Dipole-Induced Rectification Across AgTS/SAM//Ga2O3/EGaIn Junctions

Article abstract of DOI:10.1021/jacs.9b02891

This Article describes the relationship between molecular structure, and the rectification of tunneling current, in tunneling junctions based on self-assembled monolayers (SAMs). Molecular dipoles from simple organic functional groups (amide, urea, and thiourea) were introduced into junctions with the structure Ag^TS/S(CH₂)_nR(CH₂)_mCH₃//Ga₂O₃/EGaIn. Here, R is an n-alkyl fragment (-CH₂-)_2?or?3, an amide group (either -CONH- or -NHCO-), a urea group (-NHCONH-), or a thiourea group (-NHCSNH-). The amide, urea, or thiourea groups introduce a localized electric dipole moment into the SAM and change the polarizability of that section of the SAM, but do not produce large, electronically delocalized groups or change other aspects of the tunneling barrier. This local change in electronic properties correlates with a statistically significant, but not large, rectification of current (r⁺) at ±1.0 V (up to r⁺ ≈ 20). The results of this work demonstrate that the simplest form of rectification of current at ±1.0 V, in EGaIn junctions, is an interfacial effect, and is caused by a change in the work function of the SAM-modified silver electrode due to the proximity of the dipole associated with the amide (or related) group, and not to a change in the width or mean height of the tunneling barrier.

Full text of DOI:10.1021/jacs.9b02891

Subscriber access provided by UNIV AUTONOMA DE COAHUILA UADEC
Article
Dipole-Induced Rectification across AgTS/SAM//Ga2O3/EGaIn Junctions
Mostafa Baghbanzadeh, Lee Belding, Li Yuan, Junwoo Park,
Mohammad H. Al-Sayah, Carleen M. Bowers, and George M. Whitesides
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02891 • Publication Date (Web): 09 May 2019
Downloaded from http://pubs.acs.org on May 10, 2019
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 38
Journal of the American Chemical Society
1
2
3
4
5
Dipole-Induced Rectification across AgTS/SAM//Ga2O3/EGaIn
6
7
8
Junctions
9
Mostafa Baghbanzadeh,a,† Lee Belding,a,† Li Yuan,a Junwoo Park,a Mohammad H. Al-Sayah,a,b
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Carleen M. Bowers,a and George M. Whitesidesa,c,d
*
aDepartment of Chemistry and Chemical Biology, Harvard University
12 Oxford Street, Cambridge, MA 02138, USA
bDepartment of Biology, Chemistry and Environmental Sciences, American University of
Sharjah, PO Box 26666, Sharjah, United Arab Emirates
cKavli Institute for Bionano Science and Technology, Harvard University
29 Oxford Street, Cambridge, MA 02138, USA
dWyss Institute of Biologically Inspired Engineering, Harvard University, 60 Oxford St.
Cambridge, MA 02138, USA
†Authors contributed equally to this work.
* To whom correspondence may be addressed: gwhitesides@gmwgroup.harvard.edu
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 38
1
2
3
4
ABSTRACT
5
6
7
8
This paper describes the relationship between molecular structure, and the rectification of
tunneling current, in tunneling junctions based on self-assembled monolayers (SAMs).
Molecular dipoles from simple organic functional groups (amide, urea, and thiourea) were
introduced into junctions with the structure AgTS/S(CH2)nR(CH2)mCH3//Ga2O3/EGaIn. Here, R is
an n-alkyl fragment (−CH2−)2 or 3, an amide group (either −CONH– or −NHCO–), a urea group
(−NHCONH–), or a thiourea group (−NHCSNH–). The amide, urea, or thiourea groups
introduce a localized electric dipole moment into the SAM and change the polarizability of that
section of the SAM, but do not produce large, electronically delocalized groups or change other
aspects of the tunneling barrier. This local change in electronic properties correlates with a
statistically significant, but not large, rectification of current (r+) at ±1.0 V (up to r+ ~20). The
results of this work demonstrate that the simplest form of rectification of current at ±1.0 V, in
EGaIn junctions, is an interfacial effect, and is caused by a change in the work function of the
SAM-modified silver electrode due to the proximity of the dipole associated with the amide (or
related) group, not to a change in the width or mean height of the tunneling barrier.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Keywords: rectification, EGaIn electrode, dipole moment, work function, molecular electronics,
self-assembled monolayers, charge tunneling.
1
ACS Paragon Plus Environment

