Replacing -CH2CH2- with -CONH- does not significantly change rates of charge transport through AgTS-SAM//Ga 2O3/EGaIn junctions

Article abstract of DOI:10.1021/ja301778s

This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a -CH₂CH₂- group with a -CONH- group changes the dipole moment and polarizability of a portion of the molecule and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure Ag^TS-S(CH ₂)_mX(CH₂)_nH//Ga₂O ₃/EGaIn (TS = template stripped, X = -CH₂CH₂- or -CONH-, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (non-shorting) junctions (when compared to n-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS～CONH～R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics.

Full text of DOI:10.1021/ja301778s

Article
pubs.acs.org/JACS
Replacing −CH2CH2− with −CONH− Does Not Significantly Change
Rates of Charge Transport through AgTS-SAM//Ga2O3/EGaIn
Junctions
Martin M. Thuo,† William F. Reus,† Felice C. Simeone,† Choongik Kim,† Michael D. Schulz,†
Hyo Jae Yoon,† and George M. Whitesides*,†,‡
†Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United
States
‡Kavli Institute for Bionano Science & Technology, School of Engineering and Applied Sciences, Harvard University, Pierce Hall, 29
Oxford Street, Cambridge, Massachusetts 02138, United States
S
* Supporting Information
ABSTRACT: This paper describes physical-organic studies of charge transport
by tunneling through self-assembled monolayers (SAMs), based on systematic
variations of the structure of the molecules constituting the SAM. Replacing a
−CH2CH2− group with a −CONH− group changes the dipole moment and
polarizability of a portion of the molecule and has, in principle, the potential to
change the rate of charge transport through the SAM. In practice, this substitution
produces no significant change in the rate of charge transport across junctions of
the structure AgTS-S(CH2)mX(CH2)nH//Ga2O3/EGaIn (TS = template stripped,
X = −CH2CH2− or −CONH−, and EGaIn = eutectic alloy of gallium and
indium). Incorporation of the amide group does, however, increase the yields of
working (non-shorting) junctions (when compared to n-alkanethiolates of the
same length). These results suggest that synthetic schemes that combine a thiol
group on one end of a molecule with a group, R, to be tested, on the other (e.g.,
HS∼CONH∼R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics.
structure 1, 2, or 3, adsorbed on so-called “ultra-flat”, template-
INTRODUCTION
■
stripped silver (AgTS) substrates, and contacted by cone-shaped
Understanding charge transport through organic molecules and
supramolecular structures is important in fields from biology1−5
to materials science.6−18 In biology, understanding the flow of
electrons in redox biochemistry requires understanding the
relation between molecular structure and rates of charge
transport. In materials science, it is important in evaluating the
potential of tunneling devices based on organic matter for use
in electronics: the concept of “wave function engineering”
that is, designing and shaping tunneling barriers by molecular
designhas been an influential and theoretically attractive, but
practically unproven, starting point for a number of concepts
proposed for molecular electronics.
HS(CH2)mCONH(CH2)nH
1, m = 10, n = 0, 1, 3, 4, 6
2, m = 11, n = 0−6
HS(CH2)mNHCO(CH2)nH
3, m = 10, n = 4
top-electrodes of the liquid eutectic of gallium and indium with
its surface film of native oxide (Ga2O3/EGaIn).19,21−23,48 The
junctions described in this paper are similar to the AgTS-
S(CH2)nH//Ga2O3/EGaIn junctions that we have described
previously,23 except for the substitution of −CONH− groups
for −CH2CH2− groups.
