Carbon nano-ribbons and their edge phonons

Article abstract of DOI:10.1016/S0038-1098(02)00186-2

Well-defined carbon nano-ribbons with a honeycomb structure have been grown on a TiC (755) surface. The width between two armchair edges is ～ 1.3 nm and the length is a macroscopic scale. The phonon modes and the electronic π bands of the carbon nano-ribbons were quantized, which is a characteristic feature of the ribbons with nano-scale widths. Additionally, we have detected the transverse optical edge phonons, of which the vibrational amplitudes are localized at the armchair edge sites.

Full text of DOI:10.1016/S0038-1098(02)00186-2

Solid State Communications 123 (2002) 33–36
www.elsevier.com/locate/ssc
Carbon nano-ribbons and their edge phonons
a,b
a,b
a,b
a,b
a,b,1
T. Tanakaa,b, , A. Tajima , R. Moriizumi , M. Hosoda , R. Ohno , E. Rokuta
,
*
C. Oshimaa,b, S. Otanic
aDepartment of Applied Physics, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
bKagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26, Nishi-Waseda, Shinjuku,
Tokyo 169-0051, Japan
cNational Institute for Researches in Inorganic Materials, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Received 8 January 2002; accepted 30 April 2002 by S. Ushioda
Abstract
Well-defined carbon nano-ribbons with a honeycomb structure have been grown on a TiC (755) surface. The width between
two armchair edges is ,1.3 nm and the length is a macroscopic scale. The phonon modes and the electronic p bands of the
carbon nano-ribbons were quantized, which is a characteristic feature of the ribbons with nano-scale widths. Additionally, we
have detected the transverse optical edge phonons, of which the vibrational amplitudes are localized at the armchair edge
sites. q 2002 Elsevier Science Ltd. All rights reserved.
PACS: 81.07.Bc; 63.22. þ m; 68.35.Ja; 82.80.Pv
Keywords: A. Nanostructures; Epitaxy; Phonons; Electron energy loss spectroscopy
Modern nano-carbon systems such as fullerenes and carbon
nano-tubes have received tremendously large attention in
the research fields of material science. The theoretical and
experimental works have clarified their interesting proper-
ties. In the practical applications, on the other hand, classical
nano-carbon systems including charcoal, activated carbon
fibers and the other non-crystalline carbons have been
widely used as absorbers of poison molecules in polluted air
and water, and they are recently applied to the anodes in the
high-capacity Li-ion batteries for a cellular telephone,
notebook-type PC, and hybrid vehicle. The difference
between the modern and classical nano-carbon systems is
large areas of edge sites, which produces the affinity of gas
adsorption and Li chemisorption.
classical nano-carbon systems because of both the simple
edge shape and the large area of edge sites [1–3]. The
theorists predicted the edge-localized states related to the
affinity of adsorption; the localized edge states of phonons
exist at armchair edges, not at the zigzag edges [1]. On the
contrary, electronic edge states of p electrons exist at zigzag
edges, not at the armchair edges [2].
From experimental point of view, however, the basic
properties of the classical nano-carbon systems are not
clearly understood because no well-defined specimens
existed so far; even atomic structures of the systems are
not clarified. In this work, we have reported well-defined
carbon nano-ribbons with the width of ,1.3 nm and the
armchair edges as shown in Fig. 1. The nano-ribbons
possess the large area of edge sites as the common feature of
the classical nano-carbon systems. In addition, we detected
the localized edge phonons, which were predicted theoreti-
cally in nano-ribbons with the armchair edges [1].
Recently, carbon nano-ribbons as shown in Fig. 1 were
discussed as a theoretical model for understanding the
*
Corresponding author. Address: Kagami Memorial Laboratory
for Materials Science and Technology, Waseda University, 2-8-26,
Nishi-Waseda, Shinjuku, Tokyo 169-0051, Japan. Tel.: þ81-3-
5286-3784; fax: þ81-3-3205-1353.
A chemical vapor deposition (CVD) method is widely
used for growth of uniform and epitaxial graphene sheets
[4]. The graphene sheets on various solid surfaces are
formed by dehydrogenation of hydrocarbons such as
ethylene, benzene, etc. Nano-ribbon-like structures were
E-mail address: 601l5085@mn.waseda.ac.jp (T. Tanaka).
1
Present address: Institutes of Scientific and Industrial Research,
Osaka University, Mihogaoka 8-1, Ibaraki, Osaka 567-0047, Japan.
0038-1098/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0038-1098(02)00186-2

