Alignment of Carbon Nanotubes by Acoustic Manipulation in a Fluidic

The acoustic radiation force and acoustic streaming, which are generated from the transmission of ultrasound through a fluid, facilitate the mobility ...
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J. Phys. Chem. C 2007, 111, 16802-16807

Alignment of Carbon Nanotubes by Acoustic Manipulation in a Fluidic Medium Wen Pei Lim, Kui Yao,* and Yifan Chen Institute of Materials Research and Engineering (IMRE), 3 Research Link, Singapore 117602 ReceiVed: May 7, 2007; In Final Form: August 1, 2007

We describe a novel solution-based acoustic manipulation technique for the parallel alignment of carbon nanotubes on solid substrates. The acoustic radiation force and acoustic streaming, which are generated from the transmission of ultrasound through a fluid, facilitate the mobility of the nanotubes and result in their alignment on the substrate surface. A theoretical model considering various forces and resultant torques exerted on the nanotubes is first proposed for analyzing the alignment through a fluid medium.

Introduction While many studies have been focused on the preparation of carbon nanotubes (CNTs) and their unique structural, mechanical, electronic, and chemical properties,1 the ability to assemble them in a controllable and predictable manner still remains an intricate challenge in nanotechnology. A key issue for the use of CNTs in electronic applications is the development of fast, reliable, and scalable alignment techniques that are capable of directing the position of the nanotubes. Both direct-growth2-4 and post-growth5-16 approaches have been used to manipulate CNTs. The direct-growth approach involves the growth of CNTs from catalyst in the desired positions using chemical vapor deposition methods,2-4 while, in post-growth strategies, the CNTs are spontaneously assembled5,6 or manipulated using external forces such as gas flow,7,8 electric9,10 and magnetic field,11,12 microfluidics,13 and the Langmuir-Blodgett technique.14 However, the above-mentioned post-growth strategy techniques usually involve complicated processes and are only suitable for individual or small-scale alignment. Although the alignment of nanotubes and nanowires has attracted immense attention, the ability to effectively mass manipulate CNTs still remains a great challenge. Motivated by the desire to develop a fast, reliable, and scalable method to manipulate CNTs for CNT-based electronics, we explore the feasibility of a solutionbased, acoustically assisted assembly process for large-scale alignment of CNTs. This approach offers significant advantages over current efforts such as random deposition15 and atomic force microscopy manipulation of individual nanowires.16 Several particle manipulation techniques utilizing ultrasound have previously been presented,17-20 but these techniques were mostly demonstrated on the manipulation of isotropic particles without considering their alignment. Yasuda et al. applied the competition between acoustic radiation force and electrostatic force to separate polystyrene spheres of different radii.17 Laurell et al. demonstrated the separation of particles by means of laminar flow and enriching the particles at defined positions using forces generated by acoustic standing wave fields.18 Recently, Jung et al. reported the elegant use of ultrasonically induced fluid forces and template topography for the selective assembly of microspheres.19 Strobl et al. used surface acoustic waves generated by interdigital transducers on a LiNbO3 substrate for the assembly of one drop of nanotube suspension * Corresponding author. E-mail: [email protected].

