Nanoscale morphology, dimensional control, and electrical properties of oligoanilines

Article abstract of DOI:10.1021/ja1014184

While nanostructures of organic conductors have generated great interest in recent years, their nanoscale size and shape control remains a significant challenge. Here, we report a general method for producing a variety of oligoaniline nanostructures with well-defined morphologies and dimensionalities. 1-D nanowires, 2-D nanoribbons, and 3-D rectangular nanoplates and nanoflowers of tetraaniline are produced by a solvent exchange process in which the dopant acid can be used to tune the oligomer morphology. The process appears to be a general route for producing nanostructures for a variety of other aniline oligomers such as the phenyl-capped tetramer. X-ray diffraction of the tetraniline nanostructures reveals that they possess different packing arrangements, which results in different nanoscale morphologies with different electrical properties for the structures. The conductivity of a single tetraaniline nanostructure is up to 2 orders of magnitude higher than the highest previously reported value and rivals that of pressed pellets of conventional polyaniline doped with acid. Furthermore, these oligomer nanostructures can be easily processed by a number of methods in order to create thin films composed of aligned nanostructures over a macroscopic area.

Full text of DOI:10.1021/ja1014184

Published on Web 07/14/2010
Nanoscale Morphology, Dimensional Control, and Electrical
Properties of Oligoanilines
†
‡
†
†
,†
Yue Wang, Henry D. Tran, Lei Liao, Xiangfeng Duan, and Richard B. Kaner*
Department of Chemistry and Biochemistry and California NanoSystems Institute, UniVersity of
California, Los Angeles, California, 90095-1969, and Fibron Technologies, Inc., Inglewood,
California, 90301-2068
Received February 18, 2010; E-mail: Kaner@chem.ucla.edu
Abstract: While nanostructures of organic conductors have generated great interest in recent years, their
nanoscale size and shape control remains a significant challenge. Here, we report a general method for
producing a variety of oligoaniline nanostructures with well-defined morphologies and dimensionalities.
1-D nanowires, 2-D nanoribbons, and 3-D rectangular nanoplates and nanoflowers of tetraaniline are
produced by a solvent exchange process in which the dopant acid can be used to tune the oligomer
morphology. The process appears to be a general route for producing nanostructures for a variety of other
aniline oligomers such as the phenyl-capped tetramer. X-ray diffraction of the tetraniline nanostructures
reveals that they possess different packing arrangements, which results in different nanoscale morphologies
with different electrical properties for the structures. The conductivity of a single tetraaniline nanostructure
is up to 2 orders of magnitude higher than the highest previously reported value and rivals that of pressed
pellets of conventional polyaniline doped with acid. Furthermore, these oligomer nanostructures can be
easily processed by a number of methods in order to create thin films composed of aligned nanostructures
over a macroscopic area.
Introduction
mance and stability of many devices are governed by how
molecules or polymer chains assemble in the solid state.
1
2,13
One-dimensional (1-D) nanostructures of molecular organic
conductors and conducting polymers have attracted growing
attention in recent years due to their solution processability,
easily tunable electronic properties, and potential use in many
novel applications.
on the structure-property relationships for this unique class of
nanomaterials. Among the family of conducting polymers,
polyaniline has been extensively studied due to its unique
oxidation-reduction chemistry, environmental stability, bio-
compatibility, and for its simple acid-base doping-dedoping
process as illustrated in Figure 1.
have demonstrated enhanced performance in applications such
as chemical sensors, catalysis, and supercapacitors due to their
The lack of order at the chain level has limited polyaniline
nanofibers from being integrated into electronic devices that
require high carrier mobility and stability such as organic field-
effect transistors and solar cells. These applications are currently
dominated by molecular semiconductors that can often self-
1-3
In addition, these materials can shed light
1
,2,14
1
assemble into highly ordered nanowires or nanoribbons.
Moreover, polyaniline nanofibers typically possess low aspect
ratios and are highly flexible, which makes the fabrication of
single-wire devices and post-synthetic alignment difficult.
In contrast to polyaniline, oligomers of aniline are capable
of forming ordered extended structures; crystals of low molec-
4
-8
Nanofibers of polyaniline
1
5-18
ular weight oligomers have already been characterized.
9
-11
However, compared to polyaniline, reports on oligoanilines are
sparse, especially in regards to generating nanoscale structures
high surface area and porous nature.
However, the perfor-
19-21
and/or improving their electrical conductivity.
Oligoanilines
†
University of California.
Fibron Technologies, Inc.
‡
(
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10.1021/ja1014184  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 10365–10373 9 10365

A R T I C L E S
Wang et al.
