Formation of high-quality self-assembled monolayers of conjugated dithiols on gold: Base matters

Article abstract of DOI:10.1021/ja110358t

This Article reports a systematic study on the formation of self-assembled monolayers (SAMs) of conjugated molecules for molecular electronic (ME) devices. We monitored the deprotection reaction of acetyl protected dithiols of oligophenylene ethynylenes (

Full text of DOI:10.1021/ja110358t

ARTICLE
pubs.acs.org/JACS
Formation of High-Quality Self-Assembled Monolayers of Conjugated
Dithiols on Gold: Base Matters
||
Hennie Valkenier,†,‡ Everardus H. Huisman,‡, Paul A. van Hal,§ Dago M. de Leeuw,‡,§ Ryan C. Chiechi,*,†,‡
and Jan C. Hummelen*,†,‡
†Stratingh Institute for Chemistry and ‡Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, The Netherlands
§Philips Research Laboratories, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands
S Supporting Information
b
ABSTRACT: This Article reports a systematic study on the
formation of self-assembled monolayers (SAMs) of conjugated
molecules for molecular electronic (ME) devices. We mon-
itored the deprotection reaction of acetyl protected dithiols of
oligophenylene ethynylenes (OPEs) in solution using two
different bases and studied the quality of the resulting SAMs
on gold. We found that the optimal conditions to reproducibly
form dense, high-quality monolayers are 9-15% triethylamine
(Et3N) in THF. The deprotection base tetrabutylammonium
hydroxide (Bu4NOH) leads to less dense SAMs and the incorporation of Bu4N into the monolayer. Furthermore, our results show
the importance of the equilibrium concentrations of (di)thiolate in solution on the quality of the SAM. To demonstrate the
relevance of these results for molecular electronics applications, large-area molecular junctions were fabricated using no base, Et3N,
and Bu4NOH. The magnitude of the current-densities in these devices is highly dependent on the base. A value of β = 0.15 Å-1 for
the exponential decay of the current-density of OPEs of varying length formed using Et3N was obtained.
’ INTRODUCTION
reproducibility of these SAMS in electrochemical analyses and by
fabricating large-area molecular junctions (LAMJs)5 of a series of
OPEs using the optimized methods presented here and measur-
ing the dependence of the tunneling-current on the thickness of
the resulting SAMs.
This Article describes the influences of the base on the quality
and density of self-assembled monolayers (SAMs) of thiolates of
oligo(phenylene ethynylenes) (OPEs) and terphenylenes for
molecular-electronic (ME) applications from the in situ depro-
tection of acetyl thioesters (Figure 1). We studied the influence
of two commonly used bases, tetrabutylammonium hydroxide
(Bu4NOH) and triethylamine (Et3N), in THF by forming SAMs
comprising oligo(phenylene ethynylene)dithiolates, monothio-
lates, and terphenyldithiolates on gold and varying the concen-
trations of the base and the thioester. We followed the
deprotection process using UV-vis and 1H NMR spectroscopy
to identify and differentiate the species that are present during
the self-assembly process and measured the thickness, composi-
tion, and quality of the resulting SAMs using ellipsometry, XPS,
and electrochemical measurements, respectively. We found that
high concentrations of Et3N (9-15% v/v) in THF reproducibly
gave the best agreement between predicted and measured
thickness of the SAMs, while the thickness of the SAMs that
were formed using Bu4NOH varied considerably with concen-
tration. The SAMs formed using Et3N were also more densely
packed than those formed from Bu4NOH. These findings high-
light the importance of careful, optimized experimental proce-
dures and an understanding of the underlying mechanisms when
forming SAMs of OPEs and phenylenes, particularly when these
SAMs will be used as the basis for further studies in ME devices
and measurements.1-4 We demonstrate the high quality and
Conjugated molecules and, in particular, OPEs and pheny-
lenes are a critical component of ME devices because they enable
the control of the tunneling of electrons by synthetic chemistry.
In these devices, the SAM is used to define the smallest
dimension of the device (i.e., the tunnel-junction between the
electrodes), while the lateral dimension can be relatively large
(up to hundreds of micrometers). Therefore, the reliable,
reproducible formation of densely packed, high-quality SAMs
over large areas is absolutely required. This is in contrast to the
formation of junctions of single molecules, for example, me-
chanically controllable break junctions and STM break junctions,
where lower coverages are often preferred over densely packed
SAMs. These issues are less evident for SAMs of alkanethiolates
because of the well-known insensitivity of the quality of these
SAMs to the conditions of formation.6 However, unlike simple
alkanes, conjugated molecules are not sufficiently air-stable as
free thiols to form SAMs and therefore must be protected,
typically as acetyl thioesters, because they are stable and easy
to prepare. Thus, SAMs are routinely formed by cleaving the
Received: November 18, 2010
Published: March 08, 2011
r
2011 American Chemical Society
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Figure 1. A schematic of the self-assembled monolayers on gold that are formed from solutions of OPE and terphenyl dithioesters in THF: without
base, a SAM of about 70% of the maximum density is formed, with 9-15% (v/v) Et3N, densely packed SAMs are formed, and with 4 equiv of Bu4NOH,
the molecules lay flat on the surface, and Bu4Nþ is incorporated in the SAM.
acetyl thioesters with a base, generating the free thiolate in situ.
The exact procedure, base, concentrations of thioester and base,
immersion times, etc., of this deprotection, however, varies
somewhat arbitrarily between laboratories and studies. This lack
of reproducible protocols and adequate characterization hinders
the meaningful interpretation of electronic measurements on
devices containing these conjugated SAMs.
is commonly used to deprotect conjugated dithioacetates in situ
for ME measurements in mechanically controllable break-
junctions27-30 and STM break-junctions.31 We chose Et3N
because of the promising results of Shaporenko et al.,16 and
the need for high-quality, densely packed SAMs of conjugated
molecules for ME devices using, for example, μ-contact
printing,32-34 conjugated polymers,5,35 Hg,36-38 eutectic
GaIn,39,40 and CP-AFM41-44 to contact the SAM.45-48
The first study on the formation of SAMs from conjugated
thiols, acetyl thioesters, and deprotected thioacetates was re-
ported by Tour et al.7 They demonstrated that the ellipsometric
thicknesses of SAMs of conjugated monothiols grown from THF
reach the expected value within 24 h, while dithiols form
multilayers.7,8 If the dithiols are first protected as acetyl thioe-
sters, the resulting SAMs do not form multilayers, but are thinner
than expected.7,9,10 When the acetyl thioesters are hydrolyzed
in situ with NH4OH, Tour et al. also observe the formation of
multilayers.7 Despite this observation, over the past 15 years, this
paper7 has been frequently and incorrectly cited as evidence of
the formation of high-quality SAMs of conjugated dithiols
formed from the in situ deprotection of acetyl thioesters with
NH4OH. Some studies report reasonable thicknesses using
NH4OH,11-15 while others report thicknesses that are higher
than the calculated value,11,16,17 and impurities attributed to the
NH4OH are reported.18 The problem with NH4OH is that it is
only effective in polar, protic solvents (usually ethanol, which is
also commonly used with alkanethiols). Conjugated, rigid-rod
molecules such as OPEs and phenylenes, however, are not
compatible with protic solvents; thus SAMs of these molecules
are typically formed from aprotic solvents (usually THF). This
incompatibility makes it impossible to control the concentration
of NH4OH (which is 30% aqueous NH3) because it is lost as
NH3, particularly while sparging with an inert gas. Despite this
fact, NH4OH is commonly used in aprotic solvents, which can
lead to irreproducible, low-quality, loosely packed SAMs. Using
NaOH as deprotecting agent causes similar problems19 and
damages the metal substrate.20
Tour et al. used Et3N to favor the thiolate form of thiophe-
nethiol over the dimeric form,7,21 but not to deprotect thioesters.
