Odd-even effects in charge transport across n-alkanethiolate-based SAMs

Article abstract of DOI:10.1021/ja509436k

This paper compares rates of charge transport across self-assembled monolayers (SAMs) of n-alkanethiolates having odd and even numbers of carbon atoms (n_odd and n_even) using junctions with the structure M^TS/SAM//Ga₂O₃/EGaIn (M = Au or Ag). Measurements of current density, J(V), across SAMs of n-alkanethiolates on Au^TS and Ag^TS demonstrated a statistically significant odd-even effect on Au^TS, but not on Ag^TS, that could be detected using this technique. Statistical analysis showed the values of tunneling current density across SAMs of n-alkanethiolates on Au^TS with n_odd and n_even belonging to two separate sets, and while there is a significant difference between the values of injection current density, J₀, for these two series (log|J_0Au,even| = 4.0 ± 0.3 and log|J_0Au,odd| = 4.5 ± 0.3), the values of tunneling decay constant, β, for n_odd and n_even alkyl chains are indistinguishable (β_Au,even = 0.73 ± 0.02 ?^-1, and β_Au,odd= 0.74 ± 0.02 ?^-1). A comparison of electrical characteristics across junctions of n-alkanethiolate SAMs on gold and silver electrodes yields indistinguishable values of β and J₀ and indicates that a change that substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has no influence on the injection current density across SAMs of n-alkanethiolates.

Full text of DOI:10.1021/ja509436k

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Article
Odd-Even Effects in Charge Transport across n-Alkanethiolate-Based SAMs
Mostafa Baghbanzadeh, Felice C. Simeone, Carleen M. Bowers, Kung-Ching Liao, Martin Thuo,
Mahdi Baghbanzadeh, Michael S Miller, Tricia Breen Carmichael, and George M Whitesides
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja509436k • Publication Date (Web): 07 Nov 2014
Downloaded from http://pubs.acs.org on November 12, 2014
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Odd-Even Effects in Charge Transport across n-Alkanethiolate-Based SAMs
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Mostafa Baghbanzadeh,a Felice C. Simeone,a Carleen M. Bowers,a KungꢀChing Liao,a Martin
Thuo,a Mahdi Baghbanzadeh,b Michael S. Miller,c Tricia Breen Carmichael,c and George M.
Whitesidesa,d*
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aDepartment of Chemistry and Chemical Biology, Harvard University,
12 Oxford Street, Cambridge, MA 02138, United States
bDepartment of Statistics, Shiraz University, Shiraz, Iran
cDepartment of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue,
Windsor, Ontario, Canada, N9B3P4
dWyss Institute of Biologically Inspired Engineering, 60 Oxford Street, Cambridge, MA 02138,
United States
* To whom correspondence may be addressed: gwhitesides@gmwgroup.harvard.edu
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ABSTRACT
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This paper compares rates of charge transport across selfꢀassembled monolayers (SAMs) of nꢀ
alkanethiolates having odd and even numbers of carbon atoms (nodd and neven) using junctions
with the structure MTS/SAM//Ga2O3/EGaIn (M = Au or Ag). Measurements of current density,
J(V), across SAMs of nꢀalkanethiolates on AuTS and AgTS demonstrated a statistically significant
oddꢀeven effect on AuTS, but not on AgTS, that could be detected using this technique. Statistical
analysis showed the values of tunneling current density across SAMs of nꢀalkanethiolates on
AuTS with nodd and neven belong to two separate sets, and while there is a significant difference
between the values of injection current density, J0, for these two series (log|J0Au,even| = 4.0±0.3
and log|J0Au,odd| = 4.5±0.3), the values of tunneling decay constant, β, for nodd and neven alkyl
chains are indistinguishable (βAu,even = 0.73±0.02 Åꢀ1, and βAu,odd= 0.74±0.02 Åꢀ1). A comparison
of electrical characteristics across junctions of nꢀalkanethiolate SAMs on gold and silver
electrodes yields indistinguishable values of β and J0, and indicates that a change that
substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM) has
no influence on the injection current density across SAMs of nꢀalkanethiolates.