placed between the substrate and a glass plate.20 However, this method is not readily scalable, and the type of substrate used is limited to piezoelectric materials. Here we report the parallel alignment of CNTs on solid substrates via a novel solution-based acoustic manipulation technique. The acoustic radiation force and the acoustic streaming of the assembly fluid, consequences of the transmission of ultrasound through a fluid, facilitate the mobility of the nanotubes in the suspension. The degree of substrate functionalization is also found to play a crucial role in the assembly. By a delicate balance between the acoustic force on the nanotubes and the electrostatic interaction between the nanotubes and the functionalized substrate, large-scale alignment of nanotubes is observed. Because an existing theoretical model for analyzing the fluidic alignment of anisotropic onedimensional nanotubes or nanowires is lacking, we report the first theoretical model on the alignment of CNTs. This model is derived from the analyses of the various forces and the resultant torques exerted on the CNTs when an acoustic wave is transmitted through the fluid. Our theoretical model elucidates the effects of various experimental parameters and is aimed to be applicable for the general alignment of one-dimensional nanomaterials on an electrically charged surface using fluidic force. We believe that this technique could be a potential candidate for the mass assembly of various types of nanotubes or nanowires, and the theoretical model could serve as a reference for analyzing the assembly of nanotubes or nanowires in a fluidic medium. Experimental Section Preparation of Aqueous CNT Dispersion. The multiwalled CNTs used in this experiment were obtained from Chengdu Organic Chemistry Co., Ltd., and are -OH functionalized and ∼2-5 µm and ∼10 nm in length and diameter, respectively. Sodium dodecyl sulfate (SDS; CH3(CH2)11OSO3Na) was used as the surfactant to solubilize CNTs in water. The negatively charged sulfonate groups of SDS would provide efficient adsorption of the nanotubes on positively charged surfaces by means of electrostatic interactions. A single-step solubilization scheme21 was used to prepare the CNT dispersion, where the CNTs were ultrasonically mixed with SDS solution (1 wt %) for 2 h. The concentration of the dispersed CNTs in SDS was 0.02 mg/mL. Preparation of APTES-Modified Substrate. A (100) Si substrate was cleaned in HF and piranha solution (H2SO4:H2O2

10.1021/jp073456c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2007

CNT Alignment by Acoustic Manipulation

Figure 1. Schematic diagram of the home-built setup for nanotubes alignment by acoustic manipulation in three different configurations: (a) the substrate was placed perpendicular to the radiation surface, (b) the substrate was bonded on top of the radiation surface and (c) a combination of the “a” and “b” configurations.

) 7:3 v/v). Silanization with aminopropyltriethoxysilane (APTES, H2N(CH2)3Si(OCH2CH3)3) was performed under a dry N2 atmosphere. APTES (∼ 4 mL) was heated at 80 °C, and the surface of the Si substrate was treated with a slow flux of APTES vapor for 2 min. After curing for 20 min at 100 °C, the silanated substrate was immersed in 0.5 M aqueous HCl solution at room temperature for 10 min, to convert the NH2 groups to NH3+ groups. The substrate was then rinsed with deionized water and blown dry under N2. Acoustic Manipulation of CNTs on APTES-Modified Substrate. Figure 1 shows a schematic diagram of the homebuilt setup designed for the subsequent acoustic assembly process. The acoustic manipulation of the suspended nanotubes was achieved via the piezoelectric ultrasound generator made of piezoelectric lead zirconate titanate (PZT) ceramic (Shanghai Institute of Ceramics) operating in thickness mode. When the alternating electrical voltage was applied, the PZT plate generated an ultrasound wave in the CNT aqueous dispersion. The acoustic field in turn induced forces, including time-invariant mean net forces, on the nanotubes through the solution, resulting in the alignment of the nanotubes. To elucidate the role of acoustic induced forces on the nanotube adsorption and alignment, the assembly experiments were set up in three different configurations, as shown in Figure 1, where the substrate was (a) placed perpendicular to one single radiation surface, (b) bonded on top of a radiation surface, and (c) a combination of the two radiation surface configurations in panels a and b. Several parameters, including the voltage and frequency of the alternating electric field applied to the piezoelectric ceramic and the surface charge density of the substrate (different substrates with contact angles from 30 to 50° prepared), were adjusted for controlling the nanotube assembly and adsorption density on the surface of the substrate. After the assembly, the substrate was removed and rinsed copiously with deionized water and dried in air. The aligned CNTs were then observed under field emission scanning electron microscopy (FESEM) (JEOL JSM6700).

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Figure 2. FESEM images of the acoustically assembled CNTs by applying an AC voltage of 40 V at (a) 5 kHz, (b) 30 kHz, (c) 60 kHz, and (d) 180 kHz to the piezoelectric transducer in accordance with the configuration in Figure 1a. Inserted arrows indicate the direction of acoustic propagation.

