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Chemistry Letters Vol.35, No.10 (2006)
Analysis of Molecular Aggregation States in Pentacene Thin Films
Prepared from Soluble Precursor
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Takahiro Akinaga, Shigekazu Yasutake, Sono Sasaki, Osami Sakata,
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Hideyuki Otsuka, and Atsushi Takahara
Graduate School of Engineering, Kyushu University, Fukuoka 812-8581
Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198
Institute for Materials Chemistry and Engineering, Kyushu University, Fukuoka 812-8581
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(Received August 4, 2006; CL-060889; E-mail: takahara@cstf.kyushu-u.ac.jp)
Molecular packing structure in pentacene films prepared
A precursor of pentacene was synthesized as reported by
4
from the Afzali’s precursor soluble in general organic solvents
was characterized by in-plane grazing-incidence X-ray diffrac-
tion (GIXD) measurements. The crystallographic ab plane of
pentacene was parallel to the surface of a Si substrate. Crystal
structure of pentacene near the substrate was identified as the
thin-film phase.
Afzali et al. The obtained precursor was dissolved in chloro-
form and spin-coated on Si substrates at 2000 rpm. The precursor
was converted to pentacene in the film state with heating at
423 K for 30 min under nitrogen atmosphere. Bulk structure
was investigated for powder samples by wide-angle X-ray dif-
fraction (WAXD) measurements with a large Debye–Scherrer
camera and an imaging plate detector at BL02B2 beamline of
JASRI/SPring-8. The wavelength, ꢀ, of incident X-rays was
0.100 nm, and exposure time was 300 s. In order to investigate
molecular packing state in the parallel direction to the substrate
surface, in-plane GIXD measurements were carried out for films
with a six-axis diffractometer and a scintillation detector at
BL13XU beamline of JASRI/SPring-8. The ꢀ of incident
X-rays was 0.128 nm. The incident angle of the X-rays to the
sample surface was 0.1 deg which was lower than the critical
angle of the samples and Si substrate.
Past decade, the research of organic semiconductors has be-
come very popular. The printing of organic semiconductor-inks
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can realize low-cost fabrication. However, most of soluble
organic semiconductor shows low-semiconducting properties.
On the other hand, some of insoluble organic semiconductors
show high-semiconducting properties. Especially, pentacene
films show semiconducting properties as high as amorphous
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silicon, reproducibly.
Recently, the method using the pentacene precursor is ex-
amined as the strategy which realizes solubility and high mobi-
lity. In 1999, M u¨ llen and Herwig reported a soluble pentacene
precursor which has tetrabromobenzene moiety as a leaving
group. In 2002, Afzali et al. reported a convenient manufactur-
ing method of the soluble pentacene precursor which has N-sul-
Figure 1 shows WAXD profiles for bulk samples of a penta-
cene precursor and pentacene obtained by completion of retro
Diels–Alder reaction for the precursor. The magnitude of scat-
tering vector q is defined as q ¼ 4ꢁsin ꢂ=ꢀ, where ꢂ is Bragg
angle. Weak reflections from the precursor which was not detect-
ed with laboratory instruments were detected in excellent S/N
ratio by synchrotron WAXD measurements. With increasing in
temperature, reflections from the precursor crystal decreased in
intensity, and those from pentacene crystal increased because
of elimination of N-sulfinylacetamide from the precusors by ret-
ro Diels–Alder reaction. Crystal structure of pentacence precur-
sor was difficult to obtain owing to difficulty in preparation of a
large single crystal and low stability. According to ref 9, some
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finylacetamide moiety as a leaving group through retro Diels–
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Alder reaction. The Afzali’s precursor is soluble in chloroform,
dichloromethane, THF, and dioxane, and it transforms to penta-
cene by heating at 393–473 K under nitrogen atmosphere
(Scheme 1). In addition, it was reported that pentacene precur-
sors which can be soluble in alcohols and be patterned by UV
light were synthesized with the other types of N-sulfinylalkyl-
amides.5 Although crystal structure in the film sate is very im-
portant for semiconductor performance, molecular aggregation
structure of pentacene has not been revealed yet for its films pre-
pared from these soluble precursors. Grazing incidence X-ray
diffraction (GIXD) measurement is a powerful technique to in-
vestigate molecular packing structure in the near-surface region
,6
ꢁ1
distinct diffraction peaks observed for pentacene at q < 10 nm
precursor x 5
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,8
of organic materials. In this paper, the authors investigated
molecular packing structure and morphology of pentacene films
prepared from the Afzali’s soluble precursor by using GIXD
measurements and field emission scanning electron microscopic
(001)
pentacene
(
FE-SEM) observation, respectively.
(
002)
10
O
N
O
heat
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15
S
-1
q / nm
CH3
-
C N S O
Figure 1. WAXD profiles measured for powder samples of a
pentacene precursor (top) and pentacene (bottom) obtained by
completion of retro Diels–Alder reaction for the precursor.
O
Scheme 1.
Copyright ꢀ 2006 The Chemical Society of Japan