Other studies use Et3N to form thiols from nonthioester
precursors.22,23 Shaporenko et al. compared SAMs formed from
biphenyldiacetylthiolate using “appropriate amounts” of Et3N to
those using NH4OH and found that the former produced SAMs
that were more densely packed.16 There are other studies on the
deprotection of conjugated OPE monothioacetates;24-26 how-
ever, they focus on other aspects (e.g., reactions between the
deprotecting agent and other functional groups) and do not
observe the difficulties that are inherent to dithiols. We chose to
study Bu4NOH as a nonvolatile analogue of NH4OH because it
’ EXPERIMENTAL SECTION
Chemicals and Synthesis. diSAc-OPE3 and diSAc-P3 were
synthesized according to literature procedures.49,11 We recently re-
ported the synthesis of compounds diSAc-OPE2 and diSAc-OPE4
elsewhere.50 The synthesis of SAc-OPE2 and SAc-OPE3 is described
in the Supporting Information.
Benzenedithiol was purchased from TCI (>95%) and benzene-
(mono)thiol from Aldrich (>99%). Tetrahydrofuran (THF) was dried
by percolation over columns of aluminum oxide and R3-11-supported
Cu-based oxygen scavengers, degassed, and stored under nitrogen.
THF-d8 was dried and degassed a prior to use. Triethylamine (Et3N)
was distilled over KOH under nitrogen or purchased from Fisher
(HPLC grade) and degassed. Tetrabutylammonium hydroxide 30-
hydrate (Bu4NOH) was purchased from Sigma-Aldrich and stored
under nitrogen.
Substrates. 150 nm Au was thermally evaporated onto freshly
cleaved mica after heating the mica for 16 h at 375 °C in vacuo. The
samples were heated for one more hour after the deposition, then
gradually cooled to room temperature, taken from the vacuum chamber,
cut into pieces, and transferred into the glovebox for immersion in a
thiolate solution. For the electrochemical measurements, commercial 1
in. glass disks coated with 200 nm Au were used (Ssens BV, Hengelo).
These were cleaned with acidic piranha (1 part 30% H2O2 added to 3
parts 98% H2SO4; Caution: Dangerous!), rinsed with water and ethanol,
and dried with a nitrogen flow prior to use.
Preparation of Solutions and Formation of SAMs. All
solutions and SAMs were prepared inside a glovebox filled with nitrogen
(<5 ppm O2). THF was used as solvent in all reported experiments.
Solutions of diSAc-OPE4 were stirred and heated to 50 °C and filtered
through a 1 μm PTFE syringe filter by gravity prior to monolayer
growth. After the given immersion time, samples were taken from
solution and immersed three times in vials with clean THF, after which
the samples were dried in the glovebox or with flowing nitrogen.
UV-Vis Absorption. UV-vis absorption spectra were measured
on a Perkin-Elmer Lambda 900 spectrometer in a 1 mm quartz cuvette
with Teflon stopper, typically in 30-60 min after preparation of the
solution, unless indicated otherwise.
1H NMR. 1H NMR measurements were recorded on a Varian
AMX400 (400 MHz) or Varian unity plus (500 MHz) NMR
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spectrometer in THF-d8. J. Young NMR tubes were used to keep the
solutions under nitrogen during the measurement. Spectra were refer-
enced to the solvent signal (3.60 ppm for THF-d8).
Ellipsometry. Ellipsometry measurements were performed using a
V-Vase from J. A. Woollam Co., Inc. in air. Measurements were acquired
from 300 to 800 nm with an interval of 10 nm at 65°, 70°, and 75° angles
of incidence. For every set of experiments, a fresh gold-on-mica sample
was measured at three or four different spots. The data from these
measurements were merged, and the optical constants were fitted. For
every SAM, three spots were measured, and the thickness of a cauchy
layer (n = 1.55, k = 0 at all λ) on top of the gold layer was fitted. The
thicknesses are reported as the average of the three spots with the
standard deviation as the error bar.
XPS. XPS measurements were performed on a X-PROBE Surface
Science Laboratories photoelectron spectrometer with an Al KR X-ray
source (1486.6 eV) and a takeoff angle of 37°. We accumulated 20 scans
for S2p, 10 for C1s, 10 for O1s, 15 for N1s, and 5 for Au4f. All reported
data are averaged over four different spots per sample. WinSpec51 was
used to fit the recorded data with a background and minimum number of
mixed Gaussian-Lorentzian singlets (C1s, N1s, O1s) or doublets
(Au4f; Δ = 3.67 eV, S2p; Δ = 1.18 eV) with a width of 1.21 eV. For
details on the estimation of the thickness of the SAMs based on XPS
results, see the Supporting Information.
Electrochemistry. Electrochemical measurements were per-
formed with an Autolab PGSTAT10 in a three-electrode liquid cell,
with the gold working electrode mounted at the bottom of the cell (area
= 0.44 cm2), a platinum disk counter electrode, and a Hg/HgSO4
reference electrode (þ0.64 V vs SHE). Capacitance measurements were
performed electrochemically by cyclic voltammetry (-0.1 to -0.4 V) in
0.1 M K2SO4 in water at 50, 100, 200, 300, and 500 mV/s. The average of
the positive and negative current at -0.25 V was plotted versus the scan
speed. The slope of this plot gives the electrochemical capacitance value
of the SAM (see Figure S13). The packing density of the SAMs was
studied in a solution of 50 μM FeK3(CN)6 and 50 μM FeK4(CN)6 as
redox couple in 0.1 M K2SO4 in water by cyclic voltammetry (0.3 to -
0.6 V) at 100 mV/s.
Large-Area Molecular Junctions. The large-area molecular
junctions were fabricated and measured as described by van Hal et al.,52
using MA1407 photoresist. SAMs were grown by immersion in 0.5 mM
solutions in THF with Et3N (for the acetyl protected compounds) for 2
days. The SAM of diSAc-OPE4 was grown from a filtered 0.1 mM
solution.
Figure 2. A schematic of the kinetics of the formation of self-assembled
monolayers on gold from solutions of (a) diSAc only, incomplete
coverage due to weak association with the surface and slow incorpora-
tion into the monolayer; (b) diS only, strong association with the gold
surface hinders self-assembly; polymeric disulfides form readily and
precipitate; and (c) diSAc plus SAcS, self-assembly occurs similarly to
diSAc-only except SAcS incorporates readily, resulting in a densely
packed monolayer.
’ RESULTS AND DISCUSSION
to form equilibrium mixtures that reflect the initial concentration
of thiolate and the relative stabilities of the various thiolates and
thioesters.53,54 For example, introducing R1-S to a solution of
R2-SAc forms a mixture of R1-SAc, R1-S, R2-SAc, and R2-S
(see Figure S2). Because this equilibration occurs on a micro-
second time scale in solution and the formation of a SAM occurs
at the interface between the solution and the substrate on a
minute time scale, the solution from which the SAM forms is
highly dependent on how much thiolate is generated from the
deprotecting agent. (The removal of thiolate by the growing
SAM has a negligible impact at the concentrations typically
used.) This means that, in the case of a dithiol, there will be a
competition between the diSAc-OPE, SAc-OPE-S, and diS-
OPE, each of which interacts differently with the substrate (see
Figure 2).
Nomenclature. We abbreviate acetyl as “Ac” and thiolates as
“S” (i.e., we omit the minus sign of S-) such that SAc is the
protected acetyl-thioester. We refer to monoacetyl-protected
monothiols with the prefix “SAc” and diacetyl-protected
dithiols with the prefix “diSAc.”
