Self-assembled monolayers of isocyanides on nickel electrodes

Article abstract of DOI:10.1002/anie.200500942

(Figure Presented) Alligator clip on a nickel electrode: Monolayers of ferrocenyl-terminated isocyanides are strongly chemisorbed on nickel surfaces (see picture). The nickel-isocyanide interaction has a parallel binding mode, and the monolayers have higher thermal stability in boiling ethanol than thiol monolayers on gold. The nickel-isocyanide monolayers could be useful in molecular electronics.

Full text of DOI:10.1002/anie.200500942

Communications
Molecular Devices
junction is not as good as the thiol–gold junction for
molecular electronic applications. Therefore, it is necessary
to develop new alligator clips for Ni substrates. According to
theoretical calculations, the isocyanide–nickel junction has
significant advantages over the thiol–gold junction and others
Self-Assembled Monolayers of Isocyanides on
Nickel Electrodes**
[
3]
because of its low charge-transport barriers. There have
been several examples of SAMs generated from isocyanides
Youngu Lee, Gustavo M. Morales, and Luping Yu*
[
9]
[10]
[11]
[9b,12]
[13]
on Au, Pt,
Pd,
Ag,
and Cr.
However, the
Self-assembled monolayers (SAMs) of thiols on Au(111)
surfaces have been examined extensively in recent years as a
platform for surface science. They have been used in a
number of applications in the areas of molecular electronics
isocyanide–nickel interaction has not yet been explored
except for vacuum-evaporated methyl isocyanide (CH NC)
3
[
14]
on Ni(100) and Ni(111) surfaces.
Herein, we report the synthesis of ferrocenylalkyl iso-
cyanides and their assembly on polycrystalline Ni surfaces.
The ferrocenyl moiety was introduced as a messenger for
convenient characterization of monolayers by electrochem-
ical methods to evaluate the surface coverage and thermal
[
1]
and surface-property modulation. In molecular electronics,
SAMs of thiol on a gold substrate have been particularly
useful in controlling the placement and orientation of
molecular electronic components such as wires, diodes,
switches, and memories. In the design of molecular electronic
components, appropriate interfaces between molecules and
electrodes are as crucial as the active organic molecules. To be
useful, these interfaces should possess low impedance and
high stability. The mismatch between the Fermi level of the
electrodes and either the HOMO or LUMO level of the
adsorbent molecule generally induces a contact barrier which
[
15]
stability of SAMs.
Additionally, the conformation and
binding mode of isocyanide on Ni surfaces, which were
characterized by reflection–absorption infrared (RAIR)
spectroscopy, are discussed.
We synthesized the ferrocenyl-terminated isocyanide
molecules 5 to test their ability to chemisorb on Ni surfaces.
In addition to isocyanides, several ferrocenyl-terminated
molecules containing reactive functional groups that can
potentially chemisorb on Ni surfaces, such as cyanide (6),
isothiocyanate (7), diselenide (9), and thiol (10), were
synthesized (Scheme 1). Polycrystalline Ni surfaces were
used as substrates for monolayer formation. Ni surfaces
were pretreated by electrochemical reduction (ꢀ1.3 V vs. Ag/
[
2]
is the source of contact impedances. So far, the thiol–gold
linkage has been mainly considered for composing molecular
electronic devices. Gold has an advantage over other surfaces
for SAM formation because of its intrinsic inertness in air and
high conductivity. However, there are still several limitations
of gold surfaces as the electrode for practical molecular
electronic applications. Theoretical calculations predicted
that the combination of sulfur and gold yields one of the
AgCl, 3m NaCl) in an aqueous solution of HClO (1m) for
4
20 minutes prior to use, since Ni is easily oxidized on exposure
[
3]
[8]
highest contact barriers for charge transport. It is also well-
to air.
[
4]
known that SAMs of thiols on gold are thermally unstable.
The molecular packing and surface coverage of the
monolayers were assessed by cyclic voltammetry. Figure 1
shows cyclic voltammograms (CVs) obtained after electro-
chemically reduced Ni electrodes had been immersed in 0.01m
ethanolic solutions of 5, 9, and 10 for 48 hours. Oxidation and
reduction peaks were observed at + 0.47 and + 0.44 V, which
correspond to the oxidation and reduction of the ferrocene
Furthermore, the stochastic behavior of thiols on gold
electrodes that results from the breakage of the adsorbent
[
5]
molecule–metal contact have also been observed. The high
diffusion mobility of gold atoms makes them less attractive
for practical applications because of the limited lifetime of the
[
6]
devices. Therefore, it is meaningful to develop a new
combination of chemical linkage groups—alligator clips—
and metal electrodes.
[
15]
moiety.
However, Ni surfaces immersed for 48 hours in
solutions of ferrocenyl-terminated molecules containing azide
(2), amine (3), cyanide (6), and isothiocyanate (7) functional
groups gave featureless CVs similar to those of bare electro-
des. These results indicate that 5, 9, and 10 were successfully
chemisorbed on the electrochemically reduced nickel surfa-
ces. The charge density obtained for SAMs of 5, 9, and 10 are
clearly different, and they follow the order of isocyanide >
thiol > diselenide.
Current semiconductor technology employs nickel, tung-
sten, and their silicon alloys as electrodes for microelectronic
devices. In addition, the ferromagnetic character of Ni
presents the possibility of expanding the application of such
[
7]
electrodes to spintronic devices. Although there are various
[
8]
reports about the thiol–nickel interaction, the thiol–nickel
The surface coverage was calculated from the area of
ferrocene redox peaks, by assuming that the surface has zero
roughness and all chemisorbed molecules are electrochemi-
[
*] Y. Lee, G. M. Morales, Prof. L. Yu
Department of Chemistry and James Franck Institute
The University of Chicago
ꢀ9
ꢀ2
cally active. A surface coverage of 1 ꢀ 10 molcm was
obtained from the electrodes modified by a 0.01m ethanolic
solution of 5. This value is approximately two times larger
than the reported value for a well-packed monolayer of
5735 South Ellis Avenue, Chicago, IL 60637 (USA)
Fax: (+1)773-702-0805
E-mail: lupingyu@midway.uchicago.edu
[
**] This research was supported by the National Science Foundation,
the NSF MRSEC program at the University of Chicago, AFOSR, and
the UC-Argonne Nanoscience Consortium.
ꢀ
10
ꢀ2 [15]
ferrocenyl alkylthiols on gold (4.6–8 ꢀ 10 molcm ). The
discrepancy for the Ni surface may arise from the rough
morphology of Ni surfaces compared to that of gold (see the
Supporting Information).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
4
228
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200500942
Angew. Chem. Int. Ed. 2005, 44, 4228 –4231

