Single electron transistors (SET) are fabricated from multiwalled carbon nanotubes (MWNT) by manipulation with an atomic force microscope. The devices consist of either a single MWNT with Au contacts at the ends or of two crossing tubes. In the latter device, the lower nanotube acted as the central island of a single electron transistor while the upper one functioned as a gate electrode. Coulomb blockade oscillations were observed on the nanotube at low temperatures. The voltage noise of the nanotube-SET was gain dependent as in conventional SETs. The charge sensitivity at 10 Hz was 6x10 exp -4 e/√Hz. Furthermore, in another device where the MWNT is suspended above the substrate between the electrodes, we measure an extremely high charge sensitivity of 6x 10 exp -6 e/√Hz at 45 Hz, comparable to the best of the conventional SETs.Peer reviewe

Ahlskog, Markus

Hakonen, Pertti J.

Paalanen, Mikko

Roschier, Leif

Tarkiainen, Reeta

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This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.Author(s): Ahlskog, Markus & Tarkiainen, Reeta & Roschier, Leif & Paalanen,Mikko & Hakonen, Pertti J.Title: Multiwalled carbon nanotubes as single electron transistorsYear: 2001Version: Final published versionPlease cite the original version:Ahlskog, Markus & Tarkiainen, Reeta & Roschier, Leif & Paalanen, Mikko & Hakonen,Pertti J. 2001. Multiwalled carbon nanotubes as single electron transistors. AIPConference Proceedings. 590. 141-144. ISBN 0-7354-0032-6 (printed). DOI:10.1063/1.1420074.Rights: © 2001 AIP Publishing. Copyright of AIP Conference Proceedings is the property of American Institute ofPhysics and its content may not be copied or emailed to multiple sites or posted to a listserv without thecopyright holder's express written permission. However, users may print, download, or email articles forindividual use.All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, andduplication or sale of all or part of any of the repository collections is not permitted, except that material maybe duplicated by you for your research use or educational purposes in electronic or print form. You mustobtain permission for any other use. Electronic or print copies may not be offered, whether for sale orotherwise to anyone who is not an authorised user.Powered by TCPDF (www.tcpdf.org)Multiwalled Carbon Nanotubes as SingleElectron TransistorsM. Ahlskog, R. Tarkiainen, L. Roschier, M. Paalanen, and P. HakonenLow Temperature Laboratory, Helsinki University of TechnologyOtakaari 3A, Espoo, FIN-02015 HUT, FinlandAbstract. Single electron transistors (SET) are fabricated from multiwalled carbon nanotubes(MWNT) by manipulation with an atomic force microscope. The devices consist of either asingle MWNT with Au contacts at the ends or of two crossing tubes. In the latter device, thelower nanotube acted as the central island of a single electron transistor while the upper onefunctioned as a gate electrode. Coulomb blockade oscillations were observed on the nanotube atlow temperatures. The voltage noise of the nanotube-SET was gain dependent as in conventionalSETs. The charge sensitivity at 10 Hz was 6x10~4 e/VHz. Furthermore, in another device wherethe MWNT is suspended above the substrate between the electrodes, we measure an extremelyhigh charge sensitivity of 6x10~6 e/VHz at 45 Hz, comparable to the best of the conventionalSETs.INTRODUCTIONCarbon nanotubes are proposed as building blocks of future nanoscale electronicdevices (for a review, see [1]). Single electron transistors (SET) are one possibility,since the Coulomb blockade has been observed especially in devices made from singlewalled nanotubes (SWNT), while in multiwalled nanotubes (MWNT) only very fewresults have been reported [2]. Field effect transistors based on semiconductingnanotubes have been demonstrated, made both from SWNTs and SWNT ropes.The atomic force microscope (AFM) plays often an essential role in the fabricationof