Article pubs.acs.org/JPCC
Effects of Surfactants on the Electronic Transport Properties of ThinFilm Transistors of Single-Wall Carbon Nanotubes Maki Shimizu,†,‡ Shunjiro Fujii,†,‡ Takeshi Tanaka,† and Hiromichi Kataura*,†,‡ †
Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8562, Japan ‡ CREST, Japan Science and Technology Agency, Kawaguchi 330-0012, Japan S Supporting Information *
ABSTRACT: We investigated the effects of surfactants on the morphologies and electronic transport properties of solution-processed thin films of semiconductor-enriched single-wall carbon nanotubes. The network morphologies were observed using atomic force microscopy, and the electronic transport properties of thin-film transistors composed of these materials were examined. The network morphologies were highly dependent on the surfactants, although the thin films were fabricated from the same nanotube using the same process. Among the four surfactants examined in this work, sodium deoxycholate produced the finest mesh structures that comprised mostly individual nanotubes. The performance of the thin-film transistor was highly correlated with the morphology of the nanotube network; however, no considerable effect of the residual surfactant was observed. Because of the fine mesh structure, both the highest on/off ratio (1 × 106) and the highest mobility (42 cm2 V−1 s−1) were obtained for the thin-film transistor produced from sodium deoxycholate.
■
INTRODUCTION Because of their intrinsic high mobility,1 single-wall carbon nanotubes (SWCNTs) have great potential for use in thin-film transistors (TFTs), which are promising for flexible electronics, printed electronics, and sensors.2−8 Recent progress in the metal/semiconductor separation technique enabled us to prepare high-purity semiconducting SWCNTs (s-SWCNTs) on a large scale,9−13 and high-performance TFTs can be fabricated using simple and low-cost solution processes.14−18 However, a TFT fabrication process using an s-SWCNT solution has not yet been established because this process is not well-understood. For the metal/semiconductor separation, for example, the density gradient ultracentrifugation method requires using a mixture of two surfactants, i.e., sodium cholate (SC) and sodium dodecyl sulfate (SDS). Thus, s-SWCNTs were obtained as an SC plus SDS aqueous solution. On the other hand, with the gel chromatography method, a surfactant can be selected from candidate materials to extract s-SWCNTs such as sodium deoxycholate (DOC), which is well-known as one of the strongest dispersants of SWCNTs. Because the dispersion abilities of these surfactants differ from one another,19 the network morphology of the obtained thin films could be different. In this work, we focused on how the surfactant affects the network morphology and on the performance of the TFT. We used four different surfactants, i.e., SDS, sodium dodecylbenzene sulfonate (SDBS), SC, and DOC. SDS, SC, and DOC are used to separate s-SWCNTs, and SDBS is often used in fabricating conductive films.9−13,20 It is important to understand the effects of these commonly used surfactants on thin-film formation. Although surfactants are essential in dispersing SWCNTs, in general, the selection of the best surfactant for TFT is not © XXXX American Chemical Society
obvious. Because the dispersion ability varies among surfactants, the bundle thickness on the formed thin film is expected to depend on the type of surfactant. It has been theoretically predicted that highly concentrated surfactant micelles in the solution induce an attractive force among SWCNTs, which accelerates the formation of bundles and affects the network structure.21,22 It is important to keep the bundles thin to obtain higher performance TFTs because the percolation threshold of the residual metallic SWCNT (mSWCNT) network in the bundled SWCNTs is considerably lower than that of individual SWCNTs.23 Therefore, a surfactant with a stronger dispersion ability should be better suited to producing a TFT that has a high on/off ratio. However, the surfactant molecules with strong dispersion abilities could adhere so strongly that the presence of residual surfactant on the SWCNTs could increase the SWCNT− SWCNT contact resistance. In this work, we prepared high-purity s-SWCNTs using the gel chromatography method.10,11 Thin films were then prepared by dropping the SWCNT solution onto and then removing it from a SiO2/Si substrate covered with a monolayer of 3-aminopropyltriethoxysilane (APTES). Back-gated TFTs were prepared, and the transfer characteristics were then measured. The thin film that was prepared using the DOC solution exhibited the finest mesh structure, which comprised mostly individual SWCNTs. Furthermore, the TFT prepared from the DOC solution exhibited the best transfer characteristics. The best value was a mobility of 42 cm2 V−1 s−1 with an Received: November 16, 2012 Revised: May 1, 2013
A
dx.doi.org/10.1021/jp3113254 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
on/off ratio of 1 × 106. This result suggests that the use of the surfactant with the strongest dispersion ability is optimal for obtaining a TFT with enhanced performance. We discuss the correlation between the film morphology and the device performance resulting from the differences in the dispersion abilities of the surfactants in a later section.
