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phys. stat. sol. (b) 245, No. 10, 2280–2283 (2008) / DOI 10.1002/pssb.200879631

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

phys. stat. sol. (b) 245, No. 10, 2280–2283 (2008) / DOI 10.1002/pssb.200879631 p s sbasic solid state physics

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Electrical properties of C4+ irradiated single-walled carbon nanotube paper

Viera Skákalová*, 1, Alan B. Kaiser2, Ursula Dettlaff1, Kai Arstila3, Arkady V. Krasheninnikov3,4, Juhani Keinonen3, and Siegmar Roth1

1 Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany 2 MacDiarmid Institute for Advanced Materials and Nanotechnology, SCPS, Victoria University of Wellington, P.O. Box 600,

Wellington, New Zealand 3 Materials Science Division, University of Helsinki, P.O. Box 43, 00014 Helsinki, Finland 4 Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, 02015, Helsinki, Finland

Received 2 May 2008, revised 25 June 2008, accepted 26 June 2008

Published online 8 September 2008

PACS 61.80.Jh, 73.63.Fg, 78.30.Na, 81.40.Rs

* Corresponding author: e-mail iwep@fkf.mpg.de

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Nano-engineering to tailor the prop-erties of carbon nanotubes by irradiation [1, 2] with high-energy ions or electrons has been demonstrated experimen-tally and by theoretical simulations. Ion irradiation modi-fies conduction in individual single-wall carbon nanotubes (SWNTs) [3] and in SWNT networks [4], with remarkable correlations between changes in Young’s modulus, Raman line frequencies and conductivity [5].

In this paper we report the effects of bombardment by ions of high energy (23 MeV) chosen so that the majority of ions will pass right through the SWNT paper. We find that there is a qualitatively different effect on the conduc-tivity compared to our earlier measurements using irradia-tion by 30 keV Ar+ and N+ ions, which came to rest and deposited essentially all their kinetic energy in the SWNT paper [4, 6]. We use carbon ions for the irradiation, which means that no impurities of different elements are intro-duced when some ions are stopped in the sample.

2 Experimental 2.1 Sample preparation Single-walled carbon

nanotube paper of 40 µm thickness was prepared by vac-uum filtration of HiPCO material suspended in 1 % SDS and subsequent washing with deionized water to remove surfactant. By use of the microgram scale balance the vol-ume density of CNT paper was estimated to 0.7 g/cm3. Taking into account the volume density of the paper, it was calculated using the SRIM (Stopping and Ranges of Ions in Matter) code [7] that 12C4+ ions of energy 23 MeV would have a penetration depth of 57 µm, sufficient to pass through the 40 µm paper. During irradiation, the sample was held by aluminium foil which also served as a mask (Fig. 1).

Four samples have been exposed to the 23 MeV 12C4+ beam. The accelerator used for this study was a 5 MV tan-dem accelerator of the Accelerator Laboratory, University of Helsinki. The irradiation conditions concerning doses, time and current are summarized in Table 1.

Single-walled carbon nanotube (SWNT) networks have been

irradiated with a 12C4+ ion beam (ion energy 23 MeV) to in-

vestigate changes to their properties. Measurements of the

Raman D-line show that the irradiation produces structural

defects throughout SWNT paper of thickness 40 µm, with a

linear increase in defects at the back side of the sample, but a

saturating tendency occurring at the front (irradiated) side.

The resistance of the samples also increases linearly with ir-

radiation dose. The temperature dependence of the conductiv-

ity of the samples is changed progressively by the irradiation:

whereas the pristine sample exhibits mixed metallic-

nonmetallic character consistent with the usual model of me-

tallic conduction interrupted by thin barriers through which

fluctuation-assisted tunnelling occurs, the temperature de-

pendence loses its metallic character and follows three-

dimensional variable-range hopping for the most irradiated

sample.

phys. stat. sol. (b) 245, No. 10 (2008) 2281

www.pss-b.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

Figure 1 Aluminium sample holder with the sample inside. The

diameter of the opening is 1 cm.

Table 1 Irradiation parameters.

Dose, ions /cm2 Time, s Current, nA

1 × 1013 132 48.5

3 × 1013 365 52.6

1 × 1014 1113 57.5

3 × 1014 4668 41

2.2 Measurement methods Raman spectra were

measured using microscope laser Raman spectroscopy with a Jobin Yvon–LabRam spectrometer. The laser exci-tation wavelength was 632 nm with spectral resolution of 4 cm–1. The electrical conductivity of the SWNT paper fixed on the surface of Si/SiO2 wafers was measured using the four-probe method by evaporating four chromium (20 nm)/gold (200 nm) strips of the same geometry for all the samples, i.e. 0.3 mm distance between the strips. A constant current was applied during the temperature de-pendence measurements from liquid helium temperature up to room temperature.

