Novel method for construction of a metal-organic monolayer-Si structure utilizing thiol-terminated monolayer covalently bonded to the surface through Si-C bonds

Article abstract of DOI:10.1246/cl.2010.768

An organic monolayer with a thiol group at its end was formed on a hydrogen-terminated (H-) Si(111) surface via Si-C covalent bond by hydrosilylation. PtCl₄^2- complex ions were then immobilized on the thiol-terminated monolayer by immersing the substrate in an aqueous solution of K₂PtCl₄ using a strong interaction between the Pt complex and thiol groups. The immobilized Pt complexes were then reduced to Pt nanocluster by NaBH₄. Precise surface structure was characterized and it is proven that Pt species were indeed deposited only on top of the thiol-terminated monolayer.

Full text of DOI:10.1246/cl.2010.768

doi:10.1246/cl.2010.768
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Published on the web June 12, 2010
Novel Method for Construction of a Metal­Organic Monolayer­Si Structure Utilizing
Thiol-terminated Monolayer Covalently Bonded to the Surface through Si­C Bonds
Hitoshi Fukumitsu,1,2 Takuya Masuda,1,2 Deyu Qu, Yusuke Waki, Hidenori Noguchi,
1
1
1,2,3
Katsuaki Shimazu, and Kohei Uosaki*
4
1,2,3
1
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
2
Innovative Center of Nanomaterials Science for Environment and Energy (ICNSEE),
National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044
International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS),
Tsukuba, Ibaraki 305-0044
3
4
Group of Functional Chemistry, Section of Environmental Materials Science, Faculty of Environmental Earth Science,
Hokkaido University, Sapporo, Hokkaido 060-0810
(
Received May 17, 2010; CL-100475; E-mail: UOSAKI.Kohei@nims.go.jp)
An organic monolayer with a thiol group at its end was
formed on a hydrogen-terminated (H­) Si(111) surface via Si­C
covalent bond by hydrosilylation. PtCl42 complex ions were
then immobilized on the thiol-terminated monolayer by
immersing the substrate in an aqueous solution of K PtCl
layer in real devices, it is essential to form an M­OM­SC
structure with a top solid metal layer.
¹
2
4
using a strong interaction between the Pt complex and thiol
groups. The immobilized Pt complexes were then reduced to Pt
nanocluster by NaBH . Precise surface structure was charac-
4
terized and it is proven that Pt species were indeed deposited
only on top of the thiol-terminated monolayer.
Metal layer formation only on top of the organic monolayer
(OM) without direct contact between the substrate and the metal
layer is essential for the structure to be used in electronic
1­3
devices. Although many attempts have been made to deposit
metal on OM to construct metal­OM­substrate sandwich
structure utilizing both gas4 and liquid phase processes,
most attempts have resulted in penetration of the metal into the
OM and the deposition of metal layer not only on top of the OM
but also at the OM/substrate interface mainly because of the
existence of defect sites in the OM and the lack of control of the
deposition rate and amount of the deposited metal.
­6
7,8
Scheme 1. A schematic illustration of surface modification step.
Here, we propose a novel method for the construction of an
M­OM­SC structure with a top platinum layer by following the
procedure shown in Scheme 1.
Si(111) wafers (n-type, 1­10 ³ cm) donated by Shin-Etsu
Semiconductor were used as a substarate for the formation of
Recently, Kolb and his co-workers reported a new approach
to construct a metal­organic monolayer­metal (M­OM­M)
sandwich structure, by first immersing the Au(111) substrate
modified with self-assembled monolayer (SAM) in an aqueous
solution of noble metal complex such as K PtCl , K PdCl , and
PhCl3, and then reducing the metal complex attached to the
SAM in a metal-free solution to form metal clusters. We also
reported the construction of M­OM­M structure using dithiol
SAM. These results show that strong interactions between
terminal groups of SAM and metal, both complexes and reduced
metals, as well as the reduction in a metal ion free solution
are the key to the successful construction of M­OM­M
junctions.
1
3­15
H­Si(111) surface by a previously reported procedure.
The
9
formation of H­Si(111) surface without oxide formation was
confirmed by attenuated total reflection Fourier transform
infrared spectroscopy (ATR FT-IR) using a parallelogram ATR
2
4
2
4
1
3,14,25
prism
(Figure 1(a)) and X-ray photoelectron spectroscopy
(Figure 2A(a)).
