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Chemistry Letters Vol.35, No.7 (2006)
Structure of Organic Thin Films Grown on Surface-modified Tantalum Oxide
Ryo Onoki,1 Shigeomi Abe,1 Keiji Ueno,ꢀ1 Hiroo Nakahara,1 and Koichiro Saiki2
1Department of Chemistry, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338-8570
2Department of Complexity Science and Engineering, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa 277-8561
(Received February 28, 2006; CL-060242; E-mail: kei@chem.saitama-u.ac.jp)
We fabricated an ordered monolayer film of a long-chain
rified by repeated recrystallization from hexane or chloroform,
respectively. C18A was dissolved in benzene and PFC18A was
dissolved in a trifluoroacetic acid:chloroform = 1:4 (v/v) solu-
tion to make the spreading solutions with the concentration of
1 ꢁ 10ꢂ4 molꢃLꢂ1. Their monolayers were spread onto an aque-
fatty acid or a perfluorofatty acid onto an anodized tantalum
oxide (Ta2O5) amorphous surface by the Langmuir–Blodgett
(LB) method, and grew a pentacene or C60 thin film on it.
Remarkable improvements of morphology and crystallinity of
the organic thin films were observed on the CH3-terminated
monolayer film of the fatty acid.
ous subphase (pH = 6.8) containing CdCl2 (3 ꢁ 10ꢂ4 molꢃLꢂ1
)
and KHCO3 (5 ꢁ 10ꢂ5 molꢃLꢂ1) in Milli-Q water (R ¼ 18:2
Mꢀ). Monolayers in the form of the cadmium salts (CdC18
and CdPFC18) were deposited by the LB method on the
Ta2O5/Ta substrates at 15 ꢄC with the surface pressure of
25 mNꢃmꢂ1 using a Lauda FW-1 film balance. The k of the
dielectric layer was only slightly reduced (less than several %)
by the deposition. Pentacene (Aldrich, 98%) or C60 (99.99%)
molecules were evaporated from a Knudsen cell onto the
Ta2O5/Ta substrate at room temperature under the ultrahigh
vacuum (ca. 10ꢂ7 Pa). The evaporation rate and the final film
thickness were set to 1.0 nmꢃminꢂ1 and 100 nm, respectively.
The wettability of the bare and modified Ta2O5 surfaces was
observed by measuring the water contact angle. Surface
morphology and crystallinity were investigated by atomic force
microscopy (AFM) and X-ray diffraction (XRD).
The anodized Ta2O5 surface was smooth, and the root-
mean-square roughness value of it was below 0.2 nm, which
was as small as that of the thermally oxidized SiOx. The ano-
dized surface showed hydrophilicity with the water contact angle
of 29ꢄ, and it was expected that the LB monolayer exposing the
outermost layer of the hydrophobic group would be easily depos-
ited on it. Actually, CdC18 and CdPFC18 monolayers were
deposited with good transfer ratios (almost unity). The water
contact angles were 103 and 104ꢄ for the CdC18 and CdPFC18
monolayers, respectively, and hydrophobic nature of these
modified surfaces was confirmed. The values of contact angles
agree well with the previously reported ones.12,13 AFM observa-
tion also showed very flat surfaces of the deposited monolayer
films. Thus, we were able to obtain CH3- or CF3-terminated
well-ordered Ta2O5 surfaces by the LB method. We think that
the LB monolayer did not change the structure during the growth
of pentacene and C60 films, because they were grown at room
temperature.
Organic semiconductors have drawn much attention due to
their electronic functionality and mechanical flexibility. Devel-
opment of organic field effect transistors (OFETs) using such
soft materials is of great importance and its investigation has
spread world-wide.1,2 In most of such OFETs, thermally oxi-
dized SiOx films on conductive Si substrates have been used as
the gate dielectric. However, the dielectric constant (k) of the
SiOx layer is rather low so that it is difficult to inject a large
amount of carriers into the organic layer with low gate voltages.
Recently, many groups have reported the characteristics of the
OFETs which were fabricated on high-k gate insulators.2,3 We
have also succeeded in fabricating OFET using an anodized tan-
talum pentaoxide (Ta2O5) layer as the high-k gate dielectric.4
Another problem still remains if we use the high-k gate di-
electric. The surface of the oxide layer is amorphous, so that a
single-crystalline organic thin film never grows on the anodized
Ta2O5 surface. It is reported that the performance of OFETs is
mainly affected by the quality of the initial organic layer on
the gate dielectric.5,6 Therefore, the improvement in the crystal-
linity of the dielectric surface is expected to reduce the defects or
the domain boundaries in the organic layer, and improve the
working characteristics of OFETs. As for the SiOx surface, orga-
nosilane self-assembled monolayers (SAMs) are used to intro-
duce the ordered structure, and improvement in the OFET per-
formance has been reported.7–10 To our knowledge, however,
no SAMs onto the Ta2O5 surface has been reported so far. In
the present study, we focused our attention on the modification
of the amorphous Ta2O5 surface by the LB films of long-chain
fatty acids, which provides an artificial two-dimensional crystal-
line film on the solid substrates.11 Then, we grew pentacene and
C60 thin films on the modified Ta2O5 surfaces, and investigated
the crystallinity of the grown films.
Figure 1 shows AFM images of the pentacene and C60 thin
films on the above-mentioned surfaces. On the bare Ta2O5 sur-
face, the pentacene film had very large domains with a dendritic
shape similar to that on the SiOx surface, and the layer-by-layer
growth was clearly observed. The domain size of the C60 film,
however, was far smaller than that of the pentacene film. The
nucleation density was very high, and there were quite a number
of domain boundaries. It was not expected to grow a C60 film
with good crystallinity on the bare surface. On the CH3-terminat-
ed Ta2O5 surface, we observed large domains of pentacene,
which had the similar size with those on the bare Ta2O5 surface.
It seemed that the nucleation density was a little smaller, but the
A 0.1 mm-thick Ta sheet was purchased from Furuuchi
Chemical Corporation. A mirror-like anodized Ta2O5 surface
was obtained following the previously reported processes.4
The Ta substrate cut from the sheet was electrolytically polished
in an aqueous HF solution (50%):concentrated H2SO4 = 1:9
(v/v) mixed electrolytic solution. The polished substrate was
anodized in a KI solution (2 ꢁ 10ꢂ4 molꢃLꢂ1) at 300 V, which
produced a 600 nm-thick anodized Ta2O5 layer on Ta. Octadec-
anoic acid (C18A, Tokyo Chemical Industry Co., Ltd.) and
perfluorooctadecanoic acid (PFC18A, Fluorochem Ltd.) were pu-
Copyright Ó 2006 The Chemical Society of Japan
Onoki, Ryo
