Visualization of Confined Electrons at Grain Boundaries in a Monolayer Charge-Density-Wave Metal

1D grain boundaries in transition metal dichalcogenides (TMDs) are ideal for investigating the collective electron behavior in confined systems. However, clear identification of atomic structures at the grain boundaries, as well as precise characterization of the electronic ground states, have largely been elusive. Here, direct evidence for the confined electronic states and the charge density modulations at mirror twin boundaries (MTBs) of monolayer NbSe2, a representative charge-density-wave (CDW) metal, is provided. The scanning tunneling microscopy (STM) measurements, accompanied by the first-principles calculations, reveal that there are two types of MTBs in monolayer NbSe2, both of which exhibit band bending effect and 1D boundary states. Moreover, the intrinsic CDW signatures of monolayer NbSe2 are dramatically suppressed as approaching an isolated MTB but can be either enhanced or suppressed in the MTB-constituted confined wedges. Such a phenomenon can be well explained by the MTB-CDW interference interactions. The results reveal the underlying physics of the confined electrons at MTBs of CDW metals, paving the way for the grain boundary engineering of the functionality. Two types of 1D grain boundaries, that is, 4|4P and 4|4E mirror twin boundaries (MTBs), are synthesized in a charge-density-wave metal monolayer NbSe2. Both the confined electronic states and the charge density modulations at the MTBs are captured via scanning tunneling microscopy measurements. These results pave the way for the grain boundary engineering of the functionality.image