1
6,25
(XPS)
1
0
Construction of metal­organic monolayer­semiconductor
(
M­OM­SC) structure11 may be more important as far as the
development of novel electronics devices is concerned. In
particular, silicon is the most attractive semiconductor substrate
because novel M­OM­SC devices can easily be incorporated
to highly advanced silicon-based electronic technology. Hence,
M­OM­SC junctions have been constructed on silicon surfaces
modified with OM by using a mercury drop as a top metal
Figure 1. ATR FT-IR spectra of (a) H­, (b) HSC11­, and (c) Ptcomplex­
SC11­Si(111) surface.
The H­Si(111) substrate was illuminated with 254 nm light
1
2
¹2
electrode. Since a mercury drop cannot be used as a top metal
(ca. 3 mW cm ) for 2 h in a deaerated mixed solution of 1-
Chem. Lett. 2010, 39, 768­770
© 2010 The Chemical Society of Japan
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decene and 11-mercapto-1-undecene25 (20:1 by mole fraction) in
an Ar-filled glove box (Scheme 1(i)). The mixed solution was
used because the formation of a monolayer using neat 11-
mercapto-1-undecene was failed with a relatively small mono-
layer coverage and silicon oxide formation. After this treatment,
peaks of C­H stretching vibration were observed in the ATR FT-
IR spectrum (Figure 1(b)) and XPS measurement revealed the
70, and 90°. As the take-off angle decreased, the intensity of
Si 2p peak decreased but those of S 2s and Pt 4f peaks slightly
increased, suggesting that the Si(111) substrate was covered with
an organic monolayer with thiol groups at its end and that Pt
complex was immobilized on top of the molecular layer. Similar
results were obtained at Ptcomplex­SC11­Si(111) surface.
More quantitative analysis can be carried out utilizing the
relation between the XPS signal intensity from a thin-film-
1
6
presence of sulfur (Figure 2B(b)), confirming the formation
of monolayer with thiol groups and no oxide formation
1
8
covered substrate and take-off angle, ª, as follows
(
Figure 2A(b)). We denote this monolayer-modified Si as
ꢀ
d
HSC11­Si(111), although the majority of the monolayer is of
decyl group.
lnðIÞ ¼ ­
þ lnðI0Þ
ð1Þ
sin ª
where I and I represent the signal intensity at a bare substrate
0
and at a substrate covered with a thin film of thickness d,
respectively, and ­ is the photoelectron mean free path within
the thin film. According to eq 1, ln(I) should be linearly related
to 1/sin ª with a slope of (¹d/­) and an intercept of ln(I0).
Figure 4(d) shows ln(relative integrated intensity between Si 2p
and S 2s peaks) (open circles) and ln(relative integrated intensity
between S 2s peak and Pt 4f peak) (closed circles) of the Pt­
SC11­Si(111) surface as a function of 1/sin ª. The distance
between the thiol group and the Si surface was calculated to be
Figure 2. XP spectra in (A) Si 2p, (B) S 2s, and (C) Pt 4f regions of (a)
H­, (b) HSC11­, and (c) Ptcomplex­SC11­Si(111) surfaces.
1
.3 nm by using slope for open circles in Figure 4d, ¹d/
The monolayer-covered substrate was then immersed in a
mM aqueous K2PtCl4 solution for 20 min and then rinsed with
water (Scheme 1(ii)). The incorporation of Pt(II) complex by
this treatment (Ptcomplex­SC11­Si(111)) was confirmed by the
presence of Pt bands in XP spectrum16 (Figures 2C(b) and
­ = ¹0.38 and mean free path, ­, for Si 2p photoelectrons in
1
9
5
the organic monolayer as 3.4 nm. This thickness is in good
agreement with the ellipsometric thickness of the HSC11-
monolayer (1.5 nm). Slope for ln(I(S)/I(Pt)) (closed circle in
Figure 4(d)) is almost 0, indicating that the position of Pt is the
same as that of S, i.e., the outermost part of the monolayer.
3
A(a)). The importance of Pt­S chemical interaction for the
attachment of Pt complex to the monolayer was proven by the
fact that no Pt 4f peak was observed in the XP spectrum of the
Pt-treated Si(111) substrate with decyl monolayer, which has no
thiol group at its end. The Ptcomplex­SC11­Si(111) substrate was
then immersed in a 0.1 M aqueous NaBH4 solution for 1 min to
reduce the Pt complex to metallic Pt (Scheme 1(iii): Pt­SC11­
Si(111)). XP spectrum of Pt­SC11­Si(111) shows the Pt 4f peak
appears at lower binding energy than that of Ptcomplex­SC11­
1
6
Si(111) (Figure 3A(b)) but still higher than that of bulk Pt,
9a
indicating the formation of Pt nanoclusters. Atomic force
microscopy (AFM) also shows the presence of Pt nanoclusters
with diameter from a few nm to 10 nm as shown in Figure 3B.
