The rate of charge tunneling is insensitive to polar terminal groups in self-assembled monolayers in AgTSS(CH2) nM(CH2)mT//Ga2O3/EGaIn junctions

Article abstract of DOI:10.1021/ja409771u

This paper describes a physical-organic study of the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self-assembled monolayer (SAM)-based large-area junctions of the form Ag ^TSS(CH₂)_nM(CH₂)_mT// Ga₂O₃/EGaIn. Here Ag^TS is a template-stripped silver substrate, -M- and -T are "middle" and "terminal" functional groups, and EGaIn is eutectic gallium-indium alloy. Twelve uncharged polar groups (-T = CN, CO₂CH₃, CF₃, OCH ₃, N(CH₃)₂, CON(CH₃)₂, SCH₃, SO₂CH₃, Br, P(O)(OEt)₂, NHCOCH₃, OSi(OCH₃)₃), having permanent dipole moments in the range 0.5 < μ < 4.5, were incorporated into the SAM. A comparison of the electrical characteristics of these junctions with those of junctions formed from n-alkanethiolates led to the conclusion that the rates of charge tunneling are insensitive to the replacement of terminal alkyl groups with the terminal polar groups in this set. The current densities measured in this work suggest that the tunneling decay parameter and injection current for SAMs terminated in nonpolar n-alkyl groups, and polar groups selected from common polar organic groups, are statistically indistinguishable.

Full text of DOI:10.1021/ja409771u

Communication
pubs.acs.org/JACS
The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups
in Self-Assembled Monolayers in AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn
Junctions
Hyo Jae Yoon,† Carleen M. Bowers,† Mostafa Baghbanzadeh,† and George M. Whitesides*,†,‡,§
†Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United
States
‡Wyss Institute for Biologically Inspired Engineering, Harvard University, 60 Oxford Street, Cambridge, Massachusetts 02138, United
States
§Kavli Institute for Bionano Science & Technology, Harvard University, 29 Oxford Street, Cambridge, Massachusetts 02138, United
States
S
* Supporting Information
but some do not (Table S1 in the Supporting Information). We
ABSTRACT: This paper describes a physical-organic
study of the effect of uncharged, polar, functional groups
on the rate of charge transport by tunneling across self-
assembled monolayer (SAM)-based large-area junctions of
the form AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn. Here
AgTS is a template-stripped silver substrate, -M- and -T are
“middle” and “terminal” functional groups, and EGaIn is
eutectic gallium−indium alloy. Twelve uncharged polar
groups (-T = CN, CO2CH3, CF3, OCH3, N(CH3)2,
CON(CH3)2, SCH3, SO2CH3, Br, P(O)(OEt)2,
NHCOCH3, OSi(OCH3)3), having permanent dipole
moments in the range 0.5 < μ < 4.5, were incorporated
into the SAM. A comparison of the electrical character-
istics of these junctions with those of junctions formed
from n-alkanethiolates led to the conclusion that the rates
of charge tunneling are insensitive to the replacement of
terminal alkyl groups with the terminal polar groups in this
set. The current densities measured in this work suggest
that the tunneling decay parameter and injection current
for SAMs terminated in nonpolar n-alkyl groups, and polar
groups selected from common polar organic groups, are
statistically indistinguishable.
have, perhaps surprisingly, observed that the tunneling current
is insensitive (within the precision of our measurements) to the
structures of a range of nonpolar terminal aromatic and
aliphatic groups with different geometries and electronic
structures.7
This paper focuses on a specific physical-organic question:
Do molecular dipoles, particularly when placed at the top
interface between a thin, electrically insulating organic film (a
SAM) and a conducting top electrode, influence the rates of
charge transport by tunneling?27,28 To answer this question, we
examined the electrical characteristics of SAM-based large-area
junctions of the form AgTSS(CH2)nM(CH2)mT//Ga2O3/
EGaIn, where -M- is either -CONH- or -CH2CH2- (depending
on which was synthetically more accessible; replacing
-CH2CH2- with -CONH- does not influence tunneling current
densities in these compounds).13,29 The interface, which exists
between the top of the SAM and the rough Ga2O3 film that
covers the EGaIn in the “conical tip” electrode,13 is a van der
Waals contact. We systematically modified the terminal portion
of the SAM with polar groups (-T, Figure 1) that are uncharged
but have significant group dipole moments (0.5 < μ < 4.5), and
measured the current density (J, A/cm2) at low bias ( 0.5 V).