34
T. Tanaka et al. / Solid State Communications 123 (2002) 33–36
Fig. 1. Schematic of the carbon nano-ribbons on TiC (755).
sometimes observed in STM images [5,6]. Fig. 2 shows one
example; carbon nano-ribbons with a 1 nm width grew at the
step edge of the large terrace, and the sheets are cut
compulsorily at the edge. If we adjust the terrace width,
hence, we expect to control the ribbon widths. Recently, we
tried to grow the nano-ribbons on various step surfaces, and
on TiC (755) we have grown successfully the well-defined
nano-ribbons with a uniform width. Unfortunately, we did
not succeed to observe a STM image of nano-ribbons
because of our technical problems, but the atomic structure
of nano-ribbons was confirmed by LEED, and the phonon
spectra were measured by high resolution electron energy
loss spectroscopy (HREELS).
macroscopic lengths as shown in Fig. 1. Similarly to the
clean TiC (111) surface [7], the topmost atoms on the
terraces of TiC (755) were Ti.
For the growth of graphene sheets, the clean surface
above 1100 K was exposed to benzene (C6H6) molecules for
,200 l. An additional C 1s peak appeared in the XPS
spectra; the peak intensity indicated monolayer thickness of
the carbon layer, and the binding energy of 285 eV exhibits
formation of graphitic C–C bonds. Fig. 3 shows a typical
LEED pattern of the graphene-covered surface. The pattern
consists of diffraction spots from the terraces and the
graphene sheets with nano-scale widths. The LEED pattern
indicates clearly the growth of a graphene sheet with the
following epitaxial relation; graphene k0001l//TiC k111l and
graphene k1200l//TiCk011l. That is to say, the direction of
the step edge line is parallel to the armchair edge direction
of the graphene sheet as shown in Fig. 1. The ribbon width is
about five times as large as the size of a C hexagon ring. The
diffraction spots of the graphene are not circular but
ellipsoidal, which shows clearly the anisotropic arrange-
ment of carbon atoms in the nano-ribbons. Though the
similar ellipsoidal diffraction spots from substrate were
observed in Fig. 3, the origin of the ellipsoidal spots is
different. Some ellipsoidal spots are formed by overlapping
of two nearest-neighbor spots due to the interference of the
beams reflected from the different terrace as shown in Fig. 2.
The additional evidence for nano-ribbon growth was
observed in both the vibrational and electronic spectra.
Since one-dimensional periodicity of the nano-ribbons
guarantees the conservation law of the wavevector k parallel
to the periodic direction, we measured the k-resolved
phonon spectrum by HREELS and the k-resolved electronic
spectra by angle-resolved ultraviolet photoemission spectro-
scopy (ARUPS).
Clean TiC (755) surfaces were prepared in the
conventional way as follows. The specimens were cut
from a large single-crystal rod with a spark-erosion-cutting
tool combined with a Laue X-ray camera. The accuracy of
the crystal orientation was below ^18. The surface was
polished mechanically to optical finish, and the clean
surface was obtained by heating up to 1700 K in UHV.
The LEED patterns indicated that the clean surface
possesses regular (111) terraces of ,1.3 nm in width and
Fig. 4 shows typical k-resolved phonon spectra of both
the carbon nano-ribbon at k ¼ 0.72 A21 and a macroscopic
˚
21
˚
graphene sheet at k ¼ 0.75 A by means of HREELS. In
contrast to the isolated intense peaks in the spectra of the
macroscopic sheet, only broad bands are observed on the
nano-ribbons. This is a clear experimental indication of
nano-ribbon formation, because of the energy resolution not
high enough to resolve the many quantized phonon
Fig. 2. An STM image of carbon nano-ribbons grown in region (A)
along a step edge between the graphene-covered terrace (B) and the
´
same lower terrace (C) [5]. The moire pattern observed on both the
terraces is due to the overlapping of two periodicities of a
monolayer graphene overlayer and TaC (111) substrate.