Figure 1. The oxidation-reduction chemistry of polyaniline. The fully reduced form of polyaniline, known as leucoemeraldine, can be half-oxidized to
-10
form emeraldine or fully oxidized to form pernigraniline. The electrically insulating emeraldine base form of polyaniline (conductivity e1 × 10 S/cm)
can be doped with a strong acid to produce the electrically conducting emeraldine salt form of polyaniline (conductivity g1 S/cm). This process can be
reversed by adding a strong base. Note that only the emeraldine oxidation state of polyaniline can be doped to form an electrically conducting polymer.
Tetraaniline is obtained when n ) 1 and therefore is the smallest oligomer unit that can fully represent the structure of polyaniline.
have generally been reported to possess low conductivities
aniline oligomers in their most conductive emeraldine salt
oxidation state. Exquisite nanoscale morphology and dimen-
sionality control is demonstrated via a simple dopant-induced
process. The conductivities of these nanostructures are measured
via single wire, ribbon, or plate measurements. The values
observed are 2 orders of magnitude higher than previously
reported pressed-pellet values for oligoanilines and are com-
-6
-2
ranging from 10 to 10 S/cm for tetraaniline in pressed pellet
2
0,21
form.
However, the bulk conductivity of a conducting
polymer is governed by both the transport properties within a
1
6,19,22,23
molecular chain and between chains.
In this regard,
oligoanilines have great potential for improvements in the
material’s interchain transport properties. Reports on the
conductivity of oligoanilines from a pressed pellet are therefore
typically low due to the fact that interchain transport is not
optimized. Oligoanilines also represent an interesting middle
ground between polyaniline and molecular semiconductors
because oligoanilines retain the chemical properties of the
polymer, for example, the acid-base doping-dedoping chem-
istry (see Figure 1), while also possessing properties typically
associated with molecular semiconductors especially in regard
to monodispersity and self-assembly.
The most important oxidation state of polyaniline is the
emeraldine base form, since it can be doped by strong acids to
create the conducting form, which can be readily and reversibly
dedoped back to the insulating emeraldine base form as shown
in Figure 1. The emeraldine oxidation state of polyaniline
possesses three benzene rings for every quinoid ring. Amine/
phenyl-capped tetraaniline, hereafter referred to as tetraaniline
2
0,21
parable to that of conventional polyaniline.
Experimental Section
Synthesis. N-phenyl-1,4-phenylenediamine and N,N′-diphenyl-
1,4-phenylenediamine were purchased from Sigma-Aldrich and TCI
Chemicals, respectively, and were used without further purification.
2
4
Tetraaniline was synthesized via a previously reported route. In
brief, iron(III) chloride dissolved in 0.1 M HCl was mixed with
stoichiometric amounts of N-phenyl-1,4-phenylenediamine (aniline
dimer) suspended in the same solvent. After 2 h of vigorous stirring,
the suspension was filtered and repeatedly washed with 0.1 M HCl
until the filtrate became clear. The solid was collected and dedoped
with 0.1 M ammonium hydroxide. The dedoped crude product was
reduced to the leucoemeraldine oxidation state with hydrazine and
recrystallized from ethanol three times in order to purify the product.
Tetraaniline was then reoxidized to the emeraldine base form with
ammonium peroxydisulfate (APS). The final product was character-
ized by UV-vis, Fourier transform infrared (FT-IR) spectroscopy,
and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-
TOF) mass spectroscopy (Figure S1). Phenyl-capped octaaniline
was synthesized by a condensation reaction in an inert atmosphere
(
see Table S1 in Supporting Information for nomenclature and
structures of aniline oligomers), therefore represents an impor-
tant model compound for polyaniline because, structure-wise,
it is the shortest oligomer that can represent the emeraldine
oxidation state. Here, we report a general and versatile method
for producing structurally ordered and well-defined oligoaniline
nanostructures via the self-assembly of tetraaniline and other
25
via a previously reported route. Phenyl-capped tetraaniline was
synthesized using the same method with tetraaniline replaced by
N-phenyl-1,4-phenylenediamine as the starting material. Charac-
terizations for these two oligomers are shown in Figures S2 and
S3, respectively.
Process for Producing Nanostructures. In a typical process,
2.0 mg of finely powdered tetraaniline is added to a solvent mixture
composed of 1.0 mL of ethanol and 4.0 mL of an aqueous acidic
solution. The mixture is shaken for a few seconds and then left
(
(
(
19) MacDiarmid, A. G.; Zhou, Y.; Feng, J. Synth. Met. 1999, 100, 131–
40.
