Mobility Enhancement in Carbon Nanotube Transistors by Screening

ARTICLE pubs.acs.org/JPCC

Mobility Enhancement in Carbon Nanotube Transistors by Screening Charge Impurity with Silica Nanoparticles Jianwen Zhao,†,‡ ChengTe Lin,§ Wenjing Zhang,§ Yanping Xu,^ Chun Wei Lee,^ M. B. Chan-Park,‡ Peng Chen,*,‡ and Lain-Jong Li*,§ †

Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Ruoshui Road 398, Suzhou Industrial Park, Suzhou, China ‡ School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore, 637459 § Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, 11529 ^ School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Ave., Singapore, 639798

bS Supporting Information ABSTRACT: Ionic surfactants have been frequently used for nanotube device fabrication owing to their high efficiency in dispersing SWNTs. Surfactants are also widely adopted for state-of-the-art density gradient ultracentrifugation- and dielectrophoresis-based nanotube separation techniques. However, the residual surfactants on nanotubes have been speculated to degrade the electrical performance of SWNT devices. Conventional methods, such as extensive washing or thermal treatment, are not able to efficiently remove the residual surfactants. In this article, we reveal that a thin layer of polydimethylsiloxane (PDMS) containing silica and ionic liquid coated on SWNT network field-effect transistors (FETs) is able to largely enhance the device mobility (about 3
4 times) and suppress their hysteresis. The enhancement in electrical properties of SWNT FETs is due to the screening of residual charges by the silica/liquid ion mixture.

’ INTRODUCTION Single-walled carbon nanotubes (SWNTs) have attracted much attention due to their great potential for postsilicon electronics. Ultrathin SWNT networks have been successfully demonstrated as promising and low-cost materials for field-effect transistors (FETs).1
8 Nevertheless, high-performance solutionprocessable SWNT networks, which are suitable for printable electronics, still demand more investigations. The major hurdle to obtaining high-performance SWNT network FETs is the difficulty in obtaining high mobility and highly semiconducting devices due to the coexistence of metallic (M) and semiconducting (S) tubes in the network.9,10 Recently, several groups have demonstrated that it is possible to readily fabricate fully semiconducting FET devices based on solution processable SWNT films using the nanotubes treated with density gradient ultracentrifugation (DGU)11
13 or modified with organic diazonium salts or organic radicals.14,15 In these approaches, ionic surfactants were still unavoidably used for the device fabrication owing to their high efficiency in dispersing SWNTs. Surfactants are also widely adopted for DGU- and dielectrophoresis-based16
18 separation techniques. However, the residual molecules have been speculated to degrade the electrical performance of SWNT devices through increasing the electrode
nanotube resistance19 or the local electrostatic environments of SWNTs.20 It has been reported that these impurity charges may be screened off by r 2011 American Chemical Society

either electrolytes or ion liquid.21,22 Here, we show that residual surfactants strongly affect the mobility of carbon nanotubes. A thin layer of polydimethylsiloxane (PDMS) containing silica/ ionic liquid coated on SWNT network FETs is able to efficiently screen off the impurity charges from surfactants and, consequently, largely enhance the device field-effect mobility and suppress their hysteresis. The Hall effect measurements also corroborate the large enhancement in mobility after coating. This simple strategy promises applications in high-performance and printable SWNT-based macroelectronics.

’ EXPERIMENTAL SECTION Nanotubes. HiPCO and arc-discharge tubes were purchased from Carbon Nanotechnolgy Inc.(USA) and Integris (USA), respectively. SDS surfactants (1 wt %) were used for dispersing these commercially available tubes. In addition to commercially available HiPCO and Integris tubes, we have also tested chemically modified tubes using SWeNT@SG 65 tubes (from SouthWest NanoTechnologies (USA)). The solution for semiconducting device fabrication Received: January 31, 2011 Revised: March 1, 2011 Published: March 16, 2011 6975

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Figure 1. (a) Schematic illustration for a back-gated transistor based on SWNT networks. (b) Transfer characteristics for a transistor before and after being coated with Sylgard@184 silicone elastomer base containing 20 wt % of EMIM-TFSI.

