  Monolayers of two-dimensional van der Waals materials exhibit novel
electronic phases distinct from their bulk due to the symmetry breaking and
reduced screening in the absence of the interlayer coupling. In this work, we
combine angle-resolved photoemission spectroscopy and scanning tunneling
microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 x
1 dimer ground state in monolayer 1T-IrTe2 that has a large band gap in
contrast to the metallic bilayer-to-bulk forms of this material.
First-principles calculations reveal that phonon and charge instabilities as
well as local bond formation collectively enhance and stabilize a
charge-ordered ground state. Our findings provide important insights into the
subtle balance of interactions having similar energy scales that occurs in the
absence of strong interlayer coupling, which offers new opportunities to
engineer the properties of 2D monolayers.
