We report the effects of potassium doping on the conductance of individual semiconducting single-walled carbon nanotube ropes. We are able to control the level of doping by reversibly intercalating and de-intercalating potassium. Potassium doping changes the carriers in the ropes from holes to electrons. Typical values for the carrier density are found to be ∼100–1000 electrons/μm. The effective mobility for the electrons is μeff∼20–60 cm2 V-1 s-1, a value similar to that reported for the hole effective mobility in nanotubes [R. Martel et al., Appl. Phys. Lett. 73, 2447 (1998)]

Bockrath, Marc

Hone, J.

McEuen, Paul L.

Rinzler, Andrew G.

Smalley, Richard E.

Zettl, A.

English

Caltech Authors - Main

RAPID COMMUNICATIONS
PHYSICAL REVIEW B 15 APRIL 2000-IIVOLUME 61, NUMBER 16Chemical doping of individual semiconducting carbon-nanotube ropes
Marc Bockrath, J. Hone, A. Zettl, and Paul L. McEuen
Department of Physics, University of California, Berkeley, California 94720
and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, 94720
Andrew G. Rinzler and Richard E. Smalley
Center for Nanoscale Science and Technology, Rice Quantum Institute,
and Department of Chemistry and Physics, MS-100, Rice University, P.O. Box 1892, Houston, Texas 77251
~Received 1 November 1999!
We report the effects of potassium doping on the conductance of individual semiconducting single-walled
carbon nanotube ropes. We are able to control the level of doping by reversibly intercalating and de-
intercalating potassium. Potassium doping changes the carriers in the ropes from holes to electrons. Typical
values for the carrier density are found to be ;100–1000 electrons/mm. The effective mobility for the
electrons is meff;20–60 cm2 V21 s21, a value similar to that reported for the hole effective mobility in
nanotubes @R. Martel et al., Appl. Phys. Lett. 73, 2447 ~1998!#.Recently, much attention has been focused on the trans-
port properties of ;1 –2 nm diam single-walled carbon
nanotubes ~SWNT’s!. The band structure of a given SWNT
depends on its chirality and is either a one-dimensional ~1D!
metal or a 1D semiconductor with a band gap on the order of
0.6 eV.2,3 Recent work has shown that semiconducting nano-
tubes can exhibit transistor action that is similar in operation
to a MOSFET.1,4 In these experiments, a gate electrode is
used to electrostatically vary the charge density in the nano-
tubes. The dependence of the conductance on the carrier den-
sity indicates that the carriers added to the tubes are holes.
Besides using a gate, another way to change the charge
density in a semiconductor is by chemical doping. Previous
work on mats of SWNT’s has shown that potassium acts as
an electron donor to the nanotubes5 and that their conduc-
tance is increased significantly by doping with potassium
vapor.6,7 However, the interpretation of these experiments is
complicated by the fact that mats consist of both semicon-
ducting and metallic nanotubes, and that transport through
the mats includes hopping between nanotubes and/or
bundles.
Here we report the controlled chemical doping of indi-
vidual semiconducting nanotube ropes with potassium. This
doping changes the carriers in the ropes from holes to elec-
trons. We estimate the number of carriers added and hence
the number of intercalated potassium atoms. In addition we
determine an effective mobility meff for the electrons, which
is found to be similar in magnitude to the value reported for
holes.1
The samples are made by placing ropes on top of Au
electrodes that have 500 nm separation8 as shown in the inset
of Fig. 1. These electrodes are on top of a degenerately
doped oxidized Si wafer.9 The degenerately doped substrate
is used as a gate. Once the samples are made, semiconduct-
ing and metallic devices are distinguished by their room tem-
perature conductance behavior with respect to gate voltage.
The conductance of metallic tubes shows relatively little de-
pendence on gate voltage, whereas the conductance of semi-
conducting nanotubes shows a very strong dependence.4,10PRB 610163-1829/2000/61~16!/10606~3!/$15.00An example of semiconducting behavior is shown in Fig. 2.
The open circles in Fig. 2 show the conductance of a 3 nm
diam rope as a function of gate voltage. At positive gate
voltages, the conductance of the rope approaches zero. As a
negative gate voltage is applied, the conductance begins to
increase as holes are added to the rope.4 This increase in
conductance is approximately linear in gate voltage. A simi-
lar slope has been found in all the samples we have studied
to within a factor of 3.
Once we have obtained samples that show the above
semiconducting behavior, we dope them with potassium. To
do this, we place the sample in a glass doping vessel, which
consists of a glass tube with wire feedthroughs, as shown in
the inset of Fig. 1. The vessel is first attached to a pump and
heated in vacuum at 200 °C to remove moisture and other
FIG. 1. Main panel: Conductance of potassium-doped semicon-
