Molecular Origin of the Odd-Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

Article abstract of DOI:10.1021/jacs.0c04288

Elucidating the influence of the monolayer interface versus bulk on the macroscopic properties (e.g., surface hydrophobicity, charge transport, and electron transfer) of organic self-assembled monolayers (SAMs) chemically anchored to metal surfaces is a challenge. This article reports the characterization of prototypical SAMs of n-alkanethiolates on gold (CH3(CH2)nSAu, n = 6-19) at the macroscopic scale by electrochemical impedance spectroscopy and contact angle goniometry, and at the molecular level, by infrared reflection absorption spectroscopy. The SAM capacitance, dielectric constant, and surface hydrophobicity exhibit dependencies on both the length (n) and parity (nodd or neven) of the polymethylene chain. The peak positions of the CH2 stretching modes indicate a progressive increase in the chain conformational order with increasing n between n = 6 and 16. SAMs of nodd have a greater degree of structural gauche defects than SAMs of neven. The peak intensities and positions of the CH3 stretching modes are chain length independent but show an odd-even alternation of the spatial orientation of the terminal CH3. The correlations between the different data trends establish that the chain length dependencies of the dielectric constant and surface hydrophobicity originate from changes in the polymethylene chain conformation (bulk), while the odd-even variation arises primarily from a difference in the chemical composition of the interface related to the terminal group orientation. These findings provide new physical insights into the structure-property relation of SAMs for the design of ultrathin film dielectrics as well as the understanding of stereostructural effects on the electrical characteristics of tunnel junctions.

Full text of DOI:10.1021/jacs.0c04288

Subscriber access provided by Uppsala universitetsbibliotek
Article
Molecular Origin of the Odd-Even Effect of Macroscopic Properties
of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?
Fadwa Ben Amara, Eric R. Dionne, Sahar Kassir, Christian Pellerin, and Antonella Badia
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04288 • Publication Date (Web): 29 Jun 2020
Downloaded from pubs.acs.org on June 29, 2020
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 12
Journal of the American Chemical Society
1
2
3
4
5
6
Molecular Origin of the Odd-Even Effect of Macroscopic Properties
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
‡
‡
Fadwa Ben Amara , Eric R. Dionne , Sahar Kassir, Christian Pellerin*, and Antonella Badia*
Département de chimie, FRQNT Quebec Centre for Advanced Materials, Université de Montréal, C.P. 6128, succursale
Centre-ville, Montréal, QC H3C 3J7 CANADA
ABSTRACT: Elucidating the influence of the monolayer interface versus bulk on the macroscopic properties (e.g., surface hydro-
phobicity, charge transport, and electron transfer) of organic self-assembled monolayers (SAMs) chemically anchored to metal sur-
faces is a challenge. This article reports the characterization of prototypical SAMs of n-alkanethiolates on gold (CH (CH ) SAu, n =
3 2 n
6–19) at the macroscopic scale by electrochemical impedance spectroscopy and contact angle goniometry, and at the molecular level,
by infrared reflection absorption spectroscopy. The SAM capacitance, dielectric constant, and surface hydrophobicity exhibit de-
pendencies on both the length (n) and parity (nodd or neven) of the polymethylene chain. The peak positions of the CH
indicate a progressive increase in the chain conformational order with increasing n between n = 6 and 16. SAMs of nodd have a greater
degree of structural gauche defects than SAMs of neven. The peak intensities and positions of the CH stretching modes are chain
length independent but show an odd-even alternation of the spatial orientation of the terminal CH . The correlations between the
2
stretching modes
3
3
different data trends establish that the chain length dependencies of the dielectric constant and surface hydrophobicity originate from
changes in the polymethylene chain conformation (bulk), while the odd-even variation arises primarily from a difference in the
chemical composition of the interface related to the terminal group orientation. These findings provide new physical insights into the
structure-property relation of SAMs for the design of ultrathin film dielectrics as well as the understanding of stereostructural effects
on the electrical characteristics of tunnel junctions.
2 n
Fc(CH )
S-functionalized metal surface.5, 23 These differences
in the electronic properties of the SAM alter the shape of the
tunneling barrier and lead to an odd-even modulation of the
Introduction
Organic self-assembled monolayers (SAMs) formed on semi-
conductor, metal or metal oxide surfaces are an integral compo-
nent of molecular electronic devices, for electrical insulation,
charge storage, and charge transport. A recent series of inves-
tigations of the charge transport properties of large-area, solid-
state tunnel junctions comprised of SAMs of the redox-active
ferrocenylalkanethiolates (Fc(CH
kanethiolates (CH
the odd-even (or parity) effect,
tion of a material’s structure and/or property due to an odd or
even number of repeat units in the molecule. Parity effects in
solid condensed phases often arise from differences in intermo-
lecular interactions and molecular packing. A classic example
is the odd–even variation of the melting points of solids of odd‐
and even‐numbered n‐alkanes and n-alkyl carboxylic acids.
SAM properties exhibiting parity effects include surface wetta-
bility, friction, and electron transfer rate. Charge tunneling
across metal/SAM/metal junctions is particularly sensitive to
the SAM structure. The presence of an odd versus even number
4
-
electron tunneling rate across the junction diode in the on state.
5
A smaller ferrocene tilt angle also reduces the steric hindrance
and maximizes the chain-chain van der Waals interactions. The
1
-3
−1
computed molecule packing energy is 1.7 ± 2.9 kJ mol lower
for SAM (smaller tilt angle) than SAM (larger tilt angle)
on gold.
seemingly impacts the performance of devices of which the
SAM is the active component. Specifically, Fc(CH SAu
SAMs of neven exhibit higher redox potentials and yield a 25%
even
odd
4
-5
2
)
n
S)
or insulating n-al-
4, 13
Such a small difference in the molecular packing
6
-9
3 2 n
(CH ) S) have sparked renewed interest in
1
0-23
which refers to the alterna-
2 n
)
1
3, 18,
larger redox-induced micromechanical actuation than nodd
.
2
1
2 n
Fc(CH ) SAu SAM-based junctions with neven present lower
leakage currents and rectify current, while those with nodd do
4
-5
not.
In the case of junctions formed by SAMs of CH
gold, whose idealized all-trans extended structure is shown in
2
4-25
3 2 n
(CH ) S on
2
6
even
Figure 1a, SAMs
yield higher tunneling current densities
odd
6
than their SAM homologues. The odd-even effect in the tun-
neling rate is attributed to a difference in the van der Waals in-
teractions at the interface between the SAM and contacting
electrode due to the different orientations of the terminal groups
odd
even
of methylenes (nodd/SAM or neven/SAM ) in the polymeth-
ylene chain determines how the organothiolate molecules or-
ganize at the supramolecular level and how they couple to the
electrodes, which in turn influence the electronic structure and
odd
even 6, 8
in SAM and SAM
.
5
, 15, 23
tunnel current density of the junction.
Clearly, it is essential to optimize the supramolecular organ-
ization of the SAM via the parity of the repeat units in its back-
bone to harness its full functionality. The perception that due to
the large body of published work everything is known about the
In Fc(CH S SAMs chemisorbed to gold or silver, the parity
2 n
)
of the polymethylene chain influences the tilt angle of the ter-
minal ferrocene unit with respect to the surface normal; there is
a difference of 5–6 between nodd and neven for n = 6–15. This
odd-even effect in the ferrocene tilt angle leads to a small parity
effect in the surface dipole, which in turn causes odd-even ef-
fects in the work function and HOMO onset energy of the
2
7
o
5
structural characteristics of SAMs, including the odd-even ef-
2
6
12, 14
fect, is false.
Early structural characterizations of SAMs
did not consider the role played by the roughness of the metal
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 12
surface on the SAM order, thereby leading to contradictory re-
In this regard, we have investigated the chain length depend-
encies of the dielectric behavior, surface wettability, and supra-
molecular order of gold-supported SAMs of CH (CH S over
1
2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
sults on the existence of a parity effect, examined a limited
2
8-30
number of odd-even homologues,
n
or focused on chains of
3
2 n
)
3
1
odd . Since the presence of multiple electrodes and interfaces
a large span of chain lengths (n = 6–19). Because parity effects
in the electrical properties of SAMs can have significant tech-
nological implications, the focus is on charge storage in the in-
terfacial double layer. We sought to identify the respective con-
tributions of the monolayer bulk (polymethylene chain order)
and exposed interface (orientational order of the chain termini)
to the relative permittivity or dielectric constant (), a key prop-
erty governing phenomena in electronics, optics, and solid-state
physics. SAM characteristics such as the surface coverage den-
sity, molecular orientation, polarizability of the terminal group,
makes it challenging to elucidate the contribution of the inher-
ent supramolecular structure of the organic SAM to the junction
properties, recent investigations of the odd-even effect have fo-
cused on SAMs chemisorbed to a metal electrode or substrate,
probing either the structure of the SAM/ambient interface (near
5
edge X-ray fine structure spectroscopy, photoemission spec-
23
20
troscopy, static contact angle, sum frequency generation
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
19
22
spectroscopy, quartz crystal microbalance ) or the electronic
structure of the SAM/metal interface (ultraviolet photoelectron
spectroscopy) . None of these studies however probed directly
5
35-36
and bond conjugation have a large effect on the value of .
the conformational order of the polymethylene chains of the
SAM; inferences are made in some cases from the odd-even ef-
fect observed in the interfacial SAM property (e.g., wettabil-
We employed nonfaradaic electrochemical impedance spec-
troscopy (EIS) to determine the capacitance (C) and  (Figure
1b). Water contact angles characterized the surface hydropho-
bicity of the SAM (Figure 1c). Both dielectric properties as well
as the surface hydrophobicity exhibit marked chain length de-
pendent parity effects. Infrared reflection absorption spectros-
copy (IRRAS) tracked the evolution of the chain gauche con-
2
0
ity) or the spectral signal(s) of the terminal functional
1
9, 32
groups
. The structural characterization of both the organic
interface and bulk of the SAM is essential for understanding
how the former relates to the latter and how each influence the
SAM properties.
formational defects and the orientation of the terminal CH
Figure 1d) as a function of the molecular length and parity. The
3
(
IRRAS and contact angle data reveal the structural origins of
the odd-even and chain length trends of dielectric properties of
the SAM.
Experimental Section
Details of the chemical reagents used, synthesis and purifica-
tion of n-alkanethiols, preparation of the gold substrates and
SAMs, EIS measurements, calculation of the complex capaci-
tance quantity, contact angle goniometry, and IRRAS analyses
are given in the Supporting Information.
Results and Discussion
Interfacial Capacitance of SAMs of CH
3
(CH ) S on Gold.
2
n
The electrochemical impedance of polycrystalline gold bead
electrodes (Figure S1a of the Supporting Information) modified
with SAMs of CH
3 2 n
(CH ) S of n = 6–19 was measured in 1.0 M
NaClO4(aq) over a frequency span of 1 MHz to 1 Hz at a struc-
3
7
turally non-perturbing potential. The SAMs are highly imper-
meable to ions and the SAM-modified gold interfaces exhibit
near ideal Helmholtz capacitor behavior (Figure S2 and Table
S1). Specifically, the complex impedance plane plots comprise
nearly vertical lines and the CH
phase angle of > 88 at low frequency (see Supporting Infor-
mation for a more detailed discussion).
3
(CH
2 n
) SAu SAMs present a
Figure 1. (a) Schematic views of all-trans extended conformers of
CH (CH ) S with an even vs odd number of CH repeat units (neven
3 2 n 2
o
3
8-39
The reactance or op-
vs nodd) chemisorbed on a gold surface. The Au-S-C bond is fixed
o
position of the SAM-modified gold interface to changes in the
current flow induced by the ac perturbation of the applied po-
tential, and characterized by the imaginary part ZIm of the com-
plex impedance Ẑ, shows dependencies on the length and parity
of the chain (Figure S3). These chain length and odd-even ef-
fects in ZIm are in turn reflected in the mathematically-related
CRe (discussed below). No odd-even effect is however observed
in the real part ZRe of the impedance, which is a measure of the
resistance of the SAM interface to the dc current flow.
at 110 and the linear backbone of the polymethylene chains is
o
tilted ~20–35 away from the surface normal (to maximize the
chain van der Waals interactions), based on current structural mod-
2
6-27, 33-34
els.
The shaded gray and red slabs denote the bulk and or-
ganic interfacial regions of the SAM. The calculated thicknesses of
the ethyl-like (neven) and methyl-like (nodd) interfacial layers are
6
0
.185 nm and 0.055 nm, respectively. The SAMs were character-
ized using (b) nonfaradaic electrochemical impedance spectros-
copy (capacitance and ), (c) contact angle goniometry (surface hy-
drophobicity), and (d) infrared reflection absorption spectroscopy
(intramolecular organization).
ACS Paragon Plus Environment

