12089
2008, 112, 12089–12091 Published on Web 07/19/2008
Toward High-Performance Solution-Processed Carbon Nanotube Network Transistors by Removing Nanotube Bundles Chun Wei Lee,† Cheng-Hui Weng,‡ Li Wei,§ Yuan Chen,§ Mary B. Chan-Park,§ Chuen-Horng Tsai,‡ Keh-Chyang Leou,‡ C. H. Patrick Poa,| Junling Wang,§ and Lain-Jong Li*,†,§ School of Materials Science and Engineering, Nanyang Technological UniVersity 50, Nanyang AVe., Singapore 639798, Department of Engineering and System Science, National Tsing Hua UniVersity, Hsinchu, Taiwan 300, Republic of China, School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, Singapore 637459, Institute of Materials Research Engineering, 3 Research Link, Singapore 117602 ReceiVed: June 20, 2008; ReVised Manuscript ReceiVed: July 09, 2008
Reported solution-processed field-effect transistor (FET) devices based on single-walled carbon nanotube (SWNT) networks have either high mobility but low on/off ratio or vice versa. Recently, Arnold et al. (Nat. Nanotechnol. 2006, 1, 60-65) have made significant improvements in obtaining semiconductor-enriched SWNTs by using density-gradient ultracentrifugation. Here, we report that removing the SWNT bundles using organic-aqueous interfacial purification can further enhance the electrical performance of SWNTFETs. The on/off ratio of the SWNT-FET is improved by ∼1 order of magnitude. By combining densitygradient ultracentrifugation and interfacial purification, it is possible to obtain high on/off ratio and high mobility of solution-processed SWNT-FETs at a promising yield. The electrical property of the single-walled carbon nanotube (SWNT) networks is dominated by the percolation path.1,2 These SWNT network devices can be printed, making low-cost inkjet processing possible. Hence, SWNTs are potential semiconducting materials for printed electronics. SWNT networks have promising applications in gas detection,3,4 biomolecular sensors,5,6 and optoelectronics7 despite the fact that solution-based SWNT printed electronics are still far from realization due to unsolved challenges in device fabrication. The major challenges include (1) low on/off ratio, (2) low effective field-effect mobility, and (3) low yield for production of field-effect transistors (FETs). The coexistence of semiconducting and metallic tubes in the available SWNTs ensembles is believed to be the key issue toward high device performance. Therefore, many efforts have been dedicated toward the enrichment of semiconducting species or the development in purifying a given SWNT ensemble into its distinct species, such as selective chemical functionalization,8 dielectrophoresis,9 deoxyribonucleic acid (DNA)-assisted dispersion,10 and polymer wrapping.11 Direct electrical breakdown12 of metallic tubes on SWNT devices has been reported to improve the on/off characteristics. Recently, Arnold et al. have made significant improvements in obtaining semiconductorenriched SWNTs by using density-gradient ultracentrifugation (DGU).13 In this contribution, we report that removing the bundles in SWNT solutions using organic-aqueous interfacial trapping purification (ITP)14 developed by Wang et al. is able * To whom correspondence should be addressed. E-mail:
[email protected]. † School of Materials Science and Engineering, Nanyang Technological University. ‡ National Tsing Hua University. § School of Chemical and Biomedical Engineering, Nanyang Technological University. | Institute of Materials Research Engineering.
10.1021/jp805434d CCC: $40.75
to enhance the on/off ratio of the obtained SWNT-FETs by ∼1 order of magnitude. By combining DGU and ITP methods, it is possible to obtain high on/off ratio and high mobility of solution-processed SWNT-FETs with promising yields. It is noted that no electrical breakdown is applied in this process. DGU has been reported to provide a scalable and nondestructive approach for sorting SWNTs.13 We adopt the DGU method, with a sodium dodecyl sulfate (SDS)/sodium cholate hydrate (SC) ratio of 1:4, to enrich the arc-discharge-produced SWNTs (the diameter of the tubes is distributed from 0.8 to 1.4 nm from statistical measurement in TEM). Figure 1a shows that, after DGU, multiple regions (layer 1 to layer 4) are visible throughout the density gradient, as indicated in the photograph. The absorption spectra for the unseparated (or as-received; indicated as Initial) and semiconductor-enriched SWNTs (layer 1) show that, for the layer 1 solution, the S22 peaks (2nd van Hove transitions of semiconducting SWNTs) are significantly enhanced relative to M11 peaks (1st van Hove transitions of metallic SWNTs), suggesting that layer 1 is relatively enriched with semiconducting tubes compared with the initial SWNTs. The obtained layer 1 solution is then readily used for subsequent debundling experiments. The organic-aqueous interfacial trapping purification (ITP) to remove the bundled tubes has been demonstrated recently.14 The large bundled tubes existing in SWNT solutions are believed to lower the on/off ratio of the devices. Thus, the ITP method is used to remove the nanotube bundles existing in the layer 1 solution for further improvement of the device performance. The procedure is as follows: toluene was added to the layer 1 SWNT suspensions, and the mixture was shaken vigorously to increase the interfacial area. The mixture was left for 10 min, and then, the aqueous phase was withdrawn carefully via syringe for use. Figure 1b demonstrates 2008 American Chemical Society
12090 J. Phys. Chem. C, Vol. 112, No. 32, 2008
Figure 1. (a) Photograph of SWNTs separated in a cosurfactant solution (1:4 SDS/SC) using density-gradient ultracentrifugation and the corresponding optical absorbance spectra for the initial and layer 1 solutions. Typical atomic force microscope images of high-density CNT networks prepared from layer 1 solutions (b) without and (c) with interfacial trapping purification.
