Self-assembled monolayers made of 6-(5-((6-((5-hexylthiophen-2-yl)ethynyl)- 9,10-bis(phenylethynyl)anthracen-2-yl)ethynyl)thiophen-2-yl)hexyl 3-(triethoxysilyl)propylcarbamate for ultrathin film transistors

Article abstract of DOI:10.1021/la3020942

A new functionalized triethoxysilane bearing an X-shaped, anthracene-based semiconducting molecule on one arm was designed and synthesized as a precursor for the preparation of a self-assembled monolayer (SAM) on a SiO₂ substrate. 3-Isocyanatopropyl triethoxysilane was reacted with a monohydroxyl-terminated X-shaped, anthracene-based semiconducting molecule in the presence of tin catalyst. The 6-(5-((6-((5-hexylthiophen-2-yl)ethynyl)-9,10- bis(phenylethynyl)anthracen-2-yl)ethynyl)thiophen-2-yl)hexyl 3-(triethoxysilyl)propylcarbamate (BATHT-TEOS) was found to be stable and sufficiently reactive to form organic monolayers on hydroxylated SiO₂ surfaces. The structures and properties of these SAMs were investigated using X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, photoluminescence (PL) spectroscopy, laser scanning confocal microscopy-PL spectrometry, and spectroscopic ellipsometry. In this work, BATHT-SAM was employed as an interfacial layer on SiO₂ to fabricate ultrathin film transistors (UTFTs, active layer thickness ～ 16.09 nm). The device UTFT-I, made of 0.06 wt % 5,5'-(9,10-bis(phenylethynyl)anthracene-2,6-diyl)bis(ethyne-2, 1-diyl)bis(2-hexylthiophene) (BATHT) solution on an n-octyltrichlorosilane-SAM/ SiO₂ layer, showed no gate effect for the carrier transport behavior; however, the device UTFT-II, fabricated on BATHT-SAM/SiO₂, exhibited field effect mobilities of 0.04 cm² V^-1 s^-1 (I_on/off ～ 6.3*- 10³ to 1.0*- 10 ⁴). This can be attributed to the effect of BATHT-SAM inducing uniform coverage and ordering of BATHT molecules as an upper layer.

Full text of DOI:10.1021/la3020942

Article
pubs.acs.org/Langmuir
Self-Assembled Monolayers Made of 6-(5-((6-((5-hexylthiophen-2-
yl)ethynyl)-9,10-bis(phenylethynyl)anthracen-2-yl)ethynyl)thiophen-
2-yl)hexyl 3-(Triethoxysilyl)Propylcarbamate for Ultrathin Film
Transistors
Dong Uk Heo,† Joo Bin Lee,† Yoon Deok Han,‡ Jinsoo Joo,‡ Hosuk Lee,§ Hosun Lee,§
and Dong Hoon Choi*,†
‡
†Department of Chemistry, Research Institute for Natural Sciences, and Department of Physics, Korea University, Seoul 136-701,
South Korea
§Department of Applied Physics, Kyung Hee University, Yongin 446-701, South Korea
ABSTRACT: A new functionalized triethoxysilane bearing an
X-shaped, anthracene-based semiconducting molecule on one
arm was designed and synthesized as a precursor for the
preparation of a self-assembled monolayer (SAM) on a SiO2
substrate. 3-Isocyanatopropyl triethoxysilane was reacted with
a monohydroxyl-terminated X-shaped, anthracene-based semi-
conducting molecule in the presence of tin catalyst. The 6-(5-
((6-((5-hexylthiophen-2-yl)ethynyl)-9,10-bis(phenylethynyl)-
anthracen-2-yl)ethynyl)thiophen-2-yl)hexyl 3-(triethoxysilyl)-
propylcarbamate (BATHT-TEOS) was found to be stable and
sufficiently reactive to form organic monolayers on hydroxylated SiO2 surfaces. The structures and properties of these SAMs
were investigated using X-ray photoelectron spectroscopy, UV−vis absorption spectroscopy, photoluminescence (PL)
spectroscopy, laser scanning confocal microscopy−PL spectrometry, and spectroscopic ellipsometry. In this work, BATHT-SAM
was employed as an interfacial layer on SiO2 to fabricate ultrathin film transistors (UTFTs, active layer thickness ∼ 16.09 nm).
The device UTFT-I, made of 0.06 wt % 5,5′-(9,10-bis(phenylethynyl)anthracene-2,6-diyl)bis(ethyne-2,1-diyl)bis(2-
hexylthiophene) (BATHT) solution on an n-octyltrichlorosilane-SAM/SiO2 layer, showed no gate effect for the carrier
transport behavior; however, the device UTFT-II, fabricated on BATHT-SAM/SiO2, exhibited field effect mobilities of 0.04 cm2
V−1 s−1 (Ion/off ∼ 6.3 × 103 to 1.0 × 104). This can be attributed to the effect of BATHT-SAM inducing uniform coverage and
ordering of BATHT molecules as an upper layer.
The first chemically prepared SAM was proposed in the
1980s, which thereafter inspired more advanced research to be
carried out on its formation and unique properties.7,8 Since
then, SAMs have been employed in numerous electronic
devices that have multiple interfaces in order to enhance the
corresponding device performance. Attempts have been made
to modify the wettability of the material surface,9 to tune the
work function of metals,10,11 to achieve cascade energy-level
alignment12,13 to make the carrier transport more efficient, and
to facilitate the fabrication of very thin dielectric layers in
TFTs.14,15
For the most common application of SAMs to OTFTs, the
SAM can modulate the surface property of the insulator
through the introduction of hydrophobic molecules. Thus,
vacuum-processed pentacene and rubrene molecules on OTS-
SAM/SiO2/Si have shown good coverage and provided robust
crystalline films.16,17
1. INTODUCTION
Organic thin film transistors (OTFTs) have attracted a great
deal of interest over the past decade because of their potential
application to low-cost printed electronics and large-area
flexible electronic devices. In particular, fabrication techniques
such as inkjet printing, microcontact printing, and roll-to-roll
processing of soluble organic semiconductors have enabled the
realization of high-performance solution-processed OTFTs in
large-scale devices.
