A new method to generate arene-terminated Si(111) and Ge(111) surfaces via a palladium-catalyzed arylation reaction

Article abstract of DOI:10.1021/ja308606t

Formation of silicon-aryl and germanium-aryl direct bonds on the semiconductor surface is a key issue to realize molecular electronic devices, but the conventional methods based on radical intermediates have problems to accompany the side reactions. We developed the first example of versatile and efficient methods to form clean organic monolayers with Si-aryl and Ge-aryl bonds on hydrogen-terminated silicon and germanium surfaces by applying our original catalytic arylation reactions of hydrosilanes and hydrogermanes using Pd catalyst and base in homogeneous systems. We could immobilize aromatic groups with redox-active and photoluminescent properties, and further applied in the field of rigid π-conjugated redox molecular wire composites, as confirmed by the successive coordination of terpyridine molecules with transition metal ions. The surfaces were characterized using cyclic voltammetry (CV), water contact angle measurements, X-ray photoelectron spectroscopy (XPS), fluorescence spectroscopy, and atomic force microscopy (AFM). Especially, the AFM analysis of 17 nm-long metal complex molecular wires confirmed their vertical connection to the plane surface.

Full text of DOI:10.1021/ja308606t

Article
pubs.acs.org/JACS
A New Method To Generate Arene-Terminated Si(111) and Ge(111)
Surfaces via a Palladium-Catalyzed Arylation Reaction
Yoshinori Yamanoi,* Junya Sendo, Tetsuhiro Kobayashi, Hiroaki Maeda, Yusuke Yabusaki,
Mariko Miyachi, Ryota Sakamoto, and Hiroshi Nishihara*
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: Formation of silicon−aryl and germanium−aryl
direct bonds on the semiconductor surface is a key issue to
realize molecular electronic devices, but the conventional
methods based on radical intermediates have problems to
accompany the side reactions. We developed the first example
of versatile and efficient methods to form clean organic
monolayers with Si−aryl and Ge−aryl bonds on hydrogen-
terminated silicon and germanium surfaces by applying our
original catalytic arylation reactions of hydrosilanes and
hydrogermanes using Pd catalyst and base in homogeneous
systems. We could immobilize aromatic groups with redox-
active and photoluminescent properties, and further applied in the field of rigid π-conjugated redox molecular wire composites, as
confirmed by the successive coordination of terpyridine molecules with transition metal ions. The surfaces were characterized
using cyclic voltammetry (CV), water contact angle measurements, X-ray photoelectron spectroscopy (XPS), fluorescence
spectroscopy, and atomic force microscopy (AFM). Especially, the AFM analysis of 17 nm-long metal complex molecular wires
confirmed their vertical connection to the plane surface.
halides (Figure 1a),10−17 we recently reported the palladium-
INTRODUCTION
■
catalyzed arylation of the triply σ-back bonded silane,
In recent years, semiconductor surfaces terminated with organic
tris(trimethylsilyl)silane, in the presence of an organic base.18
compounds have attracted considerable interest, because of the
Arylated products were obtained in good to high yields, without
scientific importance of heterointerfaces. The attachment of
breaking the weak Si−Si back bonds (Figure 1b). The surface
functional monolayers onto silicon surfaces is an attractive
modification was directed by the model reactions of aryl iodides
approach for the construction of novel interfaces for molecular
and tris(trimethylsilyl)silane, which is a molecular model for
electronics applications. The organic modification of hydrogen-
the Si−H group on hydrogen-terminated silicon(111) surfaces
terminated silicon surfaces with Si−C bonds provides electronic
(Si(111)−H). Because this transformation occurs under mild
coupling between organic functionalities and semiconductors
conditions and is compatible with a large number of terminal
without the interference of interfacial oxide thin films.1−4
group functionalities, it is of interest to determine if this
Various modification methods have been investigated for the
homogeneous catalytic process can be utilized to a heteroge-
formation of Si−C bonded organic monolayers on silicon
neous reaction to produce a clean aromatic monolayer on
surfaces, including the hydrosilylation of alkenes or alkynes.
