APPLIED PHYSICS LETTERS
VOLUME 79, NUMBER 13
24 SEPTEMBER 2001
Gold nanoparticle single-electron transistor with carbon nanotube leads
Claes Thelander,a) Martin H. Magnusson, Knut Deppert, and Lars Samuelson
Solid State Physics/Nanometer Consortium, Lund University, Box 118, S-221 00 Lund, Sweden
Per Rugaard Poulsen, Jesper Nygård, and Jørn Borggreen
Niels Bohr Institute, Ørsted Laboratory, Universitetsparken 5, 2100 Copenhagen, Denmark
共Received 5 February 2001; accepted for publication 31 July 2001兲
We have used carbon nanotubes to electrically contact a 7 nm gold particle by scanning-probe
manipulation. The result was a single-electron transistor showing a periodic modulation of the
current as a function of gate voltage for temperatures up to ⬃200 K, with the particle responsible
for the main features of the electron transport. This interpretation could be verified when the particle
was removed and the two nanotubes were moved into electrical contact. © 2001 American
Institute of Physics. 关DOI: 10.1063/1.1405154兴
Carbon nanotubes are exceptional molecules, having
outstanding electrical and mechanical properties.1 Recent experiments have probed the electrical transport through nanotubes, and it has been shown that metallic nanotubes can
function as nanoscale wires.2,3 While the transport properties
of nanotubes themselves have been studied in detail, no experiments have so far investigated the transport in structures
where nanotubes are actually used as passive leads to contact
other nanosized objects. In the present letter, we address this
important aspect experimentally.
The mechanical strength and flexibility of nanotubes allow manipulation of single tubes using scanning-probe
techniques.4 –7 We have utilized this to form a structure
where two nanotubes, with lengths greater than 1 ␮m, were
used as leads to contact a single 7 nm gold particle. The
transport properties of this combined structure were investigated at various temperatures. In particular, we found singleelectron transistor 共SET兲 behavior up to 200 K. The large
difference between the length of the tubes and the diameter
of the particle means that the transport was mainly governed
by the latter. Thus, Coulomb blockade in the tubes, which is
only effective below a few K, can be disregarded and their
main function was to provide contact between the macroscopic electrodes and the nanoparticle.
Carbon nanotube devices were fabricated by chemicalvapor deposition 共CVD兲 following the method developed by
Soh et al.8 Areas with Fe- and Mo-containing catalyst material were patterned by electron-beam lithography on a silicon
substrate with a 355 nm thermal oxide top layer.9 In the
following CVD step, a methane flow rate of
5200 bar cm3/min was maintained for 10 min at 900 °C. The
result is most often individual tubes, with diameters around
1–3 nm and lengths up to several micrometers, spreading out
from the catalyst areas. Most tubes appear to be single
walled as observed by transmission electron microscopy. The
catalyst areas were then covered with an evaporated metal
layer consisting of Cr 共6 nm兲 and Au 共15 nm兲. This layer
extended 3 ␮m beyond the catalyst areas, resulting in electrodes separated by 1 ␮m. The doped silicon substrate was
used as a third, back-gate electrode.
a兲
Electronic mail: claes.thelander@ftf.lth.se
Figure 1共a兲 shows a part of the 3-nm-thick, individual
carbon nanotube that was used in the experiment. The initial
resistance of the tube—including contact resistance—was 21
k⍀ and showed no gate dependence at room temperature,
i.e., the tube was metallic. When the tube was strained with
the tip of an atomic-force microscope 共AFM兲, it began to
slide under one of the electrodes, until approximately 1 ␮m
of the tube had been pulled out. 共This is often observed for
different kinds of tubes and electrode metals. For details of
AFM manipulation, see Ref. 10.兲 In order to cut the tube, and
to prevent further sliding, a certain strain in combination
with a sharp angle was required. These conditions could be
realized by first shaping the tube into a loop 关Fig. 1共b兲兴 and
then positioning the AFM tip inside this loop. A very sharp
angle was created as the tip strained the loop and the tube
was finally cut 关Fig. 1共c兲兴. When the two parts were put back
into contact, resistances down to 500 k⍀ were observed. The
resistance seemed to depend on the contact area between the
tubes, on the force that was used to press the tubes together,
and also on what parts of the tube that were connected. The
FIG. 1. AFM images showing the assembly of a gold nanoparticle SET. 共a兲
3-nm-thick carbon nanotube connects the electrodes to the left and right. 共b兲
and 共c兲, the tube is cut using the tip of the AFM. 共d兲 and 共e兲, magnifications
of the area marked in 共c兲. The tubes are shaped into a slit with a narrow gap.
