Article

pubs.acs.org/JPCC

Organization of Alkane Amines on a Gold Surface: Structure, Surface

Dipole, and Electron Transfer
Ezequiel de la Llave,† Romain Clarenc,† David J. Schiffrin,‡ and Federico J. Williams†,*
†
Departamento de Química Inorgánica, Analítica y Química-Física, INQUIMAE-CONICET, Universidad de Buenos Aires, Buenos
Aires, Argentina C1428EHA
‡
Chemistry Department, University of Liverpool, Liverpool, U.K. L69 7ZD
*
S Supporting Information

ABSTRACT: Surface molecular self-assembly is a fast

advancing field with broad applications in molecular
electronics, sensing and advanced materials. Although a large
number of practical systems utilize alkanethiols, there is
increasing interest in alkylamine self-assembled monolayers
(SAMs). In this article, the molecular and electronic structure
of alkylamine SAMs on Au surfaces was studied. It was found
that amine-terminated alkanes self-assemble, forming a
compact layer with the amine headgroup interacting directly
with the Au surface and the hydrocarbon backbone tilted by
around 30° with respect to the surface normal. The dense
layers formed substantially decrease electron tunneling across the metal/solution interface and form a dipole layer with positive
charges residing at the monolayer/vacuum interface.

■ INTRODUCTION
Rapid growth in the understanding of the interaction of organic
Amine self-assembled monolayers are not only important for
the synthesis of Au NPs as discussed above but also in
layers with metallic and semiconducting surfaces has taken molecular electronics, where they provide an alternative to
place in the past two decades. This growing interest arises from thiol-based SAMs with a corresponding greater resistance to
their broad range of applications in molecular electronics, oxidation.13
sensing, and advanced materials. Foremost examples of this Despite their importance, amine self-assembled monolayers
chemistry are self-assembled monolayers (SAMs), which over Au surfaces have not been extensively studied. The
provide simple systems to modify interfacial properties of structure and nature of long-chain aliphatic amine SAMs have
metals and semiconductors. Different functionalities with been investigated only on gold,14,15 YBa2Cu3O7,16 and stainless
specific affinity for a substrate have been used for binding steel surfaces.17 Xu et al.14 reported the only evidence of an
molecules to specific metals, metal oxides, and semiconductors, ordered monolayer of decylamine on Au surfaces using Fourier
such as thiols, disulfides, phosphates, selenols, alcohols, transform infrared external reflectance spectroscopy (FTIR-
isocyanides, and amines. The most extensively studied class ERS). Dilshad et al.15 demonstrated the formation of very
of SAMs on metals, however, is derived from the adsorption of stable SAMs of C12 and C16 alkane amines on Au nanoparticles
alkanethiols.1,2 and Au−Cu nanoalloys. Ritchie et al.16 found that alkyl- and
Elusive at first, synthetic strategies to obtain metal nano- arylamines self-assemble on the cuprate superconductor
particles (NPs) with controlled particle size distributions have YBa2Cu3O7 surface, demonstrating that their spontaneous
been achieved using SAMs of dodecanethiol as a capping adsorption produced stable and robust monolayer films with
agent.3 Although most NP functionalization has been carried no apparent damage to the bulk properties of the underlying
out with thiol/thiolated ligands, the preparation of amine- material. Finally, Ruan et al.17 reported the formation of SAMs
capped Au NPs using primary amines,4 aromatic amines,5 of alkylamines adsorbed from solution onto the electrochemi-
diamines,6 octadecylamine,7 oleylamine,8 or amino acids9 has cally reduced surface of stainless steel.
been reported. Amines have also been used as digestive The present work provides detailed physical insight into the
ripening agents to narrow the NP size distribution.10−12 Sahu molecular and electronic structures of dodecylamine (C12N)
and Prasad recently used dodecanethiol and dodecylamine as and hexadecylamine (C16N) SAMs on gold surfaces. The study
ripening agents for the synthesis of Ag, Au, and Pd NPs11 and was carried out using X-ray and UV photoelectron spectros-
found that the natures of the metallic core and of the ripening
agent influenced the final particle size distribution. Klabunde Received: October 10, 2013
and co-workers also found that the addition of dodecylamine as Revised: November 26, 2013
a ripening agent generates nearly monodisperse gold NPs.12 Published: December 11, 2013

