High-performance n-type organic transistor with a solution-processed and exfoliation-transferred two-dimensional crystalline layered film

Article abstract of DOI:10.1021/cm301775c

High-performance n-type organic field-effect transistors (OFETs) based on 2-dimensional (2D) crystalline layered films of the novel dicyanodistyrylbenzene (DCS) derivative (2Z,2'Z)-3,3'-(1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile) (CN-TFPA) were fabricated using a simple solution process. The OFETs showed electron mobilities of up to 0.55 cm² V^-1 s^-1, which was attributed to the appropriate electron affinity and dense molecular packing in the well-ordered 2D terrace structure. Because of the easy exfoliation capabilities of the CN-TFPA 2D crystalline layers, 2-10 CN-TFPA molecular monolayers could be successfully transferred onto the substrates, enabling the fabrication of ultrathin OFET devices with an active layer thickness of ～30 nm.

Full text of DOI:10.1021/cm301775c

Article
pubs.acs.org/cm
High-Performance n‑Type Organic Transistor with a Solution-
Processed and Exfoliation-Transferred Two-Dimensional Crystalline
Layered Film
Sang Kyu Park,† Jong H. Kim,† Seong-Jun Yoon,† Oh Kyu Kwon,† Byeong-Kwan An,‡
and Soo Young Park*,†
†Creative Research Initiative Center for Supramolecular Optoelectronic Materials and WCU Hybrid Materials Program, Department
of Materials Science and Engineering, Seoul National University, ENG 445, Seoul, 151-744, Korea
‡Department of Chemistry, The Catholic University of Korea, Bucheon-si, Geyonggi-do 420-753, Korea
S
* Supporting Information
ABSTRACT: High-performance n-type organic field-effect transistors (OFETs) based
on 2-dimensional (2D) crystalline layered films of the novel dicyanodistyrylbenzene
(DCS) derivative (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)-
acrylonitrile) (CN-TFPA) were fabricated using a simple solution process. The OFETs
showed electron mobilities of up to 0.55 cm2 V−1 s−1, which was attributed to the
appropriate electron affinity and dense molecular packing in the well-ordered 2D
terrace structure. Because of the easy exfoliation capabilities of the CN-TFPA 2D
crystalline layers, 2−10 CN-TFPA molecular monolayers could be successfully
transferred onto the substrates, enabling the fabrication of ultrathin OFET devices
with an active layer thickness of ∼30 nm.
KEYWORDS: single-crystalline organic field-effect transistor, n-type semiconductor, two-dimensional single-crystal, solution-process,
mechanical cleavage
withdrawing groups essential for the lowering of the LUMO
level in n-type building blocks may destroy the highly crystalline
structure or decrease the degree of molecular ordering, owing
to their bulkiness and the presence of electrostatic forces. As a
result, only a limited number of n-type π-conjugated systems
have been introduced so far; these include perylenediimide,4
naphthalenediimide,5 and oligothiophene.6 Very recently, π-
conjugated dicyanodistyrylbenzene (DCS)-type derivatives
with electron-withdrawing trifluoromethyl (−CF3) units have
also proven their promising application potential toward high-
performance n-type OFETs, because of their unique structural
features.7−9 The “twist elasticity” behavior of the DCS
backbone, which exhibits large torsional changes in the π-
conjugated backbone from the isolated solution state to the
aggregated solid state, played an important role in achieving a
coplanar structure, leading to close π−π contacts in the self-
assembled condensed state.10 In addition, the electron-
withdrawing −CN and −CF3 groups not only decreased the
LUMO to a suitable level, but also stabilized the laterally
adjacent molecules by building up specific noncovalent
intermolecular interactions, which led to tight and dense
molecular packing.11
INTRODUCTION
■
In recent years, single-crystalline organic semiconductors have
been intensively studied as a promising alternative to
amorphous silicon-based materials in a wide range of electronic
applications, especially organic field-effect transistors
(OFETs).1 Single-crystalline organic semiconductors are
particularly suited to OFETs because of characteristics such
as their high carrier mobility, which derives from the lack of
