Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate

Jan 1, 2014 - †Department of Materials Science and Engineering and ‡Department of Electrical and Computer Engineering, University of Wisconsin-Mad...
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Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate Dielectrics Feng Xu,† Meng-Yin Wu,‡ Nathaniel S. Safron,† Susmit Singha Roy,† Robert M. Jacobberger,† Dominick J. Bindl,† Jung-Hun Seo,‡ Tzu-Hsuan Chang,‡ Zhenqiang Ma,‡ and Michael S. Arnold*,† †

Department of Materials Science and Engineering and ‡Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: Field-effect transistors (FETs) that are stretchable up to 50% without appreciable degradation in performance are demonstrated. The FETs are based on buckled thin films of polyfluorene-wrapped semiconducting single-walled carbon nanotubes (CNTs) as the channel, a flexible ion gel as the dielectric, and buckled metal films as electrodes. The buckling of the CNT film enables the high degree of stretchability while the flexible nature of the ion gel allows it to maintain a high quality interface with the CNTs during stretching. An excellent on/off ratio of >104, a field-effect mobility of 10 cm2·V−1·s−1, and a low operating voltage of 104, a field-effect mobility of 10 cm2·V−1·s−1, and a low operating voltage of 25% (Supporting Information, Figure S2). Thus, we limit stretching to 25% when inducing buckling in this case. To fabricate FETs from these buckled films, first the PDMS substrate is restretched to 15% strain (Figure 1a). Au films with 683

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S5). The capacitance gradually decreases with the applied tensile strains. The mobility of ∼10 cm2/(V·s) determined from the transconductance and capacitance data fluctuates slightly but does not decrease under stretching up to 18% (Figure 2c). However, for a strain of 20%, the on-current and mobility decrease significantly, by roughly 80%, with further reductions in performance for strain >20%. To study the fatigue properties of the devices, 1000 stretching/releasing cycles in the strain range of 0−10% are performed on another device. Although the on/off ratio and mobility of the device fluctuate to some degree, the device still shows excellent performance without electrical or mechanical damage, indicating excellent stability (Figure 2d). We next describe FETs fabricated from CNTs transferred to strained PDMS (Figures 3−5). Here, we employ a well-known buckling approach6 by prestraining the PDMS substrate before transferring the CNT film, as shown in Figure 3a. To achieve reliable transfer, the prestrained PDMS substrate is quickly peeled off the glass substrate to enhance the adhesion of CNTs to PDMS.30 After releasing the strain in the PDMS substrate, the CNT film buckles (Figure 3b). By transferring to a prestrained PDMS substrate, the degree of buckling increases allowing for greater FET stretchabiltiy beyond that intrinsically accommodated via CNT−CNT sliding. As an example, the prestrain used here is 50%. The wrinkles vary in height from 10 to 100 nm, within the same film, and have an average period of ∼200 nm (Supporting Information Figure S6). Figure 3c, d, and e show scanning electron microscope (SEM) images of an already buckled CNT film on PDMS subsequently restrained by 25%, 50%, and 65%, respectively, and imaged in the stretched state. Initially, the wrinkles extend perpendicular to the direction of tensile strain. However, these wrinkles gradually disappear with increased strain (Figure 3c) and eventually develop to extend parallel to the strain (Figure 3d). Interestingly, no cracks develop in the CNT film even when the PDMS substrate is restretched to 65% (Figure 3e). To characterize the change in the morphology of the CNT films further, we utilize polarized Raman spectroscopy and quantify the Raman G-band intensity ratio, I⊥/I||, for optical excitation and detection polarized perpendicular to the direction of prestrain (I⊥) and parallel to it (I||). For as-cast CNT films, I⊥/I|| =1. Following CNT film transfer to prestrained PDMS and release, I⊥/I|| increases to ∼6 (Supporting Information, Figure S7). The increase can be attributed to (a) reorientation of CNTs normal to the substrate in the wrinkles, which reduces I||, (b) the compression of the CNT network due to release of the prestrain, which results in a net CNT alignment perpendicular to the direct of prestrain and thereby increases I⊥ and decreases I||, and (c) a stretching of the CNT network transverse to the direction of the prestrain during the release, due to the high Poisson ratio of PDMS (∼0.5), which also increases I⊥ and decreases I||. The latter stretching of the CNT network in the transverse direction likely results in an irreversible deformation, as evidenced by the observation of wrinkles that extend parallel to the direction of prestrain when the substrate is brought back to a strain of 50% (Figure 3d). I⊥/I|| decreases with increasing strain to 50%, during which the deformation and alignment are reversed with respect to the deformation and alignment induced during the initial release. CNT FETs are fabricated using the same procedures shown in Figure 1a. Optimized output and transfer characteristics of the CNT FETs prior to restretching are displayed in the