silicon surfaces. Because of the attractive electrochemical
The immobilization of aryl groups on oxide-free silicon is less
properties of ferrocene-containing molecule (1), heteroaro-
common, although it is useful because the common aryl groups
are chemically and thermally stable, structurally rigid, and
electronically conductive, and thus suitable for controlling the
geometric configuration and electronic interaction of the
matic molecule (2), photoluminescence properties of anthra-
cene-containing molecule (3), and good metal-coordination
abilities of terpyridine-containing molecule (4), we showed in
this study that they can be effectively immobilized onto
attached molecules with the semiconductor substrate. In the
Si(111)−H and Ge(111)−H to form a monolayer (Figure 1c).
conventional methods, the Si−aryl bond formation is generally
performed using aryl diazonium salts5−8 or via the electro-
grafting of iodonium salts.9 However, not monolayer formation
but multilayer formation is facilitated by the high reactive
radical intermediates used in these methods. Establishing that
there is a covalent bond between the aryl group monolayer and
the electrode surface is a complex task.
■
MATERIAL AND METHODS
Chemicals. All synthetic procedures were carried out in an inert
atmosphere. Compounds 119 and 420 were prepared according to
known methods. Compound 3 was synthesized by Suzuki coupling.
In the course of our studies on the transition-metal-mediated
Received: August 30, 2012
transformation of hydrosilanes and hydrogermanes with aryl
Published: November 26, 2012
© 2012 American Chemical Society
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immersed at a constant temperature. After immersion, the samples
were removed from the solution, and successively rinsed ultrasonically
with 1,4-dioxane, ethanol, and ultrapure water (Milli Q) for 5 min.
The modified wafers were dried in a stream of argon gas.
Preparation of Fe(tpy)2 Oligomer Wires. The tpy-terminated
wafers prepared in our method using a Pd catalyst were immersed in a
0.1 M ethanol solution of Fe(BF4)2 at room temperature for 1 h, and
subsequently in a 0.1 mM chloroform solution of 4′,4‴′-(1,4-
phenylene)bis(2,2′:6′,2″-terpyridine) (tpy−C6H4−tpy) for 23 h at
room temperature to make bis(terpyridine) complexes. These two
processes were repeated for the preparation of the Fe(tpy)2 oligomer
wires.
Surface Characterization. Electrochemical measurements were
carried out using a modified silicon(111) or germanium(111) wafer as
working electrode (electrode area: 0.264 cm2), a Pt wire counter
electrode, and an Ag+/Ag (10 mM AgClO4 in 0.1 M Bu4NClO4/
CH3CN) reference electrode in a standard one-compartment cell in
the dark. Cyclic voltammetry was performed using an ALS 750A or an
ALS 650B electrochemical analyzer. Contact angle measurements were
made by providing a 1 μL drop of Milli-Q water from a microsyringe
onto the surface of the sample and observing with an optical
microscope. The image of the static water droplet was captured using a
digital camera. Several measurements were taken on each side of the
droplet for each sample, and the average water contact angle was
calculated for the 20 individual samples. X-ray photoelectron
spectroscopy (XPS) measurements were performed using a
PHI5000 VersaProbe. Al Kα (12 kV, 25 mA) was used as the X-ray
source. The analyses were carried out at photoelectron angles (relative
to the sample surface) of 5°. Calibration of the spectra for ferrocene-
containing film was achieved with C 1s spectrum at 285.0 eV (Figure
S2, Supporting Information). That for 1H-imidazol-5-yl group
immobilized silicon was achieved with Si 2p spectrum at 99.2 eV
(Figure 3). The emission spectra and excitation spectra were recorded
with a Hitachi FL-4500 spectrometer. AFM measurements were
performed in high amplitude mode (tapping mode) using an Agilent
Technologies 5500 scanning probe microscope, and a silicon cantilever
PPP-NCH or PPP-NCL (Nano World).
