materials and have been proposed
for novel applications in nanoelectronics and quantum information technology.
Due to their complexity and diversity, the internal atomic and electronic
structure of domain boundaries that governs the electronic properties as well
as the kinetics of domain switching remains far from being elucidated. By using
scanning tunneling microscopy and spectroscopy (STM/S) combined with density
functional theory (DFT) calculations, we directly visualize the atomic
structure of domain boundaries in two-dimensional (2D) ferroelectric beta'
In2Se3 down to the monolayer limit and reveal a double-barrier energy potential
of the 60{\deg} tail to tail domain boundaries for the first time. We further
controllably manipulate the domain boundaries with atomic precision by STM and
show that the movements of domain boundaries can be driven by the electric
field from an STM tip and proceed by the collective shifting of atoms at the
domain boundaries. The results will deepen our understanding of domain
boundaries in 2D ferroelectric materials and stimulate innovative applications
of these materials.
