  We present here a method of probing the wave functions of a degenerate shell
in a triangular graphene quantum dot, triangulene, using a localized
substitutional impurity. We demonstrate its applicability to the example of
aza-triangulenes. Using the analytical solution for degenerate states of an
all-carbon triangulene as a basis for a triangulene containing a nitrogen
impurity, we predict the structure of the zero energy shell in the presence of
this impurity. We show that the impurity allows probing of the wave functions
of a degenerate shell on a carbon site where it is located. We confirm our
predictions by a comparison with the tight-binding and ab-initio calculation as
well as with experiment.