T. Tanaka et al. / Solid State Communications 123 (2002) 33–36
35
Fig. 5. Experimental and theoretical phonon dispersion relation of
the carbon nano-ribbons. The open circles represent the observed
edge modes, and the gray area represents the detected phonon
bands. The filled circles indicate the observed Rayleigh waves
related to the substrate [9]. The calculated results are shown for the
phonon modes with the perpendicular polarization (broken curve)
except the edge mode (solid curve) and the horizontal polarization
(dotted curve). The force constant model for the calculation is
explained in the text.
Fig. 3. (a) LEED pattern of carbon nano-ribbon on TiC (755).
E0 ¼ 75 eV. (b) The schematic view of picture (a). The diffraction
spots ‘G’ depicted by open circles come from carbon nano-ribbon,
and all the other ones ‘T’ depicted by solid circles are the spots of
the substrate.
branches, which result from the nano-scale width of the
nano-ribbons. The two bands of the phonons with
perpendicular displacement around 40 and 75 meV indicate
the softened graphene lattice, which is observed in
monolayer graphene on different substrate [8]. The Rayleigh
mode at ,20 meV denoted by R in Fig. 4 is acoustic phonon
mainly localized in the outermost layer of TiC substrate [8].
For comparison, the theoretical density of states (DOS)
of nano-ribbons at k ¼ 0.72 A21 is depicted as the spectrum
˚
(b) in Fig. 4. The broken curve (A) is the vibrational modes
with perpendicular polarizations to the layer, and the solid
curve (B) is the vibrational ones with horizontal polari-
zations. The phonons of nano-ribbons of ,1.3 nm in width
with armchair edges were calculated by using a force
constant model. Since the previous calculation [1] did not
contain softening of a graphene lattice, which occurred in
this system, we calculated them on the basis of a model
proposed by Aizawa et al. [8]. The spectrum (b) was finally
obtained by the convolution of the calculated DOS and a
Gaussian function with a 4 meV width expressing the
energy resolution. The spectrum (b) explains well the
observed continuous spectrum, which supports strongly
the quantization of the phonons.
Furthermore, we found a tiny phonon peak, of which the
vibrational energy agrees with the calculated one of the edge
states depicted by the dotted line in Fig. 4. The observed
energy is also consistent with the other experimental data
concerning the edge phonons of an h-BN sheet [9].
Fig. 4. (a) Typical vibrational spectrum of the nano-ribbons at
k ¼ 0.72 A21. (b) The calculated spectra. The curve A depicted by a
˚
broken curve is the vibrational modes with perpendicular
polarizations, and curve B depicted by a solid curve is the one
with the horizontal polarizations. The dotted line indicates the
energy position of the edge phonon. (c) Typical vibrational
Fig. 5 shows the phonon dispersion relation of the nano-
ribbons obtained from angle resolved HREEL spectra. The
observed bands depicted by gray area agree with the
calculated quantized branches depicted by dotted and
21
˚
spectrum of a macroscopic graphene sheet at k ¼ 0.75 A
.
Rayleigh modes are indicated by R.

36
T. Tanaka et al. / Solid State Communications 123 (2002) 33–36
broken curves. In addition, some data points plotted by open
circles agree well with the branches of the edge states shown
by solid curves. According to the theoretical calculation,
appearance of edge states supports the edge of the nano-
ribbons with armchair edges. The theory was also confirmed
in the electronic states. The quantized electronic p states
References
[1] M. Igami, M. Fujita, S. Mizuno, Appl. Surf. Sci. 130–132
(1998) 870.
[2] M. Fujita, K. Wakabayashi, K. Nakada, J. Phys. Soc. Jpn 65
(1996) 1920.
[3] K. Wakabayashi, M. Fujita, H. Ajiki, M. Sigrist, Phys. Rev. B
59 (1999) 8271.
and no edge p states were observed in ARUPS
measurements [10].
[4] C. Oshima, A. Nagashima, J. Phys.: Condens. Matter 9 (1997)
1.
In summary, all the observed data of LEED, AES, XPS,
ARUPS and HREELS are consistent with the growth of the
carbon nano-ribbons with ,1.3 nm widths. The quantiza-
tion of the phonons and electronic p states supports the
nano-scale width. The detection of the edge phonon and no
appearance of edge p state indicate armchair edges of the
ribbons on TiC (755), which are consistent with LEED
results.
[5] A. Nagashima, H. Itoh, T. Ichinokawa, S. Otani, C. Oshima,
Phys. Rev. B 50 (1994) 4756.
[6] R. Koch, O. Haase, M. Borbonus, K.H. Rieder, Phys. Rev. B
45 (1992) 1525.
[7] C. Oshima, M. Aono, S. Otani, Y. Ishizawa, Solid State
Commun. 48 (1983) 911.
[8] T. Aizawa, R. Souda, S. Otani, Y. Ishizawa, C. Oshima, Phys.
Rev. B 42 (1990) 11469.
[9] T. Tanaka, C. Oshima, in press.
[10] R. Ohno, M. Hosoda, T. Tanaka, S. Otani, C. Oshima, in press.
Acknowledgments
This work was supported by special coordination funds
for Promoting Science and technology, by the Ministry of
ESSC and by Research for the Future in JSPS.