5 Enabling Materials By Dimensionality: From 0D to 3D Carbon-Based. . .
151
leading to adsorption of the carrier gas is avoided. C 60 internal dynamics is frozen
unlike ordinary heating that dramatically increases the molecular vibrations.
– Aerodynamical acceleration. The highly diluted C 60 plus carrier gas mixture
fluxes via isentropic expansion out of the injection cell into vacuum through
a nozzle (see Fig. 5.11a). Fullerene KE can be tuned not only by changing
the carrier gas but also modifying the seeding parameters, such as the source
temperature and the gas inlet pressure. In this way C 60 KEs of 10–15 eV using
He carrier gas can be achieved, while up to 30–40 eV using H 2 .
– Collimation of the diluted mixture towards the copper reconstructed surface. The
substrate temperature can be increased as well from RT conditions.
– Thermal activated growth of graphene islands by increasing the substrate
temperature to 645 ◦ C.
To our surprise, we found out that the substrate temperature must be raised to
synthesize graphene islands as C 60 high-energy deposition on Cu, even at the highest
KE reachable by SuMBE, does not lead to immediate C 60 cage rupture at variance
with SiC growth on silicon (as confirmed by our nonadiabatic molecular dynamics
simulations that find a KE cage breaking on copper higher than 40 eV). It seems
that the excess of energy made available by C 60 supersonic impacts is spent for
rearranging fullerene positions in a very stable 4 × 4 pattern on the copper surface,
which eventually induces a tighter interaction characterized by charge transfer
between a number of carbon atoms of the organic molecule with the directly facing
copper adatoms [51]. This covalent interaction at the C 60 -Cu interface, following the
4 × 4 reconstruction, leaves its signature in our in situ core-level analysis, resulting
in a spectral shift of about −0.5 eV and in the emergence of a new feature with
respect to films deposited by standard MBE technique, at variance characterized
by C 60 clustering. These spectral characteristics, present at all beam KEs, at any
surface coverage from 0.3 to 1 equivalent monolayers (ML), without remarkable
differences for temperatures up to 445 ◦ C, have been interpreted as the proof of a
significant deformation, operated by the impinging C 60 molecules, of the copper
superficial layers into a cup shape with removal of a number of copper adatoms and
formation of stable bonds with those surrounding the deformed or partially broken
cage.
Moreover, we did not find evidence of C 60 cage rupture by SuMBE deposition
on single- or polycrystal copper in all range 10 to 40 eV, changing the carrier gas,
even increasing the substrate temperature during the C 60 -Cu collision up to 565 ◦ C.
Nevertheless, the 4 × 4 rearrangement of fullerenes on the copper surface induced
by the collision creates favourable conditions for cage unzipping via thermally
activated processes.
In fact, high-energy 4 × 4 deposition on single- or polycrystal substrates kept at
RT, followed by a temperature increase to 550 ◦ C (645 ◦ C) when using He (H 2 ) as
carrier gas, resulted in a dramatic change of the C 60 typical spectral patterns (as
seen in Fig. 5.11b). In particular, the main peak in the C1s core-level spectrum, not
compatible with the presence of unbroken C 60 , shows the typical asymmetry of
defected graphene nano-islands, while the valence spectrum loses all the features
151
leading to adsorption of the carrier gas is avoided. C 60 internal dynamics is frozen
unlike ordinary heating that dramatically increases the molecular vibrations.
– Aerodynamical acceleration. The highly diluted C 60 plus carrier gas mixture
fluxes via isentropic expansion out of the injection cell into vacuum through
a nozzle (see Fig. 5.11a). Fullerene KE can be tuned not only by changing
the carrier gas but also modifying the seeding parameters, such as the source
temperature and the gas inlet pressure. In this way C 60 KEs of 10–15 eV using
He carrier gas can be achieved, while up to 30–40 eV using H 2 .
– Collimation of the diluted mixture towards the copper reconstructed surface. The
substrate temperature can be increased as well from RT conditions.
– Thermal activated growth of graphene islands by increasing the substrate
temperature to 645 ◦ C.
To our surprise, we found out that the substrate temperature must be raised to
synthesize graphene islands as C 60 high-energy deposition on Cu, even at the highest
KE reachable by SuMBE, does not lead to immediate C 60 cage rupture at variance
with SiC growth on silicon (as confirmed by our nonadiabatic molecular dynamics
simulations that find a KE cage breaking on copper higher than 40 eV). It seems
that the excess of energy made available by C 60 supersonic impacts is spent for
rearranging fullerene positions in a very stable 4 × 4 pattern on the copper surface,
which eventually induces a tighter interaction characterized by charge transfer
between a number of carbon atoms of the organic molecule with the directly facing
copper adatoms [51]. This covalent interaction at the C 60 -Cu interface, following the
4 × 4 reconstruction, leaves its signature in our in situ core-level analysis, resulting
in a spectral shift of about −0.5 eV and in the emergence of a new feature with
respect to films deposited by standard MBE technique, at variance characterized
by C 60 clustering. These spectral characteristics, present at all beam KEs, at any
surface coverage from 0.3 to 1 equivalent monolayers (ML), without remarkable
differences for temperatures up to 445 ◦ C, have been interpreted as the proof of a
significant deformation, operated by the impinging C 60 molecules, of the copper
superficial layers into a cup shape with removal of a number of copper adatoms and
formation of stable bonds with those surrounding the deformed or partially broken
cage.
Moreover, we did not find evidence of C 60 cage rupture by SuMBE deposition
on single- or polycrystal copper in all range 10 to 40 eV, changing the carrier gas,
even increasing the substrate temperature during the C 60 -Cu collision up to 565 ◦ C.
Nevertheless, the 4 × 4 rearrangement of fullerenes on the copper surface induced
by the collision creates favourable conditions for cage unzipping via thermally
activated processes.
In fact, high-energy 4 × 4 deposition on single- or polycrystal substrates kept at
RT, followed by a temperature increase to 550 ◦ C (645 ◦ C) when using He (H 2 ) as
carrier gas, resulted in a dramatic change of the C 60 typical spectral patterns (as
seen in Fig. 5.11b). In particular, the main peak in the C1s core-level spectrum, not
compatible with the presence of unbroken C 60 , shows the typical asymmetry of
defected graphene nano-islands, while the valence spectrum loses all the features