Figure 2. (a) Effect of the wt% of EMIM-TFSI added to the Sylgard@184 silicone elastomer matrix (1 g) on the mobility enhancement ratio for the SWNT transistors. Note that the SWNT network transistors tested were with an initial mobility at ∼5
8 cm2/(V s). (b
d) Transfer curves for the devices before and after being coated with (b) pure PDMS, (c) PDMS with silica nanoparticles, and (d) PDMS with silica nanoparticles and EMIMTFSI.

was based on controlled chemical modification of CoMoCat SWNTs with radicals produced by an organic radical initiator, 1,10 -azobis(cyanocyclohexane) (ACN), as reported.14 In brief, 0.3 mg of SWNTs was dispersed in 30 mL of DMF solution via probe ultrasonication for 30 min (Sonics & Materials Inc., model VCX 130). A 1 mL portion of DMF solution containing 25 mg/ mL ACN was then added to 10 mL of the SWNT suspension, followed by 30 min ultrasonication. After reaction with ACN, the suspension was filtered through a 0.25 μm PTFE membrane, followed by repeated washing with high-purity DMF and acetone (reagent grade) to remove the residuals. The powders collected from the PTFE membrane were redispersed in a 2 wt % of cosurfactants, which consists of sodium dodecyl sulfate (SDS) and sodium cholate hydrate (SC) (weight ratio = 1:4). The nanotube bundles in the suspension were removed by centrifugation at 20 000 rpm for 90 min. The resulting supernatant was then used for fabrication of thin-film field-effect transistors (FETs). Device Fabrication. The SWNT FETs (SNFETs) were fabricated by drop-casting the suspension of modified SWNTs across two Au electrodes (100 nm thick) prepatterned on top of a

SiO2/Si substrate to form a conducting channel ∼50 μm long and ∼25 μm wide. The gate dielectrics SiO2 is 300 nm thick. For the drop-cast procedure, 25 μL of the SWNT suspension was dropped onto the devices, followed by drying at room temperature and rinsing with deionized water. The procedure was repeated until the density of the SWNTs is high enough to reach the desired current level. Furthermore, all electrical measurements were carried out in ambient conditions using a Keithley semiconductor parameter analyzer, model 4200-SCS. Preparation of the Silicone/EMIM-TFSI Mixture. To investigate the mobility enhancement effect, the PDMS elastomer, Sylgard@184 silicone elastomer base (1 g, Dow Corning Corporation, Midland, MI, USA), was first mixed with various amounts of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI)) under continuous stirring for 15 min before coating. The mixture was then coated on the SWNT network transistor directly in ambient. For the mechanism study part, silica nanoparticles (particle size, 7 nm; surface area, 390 ( 40 m2/g) are from Sigma-Aldrich. Pure PDMS, poly(dimethylsiloxane), methoxy terminated (average Mw ∼ 27 000), is also purchased from Sigma-Aldrich. 6976

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Figure 3. Transfer curves for an SWNT network transistor before and after being coated with a PDMS layer (with silica nanoparticles and EMIM-TFSI) at the center of the channel.

’ RESULTS AND DISCUSSION We use the FET device based on the SWNT networks, prepared using the tubes modified by organic radicals,14 as an example to demonstrate the mobility enhancement effect. Figure 1a schematically shows the bottom-gated device structure. Figure 1b displays the transfer curves (drain current, Id, vs gate voltage, Vg) of a typical SWNT network transistor before and after laying down a layer of Sylgard@184 silicone elastomer base mixed with 20 wt % of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI; chemical structure in Figure 2a). It is noted that the Sylgard@184 silicone elastomer base is a transparent and viscous fluid, composed of PDMS (>60%) and dimethylvinylated and trimethylated silica nanoparticles, and it becomes opaque after adding EMIM-TFSI. The mobility of the pristine device increased significantly from 8.1 to 28.5 cm2/(V s) after covering with the silicone/EMIMTFSI mixture. The output characteristics for this device are shown in Figure S1 (Supporting Information). The field-effect mobility was extracted based on the slope of the transfer curve, as described elsewhere,23
25 and we took the Vg scan from positive to negative voltage for mobility extraction. Such a mobility enhancement effect was observed on the transistor devices with a wide range of initial mobility from 0.015 to ∼10.8 cm2/(V s), as shown in Figure S2 (Supporting Information). This is also similarly observed for the devices prepared from other sources of SWNTs (without being modified with organic radicals), including commercially available CoMoCAT, HiPCO, and arcdischarge based tubes. The mobility enhancement is unique to the silicone/EMIM-TFSI mixture. Other types of coatings, such as water, organic solvents, epoxy resins, thermal glues, polyethylamine, polymethacrylic acid, and different photoresists, have been tested, but all these materials generally caused a decrease in the device mobility due to various reasons, such as increase in scattering sites or electronic doping (see Figure S3 in the Supporting Information for the typical experimental data). Figure 2a shows that the mobility enhancement ratio (the enhanced mobility to the initial mobility) for the SWNT network transistors strongly depends on the amounts of EMIM-TFSI added to the Sylgard@184 silicone elastomer base. Experimentally, the mobility enhancement effect occurs only when the silicone liquid mixture turns opaque after adding EMIM-TFSI. As no cross-linking reagent was added to the silicone elastomer base, the induced opacity is not related to the gelation or crosslinking of PDMS polymer chains. The mixture still remains as a viscous fluid. The opacity likely resulted from the aggregation of the silica nanoparticles in the mixture, which will be discussed later.