In the past, many research groups have observed that the
performance of OTFTs can be significantly improved by
treating the insulator surface with self-assembled monolayer
(SAMs) using n-octyltrichlorosilane (OTS), n-octadecyltri-
chlorosilane (ODTS),1,2 aminopropyltrimethoxysilane,3 fluo-
roalkyltrichlorosilane (FTS),4 and phenyltrichlorosilane
(PTCS).5 The modification of the dielectric surface via a
SAM provides alternative means for the crystalline semi-
conducting molecules to achieve optimum crystallinity and
edge-on molecular ordering for the facilitation of charge carrier
transport in TFTs.6
Received: May 23, 2012
Revised: June 28, 2012
Published: June 29, 2012
© 2012 American Chemical Society
10948
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955

Langmuir
Article
6-(5-((6-((5-Hexylthiophen-2-yl)ethynyl)-9,10-bis(phenylethynyl)-
anthracen-2-yl)ethynyl) thiophen-2-yl)hexan-1-ol, 5. An oven-dried
250 mL RBF was charged with a magnetically stirred solution of 3
(0.90 g, 1.4 mmol), bis(triphenylphosphin) palladium(II) dichloride
(0.049 g, 0.069 mmol), and copper iodide (0.013 g, 0.069 mmol) in a
mixture of freshly distilled THF (80 mL), triethylamine (35 mL), and
diisopropylamine (25 mL). 6-(5-Ethynylthiophen-2-yl)hexan-1-ol
(0.434 g, 2.08 mmol) was then added and the mixture heated at 80
°C for 6 h. After completing the reaction, the solution was poured into
methanol and the precipitates collected. The crude solid was purified
by silica-gel column chromatography using a mixture of MC and
hexane (1:2 v/v) as an eluent. Further purification via precipitation
afforded pure compound 5 (yield = 0.76 g, 71%). 1H NMR (400 MHz,
CDCl3): δ (ppm) 8.74 (s, 2H), 8.58 (d, J = 9.00 Hz, 2H), 7.82 (d, J =
7.84 Hz, 4H), 7.65 (d, J = 9.00 Hz, 2H), 7.48 (m, 6H), 7.22 (d, J =
3.52 Hz, 2H), 6.73 (d, J = 3.88 Hz, 2H), 3.68 (q, J = 6.64 Hz, 2H),
2.86 (m, 4H), 1.74 (m, 4H), 1.59 (m, 2H), 1.43 (m, 6H), 1.33 (m,
4H), 1.25 (t, J = 5.12 Hz, 2H), 0.92 (t, J = 6.68 Hz, 3H). 13C NMR
(100 MHz, CDCl3): δ (ppm) 187.33, 132.54, 131.83, 131.49, 128.57,
124.44, 123.09, 118.21, 104.75, 85.91, 85.55, 62.94, 32.62, 31.53,
30.18, 28.78, 25.46, 22.57. LR-MS (FAB) m/z (M+): calcd for
C54H46OS2, 774.30; found, 774.57. Anal. Calcd for C54H46OS2: C,
83.68; H, 5.98; S, 8.27. Found: C, 83.55; H, 6.08; S, 8.20.
Recently, field-effect transistors (FETs) that use only a
semiconductor-containing SAM as an active layer have been
demonstrated and combined into integrated circuits after
preparing dimethylchlorosilane-functionalized oligothio-
phenes.18−22 This single molecular layer that contains semi-
conducting moieties can be fabricated onto the insulator surface
quite efficiently.
A few more examples of SAM-based FETs based on
hydroxyl-functionalized acenes and thiophenes have already
been reported.23−27 Although the available literature suggests
that the use of merely a monolayer of an organic semiconductor
can control the molecular alignment and significantly decrease
the material expense for a single device, spontaneous self-
assembly remains challenging for realizing large-area and
practical applications for these devices.
In this work, we present new solution-processed OTFTs
fabricated onto a BATHT-SAM/SiO2/Si gate insulator.
BATHT was deposited onto BATHT-SAM/SiO2/Si by spin-
coating using an unusually dilute solution (0.06 wt %) for an
active layer. The semiconducting moiety in BATHT-SAM is
identical to the BATHT molecules used as the upper coated
active layer. The absorption and photoluminescence (PL)
spectral behaviors of the fabricated BATHT-SAM on the
substrate were investigated in detail.
Ultrathin film transistors (UTFTs) from the BATHT-SAM/
SiO2/Si were subsequently fabricated and the device perform-
ances measured and compared to those of TFTs fabricated
onto OTS-treated SiO2. The combination of the BATHT
monolayer (t = 2.10 nm) and upper film (t = 16.09 nm) coated
with BATHT provided successful charge transport properties in
the UTFT devices. The output and transfer curves were
measured to yield a saturated mobility of around 0.04 cm2 V−1
s−1 (Ion/off ≈ 6.3 × 103 to 1.0 × 104).
6-(5-((6-((5-Hexylthiophen-2-yl)ethynyl)-9,10-bis(phenylethynyl)-
anthracen-2-yl)ethynyl) thiophen-2-yl)hexyl 3-(Triethoxysilyl)-
Propylcarbamate, 7. Compound 5 (0.20 g, 0.258 mmol) and
dibutyltin dilaurate (0.46 mL, 0.774 mmol) were dissolved in 13 mL of
distilled THF under an argon atmosphere. 3-Isocyanatopropyl
triethoxysilane (ICPTEOS; 0.19 mL, 0.77 mmol) was added dropwise
to the mother solution before the mixture was kept at 65 °C for 3 h.