Results and Discussion (A) Acoustic Manipulation Using the Configuration in Figure 1a. For the acoustic manipulation of CNTs using the configuration in Figure 1a, Figure 2 shows the representative SEM images (JEOL JSM6700) of the acoustically aligned nanotubes with constant voltage (40 V) of different frequencies applied to the piezoelectric plate. The density of the adsorbed nanotubes increases with the vibration frequency of the piezoelectric plate from 5 to 180 kHz (Figure 2 a-d). The orientation of the nanotubes is consistent with the direction of the acoustic propagation. A slight improvement in the alignment of the nanotubes is also observed when the vibration frequency increases from 5 to 100 kHz. However, such alignment is lost at a higher vibration frequency of ∼180 kHz (Figure 2d). The voltage applied to the ceramic plate was also varied at a constant vibration frequency. As shown in Figure 2c and Figure 3, alignment was achieved for all the applied voltages (30, 40, 50, and 60 V) at a constant vibration frequency of 60 kHz. One challenge faced when aligning the nanotubes is the large curvature shown by the nanotubes, which unfavorably affects the alignment. The nanotubes assembled in the absence of acoustic field also reveal similar curvature, indicating that the curvature is not caused by the acoustic wave. Various types of multiwalled CNTs obtained from different suppliers have also been employed (data not shown here), but all the CNTs exhibited similar curvature. In order to quantitatively measure the parallel alignment effect of the nanotubes, the orientations of the nanotubes in the FESEM images were analyzed and compared to the axis of the acoustic wave propagation. Under the condition of an applied voltage of 40 V at 60 kHz, 60% of the straight nanotubes were within 10° deviation from the direction of the acoustic wave, and more than 80% of the straight nanotubes were within 45° deviation, as shown typically in Figure 2c. (B) Acoustic Manipulation Using the Configuration in Figure 1b. With the acoustic vibration perpendicular to the substrate surface as shown in Figure 1b, the nanotube adsorption density on the substrate is substantially improved, compared to the assembly under the same conditions except in the absence of the acoustic field (Figure 4). Although it is observed that vibrations in the direction perpendicular to the substrate surface

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Figure 3. FESEM images of the nanotubes assembled with the piezoelectric transducer vibrating at a constant frequency of 60 kHz, (a) 30 V, (b) 50 V, and (c) 60 V in accordance with the configuration in Figure 1a. Inserted arrows indicate the direction of acoustic propagation. The result corresponding to 40 V at 60 kHz is shown in Figure 2c.

Figure 4. Representative FESEM images of adsorbed nanotubes on Si substrates bonded to a piezoelectric transducer vibrating under an applied voltage of 40 V and at a frequency of (a) 60 kHz and (b) 100 kHz in accordance with the configuration in Figure 1b. (c) FESEM image in the absence of acoustic vibration.

Figure 5. FESEM images of the nanotubes assembled on the surface of the Si substrates, with the two piezoelectric transducers vibrating under an applied voltage of 40 V at (a) 30 kHz, (b) 60 kHz, and (c) 100 kHz, in accordance with the configuration in Figure 1c. Inserted arrows indicate the direction of acoustic propagation.

do not produce any directional preference in the nanotube alignment, this method has demonstrated a convenient way to increase the adsorption of nanotubes on the substrate. It is clear here that only the transmission of acoustic waves parallel to the substrate surface is responsible for the observed nanotube alignment (configuration “a” in Figure 1). Theoretical details will be discussed in later sections. (C) Acoustic Manipulation Using the Configuration in Figure 1c. With the intention to achieve both aligned and higher adsorption density of nanotubes, acoustic waves parallel and perpendicular to the substrate surface were applied concurrently, as shown in Figure 1c. Figure 5 shows the representative SEM images of the assembled nanotubes obtained with the two piezoelectric plates vibrating concurrently at the same applied voltage (40 V) and frequencies (30, 60, and 100 kHz). While alignment is achieved to a certain extent with both plates vibrating at relatively lower frequencies of 30 kHz (Figure 5a) and 60 kHz (Figure 5b), random nanotube orientation is

