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

摘要

Optical bandgap properties of high-quality few-layer topological insulator Bi_2Se_3 thin films are investigated using broadband absoφtion spectroscopy. We provide direct optical evidence for blue shift in the bulk bandgap of Bi_2Se_3 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 Bi_2Se_3. 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.