123109-2
Park et al.
Appl. Phys. Lett. 98, 123109 ͑2011͒
graphene patterns are relatively uniform in the laser-
irradiated areas, also demonstrating the uniformity of the
fabricated graphene patterns.
It is noteworthy that the graphene patterns were fabri-
cated with a single scan on the nickel foil without annealing,
cooling, or other supportive procedures such as mask pat-
terning. A 10 m wide line pattern was directly written at a
scan speed of ϳ50 m/s on the nickel foil. This means that
2
the graphene pattern with a size of 10ϫ10 m can be rap-
idly grown in 0.2 s, which implies that the growth rate is
several thousand times faster than that of conventional ther-
mal CVD methods ͑up to 20 min͒.
It is believed that the
rapid heating and cooling rates at the local region of the
metal foil contribute to the rapid growth of graphene pat-
terns. The fast heating and cooling processes play significant
roles in the rapid growth of graphene patterns and can en-
hance the graphene growth.
The heating and cooling pro-
2
cesses for a 10ϫ10 m area ͑a unit area of graphene pat-
tern͒ have to be finished within 0.2 s because the line
patterns with 10 m wide are fabricated at the scan speed of
FIG. 2. ͑Color͒ ͑a͒ Optical micrograph of a graphene pattern on a nickel foil
and ͑b͒ its Raman spectrum at “A” region. Raman intensity mapping of
5
0 m/s ͑the cooling rate faster than 5000 °C/s͒. These
c͒ G-bnad ͑1582 cm−
͒ and ͑d͒ 2D-band ͑2691 cm ͒ ͑wavelength
1
−1
͑
heating and cooling processes for a unit area of graphene
pattern are also several thousand times shorter than those of
conventional thermal CVD methods. Considering inelastic
electron–phonon coupling ͓͑ϳ1 ps ͑Ref. 16͔͒, a focused la-
ser beam with high intensity can increase the local tempera-
ture of the nickel foil very rapidly up to the growth tempera-
=
514.5 nm͒.
an objective lens ͑Mitutoyo Corp., NIR 5ϫ, focal length
40 mm͒. The laser beam was defocused on the nickel foil
=
with a spot diameter of 20 m to reduce the power sensitiv-
ity. Laser power was controlled at about 5 W to induce the
local temperature rise for graphene growth. For direct writ-
ing of graphene patterns, the substrate was moved precisely
by a motorized stage with respect to the laser beam. The scan
speed of the motorized stage was around 50 m/s.
ture ͑1000 °C in CVD methods,
͒. After laser irradiation,
the local temperature can also drop rapidly through thermal
conduction in the entire nickel foil placed in a room tempera-
ture environment. Therefore, localized laser heating has
much faster heating and cooling rates than those in conven-
tional thermal CVD methods using global heating and cool-
ing process. The single-step fabrication of graphene patterns
by LCVD utilizes a rapid heating and cooling mechanism in
highly localized regions, which can dramatically reduce the
process time. The graphene patterns were only observed fol-
lowing the laser scanning path on the illuminated-side of
nickel foil, which is different from conventional CVD meth-
ods, in which graphene layers are observed on both sides of
For transferring graphene to SiO /Si substrates,
2
graphene-patterned nickel foils were first spin-coated with a
poly͑methyl methacrylate͒ ͑PMMA͒ layer at 2500 rpm. The
nickel foils in nickel/graphene/PMMA layers were then wet
etched using a commercially available etchant ͑nickel
etchant Type 1, Transience Co.͒ for 30–60 min, resulting in a
graphene/PMMA film floating on the etchant. This film was
then collected manually and rinsed in de-ionized water be-
fore transferring it onto a SiO /Si substrate ͑with a 300 nm
2
SiO top layer͒. Finally, the PMMA layer in the graphene/
2
Figures 3͑a͒ and 3͑b͒ show the optical micrographs of
PMMA layers was dissolved in acetone, leaving the
the graphene layers on a SiO /Si substrate. The inset in Fig.
2
graphene patterns on the SiO /Si substrates.
2
3͑a͒ shows the line patterns which consist of both monolayer
Figure 2͑a͒ shows the optical micrograph of a line pat-
tern of graphene which was fabricated by LCVD on a nickel
foil. It is not easy to distinguish between the graphene pat-
tern and the Ni substrate because the graphene pattern is
and multilayer graphene. The thin top SiO layer under the
2
graphene patterns can enhance the optical sensitivity to hu-
man eyes, which allows characterizing the number of
graphene layers. Color difference can be observed with re-
2
“
A” in Fig. 2͑a͒, clearly shows the evidence of a monolayer
shown in Figs. 3͑a͒ and 3͑b͒. The color of the monolayer
graphene pattern in Fig. 3͑a͒ is lighter than that of the
in Fig. 3͑a͒ is highly transparent and its Raman spectrum in
Fig. 3͑c͒ also confirms a monolayer of the as-transferred
graphene. Figure 3͑d͒ shows the Raman spectra of the “b”
and “c” regions in Fig. 3͑b͒, which represents the bilayer and
multilayer characteristics of the graphene, respectively. It
should also be noted that the experimental condition to pro-
duce monolayer graphene patterns is very sensitive to the
scan speed. A lower scan speed is more suitable for the fab-
rication of multilayer graphene.
−1
graphene. Both the symmetric 2D-band ͑2691 cm ͒ with an
−
1
full width at half maximum of ϳ35 cm and the high ratio
−
1
of the peak intensities of 2D- to G-bands ͑1582 cm ͒
͑
ratio=3–4͒ confirm the presence of a monolayer graphene
pattern.
The absence of D-band in this spectrum also
Raman mapping in Figs. 2͑c͒ and 2͑d͒ shows that the inten-
sities of the G- and 2D-band signals are clearly distinguished
from the surrounding nonirradiated areas, thus demonstrating
the precisely controlled locality of the graphene pattern in
Atomic force microscopy ͑AFM͒ was carried out to
the LCVD process. The Raman intensity distributions of the
study the surface topography of the graphene patterns on the
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1
Park
