Variable-Range Hopping Conduction in the Assembly of Boron-Doped

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2007, 111, 11763-11766 Published on Web 07/21/2007

Variable-Range Hopping Conduction in the Assembly of Boron-Doped Multiwalled Carbon Nanotubes Shunji Bandow,*,‡ Shigenori Numao,† and Sumio Iijima‡ Department of Materials Science and Engineering, Meijo UniVersity, 1-501 Shiogamaguchi, Tenpaku, Nagoya 468-8502, and The Graduate UniVersity for AdVanced Studies, Institute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585 ReceiVed: June 1, 2007; In Final Form: July 6, 2007

Temperature dependence of the electric resistance was measured in the temperature range between 5 and 290 K for the assembly of the boron-doped multiwalled carbon nanotubes (B-doped MWNTs). The observed temperature dependence was explained by the mechanism of three-dimensional variable-range hopping (3DVRH) conduction, and the characteristic temperature T0 of 3D-VRH was varied between 1.5 and 625 K. This change of T0 was systematically explained by the change of the electronic density of states at the Fermi level N(0) of B-doped MWNTs. In addition, the boron-doping-induced electron spin resonance (ESR) signals were detected at g ) 2.001 and 2.002, and the relative values of the ESR intensities were consistent with the enhancement factors of N(0) derived from 3D-VRH. These facts were explained by the substitutional doping of the boron atoms to the sp2-bonded carbon network that organizes the nanotube wall.

Introduction Nowadays, the carbon nanotubes and related materials are considered to be a key material in the nanotechnological research. The CVD (chemical vapor deposition)-grown nanotube forests1,2 are applicable to the supercapacitor,3 and the nanotube thin film behaves as transparent for intense light having a wavelength in near-infrared regime,4 which can be used as the noise filter for the optical-fiber telecommunication. The nanotube-related material of carbon nanohorns5 has wider interior space than the nanotube. This characteristic is an advantage for the encapsulation of the large molecules; therefore, it is considered to apply the nanohorns to build up a drug delivery system6,7 that should require less interaction between the incorporated molecules and the carbon wall. On the other hand, the band gap modulation of the nanotube has been reported for the semiconducting single-walled carbon nanotubes incorporating the Gd@C82 metallofullerene molecules, indicating a band gap narrowing of ∼0.4 eV.8 As stated above, nanosized carbon materials have large potential, and the fundamental studies regarding the interaction with the guest molecules and also the atomic defects become more important for future applications. Because the gas molecules would, more or less, modify the electronic feature of the nanosystem,9 the recent high-resolution transmission electron microscopy has revealed defect formation in the sp2bonded carbon network.10 Similarly, the substitution of the carbon atom to the other atom would modify the electronic structure around the Fermi level, resulting in the drastic change in the electronic properties. In this study, boron was selected * To whom correspondence should be addressed. E-mail: bandow@ ccmfs.meijo-u.ac.jp. † Institute for Molecular Science. ‡ Meijo University.

10.1021/jp0742362 CCC: $37.00

as the dopant to the nanotube wall, and the temperature dependence of the electric resistance was measured for the pellet-formed samples by changing the doping quantity. Experimental Section Boron-doped MWNTs (multiwalled carbon nanotubes) were prepared by the vaporization of a boron-containing carbon rod in a RF (radio frequency) plasma. Details of the preparation method were described in the paper already published;11 thus, we briefly describe here the sample identification codes used in this study. B0NT means that the MWNTs were produced by the RF vaporization of a needle-shaped pure carbon rod, and B5NT is the sample from a needle-shaped composite rod stuffed with a 5% of boron-containing carbon powder into a 1 mm diameter cylindrical hole made at the center of a 5 mm diameter carbon rod, and so on. Therefore, it should be noted here that the number attached behind “B” does not mean the doping quantity of boron to MWNT. It is just a boron concentration in the carbon powder for the MWNT preparation. The MWNTs thus obtained were put into a 2 mm diameter cylinder and were hand-pressed by a piston rod in order to make a pellet-formed assembly. After taking the pellet out from the cylinder, the edge region of the pellet was cut by a razor blade in order to form the nearly rectangular shape. The pellet prepared in this way was mounted on a glass substrate by using a small amount of Apiezon N grease. Then, the 4 electric leads (50 µm in diameter of Au wire) were connected with the sample by using Au paste. Sizes of the pellet and the distance between the voltage leads were measured by using an optical microscope. The sizes of the samples were typically in the range of 0.20.3 mm in thickness, 0.4-0.8 mm in width, and 0.4-0.5 mm for the distance between the voltage leads. The resistivities at room temperature were systematically varied between ∼12 and © 2007 American Chemical Society

