  The growth of iron-containing nanostructures in the process of focused
electron beam-induced deposition (FEBID) of Fe(CO)$_5$ is studied by means of
atomistic irradiation-driven molecular dynamics (IDMD) simulations. The
geometrical characteristics (lateral size, height and volume), morphology and
metal content of the grown nanostructures are analyzed at different irradiation
and precursor replenishment conditions corresponding to the electron-limited
and precursor-limited regimes (ELR & PLR) of FEBID. A significant variation of
the deposit's morphology and elemental composition is observed with increasing
the electron current from 1 to 4 nA. At low beam current (1 nA) corresponding
to the ELR and a low degree of Fe(CO)$_5$ fragmentation, the nanogranular
structures are formed which consist of isolated iron clusters embedded into an
organic matrix. In this regime, metal clusters do not coalesce with increasing
electron fluence, resulting in relatively low metal content of the
nanostructures. A higher beam current of 4 nA corresponding to the PLR
facilitates the precursor fragmentation and the coalescence of metal clusters
into a dendrite-like structure with the size corresponding to the primary
electron beam. The IDMD simulations enable atomistic-level predictions on the
nanoscopic characterization of the initial phase of nanostructure growth in the
FEBID process. These predictions can be verified in high-resolution
transmission electron microscopy experiments.
