A General Method for Growing Two-Dimensional Crystals of Organic Semiconductors by “Solution Epitaxy”

Article abstract of DOI:10.1002/anie.201602781

Two-dimensional (2D) crystals of organic semiconductors (2DCOS) have attracted attention for large-area and low-cost flexible optoelectronics. However, growing large 2DCOS in controllable ways and transferring them onto technologically important substrates, remain key challenges. Herein we report a facile, general, and effective method to grow 2DCOS up to centimeter size which can be transferred to any substrate efficiently. The method named “solution epitaxy” involves two steps. The first is to self-assemble micrometer-sized 2DCOS on water surface. The second is epitaxial growth of them into millimeter or centimeter sized 2DCOS with thickness of several molecular layers. The general applicability of this method for the growth of 2DCOS is demonstrated by nine organic semiconductors with different molecular structures. Organic field-effect transistors (OFETs) based on the 2DCOS demonstrated high performance, confirming the high quality of the 2DCOS.

Full text of DOI:10.1002/anie.201602781

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201602781
German Edition: DOI: 10.1002/ange.201602781
Organic Semiconductors
A General Method for Growing Two-Dimensional Crystals of Organic
Semiconductors by “Solution-Epitaxy”
Chunhui Xu, Ping He, Jie Liu, Ajuan Cui, Huanli Dong, Yonggang Zhen, Wei Chen, and
Wenping Hu*
Dedicated to the 60th anniversary of Institute of Chemistry, Chinese Academy of Sciences
[13a,b]
Abstract: Two-dimensional (2D) crystals of organic semi-
conductors (2DCOS) have attracted attention for large-area
and low-cost flexible optoelectronics. However, growing large
2) excellent flexibility, 3) solution processability,
provid-
and solution-
techniques, for large-area, low-cost electronics.
[
13c]
ing versatile routes, such as inkjet printing
[
14]
shearing
2
DCOS in controllable ways and transferring them onto
However, over the past decade, research on 2DCOS is
seriously hampered by the lack of understanding of, and
absence of the corresponding technologies for, controllable
crystal growth. By careful molecular design, Jiang et al. cast-
assembled 2DCOS of 1,4-bis((5ꢀ-hexyl-2,2ꢀ-bithiophen-5-
yl)ethynyl)benzene (HTEB) with length and width of hun-
dreds micrometers and demonstrated that the charge trans-
port in organic field-effect transistors (OFETs) could occur in
technologically important substrates, remain key challenges.
Herein we report a facile, general, and effective method to grow
2
DCOS up to centimeter size which can be transferred to any
substrate efficiently. The method named “solution-epitaxy”
involves two steps. The first is to self-assemble micrometer-
sized 2DCOS on water surface. The second is epitaxial growth
of them into millimeter or centimeter sized 2DCOS with
thickness of several molecular layers. The general applicability
of this method for the growth of 2DCOS is demonstrated by
nine organic semiconductors with different molecular struc-
tures. Organic field-effect transistors (OFETs) based on the
[15]
single molecular layer near the gate insulator. Zhang et al.
grew fullerene 2DCOS with a lateral size of several micro-
[
16]
meters, and He et al. grew few-layer 2DCOS of dioctyl-
benzothienobenzothiophene (C8-BTBT) by van der Waals
[
17]
2
DCOS demonstrated high performance, confirming the high
epitaxy up to 80 micrometers. These results demonstrated
[
18]
quality of the 2DCOS.
the promising opportunity of preparing 2DCOS. Herein,
we report a novel approach of “solution epitaxy” to grow
2DCOS on water surface, wherein water surface acts as
a molecularly flat and defect-free substrate.
2
D
crystals are promising functional materials for next-
generation electronics due to their long range order on the
macroscopic scale in spite of being only a few atomic/
Our method for growth of 2DCOS is depicted in Figure 1.