20) Wei, Z.; Faul, C. F. J. Macromol. Rapid Commun. 2008, 29, 280–
1
2
92.
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Ammu, S.; Manohar, S. K. J. Am. Chem. Soc. 2009, 131, 12528–
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2529.
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Met. 1995, 72, 65–71.
1
0366 J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010

Properties of Oligoanilines
A R T I C L E S
Figure 2. (a) Photographs show a mixture of tetraaniline, ethanol, and 0.1 M HCl at the beginning of the process (left vial) and the green tetraaniline
nanowire dispersion produced at the end of 4 days (right vial). Both vials were agitated prior to taking the photos. (b) A scanning electron microscope (SEM)
image of the as-synthesized tetraaniline powder. (c) UV-vis spectra obtained at the given time intervals during the course of nanowire assembly. (d) An
SEM image of a network of tetraaniline nanowires. The molecular structure of tetraaniline is shown across the top of the image. (e) A transmission electron
microscope (TEM) image of a single tetraaniline nanowire; and (f) the results of the same process carried out in a mixture of ethanol and water in the
absence of doping acids. Scale bars: 1 µm.
undisturbed for 4 to 5 days. Less ethanol is used for N-phenyl-
,4-phenylenediamine and N,N′-diphenyl-1,4-phenylenediamine
spectra of the nanowire dispersion were taken with a quartz cuvette
that has a 1 mm light path-length on a Shimadzu UV-3101 PC
UV-vis-NIR Scanning Spectrophotometer.
1
than tetraaniline since they have a higher solubility in ethanol. The
duration of the processes varied slightly depending on the oligomer.
Molecular structures and common abbreviations for the aniline
oligomers are summarized in Table S1 along with the detailed
conditions for their nanostructure formation. Each reaction mixture
was purified by dialysis against deionized water for approximately
Single Wire Measurements. Bottom-contact devices were
fabricated for all single wire, ribbon, and plate measurements. Gold
electrodes were thermally deposited on a Si/SiO
2
substrate with a
300 nm SiO layer to create channels that are 2-10 µm in length.
2
One drop of oligoaniline dispersion was drop-cast onto the
electrodes, and the nanowires, nanoribbons, or rectangular nano-
plates were allowed to settle for a few seconds. The rest of the
droplet was then quickly removed with flowing nitrogen and the
devices blown thoroughly dry. Measurements were carried out
directly after deposition using a standard probe station under
ambient conditions.
1
day. A small amount of granular debris often precipitated out
and was visible at the bottom of the vial. The debris can be readily
removed by agitating the vial since the granular debris settles out
from the dispersion quickly, while the nanostructures remain well-
dispersed for a lengthy period of time. As a result, the top nano-
structured dispersion can be collected into a different container and
the debris that accumulates at the vial bottom discarded.
Microscopy. Scanning electron microscopy (SEM) and transmis-
sion electron microscopy (TEM) samples were prepared by drop-
casting 1 to 2 drops of a diluted oligoaniline dispersion onto a piece
of silicon wafer or a TEM grid, respectively. SEM images were
taken with a JEOL JSM-6700 Field Emission scanning electron
microscope, and TEM images were taken with a FEI/PHILIPS CM
Results and Discussion
Tetraaniline synthesized by previously reported methods
possesses a granular, agglomerated morphology (Figure 2b).
However, nanostructures of tetraaniline can be grown by adding
this granular powder to a mixture of 0.1 M HCl and ethanol,
which is then left undisturbed for 4 to 5 days. During this time,
large particles are observed that initially precipitate to the bottom
of the vial and gradually turn into the green, doped form of
tetraaniline. This material slowly disperses into the surrounding
medium. At the end of this process, only doped tetraaniline is
observed floating in the solvent. The mixture forms a stable
dispersion upon simple agitation (Figure 2a). SEM and TEM
images of the resulting dispersion show that long, nonwoven
nanowires with high aspect ratios are created during this process
(Figure 2d,e). The nanowires typically range in diameter from
1
20 transmission electron microscope.
Other Characterization Techniques. Powder X-ray diffraction
patterns of the tetraaniline nanostructures were taken on a Pana-
lytical X’Pert Pro X-ray powder diffractometer with a scan rate of
2
deg/min. Sodium chloride was used as an internal standard for
the crystallite size calculations using the Scherrer equation. MALDI
samples were prepared with 2,5-dihydroxybenzoic acid (DHB) as
the matrix and the spectra were taken on an Applied Biosystems
Voyager-DE-STR MALDI-TOF spectrometer. FT-IR samples were
prepared with FT-IR grade KBr and the spectra were taken with a
JASCO FT/IR-420 spectrometer. UV-vis-NIR (near-infrared)
J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010 10367

A R T I C L E S
Wang et al.