This project had two objectives. (i) We wished to make a
controlled perturbation to the structure of the n-alkanethiolates
(which have been the predominant subject of studies of
processes involving SAMs), and to determine the influence of
this perturbation on the rates of charge transport across these
SAMs. The amide group (−CONH−) is one of the best
understood in organic chemistry,49,50and while not isostructural
with −CH2CH2− is similar in size, and is known, from prior
We,19−23 and others,24−47 are developing experimental
systems for investigating charge transport by tunneling across
self-assembled monolayers (SAMs) as a function of the
structure of the molecules making up the SAMs. An ideal
system would offer (i) convenience and reproducibility, (ii)
robustness (i.e., the ability to generate statistically significant
numbers of data rapidly), and (iii) versatility (i.e., the ability to
modify, easily and rapidly, the structure of the organic part of
the junction through synthesis). This paper describes the
measurement of current density (J, amps/cm2), as a function of
applied bias (V), in molecular junctions comprising self-
assembled monolayers (SAMs) formed from thiols having the
Received: February 27, 2012
Published: June 7, 2012
© 2012 American Chemical Society
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Article
work, to be compatible with the formation of SAMs.51−55 (ii)
Perhaps more importantly, we wished to develop a system of
SAMs to use in studies of charge transport that was more easily
modified structurally than are derivatives of n-alkanethiolates.
Preparing compounds of the structure HS(CH2)nR, where n =
10−20, and R is a group that we might wish to select, with as
few restrictions as possible, from the full range of organic and
organometallic groups, can be synthetically arduous. By
comparison, compounds of the structure HS(CH2)∼10CONH-
(CH2)nR are relatively simple to make, since amide-forming
reactions are among the most versatile in organic synthesis in
their ability to couple different groups. We knewfrom other
workthat amide groups are compatible with SAMs, and that
the literature contains data suggesting that they are more stable
(perhaps because of interchain hydrogen bonding) than are
simple n-alkanethiolate-containing SAMs.51−55
Examination of current as a function of voltage for these
amide-containing SAMs yielded two important results: (i)
substituting an amide moiety, −CONH−, for an ethylene
moiety, −CH2CH2−, resulted in no significant change in
current density, and (ii) introducing the amide group into the
SAM raised the yield of non-shorting junctions from ∼80−
90%23 to ∼100%. The former results indicate that even a large
(from the vantage of organic chemistry) change in the
electronic structure, dipole moments, and other properties of
the SAM does not significantly influence the rate of charge
transport by tunneling. It also provides a reality check on the
idea that “wave function engineering” may provide an easy
method of designing new materials with currently unprece-
dented charge-transport properties. The latter result suggests
that amides (and perhaps other functional groups capable of
interchain hydrogen bonding) may provide the structural basis
for a useful strategy to use in improving the robustness and
practicality of SAM-based tunneling junctions and other
devices.
In this equation, J (A/cm2) is the current density flowing
between the electrodes, d is the length of the molecule (in
either Å or number of non-hydrogen atoms in the extended
chain, n), J0 is the current density in the hypothetical case of a
junction with a SAM of zero thickness, but still including the
contribution of all interfaces in the junction, and β (either Å−1
or per number of non-hydrogen atoms, n−1) is an attenuation
factor related to the shape and height of the tunneling barrier
posed by the SAM.
We19,21−23 and others66−68 have previously reported that J
through SAMs of n-alkanethiols is approximately log-normally
distributed (albeit often with long, asymmetrical tails and
significant outliers), rather than normally distributed, and have
suggested that variations from junction to junction in thickness
and in the number or type of defects in the SAM and electrodes
would lead to a normal distribution in the effective thickness, d,
of the SAM.19,21−23,66,69 Since J is exponentially dependent on a
normally distributed parameter (eq 1), J itself should be log-
normally distributed.
We have reviewed the literature on charge transport through
SAMs of alkanethiols, and found a consensus for the value of β
= 0.8−0.9 Å−1 (1.0−1.15 nC−1) across many techniques;70 we
also found a much looser consensus for a value of J0 (J0 ≈ 10−
103 A/cm2) in junctions of the form metal-SAM//(protective
layer)/liquid metal. (We discuss the significance of the
“protective layer” in another paper.48)
We have demonstrated a statistically significant difference in J
between alkanethiols with odd and even numbers of carbon
atoms (the so-called “odd−even” effect).23 Specifically, J for
odd-numbered alkanethiols is roughly 1 order of magnitude
smaller than what one would predict for the same thickness
from an interpolation of J for even-numbered alkanethiols. In
this work, we infer that this “odd−even” effect persists in SAMs
containing amide moieties.