Angewandte
Chemie
suggests that chemisorbed monolayers are
chemically and thermodynamically stable
under the given experimental conditions.
When Ni surfaces not treated reduc-
tively were used, the surface coverage was
ꢀ10
ꢀ2
1
.9 ꢀ 10 molcm , five times lower than
that of the electrochemically reduced Ni
surfaces. The surface coverage of the mono-
layer on the nonreduced Ni surfaces
decreased by over 50% of the original
ꢀ1
value after 100 repetitive scans at 0.1 Vs .
The instability of the monolayers on non-
reduced Ni surfaces may be a result of the
weak interaction from the physisorption of
isocyanides on nickel oxide rather than
chemisorption.
Figure 2 shows the CVs of electrochemi-
cally reduced Ni electrodes modified with 5
at different scan rates. The peak splitting
ox
p
red
p
(
DE = E ꢀE ) increased with increasing
p
scan rate and was larger than that obtained
under the same conditions for a monolayer
of 5 on polycrystalline gold (data not
shown). These phenomena are not consis-
tent with the theory for a reversible redox
Scheme 1. Synthesis of ferrocenyl-terminated molecules containing various functional
groups. a) NaN , DMF, RT, 12 h, 92%; b) lithium aluminum hydride, Et O, RT, 2 h, 80%;
3
2
c) ethyl formate, reflux, 18 h, 93%; d) POCl , triethylamine, THF, 08C, 2 h, 60%; e) NaCN,
3
DMF, RT, 12 h, 95%; f) di-2-pyridyl thionocarbonate, CH Cl , RT, 1 h, 81%; g) KSeCN,
2
2
[
16]
CH CN, 08C to RT, 30 min, 92%; h) NaH, DMF, RT, 24 h, 66%; i) thiourea, EtOH, reflux,
3
couple confined to a surface.
However,
1
8 h and then KOH, reflux, 2 h, 60%.
DE approaches zero at low scan rates, as
p
Figure 1. Cyclic voltammograms of monolayers of isocyanide 5, disele-
nide 9, and thiol 10 self-assembled on electrochemically reduced nickel
ꢀ1
surfaces. Electrolyte: 0.1m nBu NPF in THF. Scan rate: 0.01 Vs
.
4
6
Figure 2. a) Cyclic voltammograms of an electrochemically reduced
nickel electrode modified with a monolayer of isocyanide 5. Electrolyte:
The electrochemical responses of monolayers of 5 were
shown to be stable when subjected to repetitive cycling at
different sweep rates. Only a small loss (5%) in the surface
coverage was observed after 100 cycles at 0.2 Vs . Notably,
surface coverage decreased by only 12% when monolayers of
ꢀ
1
0
.1m nBu NPF in THF. Scan rate: 0.01, 0.05, 0.1, 0.25, and 0.5 Vs .
4 6
b) Plot of current density versus scan rate (~ oxidation,
&
reduction)
ꢀ
1
for a monolayer of isocyanide 5 at an electrochemically reduced nickel
surface and recorded in 0.1m nBu NPF /THF. Inset: enlargement of
4
6
5
were sonicated in THF for 10 minutes. This observation
the low-scan-rate region; the line is intended to guide the eye.
Angew. Chem. Int. Ed. 2005, 44, 4228 –4231
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
expected for a reversible surface-confined redox couple. This
the RAIR spectrum. While a strong peak for isocyanide
stretching is observed in the transmission spectrum at
behavior can be attributed to a slow electron transfer rate on
the timescale of the experiment, where at low scan rates the
redox centers remain nearly at equilibrium with the electrode
surface.
Another important piece of evidence for a surface-
confined faradaic reaction is the linearity between the peak
ꢀ
1
2147 cm , the RAIR spectrum on nickel does not show the
corresponding peak. In contrast, the isocyanide stretching
ꢀ
1
peak of the monolayers of 5 on gold occurs at 2213 cm . This
result is in agreement with previous reports of isocyanides on
gold, and the peak has been assigned to the absorption of
isocyanide bonded to the metal surface through the carbon
current (J ) and the scan rate (n), as expressed by Equa-
p
16]
[
[9]
tion (1).
atom of the isocyanide. Considering the surface selection
[
17]
rules for RAIR spectroscopy, the absence of an isocyanide