11764 J. Phys. Chem. C, Vol. 111, No. 32, 2007

Figure 1. Temperature dependence of the electric resistivity F for the pellet-formed MWNTs. Explanation of the sample codes (B0NT, B5NT, and so on) is in the text. The inset represents the room-temperature F (FRT) against the boron concentration in the composite rod for the MWNT preparation.

0.8 Ω‚mm, depending on the boron concentration. Electric current was set at 100 µA for the samples with the resistivity smaller than several Ω‚mm and at 10 µA for others. ESR (electron spin resonance) spectra were recorded at room temperature by using a Bruker EMX8/2.7 spectrometer operated at the X-band (9.42 GHz). For the ESR measurements, MWNTs, not a pellet form, were put into the quartz tube (5 mm in diameter) and vacuum-sealed. The magnitude of the spin magnetic susceptibility was determined by the double integration of the ESR signals, which were scaled by that of a reference material of DPPH (1,1-diphenyl-2-picrylhydrazyl). Results and Discussion Temperature dependence of the electric resistivity F indicated the semiconductor-like behavior as shown in Figure 1, that is, F was increased with decreasing temperature, and the magnitude of F at room temperature (FRT) showed a tendency to decrease with increasing the boron concentration in the composite rod for the sample preparation (see inset in Figure 1). The latter tendency can be considered as a result of increasing the metallic character of individual MWNTs, and the mechanism of the former temperature dependence can be explained by the Mott’s variable range hopping (VRH) conduction,12 which is frequently seen in the assembly of the small conducting materials or particles.13-17 In the Mott’s mechanism, the conduction electrons have a tendency to localize in the individual conducting particles, and therefore, the localization length ξ of the conduction electrons is smaller than the particle size. The electric conduction for the assembly of the particles takes place by the hopping of the electrons between the particles. Here, the electron hopping is not necessarily to the neighboring particles but requires the energy level matching between the hopping sites within the energy of kBT (kB is the Boltzmann’s constant and T is temperature). In the VRH, the hopping probability can be represented by an expression of exp(-2r/ξ)‚exp(-∆/kBT); here, the parameter of ξ is on the same order of magnitude with the particle size, r is the distance between the particles to hop, and the magnitude of the energy level difference is ∆. In the assembly of the particles, there are a number of particles associated with the hopping conduction, and this number is

Letters

Figure 2. T-1/4 plot for the normalized electric resistivity. Electric resistivities were normalized by the values at room temperature, FRT, and the temperature dependences can be explained by the mechanism of the three-dimensional variable range hopping (3D-VRH) conduction represented by F/F0 ) exp(T0/T)1/4, where F0 is constant. The values of T0 determined from the slopes (see the solid lines) are noted in the figure.

proportional to rd (d represents the dimension for the hopping); therefore, ∆ is inversely proportional to rdN(0), where N(0) is the electronic density of states at the Fermi level. Then, we get the hopping probability of exp(-2r/ξ)‚exp{-1/(rdN(0)kBT)}. By differentiating this expression with respect to r, we get that the maximum probability of the electron hopping taking place at the length δ (hopping length) of

δ)

(

dξ 2N(0)kBT

)

1/(d+1)

(1)

By using eq 1, the electric conductivity σ can be represented by the expression of σ ) σ0exp(-δ/ξ) ≡ σ0exp(-T0/T)1/(d+1), and therefore, the F can be written by F ) 1/σ ) F0exp(T0/T)1/(d+1), where σ0 and F0 are the constant values. In this expression, the characteristic temperature T0 of threedimensional (3D) VRH can be written as