nanotube based electronic devices. Besides using the AFM for locating and imagingan individual tube, it can be used more actively for manipulating nanotubes [2]. Ourgroup has developed a method for the manipulation of nanoscale particles using theAFM in non-contact mode. This method has the advantage that manipulation andimaging can be performed simultaneously. It is possible to move with this method amultiwalled nanotube over a distance of several micrometers. It is also possible topush a tube over an obstacle with a height of several tens of nanometers. Armed withthese capabilities we have fabricated two types of nanotube-SETs: One made fromtwo nanotubes where a MWNT has been pushed on top of another (Device 1) and onewhere a MWNT has been pushed on top of two adjacent gold electrodes (Device2).CP590, Nanonetwork Materials, edited by S. Saito et al.© 2001 American Institute of Physics 0-73 54-0032-6/017$ 18.00141Results and DiscussionFigure l(a-c) shows AFM images of how a crossing from two MWNTs separated by afew micrometers is made. The moved MWNT is ultimately positioned on top of theother nanotube. The other end of this MWNT is hanging above the substrate by 30-40nm:s. In Figure l(d) is shown how this "nanotube cross" is contacted with Auelectrodes, resulting in a three terminal device where the upper MWNT functions as agate electrode [3]. We call this device Device 1. In the other type of a device (Device2)a MWNT has been positioned on top of two 25 nm thick Au electrodes very close toeach other. The section of the nanotube between the electrodes is 275 nm:s long and isseparated from the SiOi substrate, that is, the tube is suspended. The diameters of allnanotubes were = 15 nm.The lower nanotube of Device 1 had a room temperature resistance RBOOK = 71 kQ,while the two-point resistance over the crossing was =10 MQ, a value significantlyhigher than those found for the resistance between crossing metallic SWNTs.Furthermore, the zero-bias resistance increased to ~ 1 GQ below 4 K. Thus we couldutilize the upper tube for gating the current in the lower tube. Figure 2 (a) displays IVcurves measured at temperatures from 300 K down to 150 mK. A Coulomb blockadedevelops fully only at subkelvin temperatures, with a gap of about 1 mV at 150 mK.Figure 2 (b) shows the source-drain current I as a function of the gate voltage Vg,applied from the upper tube. From the shape of the Coulomb oscillations in the I vs.Vg curves, it is concluded that RI = RI, where RI and R2 are the junction resistances atthe opposite ends of the nanotube. We estimate [3] the corresponding capacitances asCi = 0.32 fF and C2 = 0.22 fF. We get for the charging energy Ec - 1/2e2/(Ci+C2+Ctube)= 0.14 meV, where we estimate the nanotube self-capacitance as Ctube = 5xlO"17 F.The gate modulation period was measured as AVg = 4 mV. We calculate the gatecapacitance to the upper tube as Cg = e/AVg = 4x10~17 F. A Fourier analysis of the gatemodulation curves in general (including those from a more remote side-gate [3])revealed only one period, indicating the existence of only one island. Particularly, thisimplies that the lower tube is not electrically split into two parts separated by atunneling junction at the point of crossing with the upper nanotube, whereconsiderable mechanical forces between the tubes are known to exist [4].1 \JirnFIGURE 1. (a-c) Manipulation with AFM in non-contact mode to make a nanotube cross, (d) Goldelectrodes deposited on the same nanotubes (Device 1).1422 0100- 1 0- 2 00 . 50 . 3150 m K- 4 - 2 0 2 4U ( m V )- 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4U ( m V );vw\/: 1 . 20 . 80 . 40 . 0V* V: :.*d ) V/U V;0 . 0 0 . 5 1 . 0 1 .