Properties of Alkanethiol SAMs Compared to SAMs
Containing Secondary Amides. It has been repeatedly
demonstrated that the strong metal−sulfur bond (168 kJ/mol)
and the favorable van der Waals interactions,71−73 supple-
mented by interchain hydrogen bonds, restrict conformational
mobility of the alkyl chains in SAMs containing secondary
amides, and generate an ordered assembly in these mono-
layers.51−55,74−76 Incorporating interchain hydrogen bonds in
SAMs decreases the density of defects, and increases thermal
stabilities of amide-containing SAMs, compared to SAMs of n-
alkanethiols.51−53,55,75,77,78 In the current study we replaced
ethylene units, −CH2CH2−, in alkanethiols with secondary
amide moieties, −CONH− or −NHCO−, where both
orientations of the amide moiety are capable of interchain
hydrogen-bonding.79
This project is complementary to a related study using the
same type of junction.56 In this other study, we used SAMs
made up of a related structure (4), also containing an amide
group. The objective of work involving 4 was different from
HS(CH2)4CONHCH2CH2R
4
that in this paper. It was designed to examine the influence of
the structure of the R group (chosen to include a number of
different aliphatic and aromatic groups) on the tunneling
current for SAMs of approximate constant thickness. It also
compared the amide-containing compounds with homologous
n-alkyl thiolates of the same length, and concluded that both
compounds tunnel currents at almost similar rates (any
difference was less than a factor of 3).
EXPERIMENTAL DESIGN
■
SAMs Containing Secondary Amides. Amides are important
functional groups throughout organic chemistry.49,50,79 From a
practical point, amides are readily synthesized,2,8 and can be used to
introduce, or couple, many functional groups into a molecular system.
Why Secondary (−CONHR−) Amides? This work focuses on
secondary amides because (i) they have the potential to form
intermolecular hydrogen bonds when incorporated into a SAM; (ii)
the absence of a second N-alkyl group leads to less interference with
self-assembly than would more hindered structures such as −CON-
(R′)R, and therefore the charge transport characteristics of SAMs
incorporating −CONH− groups can be compared to those of n-
alkanethiolate SAMs (which are often seen as a baseline/standard
system); and (iii) although a −CONH− group has only small
BACKGROUND
■
Charge Transport in Insulating Organic Molecules.
The current consensus in the field of molecular electronics is
that charge transport in SAMs of insulating organic molecules
proceeds via non-resonant, through-bond tunnel-
ing.20,23,34,42,57−64 This behavior is typically modeled by a
simple form of the Simmons equation (eq 1).65
J = J0e−βd
(1)
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structural/steric differences from a −CH2CH2− group, amides have a
large (μ ≈ 4 D)80−84 group dipole moment. Tertiary amides, are
sterically larger than −CH2CH2− groups, and cannot be directly
compared to n-alkanethiols. Primary amides place a polar −CONH2
group at the interface with the Ga2O3 film, and as such can perhaps
not be directly compared with n-alkanethiolates.
Scheme 1. Synthesis of Mercapto-N-alkylamides (m = 10 or
11) from the Respective Primary Bromocarboxylic Acid via a
a
Bromoalkylamide
Position of the Amide. We synthesized thiols, HS-
(CH2)mCONH(CH2)nH, with the amide moiety separated from the
thiol group by 10 or 11 methylene units (m = 10 or 11). We chose a
C10 or C11 spacer to allow for a well-ordered region between the amide
moiety and the thiol.73,85
Use of n-Alkanethiolate SAMs as Bracketing Standards. Data
were collected over long intervals of time (days to months apart), and
random and/or systematic errors (environmental and seasonal
variations, differences among operators, changes in equipment) were
probably unavoidable. To make sure that data collected at different
times by different operators were comparable, we used two n-
alkanethiols (octadecanethiol and dodecanethiol) that bracket the
values of J(V) of interest in this work to calibrate values obtained with
new compounds or from multiple users; we call these thiols (C12 and
C18) “bracketing standards”. We took measurements of these two
SAMs periodically during collection of data on amide-containing
SAMs, and compared these data to those previously reported21,23
using the EGaIn/Ga2O3 top electrode. Measurements using the
standards were collected randomly throughout the study, with the first
3−5 tunneling junctions measured in any set of experiments being
from the two standards, before starting to measure the amides.