We define high-quality SAMs as being free of pinholes, as
determined by electrochemical measurements. We define den-
sely packed as a SAM in which the thickness measured by
ellipsometry is reproducibly within 80-100% of the maximum
theoretical thickness (i.e., the length of the molecule þ2.3 Å for
the S-Au bond). We use the term cyclic voltammetry (CV) only
to refer to three-electrode electrochemical measurements in
solution.
Forming SAMs from Thioesters of OPEs. When generating
thiolates from thioesters to form monolayers, both the mechan-
isms of the deprotection of acetyl thioesters and the formation of
SAMs must be considered. A mixture of thioesters, in the
presence of a thiolate (i.e., R-S, not R-SH), will metathesize
The formation of a SAM occurs in three basic steps: (i)
association between molecules and substrate where the mole-
cules lie down on the surface, (ii) reorganization of these
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Figure 3. UV-vis absorption spectra of 0.3 mM diSAc-OPE3 in THF
with 0 equiv (black, -), 1 equiv (blue, - - -), 2 equiv (green,
), 3
3 3 3
- -), 4 equiv (red, - - -), and 6 equiv Bu4NOH
3
equiv (orange, -
3
(dark red, -), showing the conversion of thioacetate into free thiolate.
The inset is a plot of the normalized intensity of the growing absorption
peak as function of the number of equivalents of Bu4NOH added for
molecules diSAc-OPE3, SAc-OPE3, and SAc-OPE2: diSAc-OPE3
reacts with 4 equiv of base, whereas SAc-OPE2 and SAc-OPE3 react
with 2 equiv of base.
molecules into islands of loosely packed standing molecules
(observed as striped phases for alkanethiolates), and (iii) growth
of these islands into a densely packed monolayer.55 The transi-
tion from lying-down to standing up is driven by the strength of
the Au-thiolate bond and can only occur if the association with
surface is weak enough to be reversible on the time scale of the
experiment (dotted lines in Figure 2a). Likewise, the growth of
the domains of standing molecules can only occur only if each
step is reversible and the densely packed monolayer is the most
thermodynamically stable.56 Thiolates associate much more
strongly than thiols or thioesters, but Au cleaves thioesters
(and thiols) to form Au-thiolates. The resulting Au-bound
thiolates dissociate as Au complexes; thus they do not participate
in further thioester metathesis (i.e., the reaction of Au with SAc-
OPE does not produce any free S-OPE).
SAMs of diS-OPE (Figure 2b) are arrested at step i: inter-
actions between the Au surface and the π-electrons in combina-
tion with two highly reactive thiolates drives the equilibrium
between the standing-up phase and lying-down too far toward
the lying-down phase for a SAM to form. By contrast, in a
solution composed mainly of diSAc-OPE3 and/or SAc-
OPE3-S, the equilibrium is pushed toward the standing-up
phase by the poor interaction of the SAc groups with the Au
surface (and the favorable packing of these groups in the SAM).
Although diSAc-OPE tends to form standing-up SAMs
(Figure 2a), the thioacetates are too bulky to fill the last vacancies
in the SAM. This can be solved by the presence of SAc-OPE-S
in the solution (Figure 2c), even in small quantities.57
Figure 4. 1H NMR spectra from 7.7 to 6.7 ppm, of 0.3 mM diSAc-
OPE3 in THF-d8 (spectrum a), with addition of 2 equiv of Bu4NOH
(spectrum b), with addition of 4 equiv of Bu4NOH (spectrum c), and
the spectrum of b with a and c subtracted (spectrum d), which shows
that in addition to diSAc-OPE3 and diS-OPE3, the asymmetric SAc-
OPE3-S persists in the solution with 2 equiv of Bu4NOH.
three and four and remained constant through equivalents five
and six, indicating that 2 equiv of Bu4NOH reacts with each SAc
in diSAc-OPE3. We repeated this experiment with SAc-OPE3,
SAc-OPE2, diSAc-OPE2, and diSAc-P3 and observed the
same result; 2 equiv of Bu4NOH remove one Ac (Figures S4 and
S5). The first equivalent hydrolyzes the Ac through simple
addition/elimination, and the second deprotonates the acetic
acid that is formed in the first step (Figure S1). The linear
dependence of the increase in intensity at 458 nm (Figure 3,
inset) and commensurate decrease at 331 nm with increasing
Bu4NOH, and the semi-isosbestic point at 363 nm, indicate that
there are no intermediate chromophores between the SAc and S
compounds. There must, however, be a monodeprotected
intermediate (SAc-OPE3-S).
To probe for this intermediate, we prepared four solutions of
0.3 mM diSAc-OPE3 in THF-d8 with 0, 1/2, 2, and 4 equiv of
1
Bu4NOH and examined them using H NMR spectroscopy
(Figure 4). The spectrum of the solution containing 4 equiv of
Bu4NOH consisted of three peaks (Figure 4c) that are shifted
upfield relative to diSAc-OPE3 (Figure 4a), which we ascribe to
diS-OPE3.58 Assuming that the solution containing 2 equiv of
Bu4NOH (Figure 4b) comprised a mixture of diSAc-OPE3,
SAc-OPE3-S, and diS-OPE3, we subtracted these two spec-
tra (a and c) from (b) to isolate the spectrum of SAc-OPE3-S
(Figure 4d). We integrated the peaks in the aromatic region
(6.7-7.7 ppm) to determine that the solution with 2 equiv of
Bu4NOH comprised 22% diSAc-OPE3, 56% SAc-OPE3-S,
and 22% diS-OPE3, a ratio of 1:2:1. We repeated this analysis
Deprotection Using Bu4NOH. We followed the deprotection
reaction by recording UV-vis spectra of 0.3 mM diSAc-OPE3
in THF during the addition of 6 equiv (relative to the number of
diSAc-molecules; i.e., 3 equiv of per SAc) of Bu4NOH. The
solutions changed from colorless to bright orange to yellow as the
free thiolates formed (Figure 3).
During the addition of the first 2 equiv of Bu4NOH, a new
absorption peak appeared at 460 nm, which we ascribe to
delocalized thiolates. This peak increased in intensity and
gradually shifted to 458 nm during the addition of equivalents
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Figure 6. A plot showing a linear increase of the thicknesses of SAMs of
OPE-dithiols (blue structures, upper-left) and OPE monothiols (red
structures, lower-right), formed after 2 days immersion time in solutions
with 9-15% Et3N, with their length. SAMs of benzenedithiol, diSAc-
OPE2, diSAc-OPE3, and diSAc-OPE4 were measured by ellipsome-
try (blue b), and those of benzene monothiol, SAc-OPE2, and SAc-
OPE3 were measured by ellipsometry (red 9) and XPS (black 0).
Figure 5. Ellipsometric thicknesses (Å) of the SAMs formed from
0.3 mM diSAc-OPE3 as function of the immersion time, with 0 equiv
(b), 1 equiv (blue 9), 2 equiv (green [), and 4 equiv of Bu4NOH (red
1). SAMs with 1 and 2 equiv give reasonable thicknesses in about 1 h.
and determined that the ratio of the solution containing 1/2
equiv of Bu4NOH was 3:1:0.59 This means that the three species
coexist as a statistical mixture in solution (due to the metathesis
of the thioesters), in a ratio determined by the amount of
Bu4NOH and with no preference for the formation of a particular
species. These observations are in agreement with the UV-vis
data, which show a single absorption growing in intensity with
the addition of Bu4NOH, meaning that the absorptions of the
thiolates of SAc-OPE3-S and diS-OPE3 are nearly identical.
