152
S. Taioli
reported for the MBE deposited C 60 film. Finally, the KVV Auger signal intensity
from carbon is not depleted with respect to the copper one by increasing the
temperature up to 800 ◦ C, meaning that the grown species is stable and not volatile.
Most importantly, evidence of fullerene cage disruption under these conditions has
been found independently of the carrier gas used and of copper surface coverage
from 0.3 to 1 ML.
The ultimate evidence of the presence of defected nanometric graphene islands
on the copper substrate is confirmed by both STM measurements and Raman
spectroscopy, reported in Fig. 5.11c, d, respectively.
These findings point towards a graphene growth model, in which the excess of
energy provided by the C 60 translational KEs does not lead immediately to C 60 cage
break and to the activation of kinetically driven chemical-physical mechanisms;
rather, at odds with the case of SiC synthesis, this energy is used to enhance surface
mobility and C 60 diffusion, regardless of the carrier gas used for the expansion.
Moreover, after fullerenes find their optimal hosting sites by lattice distortion of
the copper surface owing to this increased mobility and tighten their interaction by
forming covalent bonds with the inorganic substrate, chemical-physical processes
that change the material topological and electronic properties can be thermally
activated by raising the substrate temperature, eventually leading to the C 60 cage
break. The increase of temperature can of course enhance the catalytic action of
copper as well as contribute to activate nonlinear excitations of vibrational motion
and to desorb physisorbed species above the first C 60 ML.
5.2.1.2 Computational Modelling: Breaking the Fullerene Cage
In this section, we report the principal computational results and modelling tools
that have been used to model the physical-chemical processes leading from C 60
impacts onto metallic surfaces to the early stages of graphene formation. A very
similar theoretical and computational framework could be used to model the
interaction with other surfaces commonly used in graphene synthesis, such as
nickel, or to understand, and thus control, the growth of other carbon-based nanoclusters. However, due to the possible formation of nickel carbide species, we
opted for single- or polycrystal copper substrates for which the chemical-physical
mechanisms underlying graphene growth turned out to be very similar.
Previous calculations based on empirical or semiempirical interaction potentials
[52, 53], aimed at investigating the stability of C 60 molecules upon impact on silicon
surfaces, indicate that KEs of several hundred of eV are required to observe cage
rupture. However, this result was in sheer contrast with our experiments [42, 43],
showing SiC formation on the surface of silicon for impinging KEs of 35 eV
at RT conditions. A detailed analysis of fullerene cage breaking conditions upon
impact on the silicon substrate at different levels of accuracy (and corresponding
different computational costs), using both classical and Born-Oppenheimer (BO)
ab initio molecular dynamics (AIMD), confirmed these previous findings. Indeed,
no cage breaking was observed for fullerene initial KEs lower than 300 eV, even
S. Taioli
reported for the MBE deposited C 60 film. Finally, the KVV Auger signal intensity
from carbon is not depleted with respect to the copper one by increasing the
temperature up to 800 ◦ C, meaning that the grown species is stable and not volatile.
Most importantly, evidence of fullerene cage disruption under these conditions has
been found independently of the carrier gas used and of copper surface coverage
from 0.3 to 1 ML.
The ultimate evidence of the presence of defected nanometric graphene islands
on the copper substrate is confirmed by both STM measurements and Raman
spectroscopy, reported in Fig. 5.11c, d, respectively.
These findings point towards a graphene growth model, in which the excess of
energy provided by the C 60 translational KEs does not lead immediately to C 60 cage
break and to the activation of kinetically driven chemical-physical mechanisms;
rather, at odds with the case of SiC synthesis, this energy is used to enhance surface
mobility and C 60 diffusion, regardless of the carrier gas used for the expansion.
Moreover, after fullerenes find their optimal hosting sites by lattice distortion of
the copper surface owing to this increased mobility and tighten their interaction by
forming covalent bonds with the inorganic substrate, chemical-physical processes
that change the material topological and electronic properties can be thermally
activated by raising the substrate temperature, eventually leading to the C 60 cage
break. The increase of temperature can of course enhance the catalytic action of
copper as well as contribute to activate nonlinear excitations of vibrational motion
and to desorb physisorbed species above the first C 60 ML.
5.2.1.2 Computational Modelling: Breaking the Fullerene Cage
In this section, we report the principal computational results and modelling tools
that have been used to model the physical-chemical processes leading from C 60
impacts onto metallic surfaces to the early stages of graphene formation. A very
similar theoretical and computational framework could be used to model the
interaction with other surfaces commonly used in graphene synthesis, such as
nickel, or to understand, and thus control, the growth of other carbon-based nanoclusters. However, due to the possible formation of nickel carbide species, we
opted for single- or polycrystal copper substrates for which the chemical-physical
mechanisms underlying graphene growth turned out to be very similar.
Previous calculations based on empirical or semiempirical interaction potentials
[52, 53], aimed at investigating the stability of C 60 molecules upon impact on silicon
surfaces, indicate that KEs of several hundred of eV are required to observe cage
rupture. However, this result was in sheer contrast with our experiments [42, 43],
showing SiC formation on the surface of silicon for impinging KEs of 35 eV
at RT conditions. A detailed analysis of fullerene cage breaking conditions upon
impact on the silicon substrate at different levels of accuracy (and corresponding
different computational costs), using both classical and Born-Oppenheimer (BO)
ab initio molecular dynamics (AIMD), confirmed these previous findings. Indeed,
no cage breaking was observed for fullerene initial KEs lower than 300 eV, even