Whenever the measured standards deviated from the literature by
more than an order of magnitude, the experiment was stopped, a new
tip formed, and data re-collected. By applying a randomized system of
measuring the standards, we minimized uncontrolled variations in the
measurements.
Management of Measurement Variability. To minimize and
manage experimental errors, we have developed a standard operating
procedure and well-defined statistical tools for this work.69 We
collected data generated by multiple users, to avoid a single-user bias
in the results. We pooled all non-shorting (working junctions only)
data from different users, and summarized the pooled data as
histograms. To these histograms, we fitted Gaussian curves using a
non-weighting algorithm to avoid biasing the data due to outliers.69
Statistical Analysis of log(|J|) Rather Than J. As noted
previously by us22,23 and others,43,66,67,86 J is log-normally distributed,
rather than normally distributedthat is, log(|J|) is (approximately)
normally distributed. We chose, therefore, to plot and fit histograms of
log(|J|), rather than J. When necessary, we used two-sample t tests69 to
determine whether the distributions of log(|J|) for two compounds had
distinguishable or indistinguishable means, at the 99% confidence
level. Since the t test assumes normality, and since the distributions we
observe sometimes deviate from normality, the statistical inferences
from t tests are suggestive but not conclusive.
a
Subsequent reaction with thiourea followed by hydrolytic cleavage of
the resulting isothiuronium salt gave the target thiol. The reverse
amide was synthesized in a similar manner. The thiols were obtained
in 45−81% yield over the four steps.
Table 1. Junction Performance Results for Measurements on
Mercapto-N-alkyl Amides of the Form
S(CH2)mCONH(CH2)nH, Abbreviated as m,n, and of the
Form S(CH2)mNHCO(CH2)nH, Abbreviated as m,n*
no. of AgTS
substrates
failed
junctions
yield
(%)
a
b
compd
junctions
N
c
[11,0]
11,1
1
3
1
2
1
2
1
1
1
1
1
1
23
52
24
24
10
51
28
26
19
12
18
15
780
2144
1008
974
1
0
0
1
0
0
0
0
0
0
0
1
[96]
100
100
96
11,2
11,3
11,4
420
100
100
[100]
100
100
100
100
93
11,5
1782
1176
1026
758
c
[10,0]
10,1
10,3
10,4
466
10,6
768
10,4*
604
a
b
Includes all junctions, both working and failed. Failure was identified
by current that reached the compliance limit of the electrometer (105
mA): the equivalent to a short circuit. Square brackets, [ ], indicate a
terminal primary amide; these compounds may not be comparable to
the −CH3 terminated (n = 1−6) compounds.
c
roughly 20 J(V) traces for each junction. A J(V) trace involved
sweeping from 0 V → +0.5 V → −0.5 V → 0 V in steps of 50
mV, with a delay of 0.2 s between each step in applied bias,
while measuring J at each bias. Thus, one J(V) trace yielded two
values of J for each value of applied bias. Using this protocol, we
collected between 400 and 2200 values (N) of J at every applied
bias for each molecule. Collecting data for each compound
required between 2 and 12 h (depending on N); in some cases,
measurements were spread out over multiple (sometimes non-
consecutive) days, although data collected over short periods of
time had fewer variations and gave more tightly clustered data.
Before collecting data for a particular amide-containing SAM,
we measured current densities across each of the two
bracketing standards from 3−5 junctions, and then randomly
repeated throughout the analysis after every 10 junctions.