four equal portions to obtain the same purity s-SWCNT solution with different surfactants. Each solution was ultracentrifuged for 6 h to precipitate all of the s-SWCNTs to the bottom of the centrifugation tube, and the upper clear portion was removed. Four types of 1 wt % SC, DOC, SDS, and SDBS aqueous solutions were applied to the centrifugation tubes instead of the removed clear portion. Each solution was then dispersed again using the homogenizer to replace the surfactants surrounding the SWCNTs. We repeated this procedure three times to completely replace the surfactant molecules. TFT Fabrication. Thin films of SWCNTs were deposited onto a 200 nm thick SiO2 layer formed on a heavily doped Si substrate. First, the surface of SiO2 was functionalized with a monolayer of APTES (Sigma-Aldrich) to improve the affinity for SWCNTs.13,24 Thirty microliters of each s-SWCNT solution was dropped onto the substrate, remained there for 90 min, and was then removed using N2 gas. The substrate was then immersed in water for 1 h to remove the residual surfactant on the SWCNTs. In this adsorption process, the APTES tends to adsorb s-SWCNTs selectively.13 However, in this work, we assume that the ratio of the m- and s-SWCNTs on each thin film is the same. Because individual m-SWCNTs are removed in the gel separation process, the purity of the original s-SWCNT solution is limited by mixed bundles of mand s-SWCNTs, which means the m-SWCNTs in the original s-SWCNT solution are already bundled with s-SWCNTs. Therefore, the purity of the s-SWCNTs cannot be improved by the APTES absorption processes. Atomic force microscopy (AFM) was used to characterize the morphology of the thin films on the Si substrates because the AFM has sufficient resolution to detect individual SWCNTs and is free from electron beam damage that seriously degrades TFT performance. Ti/Au (30 nm/70 nm) electrodes were deposited on the film as a source and a drain using a metal mask, and the heavily doped Si substrate was used as a back gate. Four different channel lengths, i.e., 25, 50, 75, and 100 μm, were prepared. All of the widths were 200 μm. The transfer characteristics were measured using a semiconductor parameter analyzer (Keithley 4200). The SWCNT density in the TFT channel depends on this procedure. We optimized this procedure to produce higher densities than the percolation threshold of the SWCNT network because the TFT performance had to be examined. We utilized the same procedure for all of the surfactants in the present work.
■
EXPERIMENTAL METHODS Dispersion of SWCNTs. The SWCNTs synthesized by high-pressure carbon monoxide processing (mud-form, raw HiPco, NanoIntegris) were used in this study. First, 100 mg of HiPco SWCNTs was dispersed in 100 mL of a 1 wt % SDS aqueous solution using a 1/2 in. tip-type ultrasonic homogenizer (Branson Sonifier 450D) with 30% output for 3 h. During the dispersion process, the SWCNT solution was stored in a glass bottle that was immersed in cooled water to prevent any undesirable heating from the ultrasonic power. After ultracentrifugation at 210 000g for 1 h, the upper 80% of the supernatant was collected as the isolated SWCNT solution for the following separation procedure. Metal/Semiconductor Separation of SWCNTs. The isolated SWCNT solution was injected into a column filled with Sephacryl S-200 gel beads (GE Healthcare). In this system, s-SWCNTs are selectively adsorbed in the gel column, whereas the m-SWCNTs are eluted by the SDS aqueous solution. After carefully washing the m-SWCNTs with an SDS solution, high-purity s-SWCNTs were eluted from the gel column using a 1 wt % DOC aqueous solution. The optical absorption spectrum was measured using a UV−vis−NIR spectrometer (Shimadzu UV-3600). Figure 1 presents the
Figure 1. Optical absorption spectra of the pristine and separated SWCNTs in an aqueous DOC solution. The M11 peaks disappeared in the s-SWCNT solution.
