(a) a structure model of the Bi₂Te₃ topological insulator, (b) a scanning tunneling microscope (STM) image (500 nm×500 nm) demonstrating the atomically flat epitaxial Bi₂Te₃ thin film. Inside is the Bi₂Te₃ lattice (2.5 nm×2.5 nm), (c) high-energy reflection electron diffraction intensity oscillation curves proving the layer-by-layer growth mode of Bi₂Te₃ thin films under the indicated parameters. The remaining figures show angle resolved photoemission spectroscopy (ARPES) intensity maps from Bi₂Te₃ thin films of 1 (d), 3 (e), and 5 (f) quintuple-layers at liquid nitrogen temperature. All panels are adopted from Ref. [118].