Figure 1. Synthetic scheme. (a) Transition metal-catalyzed arylation of
hydrosilanes and hydrogermanes (previous work). (b) Pd-catalyzed
arylation of tris(trimethylsilyl)silane (model reaction). (c) Pd-
catalyzed surface reaction to immobilize aromatic group directly on
hydrogen-terminated silicon(111) or germanium(111) surfaces (this
work).
1
These compounds were identified using H and 13C NMR and EI−
MS. Compounds 2 and L (tpy−C6H4−tpy) were purchased from TCI
and Sigma-Aldrich, respectively.
Preparation of Hydrogen-Terminated Silicon(111) Surface.
The hydrogen-terminated Si(111) surface was prepared using a
modification method based on our previous reports.21−24 N-type
silicon(111) wafers (for the immobilization of 1, P doped, 0.001−
0.013 Ω cm, E & M Co. Ltd. and for the immobilization of 3, Sb
doped, 0.01−0.03 Ω cm, SGI Japan Ltd.) and P-type silicon(111)
wafers (for the immobilization of 2 and 4, B doped, ≤0.005 Ω cm, E &
M Co. Ltd.) were cut into squares (15 × 15 mm). The wafers were
cleaned ultrasonically in ultrapure water (Milli Q), ethanol, acetone,
and ultrapure water (Milli Q) for 5 min to remove contaminations.
The silicon wafers were then etched in a 1% HF aqueous solution at
room temperature for 2 min and further etched in a 40% NH4F
aqueous solution at 70 °C for 2 min to remove surface oxides and
make the surfaces atomically flat. Finally, the wafers were taken out
and rinsed thoroughly with ultrapure water, and then dried in a stream
of argon gas. The samples for AFM measurements and for
constructing the oligomer wires were hydrogen-terminated as
follows.25 The silicon wafers were immersed in piranha solution
(conc. H2SO4:30% H2O2 = 3:1) at 120 °C for 10 min and stored in
ultrapure water (Milli Q). Just before use, the oxidized silicon wafers
were immersed in 40% NH4F aqueous at room temperature for 10
min, and then rinsed thoroughly with ultrapure water and dried in a
stream of argon gas.
Preparation of Hydrogen-Terminated Germanium(111) Sur-
face. The hydrogen-terminated Ge(111) surface was prepared using a
modification method based on the literature.26 P-type germa-
nium(111) wafers (B doped, 0.1−1.0 Ω cm, Tak materials Co. Ltd.)
were cut into squares (15 × 15 mm). The wafers were cleaned
ultrasonically in ultrapure water (Milli Q), ethanol, acetone, and
ultrapure water (Milli Q) for 5 min to remove contaminations. The
germanium wafers were then immersed in a 47% HF aqueous solution
at room temperature for 10 s, and then rinsed with ultrapure water.
This procedure was repeated a total of five times before drying the
samples in a stream of argon gas.
RESULTS AND DISCUSSION
■
For applying the homogeneous catalytic reaction to the surface
modification, preliminary studies were carried out to optimize
the immobilization of 1 on a hydrogen-terminated silicon
surface. The hydrogen-terminated silicon surface was prepared
by etching an oxidized Si surface in an HF and NH4F solution
as described in our previous reports.21−24 The surface coverage
was estimated using cyclic voltammetry. As shown in Table 1,
when the freshly prepared Si(111)−H surface was immersed in
a mixture of 4-iodophenylferrocene (1), (i-Pr)2EtN, Pd(P(t-
Bu)3)2, and 1,4-dioxane at 100 °C for 20 h, a modified surface
with a Si−C bonded organic monolayer with good surface
coverage was obtained (entry 5 in Table 1 and Figure 2a). The
modified surface was rinsed successively with 1,4-dioxane,
ethanol, and ultrapure water in an ultrasonic bath for a time of
5 min for each solvent. Longer rinsing did not decrease the
surface coverage, indicating that the molecules were strongly
attached to the surface and that there was no abrasion of the
surface during the ultrasonic treatment. Other palladium
catalysts, such as Pd(dba)2 or Pd(PPh3)4, were less effective
for this transformation (entries 3 and 4). As a control, the
Si(111)−H surface was exposed to a mixture of 1 and base in
the absence of a catalyst at 100 °C for 24 h. No immobilization
of 1 was observed under these conditions (entry 6 in Table 1
and Figure 2a). The quantity of palladium catalyst used is
important, and the formation of Pd nanoparticles (ca. 50 nm)
on the surface was confirmed at intervals using AFM when the
concentration of the palladium catalyst was higher (>ca. 0.8
Immobilization of Aryl Groups onto Hydrogen-Terminated
Silicon(111) and Germanium(111) Surface. The fresh hydrogen-
terminated silicon(111) or germanium(111) wafers were added to a
mixture of 1,4-dioxane, Pd(P(t-Bu)3)2 (0.08 mM), (i-Pr)2EtN (0.3 M),
and 1, 2, 3, or 4 (8 mM) under a nitrogen atmosphere, and were
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a
Cyclic voltammograms for monolayer films of ferrocene on
N-type Si(111) and P-type Ge(111) in Bu4NClO4/CH3CN are
shown in Figure 2. The redox peaks did not change at all, even
when the potential sweep was repeated 200 times (Figure 2b).