© 2013 American Chemical Society 468 dx.doi.org/10.1021/jp410086b | J. Phys. Chem. C 2014, 118, 468−475
The Journal of Physical Chemistry C Article

Figure 1. (a) N 1s, (b) C 1s, and (c) Au 4f XP spectra corresponding to the bare Au substrate, dodecylamine (C12N), and hexadecylamine (C16N)
SAMs.

copies (XPS and UPS, respectively), scanning tunneling immersion in 50 mM toluene solution of the amines in toluene
microscopy (STM), cyclic voltammetry (CV), and electro- for 2 h, followed by extensive rinsing with toluene. The same
chemical impedance spectroscopy (EIS). We found that C12 treatment was carried out for the electrodes used in the
and C16 amine SAMs form compact layers with the amine electrochemical experiments.
functionality interacting directly with the Au substrate. This Photoelectron Spectroscopy. XP spectra were acquired
compact layer blocks the surface for electron-transfer reactions on grounded conducting substrates at a constant pass energy of
and results in a work function decrease similar to that observed 20 eV using a Mg Kα (1253.6 eV) source operated at 12.5 kV
for alkanethiol SAMs of similar chain lengths. and 20 mA at a detection angle of 30° with respect to the

■ EXPERIMENTAL METHODS
XPS and UPS measurements were performed using an
sample normal. The binding energies quoted are referred to the
Au 4f7/2 emission at 84.0 eV. Atomic ratios were calculated
from the integrated intensities of core levels after instrumental
ultrahigh-vacuum (UHV) chamber equipped with a home- and photoionization cross-section corrections. UP spectra were
built transfer system that allows easy and rapid controlled acquired with normal detection at a constant pass energy of 2
transfer of the sample between the UHV environment and the eV using a He I radiation source (21.2 eV). Samples were
liquid phase containing the amine solutions at atmospheric biased at −8 V to resolve the secondary electron cutoff in the
pressure.18 The self-assembled monolayers were prepared in UP spectra. Work function values were determined from the
the reactor preparation chamber that was interfaced to the main width of the UP spectra as discussed below.
analysis UHV chamber. This experimental setup permits ex situ STM Imaging. Observations were performed in the ambient
electron spectroscopic measurements on samples initially environment using an Agilent 5500 scanning tunneling
cleaned in UHV that are not exposed to the laboratory microscope (Agilent Technologies) isolated from vibrations,
atmosphere when transferred to the preparation chamber, thus air turbulence, and acoustic noise. Images were recorded in
allowing full spectroscopic characterization of the sample before constant-current mode (1 nA) with a sample bias of 0.1 V and a
and immediately after monolayer formation. tip scan speed between 0.2 and 0.8 μm·s−1. Tips were made
Monolayer Formation. The Au sample was Ar+ sputtered from a 0.25-mm-diameter Pt0.8Ir0.2 wire.
(E = 1000 eV) and annealed (T = 625 K) in subsequent cycles Electrochemical Measurements. Electrochemical meas-
until no impurities were detected by XPS. The spectroscopi- urements were carried out using an Autolab potentiostat−
cally clean Au sample was then transferred from the UHV galvanostat equipped with a frequency response analyzer (FRA)
environment to the preparation chamber without exposure to module (Eco Chemie, Utrecht, The Netherlands). Impedance
the laboratory atmosphere. It was then placed in contact with a studies were carried out with a 10 mV (root-mean-square)
50 mM solution of dodecylamine (C12N, Fluka, puriss 99.5%) amplitude potential perturbation, and the spectra were
or hexadecylamine (C16N, Aldrich, 98%) in toluene (SeccoSolv, collected between 1 and 100 kHz. A standard three-electrode
Merck) for 2 h at room temperature by forming a meniscus electrochemical cell was employed with a platinum mesh
with the solution under an Ar atmosphere. The amine solution counter electrode and a Ag/AgCl/3 M KCl reference electrode.
was then removed, and the sample was extensively rinsed with All potentials are quoted with respect to this electrode.
pure toluene under a constant flow of Ar. STM images and Solutions were prepared with deionized H2O from a Milli-Q
electrochemical measurements were acquired on 250-nm gold purification system (Millipore Products, Bedford, MA). All
films evaporated on a thin layer of chromium supported on a other chemicals used were of the highest analytical grade
available.