the typical AFM image for the device drop-cast with layer 1, where large bundles are present. By contrast, for the device prepared by drop-casting a similar density of ITP-treated layer 1, no large bundles are found, as shown in Figure 1c. These AFM images strongly suggest that the interfacial trapping is efficient in removing large bundled tubes. The SWNT-FETs were fabricated in a bottom contact device geometry, where a highly p-doped silicon wafer with a 300nm-thick SiO2 layer was used as the back gate and Au electrodes with a channel length of 200 µm were patterned on top of it. Drop-casting of arc-discharge-produced SWNTs was then applied to form networks, as illustrated in the inset of Figure 2a. Before performing the conductivity measurement of the SWNT-FETs in the percolation region, we verify that the resistances of the devices are predominantly contributed to the network by fabricating various channel lengths of devices as described in the literature.2 The Au-SWNT contact contributes about 12% of the total resistance from the device with a 200 µm channel length, and this ensures that the electrical behaviors are dominated by the networks. Parts a and b of Figure 2 demonstrate the typical transfer characteristics, forward and reverse sweeps of drain current (Id) vs gate voltage (Vg), and output characteristics, Id vs drain voltage (Vd), respectively, for a SWNT-FET drop-cast from the layer 1 + ITP solution. We summarize in Figure 2c the histogram of device percentage with various on/off ratios for the devices made from initial, layer 1, and layer 1 + ITP solutions, respectively. The sample size for each type of SWNT devices is 60. The on/off ratio obtained from layer 1 is significantly higher than that from the initial SWNTs, corroborating that the separation efficiently increases the semiconducting-to-metallic tube ratio of layer 1 (Figure 1a). Also, the on/off ratio of the device is significantly improved (∼1 order) after treating with the ITP method (removal of bundles). Parts a and b of Figure 3 summarize the relations of measured on/off ratio and off-current (Ioff) with the increased on-current
Letters
Figure 2. (a) Typical transfer characteristics, Id vs Vg, and (b) output characteristics, Id vs Vd, for a SWNT-FET drop-cast from the layer 1 + ITP solution. (c) The histogram of device percentage with various on/off ratios for the devices made from the initial, layer 1, and layer 1 + ITP solutions, respectively.
Figure 3. The relations of (a) on/off ratio and (b) off-current with the increased on-current (Ion). The (c) on/off ratio and (d) extracted mobility versus the effective tube density for the samples layer 1 and layer 1 + ITP. (e) The sheet conductance vs effective tube density for the devices made from layer 1 and layer 1 + ITP. (f) The fitting of the sheet conductance vs tube density relations using the percolation theory, where the percolation threshold of the device is 30 tubes/µm2.
