Optical evidence for blue shift in topological insulator bismuth selenide in the few-layer limit

Optical bandgap properties of high-quality few-layer topological insulator Bi2Se3 thin films are investigated using broadband absorption spectroscopy. We provide direct optical evidence for blue shift in the bulk bandgap of Bi2Se3 as it approaches the two-dimensional limit. The blue shift is robust and observed in both protected (capped) and exposed (uncapped) thin films. The behavior is strongest below six quintuple layers (QLs), particularly at the 2 and 3 QL level, where finite-size effects are known to be most significant in Bi2Se3. A further bandgap increase is observed in all films that we attribute to the Burstein-Moss effect. Our result provides additional insights into the scaling behavior of topological materials. The bandgap increase has a significant impact on the electronic and optoelectronic applications of topological insulators.