observed at the higher frequency of 100 kHz (Figure 5c). However, even with low frequencies, the alignment is not as effective as that observed using configuration “a” (Figure 1). It is believed that the interactions between the two perpendicular acoustic waves interfere with the radiation and drag forces in the horizontal direction and hence upset the alignment. Higher frequency waves may worsen the alignment due to more disordered fluidic forces along the horizontal plane. Nevertheless, the assembly density of the nanotubes is substantially improved with the increase in frequency. (D) Nanotube Assembly on Substrates with Different Charge Densities. Substrates with different degree of APTES functionalization were obtained by varying the duration of functionalization. The homogeneity and degree of functionalization were checked with contact angle measurements (rame´hart 100-00). As shown in Figures 2c and 6, while the amount of adsorbed nanotubes increased with increase in the surface’s charge density, nanotube alignment is lost at higher adsorption

CNT Alignment by Acoustic Manipulation

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Figure 7. Schematic depiction of a nanotube attached to the substrate surface at one end and forces/torques exerted on the nanotube from the incident acoustic wave and electrostatic interaction.

angle from the positive x-axis. The magnitude of the radiation force exerted on an object is given as

FR ) IYRSc

(1)

23

Figure 6. Representative FESEM images of the nanotubes adsorbed on different Si substrates with different degrees of surface functionalization. Time of functionalization and average measured contact angle are (a) 1 min, 34° and (b) 5 min, 51°, respectively. Nanotube adsorption on the substrate with 2 min of functionalization and a corresponding contact angle of 46° is shown in Figure 2c. Inserted arrows indicate the direction of acoustic propagation.

rates. These results strongly suggest that electrostatic interactions play an important role in the assembly and alignment process. (E) Theoretical Analyses on the Acoustic Alignment of Nanotubes. The transmission of ultrasound through a liquid produces different ultrasound-induced forces: linear forces such as added mass, drag, lift, and Basser forces, and nonlinear ones due to radiation pressure and drag exerted by acoustic streaming.22 Only the nonlinear forces have non-zero time-averages and so provide a mean component of the alignment force. Hence, here we consider only the effects of the nonlinear forces on the nanotubes. In our model, the nanotubes are assumed to have a straight cylindrical shape. The curved nanotubes are more complicated and may also not be suitable for future device applications involving horizontal alignment, and hence will not be theoretically discussed here. Assuming that the time-average force exerted on a nanotube that is suspended freely in the fluid is uniform throughout its entire length, the nanotube will simply move along the fluid flow direction without rotation since zero torque is generated. However, when one end of the nanotube touches and is adsorbed onto the substrate, such forces will result in the rotation of the nanotube. Therefore, here we consider a nanotube with one end anchored to the substrate surface, which is assumed to be a cylinder of length L and diameter D. A spherical polar coordinate system (r,θ,φ) as shown in Figure 7, in which the origin of the system is located at the anchored end of the nanotube, is used. The variable r represents the radial distance of an arbitrary point P on the nanotube from the origin, φ is the zenith angle from the positive z-axis, and θ is the azimuth

where Sc is the cross-sectional area, I is the mean intensity of the ultrasound, and YR is a dimensionless factor related to the properties of the fluid medium and shape of the object. Consider an infinitely small segment dr of the nanotube at point P, the radiation force dFR on the segment can be expressed as

dFR ) YRI xcos2 φ + sin2 φ sin2 θ Ddr

(2)

The acoustic field in the boundary layer at the surface of an object will generate a parallel acoustic streaming motion, and the interaction between this mean flow and the object will lead to a drag force that is proportional to the object’s surface area.22 For a nanotube, the drag force is given as

dFST ) YSTf(θ,φ)IDdr

(3)

where YST and f(θ,φ) are two factors related to the property (particularly the viscosity) of the fluid medium and the orientation of the nanotube, respectively. In addition to the forces generated by the ultrasound, the attractive electrostatic interaction between the nanotube and the substrate surface is also deemed important for the assembly process. In the case of a nanotube anchored on a infinitely large substrate, an uniform electric field strength of σ / 2ro, where σ is the surface charge density of the substrate,24 directed normally away from the surface, can be assumed. The electrostatic force dFES exerted on dr of the nanotube is hence

dFes )

