Figure 3. Evaluation of the crystal structure and electronic structure of 2D chalcogenide crystal arrays.

(a) Typical transmission-mode OM image of 7 × 10 Bi₂Se₃ nanoplate array with the same orientation on a transparent mica substrate. Scale bar, 20 μm. (b) Histogram of the orientation distribution of Bi₂Se₃ nanoplates in a. (c) TEM image of a patterned Bi₂Se₃ crystal array transferred to a lacey carbon support film on a TEM copper grid. Scale bar, 5 μm. (d) HRTEM image obtained from the same Bi₂Se₃ crystal array, with corresponding fast Fourier transformation pattern shown in the inset. Scale bar, 1 nm. (e) Full valance spectra contrast map of a 2D Bi₂Se₃ crystal array measured by micro-ARPES with beam spot size of <1 μm and a 1-μm step scan. Scale bar, 20 μm. (f) Electronic band structure of individual Bi₂Se₃ crystals, showing a direct bulk gap and surface states consisting of a single Dirac cone, a hallmark of the topological insulator. The conduction band (CB), valence band (VB), surface state (SS) and Dirac point (DP) are indicated, together with the Fermi energy (E_F), the energy level of Dirac point (E_D) and wave vector k. (g) Hall mobility (μ_n) and carrier density (n_2D) as a function of temperature for an individual Bi₂Se₃ crystal with the thickness of ∼30 nm. Inset: OM image of a Hall-bar device fabricated on an individual Bi₂Se₃ crystal from a 2D array. Scale bar of inset, 5 μm. (h) Statistic distribution of room temperature mobility of five devices based on the 2D Bi₂Se₃ crystal array, indicating high mobility and reasonable uniformity of 2D crystals.