Yield of Junctions Incorporating SAMs with Internal
Amides. For the series of secondary amides with m = 10 or 11,
we observed only two shorting junctions out of 287 (N = 11
302) total junctions measured (99% yield, Table 1). By
contrast, the yields of working junctions incorporating SAMs of
n-alkanethiols (S(CH2)n−1CH3, n = 9−18) averaged ∼80−
RESULTS AND DISCUSSION
■
Synthesis of Amides. Scheme 1 summarizes the general
proceduretwo consecutive two-step reactionswe used to
synthesize all molecules (see Supporting Information for
details). Synthesis followed by chromatographic purification
gave the target thiols in 45−81% yields over four steps, and in
high purity (as determined by 1H NMR). As previously
discussed,23 we purified the thiols carefully, by column
chromatography using 15% ethyl acetate in n-hexanes as eluant,
before forming SAMs. The purified thiols were stored under N2
and refrigerated when necessary. When, or if, the molecules
degraded, they were re-purified by column chromatography,
1
and purity was confirmed by H NMR. For convenience, we
abbreviated the names of the compounds using the assignments
in Table 1. For each compound investigated, we measured
between 10 and 52 junctionsindividual points of contact
between a Ga2O3/EGaIn tip and the SAMand collected
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90%.23 We attribute the high yields of junctions derived from
amide-containing SAMs to the formation of a network of
intermolecular hydrogen bonds extending across the
SAM.51−53,55,74,77,87−90
in the amides (Figure 2). A crude estimate of the difference
between values of ⟨log |J|⟩ for a hypothetical, common,
Estimates of β and J0 for Alkyl Amides. For these amide-
containing SAMs, we do not have data over a sufficiently broad
range of lengths to generate confident estimates of β and J0 by
fitting ⟨log |J|⟩ vs n to the Simmons equation.91 We exclude
from our analysis compounds 10,0 and 11,0, since they
terminate in a −CONH2 group rather than a −CH3 group. We
draw three qualitative conclusions from the data. (i) Within the
uncertainty of the measurements, it is not possible to
distinguish between n-alkanethiolates and amides with the
same length (Figure 1). This statement is not the same as an
assertion that there is no difference, only that within the
uncertainties of these measurements (≤ 1 unit in log |J|), we
cannot distinguish them. (ii) It appears that the difference
between chains with odd and even numbers of atoms in the
backbone of the chain, observed previously,23 is still preserved
Figure 2. Plot of ⟨|J|⟩, at V = −0.5 V, vs the total length of the
molecule (i.e., the number of bonds in the longest trans-extended
chain) for alkyl amides with m = 11. All molecules with an even length
(filled squares) gave higher current densities than those having an odd
length (open squares). The two inserts are histograms of log |J|,
plotted against counts, for 11,2 and 11,3 compounds to illustrate the
relationship between the standard deviation in the data and the spread
in the histograms of the data. The structures of the different molecules
are given below the x-axis. The primary amide, 11,0, is marked with
square brackets, [ ], as a reminder that this interface is different from
the other −CH3 terminated compounds and as such, perhaps, cannot
be compared directly. The two lines are inserted to highlight how the
molecules segregate into two groups of “odd” and “even” lengths.
extrapolated length suggests that for these amides, the value
of ⟨log |J|⟩ is higher by by a factor of ∼1 (with an unquantified
uncertainty) for the even-numbered compounds than for the
odds (albeit from fitting only five data); for comparison, the
value of ⟨log |J|⟩ previously estimated from more precise data
for n-alkanethiolates, the evens are higher than the odds by a
factor of ∼1. (iii) The small number of data do not support a
useful estimate of β. If, however, we constrain the amide |J0,even
|
to have a value similar to that estimated for the n-alkanethiols
(J0,even ≈ 5.4 × 102 A/cm2),23 since the values for J for amide-
containing and n-alkyl thiolates SAMs are indistinguishable,
then the values for β must be similar (we calculated βeven ≈ 1.0
n−1, where n is the total number of non-hydrogen atoms in the
trans-extended form of the amides).
Substituting an Amide for an Ethylene Unit Has No
Discernible Effect on σlog. We also compared the log-
standard deviation, σlog, of data derived from amides to those
derived from n-alkanethiols (Table 2). We observed no
consistent difference in σlog between the two classes of
compounds. This result tentatively suggests that the main
Figure 1. Plot of ⟨log |J|⟩, at V = −0.5 V, vs the number of atoms in the
molecular backbone for three types of SAMs: triangles, 10,n, amides n
= 0, 1, 3, 4, 6; squares, 11,n, amides n = 0−5); and circles, 11,n, n-
alkanethiols n = 12−18). Closed symbols (top panel) represent
compounds with an even length (in terms of the number of non-
hydrogen atoms in the molecular backbone), while open symbols
(bottom panel) represent compounds with an odd length. The dashed
line highlights the best linear fit for the n-alkanethiols and is given as a
guideline, and as an aid in comparing the behavior of the amides.