3 Results and discussion Raman spectra were measured on both surfaces of the samples: the front side (directly irradiated) as well as the back side. It is well known [8] that increasing defect concentration in carbon structures with sp2 electron orbital configuration causes an increase of the Raman D line, in our case with the peak maximum at about 1313 cm–1 (excitation light wavelength of 633 nm) and, therefore, we use this as an indication of the structural damage caused by ion irradiation. To com-pare the spectra of the samples with different doses of ion irradiation, the spectra were normalized to the intensity of the second order mode related to the D line, the D* mode at 2614 cm–1. Although the intensity of this mode depends on the helicity of the tubes [9], it shows only little depend-ence on the defect concentration [10]. In our case if nor-malized to G mode, both D and D* intensities increase upon irradiation but while the D* mode rises by only 30%, the D mode increases more than 10 times compared to that of the pristine sample. The relative changes of the intensity of the D line normalized to that of the D* line are plotted versus dose of ion irradiation for the front and the back sides of the samples in Fig.2A. Since the increase of the D

Figure 2 (A) Relative increase of intensity of the D line meas-

ured on the front (violet triangles) and the back (pink circles) sur-

faces of the samples and (B) electrical resistivity at room tem-

perature versus dose of C4+ ion irradiation. line is so large, normalizing by the G+ line intensity in-stead of the D* line yields almost identical plots in Fig. 2A

Figure 3 shows the displacement per atom (DPA) as a function of sample depth for two irradiation doses, as cal-culated with the code SRIM [7]. The threshold energy for displacing carbon atom was equal to 15 eV [2]. Based on our SRIM calculations, one can expect that the amount of damage caused by C4+ ions with the energy of 23 MeV should be higher at the back side. This is confirmed by the measured data, which show a greater increase in D line in-tensity at the back side of the sample (Fig. 2A).

Figure 2B shows how the resistivity of the samples in-creases approximately linearly with the irradiation dose following the relation:

ρ = 0.00286 + 5.4 x 10–17 d Ωcm, (1)

where d is the dose of 23 MeV C4+ ions per cm2. It is clear that the defects introduced by irradiation drastically in crease the resistance so that the resistance at higher doses is strongly dominated by these irradiation defects. We now turn to an analysis of the temperature depend-ence of the conductivity σ(T) of the samples (plotted in Fig. 4) to give an indication of the conduction mechanisms oc-curring, following our earlier modelling of conduction in SWNT paper irradiated with 30 keV Ar+ and N+ ions [4, 6]. For the pristine sample (before irradiation), there is no sig-nificant difference (Fig. 4) for decreasing temperature and increasing temperature at double the current (0.5 mA and

0 5 10 15 20 25 30

5

10

15

20

B

Dose of C4+

(1013

ion/cm2

)

Re

sis

tivity (

mΩ.c

m)

0

2

4

6

8

Irradiated side

Back side

(Id-Io)/I o

A

2282 V. Skákalová et al.: Electrical properties of C4+ irradiated single-walled carbon nanotube paper

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-b.com

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ysic

ap s sstat

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i b

Figure 3 SRIM calculations of the damage created by C4+ ions

with the energy of 23 MeV in the SWNT paper with a density of

0.7 g/cm3 and thickness of 40 µm.

Figure 4 Temperature dependence of the conductivity of the

SWNT paper samples, with the dose of C4+ ions given beside the

data sets in units of 1013 ions/cm2; two data sets are shown for the

sample prior to irradiation, one with a current of 0.5 mA for de-

creasing temperature (red circles), and the other at 1 mA for in-

creasing temperature (dark yellow squares). The lines through the

data points are fits to Eq. (2) for data sets 0 and 1, and to Eq. (3)

for the others, as explained in the text.

1 mA respectively), illustrating the absence of hysteresis effects and dependence on current level.

The pristine data show the characteristic features of highly conducting SWNT networks [6] where conduction is dominated by the metallic nanotubes, with a finite con-ductivity still present in the zero-temperature limit, and a turnover to metallic sign for the conductivity temperature dependence as temperature increases above approximately 240 K. However, the increase of conductivity with tem-perature up to 240 K indicates the presence of thermally-assisted conduction, ascribed to tunnelling through thin barriers that allow conduction even at zero temperature. An

excellent fit to the data for the pristine data sets, and for the lowest-dose irradiated sample which shows similar behav-iour, is given by our model [6] of quasi-one-dimensional (quasi-1D) metallic conduction [11] with fluctuation-assisted tunnelling [12] though defect barriers between and along nanotubes:

1exp exp

( )

b m

s

T TB A

T T T Tσ

⎛ ⎞ ⎛ ⎞= + −⎜ ⎟⎜ ⎟+ ⎝ ⎠⎝ ⎠. (2)

Here A and B are coefficients depending on geometrical

factors that can often be taken as independent of tempera-

ture [6]. The order of magnitude of typical barrier energies

is indicated by the value of kBTb and the extent of the re-

maining tunnelling conductivity at low temperatures is in-

dicated by the ratio Ts /Tb. The energy kBTm is that of the

phonons that dominate scattering of the carriers [12]. The

values of the fit parameters (listed in Table 2) are compa-

rable to those of earlier samples, corresponding to small

tunnelling barriers kBTb of approximately 2 meV, and zone-

boundary phonon energies kBTm as expected [6].