■
RESULTS AND DISCUSSION Figure 2 presents typical AFM images of the thin films formed from the (Figure 2a) SDS, (Figure 2b) SDBS, (Figure 2c) SC, and (Figure 2d) DOC solutions (more AFM images of the thin films are presented in Supporting Information Figures S1−S3). We can observe clear differences in the network topologies between the films. The networks formed from the SDS and SDBS solutions were constructed from relatively thick bundles of SWCNTs. The heights of the bundles ranged from 2 to 6 nm, whereas the average diameter of the HiPco SWCNTs is known to be approximately 1 nm. The bundles in the film formed from SDS are straight and locally parallel, similar to short, flat cables. The network density is inhomogeneous. For SDBS, the configuration was considerably different from that of SDS. Long, bundled ropes formed a homogeneous and continuous but coarse (approximately 10 μm) mesh. However,
optical absorption spectra of the pristine and s-SWCNTs in an aqueous DOC solution. The M11 (400−600 nm) label corresponds to the absorption band of m-SWCNTs, and S11 (900−1350 nm) and S22 (500−900 nm) correspond to the absorption band of the s-SWCNTs. The absorption peaks in the range from 500 to 600 nm represent a mixture of metallic and semiconducting SWCNTs. Although assigning the absorption peaks is difficult in this range, it is clear that the s-SWCNT solution has smaller M11 absorption peaks (400− 600 nm) than the pristine solution, which is due to the absence of m-SWCNTs. Replacement of Surfactants. Because the s-SWCNTs were collected using a DOC solution, the s-SWCNTs were dispersed in the DOC solution at this stage. For the following experiments, this original SWCNT solution was divided into B
dx.doi.org/10.1021/jp3113254 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
is an important factor in this process. After removing the concentrated solution and completely drying the substrate, the second step is the washing process, in which the majority of the surfactants are dissolved in water and removed from the network. The bundles could be formed during either step because the concentration of the surfactant changes drastically during these steps. The surfactant concentration has an optimum value, and bundles can be formed at both low and high concentrations.26,27 In the low-concentration condition, the SWCNTs are not suspended in the solution and form bundles because the number of the surfactant molecules is not sufficient for maintaining isolated SWCNTs. However, even in the high-concentration condition, aggregation occurs because the free micelles of the surfactant induce an attractive force between the SWCNTs. When the distance between the SWCNTs becomes less than the size of a free surfactant micelle, the micelles cannot remain between the SWCNTs and are excluded. The attractive force is then induced by osmotic pressure, and the SWCNTs are forced together. This attractive force is called the depletion force.28 During the drying process, the density of the SWCNTs in the solution and the concentration of the surfactant become very high. This condition results in a strong depletion force. In the second washing process, the surfactant concentration becomes very low, and the SWCNTs can easily aggregate and form more bundles if the APTES cannot strongly catch the SWCNTs. The magnitude of the attractive force between the SWCNTs depends on the Coulomb repulsion between micelles, i.e., the dispersion ability of the surfactant. DOC and SC are typical surfactants that have strong dispersion abilities.19,26 In particular, DOC can disperse SWCNTs more efficiently than SC, which has one more hydroxyl group than DOC. On the other hand SDBS can disperse SWCNTs more efficiently than SDS because the benzene ring in SDBS strongly interacts with the SWCNTs.29 Consequently, the dispersion ability varies as DOC > SC > SDBS > SDS, and thus, the attractive force between the SWCNTs is believed to change as SDS > SDBS > SC > DOC. Furthermore, because the micelle size of SDS is considerably larger than that of DOC,30,31 a greater depletion force is expected in the SDS solution.