Page 3 of 12
Journal of the American Chemical Society
(a)
n = 6
n = 7
n = 8
n = 9
(b) 1.2
1
2
3
4
5
6
7
8
9
-
1.2
1.0
1
.1
.0
-
n = 10
n = 11
n = 12
n = 13
n = 14
n = 15
n = 16
n = 17
n = 18
n = 19
1
-0.8
0.9
.8
0.7
.6
-0.6
-0.4
-0.2
0
0
0.0
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1 MHz
1 Hz
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
6
7
8
9
10 11 12 13 14 15 16 17 18 19
-
2
number of methylenes, n
CRe / F cm
Figure 2. Nonfaradaic EIS of gold electrodes modified with CH
a) Complex plane plot of the imaginary CIm vs real CRe part of the complex capacitance Ĉ. Symbols are the experimental data and the solid
curves represent the fits of the data to the equivalent circuit (shown in inset), consisting of the solution resistance R in series with the film
3 2 n
(CH ) S (n = 6–19) in 1.0 M NaClO4(aq) (details in Supporting Information).
(
s
-1
capacitance. (b) Plot of the reciprocal of the capacitance C as a function of the number of methylene units n. Each data point is the mean
2
values of at least 10 electrodes. Error bars represent the 95% confidence intervals. Dashed blue lines are linear regressions: r = 0.9993 for
2
n
even and r = 0.9986 for nodd. Full black lines are guides to the eye.
38
Shown in Figure 2a are plots of the imaginary part CIm versus
and in other instances, this can be ascribed to the short-chain
49-50
real part CRe of the impedance-derived complex capacitance Ĉ
(eq S2–S5). CRe corresponds to the capacitance and CIm to losses
odd-even homologues investigated (n  9)
.
More recently, Jiang and coworkers observed an odd-even
chain length dependency in the capacitance of CH (CH
SAMs on silver in large-area junctions with Ga /EGaIn. The
results presented in Figure 2b indicate that the parity effect in
the film capacitance is not limited to (i) solid-state electrical
measurements on SAMs sandwiched between metal contacts, it
also manifests itself in wet electrochemical conditions and (ii)
SAMs formed on silver, which present significant structural dif-
4
0
in the form of energy dissipation. The complex capacitance is
better suited to the analysis of insulating dielectric layers than
the complex impedance as it highlights the electric charge stor-
age characteristics of the interface rather than its resistive con-
3
2 n
) S
9
2
O
3
4
1-42
tributions.
The complex capacitance plane plots immedi-
ately show that (i) the static capacitance (i.e., low-frequency in-
4
2
tercept of the semi-circle on the real axis) decreases with in-
creasing number of methylene units n and (ii) the magnitude of
the static capacitance is the same for a SAM of nodd and its
longer chain even neighbor (neven = nodd+1). Bode plots of CRe
26-28
ferences to SAMs on gold . Both systems present a parity
effect in their interfacial charge storage capacity, but a compar-
ison of the measured impedance reveals significant differences
in the electrical transport characteristics of the metal-SAM-
metal junction versus the ionic insulator properties of the SAM
(
Figure S4) display the same chain length and odd-even depend-
encies at frequencies ≲ 1 kHz, where CRe is independent of fre-
quency.
38
investigated here. We and others have shown that densely-
The impedance data was fit to the series RC-type electrical
circuit used to describe the impedance response of ion-blocking
3 2 n
packed CH (CH ) SAu SAMs effectively behave as blocking
layers to ion permeation in electrolyte solution under non-fara-
1
8, 38, 43-44
SAMs (Figure 2a, inset),
to extract the exact static ca-
daic electrochemical conditions. By contrast, SAM-based junc-
9
pacitance value, referred to as C (Figure S4 and Table S2). The
addition of an extra monolayer film resistance (RSAM) in parallel
to the capacitance does not improve the fits of the data and gives
tions (e.g.,
Ag-SAM-Ga
2
O
3
/EGaIn
and
Au-SAM-
5
1
PEDOT:PSS-Au ) present complex impedance responses as-
sociated with electronic conduction and that cannot be de-
8
2
42, 52
very large values of ≳ 10  cm , with fitted errors ≳ RSAM, for
the resistance to ion transport in the SAM across the chain
lengths n = 6-19 investigated (Figure S5), attesting to the highly
scribed by a series RC circuit.
The resistance of the
CH (CH ) SAg SAM to electron transport across the tunnel
3
2 n
3
8
junction increases with n and is 10 (n=16) to 10 (n=6) lower
39
ionic insulating character of the SAMs. In other words, RSAM
in magnitude (Supporting Information of ref 9) than the re-
44-45
is physically meaningless for the SAMs investigated here,
sistance to ion transport measured herein for CH
SAMs. The charge transport resistance of the CH
3
(CH
(CH
2
)
)
n
SAu
SAg
and these can be considered to be effectively free of defects,
3
2
n
meaning that current leakage at monolayer defect sites is negli-
gible.
SAM also exhibits an odd-even effect that follows the electron
tunneling current density – nodd of lower charge transport re-
sistance yields a larger tunneling current density than nodd-1 of
-1
C is plotted as a function of n in Figure 2b. CH
3
(CH
2
-1
)
n
SAu
9
even
odd
higher charge transfer resistance. The ion transport resistance
SAMs and SAMs exhibit the linear increases of C with n
of the insulating CH
parent dependence on n (Figure S5). While defect-free
CH (CH S SAMs block ion transport in the potential stability
3 2 n
(CH ) SAu SAMs does not exhibit an ap-
expected from a linear increase of the film thickness with chain
4
6-48
-1
length (Figure S6).
Superimposed on the increase in C is
3
2 n
)
the parity effect evident in the complex capacitance plane plots.
The capacitances of nodd / (nodd+1) neighbors are statistically in-
distinguishable, while there is a statistically significant increase
3
7
region, they allow electron transport because the charge carri-
6
ers travel through the alkyl chain backbone of the SAM . The
-
1
different mechanisms of electron and ion transport within the
SAM explain its distinct dielectric response in electrical versus
electrochemical environments.
in C between neven and (neven+1) (99% confidence level, Table
-
1
S2). A (quasi-)linear increase in C with chain length has been
31, 38, 49-50
observed in previous electrochemical investigations.
However, none of these studies report an odd-even effect. In
SAM Dielectric Constant and Odd-Even Effect in the
SAM Capacitance. There is no evidence that SAMs of neven are
31,
some cases, this is simply due to the use of chains of only nodd
,
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 12
more leaky or permeable to ions, and therefore exhibit a higher
capacitance with respect to their chain length, than SAMs of
odd. So what is at the origin of the observed parity effect? To
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
.4 (a)
n
-
1
answer this question, we look to the linear dependence of C on
2.3
3
1, 49-50
n.
It is consistent with the behavior of an ideal spherical
capacitor (i.e., the electrode is a spherical bead), with the gold
surface acting as one of the concentric capacitor shells and the
physisorbed ions at the SAM/electrolyte interface as the other
shell:
2.2
2.1