31,32
7
0 to 200 nm and are up to hundreds of micrometers long. A
organization.
The transition in oxidation state from the initial
UV-vis-NIR spectrum of the resulting tetraaniline nanowires
emeraldine base form of tetraaniline to the final emeraldine salt
form was monitored in situ via UV-vis-NIR spectroscopy
during the course of nanowire assembly. UV-vis-NIR spectra
were collected 5 h into the process and then every 15 h until
the relative peak ratios stopped changing (Figure 2c). At 5 h,
agglomerates that resemble the morphology of the as-synthesized
tetraaniline are observed, and nanowires become the dominant
product after 65 h. Three peaks are present. The peak at 290
nm corresponds to the π f π* transition. This transition
in the emeraldine salt form dispersed in water is shown as the
“65 h” curve in Figure 2c, and a full spectrum that extends into
the near-infrared (NIR) up to 3000 nm is shown in Figure S4.
The as-synthesized dedoped tetraaniline in its emeraldine base
form dissolved in ethanol is presented in Figure S1c and is
7
,24
consistent with previous work on tetraaniline.
Literature reports indicate that π-conjugated molecules can
form well-defined morphologies and molecular arrangements
through noncovalent interactions such as π-π stacking and
hydrogen bonding. Controlled aggregation of molecules through
these noncovalent interactions in a nonsolvent or in a mixture
of a nonsolvent and a good solvent can lead to the formation
33,34
typically appears between 350-360 nm for doped polyaniline;
however, it undergoes a blue shift to 290 nm due to the much
shorter conjugation length of tetraaniline compared to polya-
niline. Absorptions at 420 nm and around 950 nm are ascribed
to the polaron f π* and π f polaron band transitions,
respectively, indicating that the nanowires are in their conductive
1
,14
of nanowires or nanoribbons.
Tetraaniline nanowires may
form in an analogous process to that known for other molecular
semiconductor nanowires such as perylene tetracarboxylic
33
emeraldine salt form. The weak peak around 1420 nm is due
35
to water absorption. As the assembly process progresses, the
1
4,26,27
diimide derivatives.
Although tetraaniline is not soluble
spectra changes significantly with a defined isosbestic point at
in an acidic aqueous solvent, it can be doped in this environment.
Tetraaniline is, however, soluble in common organic solvents
such as ethanol. The solubility and the degree of aggregation
of tetraaniline in different liquids can be easily tuned by
adjusting the ratio of the two solvents/nonsolvents. Upon
increasing the aqueous component, the enhanced solvent polarity
can create solvophobic association between the aromatic rings,
which promotes extended π-π stacking, in a similar manner
3
25 nm. The drastic decrease in the ratio of the 290 nm peak to
the 420 nm peak indicates that the π f π* transition energy is
greatly reduced as the tetraaniline molecules equilibrate into a
more preferred aggregate phase with extended π-π stacking
as nanowires form. A similar effect has been observed in
nanostructures of molecular conductors. On the other hand,
the ratio of the 420 nm peak to the 290 nm peak increases
throughout the process, indicating an increase in doping level
as more polarons are injected into the π* band as assembly
progresses. Simultaneously, the broad peak at ∼900 nm which
extends over several hundreds of nanometers from the visible
to the NIR region also increases in intensity compared to the
290 nm peak. The broadening of this asymmetric peak is
associated with the increasing linearity of polyaniline which
3
0
2
6,28
to how surfactants and amphiphilic molecules assemble.
Tuning the solvent ratio thus allows for controlled aggregation
and stacking of tetraaniline molecules. With a solvent mixture
of 4:1 (v/v) of an aqueous solution of HCl (a nonsolvent) and
ethanol (a good solvent), tetraaniline molecules assemble into
an extended superstructure consisting of nanowires. During this
process, bulk tetraaniline powder slowly disperses into the
solvent to form a suspension of tetraaniline nanowires. Control
experiments carried out in a solely acidic aqueous environment
lead to poorly defined structures (see Figure S5), which
illustrates the importance of achieving the correct solvent
polarity from a mixture of solvents.
3
3,34,36
leads to increased polaron delocalization.
The increase
in absorption of this broad peak during tetraaniline nanowire
assembly suggests that the intermolecular stacking between
aniline tetramers leads to delocalization of carriers because of
the enhanced π-π orbital overlap. Therefore, the overall change
in the ratios of peak absorptions indicates that tetraaniline
molecules rearrange themselves from the initially agglomerated
morphology into an extended, more thermodynamically favor-
able array with better carrier transport during the process of
nanowire assembly.