πDσnσ dr 2ro

(4)

where σn is the surface charge density of the nanotube, and r and 0 are the relative dielectric constant of the fluid medium and the absolute dielectric constant of vacuum, respectively. While FR and FST are in the x-direction, FES is in z-direction. In order to evaluate their contribution to the rotation of the anchored nanotubes, we need to determine the forces and hence the resultant torques along the θ direction, and compare them with that along the φ direction. On the basis of Figure 7, the

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force components of dFR and dFST that give rise to a force dFθ along the θ direction are given as

From eqs 7 and 8, the following relationship between Mθ and Mφ is established:

dFθ ) (YR sin θ xcos2 φ + sin2 φ sin2 θ + YST sin θf (θ,φ))IDdr (5)

πDσnσL tan θ Mθ ) tan θ tan φ M cos φ φ 40r

On the other hand, from the geometric relationship, the sum of the force components of dFR and dFST, and that of dFES will give rise to a force dFφ along φ and is given as

The alignment of the nanotube is mainly driven by Mθ, and, according to eq 10, Mθ is directly proportional to the square of the applied voltage V and the square of the angular frequency ω. An increase in frequency also leads to a decrease in the denominator on the right side of eq 10 when ωτ < π, which correlates well with the experimental observations where the frequency is below 200 kHz (corresponding to ∼π/8). This explained the observed improvement in nanotube alignment with increased voltage and frequency of the electric field applied to the transducer in configuration “a” in Figure 1. However, the loss in alignment at the higher frequency of 180 kHz (Figure 2d) cannot be explained solely with eq 10. We believe that the anchored nanotube must first rotate to its aligned position along the acoustic wave direction before irreversibly attaching on the surface of the substrate. An increase in V, ω, and the resulting acoustic intensity in the fluid not only increases Mθ but also Mφ, according to eqs 11 and 12. Thus, an overly large Mφ may result in the rapid deposition of nanotubes without adequate chance for alignment. The loss in alignment observed at high charge densities of the substrate surface (Figure 6 and Figure 2c), where the electrostatic interaction between the nanotube and the substrate surface is strong, can also be due to an excessively large Mφ in eqs 11 and 12. Since the increase in electrostatic force contributes only to Mφ but not Mθ, the possibility of obtaining well-aligned nanotubes is lower with higher surface charge density. From eq 12, it can be noted that the effect of the enhancement of acoustic intensity on increasing Mφ and Mθ is strongly dependent on the orientation of the nanotube. The ratio between ∆Mθ and ∆Mφ is given as tan θ/cos φ, that is, the change in Mθ is larger than that of Mφ at a larger θ. However, as the nanotube is aligned along the acoustic transmission direction, θ and the alignment torque decrease, and, consequently, the rotation momentum of the nanotube will decrease and eventually be lost when the nanotube orientation approaches θ ) 0° (see also eq 10). The increase in φ with the nanotube approaching the full attachment onto the substrate (φ ) 90°) will render the alignment more difficult as a result of the increased torque from the electrostatic interaction (the first term in eq 11). Therefore, it is critical to align the nanotube before φ approaches 90°. The observed improvement in nanotube adsorption density in the presence of mechanical vibration using configuration “b” in Figure 1 is possibly attributed to the improved probability of contact between the nanotubes and the moving substrate surface.

dFφ )

(

πσnσ sin φ + 2r0

cos θ cos φ xcos2 φ + sin2 φ sin2 θ YRI + cos θ cos φf

)

(θ,φ)YSTI Ddr (6) Thus, the torque Mθ of the nanotube with a total length L (i.e., φ is constant) is

Mθ )

