Electrochemically grafted single molecule junctions exploiting a chemical protection strategy

Article abstract of DOI:10.1016/j.electacta.2016.10.095

Diazonium grafted oligo(phenylene ethynylene) (OPE) monolayers suitable for single molecule conductance studies have been prepared. The surface-bound OPE is prepared from the amino tolan, 4-(2-(4-(2-(triisopropylsilyl)ethynyl)phenyl)ethynyl)benzamine, which is converted to the diazonium derivative in situ, and subsequently attached to a gold substrate using conventional cathodic electrochemical grafting. Multilayer formation is avoided by the presence of the bulky triisopropylsilyl (TIPS) protected ethynyl group. After removal of the TIPS group by treatment with fluoride, the film, now bearing exposed terminal acetylene moieties, is further functionalised by reaction with 1-iodo-4-acetylthiobenzene using Sonogashira coupling chemistry, to create sites bearing a strongly aurophilic top-contacting group. The success of these surface reactions is confirmed using Raman spectroscopy (gap-mode and SHINERS, employing gold nanoparticles and SiO₂ coated gold nanoparticles, respectively). It is shown that such thiolate-modified, diazonium-grafted monolayers are suitable for STM-based molecular junction conductance measurements.

Full text of DOI:10.1016/j.electacta.2016.10.095

Electrochimica Acta 220 (2016) 436–443
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemically grafted single molecule junctions exploiting a
chemical protection strategy
Jinghong Lianga, Robert E.G. Smithb, Andrea Vezzolib, Liqiang Xiea, David C. Milanb,
*
Ross Davidsonc, Andrew Beebyc, Paul J. Lowd, Simon J. Higginsb, Bingwei Maoa,
,
Richard J. Nicholsb,
*
a
State Key Laboratory of Physical Chemistry of Solid Surfaces and College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 7ZD, United Kingdom
Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom
b
c
d
School of Chemistry and Biochemistry, University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia
A R T I C L E I N F O
A B S T R A C T
Article history:
Diazonium grafted oligo(phenylene ethynylene) (OPE) monolayers suitable for single molecule
conductance studies have been prepared. The surface-bound OPE is prepared from the amino tolan,
4-(2-(4-(2-(triisopropylsilyl)ethynyl)phenyl)ethynyl)benzamine, which is converted to the diazonium
Received 1 July 2016
Received in revised form 11 October 2016
Accepted 15 October 2016
Available online 17 October 2016
derivative in situ, and subsequently attached to
a gold substrate using conventional cathodic
electrochemical grafting. Multilayer formation is avoided by the presence of the bulky triisopropylsilyl
(TIPS) protected ethynyl group. After removal of the TIPS group by treatment with fluoride, the film, now
bearing exposed terminal acetylene moieties, is further functionalised by reaction with 1-iodo-4-
acetylthiobenzene using Sonogashira coupling chemistry, to create sites bearing a strongly aurophilic
top-contacting group. The success of these surface reactions is confirmed using Raman spectroscopy
(gap-mode and SHINERS, employing gold nanoparticles and SiO2 coated gold nanoparticles,
respectively). It is shown that such thiolate-modified, diazonium-grafted monolayers are suitable for
STM-based molecular junction conductance measurements.
Keywords:
STM
single molecule conductance
diazonium grafting
SHINERS
Raman spectroscopy
ã 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
1. INTRODUCTION
structures that take the form, for instance, of atomically flat
surfaces and STM tips, nano-fabricated contacts or mechanically
The field of molecular electronics has attracted a great deal of
interest over the past two decades, driven by the development of
methods for reliably characterising the electrical properties of
molecules within large area planar and even single molecule
junctions. As well as the basic conductance and current-voltage
characteristics, topics of on-going research include molecular
rectification, conductance switching, electrochemical gating,
negative differential resistance, transistor-like behaviour and
thermoelectric properties. However, obtaining reliable contacting
of molecules within the electrical junctions remains an overriding
theme that can impact significantly on all of these phenomena.
Gold has featured as by far the most common material for making
electrical contacts to molecules. This is due to (i) its lack of surface
oxide under standard ambient conditions, (ii) favourable electrical
properties of gold and (ii) the ease of preparation of electrode
controlled and STM-based break junctions. However, equally
importantly, there is also an extensive variety of versatile
chemistry for forming gold-molecule contacts. Chemisorbed
anchor groups have featured most prominently with key examples
including thiol, thiomethyl ether, pyridyl, amine, dimethyl
phosphine and carboxylate functional groups [1–5]. An attractive
alternative to these has been the direct “grafting” of molecules to
metal (M) electrodes through the formation of metal-carbon (M-C)
linkages, with the expectation that such coupling would increase
the electrical transmission of such junctions, which can be a
limiting feature in junction conductance.
Direct Au-C anchoring has been achieved through the
spontaneous in-situ cleavage of trimethyltin terminal groups on
molecular junction targets [6]. In this way, simple polymethylene
or aromatic molecular bridges could be linked by gold contacts
within an STM break junction configuration [6]. In a related
approach, fluoride-induced cleavage of trimethylsilyl (TMS)
protecting groups from alkyne groups has also been shown to
* Corresponding authors.