Figure 2. AFM images and height profiles (the green line in the AFM image) of the typical SWCNT networks formed using (a) SDS, (b) SDBS, (c) SC, and (d) DOC. Scale bar = 1 μm. Inset in panels c and d: magnified view of the thin film. Scale bar = 400 nm.
for the films formed from SC and DOC, fine mesh structures were observed. The thicknesses of the bundles were approximately 2 and 1 nm for SC and DOC, respectively, which were smaller than those of SDS and SDBS. For the film formed using DOC, the SWCNTs were straight and thought to be mostly individual because the height of the network was the same as the mean diameter of the SWCNT. To understand the difference in the network morphology for the different surfactants, it is important to consider the film formation process. The formation of thin films is believed to consist of two steps. The first step consists of the drying process, in which the dropped solution is concentrated. In this step, the SWCNTs are physically adsorbed by APTES molecules on the substrate.25 Because the SWCNTs contact the functionalized substrate more frequently when the SWCNT concentration is increased, the concentration of the SWCNTs
Figure 3. (a−d) Schematic images of the network with different attractive interactions in the solution just before removing it. As demonstrated in this figure, more SWCNTs are aligned in a parallel fashion as the attractive interaction increases. The figures are based on the theoretical prediction (Figure 4 in ref 22) of SWCNT networks with different attractive interactions and are drawn to connect this prediction to our experimental results, presented in Figure 2. (a) Aligned short bundles (SDS). (b) Continuously connected bundles (SDBS). (c) Locally aligned SWCNTs (SC). (d) Random network of SWCNTs (DOC). (e and f) Side-view images of the lateral capillary force, which works when the substrate is dried. (e) The SWCNTs form bundles by inhomogeneous lateral capillary forces (SDS, SDBS). (f) The networks maintain a random network (SC, DOC) because the capillary forces cancel each other. C
dx.doi.org/10.1021/jp3113254 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
Figure 4. Transfer characteristics of the SWCNT-TFT device corresponding to Figure 2: (a) SDS, (b) SDBS, (c) SC, and (d) DOC. The channel length is 25, 50, 75, and 100 μm, respectively. Inset: the channel length dependences on the on- and off-currents. The figure also includes the device structure.
transfer characteristics to evaluate the effect of the surfactant molecules remaining in the network and the effect of the network morphology. Parts a−d of Figure 4 present the transfer characteristics of the TFTs prepared using the four types of sSWCNT thin films described in Figure 2. Here, the devices with on-currents of approximately 10 μA were selected to compare the device performances. The results for the other devices are presented in the Supporting Information with their corresponding AFM images. The source-drain current was measured by applying a bias voltage of −1 V and sweeping the back-gate voltage from −50 to 50 V. The gate leakage currents of these devices were less than 100 pA. Although the sSWCNTs and fabrication processes were the same for all of the TFTs, except for the type of surfactant, clear differences in the transfer characteristics were observed. The on- and off-currents were determined from the transfer characteristics and are indicated in the insets of Figure 4, parts a−d. In all devices, the on-current gradually decreases as the channel length increases. In contrast, the off-current strongly depends on the devices. The off-current is approximately 100 pA or lower in the cases of SC and DOC for all of the channel lengths. However, the offcurrents are larger (1 to ∼100 nA) when the devices are fabricated from the SDS and SDBS solutions. This result may have occurred for the following two reasons. The first reason is the existence of residual m-SWCNTs because the purity of the s-SWCNT is approximately 95%.23 For SDS and SDBS, the networks were formed by thick bundles, as illustrated in Figure 2. If the bundle contains even one m-SWCNT, the bundle behaves as a metallic path in the network. Because the purity of s-SWCNTs is the same for all TFTs, the thicker bundles have a higher probability of containing a m-SWCNT. For example, when the diameter of the bundle is 5 nm, the bundle contains 20 SWCNTs. For a 95% purity of s-SWCNTs, only 36% of the bundles can be purely semiconducting, and 64% behave as metals.36 Consequently, the TFTs fabricated from the SDS and SDBS solutions are easily affected by m-SWCNTs and should have a higher off-current. The second reason is the gate screening effect. It is wellknown that the back-gate potential can only work for the SWCNTs that neighbor the gate insulator. Therefore, the SWCNTs in the thick bundle cannot feel the gate field and cannot be in the on and off states. Under ambient conditions, oxygen molecules adsorb onto the SWCNTs and dope hole carriers into the SWCNTs. Thus, the screened SWCNTs