This showed that the modified surface prepared using this
method had excellent durability, which resulted from the
combination of the 4-ferrocenylphenyl groups with the silicon
substrate via covalent Si−C bonds. A pair of anodic and
cathodic waves was observed, with peak potentials located at
approximately 0.1 V versus Ag+/Ag. The anodic and cathodic
peak currents exhibited a linear dependence on the scan rate,
indicating a surface confined redox process. By integrating the
cathodic waves in the voltammograms, the surface coverage Γ
was estimated to be 2.6 × 10−10 mol/cm2 (Figure 2c). The
monolayer coverage was estimated to be ca. 58%; this value was
related to the unit cell size of Si(111) (12.77 Å2)28 as compared
to the diameter of the 4-ferrocenylphenyl group (38.0 Å2), and
the maximum coverage of 4-ferrocenylphenyl groups on a
silicon surface can be estimated to be 4.5 × 10−10 mol/cm2. The
coverage of 4-ferrocenylphenyl groups on the germanium
surface was similarly estimated to be ca. 15% (Figure 2d).
Water contact angle measurement and AFM were employed
to investigate the properties of the modified silicon substrate.
The silicon surfaces modified with 4-ferrocenylphenyl mono-
layer were more hydrophilic as shown by smaller contact angles
obtained with H2O (45 2°), as compared to those measured
Table 1. Optimization of Immobilization Process
surface
coverage Γ
temperature
(×10−10
b
entry
E
Pd(0) catalyst
solvent
THF
(°C)
mol/cm2)
1
2
3
4
5
6
7
Si
Si
Si
Si
Si
Si
Ge
Pd(P(t-Bu)3)2
Pd(P(t-Bu)3)2
Pd(dba)2
20
50
0.06
0.24
0.12
0.10
2.6
THF
THF
50
Pd(PPh3)4
THF
50
Pd(P(t-Bu)3)2
1,4-dioxane
1,4-dioxane
1,4-dioxane
100
100
100
c
c
−
−
Pd(P(t-Bu)3)2
0.69
a
Reaction conditions: Si(111)−H or Ge(111)−H (15 × 15 mm) was
immersed in a solution (7.5 mL) of catalyst (3.0 mg), (i-Pr)2EtN (0.38
mL), and 1 (0.06 mmol) for 20 h. Average of several runs. No
aromatic compound was observed on the surface in the absence of Pd
catalyst.
b
c
on hydrogen-terminated silicon (82
5°) (Figure S3,
Supporting Information).29 The water contact angles measured
in the present study were approximately consistent with other
ferrocene-terminated SAMs.30,31 The surface homogeneity of
the modified silicon surface was confirmed in the AFM images
(Figure S4, Supporting Information). The modified surface had
terraces with the same width and atomic-thick steps similar to
the hydrogen-terminated Si(111) surface. These results
demonstrated that the prepared surfaces had uniform
monolayer films.