The mixture was poured into 200 mL of distilled hexane to collect the
precipitates, with this precipitation procedure repeated for further
purification. The product was dried under vacuum at room
temperature for 24 h and stored in a vacuum desiccator before
being used further (yield = 0.10 g, 38%). 1H NMR (300 MHz,
CDCl3): δ (ppm) 8.77 (s, 2H), 8.62 (d, J = 9.06 Hz, 2H), 7.82 (d, J =
7.98 Hz, 4H), 7.63 (d, J = 9.06 Hz, 2H), 7.51 (m, 6H), 7.22 (d, J =
3.57 Hz, 2H), 6.73 (d, J = 3.57 Hz, 2H), 4.05 (t, J = 6.86 Hz, 2H), 3.85
(q, J = 7.14 Hz, 6H), 3.17 (t, J = 6.86 Hz, 2H), 2.83 (t, J = 7.68 Hz,
4H), 1.65 (m, 8H), 1.34 (m, 10H), 1.25 (t, J = 6.87 Hz, 9H), 0.93 (t, J
= 7.14 Hz, 3H), 0.66 (t, J = 8.52 Hz, 2H). 13C NMR (75 MHz,
CDCl3): δ (ppm) 149.23, 132.78, 132.08, 128.82, 123.36, 118.47,
103.26, 93.37, 86.07, 85.72, 58.69, 32.14, 31.79, 28.97, 27.86, 22.82,
18.53, 14.33, 13.82. LR-MS (FAB) m/z (M+): calcd for
C64H67NO5S2Si, 1021.42; found, 1021.43. Anal. Calcd.for
C64H67NO5S2Si: C, 75.18; H, 6.61; N, 1.37; S, 6.27. Found: C,
76.43; H, 6.76; N, 1.45; S, 5.68.
2. EXPERIMENTAL SECTION
2.1. Synthesis. All commercially available starting materials and
solvents were purchased from Aldrich, TCI, or Acros Co. and were
used without further purification. 2,6-Dibromo-9,10-bis-
(phenylethynyl)anthracene (1), 2-ethynyl-5-hexylthiophene (2), and
6-(5-ethynylthiophen-2-yl)hexan-1-ol (4) were synthesized using the
following literature procedures.28−30
2-((6-Bromo-9,10-bis(phenylethynyl)anthracen-2-yl)ethynyl)-5-
hexylthiophene, 3. An oven-dried 250 mL round-bottomed flask
(RBF) was charged with a magnetically stirred solution of 2,6-
dibromo-9,10-bis(phenylethynyl)anthracene, 1 (3.00 g, 5.6 mmol),
bis(triphenylphosphine) palladium(II) dichloride (0.196 g, 0.28
mmol), and copper iodide (0.053 g, 0.28 mmol) in a mixture of
freshly distilled THF (80 mL), triethylamine (35 mL), and
diisopropylamine (25 mL). 2-Ethynyl-5-hexylthiophene (0.97 g, 5.03
mmol) was then added and the mixture heated at 80 °C for 6 h. After
completing the reaction, the solution was poured into methanol and
the precipitates collected. The crude solid was purified by silica-gel
column chromatography using a mixture of methylene chloride (MC)
and hexane as an eluent (MC/hexane = 1:3 v/v). Further precipitation
afforded pure 2-((6-bromo-9,10-bis(phenylethynyl)anthracen-2-yl)-
1
2.2. Instrumental Analysis. H NMR spectra were recorded on
Varian Mercury 400 and 300 MHz NMR spectrometers using
deuterated chloroform purchased from Cambridge Isotope Labo-
ratories, Inc. Elemental analyses were performed using an EA1112
(Thermo Electron Corp.) elemental analyzer. Low-resolution mass
spectrometry (LR-MS) was performed using a JMS-700 MStation
mass spectrometer (JEOL, Tokyo, Japan; resolution of 60 000 and m/
z range at full sensitivity of 2400).
2.3. UV−Vis Absorption and PL Spectroscopy. Absorption
spectra of the solution (solvent = 1 × 10−5 mol L−1 chloroform) and
film samples were obtained using a UV−vis spectrometer (HP 8453,
photodiode array type) in the wavelength range 190−1100 nm. PL
spectra of the solutions and films were acquired at room temperature
using a Hitachi F-7000 FL spectrophotometer.
2.4. Laser Confocal Microscopy−PL Spectroscopy (LCM-PL).
Nanoscale PL images and spectra were taken for the BATHT-SAM
and its surface measured spot by spot using a customized laser
confocal microscope (Nano-Focus Ltd., Albatross) built around an
inverted optical microscope (Axiovert 200, Zeiss GmbH). The 488-nm
line of an unpolarized argon (Ar) ion laser was used for PL excitation.
The BATHT-SAM was placed on a glass substrate, which was then
mounted onto an xy-scan stage (PSIA XE-120) of the confocal
microscope. The spot size of the focused laser beam on the sample was
1
ethynyl)-5-hexylthiophene, 3 (yield = 1.1 g, 30%). H NMR (400
MHz, CDCl3): δ (ppm) 8.72 (d, J = 1.96 Hz, 1H), 8.67 (s, 1H), 8.51
(d, J = 9.00 Hz, 1H), 8.44 (d, J = 9.00 Hz, 1H), 7.78 (m, 4H), 7.63 (m,
2H), 7.47 (m, 6H), 7.22 (d, J = 3.52 Hz, 1H), 6.73 (d, J = 3.52 Hz,
1H), 2.85 (t, J = 7.84 Hz, 2H), 1.74 (m, 2H), 1.41 (m, 6H), 0.92 (t, J =
7.04 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ (ppm) 149.01, 132.85,
132.56, 131.78, 128.60, 127.28, 124.39, 122.98, 121.77, 120.29, 117.43,
103.28, 93.04, 85.56, 31.55, 30.29, 28.73, 22.59, 14.10. LR-MS (FAB)
m/z (M+): calcd for C42H31BrS, 646.13; found, 648.10. Anal. Calcd for
C42H31BrS: C, 77.89 ; H, 4.82 ; S, 4.95. Found: C, 77.80; H, 4.91; S,
5.01.
