Site-specific growth and density control of carbon nanotubes by direct deposition of catalytic nanoparticles generated by spark discharge
© Na et al.; licensee Springer. 2013
Received: 31 July 2013
Accepted: 25 September 2013
Published: 4 October 2013
Catalytic iron nanoparticles generated by spark discharge were used to site-selectively grow carbon nanotubes (CNTs) and control their density. The generated aerosol nanoparticles were deposited on a cooled substrate by thermophoresis. The shadow mask on top of the cooled substrate enabled patterning of the catalytic nanoparticles and, thereby, patterning of CNTs synthesized by chemical vapor deposition. The density of CNTs could be controlled by varying the catalytic nanoparticle deposition time. It was also demonstrated that the density could be adjusted by changing the gap between the shadow mask and the substrate, taking advantage of the blurring effect of the deposited nanoparticles, for an identical deposition time. As all the processing steps for the patterned growth and density control of CNTs can be performed under dry conditions, we also demonstrated the integration of CNTs on fully processed, movable silicon microelectromechanical system (MEMS) structures.
KeywordsCarbon nanotubes Thermophoresis Patterning Spark discharge Chemical vapor deposition
Carbon nanotubes (CNTs) have attracted an enormous amount of attention from many researchers, who have found numerous device applications [1–3] taking advantage of their unique properties. Integrating CNTs into devices inevitably requires control of their location and/or density [4, 5]. Controlling the synthetic location has been achieved mainly by depositing the metal catalysts in a controlled and patterned way for the following chemical vapor deposition (CVD) process.
Typically, patterning catalytic metals has been achieved using the lift-off technique, which consists of a conventional photolithography process and thin film deposition . Alternative patterning methods such as soft lithography  or depositing catalytic thin films through shadow masks  have also been introduced. In these methods, however, either the catalytic film deposition requires a high-vacuum system [6, 8] or the number of process repetitions is limited by the low durability of the stamp . Although electroplating or electroless plating techniques [9–11] can be used to grow CNTs site-selectively and to control the density of the CNTs, these wet process approaches are not suitable for fully processed, movable silicon microelectromechanical system (MEMS) structures.
In this study, we used the spark discharge method to generate catalytic aerosol nanoparticles for CNT synthesis and patterned the particle-deposited area using a shadow mask and the thermophoresis effect [12, 13]. With the patterned nanoparticles, site-specific growth of CNTs was demonstrated. Moreover, the blurring effect caused by adjusting the gap distance between the shadow mask and the substrate allowed us to deposit the catalytic nanoparticles with adjustable and location-specific densities so that the density of synthesized CNTs could be controlled differently on a single substrate. The spark discharge technique is performed using simple equipment without any high-vacuum system, and it generates and deposits the catalytic nanoparticles under dry conditions and atmospheric pressure. In addition, the shadow mask can be used repeatedly without clogging or chemical damage, and all the fabrication steps including thermophoresis are scalable to wafer scale. Furthermore, it is possible to integrate CNTs directly onto microstructures with high aspect ratios utilizing the shadow masking technique, which is difficult with conventional photolithographic patterning of a catalytic layer.
The exemplary applications of the suggested process could be field emission devices and gas sensors. Many of field emitters adopt CNTs for their electron emission tips, and the density of CNTs in this case is directly related to the current density of the device. Hence, it is important to adjust the density of CNTs which enables the device to get the desired field emission performance [14, 15]. So the suggested process which can control the density of CNTs may be used as in this application. In addition, a gas sensor is usually fabricated as resistor type where the target gas is absorbed onto CNTs and changes the resistance of CNTs connecting the electrodes. Because the sensitivity of the sensor and the density of CNTs are closely related, it is needed to adjust the density of CNTs ; thus, this process could be also used to fabricate the gas sensor with enhanced sensitivity.
We were able to analyze the size distribution of the nanoparticles before deposition through a scanning mobility particle sizer (SMPS). The aerosol that flowed into SMPS through nitrogen at 500 sccm was analyzed for 150 s to measure the size and number of the nanoparticles, and the measurement was repeated five times to calculate the average value. Through this analysis, we were able to find the size distribution of nanoparticles in the aerosol; the diameter of the nanoparticles was distributed from 4.5 to 165.5 nm, and the mean diameter was 40.8 nm.
CNTs were synthesized by thermal CVD in a furnace. The SiO2 substrate was separated from the shadow mask and loaded into the quartz tube of the furnace for thermal CVD at a pressure of several millitorr. Nitrogen gas was passed through the quartz tube to prevent the oxidation of the iron catalyst and to clean the inside while the temperature was increasing up to 700°C. When the temperature stabilized, the carrier gas was replaced with a mixture of ammonia gas and acetylene gas for 10 min. In order to grow CNTs vertically, a mixture ratio of 3:1 was used, i.e., 90 sccm of ammonia gas and 30 sccm of acetylene gas .
Results and discussion
As shown in Figure 3b, there were CNTs of low density with an unclear pattern when the deposition time was less than 10 min. However, with over 10 min of catalytic nanoparticle deposition time, vertically aligned CNTs were grown with high density forming a clear line pattern. Moreover, we found that the density of CNTs decreased and pattern fidelity deteriorated due to CNTs grown outside the pattern as shown in Figure 3d when the catalytic nanoparticle deposition time was over 40 min. In conventional synthesis result using Fe thin film catalyst, when the Fe thin film deposited is too thin or thick, the quality of CNTs such as density, directionality, and length becomes worse . For a similar reason, it is considered that the density of CNTs is decreased by depositing an excessive amount of catalytic Fe nanoparticles through the spark discharge method. The longer deposition time may also cause an excessive blurring effect of line patterns, increasing the number of CNTs grown outside the pattern and making the pattern fidelity worse. It is concluded from this experiment that there would be an optimized deposition time for clear pattern boundaries and high density of CNTs in the proposed method, and the excessive deposition of catalytic particles resulted in blurred boundary of CNT pattern and reduced density of the CNTs grown.
In conclusion, we demonstrated for the first time that the nanoparticles generated using the spark discharge method can be used successfully as catalysts for the growth of CNTs. The nanoparticles were transferred onto the desired area on a substrate by thermophoresis and were patterned using a shadow mask to realize patterned growth of CNTs. The nanoparticle deposition time determines the final density of the grown CNTs, and vertically aligned growth of CNTs was achieved after 10 min of nanoparticle deposition in our experiment. An alternative approach to changing the density of CNTs was to change the gap between the shadow mask and the substrate, and a patterned line of CNTs with gradually varying density along the line could be formed by tilting the shadow mask. The proposed all-dry process could also be applied to completely fabricated micromechanical structures, as demonstrated by site-specifically growing the CNTs on the released high-aspect-ratio microstructures.
Chemical vapor deposition
Scanning electron microscopy
This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (grant no. NRF-2011-0030206) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2043661).
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