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Swearer et al., J Nanomater Mol Nanotechnol 2014, 3:2
http://dx.doi.org/10.4172/2324-8777.1000143
Journal of Nanomaterials &
Molecular Nanotechnology
Research Article
Self-Assembling 1,4,5,8-Naphthalentetracarboxylic DiimideMicrowires for Optoelectronic
Devices
Dayne F Swearer1, Noah M Johnson1, Arben Kojtari1 and HaiFeng Ji1*
Abstract
The widespread use of organic electronic devices requires simple
yet effective synthesis techniques to create materials with low-cost
and high performance. We report the solution phase crystallization
of 1,4,5,8-naphthalenetetracarboxylic diimide (NDI) microwires
from dimethylsulfoxide. These microwires were characterized by
means of scanning electron microscopy, thermal analysis, and
infrared and fluorescence spectroscopies. The microwires were
also characterized for their electrical properties and were shown
to exhibit an n-type semi-conductive property as well as photoresponsive changes in conductivity. The electron mobility within the
NDI microwires was determined to be lower than previous literature
reports, yet the combination of photo-responsive changes in
electrical conductivity may hold promise of photo-sensors, organic
light-emitting diodes, and other optoelectronic devices.
Keywords: 1,4,5,8-naphthalenetetracarboxylic diimide; NDI;
Microwire; Optoelectronic; Mobility
Introduction
The last decade has seen immense interest in self-assembled
supramolecular structures for organic electronic applications [1-7]
and in particular aromatic diimides derivatives [8]. Thin films and
other nanostructures of Perylene tetracarboxylicdiimide (PTCDI) [911], and NDI [12-18] (Figure 1), along with their derivatives, have
been well studied for a range of applications from organic field effect
transistors (OFET) and organic light emitting devices (OLED) to
organic photovoltaic’s (OPVs). Both PTCDI and NDI are well known
for their interesting photo-luminescent and electronic properties.
Recently, some attention has been paid to electronic devices built
from these supramolecular assemblies because of the unique
properties of such nanomaterials [16,19-21]. Previously, we reported
the physical properties and electronic characteristics of a single
unsubstituted PTCDI nanowire prepared from vapor deposition
[18]. In our previous attempts, NDI nanowires prepared in the gas
phase are not uniform or crystalline [22], and thus are not conductive
or semi-conductive, which limits their applications. The electronic
a SciTechnol journal
properties of a single NDI nanowire have not yet been reported. In
this study, we report the self-assembly of crystalline NDI microwires
through π-π stacking and hydrogen bonding from solution and the
characterization of the physical and electronic properties of the
structures.
Experimental Section
NDI was purchased from TCI Chemical and used as received.
Anhydrous DMSO (99.7%) was purchased from Acros Organics and
was used without further purification. NDI was dissolved in DMSO
at various concentrations. NDI microwires were prepared from
evaporation of DMSO in an Omni-Lab VAC 101965 glove box system
under nitrogen, the atmosphere within a glove box was maintained
under 0.2 ppm moisture content. Wires were self-assembled in glass
vials through evaporation of DMSO with a starting volume of 5mL of
NDI solution in a 20mL sample vial.