2.0
d
1
1
C


r(r  d) 4πε ε
o
SAM
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(1)
1.9
110
108
106
104
where r is the radius of the gold bead (r = 1.4 mm), d is the
thickness of the SAM dielectric medium separating the two
shells (1.0 ≲ d ≲ 2.5 nm), SAM is the relative permittivity or
(b)
o
dielectric constant of the SAM, and  is the permittivity of free
-
12
-1 53
5
6
space (8.85419 × 10 F m ). Since r ≅ (10 –10 )d, r + d ≅
r, and eq 1 simplifies to eq 2 after rearrangement to give the
capacitance expressed per unit area:

d
C
1

nodd
neven
ε ε
o
SAM
(
2).
1
02
00
-
1
odd
The different linear dependencies in C –n between SAM
even
and SAM
suggest a difference in their respective dielectric
1
constants. Instead of determining an average value of  for
6
7
8
9
10 11 12 13 14 15 16 17 18 19
number of methylenes, n
even
odd
−1
−1
SAM
data,
and SAM from the slopes of the C –n (or C –d)
9, 31, 38, 54-55
a value was obtained for each chain length to
reveal the structure-related variation of SAM (Figure 3a and Ta-
ble S3). Our calculations of SAM include the weak odd-even
fluctuation of 0.2 Å in the SAM thickness (Figure S7) from
the different orientations of the chain termini (Figure 1a). The
impact of the parity effect in d on SAM is relatively small. The
mean values of SAM span 1.92 to 2.38. Published values of 
determined by electrochemical or optical methods for
Figure 3. Comparison of the chain length dependencies of (a) SAM
dielectric constant SAM determined from the capacitance using eq
and film thickness d calculated assuming a 30 tilt from the sur-
o
2
face normal of all-trans extended alkyl chains and Au-S layer thick-
6
, 48
ness of 0.19 nm
and (b) static contact angle of water 
w
. Error
bars represent the 95% confidence intervals. Solid black lines and
dashed blue curves are guides to the eye. Each data point for  is
w
the mean value of at least 4 different SAMs.
3
1, 38, 48, 50, 56
CH
3 2 n
(CH ) SAu SAMs range from 2.0 to 3.0.
We therefore conclude that the observed odd-even effect in
the capacitance originates from an odd-even difference in the
SAM dielectric constant. On moving up in n from neven to nodd
an increasing film thickness d and a lower SAM move C (C ) in
the same direction, i.e., to a lower (higher) value. Moving on
from nodd to neven, a higher SAM compensates the decreasing ef-
fect of an increasing d, resulting in little or no change in C.
Thus, due to the opposing effects of d and  on C (eq 2), the
capacitance of the longer-chain SAM
nearly) the same as that of its shorter-chain SAM neighbor
of lower .
First and foremost, the plot of SAM versus n indicates that 
even
odd
of SAM is systematically higher than  of SAM over the
entire range of chain lengths investigated (n = 6 –19). A least-
squares polynomial fit of the SAM data to separate the contribu-
tion of the chain parity from that of the chain length yields an
odd-even variation of 0.16 (Figure S8). An odd-even variation
,
-
1
of 0.4 was measured in  for analogous SAMs of CH
3
(CH
/EGaIn. We also conducted
EIS measurements on selected chain lengths (n = 9–12) using a
different electrolyte, i.e., 1.0 M NaCH SO4(aq), to verify that the
2 n
) S
9
2 3
on silver in junctions with Ga O
even
of higher  remains
odd
(
3
dielectric behavior observed in NaClO4(aq) is not electrolyte-
specific. Indeed, the odd-even difference of SAM of 0.17 deter-
In addition to a parity effect, the plot of SAM versus n shows
a linear increase between n = 6 and 15,  = 0.033 ± 0.002 per
for neven and  = 0.032 ± 0.002 per CH for nodd, followed
2
by a tendency towards limiting values for n > 15. The dielectric
mined in NaCH
3
SO4(aq) (Figure S9) is similar to that obtained in
-
NaClO4(aq). The symmetric ClO
4
anion has a zero-dipole mo-
possesses a calculated permanent dipole
moment of ~6 D (see Supporting Information). One might ex-
CH
2
-
3 4
ment, while CH SO
response of saturated hydrocarbons is the same at low and high
-
pect a stronger interaction of the CH
groups (surface dipoles) of the SAMs compared to the ClO
3
SO
4
with the terminal
35
frequency. The optical (high-frequency) dielectric constant of
-
4
,
CH
(
3
(CH
2
)
n
SH liquids increases near-linearly with chain length
( =
The variation of the optical
SH with chain length,  ≅
, is an order of magnitude smaller than that ob-
and hence, a difference in the odd-even capacitances deter-
mined in the presence of these two electrolyte anions. The ab-
sence of such a difference suggests that the observed parity ef-
fect in the capacitance does not originate from differences in the
dipole interactions between the electrolyte anions and the ter-
Figure S10) due to the higher dielectric constant of CH
2.164) versus CH
dielectric constant of CH
0.0035 per CH
2
58-59
3
( = 1.399).
2 n
(CH )
3
2
served here in the static (low frequency) dielectric constant of
the SAM, suggesting that the chain length dependence of SAM
2
does not stem from the number of CH ’s in the molecule. More-
odd
minal groups, whose different orientations in SAMs and
even
SAMs
ments.
(Figure 1a) yield different interfacial dipole mo-
1
1, 20, 57
ACS Paragon Plus Environment

Page 5 of 12
Journal of the American Chemical Society
over, the observed increase in SAM with n is not due to an in-
crease in the surface coverage density, as predicted by theoreti-
an evolution of the molecular organization and conformational
order within the SAM with chain length. The next section ad-
dresses these structural characteristics.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
5, 60-61
cal calculations,
since the chemisorbed thiolate coverage
does not change with chain length or parity over n = 6–19 (re-
Odd-Even Effects in the SAM Molecular Organization.
We employed IRRAS to relate the odd-even chain length de-
pendence of the SAM dielectric constant to changes in the
polymethylene chain conformation and orientation in the bulk
and at the interface of the SAM. IRRAS is performed at a graz-
ing angle of incidence with p-polarized light to selectively en-
hance the electric field normal to the metal surface. Hence, only
the vibrational modes that have a component of their dipole
change perpendicular to the surface, and interact strongly with
6
2-64
ductive desorption data
and high resolution AFM imag-
6
5
ing ).
Odd-Even Effect in the SAM Hydrophobicity. The odd-
even chain length dependence of the water contact angle (
on the CH (CH SAu SAMs formed on thermally-evaporated
w
)
3
2 n
)
gold thin films (Figure S1b) parallels that of SAM (Figure 3b),
odd
indicating that the same molecular effects are at play. SAMs
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
presenting a "methyl-like" surface exhibit higher contact angles
69
greater surface hydrophobicity) than SAMseven of comparable
the electric field, are detected. This metal surface selection
(
rule yields unique information about the spatial geometry of the
adsorbate.
chain length exposing an "ethyl-like" surface (Figure 1a), in
16, 66
agreement with previous reports.
The odd-even variation of
o

