Figure 4

Vibration modes and Raman measurement. (a) Schematic diagram of the four Raman-active modes and two IR-active modes of the Bi₂Te₃ quintuple. The purple, blue and green colors represent the Bi, Te² and Te¹ atoms respectively. The vertical arrows represent out of plane vibration. The horizontal arrows represent in plane vibration that parallel to the page. The −, + signs represent in plane vibrations that are perpendicular to the page. (b,c) Micro-Raman spectra of n-type Bi₂Te_2.7Se_0.3 without and with the presence of the A_1u ² mode as function of different exfoliation times compared to the bulk sample. The open symbols represent the raw data and the solid lines through the raw data are the fits. The individual Lorentzian fits are shown below each spectrum. The red bars indicate the position of the optical phonon peaks of bulk n-type Bi₂Te_2.7Se_0.3. The gray (green) bar in (c) indicate the peak position of the IR-active A_1u ¹ (A_1u ²) mode in bulk Bi₂Te₃.