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Keywords: Oddꢀeven effect; tunneling; interface; selfꢀassembled monolayer; EGaIn, molecular
electronics, charge transport
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INTRODUCTION
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Understanding charge tunneling through thin molecular films is important to
understanding the movement of charge in organic insulators, and more broadly in nanoscience.1ꢀ7
Using selfꢀassembled monolayer (SAM)ꢀbased largeꢀarea junctions with the structure
AgTS/S(CH2)nT//Ga2O3/EGaIn (one type of soꢀcalled “EGaIn junction”),8,9 we were able to study
the influence of the changes in the structure of the SAM on the tunneling current density across
it.10ꢀ14 (Here AgTS is a templateꢀstripped silver substrate; S(CH2)n is the nꢀalkanethiolate SAM
covalently bonded to the AgTS substrate, T is the terminal functional group; and EGaIn is the
liquid alloy of gallium and indium, covered with a thin, electrically conductive surface oxide—
mostly Ga2O3—that forms spontaneously upon exposure to air.15,16) Those studies showed that
alterations in the structure of the insulating organic layer, S(CH2)nT,‒‒for example, changing the
anchoring group of the SAMs10 or introducing polar organic groups either into the backbone of a
polyethylene chain11 or at the van der Waals T//Ga2O3 interface12ꢀ14‒‒do not influence the rates
of charge transport at a level that is statistically significant (less than a factor of three). In
surprising contrast, we and others observed that the addition of one CH2 group to the alkyl
chain‒‒a change that, in essence, simply interchanges an exposed terminal methyl group for an
ethyl group‒‒appears to be sufficient to make a small but statistically significant difference in
the electronic properties of the junction.17ꢀ20 This phenomenon is one manifestation of the soꢀ
called oddꢀeven effect,21 and its origin is a continuing puzzle in understanding SAMꢀbased
tunneling junctions.
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In this work, we focused on three specific questions about the oddꢀeven effect: i) Is there
indeed‒‒when examined using current technology‒‒a systemic difference in the rate of charge
transport across SAMs of nꢀalkanethiolates (SCn) with an odd number of carbons (nodd) and
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SAMs with an even number of carbons (neven)? (Much of the historical evidence for oddꢀeven
effects in charge transport has come from earlier and less accurate studies,17 and from studies of
phenomena such as wetting, which are of unknown relevance to quantum tunneling.21) ii) If there
is an oddꢀeven effect, then, mechanistically, how can such a small change in the structure of a
SAM influence tunneling probabilities? iii) How does changing the substrate supporting the
SAM from Au to Ag—a change that also changes the tilt angle of nꢀalkanethiolates as well as
properties such as wetting and work function of electrode—influence tunneling current density
across SAMꢀbased largeꢀarea junctions?
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There are three plausible explanations for the observed oddꢀeven effect in the rates of
charge transport: i) Changes in the nature of the van der Waals interface between the SAM and
the top electrode—changes that are presumed to be responsible for the oddꢀeven effect in
wetting21—might also influence the ability of the SAM and the Ga2O3 to “wet” and conform to
one another, or to condense a water film, or to adsorb adventitious contaminants at the interface.
ii) Differences in rates of charge transport might reflect the differences in the orientation of the
terminal group at the interface of the SAM with the Ga2O3 (e.g., CH2CH3 vs. CH3, Figure S2),
which might influence the strength of electronic coupling between the HOMOs of the nꢀalkyl
groups and the Ga2O3. iii) There might be a systematic difference in the thickness of the SAM
between odd and even nꢀalkanethiolates. (The tunneling path might have a throughꢀspace
component and thus be sensitive to the thickness, independently of the length of the nꢀalkyl
chain.) A fourth (in principle) possibility—differences in the electric dipole moment along the
surface normal, which would be induced by the oddꢀeven differences in the orientation of the
terminal ethyl (‒CH2CH3) group—seems unlikely to be important since the difference is
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undoubtedly small, and introducing even large dipoles into the interface has little or no
effect.12,13
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In our previous studies, although we were able to identify an oddꢀeven effect in tunneling
current density, J (A/cm2) at ±0.5 V (i.e., the results from SAMs with neven and nodd belonged to
separate data sets and demanded separate fits to eq. 1), we were not able—because the logꢀ
standard deviation (σlog) in those studies was large (σlog = 0.6ꢀ1.4) relative to those we now
generate (σlog = 0.3ꢀ0.6)—to determine the mechanism of the oddꢀeven effect, or to establish
whether there was a statistically significant difference in the two parameters, β, and, J0, of the
simplified Simmons equation (eq. 1).17 In Eq. 1, J(V) is current density (current divided by the
geometrical contact area; A/cm2) at an applied voltage V.22,23
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J(V) = J0(V)eꢀβd = J010ꢀβd/2.303
(1)
In this study, as in prior work, we have focused on trends in current densities as a
function of the number of methylene (ꢀCH2ꢀ) units in SAMs of structure M(Au or
Ag)/SCn//Ga2O3/EGaIn, rather than on absolute values of these measurements. We have
described previously details of the procedure we used.15 Here we used soꢀcalled “flattened”
EGaIn tips, V = ±0.5V and templateꢀstripped (TS) Ag and Au substrates (Figure 1).8,9,15 The
fabrication of the conical tip—a process that stretches the EGaIn until it breaks—results in a
rough Ga2O3 surface. Using “selected”14 or “flattened”15 tips yield data with slightly narrower
distribution than tips with rougher surfaces. We do not know if flattening influences the
thickness of the oxide.