w
of 1.6 (Figure S11) is attributable to a difference in the sur-
face free energies of SAMs (20 mJ m- ) and SAMs (21
mJ m- ) related to the terminal group orientations. On the
other hand, the increase of  from n = 6 to 16 reflects a change
in the bulk phase state of the SAM. Our IRRAS data, pre-
sented in the next section, and that of Porter and coworkers,
indicate an evolution of the CH (CH SAu SAMs from a con-
Spectra for selected chain lengths of SAMs on gold thin films
odd
2
even
are presented in Figure S12. These spectra resemble those al-
2
30
28-29, 31, 70
ready published.
The region of interest is that of the CH
-
1
stretches between 2800 and 3000 cm .
w
1
6
SAM Interface. We begin by assessing the orientation of the
3
1
terminal CH
3
groups (Figure 4a). The intensity of the symmetric
)
n
-1
3
2
CH stretching mode, 
3
s
(CH
3
), at 2878 cm , whose transition
formationally-disordered liquid-like phase at short chain
lengths to a crystalline-like environment at n ≥ 16 due to an in-
crease in the cohesive, interchain van der Waals interactions.
Additionally, there is a variation in the chain tilt angle (from
28
dipole moment is colinear with the C-CH
3
axis, is systemati-
cally larger for nodd than neven. This tendency indicates that the
permanent dipole of the CH group is oriented more parallel to
the surface normal for nodd than neven (Figure 1a). The antisym-
3
2
2–30° for n = 6 to 30–35° for n ≥ 12) and tilt direction with
-1
metric stretching mode, 
a 3
(CH ), at 2965 cm , shows the op-
chain length (grazing incidence X-ray diffraction and molecular
posite trend because its transition dipole is perpendicular to the
10, 67-68
dynamics simulations).
w
Analogous to  , the observed in-
28
3
C-CH bond and parallel to the C-C-C plane, thus corroborat-
crease in SAM as a function of n for 6 ≤ n ≤ 15 likely reflects
3
ing the change in the orientation of the terminal CH between
2
1
1
.0
.5
.0
2966
(b)
(
a)
s
a
a
s
2965
a
nodd
neven
~
2964
~
a
s
2879
2878
s
0.5
1.5
0.8
(c)
(d)
0.6
0.4
0.2
0.0
2
922
1.0
0.5
Gauche
2921
~
6
8
10 12 14 16 18
n
~
2920
~
~
2919 Trans
6
0.0
7
8
9 10 11 12 13 14 15 16 17 18 19
6
7
8
9 10 11 12 13 14 15 16 17 18 19
number of methylenes, n
number of methylenes, n
Figure 4. IRRAS of CH
3
(CH
2
)
n
SAu SAMs. Chain length dependence of (a) 
a
(CH
3
) and 
s
(CH
3
) peak intensities, (b) 
a
(CH
3
) and 
s 3
(CH )
peak positions, (c)  (CH
a
2
) peak position, and (d) 
s
(CH ) peak intensity. Inset in (c) is a plot of the difference between the experimental
2
positions and those determined from the polynomial fit of the neven data points. Each data point is the mean value of 5–12 independently
prepared SAMs. Error bars represent the 95% confidence intervals. Dashed blue curves in (c) are second order polynomial fits of the data.
Full and dotted lines are guides to the eye. Gray zone in (d) indicates region of odd-even fluctuation.
ACS Paragon Plus Environment

Journal of the American Chemical Society Page 6 of 12
3 2 n
odd and neven. The magnitude of this odd-even variation is con- whole series of CH (CH ) S. It is important to note that the re-
n
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
stant with chain length, analogous to the constant odd-even al-
ported peak positions are the average of multiple spectra ac-
quired on independently prepared SAMs and that the wave-
number shifts, while small, are telling. The experimentally de-
ternations of SAM and 
(CH ) and  (CH ) peak intensities, also observed in previous
is significant because it signals the presence of
w
(Figure 3). The parity effect in the

s
3
a
3
2
8, 30, 71-72
odd
-1
studies,
termined peak positions of SAMs are 0.7 cm higher for n
-
1
orientational order at the interfaces of the SAMs prepared
herein. Additionally, both the symmetric and antisymmetric
= 7 and 9 and 0.4 cm higher for n = 11–19 than the “hypo-
thetical” positions (i.e., positions in the absence of an odd-even
odd
even
CH
3
stretching modes exhibit an odd-even oscillation of their
effect) of the SAMs on the fitted SAMs curve (Figure 4c,
inset), pointing to a greater number of gauche bond conformers
peak positions (Figures 4b and S12d), a characteristic hereto re-
7
3
odd
even
ported only for CH
3
O(CH
cm difference in the  (CH
and a 1.1 ± 0.2 cm difference for 
could originate from: (i) the different interactions/chemical en-
2
)
n
SAu SAMs. There is a 0.7 ± 0.2
) peak positions of neven and nodd
(CH ). This odd-even effect
in SAMs
(CH ) peak position validates the general conclusions of pre-
vious theoretical calculations: SAMs have a lower percent-
versus SAMs . This odd-even effect in the
-
1
s
3

a
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
-1
even
a
3
odd
age of gauche defects than SAMs (i.e., 1.5% lower for n =
odd
even
vironments of the CH
3
groups in SAM and SAM and as-
11–14) and are consequently structurally and thermally more
…
odd 10
sociated intermolecular inductive effect (CH
3
CH
3
in nodd and
stable than SAMs . A greater conformational order should
…
73-74
even
CH
disorder near the chain termini,
tion potentials of molecules (e.g., water) physisorbed to SAMs
3
CH
2
in neven),
(ii) a difference in the conformational
favor closer packing of the polymethylene chains in SAMs
and lead to a higher dielectric constant.
10, 29
or (iii) the different interac-
We also followed the intensities of the CH
as they provide information on the orientation of the polymeth-
ylene chains. The  (CH ) and  (CH ) modes are both perpen-
dicular to the main molecular axis. We focus on the  (CH )
2
stretching modes
7
5
of nodd versus neven . We cannot at present say which of these
three possibilities is the operative mechanism.
s
2
a
2
2
Bulk of the SAM. The peak positions of the CH stretching
s
2
modes are qualitative indicators of the degree of conformational
disorder, in the form of gauche bond defects, within hydrocar-
bon-based molecular assemblies, with increasing wavenumber
peak as it is less affected by interfering absorption from other
vibrational modes than the  (CH ) peak. As expected, the in-
tensity increases with the number of CH units in the SAM
a
2
2
7
6
corresponding to increasing disorder. The symmetric mode,
backbone (Figure 4d). Small local fluctuations of the experi-
mental data are also apparent. Notably, the peak intensity of the
shorter nodd is statistically indistinguishable from that of its
longer neven neighbor for n = 7–16. Both the percentage of
gauche defects and average tilt angle of the polymethylene
-
1


s
(CH
2920 cm both show similar chain length dependencies, but
we focus on  (CH ) (Figure 4c) because it is more sensitive to
gauche bond defects than 
exponential-like decrease in wavenumber with increasing n, as
2
), at 2850 cm and antisymmetric mode, 
a 2
(CH ), at
-
1
a
2
31, 77
s
(CH
2
) (Figure S13).
There is an
2
chains in the SAM influence the CH stretching peak intensity
odd 31
77, 81
odd
observed previously by Porter and coworkers for SAMs
The peak positions of 2922.2 and 2853.2 cm for the shortest
.
for a given n.
respect to its chain length is consistent with the greater orienta-
tional disorder denoted by the higher CH stretching peak posi-
tions (Figures 4c and S13). Gauche conformers partially ran-
domize the orientation of the monolayer CH groups relative to
the surface (interruption of the all-trans conformation), increas-
ing the intensities of the CH
stretching modes.77 The molecular
The greater 
s 2
(CH ) intensity of SAM with
-
1
SAM of n = 6 point to significant conformational disorder,
2
-
1
while the values of 2919.2 and 2850.5 cm for n = 18 are indic-
ative of polymethylene chains in a predominantly trans-ex-
2
3
1, 70, 77-78
tended conformation for the longest SAMs.
This de-
crease of the CH stretching peak positions with increasing n
2
2
reflects an ordering of the polymethylene chains and evolution
of the SAM phase state from a liquid- to solid-like environ-
ment. Limiting values of the peak positions are attained at n ≥
dynamics simulations that predict a greater conformational dis-
order for nodd versus neven calculate smaller average tilt angles
31
o
10
for nodd compared to neven (i.e., ~3–4 smaller for n = 11–14).
1
6 for 
ulations by Ramin and Jabbarzadeh of the chain length depend-
ence of the structural properties of CH (CH S SAMs on
a
(CH
2
) and n ≥ 14 for 
s
(CH
2
). Molecular dynamics sim-
A more vertical orientation of the chains would accommodate
odd
the greater number of gauche bonds found in SAMs by in-
3
2
)
n
0
creasing the chain-to-chain spacing (without the need for a
lower chemisorbed surface coverage). A smaller chain tilt angle
with respect to the surface normal would lower the value of the
1
Au(111) are concordant with the observed trend. Their calcu-
lated percentage of gauche bonds along the molecule decreases
from 12% for n = 6 to 4% for n = 11 and remains constant at
1.5% for n > 15. Densely-packed long chain SAMs contain
gauche bonds due to thermal motions (concentrated near the
peak intensity (HCH planes of the CH
to the surface when the tilt angle is smaller).
2
groups are more parallel
1
0
31, 77
We deduce
that the contribution to the measured peak intensities of an odd-
even difference in the number of gauche defects is greater than
that of an odd-even variation in the orientation of the polymeth-
2
9
chain termini) and conformational defects at SAM crystal
edges from the limited size (5–15 nm) of the CH
3
(CH
2
)
n
SAu
domains formed at room temperature 7, 79-80.
6
s
(CH
2
a 2
) and  (CH )
ylene chains. The relative intensity of 
bands provide additional information on the twist angle of the
polymethylene chains because their transition dipole moment is
Once the degree of intramolecular order stabilizes for chain
lengths of n ≥ 16, the SAM supramolecular structure (including
28
parallel and perpendicular to the C-C-C plane, respectively.
molecular packing defects), and consequently SAM and 
w
,
The intensity ratio does not vary with n nor with the parity of
cease to vary (or vary less) with n.
2
8
the chain (Figure S14), in agreement with previous work, in-
dicating the absence of a chain length dependence of the twist
angle.
a 2
Most importantly, the odd-even effect in the  (CH ) peak
-
1
positions parallels the one seen in the C –n plot (Figure 2b).
Polynomial (empirical) fits of the data show that neven and nodd
exhibit a similar evolution with chain length, but with the peak
Bulk or Interface? We employ the chain length trends in the
molecular conformational order and orientation within the
SAM to discriminate between bulk and interface effects, re-
spectively, to the odd-even effects in the SAM wettability and
odd
positions of SAMs shifted to higher wavenumber across the
ACS Paragon Plus Environment