Forming SAMs of OPEs Using Bu4NOH. We examined the
influence of the concentration of Bu4NOH on the formation of
SAMs of OPEs by immersing Au substrates in 0.3 mM solutions
of diSAc-OPE3 and either 0, 1, 2, or 4 equiv of Bu4NOH and
measuring the thicknesses of the resulting SAMs using ellipso-
metry (Figure 5). These values are based on the assumptions that
the index of refraction of a monolayer of conjugated molecules is
1.55,7,11 and that the absorption of the SAMs can be neglected;
they are accurate to (2 Å. Lower values indicate a SAM that is
less densely packed, but we cannot determine the cause (i.e., the
structure of the SAM) using only ellipsometry.9
Within 2 h, the thicknesses measured for the SAMs formed
using 1 or 2 equiv of Bu4NOH matched the predicted value (24.9
Å),60 indicating the presence of a densely packed monolayer
composed of S-OPE3-SAc and S-OPE3-S. The thicknesses
measured for SAMs that were immersed for longer than 2 h were
irreproducible and usually larger than the predicted value, which
may indicate the formation of multilayers as Tour et al. reported
for NH4OH.7 Regardless of the cause, this observation means
that SAMs formed from solutions with Bu4NOH lack the
reproducibility required for ME devices.
oxygen (Figure S8). It is unlikely, however, that micrometer-scale
precipitates influence the formation of the SAMs, which happens
on molecular length scale.
To broaden the scope of our results, we repeated the UV-vis
and ellipsometry experiments for diSAc-P3 (deprotected with
Bu4NOH) and found the same results as for diSAc-OPE3 (see
Figures S6 and 7). Furthermore, these experiments allowed us to
relate our results to previous studies of Krapchetov et al.61 on
diSAc-P3 deprotected with NH4OH (see the Supporting
Information). The published data support our hypothesis, which
is depicted in Figure 2.
Forming SAMs of OPEs Using Et3N. To the best of our
knowledge, the only report on using Et3N to form SAMs from
acetyl thioesters of conjugated dithiols is from Shaporenko
et al.16,62 They observed densely packed SAMs, but did not
describe the exact conditions used. This may be because the
quality of SAMs of OPEs formed using Et3N is much less
sensitive to an excess of base than, for example, SAMs formed
using Bu4NOH. We investigated the effects of the concentration
of Et3N on the formation of SAMs from diSAc-OPE2, diSAc-
OPE3, diSAc-OPE4, SAc-OPE2, and SAc-OPE3 by immer-
sing Au substrates in solutions of different concentrations of
Et3N and 0.5 mM of each of these compounds in THF (except
diSAc-OPE4, which saturated above 0.1 mM)63 and measuring
the thicknesses of the resulting SAMs by ellipsometry (Table
S1). Using 3% (v/v) Et3N with diSAc-OPE3, we observed
reasonable thicknesses, but SAMs of diSAc-OPE4 were too
thin. For 9-15% (v/v) Et3N, we measured a linear increase in
the thicknesses of the SAMs with increasing length of molecules
(Figure 6), in contrast to SAMs grown from solutions with
Bu4NOH or without base (Figure S5).
In the absence of Bu4NOH (0 equiv), we measured thick-
nesses of 14 Å after 100 min and 15 Å after 24 h, which is about
70% of the predicted value. This result indicates that, as expected,
diSAc-OPE3 can form monolayers without in situ deprotection,
but that these SAMs are not densely packed (Figure 2a). Using 4
equiv of Bu4NOH, we measured a thicknesses of only 10 Å,
meaning that diS-OPE3 forms worse monolayers than does
diSAc-OPE3. Upon immersing the Au substrates in the solu-
tions with 4 equiv of Bu4NOH, an orange precipitate formed that
we ascribe to the formation of insoluble disulfide precipitates. We
observed this precipitate in the NMR tube after exposure to
We followed the deprotection reaction using Et3N by acquir-
ing UV-vis spectra for a 0.3 mM solution of diSAc-OPE3 in a
12% solution of Et3N in THF after 5 min, 24 h, and 48 h (Figure
S9). The spectrum did not change appreciably, and there were no
peaks at 458 or 460 nm, indicating a lack of thiolate in solution.
Unlike Bu4NOH, Et3N is a hindered, non-nucleophilic base, and
therefore the mechanism of the deprotection probably proceeds
through the generation of a ketene. Ketenes are normally
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Table 1. Composition of the SAMs: X-ray Photoelectron Spectroscopy Measurements
integrated intensitiesa
normalized intensities
conditions
Au4f
C1s CxHy
C1s CdO S2p S-Au S2p S-R
O1s
N1s
C1s per OPE C-atomb
0.5 mM diSAc-OPE3 in THF
83.98 eV 283-287 eV
288 eV
162 eV
164 eV
532 eV 403 eV
283-287 eV
no base, 70 h
65 020
54 227
60 156
39 315
29 010
977
1229
633
39
28
30
33
5
39
72
43
40
17
43
55
17
21
17
15% Et3N, 46 h
3 mM Bu4NOH, 40 min
(1) no base, 70 h, (2) 20 mM Bu4NOH, 80 sc
(1) 15% Et3N, 49 h, (2) 20 mM Bu4NOH, 7 minc
860
29
12
12
15
25
696
20
20
20
a These areas are divided by the sensitivity factor: 1 for C1s, 1.79 for S2p, 2.49 for O1s, and 1.68 for N1s. For Au4f, the total area is reported (not divided
by a sensitivity factor). b The total area of the CxHy C1s signal is divided by the number of C-atoms in the OPE core. We correct for the presence of other
C-atoms: (1) from the acetyl group, by assuming that the intensity of CH3 is the same as CdO, and (2) from Bu4Nþ, by assuming that the intensity of
the N1s signal only comes from Bu4Nþ, and that the 16 C atoms of Bu4Nþ each contribute with the same intensity to the C1s signal. c SAMs were
prepared in two steps: the first step is the immersion in OPE-solution, and the second step is the treatment with a 20 mM Bu4NOH solution in THF
(without OPE molecules).
generated by treating acid chlorides/bromides with a non-
nucleophilic base64,65 (e.g., acetyl chloride and Et3N), which
abstracts an R-proton and eliminates chloride/bromide, re-
forming the carbonyl with a cumulative double bond (Figure
S3). While thioesters normally do not form ketenes, here the
thiolate leaving-group is sufficiently delocalized to make it stable
enough to form a small amount of ketene (and the commensu-
rate OPE thiolate). Taking into account the fact that (loosely
packed) SAMs can form directly from acetyl thioesters, we
propose the following mechanism for the formation of SAMs
from diSAc-OPE3 and Et3N: diSAc-OPE3 adsorbs to the Au
substrate where it is slowly converted to SAc-OPE3-S-Au,
eventually forming a very sparse monolayer comprising small
domains of standing-up phases, disordered phases, and lying-
down phases (which we observe as a smaller ellipsometric
thickness than the predicted value). The slow reaction between
Et3N and diSAc-OPE3 in solution produces an amount of
SAc-OPE3-S that is small enough that effectively no diS-
OPE3 is formed. This small amount is below the detection limit
of our UV-vis experiments. The SAc-OPE3-S that does form,
however, associates strongly with the Au substrate, displacing
adsorbed diSAc-OPE3 and incorporating itself into the stand-
ing-up phase, forming a densely packed SAM as drawn in
Figure 2c that we observe as an ellipsometric thickness that
equals the predicted value. These are the ideal conditions for self-
assembly because each step is highly reversible: diSAc-OPE3
forms a weakly adsorbed lying-down phase, and the low con-
centration of SAc-OPE3-S prevents driving the equilibrium too
far toward the (disordered) chemisorbed state. Over the course
of 24-48 h, this process forms a high-quality, densely packed
monolayer.