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Figure 1. Schematic structure of a AuTS/SAM//Ga2O3/EGaIn junction. Here, d represents
the length of the tunneling barrier established by the alkyl chains in the SAM. Assuming
a throughꢀbond mechanism, d coincides with the length of the alkyl chain. J0 is defined as
the current density across a hypothetical junction where d = 0.
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Surfaces of Au and Ag have similar work functions (ФAu = 4.9 eV and ФAg = 4.4 eV).24
The tilt angle of the backbone from the surface normal for nꢀalkanethiolate monolayers on
Au(111) (~30°) is larger than that on Ag(111) (~10°) (Figure S2).25 Accordingly, comparing the
rate of charge transport across the nꢀalkanethiolate SAMs on Au and Ag substrates makes it
possible to separate the influence of several characteristics of a SAM—especially the tilt angle
(and thus the relative thickness of the SAM along the surface normal) of the SAM, and the SAM
packing density (21.7 Å2/molecule on Au and 19.1 Å2/molecule on Ag),26 on rates of tunneling.
In particular, we hoped to answer two questions in this comparative study: i) Does the charge
move along the shortest distance between the two electrodes (a throughꢀspace path)? or ii) does
the charge follow the molecular orbitals along the backbone, regardless of the tilt of the nꢀalkyl
group (a throughꢀbond path)?27,28 Density of chains, tilt angle, and the properties of the interface
(T//Ga2O3, T = —CH3, —CH2CH3) are obviously convoluted, and no simple comparison of
SAMs on Au and Ag will distinguish them all unambiguously; our results do, nonetheless, both
allow some possibilities to be excluded, and help to frame questions for future study.
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We observed a statistically significant oddꢀeven effect in the rates of tunneling across
SAMs of nꢀalkanethiolate on AuTS, where nꢀalkanethiolates with nodd gave higher current
densities (J(V)) than did SAMs of nꢀalkanethiolate with neven. We show that the tunneling current
densities across SAMs of nꢀalkanethiolates with nodd and neven on AuTS fit in separate sets of data,
when analyzed using the simplified Simmons model (eq. 1); that is, analyzing the two series
separately led to more consistent interpretations of the results than analyzing them together. In
contrast, charge tunneling across SAMs of nꢀalkanethiolate on AgTS does not show an oddꢀeven
effect that we can detect in either
β or J0.
BACKGROUND
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Odd-Even Effects in Tunneling across SAMs. The oddꢀeven effect in SAMs has been
observed (and reviewed21) in a number of properties, including wetting by liquids,29ꢀ32 molecular
packing,33 and electronic,19 frictional ,34 and electrochemical behaviors.35 Oddꢀeven effects have
been studied in the surface treatment of dielectric materials for pentaceneꢀbased fieldꢀeffect
transistors,36 and in the rate of charge transport across carbonꢀchainꢀbased molecular devices.20
Oddꢀeven alternation in rectification of tunneling current density has also been reported.18 Using
EGaInꢀbased top electrodes in molecular junctions, we have studied oddꢀeven effects in charge
transport across nꢀalkanethiolate SAMs on AgTS substrates.17,11 Using statistical tools, we
showed that the data for current densities for SAMs with neven and nodd belong to separate data
sets.37
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Theory of Charge Tunneling across SAMs. SAMs of alkanes are considered to be good
electrical insulators for current transport at low bias (we use ±0.5 V), and charge transport takes
place by hole tunneling.38,39 When such molecules are placed between two electrodes, the charge
transport has usually been interpreted (in the absence of a more detailed model) in terms of a
rectangular barrier using Eq. 1.22 The injection current density at an applied voltage V, J0(V),
gives, ideally, the current flowing across a hypothetical junction in which the SAM has zero (d =
0) thickness, but the characteristic of the interfaces between the SAM and the electrodes remain
the same as those when there is a SAM. The tunneling decay constant β (Åꢀ1) contains, according
to the Simmons model, information about the shape of the tunneling barrier, which, in principle,
is determined by the molecular/electronic structure of the backbone in the SAM. The width of
the barrier across which charges move is d (Å), and is usually assumed to be determined by the
molecular structure, not the shortest path between the two electrodes, but this assumption is not
based either on solid empirical or theoretical grounds.