grain boundaries and the minimal number of charge trap sites,
and their merits in terms of ease of processing and deposition
on flexible substrates. In addition, the solution-processed self-
assembly of organic semiconductors offers opportunities for the
low-cost and large-area processing of next-generation elec-
tronics. Simple solution-processed single-crystalline OFETs
(SC-OFETs) have therefore attracted much interest;2 to date, a
variety of high-performance p-type SC-OFETs fabricated via
various solution processes have been successfully demonstrated,
and some of them have shown performances far superior to
that of amorphous silicon.3
In contrast with solution-processed p-type SC-OFETs,
however, it remains a challenge to fabricate solution-processed
n-type SC-OFETs, because the building blocks for high-
performance n-type SC-OFETs inherently have much more
complicated molecular design prerequisites; specifically, they
should show not only dense molecular packing and good
solubility, but also high electron affinity (or low lowest
unoccupied molecular orbitals (LUMOs)) for an efficient
electron injection from electrodes. In particular, the electron-
In the course of the continuous systematic syntheses of the
DCS-backbone building blocks for the n-type SC-OFET
Received: June 7, 2012
Revised: August 1, 2012
Published: August 1, 2012
© 2012 American Chemical Society
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spectrophotometer. Differential scanning calorimetry (DSC) was
recorded using a Perkin-Elmer DSC7.
materials, we found that one of our newly synthesized DCS
derivatives, (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(3,5-bis-
(trifluoromethyl)phenyl)acrylonitrile) (CN-TFPA) (Scheme
1), possessed a strong tendency to form well-ordered two-
Sample Preparation. The nanoparticle suspension samples were
prepared by typical simple precipitation method employing tetrahy-
drofuran and distilled water as solvent media. The CN-TFPA was
dissolved in tetrahydrofuran with a concentration of 5 × 10−3 mol L−1.
Then distilled water was slowly injected in various volume fractions
with total concentration of 5 × 10−6 mol L−1. The suspensions were
left for at least 2 h before absorption spectra measurements. The bulk
single-crystals of CN-TFPA were obtained via a solvent diffusion
crystal growth method using ethyl acetate/methanol.
Scheme 1. Synthetic Procedure to Obtain CN-TFPA
Device Fabrication. To prepare the substrates, we rinsed the
substrates with acetone and isopropyl alcohol for 10 min under
ultrasonication. After rinsing, 10 min of UV (360 nm) O3 treatment
was applied.
dimensional (2D) crystals in large areas directly on the
substrate, via a simple solution-processed self-assembly process.
In this Article, we report that the 2D crystalline layer structure
of CN-TFPA, which is conceived as a new, advantageous
feature of the DCS π-conjugated system to build up high n-type
conducting channels, can serve as an effective scaffold for the
fabrication of solution-processed, high-performance, ultrathin
n-type SC-OFETs; this is due to its dense molecular packing,
low LUMO levels, and the fact that it can be easily exfoliated to
give thin-layered molecular assembly sheets.
For simple solution processable single crystalline field-effect
transistor fabrication, we employed the solvent evaporation crystal
growth technique. We leaned an SiO2/Si wafer on the inner wall of a
20 mL vial containing a 0.05 wt % CN-TFPA solution (in
dichloromethane), under ambient conditions, as depicted in
Supporting Information, Figure S3. CN-TFPA single crystal films
were spontaneously grown (directly and slowly) on the substrates,
which resulted in firm contact between substrates and the crystal
films.12 The resulting crystals were found to have two different surface-
induced solid-state packing structures (G-phase and B-phase, vide
infra). After crystal formation, the substrates were annealed at 50 °C
for 1 h, to eliminate any residual solvent. 50 nm-thick source-drain Au
electrodes were thermally deposited directly onto the crystalline layers
through the metal mask, to fabricate top-contact SC-OFETs (the
deposition rate was 0.1−0.2 Å/s) (Supporting Information, Figure
S4).
