  Graphene and carbon nanotubes (CNTs) share the same atomic structure of
hexagonal carbon lattice. Yet, their synthesis differs in many aspects,
including the shape and size of the catalyst. Here, we demonstrate a
floating-catalyst chemical vapor deposition (FCCVD) technique for
substrate-free, single-step growth of CNT-graphene heterostructures (CNT-G-H)
using ethylene as a carbon source. The formation of CNT-G-H is directly
evidenced by lattice-resolved (scanning) transmission electron microscopy
(STEM) and electron diffraction experiments, corroborated by atomic force
microscopy (AFM). Our experiments show the relative number density of
graphene-nanoflakes can be tuned by optimizing the synthesis conditions. Since
in the applied process the formation of the structures take place in
gas-suspension, the as-synthesized CNT-G-H films can be deposited on any
surface in ambient temperature with an arbitrary thickness. Moreover, this
process of CNT-G-H synthesis with strong universality has also been realized in
multiple systems of ethylene-based FCCVD with various catalysts and set-ups.