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RESULTS AND DISCUSSION
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We collected J(V) using “flattened” conical tips. The procedures for formation and use of
these flattened tips are described in detail elsewhere.15 (See the Supporting Information for
details of experimental design.)
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Charge Transport across n-Alkanethiolates on AgTS and AuTS Substrates. Values of J are
approximately logꢀnormally distributed (Figures S3 and S4; Supporting Information). For each
SAM, we could fit, by nonlinear leastꢀsquares fitting,37 Gaussian curves to the distributions of
log|J|. Figures S3 and S4 show the histograms for the values of log|J| (J, Acmꢀ2) for SAMs of nꢀ
alkanethiolates on AgTS and AuTS. Table S1 summarizes the values of log|J| at V = +0.5 V for
SAMs of nꢀalkanethiolates (SCn) on AgTS and AuTS with both odd and/or even numbers of
carbons (nodd and neven). Table 1 summarizes the values for injection current density, log|J0|, and
tunneling decay coefficient, β, for the nꢀalkanethiolates with neven and nodd on both Ag and Au
substrates. Figure 2 shows plots of linear regression of log|J| versus molecular length (number of
carbons) for SAMs of nꢀalkanethiolate on Ag (Figure 2a) and Au (Figure 2b) with neven and nodd.
Figure S5 shows plots of log|J|, at V = +0.5V, versus d(Å) for different lengths of nꢀ
alkanethiolates on both AgTS and AuTS substrates. We calculated the molecular length from the
sulfur atom to the distal hydrogen atom of the final methyl group in contact with Ga2O3 (Figure
1). Despite the potential for differences in the structural order for SAMs with short (n < 9) and
long chains (n > 9),40 we do not detect these differences in the current density or the yields of
working junctions. The values of J0 and β for SAMs of alkanethiolate on AgTS with neven are in
agreement with our report using flattened Ga2O3/EGaIn conical tips.15 The β value (0.74 Åꢀ1) for
nꢀalkanethiolates with neven on AuTS is in good agreement with reported values of β (0.6ꢀ1 Åꢀ1)
across a wide range of measurement techniques.2
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Table 1. Values for injection current density, log|J0|, and tunneling decay coefficient, β,
for the nꢀalkanethiolates with neven and nodd on both AgTS and AuTS substrate.
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Length (Å)b
log|J0|
Length (nC)c
Ma/odd or even
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ꢀ1
β (Åꢀ1)
log|J0|
3.4±0.3
3.3±0.3
3.6±0.3
4.1±0.3
β (nC )
AgTS/even
AgTS/odd
AuTS/even
AuTS/odd
3.7±0.3
3.6±0.3
4.0±0.3
4.5±0.3
0.73±0.02
0.71±0.02
0.73±0.02
0.74±0.02
0.93±0.02
0.91±0.02
0.93±0.02
0.94±0.02
aThe metal substrate. bMolecular length from the sulfur atom to the distal hydrogen atom of the
final methyl group in contact with Ga2O3 (Figure 1). cLength based on the number of carbons of
SCn.
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Figure 2. log|J| at V = +0.5V versus number of carbons for SAMs of nꢀalkanethiolates (SCn) on
a) AgTS and b) AuTS. Solid line (for nꢀalkanethiolate SAMs with neven) and dashed line (for nꢀ
alkanethiolate SAMs with nodd) represent the linear regression analyses which gives β/2.303
(slopes) and log|J0| (intercepts at d = 0). Error bars represent the standard deviation for the log|J|
values (0.1≥σlog≥ 0.6).
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For SAMs on AuTS, junctions comprising SAMs of nꢀalkanethiolates with nodd
consistently showed higher values of current densities than their analogs with neven (Figures 2b
and 3). Based on a careful statistical analysis, we concluded, with 95% probability, that there is a
significant difference between two regression lines (Figure 2b). We determined that the
difference in the values of the intercepts (log|J0|) of the two lines is statistically significant, but
the slopes (β) are indistinguishable (for details of statistical analysis; see Supporting
Information). We did not, however, observe any significant difference in the trends of log|J| (i.e.,
oddꢀeven alternation) for SAMs of nꢀalkanethiolates with neven or nodd on AgTS (Figure 2a).
