Conductivity enhancement of multiwalled carbon nanotube thin film via thermal compression method
© Tsai et al.; licensee Springer. 2014
Received: 27 June 2014
Accepted: 15 August 2014
Published: 29 August 2014
For the first time, the thermal compression method is applied to effectively enhance the electrical conductivity of carbon nanotube thin films (CNTFs). With the assistance of heat and pressure on the CNTFs, the neighbor multiwalled carbon nanotubes (CNTs) start to link with each other, and then these separated CNTs are twined into a continuous film while the compression force, duration, and temperature are quite enough for the reaction. Under the compression temperature of 400°C and the compression force of 100 N for 50 min, the sheet resistance can be reduced from 17 to 0.9 k Ω/sq for the CNTFs with a thickness of 230 nm. Moreover, the effects of compression temperature and the duration of thermal compression on the conductivity of CNTF are also discussed in this work.
Carbon nanotubes (CNTs) have attracted much attention because of their high aspect ratio, large current capability, high mechanical strength, good chemical inertness, and high thermal conductivity [1, 2]. CNT can be produced by numerous techniques such as chemical vapor deposition (CVD) method , arc-discharge method , and laser ablation method . Among these methods, the CVD method is the most attractive way because of the possibility for mass production, selective growth, and well controllability in length. However, a high-temperature process is necessary for the growth of high-quality CNT via CVD method, and it is the high-temperature process that restricts some applications of CVD-grown CNTs. Therefore, the CNT solution is regarded as another way to realize a low-temperature and large-area process while the high-temperature process for the CNT growth is isolated from the deposition of CNT solution. The CNT solution can be then deposited onto a substrate to form a carbon nanotube thin film (CNTF) by various methods [6–8]. Nevertheless, the conductive resistance of a pristine CNTF is still too high to meet the requirements in practical use nowadays. And the high resistance of CNTF is majorly attributed to the defects of tubes and the junctions between CNTs as well as the latter dominated the overall conductance [9, 10].
To improve the conductivity of pristine CNTF, B. Pradhan et al.  have introduced a composite of CNT and polymer to increase mobility for carrier transport. Y. S. Chien et al.  have reported the laser treatment on a Pt-decorated CNTF for enhancing the efficiency of the dye-sensitized solar cells. Also, M. Joo and M. Lee  applied the laser treatment on a solution-deposited CNTF for improving its conductivity. Although these reported literatures made some progress on the enhancement of conductivity for CNTFs, the complex processes, expensive equipments of laser systems, and contamination issues might restrict the applications of such reported CNTFs in future devices. In this work, a simple, low-cost, and low-temperature method of thermal compression is utilized to effectively enhance the electrical conductivity of CNTFs for the first time. The effects of compression temperature and the duration of thermal compression on the conductivity of CNTF are also discussed. In addition, a possible mechanism for the structure transformation of CNT is suggested to understand the influence of thermal compression on the conductivity of CNTF.
The multiwalled CNTs were grown at 700°C via a thermal chemical vapor deposition system under the acetylene, nitrogen, and hydrogen ambience. The as-grown CNTs were scraped off from the substrate, and then the derived 0.03-g CNTs were suspended in a mixture of concentrated H2SO4 (95%), HNO3 (70%), and deionized water for 15 min at 140°C to enhance the solubility of CNTs in the following solvents. The filtered CNTs were rinsed by deionized water to remove the acidic residues. Afterwards, these acid-treated CNTs were dissolved in a mixture of ethanol and ethylene glycol and then ultrasonicated in ice bath for 3 h. After centrifugalizing, a homogeneous CNT solution with an approximate 0.5-mg/ml concentration of CNTs was sprayed onto glass substrates (Eagle 2000, Corning Display Technologies Taiwan Co., Ltd, Taipei, Taiwan) at 200°C to form the CNTFs. The thickness of CNTF could be adjusted by varying the spray times, and therefore, the 110-nm-thick and 230-nm-thick CNTFs on the glass substrates were obtained, respectively. Subsequently, two glass substrates, one was deposited with CNTF and the other was a bare glass substrate, were face-to-face compressed with a force of 100 N. The thermal compression temperature was varied from room temperature to 400°C, and the compression duration changed from 0 to 50 min.
