Preparation and ambipolar transistor characteristics of co-crystal microrods of dibenzotetrathiafulvalene and tetracyanoquinodimethane

Article abstract of DOI:10.1039/c3tc30112e

Co-crystal microrods of dibenzotetrathiafulvalene (DBTTF) and tetracyanoquinodimethane (TCNQ) molecules with the mixed-stack structure were prepared via a facile solution process and fully characterized. The as-prepared microrods were directly used for fabricating prototype devices, which exhibited typical ambipolar charge transport characteristics. The result indicated that the co-crystal microrods of DBTTF-TCNQ were potentially useful for miniaturized devices.

Full text of DOI:10.1039/c3tc30112e

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Materials Chemistry C
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Preparation and ambipolar transistor characteristics of
co-crystal microrods of dibenzotetrathiafulvalene and
tetracyanoquinodimethane†
Cite this: J. Mater. Chem. C, 2013, 1,
2286
a
a
a
Hao-Di Wu,abc Feng-Xia Wang, Yan Xiao and Ge-Bo Pan
Received 5th December 2012
Accepted 8th February 2013
*
DOI: 10.1039/c3tc30112e
www.rsc.org/MaterialsC
Co-crystal microrods of dibenzotetrathiafulvalene (DBTTF) and tet- by Zhang et al. via a drop-casting method.13 On the other hand,
racyanoquinodimethane (TCNQ) molecules with the mixed-stack one-dimensional (1D) structures might play more important
structure were prepared via a facile solution process and fully char- roles as interconnects and functional units in miniaturized
acterized. The as-prepared microrods were directly used for fabri- devices in comparison with two-dimensional (2D) ones. Never-
cating prototype devices, which exhibited typical ambipolar charge theless, the drop-casting method fails to control the dimension
transport characteristics. The result indicated that the co-crystal and shape of structures.
microrods of DBTTF-TCNQ were potentially useful for miniaturized
devices.
Dibenzotetrathiafulvalene
(DBTTF)–tetracyanoquinodi-
methane (TCNQ) represents a typical CT complex. The crystal
structure of DBTTF–TCNQ, rstly reported by Hayao in 1981,
belongs to the triclinic lattice with the mixed-stack in the [011]
direction.20 The devices based on bulk single crystals of DBTTF–
TCNQ exhibit good-performance transistor and photovoltaic
characteristics.10,21 Moreover, theoretical study has indicated
that the DBTTF–TCNQ co-crystals may have high mobility for
holes (80 cm2 Vꢀ1 sꢀ1) and electrons (60 cm2 Vꢀ1 sꢀ1) along the
stacking direction at room temperature.22
Herein, we report a facile solution method which has
combined precipitation and self-assembly processes for the
preparation of co-crystal microrods of DBTTF–TCNQ. The
chemical structures of DBTTF and TCNQ are shown in Fig. S1.†
The as-prepared co-crystal microrods were fully characterized
and directly used for fabricating prototype eld-effect transis-
tors with the bottom-gate bottom-contact geometry. The results
indicate that the co-crystal microrods have well-dened shapes
and smooth surfaces and the devices based on them exhibit
typical ambipolar charge transport characteristics.
In a typical synthesis, 0.5 mL chloroform solution of DBTTF
(ꢁ4 mM) was fast injected into 0.5 mL chloroform solution of
TCNQ (ꢁ4 mM). The mixed solution was shaken vigorously for
30 s and maintained for 5 minutes. Then, 3 mL n-hexane was
added slowly to the surface of the mixed chloroform solution.
The resulting two-layer mixture was stored at room temperature
for 12 h without disturbance. The nal solution was transparent
and the brown precipitates were collected at the bottom. For the
fabrication of devices, the precipitates could be dispersed by
shaking the mixed solution for several seconds. Fig. 1 shows the
typical scanning electron microscope (SEM) images of the
microrods deposited on Si substrates. The microrods have
Functional organic nano/microstructures have attracted
increasing interest due to their great potential as building
blocks of bottom-up assembled highly integrated miniaturized
devices.1–4 Up to now, most organic structures are single
component, i.e., consisting of one type of molecule. Meanwhile,
charge-transfer (CT) complexes, representing a typical two-
component system, are widely studied owing to their unique
conduction and magnetism.5–7 Moreover, recent studies indi-
cate that they are ideal building blocks as active layers and
source/drain electrodes in electronic devices.8–14 It should be
noted that most CT complexes have been prepared in bulk
single-crystalline form,15–18 which are fragile and difficult to
process in nano/microdevices.
