Strong electron-electron Coulomb interactions in materials can lead to a vast
range of exotic many-body quantum phenomena, including Mott metal-insulator
transitions, magnetic order, quantum spin liquids, and unconventional
superconductivity. These many-body phases are strongly dependent on band
occupation and can hence be controlled via the chemical potential. Flat
electronic bands in two-dimensional (2D) and layered materials such as the
kagome lattice, enhance strong electronic correlations. Although theoretically
predicted, correlated-electron phases in monolayer 2D metal-organic frameworks
(MOFs) - which benefit from efficient synthesis protocols and tunable
properties - with a kagome structure have not yet been realised experimentally.
Here, we synthesise a 2D kagome MOF comprised of 9,10-dicyanoanthracene
molecules and copper atoms on an atomically thin insulator, monolayer hexagonal
boron nitride (hBN) on Cu(111). Scanning tunnelling microscopy (STM) and
spectroscopy reveal an electronic energy gap of ~200 meV in this MOF,
consistent with dynamical mean-field theory predictions of a Mott insulating
phase. By tuning the electron population of kagome bands, via either
template-induced (via local work function variations of the hBN/Cu(111)
substrate) or tip-induced (via the STM probe) gating, we are able to induce
Mott metal-insulator transitions in the MOF. These findings pave the way for
devices and technologies based on 2D MOFs and on electrostatic control of
many-body quantum phases therein.Comment: 19 pages, 4 figure