  Several proposed applications and exotic effects in topological insulators
rely on the presence of helical Dirac states at the interface between a
topological and a normal insulator. In the present work, we have used
low-energy angle-resolved photoelectron spectroscopy to uncover and
characterize the interface states of Bi$_2$Se$_3$ thin films and
Bi$_2$Te$_3$/Bi$_2$Se$_3$ heterostuctures grown on Si(111). The results
establish that Dirac fermions are indeed present at the
topological-normal-insulator boundary and absent at the
topological-topological-insulator interface. Moreover, it is demonstrated that
band bending present within the topological-insulator films leads to a
substantial separation of the interface and surface states in energy. These
results pave the way for further studies and the realization of
interface-related phenomena in topological-insulator thin-film
heterostructures.
