Selective Deposition and Alignment of Single-Walled Carbon Nanotubes Assisted by Dielectrophoresis: From Thin Films to Individual Nanotubes
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Dielectrophoresis has been used in the controlled deposition of single-walled carbon nanotubes (SWNTs) with the focus on the alignment of nanotube thin films and their applications in the last decade. In this paper, we extend the research from the selective deposition of SWNT thin films to the alignment of small nanotube bundles and individual nanotubes. Electrodes with “teeth”-like patterns are fabricated to study the influence of the electrode width on the deposition and alignment of SWNTs. The entire fabrication process is compatible with optical lithography-based techniques. Therefore, the fabrication cost is low, and the resulting devices are inexpensive. A series of SWNT solutions is prepared with concentrations ranging from 0.0125 to 0.2 mg/ml. The alignment of SWNT thin films, small bundles, and individual nanotubes is achieved under the optimized experimental conditions. The electrical properties of these samples are characterized; the linear current–voltage plots prove that the aligned SWNTs are mainly metallic nanotubes. The microscopy inspection of the samples demonstrates that the alignment of small nanotube bundles and individual nanotubes can only be achieved using narrow electrodes and low-concentration solutions. Our investigation shows that it is possible to deposit a controlled amount of SWNTs in desirable locations using dielectrophoresis.
KeywordsSingle-walled carbon nanotubes (SWNT) Dielectrophoresis (DEP) Thin film Nanotube bundle Individual nanotube Deposition
Carbon nanotube (CNT) is a unique form of carbon material with remarkable physical, chemical, and electrical properties . It has attracted considerable attention in the last 20 years. CNTs have a high potential in a great number of applications, especially in nanoelectronics and biomedical sensors. A wide variety of electronic devices based on individual single-walled carbon nanotubes (SWNTs) or SWNT thin films have been successfully developed and used as sensors [2, 3, 4], field-effect transistors [5, 6], conductive interconnects [7, 8], and energy storage systems [9, 10]. A critical step to obtain these practical devices is to deposit well-organized and highly aligned SWNTs in desired locations. Recently, researchers have developed a number of methods to align SWNTs: using moving fluids to organize nanotubes , introducing gas flows in reactors or channels , withdrawing microfluidic channels from solutions , spin coating nanotube dispersions with controlled speeds [14, 15], and magnetic capturing of nanotubes . However, many of these techniques have limitations and restrictions, because they require either intensive preparation processes or assisting materials with special properties. Therefore, their applications are relatively limited.
In comparison, dielectrophoresis, a simple but versatile method, has proven to be effective in aligning SWNTs in small and large scales [17, 18]. This method can be conducted at room temperature with low voltages. In addition, a number of parameters such as solution concentration, deposition time, AC source amplitude, and frequency can be adjusted to optimize the quality of the aligned SWNTs. More importantly, dielectrophoresis can be easily incorporated into device fabrication [19, 20] and eventually used in wafer-level-controlled deposition . Recently, devices based on dielectrophoresis-aligned SWNTs have been developed and used as biocompatible substrates for cell growth , bacteria capturing chips , gas sensors , and memory devices . Numerical studies have also been performed to provide theoretical support of the process [26, 27]. However, most of these research efforts are focused on the alignment of SWNT thin films. Even though the controlled assembly of single SWNT bundles has been studied by using various voltage magnitudes and types , a thorough investigation into electrode geometry and solution concentration is still necessary to achieve the precise alignment of individual SWNTs and small nanotube bundles.
In this paper, we examine the selective deposition of SWNTs with dielectrophoresis to obtain aligned nanotubes in the form of thin films, small bundles, and individual nanotubes. These different results are achieved by changing a number of parameters in the dielectrophoresis process and the SWNT samples. Pristine SWNTs are treated with acids for surface functionalization and diluted with deionized (DI) water to obtain different concentrations. The SWNT thin films are deposited and aligned using a large-width electrode design; the alignment of nanotube bundles and individual nanotubes is achieved by using a small-width “teeth”-like electrode design. The “teeth”-like electrodes are used to induce concentrated and highly directional electric field in between two opposite “teeth”. Consequently, the electric field attracts SWNTs to this location and rotates them to follow the electric field lines. The electrodes are fabricated with optical lithography and wet etching; expensive equipment, such as electron-beam writer, commonly used in the fabrication of individual nanotube devices is avoided. The dielectrophoresis experiments are conducted at room temperature. Scanning electron microscopy (SEM) inspection shows that the SWNTs are well aligned in desired locations. Electrical characterization of these SWNT devices demonstrates that they have linear current–voltage (I–V) curves, and their resistance is dependent on the SWNT solution concentration. The fabrication steps, the dielectrophoresis process, the quality of the aligned nanotubes and thin films, and the characterization results are described and discussed in this paper.
where V is the volume of the particle, εp and εm are the permittivities of the particle and the medium, E is the external electric field strength, AL ≈ 4r2/l2[ln (l/r) − 1] is the depolarization factor, and r, l are the radius and length of the ellipsoidal particle, respectively.
