  The intriguing properties, especially Dirac physics in graphene, have
inspired the pursuit of two-dimensional materials in honeycomb structure. Here
we achieved a monolayer transition metal monochalcogenide AgTe on Ag(111) by
tellurization of the substrate. High-resolution scanning tunneling microscopy,
combined with low-energy electron diffraction, angle-resolved photoemission
spectroscopy, and density functional theory calculations, demonstrates the
planar honeycomb structure of AgTe. The first principle calculations further
reveal that, protected by the in-plane mirror reflection symmetry, two Dirac
node-line Fermions exist in the electronic structures of free-standing AgTe
when spin-orbit coupling (SOC) is ignored. While in fact the SOC leads to the
gap opening, and resulting in the emergence of the topologically nontrivial
quantum spin Hall edge state. Importantly, our experiments evidence the
chemical stability of the monolayer AgTe in ambient conditions. It is possible
to study AgTe by more ex-situ measurements and even to apply it in novel
electronic devices.