For thermally evaporated vacuum deposited poly crystalline FETs
fabrication, we introduced an octadecyltrichlorosilane (ODTS) layer
for reduced charge trap sites as well as for domain enlargement. ODTS
was treated in vapor phase in a vacuum oven; then the substrates were
brought into nitrogen filled glovebox. Thirty nm thick CN-TFPA
active layers were thermally deposited with deposition rate of 0.1−0.2
Å s−1 and different substrate temperatures (TSUB) (RT, 50, and 70 °C),
under a vacuum of 7 × 10−7 Torr.
To fabricate the transfer-printed thin-crystal OFETs via a
mechanical cleavage method, we prepared mother crystals of 2D
crystalline layers for the G/B-phase, using the solvent evaporation
crystal growth technique. Scotch tape (3M) was attached to the top of
the 2D crystalline layers, and pressed gently using fingers. By detaching
the Scotch tape, we could obtain exfoliated, flaky, thin crystals from the
mother G/B-phase crystals. Finally, we pressed the Scotch tape with
the thin crystals onto the other SiO2/Si substrate, so that we could
transfer the thin crystals by detaching the Scotch tape. All of these
procedures were carried out under ambient conditions. Finally, we
took the samples into the nitrogen-filled glovebox, and thermally
evaporated the Au electrodes, as presented in the fabrication of SC-
OFETs section.
EXPERIMENTAL SECTION
■
Materials. CN-TFPA was synthesized via Knoevenagel condensa-
tion between 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile and ter-
ephthalaldehyde, as shown in Scheme 1. All chemicals were purchased
commercially and used without further purification. Terephthalalde-
hyde (0.8 g) and 2-(3,5-bis(trifluoromethyl)phenyl)acetonitrile (3.0 g)
were dissolved in 50 mL of t-butyl alcohol. A 0.84 mL aliquot of
tetrabutylammonium hydroxide was slowly dropped into the solution,
using a reaction temperature of 50° and a reaction time of 2 h. The
precipitated product, (2Z,2′Z)-3,3′-(1,4-phenylene)bis(2-(3,5-bis-
(trifluoromethyl)phenyl)acrylonitrile), was then filtered and purified
by flash column chromatography with dichloromethane. For further
purification, the product was recrystallized in a dichloromethane/
methanol solution. Finally, sublimation purification was conducted
under high vacuum (below 1 × 10−6 Torr), and a bright greenish-
yellow powder was obtained as the product. The chemical structure of
CN-TFPA was confirmed using 1H NMR, 13C NMR, elemental
analysis (EA), and mass spectroscopy (vide infra).
1H NMR (300 MHz, CDCl3) δ [ppm]: 8.13 (s, 4H, Ar−H), 8.10 (s,
4H, Ar−H), 7.95 (s, 2H, Ar−H), 7.69 (s, 2H, vinyl-H). 13C NMR
(500 MHz, CDCl3) δ [ppm]: 143.70, 136.55, 135.58, 133.60, 133.33,
133.06, 132.79, 130.55, 126.42, 124.20, 123.46, 122.02, 116.81, 111.23.
MS (FAB) (m/z): Calculated for C28H12F12N2: 604.08, Found:
604.08. Anal. Calculated for C28H12F12N2: C, 55.64; H, 2.00; F,
37.72; N, 4.63. Found: C, 55.71; H, 1.98; N, 4.63.