Page 3 of 38
Journal of the American Chemical Society
1
2
3
4
INTRODUCTION
5
6
7
8
Interest in molecular electronics has developed, in part, based on the proposition that
organic synthesis would enable so-called “wave-function engineering,” and that current-voltage
characteristics of junctions containing organic compounds could be tailored through
modifications of molecular structure.1-7 One of the original stimuli in this field was a paper by
Aviram and Ratner,8 which proposed that a single organic molecule with a π-donor and a π-
acceptor separated by an insulating sigma bridge—a so-called donor-sigma-acceptor (D−−A)
system—could, under an applied field, rectify current. This proposal—based on a general
consideration of rectification in terms of a favorable molecular orbital framework—has been
interpreted to be in agreement with results obtained in a number of studies using molecular
rectifiers,9-17 although not with systems incorporating ferrocene13;18-26 and bipyridyl27 groups,
where rectification seems to reflect a combination of hopping and tunneling. The mechanisms of
rectification in most systems involving tunneling are still being established.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Molecular orbital theory formed the basis of the Aviram-Ratner proposal for rectification.
In this model, the alignment of donor and acceptor energy levels in the molecule, in addition to
those of the electrodes, contributes to the passage of current. This model suggested that the
internal energetic topography of the molecule is capable of influencing the tunneling barrier, and
that effect is dependent on the sign of the applied bias. The design of an Aviram-Ratner-type
rectifier proved to be difficult to implement experimentally, but many (and many types of)
molecular rectifiers and diodes have been demonstrated in Langmuir-Blodgett films,28-29 in
SAMs,20-21;23;27;30-34 and in single-molecule devices.35-37 Rectification is also possible in a purely
tunneling system (e.g. not a combination of hopping and tunneling21) if the energy barrier is
sufficiently asymmetric.17;37-39
2
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 38
1
2
3
4
5
6
7
8
9
Our objective for this work was to understand if the presence of a simple dipolar group
within a SAM—in the absence of accessible delocalized molecular orbitals—would alter the
shape of the tunneling barrier (and perhaps the work function of the electrodes) sufficiently to
induce the rectification of current, and/or to alter tunneling current density (Figure 1). We also
wished to test the influence of the position of this polar group in the SAM—relative to each
electrode—using junctions of the form AgTS/SAM//Ga2O3/EGaIn (here AgTS is a template-
stripped silver substrate, and EGaIn is eutectic gallium-indium alloy, with its surface film of
electrically conducting Ga2O3).40-42
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The dipole moment induced in an n-alkanethiolate-based SAM under an external electric
field is assumed to be negligible.43 Among the possible dipolar groups that can generate
permanent dipole moments in SAMs we chose amide, urea, and thiourea groups. These
functional groups are relatively redox-inert, and will not strongly perturb the structure of the
HOMO or LUMO, but have a substantial dipole moment (~4-5 Debye)44-49. We incorporated
these groups systematically in place of –(CH2)2– or –(CH2)3– groups in the SAM, while keeping
both interfaces (Metal/SAM and SAM//Ga2O3) unaltered. We examined tunneling currents
through two homologous series of molecules (Figure 2) with structures (i)
HS(CH2)mXY(CH2)nH, where –XY– is –CH2CH2–, –CONH– and –HNCO–, and m + n = 12
such that 1 ≤ m ≤ 11, and (ii) HS(CH2)2XYZ(CH2)6CH3, where –XYZ– is –CH2CH2CH2–, –
CH2CONH–, –NHCONH–, –NHCSNH–, or –NHCOCH2–.
3
ACS Paragon Plus Environment

Page 5 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 1. (A) Representation of small molecules aligned in an external electric field, based on
the direction of their dipole moments. (B) Schematic representation of an alkanethiol SAM,
within an electrical circuit made from the EGaIn junction, where the –CH2CH2– groups (blue)
act as resistive elements in the circuit. The red box shows the dipolar functional groups used in
this work. Approximations of their associated dipole moments are represented by grey arrows.
The black box (above) shows the convention that we use for drawing a dipole moment. The net
dipole moment (μnet) can be decoupled into its x- and y-components (μx and μy) relative to the
mean plane of the surface of the electrode, and points towards the region of positive electrical
charge.
4
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 38
1
2
3
4
5
6
7
8
9
Dipole Moment Convention. This paper uses the convention used by physicists for
drawing dipole moments. That is, we draw the dipole as an arrow that points towards the region
of positive electrical charge; this arrow represents the direction with which a polar molecule will
align in an external electric field (Figure 1A). Chemists, in contrast (and despite the IUPAC
definition), usually draw dipole moments in the opposite direction, where the arrowhead points
towards the region of negative electrical charge.50
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
BACKGROUND
Definition of the Rectification Ratio. We define the rectification ratio (r) as the absolute
value of the larger current density at a particular voltage divided by the absolute value of the
current density at the opposite bias but the same magnitude of voltage. We also include an
indication of polarity: r+ = |J(+V)|/|J(-V)| or r - = |J(-V)|/|J(+V)|; in this definition, r is always ≥
1. In EGaIn junctions, the bottom electrode (AgTS in this study) is always grounded, and the sign
of the voltage is defined by the polarity of the EGaIn electrode; “+V” means the polarity of
EGaIn is positive and it (or, more precisely, its electrically conducting Ga2O3 surface film) is
oxidizing relative to the Au or Ag electrode, and “-V” means the polarity of the Ga2O3/EGaIn
electrode is negative and it is reducing, relative to that electrode.
Factors unrelated to the molecular structure might—in principle—induce rectification,
due perhaps to i) the presence of oxides or contaminants on the surface of the electrodes; ii) a
difference in the work function of the electrodes; iii) asymmetric contact at electrode-molecule
interfaces, or iv) experimental uncertainty. Junctions composed of SAMs of
n-alkylthiolates on gold and silver surfaces, and using EGaIn as the top electrode, seem to
produce r+ values of up to 3 ± 2. We attribute these values to differences between the bottom and
top electrodes, including differences in the interfaces between the SAM and the Au or Ag
5
ACS Paragon Plus Environment