central goal in the field of molecular electronics is to
Aunderstand relationships between rates of charge transport
and molecular structure.1−13 Using self-assembled monolayer
(SAM)-based large-area junctions having the structure AgTSS-
(CH2)nM(CH2)mT//Ga2O3/EGaIn, where AgTS is a template-
stripped silver substrate and EGaIn is eutectic gallium−indium
alloy, we explored the influence of the “terminal” group, T, and
“middle” groups, M, of the SAM on the tunneling current. One
current focus for our work is the importance of the two
interfaces: AgTS-SR and T//Ga2O3.14 This paper focuses on the
latter interface and examines the influence of the group T on
the rate of charge transport.
Figure 1. Molecules with polar terminal groups and n-alkanethiols (as
standards) used to form SAMs.
Our results7,15−17 (and those obtained using other types of
junctions9,11,18−26) have not led to a single, broad conclusion
about this matter: a few groups T (e.g., ferrocene)16,17 seem to
change the rate of charge transport (relative to a methyl group),
Received: September 23, 2013
Published: December 18, 2013
© 2013 American Chemical Society
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Communication
We compared the current densities of these compounds with
those of hypothetical n-alkanethiolates of the same lengths
(estimated by a well-defined model: the simplified Simmons
equation, vide infra). Surprisingly, we found that the presence
of a range of molecular dipoles embedded in “simple” (e.g.,
non-redox-active) organic functional groups at the SAM//
Ga2O3 top interface does not significantly change the rate of
charge tunneling.
We estimated the molecular length (Å) of the target
molecules through modeling using ChemDraw 3D software,
assuming an extended, all-trans conformation (Figure 1). The
lengths of the molecules were calculated from the sulfur
headgroup to the most distal atom (hydrogen or other
elements) of the group T. The central strategy in this study
is to compare electrical characteristics of molecules with
different structures, regardless of length. The well-defined
values of β and Jo of n-alkanethiolates in the EGaIn-based
tunnel junctions make it possible to estimate the current
density for a hypothetical n-alkanethiolate of the same length as
the target molecule (with a length ranging from methyl to n-
octadecyl), and thus to compare current densities of target
molecules with those of hypothetical n-alkanes of the same
length. This strategy makes it possible to compare a number of
molecules with different structures and lengths, and SAMs of
different thicknesses, and avoids (or minimizes) laborious
syntheses of molecules with constant lengths.
The simplified Simmons equation (eq 1) is commonly used
to summarize measurements of rates of charge transport
J(V) = Jo(V) e−βd
(1)
through molecular junctions. The underlying assumption in this
equation is that the tunneling barrier is rectangular (or
approximately so). This assumption certainly does not describe
a SAM exactly, but the extent to which the obvious differences
between theory and reality are important in rationalizing trends
in J(V) with structure remains undefined. In eq 1, J(V) is
current density (current divided by the geometrical area
determined by a microscopy, A/cm2) at an applied voltage V.
Jo(V) is the injection current at an applied voltage V; it is the
current density for a hypothetical junction in which the SAM
has no thickness but the interfaces are those characteristic of a
SAM-contacting junction. The tunneling decay parameter β
(Å−1) contains information about the height of the barrier; it is
determined by the molecular structure of the backbone in the
SAM. The width of the barrier across which charges traverse is
d (Å).