The modified surface was also characterized using XPS,
because useful information on the nature of the grafted layer
was obtained (Figures S2 and S5, Supporting Information). A
very weak Si 2p signal for silicon oxide (ca. 103 eV) was
measured, due to the extended handling. Peaks for Fe 2p
photoelectrons were observed at binding energies of 708.1 and
720.3 eV for the modified surface, indicating that the organic
monolayer formed successfully on the silicon surface. These
values agree well with those previously reported in the
literature.31 Peaks for C−Si (at approximately 284−285 eV)
and C−C species are overlapped in the C 1s spectra and were
not resolved under these measurement conditions.
Figure 2. Electrochemical characterization of modified silicon or
germanium surface. (a) CVs for entry 5 (Si(111)−1) and entry 6 in 1
M Bu4NClO4/CH3CN. (b) CVs for entry 5 (Si(111)−1) in 1 M
Bu4NClO4/CH3CN with 200 scan cycles. (c) Characterization of
Si(111)−1. (d) Characterization of Ge(111)−1. Cyclic voltammo-
grams for (c) and (d) were carried out in 1 M Bu4NClO4/CH3CN.
The scan rates were from 0.025 to 0.5 V s−1. (Inset) Plots of anodic
and cathodic peak currents versus the scan rates.
To overcome these problems with the XPS measurements,
1H-imidazol-5-yl groups were immobilized on a hydrogen-
terminated silicon surface (Figure 3a). The XPS spectra for the
chemical species resulting from the treatment of 5-iodo-1H-
imidazole (2) with H−Si(111) are shown in Figure 3. Figure 3b
shows high-resolution C 1s spectra for the modified surface.
Three peaks were observed; the peak at 284.7 eV represented
the C 1s binding energy of hydrocarbon, and the other two
peaks, located at 284.0 and 286.0 eV, in the shoulder of the
main peak were assigned to Si−C and C−N species,
respectively.32 Figure 3c shows high-resolution Si 2p spectra
for the modified surface. The spectra were divided into three
components. The peak centers located at 99.2, 99.7, and 100.1
eV represented two elemental Si and Si−C, respectively. These
results suggested that the aromatic groups were attached on the
mM) as shown in Figure S1, Supporting Information.27
Although it was difficult to set palladium on surface to zero
completely, XPS clearly showed that the amount of palladium
on surface sharply decreased when palladium concentration was
changed from 0.8 to 0.08 mM. The quantity of ferrocenyl
(Fe2+) group was close to the same under both reaction
conditions (Figure S2, Supporting Information). Under the
same reaction conditions with entry 5, Ge(111)−H was also
reacted with 1, and a ferrocene-modified Ge surface was
obtained (entry 7).26 Because a Ge−Ge bond is longer than a
Si−Si bond, the influence of a horizontal molecule becomes
small when a self-assembled monolayer (SAM) is formed on
the surface.
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Tada and Ara reported that fluorescein isothiocyanate (FITC)
was immobilized on silicon via three steps,37,38 these
observations demonstrated the possibility of potential applica-
tions in light emitting devices.
Finally, to further demonstrate the scope of this approach, we
attempted to construct redox-active metal complex oligomer
wires on a hydrogen-terminated Si(111) surface using
subsequent complexation as described in previous reports
from our laboratory.39−45 The preparation of the oligomer
wires was carried out as follows (Figure 5a). Because the
coordination of Fe2+ with tpy−C6H4−tpy was blocked when
there was a high density of tpy−C6H4 groups on the silicon
surface, the hydrogen-terminated Si(111) surface was initially
treated with 4′-(4-iodophenyl)-2,2′:6′,2″-terpyridine (tpy−
C6H4−I, 4) in the presence of Pd catalyst at 100 °C for 3 h
as tpy−C6H4 group can be sparsely fixed on the surface. The
tpy-covered substrate was then immersed in a 0.1 M ethanol
solution of Fe(BF4)2 at room temperature for 1 h, and
subsequently immersed in a 0.1 mM chloroform solution of
4′,4‴′-(1,4-phenylene)bis(2,2′:6′,2″-terpyridine) (tpy−C6H4−
tpy, L) for 23 h. The [Fe(tpy)2]2+ oligomer wires were
constructed by repeating these two processes (the coordination
of the Fe ions and the ligand).
