Chemistry Reference
In-Depth Information
(RGO) in DMF/H
2
O. To make a stable dispersion of RGO, a further
amount of DMF is added to the suspension. It is useful to distinguish this
single-layer material (RGO) from the SG obtained by micromechanical
cleavage of graphite or other means since RGO may yet contain some
residual oxygen functionalities. Gram quantities of single-layer graphene
have been obtained by a solvothermal procedure using sodium and
ethanol [27]. Exfoliation of graphite in N-methylpyrrolidone or a
surfactant/water solution employing ultrasonication yields stable SG
dispersions [28-29].
SG films are produced on the Si- terminated (0001) face of single-
crystal 6H-SiC by thermal desorption of Si [30-32]. In this procedure, the
substrates are subjected to electron bombardment in ultrahigh vacuum
to 1000°C to remove oxide contaminants and then heated to temperatures
ranging from 1250 to 1450°C for 1-20 min. SG is prepared more
conveniently using chemical vapor deposition (CVD) by decomposing
hydrocarbons on films or sheets of transition metal such as Ni, Cu, Co
and Ru [33]. We have grown graphene layers on different transition
metal substrates by decomposing a variety of hydrocarbons such as
methane, ethylene, acetylene and benzene, the number of layers varying
with the hydrocarbon and reaction parameters. In our experiments,
nickel (Ni) and cobalt (Co) foils with thickness of 0.5 mm and 2 mm
respectively were used as catalysts. These foils were cut into 5 × 5 mm
2
pieces and polished mechanically and the CVD process carried out by
decomposing hydrocarbons around 800-1000°C. By employing a nickel
foil, CVD was carried out by passing methane (60-70 sccm) or ethylene
(4-8 sccm) along with a high flow of hydrogen around 500 sccm at
1000°C for 5-10 minutes. With benzene as the hydrocarbon source,
benzene vapor diluted with argon and hydrogen was decomposed at
1000°C for 5 minutes. On a cobalt foil, acetylene (4 sccm) and methane
(65 sccm) were decomposed at 800 and 1000°C respectively. In all
these experiments, the metal foils were cooled gradually after the
decomposition. Figure 3 shows high resolution TEM images of graphene
sheets obtained by CVD on a nickel foil. Figure 3(a) shows graphenes
obtained by the thermal decomposition of methane on the nickel foil
where as 3(b) shows graphene obtained by thermal decomposition of
benzene. The insets in Figs. 3(a) and 3(b) show selected area electron