We prepared SAMs on AgTS substrates, and formed and
characterized the molecular junctions with an “unflattened”
conical Ga2O3/EGaIn tip7,29 (see the Supporting Information
for more detail). Histograms (based on 342−548 J−V scans) of
values of J(V) exhibited approximately log-normal distributions
(Figure S1 in the Supporting Information); fitting Gaussian
curves to these histograms yielded mean values and standard
deviations (σlog) of log|J(V)|. Table S2 in the Supporting
Information summarizes the data from the junction measure-
ments. Mean and median values for the histograms of J(V)
were indistinguishable (this observation indicates that outliers
do not distort the mean values calculated using Gaussian curve
fit to the data36).
Comparisons of the tunneling rates of SAMs with the
structure S(CH2)nM(CH2)mT, and of standards with T = -CH3
in Figure 2A, led to the conclusion that current densities (log|
Jpolar|) for the S(CH2)nM(CH2)mT were statistically indistin-
Our study on the rates of charge transport is a physical-
organic study: that is, one focusing on trends in current
densities as a function of variations in the structures of
molecules, rather than on absolute values of these measure-
ments. Values of β for n-alkanethiolates have been widely
reproduced (across many users and different research groups)
and provide a reproducible internal standard.13
Molecular dipoles, introduced into the SAMs by molecules
having polar functional groups, are readily dissipated or
canceled by disorder in the SAMs and by interactions with
the electrodes and/or with neighboring molecules. Several
studies30−35 previously reported depolarization in SAMs
formed with dipolar molecules: for example, Gershevitz et
al.34 reported that changes in molecular dipoles in the SAM are
due to changes in molecular conformation and order of the
molecular layers.
guishable from current densities (log|JCH |) for n-alkanethiolate
3
standards of the same (hypothetical) thicknesses; log|JCH | was
3
estimated through substitution of the estimated length (Å) of a
polar molecule, S(CH2)nM(CH2)mT, into the algebraic
equation of the linear least-squares fit for n-alkanethiolate
standards. The difference in log|J| (Δlog|J| = log|Jpolar| − log|
JCH |) was ≤0.5, or less than a factor of 3 in |J|. The typical range
3
of σlog observed in the EGaIn-based junctions is ∼0.1−0.5,
which corresponds to Δ|J| ≤ ×3: two values of J(V) less than
×6 apart (Δlog|J(V)| ≤ 0.8) cannot therefore be distinguished
without other information.
The rate of charge tunneling through AgTSS(CH2)nM-
(CH2)mT//Ga2O3/EGaIn junctions was independent of the
structure of polar functional groups T (1−14, Figure 1), and
there was no correlation between the molecular dipole of T and
variations in the values of log|J| (Figure 2B). The polar
functional groups T also had no effect on the symmetry of J−V
curves: the rectification ratios (r,r = |J(+V)|/|J(−V)|) were
∼1.0−1.3 (Table S2 in the Supporting Information), regardless
of the structure or dipole moment of T. Similar values are
observed for n-alkanethiolates. We regard these values as
indicating “no rectification”.
The absence of rectification is an interesting result. We
imagined that the application of a strong applied electric field
(ca. 250 MV/m, assuming that the distance between top and
bottom electrodes is 2 nm at 0.5 V) might orient (at least
partially) the dipoles at the top interface at one polarity, leading
We emphasize that the association of a dipole moment with a
terminal group T does not translate into an electrostatic field
perpendicular to the SAM at the top interface. Antiparallel
orientation of the polar groups can result in an effective
cancellation of dipolar fields. The effect of the imposed
electrical potential and electrostatic field on this orientation of
polar groups in the SAM is also not known.