Fluorescence spectra were obtained with a Hitachi F-7000
fluorescence spectrometer. Attenuated total reflection infrared (ATRIR) spectra and cross-polarized optical microscopy (CPOM) were
obtained on a Smith Polarized IR microscope. Thermal analyses were
obtained with a thermo-gravimetric/differential thermal analyzer (TG/
DTA 6300, SII Nanotechnology Inc). Scanning electron microscopy
(SEM) images were obtained using a Hitachi TM-1000 tabletop
microscope. Elemental composition was found on a Zeiss Supra 50
VP equipped with an Oxford energy-dispersive X-ray spectroscopy
(EDS). Photon sensing experiments were performed with a microwire
resistor device mounted inside a electromagnetic shielding box which
was then placed inside the chamber of the Hitachi F-7000 fluorescence
spectrometer. The shielding box was left uncovered with the topside
directly facing the Xe lamp of the fluorescence spectrometer. CurrentVoltage (I-V) curves were measured on a Keithley 2636A dual channel
system source meter before and after the devices were exposed 355nm
light from a Xe lamp. The two gold electrodes at the distance of
10μm from each other were patterned by a lift-off photolithography
technique on a 300 nm thick SiO2 layer on a highly p-type-doped
Si wafer. An NDI microwire was transferred onto the wafer across
the two gold electrodes. Optical microscopy was used to check for a
suitable wire across the electrodes to ensure that the only conductive
path was through the single mesowire. The two gold electrodes are
used as a source and a drain, the SiO2 serves as a gate oxide, and the
*Corresponding authors: Dr. Hai-Feng Ji, Department of Chemistry,
Drexel University, Philadelphia, PA 19104, USA, Tel: 01-215-895-2562;
Fax: 01-215-895-1265; E-mail: [email protected]
Received: January 23, 2014 Accepted: March 12, 2014 Published: March 16,
2014
International Publisher of Science,
Technology and Medicine
Figure 1: Chemical structure of NDI.
All articles published in Journal of Nanomaterials & Molecular Nanotechnology are the property of SciTechnol, and is protected
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Citation: Swearer DF, Johnson NM, Kojtari A, Hai-Feng J (2014) Self-Assembling 1,4,5,8-Naphthalentetracarboxylic DiimideMicrowires for Optoelectronic
Devices. J Nanomater Mol Nanotechnol 3:2.
doi:http://dx.doi.org/10.4172/2324-8777.1000143
p-type Si wafer is a gate electrode. Gate voltage experiments were
performed using a -2V to +2V sweep under a 10V bias on a Keithley
2636A dual channel system source meter.
Results and Discussion
Evaporation of a 1.0×10-3M NDI in DMSO solution afforded
self-assembled crystalline NDI microwires of several hundred
microns in length (Figure 2). The size of NDI microwires was found
to be kinetically controlled as a function of evaporation rate; slower
evaporation rates led to much larger wires, but fast evaporation
of solution led to aggregation on the surface and no well-defined
nanostructure. Typical time frames for full evaporation of DMSO
solution were between 48-72 hrs. Dilute solutions of 1×10-4, 1×10-5
and 1×10-6 M concentrations were also examined but were found not
help the formation of microwires; lower concentration solutions were
found to reduce or in hinder wire formation. Higher concentrations
than 1mM have not been investigated due to limited solubility of NDI
in DMSO. Because wires at microscale are easy to handle than those
at nanoscale, all the characterization of the NDI crystalline wires were
conducted on NDI microwires that were prepared from evaporation of
DMSO in a 1mM NDI solution. NDI has very low solubility in many
common organic solvents and water. One caveat for self-assembly
of this system is the highly hygroscopic nature of DMSO. If the same
experimental methods are conducted outside an inert atmosphere,
anhydrous DMSO absorbs atmospheric water vapor, precipitating the
dissolved NDI out of solution rather than allowing NDI to self-assemble
slowly. These microwires were characterized for composition, electrical
and optical properties, as well as optical sensing applications.
triphenylene, however, can be determined from other techniques, such
as polarized optical microscopy. Cross-polarized optical microscopy
image and a polar plot of wire brightness as a function of angle show
the NDI wires are crystalline (Figure 4) [23]. There are four repeats in
a complete circle. The maximum or minimum brightness of the image
repeats every 900. When the long axis is parallel to the polarization
light (0o), the brightness is the minimum. The brightness increases
when the angle increases from 0o to 45o. The periodical change of the
brightness vs. the angle shows that the NDI nanowires are birefringent
single crystals [23], i.e. the crystal has two distinct indices of refraction
and it splits one ray of light into two rays. The strong birefringence is
associated with the plane-to-plane stacking (H-aggregation) [24] of
NDI molecules in the nanowires.