Characterization. 1H NMR was recorded using a Bruker, Avance-
300 (300 MHz) instrument, in a CDCl3 solution. 13C NMR was
recorded using a Bruker, Avance-500 (500 MHz) instrument, in a
CDCl3 solution. Elemental analysis was conducted on CN-TFPA using
a CE Instruments, EA1110 elemental analyzer. The mass spectrum of
CN-TFPA was measured using a JEOL, JMS-600W mass spectrom-
eter. Out-of-plane X-ray diffraction measurements were performed
using a D8-Advance X-ray diffractometer (Bruker Miller Co.,
Germany), and the operating conditions were a step size of 0.02, a
scan rate of 3 degree/min, a 40 kV generator voltage, a 40 mA tube
current, and room temperature (Cu std target λ = 1.5418 Å). Atomic
force microscopy (AFM) was performed using a PSIA instrument XE-
150 and a Bruker instrument multimode with a NanoScope V
controller. AFM topographic and phase images were recorded
simultaneously in either contact or noncontact mode. UV−visible
absorption spectra were recorded on a Shimadzu, UV-1650 PC
spectrometer. The photoluminescence (PL) spectra of each solid-state
phase were measured using a Varian, Cary Eclipse, fluorescence
Transistor Measurements. The I−V characteristics of all of the
OFETs were measured in a nitrogen-filled glovebox, using a Keithley
4200 SCS instrument connected to a probe station. The mobility
values and Vth were obtained from the transfer curve in the saturated
regime. For the charge carrier mobility calculations, we checked the
source-drain channel width and the length of the individual devices
using an optical microscope.
RESULTS AND DISCUSSION
■
CN-TFPA was prepared in a one-step Knoevenagel con-
densation reaction (Scheme 1). Because of the unique twisting
conformation and polar substituents of CN-TFPA, it showed
good solubility in common organic solvents such as dichloro-
methane, chloroform, and 1,2-dichloroethane; this solubility
was attained despite the absence of the long alkyl chains that
are commonly used to achieve good solubility in the rigid π-
conjugated system, but are inactive and undesirable for the
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tuning of the electronic properties. Upon slow solvent
evaporation, CN-TFPA molecules in a 0.20 wt % 1,2-
dichloroethane solution readily self-assembled into a large,
single crystal with terraced surfaces (Figure 1a). The step-
plane) is predicted to be much weaker than that along the
direction c (in-plane), because of the smaller number of H-
bonding sites (8 vs 4 for H-bonding along directions c and a,
respectively), and the intrinsically weak nature of the H-
bonding between the F in the−CF3 group and the phenyl
HC−.14 Consequently, this strong anisotropic intermolecular
interaction between the plane bc and the direction a led to thin
2D lamellar CN-TFPA supramolecular structures with low-
energy surfaced layers. It is worthy of note that both the −CN
and −CF3 units play a decisive role not only in achieving tight
molecular packing and constructing a 2D layer structure via
specific H-bonding, but also in efficiently stabilizing the LUMO
level of CN-TFPA (HOMO: −6.98 eV, LUMO: −4.14 eV,
bandgap: 2.84 eV, calculated from ultraviolet photoemission
spectroscopy (UPS) and the optical band gap of vacuum
deposited film, see Supporting Information, Figure S5a) via
their electron-deficient characters; this stability is desirable for
an efficient electron injection from the metal electrodes.
To form thin, large-area, well-ordered 2D crystalline layered
CN-TFPA films for SC-OFET applications, we employed a
simple “solvent evaporation crystal growth technique”
(Supporting Information, Figure S3), in which the SiO2/Si
substrates were leaned on the inner wall of a 20 mL vial of
0.20−0.05 wt % CN-TFPA solution in 1,2-dichloroethane or
tetrahydrofuran, during a slow solvent evaporation process in
ambient conditions. Ultrauniform 2D CN-TFPA crystalline
layer films with a thickness of 100−200 nm (approximately
60−130 layers) were grown directly on the substrate with x-y
dimensions of a few millimeters; the films exhibited a green
fluorescence emission (Figure 2a), similar to that shown in bulk
single crystals of CN-TFPA. Interestingly, it was also found that
blue fluorescent CN- TFPA crystalline films (Figure 2b) could
be developed and isolated when dichloromethane or chloro-
form was used as a solvent medium; this was probably due to
the different solvent properties, such as the polarity, boiling
point, and vapor pressure.15 The obtained blue fluorescent CN-
Figure 1. Fluorescent image of a single CN-TFPA crystal, taken using
an optical microscope (circled image insets: birefringence image of
same crystal using polarized optical microscope) (a). AFM image
(scale bar: 1 μm) of a single CN-TFPA crystal (insets: thickness
profile along dashed line) (b). Molecular stacking observed from
single-crystal XRD analysis (the directions of the intermolecular
interactions are illustrated using white arrows) (c, d).
terrace morphology was observed using atomic force
microscopy (AFM), and was found to have a step size of
approximately 1.4 nm, as shown in Figure 1b. The crystal films
showed strongly anisotropic characteristics, as indicated by the
uniform extinction of the reflected light intensity when the
crystal films were rotated 45° to the crossed polarizers
(Figure 1a inset).