■
glass substrate (Arrandee, Werther, Germany). Substrates were
prepared by annealing for 5 min in a butane/propane flame to a
dark red color. After annealing, the substrates exhibited large RESULTS AND DISCUSSION
atomically smooth (111) terraces separated by steps of XPS Measurements. The attached monolayers were
monatomic height. SAMs on these surfaces were prepared by studied by X-ray phototelectron spectroscopy (XPS). Survey
469 dx.doi.org/10.1021/jp410086b | J. Phys. Chem. C 2014, 118, 468−475
The Journal of Physical Chemistry C Article

scans confirmed the presence of only O, N, C, and Au. Figure 1 the thiol SAMs, explaining the similarities in α between thiol
shows high-resolution XP spectra for the N 1s, C 1s, and Au 4f and amine SAMs (given that other factors are very similar for
regions corresponding to the bare gold substrate (Au), the two groups of molecules).
dodecylamine (C12N), and hexadecylamine (C16N) self- The N/C ratio, after instrumental and photoionization cross-
assembled monolayers, respectively. section corrections, for the C12N and C16N SAMs were 1:17
The bare Au substrate shows no N 1s or C 1s XP signals, and 1:26, respectively. These ratios are lower than the expected
corroborating that the initial state of the sample prior to values of 1:12 (C12N) and 1:16 (C16N), as the N 1s signal is
monolayer formation corresponds to a clean Au surface. The N attenuated by the hydrocarbon chain, resulting in a lower
1s XP spectra of the C12N and C16N SAMs show a broad signal intensity value. This is a clear indication that the molecules lie
with a pronounced shoulder at higher binding energies that can with the −NH2 moiety interacting with the Au surface.
be fitted with two major components. The low-binding-energy Moreover, the N/C ratio for C12N measured at grazing
component centered at 399.5 eV is assigned to the −NH2 detection (80° with respect to the surface normal) decreases to
moiety, whereas the high-binding-energy peak at 401.5 eV is 1:20, providing additional evidence of a direct interaction
due to protonated amine −NH3+, in excellent agreement with between the −NH2 group and the underlying Au substrate. The
previously reported values.19 In both cases, the ratio of surface coverage estimated from the N 1s and Au 4f intensities
unprotonated to protonated species is approximately 7:3. The was approximately 4 × 1014 molecules/cm2 for both amines,
C 1s spectra corresponding to C12N and C16N show one broad giving a molecular density of approximately 25 Å2 molecule−1,
signal centered around 285.0 eV, which is due to the methylene closely resembling the estimated surface coverage of alkanethiol
carbon atoms present in the SAMs. The C16N SAM presents a molecules self-assembled on Au(111) surfaces.22 Therefore, the
larger C 1s signal, as expected from the larger number of C XPS results suggest that amine alkane molecules self-assemble
atoms per molecule present in the monolayer. In addition, the on Au surfaces with a structure similar to that observed for
O 1s XP spectra (not shown) also indicated the presence of alkanethiol SAMs, that is, the molecules are adsorbed with the
OH− anions in an approximately 1:1 ratio with the protonated −NH2 group on the Au surface tilted approximately 30° with
amine −NH3+, suggesting the formation of ion pairs. respect to the surface normal and packed with a molecular
The Au 4f spectra show the characteristic Au 4f7/2 (84.1 eV) surface density of approximately 25 Å2 molecule−1, as shown in
and Au 4f5/2 (87.8 eV) doublet in a 4:3 intensity ratio.20 The Scheme 1.
thickness (d) of the self-assembled monolayer can be estimated