Page 7 of 12
Journal of the American Chemical Society
dielectric constant. The length of the CH
fects these macroscopic properties irrespective of the chain par-
ity. The SAM surface becomes more hydrophobic ( in-
3
(CH
2
)
n
S molecule af-
2 3
Assuming that the CH and CH groups occupy the same vol-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
ume, the refractive indices of the individual groups yield an op-
tical dielectric constant of 1.761 for the ethyl-like terminal
w
5
9
creases) and SAM increases with n as the degree of conforma-
group (see Supporting Information for the calculation). The
optical dielectric constant of the terminal methyl group is 1.399,
tional order of the polymethylene chains in the monolayer bulk
odd
59
increases. Since SAM has more gauche bond conformers
while that of the remaining polymethylene chain is 2.164. Cal-
even
(
greater conformational disorder) than SAM , its surface
culations by Ratner et al. indicate that the dielectric constant of
a bilayer film is dominated by the behavior of the layer of the
material with the lowest dielectric constant, even if it is much
even
should be more hydrophilic than that of SAM of comparable
chain length. The opposite behavior is however observed,  of
of neven, reaffirming that the parity effect in the SAM
hydrophobicity originates from the orientation of the chain ter-
mini at the SAM interface (Figure 1a). When the terminal CH
is oriented parallel to the surface normal (nodd), the SAM hydro-
phobicity and  increase compared to when it is tilted away
from the surface normal (neven) due to the lower energy of the
w
35
thinner than the accompanying layer of material with higher .
n
odd > 
w
What this means is that the lower interface dominates over the
higher bulk of the SAM. The ions and water molecules located
at the capacitor plate formed by the SAM/electrolyte interface
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
(Figure 2a, inset) are physisorbed onto an ethyl-like surface of
w
even
higher dielectric constant in the case of the SAM
versus a
3
0
odd
methyl-like versus ethyl-like surface. The bulk and interface
methyl-like surface of lower dielectric constant in SAM . The
estimated difference in the optical dielectric constants of the
ethyl and methyl groups of 0.36 is of comparable order of mag-
nitude to the odd-even difference in SAM of 0.16 derived from
the capacitance. The calculated difference represents the ex-
treme case of pure methyl and ethyl surfaces. It does not take
into account the contribution of the monolayer bulk to SAM or
the orientation of the terminal groups in the SAMs. Even in the
of the SAM move  in opposite directions.
w
By contrast, the value of SAM moves in the direction expected
from the degree of chain conformational order, i.e., SAM of
odd
even
SAM < SAM of SAM . This does not mean that the parity
effect in the SAM dielectric constant is dictated by the bulk.
a 2
First, the amplitude of the odd-even variation in the  (CH )
peak position decreases with n (Figure 4c, inset), contrary to the
parity effect in SAM which remains constant across the entire
range of chain lengths investigated. If the odd-even alternation
of SAM stems from the bulk, it should show a reduction in mag-
nitude with n. Second, we can estimate the magnitude of the
odd
even
idealized representations of Figure 1a, SAM and SAM do
not present neat methyl and ethyl groups, respectively, due to
the imperfect interfacial orientations of the terminal groups in
the SAMs. Consequently, the effective dielectric constant of the
odd
terminal methyl in SAM would be somewhat larger than the
odd
decrease in  of SAM that would arise from the difference in
even
calculated value and that of the ethyl group in SAM
would
even
the number of chain gauche defects between SAM
and
be lower, thereby yielding a smaller odd-even difference in
SAM. Additionally, the presence of gauche bond conformers in
odd
SAM using the chain length dependence of the 
a
(CH
2
) peak

position as a metric. If we equate the change in SAM of 0.312
the polymethylene chains, defects at the SAM domain bounda-
ries, and the contribution of the bulk layer (whose chemical
composition, and hence bulk, is the same for odd/even neighbors
of similar chain lengths) to SAM should also lower the differ-
between n = 6 and 16 to the corresponding change in the
-
1
a 2
 (CH ) peak position of 3.1 cm , then the average shift of 0.53
-1
cm for nodd (7–15) with respect to its hypothetical even posi-
tion suggests that SAM would be lowered by an average of 0.049
odd
even
ence measured experimentally between SAM and SAM
.
even
from its hypothetical value on the trend line of SAMs . The
Having ruled out dipole-governed interactions, we ascribe the
parity effect in the SAM dielectric constant mainly to the dif-
ferent dielectric constants/chemical compositions of the interfa-
cial layers of the terminal moieties (interface).
odd
dielectric constants of SAMs of n = 7–15 determined from
the SAM capacitance are 0.16±0.02 lower (i.e., 3.3 fold lower)
than the hypothetical even values of . The organic interface
layer must account for the majority of the experimentally ob-
served parity effect in SAM
Based on the parallels between the odd-even chain length de-
pendencies of the CH and CH stretching modes and global
.
Conclusions
We have used surface IR spectroscopy to draw conclusions
about the state of the CH (CH ) SAu SAM (dis-)order and as-
3
2
3
2 n
SAM dielectric constant SAM, we propose a bilayer stack model
(Figure 1a), which assigns separate dielectric constants to the
bulk (or interior) of the SAM (bulk) and to the layer of terminal
groups exposed at the SAM/ambient or SAM/electrolyte solu-
sociated interface with the number of methylene units (odd and
even) in the SAM backbone. This structural information allows
us to elucidate the relative contributions of the bulk versus in-
terface of the SAM to the dielectric properties determined by
electrochemical impedance spectroscopy.
3
5
tion interface (interface). The increase of  with n (odd or even)
reflects an increase in bulk due to the progressive ordering of the
polymethylene chains and decrease of the gauche defects within
Odd-Even Chain Length Dependence of the Chain Con-
formational Order and Orientation. The chain conforma-
tional order of the CH (CH ) SAu SAM improves with molec-
3
the SAM improving the chain packing. The constant CH peak
3
2 n
intensities and positions (Figure 4 a,b) imply that the terminal
ular length and stabilizes at n ≥ 16, consistent with the findings
even
31
group orientations and chemical environments in SAMs and
of Porter et al. The CH peak positions also reveal an unprec-
2
odd
SAMs are independent of chain length for a given parity. The
edented odd-even difference in the degree of gauche bond de-
fects across the whole series of CH (CH ) S investigated, with
30
different water contact angles (Figure 3b) and friction forces
measured for SAMs
3
2 n
even
odd
odd
and SAMs corroborate the distinct
SAM
SAM
having more gauche conformers than its shorter
neighbor. Previous temperature-dependent current–
even
chemical nature of the exposed chain termini. These odd-even
differences in the chemical composition of the SAM interface
influence interface, analogous to the SAM hydrophobicity.
voltage and IR spectroscopy measurements on monolayers of
polymethylene chains chemically bound to oxide-free silicon
indicate that thermally-induced intramolecular disorder de-
creases the electron transport efficiency along the molecules
ACS Paragon Plus Environment