E-mail addresses: bwmao@xmu.edu.cn (B. Mao), nichols@liv.ac.uk (R.J. Nichols).
http://dx.doi.org/10.1016/j.electacta.2016.10.095
0013-4686/ã 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

J. Liang et al. / Electrochimica Acta 220 (2016) 436–443
437
promote the in-situ formation of Au-C-linked molecular junctions
[7]. Direct Au-C bonds are strong [8], and such junctions have
produced junction conductance values up to 100 times greater
than those with more conventional anchoring groups such as thiols
[6]. Another popular method for achieving direct M-C linkage of
adsorbates to metal, semiconductor or carbon electrodes involves
diazonium chemistry. Surface attachment can be achieved by in-
situ electrochemical reduction of diazonium-terminated molecular
targets, which graft to the cathodically polarised electrode through
other molecular terminus was through a conventional thiol link to
the gold STM probe tip. In this way, the thiol link to the gold STM tip
could be repeatedly made, broken and remade in STM evaluation of
the molecular junction conductance. Cyclic voltammetry and
Raman spectroscopy has also been used to characterise the
formation of the molecular layers.
2. EXPERIMENTAL DETAILS
the resulting radical intermediate (Ar-N2+ + eꢀ ! Ar
ꢁ
+ N2(g)). This
2.1. General synthetic details
approach was pioneered by Pinson and co-workers in 1992 for the
direct attachment of aryl groups to carbon electrodes [9]. Such
attachment methodologies based on electrografting of diazonium
derivatives have found wide application in electrochemistry and
beyond, including, for example, the formation of molecular films
for surface functionalisation and polymer growth, electron transfer
studies, solubilisation of carbon nanotubes, corrosion protection,
surfaces for the immobilisation of nanoparticles, bio- and chemo-
sensing systems and surfaces with chemically controlled wetta-
bility (see [10] and references therein). Diazonium grafting has also
been used for forming carbon-aryl-metal [11–13] and silicon-
molecule junctions [14,15] for planar molecular electronics
junctions and also in commercial electronic applications [16].
Electrochemical scanning tunnelling spectroscopy has been used
to investigate charge transport through redox-active molecules
bound to the surface through direct Au-C linkage achieved through
diazonium chemistry [17]. More recently, single molecule junc-
tions have been formed through the electrochemical reduction of
molecules with diazonium termini at both ends [8]. Although these
Au-C-anchored molecular junctions did not have appreciably
higher conductance than equivalent junctions anchored with
amine termini, they did exhibit greater stability and could be
stretched to greater lengths in STM-tip retraction experiments [8].
It is well recognised that diazonium compounds have potential
for grafting onto a wide variety of contacts for molecular
electronics [11–16], however without protection of the starting
diazonium salts multilayers can form, and oligomer chains with a
complex stoichiometry are likely to result. Grafting of well-defined
monolayers would then be difficult to achieve for either
applications in large area molecular electronics or for single
molecule measurements. Defined stoichiometry is of course
necessary for a robust understanding of the transport properties
of metal-carbon junctions. This well-known issue with diazonium
grafting arises because of the high reactivity of the electro-
generated radicals, which then show a propensity to react with
existing surface-grafted materials [10,18,19]. The use of sterically-
hindered diazonium salts [20] or protection-deprotection chemis-
try [21] has proven to be effective in suppressing multilayer
formation in the electrografting of aryldiazonium salts. Notably
Leroux et al. have employed aryldiazonium salts with protecting
groups at the other terminus (trimethylsilyl, triethylsilyl, and
triisopropylsilyl were evaluated) in their grafting to carbon
substrates [21]. The size of the protecting-head group controlled
the spacing of the electrochemically grafted molecules and, once
the protecting group was removed, an ethynylaryl monolayer was
revealed which could be subsequently functionalised using “click
chemistry” [22]. Other protecting group chemistry, for instance
tert-butyloxycarbonyl (BOC) protection of amine termini [23], have
also been employed in diazonium grafting. The triisopropylsilyl
(TIPS) protecting group has also been examined previously to make
large-area molecular junctions [24].
NMR spectra were recorded in deuterated solvent solutions on a
Varian VNMRS-600 spectrometer and referenced against solvent
resonances (1H, 13C). MS(ASAP) data were recorded on a Xevo QTOF
(Waters) high resolution, accurate mass tandem mass spectrome-
ter equipped with Atmospheric Pressure Gas Chromatography
(APGC) and Atmospheric Solids Analysis Probe (ASAP). NMR
spectra are shown on the supporting information. Microanalyses
were performed by Elemental Analysis Service, London Metropol-
itan University, UK. Analytical grades of solvents were used. ((4-
Bromo phenyl)ethynyl)triisopropylsilane [25] and 4-ethynylani-
line [26] were synthesised according to literature methods. All
other chemicals were sourced from standard suppliers.
2.1.1. Synthesis of 4-(2-(4-(2-(triisopropylsilyl)ethynyl)phenyl)
ethynyl)benzamine (1)
((4-Bromophenyl)ethynyl)triisopropylsilane (2.00 g, 5.95 mmol)
and 4-ethynylaniline (0.70 g, 6.00 mmol) were added to a round
bottom flask that was degassed and filled with dry THF (50mL) and
Et3N (10 mL). The solutionwasdegassedby threefreeze-pump-thaw
cyclesbeforetheadditionofCuI(0.11 g, 0.59 mmol)and PdCl2(PPh3)2
(0.41 g, 0.59 mmol). The solution was heated to reflux for 16 hours
under nitrogen, and the solvent was then removed in vacuo. The
remaining solid was dissolved in hexane and filtered, the filtrate was
eluted on a silica column with CH2Cl2:hexane (1:1), collecting the
yellow band. The solvent was removed to give a yellow oil. Yield:
1.57 g(71%).1HNMR(CD2Cl2)
(d, J = 8 Hz, 2H), 3.94 (s, 2H),1.14 ppm (s, 21H, assigned to unresolved
multipletfromtheTIPS group hydrogens).13C NMR (CD2Cl2)
: 147.3,
d:7.45 (s, 4H), 7.34 (d, J = 8 Hz, 2H),6.65
d
132.9,131.8,130.9,123.9,122.5,114.5,106.7, 92.4, 92.2,18.4,11.3 ppm.