Statistical analysis, similar to that carried out for SAMs on AuTS, for SAMs of nꢀalkanethiolates
with neven or nodd on AgTS showed that the two lines are indistinguishable and values of the
injection current density, log|J0|, and of tunneling decay coefficient, β, for the nꢀalkanethiolates
with neven were indistinguishable from log|J0|, and β for nꢀalkanethiolates with nodd.
For SAMs on AuTS, values of log|J| of most of the nꢀalkanethiolate SAMs with neven (SCn)
were within 0.1 of the values of log|J| of their next homolog with nodd (SCn+1) (Figure 3a). Figure
3b shows that the ꢁlog|J| (ꢁlog|J| = log|Jobserved| ꢀ log|Jfitted|) for SAMs with nodd are constantly ≤ 0
while ꢁlog|J| for SAMs with neven are constantly ≥ 0.
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To examine the statistical difference between the tunneling current density values, log|J|,
of SAMs of nꢀalkanethiolates on AuTS we calculated the 95% confidence intervals (CI) for the
whole set of log|J| values (for details see SI). Error bars in Figure 3 show the 95% CI for the
log|J| values on AuTS substrate (i.e., the smaller the overlap of bars, or the larger the gap between
bars, the stronger the evidence for a true difference).41 Figure 3b shows an interesting
phenomenon; that is, although the oddꢀeven alternation can be observed for the whole series of
SCn (n = 5ꢀ18), it is only statistically significant for 10≤n≤14. We rationalize this difference as a
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reflection of the conclusion—drawn largely from spectroscopic studies42—that SAMs with
longer SAMs (n ≥ 10) are crystalline and well ordered, while those with 9 ≥ n are disordered,
probably because of weak interactions between the shorter alkyl chains. The SAMs with long
chain—n ≥ 15—may be less ordered and have chains with gauche configurations (similar
observations have been reported for some other types of oddꢀeven effect18,43). Our data are
consistent with suggestions that the magnitude of oddꢀeven effect depends on the degree of order
of the SAM.
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One of the factors that, potentially, can influence the quality of the SAM is the incubation
time (i.e., the time that metal substrate stayed in a solution of the thiols (1 mM in toluene)). To
examine the effect of incubation time, we prepared SAMs of SC10 and SC11 on both AgTS and
AuTS substrates with incubations of 3 h, 1 day, 2 day, and 7 day. The rate of charge tunneling for
SAMs formed over 3 h is not distinguishable from the 7ꢀday sample (Table S2).
Comparison between Au and Ag. To examine the influence of geometry of SAMs on tunneling
transport, we compared J(V) of nꢀalkanethiolates on AuTS and AgTS. Figure 4 shows plots of
log|J| at V= +0.5 versus molecular length (in number of carbons) for nꢀalkanethiolate SAMs with
neven and nodd on both AuTS and AgTS. Statistical analysis, similar to that which we have carried
out for the oddꢀeven effect, showed that SAMs with neven on both AuTS and AgTS result in
indistinguishable values of log|J0|and β (Table 1, Figure 4a). When, however, we compared the
tunneling current density between SAMs with nodd on AgTS and AuTS, we observed a significant
difference in values of log|J0|, while the values of β were indistingushable (Table 1, Figure 4b).
These comprisons lead to two important conclusions:
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Figure 3. Oddꢀeven effect in charge transport across nꢀalkanethiolates on AuTS. a) Plot of
log|J| at V = +0.5V versus number of carbons for SAMs of nꢀalkanethiolates on AuTS ( SCn,
n=5ꢀ18). The solid line represents the linear regression analysis to all the data. Error bars
represent 95% CI for the log|J| values. b) Difference (ꢁlog|J|) between the observed current
density (log|Jobserved|) and the calculated current density (log|Jfitted|) based on the regression line
(ꢁlog|J| = log|Jobserved| ꢀ log|Jfitted|). Error bars represent standard deviation (light dotted) and
95% CI (solid) for the log|J| values.
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i) They indicate that replacing Au/SR with Ag/SR (for SAMs with neven)—a change that
substantially alters the tilt angle of the alkyl chain (and, therefore, the thickness of the SAM)29—
had no influence (to the precision that we can measure) on the injection current density across
these SAMs (Figure 4a).