Page 7 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
junction and the SAM and the Ga2O3/EGaIn electrode, and the polarizability of the sulfur atom
which is covalently bound to the Ag or Au (bottom) electrode.51 We do not consider a value of
this size to demonstrate the occurrence of rectification due to electronic features of the SAM. We
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
use this value of |r|—a value generated for SAMs of n-alkanethiolates—as a threshold for
determining significant r values in other molecular systems. In practice, and with a degree of
subjectivity, we consider any value of r < 5 to be too small to be reliably assigned to (or
mechanistically interpreted in terms of) molecular rectification,40 although smaller values of r are
routinely observed (and reported) even with the structurally simplest SAMs.
Factors that may Lead to Rectification of Current in a Tunneling Junction. The
proposal of Aviram-Ratner for D−−A rectifiers8 is based on a specific molecular orbital
framework in which the frontier molecular orbitals of the D/A system align energetically with
the Fermi levels of the electrodes at one particular bias; application of the opposite bias does not
lead to such alignment of energy levels, with the result of asymmetry in the I/V trace.
Experimental evidence for this type of rectifier (with r+ of ~ 5) was claimed by Metzger and
coworkers52 using a D−−A system (γ-hexadecylquinolinium tricyanoquinodimethanide). The
authors suggested two origins for rectification: i) the position of the frontier molecular orbitals
relative to the Fermi levels of the electrodes and, ii) an asymmetric drop in the electrostatic
potential across the molecule due to the presence of a large dipole moment.52-53
Ratner and coworkers suggested,54 as an alternative source of rectification, that
asymmetry in I/V curves could be achieved from perturbations in the electrostatic potential
profile of a molecular junction; these changes in electrostatic potential could take place across
the molecular bridge or at the metal/molecule interfaces.17;51 38;55 For this type of rectification
two mechanisms have been suggested. i) A dipole-induced mechanism. In this mechanism the
6
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 38
1
2
3
4
5
6
7
8
9
profile of the potential generated by the applied bias is perturbed by an electrical dipole
(permanent or induced) localized in a molecule or group29;35;55-57. Accordingly, the energy levels
of the molecular orbitals (especially the HOMO for hole tunneling) change with the bias. Bias in
one direction might bring a molecular orbital closer to resonance with the Fermi level, EF, of an
electrode, and reduce the tunneling barrier, while a bias of the same magnitude in the opposite
direction would have an inverse effect and increase the tunneling barrier. ii) Molecular
asymmetry. Rectification might appear in a molecular orbital structure that is coupled differently
to the two electrodes. Such asymmetric couplings might lead to an asymmetric electric potential
profile along the molecule, and lead to rectification (although a detailed mechanism of
rectification has not been clearly identified).38;51;55
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EXPERIMENTAL DESIGN
Structural Design of Dipole-Embedded Alkylthiolates. To determine the influence of
an embedded dipole moment (that is, a dipole in an otherwise non-polar SAM) on the
rectification of current, we compared two series of compounds (Series I and II; Figure 2). In
Series I, we placed an amide group, –CONH–, at each position along the backbone of a C14-
alkylthiolate chain to determine if the position of the dipole, and (perhaps) the inter-chain H-
bonding between amide groups, has an effect on the rate of tunneling currents at either positive
or negative bias. We designed the structural perturbations to include a minimum of at least one
methylene group (–CH2–) as spacer between the amide group and the electrodes; this spacer
serves to isolate (at least by direct conjugation58) the amide/urea group and its dipole from the
sulfur atom and the electrode. 27;59-62 The mixture of interactions between HOMOs is, however,
apparently more delocalized than anticipated by direct interaction of π-orbitals, and we have not
quantified this interaction.
7
ACS Paragon Plus Environment

Page 9 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. Schematic representation of the structural variations upon the inclusion of the amide
group in the backbone of C14-alkanethiol (Series I) and the inclusion of the amide, the urea, and
the thiourea groups in the backbone of C12-alkanethiol (Series II). “m” indicates the number of
methyl groups between the sulfur anchoring atom and the amide group and “n” indicates the
number of methyl groups between the amide group and the terminal hydrogen atom in contact
with the top electrode.
8
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 38
1
2
3
4
5
6
7
8
9
In the second series of molecules (Series II, Figure 2), we embedded different functional
groups and changed the orientation of the amide group by replacing −CONH– with −NHCO–.
We also changed the chemical structure, polarizability and dipole moment of the polar group by
introducing urea (–NHCONH–), and thiourea groups (−NHCSNH–), and compared their charge-
transport characteristics. To understand the influence of a urea or thiourea moiety on charge
transport, we kept the overall length of the molecule equivalent to dodecanethiol (C12) (Figure
2); that is, we kept the width of the tunneling barrier very close to the same, so the change in
current density, if any, should allow us to recognize a change in tunneling current that correlates
with the chemical (or electronic) structure of the molecule or the dipole groups embedded in it.
Electronic Influence of Dipolar Groups. Amide ( ~ 4 D),44-46 urea ( ~ 4-5 D),47-49
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
and thiourea ( ~ 4-5 D)47;63 groups have large dipole moments and high polarizabilities,
relative to those associated with the –(CH2)n=2,3– groups ( ~ 0 D) they replace.64 This change in
electronic structure can, in principle, induce changes in rates of charge transport.54 Amide and
urea/thiourea groups also introduce a different orientation of the dipole vector relative to the
(assumed) direction of charge tunneling (Figure 1B). Thus, the inclusion of an
amide/urea/thiourea group in the structure of a tunneling junction might result in a significant
change in the electrostatic potential along the charge-transport pathway, and change either the
rate of charge tunneling through the junction, or the rectification ratio (r) (that is, relative the
rates of tunneling in opposite directions at the same absolute value of voltage |V|).
Structure of SAMs Containing Amide and Urea Groups. We65 and others66-68
observed that the replacement of –CH2CH2– by an amide group (either a –CONH– or a
–NHCO–) enhances the stability of the SAM, possibly through inter-chain H-bonding. Structural
studies on amide-66-68 and urea-69-71 containing SAMs have shown that the C=O and N—H of the
9
ACS Paragon Plus Environment