ARTICLE

Figure 4. Transfer curves for an SWNT network transistor before and after being coated with a PDMS layer (with silica nanoparticles and EMIM-TFSI) at the center of the channel.

It is arguable that the mobility increase could be due to the changes in environmental dielectrics, that is, from the dielectric constant ∼1 (in air) to ∼2.5 (PDMS). Dielectric-enhanced mobility has been reported for graphene-based transistors.26,27 However, the results in Figure 2b (coating with pure PDMS) clearly suggest that the dielectric change is also not a dominating factor. To further clarify the causes of mobility enhancement, we confirmed that PDMS added with only silica nanoparticles (without EMIM-TFSI) (Figure 2c) or PDMS with EMIM-TFSI (without silica) (data not shown) both reduced the device mobility. The mobility enhancement was only observed when the PDMS layer was mixed with both silica nanoparticles and EMIM-TFSI, as shown in Figure 2d. Taking into account all the observations described, the aggregation of silica nanoparticles induced by liquid ions EMIM-TFSI seems to play the key role in enhancing the mobility, whereas the PDMS layer serves as the matrix to host silica nanoparticles and liquid ions. To further understand the mechanism, we tested the coating at the center of the conducting channel without covering the metal
SWNT junctions. As shown in Figure 3, the mobility can still be enhanced even if the coating is limited to the conducting channel, suggesting that the observed effects are essentially related to conducting channels (SWNT networks). Our results in Figure 1 have revealed that the mobility enhancement occurs simultaneously with the hysteresis change. Figure 4 shows that the mobility enhancement ratio was less significant when the hysteresis of the pristine transistor (before coating) was smaller, where the hysteresis was quantified based on the method reported elsewhere.28 It has been established that the hysteresis in SWNT transistors is caused by the trapped charges between SWNTs and environmental dielectrics28,29 or charges introduced by residual surfactants.30,31 We have verified that the coating with pure PDMS polymers does not change the hysteresis of our devices (Figure 2b). Hence, the observed hysteresis reduction did not result from the change of environmental dielectrics. Instead, it is likely due to suppression of the influences from the residual surfactants. This notion is further corroborated by a separate experiment that thorough water rinsing to reduce the residual surfactants led to a lower enhancement ratio of ∼2.3 as compared with that from devices without thorough rinsing, ∼4.2. Furthermore, we annealed the pristine devices at 260 C in air for various time periods to burn off (or oxidize) the residual surfactants. As expected, with the increasing annealing time, the devices exhibit a lower mobility enhancement ratio after coating with silicone/EMIM-TFSI (see Figure S4 in the Supporting Information). 6977

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Table 1. Hall Effect Measurements Based on the Van der Pauwl Method Were Performed to Extract the Hall Mobility, Carrier Concentration, and Sheet Resistance of the 1 cm  1 cm SWNT Networksa mobility (cm2/(V sheet resistance (Ω/ sampleprocess s)) sq) 1

before after

2

before after

hole concn (/ cm2)

8.07

8.51  105

9.45  1011

24.71

7.35  105

4.32  1011

7.63

2.19  10

6

8.58  1011

25.40

2.43  10

6

1.33  1011

3

before

2.73

2.56  10

1.04  1012

4

after before

4.44 1.67

3.15  106 1.20  106

4.81  1011 3.18  1012

after

3.41

1.29  106

1.45  1012

1.46

4.37  10

5

1.05  1013

6.24

6.80  10

5

1.69  1012

5

before after

6

a

The decrease of hole concentration after coating suggests that the screening of the negatively charged surfactants occurs.

