Single-crystalline, size, and orientation controllable nanowires and ultralong microwires of organic semiconductor with strong photoswitching property

Article abstract of DOI:10.1021/ja077600j

Single-crystalline, precise size-controlled nanowires and ultralong microwires with lengths reaching several millimeters of organic semiconductor 1 were prepared in large scale by cast assembly. The size and density of the nanowires and microwires could be controlled by simply adjusting the concentration of 1 in casting solutions. More importantly, the formation of these nanowires and microwires showed no substrate and solvent dependence and was orientation controllable. Highly reproducible and sensitive photo response characteristics were observed in these nanowires and microwires. Fast and reversible photo-switchers based on multiple or individual single-crystal microwires were fabricated via "multi times gold wire mask moving" technique with switch ratio over 100.

Full text of DOI:10.1021/ja077600j

Published on Web 02/29/2008
Single-Crystalline, Size, and Orientation Controllable
Nanowires and Ultralong Microwires of Organic
Semiconductor with Strong Photoswitching Property
Lang Jiang,†,‡ Yanyan Fu,†,‡ Hongxiang Li,*,† and Wenping Hu*,†
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute
of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China, and Graduate School of
Chinese Academy of Sciences, Beijing, 100039, China
Received October 2, 2007; E-mail: lhx@iccas.ac.cn; huwp@iccas.ac.cn
Abstract: Single-crystalline, precise size-controlled nanowires and ultralong microwires with lengths reaching
several millimeters of organic semiconductor 1 were prepared in large scale by cast assembly. The size
and density of the nanowires and microwires could be controlled by simply adjusting the concentration of
1 in casting solutions. More importantly, the formation of these nanowires and microwires showed no
substrate and solvent dependence and was orientation controllable. Highly reproducible and sensitive photo
response characteristics were observed in these nanowires and microwires. Fast and reversible photo-
switchers based on multiple or individual single-crystal microwires were fabricated via “multi times gold
wire mask moving” technique with switch ratio over 100.
tors,4 especially for single-crystalline ones, because of the weak
intermolecular interactions between organic semiconductors,
Introduction
Nanowires and nanorods can act as both active devices and
interconnects and have attracted particular attention recently in
nanoelectronics.1 It is especially true for single-crystalline
nanowires and nanorods of semiconductors that combine the
merits of single crystal (no defect and revealing the intrinsic
property of material) and the applications of semiconductors
into nanostructures. To date, many studies based on carbon
nanotubes2 and nanowires and nanorods of inorganic semicon-
ductors have been reported.3 Compared with inorganic semi-
conductors, organic semiconductors offer great chemical struc-
ture variety and property tunability and are promising candidates
for molecular electronics. However, there has been little in the
literature about nanowires and nanorods of organic semiconduc-
complexities of the self-assembly process, and difficulties to
control organic molecular packing.
In addition to the syntheses difficulty of organic semiconduc-
tor nanowires and nanorods, there are other challenges in organic
semiconductor nanowires and nanorods: one is to synthesize
large quantity and large-scale nanowires and nanorods easily
and repeatedly at low cost. Second is to control the size and
orientation of these organic semiconductor nanwires and na-
norods. To synthesize nanowires and nanorods across multiple
length scales (from nanometer to micrometer) and to control
the orientation of these nano-objects not only have great
scientific fundamentality but also are the crucial step for their
applications.5 Usually small molecules self-assemble to ordered
structure from solution in nanoscale while in micrometer they
exhibit disordered structure. Additionally, on the micrometer
scale these nano-objects normally adopt random position and
orientation. Although indirect, slow, and multistep patterning
approaches have been proposed, they have lots of disadvantages,
† Institute of Chemistry, Chinese Academy of Sciences.
‡ Graduate School of Chinese Academy of Sciences.
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J. AM. CHEM. SOC. 2008, 130, 3937-3941
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Jiang et al.
