Honeycomb AgSe Monolayer Nanosheets for Studying Two-dimensional Dirac Nodal Line Fermions

Two-dimensional (2D) materials are interesting for both fundamental properties and potential applications. Among them, the recently fabricated transition-metal monochalcogenides (TMMs) possess Dirac nodal line fermions and can serve as a model to explore topologically nontrivial quantum spin Hall edge states. Here, we successfully fabricated a flat AgSe monolayer nanosheet via direct selenization of a Ag(111) substrate at room temperature. The AgSe monolayer is characterized at the atomic scale by high-resolution scanning tunneling microscopy combined with low-energy electron diffraction, angle-resolved photoemission spectroscopy, and density functional theory (DFT) calculations. The results show that the AgSe monolayer endowed with a honeycomb lattice grows incommensurately on the Ag(111) substrate and exhibits a semiconductor behavior. In addition, DFT calculations reveal that the free-standing AgSe monolayer possesses two 2D Dirac nodal line fermions protected by mirror reflection symmetry without considering the spin–orbit coupling. The realization and investigation of the flat honeycomb AgSe monolayer extend the scope of 2D TMMs and provide an opportunity to study 2D Dirac nodal line fermions.