10949
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955

Langmuir
Article
Figure 1. Structures of TFT devices. (a) UTFT-I: BATHT/OTS-SAM/SiO2/Si and (b) UTFT-II: BATHT/BATHT-SAM/SiO2/Si.
Scheme 1. Synthetic Route to BATHT-TEOS, 7
estimated to be about 190 nm. The scattered light was collected with
the same objective lens and with the excitation laser light filtered out
using a long-pass edge-filter. The measured PL signal was focused to a
multimode fiber (core size = 100 μm) that acted as a pinhole for the
confocal detection. The other end of the multimode fiber was
connected to the photomultiplier tube for the PL image, or the input
slit of a 0.3-m-long monochromator (Acton Spectra Pro 300i)
equipped with a cooled charge coupled device (Acton PIXIS 100) for
measurement of the PL spectra. For all confocal PL measurements
performed in this work, the laser power incident on the sample and the
acquisition time for each PL spectrum were fixed at 3 μW and 1 s,
respectively.
2.5. Contact Angle Measurements. A Phoenix 300Plus
instrument with an automatic liquid dispenser was employed to
measure contact angles at ambient temperature. Static water contact
angles were measured using a 4-μL droplet of deionized water at three
different locations on a film. Data are reported as the average of these
individual measurements.
the corresponding solutions and the surface coverage and roughness of
the SAM on the SiO2/Si dielectric layer investigated.
2.9. TFT Fabrication. To fabricate the TFTs and characterize their
performance, a bottom-gate top-contact device geometry was
employed. To fabricate the active layer, a solution of BATHT was
prepared with unusually low concentration (∼0.06 wt %). The
solution was spin-coated onto OTS-treated SiO2/Si (UTFT-I) or
BATHT-SAM/SiO2/Si (UTFT-II) (see Figure 1).
Source and drain electrodes were then thermally evaporated (100
nm) through a shadow mask with a channel width and length of 1500
and 50 μm, respectively.
All field-effect mobilities were extracted from the saturation regime
using the relationship μsat = (2IDSL)/(WC(VG − Vth)2), where IDS is
the saturation drain current, C is the capacitance of the dielectric, VG is
the gate bias, and Vth is the threshold voltage. The device performance
was evaluated using a 4200-SCS semiconductor characterization
system under ambient conditions.
2.6. Spectroscopic Ellipsometry. Ellipsometric angles Ψ and Δ
were measured using spectroscopic ellipsometry with incidence angles
of 65°, 70°, and 75°. A variable angle spectroscopic ellipsometer
(Woollam Inc., VASE model) was employed with an autoretarder in
the spectral range between 1.0 and 5.0 eV.
2.7. X-ray Photoelectron Spectroscopy (XPS). An ESCALAB
220i-XL X-ray photoelectron spectrometer was employed to perform
elemental analysis of the film surface (Ep = 1486.7 eV). The typical
pressure of the chamber during analysis was less than 6 × 10−10 mbar.
All XPS spectra were measured with a pass energy of 58.7 eV and an
energy step of 0.125 eV. The data were collected at 45° to the normal
of the sample surface.
3. RESULTS AND DISCUSSION
3.1. Synthesis and SAM Formation. The two-dimen-
sional (2-D) π-conjugated molecule 5,5′-(9,10-bis-
(phenylethynyl)anthracene-2,6-diyl)bis(ethyne-2,1-diyl)bis(2-
hexylthiophene) (BATHT) and its analogue, 5, which bears a
hydroxyl group, were synthesized to prepare BATHT-TEOS.
The monohydroxyl-terminated BATHT, 5, was synthesized by
deprotecting the acetate group of compound 4. Compound 5
was tethered to one arm of triethoxysilane via a simple
carbamate forming reaction. ICPTEOS, 6, is very reactive
toward a hydroxyl group, yielding carbamate-containing
BATHT molecules in the presence of dibutyltin dilaurate
(DBTDL) in dried THF at 60 °C (Scheme 1). Using the
synthesized BATHT-TEOS, hydrolysis and condensation
2.8. Atomic Force Microscopy (AFM). An advanced scanning
probe microscope (XE-100, PSIA) operating in a tapping mode with a
silicon cantilever was used to characterize the surface topographies of
the samples. The monolayer and spin-coated films were prepared using
10950
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955

Langmuir
Article
reactions were performed in dried toluene on the SiO2/Si
insulator layer (temperature = 100 °C, concentration of
BATHT-TEOS solution = 0.6 mM). Upon grafting of the
BATHT-TEOS onto the SiO2, any unreacted BATHT-TEOS
was thoroughly removed by sonication.
3.2. Surface Characterization of BATHT-SAM on SiO2/
Si. After fabricating BATHT-SAM on SiO2/Si, the placement
of BATHT molecules on SiO2/Si was confirmed by XPS,
spectroscopic ellipsometry, contact angle measurements, AFM,
absorption spectroscopy, and LCM-PL.
First, XPS provided the elemental composition within the
substrate-bearing BATHT moieties (scan range = 0−1200 eV).
The samples were prepared by immersing SiO2/Si substrate
into the BATHT-TEOS solution (solvent = 0.6 mM dried
toluene) at 100 °C for 5 h before being dried at room
temperature for 6 h under vacuum. XPS spectra of BATHT-
TEOS-treated SiO2/Si and bare SiO2/Si are shown in Figure 2.
measured (work of adhesion = 82.14 mN/m) as a result of the
imposed hydrophobicity on the surface of the SiO2/Si.