ducting sample vs time. At point A heat was applied to the sample
to de-intercalate potassium from the doped sample. At point B heat-
ing was stopped temporarily. At point C further heating resulted in
the sample becoming insulating. Heat was then applied to the po-
tassium. At point D, the vapor pressure of the potassium is suffi-
cient to begin doping the sample. At point E, heating is stopped and
the entire vessel begins to return to room temperature. Inset: dia-
gram of doping vessel and device geometry.R10 606 ©2000 The American Physical Society
RAPID COMMUNICATIONS
PRB 61 R10 607CHEMICAL DOPING OF INDIVIDUAL . . .volatile impurities. It is then transferred to an inert atmo-
sphere glove box, where a small amount of potassium is
placed into the tube. Finally, the vessel is removed from the
glove box, pumped out, and sealed with a torch. Doping is
accomplished by differentially heating the two ends of the
vessel to temperatures Ts ~for the sample end! and TK ~for
the potassium end!. We have performed doping experiments
on several samples; all yielded similar results. However, all
of the data shown here are taken from s single sample.
While heating the sample in vacuum, we find that the
conductance of undoped semiconducting nanotubes de-
creases dramatically until they become insulating. Although
we do not fully understand this behavior, we have considered
two possibilities. One is that the semiconducting nanotubes
become insulating as a result of changes in the local electro-
static environment of the nanotube ~e.g., neutralization of
trapped oxide charge!. Another possibility is that they may
be doped by molecular species that are adsorbed from the air,
which are removed by heating in vacuum. A more complete
understanding of this will require further work.
Following the bake-out, we have found that heating the
potassium briefly with a flame causes the conductance of
semiconducting nanotubes to increase from zero to a finite
value. Since the potassium donates electrons to the nano-
tubes one expects to be able to observe n-type behavior in
the doped nanotubes. Indeed, we find that this is the case,
as shown in Fig. 2. The data plotted with filled squares in
Fig. 2 show the conductance of the doped rope as a function
of gate voltage after exposure to potassium vapor. Unlike
what is observed for intrinsic nanotubes, the conductance of
the doped nanotubes increases with increasing gate voltage.
This implies that the carriers are electrons, rather than
holes.11
We have found that this doping is reversible. The main
FIG. 2. Open circles: Conductance vs gate voltage for an un-
doped nanotube rope. The conductance increases with decreasing
gate voltage, indicating p-type behavior. The left inset shows a
schematic band-structure diagram corresponding to this situation.
Filled squares: Conductance of a different sample after doping with
potassium. Filled circles: same sample after a cycle of de-
intercalation and re-intercalation at a lower doping level. The con-
ductance increases with increasing gate voltage, indicating n-type
behavior. The right inset shows a schematic band-structure diagram
corresponding to this situation.panel of Fig. 1 shows the conductance of a semiconducting
sample after the initial potassium doping. No gate voltage is
applied, unless otherwise noted. At point A, the conductance
is relatively stable at a value of ’0.6 mS. At this point, we
applied heat to the sample (Ts;400 K!. As time evolves, the
sample becomes less conductive as the potassium is de-
intercalated and electrons are removed.6,7 We stopped heat-
ing the sample at point B, at which time the conductance
begins to stabilize at a value of ;0.1 mS. Further heating
caused the sample to become insulating ~not shown!, imply-
ing that the majority of the intercalated potassium has been
removed.
To add potassium to the sample in a controlled way, we
heated both the sample and the potassium simultaneously
using the heaters. Throughout this procedure, the sample was
maintained at a slightly higher temperature than the potas-
sium (Ts2TK;20 K! to avoid gross deposition of potassium
on the sample. At point C, TK5400 K, while Ts5420 K. At
this point, the vapor pressure of the potassium is insufficient
to dope the sample, and it remains insulating. At point D,
TK5450 K, and the vapor pressure of the potassium be-
comes large enough to begin doping the sample. As time
evolves, the conductance increases. At point E both the
sample and the potassium are allowed to cool to room tem-
perature. Upon cooling down from point E, the conductance
decreases until the temperature stabilizes at room tempera-
ture, and the conductance stabilizes at G;0.12 mS. The
filled circles in Fig. 2 show the conductance of the sample as
a function of gate voltage after this procedure. At negative
gate voltages, the sample is insulating. At a threshold voltage
V th;23.8 V, the conductance begins to increase, rising ap-
proximately linearly with gate voltage.
We now discuss these results. First, we estimate the num-