Figure 5 Demonstration of 3D-VRH behaviour of conductivity

as a function of temperature at the highest dose of 30 × 1013

ions/cm2 (except at the lowest temperatures); the other data sets

show significant lessening of slope at high and low temperatures.

We focus here on the effect of irradiation by 23 MeV

C4+ ions. As we have seen, for the lowest dose of 1013 ions/cm2, the irradiation reduces the magnitude of conduc-tivity while the qualitative behaviour of Eq. (2), including the changeover to metallic temperature dependence near room temperature, is retained. However, for higher doses, Eq. (2) can no longer account for the data which is then well explained by a three-dimensional variable-range hop-ping (3D-VRH) term in parallel with the fluctuation-assisted tunnelling term (the quasi-1D metallic contribu-tion now making no significant contribution). This is the behaviour we found for SWNT paper thicker than the ion penetration depths [4, 6]. For parallel conduction, we add the conductances:

0 50 100 150 200 250 300

0

100

200

300

400

30

10

1

3

0

Co

nd

uctivity (

S/c

m)

Temperature (K)

0

0.3 0.4 0.5 0.6 0.7

1

10

100

30

10

31

Conductivity (

S/c

m)

1 / T 1/4

(K −1/4

)

0

phys. stat. sol. (b) 245, No. 10 (2008) 2283

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Original

Paper

1/4

0( ) exp expb

s

T TT C H

T T Tσ

⎛ ⎞⎛ ⎞ ⎛ ⎞= − + − ⎜ ⎟⎜ ⎟⎜ ⎟+ ⎝ ⎠⎝ ⎠ ⎝ ⎠, (3)

where C and H are approximately temperature-independent

parameters indicating the contributions of each term, and

T0 is the Mott hopping parameter that depends on the local-

ization length and density of states for the carriers involved

in the hopping [13, 14]. This model accounts very well for

the conductivity for the three most highly-irradiated sam-

ples, as shown by the fits in Fig. 4, with values of the fit

parameters listed in Table 3. The fluctuation-assisted tun-

nelling term becomes smaller until for the most highly ir-

radiated sample the conductivity is well accounted for by

the 3D-VRH term alone (with a large value of T0, i.e.

highly localized carriers), as shown directly by the linearity

of the corresponding plot of log(σ) versus 1/T 1/4 in Fig. 5

(except possibly at very low temperatures). The value of

the barrier energy kBTb increases with irradiation as ex-

pected, while the value of Ts shows no significant change.

Table 2 Fit parameters for fits of Eq. (2) to the highest conduc-

tivity samples.

Dose

(ions /cm2)

A–1

(S/cm)

Tm

(K)

B–1

(S/cm)

Tb

(K)

Ts

(K)

0 23 1770 390 23 12

1 × 1013 18 1800 330 27 12

Table 3 Fit parameters for fits of Eq. (3) to the lowest conductiv-

ity samples.

Dose

(ions /cm2)

H

(S/cm)

T0

(K)

C

(S/cm)

Tb

(K)

Ts

(K)

3 × 1013 330 330 88 42 11

1 × 1014 235 210 47 70 11

3 × 1014 475 7600 0

4 Conclusion We have shown that the room-temperature resistivity of SWNT networks of thickness 40 µm irradiated by 23 MeV C4+ ions increases approximately linearly with dose, mirroring the increase of the defect density determined from Raman D-line intensity measure-ments. The increase of D-line intensity at the back side confirms that a large fraction of C4+ ions penetrate across the sample leaving damage distributed throughout the whole sample. Due to the very high energy of the ions, a larger defect rate is observed at the samples’ back surfaces, where the kinetic energy of the penetrating ions is reduced significantly so that the cross section for defect production is larger. The temperature dependence of conductivity is well described by quasi-1D metallic conduction interrupted by fluctuation-assisted tunnelling through small barriers for the pristine and lightly-irradiated samples. At higher C4+ ion doses, 3D variable-range hopping makes a significant contribution to conduction, and dominates the total con-ductivity at the highest dose. This dominance of 3D VRH

conduction was also what we observed in thin transparent SWNT networks irrradiated with 30 keV Ar+ and N+ ions that also penetrated through these very thin samples [14]. In contrast to the case of 23 MeV C4+ ions, lower en-ergy 30 keV Ar+ and N+ ions incident on SWNT paper of thickness approximately 50 µm penetrate at most a few µm and Raman D-line measurements confirm that none reach the back side of the SWNT paper [4]. In that case, we con-cluded that annealing by heat propagating along SWNTs from the impact region affected a region of the sample much larger than the surface impact layer itself, leading (at low doses) to an increase in conductivity in this region due to annealing of small barriers along SWNTs, consequent delocalization of charge carriers, and removal of impurity dopants that increased the carrier density. This scenario ac-counted for the peaks in conductivity observed as a func-tion of ion irradiation dose [4, 5]. For the case of 23 MeV C4+ ions in this paper, however, the whole of the sample is affected so there are no highly-conducting layers left undamaged and VRH makes the dominant contribution to resistance across the whole sam-ple at the highest doses.

Acknowledgements This work was supported by EC pro-

jects CANAPE and SANES and the Slovak national project

APVV-0628-06.

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