Figure 3. High-resolution XPS of Si(111) substrates after grafting of
1H-imidazol-5-yl group. The detector is 5° from the surface plane. (a)
Structure of Si(111)−2. (b) C 1s region. (c) Si 2p region.
silicon surface through Si−C covalent bonds, and that any
possible physisorption or alternative grafting onto the surface
could be ruled out.
To give a further surface modification, the hydrogen-
terminated Si(111) surface was treated with 9-(4-iodophenyl)-
anthracene (Ant−C6H4−I, 3) in the presence of Pd catalyst at
100 °C for 20 h, using the method mentioned above (Figure
S6, Supporting Information). Contact angle measurements
confirmed the presence of terminal anthryl groups on the
modified surface. The water contact angle obtained for the
Cyclic voltammetry was performed for the [Fe(tpy)2]2+
oligomer wire-modified Si surface (denoted here as Si(111)−
4−(FeL)n) to confirm the stepwise formation of wires on the Si
surface (Figure S8, Supporting Information). The CV plots
shown in Figure 5 exhibited redox peaks corresponding to the
[Fe(tpy)2]3+/2+ couple in oligomer wires, at a potential of
approximately 0.8 V versus Ag+/Ag. The current waves of the
peaks increased as the number of immersion cycles (n)
increased. The coverage estimated from the cathodic peaks was
proportional to n, which suggested that the preparation of the
oligomer wires was quantitative. The anodic and cathodic peak
currents of Si(111)−4−(FeL)10 exhibited a linear dependence
on the scan rate (Figure S9, Supporting Information),
confirming that the molecular wires were confined on the
surface.
modified surface (73
3°) agreed well with previously
reported values for anthracene-terminated SAMs.33 Anthracene
derivatives have been recognized as attractive materials because
of their photochemical and electrochemical properties, as well
as their potential as a medium for photoconductive and
electroluminescent materials.34−36 The fluorescence at approx-
imately 420 nm, which derived from the Ant−C6H4 moiety,
appeared with the excitation at 254 nm (Figure 4). This
The [Fe(tpy)2]2+ oligomer wire growth process was observed
using AFM. Figure 6a shows representative AFM images of
Si(111)−4, Si(111)−4−(FeL)2, Si(111)−4−(FeL)4, and
Si(111)−4−(FeL)10. In these four images, a number of convex
structures were observed, which appeared to correspond with
the formation of the Fe(tpy)2 oligomer wires. The heights of
these structures were 1.2 0.2, 4.3 0.2, 7.3 0.4, and 17.0
0.5 nm (Figure 6b); these values were consistent with the
theoretical height of Si(111)−4−(FeL)n (ca. 1.6 nm × n-layers
(oligomer wire moiety) + 1.1 nm (surface anchoring moiety)).
The lengths of molecular wires were estimated by MM2
predictions. These results also suggested that oligomer wires
were grown vertically on the Si surface, and that tpy−C6H4−I
was immobilized on the Si surface via the Pd-catalyzed reaction
(Figure S10, Supporting Information).
Figure 4. Preparation and characterization of Si(111)−3. (a)
Immobilization of 9-(4-iodophenyl)anthracene (3) on Si(111).
Si(111)−H was immersed in a 1,4-dioxane solution (7.5 mL) of
Pd(P(t-Bu)3)2 (3.0 mg), (i-Pr)2EtN (0.38 mL), and 3 (0.06 mmol) at
100 °C for 20 h. (b) Comparison of fluorescence spectra (254 nm
excitation): (i) Si(111)−3, (ii) Si(111)−H, (iii) Ant−C6H4−I (3) in
the solid state.