ATR-IR spectroscopy, TG/DTA, and EDS were conducted
to compare the physical properties of the self-assembling wires
and the powdered NDI. Figures 5 and 6 show the similarity in
IR spectra and TG/DTA results between the NDI powder and
microwires. The similarity in NDI powder and microwires from
both IR and TG/DTA does not provide information whether the
structure of NDI nanowires is different from those in the powders
because hydrogen bonding and the π-π interaction between
NDI molecules exist in both NDI powders and wires. The useful
information we draw from EDS (Figure 7) results is that DMSO
X-ray diffraction (XRD) and transmission electron microscopy
(TEM) are typically used to obtain the crystalline structures of the
organic and inorganic materials. However, despite extended lengths,
the diameter of our NDI nanowires is less than 50µm and therefore
too small for single-crystal XRD. In another attempt, TEM electron
diffraction showed no clear but a vague ring pattern (Figure 3) because
the self-assembly of NDI assembly can be readily disintegrated under
high electron beam voltage. The crystallinity of the 1D structures of
Figure 4: Cross polarized optical microscopy images of an NDI microwire.
Single crystal NDI wire at [A] 45˚, [B] 30˚, [C] 0˚ angles from polarizers.[D]
Shows the intensity as a function of angle rotated completely around 360˚
% Transmittance
100
Figure 2: (Left) SEM images of a single NDI microwire which has self assembled from DMSO; this wire was approximately 1mm long. (Right) View
of multiple wires grown during a single process.
80
60
40
4000
Figure 3: TEM images (left) and electron diffraction pattern of a NDI microwire
Volume 3 • Issue 2 • 1000143
NPDI Powder
NPDI Wires
3500
3000
2500
2000
1500
1000
500
Wavelength (cm-1)
Figure 5: Infrared Spectra of NDI powder and wires.
• Page 2 of 5 •
Citation: Swearer DF, Johnson NM, Kojtari A, Hai-Feng J (2014) Self-Assembling 1,4,5,8-Naphthalentetracarboxylic DiimideMicrowires for Optoelectronic
Devices. J Nanomater Mol Nanotechnol 3:2.
doi:http://dx.doi.org/10.4172/2324-8777.1000143
80
100
60
40
40
20
20
0
NPDI Powder
NPDI Wires
60
DTA (µV)
Weight Loss (%)
80
NPDI Powder
NPDI Wires
100
200
0
300
400
500
100
Temperature (ºC)
200
300
400
500
Temperature (ºC)
Figure 6: (Left) Thermogravimetric Analyses of NDI powder and wires. (Right) differential thermal analysis of NDI wires and powder. Heating rate was
5°C•min-1 with a nitrogen flow (100 mL•min-1).
molecules are not included in the NDI microwiressince sulfur
atoms are not observed.
Although the IR and TG/DTA techniques are not sensitive enough
to distinguish the molecular environments of the NDI molecules in
the forms of powders and microwires, the fluorescence spectrum of
NDI microwires, on the other hand, showed significant differences
from that of NDI powder as seen in figure 8. The NDI powder shows
one emission peak at 446 nm, while NDImicrowires show one peak at
476 nm. The redshift may be attributed to the formation of an ordered
crystal network in the microwires.A weak emission peak at 580nm
in the microwires can be attributed to an excimer within the crystal
structure of the NDI wires [25].
Gate voltage tests were made to measure the effect of an
external magnetic field on the conductivity of the wire. Our NDI
single microwire metal-oxide-semiconductor field-effect transistor
(MOSFET) is of a depletion type, since the channel exists at zero
gate-source voltage. To determine the output characteristics of the
device, the drain-source voltage, VDS, was swept between -2V to +2
V, and the gate-source voltage, VGS, was kept constant throughout the
sweep. The experimental output characteristics of the device (Figure
9 left) show that NDI wires behave like n-type transistor. At 0V bias,
the electrical conductivity was determined to be 4×10-7 S cm-1. The
conductivity of this wire has been shown to decrease with testing and
time. This process is believed to be the result of degradation of the
crystal structure within the wire, however whether this degradation
is of chemical or physical nature will be the focus of future studies on
long-term device stability.