A 7 nm Au particle 共indicated by a small arrow兲 is moved to electrically
connect the nanotubes. 共f兲 The particle has been pushed out of the slit and
the two tubes are moved into electrical contact 共indicated by the larger
arrow兲.
0003-6951/2001/79(13)/2106/3/$18.00
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© 2001 American Institute of Physics
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Appl. Phys. Lett., Vol. 79, No. 13, 24 September 2001
FIG. 2. 共Color兲 共a兲 Plot of dI/dV SD as a function of both bias voltage and
gate voltage for the tube–particle–tube system. Brighter areas correspond to
a higher dI/dV SD . (T⫽4.2 K). 共b兲 Magnification of an area in between two
larger Coulomb blockade areas, marked with a rectangle in the previous
plot. 共c兲 Oscillations in the current as a function of gate voltage are observed
for temperatures up to ⬃200 K. The curves are offset from 200 pA 共94 K兲 to
1 nA 共212 K兲 for clarity (V SD⫽50 mV).
last may have been due to local contamination of the tubes
by residues from the processing. Finally, the area around the
tubes was thoroughly mapped with the AFM. The sample
was then transferred to the deposition chamber of an aerosol
particle generator where 7 nm Au particles were deposited
onto the sample.11 Once more, the area around the tubes was
mapped with the AFM, and by comparing with the previously recorded images we were able to identify and select a
suitable gold particle.
In order to establish electrical contact to the particle, a
100-nm-long slit, only about 5–10 nm wide, was formed by
the two tubes, and a bias of 500 mV was applied between
them 关Figs. 1共d兲 and 1共e兲兴. As soon as the gold particle was
moved into the slit, current was detected in the system. Figure 1共e兲 was scanned with a very weak tip–sample interaction and shows the particle in its active position. For this
particular case the resistance of the system was 50 M⍀.
The sample was then cooled to 4.2 K. Figure 2共a兲 is a
differential conductance plot (dI/dV SD) as a function of both
bias voltage (V SD) and gate voltage (V G ) for the system. The
plot shows a regular pattern of diamond-shaped areas with
zero conductance. These are regions of Coulomb blockade
corresponding to a charging energy12 of 60⫾5 meV. The
plot suggests that one small island controlled the transport in
the system. This island coupled rather weakly to the gate, as
only six blockade regions show up for a span of 12 V on the
gate. However, there is also a fine structure present that has a
much stronger coupling to the gate 关Fig. 2共b兲兴. This structure
has a smaller energy scale and may be interpreted as charging effects in larger islands. The diamond patterns give directly the gate coupling, ␣ ⫽C G /C ⌺ , where C G is the capacitance between the island and the gate and C ⌺ is the total
capacitance of the island. With 2⌬V SD as the vertical diagonal in a diamond and ⌬V G being the horizontal diagonal, we
have ␣ ⫽⌬V SD /⌬V G . This results in ␣ ⫽0.03 for the large
Thelander et al.
2107
FIG. 3. 共Color兲 共a兲 dI/dV SD plotted as a function of both bias voltage and
gate voltage for the tube–tube system. Brighter areas correspond to a higher
dI/dV SD , blue areas correspond to a negative dI/dV SD . (T⫽4.2 K) 共b兲 A
magnification of the area marked with a rectangle in the previous plot shows
that the fine structure is still present with the same general appearance as in
Fig. 2共b兲.
diamonds 关Fig. 2共a兲兴, while the smaller structure 关Fig. 2共b兲兴
corresponds to ␣ ⬃0.3, i.e., a ten times stronger coupling.