These equations indicate that the dielectrophoresis of SWNTs is affected by many factors including the dimensions of the nanotubes, the properties of the medium, and the strength of the electric field. In our investigation, the following parameters are adjusted to control the alignment of the nanotubes: bias voltage, frequency, deposition time, width of the electrodes, and nanotube solution concentration.
Previous research demonstrates that the polarization along the longitudinal direction is much higher than that along the transverse direction for metallic SWNTs, but comparable for semiconducting SWNTs . This is because the metallic SWNTs have a larger Re[fcm], and the dielectrophoresis force exerted on them is much stronger than that experienced by the semiconducting SWNTs. Therefore, we expect that the metallic SWNTs dominate the movement of SWNT bundles in the dielectrophoresis process in our experiments.
The pristine SWNTs (outer diameter: 2 nm, length: 10 μm, purity: >90%), in the form of powder, are purchased from SES Research Inc. (Houston, TX). Because dielectrophoresis requires that the SWNTs used in the process are free to move and rotate in a medium, the pristine SWNTs are first treated with chemicals to increase the solubility in water. This step is achieved by the surface functionalization of SWNTs using a mixture of strong acids H2SO4:HNO3(volume ratio 3:1) . The acid treatment induces the covalent attachment of carboxylic (–COOH) groups on the surfaces and open ends of SWNTs. As a result, the SWNTs can be uniformly dispersed in DI water and remain stable for a long period of time (10–12 months). The nitric acid in the mixture can also purify the SWNTs by removing amorphous carbon, carbon particles, and other impurities . Next, the functionalized SWNTs are diluted with DI water and filtered with a polyvinylidene fluoride (PVDF) filtration membrane (with an average pore diameter of 0.22 μm) repeatedly for 5–6 times until the pH value of the dispersion reaches five. After dilution, the SWNT dispersion is treated with an ultrasonic process for 30 min and followed by the PVDF filtration to collect the purified and functionalized SWNTs. Last, the collected SWNTs are diluted with DI water to obtain different concentrations: 0.2, 0.1, 0.05, 0.025, and 0.0125 mg/ml.
The electrodes are fabricated on 4-inch silicon wafers. The wafers are covered by a 200-nm-thick thermal-grown SiO2 insulating layer. Metal layers of Cr (100 nm, adhesion material) and Au (200 nm, electrode material) are coated on the wafer surface with sputtering. One of the biggest advantages of dielectrophoresis in nanotube alignment is its potential in wafer-level-controlled deposition, which is compatible with the parallel micro/nanofabrication processes used in the semiconductor industry. Therefore, series (and often expensive) nanofabrication processes using equipment such as electron-beam lithography and scanning-probe lithography should be avoided. In our investigation, the entire fabrication process is compatible with the traditional microfabrication technology. We use optical lithography with a hard contact aligner, a positive photoresist (Shipley S1813), and controlled wet etching to obtain electrodes with sharp tips which can dramatically strengthen the local electric field. The parameters for the etching steps are 120 s for Cr etching using a standard chromium mask etchant and 20 s for Au etching using a gold etchant (Type TFA from Transene Inc.). After the controlled etching process, the actual widths of the electrodes are reduced to 2–3 (designed width: 5 μm) and 0.5–1 μm (designed width: 3 μm).
Results and Discussion
The selective deposition and alignment of nanotube bundles and individual nanotubes are highly repeatable. This method can be used in the development of individual nanotube devices and systems. This may give nanotube-based electronics a wide range of new opportunities. For example, in addition to the traditional electronic applications as field-effect transistors and circuits, devices with a controlled amount of SWNTs can be used as high-performance sensors for chemical sensing, gas detection, and DNA analysis. Even though the fabrication and deposition steps presented in this paper are still used for small-scale processes, they can be easily extended to large-scale production. Furthermore, because the entire process is compatible with the traditional microfabrication technology, it has a high potential to be used in wafer-level fabrication to produce identical devices across the entire surface of the substrate.
In conclusion, we have successfully deposited and aligned nanotube thin films, small bundles, and individual nanotubes at desired locations. The different alignment results are achieved by using various electrode designs and SWNT solutions with various concentrations. In general, electrodes with large widths (400 and 5 μm) generate an eventually distributed electric field with parallel field lines; the aligned SWNTs are in the form of thin films or sparsely distributed bundles. For electrodes with small width (3 μm), the electric field is highly concentrated and can induce individual nanotube deposition and alignment. The electrical characterization shows that the aligned SWNTs have a highly linear current–voltage relationship, which proves that the aligned SWNTs are mostly metallic nanotubes. In addition, the fabrication process is compatible with the traditional microfabrication technology and has a high potential to be used in the wafer-level fabrication in the future. The method presented in this paper can be used in a wide range of applications, especially in the development of individual nanotube-based devices.
This work was supported by the Washington State University New Faculty Seed Grant (award ID: 110300_001).
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