2
5
Figure 4. XP spectra of Pt­SC11­Si(111) surface in (a) Si 2p, (b) S 2s,
and (c) Pt 4f regions measured at take-off angles of 10, 30, 50, 70, and 90°
and (d) ln(relative integrated intensity between Si 2p and S 2s peaks) (open
circles) and ln(relative integrated intensity between S 2s and Pt 4f peaks)
(
closed circles) as a function of (1/sin ª).
Figure 3. (A) XP spectra in Pt 4f region of (a) Ptcomplex­SC11­Si(111)
and (b) Pt­SC11­Si(111) surfaces and (B) AFM image of Pt­SC11­Si(111)
surface. 1 ¯m © 1 ¯m.
Several groups have reported that vibrational spectroscopy
is quite useful to probe the perturbation of organic monolayer
structure caused by the penetration of metal into the monolayer.
2
0
21
The relative position of each element in the vertical
direction was determined by the angle-resolved (AR) XPS
Richter et al. and Jun and Zhu investigated the structure of
octadecyltrichlorosilane (OTS) SAM on SiO surface affected
2
1
7
measurements. Figure 4 shows the XP spectra of Pt­SC11­
Si(111) surface in (a) Si 2p, (b) S 2s, and (c) Pt 4f regions
measured at take-off angle, which is defined as the angle
between the sample surface and the analyzer axis, of 10, 30, 50,
by the vapor metal deposition by IR spectroscopy. In both cases,
the introduction of conformational disorder in the alkyl chain
caused by the penetration of the deposited metal was observed.
2
2
Bittner et al. carried out sum frequency generation (SFG)
Chem. Lett. 2010, 39, 768­770
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spectroscopy of alkanethiol SAM on Au(111) surface during
copper electrodeposition. They showed that even very small
amounts of the electrochemically deposited copper can lead to a
remarkable change in the CH region of the SFG spectrum due to
an interaction between the penetrated metal and the alkyl chain.
Recently, we also reported that gauche defects were introduced
to alkyl monolayers, which were formed on H­Si(111) surface
via Si­C bond as of this study, when metals were deposited
by thermal evaporation and sputtering as evidenced by the
In conclusion, M­OM­SC structure without metal penetra-
tion into the monolayer is successfully constructed on a H­
Si(111) surface by the following steps. 1. An organic monolayer
with a thiol terminal group was formed on a H­Si(111) surface
via Si­C bond by UV-induced hydrosilylation. 2. Pt nanoclusters
were formed on the monolayer by first immersing the mono-
layer-covered substrate in an aqueous solution containing
PtCl4 , followed by the reduction of Pt complex in a Pt-free
solution. Results of vibrational spectroscopy and ARXPS show
the presence of Pt only on top of the monolayer. Further metal
deposition to form continuous metal layer by electro- and
electroless deposition using Pt as seeds and study of electron-
transport properties of the M­OM­Si(111) structure, which are
required for the application to electronic devices, are under
progress.
2¹
+
¹
appearance of methylene bands (d and d ) in the SFG
spectrum, clearly showing the penetration of metal into mo-
6
lecular layer.
Since SFG is quite sensitive to the small change of the
conformational order of monolayers, SFG measurements were
carried out to prove that Pt did not penetrate into the monolayer
and thus Pt is present only on top of the monolayer. The SFG
system used in the present study has been described in detail
This work was supported in part by a Grant-in-Aid for
Scientific Research (No. 1820501607), the Global COE Pro-
gram (Project No. B01: Catalysis as the Basis for Innovation in
Material Science) and Promotion of Novel Interdisciplinary
Fields Based on Nanotechnology and Materials, from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
6
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elsewhere.
Figure 5(a) shows an SFG spectrum in the C­H
stretching region of HSC11­Si(111) surface. The three peaks
¹
1
observed at 2876, 2937, and 2963 cm can be assigned to the
+
C­H symmetric vibration (r ), Fermi resonance (FR) between
+
r
and C­H bending overtone, and the C­H asymmetric
¹
vibration (r ), respectively, of the terminal methyl (CH ) group
of decyl monolayer. No peaks due to the C­H stretching
vibration of methylene (CH2) group around 2850 (d ) and
3
2
4
References and Notes
+
1
2
3
4
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¹1
¹
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4
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1
3
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1
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1
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2
2
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Supporting Information is available electronically on the CSJ-Journal Web
site, http://www.csj.jp/journals/chem-lett/index.html.
Chem. Lett. 2010, 39, 768­770
© 2010 The Chemical Society of Japan
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