Scheme 1. Chemical Structure of Organic Semiconductor 1
such as involving surface-invasive steps and chemically limiting
procedure and not being easily applicable on different scales.6
The third great challenge is the application of organic semi-
conductor nanowires and nanorods: device fabrication. Usually,
organic nanowires and nanorods are prepared through solution-
based self-assembly (e.g., solution deposition), template-based
methods, and vacuum sublimation.4 By using these methods,
we need to move the nanostructures to substrate for device
fabrication. Currently, several methods have been developed
to transfer nanostructures to substrates, namely mechanical
transfer7 and microfluidic,1d,8 Langmuir-Blodgett,9 and stamp
techniques.10
Cast assembly is a method that can synthesize self-assembled
nanostructures by casting a solution onto the substrate. It is low
cost and can prepare nanostructures in large quantity and large
area easily. More importantly, through cast assembly nano-
structures can adhere to the substrate directly and further be
fabricated to device in situ, which provides great advantage in
device fabrication. Herein, we reported the synthesis of single-
crystalline nanowires and ultralong microwires of organic
semiconductor in large scale through cast assembly. The size
and orientation of these wires were easily controlled by changing
experimental conditions. Moreover, these nanostructures adhered
to the substrate tightly and were fabricated to devices in situ.
The two-end devices based on single or multiple microwires
suggested these nanostructures had strong photo response and
could be used as a photoswitcher.
Figure 1. Morphology of cast products of organic semiconductor 1 on
glass substrate. (a-d) SEM images of cast products: (a) nanowires, (b)
microwires, (c) ultralong microwires, and (d) nanowires in large area. (e)
Optical image of microwires in large area.
Results and Discussion
Figure 2. (a) TEM image and (b) SAED pattern of individual nanowire.
Organic semiconductor 2-anthracen-9-ylmethylene malono-
nitrile (1; Scheme 1) was synthesized by reacting 9-anthralde-
hyde with malononitrile in the presence of piperidine in medium
yield.11 When a solution of 1 in methylene chloride was cast
on substrates, nanowires and microwires were obtained. Figure
1shows the morphologies of nanowires (Figure 1a) and mi-
crowires (Figure 1b) cast on glass substrate. The width and
height of nanowires are usually ∼50-100 nm and ∼50-80 nm
(Supporting Information), respectively, while the diameter of
microwires changes from hundreds of nanometers to several
micrometers and the length reaches several millimeters (Figure
1c). Because the products are obtained by cast assembly, they
are easily synthesized in a large area as shown in Figure 1d,e.
Transmission electron microscopy (TEM) image of individual
nanowires and microwires and its corresponding selected area
electron diffraction (SAED) patterns indicate that the whole
nanowires and microwires are single crystals (Figure 2).
Experimental results showed that the size and density of
nanowires and microwires of compound 1 could be controlled
through simply adjusting the concentration of 1 in casting
solutions (Figure 3). When 1 was at low initial concentration
(0.007 M in methylene chloride), dispersed nanowires were
obtained, and the diameter of these nanowires was close to 50
nm (Figure 3a). With increasing initial concentration (0.07 M),
dense nanowires with well-defined shapes and diameter (∼80
nm) were formed (Figure 3b and Supporting Information). With
further increasing initial concentration (close to saturation, ∼0.1
M), ultralong microwires with diameters changing from hun-
dreds of nanometers to several micrometers and lengths reaching
several millimeter were synthesized (Figure 3c). UV-vis spectra
of these nanowires and microwires showed that the absorption
of 1 was red shifted with increasing size (Figure 3d), which is
definitely a direct proof of size effects.12 This phenomena can
be explained according to Kubo theory,13 ∆E ) (4EF)/(3N)
(6) Garcia, R.; Martinez, R. V.; Martinez, J. Chem. Soc. ReV. 2006, 35, 29-
38.
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10233.
