  Optical band gap properties of high-quality few-layer topological insulator
Bi2Se3 thin films grown with magnetron sputtering are investigated using
broadband absorption spectroscopy. We provide direct optical evidence of a
rigid blue-shift to up to 0.5 eV in the band gap of Bi2Se3 as it approaches the
two-dimensional limit. The onset of this behavior is most significant below six
quintuple layers. The blue shift is very robust and is observed in both
protected (capped) and exposed (uncapped) thin films. Our results are
consistent with observations that finite-size effects have profound impact on
the electronic character of topological insulators, particularly when the top
and bottom surface states are coupled. Our result provides new insights, and
the need for deeper investigations, into the scaling behavior of topological
materials before they can have significant impact on electronic applications.
