Bandgap reduction at wrinkles on the cleavage surfaces of GeSe single crystals

Abstract

Germanium selenide (GeSe), a van der Waals semiconductor, is heavily investigated due to its suitable bandgap of ∼ 1.2 eV for various applications. Herein, we investigate the atomic and electronic structures of GeSe single crystals using scanning tunneling microscopy/spectroscopy (STM/S). STM topography shows a rectangular lattice at GeSe surfaces, which are attributed to Se atom. We find that the highly localized state of an STM tungsten tip can allow for detection of the hidden Ge atoms, showing the zigzag structures of Se and Ge atoms in topography. It is also found that the cleavage surface of GeSe contains various wrinkles formed during mechanical exfoliations. Interestingly, STS spectra indicate that the bandgap is reduced to ∼ 1.0 eV at a wrinkle due to local strain. The uniaxial strain at the wrinkle is indeed estimated to be ∼ 7.4%, explaining such bandgap reductions. Our results provide important information for understanding the properties of GeSe.