Size and shape control of 1-D organic nanostructures remains
a significant challenge within the field of organic conductors
and continues to draw increasing interest. For example, studies
have shown that using different doping acids during the synthesis
of polyaniline nanofibers can change their diameter. When
synthesized in 1.0 M HCl, polyaniline nanofibers typically have
The nanowire growth process requires approximately 4 days
in order for directional molecular self-assembly to occur which
has been suggested as a factor in forming ordered nanostructures
14,26,27,29
forothermoleculessuchasperylenediimideandpentacene.
Noncovalent forces such as hydrogen bonding can modulate
27,30
the directional nature of nanowire formation.
Although high
aspect ratio nanowires are obtained in a solvent mixture of
ethanol and 0.1 M HCl, an identical process carried out in a
mixture of ethanol and water without any dopant acid only yields
agglomerates (Figure 2f). Tetraaniline remains in its initial
emeraldine base oxidation state in water, but becomes the
protonated emeraldine salt form when the process is performed
in an acid solution. This may be a result of extra hydrogen
bonding associated with the protonation which has been
proposed as a driving force to regulate supramolecular
diameters around 30 nm, while in 1.0 M camphorsulfonic acid
37
(
CSA), the diameters average 50 nm. For oligomers, their
organization into extended supramolecular structures can be
controlled by using different dopants that have different sizes.
As shown in Figure 2, 1-D nanowires are formed when HCl is
(
31) Stejskal, J.; Sapurina, I.; Trchova, M.; Konyushenko, E. N.; Holler,
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0368 J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010

Properties of Oligoanilines
A R T I C L E S
Figure 3. (a) An SEM image of tetraaniline nanoribbons obtained when 1.0 M HNO
3
is used as the doping acid. The inset shows a TEM image of a single
ribbon. (b) An edge-on view of the nanoribbons. (c) An SEM image of rectangular-shaped tetraaniline nanoplates produced using 0.5 M HClO
doping acid. The inset shows a TEM image of a nanoplatelet. (d) An SEM cross-sectional view of the nanoplates. (e) An SEM image of nanoflowers
produced using 1.0 M H SO as the doping acid; and (f) an SEM image of interconnected nanofibers obtained when 1.0 M CSA is used as the doping acid.
4
as the
2
4
Scale bar: 100 nm for (d), 1 µm for all the others.
used as the doping acid; however, as the dopant is changed to
supramolecular structure observed for polyaniline since indi-
vidual dopants only have a local effect on the polymer chains
HNO
HClO
shaped nanoplates (Figure 3c,d), while H
3
, 2-D nanoribbons are obtained (Figure 3a,b). Using
induces the formation of well-defined 3-D rectangular-
SO produces tetra-
3
8
4
(Figure 4a). Thus, changing the dopant acid during the
synthesis of polyaniline nanofibers primarily changes diameter,
but has little effect on superstructure. However, oligoanilines
such as tetraaniline are discrete, compact molecules. Conse-
quently, the size of the dopant anion has a much more profound
effect on the packing distances between tetraaniline molecules
(Figure 4b). This effect can be observed in the X-ray diffraction
(XRD) patterns for the different tetraaniline nanostructures
(Figure 4d). For each of the four structures, the most intense
peak in the X-ray diffraction pattern is centered around 2θ )
20°, which has been ascribed to the periodicity parallel to the
2
4
aniline molecules that pack into nanoflowers consisting of well-
organized clusters of 2-D nanosheets (Figure 3e).
The exquisite dopant-induced nanoscale control of the size
and shape of aniline-based oligomers and polymers can be
explained by examining and comparing the structural differences
between the polymer and the oligomer. When polyaniline or
oligoaniline is doped with an acid, the imine nitrogens become
protonated and the negatively charged dopant anions reside near
the positively charged nitrogens due to ionic attractions (Figure
3
9-42
1
). The size of the dopant anions thus influences the interchain
polymer chain.
When HCl is used as the doping acid to
-
packing distance and ultimately the supramolecular morphology.
Polyaniline molecular chains are long and semirigid in sections;
thus, dopant anion size does not have a large effect on the
induce nanowire formation, Cl ions remain between the chains,
(38) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995, 69, 85–92.
J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010 10369

A R T I C L E S
Wang et al.
defined, agglomerated tetraaniline nanofibers are obtained
(
Figure 3f). This is likely due to the fact that CSA anions are
bulky in size and they prevent oligomers from packing into
ordered and discrete supramolecular structures (Figure 4c). Their
lack of packing order is evident in the corresponding XRD
pattern, which does not have well-defined peaks and thus
suggests an amorphous structure (Figure 4d). Note that, aside
from the amorphous CSA-doped agglomerated nanofibers, the
intensity of the X-ray diffraction patterns indicate that the nano-
wires are the least crystalline form of nanostructure, while the
rectangular nanoplates and nanoribbons are far more crystalline.
