  A flat energy dispersion of electrons at the Fermi level of a material leads
to instabilities in the electronic system and can drive phase transitions. Here
we introduce a method to induce a flat band in two-dimensional (2D) materials.
We show that the flat band can be achieved by sandwiching the 2D material by
two cesium (Cs) layers. We apply this method to monolayer graphene and
investigate the flat band by a combination of angle-resolved photoemission
spectroscopy experiment and the calculation. Our work highlights that charge
transfer, zone folding of graphene bands and the covalent bonding between C and
Cs atoms are at the origin of the flat energy band formation. The presented
approach is an alternative route for obtaining flat band materials to twisting
bilayer graphene which yields thermodynamically stable flat band materials in
large areas.
