113103-2
Yu et al.
Appl. Phys. Lett. 93, 113103 ͑2008͒
FIG. 2. ͑Color online͒ TEM images of graphene. ͑a͒ Low magnification
image with step shaped edges, highlighted by red dash lines. Inset shows the
SAED pattern of the graphene film. ͑b͒ HRTEM image of wrinkles in the
graphene film, apparently of 3–4 layers.
FIG. 3. ͑Color online͒ Raman spectra of segregated carbon at Ni surface
the mobility before they can diffuse. With a wide range of
medium cooling rates, a finite amount of carbon can segre-
gate at the surface. The extremely slow cooling rate allows
carbon with enough time to diffuse into the bulk, so there
will not be enough carbon segregated at the surface.
with different cooling rates.
͑ϳ1580 c cm−1͒ when the number of graphene layers be-
comes less than 4. ͑3͒ The position of G peak moves to lower
wave-number when the number of graphene layers increases
͑from 1587 cm−1 for monolayer to 1581 cm−1 for HOPG͒.
͑4͒ The profile of D peak ͑ϳ1360 cm−1͒ reflects the defect
density ͑the absence of D peak corresponds to very low de-
fect density͒. Analysis of the Raman spectra ͑Fig. 3͒ shows
that the cooling rate significantly affects the amount and
quality of the carbon segregated at Ni surface. With a low
cooling rate ͑0.1 °C/s͒, no carbon peak is seen in the Raman
spectrum ͑in the wavenumber range 1000–3000 cm−1͒, in-
dicating few carbon atoms were segregated at the surface, as
the carbon atoms near the surface have enough time to dif-
fuse into the bulk of the Ni substrate. With a medium cooling
rate ͑ϳ10 °C/s͒, two prominent peaks appeared at ϳ1583
and ϳ2704 cm−1, corresponding to the G and 2D bands, and
the higher peak intensity for the 2D peak relative to G peak
suggests that few ͑four or less͒ layers of graphene formed. A
faster cooling can reduce the rate of carbon migration from
near the surface into the bulk and thus enhance the carbon
segregation at the surface. With further increase in the cool-
ing rate ͑up to ϳ20 °C/s͒, a D band at ϳ1360 cm−1 in the
Raman spectrum appeared in addition to the G and 2D bands
͑Fig. 3͒, suggesting that although a significant amount of
carbon atoms can segregate at the surface in a short time,
they may not have enough time to reach a state with a good
crystallinity. These results suggest that several layers of high
quality graphene can be synthesized on Ni surface with op-
timized medium cooling rates, while higher cooling rates re-
sult in the formation of graphite with more defects.
Transferring graphene from metal substrates to insulators
is a critical step for realizing electronic applications. Trans-
ferring graphene from metal to insulator substrates and the
effect of such transfer on the graphene quality have not been
reported previously. Silicone rubber ͑polymerized siloxanes͒
was used as the media to transfer 5ϫ5 mm2 graphene as-
grown on a Ni substrate to a glass plate. After graphene
synthesis on metal, a thin layer of silicone was applied on the
graphene film, then covered with a glass plate to form a four
layer sandwich structure ͑Ni/graphene/silicone rubber/glass͒.
After a 24 h cure, the silicone rubber was solidified and the
metal substrate was etched away with diluted HNO3 solu-
tion. The transferred graphene is transparent to the eye.
However, using an optical microscope with polarized light,
In our experiments, polycrystalline Ni foils with thick-
ness of 0.5 mm and purity Ͼ99.99% from Alfa Aesar were
cut into 5ϫ5 mm2 pieces, followed by a mechanical polish.
Precursor gases were CH4:H2:Ar=0.15:1:2 with a total gas
flow rate of 315 SCCM ͑cubic centimeter per minute at STP͒
and pressure at 1 atm, with H2 introduced 1 h before the CH4
and Ar. Carbon dissolution time is 20 min at 1000 °C.
Samples were cooled down by mechanically pushing the
sample holder to a lower temperature ͑in the range of
30–500 °C͒ zone in Ar atmosphere. Cooling rates were
monitored by a thermal couple on the sample holder. Differ-
ent cooling rates, corresponding to fast ͑20 °C/s͒, medium
͑10 °C/s͒, and slow ͑0.1 °C/s͒, were employed, and the
structural characteristics of graphene formed on Ni substrates
were studied by transmission electron microscopy ͑TEM͒
and Raman spectroscopy ͑excited by an Argon laser operat-
ing at 514 nm͒.
Samples for TEM were prepared by detaching the
graphene films from Ni in HNO3 solution, followed by rins-
ing with de-ionized water. The films float on water owing to
the hydrophobic nature of graphene. The films, found to be
almost transparent, can nonetheless be distinguished from
water by their different reflectivity. Copper grids with Fam-
var films were used to dredge up the graphene films, which
were then dried in air naturally. In Fig. 2͑a͒, the red dash
lines highlight edges of a graphene film, with step features
that can be attributed to graphene cracking along certain
crystalline directions. The selected area electron diffraction
͑SAED͒ pattern along ͓001͔ direction clearly shows the
graphite lattice structure, and typically 3–4 layers of
graphene were observed at the wrinkles and edges of the
films as shown by the high resolution TEM ͑HRTEM͒ image
Using Raman spectroscopy ͑with excitation wavelength
at 514 nm͒, we have characterized the quality of the films
and the numbers of graphene layers segregated on Ni sub-
strates with different cooling rates ͑Fig. 3͒. Generally, four
distinct features in the Raman spectrum can be used to
characterize graphene and distinguish it from bulk graphite
͑such as HOPG͒.16,17 ͑1͒ The 2D peak at ϳ2700 cm−1 is
symmetric for graphene but has a bump at the left side for
HOPG. ͑2͒ The height of 2D peak is higher than G peak
Yu, Qingkai