Such observations are consistent with the physical appearances
of the nanostructures. Nanoflowers also produce intense dif-
fraction peaks despite the fact that they do not appear as ordered
as the nanoplates or nanoribbons. This can be attributed to the
crystallinity of the stacked 2-D sheets/“petals” that comprise
the nanoflowers.
The Scherrer equation has been applied to assess the
crystallite sizes of the four different morphologies. Using sodium
chloride as an internal standard, the crystallite sizes were
calculated to be 17 Å for the nanowires, 44 Å for the
nanoribbons, 91 Å for the rectangular nanoplates, and 25 Å for
the nanoflowers. Since the calculated crystallite sizes for the
nanowires and the nanoflowers are well below the 50 Å accuracy
limit of the Scherrer equation, the data for these materials should
be considered qualitative. The rectangular nanoplates have the
longest crystallite coherence length, approximately one-tenth
of their average physical thickness, further confirming their high
crystallinity. The calculated crystallite size of the nanoribbons
falls between the nanocrystalline nanowires/nanoflowers and the
more crystalline nanoplates, in agreement with the XRD data.
In addition, well-defined 2θ peaks around 26° that correspond
to classical π-π stacking between aromatic rings are observed
for the more crystalline nanoribbons, nanoplates, and nano-
flowers, further demonstrating that π-π stacking is likely an
important driving-force for nanostructure assembly. This is also
supported by the previously discussed in situ UV-vis-NIR
analysis (Figure 2c), where a greatly reduced π f π* transition
energy is observed due to the formation of extended π-π
stacking as the nanostructures assemble.
Figure 4. (a) An illustration of the polymer chain conformation typically
found which is compactly coiled when a small doping acid such as HCl is
used (left); the chain becomes more expanded when doped with a bulky
doping acid such as CSA and processed in certain solvents (right). (b)
The dopant anions likely reside between the tetraaniline molecular chains;
therefore, the packing distance will be affected by the size of the anions.
3
8
(
c) Tetraaniline molecular chains do not pack well when a bulky doping
The assembly methodology presented here appears to be a
general and versatile route for producing nanostructures of other
aniline oligomers. For example, nanowires of aniline dimer (N-
phenyl-1,4-phenylenediamine) and phenyl-capped aniline dimer
acid such as CSA is used as the tetramers are prevented from arranging
themselves into extended structures; and (d) X-ray diffraction patterns of
nanowires (top), nanoribbons (second), rectangular nanoplates (third),
nanoflowers (fourth), and agglomerated nanofibers (bottom).
(
N,N′-diphenyl-1,4-phenylenediamine) are obtained by the same
which results in a peak at 2θ ) 20.00° corresponding to a
method when HCl is used as the doping acid, and phenyl-capped
tetraaniline forms a mixture of nanowires and narrow nanorib-
bons under the same conditions (Figure 5a-c). Unfortunately,
when the same approach is applied to phenyl-capped octaaniline,
only agglomerates are obtained (Figure 5d), likely due to the
fact that the increase in chain length results in a molecule with
more polymer-like characteristics so that its size and shape
cannot be as readily controlled by this process.
d-spacing of 4.22 Å. This peak shifts to 2θ ) 19.87° for HNO
3
doped nanoribbons, which indicates an increase in d-spacing
-
to 4.46 Å. This is consistent with the fact that NO
3
is a larger
dopant anion than Cl . Nanoflowers doped by H SO and
rectangular nanoplates doped by HClO have even larger
d-spacings of 4.62 and 4.63 Å, respectively, as HSO
-
2
4
4
-
4
and
4 3
ions are larger than NO and Cl . To further illustrate
-
-
-
43
ClO
the impact of dopant anion size, when a very large doping acid
such as camphorsulfonic acid (CSA) is employed, only poorly
Electrical transport studies on HCl-doped nanowires, HNO
3
-
doped nanoribbons, and HClO -doped rectangular nanoplates
4
were performed in order to determine the conductivities of each
of these nanostructures. Previous studies on the conductivity
of doped tetraaniline measured on pressed pellets have proven
(
(
(
39) Tang, Q.; Wu, J.; Sun, X.; Li, Q.; Lin, J. Langmuir 2009, 25, 5253–
257.
40) Anilkumar, P.; Jayakannan, M. J. Phys. Chem. B 2009, 113, 11614–
1624.