In spite of their excellent opto/electronic properties, studies
on nano/microstructures of CT complexes are very limited. This
is possibly because fabrication of nano- and microscale struc-
tures of CT complexes with well-dened shapes is still a great
challenge. Recently, nanosheets of fullerene–ferrocene19 and
fullerene–cobalt porphyrin were prepared by Miyazawa et al.14
Microribbons of sulfur-bridged annulene–TCNQ were prepared
aSuzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences,
Suzhou 215123, P. R. China. E-mail: gbpan2008@sinano.ac.cn; Fax: +86-512-
62872663
bGraduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China
cInstitute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
† Electronic supplementary information (ESI) available: Experimental details,
chemical structures of DBTTF and TCNQ, schematic structures of triclinic
co-crystal of DBTTF–TCNQ. See DOI: 10.1039/c3tc30112e
2286 | J. Mater. Chem. C, 2013, 1, 2286–2289
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Fig. 3 (a) XRD pattern of the DBTTF–TCNQ microrods along with those of the
starting powders of DBTTF and TCNQ. (b) TEM image of a single DBTTF–TCNQ
microrod. The inset is the corresponding SAED image. The arrow indicates the
direction of the microrod.
Fig. 1 (a) and (b) SEM images of the DBTTF–TCNQ microrods recorded at
different magnifications.
well-dened shapes (i.e., rectangular cross-section) and smooth
surfaces (Fig. 1b). The width of the microrods is 1–2 mm, the
thickness is 0.5–1 mm, and the length is tens of microns. In
addition, the microrods could be produced on a large scale
when larger quantities of DBTTF and TCNQ solutions are mixed.
To conrm the existence and locate the distribution of
DBTTF and TCNQ molecules in the microrods, a scanning
transmission electron microscope (STEM) mapping analysis was
performed (Fig. 2). Both nitrogen and sulfur atoms are detected
in the same microrod, as shown in Fig. 2b and c, respectively. It
is clear that the distributions and concentrations of nitrogen
and sulfur are uniform across the whole microrod. This
nding indicates that nitrogen and sulfur atoms are uniformly
dispersed in the microrod. This is powerful evidence that the co-
crystal microrods consist of DBTTF and TCNQ molecules.
Fig. 3a represents the X-ray diffraction (XRD) pattern of
microrods along with those of the starting powders of DBTTF
and TCNQ. It can be seen that the XRD pattern of microrods is
different from those of the DBTTF and TCNQ. The two peaks
from microrods could be well indexed to the crystal planes of
(11ꢀ1) and (22ꢀ2) of DBTTF–TCNQ co-crystal with the triclinic
lattice.20 Moreover, the intensity of the (11ꢀ1) plane is much
stronger compared to the other peaks, indicating a preferred
growth along (11ꢀ1). Fig. 3b shows a typical transmission
electron microscope (TEM) image of a single microrod and the
corresponding selected area electron diffraction (SAED) image.
The TEM image reveals the formation of solid structures with
smooth surfaces. The well-dened pattern of SAED could be
recorded through the whole microrods (i.e., a single crystal) and
indexed as the [011] direction. This result is in good agreement
with the XRD pattern because the [011] direction is parallel with
the (11ꢀ1) plane and the (11ꢀ1) and (22ꢀ2) peaks are greatly
enhanced. Based on the above analysis, it is concluded that the
microrods are composed of DBTTF and TCNQ and have the
triclinic lattice reported by Kobayashi and Nakayama.20 In
addition, the DBTTF and TCNQ molecules stack alternately
along the [011] direction in the microrods (Fig. S2†).
