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compressive restoration of the conductivity above
10% strain for the metallic films. On the basis of all
of these observations, we suggest that metallic
SWCNT films are more durable as flexible transparent
EXPERIMENTAL METHODS
Sample Preparation. Polymer substrates of polydimethylsilox-
ane (PDMS) were prepared at a 10:1 monomer/cross-linker ratio.
The two components were mixed, poured into a mold, and
placed in a vacuum oven for 1 h to remove air bubbles. Without
breaking vacuum, the mold was then baked at 80 0C for 2 hours.
Substrates with dimensions 75 mm x 25 mm x 1.5 mm were cut
from the mold and cleaned with ethanol. The two ends of a
substrate were then clamped in a strain stage and a controlled
stretch was applied up to a specific strain. Two SWCNT pieces of
approximately 2 mm x 10 mm were cut from the filter paper
and placed SWCNT-face down onto the prestrained substrate,
one oriented with the long edge normal to the direction of
strain and the other oriented parallel. The sample was then
subjected to a gentle acetone wash over the course of 1 h to
remove the filter paper. This was followed by a 20 min ethanol
rinse to remove any residual surfactant, and the test specimen
was then placed in a vacuum for several hours to expedite
drying. Once dry, the strain on the substrate was carefully re-
leased at a controlled and consistent rate, compressing the two
films. The sample was then positioned on a mask and placed in a
deposition chamber where 10 nm of chromium followed by
100 nm of gold was sputtered onto the sample without break-
ing vacuum (Figure 4f). Pieces of the SWCNT films were also
deposited in a similar manner on quartz to measure spectra,
scattering, and thickness.
Characterization. UV-vis-NIR spectroscopy was performed
on a commercial spectrometer, and optical transparency anal-
ysis was performed on an optical microscope equipped with a
spectrometer. Multiple spectra were captured at various loca-
tions across the surface of each film. An atomic force micro-
scope (AFM) configured in tapping mode was used to collect
several images at various points along the edges of each film,
and the thickness was determined by analyzing the quartz-
SWCNT step height normal to the edge. Impedance spectros-
copy, configured for a 2-probe capacitive frequency (CF) test
and calibrated against a 1.01 pF capacitor, was used to measure
the complex impedance spectra for each sample. The CF
measurement uses a 250 mV amplitude over the frequency
interval 1-1000 kHz. A reflection optical microscope was used
to collect images of all tested gaps, and these images were used
to compute geometrical factors to convert impedance to sheet
resistance. A 4-point probe was then used to measure the zero-
frequency sheet resistance of each sample, calibrated against
an ITO standard. This 4-point measurement was used for the
final calibration of each impedance trace from the low fre-
quency plateau in the real part of the response. For cyclic strain
experiments, optical measurements of the in-plane geometry as
a function of strain were used to compute strain-dependent
geometric factors. Measurements of the conductivity of the
gold electrodes under cyclic strain showed negligible effects
associated with strain-induced changes in electrode structure,
contact, and topography. Static small-angle light scattering
measurements were performed with a 30 mW He-Ne laser
directed through unstrained films deposited on thin quartz. The
scattered light was imaged on a screen equipped with a beam
stop using a thermoelectrically cooled CCD, and measured
background scattering from the quartz substrate was sub-
tracted from the total intensity. The signal was then circularly
averaged and reduced by the thickness of the film. Increased
scattering in the metallic film (Figure 1 d) is a consequence of the
shape of the real part of the dielectric response and the near-
resonant (M11) laser line.39 Unless otherwise indicated, error
bars are the size of the data markers and represent the max-
imum instance of two standard deviations in uncertainty.HARRIS ET AL.
conductors because (i) the metallic films themselves
are more mechanically robust and (ii) the impeda-
nce between contacted metallic SWCNT bundles is
smaller.
To image the membranes with transmission electron micro-
scopy (TEM), SWCNT films on quartz or PDMS were first coated
with evaporated Pt/C at a shadowing angle of approximately
260 (the height of an object is thus half of the length of its
shadow). A thin evaporated coating of carbon was then applied
at normal incidence. To detach the decorated nanotube mem-
branes from the supporting substrates, a drop of aqueous
poly(acrylic acid) (PAA) solution was placed on the film, allowed
to dry overnight at 55 C, and then removed the following day.
Slow removal of the PAA typically resulted in a clean detach-
ment of the membrane from the substrate. The detached
sample was then placed PAA side down onto a water bath,
dissolving the PAA and floating the SWCNT membrane on the
water surface. After gentle washing, the film was retrieved on a
copper 600 mesh grid. TEM was performed using a Philips
EM400T operated at 120 kV in bright-field mode with an
objective aperture to enhance contrast. Images were recorded
with a Cantega 2k CCD camera. In this mode, the darkness of a
point in the image is correlated to the total mass (integrated
thickness) at that location.
Acknowledgment. We thank Jan Obrzut, Chris Stafford,
Jun Chung, Benjamin Forsythe, Steven Ruckdashel, Matthew
Mumm, and Abby Mattson for assistance. E.K.H. acknowledges
the support of the NSF through CMMI-0969155 (J.M.H.) and the
DOE through DE-FG36-08GO88160 (G.R.S.I.).
Supporting Information Available: Details of the SWCNT
separation scheme and the measured SWCNT length distribu-
tions, with AFM images. This material is available free of charge
via the Internet at http://pubs.acs.org.
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Harris, J.; Iyer, S.; Bernhardt, A.; Huh, J. Y.; Hudson, S.; Fagan, J. et al. Electronic Durability of Flexible Transparent Films from Type-Specific Single-Wall Carbon Nanotubes, article, December 11, 2011; United States. (https://digital.library.unt.edu/ark:/67531/metadc836378/m1/6/: accessed March 29, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.