MS(ASAP): m/z 373.220 [M]+, 374.213 [M + H]+. Anal. Calc. For
C25H31NSi: C, 80.37; H, 8.36; N, 3.75%. Found: C, 80.27; H, 8.16; N,
3.85%.
Compound 1 used to form Layer-1 (see Scheme 1).
2.1.2. Synthesis of S-(4-iodophenyl)ethanethioate)
A solution of 1,4-diiodobenzene (1.65 g, 5 mmol) in dry toluene
(40 mL) was degassed by bubbling Ar through for 30 minutes in a
Schlenk flask. CuI (95 mg, 0.5 mmol), KSAc (0.685 g, 6 mmol) and
1,10-phenanthroline (180 mg, 0.1 mmol) were added while keeping
the flask in a gentle stream of dry argon and the resulting
suspension was stirred for 24 h at 100 ꢂC, after which time the
colour changed to deep brown. Upon cooling, 50 mL of H2O was
added and the organic layer was separated. The aqueous layer was
extracted with CH2Cl2 (2 ꢃ 25 mL), the combined organics washed
with brine and dried over MgSO4. The solvent was removed to yield
a red crude product that was purified by flash column chromatog-
raphy on silica (20% CH2Cl2 in hexanes as eluent) to give the title
compound as a pale yellow solid (0.581 g, 42%). 1H NMR (400 MHz,
In this study, diazonium grafting to gold substrates, accompa-
nied by protection-deprotection chemistry, has been deployed in
the fabrication of metal-molecule-metal junctions for single
molecule electronics. Such junctions were linked to the gold
electrode through a direct Au-C bond, while the attachment at the
CDCl3) d= 7.65 (d, J = 8.3 Hz, 2H, Ph), 7.03 (d, J = 8.5 Hz, 2H, Ph), 2.32

438
J. Liang et al. / Electrochimica Acta 220 (2016) 436–443
Scheme 1. Cathodic electrochemical grafting of compound 1 (left) to form Layer-1 (right).
(s, 3H, CH3).13C NMR (100 MHz, CDCl3)
d
= 193.2,138.3,135.9,127.6,
2.3. STM Conductance Measurements
95.9, 30.3. m/z (HRMS, CI, NH3) 295.9609 [M + NH4]+, C8H11NIOS
calc. 284.9606.
Measurements of single molecule conductance were performed
using the I(s) method (where I = current and s = distance) which has
been described in detail previously [27]. In brief, the gold STM tip is
taken to high tip-substrate current values (e.g. 30 nA) so that the
STM tip approaches close to the metal surface. From this position of
close approach the STM tip was then rapidly retracted 2 nm with a
scan duration of 10 ms. This process of approaching close to the
substrate and then rapidly retracting is repeated many times and
the resulting I(s) scans which show successful metal-molecule-
metal junction formation are recorded and analysed. These
junction formation traces are collected together in histograms to
determine the most probable conductance value of a single
molecule at a given bias voltage (U).
2.2. Surface Functionalisation
2.2.1. Layer-1 preparation
The formation of Layer-1 by cathodic electrografting is
described in the Results and Discussion section (see Scheme 1
and accompanying text).
2.2.2. Layer-1 Desilylation
A gold slide with the electrografted Layer-1 was immersed in
5 mL 1.0 M TBAF in THF solution under Ar atmosphere, and
sonicated for 10 minutes at room temperature. The sample was
then rinsed with copious ethanol and acetone, and used in the
following step.
All the STM experiments and molecular conductance determi-
nations were performed with a ‘Molecular Imaging’ (Keysight)
STM. STM tips were prepared by cutting a
f0.25 mm gold wire
2.2.3. General procedure for Sonogashira surface functionalisation to
form Layer-2
The desilylated Layer-1 on Au was put in a solution of bis
(acetonitrile)dichloropalladium(II) (0.013 g, 0.05 mmol), triphenyl-
phosphine (0.039 g, 0.15 mmol), cuprous iodide (CuI, 0.005 g,
(Goldfellow, 99.99%) for each experiment. The molecules were
used as received from Sigma-Aldrich (reagent grade) and the
synthesis and characterization of 1 is as described in Section 2.1.1.
Commercial gold on glass plates (from Arrandee1) were used for
all measurements. These samples were used as the working
electrode for diazonium grafting by connecting with a gold wire.
After flame annealing they were immediately immersed in an
electrochemical cell with 5 mM compound 1 + 5 mM NaNO2 + 10
mM HClO4 + 0.1 M TBAB in acetonitrile solution and surface
functionalisation was achieved as described in the “Results and
Discussion” section.
0.026 mmol)
and
S-(4-iodophenyl)ethanethioate
(0.139 g,
0.5 mmol) in 20 mL of redistilled and degassed diisopropylamine
and sonicated for 1 h under argon, over which time the
temperature increased to 45 ꢂC. The sample was then rinsed with
copious ethanol and acetone, and used in the STM and Raman
characterisation.
Using this surface functionalisation scheme it is possible that
the S-(4-iodophenyl)ethanethioate might react directly with the
bare gold surface if the base layer was pin-holed or poorly packed,
but the data (for instance the cyclic voltammograms with
ferrocyanide shown in Fig. 2) show this not to be the case. Even
in the eventuality of small pinholes and even if the reagent were to
in-fill the ‘deprotected’ monolayer it would be unable to react
further in the Sonogashira reaction and would be buried too deeply
to interact with the gold nanoparticles.
