Supplementary Information for
Highly Stretchable Carbon Nanotube Transistors Enabled by Buckled
Ion Gel Gate Dielectrics
Meng-Yin Wu1,a), Juan Zhao2,3,a), Feng Xu3, Tzu-Hsuan Chang1, Robert M. Jacobberger3,
Zhenqiang Ma1, and Michael S. Arnold3,b)
1
Department of Electrical and Computer Engineering, University of Wisconsin-Madison,
Madison, Wisconsin 53706, United States
2
School of Optoelectronic Information, University of Electronic Science and Technology of
China, Chengdu 610054, China
3
Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison,
Wisconsin 53706, United States
a)
Meng-Yin Wu and Juan Zhao contributed equally to this work.
b)
Author to whom correspondence should be addressed. Electronic mail: michael.arnold@wisc.edu.
Preparation of polymer wrapped (7, 5)-enriched semiconducting single-walled carbon
nanotubes:
We adapt the experimental methods of Nish et al.1 and Hwang et al.2 to post-synthetically
isolate semiconducting carbon nanotubes. 70 mg of carbon nanotube powder (SouthWest
Nanotechnologies, SG65i) is added to 70 ml of 2 mg ml-1 poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO,
American Dye Source) in toluene. A horn ultrasonicator (diameter of 12.7 mm) is used to
aggressively agitate the mixed solution at 40% of maximum power for 30 minutes to unbundle
carbon nanotubes and wrap them with PFO. A swing bucket rotor separates the well-wrapped,
isolated nanotubes from unwanted bundles, unselected nanotubes, and other impurities at 300,000
g for 15 min. The top 90% of supernatant is carefully decanted, which contains the enriched (7, 5)
PFO wrapped semiconducting nanotubes. The collected supernatant is filtered through a 5.0 µm
PVDF membrane to further remove micro- and millimeter-sized impurities. The nearly clear PFO
nanotube solution is vacuum-distilled to dry all the toluene. Then, the resulting PFO nanotube
solid is re-dissolved into hot tetrahydrofuran (THF). This process aids in the removal of excess
PFO because PFO wrapped nanotubes have lower solubility in THF than PFO. The solution is then
centrifuged in a fixed-angle rotor at 50,000 g at 4 °C for 24 hours. The pellet consists of nanotube
enriched solid. Dispersion in THF followed by ultracentrifugation is repeated until the PFO to
nanotube mass ratio in the solid is less than 2 to 1, as determined by optical absorption
spectroscopy. After, the pellet is vacuum dried to remove THF and the PFO wrapped (7, 5) tubes
are re-dispersed into ortho-dichlorobenzene (ODCB). The ODCB nanotube solution is briefly
ultrasonicated for 30 seconds right before casting the nanotube film by using a microtip at 10% of
the maximum amplitude.
Preparation of ion gel:
The preparation of the ion gel is adapted from Pu et al.3 A triblock copolymer poly(styreneblock methylmethacrylate-block-styrene) (PS-PMMA-PS, Polymer Source Inc. MPS = 4.3 kg/mol,
MPMMA = 12.5 kg/mol, MW = 21.1 kg/mol) is mixed with an ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide (Sigma Aldrich), and dissolved in ethyl
acetate (Sigma Aldrich). The mass ratio of polymer, ionic liquid, and solvent is 0.7:9.3:20. The
mixed solution is stirred in glove box at 1000 rpm for 4 hours and then filtered using a 200 nm
poly(tetrafluoroethylene) filter. The solution is stored in inert nitrogen environment.
Device fabrication:
The PDMS substrates are prepared using Sylgard® 184 Silicone Elastomer (Dow Corning)
by mixing the “base” and “curing agent” at a 10:1 ratio. The mixture is spin-cast onto a plastic
petri dish at 200 rpm for 2 minutes. The PDMS is cured in oven at 75˚C for three hours, leading
to a PDMS membrane with a thickness of 150 to 200 µm. The PDMS is cut into 1 cm × 4 cm
rectangles. The substrates are rinsed in isopropyl alcohol, air-dried, and annealed on a hotplate at
100 ˚C to remove residual solvent. The substrates are taped (Single Sided Kapton® Polyimide
Tape) on one end to a glass slide, elongated by 80%, and then taped to the glass slide on the other
end. The PDMS on glass substrate is set on the surface of a hotplate at 130 ˚C in a nitrogen glove
box. Next, 5 µl of carbon nanotube solution (see above) is deposited and spread across the substrate
using a hand-drawn doctor-blade tool to cast a homogeneous carbon nanotube film. Then,
electrodes consisting of Cr/Au/Cr (4 nm/26 nm/4 nm) are thermally evaporated as the source, drain,
and gate electrode, through a shadow mask at a high vacuum (~3×10-7 torr). A Q-tip wetted with
chlorobenzene is used to wipe and remove the nanotube film outside of the area defined by the
source and drain electrodes. Finally, the ion gel is manually drop-cast on top of the channel area
and the whole substrate is then annealed at 65 ˚C for 30 minutes and at 105 ˚C for one hour to
remove residual solvent and water.