Page 11 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
H-bonded amides or ureas are oriented approximately parallel to the metal surface (Figure 2).
The formation of intermolecular hydrogen bonding between the chains in the SAM causes a
change in the tilt angle72 (the tilt angle decreases from ~24 to 18 on gold) in order to
accommodate a conformation favorable for formation of the C=O---H—N H-bond between
molecules. This change in the tilt angle—concurrent with a change in the inter-chain distance
caused by the H-bonding networks—leads to a change in the order and conformation of the alkyl
chains in the SAM.65 The change in the tilt and twist angles is also dependent on the position of
the amide group relative to the metal-thiol interface, and on the overall length of the alkyl chain
in the overlayer.65-68 The structure of SAMs containing a urea group is slightly more
complicated; each urea group has been suggested to form four hydrogen bonds—two acceptors
and two donors—with neighboring urea groups.69-71
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
EXPERIMENTAL METHODS
Synthesis of Urea- and Thiourea-Containing Alkylthiolates. Amide-, urea- and
thiourea-based compounds were synthesized using a previously reported protocol (see SI for
details).73 For convenience, we abbreviated the names of the compounds using the assignments
in Figure 1.
Formation of the SAMs on Silver Bottom Electrodes. SAMs of amide-, urea- and
thiourea-based compounds were formed on the template-stripped surface of silver (AgTS)74 using
a thiol anchoring group. We immersed the surface in a nitrogen-purged ethanolic solution of the
thiol (3 mM). After 12 hours of incubation under a nitrogen atmosphere, we rinsed the SAMs
with ethanol (30 mL) and dried them by evaporation under a gentle stream of nitrogen. We used
SAMs for electrical measurements immediately after drying.
10
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 38
1
2
3
4
5
6
7
8
9
Measurements of Work Function (WF). We performed measurements of work function
using a K-Alpha XPS system. This system is equipped with an X-ray source with energy 1486.6
eV. A biasing stage held the samples in electrical contact with the analyzer; -30 V applied to the
stage accelerated the secondary electrons and helped to detect their cut-off. We recorded the
secondary electron cut-off from 32 to 38 eV (kinetic energy), and the Fermi edge from -35 to -25
eV (binding energy). We extrapolated the secondary electron cut-off to the x-axis to determine
the vacuum level (Ev). Using the vacuum energy and Fermi level (Ef), we calculated the WF (WF
= Ev - Ef) of the SAM-modified Ag surfaces, which is the energy required to eject a
photoelectron from the surface of the metal-sulfur interface, through the monolayer, to the
detector located within a vacuum chamber at 1x10-8 mbar. Work functions calculated in this way
are generally used to represent the Fermi level of the modified metal surface: that is, the energy
level of the electrons in the highest energy occupied states at the SAM-modified metal surface.
Measurement of Current Density. We measured the current density along SAMs of the
molecules (Series I and II; Figure 2) on AgTS surfaces in steps over a range from + 1.0 V to -1.0
V (the Ag electrode was always grounded, and potentials are referenced to ground potential). For
each SAM, the values of log|J(V)| are approximately normally distributed and could be fit to
Gaussian curves. (We thus assume—incorrectly, at some level—no systematic error in the
experiments—that is, only random noise—and a single peak. In fact, although a majority of the
data are compatible with this assumption, some compounds seem to yield data with intrinsic
heterogeneity; see the SI for the raw data). Figures S1-S6 (in Supporting Information) show the
histograms for the values of log|J(V)| (J, A/cm2) for SAMs of n-alkylthiolates and amide-
containing compounds on AgTS. The Supporting Information summarizes details of electrical
measurements.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
11
ACS Paragon Plus Environment

Page 13 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Statistics. For every molecule used in this work, we collected data on three separate
chips (separately prepared SAMs). We analyzed at least ten (and maximum twenty three)
junctions for each chip and collected 21 J(V) traces per junction. Each J(V) trace involves a
forward-bias sweep and a reverse-bias sweep; see supporting information for details. This
procedure provides a minimum of 420 measured values of current density (J) for each applied
voltage. The histograms take into account every measured value of J at a given voltage. We fit
the histograms with Gaussian curves to obtain the log-mean and log-standard deviations. The
plots of log|J|-V are derived from the average of all traces. We calculate the values
of rectification (r+) for each molecule by averaging the values at |J(+V)|/|J(-V)| for every
measured J(V) trace (i.e., the reported value of r+ is an average of all values of r+ that we
measured). The corresponding histograms of r+ were fitted to Gaussian functions to obtain the
log-mean and the log-standard deviation.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
RESULTS AND DISCUSSION
At ±0.5 V the Presence of an Amide, Urea, or Thiourea Bond does not Influence
Rates of Charge Tunneling. We reported previously that the inclusion of an amide group (–
CONH– or –NHCO–) in place an ethylene group (–CH2CH2–), in alkylthiolates, 11 or 12 atoms
from the sulfur anchoring group, has no statistically significant effect on the rate of charge
tunneling at ± 0.5 V.73;75-77 Here, we made systematic substitutions along the backbone of a C14-
alkylthiolate (Figure 2, Series 1) by replacing an ethylene group (–CH2CH2–) with an amide
group (–CONH–) at each position. The values of J (± 0.5 V) for the n-alkylthiolates and amide-
containing compounds examined here are not distinguishable (at the precision of our
measurements; Figures S1, S7, Table S1); that is, the values of <log|J(V)|> for amide-containing
compounds are not distinguishable from that of a C14-alkylthiolate. Similarly, the values of J (±
12
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 14 of 38
1
2
3
4
5
6
7
8
9
0.5 V) for the amide-, urea-, and thiourea-containing C12-alkylthiolates of Series II are not
distinguishable from a C12-alkylthiolate standard (Table 1, Figure S2, S3, S7; see SI for
additional details, including apparent trends in log|J| at ±0.5 V).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Measurements of J(V) at an applied bias of ±0.5 V resulted in almost symmetric voltage
profiles at forward and reverse biases for all compounds; that is, none of the compounds
measured showed a rectification ratio larger than 2.5 (Tables S1 and S3). In our judgment,
rectification ratios that are < 5.0 should not, in the absence of other information, be interpreted to
originate from the SAM and its molecular orbitals or electrical characteristics.53 These data
indicate that incorporation of chemical linkages such as amide or urea groups—even with large
dipoles ( ~ 5 Debye)—into SAMs across the pathway of a tunneling current does not cause
rectification of current at ± 0.5 V.
At ± 1.0 V, the Presence of an Amide, Urea, or Thiourea Bond Can Influence the
Current Density, and Result in Rectification. Increasing the applied voltage to ±1.0 V
produced rectification of current; the magnitude of rectification depended on the location and
direction of the dipolar group (Table 1, Figures S2, S3, S7, S8). For the molecules in Series I, we
observed a small (relative to that observed with Fc and BIPY terminal groups at the SAM/Ga2O3
interface) but statistically significant rectification ratio (r+ = |J(+1.0 V)|/|J(-1.0 V)| > 5) when
the dipole—the amide group—was located close to the bottom (AgTS) electrode, and in
particular, when the aliphatic spacer between the sulfur anchoring atom and the dipole group was
no more than three carbon atoms in length.
By comparing the molecules in Series I that rectify current, and those that do not, we
conclude that rectification is caused predominantly by a larger increase in tunneling current at
positive bias (EGaIn is oxidizing) than at negative bias (EGaIn is reducing).
13
ACS Paragon Plus Environment