Journal of the American Chemical Society
silver, for which the methyl orientation is similar in SAMs
Page 8 of 12
and the measured tunneling current.82 The greater number of
gauche conformational defects in SAM may therefore con-
even
odd
odd 28
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
and SAMs . Gupta and Abbott thus concluded that the ori-
tribute to the lower tunneling current densities observed for
entation of the terminal groups within the SAMs formed on gold
plays an important role in determining the azimuthal orienta-
tions of liquid crystals on these surfaces. They hypothesized
that if a flexoelectric polarization of the liquid crystals is the
cause of the observed parity effect, the different orientations of
odd
junctions of CH
SAM homologues.
3
(CH
2
6
)
n
SAu SAMs
with respect to their
even
The different spatial orientations of the terminal methyl
even
odd
groups in SAM and SAM manifest themselves as an odd-
even alternation of the CH peak intensities. Additionally, the
constant spectral intensities in SAMs
even
odd
the methyl groups in SAM and SAM must change the di-
electric constant of the SAM in contact with the liquid crys-
3
even
odd
and in SAMs , also
83
7
1
tals, just as these alter the surface hydrophobicity. Our results
reported by Chen et al. down to n = 4, indicate that the termi-
nal group orientations are not affected by the chain length de-
pendent conformational disorder. This chain length independ-
ent trend explains why the parity effects observed in the
experimentally validate their hypothesis.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
In summary, the odd-even effect in the SAM dielectric con-
stant derived from the interfacial capacitance highlights the dis-
tinct physicochemical properties that the orientation of the
chain termini can impart to the surface/interface. The findings
have implications for charge storage at/charge transfer across
interfacial molecular assemblies in applications such as gate di-
electrics for organic field-effect transistors, molecular tunneling
junctions, and capacitive biosensors as well as for theoretical
studies on the structure and stability of SAMs.
CH
3
(CH
2
)
n
SAu SAM dielectric constant, surface hydrophobi-
(CH SAu
city, and in the electron transport rate across CH
3
2 n
)
6
SAM-based junctions , which are all ascribed to an odd-even
difference in the terminal group orientation, persist down to
short chain lengths. It challenges a common view, derived from
the bottom-up organization of tilted, all-trans polymethylene
chains, that gauche defects in the chain randomize the spatial
orientation of the terminal group, thereby blurring the distinc-
tion between the surfaces presented by CH
3
(CH
2
)
n
S of neven and
ASSOCIATED CONTENT
2
0
n . Our data sets imply that disorder in the bulk of the SAM
odd
Supporting Information. Experimental details, synthesis and pu-
rification of n-alkanethiols, AFM images of the gold surfaces, sta-
tistical analyses, calculations, IRRAS spectra, and additional data
(PDF). This material is available free of charge via the Internet at
http://pubs.acs.org.
does not preclude the preferential orientation of the chain ter-
mini at the surface/interface. This finding has immediate rami-
fications for the interpretation of the odd-even chain length de-
pendence of SAM interfacial properties, in which the diminu-
tion or absence of odd-even fluctuations at shorter chain lengths
is ascribed to increased chain conformational defects and the
AUTHOR INFORMATION
Corresponding Authors
2
0
consequent loss of interfacial order.
Capacitance Spectroscopy Directly Reveals Odd-Even
Chain Length Differences. The complex impedance response,
of the as-prepared SAMs is consistent with ionic insulating be-
havior. Conversion of the complex impedance to the complex
capacitance brings to the fore the charge storage in the interfa-
cial double layer. The capacitance spectra of the insulating
*
*
antonella.badia@umontreal.ca
c.pellerin@umontreal.ca
Author Contributions
‡These authors (F.B.A. and E.R.D.) contributed equally.
CH
3
(CH
2
)
n
SAu SAMs display outrightly the chain length (ex-
pected) and odd-even dependencies of the SAM capacitance.
The capacitance of the longer chain neven is statistically indistin-
guishable from that of its shorter nodd neighbor.
Notes
The authors declare no competing financial interest.
Odd-Even Effect in the SAM Dielectric Constant. The
SAM dielectric constants derived from the capacitance values
using an ideal capacitor model exhibit a chain length depend-
ency and a constant odd-even oscillation of 0.139, with  of
ACKNOWLEDGMENT
This research was supported by the Natural Sciences and Engineer-
ing Research Council of Canada through grant numbers RGPIN-
03588-2014 (A.B.), RGPAS-462153-2014 (A.B.), and RGPIN-
even
even
odd
odd
SAM
SAM
  of SAM . The higher dielectric constant of
raises its capacitance to the value of its shorter-chain
0
4014-2015 (C.P.). F.B.A. thanks the Tunisian Ministry of Higher
SAM neighbor of lower dielectric constant. The odd-even
chain length dependence of SAM cautions against the use of a
single dielectric constant in ellipsometric measurements of
Education and Scientific Research for a scholarship. The authors
thank Dr. Violeta Toader (FRQNT Quebec Centre for Advanced
Materials) for the synthesis and purification of n-alkanethiols used
in this study.
3
1, 46-48
SAM thicknesses.
Orientation-Induced Changes in the Chemical Composi-
tion of the SAM Interface are the Origin of the Odd-Even
Effect in the SAM Dielectric Constant and Surface Hydro-
phobicity. The observed odd-even difference in the SAM ca-
pacitance is primarily due to differences in the dielectric con-
stants of the ultrathin layers of the terminal moieties exposed at
REFERENCES
1.
Self-Assembled Monolayers in Organic Electronics. Chem. Soc. Rev.
017, 46 (1), 40-71.
2. Vilan, A.; Aswal, D.; Cahen, D., Large-Area, Ensemble
Molecular Electronics: Motivation and Challenges. Chem. Rev. 2017,
17 (5), 4248-4286.
Fabre, B., Functionalization of Oxide-Free Silicon Surfaces
with Redox-Active Assemblies. Chem. Rev. 2016, 116 (8), 4808-4849.
Nerngchamnong, N.; Yuan, L.; Qi, D.-C.; Li, J.; Thompson,
D.; Nijhuis, C. A., The Role of van der Waals Forces in the
Casalini, S.; Bortolotti, C. A.; Leonardi, F.; Biscarini, F.,
2
even
odd
the surfaces of SAM
and SAM . More than two decades
o
ago, Gupta and Abbott reported a dramatic 90 rotation in the
macroscopic azimuthal orientations of nematic liquid crystals
1
3
.
even
odd
3 2 n
deposited on SAMs versus SAMs of CH (CH ) S formed
8
3
on gold surfaces with an anisotropic topography. No odd-even
effect was observed in the liquid crystal anchoring on SAMs on
4
.
ACS Paragon Plus Environment

