Surface Science 507–510 (2002) 577–581
www.elsevier.com/locate/susc
Scanning tunneling microscopy and spectroscopy
of Y-junction in carbon nanotubes
Z. Klusek
a
a,*
, S. Datta a, P. Byszewski
b,c
, P. Kowalczyk d, W. Kozlowski
d
Advanced Materials Research Institute, University of Northumbria, Ellison Building, Ellison Place, Newcastle upon Tyne, NE1 8ST, UK
b
Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland
c
Institute of Vacuum Technology, Dluga 44/50, 00-241 Warsaw, Poland
d
Department of Solid State Physics, University of Lodz, 90-236 Lodz, Pomorska 149/153, Poland
Abstract
Scanning tunneling microscopy and spectroscopy were used to observe the variations of electronic structure on the
carbon nanotube Y-junction. The electronic structure derived from tunneling conductance maps showed natural metallic–metallic and semiconducting–metallic transitions close to the Y-junction, and the occurrence of metallic character
of the whole Y-junction with the presence of additional resonant state at energy of 0.2 eV below the Fermi level. It is
suggested that the state originates from heptagons forming the junction. Ó 2002 Elsevier Science B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopies; Carbon; Surface electronic phenomena (work function,
surface potential, surface states, etc.)
1. Introduction
The introduction of polygons with five and seven carbon atoms to the graphene sheet makes it
possible to connect carbon nanotubes with different chiral vectors [1], allowing the formation of
intramolecular electrically active junctions. When
pentagon and heptagon are located at the opposite
sides of the nanotube structure the axis of the two
connected tubes makes an angle depending on
their diameter up to about 30°. The angle can be
smaller when the pentagon and heptagon are closer to one another. It is even possible to connect
tubes of different diameters without bending them.
*
Corresponding author. Tel.: +44-191-227-3838; fax: +44191-227-3684.
E-mail address: [email protected] (Z. Klusek).
Furthermore, it was shown that the more complex
intramolecular junction like Y-type could be
evolved from two bend junctions with different
electronic characters (M–M, M–S, S–S, where S is
semiconductor, M is metal) [2,3]. In this case, the
Y-junction can be considered as three nanotubes
joined via two triangular central spacers and the
heptagons were found to fit well into the hexagonal network.
The electronic properties of intramolecular
junctions are currently the focus of considerable
interests. This is due to a theoretical analysis suggesting that the electronic structure of a junction
differs considerably from that of the nanotube
‘‘bulk’’ region [1,4], and results in M–S transition
and additional resonant states close to the Fermi
level. The implications of these studies are especially important for molecular electronics devices
[2,5–7].
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 3 1 3 - 4
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Z. Klusek et al. / Surface Science 507–510 (2002) 577–581
Although, tunneling spectroscopy results for
carbon nanotubes and carbon nanotubes with topological defects were presented previously [8–14],
only a few experimental data in the high-energy
range were published, especially for Y-junctions.
The goal of this study is to gain a detailed understanding of the Y-junction tunneling spectra at
2 eV around the Fermi energy. These experiments
are intended to identify the electronic band structure of the junction and additional resonant states
around the Fermi level, if they exist. The results
obtained will lead to a better understanding of the
electronic structure of carbon nanotube defects,
and electronic structure of nanotubes in general.
2. Experimental
A few micrograms of carbon nanotubes prepared in the electric arc were sonicated for 30 min
in 2 cm3 of 1,2-dichloroethane. Then a droplet of
the solution was deposited on freshly cleaved
(0 0 0 1) basal plane of highly oriented pyrolitic
graphite surface. After the treatment the sample
was loaded in the UHV STM chamber. The STM/
STS experiments were performed at room temperature with a VT-STM/AFM system in UHV
condition (Omicron GmbH). The tips used were
prepared by mechanical cutting from the 90%Pt–
10%Ir alloy wires. Although, Au(1 1 1) surface is
widely used as a substrate for nanotube deposition
[8–14] we decided to use graphite. This was to
avoid a small charge transfer that takes place between the gold substrate and the nanotube due to
the difference in work functions. It results in a shift
of the Fermi level [10,15], and dI=dV asymmetry
with respect to the bias voltage. However, similar
effects can also be caused by particular tip geometry or point contact during STS measurements
even on graphite [15].