2.4. Raman spectroscopy
We employed SHINERS in Au(111)|Molecular Layer|Au@SiO2-NP
sandwich structures. The 55 nm Au NPs obtained by a citrate
reduction method were further coated with a pinhole-free thin
silica film (thickness 2–3 nm) prepared as described in the
literature [28]. Gap-mode Raman spectroscopy is also used to
characterise the monolayers with the aid of gold nanoparticles not
coated with a SiO2 shell. Some differences in spectra between gap-
mode Raman and SHINERS, such as band shifts, may be expected
For the STM imaging of Layer-2 it was immersed into a dilute Au
NPs solution overnight, and then thoroughly rinsed and dried in a
nitrogen flow, before being imaged with STM.

J. Liang et al. / Electrochimica Acta 220 (2016) 436–443
439
suppressed on the electrografted surface. The peak-to-peak
separation on the bare gold electrode in these experiments was
found to be larger than expected, and surface preparation-
dependent. Similar observations for this particular couple on
graphite electrodes have been made previously, and attributed to
an inner sphere-type mechanism operating, in contrast to other
familiar anionic couples such as [IrCl6]3ꢀ/2ꢀ [30–32].
Layer-1 features a terminal triisopropylsilyl (TIPS) protecting
group that needs to be removed prior to the introduction of a
suitable group for forming robust metal-molecule-metal electrical
junctions. The deprotection is achieved by treatment of the
modified electrodes with tetra-n-butylammonium fluoride (TBAF)
in THF (Scheme 2). In a subsequent step, the Sonogashira reaction
was used to introduce
a benzenethiolate terminus to the
monolayer (protected as the thioacetate) and it was these modified
monolayers (Layer-2) that were used for the formation of metal-
molecule-metal junctions in the subsequently-described STM
conductance measurements.
Fig. 1. Cyclic voltammograms for the grafting of diazonium salts of 1 onto the gold
surface at a scan rate of 50 mV/s in an acetonitrile solution containing 5 mM
1 + 5 mM NaNO2 + 10 mM aq. HClO4 + 0.1 M TBAB. TBAB served as the supporting
electrolyte, a platinum mesh as the counter electrode, and SCE as the reference
electrode.
It has been shown that thioacetate groups can bind to gold in
ambient conditions with spontaneous deprotection [33]. With this
in mind, the modified thioacetate terminated surfaces of Layer-2
have been exposed to solutions of ꢄ20 nm diameter gold nano-
particles. STM images (Fig. 3) show the decoration of such surfaces
with gold nanoparticles. Such a partial surface coverage with gold
nanoparticles is beneficial for the gap-mode Raman characterisa-
tion (see below), which would have been more difficult to interpret
for densely packed or multilayer gold nanoparticle coverage.
Molecular characterisation of the surface functionalisation
steps was achieved with Raman spectroscopy. Fig. 4 shows a
cartoon illustration of the gap-mode and SHINERS (shell-isolated
nanoparticle-enhanced Raman spectroscopy) methods used. The
gap-mode method relies on the adsorption of gold nanoparticles
directly onto the top of the monolayer under investigation, while
SHINERS [34–36] employs gold nanoparticles coated with a thin
SiO2 shell. Control experiments were also undertaken for the gold
nanoparticles on a clean gold substrate. Gap-mode Raman uses
bare Au NPs, which can induce charge transport between the NPs
and target molecules, which may also shift the frequency or
intensity, giving a strong Raman signal as for surface enhanced
Raman spectroscopy (SERS). SHINERS uses “insulated” NPs, which
are not expected to promote charge transport between the NP and
the molecules. Consequently, the SHINERS spectrum is weaker
than comparable gap-mode Raman spectra, however the SHINERS
signal can be simpler and clearer to interpret. For these reasons we
use both techniques.
since the former case employs “bare” gold nanoparticles while the
latter uses SiO2 capped nanoparticles.
Raman measurements were performed with a HR800 confocal
Raman spectrometer (Horiba JY, France). The excitation wave-
length was 632.8 nm from a He–Ne laser. A 50-times magnification
long-working-distance (8 mm) objective was used to focus the
laser on the sample surface and to collect the scattered light in a
backscattering geometry. A laser power of 0.15 mW was used for
recording conventional Raman spectra of 1 and also gap-mode
Raman experiments, while a power of 1.5 mW was used for
SHINERS measurements. In all cases spectra were acquired over a
10 second period and the resolution used in spectra acquisition was
ca. 3 cmꢀ1
.
3. RESULTS AND DISCUSSION
Scheme 1 shows the initial stage of the surface grafting. The
gold samples were flame-annealed and immediately immersed in
an electrochemical cell containing compound 1 (5 mM), NaNO2
(5 mM) and aq. HClO4 (70%, 50
butylammonium bromide (TBAB, 0.1 M) in acetonitrile. Cathodic
electrochemical grafting was achieved by running cyclic
mL) in a solution of tetra-n-
a
voltammogram (CV) at 50 mV/s between 0.6 and ꢀ0.7 V (vs.
SCE). Fig. 1 shows the first and second CV cycles during the
electrografting progress, with the current falling dramatically after
the first cathodic cycle indicating rapid passivation of the electrode
surface and signifying grafting of the surface layer. The significant
cathodic current starting at ꢀ0.5 V is also observed in our other
studies of diazonium grafting, and we attribute this to the
reduction of water. The reduction wave between 0.2 and ꢀ0.4 V
is mostly likely to arise from the reduction of oxygen dissolved in
solution. Obviously, for these reactions to appear in the second
cycles, the oxygen as well as H2O molecules have to be penetrate
the molecular layer, even if the layer can prohibit the redox
reactions of large ferrocyanide molecules in the electrolyte (see
later).