Page 15 of 38
Journal of the American Chemical Society
1
2
3
4
5
Table 1. Current density (log|J|) values and rectification ratios observed for SAMs composed of
Series I compounds in a Ag/SAM//Ga2O3/EGaIn junction.
6
7
8
+1.0 V
log|J|
-1.0 V
log|J|
Compound
log|r+|a
|r+|
log
log
log
C14
-1.6
-1.2
-1.1
-0.9
-0.8
-1.3
-1.2
-1.0
-1.4
-1.6
-1.2
-1.2
-1.0
-1.3
0.1
-2.2
-2.0
-2.4
-2.1
-1.5
-1.7
0.5
0.5
0.2
3.6
7.9
18.1
15.1
3.8
2.1
3.2
3.0
3.1
1.6
3.0
1.6
1.8
1.3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
SC1-CONH-C11
SC2-CONH-C10
SC3-CONH-C9
SC4-CONH-C8
SC5-CONH-C7
SC6-CONH-C6
SC7-CONH-C5
SC8-CONH-C4
SC9-CONH-C3
SC10-CONH-C2
SC11-CONH-C1
SC10-NHCO-C2
0.1
0.1
0.2
0.1
0.1
0.3
0.3
0.1
0.4
0.2
0.1
0.4
0.2
0.2
0.2
0.3
0.3
0.2
0.2
0.4
0.2
0.3
0.5
0.2
0.3
0.1
0.88
1.26
1.18
0.58
0.33
0.49
0.46
0.49
0.20
0.47
0.20
0.26
0.11
0.25
0.04
0.29
0.31
0.12
0.09
0.20
0.20
0.20
0.33
0.08
0.21
0.51
-1.8
-1.4
-1.9
-1.7
-1.8
-1.4
-1.5
-1.4
SC3-CONMe-C9
+
ar = <|J(+1.0 V)|/|J(-1.0 V)|>.
14
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 16 of 38
1
2
3
4
5
6
7
8
9
For the amides that rectify, at positive bias, the increase in tunneling current from +0.5 V to +1.0
V is larger (average log|J(+1.0 V)| - log|J(+0.5 V)|) = 1.5 A/cm2) than the increase observed for
C14 (average log|J(+1.0 V)| - log|J(+0.5 V)|) = 0.8 A/cm2). The amides that do not rectify,
however, show an increase in current at positive bias that is approximately the same as that
observed for C14 (log|J(+1.0 V)| - log|J(+0.5 V)|) = 0.9 A/cm2).At negative bias the increase in
tunneling current observed from -0.5 V to -1.0 V for both rectifying amides (average log|J(+1.0
V)| - log|J(+0.5 V)|) = 0.8 A/cm2) and non-rectifying amides (average log|J(+1.0 V)| - log|J(+0.5
V)|) = 0.6 A/cm2) is nearly the same as that observed for C14 (log|J(+1.0 V)| - log|J(+0.5 V)|) =
0.6 A/cm2). Clearly, these results (displayed graphically in Figure S11 in the supporting
information) demonstrate that rectification is the result of an increased rate of tunneling at
positive bias.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The molecules in Series II were designed to preserve the length of the aliphatic spacer
(between the sulfur anchoring atom and the dipole) that results in (or is correlated with)
rectification (m = 2, 3), while changing the dipolar group. We observed that urea and thiourea
groups all rectified current (Figure 3, S9, S10, Table 2, S2). Amide groups oriented in the
opposite direction of those in Series I (i.e., C3-NHCO-C7 and C2-NHCO-C8), however, did not
rectify current. The only obvious difference in polarity between the Cm-NHCO-Cn amides and
the Cm-CONH-Cn amides, is that the y-component of their dipoles must be aligned in opposing
directions. Although the conformation of the chains and the exact orientations of the amide
groups and of the associated dipoles is not known, no plausible configuration of the SAM (for a
trans-extended conformation of the organic molecule, but independent of the tilt angle) reverses
the direction of the component of the amide dipole relative to the mean plane of the surface.
15
ACS Paragon Plus Environment

