Conjugated Polymers and Carbon Nanotubes
J. Phys. Chem. B, Vol. 106, No. 12, 2002 3129
(2.66 g, 83%) were collected via filtration and dried under
vacuum. m.p. > 200 °C; H NMR (360 MHz, CDCl3): δ )
C31H43NO2 (461.7): C, 80.65; H, 9.39; N, 3.03; found: C,
78.69; H, 9.17; N, 2.83. Calculated for HCl protonated once
every three repeat units: C, 78.58; H, 9.22; N, 2.96. The
molecular weights and polydispersity (PDI) of PPyPV (Mw )
24 246; PDI ) 1.8) were determined in THF by using an SEC
instrument as described above. The GPC measurements of the
polymer shows that the number-average molecular weight (Mn)
is 13 543, corresponding to 29 repeating units.
1
7.75-7.73 (m, 3H), 7.66-7.63 (m, 12H), 6.68 (s, 1H), 5.34 (d,
J ) 12.7 Hz, 2H), 2.99 (t, J ) 5.8 Hz, 2H), 1.32-1.19 (m,
5H), 1.13-1.04 (m, 5H), 0.90 (t, J ) 6.7 Hz, 3H); 13C NMR
(90 MHz, CDCl3): δ ) 150.4, 134.7, 134.2, 134.14, 134.09,
130.0, 129.95, 129.88, 118.5, 117.5, 116.3, 115.8, 67.8, 31.7,
29.2, 29.1, 28.6, 25.7, 22.5, 14.0.
Poly{(m-phenylenevinylene)-co-[(2,5-dioctyloxy-p-phenyle-
ne)vinylene]} (PmPV). A 5% NaOEt in EtOH solution (2.8 g,
6 mmol) was added dropwise to a solution of 4 (1.91 g, 2 mmol)
and isophthaldehyde (0.27 g, 2 mmol) in a mixture of anhydrous
EtOH (20 mL) and THF (20 mL) at ambient temperature. The
reaction mixture was stirred for an additional 24 h. The resulting
polymer was precipitated twice from MeOH and then dried to
afford crude PmPV (0.55 g, 60%) as a yellow resin. 1H NMR
(360 MHz, CDCl3): δ ) 7.60-7.30 (m, 6H), 7.20-6.50 (m,
4H), 4.09 (t, J ) 6.5 Hz, 2H (trans-fragment)), 3.51-3.49 (m,
2H (cis-fragment), 1.90 (p, J ) 6.5 Hz, 2H), 1.56 (br, 2H),
1.25 (m, 8H), 0.86 (br, 3H). A sample of the crude PmPV (0.55
g) and I2 (0.002 g) were refluxed in PhMe (20 mL) for 4 h.
The solvent and iodine were evaporated off poly{(m-phenyle-
nevinylene)-co-[(2,5-dioctyloxy-p-phenylene)vinylene]} under
reduced pressure and the product was dissolved in CHCl3 and
precipitated out with MeOH. The resulting precipitate was dried
to afford PmPV as a yellow resin (0.5 g). 1H NMR (360 MHz,
CDCl3): δ ) 7.64 (s, 1H), 7.55-7.46 (m, 4H), 7.37 (t, J ) 7.5
Hz, 1H), 7.21 (s, 1H), 7.16 (s, 3H), 4.09 (t, J ) 6.5 Hz, 2H),
1.90 (p, J ) 6.5 Hz, 2H), 1.55 (p, J ) 6.5 Hz, 2H), 1.35 (m,
8H), 0.87 (t, J ) 6.5 Hz, 3H); 13C NMR (90 MHz, CDCl3): δ
) 151.0, 138.3, 128.5, 126.9, 125.7, 124.0, 111.1, 69.8, 31.8,
29.4, 26.3, 22.7, 14.1; calcd for C32H44O2 (460.7): C 83.43, H
9.63; found: C 82.59, H 9.72.
Preparation of the SWNT/Polymer Complex. SWNTs were
produced by the HiPco method3 and used as received from Rice
University. SWNTs (0.3 mg) were added to a solution of the
polymer in CHCl3 solution (1 mg in 5 mL). Sonication for 30
min gave a stable transparent solution.
UV-vis and Photoluminescence Measurements. The poly-
mer which was used in these experiments was synthesized
shortly before use. The nanotubes were added to the chloroform
solutions of the polymer in CHCl3 right before use. The solutions
were placed in a 1 mm path length quartz cell. The solid-state
samples were produced shortly before they were tested in order
to reduce polymer oxidation. The solid-state samples were
created by spin-coating the composite material onto a cleaned
glass substrate. The UV-vis experiments were carried out on
a Varian Cary 100 Bio spectrophotometer. The photolumines-
cence spectroscopy was carried out using a Fluorolog-3 (Instru-
ment S. A. & Co.) using a xenon lamp for the excitation source.