T0 )

3 1 ∼ 3 2kBN(0)ξ kBN(0)ξ3

(2)

Hence, eq 2 represents that T0 is inversely proportional to N(0), if ξ is constant. In Figure 2, we plotted the T-1/4 dependence of the electric resistivity normalized by FRT, which clearly suggests that the observed features are resultant from the 3D-VRH conduction. Here, the slopes of the lines indicate T0, and they have different values between the samples. As noticed above, the conduction electrons are confined in the system considered so that, in the present case, ξ should be on a comparable order with the nanotube size. In fact, ξ for undoped MWNTs (B0NT) can be estimated at 3.4 nm by using the N(0) for graphite, ∼3 × 1032 states/(erg‚cm3).18 This value for ξ is quite smaller than that for the nanotube length, which reaches several µm, while the outermost tube diameters (∼10 nm) are of comparable magnitude with ξ. Hence, we consider that ξ depends on the outermost tube diameters, whose distributions were almost the same between the samples used in this study (see Figure 1 of ref 11). This fact means that ξ can be treated as a constant value for all of the samples, and therefore, the change of T0 should associate with a variation of N(0). In order to evaluate quantitatively the variation of N(0), we took the inverse ratio of T0 with respect to a reference value, and

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J. Phys. Chem. C, Vol. 111, No. 32, 2007 11765

TABLE 1: Parameters for the Variable-Range Hopping Conduction Determined by the Temperature Dependence of the Electric Resistivity and ESR Measurementsa T0 (K) χs (emu/g) 1/(T0/T0B5NT) χs /χsB5NT

B0NT

B5NT

B10NT

B15NT

625

111 4 × 10-10 1 1

2.6 1.8 × 10-8 43 45

1.5 3.1 × 10-8 74 78

T0 and χs (equal to χs1 + χs2) are from Figures 2 and 3b, respectively. These ratios are normalized by the values for B5NT, where the boron-doping-induced weak ESR signal was first observed. a

Figure 3. Electron spin resonance spectra (a) and the spin magnetic susceptibilities (b). ESR was measured at room temperature, and the simulated components (thin and thick solid lines) with g ) 2.015, 2.005, 2.002, and 2.001 are indicated in (a). The ESR components at g ) 2.001 and 2.002 (thick solid lines) are the boron-doping-induced signals, and those at g ) 2.005 and 2.015 (thin solid lines) are the signals from undoped MWNTs. The sums of the spin magnetic susceptibilities of the boron-doping-induced signals (χs1+χs2) are shown in (b).

they are indicated in Table 1 by setting the T0 for B5NT (T0B5NT) as a reference. These values are increased monotonically with increasing the boron concentration in the composite rod for the sample preparation. In the ESR measurements, the doping-induced ESR signals can be seen at g ) 2.001 and 2.002 (see the thick, solid, simulated lines in Figure 3a), and their intensities behave Paulilike at the temperatures above ∼130 K.11,19 In Figure 3b, the total spin magnetic susceptibilities (χs) of the boron-dopinginduced spectral components are indicated as a function of the boron concentration in the composite rod for the sample preparation. Furthermore, the numerical values of χs are listed