41) Wang, J.; Wang, J.; Dai, Z.; Wang, Z.; Zhang, F. Synth. Met. 2009,
5
-
6
-2
1
to be disappointing: the conductivity ranges from 10 to 10
S/cm depending on the synthetic method and doping acid
1
59, 1583–1588.
20,21
used.
Our studies confirm that bulk tetraaniline has a
(
42) Winokur, M. J.; Mattes, B. R. Macromolecules 1998, 31, 8183–8191.
43) Conway, B. E. Ionic Hydration in Chemistry and Biophysics; Elsevier:
Amsterdam, 1981; pp 59-74.
-
3
conductivity of only 3 × 10 S/cm. However, pressed-pellet
(
measurements represent the lower boundary for conductivity
1
0370 J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010

Properties of Oligoanilines
A R T I C L E S
Figure 5. SEM images of (a) aniline dimer nanowires; (b) phenyl-capped aniline dimer nanowires; (c) phenyl-capped aniline tetramer nanowires and
nanoribbons; and (d) phenyl-capped octaaniline agglomerates produced under the same experimental conditions that form tetraaniline nanostructures. Scale
bar: 1 µm. Insets show the molecular structures of the corresponding aniline oligomer.
Figure 6. (a) An SEM image of a representative bottom-contact device fabricated from a single tetraaniline nanowire on a Si substrate covered by a 300
nm thick SiO
1 µm.
2
dielectric layer. The channel lengths between the Au electrode pairs were varied from 2 to 10 µm; a 2 µm channel length is shown. Scale bar
)
of oligoanilines due to high contact resistance between the
numerous nanostructure-nanostructure junctions within the pellet
that can cause the conductivity to appear lower than the intrinsic
conductivity of a single wire or ribbon. Measuring the conduc-
tivity of a single nanowire, nanoribbon or nanoplate therefore
represents a method to determine the intrinsic conductivity of
oligoaniline nanostructures.
nanowire. The nanowire was drop-cast across the source and
drain electrodes from a freshly prepared dispersion of tetraaniline
nanowires in water. The nanowires were allowed to deposit for
a few seconds, and the rest of the droplet was quickly blown
away by nitrogen flow, followed by taking an immediate
measurement. This method of deposition reduces the amount
of solvent impurities that would be left behind from solvent
drying, which would decrease the quality of the measurements.
Tetraaniline nanoribbon and nanoplate devices were fabricated
Figure 6a shows a representative SEM image of an Au
bottom-contact device constructed from a single tetraaniline
J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010 10371

A R T I C L E S
Wang et al.
Figure 7. (a) A schematic representation of evaporation-induced (dewetting) deposition and alignment between the solvent front and the substrate. (b) An
optical micrograph taken in dark field shows a small section of the ring. The nanowires align in the drying direction. (c) A dark field optical micrograph
showing a closer view of (b).
in the same fashion. Figure 6b shows a typical I-V curve for
a single nanowire device through a two-probe measurement.
All measurements were carried out under ambient conditions
with a standard semiconductor probe station. The linearity of
the curve indicates that the tetraaniline nanostructures exhibit
ohmic behavior. The conductivity of each nanostructure was
calculated using the slope of the I-V plot along with the length
of the nanostructure that bridges the source and drain electrodes.
The actual length and width (or diameter in the case of a
nanowire) of each channel were measured by SEM, and the
thickness of the ribbon or plate was acquired through tilted SEM
measurements assisted by AFM analysis when needed. The
conductivity of a single nanowire was found to be as high as
it is evident that the conductivities we report here could likely
be further improved if the contact quality and other issues are
addressed; for instance, fabricating four-probe top-contact
devices to reduce contact resistance. Despite these issues, the
conductivity of the nanoribbon exhibits a 2 orders of magnitude
increase from the highest value reported previously for tetraa-
2
1
niline, and a 3 orders of magnitude increase from the method
24
we followed for synthesizing bulk aniline tetramer. It is worth
emphasizing that the conductivity of the nanoribbons rivals that
of conventional polyaniline, which has a conductivity on the
0
1
6,21,44
order of 10 -10 S/cm.
Such results clearly demonstrate
that oligoanilines can be as highly conducting as the polymer
once the interchain transport and the interdomain transport
components of bulk conductivity are optimized. The improved
interchain transport as a result of the increased intermolecular
order is evident from the in situ UV-vis-NIR analysis and the
XRD patterns. The conductivity may be further enhanced if a
balance between the oligomer chain length and interchain
packing is achieved.