Kyrylyuk and van der Schoot calculated the network topology of SWCNTs in solution under various attractive interactions.22 By applying their model, the network formation process can be described as follows. Parts a−d of Figure 3 represent the schematics of the SWCNT networks formed on the substrate in the first step under the different attractive force conditions. In aqueous solution, SWCNTs are thought to be dispersed individually for all surfactants due to sufficient ultracentrifugation. Figure 3d presents the case of a weak attractive force, such as DOC dispersion. In this case, an individual random network can be formed with the morphology being maintained in the dispersion because APTES traps the SWCNTs strongly enough against the attractive force between SWCNTs. When the attractive force slightly increases (Figure 3c), as in SC dispersion, the SWCNTs begin to align locally in the same direction because the attractive force tends to induce a parallel arrangement of SWCNTs to increase the contact area.21,32 The SWCNTs start to form thin bundles against the interaction with APTES but maintain a homogeneous and random network. For a large attractive force, as in the SDBS case, APTES cannot trap SWCNTs stiffly enough and thus form thick bundles to create a larger contact area.33 The network becomes inhomogeneous and contains large mesh structures, as illustrated in Figure 3b. For the largest attractive force (Figure 3a), such as the SDS dispersion, the SWCNTs form the thickest bundles. In this case, the continuous network structure is broken, and short bundles are formed to maximize the contact area between SWCNTs.34,35 In the above processes, the remaining surfactant molecules around SWCNT may prevent the formation of tight bundles. After the washing process (the second step), however, the capillary force produces tight bundles in the final drying process, as illustrated in Figure 3e. Thus, flat cable-like bundles were formed from the SDS solution, and a large mesh structure was produced from the SDBS solution. Although the capillary force was applied similarly to the SC and DOC networks, they maintained homogeneous network structures. Because their networks were fine and uniform, capillary forces were applied uniformly and canceled each other, as illustrated in Figure 3f. In this case, APTES could maintain the random network structures. The water washing process is not perfect, and the surfactant molecules remain in the network and affect the electrical contacts between the SWCNTs. It is very difficult to directly measure the contact resistance between SWCNTs. Instead of a direct measurement, we prepared TFTs and measured their D
dx.doi.org/10.1021/jp3113254 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
Article
individual SWCNT networks kept the percolation threshold of the m-SWCNTs at a high level for SC and DOC, and they did not reach the percolation threshold when the on-current reached 10 μA. Figure 5b presents the mobility as a function of the oncurrent. Here, the mobility was calculated using a simple parallel plate model, although each device has different network morphologies and coverages. Every surfactant exhibits the same tendency: the mobility increases as the on-current increases. For SC and DOC, the mobility is proportional to the oncurrent, as indicated by the black dotted line, which satisfies the definition of the mobility. This result indicates that the parallel plate model represents these TFTs well. However, for SDS and SDBS, the mobility is distributed more widely and is mostly lower than the black dotted line. This result indicates that the parallel plate model was insufficient in describing these TFTs because the coverage of the SWCNT film is low, as illustrated in Figure 2. The actual capacitance is lower than the capacitance calculated using the parallel plate model.38 As discussed above, the differences in the transfer characteristics of the TFTs fabricated using four different surfactants can be well-explained by the differences in the network morphology of the SWCNTs. We did not need to introduce any effect of the residual surfactant molecules on the electric contact between SWCNTs. Actually, the best TFT performance for DOC, i.e., 42 cm2 V−1 s−1 in mobility with a 1 × 106 in on/off ratio, is higher than the mobility of the TFT fabricated using Nmethylpyrrolidone to disperse SWCNTs without using surfactants.16,23 This result suggests that the washing process works well to remove surfactant molecules and that the amount of the residual surfactant is not significant enough to affect the SWCNT−SWCNT contact resistance. In conclusion, the surfactant with higher dispersion ability should be selected to obtain better thin-film morphology in the fabrication of TFTs.