Determining the Composition of the SAMs. We used X-ray
photoelectron spectroscopy (XPS) to determine the composi-
tion of SAMs formed from benzenethiol, SAc-OPE2, and SAc-
OPE3 (see Figure S11). We found only one S2p doublet at 162
eV for all of the SAMs formed from monothiols, indicating that
all of the sulfur atoms in the SAM are bound to Au. From the
same data, we estimated the thicknesses of the SAM by deter-
mining the ratio of the areas of the peaks corresponding to C1s
and Au4f and found good agreement with those determined by
ellipsometry (Figure 6, see Table S2 for more details). We varied
the conditions of the formation of the SAMs from diSAc-OPE3
to determine the influence of the ratios of diSAc-OPE3, SAc-
Figure 7. C1s (a,c,e) and S2p (b,d,f) XPS signals for SAMs from
diSAc-OPE3 without base (a and b), with 15% Et3N (c and d), and with
6 equiv of Bu4NOH (e and f). Fits are shown as colored lines. In the
SAM grown without base, acetyl groups are clearly present. Less acetyl
groups are present in SAM grown with Et3N. Both SAMs clearly contain
both S atoms bound to gold and S atoms that are not bound to gold.
There is no evidence for S atoms that are not bound to Au in the SAM
grown using Bu4NOH.
OPE3-S, and diS-OPE3 that are present in solution. We
measured SAMs formed from three solutions: 0.5 mM diSAc-
OPE3 and (i) no base, (ii) 15% Et3N, and (iii) 6 equiv of
Bu4NOH (Table 1 and Figure S12). We chose these three
conditions because (i) serves as a control and (ii) and (iii) are
the boundary conditions for the formation of thiolate: no
dithiolate and no monothiolate, respectively. The thicknesses
measured by XPS were 13.1 Å for no base, 17.5 Å for 15% Et3N,
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Journal of the American Chemical Society
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bilayer or multilayer because these would be apparent by
ellipsometry. We could not identify any signals for nitrogen in
the XPS data, indicating that none of the Et3N was incorporated
into the SAM.
The XPS data for SAMs formed using 6 equiv of Bu4NOH
show peaks for sulfur that are too small to draw any conclusions
from (other than the fact that there is not much sulfur in the
sample). No peaks corresponding to the carbon and oxygen of
the carbonyl are visible, but there is a large nitrogen peak. This
means that the SAMs formed from 6 equiv of Bu4NOH (i.e., pure
diS-OPE3) are not only significantly less dense than the other
two samples, but also incorporate the counterion of the base
(ratio OPE:Bu4Nþ ≈ 1). We treated the other two SAMs (no
base and 15% Et3N) with Bu4NOH and measured them again.
The resulting spectra show thicker SAMs with smaller signals for
the carbonyl carbon and oxygen and the appearance of a nitrogen
peak. We conclude that Bu4NOH can be used to remove acetyl
thioesters from the surface of a SAM, but that Bu4Nþ ions are
incorporated into the SAM (probably as counterions to the
thiolates). Nevertheless, these results show that we cannot only
determine the composition of head groups in SAMs from
dithioacetates, but that we can control this composition.
Electrochemical Characterization of the SAMs. We mea-
sured the density of SAMs on Au discs formed from 0.3 to
0.5 mM diSAc-OPE3 in THF and (i) no base, (ii) 1 equiv of
Bu4NOH, (iii) 4 equiv of Bu4NOH, and (iv) 15% (v/v) Et3N
using cyclic voltammetry (CV). We acquired CV data for these
SAMs in a 0.1 mM aqueous K2SO4 electrolyte, containing a
Fe2þ/Fe3þ redox couple, using the Au discs as the working
electrode. We compared the redox waves to those for freshly
cleaned Au (Figure 8). There were no redox waves present for
SAMs formed using Et3N, indicating that these SAMs were free
of pinholes; that is, they are densely packed and high quality,
covering 100% of the Au disk. Thus, with Et3N we meet the most
important requirement of SAMs for applications in molecular
electronics: a densely packed and defect-free SAM reduces the
formation of shorts.
Figure 8. Cyclic voltammograms of 50 μM FeK3(CN)6 and 50 μM
FeK4(CN)6 as a redox couple in 0.1 M K2SO4 in water at 100 mV/s with
Hg/HgSO4 as reference electrode, a Pt disk counter electrode, and gold
disk working electrode (0.44 cm2) with diSAc-OPE3 and 4 equiv of
Bu4NOH (red, -
- -), diSAc-OPE3 and 1 equiv of Bu4NOH
3
3
(blue, - - -), diSAc-OPE3 and 15% Et3N (green,
), diSAc-
3 3 3
OPE3 without base (black, - - -), bare gold (orange, -), and gold
immersed in 15% Et3N in THF without OPE (gray, - - -). Only
the SAM of diSAc-OPE3 formed using 15% Et3N completely blocks
the Fe2þ/Fe3þ redox signal, indicating a densely packed SAM that is free
of pinholes.
and 9.8 Å for 6 equiv of Bu4NOH; we observed the same trend by
ellipsometry (see notes of Table S2 for more details).
The XPS spectra of the SAM formed without base shows two
sulfur peaks, one bound to Au (162 eV) and one unbound sulfur
atom (164 eV) as shown in Figure 7. The integral of the peak
corresponding to the sulfur atoms that are bound to gold is
smaller than that for the unbound sulfur atoms because the signal
for the buried sulfur atoms is attenuated by the SAM. The
integrals of the peaks corresponding to the carbon (C1s, 288 eV)
and oxygen (O1s, 532 eV) of the carbonyl are equal to that of
unbound sulfur (Table 1). We did not observe any other sulfur or
oxygen peaks; thus we conclude that the SAM is composed of
SAc-OPE3-S bound to Au. The SAM formed using 15% Et3N
again shows two unequal sulfur peaks (162 and 164 eV), but the
peak for S-Au was significantly smaller. This increased attenua-
tion is again due to the increase in the density of the SAM formed
using 15% Et3N as compared to that formed without base. The
integral of the peak corresponding to the carbonyl carbon (288
eV) is 70% of that of the oxygen (532 eV) and only 40% of that of
unbound sulfur. Because there are only two sulfur peaks (and no
peaks for SdO at 168 eV), it is unlikely that this excess oxygen is
from oxides of sulfur. By comparing the integral of the peak
corresponding to the carbonyl C1s to those of the carbon atoms
in the OPE backbone and the signal of unbound sulfur, we
conclude that 40-60% of the sulfur atoms that are not bound to
Au are protected by acetyl groups. (The presence of free thiols
may be causing the apparent increase in the amount of oxygen by
inducing surreptitious, oxygen-rich compounds to adsorb to the
surface.) By comparison, Shaporenko et al. estimated this value
to be 10-20% for SAMs of diSAc-biphenyls.16 We cannot
exclude the presence of small amounts of additional molecules
bonded to the surface of the SAM through disulfides from our
XPS data because the S2p signal from disulfides cannot be
distinguished from the S2p signals from unbound sulfur atoms
(e.g., SH or SAc). However, we can exclude the formation of a
The CV data for solutions comprising only Et3N (no OPE) are
identical to those for clean Au and have the largest current-
density (J, 175 and 180 μA cm-2, respectively). The second-
largest J value is for SAMs formed using no base (132 μA cm-2
)
and 4 equiv of Bu4NOH (136 μA cm-2). J is smaller for SAMs
formed using 1 equiv of Bu4NOH (91 μA cm-2). These data
agree with the data obtained using ellipsometry, and we can
conclude that the thicknesses measured by ellipsometry that
were lower than the predicted values correspond to SAMs that
are less dense than 100% coverage.