- 8 - 4 0 4 8V g ( m V ) V g ( V )FIGURE 2. (a) IV characteristics of Device 1. (b) Gate modulation from the upper tube of the currentof Device 1 (at U = 0.4 mV) (c) IV characteristics of Device2 at T = 150 mK (d) Gate modulation ofDevice2, applied from a side gate. A and B refer to the points of maximum and minimum gain wherethe noise data of Fig. 3(b) was measured .The IV characteristics of Device2 is shown in Fig. 2 (c). The nanotube had a roomtemperature resistance of 28 kQ. As opposed to the usual case of a Coulomb blockadeat low voltages (and low temperatures), this nanotube exhibited increased conductancearound zero bias, which we attribute to resonant tunnelling. Two weakly quantizedsteps are seen, therefore this tube can not be said to be fully ballistic. The ballisticityof freestanding samples are likely to be enhanced for several reasons; besides theabsence of contact with impurity states of the surface, the plasmon speed, which issensitive to the permittivity of the substrate, is increased. We measured a totalresistance of = 40 kQ outside the Coulomb blockade regime. The junction resistanceof the nanotube-Au contacts are thus less than the quantum of resistanceRq = 26 kQ, which means that the Coulomb blockade can not fully develop.Consequently, the Coulomb oscillations that we measure are smoothened.We have measured the low frequency noise characteristics of these nanotubedevices as charge detectors. The frequency dependence of the cross was roughly 1/fwhile Device2 had more of a 1/f2 character (f < 50 Hz). The current noise wasmeasured for Device 1 over one period of the gate modulation curve (at a bias of Ub =0.4 mV), The 1/f noise at 10 Hz over one gate modulation period is shown in Fig. 3(a).As expected for a SET, the noise level varied with the gain of the nanotube device.The input equivalent charge noise qn is obtained from the measured current noise inaccording to the formula qn= Cgin/(9I/9Vg). We obtain as the minimum charge noise at10 Hz 6x10~4 e/VHz (using in = 1 pA, 9I/5Vg = 3.5 nA/V) which corresponds to atypical value for a metallic SET device.1431CfID'13-2.2 -2.1 -2.0 -1.9 -1.8 -1.72x10"*4 6 8 1 0 2 0 4 0f(Hz)FIGURE 3. (a) Current noise (at 10 Hz) of Device 1 measured over one Coulomb blockade oscillation.The up- and down-arrows refer to maximum and minimum gains, respectively, (b) Current noise ofDevice2 measured at minimum (lower) and maximum gain (upper trace). The right axis gives theequivalent charge noise 8# for the upper trace.Similar modulation of the noise was seen in Device2. At a frequency of 45 Hz weobtain the charge noise qn as 6x10~6 eA/Hz, which is comparable to the best metallicSET devices reported to date [5]. Theoretically the minimum noise level for a SET isSQmin = /JCAfRq/Rr, where C is the total capacitance, Af the frequency range and RT isthe tunneling resistance [6]. Taking Rq/4RT = 1 and assuming no cotunneling, weobtain the minimum noise as IxlO"6 e/VHz. This implies that white noise woulddominate over 1/f noise above 3 kHz.In summary, we have demonstrated the good performance of multiwalled carbonnanotubes as building blocks of nanoscale electronic devices. Under properconditions, such as separating the tube from the substrate, it is possible to minimizethe noise level and in other aspects as well to approach the theoretical limits ofperformance.ACKNOWLEDGMENTSWe thank C. Journet and P. Bernier from Universite Montpellier II for supplying uswith the nanotube material and F. Hekking, E. Sonin, and A. Zaikin for usefuldiscussions.REFERENCES1. Nygard J. et al., Appl Phys. A 69, 297 (1999).2. Roschier L. et al., Appl Phys. Lett. 75, 728 (1999).3. Ahlskog M. et al., Appl Phys. Lett. 77, 4037 (2000).4. Hertel T., Walkup R., and Avouris P., Phys. Rev. B 58, 13870 (1998).5. Krupenin V.A. et al., J. Low Temp. Phys. 118, 287 (2000).6. Korotkov A. N. et al. in Single-Electron Tunneling and Mesoscopic Physics, editedby H. Koch and H. Lubbig, Springer, Berlin, 1992, p. 45.144Copyright of AIP Conference Proceedings is the property of American Institute of Physics and its content maynot be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express writtenpermission. However, users may print, download, or email articles for individual use.

