The document discusses the manipulation of individual carbon nanotubes using an atomic force microscope to create nanodevices, including field-effect transistors (FETs) and rings. It highlights the electrical properties of these nanotubes and presents techniques for producing high-yield rings of single-wall nanotubes. The results indicate significant control over the nanotubes' position and properties, enabling advanced applications in nanoelectronics.

1. E L S E V I E R Microelectronic Engineering 46 (1999) 101-
104
Manipulation of Carbon Nanotubes and Properties of Nanotube
Field-Effect Transistors and
Rings
IvIICE:)~-~'CTIROlt~
H.R. Shea, R. Martel, T. Hertel, T. Schmidt, and Ph. Avouris*
I B M Research Division, T.J. Watson Center, P.O. Box 218,
Yorktown Heights, NY 10598, U.S.A.
Using the tip o f an atomic force microscope, we have
manipulated individual carbon nanotubes on a patterned
substrate, and have fabricated model nanodevices, including a
room temperature field-effect transistor with a
channel only 1.6 nm wide, as well as single-electron transistors.
W e have also developed a technique to produce
rings o f single-wall nanotubes with a very high yield.
1. I N T R O D U C T I O N
Carbon nanotubes (NT) are a novel class of
nanostructures consisting of one or several graphene
sheets rolled into a single seamless hollow cylinder,
or several concentric cylinders, respectively.
Depending on the width o f the graphene sheet and
the angle at which it is rolled, the N T can be either a
semiconductor or a metal. In addition to these
intriguing electrical properties, nanotubes have a

2. very high tensile strength and are extremely rigid.
Because o f these characteristics, a great number o f
applications for NTs have been proposed, such as
1D nanowires and switching elements in nano-
devices [1].
In this paper we discuss first the manipulation o f
nanotubes on a patterned substrate, then field-effect
transistors (FETs) and single-electron transistors
(SETs) in which nanotubes are the channel. Finally
we demonstrate the capability o f making rings o f
nanotubes.
2. M A N I P U L A T I O N
There is a strong Van der Waals attraction
between nanotubes, and between nanotubes and the
substrate they are deposited upon. Because o f the
high binding energy with the substrate (~0.8 eV//~
for a 1 0 0 / ~ diameter multi-wall tube [2]) the tube
can be pinned in a highly strained (bent)
configuration despite its high Young's modulus (~1
TPa). W e can manipulate the N T position at room
temperature by applying lateral forces with the tip o f
an atomic force microscope (AFM). W e found the
shear stress on surfaces such as H-passivated silicon
is high, of the order o f 10 7 N/m, such that not only
the position but also the shape o f the N T can be
controlled [2].
a ) - ~ { ~ , " b ) , , :i:: ::'
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3. Fig. 1: AFM manipulation o f a single multi-wall
nanotube. Initially the N T is located on an insulating
part o f the sample. In a stepwise fashion, it is
dragged onto the 80 A high metal thin film wire, and
finally stretched across the oxide barrier.
0167-9317/99/$ - see front matter © 1999 Elsevier Science B.V.
All fights reserved.
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I02 H.R. Shea et al. / Microelectronic Engineering 46 (1999)
101-104
To perform the manipulation, we alternate
between the non-contact mode of the AFM for
imaging without moving the tubes, and the contact
mode for displacing the tubes. While the forces in
non-contact mode are very small (in the pN range),
vertical loads of 10-50 nN are necessary to move the
tube.
Fig. 1 is a sequence of manipulation steps to
position a multi-wall NT over an oxide barrier. The
tube is moved from an insulating substrate (SiO2)
onto a tungsten thin film wire (~80 • high) and then
stretched across an insulating WOx barrier. Such a
setup enables us to perform transport measurements
of the NT.
3. F I E L D - E F F E C T T R A N S I S T O R
We have used both single-wall and multi-wall
nanotubes (SWNTs and MWNTs) as the channel of
an FET. The device consists of a single nanotube

4. bridging two Au electrodes deposited on a 140 nm
thick gate oxide film over a doped Si wafer, which is
used as a back gate. The source-drain current Isd
between the electrodes was measured as a function
of source-drain voltage V~d and gate-source voltage
Vg.
Nanotube
50
4O
20
10
0
- i | i i | i i I -
V ~ = ] O 0 m V 10-61 - , , , ' ' 1
i,~O~o ~" Io-~I- t 4
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¢ ~ . . . ~ l b 2 ~ - 2 0 2 4 6
~ ~ vo {v)
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vQ (v)
Fig. 2: A field-effect transistor based on a single
nanotube.