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Sfeir, M. Y.; O’Brien, S. P.; Hone, J. Nano Lett. 2005, 5, 1515-1518. (b)
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A R T I C L E S
Figure 5. (a) SEM image and (b) optical image of highly ordered
microwires formed by slow solvent evaporation.
into nice nanowires and microwires with further solvent
evaporation (Figure 4b,c). According to the above formation
mechanism of nanowires, the concentration dependence of the
morphology of 1 is well understood. Since the self-assembly
of different concentration solutions is in the same environment,
if we ignore the effect of concentration to the evaporating rate
of solvent, we can assume the evaporating rates of the solutions
(with different concentrations) are same. Thus, when different
concentrations of solutions (30 µL) were drop cast to the
substrates, higher initial concentration solution reaches super-
saturation (at which the nuclei was begun) faster than the lower
one. Hence, these nuclei have a longer time to grow, and this
leads to the formation of microwires (Figure 3c and Supporting
Information). To further confirm this assumption, we monitored
the in situ growth process of microwires in solution by optical
microscope (Supporting Information). From the growth process,
we can see that, unlike the normal solution-based self-assembly
in which the self-assembly is begun in solution and nano-objects
are deposited on substrate, the nucleation first occurs on the
side of the substrate and with the solvent evaporating microwires
grow up directly from the nucleation on the substrate.
Figure 3. (a-c) SEM images of cast products of compound 1 prepared at
different concentrations from CH2Cl2 solution. (a) 0.007, (b) 0.035-0.07,
and (c) 0.1 mol/L. (d) UV-vis spectra of 1 in CH2Cl2 solution (5 × 10-5
M), nanowires and microwires on quartz substrates.
As we know, for a self-assembly system, the formation of
the product is usually affected by substrates and solvents (low
qualified and even no nanostructures will be formed while
changing the substrates or solvents), which limit the application
of the materials.17,18 Fortunately, the formation of nanowires
and microwires of compound 1 exhibited no significant differ-
ence on different substrates (glass, Si, Si/Cu (evaporated Cu
film on silicon), Au). When we changed the solvents of the
casting solution (hexane, toluene, methylenechloride, THF; from
no polarity to high polarity), nanowires and microwires were
also obtained. The excellent nanowires and microwires obtained
on all kinds of substrates and solvents definitely confirmed the
substrates’ and solvents’ independent ability of the nanowires
and microwires and the greatest self-assembling property of
organic semiconductor 1.
Considering the formation mechanism of nanowires and
microwires, the merits of cast assembly and the unique self-
assembly property of organic semiconductor 1, we tried to
control the orientation of these nanowires and microwires. When
a solution of 1 (1:2, CH2Cl2/petroleum ether) was added to a
beaker that was covered by a perforated polyethylene sheet (the
roles of polyethylene sheet are (1) to prevent the dust in air to
pollute the solution and (2) to slow the evaporation rate of
solvent), highly ordered microwires were obtained on the wall
of the beaker with solvent evaporating (Figure 5a,b). The
formation process is that nucleation first occurs at the air/
Figure 4. SEM images of the time-dependent growth process of the cast
nanowires of compound 1.
1/d3, where ∆E is the energy gap of particles, EF is the Fermi
energy, N is the molecule number of the particle included, and
d is the diameter of the nanostructures. With the particle size
reducing, NV, ∆Ev, it results in a widened energy gap, which
leads to the blue shift of the absorption spectrum.
The formation of nanowires and microwires of 1 might take
place via a crystallization process14-16 including nucleation and
growth. First, solute molecules exist as monomer in the dilute
solution. Then, with solvent evaporating, solution concentration
reaches supersaturation and molecules begin to nucleate on the
substrate. All nuclei clearly tend to stack in one direction as
the embryonic form of nanowires (Figure 4a) and finally evolve
(12) (a) Tian, Z. Y.; Chen, Y.; Yang, W. S.; Yao, J. N.; Zhu, L. Y.; Shuai, Z.