Multiwalled carbon nanotubes as single electron transistors

Aaltodoc

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail.
Author(s): Ahlskog, Markus & Tarkiainen, Reeta & Roschier, Leif & Paalanen,
Mikko & Hakonen, Pertti J.
Title: Multiwalled carbon nanotubes as single electron transistors
Year: 2001
Version: Final published version
Please cite the original version:
Ahlskog, Markus & Tarkiainen, Reeta & Roschier, Leif & Paalanen, Mikko & Hakonen,
Pertti J. 2001. Multiwalled carbon nanotubes as single electron transistors. AIP
Conference Proceedings. 590. 141-144. ISBN 0-7354-0032-6 (printed). DOI:
10.1063/1.1420074.
Rights: © 2001 AIP Publishing. Copyright of AIP Conference Proceedings is the property of American Institute of
Physics and its content may not be copied or emailed to multiple sites or posted to a listserv without the
copyright holder's express written permission. However, users may print, download, or email articles for
individual use.
All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and
duplication or sale of all or part of any of the repository collections is not permitted, except that material may
be duplicated by you for your research use or educational purposes in electronic or print form. You must
obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or
otherwise to anyone who is not an authorised user.
Powered by TCPDF (www.tcpdf.org)
Multiwalled Carbon Nanotubes as Single
Electron Transistors
M. Ahlskog, R. Tarkiainen, L. Roschier, M. Paalanen, and P. Hakonen
Low Temperature Laboratory, Helsinki University of Technology
Otakaari 3A, Espoo, FIN-02015 HUT, Finland
Abstract. Single electron transistors (SET) are fabricated from multiwalled carbon nanotubes
(MWNT) by manipulation with an atomic force microscope. The devices consist of either a
single MWNT with Au contacts at the ends or of two crossing tubes. In the latter device, the
lower nanotube acted as the central island of a single electron transistor while the upper one
functioned as a gate electrode. Coulomb blockade oscillations were observed on the nanotube at
low temperatures. The voltage noise of the nanotube-SET was gain dependent as in conventional
SETs. The charge sensitivity at 10 Hz was 6x10~4 e/VHz. Furthermore, in another device where
the MWNT is suspended above the substrate between the electrodes, we measure an extremely
high charge sensitivity of 6x10~6 e/VHz at 45 Hz, comparable to the best of the conventional
SETs.
INTRODUCTION
Carbon nanotubes are proposed as building blocks of future nanoscale electronic
devices (for a review, see [1]). Single electron transistors (SET) are one possibility,
since the Coulomb blockade has been observed especially in devices made from single
walled nanotubes (SWNT), while in multiwalled nanotubes (MWNT) only very few
results have been reported [2]. Field effect transistors based on semiconducting
nanotubes have been demonstrated, made both from SWNTs and SWNT ropes.
The atomic force microscope (AFM) plays often an essential role in the fabrication
of nanotube based electronic devices. Besides using the AFM for locating and imaging
an individual tube, it can be used more actively for manipulating nanotubes [2]. Our
group has developed a method for the manipulation of nanoscale particles using the
AFM in non-contact mode. This method has the advantage that manipulation and
imaging can be performed simultaneously. It is possible to move with this method a
multiwalled nanotube over a distance of several micrometers. It is also possible to
push a tube over an obstacle with a height of several tens of nanometers. Armed with
these capabilities we have fabricated two types of nanotube-SETs: One made from
two nanotubes where a MWNT has been pushed on top of another (Device 1) and one
where a MWNT has been pushed on top of two adjacent gold electrodes (Device2).
CP590, Nanonetwork Materials, edited by S. Saito et al.
© 2001 American Institute of Physics 0-73 54-0032-6/017$ 18.00
141
Results and Discussion
Figure l(a-c) shows AFM images of how a crossing from two MWNTs separated by a
few micrometers is made. The moved MWNT is ultimately positioned on top of the
other nanotube. The other end of this MWNT is hanging above the substrate by 30-40
nm:s. In Figure l(d) is shown how this "nanotube cross" is contacted with Au
electrodes, resulting in a three terminal device where the upper MWNT functions as a
gate electrode [3]. We call this device Device 1. In the other type of a device (Device2)
a MWNT has been positioned on top of two 25 nm thick Au electrodes very close to
each other. The section of the nanotube between the electrodes is 275 nm:s long and is
separated from the SiOi substrate, that is, the tube is suspended. The diameters of all
nanotubes were = 15 nm.
The lower nanotube of Device 1 had a room temperature resistance RBOOK = 71 kQ,
while the two-point resistance over the crossing was =10 MQ, a value significantly
higher than those found for the resistance between crossing metallic SWNTs.
Furthermore, the zero-bias resistance increased to ~ 1 GQ below 4 K. Thus we could
utilize the upper tube for gating the current in the lower tube. Figure 2 (a) displays IV
curves measured at temperatures from 300 K down to 150 mK. A Coulomb blockade
develops fully only at subkelvin temperatures, with a gap of about 1 mV at 150 mK.
Figure 2 (b) shows the source-drain current I as a function of the gate voltage Vg,
applied from the upper tube. From the shape of the Coulomb oscillations in the I vs.
