183106-3
C. M. Lee and J. Choi
Appl. Phys. Lett. 98, 183106 ͑2011͒
improvements over that with a nickel overlayer deposited
for 3 min ͓Fig. 2͑a͔͒, especially with annealing at 300 °C.
This is attributed to the formation of nickel aggregates and
the diffusion of nickel below the nanographene layer with
relatively thick nickel overlayers. Variations in annealing
temperature-dependent transmittance of the nanographene
films with a nickel overlayer deposited for 7 min was rela-
tively small compared to that obtained with a nickel over-
layer deposited for 3 min. This also supports the contention
that nickel diffused underneath the film.
Figure 3 shows the Raman spectra taken from samples
annealed at 300 ͑black͒, 400 ͑blue͒, and 500 °C ͑red͒ for 3
min ͓Fig. 3͑a͔͒ and 7 min ͓Fig. 3͑b͔͒, after etching off the
nickel overlayer. The Raman spectrum of the as-deposited
nanographene film shown in Fig. 3 ͑green͒ shows a signifi-
cantly higher ϳ1350 cm−1 D-band intensity compared to
that of the 1600 cm−1 G-band peak. With annealing treat-
ments at higher temperatures, the overall peak intensities re-
duced. This is consistent with higher transmittance at higher
treatment temperature ͑Fig. 2͒. In particular, the G-band peak
intensity becomes higher relative to the D-band peak. This
clearly indicates that the crystallinity of the nanographene/
nanographitic carbon films improves with higher annealing
temperatures. The intensity ratio of the D-band to the
G-band, plotted in Fig. 3͑c͒, was obtained by first fitting the
curve with three peaks ͑gray curves͒, as shown in Figs. 3͑a͒
and 3͑b͒, to get intensities estimated. The intensity ratio de-
creases with temperature except in the case of the heat-
treated nanographene film at 300 °C with nickel postdepos-
ited for 7 min.
nickel is not be readily etched, such as below the nan-
ographene layer. The EDS ͑Fig. 4͒, Raman ͑Fig. 3͒, and
transmittance ͑Fig. 2͒ studies strongly suggest that the nickel
may be placed under the nanographene, particularly when
thicker nickel overlayers are applied prior to the annealing
treatments ͑and then removed͒. Based on our observations,
we hypothesize a two step diffusion process through the nan-
ographene overlayer. We envision that first nickel aggregates
on nanographene layer as the temperature increases. This
aggregation will favor defect sites and the graphene edges.
At the same time, nickel diffuses largely at the nanographene
boundaries ͑grain boundary diffusion͒. Then, the nickel is
further diffused into the space between the graphene layer
and the substrate caused by the relatively weak bonding be-
tween them. Definitive assignment of the graphitic-carbon
nickel structure awaits careful structural characterization at
each stage of the annealing process.
In summary, films of nanographene were directly depos-
ited on glass at 750 °C without using any extrinsic metal
catalyst. The as-deposited films consist of nanographene with
a crystalline size of ϳ15 nm. The crystalline size of the
nanographene film almost linearly increased to ϳ20 nm, 21
nm, and 23 nm by annealing with the nickel postdeposited on
nanographene film at 300 °C, 400 °C, 500 °C, respectively.
Direct deposition of nanographene on glass at low tempera-
ture is feasible and the crystallite size is controllable.
This research was supported by the Basic Science Re-
search Program through the National Research Foundation of
Korea ͑NRF͒ funded by the Ministry of Education, Science
and Technology ͑MEST͒ ͑Grant No. 2010-0005706͒.
From the intensity ratio, the in-plane crystallite length
scale was estimated using the relation of LC͑nm͒
4
−1
=͕560/͓E͑eV͔͒ ͖͑ID/IG͒ ͑Ref. 11͒ where ID and IG are the
intensity of the D-band and G-band, respectively. These es-
timates of crystallite size are plotted in Fig. 3͑c͒. The crys-
talline length of as-deposited nanographene/nanographitic
carbon film is ϳ15 nm. The as-deposited film can be con-
sidered nanographitic carbon because of the relative high in-
tensity ratio of ͑ID/IG͒. After annealing at 500 °C, the crys-
talline size increased to ϳ23 nm. The crystalline size
increased almost linearly with the annealing temperature
with the thinner nickel overlayers ͑3 min—blue͒.
The chemical composition of the samples after the
etching-process was undertaken, using EDS. As shown in
Fig. 4, the EDS spectra show a characteristic nickel feature,
at 0.84 keV, in addition to the several features related to the
glass substrate ͓O ͑0.52 keV͒, Al ͑1.48 keV͒, and Si ͑1.74
keV͔͒. The samples heat-treated at 300 °C did not show any
measurable nickel features, after the etching-process, sug-
gesting that the nickel was completely etched away. The
samples heat-treated at temperatures higher than 300 °C ex-
hibited nickel features even after the etching-process. This
suggests that the nickel was located at a position where
1S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zhang, J. Blakrishnan, T. Lei,
H. R. Kim, Y. I. Song, Y.-J. Kim, K. S. Kim, B. Ozylimaz, J.-H. Ahn, B.
2Y. Lee, S. Bae, H. Jang, S. Jang, S.-E. Zhu, S. H. Sim, Y. I. Song, B. H.
3K.-S. Kim Y. Zhao, H. Jang, S. Y. Lee, J. M. Lim, K. S. Kim, J.-H. Ahn,
4K. V. Emtsev, A. Bostwick, K. Horn, J. Jobst, G. L. Kellogg, L. Ley, J. L.
McChesney, T. Ohta, S. A. Reshanov, J. Rohrl, E. Rotenberg, A. K.
Schmid, D. Waldmann, H. B. Weber, and T. Seyller, Nature Mater. 8, 203
͑2009͒.
5K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V.
Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666 ͑2004͒.
͑2006͒.
9R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T.
Stauber, N. M. P. Peres, and A. K. Geim, Science 320, 1308 ͑2008͒.
10S. Gaddam, C. Bjelkevig, S. Ge, K. Fukutani, P. A. Dowben, and J. A.
11L. G. Cançado, K. Takai, T. Enoki, M. Endo, Y. A. Kim, H. Mizusaki, A.
Jorio, L. N. Coelho, R. Magalhaes-Paniago, and M. A. Pimenta, Appl.