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Journal of the American Chemical Society
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Junctions with Structure AgTS-SAM//Ga2O3/EGaIn,
Based on SAMs with Amides, Do Not Rectify. Aviram
and Ratner proposed,92 and others have claimed,93−101 that
diode-like rectification of current can result from two accessible
molecular orbitalsan electron acceptor and an electron
donorarranged in series along the path of charge transport.
We have shown a simpler way of achieving rectification than
the Aviram−Ratner approach: placing a single accessible
molecular orbital asymmetrically between two electrodes (i.e.,
at the terminus of an alkanethiol chain).21,22 Others have
proposed that even an accessible molecular orbital is not
requiredthat a dipole with a component along the axis of
charge transport is sufficient to cause rectification by “tilting”
the tunneling barrier and breaking the symmetry between the
wave functions of electrons approaching the barrier from the
left and right.101 An amide has a large dipole moment (μ ≈ 4
D).81,83,84 Placing an amide within the SAM does not produce
an easily quantifiable change in that component of the dipole
moment that might be relevant to the shape of the tunneling
barrier for a number of reasons (the orientation of the dipole,
partial cancellation of dipoles on adjacent molecules, and
uncertainties concerning the path of the electron during
tunneling, among others). Nonetheless, it seemed worthwhile
to test our set of amide-containing SAMs for rectification. We
calculated the raw rectification ratio, r, as the ratio of the
current density at opposing values of applied bias,
Table 2. Comparison of Log-Means, ⟨log |J|⟩, and Log-
Standard Deviations, σlog, for SAMs Derived from the
Amides with Those from n-Alkanethiols of Analogous
Lengths
⟨log |J|⟩ σlog
compd
amides
S(CH2)nH
11,0
11,1
11,2
11,3
11,4
11,5
10,0
10,1
10,3
10,4
11,6
−4.4 0.9
−3.4 0.2
−5.2 0.4
−4.1 0.9
−5.7 0.5
−4.3 0.7
−2.4 0.1
−4.2 0.6
−5.6 0.4
−3.4 1.2
−4.0 0.3
−3.9 0.7
−3.7 1.1
−4.9 1.1
−4.3 0.5
−5.8 0.2
−5.3 0.7
−2.5 0.8
−3.9 0.7
−4.9 1.1
−4.3 0.5
−5.3 0.7
advantages of having an internal amidehigh yields of working
junctions and ease and flexibility in synthesiscomes without a
trade-off in the spread of log(|J|).
Reversing the Orientation of the Amide, −CONH− vs
−NHCO−, Does Not Make a Significant Difference to
Either ⟨log |J|⟩ or σ. We synthesized and measured one
compound with the amide moiety reversed [10,4*, S-
(CH2)10NHCO(CH2) CH3)]. Figure 3 compares the histo-
r = J(+V)/J(−V)
3
(2)
grams of 10,4* and 10,4. Reversing the orientation of the
amide moiety lowered the mean value of ⟨log |J|⟩ by 0.6 log unit
(ΔJ = 100.6 A/cm2), but this difference is not statistically
significant (p > 0.1), and the two values are not distinguishable.
For all molecules synthesized, we calculated the value of r, at
V =
0.5 V, for each trace, plotted values of log |r| in
histograms (r, like J, is log-normally distributed), and fitted
Gaussians to the histograms to determine the mean and
standard deviation of log |r|. All of the amides gave values of
log |r| that were close to zero, and similar to those of n-
alkanethiols in magnitude and polarity (Table 3). Figure 4 and
Table 3 illustrates that there was no significant effect of position
(varied by one carbon) and/or structure on the rectification
ratio. The small rectification at positive bias observed with these
amide molecules is comparable to that observed with n-
alkanethiolate SAMs (Table 3, column 6) and could arise from
Table 3. Rectification Ratios Observed for All Amides
a
Containing SAMs in an AgTS-SAM//Ga2O3/EGaIn
c
|+r|
b
amide
NJ
⟨log |J|⟩
σ
amides
S(CH2)nH)
11,0
11,1
11,2
11,3
11,4
11,5
10,0
10,1
10,3
10,4
10,6
10,4*
780
2144
1008
974
−4.4
−3.4
−5.2
−4.1
−5.7
−4.3
−2.4
−4.2
−5.6
−3.4
−4.0
−4.0
0.9
0.2
0.4
0.9
0.5
0.7
0.1
0.6
0.4
1.2
0.3
0.6
1.2 1.2
1.3 1.3
1.4 1.4
1.1 1.4
1.3 1.5
1.1 1.2
1.2 1.1
1.6 1.6
1.2 1.3
1.2 1.2
1.1 1.2
1.3 1.2
1.3 1.2
1.3 1.4
1.4 1.5
1.3 1.4
1.2 1.3
1.5 1.5
1.1 1.2
1.3 1.2
1.4 1.5
1.3 1.4
1.6 1.6
1.3 1.4
420
1782
1176
1026
758
466
768
604
Figure 3. Histograms of log |J| for two alkyl amides, 10,4 and 10,4*,
compounds in which the orientation of the amide has been reversed
but the total length of the molecule is the same. The spreads in the
distributions for the two compounds overlap, making the averages of
log |J| indistinguishable.
