Vertical organic nanowire arrays: Controlled synthesis and chemical Sensors

Full text of DOI:10.1021/ja809360v

Published on Web 02/18/2009
Vertical Organic Nanowire Arrays: Controlled Synthesis and Chemical
Sensors
Yong Sheng Zhao, Jinsong Wu, and Jiaxing Huang*
Department of Materials Science and Engineering, Northwestern UniVersity, EVanston, Illinois 60208-3108
Received December 1, 2008; E-mail: jiaxing-huang@northwestern.edu
Vertical arrangement of nanowire materials has been found to
by the evaporation temperature, while the length can be tuned from
500 nm to over 10 µm by the deposition time (see Figure S1 for
more).
Figure 2A is a transmission electron microscope (TEM) image
showing the smooth surface and uniform width of a typical nanowire
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be advantageous for applications such as solar cells, batteries,
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electrochemical sensors, field emitters, light emitting diodes, and
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transistors and has led to the discoveries of nanolasers and
nanopiezotronics. So far, the predominating body of work has
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focused on inorganic materials. Organic nanowires can bring new
opportunities since they are composed of molecular building blocks
with relatively weaker interactions, thus allowing highly flexible
structural tunability. Organic nanostructures have emerged to play
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increasingly important roles in miniaturized optoelectronic devices.
Here we report the direct growth of vertically aligned organic
nanowires of 1,5-diaminoanthraquinone (DAAQ) on solid surfaces.
The crystal lattice and parameters of the nanowires and their growth
direction were determined by extensive electron diffraction studies.
The fluorescence of DAAQ nanowires can be quenched when
exposed to acidic vapor and recovered by basic vapor. The ease of
the oriented vertical growth should allow the direct integration of
these nanowires into photonic sensing devices.
DAAQ (Figure 1, inset) belongs to a class of quinoid compounds
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that have been used in the dye industry for over a century. Vertical
Figure 2. (A) TEM image of the DAAQ nanowires. Scale bar is 200 nm.
(
B) A set of electron diffraction patterns of the same nanowire acquired
while rotating the nanowire along its [100] axis. For example, a 34.6° of
axial rotation switched the pattern from along [001] to [011] zone axes,
while a 35.6° rotation in the opposite direction switched it to one along the
Figure 1. SEM images (top: 40° tilted view; bottom: cross-sectional view)
showing the vertically aligned DAAQ nanowire arrays prepared at (A) 160
°
C, for 5 min and (B) 180 °C, for 5 min. All scale bars are 1 µm. The inset
[
0-11] zone axis. The angles in parentheses are theoretical values calculated
from the reconstructed unit cell. (C) Cross-sectional and (D) tilted TEM
views of a single DAAQ seed grown on SiO coated copper grid. Scale
shows the molecular structure of the DAAQ molecule.
x
arrays of DAAQ nanowires on solid substrates were prepared by a
bars are 50 nm. Insets: electron diffraction patterns taken from the seeds.
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physical vapor transport method. In a typical synthesis, a thin
coating of DAAQ was made on the inner wall of a round bottle
flask by spreading an ethanol solution. This ensured uniform heating
over the entire sample area in the subsequent sublimation steps.
Then the flask was heated to a predetermined temperature of
product. To determine the crystal structure and the growth direction
of the DAAQ nanowires, we collected the orientation dependent
electron diffraction patterns from a few single nanowires. Figure 2B
shows a set of patterns obtained by continuously rotating a single
nanowire along its axis. These patterns allowed reconstruction of the
reciprocal lattice for determining the lattice type and parameters (see
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50-200 °C to vaporize the powders. The vapor of DAAQ can be
transported downstream by air, nitrogen, or diffusion under vacuum
to deposit on a solid substrate (e.g., Si, glass, Au, Al, Fe, Cu foils,
or W needle) placed downstream. Figure 1 shows the typical
scanning electron microscope (SEM) images of the vertically
aligned nanowire arrays prepared on Si wafers. The length and
diameter of the nanowires were controlled by the growth conditions.