5. The top of Fig. 2 is a schematic diagram of the
nanotube FET. The lower section shows the room
temperature transfer characteristics Isd-V~ for several
values of Vsa for a 1.6 nm diameter SWNT. We
observe clear transistor action. For Vg<0 the Isd-V~d
curves (not shown) are linear, but become
increasingly non-linear for Vg>0, up to a point where
the current becomes immeasurably small for Vg>>0.
The inset in Fig. 2 shows the FET conductance vs.
Vg: it can be modulated by over 5 orders of
magnitude.
The nanotube FET behaves very much like a p-
channel MOSFET, and transport is dominated by
holes. The saturation at negative Vg is in part due to
the large (~ 1 MD) contact resistance.
A key issue is the origin of the holes: one
possibility is that the carrier concentration is inherent
to the nanotube. Another possibility is that most of
the holes are injected at the gold-NT contact because
of the difference in work function between the two
materials, as suggested by Tans et al. [3]. Our results
favor the first explanation, and we determine the 1D
hole density to be 9x106 cm l . This value
corresponds to 1 hole per 250 carbon atoms,
suggesting the tube is degenerate and/or highly
doped, perhaps as a results of its processing [4].
Devices made with MWNTs rather than SWNTs
typically exhibit no gate action. However, we have
successfully used deformed MWNTs as the channel
of a room temperature FET, although with a
conductance modulation only of order 2 [4].

6. 4. S I N G L E - E L E C T R O N T R A N S I S T O R
Because of their very small size, and the high
contact resistance, our FET devices exhibit Coulomb
blockade behavior at low temperatures, when the
charging energy e2/2C to add an electron to the tube
is much larger than kBT, where C is the capacitance
of the tube, and T temperature [5]. Fig. 3 is a plot of
Isa vs. Vg of a bundle of SWNT draped over two gold
electrodes 0.4 gm apart at 290 K, 77 K, and 4 K for
Vsa = 20 mV, 50 mV, and 5 mV, respectively. The
insets are Isd-Vsa curves at selected gate voltages.
The 290 K data show FET behavior and linear I-
V curves, with a minimum device resistance of 250
kD. At 4 K, the I-V curves have a pronounced
Coulomb gap, and there is very sharp nonmonotonic
structure in I~d vs. Vg corresponding to the lining up
of states in the tube with the Fermi level of one of
H.R. Shea et al. I Microelectronic Engineering 46 (1999) 101-
104 103
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290K Jl
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4 0 o 4 0 ~ , . . j
0 o L I V s d ( I m V ) , I I " I I ~
-10 -5 0 5 10
Gate Voltage (V)

7. 100J - I /~ ET'emperatur'e = 77K I ' l- 'I . , - " A i
v 8 0 ~ 40
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40~-- I ~ / ' ~ - I'AI~ - , I , l [ H
, v11 ,o,oo 4o I 2 ~mV) O
-20 -15 -10 -5 0 5
Gate Voltage (V)
' b I ' ,
, v , , , ,,
20 -40 o 40
' I I V~d ( m Y ) 0 I .U,,,
-20 -16 -12 -8
Gate Voltage (V)
Fig. 3: Isd-Vg curves for a bundle o f S W N T device at
290 K, 77 K, and 4.2 K. The device behaves like an
FET at 290 K, and like an SET at 4.2 K.
the leads. At 4 K, the device acts as a single-electron
transistor. The 77 K data show features due to both
FET and Coulomb blockade behavior.
Data at 4 K from other of our S W N T devices
also show clear structure corresponding to adding
one electron at a time to the tube. Each diamond in
the grayscale dI/dVsd vs. Vg and Vsd plots, as shown
in Fig. 4a, corresponds to changing the number o f
electrons on the tube by one. For a single metallic
island, the diamonds all have the same shape. In our
nanotube SETs, the Coulomb gap oscillates