Figure 2. Device performance as a function of tensile strain and fatigue. (a,b) Typical transfer characteristics (VD = −0.1 V) of the devices under strain up to 20%. (c) On and off currents and mobility as a function of applied strain. (d) On and off currents and mobility of another device with repeated stretch-and-release in the strain range of 10%, as a function of cycle.

the FET channel length matches the strain applied to the PDMS. The electrical transfer characteristics are measured simultaneously during the stretching process (Figure 2a,b). The devices exhibit fairly consistent performance under strain of up to 18%: the off-current is nearly constant while the on-current and transconductance are reduced by 17% and 13% with respect to the unstrained values, respectively (Figure 2a and Supporting Information, Figure S4). To determine the mobility of the transistors during the stretching process, the capacitance of the ion gel is also measured (Supporting Information, Figure 684

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Figure 3. FETs fabricated from CNTs transferred to strained PDMS. (a) Schematic showing the transfer and buckling of CNT films. (b) SEM image of the CNT film after release of strain and buckling. (c−e) SEM images of buckled CNT film restretched to 25% (c), 50% (d), and 65% (e).

Figure 5. Demonstration of stretchabiltiy. FETs fabricated from CNTs transferred to strained PDMS twisted between tweezers (a), conformally wrapped around a glass rod (b), and adhered to a textile and stretched to 30% (c,d). Corresponding transfer characteristics are presented in the Supporting Information, Figure S11. Figure 4. Performance of device stretched over 50%. (a) Typical transfer characteristics (VD = −0.5 V) under strain up to 57.2%. (b) On and off currents and mobility as a function of applied strain.

invariant mobility and on/off current ratio to strain >50% is observed for all devices tested (N = 4). A CNT FET is mainly composed of three electronic components including a dielectric layer, electrodes, and a CNT channel. To make the CNT FETs stretchable, each component must be stretchable. To better understand the failure of the CNT FETs, we next examine the stretchability of all three components in our devices individually. Both the buckled CNT film and metal electrodes are highly stretchable, and their interface is well-maintained even up to 75% strain (Supporting Information, Figure S10). On the other hand, the ion gel film is observed to fracture in some cases for strain as low as 45% (Supporting Information, Figure S5). It should therefore be possible to further improve the performance of the CNT FETs for >50% strain by engineering the ion gel’s composition and how it is implemented in the device stack. It should be noted that the FETs demonstrated here also accommodate 10% strain in the transverse direction. A higher transverse strain should be possible in the future by employing biaxial buckling strategies.

Supporting Information, Figure S8. A field-effect mobility of 6.9 cm2/(V·s) and on/off current ratio of 3 × 104 are measured. The small decrease in mobility is attributed to an increase of the effective channel length (in other words a longer arc-length along the buckled CNT film) due to the large prestrain (50%), as compared to those used in Figure 1. Figure 4a presents typical electrical characteristics of the CNT FETs as a function of applied strain. The gate capacitance reversibly decreases with strain (Supporting Information, Figure S5), and therefore the transconductance decreases, as well (Supporting Information, Figure S9); however, the devices exhibit stable operation with fairly invariant mobility and on/off current ratio up to 50.8% stain (Figure 4b), only first decreasing significantly when the applied strain increases to 57.2%. It should be noted that there is some device to device variation in initial performance prior to restretching, depending on the doctor-blade draw speed, which we control by hand. Nonetheless, a stable operation with fairly 685

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In summary, highly stretchable transistors are developed based on buckled CNT films with ion gel dielectrics. The devices exhibit an excellent on/off ratio of ∼104 and a low operating voltage of less than 2 V. Using a prestrain during CNT film transfer of 50% enables the fabrication of FETs that operate in a large tensile strain range of 0−50%. Operation under strain of 75% and potentially beyond will be possible with further engineering of the ion gel and the device stack. In general, such deformable FETs based on highly type-sorted semiconducting CNTs promise to exploit CNTs’ high current carrying capacity, outstanding mechanical resilience, and excellent air and chemical stability and thereby facilitate the development of new technologies like stretchable displays, wearable and conformal devices, and electronic skins.

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ASSOCIATED CONTENT

S Supporting Information *

Additional supporting figures and information. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.X. and M.S.A. acknowledge primary support for this work by the U.S. Army Research Office, W911NF-12-1-0025. M.-Y.W. is supported by the University of Wisconsin-Madison Center of Excellence for Materials Research and Innovation (DMR1121288). N.S.S. and R.M.J. are supported via the DOE Office of Science Early Career Research Program (Grant no. DESC0006414) through the Office of Basic Energy Sciences. R.M.J. also acknowledges support from the Department of Defense (DOD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. S.S.R. and D.J.B. acknowledge support from the National Science Foundation CBET-1033346 and DMR-0905861, respectively. T.-H.C., J.-H.S., and Z.M. are supported by AFOSR under grant FA9550-091-0482.

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