For further precise characterization, spectroscopic ellipsom-
etry measurements were performed for the ultrathin films on
the SiO2/Si with incidence angles of 65°, 70°, and 75° using a
variable angle spectroscopic ellipsometer (VASE) with an
autoretarder in the spectral range 1.0−5.0 eV. In order to carry
out the VASE study, two samples on SiO2/Si were fabricated:
(a) BATHT-SAM/SiO2/Si (Sample I) and (b) spin-coated
BATHT solution (0.06 wt %)/BATHT-SAM/SiO2/Si (Sample
II). The use of multiple incident angles increases the accuracy
in determining the dielectric function of the layer from the
ellipsometric angles Ψ and Δ. The ellipsometric angles were
fitted using a parametric optical constant (POC) model.31
Assuming three phases for the BATHT-SAM layer, SiO2, and
Si substrate, the refractive index and extinction coefficient of
the BATHT-SAM were estimated using layer modeling
analysis. In the case of a molecular layer of BATHT on a
BATHT-SAM, four phases were assumed: BATHT layer,
BATHT-SAM layer, SiO2, and Si substrate, and the refractive
index and extinction coefficient of the BATHT layer was
estimated with a fixed thickness for the BATHT-SAM using
layer modeling analysis.
A POC model was adopted for the line shape of the dielectric
function (ε* = ε1 + iε2) of the organic layer,31 where the
complex refractive index, (n + ik), is the square root of the
dielectric function ε. Figure 3c summarizes the layer models for
(a) Sample I and (b) Sample II.
To model the surface roughness layer, we used an effective
medium approximation with components comprised of a
mixture of the main layer and voids. The effective dielectric
function was calculated according to the Bruggeman effective
medium approximation (EMA):32
εmain − εeff
εvoid − εeff
εvoid + εeff
fmain
+ fvoid
= 0
εmain + 2εeff
(1)
where the effective dielectric function of the roughness layer is
denoted by εeff and dielectric functions (volume fractions) of
the main layer and voids are denoted, respectively, as
εmain(fmain) and εvoid(fvoid). We fixed fvoid = 0.5 for simplicity.
Figure 3a,b shows the measured complex refractive indices of
the films for (a) Sample I and (b) Sample II. The standard
critical point (SCP) model assumes a simple parabolic
dispersion relationship for the valence and conduction bands
of semiconductors, as developed by Cardona33 and Aspnes.34
This model provides accurate critical point (CP) parameters
such as the energy threshold, broadening, amplitude, and
excitonic phase angle and can be applied to polymers.32 The
SCP line shape equation is given by35
Figure 2. XPS spectra of bare SiO2/Si (A) and BATHT-TEOS treated
SiO2/Si substrate (B). Insets: Image of water droplet (4 μL) on the
substrate and magnified spectra in the range of 390−409 eV.
ε(E) = C − AeiΦ(E − Eth + iΓ)n
(2)
From the sample treated with the BATHT-TEOS solution,
there can be observed a significant intensity increase in the
C(1s) peak at 284 eV and the existence of an N(1s) peak at 399
eV (see Figure 2B, inset). This therefore provides evidence that
the BATHT-TEOS was successfully grafted onto the SiO2/Si
substrate.
There is also further corroborative evidence of SAM
deposition onto the SiO2/Si surface. For example, significant
differences in the measured contact angles can be observed for
the two above-mentioned samples (see the insets of Figure 2).
After BATHT-SAM deposition, a contact angle of 82° was
where the CP is described by the amplitude A, threshold energy
Eth, broadening Γ, and excitonic phase angle Φ. The exponent n
takes the values of −1/2 for one-dimensional (1-D), 0
[logarithmic, i.e., ln(E − Eth + iΓ)] for 2-D, and 1/2 for 3-D
CPs. Discrete excitons are represented by n = −1. Here, the
excitonic phase angle, Φ, represents coupling between the
discrete exciton states and continuum band states. To remove
the background contribution, the second derivative of the
dielectric function with respect to energy (d2ε/dE2) is fitted
using the SCP model. The best matched lowest optical gap
10951
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955

Langmuir
Article
Figure 3. Refractive index and extinction coefficient vs photon energy. The best match complex refractive index of the films for fabricated (a) Sample
I and (b) Sample II. (c) Configurations of the multilayer in the samples: (i) Sample I and (ii) Sample II.
Figure 4. UV−vis absorption (A) and photoluminescence (B) spectra of a solution and film of compound 5, and those of BATHT-SAM on glass:
dashed−dotted−dashed line (solution of 5), dotted line (film of 5), solid line (BATHT-SAM/glass), and dashed line (BATHT (0.06 wt
%)/BATHT-SAM/glass). (C) LCM-PL spectra of BATHT-SAM/glass with each measurement position. Inset: PL image of BATHT-SAM on glass
substrate indicating the separately measured spots with yellow circles.
energies are 2.322 and 2.288 eV for (a) Sample I and (b)
Sample II, respectively.
broadening and a bathochromic shift to reduced bandgap
energy.
Figure 4A shows the UV−vis absorption spectra of BATHT-
SAM, BATHT/BATHT-SAM/glass, and solution and film
samples of compound 5. From this figure it can be seen that the
solution spectrum of 5 shows absorption maxima at 461 and
489 nm, whereas that of the film of 5 shows increased
In a previous study carried out by us, a film of BATHT was
found to exhibit J-aggregation behavior, with absorption
maxima at 481 and 504 nm showing highly resolved vibronic
transitions.28 On the other hand, the spectrum for BATHT-
SAM/glass shows no spectral characteristics that are specific to
10952
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955

Langmuir
Article
Figure 5. AFM topographic images (10 μm × 10 μm) of A: bare SiO2/Si, and B: BATHT-SAM/SiO2/Si (after 5 h immersion of silicon wafer in
BATHT-TEOS solution).
Figure 6. AFM topographic images (10 μm × 10 μm) of A: BATHT/BATHT-SAM/SiO2/Si, B: BATHT/OTS-SAM/SiO2/Si, and C: BATHT/
SiO2/Si.
this aggregation behavior. This may be attributed to the
anchoring of one end of the peripheral group to the glass
substrate which thus prevents J-aggregation behavior from
occurring. When the extremely thin film of BATHT was coated
onto the BATHT-SAM, the peak at 522 nm was clearly
observed; therefore, BATHT molecules in the upper layer were
associated via a J-aggregation manner.
