Growth of Mesoscale Ordered Two-Dimensional Hydrogen-Bond Organic Framework with the Observation of Flat Band

Flat bands (FBs), presenting a strongly interacting quantum system, have drawn increasing interest recently. However, experimental growth and synthesis of FB materials have been challenging and remained elusive for the ideal form of monolayer materials where the FB arises from destructive quantum interference as predicted in 2D lattice models. Here, we report surface growth of self-assembled monolayer of 2D hydrogen-bond (H-bond) organic frameworks (HOFs) of 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) on Au(111) substrate and the observation of FB. High-resolution scanning tunneling microscopy/spectroscopy (STM/STS) shows mesoscale, highly-ordered and uniform THPB-HOF domains, while angle-resolved photoemission spectroscopy (ARPES) highlights a FB over the whole Brillouin zone (BZ). Density-functional-theory (DFT) calculations and analyses reveal that the observed topological FB arises from a hidden <i>electronic</i> breathing-Kagome lattice without <i>atomically</i> breathing bonds. Our findings demonstrate that self-assembly of HOFs provides a viable approach for synthesis of 2D organic topological materials, paving the way to explore many-body quantum states of topological FBs.