from the Au 4f intensity attenuation (I/I0, where I is the Scheme 1. Linear Alkane Amine Molecules Self-Assemble on
substrate intensity of the SAM-covered surface and I0 is that of Au Surfaces, Forming a Highly Dense Layer Tilted
the bare substrate) from the expression I = I0 exp(−d /λι cos Approximately 30° with Respect to the Surface Normal
θ),20 where θ is the angle of detection with respect to the
surface normal and λι is the photoelectron attenuation length,
which is equal to 36.6 Å for self-assembled alkane chains.20
From the decrease in the Au 4f signal after C12N and C16N
adsorption, overlayer thicknesses of 12 and 17 Å, respectively,
can be estimated. These values can be compared to the
calculated molecular lengths of the amines [14.2 Å for C12N
and 20.6 Å for C16N, as determined by geometry optimization
of the isolated molecules in a vacuum using density functional
theory (DFT) as implemented in the Quantum Espresso
code21], indicating that these amines form packed monolayers
with tilt angles (α) of approximately 30° ± 10° with respect to
the surface normal. Although the tilt angle estimation using the UPS Measurements. Figure 2 compares the UP spectra of
XPS-calculated thickness yields approximate values with a C12N and C16N self-assembled monolayers on Au with that of a
relatively large error bar, more precise polarization modulation clean gold surface. Figure 2a shows the secondary electron
infrared reflection absorption spectroscopy measurements cutoff, whereas Figure 2b shows the region around the Fermi
(results not shown) resulted in similar tilt angle values, edge.24 The UP spectrum corresponding to the bare Au
supporting the XPS tilt angle estimation. substrate shows peaks corresponding to the 5d bands, in
Table ESI 1 (Supporting Information) collects some relevant agreement with the spectrum previously reported for clean Au
estimates of α taken from the literature for alkanethiol SAMs surfaces.24 The adsorption of these amines does not result in
on Au derived from different experimental techniques. The discernible new features in the UP spectra, resulting only in the
average tilt angle estimate is around 35°, and no clear trend attenuation of the signals arising from the underlying Au
relating the tilt angle for C12 and C16 thiols is discernible. substrate, also in agreement with the reported spectra of
Therefore, the values of amine SAM tilt angles estimated in this alkanethiols SAMs.25
work are comparable to those observed for the same-length The Au work function (Φ) was calculated from the width
alkanethiol SAMs on Au.22 The value of α results from the (W) of the spectrum corresponding to the bare Au substrate
interplay of spacing at the head groups dictated by the atomic according to Φ = 21.2 eV − W,26 and a value of Φ = 5.1 eV was
arrangement of surface Au atoms and by chain−chain van der obtained (W = 16.1 eV), in excellent agreement with previous
Waals interactions that are strongly dependent on chain determinations for polycrystalline gold.24 Figure 2a shows that
length.23 In addition, dipole−dipole interaction will impinge the adsorption of dodecyl and hexadecylamine shifts the
on the tilt angle to achieve maximum lateral interactions in the position of the secondary electron cutoff, giving rise to changes
SAM. The present UPS results (see below) demonstrate that in the work function of −1.2 and −1.3 eV, respectively, with
the surface dipole in the amine SAMs is very similar to that in respect to that of the clean Au substrate. These results are in
470 dx.doi.org/10.1021/jp410086b | J. Phys. Chem. C 2014, 118, 468−475
The Journal of Physical Chemistry C Article

