Reliable Diameter Control of Carbon Nanotube Nanobundles Using Withdrawal Velocity
© The Author(s). 2016
Received: 24 February 2016
Accepted: 26 August 2016
Published: 1 September 2016
Carbon nanotube (CNT) nanobundles are widely used in nanoscale imaging, fabrication, and electrochemical and biological sensing. The diameter of CNT nanobundles should be controlled precisely, because it is an important factor in determining electrode performance. Here, we fabricated CNT nanobundles on tungsten tips using dielectrophoresis (DEP) force and controlled their diameters by varying the withdrawal velocity of the tungsten tips. Withdrawal velocity pulling away from the liquid–air interface could be an important, reliable parameter to control the diameter of CNT nanobundles. The withdrawal velocity was controlled automatically and precisely with a one-dimensional motorized stage. The effect of the withdrawal velocity on the diameter of CNT nanobundles was analyzed theoretically and compared with the experimental results. Based on the attachment efficiency, the withdrawal velocity is inversely proportional to the diameter of the CNT nanobundles; this has been demonstrated experimentally. Control of the withdrawal velocity will play an important role in fabricating CNT nanobundles using DEP phenomena.
Carbon nanotubes (CNTs) have intrinsically large surface areas (700–1000 m2 g−1), chemical stability, and excellent mechanical and electrical properties, including an extremely high conductance and aspect ratio . For these reasons, CNTs are widely used as electrode materials for improving biocompatibility and additional functional properties [2–5]. Based on these favorable characteristics, the use of CNTs for biosensor applications, in the form of modified electrode [6–8], polymer-CNT composites , and CNT membranes  has received much attention.
CNT nanobundles have contributed to the development of nanotechnology providing many advantages in nanoscale imaging, fabrication, and electrochemical and biological sensing due to their superior geometric, electronic, chemical, and mechanical properties [11–16]. Various methods have been developed to attach or directly grow CNTs on the apex of micro-tips, such as a tungsten tip or an atomic force microscope (AFM) tip [17–23]. However generally, these techniques are time-consuming and not viable as commercial techniques. To overcome these limitations, new fabrication methods for attaching CNTs on a tip apex have been introduced using dielectrophoresis (DEP) phenomena [24–26] which are non-invasive, non-destructive, and active methods for the manipulation, alignment, and separation of particles at the microscale [27–29].
Generally, it is important to control the diameter of the electrode because it can determine electrode performance. Parameters such as withdrawal velocity, applied voltage and frequency, and CNT concentration are important factors in determining the electrode diameter. However, few studies exist regarding control of the diameter of CNT nanobundles attached to a tip. Kim et al. performed a numerical simulation of CNT nanobundles attached to an AFM tip, excluding any finite-size effect of CNTs on the DEP force and torque . Wei et al. showed that the diameter of CNT nanobundles grew asymptotically as the voltage increased from 5 to 19 V, and tip wetting and geometry played a key role using eight-type scanning probe microscopy (SPM) probes . However, no previous study has reported the effects of the withdrawal velocity, which is a reliable parameter to control the diameter of CNT nanobundles.
In this study, we fabricated CNT nanobundles using DEP phenomena. The withdrawal velocity of a tungsten tip was controlled precisely using a one-dimensional (1-D) motorized stage, and the fabricated CNT nanobundles were observed by scanning electron microscope (SEM). The effect of the withdrawal velocity on the diameter of CNT nanobundles was investigated theoretically and experimentally.
Preparation of Sharpened Tungsten Tips
To obtain a smooth and sharp tip apex without the “neck-in” phenomenon, the tungsten wire was steadily shuttled up and down during the electrochemical reaction. Figure 1b shows a SEM image of the fabricated sharpened tungsten tip obtained by this process.
Preparation of the CNT Suspension
Experimental Setup for Fabricating CNT Nanobundles
Results and Discussion
The Force Acting on CNTs at the Liquid–Air Interface
where A, d, and x are the Hamaker constant, the diameter of the CNTs, and the space between the CNTs, respectively. Generally, the Hamaker constant representing the attraction force between molecules, SWNTs in this case, is assumed to be ~0.84 × 10−19 J [33, 34]. Thus, the van der Waals force is about F vdW = 168 nN/μm, assuming that the diameter of the CNTs is about 10 nm (Fig. 2) and the interlayer spacing of the CNTs is x = 0.34 nm.
When CNTs are removed from a liquid–air interface, they experience a shear force (~capillary force, F cap) caused by the surface tension of a liquid. If this force acting on the CNTs is larger than the adhesion force between the CNTs and CNT nanobundle, CNTs cannot escape from the suspension and are finally washed out. Dujardin et al. found that the capillary force acting on CNTs was about 130~170 mN/m at a water-based interface with the surface tension of which was 70 mN/m at room temperature . When the diameter of the CNTs is about 20 nm (Fig. 2), the maximum capillary force acting on CNTs is about F cap = 10 nN. This value is much smaller than the adhesion force between the CNTs and a synthesized CNT nanobundle (F vdW = 200 nN/μm). Therefore, most of the CNTs were attached to a synthesized CNT nanobundle and were used to constitute a CNT nanobundle.
Effect of Withdrawal Velocity on CNT Nanobundle Diameter
Under the same conditions (electrode size, electric field, and concentration of CNT suspension), the same amount of CNTs gather in the same area of the meniscus. Generally, one of the most important parameters to determine the diameter of CNT nanobundles is the attachment efficiency. The attachment efficiency is the ratio of particle numbers striking the collector to that around the collector and proportional to the diameter of the collector . In this study, a synthesized CNT nanobundle (the tip of the CNT nanobundle being synthesized) acted as a collector.
In conclusion, a non-uniform electric field (positive DEP) was used to manipulate CNTs on a tungsten tip to fabricate a high-aspect-ratio CNT nanobundle. The withdrawal velocity of the tungsten tip was controlled automatically and precisely using a 1-D motorized stage. When the tungsten tip was pulled away from liquid–air interface, the capillary and van der Waals forces determined the diameter of the CNT nanobundle. Most of the CNTs attached to an electrode can be used to constitute a CNT nanobundle, because the capillary force acting on the CNTs is much smaller than the van der Walls force between the CNTs. It was determined that the withdrawal velocity of the tungsten tip was inversely proportional to the CNT nanobundle diameter, based on attachment efficiency. Finally, this was confirmed by experimentally controlling the withdrawal velocity from 1 to 10 μm/s under a constant AC electric field. We anticipate that the control technique using the withdrawal velocity could provide an important method for fabricating CNT nanobundles.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NO. 2015R1A2A1A14027903) and the Korea National University of Transportation in 2015.
JHS, TA, and WC designed experiments. JHS and KK carried out the experiments, and JHS wrote this manuscript and created the figures. JHS and WC analyzed the force acting on CNTs and the effect of withdrawal velocity on the CNT nanobundle. All authors discussed the results and commend on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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