Diamond & Related Materials

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1 Diamond & Related Materials 18 (2009) Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: The separation of different conducting multi-walled carbon nanotubes by AC dielectrophoresis Chehung Wei a,, Ting-You Wei a, Cheng-Hao Liang a, Fong-Cheng Tai b a Department of Mechanical Engineering, Tatung University, Taipei 104, Taiwan b Sustainable Environment Research Center, National Cheng Kung University, Tainan 709, Taiwan article info abstract Available online 9 September 2008 Keywords: Dielectrophoresis Purification Multi-walled carbon nanotube Electric property A microfluidic device was fabricated to separate different conducting parts of multi-walled carbon nanotubes (MWCNTs). The device consists of a curing PDMS fluidic-flow chamber covered on electrode coated glass. The electrode was designed to generate non-uniform electric field by patterning via lithography. A range of frequencies with low applied voltage was utilized to induce different sign of dielectrophoresis force. Two different separation schemes based on positive and negative dielectrophoresis were employed to separate different conducting parts in unsorted MWCNTs. From Raman spectroscopy, conducting MWCNTs collected by positive dielectrophoresis showed little variation in the intensities of D-band and G-band ratio while the less conducting MWCNTs collected by negative dielectrophoresis showed decreasing intensities in these positions. The ID/IG ratio in the samples collected by both separation schemes is decreasing compared to the unsorted samples. The electric properties of the samples were characterized by a dielectrophoresis frequency spectra method. The conductance in positive dielectrophoresis collected sample is the greatest while the conductance in negative dielectrophoresis collected sample is the smallest. The trend in the conductance in unsorted and sorted samples is confirmed by current-voltage measurements Elsevier B.V. All rights reserved. 1. Introduction 2. Experimental procedure Carbon nanotubes (CNTs) have recently attracted extensive attention due to their unique mechanical, thermal and electrical properties [1 3]. In optoelectronics applications like field-effect transistor or memory, metallic or semicoducting CNTs have been utilized to enhance the performance of the device [4 7]. For good performance, the uniformity in the material is very important. Several chemical or physical purification processes based on chemical moieties [8,9], electric field [10], or density gradients [11] are available to purify CNTs. Even though each method has its own merit and drawback, purification based on the electric field has the advantages of simple fabrication and easy integration. Dielectrophoresis (DEP), a phenomenon resulted from the non-uniform electric field, has been used to deposit large number CNTs in field-effect transistors [12,13] and separate metallic from semiconducting CNTs [14]. In this study, a method based on AC dielectrophoresis was developed to separate different conducting parts of the multi-walled CNTs (MWCNTs). Also the electric property of the sorted and unsorted samples was evaluated by a dielectrophoresis frequency spectra method and current-voltage curves. Corresponding author. address: cwei@ttu.edu.tw (C. Wei). The dielectrophoresis chip used for separation was fabricated via MEMS technique. The advantages of MEMS for dielectrophoresis chip fabrication are the reduction of applied voltage, less Joule heating due to high surface to volume ratio and ease of scaling up for mass production. The DEP chip was made of two parts. The electrode was made from a Corning 1737 glass sputtered with Cr and patterned to an interdigitated triangular shape through lithography. The fluidic chamber was made from a curing PDMS fabricated from a SU-8 mode. After the surface treatment of the fluidic chamber via oxygen plasma (Harrick, Scientific Plasma Cleaner), these two parts were bonded together as a dielectrophoresis chip. The fabrication procedure was shown in Fig. 1. The solutions of MWCNTs was prepared by dispersing MWCNTs (NanoAmor, 95%, OD nm) g into DI water and mixed with alcohol 30 g and surfactant (Dodecylbenzenesulfonicacid, sodium salt) 1 g. The solution was then pipetted onto the chip. AC electric fields were generated from a function generator (Wavetek 195) for applied voltage 8 V under frequencies ranging from 25 Hz to 10 MHz. The signal was applied to the electrode to generate dielectrophoresis reaction. The bonding structure in MWCNTs for unsorted and sorted samples was characterized by Raman spectroscopy which was carried out by a Renishaw micro-raman, with nm laser excitation. The characterization of the electric property /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.diamond