Scheme 2. Schematic Energy Level Diagrams Corresponding

to the SAM/Au Interfacea

Figure 2. (a) Secondary electron cutoff and (b) 5 eV below the Fermi
edge of the UP spectra of the bare gold substrate (Au, black curve) and
the dodecylamine (red curve) and hexadecylamine (blue curve) SAMs.

line with previous work on self-assembled monolayers of

alkanethiols on Au, Ag,27 and Cu,28 which showed that
chemisorbed alkanethiols cause a major decrease in the metal
a
ΦAu is the Au work function, ΔΦ is the work function change after
SAM formation, and ΦSAM is the SAM-modified Au surface work
work function. In particular, alkanethiol SAMs up to 18 carbon
function.
atoms in length are known to change the work function of Au
by between −1 and −1.4 eV.25,29,30
Work function (Φ) values are determined by the chemical facilitates robust self-assembly, but the interaction is so strong
potential of the electron in the metal resulting from its that the surface is reconstructed and pits are formed. The
interaction with the bulk of the metal and the surface potential absence of pits at the step edges or at terraces in amine SAMs
due to dipoles present on the surface.31 Therefore, changes in suggests that the binding energy of amines to gold substrates is
the metal work function can be due to changes in the bulk considerably lower than that of thiols and not sufficiently large
properties of the metal or in the surface potential by to displace Au atoms from the surface, a result consistent with
modifications of the surface layer dipole. Formation of a the DFT calculations of Hoft and co-workers.32
SAM does not alter the metal bulk properties, and therefore, Electrochemical Measurements. The formation of
work function changes (ΔΦ) can be interpreted solely in terms compact SAMs of alkylamines on Au as determined from the
of changes in the surface potential due to adsorption of the XPS data was further confirmed by the inhibition of electron-
SAM. Simple electrostatics dictates a linear relationship transfer (ET) rates across the absorbed films. Cyclic
between the change in the work function and the adsorbate- voltammetry (CV) and electrochemical impedance spectrosco-
induced dipole on the surface.29 py (EIS) measurements were carried out for 1 mM Fe(CN)64−
The decrease of the underlying metal work function can be in 0.1 M NaF on both bare gold and gold functionalized with
considered as resulting from a dipole sheet with negative C12N and C16N SAMs. Figure 4 compares the cyclic
charges residing at the metal/monolayer interface and positive voltammograms for the Fe(II)/Fe(III) redox couple in the
charges residing at the monolayer/vacuum interface. Alka- absence and presence of amine SAMs. The large inhibition of
nethiol self-assembled monolayers (Au−S−R) result in a ET observed is a clear indication of the formation of a compact
decrease of the work function of the Au substrate because amine monolayer, in agreement with the well-known blocking
the effective R−S dipole is larger than the Au−S dipole. In fact, behavior of aliphatic thiol SAMs of similar chain length that are
DFT calculations indicate that the Au−S bond gives rise to a known to form highly dense monolayers.33,34
negligible dipole moment.29 As mentioned above, the present There is, however, a clear difference between the present
results indicate that replacing the S headgroup by the amine results and extensive previous work on thiols. The data in
functionality results in a work function decrease similar to that Figure 4a show a voltammetric wave at ∼0.225 V superimposed
caused by the SAM. This indicates that amine SAMs (Au− on an irreversible Fe(II) oxidation process. A detail of these
NH2−R) have a dipole perpendicular to the surface, with the features corrected for double-layer charging effects is shown in
positive charges residing at the monolayer/vacuum interface Figure 4b. These correspond to the response of a micro-
and negative charges at the metal/monolayer interface similar electrode array in which the spacing between the elements is
in intensity to the dipole of alkanethiol SAMs (Au−S−R) of greater than the spherical diffusion characteristic length. It is
the same chain length, as shown in Scheme 2. proposed that this feature originates from small imperfections
STM Imaging. STM images of C12N SAMs on gold in the SAM, equivalent to pores that allow penetration of the
substrates are shown in Figure 3a,b. Both images show terraces electroactive ions close to the electrode surface, thus resulting
separated by clearly resolved monatomic steps. The absence of in highly localized fast electron transfer and giving rise to a
dark regions on the Au terraces indicates that C12N SAM response similar to that of ultramicroelectrodes.35
formation does not result in the creation of pits and defects. The voltammetric response of these electrodes was modeled
This is in sharp contrast with the typical monatomic or as a parallel reaction involving a mass-transfer-limited reaction
diatomic depth vacancy islands (pits) found upon alkanethiol following spherical diffusion taking place at the SAM
SAM formation on Au.2 Formation of a strong Au−S bond imperfections and an inhibited ET reaction across the organic
471 dx.doi.org/10.1021/jp410086b | J. Phys. Chem. C 2014, 118, 468−475
The Journal of Physical Chemistry C Article

Figure 3. STM images of dodecylamine self-assembled monolayer on gold surfaces: (a) 154 × 154 nm2 and (b) 50 × 50 nm2.

SAM to the metal substrate, the ratio of measured k0′ values for
the C12N and C16N SAMs should have been approximately 50,
whereas a ratio of only ∼1.6 was observed (Table ESI 2,
Supporting Information). The probable reason for this
discrepancy is the assumption of total exclusion of the ions
from the outer layers of the SAM, and some degree of
penetration enhanced by the interfacial field probably takes
place.
Electrochemical impedance spectroscopy was further em-
ployed to probe the electrochemical properties of these
interfaces. Figure 5 presents typical Nyquist diagrams for
dodecylamine and hexadecylamine SAMs in contact with 1 mM
Fe(CN)63− in 0.1 M NaF measured at the half-wave potential.
Figure 4. (a) Cyclic voltammogram of 1 mM Fe(CN)63− in 0.1 M
NaF for bare gold (black line) and dodecylamine (red curve) and
hexadecylamine (blue curve) SAMs (scan rate = 1.00 mV/s). (b)
Detail of the voltammetric feature at 0.225 V including a comparison
of experimental data for Fe(CN)64− oxidation with results from the
NLR analysis based on equation ESI 1 (Supporting Information) for
C12N (red dots) and C16N (blue dots) SAMs (see below).

layer with a rate constant determined by the SAM thickness.