slightly increase the off-current and never contribute to the oncurrent. Actually, the off-current exponentially decreases as the channel length becomes longer because the long channel length reduces the percolation transport pathways.37 However, the off-currents of the devices fabricated using DOC and SC were low and did not change with the channel length. As illustrated in Figure 2, parts c and d, fine meshes were observed in the AFM images. In these cases, because of the sufficiently high purity of s-SWCNT, the m-SWCNTs are not percolated and the gate electric field applied from the backgate is terminated on the thin films.38 All of the s-SWCNTs can feel the electric field uniformly, and these thin films can behave as a conductive two-dimensional (2D) plate because of the high coverage of SWCNTs. Therefore, the coupling efficiency between the gate field and thin film is considered to be very high. The applied gate field effectively removed entire carriers on the thin film, and the low off-current was realized. In the SWCNT-TFT, the on/off ratio tends to decrease as the on-current increases by increasing the SWCNT density because the contribution of m-SWCNTs is more effective in the high-density network near the percolation threshold.37 To understand the effect of the network morphology on the device performance, we examined the on-current dependence of the on/off ratio. Figure 5a presents the on-current values for 10
■
CONCLUSIONS We prepared four different s-SWCNT solutions using different surfactant molecules and investigated the effects of the surfactants on the formation of networks and their transport properties. Most likely because of the difference in their dispersion ability, significant differences in the bundle sizes and network topologies were observed. Furthermore, this difference greatly affected the transfer characteristics of the TFTs prepared using the s-SWCNT solutions. High on/off ratios and high mobilities were obtained for SC and DOC, which have high dispersion abilities. We concluded that the surfactant with the higher dispersion ability is ideal for producing better thin films for the fabrication of TFTs, at least for the four surfactants used in this work.
Figure 5. (a) On/off ratio as a function of the on-current, which was measured from the transfer characteristics. (b) Mobility as a function of the on-current. Inset: magnified view of the gray dotted square. The legends are the same as in panel a. The channel length is 50 μm.
independent TFTs for each surfactant as a function of its on/off ratio. For this plot, we collected data from TFTs with channel lengths of 50 μm. For SDS and SDBS, the on/off ratios increased as the on-current decreased, as expected. The on/off ratios of the TFTs formed with SDS and SDBS with an oncurrent of 10 μA are on the order of 101 and 103, respectively. This result suggests that the contribution of the m-SWCNTs is more significant in the higher density network formed from SDS and SDBS, which is primarily due to the thick bundled structures. As mentioned above, there are also many bundled sSWCNTs that cannot feel the gate electric field. This result also reduces the on/off ratio. Consequently, the thick bundled structure is the primary reason why we could not fabricate a low off-current device using SDS and SDBS. However, for SC and DOC, the on/off ratio did not depend on the on-current and remained at approximately 105, even when the on-currents reached 10 μA. This difference can also be explained by the difference in the network structures. The decrease of the percolation threshold in the network formed by the adhesive and thick-bundled SWCNT is calculated and discussed in refs 22 and 33, and the same can be said for the m-SWCNTs on the thin films fabricated by SDS and SDBS. Thin bundles or
