Ultralong nanobelts self-assembled from an asymmetric perylene tetracarboxylic diimide

Article abstract of DOI:10.1021/ja071903w

Ultralong nanobelts (>0.3 mm) have been fabricated from an asymmetric perylene tetracarboxylic diimide (PTCDI) molecule via a seeded self-assembly processing. The long length of nanobelts facilitates the construction of two-electrode devices employing the nanobelt as channel material, and the long-range one-dimensional π-π molecular stacking allows for efficient conductivity modulation through surface doping. A combination of these two characters enables efficient electrical sensing of reducing VOCs using the nanobelt. As examined for hydrazine, more than 3 orders of magnitude increase in current was observed for a single nanobelt when exposed to the saturated vapor of hydrazine. Copyright

Full text of DOI:10.1021/ja071903w

Published on Web 05/17/2007
Ultralong Nanobelts Self-Assembled from an Asymmetric Perylene
Tetracarboxylic Diimide
Yanke Che, Aniket Datar, Kaushik Balakrishnan, and Ling Zang*
Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901
Received March 18, 2007; E-mail: lzang@chem.siu.edu
One-dimensional (1D) self-assembly of planar aromatic (semi-
conductor) molecules into well-defined nanowires or nanobelts has
gained increasing interest1 since such an approach may extend the
optoelectronic properties and applications intrinsic to organic
semiconductors from film-based materials into the size ranges and
architectures that have never before been achieved. Among the small
number of molecules successfully explored in such 1D self-
assembly,2-8 symmetric molecules modified with all identical side
chains represent the majority of building block candidates.2-6 In
contrast, there have been much less asymmetric molecules employed
in the fabrication of well-defined nanowires or nanobelts,1b,2,8
although asymmetric molecules may provide more options and
adaptability for the surface modification to approach optimized
sensing sensitivity and selectivity for the nanomaterials thus
fabricated.8b
In this Communication, we report for the first time the fabrication
of ultralong nanobelts (>0.3 mm) from an asymmetric perylene
tetracarboxylic diimide (PTCDI) molecule as shown in Chart 1.
PTCDIs represent a unique class of molecules that demonstrate
extremely high thermal and photostability and have been employed
in a wide variety of film-based optoelectronic devices.3,9 While well-
defined nanowires have recently been fabricated from various
symmetric PTCDIs,3,5,10 which possess identical linear side chains
at the two imide positions, there is no such 1D self-assembly
reported on the asymmetric PTCDIs. The new self-assembly
approach reported herein on the asymmetric PTCDI will open
broader options for both molecular design and engineering to
improve the fabrication of 1D nanomaterials. Moreover, the
millimeter long nanobelt fabricated in this study will enable more
expedient construction of integrated nanoelectronic devices, for
which deposition of a wire across multiple parallel electrodes is
usually demanded.11
Figure 1. (A) A large-area SEM image showing the growth of long
nanobelts from the central seeding particulate aggregates. (B) A zoom-in
image showing the belt morphology and uniform size of the nanobelts.
Chart 1
crystalline growing process, typically over 5 days, in a 1:1 water/
ethanol solvent (Supporting Information). Such a slow crystalliza-
tion process allows for more organized molecular stacking and more
extended growth along the fibril long axis. Most of the fibers are
more than 0.3 mm in length, and some of them even reach ca. 0.5
mm (Figure S4). AFM imaging over a single fiber revealed the
belt-like morphology, with a width-to-thickness aspect ratio of ca.
10:1 (Figure S6). Such nanobelt conformation was also demon-
strated by small-area SEM imaging (Figure S5), where twisted
nanobelts are clearly unveiled, showing the same width-to-thickness
aspect ratio as that imaged by AFM. The extended 1D self-assembly
is likely dominated by the strong π-π interaction between the
PTCDI scaffolds, as indicated by the X-ray diffraction (Figure S8),
for which the typical π-π stacking peak (with d-spacing ) 3.57
Å) was observed. The first and third diffraction peaks (d-spacing
) 38.2 and 12.6 Å) are assigned to the edge-to-edge distances
between two adjacent PTCDI molecules at the longitudinal and
transverse directions, while the second peak is likely due to the
secondary diffraction of the longitudinal molecular arrangement
since the d-spacing is about half of the value of the first peak.
The polyoxyethylene side-chain attachment makes molecule 1
highly soluble in hydrophilic solvents such as ethanol. Taking
advantage of the miscibility between alcohol and water, the
solubility (or self-assembly) of the molecule can feasibly be
controlled by adjusting the volume ratio of water and alcohol
(detailed in the Supporting Information). Upon increasing the water
component, the increase in solvent polarity will force solvophobic
association between the alkyl side chains, in a similar manner of
1D self-assembly of surfactants and other amphiphilic molecules.12-14
Such hydrophobic interdigitation will bring the molecules in
proximity where the π-π interaction dominates the molecular
packing configuration. The π-π molecular stacking is likely
facilitated by the stretching-out conformation of the polyoxyethylene
side chain, which is favored in hydrophilic solvent.8a Indeed,
ultralong nanobelt structure was obtained from the self-assembly
of compound 1 in water/ethanol solution at an appropriate volume
ratio, ca. 1:1.
As demonstrated by the large-area SEM imaging shown in Figure
1A, ultralong fibril structure was formed through a slow 1D
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C O M M U N I C A T I O N S
migrate along the long axis of the nanobelt, due to the long-range
band-like π-electron delocalization along the nanobelt, leading to
significant enhancement in current. Indeed, over 3 orders of
magnitude increase in current was observed for the nanobelt upon
exposure to the saturated vapor of hydrazine. Such high current
modulation, along with the ultralong length, will make the nanobelts
ideal building blocks in optoelectronic nanodevices.
In summary, millimeter long nanobelts have been fabricated from
an asymmetric PTCDI molecule through a seeded self-assembling
method. The long length of nanobelts facilitates the construction
of two-electrode devices employing the nanobelt as channel
material; the long-range π-π molecular stacking allows for efficient
conductivity modulation through surface doping. A combination
of these two characters will enable broad optoelectronic applications
with these long nanobelts.
Figure 2. (A) UV-vis absorption spectra of 1 (0.6 mM) molecularly
dissolved in ethanol (dotted line) and in the aggregate state dispersed in
1:1 water/ethanol (solid line). (B) Absorption spectra of the aggregate shown
in (A) at different aging times: 0, 5, 24, 48, 72 h. A 2 mm cell was used.
Acknowledgment. This work was supported by NSF (CMMI
0638571), ACS-PRF (45732-G10), and NSFC (20520120221). We
thank Dr. Tiede for help with the XRD measurement.
Supporting Information Available: Synthesis, I-V, spectroscopy,
and microscopy measurements. This material is available free of charge
via the Internet at http://pubs.acs.org.
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The extended π-π stacking along the long axis of the nanobelt
would enable efficient long-range charge migration due to the
effective intermolecular π-electron delocalization15 and thus allow
increasing the electrical conductivity of the nanobelt through
external charge doping. Figure 3 shows the current-voltage (I-
V) measurement of a single nanobelt in the presence and absence
of hydrazine vapor, a strong reducing reagent that is capable of
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ox
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vs SCE). Under an applied bias, the doped electrons will rapidly
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