After grafting, the Layer-1 functionalized samples were
thoroughly washed by ultrasonication in ethanol, dried under a
nitrogen flow and then transferred into a sealed flask. Cyclic
voltammetry can be used as an effective method for assessing the
barrier properties of diazonium-grafted organic monolayers on
surfaces [29]. Cyclic voltammograms in aqueous potassium
ferrocyanide were used to ascertain the formation of a blocked
monolayer and these are shown in Fig. 2. A clear voltammetric
wave is seen on the bare gold electrode and this is totally
Fig. 2. Cyclic voltammograms of a Au(111) electrode in 1 mM K4Fe(CN)6 + 0.1 M KCl
before (red line) and after (black line) electrografting, scan rate at 50 mV/s.

440
J. Liang et al. / Electrochimica Acta 220 (2016) 436–443
Scheme 2. Desilylation chemistry and Sonogashira coupling chemistry to synthesise Layer-2.
Fig. 5 shows normal Raman spectra of 1 and gap-mode Raman
[27]. As the junction is extended the plateaus rapidly decay as the
molecular junction is cleaved. This presumably occurs at the Au
(STM tip)-S interface, as the gold-thiol chemisorption bond will be
weaker than the Au-C linkage at the substrate. In this respect we
note that gold-thiol bonds have been quoted by Pensa et al. as
having bond energies of 40–50 kcal molꢀ1 (1.7–2.2 eV) [42]. On the
other hand Cheng et al. [6] find that the energy required to rupture
the Au–C bond in Au-(CH2)6-Au molecular single junctions (bound
to a single gold atom as a model of the electrode) is 3.0 eV. This
highlights the strength of the Au-C bond.
Many plateau containing I(s) traces are represented in histo-
grams of all-current points, with the histogram peak taken as the
single molecule conductance. A histogram of such junctions
formed from Layer-2 is shown in Fig. 7. A clear peak is seen at 3.0
nS. Examples of individual I(s) traces and break-off distance
analysis is given in the supporting information. By comparison
with the 3 nS conductance recorded here, a similar OPE molecular
wire with three phenylene rings (1,4-bis[4-(acetylsulphanyl)
phenylethynyl]-2,6-dimethoxybenzene), which differs by having
thiol groups at both termini and two solubilising methoxy
substituents on the central phenyl ring, showed a conductance
of slightly less than 2 nS with gold contacts [43]. Comparable
amine terminated OPE analogues with three phenylene rings also
give comparable conductance values centred around 2.5 nS, but
with broad conductance histogram distributions [44]. These
comparisons show that the single Au-C anchoring in Layer-1 does
not give appreciably higher junction transmission when compared
to similar length OPE analogues with either thiol or amine
anchoring groups. The influence of contacting groups on molecular
spectra of Layer-1 and Layer-2, alongside a control experiment for
the gold nanoparticles adsorbed on a blank gold substrate. Key
spectral bands are summarized in Table 1. Layer-1 shows spectral
bands that agree well with those of pure compound 1. Importantly
Layer-1 features in addition
a
band at ꢄ400 cmꢀ1 which
corresponds to the Au-carbon bond stretch of the surface-attached
compound. [37,38] Layer-2, on the other hand, features additional
bands associated with the terminal thioacetate moiety, confirming
success of the Sonogashira post-grafting step. SHINER spectra
further confirm the success of the surface molecular functional-
isation reactions, with new spectral bands apparent after addition
of the thioacetate moiety (Fig. 6 and Table 2). Band assignments
follow those that given in references [39–41].
Layer-2 was used for single molecule electrical measurements
recorded with an STM. The metal-molecule contacts here are the
direct Au-C bond formed at the substrate and the thiol group at the
other terminus. Note that Au-S covalent binding has been shown to
spontaneously occur for thioacetate termini upon contact with
gold even without the use of base [33]. The so-called I(s) method
has been used to generate molecular junctions and measure their
conductance [27]. In this method the gold STM tip is lowered into
the molecular layer, to high values of set-point current (I0). The
STM tip is then rapidly retracted and current decay (I) as a function
of tip retraction distance (s) is recorded. A proportion of these
retraction events lead to the formation of gold-molecule-gold
junctions and these are recognised as plateaus in the I(s) traces
characteristic of current flow through intact molecular bridges
Fig. 4. (a) Schematic diagram of gap-mode SERS (Au(111)/Molecular layer/Au NP)
and (b) SHINERS (Au(111)/Molecular layer/Au@SiO2 NP) on the Sonogashira-
modified surfaces. Colour code: blue line = Au-C surface contact, red circle = thiol or
thioacetate molecular headgroup, grey shell = SiO2 coating of SHINERS particle and
pink rectangle = molecular backbone (with and without headgroup).
Fig. 3. An STM image of Layer-2 decorated with 20 nm gold nanoparticles.

J. Liang et al. / Electrochimica Acta 220 (2016) 436–443
441
Fig. 5. Gap-mode Raman spectra of Layer-1, Layer-2 on gold substrates and a control spectrum of gold nanoparticles on a blank gold substrate. The upper curve is a normal
Raman spectrum of 1.
Table 1
Summary of key Raman spectral bands of compound 1 and in gap-mode spectra of Layer-1 and Layer-2. Band assignments follows those given in references [39–41]. Also see
the supporting information for assignment of the 481 cmꢀ1 band.