Our proposed mechanistic explanation for the Si−H surface
arylation process is described in Figure 7. Initially, the
dissociation of a phosphine ligand formed the monoligating
Pd complex A. The resultant Pd(P(t-Bu)3) would be capable of
undergoing oxidative addition with H−Si on the surface,
generating the σ-bond metathesis intermediate B with aryl
iodide, and then forming Si−C(sp2) bonds with the reforming
active Pd catalyst in the presence of base. We do not think that
dangling bonds (silicon radical: Si•) participated in the Si−C
difference suggested that anthrylphenyl group was attached on
the Si(111) surface as compared to the fluorescence of Ant−
C6H4−I (3). The spectrum of Si(111)−3 displayed broader
than that in the solid state (Figure 4b) or in solution (Figure
S7, Supporting Information) due to a ground state π−π
stacking interaction in addition to excimer formation. Because
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Figure 5. Preparation of Si(111)−4 and resulting oligomer wires. (i) Immobilization of 4′-(4-iodophenyl)-2,2′:6′,2″-terpyridine (4) on Si(111).
Si(111)−H was immersed in a 1,4-dioxane solution (7.5 mL) of Pd(P(t-Bu)3)2 (0.3 mg), (i-Pr)2EtN (0.38 mL), and 4 (0.06 mmol) at 100 °C for 3
h. (ii, iii) Preparation of Fe(tpy)2 complex oligomer wires, Si(111)−4−(FeL)n. Inset: Cyclic voltammograms for Si(111)−4−(FeL)n (n = 1−4)
taken at a scan rate of 0.1 V s−1 in 1 M Bu4NClO4/CH2Cl2 (left) and coverage of Si(111)−4−(FeL)n versus the number of coordination cycles
(right).
Figure 6. (a) Representative AFM images of Si(111)−4, Si(111)−4−(FeL)2, Si(111)−4−(FeL)4, and Si(111)−4−(FeL)10. (b) Histogram of height
analyses of Si(111)−4, Si(111)−4−(FeL)2, Si(111)−4−(FeL)4, and Si(111)−4−(FeL)10.
bond formation, because tris(trimethylsilyl)silyl radical is an
coverage. Although a few studies have investigated transition
metal-catalyzed transformation onto hydrogen-terminated
silicon surfaces,47−49 problems result from substantial oxidation
and metal deposition at the surface. The present results showed
that covalently bonded aryl monolayers could be easily
produced on Si(111) and Ge(111) surfaces via the chemical
reaction of various aryl iodides with Si(111)−H or Ge(111)−H
in the presence of a transition-metal catalyst. The attachment of
these molecules was persistent and strong. The modified
surfaces were chemically stable and could be stored for several
weeks without measurable decomposition. This approach
underpins the development of the stepwise solid-phase-like
effective reducing species for organic halides.46
CONCLUSIONS
■
This palladium-catalyzed grafting process was effective for 4-
iodophenylferrocene (1), 5-iodo-1H-imidazole (2), 9-(4-
iodophenyl)anthracene (3), and 4′-(4-iodophenyl)-2,2′:6′,2″-
terpyridine (4), and gave the desired surface functionalization.
The availability of a synthetic approach is key to the
development of flexible surface synthesis strategies. Cyclic
voltammetry, X-ray photoelectron spectroscopy, and AFM were
used to characterize the surface and to estimate the surface
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Figure 7. Proposed mechanistic pathway of Pd-catalyzed Si−H
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ASSOCIATED CONTENT
* Supporting Information
Compound characterization data and additional character-
ization data of the modified silicon(111) surface. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
yamanoi@chem.s.u-tokyo.ac.jp; nisihara@chem.s.u-tokyo.ac.jp
■
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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Mater. 2000, 12, 1457−1460.
The present work was financially supported by Grant-in-Aids
from the Ogasawara Foundation for the Promotion of Science
& Engineering, Scientific Research on Innovative Areas
“Coordination Programming” (area 2107, no. 21108002), and
the Global COE Program for “Chemistry Innovation through
the Cooperation of Science and Engineering” from the Ministry
of Education, Culture, Sports, Science, and Technology, Japan.
We thank the Research Hub Advanced Nano Characterization
(School of Engineering, The University of Tokyo) for the X-ray
photoelectron spectroscopy measurements and also thank
ADEKA Corp. for their financial support throughout this work.
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