Full Paper
Supramolecular polymer-SWNT complexes of P1 and P2
spectrum for P2-SWNT in THF also shows sharp peaks in the
S11 and S22 regions, which suggests that the electron-poor P2
likewise exfoliates sc-SWNTs in THF. However, the presence of
a broad, featureless, and relatively intense absorption back-
ground in this spectrum indicates the presence of m-SWNTs,
which is confirmed by the presence of peaks in the M11 region
(and is consistent with the dark-brown/black color of the P2-
SWNT solution in THF, Figure S4, Supporting Information). The
overlap of polymer absorption with the M11 region precludes
a detailed analysis of the specific m-SWNT chiralities present in
the sample. In toluene and the 1:1 THF/toluene co-solvent mix-
ture, the P2-SWNT dispersions have no discernible absorbance
features outside of the polymer absorbance, which suggests
that P2 does not form a stable colloidal dispersion with SWNTs
in either of these solvents. As a consequence, these solvent
options were not pursued in subsequent studies. A control
sodium dodecylbenzenesulfonate (SDBS) dispersion was pre-
were prepared with raw HiPCO SWNTs by following previously
[
47]
reported procedures. A few different polymer/SWNT weight
ratios were investigated and it was found that a ratio of 1.5:1
polymer/SWNT produced the best dispersions for both P1 and
P2 (see the Supporting Information, Figure S4). Additionally,
THF, toluene, and a 1:1 mixture of these solvents were chosen
for dispersion selectivity studies. The optimized dispersion pro-
tocol involved dissolving 15 mg of polymer in 20 mL of solvent
before adding 10 mg of SWNTs. The mixture was sonicated for
2
h in a bath sonicator chilled with ice before being centri-
fuged at 8,346 g for 30 min. The supernatant was carefully re-
moved, filtered through a Teflon filtration membrane with
0
.2 mm diameter pores, and the resulting polymer-SWNT resi-
due (“bucky paper”) was washed with CHCl until the filtrate
3
did not exhibit any observable fluorescence when excited at
3
65 nm with a hand-held UV lamp. The bucky paper was re-
dispersed in 5 mL of solvent, sonicated for 1 h in a bath sonica-
tor chilled with ice, and centrifuged again. The resulting poly-
mer-SWNT dispersions were stable on the bench top for at
least several months, with no observable flocculation.
pared in D O and showed no SWNT selectivity (see the Sup-
2
porting Information, Figure S9).
To further investigate the differences in nanotube popula-
tions dispersed by P1 and P2, resonance Raman spectroscopy
was performed. This technique allows for the examination of
To investigate the polymer-SWNT dispersions, we initially
performed UV/Vis-NIR absorption spectroscopy shown in
Figure 2. Nanotube absorption features depend on their re-
[49]
both m- and sc-SWNT species within a given sample, and uti-
lizes laser excitation wavelengths that overlap with the van
Hove singularities present in the 1D density of states for a par-
[50]
ticular SWNT. As the electronic transitions depend on nano-
tube chirality and diameter, only a subset of the total nano-
tube population will be observed for each individual excitation
[51]
wavelength.
Thin film samples were prepared from the polymer-SWNT
complexes by drop-casting the dispersions onto silicon wafers.
A reference SWNT sample was also prepared by sonicating
a small amount of the SWNT starting material in CHCl and
3
making a solid film with the same drop-casting method.
Raman spectra were collected using three excitation wave-
lengths: 514, 633, and 785 nm. These excitation wavelengths
have previously been shown to be adequate for characterizing
the electronic character of HiPCO SWNT samples, as both m-
[52]
and sc-SWNTs can be separately probed. Figure 3 shows the
radial breathing mode (RBM) regions from the three samples
at each excitation wavelength (full Raman spectra are provided
in the Supporting Information, Figure S6). All Raman spectra
were normalized to the G-band at ~1590 cmꢀ and offset for
clarity. Upon excitation at 514 nm, two dominant RBM features
are observed in the Raman spectrum: a broad feature arising
Figure 2. UV/Vis-NIR absorption spectra for P1-SWNT (red) and P2-SWNT
(blue) in THF.
1
spective diameters and chiralities, and arise from the interband
transitions of the van Hove singularities, resulting in specific
nanotube chiralities with specific transition energies. The ab-
sorbance features in the observed range can be grouped into
three categories: two semi-conducting regions, S11 (830–
ꢀ
1
from sc-SWNTs centered at 180 cm , and several sharp peaks
ꢀ
1
[53]
from 225 to 290 cm arising from m-SWNTs. The P1-SWNT
sample shows a single peak in the sc-SWNT region, confirming
that m-SWNTs are not present in the dispersions prepared
using this polymer. Meanwhile, the P2-SWNT sample exhibits
peaks corresponding to both sc- and m-SWNTs. This observa-
tion is corroborated by analysis of the G-band region at this
excitation wavelength, which is shown in the inset of Fig-
ure 3 A. The G-band consists of two peaks: a lower frequency
1
600 nm) and S (600–800 nm), and a metallic region, M11
22
[
48]
(440–645 nm). The absorption spectrum for P1-SWNT in THF
shows sharp peaks in the S11 and S22 regions, suggesting that
the electron-rich P1 efficiently exfoliates sc-SWNTs in THF. This
is corroborated by the intense green color of the P1-SWNT so-
lution in THF (Supporting Information, Figure S5). The absence
of a broad, featureless absorption background in the spectrum
indicates effective nanotube exfoliation and the removal of m-
ꢀ
+
ꢀ
G and a higher frequency G . For sc-SWNTs, both the G and
+
ꢀ
G
have Lorentzian line shapes, but for m-SWNTs the G exhib-
[
43]
[54]
SWNTs, consistent with previous reports.
The absorption
its a broader Breit–Wigner–Fano (BWF) line shape. A broad
Chem. Eur. J. 2016, 22, 1 – 8
3
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