To elucidate the 2D terrace structure of the CN-TFPA
molecules in the single-crystal state, X-ray diffraction (XRD)
analysis was performed on a single-crystal grown via solvent-
diffusion crystal growth method. The molecular conformation
and stacking structure of CN-TFPA are depicted in Figure 1c,
d. It was clearly seen that the CN-TFPA had a quasi-planar
molecular conformation in the condensed crystal state,
although the twisted structure was more favored in the isolated
state (see also Supporting Information, Figure S2), owing to its
unique “twist elasticity” behavior. The quasi-planar CN-TFPA
molecules bearing abundant electron-deficient groups were
responsible for the strong π−π stacking interactions and the
close interlayer distance of 3.09 Å along the b direction.13 In
addition, the continuously aligned H-bonding between the N of
the cyano group and the two HC− of the laterally adjacent
phenyl groups along direction c (8 different −N···HC−
interactions (2.65, 2.72 Å) per molecule) (Figure 1d) played
an essential role in producing the 2D crystal growth. The CN-
TFPA molecules formed a layer-by-layer lamellar structure that
was created via H-bonding between the F of the −CF3 groups
and the HC− of the adjacent phenyl groups along the direction
a (4 different −F···HC− interactions (2.59 Å) per molecule),
which was perpendicular to the plane bc. However, the
intermolecular H- bonding along the direction a (out-of-
Figure 2. Fluorescent images of 2D crystalline CN-TFPA layers grown
by solution evaporation crystal growth (scale bar: (a) 100 μm, (b) 50
μm). Molecular monolayer and bilayer from B-phase 2D sheet,
measured using AFM (scale bar: 1 μm, inset: thickness profile along
white dashed line) (c). Out-of-plane XRD analysis of the B-phase
(blue line), G-phase (green line) and bulk single crystal (red line)
(insets: schematic drawings of molecular stacks of each phase, with
slipped angles to the plane bc) (d).
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TFPA crystalline films also exhibited a neat step-terrace
structure (Figure 2c) (hereinafter green and blue fluorescent
CN-TFPA crystalline layers are denoted as G-phase and B-
phase, respectively). In the out-of-plane XRD analysis of the G-
phase and B-phase crystalline layers, these two layers were
found to have a different slipped angle for the DCS π-
conjugated backbone (Figure 2d). The plane of the DCS
backbone in the G-phase was slipped with respect to the plane
bc with a dihedral angle of about 47.2° (d100: 1.37 nm), whereas
that in the B-phase was more perpendicular to the plane bc,
with a dihedral angle of about 58.8° (d100: 1.63 nm). In terms of
carrier mobility, the B-phase was expected to be more efficient,
since the π-electronic communication among neighboring
molecules in the B-phase was better than that in the G-
phase. By comparing out-of-plane X-ray diffraction peaks of
bulk single-crystal with peaks of 2D crystalline layers (Figure
2d), we could assign that the G-phase 2D terrace structure have
same packing motif with that of bulk single-crystal. Moreover,
we could also observe coincidence between the PL spectra of
bulk-crystals grown in solution and G-phase 2D terrace
structure grown via solvent evaporation crystal growth
technique because of the equivalent molecular packing states,
as shown in Supporting Information, Figure S5b.
