  Devices made from two-dimensional (2D) materials such as graphene or
transition metal dichalcogenides possess interesting electronic properties that
can become accessible to experimental probes when the samples are protected
from deleterious environmental effects by encapsulating them between hexagonal
boron nitride (hBN) layers. While the encapsulated flakes can be detected
through post-processing of optical images or confocal Raman mapping, these
techniques lack the sub-micrometer scale resolution to identify tears,
structural defects or impurities, which is crucial for the fabrication of
high-quality devices. Here we demonstrate a simple method to visualize such
buried flakes with sub-micrometer resolution, by combining Kelvin force probe
microscopy (KPFM) with electrostatic force microscopy (EFM). KPFM, which
measures surface potential fluctuations, is extremely effective in spotting
charged contaminants within and on top of the heterostructure, making it
possible to distinguish contaminated regions in the buried flake. When applying
a tip bias larger than the surface potential fluctuations, EFM becomes
extremely efficient in highlighting encapsulated flakes and their sub-micron
structural defects. We show that these imaging modes, which are standard
extensions of atomic force microscopy (AFM), are perfectly suited for locating
encapsulated conductors, for visualizing nanometer scale defects and bubbles,
and for characterizing their local charge environment.