a
All the rectification ratios are in the positive bias. For comparison
purposes, the values of rectification ratios for n-alkanethiols of
analogous length are given. Numbering derived from Table 1 and
Scheme 1. r = J(+0.5 V)/J(−0.5 V).
b
c
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bias. The junction at one bias thus acts as a reference for the
same junction at the opposite bias.
CONCLUSIONS
■
Homologous SAMs Having −CH2CH2−, −CONH−, and
−NHCO− Groups Support Statistically Indistinguishable
Tunneling Currents. The primary objective of this work was
to compare isostructural compounds (1, 2, 3) with one another,
and with n-alkanethiolates of the same length, in junctions of
the structure AgTS-SAM//Ga2O3/EGaIn. This work benefits
from a comparison with another series of amides of structure 4;
these compounds differ in the position of the amide group,
which is closer to the silver electrode in 4 than in 1−3.56 We
draw three conclusions. (i) The amide-based compounds are
easier to synthesize than those with an all-carbon backbone,
and allow easy synthetic access to a wide range of compounds
with which to test hypotheses relating structure to tunneling
current. (ii) The presence of the dipole moment embedded in
the SAM by the amide has no apparent influence on either the
tunneling currents, or, perhaps more significantly for the theory
of these systems, on their rectification ratios (r): the values of r
for amide-containing and all-hydrocarbon compounds are
indistinguishable. (iii) The orbital structure of the −CONH−
group is thus, apparently, not sufficiently different from that of
a −CH2CH2− group to influence the rate of charge transport
by tunneling across these junctions significantly.
The quality of the data we report in this paper is somewhat
more broadly distributed than that in previous papers focused
on n-alkanelthiolates, and also on amides of structure 4.56 We
do not know the reason for this difference yet.
The most important problem in the design of compounds
1−3 is the long −(CH2)10−11−chain connecting the thiol and
the amide group. This length limits the size of the groups, R,
that can be placed on the other side of the amide, since for large
R groups, values of J(V) become too small to measure reliably
with our electrometer. We are thus constrained to use relatively
small R groups, where the molecular order of these groups in
the SAM is not established, but is almost certainly less than in
n-alkanethiolates.51−54,77
Whatever the reason, the data in Figures 1 and 2 allow us to
say that the alkanethiolates and homologous amides are
statistically indistinguishable, but do not allow good estimates
of values of β for the amides. Making the assumption that
values of Jo are similar for amides and for n-alkanethioates,
however, gives similar values of β (β ≈ 1 n−1).
Figure 4. Histograms of log |+r|, where r = J(0.5 V)/J(−0.5 V), for
compounds with amides at near the terminal group closer to the top
electrode and for n-alkanethiols. All compounds have 15 non-H atoms
in the molecular backbone (excluding sulfur). The first two histograms
illustrate the effect of having an amide, and/or shifting the amide by a
single carbon, while the bottom histogram is from an n-alkanethiol of
similar length (same barrier width) for reference.