stretching peak in the RAIR spectrum suggests that the
chemisorbed isocyanides are bonded through both their
carbon and nitrogen atoms, and that they adopt a position
parallel to the surface. This observation is consistent with
2
2
I
A
p
n F Gn
4 RT
ð1Þ
jp ¼
¼
The plot of anodic and cathodic current density against
[
14a]
the scan rate is nearly linear at low scan rates, which indicates
the existence of a surface-confined faradaic reaction (inset in
Figure 2b). At faster scan rates, a deviation from the linearity
was observed which is common for slow-electron-transfer
kinetics (Figure 2). Along with previous CV results, this
electrochemical behavior confirms the fact that a monolayer
of 5 was formed on Ni surfaces.
previous studies of methyl isocyanide on Ni surfaces.
We examined the thermal stability by immersing Ni
[
18]
electrodes modified with 5 into boiling ethanol (788C). The
surface coverage (G) of the monolayers of 5 was monitored by
cyclic voltammetry as a function of boiling time (Figure 4).
Reflection–absorption infrared (RAIR) spectroscopy has
been used to investigate the chemisorption and binding mode
[
15]
of SAMs on various metal surfaces.
Figure 3 shows the
Figure 4. Thermal stability profiles of nickel–isocyanide 5 ( ) and
&
gold–thiol 10 (~) as a function of time in boiling ethanol (788C).
After three days in boiling ethanol, the surface coverage
decreased by about 50%. Notably, SAMs of 10 on a gold
surface lost over 50% of their surface coverage after one hour
in boiling ethanol. The nickel–isocyanide interaction in the
present study clearly shows higher thermal stability than the
gold–thiol interaction. The peak shape and position in the
voltammograms before and after the heating process are
constant, which indicates that this experimental condition
does not lead to decomposition of the isocyanide compounds
on the surface.
Figure 3. FTIR spectra of 5: a) transmission on a KBr plate and
b) RAIR spectra of a self-assembled monolayer on a nickel surface.
In conclusion, ferrocenyl-terminated isocyanides were
synthesized and tested for surface reaction with Ni substrates.
SAMs generated from isocyanides on Ni surfaces were
characterized by cyclic voltammetry and RAIR spectroscopy.
It was found that SAMs of isocyanides on Ni surfaces have a
parallel binding mode and high thermal stability in boiling
ethanol. The results indicate the presence of strong chem-
isorption between isocyanides and the metallic Ni surface.
This work is an indication that molecular electronic compo-
nents with isocyanide groups should be synthesized so that
they can be immobilized on substrates more relevant to
industrial standards.
transmission (a) and RAIR (b) spectra for 5 on a KBr plate
and a Ni surface, respectively. In the transmission mode, the
characteristic peaks of the hydrocarbon chain and ferrocene
unit of 5 appear at 2932 (CꢀH symmetric stretching), 2855
(
CꢀH asymmetric stretching), 3093 (CꢀH stretching in
ferrocene), 1464 (CꢀC stretching from the substituted ferro-
ꢀ1
cene), and 1350 cm (CꢀH bending from the substituted
ferrocene; Figure 3a). These peaks are also observed in the
RAIR spectrum (Figure 3b). This result clearly shows the
formation of monolayers of 5 on Ni surfaces. In addition, the
binding mode of isocyanides on Ni surfaces was deduced from
4
230
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chemie
Experimental Section
Experimental details for the synthesis of all compounds, monolayer
preparation, cyclic voltammetry, FTIR spectroscopy, and AFM
measurements can be found in the Supporting Information.
Received: March 15, 2005
Published online: June 2, 2005
Keywords: chemisorption · molecular devices · monolayers ·
.
nickel · self-assembly
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