G. Angew. Chem., Int. Ed. 2004, 43, 4060-4063. (b) Hu, J.; Guo, Y.; Liang,
H.; Wan, L.; Jiang, L. J. Am. Chem. Soc. 2005, 127, 17090-17095.
(13) Kubo, R. J. Phys. Soc. Jpn. 1962, 17, 975-986.
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Rosmalen, G. M. Cryst. Res. Technol. 2003, 7-8, 555-574.
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(b) Anwar, J.; Boateng, P. K. J. Am. Chem. Soc. 1998, 120, 9600-9604.
(c) Galkin, O.; Vekilov, P. G. J. Am. Chem. Soc. 2000, 122, 156-163.
(16) (a) He, G. W.; Bhamidi, V.; Tan, R. B. H.; Kenis, P. J. A.; Zukoski, C. F.
Cryst. Growth Des. 2006, 6, 1175-1180. (b) Xiong, J. Y.; Liu, X. Y.;
Chen, S. B.; Chung, T. S. J. Phys. Chem B 2005, 109, 13877-13882.
(17) Rabani, E.; Reichman, D. R.; Geissler, P. L.; Brus, L. E. Nature 2003,
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sensitivity of our single-crystalline devices to light is under-
standable. On the basis of the high photosensitivity of the
microwires, single-crystalline photoswitchers of single or mul-
tiple microwires were fabricated (Figure 6b). With the photo
irradiation on/off the devices could work between low and high
impedance states fast and reversibly with switching ratio over
100.
Experimental Section
Compound 1 was synthesized as follows: A solution of 0.393 g
(1.9 mmol) of 9-anthraldehyde in 50 mL of CH3CN was added to 0.372
g (5.6 mmol) of malononitrile and a drop of piperidine. The mixture
was heated to reflux for 0.5 h. After cooling, the solvent was evaporated.
The residue was purified by column chromatography on silica gel by
eluting with CH2Cl2, affording 0.3 g of compound 1. Yield: 62%. MS
1
(EI) m/z, 254. H NMR (300 MHz, CDCl3): δ 8.94 (s, 1H), 8.66 (s,
4H), 8.09 (d, 2H), 7.94 (d, 2H), 7.70-7.54 (m, 4H).
The substrates used here were successively cleaned with pure water,
hot acetone, hot ammonia/hydrogen peroxide solution (ammonia/
hydrogen peroxide/water ) 1:1:5), pure water, pure ethanol, and finally
with argon plasma treated for 15 s. After that, 30 µL of solution at
different concentrations such as 0.007, 0.07, and 0.1 M was prepared,
and these solutions were cast on 1 cm × 1 cm substrates (solvent could
be dichloromethane, THF, toluene, acetonitrile, n-hexane, etc.) for
nanowires and microwires. The products were characterized by XRD
(D/max2500), AFM (a Nanoscopy IIIa), SEM (Hitachi S-4300 SE),
and TEM (JEOL 2010).
Two-end devices were fabricated as follows: A solution of 1 was
cast assembled on a titled Si/SiO2 substrate, and ordered microwires
were obtained. Then Au electrodes were thermally evaporated by laying
a micrometer-sized Au wire on the microwires as the mask to obtain
a gap between two electrodes. Five-nanometer Ti was deposited under
the Au pad electrodes to increase the adhesion between the Au
electrodes and the SiO2 insulator. After the first deposition of electrodes,
the Au wire mask was moved slightly so that a portion of the gap area
could be exposed to Au vapor again. Subsequently, Au electrodes were
deposited for the second time, third time, and so forth, until the
electrodes were linked by microwires. Photo response characteristics
of the devices were recorded with a Keithley 4200 SCS and a
Micromanipulator 6150 probe station in a clean and metallic shielded
box at room temperature in air.