Results and discussion
In summary, the carrier transports with a high conductivity are obtained due to the lower junction barrier at the joints of linked CNTs after the thermal compression. Therefore, the sheet resistance of the 230-nm-thick CNTF decreases to 0.9 k Ω/sq with the compression temperature of 400°C and the compression force of 100 N for 50 min. Moreover, the sheet resistance of the 110-nm-thick CNTF can be reduced by over 30 times after the thermal compression to 1.1 k Ω/sq. These results for the multiwalled CNT thin films are impressive and indicate that the thermal compression method is an effective way to enhance the conductivity of CNTF. The highly conductive CNTFs after the thermal compression with the simple, low-cost, and low-temperature processes facilitate the applications of such CNTFs in the electrodes of supercapacitors, fuel cell, photovoltaic cells, and so on.
W-LT (Wan-Lin Tsai) received the B.S. degree in Electronics Engineering from National Chiao Tung University (National Chiao Tung University), Hsinchu, Taiwan, in 2004. He is currently pursuing the Ph.D. degree at the Department of Electronics Engineering in National Chiao Tung University, Hsinchu, Taiwan. His research interests include carbon nanotube and graphene in the applications of biosensor, field emission, and electronic devices.
K-YW (Kuang-Yu Wang) received the B.S. degree in Materials Science and Engineering from National Tsing Hua University (National Tsing Hua University), Hsinchu, Taiwan, in 2006. He is presently a Ph.D. student at the Department of Electronics Engineering in National Chiao Tung University (National Chiao Tung University), Hsinchu, Taiwan. His research interests include nanomaterials and biosensors.
Y-JC (Yao-Jen Chang) is currently pursuing the Ph.D. degree at the Department of Electronics Engineering in National Chiao Tung University (National Chiao Tung University), Hsinchu, Taiwan. His research interests include 3D IC, chip bonding, and electronic devices.
Y-RL (Yu-Ren Li) received the B.S. degree in Physics from National Cheng Kung University (National Cheng Kung University), Tainan, Taiwan, in 2005. She is presently a Ph.D. student at the Department of Electronics Engineering in National Chiao Tung University (National Chiao Tung University), Hsinchu, Taiwan. Her research interests include metal oxide, nanomaterials, and UV detectors.
P-YY (Po-Yu Yang) received his B.S. degree from the Institute of Display in National Chiao Tung University, Hsinchu, Taiwan, in 2007. He received the Ph.D. degree at the Department of Electronics Engineering in National Chiao Tung University (National Chiao Tung University), Hsinchu, Taiwan, in 2011. He now works in Taiwan Semiconductor Manufacturing Company, Hsinchu, Taiwan. His research interests include the applications of the devices with zinc-oxide-based nanostructures synthesized at low temperature.
K-NC (Kuan-Neng Chen) is a professor of the Department of Electronics Engineering in National Chiao Tung University (National Chiao Tung University), Hsinchu, Taiwan. He received his Ph.D. degree in Electrical Engineering and Computer Science and his M.S. degree in Materials Science and Engineering from Massachusetts Institute of Technology (MIT), respectively. Prior to the faculty position, he was a research staff member and project leader at the IBM Thomas J. Watson Research Center. His current research interests are three-dimensional integrated circuits (3D IC), through-silicon via (TSV) technology, wafer bonding technology, and heterogeneous integration.
H-CC (Huang-Chung Cheng) is a professor of the Department of Electronics Engineering in National Chiao Tung University (National Chiao Tung University), Hsinchu, Taiwan. He received the B.S. degree in physics from National Taiwan University in 1977 and the M.S. and Ph.D. degrees from the Department of Materials Science and Engineering, National Tsing Hua University (National Tsing Hua University), Hsinchu, Taiwan, in 1979 and 1985, respectively. He has published nearly 500 technical papers in international journals and conferences and also held more than 50 patents. His current research interests are in the areas of high-performance TFTs, novel nanowire devices, non-volatile memories, three-dimensional integrations, novel field emission displays, biosensors, and photoelectronic device.
The authors thank the National Science Council of the Republic of China for their support under the Contract NSC 101-2221-E-009-077-MY3. Thanks are also due to the Nano Facility Center (NFC) in National Chiao Tung University for the technical supports.
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