Charge Density Waves in Single Layer 2H-NbSe₂

Bulk 2H-NbSe₂ is metallic at room temperature, but exhibits (3×3) charge density wave (CDW) order below 30 K. Recent work has shown that this CDW modulation persists even at the single layer limit. In this work, single layer 2H-NbSe₂ is grown by molecular beam epitaxy on epitaxial graphene/SiC(0001) substrate. In situ scanning tunneling microscopy (STM) shows layer-by-layer growth, resulting in van der Waals heterostructures of single layer 2H-NbSe₂/graphene and bilayer 2H-NbSe₂/graphene. The (3×3) CDW order is observed in both cases below 20 K, as well as a gap-like feature at the Fermi level as revealed by tunneling spectroscopy. This indicates that this CDW modulation is independent of layer thickness and stacking, though there are modifications of the conduction band for the bilayer heterostructures. These findings and their implication on the collective electronic states of single layer 2H-NbSe₂ will be discussed at the meeting.