Moiré superlattice has recently been found in topological insulators, which can lead to periodic modulation on the electronic structure. In this work, we report the low-temperature scanning tunneling microscopy study of Sb₂Te₃ films grown on graphitized 4H-SiC. We find that substrate temperature can strongly influence the rotation angles between Sb₂Te₃ film and graphene substrate. Three kinds of moiré patterns are observed at the first quintuple layer Sb₂Te₃ film under different substrate temperatures. One shows complicated patterns with a rotation angle of nearly 0° relative to the substrate, another just exhibits simple 1 × 1 structure with a rotation angle of 30°. Other rotation angle like 8.2° is observed at higher substrate temperature as well, which is relatively rare. Comparison of the dI/dV curves from Sb₂Te₃ films with different moiré patterns indicates that the superstructure can offer degrees of freedom in tailoring electronic structure. This work may stimulate the further study on the moiré modulation to the electronic properties of topological insulators.

Our scanning tunneling microscopy (STM) study observes, for the first time, twin domain boundary (TDB) formations on the surface of WTe₂ single crystal, which is glued by solidifying indium to Si substrate. In these TDB regions, a large inhomogeneous strain field, especially a critical shear strain of about 7%, is observed by geometric phase analysis. This observation does not obey the old believe that a small mechanical stress is sufficient to drive thermally-induced TDB formations in two-dimensional materials. To resolve the contradiction, we perform density functional theory calculations combined with elasticity theory analysis, which show that TDBs on WTe₂ are entirely displacement-induced, for which a critical strain is necessary to overcome the onset barrier.

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.

The growth and characterization of single-crystalline thin films of topological insulators (TIs) is an important step towards their possible applications. Using in situ scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we show that moderately thick Sb₂Te₃ films grown layer-by-layer by molecular beam epitaxy (MBE) on Si(111) are atomically smooth, single-crystalline, and intrinsically insulating. Furthermore, these films were found to exhibit a robust TI electronic structure with their Fermi energy lying within the energy gap of the bulk that intersects only the Dirac cone of the surface states. Depositing Cs in situ moves the Fermi energy of the Sb₂Te₃ films without changing the electronic band structure, as predicted by theory. We found that the TI behavior is preserved in Sb₂Te₃ films down to five quintuple layers (QLs).