Vg curves, it is concluded that RI = RI, where RI and R2 are the junction resistances at
the opposite ends of the nanotube. We estimate [3] the corresponding capacitances as
Ci = 0.32 fF and C2 = 0.22 fF. We get for the charging energy Ec - 1/2e2/(Ci+C2+Ctube)
= 0.14 meV, where we estimate the nanotube self-capacitance as Ctube = 5xlO"17 F.
The gate modulation period was measured as AVg = 4 mV. We calculate the gate
capacitance to the upper tube as Cg = e/AVg = 4x10~17 F. A Fourier analysis of the gate
modulation curves in general (including those from a more remote side-gate [3])
revealed only one period, indicating the existence of only one island. Particularly, this
implies that the lower tube is not electrically split into two parts separated by a
tunneling junction at the point of crossing with the upper nanotube, where
considerable mechanical forces between the tubes are known to exist [4].
1 \Jirn
FIGURE 1. (a-c) Manipulation with AFM in non-contact mode to make a nanotube cross, (d) Gold
electrodes deposited on the same nanotubes (Device 1).
142
2 0
10
0
- 1 0
- 2 0
0 . 5
0 . 3
150 m K
- 4 - 2 0 2 4
U ( m V )
- 0 . 4 - 0 . 2 0 . 0 0 . 2 0 . 4
U ( m V )
;vw\/: 1 . 20 . 80 . 4
0 . 0
V* V: :
.*
d ) V/U V;
0 . 0 0 . 5 1 . 0 1 .
- 8 - 4 0 4 8
V g ( m V ) V g ( V )
FIGURE 2. (a) IV characteristics of Device 1. (b) Gate modulation from the upper tube of the current
of Device 1 (at U = 0.4 mV) (c) IV characteristics of Device2 at T = 150 mK (d) Gate modulation of
Device2, applied from a side gate. A and B refer to the points of maximum and minimum gain where
the noise data of Fig. 3(b) was measured .
The IV characteristics of Device2 is shown in Fig. 2 (c). The nanotube had a room
temperature resistance of 28 kQ. As opposed to the usual case of a Coulomb blockade
at low voltages (and low temperatures), this nanotube exhibited increased conductance
around zero bias, which we attribute to resonant tunnelling. Two weakly quantized
steps are seen, therefore this tube can not be said to be fully ballistic. The ballisticity
of freestanding samples are likely to be enhanced for several reasons; besides the
absence of contact with impurity states of the surface, the plasmon speed, which is
sensitive to the permittivity of the substrate, is increased. We measured a total
resistance of = 40 kQ outside the Coulomb blockade regime. The junction resistance
of the nanotube-Au contacts are thus less than the quantum of resistance
Rq = 26 kQ, which means that the Coulomb blockade can not fully develop.
Consequently, the Coulomb oscillations that we measure are smoothened.
We have measured the low frequency noise characteristics of these nanotube
devices as charge detectors. The frequency dependence of the cross was roughly 1/f
while Device2 had more of a 1/f2 character (f < 50 Hz). The current noise was
measured for Device 1 over one period of the gate modulation curve (at a bias of Ub =
0.4 mV), The 1/f noise at 10 Hz over one gate modulation period is shown in Fig. 3(a).
As expected for a SET, the noise level varied with the gain of the nanotube device.
The input equivalent charge noise qn is obtained from the measured current noise in
according to the formula qn= Cgin/(9I/9Vg). We obtain as the minimum charge noise at
10 Hz 6x10~4 e/VHz (using in = 1 pA, 9I/5Vg = 3.5 nA/V) which corresponds to a
typical value for a metallic SET device.
143
1Cf
ID'13
-2.2 -2.1 -2.0 -1.9 -1.8 -1.7
2x10"*
4 6 8 1 0 2 0 4 0
f(Hz)
FIGURE 3. (a) Current noise (at 10 Hz) of Device 1 measured over one Coulomb blockade oscillation.
The up- and down-arrows refer to maximum and minimum gains, respectively, (b) Current noise of
Device2 measured at minimum (lower) and maximum gain (upper trace). The right axis gives the
equivalent charge noise 8# for the upper trace.
Similar modulation of the noise was seen in Device2. At a frequency of 45 Hz we
obtain the charge noise qn as 6x10~6 eA/Hz, which is comparable to the best metallic
SET devices reported to date [5]. Theoretically the minimum noise level for a SET is
SQmin = /JCAfRq/Rr, where C is the total capacitance, Af the frequency range and RT is
the tunneling resistance [6]. Taking Rq/4RT = 1 and assuming no cotunneling, we
obtain the minimum noise as IxlO"6 e/VHz. This implies that white noise would
dominate over 1/f noise above 3 kHz.
In summary, we have demonstrated the good performance of multiwalled carbon
nanotubes as building blocks of nanoscale electronic devices. Under proper
conditions, such as separating the tube from the substrate, it is possible to minimize
the noise level and in other aspects as well to approach the theoretical limits of
performance.
ACKNOWLEDGMENTS
We thank C. Journet and P. Bernier from Universite Montpellier II for supplying us
with the nanotube material and F. Hekking, E. Sonin, and A. Zaikin for useful
discussions.
REFERENCES
1. Nygard J. et al., Appl Phys. A 69, 297 (1999).
2. Roschier L. et al., Appl Phys. Lett. 75, 728 (1999).
3. Ahlskog M. et al., Appl Phys. Lett. 77, 4037 (2000).
4. Hertel T., Walkup R., and Avouris P., Phys. Rev. B 58, 13870 (1998).
5. Krupenin V.A. et al., J. Low Temp. Phys. 118, 287 (2000).
6. Korotkov A. N. et al. in Single-Electron Tunneling and Mesoscopic Physics, edited
by H. Koch and H. Lubbig, Springer, Berlin, 1992, p. 45.
144


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not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written
permission. However, users may print, download, or email articles for individual use.


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