Dozens of microliters of solution of molecular materials are
dropped onto water surface (Figure 1a), which then spread
quickly over the whole water surface through surface tension
(Figure 1b). Then, the molecules of organic semiconductors
begin to aggregate (Figure 1c) through strong p–p molecular
interactions, and evolve into micrometer-sized 2DCOS (Fig-
ure 1d). Using the micrometer-sized 2DCOS as seed crystals,
dropping new solution on them (Figure 1e), causes epitaxial
growth of the small 2DCOS, finally 2DCOS with sizes
a millimeter or even over a centimeter are obtained (Fig-
ure 1 f). For better control of the crystal growth, it is crucial to
choose a solvent and a solution concentration for this
[
1–3]
molecular layers thick.
crystal,
Mo, W; X = S, Se, Te, etc.),
considered to be good alternatives with band gaps, for
structural analogues of graphene. Two-dimensional crystals
of organic semiconductors (2DCOS) have demonstrated
unique advantages of 1) ideal band gaps and specific functions
Graphene is the foremost 2D
[
4–6]
and transition-metal dichalcogenides (MX , M =
2
[
7–9]
[10,11]
and black phosphorus
are
[12]
(
electronic, optical, or magnetic) by molecular design,
[
*] C. Xu, Dr. P. He, Dr. A. Cui, Dr. H. Dong, Dr. Y. Zhen, Prof. W. Hu
Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Science (ICCAS)
“
solution-epitaxy” that avoids polycrystalline growth. Mole-
Beijing 100190 (China)
E-mail: huwp@iccas.ac.cn
cules capable of engaging in multi-molecular interactions,
such as p–p, hydrogen bond, CÀH–p or CÀS bonds are also
Dr. J. Liu, Prof. W. Hu
[
19]
Tianjin Key Laboratory of Molecular Optoelectronic Sciences,
Department of Chemistry, School of Science
Tianjin University & Collaborative Innovation Center of Chemical
Science and Engineering (Tianjin)
important.
Figure 2 display images of optical, transmission electron
microscopy (TEM), and selected-area diffraction pattern
(SAED) of 2DCOS assembled through this method. The
Tianjin 300072 (China)
perylene 2DCOS up to several hundreds of micrometers are
available as shown in Figure 2a. The straight baseline and the
sharp diffraction peaks in X-ray diffraction (XRD) pattern
demonstrate the high quality of the 2DCOS (Figure S1 in the
Supporting Information). The corresponding secondary dif-
fraction peaks are also observed, manifesting the layer-by-
Prof. W. Chen
Department of Chemistry and Department of Physics, National
University of Singapore
3
Science Drive 3, 117543 Singapore (Singapore)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602781.
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layer growth of organic semiconducting molecules on sub-
strate. The quality of the 2DCOS can be further evaluated by
the full-width-at-half-maximum (FWHM) of the rocking
curve (Figure S2). The narrow width testifies highly ordered
alignment of planes in the out of-plane direction. The rocking
curves at 2q = 8.798 shows FWHM at 0.0298, which is
comparable with the values of dinaphtho[2,3-b:2’,3’-f]thieno-
[
3,2-b]thiophene (DNTT) crystals grown by physical vapor
[20]
transport.
A bright-field TEM image of a randomly
selected perylene 2DCOS on copper grid is shown in Fig-
ure 2b and its corresponding SAED is shown in Figure 2c.
Typical TEM images of 2DCOS show uniform morphology.
SAED at different parts show identical diffraction patterns
(
2
Figure S3), confirming the single crystalline nature of the
DCOS. Lattice constants calculated from SAED patterns is
b = 10.7 ꢁ, c = 11.5 ꢁ, which is consistent with that of the
[
21]
bulk crystal. Interestingly, it is found that this method is
wide applicable for assembling 2DCOS of other organic
semiconductors, for example, C6-DPA (Figure 2d–f), C6-
PTA (Figure 2g–i), and C6-DBTDT (Figure 2j–l), and very
nice 2DCOS are obtained with smooth surfaces over tens of
micrometers (as well as other compounds Figure S4,5)
demonstrating the general applicability for 2DCOS growth.
Powder XRD of the 2DCOS show intense peaks, for example,
C6-DBTDT at 2q = 3.388, which correspond to d-spacing of
Figure 1. Schematic diagrams of growing 2DCOS, a) Dropping solu-
tion onto water surface, b) solvent evaporation resulting in molecular
aggregation, c) molecular assembly by p–p interaction, d) self-assem-
bled small 2DCOS on water surface, e) epitaxy growth based on small
2
DCOS, f) large 2DCOS on water surface.