Mode/cmꢀ1
1
Layer-1
Layer-2
v(CRC)
2211
2155
1600
2214
2148
1592
2214
2148
1597
1560
Benzene ring stretching
¼
CꢀꢀNH2 stretching
1226
1180
1137
Ring modes
1180
1137
1180
1139
1078
1058
481
Ring modes
SꢀꢀCꢀꢀO bending mode of terminal thioacetate (see supporting information).
v(AuꢀꢀC)
404
402
junction conductance has been a popular topic of investigation in
single molecule electronics [2,45,46]. In general, contact chemistry
that alters electronic coupling between molecular bridges and
their contacts, and/or alters energy spacing between the contact
Fermi energies and the HOMO or LUMO frontier orbitals may be
expected to impact on the conductance. The similar conductance
we report here between the Au-C terminated Layer-1 and similar
length OPE analogues with either thiol or amine anchoring groups
contacting with Au-C at one end and Au-S at the other. Indeed,
in agreement with our current observation, Hines et al. have found
that gold-biphenyl-gold molecular junctions with direct Au-C
bonding formed by in-situ diazonium grafting do not give
appreciably higher conductance than analogous junctions with
amine surface contacts [8], although the former junctions were
found to be significantly more stable. Nevertheless, this recognised
high stability of diazonium grafted monolayers would be expected
to confer advantages to the metal-molecule junctions compared to
the more fluxional contacts formed by amines or thiols [8].
may be related to Layer-1 possessing
a non-symmetrical
4. CONCLUSIONS
In summary, diazonium grafted oligo(phenylene ethynylene)
(OPE) monolayers have been prepared from an amine terminated
molecular wire using cathodic electrochemical grafting and
protecting group chemistry to avoid multilayer formation.
Table 2
Summary of key Raman spectral bands of compound 1 and in SHINERS spectra of
Layer-1 and Layer-2. Band assignments follow those given in references [39–41].
Band/cmꢀ1
1
Layer-1
Layer-2
v(CRC)
2211
2155
1600
2216
2157
1600
2214
Benzene ring stretching
1600
1561
¼
CꢀꢀNH2 stretch
1226
1180
1137
Ring modes
1182
1140
1178
1139
1077
1056
Fig. 6. SHINERS spectra of Layer-1 and Layer-2 on gold substrates and a control
spectrum of gold nanoparticles on blank gold substrates. The upper curve is a
normal Raman spectrum of 1.
Ring modes

442
J. Liang et al. / Electrochimica Acta 220 (2016) 436–443
Fig. 7. (a) A schematic diagram of the STM conductance measurements, (b) conductance histograms and (c) 2D conductance and distance histograms of Layer-2 (3.0 nS). The
2D histogram shows the break-off distance around 1.7 nm (Layer-2). The histogram consists of 336 traces. The setpoint current used was 30 nA (giving an initial calibrated tip-
substrate distance, s0 ꢅ1 nm) and the STM tip retraction rate was 20 nm/s.
Following monolayer electrografting deprotection is achieved by
desilylation chemistry and then Sonogashira coupling chemistry is
used to append a phenyl-thioacetate terminal group to the
monolayer. The success of these surface reactions is confirmed
using Raman spectroscopy (gap-mode and SHINERS). It is shown
that such monolayers are suitable for STM based molecular
junction conductance measurements.
[5] V. Kaliginedi, A.V. Rudnev, P. Moreno-Garcia, M. Baghernejad, C.C. Huang, W.J.
Hong, T. Wandlowski, Promising anchoring groups for single-molecule
conductance measurements, Physical Chemistry Chemical Physics 16 (2014)
23529–23539.
[6] Z.L. Cheng, R. Skouta, H. Vazquez, J.R. Widawsky, S. Schneebeli, W. Chen, M.S.
Hybertsen, R. Breslow, L. Venkataraman, In situ formation of highly conducting
covalent Au-C contacts for single-molecule junctions, Nature Nanotechnology
6 (2011) 353–357.
[7] W.J. Hong, H. Li, S.X. Liu, Y.C. Fu, J.F. Li, V. Kaliginedi, S. Decurtins, T.
Wandlowski, Trimethylsilyl-Terminated Oligo(phenylene ethynylene)s: An
Approach to Single-Molecule Junctions with Covalent Au-C sigma-Bonds,
Journal of the American Chemical Society 134 (2012) 19425–19431.
[8] T. Hines, I. Díez-Pérez, H. Nakamura, T. Shimazaki, Y. Asai, N. Tao, Controlling
Formation of Single-Molecule Junctions by Electrochemical Reduction of
Diazonium Terminal Groups, Journal of the American Chemical Society 135
(2013) 3319–3322.
[9] M. Delamar, R. Hitmi, J. Pinson, J.M. Saveant, Covalent Modification of Carbon
Surfaces by Grafting Of Functionalized Aryl Radicals Produced From
Electrochemical Reduction of Diazonium Salts, Journal of the American
Chemical Society 114 (1992) 5883–5884.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
We gratefully acknowledge Professor Wu and Mr. Xia-Guang
Zhang, Xiamen University, for the simulation of Raman spectra for
HCC-C6H4-S-COCH3 and HCC-C6H4-S-Au5 in the supporting infor-
mation and advice in assignment of the ꢄ480 cmꢀ1 spectral band.
RJN and SJH thank EPSRC for funding (grant EP/H035184/1 and EP/
K007785/1). This work was also supported by the grant “Electro-
nanomat” (Molecular Scale Electrochemistry and Nontraditional
Electrochemical Materials Science, project reference 318990) from
the European Union (Seventh Framework Programme, Marie Curie
Programme). AB and RJD also gratefully acknowledge the EPSRC
(EP/K007548/1) for funding this work. PJL gratefully acknowledges
support from the ARC (DP140100855) and the award of a Future
Fellowship (FT120100073).
[10] S. Mahouche-Chergui, S. Gam-Derouich, C. Mangeney, M.M. Chehimi, Aryl
diazonium salts: a new class of coupling agents for bonding polymers,
biomacromolecules and nanoparticles to surfaces, Chemical Society Reviews
40 (2011) 4143–4166.
[11] A.J. Bergren, K.D. Harris, F.J. Deng, R.L. McCreery, Molecular electronics using
diazonium-derived adlayers on carbon with Cu top contacts: critical analysis
of metal oxides and filaments, Journal of Physics-Condensed Matter 20 (2008)
374117.
