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Substitutional boron-doping of carbon nanotubes q

R. Czerw a, P.-W. Chiu b, Y.-M. Choi c, D.-S. Lee c, D.L. Carroll a,*, S. Roth b,Y.-W. Park c

a School of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USAb Max-Planck-Institut fuur Festkoorperforschung, Stuttgart, Germany

c Department of Physics and Condensed Matter Research Institute, Seoul National University, Seoul, South Korea

Received 8 May 2002; accepted 25 May 2002

The substitutional placement of boron within the

lattice of carbon nanotubes yields quite different trans-

port properties for single walled nanotubes (SWNTs) as

compared to multi-walled nanotubes (MWNTs). Boron

doping of the MWNTs results in an acceptor state in

the local density of states (LDOS) that lies near the

Fermi level and can be directly correlated with features

in the thermoelectric power (TEP) of B-doped MWNT

mats. Transport measurements of individual B-doped

MWNTs exhibit features associated with variable range

hopping. In contrast, B-doping of SWNTs results in

features in the density of states further from the Fermi

level, and transport of the SWNTs shows an unusual

variability in rectification not observed in the MWNTcase. This suggests that boron has been introduced into

the lattice of these two morphologies of nanotubes in

very different ways.

Interest in the electrical transport properties of both

single-walled and multi-walled carbon nanotubes stems

primarily from potential applications in nanoelectronics

[1]. Initially, nanoscale electronic architectures will be

similar to those in use today. Thus, one expects metallic

conduits along with heterojunctions formed from doped

nanomaterials in analogy to bulk Si devices connected

with metal interconnect lines. Further, it is clear that the

effects of lattice impurities are of fundamental interest in

understanding transport phenomena in these unusual

topological objects. However, the direct substitutional

doping of carbon nanotubes is quite difficult. Their low

dimensional structure does not provide an energetically

favorable environment for most impurity atoms. There

are two promising candidates, boron [2] and nitrogen

[3], both of which seem happy to reside within the car-

bon lattice. The behavior of boron in SWNTs and in

MWNTs, as evidenced through transport and tunnel-

ing spectroscopy, appears to be quite different. In this

work, we describe several important differences in B-

doped MWNTs and B-doped SWNTs.

For these studies arc growth methods were used ex-

clusively. Pure carbon nanotubes were arc grown using

methods described in detail elsewhere [4]. Transmission

electron microscopy (TEM) showed a diameter distri-

bution to be centered around 20 nm with tubes as small

as 3 nm and as large as 40 nm with tube lengths typically1 lm. The primary impurities were carbonaceous ma-

terials and polyhedral particles and an scanning tun-

neling microscopy (STM) image of a typical MWNT

bundle is shown in Fig. 1a.

B-doped MWNTs were also grown using arc methods

as described in the literature [5]. TEM characterization

showed these materials to have typical tube diameters of

20 nm with a range of 540 nm. Selected area diffraction

confirms that these tubes possess predominantly zig-zag

chiralities [6]. Tunneling microscopy and spectroscopy,

coupled with electron energy loss spectroscopy (EELS),

has been used to demonstrate that the boron is in-

corporated into the lattice as islands of BC3 [7]. The

impurities in the growth materials were found to be

polyhedral particles (also seen in Fig. 1b) and small

concentrations of carbonaceous material. No catalysts

were used in the growth of either of these MWNT, arc-

produced materials.

Finally, B-doped SWNTs were produced by using the

same process of arc growth as outlined in the literature

for pure SWNTs. However, in the case of B-doping,

pure boron was mixed with Ni/Y catalysts and carbon

and packed into the center of the graphite anode rod.

The raw growth materials resulting from the boron-rich

Current Applied Physics 2 (2002) 473477

www.elsevier.com/locate/cap

q Original version presented at QTSM & QFS 02 (Multi-lateral

Symposium between the Korean Academy of Science and Technology

and the Foreign Academies), Yonsei University, Seoul, Korea, 810

May, 2002.* Corresponding author.

E-mail address: [email protected](D.L. Carroll).

1567-1739/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved.

PII: S 1 5 6 7 - 1 7 3 9 ( 0 2 ) 0 0 1 0 6 - 2
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growth plasma are much more tightly packed around

the cathode than typical pure SWNT growth. Interest-

ingly, we note that atomic resolution of this material

shows the existence of chiral nanotubes. Unlike the

atomic scale imaging of the zig-zag B-doped MWNTs,

the B-doped SWNTs also show a variety of super-

structures across the surface. In fact, these superstruc-

ture patterns seem more the rule in imaging these

materials with STM. A typical superstructure from a B-

doped SWNT is seen in Fig. 1c. This image was acquired

at 50 pA and a bias voltage of 1 V with the samples in

ultrahigh vacuum at room temperature.

As noted in the above references, the B-dopedMWNTs are predominately zig-zag and are heavily

doped with boron. Previous studies have shown that an

acceptor-like state is introduced into the band structure

of these metallic tubes. This state derives from the

formation of BC3 islands and its position depends on

the distribution of island sizes within the matrix. The

average position of the acceptor state in this material

was around 25 meV as reported previously. To un-

derstand the average doping behavior of the boron in

the MWNTs and how the large feature near the Fermi

level might effect transport, the TEP was measured

[8,9]. Since the TEP is a zero current transport coeffi-

cient, it can probe the intrinsic properties of the indi-

vidual nanotubes without influence of the randomly

entangled morphology of the mats in which they are

normally produced [10]. Generally, both SWNTs and

MWNTs mats show a positive and moderately large

TEP over the temperature range of 0 to 300 K, with

temperature dependencies that approach zero as T0 ! 0

[11].