The latter was considered to follow a classical Butler−Volmer
model, given by j = −Fk0′c exp[−αf(E − E0′)], where k0′ is the
formal electrochemical rate constant, α is the transfer
coefficient, f = F/RT, and E0′ is the formal potential of the
Fe(II)/Fe(III) couple taken here as the half-wave potential,36
whereas the former corresponded to a mass-transfer-limited
process to independent microelectrodes. Figure 5. Impedance-plane (Nyquist) plots for dodecylamine (■) and
The data were analyzed by nonlinear regression, and the hexadecylamine (●) SAMs in the presence of 1 mM Fe(CN)63− in 0.1
results are shown in Figure 4b, indicating that the simple model M NaF at 0.225 V. The superimposed solid lines correspond to the
proposed correctly describes the voltammetric response. Full fitted circuit.
details of these calculations are given as Supporting
Information, with the parameters derived from these
calculations reported in Table ESI 2 (Supporting Information). The impedance spectra of the SAM-modified electrodes was
The values of k0′ calculated were (5.8 ± 0.3) × 10−5 and (3.7 ± analyzed considering two main parallel circuit components
0.06) × 10−5 cm s−1 for the C12N- and C16N-functionalized reflecting the two modes of charge transfer discussed above,
electrodes, respectively. The decrease in the rate constant namely, through-film tunneling and charge transfer at film
values when the length of the alkyl chain was increased from 12 imperfections that allow electrolyte penetration. The equivalent
to 16 methylene groups was much less than expected. The rate circuit employed for modeling is shown in the inset to Figure 5,
constant for electron transfer across a SAM, kET, decays with which satisfactorily fitted the experimental results (Figure 5).
distance according to kET ∼ exp[−β(d − d0)],37 where d is a R1, the uncompensated solution resistance, was obtained by
distance parameter and β is the electronic decay coefficient, extrapolation of the dependence of Z″ on Z′ to infinite
typically in the range of 1 per methylene group for n-alkanes.38 frequency. At low frequencies, a response characteristic of a
Thus, if electron transfer occurred from the outer layer of the parallel RC component (R4−C1) is observed. The values of the
472 dx.doi.org/10.1021/jp410086b | J. Phys. Chem. C 2014, 118, 468−475
The Journal of Physical Chemistry C Article

