Surface-Induced Enhanced Band Gap in the (0001) Surface of Bi₂Se₃ Nanocrystals: Impacts on the Topological Effect

The (0001)-oriented hexagon-shaped nanocrystal plates of Bi2Se3 are synthesized by the hot-injection method using a nontoxic solvent. The Bi2Se3 hexagonal plates have a thickness of around 40 nm with a lateral dimension of 600 nm. The selected-area high-resolution transmission electron microscopy images show hexagonal lattice fringes with a lattice spacing of the (0001)-oriented hexagonal crystal structure. These nanocrystals exhibit a band transition energy of ∼0.66 eV, which is 3 times that of Bi2Se3 single crystals. The first-principles density functional theory (DFT) calculations demonstrate that the expanded band gap (Eg) of these Bi2Se3 nanocrystals is due to the phenomenal surface band dispersion and the competition between covalent and van der Waals interactions that determines the topological state. The DFT calculations provide evidence for expansion of the (0001) surface Eg of Bi2Se3 from 0.3 to 0.63 eV with an increase of the surface for the same thickness. These Bi2Se3 nanocrystals dispersed between the Ag contact pads exhibit thermally activated behavior with Poole–Frenkel-type conduction due to the electron trapping/detrapping barriers between the nanocrystals. These results will further advance the understanding of fundamental charge-transport mechanisms in Bi2Se3, which can also be an essential parameter in the development of various electronic applications such as resistive memory switching and sensing devices.