Based on these observations, we attribute the larger blockade
structure to the particle, and the complicated fine structure to
single-electron charging effects in the nanotube leads. This
would be consistent with the difference in gate coupling, as
C G /C ⌺ should be larger for a one-dimensional tube section
compared to a zero-dimensional particle with the current
relative dimensions.
A charging energy of E C ⫽60 meV for the larger blockade areas is in good agreement with an estimation of E C
⫽e 2 /2C S ⫽68 meV, resulting from the self-capacitance, C S
⫽2 ␲ ␧ 0 ␧ r D, of a sphere with a diameter D⫽7 nm 共estimated dielectric constant ␧ r ⬃3兲. The bright diagonal stripes
in Fig. 2共a兲 surround areas with constant charge on the smallest island in the system, and are indications of a strong asymmetry in the resistance of the two tunnel junctions that connect this island.13 Since the sum of the two tunnel resistances
connecting the particle was 50 M⍀, while the upper limit for
the total resistance of the nanotube leads was only 500 k⍀,
this observation supports the suggestion that the gold particle, rather than an intratube section, was the smallest island.
The sample was then slowly warmed to room temperature. Figure 2共c兲 shows that oscillations in the current as a
function of gate voltage persisted for temperatures up to
around 200 K. The sample was transferred back to the AFM
to resume the manipulation. First, the particle was carefully
pushed aside, and then the two nanotube parts were moved
into contact at the point where the particle had been 关Fig.
1共f兲兴. At a resistance of 50 M⍀, the sample was once more
removed from the AFM and cooled to 4.2 K. A dI/dV SD plot,
shown in Fig. 3共a兲, was recorded, but no periodic features
can be observed on the scale of the blockade areas in Fig.
2共a兲. Instead, the plot reveals many areas with negative dif-
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2108
Thelander et al.
Appl. Phys. Lett., Vol. 79, No. 13, 24 September 2001
ferential conductance, indicating discrete energy states in the
tube sections coming into and out of alignment. This was an
effect that seems to have been suppressed when the gold
particle was a part of the system, where only weak traces of
negative dI/dV SD were observed. An explanation could be
that tunneling of electrons between the nanotube leads, from
one discrete energy level to another, was then convoluted
with a quasicontinuum of energy levels in the metal particle.
However, the fine structure is still present in Fig. 3共b兲, with
approximately the same gate coupling and energy scale as
for the tube–particle–tube case. In addition, we see that the
distortion of the Coulomb blockade areas in Fig. 2共a兲 for
positive gate voltages could be related to the irregular behavior of the nanotube contacts in the corresponding area in
Fig. 3共a兲.
Based on these observations, we conclude that the gold
particle was responsible for the large-scale features in Fig.
2共a兲. Despite a complex behavior of the transport in the
nanotube leads, as a result of either the manipulation or of
the fact that 1 ␮m of the tube has a history of being covered
with Cr and Au, the system still shows periodic gate oscillations. Consequently, carbon nanotubes can be used as leads
in a SET without negatively affecting the performance of the
transistor. For room-temperature operation, a charging energy in excess of 100 meV is needed, corresponding to a
particle with a diameter of less than 4 nm. This size range
can be reached with the nanoparticle and cluster fabrication
techniques available today and will not hinder such a required miniaturization. Neither do we expect to be limited by
the contacts, as carbon nanotubes can be fabricated with diameters down to 1 nm. The challenge lies instead in establishing electrical connections between objects on this scale.
In our approach we need to push the limits of scanning-probe
manipulation; an alternative could be to use chemical
methods.14 More importantly, leads of carbon nanotubes may
come to play a key role in many other interesting systems.
The gold particle can be replaced with a magnetic, a semiconducting, or a superconducting particle, or perhaps a molecule. However, interpretation of data obtained from lowtemperature studies of such systems will require a moredetailed understanding of the transport in the nanotube leads.
This work has been supported by grants from the Natural
Science and the Engineering Science Research Councils in
Sweden, the Swedish Foundation for Strategic Research, and
the Danish Natural Science Founation.
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