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ii) The oddꢀeven effect in charge tunneling across the SAMs of nꢀalkanethiolates is related
to the orientation of the terminal methyl group. This study showed that SAMs of nꢀ
alkanethiolates with neven on AuTS behave, in EGaIn junctions, similarly to the SAMs with neven
and nodd on AgTS, and that only SAMs with nodd on AuTS conduct differently (Figure 4 and Table
1). The significant structural difference between SAMs with nodd on AuTS with the other SAMs
studied in this comparison is the orientation of terminal ethyl group: for SAMs with neven on
AuTS, and SAMs with neven and nodd on AgTS, the surface consists predominantly of methyl
groups. Based on highꢀresolution electron energy loss spectroscopy (HREELS) studies,43 the
terminal CH2ꢀCH3 moiety of the SAMs with nodd on AuTS is tilted away from the surface normal,
producing a surface that is composed of a mixture of methyl and methylene groups (that is, an
“ethyl” rather than “methyl” surface29) (Figure S2). The same behavior was reported in wetting
properties of SAMs on Ag and Au; SAMs of nꢀalkanethiolate on Ag showed no29 or slight44 oddꢀ
even effect; the effect was more distinguishable on Au.45
An alternative hypothesis that might rationalize the fact that no oddꢀeven effect is observed
on AgTS could be that there are differences in the order of the SAMs on these substrates, with
SAMs on AgTS being more disordered than SAMs on AuTS. Differences in these SAMs are due
primarily to differences in the tilt angle of the monolayers, and to the resulting differences in the
density of molecules per unit area. Since the templateꢀstripped metal substrates have similar
values of surface roughness, we do not expect significant differences in the degree of order
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between SAMs on AgTS and AuTS due to surface roughness. Moreover, the narrow distribution of
data, high yields in nonꢀshorting junctions, and good reproducibility46 on AgTS substrates provide
empirical evidence that there is no difference between the order of the SAMs on AgTS and AuTS.
CONCLUSION
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The Odd-Even Effect in Charge Transport by Tunneling Is Real. Our statistical
analysis for SAMs of nꢀalkanethiolates on AuTS showed i) the values of tunneling current density
across SAMs of nꢀalkanethiolates with nodd and neven belong to two separate data sets, and ii)
while there is a significant difference between the values of J0 for these two series, the values of
β for nodd and neven alkyl chains were indistinguishable. We conclude that there is an oddꢀeven
effect in tunneling transport across SAMs on AuTS; an oddꢀeven effect, however, was not
observable (at the precision that we could measure) for the series on AgTS.
The Geometry of the n-Alkyl Chain Determines the Odd-Even Effect. The contrast of
junction measurements between SAMs of nꢀalkanethiolates on gold (where we observed an oddꢀ
even effect) and on silver (where we did not observe a significant trend) indicates that the oddꢀ
even effect in tunneling transport corresponds to the composition (“methyl” vs. “ethyl”) of the
surface of the SAM (Figure S2).
The Tunneling Path Is “along Single Molecule” rather than “across Multiple
Molecules”. Our results demonstrate that although a SAM on gold is thinner than one on silver,
the values of J0 and β for SAMs of nꢀalkanethiolates with neven on AuTS and AgTS are statistically
indistinguishable (Figure 4a). We suggest—based on this observation—that charge carriers
travel through the backbone of the alkyl chain in the SAM, rather than across the shortest (and
thus across multiple molecules of nꢀalkanethiolate) distance between the two electrodes.
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Figure 4. log|J| at V = +0.5V versus number of carbons for SAMs of nꢀalkanethiolates a) with an
even number of carbons on AuTS and AgTS and b) with an odd number of carbons on AuTS and
AgTS. Solid line (for nꢀalkanethiolate SAMs on AgTS) and dashed line (for nꢀalkanethiolate
SAMs AuTS) represent the linear regression analyses which give β/2.303 (slopes) and log|J0|
(intercepts at d = 0). Error bars represent the standard deviation for the log|J| values.
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The SAM//Ga2O3 Interface Is Responsible for the Odd-Even Effect on AuTS. Since
the value of J0 reflects the characteristics of the interfaces between the SAM and the electrodes,
rather than the shape of the most insulating part of the tunneling barrier, significant differences in
values of injection current densities, J0, and indistinguishable values of tunneling decay, β, for
SAMs of nꢀalkanethiolates with nodd and neven imply that the oddꢀeven effect is influenced by
some property of the interfaces (e.g., Au/S or SAM//Ga2O3). We suggest that a determining
contribution of the SAM//Ga2O3 interface to oddꢀeven effects is more plausible than one for the
Au/S interface for two reasons: i) Our previous studies,10,47 and the current study (Figure 4a),
showed that changing the bottom interface does not significantly influence the tunneling current
densities. ii) Results of this study indicate that changing the terminal group from CH3 to CH2CH3
correlates with a change in the injection current density (Figures 2b and 4b).