main circuit parameters obtained are presented in Table 1. The circuit components are not necessarily independent of each
through-film charge-transfer resistances (RCT) corresponding to other. This becomes evident when attempting to include
nonlinear effects into the diffusional Warburg impedance at the
Table 1. Equivalent Circuit Parameters Calculated from the pores. Although this has been successfully carried out for two-
Data in Figure 5 dimensional single-walled carbon nanotube networks,46 the
complications introduced by the SAM make the extension of
parametera C12N C16N
this analysis very difficult. For these reasons, the simplest
R1 (Ω cm−2) 8.68 9.50 possible approach to model the SAM imperfections was
C1 (μF cm−2) 1.77 ± 0.06 1.2 ± 0.1 followed.
R4 Ω cm−2 (1.21 ± 0.04) × 104 (1.5 ± 0.1) × 104 R2 (Figure 5) is the electrolyte resistance within the pore,
R3 (Ω cm−2) (5.22 ± 0.04) × 104 (1.02 ± 0.07) × 104 and the combination R3−CPE1 represents the kinetics of
(1 − θ) 0.9997 0.9987 electron transfer at the bottom regions of the pore. In this
a
R1 was calculated separately in the very high frequency region (see model, R3 is the charge-transfer resistance within the pores, and
text) to reduce the number of parameters to calculate and ensure CPE models the leaky interfacial capacitance associated with
correct convergence of the limiting values for the impedance the electron-transfer reaction at the pores. Because this part of
components. (1 − θ) is the coverage of the electrode by the amine the equivalent circuit makes a significant contribution only at
SAMs and was calculated from eq 1 (see below). frequencies higher than 500−700 Hz, the diffusional
component will be small, and therefore, the simple parallel
the rate constants obtained from Figure 4b36 are 9.2 and 14.5 × R 3 −CPE 1 model was adopted. Any additional circuit
103 Ω cm2 for the C12N and the C16N SAMs, respectively. R4 component combination such as a Warburg component with
has the same physical meaning as RCT. The value of this a resistance in parallel to model deviation from linearity45,46
quantity for the C16N SAM calculated from the impedance data could not fit the data satisfactorily. This is not surprising
(R4, Table 1) is close to the voltammetric value, but that for because a simple pore model cannot accurately describe a
C12N is 30% higher. disordered region in the SAM.
The order of magnitude of the film capacitance obtained is In a separate experiment, the impedance in the absence of
correct. For a parallel-plate condenser, the capacitance per unit the amine SAMs was measured at the same potential, and a
−2
area is C = εε0/d, where ε is the film dielectric permittivity, ε0 is CT = 13 Ω cm
charge-transfer resistance of Rbare was found. The
the permittivity of free space, and d is the length of the coverage by amines (1 − θ, where θ is the surface coverage by
molecule. From the molecular dimensions calculated above, the holes) was then calculated from
dielectric permittivities were 2.85 and 2.82 for the C12N and ⎛ R bare ⎞
C16N films, respectively. These values are higher than those 1 − θ = 1 − ⎜⎜ CT ⎟⎟
obtained from the refractive index of a hydrocarbon chain, ⎝ R3 ⎠ (1)
2.02−2.04.39 The presence of the amino group leads, however,
to a higher static relative dielectric permittivity for the amines, on the assumption that the rate constant at the bottom of the
for instance, to a value of 3.1 for dodecylamine.40 Thus, C1 pores has the same value as that for the free surface because this
estimated from the impedance results is consistent with the impedance component refers only to the surface exposed areas
available dielectric data for these amines. The values of the (see Table 1).
capacitances are also in agreement with those reported for Both the cyclic voltammetry and ac impedance measure-
alkanethiol SAMs and show comparable dependencies with the ments demonstrate that the electrochemical properties of an
chain lengths of these systems.41 electrode with an amine SAM can be described as a well-
The second semicircle present at higher frequencies relates to packed, highly dense monolayer SAM containing a group of
the R2−R3−CPE1 combination. This is due to electron transfer ultramicroelectrodes formed by pinholes. This characteristic
at the defects present in the SAM, which can be modeled as behavior arises from mass transfer through pinholes and
microelectrodes. Their impedance response has been the imperfections present in the SAM. As a consequence, fast
subject of much debate and, in particular, the influence of electron transfer takes place only in the small exposed areas of
interacting diffusional fields that distort the hemispherical mass the gold electrode that are not blocked by the SAM47−49 with
transfer geometry, leading, in some cases, to linear diffusion at simultaneous electron transfer taking place across the attached
very low frequencies. Microarray electrodes have been used as a amine layer. Electron transfer in the defect regions occurs, but
model of pinholes in alkanethiol SAMs, and the theory for at a range of distances, and therefore, the SAM coverage values
these systems was developed by Finklea et al.42 and others.43,44 calculated from eq 1 (see Table 1) represent an approximation
Because of its simplicity, the model by Finklea et al. has been of an average property. Despite these uncertainties, the
used extensively to analyze the impedance response of defects coverages estimated are well above 99%, thus confirming the
in SAMs.45 The problem of extracting physically meaningful high packing density that the XP results indicate.
information by spectral deconvolution of equivalent circuit
components that make a small contribution to the interfacial
response was highlighted by Finklea et al.42 This is particularly
■ CONCLUSIONS
C12 and C16 amine-terminated linear alkanes self-assemble into
difficult for the present results, because the information on the packed monolayers with the amine functional group interacting
microarray domains has to be extracted from data in the low- directly with the Au surface tilted at ∼30° with respect to the
frequency region (say, at less than 20 Hz), as is clear from surface normal. Furthermore, formation of these monolayers
several examples in the literature45 and for which the C1−R4 results in Au work function decreases of 1.2 and 1.3 eV,
contribution dominates. respectively, comparable to the changes observed for
Another complication of complex systems such as the alkanethiols of similar length. This observation indicates that
present one is that the different contributions of the equivalent both thiol- and amine-based SAMs result in similar dipole
473 dx.doi.org/10.1021/jp410086b | J. Phys. Chem. C 2014, 118, 468−475
The Journal of Physical Chemistry C Article

