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

TL;DR

This study provides optical evidence of a significant blue shift in the band gap of Bi2Se3 topological insulator films as they become very thin, especially below six layers, impacting their electronic properties.

Contribution

It presents the first direct optical observation of a blue shift in the band gap of few-layer Bi2Se3, highlighting finite-size effects on topological insulators.

Findings

Blue shift up to 0.5 eV observed in thin films

Significant effects below six quintuple layers

Blue shift consistent across capped and uncapped samples

Abstract

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.