2 Results and discussion 3.1. The purification of MWCNTs by dielectrophoresis The dielectrophoresis (DEP) force is generated from a non-uniform electric field. When a polarizable object is under a non-uniform electric field, a dipole moment is induced and the object will move towards the maxima or the minima of the electric field depending on its relative polarizability to the medium. A quantitative expression for the dielectrophoresis force can be written as follows [15]: F DEP = 1 4 vre½kðwþšjjej2 = 1 ~ 4 vre ɛp ~ ɛ m ~ ɛp 2 ~ jjej 2 ð1þ ɛ m where E is the electric field, v is the dimensional constant of the particle, Re[K(w)] is the Clausius Mossotti factor and ε m and ε p are the complex permeability of the medium and the particle, respectively. In Eq. (1), the particle denotes CNT and the medium is the mixture of alcohol, surfactant and DI water. The expressions for ε m and ε p are as follows: ~ ɛm =ɛ m j σ m w ;~ ɛ p =ɛ p j σ p w ; ð2þ where j is the complex number, ε is the permittivity, σ is the conductivity, and w is the applied frequency. The frequency dependence of the DEP force is contained in the Clausius Mossotti factor [16] Re½KðwÞŠ= ɛ pɛ m 3 ɛ m σ p ɛ p σ m ɛ p 2ɛ 2 ð3þ m τ MW σ p 2σ m 1w2 τ 2 MW where τ MW = ɛpɛm σ is Maxwell Wagner charge relaxation time constant p2σ m which characterize the decay of dipolar distribution of free charge in a spherical surface. In high frequency limit Fig. 1. The fabrication of dielectophoresis chip. of the unsorted and sorted samples was calculated based on a dielectrophoresis frequency spectra method and the current voltage method. The current voltage (I V) curves of those samples were measured by a Keithley 2400 source meter and recorded by a Keithley 2700 data acquisition system. Re½KðwÞŠ ɛ pɛ m for w : ð4þ ɛ p 2ɛ m And at low frequency Re½Kw ð ÞŠ σ pσ m for w 0: ð5þ σ p 2σ m Obviously at low frequencies the DEP force is stronger for conductive particles whereas at high frequencies the dielectric properties dominate. Under positive DEP, the conducting CNT will Fig. 2. The strength of electric field of the interdigitated triangular electrode. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3 334 Table 1 The sign of DEP for unsorted MWCNTs at different frequency Unsorted MWCNTs Frequency 25 Hz 50 Hz 75 Hz 100 Hz 1 khz 100 khz 1 MHz Sign of DEP move to the maxima of the electric field and the less conducting CNT will move to the minima of the electric field under negative DEP. To identify the sign of the DEP force, it is necessary to identify the maxima and the minima of the electric field. The distribution of the strength of the electric field in an interdigitated triangular electrode was shown in Fig. 2 by a numerical simulation via CFD ACE (CFD research cooperation). In this patterned electrode, the maxima of the electric field occur in the sharp edge of the electrode (shown in red) and the minima occur between the maxima (shown in blue). This simulation provides a guideline to identify the sign of the dielectrophoresis. AC signals were applied at voltage 8 V under different frequencies. The sign of the dielectrophoresis was determined based on whether MWCNTs collected on the maxima or the minima in the electric field and the results for unsorted sample were listed in Table 1. Two micrographs for collected samples at 50 Hz and 1 khz were shown in Fig. 3(a) and (b) for demonstration purpose. The collected samples under positive DEP and negative DEP were compared with Fig. 4. The Raman spectra for unsorted MWCNTs compared with (a) positive DEP sorted (b) negative DEP sorted sample. the unsorted samples via Raman spectra and the results were shown in Fig. 4. The spectra shown that the relative intensities of the disorder band (D-band) at 1335 cm 1 and the graphite band (G-band) at 1585 cm 1 exhibit little variation for samples collected by positive Fig. 3. The micrographs for collection of MWCNTs for (a) 50 Hz by negative dielectrophoresis and (b) 1 MHz by positive dielectrophoresis for applied voltage 8 V. Fig. 5. The ID/IG ratios for unsorted, positive DEP sorted and negative sorted MWCNTs.

4 335 Table 2 The sign of DEP for positive DEP sorted MWCNTs at different frequency Positive DEP sorted MWCNTs Frequency 50 Hz 100 Hz 1 khz 32 khz 100 khz 1 MHz 10 MHz Sign of DEP dielectrophoresis (conducting) but is decreasing in samples collected by negative dielectrophoresis (less conducting). The ID/IG ratios for unsorted, positive DEP and negative DEP sorted samples were shown in Fig. 5. The ratio for both sorting schemes is decreasing compared to the unsorted sample The characterization of electric properties of unsorted and purified MWCNTs The electric properties of the MWCNTs are characterized by a procedure based on the Clausius Mossotti factor frequency spectra [17]. This method is similar to the zero force measurement [18 20] and is based on the assumptions that the property of MWCNTs is homogeneous and the shape effect is negligible. The essence of this method is based on the fact that the cross-over frequency in DEP frequency spectra is a