Device characterization:
To measure electrical performance of the FETs, both ends of the released PDMS substrates
are clamped on a stretching stage with two sliders driven by two right and left-hand threaded
screws. The elongation of the PDMS is measured using a digital caliper. Current-voltage curves
are measured by using Keithley source meter (model 2636A) in air. The ion gel capacitance is
measured by using an Agilent LCR Meter (4284A). SEM images are taken from a LEO-1550VP
field-emission scanning electron microscope at 3 kV of accelerating voltage. Optical
interferometry (Zygo NewView 6K) is used to measure the amplitude of the buckles in the released
ion gel film.
Carbon nanotube quantum capacitance:
The ion gel capacitance and nanotube quantum capacitance are connected in series.
−1
C = ((𝐶𝑞 × 𝜌)−1 + 𝐶𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑
)−1
where the nanotube quantum capacitance 𝐶𝑞 is 4 × 10-6 µF cm-1 and ρ is the linear density of
nanotubes.4 Using a mass density for the polymer wrapped (7, 5) nanotube film of 1.36 g cm-3 and
a mass ratio of polymer to nanotubes of 1:1,5 the average tube-tube spacing is 1.84 nm, giving a ρ
of 540 nanotubes µm-1.
Amplitude of the wrinkles in the ion gel film
Fig. S1 shows the optical interferometry image of a released ion gel film. The amplitude of the
wrinkles varies from 2 to 4 µm.
Figure S1. Optical interferometry image of a released ion gel film.
Capacitance of ion gel versus applied strain
Fig. S2 shows the capacitance of the ion gel, measured between two metal thin films, as a function
of the applied tensile strain. The capacitance slightly increases as the elongation increases from
0% to 90% and start to decrease beyond the 90% elongation.
Figure S2. Capacitance of the ion gel as a function of applied strain.
Buckled ion gel under various strain
Fig. S3 shows optical images of an ion gel film under various applied elongation. The wrinkles are
gradually flattened and disappear with increasing elongation. The remaining local buckling at
100% strain can be attributed to the inhomogeneous thickness of ion gel film.
Figure S3. Optical images of buckled ion gel film under various elongation.
Delamination of ion gel
Fig. S4 shows the delamination of the ion gel after elongation to 90% and release. This
delamination can cause poor contact between the ion gel and the metal electrodes, which reduces
device performance.
Figure S4. Delamination of ion gel film.
Conductance of carbon nanotube film versus applied strain
Fig. S5 shows the conductance of carbon nanotube film as a function of applied strain. The applied
voltage is VD= -20 V. The conductance remains invariant at 95% elongation.
Figure S5. Conductance of nanotube film as a function of applied strain.
Conductance of metal electrode film versus applied strain
Fig. S6A shows the conductance of metal film as a function of applied strain. The conductance
remains invariant up to 50% strain, and then increases as the strain is further increased to 100%.
Beyond 100% strain, the conductance starts to decrease. The increasing of conductance can be due
to the unwrinkling of the metal film while the decay of conductance is from the additional cracks,
as shown in Fig. S6B.
Figure S6. A) Conductance of metal electrode film as a function of applied strain. B) Optical
images of buckled metal electrode film under varies applied strain.
1
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J. Nicholas, Journal of the American Chemical Society 130 (11), 3543 (2008).
3
Jiang Pu, Yohei Yomogida, Keng-Ku Liu, Lain-Jong Li, Yoshihiro Iwasa, and Taishi Takenobu, Nano Lett
12 (8), 4013 (2012).
4
Sami Rosenblatt, Yuval Yaish, Jiwoong Park, Jeff Gore, Vera Sazonova, and Paul L. McEuen, Nano Lett 2
(8), 869 (2002).
5
Meng-Yin Wu, Robert M. Jacobberger, and Michael S. Arnold, Journal of Applied Physics 113 (20),
204504 (2013).
2