ber of potassium atoms added to the sample. When the gate
voltage is zero, the charge density in the ropes is determined
by their local electrostatic environment, which includes the
ionized potassium donors. If we make the assumption that
when the device is off that the electrons in the rope are
depleted, then the electron density ne at Vg50 will be given
by ene5CLuV thu. Here, CL is the capacitance per unit length
of the rope to the gate electrode and V th is the threshold
voltage at which the device conducts. Measurements of me-
tallic nanotubes in a similar geometry in the Coulomb block-
ade regime give a typical value CL;20 e2/V mm53.2
310212 F/m.9 With the approximation that at Vg50 each
potassium atom donates an electron to the rope, we have
nK;ne . Thus from the data shown in Fig. 2 as filled circles
we conclude that nK;100 mm21. For the higher conduc-
tance data ~filled squares! we estimate that the number of
carriers is nK;700 mm21 from the ratio of the conductance
between the two doping levels at Vg50. These doping levels
correspond to a few potassium atoms per 1000 carbon atoms.
In comparison to the KC8 stoichiometric doping levels re-
ported by Lee et al. and Grigorian et al.,6,7 this is therefore,
very light doping.
We note that similar to the case of p-type semiconducting
nanotubes,1 the increase in conductance is approximately lin-
ear in gate voltage. We can thus determine an effective mo-
bility for the carriers: meff5L2/C dG/dVg , where L is the
length of the sample, C is the capacitance, G is the conduc-
tance, and Vg is the gate voltage. For the undoped sample in
RAPID COMMUNICATIONS
R10 608 PRB 61MARC BOCKRATH et al.Fig. 2 we find that the hole mobility is given by meff;60
cm2 V21 s21 in agreement with values reported by others.1
For the doped sample, the higher conductance data in Fig. 2
~solid squares! yield meff;20 cm2 V21 s21, while the lower
conductance data ~filled circles! yield meff;60 cm2 V21 s21.
These values for the electron effective mobility are thus
similar to typical values for the hole effective mobility. This
indicates that the scattering mechanism for electrons and
holes is likely similar.
In conclusion, we report the chemical doping of indi-
vidual semiconducting nanotube ropes with potassium. We
find that this chemical doping changes their conductance
from zero to a value on the order of 1 mS. From the gate
voltage dependence of the conductance, we deduce that the
charge carriers are electrons. This is explained by a model inwhich the potassium donates electrons to the nanotube rope,
resulting in mobile charge carriers at the Fermi level. The
effective mobility for the electrons is similar in magnitude to
the effective mobility of holes. These experiments open the
way toward other experiments that require controlled doping,
such as making nanoscale p-n junctions.12
We would like to thank Steven G. Louie and Marvin Co-
hen for helpful discussions, and G.Y. McLean for experi-
mental assistance. This work was supported by D.O.E. ~Ba-
sic Energy Sciences, Materials Sciences Division, the sp2
Materials Initiative!. Nanotube production at Rice University
was supported by the Office of Naval Research, the Robert
A. Welch Foundation, and the Texas Advanced Technology
Program.1 R. Martel, T. Schmidt, H. R. Shea, T. Hertel, and P. Avouris,
Appl. Phys. Lett. 73, 2447 ~1998!.
2 T. W. Odom, H. Jin-Lin, P. Kim, and C. M. Lieber, Nature ~Lon-
don! 391, 62 ~1998!.
3 J. W. G. Wildoer, L. C. Venema, A. G. Rinzler, R. E. Smalley,
and C. Dekker, Nature ~London! 391, 59 ~1998!.
4 S. J. Tans, R. M. Verschueren, and C. Dekker, Nature ~London!
393, 49 ~1998!.
5 A. M. Rao, P. C. Eklund, S. Bandow, A. Thess, and R. E. Smal-
ley, Nature ~London! 388, 257 ~1997!.
6 R. S. Lee, H. J. Kim, J. E. Fischer, A. Thess, and R. E. Smalley,
Nature ~London! 388, 255 ~1997!.
7 L. Grigorian, G. U. Sumanasekera, A. L. Loper, S. Fang, J. L.
Allen, and P. C. Eklund, Phys. Rev. B 58, R4195 ~1998!.
8 S. J. Tans, M. H. Devoret, H. Dai, A. Thess, R. E. Smalley, L. J.Geerligs, and C. Dekker, Nature ~London! 386, 474 ~1997!.
9 D. H. Cobden, M. Bockrath, P. L. McEuen, A. G. Rinzler, and R.
E. Smalley, Phys. Rev. Lett. 81, 681 ~1998!.
10 We note that none of the procedures described hereafter have a
significant effect on metallic tubes, and thus we will focus ex-
clusively on the properties of semiconducting tubes.
11 We have never seen n-type behavior in undoped samples, even at
large positive gate voltages. We believe that this is because in
the back gated geometry the contact region is screened from the
gate by the leads. Thus, the contact regions remain p-type re-
gardless of the gate voltage, preventing the injection of charge
into the n-type region.
12 K. Esfarjani, A. A. Farajian, Y. Hashi, and Y. Kawazoe, Appl.
Phys. Lett. 74, 79 ~1999!.


Chemical doping of individual semiconducting carbon-nanotube ropes

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Chemical doping of individual semiconducting carbon-nanotube ropes

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