one-to-one function of the electric properties of the medium and the particle. Therefore once the electric property of the medium is known, the electric property of the particle can be calculated from the cross-over frequency. The information of the crossover frequency is obtained from the dielectrophoresis frequency spectra where the sign change of DEP occurs. The DEP frequency spectra for positive DEP sorted and negative DEP sorted MWCNTs were listed in Tables 2 and 3, respectively. As in the tables, the sign for positive DEP sorted sample is almost the opposite to that of negative DEP sorted sample. Moreover, the unsorted sample (Table 1) has a similar trend as the negative DEP sorted sample which implies these two materials should exhibit similar electric behavior. The simulations of Clausius Mossotti factor frequency spectra based on these experimental results were shown in Fig. 6. The electric properties of unsorted, positive DEP sorted, negative DEP sorted samples and the medium were listed in Table 4. The conductivity of the unsorted sample lies between the positive DEP sorted sample (conducting) and the negative DEP sorted sample (less conducting). Moreover, the permittivity of the unsorted and the negative DEP sorted sample is high. These results are further examined through a current voltage (I V) measurement for unsorted, positive DEP sorted and negative DEP sorted samples and the result is shown in Fig. 7. In that figure, the relative order of the conductivity in unsorted and sorted sample is consistent with Table 4. Based on these results, the electric property of the purified MWCNTs from positive DEP sorted is more conducting than the negative DEP sorted and unsorted samples. 4. Conclusions In this paper, dielectrophoresis has been utilized to separate the conducting and less conducting parts from commercial MWCNTs. The purification process can be achieved by either positive DEP or negative DEP. The samples collected by negative DEP schemes show a decrease in Fig. 6. The CM factor frequency spectra for (a) unsorted (b) positive DEP sorted (c) negative DEP sorted MWCNTs for applied voltage 8 V. Table 3 The sign of DEP for negative DEP sorted MWCNTs at different frequency Negative DEP sorted MWCNTs Frequency 50 Hz 100 Hz 1 khz 35 khz 100 khz 1 MHz 10 MHz Sign of DEP Table 4 The electric properties of unsorted, positive DEP sorted, negative DEP sorted MWCNTs Electric properties unsorted positive DEP sorted negative DEP sorted medium σ m (S/m) ɛ m /ɛ ɛ 0 : the free space permittivity= C 2 /N m.

5 336 Acknowledgement The financial support provided by National Science Council under grant E and Tatung University under grant B93- M is greatly acknowledged. References Fig. 7. The I V curves for unsorted, positive DEP sorted, negative DEP sorted MWCNTs. the intensity of D-band and G-band. The ID/IG ratio for positive or negative DEP collected sample is decreasing compared to unsorted sample. The electric properties of the unsorted, positive DEP sorted and negative DEP sorted were calculated based on a Clausius Mossotti factor frequency spectra method. The positive sorted MWCNTs have the highest conductivity among all these samples and this is verified by the I V measurement. This work shows that dielectrophoresis not only can be applied to purify MWCNTs but also can be utilized to characterize the electric property of the unsorted and sorted MWCNTs. [1] S. Iijima, Nature 354 (1991) 56. [2] Y. Lin, F. Lu, Y. Tu, Z. Ren. Nano Lett. 4 (2) (2004) 191. [3] R. Saito, G. Dresselhaus, M.S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, [4] R.H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297 (2002) 787. [5] P. Avouris, Chem. Phys. 281 (2002) 429. [6] H. Dai, Phys. World. 3 (2000) 43. [7] H. Dai, J.H. Hafner, A.G. Rinzler, D.T. Colbert, R.E. Smalley, Nature 384 (1996) 147. [8] H.P. Li, B. Zhou, Y. Lin, L.R. Gu, W. Wang, K.A.S. Fernando, S. Kumar, L.F. Allard, Y.P. Sun, J. Am. Chem. Soc. 126 (2004) [9] M.S. Strano, C.A. Dyke, M.L. Usrey, P.W. Barone, M.J. Allen, H.W. Shan, C. Kittrell, R.H. Hauge, J.M. Tour, R.E. Smalley, Science 301 (2003) [10] H.Q. Peng, N.T. Alvarez, C. Kittrell, R.H. Hauge, H.K. Schmidt, J. Am. Chem. Soc. 128 (2006) [11] M.S. Arnold, S.I. Stupp, M.C. Hersam, Nano Lett. 5 (2005) 713. [12] X.Q. Chen, T. Saito, H. Yamada, K. Matsushige, Appl. Phys. Lett. 78 (23) (2001) [13] J. Li, Q. Zhang, D. Yang, J. Tian, Carbon 42 (2004) [14] R. Krupke, F. Hennrich, H. von Lohneysen, M.M. Kappes, Science 301 (2003) 344. [15] T.B. Jones, Electromechaincs of Particles, Cambridge University Press, [16] L. Benguigui, I.J. Lin, J. Appl. Phys. 53 (1982) [17] C. Wei, C.H. Liang, T.Y. Wei, Proc. of SPIE 7039 (2008) 7039H. [18] M. Fikus, P. Marszalek, S. Rozycki, J.J. Zielinsky, Stud. Biophys. 119 (1987) 73. [19] P. Marszalek, J.J. Zielinsky, M. Fikus, Bioelectrochem. Bioenerg. 22 (1989) 289. [20] P. Marszalek, J.J. Zielinsky, M. Fikus, T.Y. Tsong, Biophys. J. 58 (1991) 982.