For example, the diameter can be tuned from 80 to over 500 nm
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Supporting Information). The reciprocal lattice parameters derived
-
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-1
from the reconstruction are a* ) 0.265 Å , b* ) 0.103 Å , c* )
-1
0.067 Å , R* ) γ* ) 90°, ꢀ* ) 97.6°, which correspond to a
monoclinic lattice with a ) 3.78 Å, b ) 9.73 Å, c ) 15.01 Å, ꢀ )
82.4°. The reconstructed unit cell was validated by the good agreement
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158 9 J. AM. CHEM. SOC. 2009, 131, 3158–3159
10.1021/ja809360v CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
between the experimental tilting angles among the zone axis and those
recalculated (in parentheses) based on the unit cell parameters
corresponding to the diffraction patterns in Figure 2B. The nanowire
axis was found to be along the [100] direction, which is the same
direction pointing from the transmitted spot to the (100) spot in the
diffraction patterns (Figure 2B). In addition, the XRD pattern of DAAQ
nanowires also showed a preferred orientation of the (100) plane
exposure to ∼5 ppm of HCl vapor in air. Figure 3A shows the
absorption spectra of the nanowire arrays after being exposed to HCl
for different lengths of time. The visible band rapidly decreased
accompanied by apparent bleaching of the red color of the sample
(Figure 3C). The photoluminescence (PL) spectrum of free DAAQ
molecules in ethanol shows an approximately symmetric peak centered
at 575 nm, whereas the nanowire arrays displays a super broadband
emission from 500 to 900 nm (Figure 3B). On exposure to HCl, the
PL intensity rapidly decreased, reaching over 90% quenching after
only 30 s. Fluorescence quenching can be seen by eye when the
samples were illuminated with blue light (460-490 nm) (Figure 3D).
In a control experiment, DAAQ powders composed of micron sized
particles did not show an apparent response even after 30 min of
exposure (Figure S4).
(Figure S2). The XRD pattern of the nanowires matched the powder
pattern except for the relative peak intensities, suggesting that DAAQ
did not undergo phase transition or chemical reaction during the vapor
transport.
Since the organic nanowires were made without catalysts or liquid
droplets as is needed in the vapor-liquid-solid synthesis for many
inorganic nanowires, the growth should be controlled by a vapor-solid
condensation process. To observe the early stage of nanowire growth,
the deposition was stopped within seconds. SEM observation of the
substrates revealed that DAAQ first deposited as well-separated
nanoparticles of ∼100 nm in diameter (Figure S3) with a density
comparable to that of the final nanowire arrays. This indicates that the
nanowires were grown on these seed nanoparticles. Crystallographic
Both the absorption and PL can be recovered in air to ∼95% of
their original intensity in ∼2 h. However, they can be rapidly reset by
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basic vapors (e.g., NH ) within 2 s. This also provides a mechanism
for detecting basic vapors. The basic vapor deprotonated the amine
groups and helped to restore the intramolecular charge transfer, leading
to the recovery of color and fluorescence (Figure S5). The nanowires
were exposed to cycles of acid and base vapors. A good consistency
in the bleaching and quenching efficiencies (Figure S6) was observed.
In summary, vertical organic nanowire arrays of DAAQ dye
molecules were prepared by a facile physical vapor transport method.
The crystal structure and growth direction of the nanowires were
determined by TEM and electron diffraction. These fluorescent
nanowires were sensitive to acidic and basic vapors. The ease of
oriented vertical growth should make it possible to directly integrate
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studies on such nanoparticles grown on SiO coated copper grids were
carried out in TEM by electron diffraction. When viewed from the
bottom, the cross section of the nanoparticles was perpendicular to
the electron beam. The electron diffraction pattern could be indexed
along the [100] zone axis (Figure 2C). When tilted off to nearly 90°,
the electron diffraction pattern (Figure 2D) again shows the (100) spot,
which confirms that the growth direction was along the [100] direction.
Since vertical nanowires were obtained on many different substrates,
it implies that the vapor may have condensed to form similarly oriented
seeds on those surfaces as well.
The intramolecular charge transfer between the neighboring amine
and carbonyl groups is responsible for the color and fluorescence of
DAAQ. In an ethanol solution, the molecules exhibit two resolved
absorption bands at 275 and 490 nm, respectively (Figure 3A, red line).
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these nanowires into photonic sensing devices.
Acknowledgment. The work was supported by a seed grant from
the NU-NSEC (NSF EEC-0647560) and an ACS-PRF grant (48678-
G10). TEM work was performed in the EPIC facility of the NUANCE
center at Northwestern.
Supporting Information Available: Materials and methods; More
SEM images of the nanowires; XRD patterns of the nanowires and bulk
DAAQ; SEM images of the nucleation stage; Sensing performance of
DAAQ powder; Pictures showing color and fluorescence changes of the
nanowires on exposure to HCL and NH ; PL quenching-recovery circles.
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This material is available free of charge via the Internet at http://
pubs.acs.org.
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