Table 1. Characterized Electrical Properties of CN-TFPA
SC-OFETs
μe [cm2 V−1
a
b
c
sample
s−1
]
Ion/Ioff
104
Vth[V]
n
d
G-phase 2D layered films 3.0 × 10−2
41
10
10
5
(1.1 × 10−2
)
)
)
)
)
)
d
B-phase 2D layered films 5.5 × 10−1
105
45
38
44
51
50
(4.6 × 10−1
exfoliated B-phase 2D
5.3 × 10−2
104−105
106
e
layered films
(2.9 × 10−2
vacuum deposited film
5.9 × 10−2
(3.8 × 10−2
9
f
(TSUB = RT)
vacuum deposited film
(TSUB = 50 °C)
vacuum deposited film
(TSUB = 70 °C)
2.3 × 10−1
106
9
f
(1.8 × 10−1
3.4 × 10−1
(2.0 × 10−1
106
9
f
a
b
Maximum and average electron mobility value. Average threshold
c
d
voltage. Number of devices tested. Devices fabricated by solvent
evaporation crystal growth technique. Device fabricated by mechan-
ical cleavage method. Device fabricated by vacuum deposition
e
f
technique with ODTS treated SiO2/Si substrate.
An investigation of the electrical performances of the G- and
B-phase 2D layer films was carried out by fabricating top-
contact SC-OFETs via the thermal deposition of Au electrodes
through metal masks (details in Experimental Section). The
SC-OFETs based on the G-phase 2D crystalline layer films
showed typical n-type OFET behaviors, with electron mobility
(μe) values as high as 0.03 cm2 V−1 s−1, and an on/off current
ratio of 104 (Figure 3a). As expected, however, μe was
Figure 4. Schematic illustrating the measurement of the anisotropic
field-effect mobility of the B-phase (scale bar: 200 μm) (a). Eight n-
channel mobility values from the single crystalline B-phase 2D layer
were measured every 45° (the value at 315° was not obtained) (red
arrow in (b): speculated direction formed by the π−π interaction) (b).
by the small blue arrow. The μe values in the directions
indicated by the large red and small blue arrows were assumed
to result from contributions from π−π stacking and H-bonding
(−CN···HC−), respectively. Importantly, the μe values along all
directions exceeded 0.15 cm2 V−1 s−1, indicating that the B-
phase CN-TFPA SC-OFETs are very promising for practical
OFET applications.
Figure 3. Measured n-type characteristics of the CN-TFPA crystal
devices (a: G-phase, b: B-phase).
Besides the dense and tight molecular packing derived from
the high-quality 2D crystals, the self-assembled CN-TFPA
molecules possessed additional unique 2D crystalline layer
properties, specifically strong in-plane but weak out-of-plane
intermolecular interactions, which made it possible to exfoliate
molecular sheets from the bulk 2D crystalline layers along the
out-of-plane direction (direction a). The exfoliated thin 2D
crystalline sheets could be used as an attractive platform for the
construction of ultrathin n-type SC-OFETs. To test this
possibility, we tried to exfoliate molecular layers from the B-
phase 2D crystalline layer films via the “mechanical cleavage
method”, using cellophane tape; this imitates the early stage
technique for obtaining graphene from graphite16,17 (details in
Experimental Section, Figure 5c). It was found that 2−10
monolayers (ca. 3−15 nm thickness) of CN-TFPA with neat
terrace structures were stripped and successfully transferred
onto the target SiO2/Si substrates, binding tightly in the plane
bc to maintain a large sheet area (ca. 100−200 μm) (Figure 5).
The AFM image and surface profile in Figure 6a show that the
CN-TFPA terrace films transferred onto the SiO2/Si substrates
dramatically improved by more than 18 times in the B-phase
SC-OFETs (μe = 0.55 cm2 V−1 s−1), most likely because of the
larger π−π overlaps, which led to enhanced electronic
communication (Figure 3b). For comparison, the poly
crystalline FETs were also fabricated by vacuum thermal
deposition, and a μe value as high as 0.34 cm2 V−1 s−1 was
obtained at TSUB of 70 °C, which is still lower than that from B-
phase 2D layered film-based SC-OFETs because of the large
amount of grain boundaries and possible impurities in the poly
crystalline state (see Supporting Information, Figure S6, 7 and
Table 1 for detailed information). The 2D crystalline
characteristics of the B-phase SC-OFETs were further
confirmed in an anisotropic field-effect mobility study. Figure
4 shows that the anisotropic dependence of the mobility values
was clearly observed when varying the measuring directions;
the μe in the direction indicated by the large red arrow was
approximately 2 times larger than that in the direction indicated
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practicable which is suitable for flexible SC-OFETs with
reasonable throughput device processing.
