  Context. Hydrogenated amorphous carbons, a-C:H, can incorporate a variety of
heteroatoms, which can lead to interesting effects. Aims. To investigate the
doping of interstellar a-C:H grains with, principally, Si, O, N and S atoms
within the astrophysical context. Methods. A search of the literature on doped
a-C:H reveals a number of interesting phenomena of relevance to astrophysics.
Results. X dopants in a-C:H:X materials can affect the sp3/sp2 ratio (X = Si, O
and N), lead to blue photoluminescence (undoped or X = N), induce
ferromagnetic-like behaviour (X = N and S) or simply be incorporated (depleted)
into the structure (X = Si, O, N and S). Si and N atoms could also incorporate
into fullerenes, possibly forming colour-centres that could mimic diffuse
interstellar bands. Conclusions. Doped a-C:H grains could explain several
dust-related conundrums, such as: 'volatile' Si in photo-dissociation regions,
S and N depletion in molecular clouds, blue luminescence, some diffuse
interstellar bands and ferromagnetism in carbonaceous grains.