To further clarify the effect of the silicone/EMIM-TFSI coating, a Hall effect measurement based on the Van der Pauwl method32 was performed to extract the Hall mobility, carrier concentration, and sheet resistance of the SWNT networks (1 cm  1 cm). Table 1 shows that the extracted Hall mobility for all these films increases after coating, consistent with the FET device results. In general, the sheet resistance slightly increases after coating, which is consistent with the observation of output characteristics in Figure S1 (Supporting Information). The sheet resistance increase is due to the decrease of the hole concentration after coating, indicating that the screening of the negatively charged surfactants also reduces the hole concentration in SWNTs. Silica nanoparticles carry negative surface charges in a neutral environment,33 and the interparticle electrostatic repulsion keeps them well-separated and dispersed in PDMS. It was reported that, with the incorporation of liquid ions, the interparticle electrostatic repulsion appeared to be inefficient, due to the extensively high ionic strength of the liquid ions and the resulting surface-charge screening between nanoparticles.33 Hence, silica nanoparticles tend to form clusters or aggregations, resulting in its opacity in PDMS as observed. We conceive that the ions from residual surfactants around SWNTs may be attracted by or trapped into the silica
liquid ion aggregations, leading to the effective screening of charge from surfactants and the enhancement of the effective mobility of the FETs.

’ CONCLUSIONS Pronounced enhancement in the field-effect mobility and decrease of hysteresis of SWNT network devices were achieved simply by coating with a layer of PDMS containing silica nanoparticles and liquid ions. The method allows us to get a 3to 4-fold enhancement of mobility. It is noteworthy pointing out that the surfactants in aqueous solutions are still the most popular and efficient system to disperse SWNTs for solution-based fabrication processes. Although the thermal annealing may be used to burn off the surfactants to a certain extent, the annealing usually degrades rather than enhances the device mobility. On the other hand, several groups have attempted to use organic solvents to replace the surfactants for device fabrication.34,35 However, organic solvents could introduce unexpected impurities,

doping effects, or more charge-trapping sites. The current absence of the practical surfactant-free processes makes our approach very useful in the fabrication of high-performance devices.

’ ASSOCIATED CONTENT

bS

Supporting Information. Electrical characterization results are available. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (P.C.), [email protected]. tw (L.-J.L.).

’ ACKNOWLEDGMENT J.Z. and C.T.L. contributed equally to this work. This research was supported by the Nanyang Technological and National Research Foundation Singapore (NRF-CRP 2
2007
02). P. C. also acknowledges the support from the A-star SERC grant (#0721010020). L.-J.L. thanks Academia Sinica and National Science Council Taiwan for support (NSC-99-2112-M-001-021MY3 and 99-2738-M-001-001). ’ REFERENCES (1) Snow, E. S.; Novak, J. P.; Campbell, P. M.; Park, D. Appl. Phys. Lett. 2003, 82, 2145–2147. (2) Artukovic, E.; Kaempgen, M.; Hecht, D. S.; Roth, S.; Gruner, G. Nano Lett. 2005, 5, 757–760. (3) Zhou, Y.; Gaur, A.; Hur, S. H.; Kocabas, C.; Meitl, M. A.; Shim, M.; Rogers, J. A. Nano Lett. 2004, 4, 2031–2035. (4) Shim, M.; Cao, M. Adv. Mater. 2006, 18, 304. (5) Unalan, H. E.; Fanchini, G. G.; Kanwal, A. A.; Du Pasquier, A. A.; Chhowalla, M. M. Nano Lett. 2006, 6, 677–682. (6) Kang, S. J.; Kocabas, C.; Ozel, T.; Shim, M.; Pimparkar, N.; Alam, M. A.; Rotkin, S. V.; Rogers, J. A. Nat. Nanotechnol. 2007, 2, 230–236. (7) Kocabas, C. Nano Lett. 2007, 7, 1195. (8) Cao, Q.; Kim, H. S.; Pimparkar, N.; Kulkarni, J. P.; Wang, C.; Shim, M.; Roy, K.; Alam, M. A.; Rogers, J. A. Nature 2008, 454, 495–500. (9) Topinka, M. A.; Rowell, M. W.; Gorden, G. D.; McGehee, M. D.; Hecht, D. S.; Gruner, G. Nano Lett. 2009, 9, 1866–1871. (10) Unalan, H. E.; Fanchini, G.; Kanwal, A.; Du Pasquier, A.; Chhowalla, M. Nano Lett. 2006, 6, 677. (11) Arnold, M. S.; Green, A. A.; Hulvat, J. F.; Stupp, S. I.; Hersam, M. C. Nat. Nanotechnol. 2006, 1, 60–65. (12) Hersam, M. C. Nat. Nanotechnol. 2008, 3, 387–394. (13) Wang, C.; Zhang, J.; Ryu, K.; Badmaev, A.; de Arco, L. G.; Zhou, C. Nano Lett. 2009, 9, 4285. (14) Zhao, J. W.; Lee, C. W.; Han, X. D.; Chen, F. M.; Xu, Y. P.; Huang, Y. Z.; Park, M. B. C.; Chen, P.; Li., L. J. Chem. Commun. 2009, 7182–7184. (15) Lee, C. W.; Han, X. D.; Chen, F. M.; Wei, J.; Chen, Y.; Park, M. B. C.; Li, L. J. Adv. Mater. 2009, 22, 1278–1282. (16) Krupke, R.; Hennrich, F.; von Lohneysen, H.; Kappes, M. Science 2003, 301, 344–347. (17) Krupke, R.; Linden, S.; Rapp, M.; Kappes, M. Adv. Mater. 2006, 18, 1468–1470. (18) Niyogi, S.; Densmore, C. G.; Doorn, S. K. J. Am. Chem. Soc. 2009, 131, 1144–1153. (19) Fu, Q.; Liu, J. Langmuir 2005, 21, 1162–1165. (20) Johnston, D. E.; Islam, M. F.; Yodh, A. G.; Johnson, A. T. Nat. Mater. 2005, 4, 589–592. 6978