Molecular-Electronic Devices from OPEs. To demonstrate
the high quality and high reproducibility of SAMs grown with
Et3N on large areas, we determined the electrical properties of
SAMs of benzenedithiol,66 diSAc-OPE2, diSAc-OPE3, and
diSAc-OPE4. This series allows a systematic study of both the
capacitance and the current-density as a function of SAM
thickness by using CV and LAMJs, respectively. For both
methods, it is critical that the SAMs are densely packed and of
high quality. In particular, CV measurements are sensitive to
pinholes. Pinholes will expose the underlying Au substrate to the
electrolyte, obfuscating the electrical properties of the SAM and
increasing the measured capacitances.
We measured, by CV, the capacitance of SAMs formed using
15% (v/v) Et3N in THF and 0.5 mM: (i) benzenedithiol,66 (ii)
diSAc-OPE2, (iii) diSAc-OPE3, (iv) diSAc-OPE4 (0.1 mM),
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Journal of the American Chemical Society
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cumbersome to make a direct comparison to these β values
because of the different areas (single-molecule for STM, hun-
dreds of molecules for CP-AFM, and areas of several micro-
meters for LAMJs) and contact geometries. However, 0.15 Å-1 is
in good agreement with the value obtained for oligophenylenes
in LAMJs (0.26 Å-1 69
and in accordance with theoretical
)
predictions (0.19 Å-1 for OPEs and 0.24-0.33 Å-1 for
oligophenylenes).70 For OPE4, we found a value very close to
that of OPE3 (log J = 2.33 ( 0.29 A cm-2 for OPE4 (one wafer)
vs 2.40 ( 0.50 A cm-2 for OPE3), a trend also observed in the
capacitance measurements. The relatively high J of the SAMs of
OPE4 might indicate the presence of defects in the SAMs of
OPE4 (inconsistent with the data obtained by ellipsometry). Lu
et al. observed a similar decrease in length dependence for a
diamine series of OPE1-7.71 They attribute this observation to a
transition in the charge transport mechanism from tunneling to a
hopping between OPE3 and OPE4.
To demonstrate the intimate connection between the me-
chanism of deprotection and the performance of ME devices, we
measured J values of SAMs of diSAc-OPE3 without base
(0.5 mM in THF, immersed for 2 days) and with 4 equiv of
Bu4NOH (0.3 mM in THF, immersed for 1 h). These data are
summarized in Figure 9, red b. For the SAMs without base, we
found a J that was a factor 10 higher, which, in accordance with
the lower apparent ellipsometric thickness, we attributed to a less
densely packed SAM. The SAMs formed using Bu4NOH re-
sulted in devices with J values that were 2 orders of magnitude
lower than that of the SAMs grown with Et3N. This is due to
either the formation of multilayers, the presence of Bu4Nþ, or the
adsorption of polymerized disulfides on top of the SAM.
Although we do not know the exact structure of this layer, these
electrical measurements demonstrate that 4 equiv of Bu4NOH
gives SAMs of low quality. We note that in these experiments,
despite the poor quality of the SAMs formed without base and
using Bu4NOH, we obtained functional (i.e., nonshorted) de-
vices that produced reasonable values of J, highlighting the
importance of using rigorously defined procedures to form SAMs
for electrical measurements. Although it is common to assume
that high-quality, densely packed SAMs form simply by treating a
diSAc molecule with any base, we show that it is not valid to
conclude that a SAM is of high quality simply from the observa-
tion of “reasonable” data in a ME device.
Figure 9. Average current-densities at 0.5 V for large-area molecular
junction devices with diameters ranging from 5 to 50 μm. (The inset is a
schematic (not to scale) of the architecture of the devices.) The blue “[”
show the average current-densities from two or three wafers each for
optimized SAMs of benzenedithiol (0.5 mM in THF, without base),
diSAc-OPE2 (0.5 mM in THF, 5-10% Et3N added), and diSAc-
OPE3 (0.5 mM in THF, 5-10% Et3N added). The characteristic
tunneling decay, β, from the linear fit (-) is 0.15 Å-1. The red “b”
show the average current-densities (one wafer each) of SAMs of diSAc-
OPE3 grown under different conditions: 0.5 mM in THF without base
gives current-densities that are higher than expected, and 0.3 mM in
THF with 4 equiv of Bu4NOH gives values that are much lower than
expected (see labels); only SAMs formed using Et3N lead to reprodu-
cible data between experiments.
and (v) Au disks immersed in 15% (v/v) Et3N in THF without
OPEs as a control (Figure S13). In these CV measurements in
the absence of Fe2þ/Fe3þ, the SAM acts as the dielectric layer for
a capacitor that is formed from the electrochemical double layer
and the Au disk. We measured a decrease in capacitance for
increasing thicknesses: 9.56, 6.53, 4.38, and 2.57 μF cm-2 for (v),
(i), (ii), and (iii), respectively (as was found for SAMs of
alkanethiols67 and oligophenylenethiols68). The only deviation
was the SAM formed from diSAc-OPE4 (2.47 μF cm-2), which
gave values nearly identical to those from diSAc-OPE3 (2.57
μF cm-2). It is possible that this is the result of defects in the
SAMs, which give higher values for capacitance, but unlikely
given the close match between the predicted and measured
values of thickness by ellipsometry. This same trend was
observed in LAMJs (see below).
We measured the current-densities (J, A cm-2) for the same
series of SAMs in LAMJs at an applied bias of 0.5 V (Figure 9).
These junctions comprise an evaporated Au bottom electrode,
the SAM, a layer of PEDOT:PSS that serves as the top-contact,
and a second evaporated Au electrode that is used to contact the
PEDOT:PSS.52 The measured values of J for OPE1-3 are
averaged over hundreds of devices with diameters of 5-50 μm
across two wafers for benzenedithiol and OPE2 and across three
wafers for OPE3. When plotted on a logarithmic scale, these data
are linear as a function of the thickness of the SAM. Using the
equation ln J = J0 - βd (where β is the characteristic decay, d is
the length of the molecules in the SAM, and J0 is the theoretical
current-density at d = 0), we calculated β from the slope of
OPE1-3 (Figure 9, blue [) to be 0.15 Å-1. Typical values of β
for thiol-terminated OPEs using STM break-junctions and CP-
AFM are 0.34 and 0.21 Å-1, respectively,50,15 and are signifi-
cantly lower than those found for alkanethiols (0.6-0.73 Å-1 in
LAMJs5,52 up to 0.9 Å-1 in many other junctions48). It is
’ CONCLUSIONS
We draw two important conclusions from this work: (i)
densely packed SAMs of linear, conjugated dithiols only form
from solutions that contain primarily monothiolate and dithioa-
cetate, but essentially no dithiolate, and (ii) the observation of
reasonable data in ME devices is not sufficient evidence to
conclude that a SAM is densely packed or of high quality. The
only way to achieve a solution of monothiolate from diSAc
molecules is to treat them with a base that forms monothiolate in
sufficiently low quantities that the effective concentration of
dithiolate in the solution is zero. We demonstrate unambiguously
that this can be done using 9-15% Et3N in THF and allowing
(the high quality) SAM to form over 24-48 h. Dense SAMs will
also form with 1 or 2 equiv of Bu4NOH, but, after more than 2 h
of immersion, multilayers will form. The use of any amount of
Bu4NOH will lead to the incorporation of some Bu4Nþ in the
monolayer, and too much Bu4NOH will drive the equilibrium to
the dithiolate, resulting in monolayers that are not upright
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Journal of the American Chemical Society
ARTICLE
oriented SAMs, but chemisorbed molecules lying flat on the
surface. For some ME measurements, it is not desirable to have a
dense monolayer. In these cases, diSAc molecules can be used
without deprotection to form reproducible, but not densely
packed SAMs with thioacetate head groups. With this new
understanding of the formation of SAMs, we can exert some
control over the headgroup, which opens possibilities for new
methods of contacting SAMs. It is our hope that the use of these
procedures will eliminate the quality of the SAM as a variable in
future work in ME.
