Role of Hollow Defects in Lattice Phase Separation and Electronic Features in Quasi-One-Dimensional Bi4I4 Crystals

The defects play a crucial role in the determination of the crystal structure and electronic properties of the matter, evoking tremendous interest in the manipulation of defects and exploration of underlying mechanisms. In this work, we applied scanning tunneling microscopy to investigate the influence of the defects on the lattice structure and electronic properties of Bi4I4 crystals. A lattice phase separation in the nanoscale is identified in the vicinity of hollow defects, which is absent in the sample without defects. The scanning tunneling spectroscopy reveals a bandgap around 0.1 eV at the terrace of (001) surface of Bi4I4, which is consistent with the angle-resolved photoemission spectroscopy results and first-principles calculations. The hollow defects modulate the local density of the state, leading to the edge state residing in the gap region. This edge state is regarded as responsible for the varied resistivity in different Bi4I4 samples in previous reports as their densities of the hollow defects are diverse. Our results shed light on the argument over the lattice phase of Bi4I4 at low temperatures as well as the factors determining the electronic properties.