The (0001)-oriented hexagon-shaped nanocrystal plates of Bi 2 Se 3 are synthesized by the hot-injection method using a nontoxic solvent. The Bi 2 Se 3 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 Bi 2 Se 3 single crystals. The first-principles density functional theory (DFT) calculations demonstrate that the expanded band gap (E g ) of these Bi 2 Se 3 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 E g of Bi 2 Se 3 from 0.3 to 0.63 eV with an increase of the surface for the same thickness. These Bi 2 Se 3 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 Bi 2 Se 3 , which can also be an essential parameter in the development of various electronic applications such as resistive memory switching and sensing devices.

The band gap (Eg) engineering and Dirac point tuning of the (0001) surface of 8QLs (quintuple layers) thick Bi2Se3 slab are explored using the first-principles density functional theory calculations by...

Topological insulators (TIs) represent a new state of quantum materials that behave as an insulator in bulk but possess metallic surface states on their edges or surfaces. [1] These surface states are protected by time-reversal symmetry, become insensitive to backscattering, and remain preserved under nonmagnetic interactions. [2] The surface states form a single Dirac cone at the timereversal invariant momentum (TRIM) "Γ" point and the helical spin texture in the Bi 2 Se 3 and Bi 2 Te 3 due to strong spin-orbit coupling (SOC) effect. [3][4][5] However, the occurrence of the Dirac cone depends on the thickness and complete set of quintuple layers (QLs), i.e., the QL is bounded by the Se1 terminated (0001) surfaces. [6] The literature reports suggest that the Dirac-like surface states of Bi 2 Se 3 occur for a thickness of 6 QLs and above, whereas for lower thicknesses the hybridization between the top and bottom surfaces creates a finite energy gap. [7,8] Similarly, for different atomic terminations of the surface states other than the complete set of the top QL, the surface band dispersion undergoes significant modifications. The different atomic terminations at various cleaves of the surface of Bi 2 Se 3 are investigated through scanning tunneling microscopy (STM) and firstprinciples electronic structure calculations, which result in a significant enhancement of the degree of spin polarization. [9] The stability of the surface of Bi 2 Se 3 single crystals, cleaved in vacuum, is examined using high-resolution synchrotron-based photoelectron spectroscopy and it has been shown that, even for 5 min exposure to the atmosphere, a Bi2 layer always forms on the surface, which is accompanied by a depletion of Se in the near-surface region and a 1.4eV decrease in the work function. The Bi2 layer remains stable in ultrahigh vacuum, but becomes unstable after prolonged exposer to air. [10] Through low energy ion scattering experiment, after cleaving the Bi 2 Se 3 surface at 80K, it is found that the Bi 2 Se 3 has a complete Bi surface termination. Density functional theory (DFT) calculations also confirm that a Bi layer on top of the bulk-terminated structure is energetically more favorable, which has profound implications on the electrical transport and long-term stability of such materials and devices. [11] The occurrence of Bi-terminated surface on the Bi 2 Se 3 is also observed in the X-ray photoelectron spectroscopy (XPS) because of the different bonding environment for the Bi atoms. [12] Nevertheless, low-temperature removal of protective Se cap was achieved, enabling the sample to be transported in the air for device fabrication. [13] The coexistence of a conducting bilayer ðSe À Bi or Bi À BiÞ on the bare surface of Bi 2 Se 3 is also observed, whose stability is explained through X-ray crystal truncation rod scattering. [14] It is also found that single crystals of Bi 2 Se 3 stored and cleaved in high vacuum prefer Se-terminated surface, whereas samples stored and cleaved in the air have a Bi termination. [12]

The lattice thermal conductivity (L) calculation using the Wiedemann–Franz law involves electrical conductivity, which introduces the error in the actual value of L. We have adopted a non-contact measurement technique...