Nanotube mats were produced by suspending each (as

grown) material into chloroform and then ultrasonically

agitating until the nanotubes were well dispersed. The

suspended materials were found to be relatively un-

damaged, after the extended ultrasonication using TEM.Each solution was allowed to settle and then was filtered

using 0.4 m Teflon filters. The remaining solution was

composed of nanotubes with little amorphous materials

and no polyhedra. Finally a thick film was built-up from

each of the materials using a Teflon filter and a poly-

imide mold (to insure equal dimensions in each case).

This resulted in a random packing of nanotubes in a

dense mat. The samples were 3 mm 5 mm 0:025 mm

in size. Several mats of each material were made and the

measurements performed several times on each to insure

reproducibility.

To carry out TEP measurements, the mat samples

were supported on a Teflon substrate and mounted on

top of two copper blocks. Silver paste was used for the

electrical contacts. Chromelconstantan thermocouples

were attached to the back of the copper blocks using GE

7031 varnish. Techniques for the TEP measurement of

carbon nanotubes are described with more detail in the

literature [12].

Fig. 2 shows the TEP as a function of temperature.

Note that for pure carbon nanotubes, the shape of the

TEP vs. temperature curve is exactly as expected from

previous reports. The B-doped mats also exhibit a large

Fig. 1. (a) STM image of undoped MWNT bundle, (b) TEM imageshowing B-doped MWNTs and polyhedral material and (c) STM im-

age of a B-doped SWNT.

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positive TEP but is much more linear in shape and has

a lower overall magnitude. The sign of the TEP for the

B-doped materials is not surprising since these nanotu-

bes have strong acceptor states and hole-like conduc-

tivity should be expected.

Further, differences between the functional forms of

the pure and B-doped TEPs should also be expected.

This is because the generally accepted physical picture

for TEP in mats is a set of semiconducting and metallic

parallel conducting paths each of which is separated by

randomly placed contact barriers between the tubes.

Unlike the pure MWNT case, in the B-doped materials,

there are no semiconducting contributions since the

majority of the tubes (if not all) are metallic and exhibitzig-zag chiralities (3) [13]. Therefore, the curve should

be more linear (metallic) in nature with features arising

from the sharp feature in the Fermi window (seen at

around 50 C in Fig. 2) [14,15]. Thus, the B-doped

MWNTs are behaving very much like degenerately

doped bulk materials in this regard.

These materials were then used to fashion typical

nanotube-field effect transistor structures as described in

the literature elsewhere. Generally, the nanotubes were

deposited on a siliconsilicon dioxide substrate with a

back gate. Goldpalladium contacts were added onto

the nanotube ends using electron beam lithography. A

typical device is shown in the atomic force image of Fig.

3, where the length of tube between the contacts is ap-

proximately 900 nm. From height measurements, the

tubes are isolated (not bundles) and this particular ex-

ample is approximately 20 nm. The TEP behavior is

clearly confirmed in the single tube transport behavior.

Little variation in conductance is seen with applied gate

voltage (not shown). Fig. 4a shows the normalized re-

sistivity as a function of temperature. The inset shows

the log plot of the conductance fit to an exponential

function with the exponent being 0.25 corresponding to

a variable range hopping mechanism. Fig. 4b shows the

IV characteristics for small applied voltages across the

source drain leads for 300 and 4 K clearly showing

metallic behavior in the tubes.

The situation with B-doped SWNTs is significantly

different. Shown in Fig. 5 is the LDOS as determined by

Fig. 2. TEP of pristine and doped MWNTs.

Fig. 3. Atomic force microscope images of a nanotube transistor.

Fig. 4. (a) Shows the resistance as a function of temperature. The insetfits the conductance to an exponential function vs. temperature and

yields an exponent of 0.25 and (b) shows the conductance at low

source-drain biases.

R. Czerw et al. / Current Applied Physics 2 (2002) 473477 475


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tunneling spectroscopy. Notice that, unlike the case

of B-doped MWNTs, the acceptor features in these

nanotubes occur at around 0.4 eV. This corresponds

quite well with calculations reported earlier and is an

indication that the local environment of the boron is not

the BC3 as in the case of the MWNTs. We further note

that these nanotubes can have a distinct band gap. The

examples shown here exhibit a band gap of0.5 eV. The

semiconducting nature of the tubes can be seen in the IV

characteristics as shown in Fig. 6. What is curious about

the conductance measurements is that they can have

different rectification features from run to run suggestingthat there may be some mobility to the doping species.

Notice that the apparent conductance gap remains

approximately the same.

In summary, the effects of boron doping in multi-

walled carbon nanotubes and SWNTs are significantly

different. For the multi-walled case, local bonding en-

vironments of the boron results in an acceptor state near

the Fermi level that strongly effects overall transport

behavior. This is seen in both TEP determinations as

well as single tube transport measurements. These na-

notubes are clearly metallic in nature and are hole-like

conductors. In contrast, B-doping in SWNTs results in

features much further up in the band structure on the

valence band side (acceptors). While this is in agreement

with earlier calculations for isolated defects, transport

measurements indicate some added variability in recti-

fication for a given tube. We speculate that this may be

a result of materials included inside the nanotube but

point out that his does not preclude boron added to the

lattice as well. Further, these B-doped SWNTs can occur

in a variety of chiralities as indicated by atomic scale

images as well as the existence of band gaps in their

electronic structure. These findings suggest that the use

of dopants in nanotubes might provide unique and in-

teresting ways to influence and control their electronic

properties.

Acknowledgements

The authors gratefully acknowledge support from:

KISTEP 98-I-01-04-A-026, MOST (Korea), AFOSR

F49620-99-1-0173 (US), DFG (Germany).

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0.4 eV.

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