  Electron-hole pair excitations in semiconductors have been predicted to be
able to give rise to a highly correlated many-body ground state, the excitonic
insulator (EI). Under appropriate conditions below a critical temperature (Tc),
strongly bound electron-hole pairs spontaneously form and undergo a phase
transition from a normal band insulator into an exciton condensate,
transforming the parent material into a novel correlated insulator. Despite
recent advances in spectroscopic tools, clear direct experimental evidence for
the EI state has been elusive and is often obfuscated by accompanying
electronic effects. Here we present the reticular bottom-up synthesis of a
Kagome lattice of [4]triangulene, a two-dimensional (2D) covalent organic
framework (COF) imbued with a deliberate excitonic instability excitons with
binding energies larger than the bandgap arising from a pair of flat bands
(FBs). Theoretical analyses based on first-principles calculations and scanning
tunnelling spectroscopy (STS) reveal quasiparticle spectral signatures mixing
valence (VB) and conduction (CB) characteristics of the FBs along with a
non-trivial semiconducting gap that can only be explained by invoking many-body
theory. Our findings spectroscopically corroborate the nature of a FB induced
exciton insulator ground state and provide a robust yet highly tuneable
platform for the exploration of correlated quasi boson physics in quantum
materials.