3. Results and discussion
The STM image of a carbon nanotube Yjunction is shown in Fig. 1. The junction consists
of a tube of 0.4 nm diameter (a) connected to
two tubes of 0.35 nm diameter (b) and 0.45 nm
Fig. 1. 220 nm 220 nm STM image of carbon nanotubes
connected via Y-junction. An arrow points to a crossing of two
nanotubes. The tunneling conductance maps measured along
the lines denoted as a, b and c are shown in the inset.
diameter (c). The diameters were estimated from
the height of tubes relative to the substrate
assuming van der Waals distance between the
substrate and the tube of 0.335 nm [16]. The horizontally measured diameters of the nanotubes
were 16, 9 and 19 nm, respectively. However, the
apparent horizontal width contains primarily tip
convolution effects [17]. Further the mechanical
deformation due to the interaction with the substrate and tip might lead to compression of nanotubes and anomalous low apparent height
[13,16,17]. The unusual small tube height in the
STM images can also be explained by the presence
of minitips on the tip apex (carbonaceous contamination––carbon fibers, graphite clumps),
which are in ohmic contact with the nanotube
during scanning process [18].
An illustrative way of presenting spatially resolved spectroscopic data is to plot the dI=dV
quantity as a function of bias voltage and position along the tube [12,14]. In Fig. 2, we show
tunneling conductance maps measured along the
lines denoted as a, b and c in the inset of Fig. 1.
From these measurements it is possible to extract
Z. Klusek et al. / Surface Science 507–510 (2002) 577–581
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Fig. 3. dI=dV curves recorded at different positions ð#1–#6Þ
along the a line. The # points are shown in the inset of Fig. 1.
Fig. 2. Tunneling conductance maps measured as a function of
bias voltage and position along the lines a, b and c. The region
0.5 V on the conductance map along the c line has been enhanced to show some details.
individual conductance curves. Fig. 3 shows dI=dV
curves taken from the vertical cross sections in Fig.
2 (a-line, tube-a) at the position marked by #1 to
#6. In the presented paper all the dI=dV curves are
shifted in vertical scale for the sake of clarity, and
the dI=dV ¼ 0 levels are marked by solid lines.
The dI=dV spectra recorded far from the Y-junction (#1, #2) reflect the metallic character of the
tube. In this case, there is no energy gap and
LDOS between the subbands is finite and roughly
constant. Within a simple tight-binding model the
separation DEsub between the onsets of conductivity singularities is given by DEsub ¼ 6cac–c =D,
where ac–c is the nearest-neighbour carbon–carbon
distance equal 0.142 nm, D is the tube diameter
and c means the p–p energy overlap between
neighbouring carbon atoms equal to 2.7 eV
[1,14,19]. Taking into account the DEsub–tube-a ¼ 3:2
eV obtained from spectroscopic measurements (#1
in Fig. 3) one gets D ¼ 0:71 nm. The discrepancy
between the diameters measured from the STM
height profile (0.35 nm) and band-edge separation (0.71 nm) probably arises from mechanical
deformation of the nanotube mainly caused by the
tip. On the spectroscopic curves recorded close to
the junction (#3 to #6) the main change is the
appearance of well-defined two singularities located at 0.8 eV below and 0.8 eV above the Fermi
level. The dI=dV spectrum still reflects the metallic
character, which is seen as a constant conductance
plateau given by the distance between two singularities DEsub–junction ¼ 1:6 eV.
The dI=dV curves recorded over the tube-c were
similar to those presented in Fig. 3, i.e. the nanotube reflected metallic character, which was
maintained up to the junction. However, it should
be mentioned that the position of the peaks was
slightly different due to the different nanotube diameter (DEsub–tube-c ¼ 2:9 eV).
