  Bi2Se3 is theoretically predicted1 2and experimentally observed2,3 to be a
three dimensional topological insulator. For possible applications, it is
important to understand the electronic structure of the planar device. In this
work, thickness dependent band structure of molecular beam epitaxy grown
ultrathin films of Bi2Se3 is investigated by in situ angle-resolved
photoemission spectroscopy. An energy gap is observed for the first time in the
topologically protected metallic surface states of bulk Bi2Se3 below the
thickness of six quintuple layers, due to the coupling between the surface
states from two opposite surfaces of the Bi2Se3 film. The gapped surface states
exhibit sizable Rashba-type spin-orbit splitting, due to breaking of structural
inversion symmetry induced by SiC substrate. The spin-splitting can be
controlled by tuning the potential difference between the two surfaces.
