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Bismuth (I) Iodide as a Novel Topological Insulator

Topological Insulators and the Relationship to Solid-State Chemistry

Topological insulators have recently caught massive attention of physical inorganic chemists as well as condensed matter physicists due to the unique physicochemical properties emerging upon transition from bulk to surface states. Exhibiting an energy band gap of classic insulator, the edge/surface states of the material acquire disipationless electric transport. The subject has been investigated by condensed matter physicists as well as mathematicians to provide a link between the experimental emerging properties and the modeled topology. Broadly, the material's physics pertains to the Quantum Hall effect relying upon two pillars: time-reversal symmetry and spin orbit coupling, the latter dependent on the elemental material composition.

Structure of β-Bi₄I₄

Low dimensional van der Waals bonded materials display a fundamental material unit, usually depicted as the simplest molecular formula obeying stoichiometry. A series of such fundamental units align in the bulk material phase due to weak van der Waals interactions. Overall, key advantages conferred by the chemical structure are the ease to scale the materials down to nanostructures under simultaneous conservation of the bulk structure and the reduction in defects amount.

Belonging to the larger class of quasi 1-dimensional van der Waals bonded materials, β-Bi₄I₄ has been recently reported as a novel topological insulator. The binary bismuth-iodine family class includes the known bismuth(III) iodide along with additional representatives such as α-Bi₄I₄, Bi₁₄I₄, Bi₁₆I₄, and Bi₁₈I₄. Having the same stoichiometric chemical formula, α-Bi₄I₄ and β-Bi₄I₄ show similar solid-state structures yet critically different physicochenical properties. Specififcally, α-Bi₄I₄ represents the trivial insulator phase, while stacking of the bismuth atoms along the b crystallographic axis in the β-Bi₄I₄ phase yield a different topological insulator phase.

Synthesis

Crystal growth of β-Bi₄I₄ was achieved through a solid-state reaction between Bi and HgI₂ in a ratio of 1:2. The mixture of solid-state precursors was sealed under dynamic vacuum in a quartz ampoule and subjected to a A temperature gradient of 250°C - 210°C in a two-zone furnace for 20 days. Needle-like blue crystals were obtained with sizes varying from a couple of mm in length and tenths of mm in diameter.

DFT Calculations

Key to modeling the topology of a material are the special points along k-vector of the Brillouin zone, accounting for the accurate depiction of the density of states emerging from the electronics of the material. Density functional theory (DFT) analyses predicted an indirect band gap of 0.158 eV in the β-Bi₄I₄ phase with the valence and conduction band maxima localized at the Γ and M k-space points, respectively. Interestingly enough, the major contributors to the band structure are bismuth's p orbitals of even and odd parity, thus giving the gerade and ungerade points of symmetry.

ARPES Measurements

The allowed electron energies in the topological insulator were probed with the well-employed angle-resolved photoemission spectroscopy (ARPES). Γ and M space points were found to exhibit binding energies of 0.3eV and 0.8eV, respectively. ARPES also probed the Fermi electron velocities along the x and y axes to be 0.1(1)×106 m s^-1 and 0.60(4)×106 m s^-1. The emerging non-trivial states of the topological insulator are expected to show at the space point where the conductive and valence bands almost cross or, in other words, display the smallest band gap. This point indeed showed a binding energy of 0.06eV as measured by ARPES. ARPES measurements on a different β-Bi₄Br₄ topological insulator phase show similarity to its iodine counterpart.