(11) de Boer, B.; Meng, H.; Perepichka, D. F.; Zheng, J.; Frank,
M. M.; Chabal, Y. J.; Bao, Z. Langmuir 2003, 19, 4272–4284.
(12) Walzer, K.; Marx, E.; Greenham, N. C.; Less, R. J.; Raithby,
P. R.; Stokbro, K. J. Am. Chem. Soc. 2004, 126, 1229–1234.
(13) Maya, F.; Flatt, A. K.; Stewart, M. P.; Shen, D. E.; Tour, J. M.
Chem. Mater. 2004, 16, 2987–2997.
(14) Kronemeijer, A. J.; Akkerman, H. B.; Kudernac, T.; Wees, B. J.;
Feringa, B. L.; Blom, P. W. M.; de Boer, B. Adv. Mater. 2008,
20, 1467–1473.
(15) Liu, K.; Li, G.; Wang, X.; Wang, F. J. Phys. Chem. C 2008,
112, 4342–4349.
(16) Shaporenko, A.; Elbing, M.; Blaszczyk, A.; von H€anisch, C.;
Mayor, M.; Zharnikov, M. J. Phys. Chem. B 2006, 110, 4307–4317.
(17) Colavita, P. E.; Miney, P. G.; Taylor, L.; Priore, R.; Pearson,
D. L.; Ratliff, J.; Ma, S.; Ozturk, O.; Chen, D. A.; Myrick, M. L. Langmuir
2005, 21, 12268–12277.
(18) Zeng, X.; Wang, C.; Batsanov, A. S.; Bryce, M. R.; Gigon, J.;
Urasinska-Wojcik, B.; Ashwell, G. J. J. Org. Chem. 2010, 75, 130–136.
(19) Cheng, L.; Yang, J.; Yao, Y.; Price, D. W., Jr.; Dirk, S. M.; Tour,
J. M. Langmuir 2004, 20, 1335–1341.
’ ASSOCIATED CONTENT
S
Supporting Information. Reaction mechanisms and the
b
results of additional UV-vis spectroscopy, 1H NMR, ellipsome-
try, and electrochemical measurements; and the syntheses of
SAc-OPE2 and SAc-OPE3. This material is available free of
charge via the Internet at http://pubs.acs.org.
(20) Berner, S.; Lidbaum, H.; Ledung, G.; Åhlund, J.; Nilson, K.;
Schiessling, J.; Gelius, U.; B€ackvall, J.; Puglia, C.; Oscarsson, S. Appl. Surf.
Sci. 2007, 253, 7540–7548.
’ AUTHOR INFORMATION
(21) Ponticello, G. S.; Habecker, C. N.; Varga, S. L.; Pitzenberger,
S. M. J. Org. Chem. 1989, 54, 3223–3224.
(22) Yu, C.; Chong, Y.; Kayyem, J.; Gozin, M. J. Org. Chem. 1999,
64, 2070–2079.
Corresponding Author
r.c.chiechi@rug.nl; j.c.hummelen@rug.nl
Present Addresses
Center for Electron Transport in Molecular Nanostructures,
Columbia University, 500 W 120th St., New York, NY 10027.
(23) Elbing, M.; Blaszczyk, A.; H€anisch, C. V.; Mayor, M.; Ferri, V.;
Grave, C.; Rampi, M. A.; Pace, G.; Samor, P.; Shaporenko, A.;
Zharnikov, M. Adv. Funct. Mater. 2008, 18, 2972–2983.
(24) Cai, L.; Yao, Y.; Yang, J.; Price, D. W., Jr.; Tour, J. M. Chem.
Mater. 2002, 14, 2905–2909.
(25) Stapleton, J. J.; Harder, P.; Daniel, T. A.; Reinard, M. D.; Yao,
Y.; Price, D. W.; Tour, J. M.; Allara, D. L. Langmuir 2003,
19, 8245–8255.
(26) Hacker, C. A.; Batteas, J. D.; Garno, J. C.; Marquez, M.; Richter,
C. A.; Richter, L. J.; van Zee, R. D.; Zangmeister, C. D. Langmuir 2004,
20, 6195–6205.
(27) Grunder, S.; Huber, R.; Horhoiu, V.; Gonzꢀalez, M. T.;
Sch€onenberger, C.; Calame, M.; Mayor, M. J. Org. Chem. 2007,
72, 8337–8344.
(28) Huber, R.; Gonzalez, M. T.; Wu, S.; Langer, M.; Grunder, S.;
Horhoiu, V.; Mayor, M.; Bryce, M.; Wang, C.; Jitchati, R.; Sch€onenber-
ger, C.; Calame, M. J. Am. Chem. Soc. 2008, 130, 1080–1084.
(29) Wu, S.; Gonzꢀalez, M. T.; Huber, R.; Grunder, S.; Mayor, M.;
Sch€onenberger, C.; Calame, M. Nat. Nanotechnol. 2008, 3, 569–574.
(30) Grunder, S.; Huber, R.; Wu, S.; Sch€onenberger, C.; Calame, M.;
Mayor, M. Eur. J. Org. Chem. 2010, 2010, 833–845.
(31) Mishchenko, A.; Vonlanthen, D.; Meded, V.; B€urkle, M.; Li, C.;
Pobelov, I. V.; Bagrets, A.; Viljas, J. K.; Pauly, F.; Evers, F.; Mayor, M.;
Wandlowski, T. Nano Lett. 2010, 10, 156–163.
(32) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998,
37, 550–575.
(33) Loo, Y.; Lang, D. V.; Rogers, J. A.; Hsu, J. W. P. Nano Lett. 2003,
3, 913–917.
(34) Niskala, J. R.; You, W. J. Am. Chem. Soc. 2009,
131, 13202–13203.
(35) Milani, F.; Grave, C.; Ferri, V.; Samor, P.; Rampi, M. A.
ChemPhysChem 2007, 8, 515–518.
(36) Slowinski, K.; Fong, H. K. Y.; Majda, M. J. Am. Chem. Soc. 1999,
121, 7257–7261.
(37) York, R. L.; Nguyen, P. T.; Slowinski, K. J. Am. Chem. Soc. 2003,
125, 5948–5953.
(38) Weiss, E. A.; Chiechi, R. C.; Kaufman, G. K.; Kriebel, J. K.; Li,
Z.; Duati, M.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2007,
129, 4336–4349.
’ ACKNOWLEDGMENT
We thank Tom Geuns and Auke J. Kronemeijer for their
assistance with the large-area molecular junction experiments
and data analysis, Kim Wimbush for his assistance with the
electrochemical experiments, and Reinder Gooijaarts for techni-
cal support. Petra Rudolf is acknowledged for the access to the
XPS. H.V. acknowledges NanoNed, funded by the Dutch
Ministry of Economic Affairs (project GMM.6973), for financial
support. P.A.v.H. and D.M.d.L. acknowledge ONE-P, FP7/
2007-2013 project 212311, for financial support.
’ REFERENCES
(1) Carter, F. L. J. Vac. Sci. Technol., B 1983, 1, 959–968.
(2) Weibel, N.; Grunder, S.; Mayor, M. Org. Biomol. Chem. 2007,
5, 2343–2353.
(3) McCreery, R. L.; Bergren, A. J. Adv. Mater. 2009, 21, 4303–4322.
(4) van der Molen, S. J.; Liljeroth, P. J. Phys.: Condens. Matter 2010,
22, 133001.