Page 17 of 38
Journal of the American Chemical Society
1
2
3
4
5
Table 2. Current density (log|J|) values and rectification ratios observed for SAMs composed of
compounds in Series II in a Ag/SAM//Ga2O3/EGaIn junction ±1.0 V.
6
7
8
9
Compound
+1.0 V
log|J|
-1.0 V
log|J|
log|r+|a
|r+|
log
log
0.4
log
0.41
SC3-NHCSNH-SC6
SC3-NHCONH-SC6
SC4-CONH-C6
SC3-NHCO-C7
SC12
-0.8
-0.7
-0.8
-0.6
-0.7
0.2
-1.6
-1.6
-1.6
-0.9
-0.9
-1.0
-0.9
-1.7
-0.9
0.86
0.98
0.90
0.27
0.14
1.18
0.92
0.22
0.33
0.20
0.07
0.03
0.04
0.37
0.32
0.10
7.2
9.5
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
0.4
0.2
0.1
0.1
0.3
0.5
0.6
0.2
0.6
0.33
0.16
0.15
0.39
0.2
6.3
1.8
1.4
SC2-NHCSNH-C7
SC2-NHCONH-C7
SC3-CONH-C7
SC2-NHCO-C8
15.5
8.3
-0.2
-0.8
-0.8
0.28
0.27
0.90
0.10
7.9
1.2
+
ar = <|J(+1.0 V)|/|J(-1.0 V)|>.
16
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 18 of 38
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Plot of the rectification ratio (r+ = <|J(+1 V)|/|J(-1 V)|>) at ±1.0 V, for Series II
compounds. The dashed line at r+ = 5 indicates the minimum value of rectification that we
believe is statistically significant and is not a consequence of artifacts. The error bars represent
the standard deviation of the mean values, and are asymmetric about the mean because of the
conversion from log scale to linear scale. The error bars are relatively large because their size
scales with: (a) the value of the log standard error and (b) the value of rectification with which
the error is associated.
17
ACS Paragon Plus Environment

Page 19 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
This difference between S(CH2)2-3CONHR and S(CH2)2-3NHCOR provided clear evidence that
the orientation of the dipole moment is important for current rectification. We do not know the
exact orientation of the dipoles of the urea and thiourea groups, relative to the AgTS surface, and
the differences in r+ for these molecules when m = 2 and m = 3 may be a consequence of the
distance between the dipole and the Ag surface, or changes in the orientation of the molecules
(and thus net dipole moment). Thus, we interpret these results to indicate that different dipolar
groups, when positioned close to the bottom electrode, can cause rectification of current.
A comparison within Series II, between the molecules that rectify current, and those that
do not, indicates that―as observed in Series I―rectification is a result of an increased rate of
tunneling at positive bias. At positive bias, the molecules that rectify current showed larger
increases in tunneling current from +0.5 V to +1.0 V (average log|J(+1.0 V)| - log|J(+0.5 V)|) =
1.4 A/cm2) than observed for C12 (log|J(+1.0 V)| - log|J(+0.5 V)|) = 0.7 A/cm2), or for the
molecules that do not rectify (average log|J(+1.0 V)| - log|J(+0.5 V)|) = 0.8 A/cm2). At negative
bias, the increase in tunneling current from -0.5 V to -1.0 V was approximately the same for
molecules that rectified (average log|J(+1.0 V)| - log|J(+0.5 V)|) = 0.8 A/cm2), molecules that did
not rectify (average log|J(+1.0 V)| - log|J(+0.5 V)|) = 0.7 A/cm2), and C12 (log|J(+1.0 V)| -
log|J(+0.5 V)|) = 0.6 A/cm2). Thus, the rectification observed with different dipolar groups is
also the result of an increased rate of tunneling at positive bias. (These results are displayed
graphically in Figure S12 in the supporting information)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The Orientation of the Amide Bond Influences the Rectification. A comparison
between the current densities at ±1.0 V across SAMs of C4-CONH-C6 (r+ = 7.9), C3-CONH-C7
(r+ = 9.5), C3-NHCO-C7 (r+ = 1.8), and C2-NHCO-C8 (r+ = 1.2) demonstrates that the ratio of
rectification is sensitive to the orientation of the amide bond (Figure 3).
18
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 20 of 38
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Log|J|-V responses for four compounds investigated in this study: a) Position of dipole
causes a rectification; C14 with an alkyl chain, C2-CONH-C10 with the amide group close to the
bottom interface, C5-CONH-C7 with an amide group in the middle of the backbone, and C9-
CONH-C3 with an amide group close to the top interface. b) The direction of dipole, when the
dipole is close to the bottom electrode, is important in observing rectification in current. c) The
direction of dipole, when the dipole is close to the top electrode, does not influence the current
density. d) The presence of intermolecular hydrogen bonds is important for the observation of
rectification.
19
ACS Paragon Plus Environment