0
.3 S/cm, and that for a nanoplate and nanoribbon was as high
as 0.5 and 1.1 S/cm, respectively. This trend is consistent with
the X-ray diffraction patterns and the crystallite coherence length
obtained using the Scherrer equation as the nanowires were the
least crystalline morphology with the smallest crystallite size,
while the nanoribbons and rectangular nanoplates were much
more crystalline and have larger crystallite sizes. The fact that
the nanoplates have lower conductivity than the nanoribbons,
despite their higher crystallinity and longer crystallite coherence
length, can be ascribed to their poor contact with the Au
electrodes since the plates are not as long or flexible as the
ribbons. For measurements on different devices, conductivities
in the range of 0.1-1.1 S/cm were obtained for nanoribbons.
Such distribution of values was also observed for the nanowires
and nanoplates. This range of variation can be partly attributed
to (1) the differences in contact quality at the Au electrode-
nanoribbon interfaces that are often associated with bottom
contact devices, and (2) solvent impurities and/or microscopic
debris trapped at the aforementioned interfaces. On this basis,
The interdomain transport in a nanowire film could be further
improved by assembling the randomly oriented oligoaniline
nanostructures into oriented functional arrays onto a variety of
substrates. This has been difficult to achieve for polyaniline
nanofibers which typically form an interpenetrating network of
nanofibers which makes individual fibers or wires difficult to
separate. Fortunately, the oligoaniline nanowires tend to be much
more rigid with high aspect ratios leading to discrete fibers.
Initial experiments indicate that they can be readily aligned using
dewetting alignment, in which nanowires that possess high
(
44) Peng, C.-Y.; Kalkan, A. K.; Fonash, S. J.; Gu, B.; Sen, A. Nano Lett.
2005, 5 (3), 439–444.
1
0372 J. AM. CHEM. SOC. 9 VOL. 132, NO. 30, 2010

Properties of Oligoanilines
A R T I C L E S
aspect-ratios can be deposited and aligned between the solvent
self-assemble in a suitable binary solvent system. Supramo-
lecular structures including nanowires, nanoribbons, rectangular
nanoplates, and nanoflowers are obtained through a process in
which the dopant induces a specific morphology in the oligoa-
nilines. The variation in structures could prove useful for next-
generation organic electronics; for example, the high surface
area nanoflowers could be used for organic supercapacitors,
while nanowires or nanoribbons could be well-suited for single-
wire/ribbon nanoelectronic devices. Two-probe electrical mea-
surements on single wires, ribbons, and plates reveal that the
conductivity of the tetraaniline nanostructures increase by 2
orders of magnitude from the highest previously reported values
for tetraaniline and now rival that of conventional polyaniline.
This increase in conductivity from bulk tetraaniline is due to
the increased intermolecular order as evidenced by XRD and
in situ UV-vis-NIR analyses. The nanostructures can be readily
assembled into thin films with orientation over a macroscopic
area via a number of methods such as dewetting or microfluidic
flow.
4
5-47
front and the substrate as shown in Figure 7a.
Through
this evaporation-induced assembly process, the tetraaniline
nanowires are drawn toward the direction of the droplet
evaporation, causing the nanowires to align in a radial fashion.
The solution in the center continuously replenishes the solvent
as it evaporates at the edges, thus increasing the nanowire
deposition density at the edge of the droplet. A ring with high
deposition density is formed as the droplet dries. Optical
microscope images of a section of the ring are shown in Figure
7
b,c, illustrating the high degree of alignment and high
deposition density. These aligned sections could be transferred
to a desired substrate for further device fabrication. In addition
4
8
to dewetting alignment, the microfluidic flow method is also
applicable for the assembly of tetraaniline nanowires (Figure
S6). Solution-based large-area alignment should facilitate
directional carrier transport and greatly lower contact resistance
4
9-51
versus their nonoriented counterparts,
which could prove
to be beneficial for high efficiency organic thin-film microelec-
tronic devices or for transparent, flexible organic electrodes.
Acknowledgment. We thank Sergey Prikhodko and Yujing Li
for help with TEM, Sabah Bux and Michael T. Yeung for assistance
with XRD, and Shan Jiang for help with graphics. Support for this
research has been provided by the UCLA based Focused Center
Research Program Functional Engineered NanoArchetectonics
center (R.B.K.), an NIH Director’s New Innovator Award Program,
part of the NIH Roadmap for Medical Research, through Grant
1DP2OD004342-01 (X.D.), and a National Science Foundation
Integrative Graduate Education and Research Training: Clean
Energy for Green Industry Fellowship DGE-0903720 (Y.W.).
Conclusions
Nanostructured aniline oligomers have been produced through
a novel, simple one-step method in which bulk oligoanilines
(
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