layers with positive charges residing at the monolayer/vacuum (9) Selvakannan, P. R.; Mandal, S.; Phadtare, S.; Pasricha, R.; Sastry,
interface. Although molecular resolution was not achieved, M. Capping of Gold Nanoparticles by the Amino Acid Lysine Renders
STM measurements showed that, unlike the formation of Them Water-Dispersible. Langmuir 2003, 19, 3545−3549.
alkanethiol-based SAMs, the formation of amino SAMs does (10) Sastry, M.; Kumar, A.; Mukherjee, P. Phase Transfer of Aqueous
not result in pit development on the Au terraces, suggesting Colloidal Gold Particles into Organic Solutions Containing Fatty
Amine Molecules. Colloids Surf. A 2001, 181, 255−259.
that the binding energy of amine to gold is lower than that of
(11) Sahu, P.; Prasad, B. L. V. Effect of Digestive Ripening Agent on
thiols. Finally, these monolayers block electron transfer Nanoparticle Size in the Digestive Ripening Process. Chem. Phys. Lett.
between redox probes in solution and the Au substrate, and 2012, 525−526, 101−104.
the values of the charge-transfer resistance measured by ac (12) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J.
impedance are in good agreement with the layer thickness Digestive-Ripening Agents for Gold Nanoparticles: Alternatives to
estimations derived from XPS. In addition, charge transfer takes Thiols. Chem. Mater. 2003, 15, 935−942.
place in a very small number of pinholes and defects present in (13) Chen, F.; Li, X.; Hihath, J.; Huang, Z.; Tao, N. Effect of
the monolayer in parallel with electron tunneling through the Anchoring Groups on Single-Molecule Conductance: Comparative
SAMs. These findings provide comprehensive physical insight Study of Thiol-, Amine-, and Carboxylic-Acid-Terminated Molecules.
into the molecular and electronic structures of amine-based J. Am. Chem. Soc. 2006, 128, 15874−15881.
SAMs on Au surfaces, which play an increasingly important role (14) Xu, C.; Sun, L.; Kepley, L. J.; Crooks, R. M.; Ricco, A. J.
in the synthesis of Au NPs and molecular electronics. Molecular Interactions between Organized, Surface-Confined Mono-

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layers and Vapor-Phase Probe Molecules. 6. In-Situ FTIR External
Reflectance Spectroscopy of Monolayer Adsorption and Reaction
ASSOCIATED CONTENT Chemistry. Anal. Chem. 1993, 65, 2102−2107.
* Supporting Information
S (15) Dilshad, N.; Ansari, M. S.; Beamson, G.; Schifrin, D. J. Amines
Addition tables as noted in text. This material is available free of as Dual Function Ligands in the Two-Phase Synthesis of Stable
charge via the Internet at http://pubs.acs.org. AuxCu(1−x) Binary Nanoalloys. J. Mater. Chem. 2012, 22, 10514−

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10524.
AUTHOR INFORMATION (16) Ritchie, J. E.; Wells, C. A.; Zhou, J.-P.; Zhao, J.; McDevitt, J. T.;
Ankrum, C. R.; Jean, L.; Kanis, D. R. Infrared and Computational
Corresponding Author Studies of Spontaneously Adsorbed Amine Reagents on YBa2Cu3O7:
*E-mail: fwilliams@qi.fcen.uba.ar. Phone: +54 11 45763380. Structural Characterization of Monolayers atop Anisotropic Super-
Notes conductor Surfaces. J. Am. Chem. Soc. 1998, 120, 2733−2745.
The authors declare no competing financial interest. (17) Ruan, C.-M.; Bayer, T.; Meth, S.; Sukenik, C. N. Creation and

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Characterization of n-Alkylthiol and n-Alkylamine Self-Assembled
Monolayers on 316L Stainless Steel. Thin Solid Films 2002, 419, 95−
ACKNOWLEDGMENTS 104.
Funding from CONICET, University of Buenos Aires, and (18) Méndez De Leo, L. P.; de la Llave, E.; Scherlis, D.; Williams, F.
́
Agencia Nacional de Promoción Cientifica y Tecnológica is J. Molecular and Electronic Structure of Electroactive Self-Assembled
gratefully acknowledged. Monolayers. J. Chem. Phys. 2013, 138, 114707.

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(19) Calvo, A.; Joselevich, M.; Soler-Illia, G. J. A. A.; Williams, F. J.
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