Page 9 of 12
Journal of the American Chemical Society
Performance of Molecular Diodes. Nat. Nanotechnol. 2013, 8 (2), 113-
118.
Ferrocenylalkanethiolates: Evens Push More Than Odds. J. Am. Chem.
Soc. 2018, 140 (32), 10063-10066.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5
.
Yuan, L.; Thompson, D.; Cao, L.; Nerngchangnong, N.;
22.
Cheng, H.-W.; Wu, Z.-P.; Yan, S.; Li, J.; Shan, S.; Wang,
Nijhuis, C. A., One Carbon Matters: The Origin and Reversal of Odd–
Even Effects in Molecular Diodes with Self-Assembled Monolayers of
Ferrocenyl-Alkanethiolates. J. Phys. Chem. C 2015, 119 (31), 17910-
17919.
L.; Porter, M. D.; Zhong, C.-J., A Simple Vaporous Probe with Atomic-
Scale Sensitivity to Structural Ordering and Orientation of Molecular
Assembly. Chem. Sci. 2019, 10 (29), 7104-7110.
23.
Cao, L.; Yuan, L.; Yang, M.; Nerngchamnong, N.;
6
.
Baghbanzadeh, M.; Simeone, F. C.; Bowers, C. M.; Liao,
Thompson, D.; Yu, X.; Qi, D.-C.; Nijhuis, C. A., The Supramolecular
Structure and van der Waals Interactions Affect the Electronic
Structure of Ferrocenyl-Alkanethiolate SAMs on Gold and Silver
Electrodes. Nanoscale Adv. 2019, 1 (5), 1991-2002.
K.-C.; Thuo, M.; Baghbanzadeh, M.; Miller, M. S.; Carmichael, T. B.;
Whitesides, G. M., Odd–Even Effects in Charge Transport across n-
Alkanethiolate-Based SAMs. J. Am. Chem. Soc. 2014, 136 (48),
16919-16925.
24.
Pradeilles, J. A.; Zhong, S.; Baglyas, M.; Tarczay, G.; Butts,
7
.
Thuo, M. M.; Reus, W. F.; Nijhuis, C. A.; Barber, J. R.; Kim,
C. P.; Myers, E. L.; Aggarwal, V. K., Odd–Even Alternations in Helical
Propensity of a Homologous Series of Hydrocarbons. Nat. Chem. 2020,
12, 475-480.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
C.; Schulz, M. D.; Whitesides, G. M., Odd−Even Effects in Charge
Transport across Self-Assembled Monolayers. J. Am. Chem. Soc. 2011,
1
33 (9), 2962-2975.
8. Chen, J.; Giroux, T. J.; Nguyen, Y.; Kadoma, A. A.; Chang,
25.
Yang, K.; Cai, Z.; Jaiswal, A.; Tyagi, M.; Moore, J. S.;
Zhang, Y., Dynamic Odd–Even Effect in Liquid n-Alkanes near Their
Melting Points. Angew. Chem. Int. Ed. 2016, 55 (45), 14090-14095.
B. S.; VanVeller, B.; Thuo, M. M., Understanding Interface (Odd–
Even) Effects in Charge Tunneling using a Polished EGaIn Electrode.
Phys. Chem. Chem. Phys. 2018, 20 (7), 4864-4878.
26.
Tao, F.; Bernasek, S. L., Understanding Odd−Even Effects
in Organic Self-Assembled Monolayers. Chem. Rev. 2007, 107 (5),
1408-1453.
9
.
Jiang, L.; Sangeeth, C. S. S.; Nijhuis, C. A., The Origin of
the Odd–Even Effect in the Tunneling Rates across EGaIn Junctions
with Self-Assembled Monolayers (SAMs) of n-Alkanethiolates. J. Am.
Chem. Soc. 2015, 137 (33), 10659-10667.
27.
Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.;
Whitesides, G. M., Self-Assembled Monolayers of Thiolates on Metals
as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103-1169.
1
0.
Ramin, L.; Jabbarzadeh, A., Odd–Even Effects on the
28.
Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.;
Structure, Stability, and Phase Transition of Alkanethiol Self-
Assembled Monolayers. Langmuir 2011, 27 (16), 9748-9759.
Parikh, A. N.; Nuzzo, R. G., Comparison of the Structures and Wetting
Properties of Self-Assembled Monolayers of n-Alkanethiols on the
Coinage Metal Surfaces, Copper, Silver, and Gold. J. Am. Chem. Soc.
1991, 113 (19), 7152-7167.
11.
Newcomb, L. B.; Tevis, I. D.; Atkinson, M. B. J.; Gathiaka,
S. M.; Luna, R. E.; Thuo, M., Odd–Even Effect in the Hydrophobicity
of n-Alkanethiolate Self-Assembled Monolayers Depends upon the
Roughness of the Substrate and the Orientation of the Terminal Moiety.
Langmuir 2014, 30 (40), 11985-11992.
29.
Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G., Temperature
Induced Reconstruction of Model Organic Surfaces. J. Electron
Spectrosc. 1990, 54-55, 1143-1152.
1
2.
Wang, Z.; Chen, J.; Oyola-Reynoso, S.; Thuo, M., The
30.
Wong, S.-S.; Takano, H.; Porter, M. D., Mapping
Porter-Whitesides Discrepancy: Revisiting Odd-Even Effects in
Wetting Properties of n-Alkanethiolate SAMs. Coatings 2015, 5, 1034-
Orientation Differences of Terminal Functional Groups by Friction
Force Microscopy. Anal. Chem. 1998, 70, 5209-5212.
1
055.
3.
31.
Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D.,
1
Nerngchamnong, N.; Thompson, D.; Cao, L.; Yuan, L.;
Spontaneously Organized Molecular Assemblies. 4. Structural
Characterization of n-Alkyl thiol Monolayers on Gold by Optical
Ellipsometry, Infrared Spectroscopy, and Electrochemistry. J. Am.
Chem. Soc. 1987, 109 (12), 3559-3568.
Jiang, L.; Roemer, M.; Nijhuis, C. A., Nonideal Electrochemical
Behavior of Ferrocenyl–Alkanethiolate SAMs Maps the
Microenvironment of the Redox Unit. J. Phys. Chem. C 2015, 119 (38),
2
1978-21991.
4. Chen, J.; Tevis, I.; Gathiaka, S.; Thuo, M., Stereo-Electronic
32.
Nishi, N.; Hobara, D.; Yamamoto, M.; Kakiuchi, T., Chain-
1
Length-Dependent Change in the Structure of Self-Assembled
Monolayers of n-Alkanethiols on Au(111) Probed by Broad-
Bandwidth Sum Frequency Generation Spectroscopy. J. Chem. Phys.
2003, 118 (4), 1904-1911.
Effects in Tunneling Junctions: Revisiting the Platform. Procedia Eng.
2016, 141, 138-143.
15.
Thompson, D.; Nijhuis, C. A., Even the Odd Numbers Help:
Failure Modes of SAM-Based Tunnel Junctions Probed via Odd-Even
Effects Revealed in Synchrotrons and Supercomputers. Acc. Chem.
Res. 2016, 49 (10), 2061-2069.
33.
Monolayers. Chem. Rev. 1996, 96 (4), 1533–1554.
34. Dubois, L. H.; Nuzzo, R. G., Synthesis, Structure, and
Ulman, A., Formation and Structure of Self-Assembled
1
6.
Chen, J.; Chang, B.; Oyola-Reynoso, S.; Wang, Z.; Thuo,
Properties of Model Organic Surfaces. Annu. Rev. Phys. Chem. 1992,
43, 437-463.
M., Quantifying Gauche Defects and Phase Evolution in Self-
Assembled Monolayers through Sessile Drops. ACS Omega 2017, 2
35.
Heitzer, H. M.; Marks, T. J.; Ratner, M. A., Maximizing the
(
5), 2072-2084.
7. Song, P.; Thompson, D.; Annadata, H. V.; Guerin, S.; Loh,
Dielectric Response of Molecular Thin Films via Quantum Chemical
Design. ACS Nano 2014, 8, 12587-12600.
1
K. P.; Nijhuis, C. A., Supramolecular Structure of the Monolayer
Triggers Odd–Even Effects in the Tunneling Rates across Noncovalent
Junctions on Graphene. J. Phys. Chem. C 2017, 121 (8), 4172-4180.
36.
Wang, D.; Fracasso, D.; Nurbawono, A.; Annadata, H. V.;
Sangeeth, C. S. S.; Yuan, L.; Nijhuis, C. A., Tuning the Tunneling Rate
and Dielectric Response of SAM-Based Junctions via a Single
Polarizable Atom. Adv. Mater. 2015, 27 (42), 6689-6695.
1
8.
Feng, Y.; Dionne, E. R.; Toader, V.; Beaudoin, G.; Badia,
A., Odd–Even Effects in Electroactive Self-Assembled Monolayers
Investigated by Electrochemical Surface Plasmon Resonance and
Impedance Spectroscopy. J. Phys. Chem. C 2017, 121 (44), 24626-
24640.
37.
Boubour, E.; Lennox, R. B., Potential-Induced Defects in n-
Alkanethiol Self-Assembled Monolayers Monitored by Impedance
Spectroscopy. J. Phys. Chem. B 2000, 104 (38), 9004-9010.
38.
Boubour, E.; Lennox, R. B., Insulating Properties of Self-
1
9.
Chen, J.; Liu, J.; Tevis, I. D.; Andino, R. S.; Miller, C. M.;
Assembled Monolayers Monitored by Impedance Spectroscopy.
Langmuir 2000, 16 (9), 4222-4228.
Ziegler, L. D.; Chen, X.; Thuo, M. M., Spectroscopic Evidence for the
Origin of Odd–Even Effects in Self-Assembled Monolayers and
Effects of Substrate Roughness. Phys. Chem. Chem. Phys. 2017, 19,
6989-6995.
39.
Dionne, E. R.; Ben Amara, F.; Badia, A., An
Electrochemical Immittance Analysis of the Dielectric Properties of
Self-Assembled Monolayers. Can. J. Chem. 2020, doi 10.1139/cjc-
2020-0005.
2
0.
Chen, J.; Wang, Z.; Oyola-Reynoso, S.; Thuo, M. M.,
Properties of Self-Assembled Monolayers Revealed via Inverse
Tensiometry. Langmuir 2017, 33 (47), 13451-13467.
40.
Randriamahazaka, H.; Asaka, K., Electromechanical
Analysis by Means of Complex Capacitance of Bucky-Gel Actuators
Based on Single-Walled Carbon Nanotubes and an Ionic Liquid. J.
Phys. Chem. C 2010, 114 (41), 17982-17988.
2
1.
Dionne, E. R.; Dip, C.; Toader, V.; Badia, A.,
Micromechanical Redox Actuation by Self-Assembled Monolayers of
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 12
4
1.
Góes, M. S.; Rahman, H.; Ryall, J.; Davis, J. J.; Bueno, P.
61.
Van Dyck, C.; Marks, T. J.; Ratner, M. A., Chain Length
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
R., A Dielectric Model of Self-Assembled Monolayer Interfaces by
Capacitive Spectroscopy. Langmuir 2012, 28 (25), 9689-9699.
Dependence of the Dielectric Constant and Polarizability in
Conjugated Organic Thin Films. ACS Nano 2017, 11, 5970-5981.
4
2.
Spectroscopy. John Wiley & Sons, Inc.: Hoboken, N.J., 2008.