Corresponding emission spectra can be seen in Figure 4B.
The spectra of BATHT-SAM and of the thin film of 5 show
very broad spectra whose emission maximum is around 622
nm, which can be assigned to excimer formation between the
BATHT molecules. The SAM prepared after 5 h of immersion
exhibited almost negligible emission at 540 nm but with a
dominant emission observed at 622 nm. The thin BATHT film
on BATHT-SAM/glass shows a clear emission peak at 540 nm,
which corresponds to the absorption band at 522 nm,
indicating that the J-aggregation peak results from a small
Stokes shift. This experiment also provides strong evidence that
the specific arrangement of the BATHT is aided by the
BATHT-SAM.
A series of PL spectra obtained using a LCM-PL
spectrometer are provided in Figure 4C. In order to investigate
the uniformity of BATHT-moieties in the monolayer, the
measured area was varied and the corresponding PL spectra
superimposed onto each other. The variation in emission
maximum wavelengths and intensities were monitored. No
significant variation in these parameters was observed for the
BATHT-SAM on the glass substrate, supportive of the notion
that there is uniformity of the BATHT-SAM on the glass
substrate. The inset in Figure 4C is an LCM-PL image which
10953
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955

Langmuir
Article
Figure 7. Electrical characterization for OTFTs of UTFT-II. A: plot of drain current (−IDS) versus drain−source voltage (VDS). B: plot of −IDS and
1/2
−IDS versus gate voltage (VG), VDS = −120 V. The performances were measured at ambient conditions.
visibly shows the uniformity through the surface of the
monolayer.
values obtained using only an ultrathin semiconducting layer.
The improved performance of the UTFT-II can mainly be
attributed to the improved crystallization on the surface of the
BATHT-SAM which contains the identical semiconducting
moieties to BATHT. This affords a densely packed geometry of
erect BATHT moieties on the surface of the gate insulator.
Overall, the good performance of the UTFT device on
BATHT-SAM/SiO2/Si may be attributed to the presence of
the single BATHT molecular layer placed between the overcoat
of BATHT molecules and SiO2, which supports a favorable,
uniform conformation of molecules suitable for carrier
transport. In addition, BATHT units generate strong
intermolecular interactions and induce large overlaps between
the π-orbitals of neighboring molecules, thus generating
efficient channels for charge transport and resulting in high
carrier mobility in the UTFT even though the thickness of the
active layer is extremely small (∼16.09 nm).
In order to provide further clear evidence of the obtained
surface, we recorded the AFM topographic image of the
BATHT-SAM that had been prepared by the 5 h immersion of
SiO2/Si into the BATHT-TEOS solution, with comparison
made to the image of the bare substrate.
As can be seen in Figure 5, the surface image is clearly
changed after BATHT-TEOS deposition compared to that of
the bare SiO2/Si substrate. Crystalline domains are observed in
the BATHT-SAM/SiO2/Si.
Figure 6A−C displays the surface topographies of the
samples of BATHT/BATHT-SAM/SiO2/Si, BATHT/OTS-
SAM/SiO2/Si, and BATHT/SiO2/Si, respectively, when a
coating of a 0.06 wt % solution of BATHT is applied to the
substrate. Because of the extremely low concentration of the
spin-coated solution, Figures 6B,C shows very poor surface
coverage, with only some islands observed to be occupied with
BATHT molecules. However, the film surface in Figure 6A
shows highly compact structures with good connectivity
between molecular crystallites of BATHT. Therefore, the
BATHT-SAM aided the formation of a good film, even when
coating with a 0.06 wt % solution of BATHT.
4. CONCLUSIONS
A new anthracene-based conjugated molecule containing
triethoxysilane that is a precursor molecule for SAM fabrication
was successfully synthesized. The molecules formed a uniform
SAM layer onto the large surface of a gate insulator and also
drove uniform alignment of BATHT molecules to the
substrate. We successfully deposited BATHT molecules onto
the surface of BATHT-SAM/SiO2/Si and fabricated an
unexpectedly compact thin-layer structure by spin coating
using an unusually dilute solution. UTFT-II, fabricated on
BATHT-SAM/SiO2/Si, exhibited intriguingly high carrier
mobility of around 0.04 cm2 V−1 s−1, with Ion/off ∼ 6.3 × 103
to 1.0 × 104.
3. 3. Organic Thin Film Transistors Made of BATHT
Deposited on BATHT-SAM/SiO2/Si. Bottom-gate, top-
contact BATHT-SAM-based TFTs were fabricated to inves-
tigate the carrier transport behavior under ambient conditions.
For fabrication of an active layer, a very dilute BATHT solution
(0.06 wt % in chloroform) was prepared for spin coating.
UTFT-I was prepared by coating the solution of BATHT onto
OTS-SiO2/Si, with the solution of BATHT coated onto
BATHT-SAM/SiO2/Si for the fabrication of UTFT-II.
The device characteristics of UTFT-I could not be observed
at all. This is likely due to the lack of BATHT molecules on the
surface (as shown by the AFM image in Figure 6B), with only
discontinuous islands of BATHT molecules in this case.
Therefore, a UTFT device could not be successfully realized on
OTS-treated SiO2/Si using the 0.06 wt % solution of BATHT.
However, UTFT-II fabricated on BATHT-SAM/SiO2/Si
exhibited typical p-channel FET characteristics (see Figure 7).