Figure 6. (a) Current-voltage characteristics of the two-end devices based
on the ordered multimicrowires under illumination (the gap of the devices
is 20 µm). The powers of the illumination (from bottom to top) are dark,
0.37, 1.12, 3.04, 5.76 mW/cm2. (Inset) Linear characteristics between photo
current and the power of illumination at bias 40 V. (b) Photoswitching (white
light, 5.0 mW/cm2, at bias voltage 50 V) characteristics of the two-end
devices.
solution/glass wall interface since the density of solution surface
is higher than that in interior solution and the glass wall provides
a lower surface energy. Then microwires grow down along the
glass wall with solvent evaporation. Furthermore, if we cast a
solution of 1 on tilted substrates, highly ordered microwires
could also be prepared. It is well known that controlling the
growth orientation of nanostructures is a very important issue
in nanoscience. The highly ordered microwires formed by 1
further proved the unique property of 1 in self-assembly and
the great advantage of these wires in device fabrication.
As an example of potential applications for these nanowires
and microwires, two-end devices were fabricated by depositing
Au gap electrodes on the microwires through “multi times gold
wire mask moving” technique7 (including individual microwire;
Supporting Information). It is interesting that all the single-
crystalline devices exhibited high sensitivity to light. Figure 6a
shows the typical current-voltage (I-V) characteristics of the
single-crystalline devices in the dark and under white light
illumination. It is obvious that the current of the devices
increases significantly with increasing light intensity. Moreover,
the current of the device increased linearly with increasing
illumination power (inset of Figure 6a, at bias 40 V), indicating
the extremely high sensitivity of the devices to light. We know
the conductivity of the semiconductor is determined by the free
carriers in conduction band (σ ) nqµ, where σ is conductivity,
n is the number of free carriers, q is the charge of electrons,
and µ is mobility). The energy gap of the microwires is about
2.3 eV estimated from its absorption spectra (Figure 3d). This
gap is narrow enough to permit the generation of substantial
numbers of charge carriers by white light. Hence, the high
Conclusions
In summary, nanowires and microwires of organic semicon-
ductor 1 were prepared by cast assembly. Because of the merits
of cast assembly, these nanowires and microwires could be
synthesized in large quantity and large scale at low cost. TEM
images of individual nanowires and microwires and their
corresponding SAED patterns indicated that the whole nanow-
ires and microwires are single crystals. Precise size-controlled
nanowires and ultralong microwires with lengths reaching
several millimeters were obtained by carefully adjusting the
concentrations of 1 in cast solutions. Control experiments and
growth process of microwires in solution monitored in situ by
optical microscope suggest the formation mechanism of nanow-
ires and microwires is crystallization process. Moreover, the
formation of nanowires and microwires showed no substrate
and solvent dependence and was orientation controllable. Highly
reproducible and sensitive photo response characteristics were
observed in these nanowires and microwires. Fast and reversible
photoswitchers based on multiple or individual microwires are
fabricated via “multi times gold wire mask moving” technique
with switch ratio over 100.
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Nanowires and Microwires of Organic Semiconductor
A R T I C L E S
Acknowledgment. We are grateful to Prof. Yuliang Li for
profound discussions, Dr. Wei Xu and Mr. Yabin Song for AFM
characterization, and Prof. Chen Wang and Dr. Meng He for
TEM and SAED analysis. We acknowledge financial support
from the National Natural Science Foundation of China
(20402015, 60771031, 20421101, 20404013), Ministry of
Science and Technology of China (973: 2006CB806200,
2006CB0N0100), and Chinese Academy of Science.
images of nanowires and microwires cast from different solvents,
SEM images of nanowires and microwires cast on different
substrates, powder XRD of microwires, device structures for
photoswitcher, current-voltage characteristics of bared Au
electrodes with and without illumination, and formation process
of microwires monitored by optical microscopy (this file is a
flash, it can be opened by Internet Explorer). This material is
available free of charge via the Internet at http://pubs.acs.org.
Supporting Information Available: Crystal structure of
compound 1 and its CIF file, AFM images of nanowires, SEM
JA077600J
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