title: The changing colors of a quantum-confined topological insulator.
abstract: 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, Eg ≃ 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 (Eg ≥ 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.
authors.affiliation: Department of Physics, Northeastern University , Boston Massachusetts, United States.
authors.forename: Anthony
authors.initials: A
authors.lastname: Vargas
authors.forename: Susmita
authors.initials: S
authors.lastname: Basak
authors.forename: Fangze
authors.initials: F
authors.lastname: Liu
authors.forename: Baokai
authors.initials: B
authors.lastname: Wang
authors.forename: Eugen
authors.initials: E
authors.lastname: Panaitescu
authors.forename: Hsin
authors.initials: H
authors.lastname: Lin
authors.forename: Robert
authors.initials: R
authors.lastname: Markiewicz
authors.forename: Arun
authors.initials: A
authors.lastname: Bansil
authors.forename: Swastik
authors.initials: S
authors.lastname: Kar