Figure 1 shows the polar molecules we examined. The choice
of the polar terminal groups allowed us to avoid the complexity
−
of ionization state (e.g., CO2H versus CO2 ) and ambiguities
arising from the counterions (e.g., CO2−Na+ versus
CO2 H3O+). For the backbone, the electrical equivalence
−
(insignificant differences in the rates of charge transport)
between internal (the M region) -CH2CH2- and -CONH-
groups allowed us to use synthetically accessible, amide-
containing compounds.7,29
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Communication
Molecular dipoles at the SAM//Ga2O3 top interface do not
seem to influence the rates of charge tunneling. The empirical
correlation between “length” and tunneling current in Figure 2
demonstrates that the rate of charge tunneling is insensitive to
the presence of molecular dipoles in uncharged, terminal
organic groups in junctions of the form AgTSS(CH2)nM-
(CH2)mT//Ga2O3/EGaIn. The interpretation of this observa-
tion is ambiguous, for at least three reasons:
(i) Complicated structural features of SAMs make it difficult
to relate rates of charge transport to the conformation of
dipoles at the SAM//Ga2O3/interface. Although it is
straightforward to estimate the magnitude of the dipole
moment of an isolated functional group, understanding the
electrostatic character of the interface between the SAM-bound
dipole group and the surface of the Ga2O3 is complicated
(especially when the magnitude of the applied electrical field
250 MV/mis high). The fact that the direction of the group
dipole is constrained (if not fixed) by the structure of the
molecule, but the magnitude of the tunneling current is
unchanged by reversing the polarity of the field (i.e., the
junction does not rectify), suggests both that the local dipoles
(in the range of 0.5 < μ < 4.5) do not influence tunneling
currents and that any ordering of them by intermolecular or
external field effects is not reflected in changes in charge
tunneling that we can detect.
(ii) The physical/mechanical structure of the SAM//Ga2O3
interface is not established. Another factor that complicates the
analysis of interfacial phenomena in these (and other) junctions
is an incomplete understanding of the nature of the path
followed by the charge. In the junctions we use, the evidence
indicates that only a small fraction (perhaps 10−4) of the
apparent geometrical area of contact of the top (Ga2O3/EGaIn)
junction is in electrical contact with the SAM.13 Although this
number is compatible with measures of contact from other
solid−solid electrical and mechanical interfaces,13 and although
it is quite reproducible (as judged by the values of σlog in Table
S2 in the Supporting Information), it leaves open the detailed
atomic-level nature of these electrically effective regions.
(iii) The contribution of the SAM//Ga2O3 interface to the
topography of the tunneling barrier remains undefined. A final
question about the interpretation of the data in Figure 2
concerns theoretical expectations. The hypothesis on which this
work was based is that a change in the magnitude of molecular
dipoles placed at the terminus of the SAM might influence the
shape of the tunneling barrier, and hence the magnitude of the
tunneling current. We infer experimentally that there is no
observable change using our technique. We cannot, however,
infer that there is no change in the tunneling barrier, or in the
electronic structure of the interfaces on changes in the group T,
only that any changes in them do not influence observed
tunneling currents.
Figure 2. (A) Plot of log|J| at −0.5 V for compounds with the structure
S(CH2)nM(CH2)mT as a function of calculated length (from sulfur to
the distal hydrogen atom closest to the EGaIn top electrode). (B) Plot
of the difference in log|J| (Δlog|J| = log|Jpolar| − log|JCH |) between
3
S(CH2)nM(CH2)mT and hypothetical n-alkanethiolate (estimated
from the simplified Simmons equation, eq 1) at −0.5 V as a function
of molecular dipole for the model structure (CH3T or CH3CH2T).
The solid line in (A) represents a linear square fit of mean values of
log|J| for n-alkanethiolates (standards in Figure 1). For the standards, β
≈ 0.71 0.04 Å−1 and Jo ≈ 104.0 0.3 A/cm2; the dotted lines represent
the range of Δlog|J| = 0.5 (corresponding to about a factor of 3
variation in J). Dipole moments were obtained from ref 37 (see the
Supporting Information).
to the generation of a local electric field and a decrease in the
net electric field gradient and thus, perhaps, to rectification. The
lack of rectification, regardless of the polarity of the dipole,
suggests that such a local electric field (if it is generated) at the
top interface has no detectable influence (by our measure) on r.
