Direct observation of trapped charges at ReSe₂ and graphene heterojunctions

The van der Waals (vdW) heterojunction often reveals unexpected characteristics distinct from conventional junctions. We investigate an emergent interface phenomenon between monolayer ReSe₂ and graphene via combined studies of scanning tunneling microscopy (STM) and density functional theory (DFT). When probing monolayer ReSe₂ on graphene at bias voltages within the ReSe₂ band gap (in-gap bias; −1.2 V to 0.5 V), strikingly, observed topograph appears just like ReSe₂ as it shows precisely the same hexagonal periodicity of ReSe₂ lattice instead of the underlying graphene lattice. To answer this puzzle, we examine the nature of charge redistribution at the confined vdW gap between ReSe₂ and graphene. DFT calculations indicate that the electron accumulation arises right below the ReSe₂ layer, while the electron depletion occurs from the graphene layer. This leads to an asymmetrically polarized two-dimensional layer of confined electron density (termed as trapped charges) in the vdW gap, which agrees well with the in-gap STM topograph. We also find that the accumulation of the trapped charge is enhanced strongly at the edge of ReSe₂.