In situ Raman spectroscopy of topological insulator Bi₂Te₃ films with varying thickness

Topological insulators (TIs) are a new state of quantum matter with a band gap in bulk and conducting surface states. In this work, the Raman spectra of topological insulator Bi₂Te₃ films prepared by molecular beam epitaxy (MBE) have been measured by an in situ ultrahigh vacuum (UHV)-MBE-Raman spectroscopy system. When the thickness of Bi₂Te₃ films decreases from 40 quintuple-layers (QL) to 1 QL, the spectral characteristics of some Raman modes appearing in bulk Bi₂Te₃ vary and a new vibrational mode appears, which has not been reported in previous studies and might be related to quantum size effects and symmetry breaking. In addition, an obvious change was observed at 3 QL when a Dirac cone formed. These results offer some new information about the novel quantum states of TIs.