(Ion) by increasing network density (or number of drops cast on the device). The standard percolation theory predicts that the conductivity σ is proportional to (N - Nc)R, where N is the density of conduction sticks (CS), Nc is the critical density of
Letters CS corresponding to the percolation threshold, and R depends only on the dimensionality of the space for the networks. Thus, Ion can be used as the index of the degree of the network density. Results show that the on/off ratio decreases with the on-current (Ion) (or network density) and the existence of bundled tubes leads to a higher Ioff value. These may be attributed to the fact that the bundled tubes are more metal-like. Also, the subthreshold swing is improved (decreased) after bundle removal (data not shown) due to the same reason (the bundled tubes are metallic in nature, which effectively reduces the gate coupling of SWNT networks). The effective field-effect mobility is extracted from the experimental transfer curves for the devices based on the equation,1 mobility µ ) (dId/dVg)/(εVdW/LoxL), where Lox is the gate dielectric thickness, W and L represent channel width and channel length, respectively, ε is the dielectric constant of gate dielectrics (thermal oxide ) 3.9), and Vd is the source-drain bias. Parts c and d of Figure 3 respectively show the on/off ratio and extracted mobility versus the effective tube density (at the low-density regime) for samples layer 1 and layer 1 + ITP. The effective tube density of SWNTs was obtained by averaging the number of tubes counted based on five separate 1 µm × 1 µm areas from AFM images. The improvement of on/off ratio using ITP is crucial for achieving high field-effect mobility. Besides, the effective field-effect mobility obtained from the devices with and without ITP in Figure 3d may be fitted with a universal curve, where no significant difference in effective field-effect mobility is observed for both cases. The reason is that, in the estimation of effective tube density from AFM images, bundled and individual tubes are equally counted as one effective CS. Note that the percolation theory only considers the number of effective CS and therefore the bundled and individual tubes contribute equally to the conductivity. For the solution-processed random network transistors demonstrated here, the effective field-effect mobility is able to achieve up to 1.5 cm2/V-s, while the on/off ratio is still kept reasonably high (>104). It is noted that the SWNT density discussed in Figure 3 is at the same order of magnitude as the predicted percolation threshold density (Nc ) 4.2362/πLc2); therefore, our discussion here is within the low-tube-density regime (close to percolation threshold or close to ideal two-dimensional film). For the devices discussed in Figure 3, the predicted Nc values are around 30 tubes/µm2 based on the measured average tube length (434 nm). We perform the fitting for the separate conductance measurements (Figure 3e) to the equation sheet conductance G0 ∝ (N - Nc)R, where Nc is set to 30 and the fitted curve is plotted in Figure 3f. We obtain the fitted value of 1.21 for the dimensional factor R, which is slightly lower than the prediction (1.33) for an ideal two-dimensional film,15 and we suspect that the variations are due to the underestimation of the number density
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12091 of tubes in AFM. This result suggests the feasibility of using percolation theory to describe the conduction properties of the SWNT networks.1,2 In summary, we have demonstrated with our arc-dischargeproduced SWNTs that the removal of bundled tubes efficiently improves the on/off ratio and subthreshold swing. The statistical yield is also presented. The obtained effective field-effect mobility here is not as superior as the highest reported13 (20 cm2/V-s) probably due to several reasons: (1) the device channel length we used is aiming for macroelectronics and therefore is longer (∼200 µm), (2) the source of nanotubes is different, and (3) the average length of our SWNT in the dispersion is short. However, the obtained results still allow us to conclude that bundle removal is critically important for device performance. The representative effective field-effect mobility in this report is around 1.5 with an on/off ratio around 104. It is suspected that the tube length may be a limiting factor for the electrical performance; therefore, it warrants further studies on the effect of tube length. Acknowledgment. This research was supported by Nanyang Technological University, Singapore. References and Notes (1) Snow, E. S.; Novak, J. P.; Campbell, P. M.; Park, D. Appl. Phys. Lett. 2003, 82, 2145. (2) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513. (3) Snow, E.; Perkins, F.; Houser, E.; Badescu, S.; Reinecke, T. Science 2005, 307, 1942. (4) Fu, D.; Lim, H.; Shi, Y.; Dong, X.; Mhaisalkar, S.; Chen, Y.; Moochhala, S.; Li, L. J. J. Phys. Chem. C 2008, 112 (3), 650. (5) Star, A.; Tu, E.; Niemann, J.; Gabriel, J.-C. P.; Joiner, C. S.; Valcke, C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 921. (6) Gui, E. L.; Li, L. J.; Zhang, K. K.; Xu, Y.; Dong, X.; Ho, X.; Lee, P. S.; Kasim, J.; Shen, Z. X.; Rogers, J. A.; Mhaisalkar, S. J. Am. Chem. Soc. 2007, 129, 14428. (7) Shi, Y.; Tantang, H.; Lee, C. W.; Weng, C.-H.; Dong, X. C.; Li, L. J.; Chen, P. Appl. Phys. Lett. 2008, 92, 113310. (8) An, K. H.; Park, J. S.; Yang, C. M; Jeong, S. Y.; Lim, S. C.; Kang, C.; Son, J. H.; Jeong, M. S.; Lee, Y. H. J. Am. Chem. Soc. 2005, 127, 5196–5203. (9) Krupke, R.; Hennrich, F.; Lohneysen, H. v.; Kappes, M. M. Science 2003, 301, 344–347. (10) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; Mclean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. Nat. Mater. 2003, 2, 338– 342. (11) Chen, F.; Wang, B.; Chen, Y.; Li, L. J. Nano Lett. 2007, 7, 3013– 3017. (12) Collins, P. G.; Arnold, M. S.; Avouris, P. Science 2001, 292, 706– 709. (13) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60–65. (14) Wang, R. K.; Reeves, R. D.; Ziegler, K. J. Am. Chem. Soc. 2007, 129 (14), 15124. (15) Stauffer, G. Introduction of Percolation Theory; Taylor & Francis: London, 1985.
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