The fluorescence was collected from the front face of the cuvette
to minimize the self-absorption effects from the optically dense
sample.
Fabrication and Interrogation of Polymer/SWNT Opto-
electronic Devices. Polymer/SWNT devices were prepared by
spin-coating a dilute solution of CHCl3, polymer, and SWNTs
onto a Si wafer that was coated with 0.2 micrometers of SiO2
and was pre-patterned with electrodes. The electrodes (200 nm
wide, 1 micrometer gap) were fabricated using standard electron-
beam lithography techniques and consisted of a 5 nm bottom
layer of Ti, coated with a 50 nm thick layer of Au. It is generally
possible to fabricate better electrical contacts to SWNTs and
SWNT ropes by depositing the electrode materials directly on
top of the nanotubes.15 However, such a process involves steps
such as the spin-coating of resist materials on top of the
nanotubes, and thus can potentially disrupt the nanotube/polymer
superstructure. The chosen method of depositing the polymer-
wrapped nanotubes directly on top of the patterned electrodes
not only leads to poorer electrical contacts, but it also presum-
ably retains the superstructure of the polymer-wrapped tubes.
In any case, the results discussed here, except where explicitly
noted, were highly reproducible across several devices.
The molecular weights and polydispersity (PDI) of PmPV
(Mw)19 000; PDI ) 1.6) were determined in THF by using an
SEC instrument equipped with a UV detector. The SEC system
was calibrated by using polystyrene standards prior to use. The
number-average degree of polymerization was estimated to be
n ≈ 25 from Mn and the molecular weight of the repeating unit,
460. The degree of polymerization of the lower molecular
weight polymer was confirmed by NMR end-group analysis.
Poly{(2,6-pyridinylenevinylene)-co-[(2,5-dioctyloxy-p-phe-
nylene)vinylene]} (PPyPV). A 2.5% solution of NaOEt (0.7
mmol) in EtOH was added dropwise to a solution of 4 (0.21 g,
0.22 mmol) and 2,6 pyridinedicarboxaldehyde (0.03 g, 0.22
mmol) in a mixture of anhydrous EtOH (1 mL) and THF (3
mL) at ambient temperature. The reaction mixture was stirred
for an additional 26 h. The resulting polymer was precipitated
twice from MeOH and then dried to afford the crude PPyPV
(0.05 g, 65%) as a yellow resin. 1H NMR (200 MHz, CDCl3):
δ ) 7.90-7.50 (m), 7.30-6.90 (m), 6.80-6.60 (m), 4.01 (brs,
2H (trans-fragment)), 3.51 (brs, 2H (cis-fragment), 1.83 (brs,
2H), 1.48 (brs, 2H), 1.64 (brs, 8H), 0.78 (brs, 3H). A sample of
the crude PPyPV (0.03 g) and I2 (11 µL of 0.01% iodine in
PhMe solution) were heated under reflux in PhMe (3.5 mL)
under argon atmosphere for 6h. The solvent and iodine were
evaporated off under reduced pressure and the product was
dissolved in CHCl3 and precipitated out with MeOH. The
resulting precipitate was dried to afford PPyPV as a yellow resin
(0.02 g). 1H NMR (200 MHz, CDCl3): δ ) 7.89 (s, 1H), 7.81
(s, 1H), 7.59 (brs, 1H), 7.40-7.15 (m, 7H), 4.01 (brs, 2H), 1.84
(brs, 2H), 1.49 (brs, 2H), 1.22 (brs, 8H), 0.80 (brs, 3H); 13C
NMR (90 MHz, CDCl3): δ ) 155.1, 148.7, 132.7, 129.2, 127.8,
126.0, 110.4, 69.5, 31.6, 29.2, 29.5, 26.1, 22.5, 13.9; calcd for
The electrodes were connected to larger, macroscopic pads
for wire-bonding to the pin-outs of a chip carrier. After the
device was assembled, the chip carrier was mounted onto the
coldfinger of an immersion cryostat that was equipped with
quartz window optical ports. The output of a quartz-halogen
lamp was filtered through a water cell to remove infrared
frequencies, and then was wavelength selected through a single-
pass monochromator, before being directed through quartz
windows of the cryostat onto the device. The optical response
of the device was then explored as a function of wavelength,
and all measurements reported here were carried out at 4 K.
One terminal of the junction was held at a constant bias (Vc ≈
(10 mV, for example), whereas the other terminal was
grounded through a picoammeter. Current through the device
was monitored as a function of whether light was incident on
the device. Because the current amplifier was not sufficiently