Optical band gap properties of high-quality few-layer topological insulatorBi2Se3 thin films grown with magnetron sputtering are investigated usingbroadband absorption spectroscopy. We provide direct optical evidence of arigid blue-shift to up to 0.5 eV in the band gap of Bi2Se3 as it approaches thetwo-dimensional limit. The onset of this behavior is most significant below sixquintuple layers. The blue shift is very robust and is observed in bothprotected (capped) and exposed (uncapped) thin films. Our results areconsistent with observations that finite-size effects have profound impact onthe electronic character of topological insulators, particularly when the topand bottom surface states are coupled. Our result provides new insights, andthe need for deeper investigations, into the scaling behavior of topologicalmaterials before they can have significant impact on electronic applications.
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