  Ultra-thin topological insulators provide a platform of realizing many exotic
phenomena such a Quantum spin Hall effect, Quantum anomalous Hall effect, etc.
These effects or states are characterized by quantized transport behavior of
edge states. Experimentally, although these states have been realized in
various systems, the temperature for the edge states to be the dominating
channel in transport is extremely low, contrary to the fact that the bulk gap
is usually in the order of a few tens of milli-electron volts. There must be
other in-gap conduction channels that won't freeze out till a much low
temperature. Here we grow ultra-thin topological insulator Bi2Te3 and Sb2Te3
films by molecular beam epitaxy and investigate the structures of domain
boundaries in these films. By scanning tunneling microscopy and spectroscopy we
find that the domain boundaries with large rotation angles have pronounced
in-gap bound states, through which one dimensional conduction channels are
suggested to form as visualized by spatially resolved spectroscopy. Our work
indicates the critical role played by domain boundaries in degrading the
transport properties.