The electron mobility was determined at to be on the order of
10-9 cm2 V-1 s-1 based directly on the electrical conductivity given in
equation 1. µe is the electron mobility as a function of the elementary
electron charge, e, number density of electrons, n, and the electrical
conductivity, σ. In the calculation of electron mobility, the number
density of electrons was assumed to be 1.
µe =
ne
σ
(1)
When NDI wire devices were exposed to the excitation maxima
Volume 3 • Issue 2 • 1000143
Figure 7: Energy Dispersive X-ray Spectroscopy of a single NDI wire.
Sample was excited at 5 keV for sulfur detection. Results show that
only C, N, and O are present in the wire. Black like shows approximate
expectation of sulfur peak.
1.2
Fluorescence Intensity
The conductance of the microwires was tested with the Keithley
Model 2636A controller by applying conductive silver paste on each
end of millimeter-long wires as the source and drain electrodes in a
manner where one probe is grounded and the other was connected to
a source measurement unit. Conductivity measurements have been
successfully made across wires of lengths over 1 millimeter. Microwire
devices show electrical current on a dielectric SiO2 surface.
NPDI powder
NPDI microwires
1.0
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
Wavelength, nm
Figure 8: Solid-state fluorescence spectra of NDI powder and NDI
microwires. Excitation wavelength λex= 355 nm. Data has been normalized to
show peak shifts rather than intensities.
λex = 355 nm the current within the wire increased as shown in figure
9 right, suggesting the device can be used for photo-detectors. This
• Page 3 of 5 •
Citation: Swearer DF, Johnson NM, Kojtari A, Hai-Feng J (2014) Self-Assembling 1,4,5,8-Naphthalentetracarboxylic DiimideMicrowires for Optoelectronic
Devices. J Nanomater Mol Nanotechnol 3:2.
doi:http://dx.doi.org/10.4172/2324-8777.1000143
50
Current (nA)
30
Current (nA)
4
0V Bias
5V Bias
10V Bias
40
20
10
0
-10
-20
2
Ambient Conditions
Exposure to 355nm Light
Photoexcitation of Mircowire
0
-2
-4
-2
-1
0
1
2
Voltage (V)
-1.0
-0.5
0.0
0.5
1.0
Voltage (V)
Figure 9: Left: I-V curve of a single NDI wire across a 10 micrometer gap as a gate voltage is applied. Right: Zero-field photo-responsive changes in
conductivity of a single NDI wire on exposure to 355 nm light. Inset shows a microwire device for the test. It is noteworthy that the microwires used in
these two experiments have different sizes so the current values are different under the same voltage.
phenomenon can be accounted for by the electronic transitions and
hence photo-generated currents between excited molecules along the
path of conduction [26].
Conclusions
Unsubstituted NDI has been shown to form self-assembled nano/
microwires through solvent evaporation. The size of NDI wires is
dependent on the concentration of the solution, evaporation rates
and atmospheric conditions. These variables allow for a tunable
system to tailor for specific applications. The process described
within allows for the production of multiple wires during single
crystallization procedures, aiding throughput in potential future
manufacturing. Microwires of NDI show n-type semiconductive
characteristics and are photo-responsive, leading to increased
conductivity along the length of the wire. Despite the low electron
mobility, the photo-responsive conductivity increases could hold
promise for a wide variety of applications such as photo-detectors,
OLEDs, photovoltaic’s, and optoelectronic devices.
Acknowledgement:
Support for this work was provided by the National Science Foundation (No.
DMR-1104835).
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• Page 4 of 5 •
Citation: Swearer DF, Johnson NM, Kojtari A, Hai-Feng J (2014) Self-Assembling 1,4,5,8-Naphthalentetracarboxylic DiimideMicrowires for Optoelectronic
Devices. J Nanomater Mol Nanotechnol 3:2.
doi:http://dx.doi.org/10.4172/2324-8777.1000143
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Author Affiliation
1
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Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA
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