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The Origin of the Odd-Even Effect Is not Completely Defined by Currently
Available Data. While we were able to identify the oddꢀeven effect with respect to J, β, and J0,
between nꢀalkanethiols with nodd and neven, we cannot pinpoint the origin of the observed oddꢀ
even effect on AuTS. We can, however, identify and assess four possibilities (although—based on
our current and previous studies—some possibilities are less plausible than others).
(i)
Differences in the Orientation of the Terminal (T) Group at the Ga2O3 Interface
(e.g., CH2CH3 vs. CH3). A larger exposure of terminal CH2CH3 group at the surface of the nꢀ
alkanethiolate SAMs with nodd results in larger van der Waals interactions (by changing the
polarizability of surface and increasing the area of contact) between the monolayer and the
EGaIn tip, and may consequently give rise to a difference in J(V) (similar to differences observed
in wetting of SAMs on Au21). If the orientation of terminal CH2−CH3 bonds is responsible for
both the oddꢀeven effect in charge transport, and the oddꢀeven effect in wetting, then increasing
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the van der Waals interactions as a result of different surface polarization is a possible origin of
the former. (These changes in van der Waals interactions could influence the contact resistance
at the SAM//Ga2O3 interface; however, we did not observe this influence in the distribution of
current density (Figures S3 and S4)). This hypothesis—that the oddꢀeven effect in charge
tunneling across SAMs terminating in ethyl and methyl groups reflects polarizability and van der
Waals interactions—is not easily reconciled with the fact that introducing groups such as
halogens or aromatics into this interface does not produce large differences in J0 in our
experiments.12,13 (Cahen and others have reported much larger differences with specific terminal
halogen substituents when using a mercury top electrode.48)
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Larger exposure of terminal CH2CH3 groups at the surface of the SAMs with nodd on
AuTS might also result in direct interaction between the EGaIn electrode and the CH2 groups,
which are adjacent to the terminal CH3 groups. This direct interaction would reduce the length of
the molecule between both electrodes effectively by one CꢀC bond (excluding the top CH3). This
hypothesis would also explain why this effect is not observed on the Ag substrate where the
surface consists predominantly of methyl groups, and the tilt is too little to expose CH2 group to
the moleculeꢀEGaIn interface. We do not know if there is a van der Waals contact between the
CH2 of the terminal ethyl group and the Ga2O3, and, thus, cannot accept or reject this hypothesis.
(ii)
Systematic Alternations in Energy of the Frontier Orbital Energies (i.e.
HOMO/LUMO Levels) of Odd and Even nꢀAlkanethiolates. Difference in the orientation of the
terminal group at the SAM interface between the Ga2O3 (CH2CH3 vs. CH3, Figure S2) might
influence the strength of electronic coupling between the HOMOs of the nꢀalkyl groups and the
Ga2O3. This hypothesis seems less plausible when we consider the fact that the HOMO in these
SAMs is concentrated at the AuꢀS interface, with little amplitude at the SAM/Ga2O3 interface.49
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We do not, however, have a sound theoretical understanding of the CH3 (or CH2CH3)//Ga2O3
interface.
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Systematic Differences in the Thickness of the SAM between Odd and Even nꢀ
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Alkanethiolates. Naively, it seemed that SAMs on gold with nodd might be thinner than SAMs
with neven. Our calculation of thicknesses of odd and even SAMs suggests that there is no
significant (<0.5 Å) systematic alternation in the thickness between nodd and neven. Figure S6b (in
the SI) shows an essentially monotonic increase in the thickness for nꢀalkanethiolate monolayers
with a tilt angle of 30° on AuTS. These calculations suggest that an oddꢀeven variation in
thickness is not the cause of the oddꢀeven alternation in J(V) (Figure S7). In addition, by
comparing the rate of tunneling across SAMs of nꢀalkanethiolates on Au and Ag, we have shown
empirically that charge transport is insensitive to the thickness of the SAMs (as measured along
the perpendicular to the mean plane of the surface of the metal.). The value of β for SAMs of nꢀ
alkanethiolates on AuTS and AgTS is indistinguishable; this observation indicates that tunneling
current densities are insensitive to the difference in thickness. We conclude that there is no
evidence to support a contribution of the thickness of the SAM to the existence of the oddꢀeven
effect.
