  Topological insulators (TIs) are a unique class of materials characterized by
a surface (edge) Dirac cone state of helical Dirac fermions in the middle of
bulk (surface) gap. When the thickness (width) of TIs is reduced, however,
interaction between the surface (edge) states will open a gap removing the
Dirac cone. Using density function theory calculation, we demonstrate the
creation of helical Dirac fermions from interfacing two gapped TI films, a
single bilayer Bi grown on a single quintuple layer Bi2Se3 or Bi2Te3. The
theoretical prediction is directly confirmed by experiment. We further show
that the extrinsic helical Dirac fermions consists of predominantly Bi bilayer
states, which are created by a giant Rashba effect due to interfacial charge
transfer. Our findings provide a promising new method to create novel TI
materials by interface engineering.