2
6.4 ꢁ (Figure S6). Lattice constants calculated from SAED
pattern (e.g., C6-DBTDT, a = 4.1 ꢁ, b = 10.9 ꢁ) are consis-
[
22]
tent with the data of bulk crystal (Figure 2l). All of the
diffraction patterns of SAED at different sites find no signs of
noticeable polycrystalline diffraction and apparent angle
rotation of the lattice direction throughout the entire
domain (Figure S7). The results confirm that the thin film is
a single crystal. C6-DBTDT adopts a herringbone packing
[23]
motif with p–p overlap between adjacent molecules (Fig-
ure S8). The p–p stacking distance is 3.545 ꢁ. Moreover,
short S–S contacts (3.530 and 3.793 ꢁ) and S–C contacts
(3.427 ꢁ) exist in another direction and form a 2D network of
intermolecular force. The strong 2D interactions may be the
[
24]
main driving force to grow large-area 2DCOS and realize
[23]
high-performance devices.
AFM) images show that the 2DCOS generally have a thick-
Atomic force microscopy
(
ness of 3–5 nm (Figure S9). For example, the thickness of C6-
DPA 2DCOS is 4.53 nm, C6-DBTDT is 5.01 nm, and PDIF-
CN is 4.91 nm. Considering the molecular length of C6-DPA
2
at 29.8 ꢁ, and that the molecules stand on a substrate, it
suggests and 2–3 molecular layer 2DCOS for C6-DPA, C6-
DBTDT, and PDIF-CN2.
Epitaxy is a powerful method to grow high-order struc-
tures on crystal surfaces with accurate lattice matching. Seed-
crystal-mediated-growth reportedly works effectively both
for organic crystals, for example, the growth of copper
phthalocyanine nanoribbon crystals) and graphene materi-
[
25,26]
als.
Pleasingly, we found that the small-size 2DCOS
obtained on the water surface could be used as seed-crystals
for new cycles of crystal growth by dropping the same organic
solution on the small-size 2DCOS on the water surface
Figure 2. Characterization of micrometer sized 2DCOS. Optical mi-
croscopy images (left), TEM images (middle), SAED patterns (right) of
(Figure 1e). As soon as the new solution is added, the small
2
DCOS of a)–c) perylene, d)–f) C6-DPA, g)–i) C6-PTA, and j)–l) C6-
2DCOS is etched a little by the new solution. Subsequent
epitaxial growth from the small 2DCOS, gives large 2DCOS
DBTDT.
2
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(
Figure 1 f). Indeed, such “solution-epitaxy” works effec-
tively, over mm sized C6-DBTDT 2DCOS are obtained
Figure 3a) as well as of C6-DPA and C6-PTA (Figure S10).
facile growth process (Figure S11). A substrate is inserted at
an angle into the water just below the water surface. With
water evaporation, the 2DCOS will fall on substrate auto-
matically, and deposit the 2DCOS tightly and smoothly on
substrate.
(
Cross-polarized optical micrographs of individual C6-
DBTDT 2DCOS were obtained. The uniform light extinction
over 7 mm in the cross-polarized optical micrographs indi-
cates the single-crystalline nature of the C6-DBTDT 2DCOS
(
1
The quality and potential application of the 2DCOS are
examined by organic field-effect transistors (OFETs).
Bottom-gate top-contact device configuration based on
individual 2DCOS is adopted. Au stripes are stamped on
Figure 3b–c). The thickness of C6-DBTDT 2DCOS is
4.13 nm, suggesting the 2DCOS are 5–6 molecular layers
Figure 3d). SAED of individual C6-DBTDT 2DCOS at
[
27]
(
2DCOS as source and drain electrodes and OTS-modified
different parts show identical diffraction patterns (Figure 3e–
i). It confirms that the single crystalline nature of the large-
size 2DCOS.