in Table 1. Since the Pauli paramagnetic susceptibility is represented by χs ) µB2N(0),20 where µB is the Bohr magneton, the magnitude of χs is proportional to N(0). Therefore, we can estimate the relative value of N(0) by taking the ratio of χs for each sample. Here, we have a clear reason to set χs for B5NT (χsB5NT) as a reference value since the boron-doping-induced signal was started to observe in the MWNTs prepared at this boron concentration. Relative values thus calculated by χs/χsB5NT are in Table 1, and these values are also increased with increasing the boron concentration. Both values of 1/(T0/T0B5NT) and χs/χsB5NT are the enhancement factors of N(0), and they are independently determined by using the data taken from the different experimental methods. As seen in Table 1, these values match consistently, and therefore, we can conclude that the decrease of T0 upon increasing the boron concentration is certainly resultant from the enhancement of N(0). This enhancement can be explained by the boron doping to the sp2-bonded carbon network that makes the hole band in the π band of graphite and also makes the acceptor band in the π* band. Graphite is the zero-gap semiconductor, and hence, such a doping-induced band near the Fermi level will enhance the N(0) of MWNTs. In the previous study,11 we examined the electron energy loss spectroscopy (EELS) in order to find evidence of boron doping to the nanotube wall. However, we could not catch the evidence of the boron doping, which is probably due to the detection limit of EELS, less than 1,000 ppm atomic. That is, the doping quantity of boron is estimated to be much smaller than 1020 atoms per mol of carbon. This situation is still the same at present, but the analyses of 3D-VRH reliably indicate that the doping quantity of boron increased when the sample was prepared by the vaporization of the composite rod with a higher boron concentration. Although the doping quantity of boron is unknown, N(0) for B15NT reaches about 70-80 times greater than that for B5NT. If we consider the enhancement factor of N(0) against the undoped MWNTs, the ratio of 1/(T0B15NT /T0B0NT) gives the factor of ca. 420 for B15NT. We consider that such enhancement of N(0) is directly connected with the decrease of the resistivity at room temperature, as shown in the inset of Figure 1, and FRT for the assembly of B15NT (∼0.8 Ω·mm) indicated the resistivity as small as that in the c-direction of graphite (∼1.2 Ω‚mm). 21 The paper reporting the superconductivity of nanotubes first appeared in 2001 by Kociak et al.,22 who found the 2 orders of magnitude resistance drop below 0.55 K. Furthermore, the superconducting features were reported in 2001 and 2006, respectively, for the thin diameter nanotubes23 and the MWNTs.24 The present finding of the enhancement of N(0) will be a guideline for the superconductivity in the nanotube system. In such a point of view, we measured the magnetic susceptibility of B20NT (F data are not shown here but are quite similar to those of B15NT) down to 1.9 K at the magnetic fields of 200, 800, and 5000 G. However, we could not detect clear evidence of the superconductivity (see details in the Supporting Information). This fact may suggest that much heavier doping of boron is necessary in order to enhance the N(0) more. Such doping of heteroatoms may gives the superconductivity in the nanotube system like that observed in the boron-doped diamond.25,26 Summary The electric resistivity (F) for the assembly of the borondoped MWNTs (pellet-formed sample) became lower as the boron concentration increased, indicating that the magnitudes of F at room temperature (FRT) were monotonically decreased

11766 J. Phys. Chem. C, Vol. 111, No. 32, 2007 from ∼12 Ω‚mm for B0NT (undoped MWNTs) to ∼0.8 Ω‚ mm for B15NT. This dependence was considered as a result of the increasing of the metallic character of individual MWNTs that probably connects with the doping quantity of boron to the nanotube wall. The magnitude of F was increased with decreasing temperature, and this temperature dependence was explained by the Mott’s 3D-VRH mechanism. From the combination analyses of the characteristic temperature T0 of 3DVRH and the boron-doping-induced spectral components of ESR, we concluded that N(0) was enhanced as the doping quantity of boron increased, reaching the enhancement factor of ∼420 for B15NT, as compared with that for undoped MWNTs. Even though such an enhancement of N(0) was observed in the present study, we could not detect clear evidence of the superconductivity in the boron-doped MWNTs. This is perhaps due to low doping quantity. In a further study, it is necessary to consider the way to enhance the doping quantity by using methods such as the ion beam implantation of boron to the MWNT and the preparation of MWNTs in the metal-containing gas. Acknowledgment. A part of this work was carried out under collaboration with the SORST project of the Japan Science and Technology Agency. Supporting Information Available: Scanning electron micrographs for the surface of the pellet-formed boron-doped MWNTs and the description for the temperature dependence of the magnetic susceptibility taken for B20NT with the figure. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Hata, K.; Futaba, D. N.; Muzuno, K.; Namai, T.; Yumura, M.; Iijima, S. Science 2004, 306, 1362. (2) Hiraoka, T.; Yamada, T.; Hata, K.; Futaba, D. N.; Kurachi, H.; Uemura, S.; Yumura, M.; Iijima, S. J. Am. Chem. Soc. 2006, 128, 13338. (3) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987.

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