[12] R.L. McCreery, H. Yan, A.J. Bergren,
A critical perspective on molecular
electronic junctions: there is plenty of room in the middle, Physical Chemistry
Chemical Physics 15 (2013) 1065–1081.
[13] R.L. McCreery, A.J. Bergren, Progress with Molecular Electronic Junctions:
Meeting Experimental Challenges in Design and Fabrication, Advanced
Materials 21 (2009) 4303–4322.
[14] D.K. Aswal, S.P. Koiry, B. Jousselme, S.K. Gupta, S. Palacin, J.V. Yakhmi, Hybrid
molecule-on-silicon nanoelectronics: Electrochemical processes for grafting
and printing of monolayers, Physica E-Low-Dimensional Systems &
Nanostructures 41 (2009) 325–344.
Appendix A. Supplementary data
[15] W. Wang, T. Lee, M. Kamdar, M.A. Reed, M.P. Stewart, J.J. Hwang, J.M. Tour,
Electrical characterization of metal-molecule-silicon junctions, Superlattices
and Microstructures 33 (2003) 217–226.
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
[16] B. Adam Johan, Z.-W. Lucas, S. Mitchell, P. Nikola, S. Bryan, L.M. Richard,
Musical molecules: the molecular junction as an active component in audio
distortion circuits, Journal of Physics: Condensed Matter 28 (2016) 094011.
[17] A.M. Ricci, E.J. Calvo, S. Martin, R.J. Nichols, Electrochemical Scanning
Tunneling Spectroscopy of Redox-Active Molecules Bound by Au-C Bonds,
Journal of the American Chemical Society 132 (2010) 2494–2495.
[18] C. Combellas, F. Kanoufi, J. Pinson, F.I. Podvorica, Time-of-flight secondary ion
mass spectroscopy characterization of the covalent bonding between a carbon
surface and aryl groups, Langmuir 21 (2005) 280–286.
electacta.2016.10.095.
References
[1] R.J. Nichols, W. Haiss, S.J. Higgins, E. Leary, S. Martin, D. Bethell, The
experimental determination of the conductance of single molecules, Physical
Chemistry Chemical Physics 12 (2010) 2801–2815.
[2] E. Leary, A. La Rosa, M.T. Gonzalez, G. Rubio-Bollinger, N. Agrait, N. Martin,
Incorporating single molecules into electrical circuits. The role of the chemical
anchoring group, Chemical Society Reviews 44 (2015) 920–942.
[19] T. Matrab, M. Save, B. Charleux, J. Pinson, E. Cabet-Deliry, A. Adenier, M.M.
Chehimi, M. Delamar, Grafting densely-packed poly(n-butyl methacrylate)
chains from an iron substrate by aryl diazonium surface-initiated ATRP: XPS
monitoring, Surface Science 601 (2007) 2357–2366.
[20] C. Combellas, F. Kanoufi, J. Pinson, F.I. Podvorica, Sterically hindered diazonium
salts for the grafting of a monolayer on metals, Journal of the American
Chemical Society 130 (2008) 8576–8577.
[3] C.C. Jia, X.F. Guo, Molecule-electrode interfaces in molecular electronic
devices, Chemical Society Reviews 42 (2013) 5642–5660.
[4] J. Hihath, N.J. Tao, The role of molecule-electrode contact in single-molecule
electronics, Semiconductor Science and Technology 29 (2014).

J. Liang et al. / Electrochimica Acta 220 (2016) 436–443
443
[21] Y.R. Leroux, P. Hapiot, Nanostructured Monolayers on Carbon Substrates
Prepared by Electrografting of Protected Aryldiazonium Salts, Chemistry of
Materials 25 (2013) 489–495.
[22] Y.R. Leroux, F. Hui, P. Hapiot, A protecting-deprotecting strategy for structuring
robust functional films using aryldiazonium electroreduction, Journal of
Electroanalytical Chemistry 688 (2013) 298–303.
[23] M.A. Ghanem, I. Kocak, A. Al-Mayouf, M. AlHoshan, P.N. Bartlett, Covalent
modification of carbon nanotubes with anthraquinone by electrochemical
grafting and solid phase synthesis, Electrochimica Acta 68 (2012) 74–80.
[24] S.Y. Sayed, A. Bayat, M. Kondratenko, Y. Leroux, P. Hapiot, R.L. McCreery, Bilayer
Molecular Electronics: All-Carbon Electronic Junctions Containing Molecular
Bilayers Made with Click Chemistry, Journal of the American Chemical Society
135 (2013) 12972–12975.
[25] K. Onitsuka, M. Fujimoto, H. Kitajima, N. Ohshiro, F. Takei, S. Takahashi,
Convergent synthesis of platinum-acetylide dendrimers, Chemistry-a
European Journal 10 (2004) 6433–6446.
[26] A.K. Flatt, Y.X. Yao, F. Maya, J.M. Tour, Orthogonally functionalized oligomers
for controlled self-assembly, Journal of Organic Chemistry 69 (2004) 1752–
1755.
[27] W. Haiss, H. van Zalinge, S.J. Higgins, D. Bethell, H. Hobenreich, D.J. Schiffrin, R.
J. Nichols, Redox state dependence of single molecule conductivity, Journal of
the American Chemical Society 125 (2003) 15294–15295.
[28] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y.
Zhou, Y. WuDe, B. Ren, Z.L. Wang, Z.Q. Tian, Shell-isolated nanoparticle-
enhanced Raman spectroscopy, Nature 464 (2010) 392–395.
[29] A.J. Downard, M.J. Prince, Barrier properties of organic monolayers on glassy
carbon electrodes, Langmuir 17 (2001) 5581–5586.