Page 21 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
When the NH group is the group closer to the bottom (AgTS) electrode (e.g.,
S(CH2)3NHCOR rather than S(CH2)3CONHR), the y-component of the dipole (µy) changes
direction, and is oriented towards the AgTS electrode (Figure 4b). This change in dipole moment
orientation was accompanied by a decrease in the rectification ratio from r+ = 9.5 (C3-CONH-C7)
to r+ = 1.2 (C2-NHCO-C8) and from r+ = 7.9 (C4-CONH-C6) to r+ = 1.8 (C3-NHCO-C7) (Figure
3, 4b).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
When we analyzed the effect of inverting the dipole close to the EGaIn electrode,
however, by comparing the current density along C11-CONH-C1 and C10-NHCO-C2, at ±1.0 V
(Figure 4c), we observed no rectification of either, and thus no influence of the orientation of the
amide bond on the rectification ratio.
The Rectification of Current Is Sensitive to the Supramolecular Structure of the
SAM. Our results (Figure 4a-d) showed that, for Series I molecules, the observed rectification
correlates with having a dipole moment close to the bottom electrode, with its y-component (µy)
oriented perpendicular to (and away from) the mean plane of the Ag (bottom) electrode. Amide
bonds, when embedded in the SAMs of alkanethiolates, form hydrogen bonds. This network of
hydrogen bonds could help to order the molecules, and thus influence the magnitude and
direction of the fixed dipole. In the absence of intermolecular hydrogen bonds the structure of the
SAM might, in principle, be disordered; disorder would undoubtedly influence the net dipole
moment.72;78 To examine the effect of hydrogen bonding on the rectification of current, we
compared the current density across SAMs of C3-CONH-C9 and C3-CONMe-C9 (Figure 4d).
These two compounds have (in principle) indistinguishable extended lengths, and differ only in
the elimination of the potential for interchain hydrogen bonding and in the substitution of CH3N
for HN in the amide group.
20
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 22 of 38
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. A schematic representation of a AgTS/SC2CONHC7//EGaIn junction: (a) at +1.0 V
applied voltage (EGaIn is oxidizing); the polarity of the electrodes is aligned with the y-
component of the electric field generated by the dipole moment (µy) of the amide bond. The
tunneling current is faster. (b) at -1.0 V applied voltage (Ag is oxidizing) the polarity of the
electrodes is mis-aligned with the y-component of the electric field generated by the dipole
moment (µy) of the amide bond. The tunneling current is slower.
21
ACS Paragon Plus Environment

Page 23 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
Substituting a hydrogen atom for a methyl group—a change that converts C3-CONH-C9 to C3-
CONMe-C9—eliminates the possibility of hydrogen-bonding; it also has the potential to change
local conformations in the SAM. Measurements of current density for these two compounds
yielded very different values of r+: C3-CONH-C9 (r+ = 15.1) and C3-CONMe-C9 (r+ = 1.3)
(Figure 4d). (We do not know the conformation of the individual molecules containing -CONH-
or -CONMe- groups.)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Proposed Origin of Dipole-Induced Rectification Figure 5 shows a schematic of a SC2-
CONH-C10 SAM on a AgTS surface, the polarity of the electrodes, and the directions of the
dipole moments of the amides within the SAM. The y-component of the dipole (µy) generates a
local electric field along the y-axis (perpendicular to the electrode surface). When the direction
of µy is aligned with the polarity of the electrodes the rate of the tunneling current is larger
(Figure 5a) than when the applied electric potential is reversed (Figure 5b; cathodic top (EGaIn)
electrode to anodic bottom (AgTS) electrode), and the direction of the electrostatic field generated
by the dipole opposes the polarity of the electrodes. We make these empirical observations from
these results. i) Rectification occurs only when the dipole of the amide (or urea/thiourea) is close
to the silver electrode. ii) Rectification reflects primarily an increase in J(V) at positive bias (that
is, with Ga2O3 oxidizing). iii) There is no rectification when the direction of the y-component of
the dipole moment (perpendicular to the plane of the electrode surface) is reversed, and points
toward the Ag surface
Thus, in the absence of a suitable detailed theory, we attribute the position-dependent
rectification of current to alignment between the electric field of the electrodes and a
configuration of the molecules in the SAM are such that the perpendicular component of the
22
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 24 of 38
1
2
3
4
5
6
7
8
9
dipole (µy) of the amide group points toward the EGaIn electrode. Rectification does not occur
when the dipolar group is separated from the bottom electrode by more than three carbon atoms.
Other groups have also reported a similar influence of the dipole moment on charge
transport using single molecule junctions (using peptides)79 and/or large area junctions (with 1,2-
diazine,38 and phenyl bromide80). The reported rectifications (r > 8) in those studies, were similar
in size (although slightly smaller) than those we observe in this study. The rectification ratios in
these reports are also dependent on the magnitude of applied voltage.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Analysis of Work Function Indicates that the Position of the Dipole Moment
Contributes to the Rectification of Current. Previous work has shown that collective
electrostatic properties of SAMs, such as net dipole moment, can modify the work function of
the surface, relative to that of the bare metal.80-83 To rationalize the mechanism of molecular
rectification, we measured the work function (WF) of the SAM-modified bottom-electrodes
using XPS (Table S3). We note here that our measurements of the vacuum level (Ev) and the
Fermi level (Ef), which we used to calculate the WF (WF = Ev – Ef), are performed on the SAM-
modified Ag-electrodes. Thus, the values that we obtain are those of the SAM-modified Ag-
surface and they represent the energy of the emitted photo-electron outside of the surface of the
SAM and not just outside the Ag/S interface. Figure 6 summarizes the values of the WF as a
function of m (the number of methylene groups between S and the –CONH- group) for Series I.
Our measurements indicate that the WF of the SAM-modified surface changes (relative to C14-
alkylthiolate SAM) as a function of the position of the -CONH- group. In particular, when -
CONH- is positioned away from bottom electrode (m > 5), the WF is less than that of a C14-
alkylthiolate SAM (3.78 eV), and when it is close to the bottom electrode (m = 1-4), the WF is
larger (by 0.21 eV to 0.42 eV) than that of a C14-alkanethiolate SAM. The molecules in Series I
23
ACS Paragon Plus Environment