The mobility was obtained from the source (S)−drain (D)
current−voltage curves (IDS vs VDS) for the well-resolved
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dhchoi8803@korea.ac.kr. Tel: +82-2-3290-3140. Fax:
+82-2-925-4284.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
saturation region. The saturated field-effect mobility, μFET
,
This research work was mainly supported by NRF20110026418
and 2012R1A2A1A01008797. This work was also supported by
Priority Research Center Program through the National
Research Foundation of Korea (NRF) funded by the Ministry
of Education, Science and Technology (NRF 20120005860)
could then be calculated from the amplification characteristics
using known equations that describe TFTs.
The annealing-free TFTs fabricated on BATHT-SAM/SiO2/
Si exhibited interestingly high carrier mobilities of around 0.04
cm2 V−1 s−1 with Ion/off ∼ 6.3 × 103 to 1.0 × 104, with such high
10954
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955

Langmuir
REFERENCES
Article
M. Bottom-up Organic Integrated Circuits. Nature 2008, 455, 956−
959.
■
(1) Kumaki, D.; Ando, S.; Shimono, S.; Yamashita, Y.; Umeda, T.;
(19) Mathijssen, S. G. J.; Smits, E. C. P.; van Hal, P. A.; Wonergem,
H. J.; Ponomarenko, S. A.; Moser, A.; Resel, R.; Bobbert, P. A.;
Kemerink, M.; Janssen, R. A. J.; de Leeuw, D. M. Monolayer Coverage
and Channel Length Set the Mobility in Self-assembled Monolayer
Field-Effect Transistors. Nat. Nanotechnol. 2009, 4, 674−680.
(20) Gholamrezaie, F.; Mathijssen, S. G. J.; Smits, E. C. P.; Geuns, T.
C. T.; van Hal, P. A.; Ponomarenko, S. A.; Flesch, H.-G.; Resel, R.;
Cantatore, E.; Blom, P. W. M.; de Leeuw, D. M. Ordered
Semiconducting Self-Assembled Monolayers on Polymeric Surfaces
Utilized in Organic Integrated Circuits. Nano Lett. 2010, 10, 1998−
2002.
(21) Andringa, A. -M.; Spijkman, M.-J.; Smits, E. C. P.; Mathijssen, S.
G. J.; van Hal, P. A.; Setayesh, S.; Willard, N. P.; Borshchev, O. V.;
Ponomarenko, S. A.; Blom, P. W. M.; de Leeuw, D. M. Gas Sensing
with Self-assembled Monolayer Field-Effect Transistors. Org. Electron.
2010, 11, 895−898.
(22) Ponomarenko, S. A.; Borshchev, O. V.; Meyer-Friedrichsen, T.;
Pleshkova, A. P.; Setayesh, S.; Smits, E. C. P.; Mathijssen, S. G. J.; de
Leeuw, D. M.; Kirchmeyer, S.; Muzafarov, A. M. Synthesis of
Monochlorosilyl Derivatives of Dialkyloligothiophenes for Self-
Assembling Monolayer Field-Effect Transistors. Organometallics
2010, 29, 4213−4226.
(23) Novak, M.; Ebel, A.; Meyer-Friedrichsen, T.; Jedaa, A.; Vieweg,
B. F.; Yang, G.; Voitchovsky, K.; Stellacci, F.; Spiecker, E.; Hirsch, A.;
Halik, M. Low-Voltage p- and n-Type Organic Self-Assembled
Monolayer Field Effect Transistors. Nano Lett. 2011, 11, 156−159.
(24) Hutchins, D. O.; Acton, O.; Weidner, T.; Cernetic, N.; Baio, J.
E.; Ting, G.; Castner, D. G.; Ma, H.; Jen, A. K.-Y. Spin Cast Self-
assembled Monolayer Field Effect Transistors. Org. Electron. 2012, 13,
464−468.
(25) Zhang, H.; Guo, X.; Hui, J.; Hu, S.; Xu, W.; Zhu, D. Interface
Engineering of Semiconductor/Dielectric Heterojunctions toward
Functional Organic Thin-Film Transistors. Nano Lett. 2011, 11,
4939−4946.
(26) Park, B.; Paoprasert, P.; In, I.; Zwickey, J.; Colavita, P. E.;
Hamers, R. J.; Gopalan, P.; Evans, P. G. Functional Self-Assembled
Monolayers for Optimized Photoinduced Charge Transfer in Organic
Field Effect Transistors. Adv. Mater. 2007, 19, 4353−4357.
(27) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K.-Y. Interface
Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20,
1371−1388.
(28) Jung, K. H.; Bae, S. Y.; Kim, K. H.; Cho, M. J.; Lee, K.; Kim, Z.
H.; Choi, D. H.; Lee, D. H.; Chung, D. S.; Park, C. E. High-mobility
Anthracene-based X-shaped Conjugated Molecules for Thin Film
Transistors. Chem. Commun. 2009, 5290−5292.
(29) Hur, J. A.; Bae, S. Y.; Kim, K. H.; Lee, T. W.; Cho, M. J.; Choi.,
D. H. Semiconducting 2,6,9,10-Tetrakis(phenylethynyl)anthracene
Derivatives: Effect of Substitution Positions on Molecular Energies.
Org. Lett. 2011, 13, 1948−1951.
Tokito, S. Significant Improvement of Electron Mobility in Organic
Thin-Film Transistors Based on Thiazolothiazole Derivative by
Employing Self-Assembled Monolayer. Appl. Phys. Lett. 2007, 90,
05350506.
(2) Lee, H. S.; Kim, D. H.; Cho, J. H.; Hwang, M.; Jang, Y.; Cho, K.
Effect of the Phase States of Self-Assembled Monolayers on Pentacene
Growth and Thin-Film Transistor Characteristics. J. Am. Chem. Soc.
2008, 130, 10556−10564.
(3) Park, J.; Jo, S. B.; Yu, Y.-J.; Kim, Y.; Yan, J. W.; Lee, W. H.; Kim,
H. H.; Hong, B. H.; Kim, P.; Cho, K.; Kim, K. S. Single-Gate Bandgap
Opening of Bilayer Graphene by Dual Molecular Doping. Adv. Mater.
2012, 24, 407−411.
