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Chemistry Letters Vol.37, No.4 (2008)
A Close-packed, Highly Insulating Organic Thin Monolayer on Si(111)
Kentaro Tanaka,Ã1;2 Taishi Tanaka,2 Takeshi Hasegawa,3 and Mitsuhiko ShionoyaÃ2
1Department of Chemistry, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602
2Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
3Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551
(Received December 12, 2007; CL-071379; E-mail: kentaro@chem.nagoya-u.ac.jp)
A long-chain dialkylated olefin, 2C18 was found to form a
an H–Si(111) under UV irradiation at 254 nm. The water contact
angle of the surface reached a maximum of 106ꢀ within 30 min,
which indicates that the Si(111) was fully covered with hydro-
phobic alkyl chains.8 The topographic images of the 2C18-
SAM-covered Si(111) (2C18–Si(111)) observed in the normal
AFM mode clearly showed atomically flat terraces and 0.3-nm
steps.9 No domains larger than tens of nanometer were observed
showing that the 2C18-SAM is homogeneous at the submicrom-
eter scale.
close-packed, highly insulating self-assembled monolayer
(SAM) on Si(111) via Si–C bond formation. The IR absorption
spectra of the 2C18-SAM show that the alkyl chains are densely
packed with an all-trans conformation. The threshold voltage to
initiate the scanned probe oxidation (SPO) of the 2C18-SAM-
covered Si(111) was ca. 9 V, which is much higher than the
previously reported voltages for organic monolayers on Si or
SiO2/Si substrates.
The packing structures of the alkyl chains of the SAMs
were determined by transmission IR spectra. In this study,
multiple-angle incidence resolution spectrometry (MAIRS)10
was adopted. In MAIRS, an unpolarized transmitted infrared
ray through thin films on a transparent material is collected at
different angles of incidence, and the transmittance single-beam
spectra are subjected to chemometric spectral resolution to
obtain in-plane (IP) and out-of-plane (OP) absorption spectra.10
In both the IP and OP spectra of 2C18–Si(111), the symmetric
and asymmetric stretching vibrations of the CH2 groups were
observed at wavenumbers assignable to the all-trans conforma-
tion (Figure S2a).16 This clearly indicates that the alkyl chains
are close-packed and crystalline in 2C18-SAM. In contrast,
absorption peaks for stretching vibrations of the CH2 groups
of C18–Si(111) (Figure S2b)16 were observed at higher wave-
numbers compared with those of 2C18-SAM, which indicates
that the alkyl chains display gauche conformation in which the
alkyl chains are disordered. In addition, the higher wavenumber
shifts of the bands in the OP spectrum from those in the IP one
indicate that the alkyl chains are bent with a roughly perpendic-
ular stance to the substrate.10b,10d,10e
The tilt angle of the straight alkyl chains of 2C18-SAM
from the surface normal was estimated to be 44 Æ 1ꢀ from the
absorbance ratio of the CH2 groups in the IP and OP spectra,10
which shows the thickness of 2C18-SAM is ca. 2 nm. Further-
more, the 3-dimensional density of the CH2 groups per unit
volume in 2C18-SAM was calculated from the absorbance of
the ꢀsCH2 band to be as large as those of close-packed LB
films of 10-layered cadmium stearate on a Ge substrate10a (see
Supporting Information).16 This result is consistent with the
observation that alkyl chains in 2C18-SAM adopt all-trans con-
formation and arranged in a close-packed manner on Si(111).
Although other olefins also exhibited homogeneous SAM
structures as observed in the AFM topographic images,9 the
alkyl chains proved to be more disordered with smaller water
contact angles and densities compared with those of 2C18-
SAM (Table S1).16
Hydrogen-terminated silicon surfaces (H–Si(111)s) are well
known to provide alkyl self-assembled monolayers (SAMs)
through robust Si–C bonding1 under mild conditions such as
light irradiation2 (Figure 1a). SAMs have great potential to be
atomically flat, close-packed,3 thermally,4 and chemically5
robust, and possess excellent electrical characteristics.6 In
addition, chemical modification of the organic building blocks,
in advance or stepwise, and hierarchical stacking of the function-
alized SAMs can be readily performed to control surface
functions.1,3 Thus, the application of SAMs on Si substrates
shows promise particularly as functionalized substrates for elec-
tronic devices that require highly insulating SAMs. Although
SAMs composed of long-alkyl chains are known to be robust
in the voltage range of 2–5 V,6,7 SAMs with higher insulating
properties have been long-awaited for insulators, gate dielec-
trics, resistors, and capacitors. Herein, we report a close-packed
organic thin monolayer on Si(111) using a dialkylated olefin,
2C18 (Figure 1b), that provides a highly insulating monolayer
on Si(111). We also found using a series of precursory olefins
(Figure 1b) that chemical structures of alkyl chains employed
have a strong correlation with the packing modes of structures
on Si(111) in relation to the insulating properties.
A 2C18-SAM was formed from dialkylated olefin 2C18 on
The packing efficiency of organic molecules spatially fixed
through covalent bonding in Si–C type SAMs on Si(111)s
largely depends not only on their structures but also on the
Figure 1. A self-assembled monolayer on Si(111): (a) a
schematic representation of alkylation on Si(111); (b) olefins
used in this study.
Copyright ꢀ 2008 The Chemical Society of Japan
Tanaka, Kentaro