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(21) Ozel, T.; Gaur, A.; Rogers, J. A.; Shim, M. Nano Lett. 2005, 5, 905–911. (22) Okimoto, H.; Takenobu, T.; Yanagi, K.; Miyata, Y.; Shimotani, H.; Kataura, H.; Iwasa, Y. Adv. Mater. 2010, 22, 3981–3986. (23) Zaumseil, J.; Sirringhaus, H. Chem. Rev. 2007, 107, 1296–1323. (24) Lee, C. W.; Weng, C.-H.; Wei, L.; Chen, Y.; Park, M. B. C.; Tsai, C.-H.; Leou, K.-C.; Poa, C. H.; Wang, J.; Li, L. J. J. Phys. Chem. C 2008, 112, 12089–12091. (25) Fu, D. L.; Okimoto, H.; Lee, C. W.; Takenobu, T.; Iwasa, Y.; Kataura, H.; Li, L. J. Adv. Mater. 2010, 22, 4867. (26) Adam, S.; Hwang, E. H.; Galitski, V. M.; Sarma, S. D. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 18392–18397. (27) Chen, F.; Xia, J. L.; Tao, N. J. Nano Lett. 2009, 9, 1621–1625. (28) Lee, C. W.; Dong, X.; Goh, S. H.; Wang, J.; Wei, J.; Li, L. J. J. Phys. Chem. C 2009, 113, 4745–4747. (29) Rinki€o, M.; Zavodchikova, M. Y.; T€orm€a, P.; Johansson, A. Phys. Status Solidi B 2008, 245, 2315–2318. (30) Yuan, S. N.; Zhang, Q.; Shimamoto, D.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Endo, M. Appl. Phys. Lett. 2007, 98, 143118. (31) Li, J. Q.; Zhang, Q.; Li, H.; Park, M. B. C. Nanotechnology 2006, 17, 668–673. (32) Van der Pauw, L. J. Philips Res. Rep. 1958, 13, 1–9. (33) Ueno, K.; Inaba, A.; Kondoh, M.; Watanabe, M. Langmuir 2008, 24, 5253–5259. (34) Song, J. W.; Kim, J. D.; Yoon, Y. H.; Choi, B. S.; Kim, J. H.; Han, C. S. Nanotechnology 2008, 19, 095702. (35) Okimoto, H.; Takenobu, T.; Yanagi, K.; Miyata, Y.; Kataura, H.; Asano, T.; Iwasa, Y. Jpn. J. Appl. Phys. 2009, 48, 06FF03.

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