■
ASSOCIATED CONTENT
S Supporting Information *
The network density dependence of transport properties and some AFM images of the thin films. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: +81-29-861-2551. Fax: +81-29-861-2786. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. E
dx.doi.org/10.1021/jp3113254 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry C
■
Article
(21) Vigolo, B.; Coulon, C.; Maugey, M.; Zakri, C.; Poulin, P. An Experimental Approach to the Percolation of Sticky Nanotubes. Science 2005, 309, 920−923. (22) Kyrylyuk, A. V.; van der Schoot, P. Continuum Percolation of Carbon Nanotubes in Polymeric and Colloidal Media. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 8221−8226. (23) Fujii, S.; Tanaka, T.; Miyata, Y.; Suga, H.; Naitoh, Y.; Minari, T.; Miyadera, T.; Tsukagoshi, K.; Kataura, H. Performance Enhancement of Thin-Film Transistors by Using High-Purity Semiconducting Single-Wall Carbon Nanotubes. Appl. Phys. Express 2009, 2, 071601. (24) Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Controlled Deposition of Individual Single-Walled Carbon Nanotubes on Chemically Functionalized Templates. Chem. Phys. Lett. 1999, 303, 125−129. (25) Opatkiewicz, J. P.; LeMieux, M. C.; Bao, Z. Infuence of Electrostatic Interactions on Spin-Assembled Single-Walled Carbon Nanotube Networks on Amine Functionalized Surfaces. ACS Nano 2010, 4, 1167−1177. (26) Blanch, A. J.; Lenehan, C. E.; Quinton, J. S. Optimizing Surfactant Concentrations for Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solution. J. Phys. Chem. B 2010, 114, 9805− 9811. (27) Wang, H.; Zhou, W.; Ho, D. L.; Winey, K. I.; Fischer, J. E.; Glinka, C. J.; Hobbie, E. K. Dispersing Single-Walled Carbon Nanotubes with Surfactants: A Small Angle Neutron Scattering Study. Nano Lett. 2004, 4, 1789−1793. (28) Bonard, J. M.; Stora, T.; Salvetat, J. P.; Maier, F.; Stöckli, T.; Duschl, C.; Forró, L.; Heer, W. A.; Châtelain, A. Purification and SizeSelection of Carbon Nanotubes. Adv. Mater. 1997, 9, 827−831. (29) Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. High Weight Fraction Surfactant Solubilization of Single-Wall Carbon Nanotubes in Water. Nano Lett. 2003, 3, 269−273. (30) Sowada, R. The Effect of Electrolytes on the Critical Micelle Concentration of Ionic Surfactants: The Corrin−Harkins Equation. Tenside, Surfactants, Deterg. 1994, 31, 195−199. (31) Matsuoka, K.; Moroi, Y. Micellar Formation of Sodium Cholate and Sodium Ursodeoxycholate (part 1). Biochim. Biophys. Acta 2002, 1580, 189−199. (32) Zakri, C.; Poulin, P. Phase Behavior of Nanotube Suspensions: from Attraction Induced Percolation to Liquid Crystalline Phases. J. Mater. Chem. 2006, 16, 4095−4098. (33) Schilling, T.; Jungblut, S.; Miller, M. A. Depletion-Induced Percolation in Networks of Nanorods. Phys. Rev. Lett. 2007, 98, 108303. (34) Green, M. J.; Parra-Vasquez, A. N. G.; Behabtu, N.; Pasquali, M. Modeling the Phase Behavior of Polydisperse Rigid Rods with Attractive Interactions with Applications to Single-Walled Carbon Nanotubes in Superacids. J. Chem. Phys. 2009, 131, 084901. (35) Ko, H.; Tsukruk, V. V. Liquid-Crystalline Processing of HighlyOriented Carbon Nanotube Arrays for Thin Film Transistors. Nano Lett. 2006, 6, 1443−1448. (36) 0.9520 = 0.36. (37) Kocabas, C.; Pimparkar, N.; Yesilyurt, O.; Kang, S. J.; Alam, M. A.; Rogers, J. A. Experimental and Theoretical Studies of Transport Through Large Scale, Partially Aligned Arrays of Single-Walled Carbon Nanotubes in Thin Film Type Transistors. Nano Lett. 2007, 7, 1195− 1202. (38) Cao, Q.; Xia, M.; Kocabas, C.; Shim, M.; Rogers, J. A.; Rotkin, S. V. Gate Capacitance Coupling of Singled-Walled Carbon Nanotube Thin-Film Transistors. Appl. Phys. Lett. 2007, 90, 023516.