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Z. Klusek et al. / Surface Science 507–510 (2002) 577–581
Fig. 4. dI=dV curves recorded at different positions (#7, #8)
along the b line. The # points are shown in the inset of Fig. 1.
Fig. 4 shows dI=dV curves taken from vertical
cross sections in Fig. 2 (b-line, tube-b) at the position marked by #7 and #8. The spectrum differs
significantly from that recorded on tube-a and
reflects the semiconducting character of this
nanotube. The spectroscopic curve dI=dV recorded far from the nanotube junction (#7) shows
onsets of LDOS located at about 0.80 eV (V1),
1.63 eV (V2), below and at 0.78 eV (C1), 1.38 eV
(C2) above the Fermi level. Since, the theoretical
band gap for a semiconducting tube is given by
DEgap ¼ 2cac–c =D [1,14,19] then taking into account the DEsub–tube-b ¼ 1:58 eV obtained from
spectroscopic measurements (V1 C1 in Fig. 4) one
gets 0.48 nm for the diameter of the tube-b,
whereas the value measured from the height profile
equals to 0.35 nm. The separations between
V
C
the singularities are given by DE21
¼ DE21
¼
cac–c =D ¼ 1=2DEgap ¼ 0:79 eV and are comparaV
ble to the measured values DE21
¼ 0:83 eV,
C
DE21 ¼ 0:6 eV. The small discrepancy probably
arises from neglecting the curvature of the nanotube and r–p hybridization in simple tight-binding
calculations [19]. On the spectroscopic curve recorded very close to the junction region (#8) the
semiconducting character is changed to metallic
(LDOS between the subbands is finite) and an
additional weak feature (RS) at about 0.25 eV
below the Fermi level appears. The presence of RS
was also observed on the curve #6 (junction region
of the tube-a), though the magnification of the
curve is required to enhance the feature. It was
shown previously that the presence of a naturally semiconducting–metallic transition could be
formed at the connection of two parts of a single
nanotube if built in topological defects (pentagon,
heptagon) allowed change of symmetry of the two
parts [2,4].
The final stage of our studies was to measure
the tunneling spectra on the Y-junction. Fig. 5
shows dI=dV curves taken from c-line at the position marked by #9 and #10. The spectra clearly
show metallic character of the junction with the
presence of additional resonant state at energy of
0.2 eV below the Fermi level––RS. The amplitude
of the RS slightly varies over the junction, and
assumes the maximal values at positions far from
the center of Y-junction.
Summing up we observed natural metallic–
metallic (tube-a, tube-c) and semiconducting–metallic (tube-b) transitions close to the Y-junction,
Fig. 5. dI=dV curves recorded at different positions (#9, #10)
along the c line. The # points are shown in the inset of Fig. 1.
Z. Klusek et al. / Surface Science 507–510 (2002) 577–581
and the occurrence of metallic character of the
whole Y-junction with the presence of additional
RS at energy of 0.2 eV below the Fermi level. In
our interpretation the RS may originate from
carbon heptagons needed to form the Y-type
junction structure. In such a model heptagons are
located between each junction branch and are used
to close hexagonal network [2,3]. As was shown
theoretically the existence of heptagons in graphene network enhances the LDOS close to the
Fermi level as results of the presence of additional
resonant states [20]. It should be emphasized that
the RS amplitude measured at different points over
the junction showed maximal values at positions
far from the center of Y-junction. The observations can be explained in terms of a decrease of the
RS amplitude with increasing the distance from
the defected region. Thus we can assume that the
resonant state near the Fermi level appears only in
the regions close to the heptagon rings located
between each junction branch. Unfortunately, we
were unable to distinguish different arrangements
of carbon atoms using the STM because this
method does not show atomic structure in crystallographic sense but the electronic structure of
the surface near the Fermi level. The presence of
various types of bonds in polygons can be detected
mainly by the tunneling spectroscopy not by tunneling microscopy. However, more research needs
to be done to clarify the influence of local surface
defects on the tunnelling spectra.