(iv)
Difference in the Dipole along the Surface Normal, which might Correlate with
OddꢀEven Differences in the Orientation of the Terminal CH3 Group (Figure S2). Different
orientations of the terminal methyl group in nodd and neven could cause an oddꢀeven alternation in
the dipole along the surface normal,50 which might contribute to an oddꢀeven effect through an
influence on the work function of the surface.19,50 Although we have no direct evidence to prove
or exclude this hypothesis, our previous studies12 13 showed that introducing even a group with a
,
large dipole moment at the SAM//Ga2O3 interface does not change the tunneling barrier enough
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to influence the rate of charge transport; thus, we think this very small difference in dipole along
surface normal seems unlikely to be important. To be proved or excluded, this hypothesis,
however, needs more investigation (e.g., using a series of odd and even SAMs having terminal
polar groups such as CF3 or OCH3 that introduce a dipole at the interface but do not change the
topography of the SAM).
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If There Is an Observable Odd-Even Effect for n-Alkanes, Why Was There no
Detectable Difference in much Larger Changes in Terminal Groups for SAMs? This paper
confirms that there is an oddꢀeven effect for nꢀalkanethiolates, but that it is small (a factor of ~3
when appropriately compared, or ꢁlogJ0 ~ 0.5), and only reliably detectable given the
availability of a substantial number of structurally related compounds, with measurements taken
carefully, and in parallel. Previous papers have shown that a range of groups with quite different
polarities, degrees of saturation, composition, and polarizability have a surprisingly small
influence on tunneling current densities.13 These two sets of observations seem at odds.
The problem in comparing these data is that they are not, strictly speaking, comparable.
Most of our studies of terminal groups did show variability, but the variation (which might have
been a factor of 10 across of series of groups) was not designed to detect differences as small as
those characterizing the oddꢀeven effect, and would require comparing groups known to have
different sizes and shapes. In those studies, we compared the current densities of compounds
having polar terminal groups (R) with those of hypothetical nꢀalkanethiolates of the same length.
If those comparisons demonstrate that the difference in log|J| (ꢁlog|J| = log|Jpolar| − log| JCH3|)
was ≤ 0.5, or less than a factor of three in |J|, we considered the compound with polar terminal
group and nꢀalkanethiol to be indistinguishable. In the current study, however, we did not
compare two single data points, but studied a series of compounds (S(CH2)nH, n = 5ꢀ18) and
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analyzed the results based on eq. 1. Although the ꢁlog|J| for each adjacent pair of nꢀ
alkanethiolate was ≤ 0.4 (Figure 3b), analyzing the two series separately led to more consistent
interpretations of the results than did analyzing them together (neven and nodd produce two
statistically distinguishable sets of data).
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The surprise in these studies was thus that substituting, for example, a terminal methyl
group for an amide, or a terminal cyclohexyl group with a phenyl, produced a change in current
density (J(V)) of approximately a factor of 4.0 (∆log|J(0.5V)| = 0.6),12 and that this change was
smaller than we had—perhaps naively— expected, while substituting a terminal methyl for a
terminal ethyl produced a change of a factor of 2.5 (∆log|J(0.5V)| = 0.4), which was larger than
we had expected. The precision of the former studies was, however, substantially less than the
latter, and the studies of polar and aromatic groups, as they were designed, would not have been
able to detect the subtle differences characterizing the odd/even effect. Differences in size,
composition, and other factors restricted our ability to make meaningful comparisons of current
densities with ∆log|J(0.5V)| ≤ 0.6.
So, what is the origin of the oddꢀeven effect? Although this paper cannot answer the
question unambiguously, it has eliminated a number of possibilities (differences in the thickness
of the SAM, certain differences in the HOMOs for the SAMs), and leaves a focus on the
contribution of the nonꢀcovalent interface between the SAM and the Ga2O3 film on the EGaIn
electrode. This interface has been characterized empirically by studies such as those described in
this paper, but have not yet been included in relevant theoretical calculations. In particular, the
height of the barrier to tunneling is probably high in the region where there is only physical
contact (relative to the rest of the SAM), but the width of this region is small. Understanding the
mechanism of charge transport across this interface, and its contribution to overall tunneling
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currents, remains an unresolved problem with this junction (and, in fact, with all junctions
involving SAMs as insulating layers between two conducting electrodes).
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Supporting Information. The experimental procedures, synthesis characterization of the
alkanethiols, histograms of J(V) data, and statistical analysis details. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMAIONS
Corresponding Author
*Eꢀmail: gwhitesides@gmwgroup.harvard.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The work at Harvard University was supported by a subcontract from Northwestern University
from the United States Department of Energy (DOE, DEꢀSC0000989). The DOE grant from
Northwestern also supported the salary for M.B., C.M.B, and K.ꢀC.L. Aspects of this work
(SAM preparation and characterization) was supported by the US Department of Energy,
Division of Materials Sciences, under Award No. DEꢀFG02ꢀ00ER45852.
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