  Efforts aimed at scaling fabrication processes to the level of single atoms,
dubbed atom-by-atom fabrication or atomic fabrication, invariably encounter the
obstacle of atomic scale cleanliness. When considering atomic fabrication,
cleanliness of the base material and purity of the source reservoir from which
atomic structures will be built are invariable constraints imposed by laws of
physics and chemistry. As obvious as such statements may be, and regardless of
the inevitable consequences for successful atomic fabrication, there is a
poignant lack of understanding of the "dirt" (contamination/impurities). Here,
we examine hydrocarbon contamination on graphene. Graphene has formed the base
substrate for many e-beam-based atomic fabrication studies and many strategies
for cleaning graphene have been presented in the literature. One popular method
is heating to high temperatures (>500 {\deg}C). It is usually inferred that
volatile hydrocarbons evaporate into the microscope vacuum system leaving
behind pristine graphene. Here, we show through direct image intensity analysis
that what appears to be clean graphene can be coated with a thin layer of
dynamically diffusing hydrocarbons. This result holds significant implications
for approaches to e-beam based atomic fabrication, updates the conceptual model
of e-beam induced hydrocarbon deposition, and may extend to hot surfaces
generally.