8. aperiodically with Vg, which is suggestive of single-
electron transport through multiple Coulomb islands
formed within the N T bundle.
The depletion o f carriers in the tube, and the
associated decrease in tube capacitance, are clearly
visible in Fig. 4b as an increase in the width o f the
blockade region at large positive gate voltage.
It has been observed by several groups that only
a small section o f some nanotubes act as a Coulomb
island [6]. One can determine the charging energy o f
a length o f N T by calculating its capacitance.
Alternatively, one can infer the length o f the part o f
the tube that is charging from the measured charging
energy. We always observe that the length o f N T that
is charging as determined from the measured
charging energy is less than the spacing between the
electrodes by a factor o f between 1.5 and 20. This
behavior is probably attributable to multiple islands
formed within a N T bundle, and possibly also to
barriers formed along the tube due to bending o f the
tube [6,7].
a)
>~
b)
3 -
2 -
1

9. 0
- 1
-2
-3
-10 -5 0 5 10
V,o (mV)
>~
- 2 0 0 - 1 0 0 0 1 0 0 2 0 0
%° (mY)
Fig. 4: Grayscale dI/dVsd vs. Vg and Vsd plots o f
tWO different single S W N T devices at 4 K. Lighter
gray corresponds to lower conductance.
5. R I N G S O F N A N O T U B E S
Because o f the strong van der Waals adhesion
force between tubes, it is possible to form well
defined rings (coils) o f single-wall nanotubes, where
104 H.R. Shea et al. / Microelectronic Engineering 46 (1999)
101-104
the strain energy to bend the tubes (proportional to
1/r 2, where r is the radius of curvature) is more than

10. offset by the bonding energy o f the tube to itself
(proportional to the length of overlap of the tube
with itself). W e have developed a technique that
forms rings with a ~50% yield starting with a
solution of straight bundles o f SWNTs. The process
is kinetically limited, and the average ring radius is
350 nm for bundles 3-4 g m in length. From TEM
and AFM images, we find that the rings walls have a
radius of between 5 and 30 nm.
Fig. 5: SEM micrograph of N T rings.
Fig. 6: AFM micrograph of a N T ring draped over
three gold electrodes.
The SEM image in Fig. 5 illustrates the high
yield o f rings produced by our procedure. The AFM
micrograph in Fig. 6 shows an N T ring deposited
over three gold electrodes. Preliminary measure-
ments o f the temperature dependence o f the ring
conductance indicate the ring is metallic, with
resistance decreasing from 15 k D at 4 K to 10 kf2 at
290 K. We see no charging effects even at 4 K.
As shown in Fig. 7, we observe conductance
fluctuations o f order e2/h in the ring at 4 K. Further
analysis is required to confirm the universal nature o f
these fluctuations. Should the conductance fluc-
tuations be universal, the data then suggest that the
phase coherence length o f the nanotubes is only
slightly smaller than the ring dimension.
1.5
o?, 1.4

11. " o
c 1.3
o
o
1.2
. . . . ! . . . . i . . . . i . . . .
. . , a i . . . . i . . . . I . • • •
-15 -10 -5 0 5
vg (v)
Fig. 7: Ring conductance in units of e2]h vs. Vg at 4
K for Vsd = 1 and 5 mV, showing conductance
fluctuations o f a probably universal character.
We thank A.G. Rinzler, R.E. Smalley, and H. Dai for
providing us with the single- and multi-wall
nanotubes.
REFERENCES
* e-mail address: [email protected]
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3. S. J. Tans, A.R.M. Verschueren, and C. Dekker,
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12. 4. R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and
Ph. Avouris, in press, Appl. Phys. Lett., Oct.1998
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mena in Nano-structures, N A T O ASI Series, vol.
294, Plenum Press, N Y 1992
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