Fig. 4a displays the Raman spectrum of microrods along
with those of the starting powders of DBTTF and TCNQ. The
Raman spectrum of microrods is almost the summed spectra of
the starting powders of DBTTF and TCNQ. The strong peaks
from DBTTF (marked by rectangles) and TCNQ (marked by
circles) can be identied in the spectrum of microrods. This
observation indicates that the microrods are composed of
DBTTF and TCNQ molecules. Fig. 4b are the ultra-violet visible
near-infrared (UV-Vis-NIR) spectra of microrods and the start-
ing powders of DBTTF and TCNQ. Compared with the spectra
of DBTTF and TCNQ, there is a new band between 700 and
2100 nm in the spectrum of microrods. The absorption centre is
at around 1400 nm. It has been reported that DBTTF–TCNQ
complex has an optical gap estimated as 0.9 eV,10 corresponding
to the wavelength of 1380 nm. Therefore, the new broad band
can be assigned as the CT band, i.e., the CT excitation from
DBTTF to TCNQ along the mixed stack.
In order to investigate the charge transporting properties of
the DBTTF–TCNQ microrods and explore their potential appli-
cations in miniaturized devices, prototype devices with the
bottom-gate bottom-contact geometry were fabricated on the
basis of individual microrods. The insets in Fig. 5a show
the schematic diagram (top-le) and the optical microscopy
Fig. 2 (a) SEM image and (b and c) corresponding STEM mapping images (b:
nitrogen; c: sulfur) of a single DBTTF–TCNQ microrod.
Fig. 4 (a) Raman spectra and (b) UV-Vis-NIR spectra of the DBTTF–TCNQ
microrods and the starting powders of DBTTF and TCNQ.
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image of the as-fabricated one-co-crystal device (bottom-right). electrodes. The result indicates that a better contact has been
Source–drain gold electrodes were fabricated by photo-lithog- formed between DBTTF–TCNQ microrod and gold electrodes
raphy and electron beam deposition of gold on Si substrate than that with bulk crystals. As a consequence, hole injection
covered with 300 nm thick SiO2. A drop of the mixed solution becomes feasible, and balanced ambipolar transport is
with dispersed microrods of DBTTF-TCNQ was directly depos- observed. Moreover, theoretical study has predicted that the
ited onto the prepatterned substrate. Randomly, some micro- DBTTF–TCNQ co-crystals have high mobility for both holes and
rods could be attached to the gold electrodes, which bridge the electrons.22 The predicted values have not been reached in the
source and drain electrodes. Fig. 5a and b show the typical present study, and higher performance can be expected by
output and transfer characteristics of the device based on an further device optimization.
individual microrod of DBTTF–TCNQ co-crystal. It can be seen
In summary, co-crystal microrods with rectangle cross-
that both the output and transfer characteristics demonstrate section and smooth surfaces are prepared via a facile solution
typical ambipolar charge transporting behavior. The electron method, fully characterized, and used for fabricating prototype
and hole mobilities calculated from the transfer characteristics devices. The result indicates that the microrods are composed
are 0.13 and 0.04 cm2 Vꢀ1 sꢀ1, respectively. These values are of DBTTF and TCNQ molecules, which are uniformly distrib-
higher than those calculated from the devices based on the uted and stack alternately along the [011] direction. Moreover, a
fullerene–cobalt porphyrin hybrid nanosheets and sulfur- characteristic broad CT band is observed at around 1400 nm.
bridged annulene–TCNQ microribbons with ambipolar The devices based on an individual microrod of DBTTF–TCNQ
characteristics.13,14
exhibit ambipolar charge transport characteristics with the
Previous studies on bulk single co-crystals of DBTTF–TCNQ electron mobility of 0.13 cm2 Vꢀ1 sꢀ1 and the hole mobility of
indicated that electron transport could be achieved when using 0.04 cm2 Vꢀ1 sꢀ1. The simple and low-cost solution method is
gold source and drain electrodes.10 Ambipolar charge transport expected to be useful for both fundamental study and practical
was achieved only when carrier injections were properly tuned application of donor/acceptor co-crystal nano-materials in
by using metallic CT complexes as source and drain elec- future miniaturized devices.
trodes.11 This is different from the present study in which
This work was nancially supported by the National Basic
ambipolar characteristics were obtained by using gold Research Program of China (no. 2010CB934100), the National
Natural Science Foundation of China (no. 21273272), and the
Chinese Academy of Sciences.
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