Si/SiO (300 nm) is used as substrate (Figure S12). The carrier
mobility is extracted from their saturated region character-
2
2
istics by the equation of IDS = (W/2L)C m(V ÀV ) . All
i
G
T
2
DCOS of perylene are obtained with size even near 1 cm
devices exhibit high performance with well-defined linear
and saturation regimes at ambient conditions and room
temperature (Figure 4, Figure S13,S14). For example, pery-
(
Figure 3j). XRD results of the large 2DCOS and SAED of
individual 2DCOS at different parts show identical diffraction
patterns as identical as that of micrometer sized crystals (inset
of Figure 3j). All features of the large size 2DCOS are
identical as those of small size 2DCOS, confirmed the high
quality of the cm size 2DCOS. Finally, it should be addressed
that a pre-requirement to apply the 2DCOS in electronic
devices is to transfer them to solid substrates efficiently. Our
2
À1 À1
lene OFETs possess an average mobility of 0.12 cm V
s
2
À1 À1
and the highest mobility up to 0.18 cm V
s
(Figure 4a,b),
[28]
among the best results of perylene based OFETs. Similarly,
2
À1 À1
the averaged mobility of C6-DPA reach 2.4 cm V s , and
2
À1 À1
the maximum value is as high as 4.0 cm V
s
(Figure 4c,d),
2
DCOS can be easily transferred to any substrate due to their
Figure 3. Characterization of mm and cm sized 2DCOS. a) Optical
microscopy image of several mm sized 2DCOS of C6-DBTDT,
b),c) Cross-polarized optical micrographs of the C6-DBTDT 2DCOS,
and the uniform color change over the entire micrographs confirms
that the 2DCOS is a single crystal, d) AFM image, indicating the
thickness of 2DCOS at 14.13 nm, 5–6 molecular layers. e) SEM and
f)–i) SAED of individual C6-DBTDT 2DCOS, the identical SAEDs at
different parts of the 2DCOS in (e) confirming its single crystal nature,
j) Optical microscopy image of cm sized 2DCOS (blue) of perylene,
inset: XRD and SAED of the crystal.
Figure 4. Transistor characteristics of the 2DCOS. Typical transfer and
output characteristics of the OFETs based on the 2DCOS, a),b) per-
ylene, c),d) C6-DPA, e),f) C6-PTA, g),h) C6-DBTDT on OTS SAM modi-
fied Si/SiO substrates. The different colored lines in (a) (c) (e),and
2
(g) correspond to the different gate voltages.
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which is 7 times higher than that of C6-DPA polycrystalline
films (Figure S15). OFETs of C6-PTA 2DCOS exhibit
W.C. acknowledges Singapore MOE Tier II grant R143-000-
559-112 and Singapore NRF-CRP grant R143-001-608-281.
2
À1 À1
averaged mobility of 0.63 cm V s , and the maximum
2
À1 À1
value is as high as 1.3 cm V
s
(Figure 4e,f), which is
Keywords: 2D materials · organic crystals · organic electronics ·
about 28 times higher than that of PTA thin films
organic field-effect transistors · self-assembly
2
À1 À1 [29]
(
0.045 cm V s ). The averaged mobility of C6-DBTDT
2
À1 À1
devices is around 1.6 cm V s , with the maximum value as
2
À1 À1
high as 2.8 cm V
s
(Figure 4g,h). Moreover, minor hyste-
resis and bias stress effects are observed in these devices (e.g.,
Figure S16), indicating the negligible charge traps between
the 2DCOS and gate dielectric. The summary of all device
performance is given in Table 1, demonstrating the high
performances of the devices and high quality of the 2DCOS.
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Table 1: Transistor performance of different kind 2DCOS.
Materials mave
mmax
(cm V
On/off ratio
VT[V]
2
À1 À1
2
À1 À1
(
cm V
s
)
s
)
6
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C8-BTBT
C6-DPA
C6-DBTDT 1.6[0.64]
C6-PTA
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DH6T
Ph-Ant
PFIF-CN2
DFH4T
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2.4[0.92]
11.2
4.0
2.8
1.3
0.18
9ꢀ10 –5ꢀ10 À1.7–À8.9
6
7
4ꢀ10 –8ꢀ10 À0.2–À1.5
3ꢀ10 –1ꢀ10 À3.3–À14.7
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0.63[0.33]
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0.011[0.0013] 0.012
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3ꢀ10 –1ꢀ10 À3.3–À17.4
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grow uniform, large-area and high quality 2DCOS. By
“
solution-epitaxy” growth crystal on water surface, 2DCOS
[
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4
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Received: March 19, 2016
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Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Organic Semiconductors
C. Xu, P. He, J. Liu, A. Cui, H. Dong,
Y. Zhen, W. Chen, W. Hu*
&&&& — &&&&
A General Method for Growing Two-
Dimensional Crystals of Organic
Semiconductors by “Solution-Epitaxy”
Water of crystallization: A novel method
named “solution epitaxy” for the growth
of 2D crystals of organic semiconductors
micrometer-sized 2DCOS on water sur-
face, and then epitaxial growth of them
into centimeter sized 2DCOS with thick-
ness of several molecular layers.
(
2DCOS) is based on the self-assemble
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!