4. Conclusions
In summary, we have studied the electronic
structure of the carbon nanotube Y-junction by
STM/STS. We found metallic–metallic and semiconducting–metallic transitions to be present close
to the Y-junction. Furthermore, the occurrence of
metallic character of the whole Y-junction with the
presence of additional RS at energy of 0.2 eV below the Fermi level was observed. It is suggested
that the resonant state originates from heptagons
forming the junction.
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Acknowledgements
This work has been supported by an European
Grant (ERDF). Gratitude is also expressed for the
partial financial support by the Polish Committee
for Scientific Research under grant no. 2P03B 061
19.
References
[1] R. Saito, G. Dresselhaus, M.S. Dresselhaus (Eds.), Physical Properties of Carbon Nanotubes, Imperial College
Press, London, 1998.
[2] G. Treboux, P. Lapstun, K. Silverbrook, Chem. Phys. Lett.
306 (1999) 402.
[3] B. Gan, J. Ahn, Q. Zhang, Q.F. Huang, C. Kerlit, S.F.
Yoon, Rusli, V.A. Ligachev, X.B. Zhang, W.Z. Li, Mater.
Lett. 45 (2000) 315.
[4] V. Meunier, Ph. Lambin, Carbon 38 (2000) 1729.
[5] Z. Yao, H.W.Ch. Postma, L. Balents, C. Dekker, Nature
402 (1999) 273.
[6] L. Roschier, J. Penttila, M. Martin, P. Hakonen, M.
Paalanen, U. Tapper, E.I. Kauppinen, C. Journet, P.
Bernier, Appl. Phys. Lett. 75 (1999) 728.
[7] C. Papadopoulos, A. Rakitin, J. Li, A.S. Vedeneev, J.M.
Xu, Phys. Rev. Lett. 85 (2000) 3476.
[8] D.L. Carroll, P. Redlich, P.M. Ajayan, J.C. Charlier, X.
Blase, A. De Vita, R. Car, Phys. Rev. Lett. 78 (1997) 2811.
[9] T.W. Odom, J. Huang, P. Kim, C.M. Lieber, Nature 391
(1998) 62.
[10] J.W.G. Wildoer, L.C. Venema, A.G. Rinzler, R.E. Smalley, C. Dekker, Nature 391 (1998) 59.
[11] P. Kim, T.W. Odom, J. Huang, C.M. Lieber, Phys. Rev.
Lett. 82 (1999) 1225.
[12] V. Meunier, P. Senet, Ph. Lambin, Phys. Rev. B 60 (1999)
7792.
[13] L.C. Venema, V. Meunier, Ph. Lambin, C. Dekker, Phys.
Rev. B 61 (2000) 2991.
[14] L.C. Venema, J.W. Janssen, M.R. Buitelaar, J.W.G.
Wildoer, S.G. Lemay, L.P. Kouwenhoven, C. Dekker,
Phys. Rev. B 62 (2000) 5238.
[15] G.I. Mark, L.P. Biro, J. Gyulai, P.A. Thiry, A.A. Lucas, P.
Lambin, Phys. Rev. B 62 (2000) 2797.
[16] G.I. Mark, L.P. Biro, J. Gyulai, Phys. Rev. B 58 (1998)
12645.
[17] P. Kim, T.W. Odom, J. Huang, C.M. Lieber, Carbon 38
(2000) 1741.
[18] F.X. Zha, R. Czerw, D.L. Caroll, Ph. Kohler-Redlich,
B.Q. Wei, A. Loiseau, S. Roth, Phys. Rev. B 61 (2000)
4884.
[19] A. Rubio, Appl. Phys. A 68 (1999) 275.
[20] M. Menon, D. Srivastava, Phys. Rev. Lett. 79 (1997) 4453.