  Two-dimensional (2D) materials have become a new territory for exploring
novel properties and functionalities because of their superior tunability. The
unprecedented tunability of 2D materials is also accompanied by many and
equally great challenges, as they are susceptible to defects and disorders. The
presence of defects and disorders induces the inhomogeneity of electronic
states, often making it difficult to directly probe the intrinsic properties of
materials. Therefore, many efforts have been devoted to improve the electronic
homogeneity, for example, by reducing the density of defects and disorders in
the materials and at the interface. However, little attention is paid to the
disorders embedded in underlying substrates. Here we investigate how individual
dopants in substrate interact with 2D materials and give rise to the
electron-hole puddles by low temperature scanning tunneling microscopy (STM)
and spectroscopy (STS). Specifically, we take the epitaxial bilayer graphene
grown on nitrogen doped silicon carbide (SiC) substrates as a model system,
which has played the essential roles in many applications and fundamental
studies. The nitrogen dopants inside SiC substrate were imaged over the
epitaxial graphene by STM for the first time. The charged nitrogen dopants from
the substrate induce the electron-lack puddles in graphene with a diameter of
~2 nm, via Coulomb interaction. The Fermi level with respect to the Dirac point
is modulated by the puddles with an amplitude of ~40 meV, causing the
electronic inhomogeneity of epitaxial graphene. Our findings on this prototype
material are expected to facilitate the quality improvement of graphene and
other 2D materials in general.