Page 25 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
have similar chemical structures and surface coverage, and the amides are not expected to alter
the Ag-S bonding energy (because they are separated by at least one -CH2- group).84
Nevertheless, molecules with a net dipole moment in the same direction as -CONH- are known
to decrease the WF relative to alkanethiolate-modified Ag. Our observation that -CONH-, when
positioned close to the Ag/S interface, increases the WF relative to a C14-alkanethiolate SAM
suggests that polarizable dipole moments, such as an imbedded amide group, can interact
electrostatically with the metal/organic interface. In this instance, the positional dependence
could be caused by the amide group withdrawing charge from the highly polarizable Agδ+-Sδ-
surface dipole (~0.6 Debye85) when they are in close proximity.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Although the amide (-CONH-) groups embed a large dipole moment into individual
molecules (Table S5), and we assume, into the SAM, the orientation of molecules within a SAM
might cause some cancellation of dipole moments,86-87 and lead to SAMs with a lower net-dipole
moment (µnet) than expected by considering only isolated molecules. The influence of orientation
on the net dipole is supported by the difference in WF measurements and rectification ratios
between C3-CONH-C9 (WF = 4.2 eV, r+ = 15.1), which we expect to be more conformationally
ordered and have better alignment of dipoles, and C3-CONMe-C9 (WF = 3.6 eV, r+ = 1.3), which
we expect to be less ordered and have poorer alignment of dipoles. (we expect both molecules to
have group dipole moments that are similar in magnitude and direction. This premise―that
conformation influences dipole alignment (and thus WF and r+), may also explain why the
rectification ratio of C2-CONH-C10 (r+ ≈ 18) is higher than that of C1-CONH-C11 (r+ ≈ 8),
although its dipole is closer to the AgTS electrode (Figure 4a). That is, the orientation of SAMs is
influenced by the location of hydrogen bonds between adjacent amide groups.88-89
24
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 26 of 38
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Main Plot: Plot of r+ (r+= <|J(+1 V)|/|J(-1 V)|> ) at 1.0 V of Series I compounds
(amides) and the work function of the silver substrate when covered with SAMs of these amides.
Inset plot; upper right: Plot of r+ and work function. The dashed line at r+ = 5.0 indicates the
minimum value of rectification that we consider statistically significant (not resulting from
artifacts unrelated to the molecules). The error bars represent the standard deviation of the mean
values, and are asymmetric about the mean because of the conversion from log scale to linear.
The error bar for C2-CONH-C9 is relatively large because its size scales with: (i) the value of the
log standard error and (ii) the value of rectification with which the error is associated. The
uncertainties in the measured work function values are 0.1 eV, and are determined by the
resolution of the XPS spectra.
25
ACS Paragon Plus Environment

Page 27 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
An even-number of methylene (-CH2-) groups between the sulfur anchoring atom and the
hydrogen bond acceptor (the carbonyl of the amide) produce more crystalline SAMs (which
presumably have better aligned dipoles), than an odd-number of methylene (-CH2-) groups.
Changes in WF correlate well with changes in the rectification ratio (r+) (Figure 6).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The relationship between WF and r+ can be rationalized by an interaction between the
dipole moments perpendicular to the SAM surface (µy) and electrons at the Fermi level. This
interaction changes depending on the distance and orientation of the –CONH– group relative to
the AgTS surface. When the amide bond (in S-(CH2)m–CONH–(CH2)n-H) is located close (m ≤ 4)
to the bottom (AgTS) electrode, the change in internal dipole increases the WF of the bottom
electrode. This increase in WF lowers the current injection barrier at +1.0 V82;90-91 (where the
AgTS electrode is reducing) and leads to a higher tunneling rate across the SAM from the AgTS
electrode to the EGaIn electrode, relative to the rate of tunneling at -1.0 V (where the EGaIn
electrode is reducing). We conclude, regardless of the mechanism, that to observe rectification in
current density, using EGaIn junctions, the internal dipole must be close to the metal-sulfur
interface (Figures 4 and 6).
The relationship between work function and rectification (inset plot, Figure 6) indicates
that only when the WF of the Ag-electrode is above a threshold of ~4 eV, do we observe
rectification. We have not yet rationalized theoretically and quantitatively why (above a WF of >
~4 eV) rectification increases linearly with the apparent WF (inset, Figure 6), since the thickness
of the SAM does not change. This increase probably reflects a change in the electronic profile of
the tunneling barrier. This observation must also be interpreted using a mechanism consistent
with the absence of rectification below ±0.5 V. We speculate that the onset of rectification at WF
~4 eV and voltage > 0.5 V might correspond to the onset of a new conduction mechanism. That
26
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 28 of 38
1
2
3
4
5
6
7
8
9
is, from direct tunneling to field emission92: or from direct tunneling to Fowler-Nordheim
tunneling.93 Using SC3-CONH-C9 as an example, we plotted the tunneling current as ln(J/V2) vs.
1/V and observed a transition from logarithmic dependency on voltage to linear dependency on
voltage; This transition took place at ~0.6 V (Figure 7). This transition to a linear dependency
correlates with the observed threshold voltage required for rectification, and could, in principle,
arise from an asymmetric interfacial barrier height caused by the change in WF94-95. Using SC3-
CONH-C9 as an example, we plotted the tunneling current as ln(J/V2) vs. 1/V and observed a
transition from logarithmic dependency on voltage to linear dependency on voltage, which took
place at ~0.6 V (Figure 7). This transition to a linear dependency correlates with the observed
threshold voltage required for rectification, and could, in principle, arise from an asymmetric
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
interfacial barrier height caused by the change in WF94-95
.
27
ACS Paragon Plus Environment

Page 29 of 38
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 7. Plot of the tunneling current ln(J/V2) as a function of 1/V for SC3-CONH-C9. A
transition from exponential dependency on voltage to linear dependency is apparent at ~0.6 V.
28
ACS Paragon Plus Environment