[30] K.K. Cline, M.T. McDermott, R.L. McCreery, Anomalously slow-electron transfer
at ordered graphite-electrodes ꢀ influence of electronic factors and reactive
sites, Journal of Physical Chemistry 98 (1994) 5314–5319.
[31] X.B. Ji, C.E. Banks, A. Crossley, R.G. Compton, Oxygenated edge plane sites slow
the electron transfer of the ferro-/ferricyanide redox couple at graphite
electrodes, ChemPhysChem 7 (2006) 1337–1344.
[32] E.P. Randviir, D.A.C. Brownson, M. Gomez-Mingot, D.K. Kampouris, J. Iniesta, C.
E. Banks, Electrochemistry of Q-Graphene, Nanoscale 4 (2012) 6470–6480.
[33] Y.K. Kang, D.J. Won, S.R. Kim, K.J. Seo, H.S. Choi, G.H. Lee, Z.S. Noh, T.S. Lee, C.J.
Lee, Self-assembled monolayer of the aromatic thioacetate on the gold surface,
Materials Science & Engineering C-Biomimetic and Supramolecular Systems
24 (2004) 43–46.
[34] J.F. Li, Y.F. Huang, Y. Ding, Z.L. Yang, S.B. Li, X.S. Zhou, F.R. Fan, W. Zhang, Z.Y.
Zhou, D.Y. Wu, B. Ren, Z.L. Wang, Z.Q. Tian, Shell-isolated nanoparticle-
enhanced Raman spectroscopy, Nature 464 (2010) 392–395.
Enhanced Raman Scattering, R.G. Cooks, E.S. Yeung (Eds.), Annual Review of
Analytical Chemistry 4 (2011) 129–150.
[36] J.F. Li, X.D. Tian, S.B. Li, J.R. Anema, Z.L. Yang, Y. Ding, Y.F. Wu, Y.M. Zeng, Q.Z.
Chen, B. Ren, Z.L. Wang, Z.Q. Tian, Surface analysis using shell-isolated
nanoparticle-enhanced Raman spectroscopy, Nature Protocols 8 (2013) 52–65.
[37] L. Laurentius, S.R. Stoyanov, S. Gusarov, A. Kovalenko, R. Du, G.P. Lopinski, M.T.
McDermott, Diazonium-Derived Aryl Films on Gold Nanoparticles: Evidence
for a Carbon–Gold Covalent Bond, ACS Nano 5 (2011) 4219–4227.
[38] Y. Fu, S. Chen, A. Kuzume, A. Rudnev, C. Huang, V. Kaliginedi, M. Baghernejad,
W. Hong, T. Wandlowski, S. Decurtins, S.-X. Liu, Exploitation of desilylation
chemistry in tailor-made functionalization on diverse surfaces, Nature
Communications 6 (2015) 6403.
[39] J.G. Kushmerick, J. Lazorcik, C.H. Patterson, R. Shashidhar, D.S. Seferos, G.C.
Bazan, Vibronic Contributions to Charge Transport Across Molecular Junctions,
Nano Letters 4 (2004) 639–642.
[40] Z. Liu, X. Wang, K. Dai, S. Jin, Z.-C. Zeng, M.-D. Zhuang, Z.-L. Yang, D.-Y. Wu, B.
Ren, Z.-Q. Tian, Tip-enhanced Raman spectroscopy for investigating adsorbed
nonresonant molecules on single-crystal surfaces: tip regeneration, probe
molecule, and enhancement effect, Journal of Raman Spectroscopy 40 (2009)
1400–1406.
[41] M. Fletcher, D.M. Alexson, S. Prokes, O. Glembocki, A. Vivoni, C. Hosten,
Surface-enhanced Raman scattering of a Ag/oligo(phenyleneethynylene)/Ag
sandwich, Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy 78 (2011) 706–711.
[42] E. Pensa, E. Cortes, G. Corthey, P. Carro, C. Vericat, M.H. Fonticelli, G. Benitez, A.
A. Rubert, R.C. Salvarezza, The Chemistry of the Sulfur-Gold Interface: In
Search of a Unified Model, Accounts of Chemical Research 45 (2012) 1183–
1192.
[43] W. Haiss, C.S. Wang, I. Grace, A.S. Batsanov, D.J. Schiffrin, S.J. Higgins, M.R.
Bryce, C.J. Lambert, R.J. Nichols, Precision control of single-molecule electrical
junctions, Nature Materials 5 (2006) 995–1002.
[44] M.T. Gonzalez, X.T. Zhao, D.Z. Manrique, D. Miguel, E. Leary, M. Gulcur, A.S.
Batsanov, G. Rubio-Bollinger, C.J. Lambert, M.R. Bryce, N. Agrait, Structural
versus Electrical Functionalization of Oligo(phenylene ethynylene) Diamine
Molecular Junctions, Journal of Physical Chemistry C 118 (2014) 21655–21662.
[45] Y.S. Park, A.C. Whalley, M. Kamenetska, M.L. Steigerwald, M.S. Hybertsen, C.
Nuckolls, L. Venkataraman, Contact chemistry and single-molecule
conductance: A comparison of phosphines, methyl sulfides, and amines,
Journal of the American Chemical Society 129 (2007) 15768–15769.
[46] B. Kim, S.H. Choi, X.Y. Zhu, C.D. Frisbie, Molecular Tunnel Junctions Based on pi-
Conjugated Oligoacene Thiols and Dithiols between Ag, Au, and Pt Contacts:
Effect of Surface Linking Group and Metal Work Function, Journal of the
American Chemical Society 133 (2011) 19864–19877.
[35] J.R. Anema, J.-F. Li, Z.-L. Yang, B. Ren, Z.-Q. Tian, Shell-Isolated Nanoparticle-
Enhanced Raman Spectroscopy: Expanding the Versatility of Surface-