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.

中文翻译：

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用于研究二维狄拉克节线费米子的蜂窝状 AgSe 单层纳米片

二维 (2D) 材料的基本特性和潜在应用都很有趣。其中，最近制造的过渡金属单硫属元素化物 (TMM) 具有狄拉克节线费米子，可以作为探索拓扑非平凡量子自旋霍尔边缘态的模型。在这里，我们通过在室温下直接对 Ag(111) 衬底进行硒化成功地制造了平坦的 AgSe 单层纳米片。AgSe 单层在原子尺度上通过高分辨率扫描隧道显微镜结合低能电子衍射、角分辨光电子能谱和密度泛函理论 (DFT) 计算进行表征。结果表明，具有蜂窝状晶格的 AgSe 单层在 Ag(111) 衬底上不相称地生长并表现出半导体行为。此外，DFT 计算表明，独立的 AgSe 单层具有两个二维狄拉克节线费米子，受镜面反射对称保护，而无需考虑自旋轨道耦合。扁平蜂窝状 AgSe 单层的实现和研究扩展了二维 TMM 的范围，并为研究二维狄拉克节线费米子提供了机会。