(5) Akkerman, H. B.; Blom, P. W. M.; de Leeuw, D. M.; de Boer, B.
Nature 2006, 441, 69–72.
(6) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides,
G. M. Chem. Rev. 2005, 105, 1103–1169.
(7) Tour, J. M.; Jones, L. R., II; Pearson, D. L.; Lamba, J. J. S.; Burgin,
T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am.
Chem. Soc. 1995, 117, 9529–9534.
(8) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; W€oll, C.
Langmuir 2002, 18, 7766–7769.
(9) Seferos, D. S.; Banach, D. A.; Alcantar, N. A.; Israelachvili, J. N.;
Bazan, G. C. J. Org. Chem. 2004, 69, 1110–1119.
(10) In contrast to the results in refs 7 and 9, the formation of SAMs
from diSAc-OPE3 without deprotection was reported to give the
expected thickness: Lau, K. H. A.; Huang, C.; Yakovlev, N.; Chen,
Z. K.; O’Shea, S. J. Langmuir 2006, 22, 2968–2971.
(39) Chiechi, R. C.; Weiss, E. A.; Dickey, M. D.; Whitesides, G. M.
Angew. Chem., Int. Ed. 2008, 47, 142–144.
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Journal of the American Chemical Society
ARTICLE
(40) Nijhuis, C. A.; Reus, W. F.; Whitesides, G. M. J. Am. Chem. Soc.
2009, 131, 17814–17827.
(41) Wold, D. J.; Frisbie, C. D. J. Am. Chem. Soc. 2001,
123, 5549–5556.
(66) No Et3N was added to the solution of benzenedithiol in THF.
(67) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J.
Am. Chem. Soc. 1987, 109, 3559–3568.
(68) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I.
(42) Engelkes, V. B.; Beebe, J. M.; Frisbie, C. D. J. Phys. Chem. B
Langmuir 1993, 9, 2974–2981.
2005, 109, 16801–16810.
(69) Kronemeijer, A. J.; Huisman, E. H.; Akkerman, H. B.; Goos-
sens, A. M.; Katsouras, I.; van Hal, P. A.; Geuns, T. C. T.; van der Molen,
S. J.; Blom, P. W. M.; de Leeuw, D. M. Appl. Phys. Lett. 2010, 97, 173302.
The β value of 0.26 Å-1 found for oligophenyelenes in LAMJs is smaller
than those reported for other geometries.
(43) Choi, S. H.; Risko, C.; Delgado, M. C. R.; Kim, B.; Brꢀedas, J.;
Frisbie, C. D. J. Am. Chem. Soc. 2010, 132, 4358–4368.
(44) Scaini, D.; Castronovo, M.; Casalis, L.; Scoles, G. ACS Nano
2008, 2, 507–515.
(45) Mantooth, B. A.; Weiss, P. S. Proc. IEEE 2003, 9, 1785–1802.
(46) Salomon, A.; Cahen, D.; Lindsay, S.; Tomfohr, J.; Engelkes,
V. B.; Frisbie, C. D. Adv. Mater. 2003, 15, 1881–1890.
(47) Haick, H.; Cahen, D. Prog. Surf. Sci. 2008, 83, 217–261.
(48) Akkerman, H. B.; de Boer, B. J. Phys.: Condens. Matter 2008,
20, 013001.
(70) Liu, H.; Wang, N.; Zhao, J.; Guo, Y.; Yin, X.; Boey, F. Y. C.;
Zhang, H. ChemPhysChem 2008, 9, 1416–1424.
(71) Lu, Q.; Liu, K.; Zhang, H.; Du, Z.; Wang, X.; Wang, F. ACS
Nano 2009, 3, 3861–3868.
(49) Stuhr-Hansen, N.; Sørensen, J. K.; Moth-Poulsen, K.; Chris-
tensen, J. B.; Bjørnholm, T.; Nielsen, M. B. Tetrahedron 2005,
61, 12288–12295.
(50) Kaliginedi, V.; Valkenier, H.; García-Suꢀarez, V. M.; Moreno-
García, P.; Hong, W.; Buiter, P.; Otten, J. L. H.; Hummelen, J. C.;
Lambert, C.; Wandlowski, Th., manuscript in preparation.
(51) WinSpec 2.09, developed at Laboratoire Interdꢀepartemental de
Spectroscopie Electronique, Namur, Belgium.
(52) van Hal, P. A.; Smits, E. C. P.; Geuns, T. C. T.; Akkerman, H. B.;
de Brito, B. C.; Perissinotto, S.; Lanzani, G.; Kronemeijer, A. J.; Geskin,
V.; Cornil, J.; Blom, P. W. M.; de Boer, B.; de Leeuw, D. M. Nat.
Nanotechnol. 2008, 3, 749–754.
(53) Um, I.; Lee, J.; Bae, S.; Buncel, E. Can. J. Chem. 2005,
83, 1365–1371.
(54) Wallace, O. B.; Springer, D. M. Tetrahedron Lett. 1998,
39, 2693–2694.
(55) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256.
(56) Impurities in the monolayer are the result of molecules
becoming kinetically trapped, which can occur in either phase; that is,
an impurity can be incorporated into the growing islands or interrupt the
growth of an island by lying down. These impurities can be other
molecules (e.g., S-Ar in a SAM of SAc-Ar-S) or molecules that are
not incorporated correctly (e.g., upside-down).
(57) SAc-OPE-SH molecules have been synthesized, but the
mechanism is the same as SAc-OPE-SAc except that SH is cleaved
faster than SAc by Au and is probably better in filling the defects of the
SAM, see: (a) Flatt, A. K.; Yao, Y.; Maya, F.; Tour, J. M. J. Org. Chem.
2004, 69, 1752–1755. (b) Niklewski, A.; Azzam, W.; Strunskus, T.;
Fischer, R. A.; W€oll, C. Langmuir 2004, 20, 8620–8624. See the
Supporting Information for a more detailed discussion on thiols.
(58) Although the resolution of this spectrum is low, we confirmed
these assignments with a higher resolution spectrum; see Figure S8.
(59) diS-OPE3 is probably present in the solution, however, in
quantities below the detection limit of 1H NMR (∼5%).
(60) The S-S distance of diS-OPE3 is 19.8 Å, corresponding to a
maximum theoretical thickness of 24.9 Å for a SAM of AuS-OPE3-
SAc and 22.1 Å for a SAM of AuS-OPE3-S.
(61) Krapchetov, D. A.; Ma, H.; Jen, A. K. Y.; Fischer, D. A.; Loo, Y.
Langmuir 2005, 21, 5887–5893. Krapchetov, D. A.; Ma, H.; Jen, A. K. Y.;
Fischer, D. A.; Loo, Y. Langmuir 2006, 22, 9491–9494. Krapchetov,
D. A.; Ma, H.; Jen, A. K. Y.; Fischer, D. A.; Loo, Y. Langmuir 2008,
24, 851–856.
(62) In two other studies from the same group, acetyl esters of
monothiols are deprotected with Et3N: (a) Shaporenko, A.; R€ossler, K.;
Lang, H.; Zharnikov, M. J. Phys. Chem. B 2006, 110, 24621–24628. (b)
Weidner, T.; R€ossler, K.; Ecorchard, P.; Lang, H.; Grunze, M.; Zharni-
kov, M. J. Electroanal. Chem. 2008, 621, 159–170.
(63) Precipitate was formed in the solution of diSAc-OPE4 over
time. To prevent this precipitate from contaminating the SAM, the
samples were placed upside-down in solution (see Figure S10).
(64) Sauer, J. C. J. Am. Chem. Soc. 1947, 69, 2444–2448.
(65) Wedekind, E. Ber. Dtsch. Chem. Ges. 1901, 34, 2070–2077.
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