∫0L rdFθ ) IDL 2

2

sin θ(YR xcos2 φ + sin2 φ sin2 θ + YSTf(θ,φ)) (7)

and the torque Mφ of the nanotube with a total length L (i.e., θ is constant) is

Mφ )

∫0

L

πDσnσL2 IDL2 rdFφ ) sin φ + cos θ cos φ 40r 2 (YR xcos2 φ + sin2 φ sin2 θ + YSTf(θ,φ)) (8)

The intensity of the acoustic sound I generated by a piezoelectric transducer vibrating in thickness mode with a characteristic impedance Z0 in a uniform fluid medium of impedance Z1 can be expressed as

I)

YaZ1e332V2ω2 2b2(Z12 + Z02 cot2({ωτ}/{2}))

(9)

where Ya is a factor related to the characteristics of the transducer and fluid, e33 is the piezoelectric force constant of the transducer in thickness mode, V is the voltage applied over the thickness b of the transducer, ω is the angular frequency of the applied voltage, and τ is the traveling time of sound within the transducer plate, that is, b/c, where c is the velocity of the acoustic wave in the plate.25 Substituting eq 9 into eqs 7 and 8, we have

Mθ )

YaZ1e332V2ω2DL2 4b2(Z12 + Z02 cot2(ωτ/2))

sin θ

(YR xcos2 φ + sin2 φ sin2 θ + YSTf(θ,φ)) (10) and

Mφ )

πDσnσL2 sin φ + 40r YaZ1e332V2ω2DL2 4b2(Z12 + Z02 cot2(ωτ/2))

cos θ cos φ

(YR xcos2 φ + sin2 φ sin2 θ + YSTf(θ,φ)) (11)

2

(12)

Conclusion We have proposed and demonstrated a convenient approach for the large-scale, parallel alignment of CNTs on a solid substrate via a solution-based acoustic manipulation technique. Using a substrate with an appropriate degree of surface functionalization, parallel alignment of CNTs on the substrate in the direction of the acoustic radiation was observed. First, a theoretical model derived from analyses of the forces and torques exerted on a CNT in fluid under acoustic radiation was established. In this model, the rotation of the nanotube is assumed to occur only when one end of the nanotube is anchored to the substrate. The key roles of the acoustic radiation force,

CNT Alignment by Acoustic Manipulation drag force due to the acoustic wave-induced fluidic streaming, and the electrostatic interaction between the surface-functionalized nanotubes and substrate in controlling the assembly have been considered. The experimental observations of the effects of various parameters on the CNTs alignment were explained using the theoretical model. In order to achieve effective alignment, the interaction between the nanotube and substrate surface must be weak to prevent too rapid adsorption of nanotubes on the surface, but sufficiently strong for good coverage density of the nanotubes. Hence, the parallel alignment of high coverage density nanotubes can be achieved via a delicate balance between the rotational torques arising from acoustic force on the nanotubes and the attractive interaction between the nanotube and surface. We have also demonstrated a convenient way to increase the adsorption of CNTs on a surface via the introduction of a mechanical vibration of the substrate. The results suggest that this solution-based alignment approach is readily scalable and potentially extendable to other material systems. It can be used as a general strategy for the horizontal organization of nanowires and nanotubes into functional devices or as electrical interconnects. The theoretical model also provides a general methodology for the analysis of the aligned assembly of nanotubes or nanowires on a substrate. Acknowledgment. We thank Dr. S. Shanmugavel and H. Liu from IMRE for valuable discussions on the theoretical model, and Dr. J. M. Miao and Dr. Q. Zhang from Nanyang Technological University, Singapore, Dr. Y. Wang from the Institute of Microelectronics (IME), and Dr. J. Y. Lin from the Institute of Chemical Engineering and Science (ICES), Singapore, for their assistance in the project. The work is financially supported by an A*STAR SERC grant under the Thematic Strategic Research Programme (TSRP) in Singapore (Grant No. 042 114 0043). References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (2) Li, S.; Yu, Z.; Yen, S.-F.; Tang, W. C.; Burke, P. J. Nano Lett. 2004, 4, 753.

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