(4) Calhoun, M. F.; Sanchez, J.; Olaya, D.; Gershenson, M. E.;
Podzorov, V. Electronic Functionalization of the Surface of Organic
Semiconductors with Self-Assembled Monolayers. Nat. Mater. 2008, 7,
84−89.
(5) Dong, S.; Tian, H.; Song, D.; Yang, Z.; Yan, D.; Geng, Y.; Wang,
F. The First Liquid Crystalline Phthalocyanine Derivative Capable of
Edge-on Alignment for Solution Processed Organic Thin-Film
Transistors. Chem. Commun. 2009, 3086−3088.
(6) Ito, Y.; Birkar, A. A; Mannsfeld, S.; Oh, J. H.; Toney, M.; Locklin,
J.; Bao, Z. Crystalline Ultrasmooth Self-Assembled Monolayers of
Alkylsilanes for Organic Field-Effect Transistors. J. Am. Chem. Soc.
2009, 131, 9396−9404.
(7) Nuzzo, R. G.; Allara, D. L. Adsorption of Bifunctional Organic
Disulfides on Gold Surfaces. J. Am. Chem. Soc. 1983, 105, 4481−4483.
(8) Sagiv, J. Organized Monolayers by Adsorption, 1. Formation and
Structure of Oleophobic Mixed Monolayers on Solid Surfaces. J. Am.
Chem. Soc. 1980, 102, 92−98.
(9) Ulman, A. Self-assembled monolayers of Thiols; Academic Press:
San Diego, CA, 1998; Vol. 24.
(10) de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh,
T.; Blom, P. W. M. Tuning of Metal Work Function with Self-
Assembled Monolayers. Adv. Mater. 2005, 17, 621−625.
(11) Mathijssen, S. G. J.; van Hal, P. A.; van den Biggelaar, T. J. M.;
Smits, E. C. P.; de Boer, B.; Kemerink, M.; Janssen, R. A. J.; de Leeuw,
D. M. Manipulating the Local Light Emission in Organic Light-
Emitting Diodes by using Patterned Self-Assembled Monolayers. Adv.
Mater. 2008, 20, 2703−2706.
(12) Zhu, L.; Tang, H.; Harima, Y.; Yamashita, K.; Aso, Y.; Otsubo,
T. Enhanced Hole Injection in Organic Light-Emitting Diodes
Consisting of Self-Assembled Monolayer of Tripod-Shaped p-
Conjugated Thiols. J. Mater. Chem. 2002, 12, 2250−2254.
(13) Chon, L.-W.; Lee, Y.-L.; Wen, T.-C.; Guo, T.-F. Self-assembled
monolayer-modified Ag anode for top-emitting polymer light-emitting
diodes. Appl. Phys. Lett. 2006, 89, 233513.
(14) Kalblein, D.; Weitz, R. T.; Bottcher, H. J.; Ante, F.;
Zschieschang, U.; Kern, K.; Klauk, H. Top-Gate ZnO Nanowire
Transistors and Integrated Circuits with Ultrathin Self-Assembled
Monolayer Gate Dielectric. Nano Lett. 2011, 11, 5309−5315.
(15) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J.
Molecular Self-Assembled Monolayers and Multilayers for Organic
and Unconventional Inorganic Thin-Film Transistor Applications.
Adv. Mater. 2009, 21, 1407−1433.
(16) Kim, D. H.; Lee, H. S.; Yang, H.; Yang, L.; Cho, K. Tunable
Crystal Nanostructures of Pentacene Thin Films on Gate Dielectrics
Possessing Surface-Order Control. Adv. Funct. Mater. 2008, 18, 1363−
1370.
(30) Bae, S. Y.; Jung, K. H.; Hoang, M. H.; Kim, K. H.; Lee, T. W.;
Cho, M. J.; Jin, J.-I.; Choi, D. H. 9,10-Bis(phenylethynyl)anthracene-
based Organic Semiconducting Molecules for Annealing-free Thin
Film Transistors. Synth. Met. 2010, 160, 1022−1029.
(31) Johs, B.; Herzinger, C. M.; Dinan, J. H.; Cornfeld, A.; Benson, J.
D. Development of a Parametric Optical Constant Model for Hg1‑x
Cdx Te for Control of Composition by Spectroscopic Ellipsometry
during MBE growth. Thin Solid Films 1998, 313−314, 137−142.
(32) Aspnes, D. E. Optical Properties of Thin Films. Thin Solid Films
1982, 89, 249−262.
(33) Cardona, M. Modulation Spectroscopy. In Solid State Physics;
Seitz, F., Turnbull, D., Ehrenreich, H., Eds.; Academic: New York,
1969; p 65.
(17) Asadi, K.; Wu, Y.; Gholamrezaie, F.; Rudolf, P.; Blom, P. W. M.
Single-Layer Pentacene Field-Effect Transistors Using Electrodes
Modified With Self-assembled Monolayers. Adv. Mater. 2009, 21,
4109−4114.
(18) Smits, E. C. P.; Mathijssen, S. G. J.; van Hal, P. A.; Setayesh, S.;
Geuns, T. C. T.; Mutsaers, K. A. H. A.; Cantatore, E.; Wondergem, H.
J.; Werzer, O.; Resel, R.; Kemerink, M.; Kirchmeyer, S.; Muzafarov, A.
M.; Ponomarenko, S. A.; de Boer, B.; Blom, P. W. M.; de Leeuw, D.
(34) Aspnes, D. E. Handbook on Semiconductors; North Holland:
Amsterdam, 1980; Vol. 2, p 109.
(35) Lautenschlager, P.; Garriga, M.; Vina, L.; Cardona, M.
Temperature Dependence of the Dielectric Function and Interband
̃
Critical Points in Silicon. Phys. Rev. B 1987, 36, 4821−4830.
10955
dx.doi.org/10.1021/la3020942 | Langmuir 2012, 28, 10948−10955