Wave Function Engineering. One of the hopes at the
beginning of the study of organic tunneling junctions based on
SAMs was that variations in the HOMO and LUMO energies
of the organic groupsin principle easily achieved through
synthesiswould allow the design and generation of tunneling
barriers with designed energetic topographies, and the
discovery of new types of tunneling behaviors. Variations in
the structure of the functional groups included in the SAMs
that include common groups (e.g., simple aromatics, amides,
saturated hydrocarbons) seem to have little effect on rates of
tunneling.56 The theory of tunneling through junctions
containing SAMs is not sufficiently developed at present to
give any guidance to these studies, and it is not clear whether
this low sensitivity is expected or not. It does, however,
empirically constrain the range of organic groups that seem
worthwhile to study, when looking for interesting influences on
tunneling currents to those that have much larger changes in
orbital energies than simple organic functional groups. The
a difference in work function of the electrodes (Ef Ag = −4.5
eV, EGaIn ≈ −4.3 eV). There is, thus, no evidence that the
value or r is different for n-alkanethiolates junctions and for
compounds where a −CH2CH2− has been replaced with a
−CONH− moiety.
The Widths of Histograms for the Measurement of
Rectification, r, Are Narrower Than Those for Histo-
grams for the Measurements of Current Density, J. A
comparison of width of histograms of current density, J, to
those of the rectification ratio, r, demonstrate again23 that the
former had much larger distributions than the latter. Figure 5
shows examples, and histograms, for three types of molecules;
two amides and an alkanethiol. The difference in the width of
the distributions in the histograms of J and r reflects, we
presume, the fact that the measurements of rectification ratios
are self-referencing,22 i.e. current density on reverse bias is
compared to current density in the same junction at forward
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Figure 5. Histograms of log |J| at opposite biases, +0.5 V (top row) and −0.5 V (middle row), above those of log |r| (bottom row), where r = J(+0.5
V)/J(−0.5 V), for two compounds with internal amides along the molecular backbone and an n-alkanethiol. The first two rows of histograms
illustrate the effect of reversing the applied bias on the distribution of the histograms. The bottom row of histograms gives the histogram of
rectification ratios for the three molecules. Comparison down each column illustrates the difference between the width of distributions for current
density at opposite applied biases and the rectification ratios.
large rectification observed with terminal ferrocene groups may
point toward a useful direction.22
Thus, whatever causes the dispersion in J(V) largely disappears
when the same junction is used for measurements at positive
and negative bias. Rectification thus appears to be less sensitive
to the details of the structure of the junction than measure-
ments of J(V).
The interplay of the R//Ga2O3 interface and the structure,
order, and heterogeneity of the SAM remains unclear. On the
one hand, the observation of the odd/even effect in n-
alkanethiolates, and alsoapparentlyin the amides 1, suggest
an important role for the interface. On the other, the absence of
a large effect on substitution of a nonpolar methyl or n-alkyl
group by a more polar primary amide [11,0] argues against a
very sensitive influence of interface on current density. This
anomaly requires further experimental work for resolution.
Rectification. The largest values of r so far reproducibly
observed have been with ferrocene (Fc). The most plausible
mechanism underlying these values is based on a difference in
mechanism of charge transport at opposite bias in these
systems (from pure tunneling to a combination of hopping and
tunneling).21,22 Other mechanisms22,99,100,102−105 are, of
course, in principle possible, and the observation of one
mechanism for Fc does not preclude different mechanisms for
other compounds. Again, however, the comparison of values of
rectification for the range of compounds now available
constrains the possible mechanisms. We do not see rectification
on embedding the amide dipole near the end of the SAM (away
from the silver electrode). Thus, simply embedding even a large
dipole in the SAM does not necessarily give large values of
rectification (the possible small values that seem consistently to
be observedvalues of r ≈ 1.2 that are arguably almost
indistinguishable from r = 1.0are probably due to features of
the junctions other than the SAM and its orbitals).
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Experimental details (materials and methods) and sample H
In the examination of compounds 1−4, as with other
compounds, the values of the standard deviation for
rectification are much less than for values of current density.
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
gwhitesides@gmwgroup.harvard.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the U.S. Department of Energy
(DE-SC0000989) for materials and for salary support (for
W.F.R. and F.S.). M.M.T. was supported by NSF Material
Research Science and Engineering Centre (DMR-0820484)
and a fellowship from the NanoScience and Engineering Centre
at Harvard University.
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