The Changing Colors of a Quantum-Confined Topological Insulator

• Published in: ACS nano 2014-02, Vol.8 (2), p.1222-1230
• Main Authors: Vargas, Anthony, Basak, Susmita, Liu, Fangze, Wang, Baokai, Panaitescu, Eugen, Lin, Hsin, Markiewicz, Robert, Bansil, Arun, Kar, Swastik
• Format: Article
• Language: English
• Subjects: Confinement; Energy gaps (solid state); Evolution; Insulators; Nanostructure; Semiconductors; Spectra; Three dimensional; Topology
• ISSN: 1936-0851; 1936-086X
• DOI: 10.1021/nn404013d

Bismuth selenide (Bi2Se3) is a 3D topological insulator, its strong spin–orbit coupling resulting in the well-known topologically protected coexistence of gapless metallic surface states and semiconducting bulk states with a band gap, E g ≃ 300 meV. A fundamental question of considerable importance is how the electronic properties of this material evolve under nanoscale confinement. We report on catalyst-free, high-quality single-crystalline Bi2Se3 with controlled lateral sizes and layer thicknesses that could be tailored down to a few nanometers and a few quintuple layers (QLs), respectively. Energy-resolved photoabsorption spectroscopy (1.5 eV < E photon < 6 eV) of these samples reveals a dramatic evolution of the photon absorption spectra as a function of size, transitioning from a featureless metal-like spectrum in the bulk (corresponding to a visually gray color), to one with a remarkably large band gap (E g ≥ 2.5 eV) and a spectral shape that correspond to orange-red colorations in the smallest samples, similar to those seen in semiconductor nanostructures. We analyze this colorful transition using ab initio density functional theory and tight-binding calculations which corroborate our experimental findings and further suggest that while purely 2D sheets of few QL-thick Bi2Se3 do exhibit small band gaps that are consistent with previous ARPES results, the presently observed large gaps of a few electronvolts can only result from a combined effect of confinement in all three directions.