REFERENCES
(1) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Ballistic Carbon Nanotube Field-Effect Transistors. Nature 2003, 424, 654− 657. (2) Ahn, J.; Kim, H.; Lee, K.; Jeon, S.; Kang, S.; Sun, Y.; Rogers, J. Heterogeneous Three-Dimensional Electronics by Use of Printed Semiconductor Nanomaterials. Science 2006, 314, 1754−1757. (3) Sun, D. M.; Timmermans, M. Y.; Tian, Y.; Nasibulin, A. G.; Kauppinen, E. I.; Kishimoto, S.; Mizutani, T.; Ohno, Y. Flexible HighPerformance Carbon Nanotube Integrated Circuits. Nat. Nanotechnol. 2011, 6, 156−161. (4) Gruner, G. Carbon Nanotube Films for Transparent and Plastic Electronics. J. Mater. Chem. 2006, 16, 3533−3539. (5) Ha, M.; Xia, Y.; Green, A. A.; Zhang, W.; Renn, M. J.; Kim, C. H.; Hersam, M. C.; Frisbie, C. D. Printed, Sub-3V Digital Circuits on Plastic from Aqueous Carbon Nanotube Inks. ACS Nano 2010, 4, 4388−4395. (6) Snow, E. S.; Perkins, F. K.; Houser, S. C.; Badescu, S. C.; Reinecke, T. L. Chemical Detection with a Single-Walled Carbon Nanotube Capacitor. Science 2005, 307, 1942−1945. (7) Star, A.; Joshi, V.; Skarupo, S.; Thomas, D.; Gabriel, J. C. P. Gas Sensor Array Based on Metal-Decorated Carbon Nanotubes. J. Phys. Chem. B 2006, 110, 21014−21020. (8) Snow, E. S.; Novak, J. P.; Campbell, P. M.; Park, D. Random Networks of Carbon Nanotubes as an Electronic Material. Appl. Phys. Lett. 2003, 82, 2145−2147. (9) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Sorting Carbon Nanotubes by Electronic Structure Using Density Differentiation. Nat. Nanotechnol. 2006, 1, 60−65. (10) Tanaka, T.; Jin, H.; Miyata, Y.; Kataura, H. High Yield Separation of Metallic and Semiconducting Single-Wall Carbon Nanotubes by Agarose Gel Electrophoresis. Appl. Phys. Express 2008, 1, 114001. (11) Tanaka, T.; Liu, H.; Fujii, S.; Kataura, H. From Metal/ Semiconductor Separation to Single-Chirality Separation of SingleWall Carbon Nanotubes Using Gel. Phys. Status Solidi RRL 2011, 9, 301−306. (12) Zheng, M.; Jagota, A.; Semke, E. D.; Diner, B. A.; McLean, R. S.; Lustig, S. R.; Richardson, R. E.; Tassi, N. G. DNA-Assisted Dispersion and Separation of Carbon Nanotubes. Nat. Mater. 2003, 2, 338−342. (13) LeMieux, M. C.; Roberts, M.; Barman, S.; Jin, Y. W.; Kim, J. M.; Bao, Z. Self-Sorted, Aligned Nanotube Networks for Thin-Film Transistors. Science 2008, 321, 101−104. (14) Miyata, Y.; Shiozawa, K.; Asada, Y.; Ohno, Y.; Kitaura, R.; Mizutani, T. Shinohara, Length-Sorted Semiconducting Carbon Nanotubes for High-Mobility Thin Film Transistors. Nano Res. 2011, 4, 963−970. (15) Wang, C.; Zhang, J.; Ryu, K.; Badmaev, A.; De Arco, L. G.; Zhou, C. Wafer-Scale Fabrication of Separated Carbon Nanotube Thin-Film Transistors for Display Applications. Nano Lett. 2009, 9, 4285−4291. (16) LeMieux, M. C.; Sok, S.; Roberts, M. E.; Opatkiewicz, J. P.; Liu, D.; Barman, S. N.; Patil, N.; Mitra, S.; Bao, Z. Solution Assembly of Organized Carbon Nanotube Networks for Thin-Film Transistors. ACS Nano 2009, 3, 4089−4097. (17) Rouhi, N.; Jain, D.; Zand, K.; Burke, P. J. Fundamental Limits on the Mobility of Nanotube-Based Semiconducting Inks. Adv. Mater. 2011, 23, 94−99. (18) Rouhi, N.; Jain, D.; Burke, P. J. High-Performance Semiconducting Nanotube Inks: Progress and Prospects. ACS Nano 2011, 5, 8471−8487. (19) Wenseleers, W.; Vlasov, I. I.; Goovaerts, E.; Obraztsova, E. D.; Lobach, A. S.; Bouwen, A. Efficient Isolation and Solubilization of Pristine Single-Walled Nanotubes in Bile Salt Micelles. Adv. Funct. Mater. 2004, 14, 1105−1112. (20) Dan, B.; Irvin, G. C.; Pasquali, M. Continuous and Scalable Fabrication of Transparent Conducting Carbon Nanotube Films. ACS Nano 2009, 3, 835−843. F
dx.doi.org/10.1021/jp3113254 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