3. Janek, R. P.; Fawcett, W. R.; Ulman, A., Impedance
Orazem, M. E.; Tribollet, B., Electrochemical Impedance
62.
Yang, D. F.; Wilde, C. P.; Morin, M., Electrochemical
Desorption and Adsorption of Nonyl Mercaptan at Gold Single Crystal
Electrode Surfaces. Langmuir 1996, 12 (26), 6570-6577.
4
Spectroscopy of Self-Assembled Monolayers on Au(111):ꢀ Evidence
for Complex Double-Layer Structure in Aqueous NaClO4 at the
Potential of Zero Charge. J. Phys. Chem. B 1997, 101 (42), 8550-8558.
63.
Yang, D. F.; Wilde, C. P.; Morin, M., Studies of the
Electrochemical Removal and Efficient Re-formation of a Monolayer
of Hexadecanethiol Self-Assembled at an Au(111) Single Crystal in
Aqueous Solutions. Langmuir 1997, 13 (2), 243-249.
4
4.
Wang, W.; Zhang, S.; Chinwangso, P.; Advincula, R. C.;
Lee, T. R., Electric Potential Stability and Ionic Permeability of SAMs
on Gold Derived from Bidentate and Tridentate Chelating
Alkanethiols. J. Phys. Chem. C 2009, 113 (9), 3717-3725.
64.
Zhong, C.-J.; Porter, M. D., Fine Structure in the
Voltammetric Desorption Curves of Alkanethiolate Monolayers
Chemisorbed at Gold. J. Electroanal. Chem. 1997, 425 (1), 147-153.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
5.
Olivier, G. K.; Shin, D.; Frechette, J., Factors Governing the
65.
Alves, C. A.; Smith, E. L.; Porter, M. D., Atomic Scale
Reversible Change in Ionic Permeability of a Low-Density Monolayer.
J. Electroanal. Chem. 2010, 639 (1–2), 50-58.
Imaging of Alkanethiolate Monolayers at Gold Surfaces with Atomic
Force Microscopy. J. Am. Chem. Soc. 1992, 114 (4), 1222-1227.
46.
Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.;
66.
Wang, Z.; Chen, J.; Gathiaka, S. M.; Oyola-Reynoso, S.;
Whitesides, G. M.; Nuzzo, R. G., Formation of Monolayer Films by the
Spontaneous Assembly of Organic Thiols from Solution onto Gold. J.
Am. Chem. Soc. 1989, 111 (1), 321-335.
Thuo, M., Effect of Substrate Morphology on the Odd–Even Effect in
Hydrophobicity of Self-Assembled Monolayers. Langmuir 2016, 32
(40), 10358-10367.
4
7.
Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M.,
67.
Fenter, P.; Eisenberger, P.; Liang, K. S., Chain-length
Comparison of Organic Monolayers on Polycrystalline Gold
Spontaneously Assembled from Solutions Containing Dialkyl
Disulfides or Alkanethiols. Langmuir 1994, 10 (6), 1825-1831.
Dependence of the Structures and Phases of CH3(CH2)n-1SH Self-
Assembled on Au(111). Phys. Rev. Lett. 1993, 70 (16), 2447-2450.
68.
Wang, Y.; Solano Canchaya, J. G.; Dong, W.; Alcamí, M.;
4
8.
Shi, J.; Hong, B.; Parikh, A. N.; Collins, R. W.; Allara, D.
Busnengo, H. F.; Martín, F., Chain-Length and Temperature
Dependence of Self-Assembled Monolayers of Alkylthiolates on
Au(111) and Ag(111) Surfaces. J. Phys. Chem. A 2014, 118, 4138-
4146.
L., Optical Characterization of Electronic Transitions Arising from the
Au/S Interface of Self-Assembled n-Alkanethiolate Monolayers.
Chem. Phys. Lett. 1995, 246 (1), 90-94.
49.
Widrig, C. A.; Chung, C.; Porter, M. D., The
69.
Hollins, P., Infrared Reflection–Absorption Spectroscopy.
Electrochemical Desorption of n-Alkanethiol Monolayers from
Polycrystalline Au and Ag Electrodes. J. Electroanal. Chem.
Interfacial Electrochem. 1991, 310 (1), 335-359.
In Encyclopedia of Analytical Chemistry, John Wiley & Sons, Ltd.:
2006; pp 1-17.
70.
Studies of Microscopic Wetting on Organic Surfaces. 1. Formation and
Structural Characterization of Self-Consistent Series of
Nuzzo, R. G.; Dubois, L. H.; Allara, D. L., Fundamental
5
0.
Ramírez, P.; Andreu, R.; Cuesta, Á.; Calzado, C. J.;
Calvente, J. J., Determination of the Potential of Zero Charge of
Au(111) Modified with Thiol Monolayers. Anal. Chem. 2007, 79 (17),
a
Polyfunctional Organic Monolayers. J. Am. Chem. Soc. 1990, 112 (2),
558-569.
6
473-6479.
1. Akkerman, H. B.; Naber, R. C. G.; Jongbloed, B.; van Hal,
5
71.
Chen, I. W. P.; Chen, C.-C.; Lin, S.-Y.; Chen, C.-h., Effect
P. A.; Blom, P. W. M.; de Leeuw, D. M.; de Boer, B., Electron
Tunneling Through Alkanedithiol Self-Assembled Monolayers in
Large-Area Molecular Junctions. PNAS 2007, 104 (27), 11161-11166.
of Underpotentially Deposited Adlayers on Sulfur Bonding Schemes
of Organothiols Self-Assembled on Polycrystalline Gold:ꢀ sp or sp3
Hybridization. J. Phys. Chem. B 2004, 108 (45), 17497-17504.
5
2.
Bonanos, N.; Pissis, P.; Macdonald, J. R., Impedance
72.
Tao, Y.-T., Structural Comparison of Self-Assembled
Spectroscopy of Dielectrics and Electronic Conductors. In
Characterization of Materials, Kaufmann, E. N., Ed. John Wiley &
Sons, Inc.: 2012; pp 1-14.
Monolayers of n-Alkanoic Acids on the Surfaces of Silver, Copper, and
Aluminum. J. Am. Chem. Soc. 1993, 115, 4350-4358.
73.
Wenzl, I.; Yam, C. M.; Barriet, D.; Lee, T. R., Structure and
53.
Springer: New York, USA, 1996.
4. Rampi, M. A.; Schueller, O. J. A.; Whitesides, G. M.,
Fujimoto, M., Physics of Classical Electromagnetism.
Wettability of Methoxy-Terminated Self-Assembled Monolayers on
Gold. Langmuir 2003, 19 (24), 10217-10224.
5
74.
Stole, S. M.; Porter, M. D., In Situ Infrared External
Alkanethiol Self-Assembled Monolayers as the Dielectric of
Capacitors with Nanoscale Thickness. Appl. Phys. Lett. 1998, 72,
1781−1783.
Reflection Spectroscopy a a Probe of the Interactions at the Liquid-
Solid Interface of Long-Chain Alkanethiol Monolayers at Gold.
Langmuir 1990, 6 (6), 1199-1202.
5
5.
Jiang, L.; Sangeeth, C. S. S.; Yuan, L.; Thompson, D.;
75.
Engquist, I.; Liedberg, B., D2O Ice on Controlled
Nijhuis, C. A., One-Nanometer Thin Monolayers Remove the
Deleterious Effect of Substrate Defects in Molecular Tunnel Junctions.
Nano Lett. 2015, 15, 6643−6649.
Wettability Self-Assembled Alkanethiolate Monolayers:ꢀ Cluster
Formation and Substrate−Adsorbate Interaction. J. Phys. Chem. 1996,
100 (51), 20089-20096.
56.
Assembly by Surface Plasmon Resonance Spectroscopy. Langmuir
996, 12 (20), 4731-4740.
7. Lee, H. J.; Jamison, A. C.; Lee, T. R., Surface Dipoles: A
Peterlinz, K. A.; Georgiadis, R., In Situ Kinetics of Self-
76.
Mendelsohn, R.; Flach, C. R., Infrared Reflection–
Absorption Spectrometry of Monolayer Films at the Air–Water
Interface. In Handbook of Vibrational Spectroscopy, Chalmers, J. M.;
Griffiths, P. R., Eds. John Wiley & Sons, Ltd.: 2002; Vol. 2, pp 1028-
1041.
1
5
Growing Body of Evidence Supports Their Impact and Importance.
Acc. Chem. Res. 2015, 48 (12), 3007-3015.
77.
Bensebaa, F.; Ellis, T. H.; Badia, A.; Lennox, R. B., Thermal
58.
Press: 2019.
9. Jung, L. S.; Campbell, C. T.; Chinowsky, T. M.; Mar, M. N.;
CRC Handbook of Chemistry and Physics. 100th ed.; CRC
Treatment of n-Alkanethiolate Monolayers on Gold, As Observed by
Infrared Spectroscopy. Langmuir 1998, 14, 2361-2367.
5
78.
Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H., Studies of the
Yee, S. S., Quantitative Interpretation of the Response of Surface
Plasmon Resonance Sensors to Adsorbed Films. Langmuir 1998, 14
(19), 5636-5648.
Temperature‐Dependent Phase Behavior of Long Chain n‐Alkyl Thiol
Monolayers on Gold. J. Chem. Phys. 1990, 93 (1), 767-773.
79.
Poirier, G. E., Characterization of Organosulfur Molecular
6
0.
Monolayer Coatings on a (111) Silicon Surface. J. Phys. Chem. C 2015,
19, 7264-7274.
Gala, F.; Zollo, G., Dielectric Properties of Self-Assembled
Monolayers on Au(111) using Scanning Tunneling Microscopy. Chem.
Rev. 1997, 97, 1117–1127.
1
80.
Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.;
Guentherodt, H. J.; Wolf, H.; Ringsdorf, H., Real-Space Observation
ACS Paragon Plus Environment

Page 11 of 12
Journal of the American Chemical Society
of Nanoscale Molecular Domains in Self-Assembled Monolayers.
Langmuir 1994, 10 (9), 2869-2871.
1. Ulman, A., Thermal Stability of Langmuir-Blodgett and
Self-Assembled Films: A Possible Scenario for Order–Disorder
Transitions. Adv. Mater. 1991, 3 (6), 298-303.
electrical transport through alkyl monolayers with vibrational and
photoelectron spectroscopies. Chem. Sci. 2012, 3 (3), 851-862.
83. Gupta, V. K.; Abbott, N. L., Azimuthal Anchoring
Transition of Nematic Liquid Crystals on Self-assembled Monolayers
Formed from Odd and Even Alkanethiols. Phys. Rev. E 1996, 54 (5),
R4540-R4543.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
8
8
2.
Shpaisman, H.; Seitz, O.; Yaffe, O.; Roodenko, K.; Scheres,
L.; Zuilhof, H.; Chabal, Y. J.; Sueyoshi, T.; Kera, S.; Ueno, N.; Vilan,
A.; Cahen, D., Structure Matters: Correlating temperature dependent
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 12
Insert Table of Contents artwork here
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
ACS Paragon Plus Environment