CONCLUSIONS
■
In conclusion, we report herein that CN-TFPA molecules
acting as building blocks for high-performance n-type SC-
OFETs showed a strong tendency to self-assemble into highly
ordered 2D crystalline terrace structures with lowered LUMO
levels; these structures are denoted here as G-phase and B-
phase layers. These layers were deposited on substrates over a
large area via a simple solution process that exploited the
unique CF3-substituted DCS molecular system. The SC-
OFETs of the B-phase CN-TFPA 2D crystalline layer films
showed electron mobilities (μe) of up to 0.55 cm2 V−1 s1−.
Small numbers (2−10) of CN-TFPA monolayers with neat
terrace structures could be exfoliated and transferred onto the
target substrates using a simple “mechanical cleavage” method.
The top-contact OFETs based on the exfoliated CN-TFPA 2D
ultrathin crystalline layers (which had thicknesses of ∼30 nm)
also exhibited excellent n-type transistor behavior.
Figure 5. Fluorescent images of exfoliated B-phase 2D crystal on
Scotch tape (scale bar: 30 μm) (a) and on SiO2 substrate after the
retransfer process (scale bar: 20 μm) (b). Schematic illustration of the
mechanical cleavage method (c).
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic information, schematic illustration of exper-
imental methods, UV−visible spectra, PL spectra and optical
microscope images of OFETs, crystallographic data (CCDC
885399). This material is available free of charge via the
Internet at http://pubs.acs.org. CCDC 885399 contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/data_
request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 6. AFM image (scale bar: 1 μm) of an exfoliated B-phase 2D
crystal on a SiO2 substrate (inset: thickness profile along dashed white
line) (a). Measured n-type transfer characteristics of the exfoliated B-
phase 2D crystal (b).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: parksy@snu.ac.kr.
Notes
The authors declare no competing financial interest.
were of good quality, and had the same terrace height (1.64
nm) as the mother B-phase 2D crystalline films (1.60 nm, d100
:
ACKNOWLEDGMENTS
1.63 nm). Top-contact OFETs based on these exfoliation-
transferred crystalline layer filmsand their mother crystals
were successfully fabricated, and their transistor properties were
investigated. Figure 6b shows that the OFETs based on these
exfoliated 2D crystalline layers with a thickness of ∼30 nm
exhibited good n-type transistor behavior, with μe values of up
to 0.053 cm2 V−1 s−1; their mother crystals exhibited μe values
of up to 0.30 cm2 V−1 s−1 (Supporting Information, Figure S8),
which were comparable to the value shown by the intact B-
phase 2D crystalline film. We believe that the lower μe values
(compared with the OFETs based on the B-phase 2D
crystalline layer films, for which μe = 0.55 cm2 V−1 s−1)
resulted from physical contact problems occurring during the
retransfer procedures. Further experimental investigation is
required to estimate the effects of changes in the thickness of
the active organic semiconducting layers on μe. Nevertheless,
the uniform 2D crystal formability and exfoliation capability of
the material with moderate n-type charge carrier transporting
characteristic suggest great potential for flexible electronics
application. By combining mechanical cleavage methods with a
crystal patterning technique based on surface treatment,18 we
believe repetitive formation of ultrathin crystal patterns will be
■
This research was supported by Basic Science Research
Program (CRI; RIAMI-AM0209(0417-20090011)) and WCU
(World Class University) project (R31-2008-000-10075-0)
through National Research Foundation of Korea funded by
the Ministry of Education, Science and Technology.
REFERENCES
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