The current density measurements for the cyano- (CN, 1
and 13) and methyl ester- (CO2CH3, 2 and 11) terminated
compounds (Figure 2A) give approximate (because of the small
numbers of points) information about the response of the
tunneling decay parameter, β, and the injection current, Jo, to
the presence of terminal polar groups. Mean values of J(V) for
two different lengths of the cyano compound and two different
The absence of an atomically detailed theory to guide the
design and interpretation of measurements is presently a
limitation in the field of molecular electronics. Early efforts to
add local detail to simple barriers (for example, the early, very
stimulating prediction by Aviram and Ratner28 concerning the
role of embedded molecular dipoles on rectification) so far
have no experimental support. Authentic rectification has been
observed across SAMs terminated in ferrocene groups,16,17 but
the origin of the rectification involves a change in mechanism
(from tunneling to hopping/tunneling), not electrostatic
influences due to molecular dipoles. At present, no theory
has successfully predicted what kind of changeeither in the
lengths of the methyl ester compound provided approximate
values of β ≈ 0.7
comparison, n-alkanethiolates (M = -CH2CH2 , T = -CH3)
showed β = 0.71 0.04 Å−1 and Jo = 104.0 0.3 A/cm2. Values of
β and Jo for SAMs terminated in alkanes and polar groups were
thus statistically indistinguishable.
The similarity in Jo between polar and nonpolar (-CH3)
terminal groups indicates that seemingly large changes in the
composition and electrostatic character at the SAM//Ga2O3
top interface (a van der Waals interface) do not change the
shape of the tunneling barrier associated with SAM-based
junctions sufficiently to influence the rate of charge transport.
1.5
0.2 Å−1 and Jo ≈ 103.8
A/cm2. For
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Journal of the American Chemical Society
Communication
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The conclusion from this work is that dipoles from a number
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amides (-CONH- or -NHCO-) in the interior (M)7,29 of a
SAM, have no (or small, i.e., less than a factor of 3) effect on
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summarized in Table S1) which suggest that the structure of
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From these and previous studies we infer the following: (i)
Familiar organic terminal groups linked at the terminal (T)
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shape of the tunneling barrier to change the rate of charge
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complexes9,16,17may influence J(V) significantly, but the
mechanism may involve hopping rather than pure tunneling.
(iii) Dipolar SAMs with much smaller bandgaps (e.g., highly
conjugated aromatic systems) than those tested here might
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seems not to be a primary determinant of the electrical
characteristics of the junction.
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ASSOCIATED CONTENT
* Supporting Information
■
S
(24) Gergel-Hackett, N.; Aguilar, I.; Richter, C. A. J. Phys. Chem. C
2010, 114, 21708.
(25) Cohen, Y. S.; Vilan, A.; Ron, I.; Cahen, D. J. Phys. Chem. C 2009,
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Detailed experimental procedures, spectroscopic data for all
new compounds, histograms of current densities, and summary
of junction measurements. This material is available free of
charge via the Internet at http://pubs.acs.org.
(26) Wang, G.; Kim, T.-W.; Jang, Y. H.; Lee, T. J. Phys. Chem. C
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AUTHOR INFORMATION
Corresponding Author
gwhitesides@gmwgroup.harvard.edu
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Conference on Nanoscience and Nanotechnology; Brisbane, Qld.,
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D.; Yoon, H. J.; Whitesides, G. M. J. Am. Chem. Soc. 2012, 134, 10876.
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(31) Cornil, D.; Olivier, Y.; Geskin, V.; Cornil, J. Adv. Funct. Mater.
2007, 17, 1143.
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■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a subcontract from Northwestern
University from the U.S. Department of Energy (DE-
SC0000989) for work on synthesis of materials and measure-
ments of charge transport. The salary for H.J.Y. and C.M.B. is
supported by the DOE fund of Northwestern University; the
salary for C.M.B. is also supported by the Bill and Melinda
Gates Foundation under award 51308.
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